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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/6630-0.txt b/6630-0.txt new file mode 100644 index 0000000..6ada29d --- /dev/null +++ b/6630-0.txt @@ -0,0 +1,5558 @@ +The Project Gutenberg eBook of Curiosities of the Sky, by Garrett P. Serviss + +This eBook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online at +www.gutenberg.org. If you are not located in the United States, you +will have to check the laws of the country where you are located before +using this eBook. + +Title: Curiosities of the Sky + +Author: Garrett P. Serviss + +Release Date: January 6, 2003 [eBook #6630] +[Most recently updated: December 27, 2022] + +Language: English + +Character set encoding: UTF-8 + +Produced by: William McClain + +*** START OF THE PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY *** + + + + +[Illustration] + + + + +Curiosities of the Sky + +by Garrett P. Serviss + + + + +Contents + + PREFACE + CURIOSITIES OF THE SKY + I. The Windows of Absolute Night + II. Star-Clouds, Star-Clusters, and Star-Streams + III. Stellar Migrations + IV. The Passing of the Constellations + V. Conflagrations in the Heavens + VI. Explosive and Whirling Nebulæ + VII. The Banners of the Sun + VIII. The Zodiacal Light Mystery + IX. Marvels of the Aurora + X. Strange Adventures of Comets + XI. Meteors, Fire-Balls, and Meteorites + XII. The Wrecking of the Moon + XIII. The Great Mars Problem + XIV. The Riddle of the Asteroids + + + + +[Illustration: Great spiral nebula in Andromeda] + + + + +Illustrations + + Great spiral nebula in Andromeda + The Milky Way + Star-cluster in Hercules + Great southern star-cluster, Omega Centauri + The Pleiades + The “Great Dipper” + Cassiopeia + The “Northern Crown” + The “Southern Cross” + Chart showing location of Tycho’s star, 1572, and Nova Persei of 1901 + Nova Persei, with its nebular rings + Lord Rosse’s nebula + Wonderful spiral in triangulum + Spiral in Ursa Major + Nebula in Cetus + The Orion nebula + The Corona + A solar “prominence” + Auroral beams seen in England + Auroral arches of an eliptic form seen in the Arctic regions + Auroral curtain seen in Scandinavia + Auroral arches seen in Scandinavia + Swift’s comet + Daniels’ comet + Brooks’ comet + Curious forms of meteorite trains + Section of the atmosphere up to 100 kilometers + A meteor photographed in flight + Looking across Coon Butte crater from northern rim + Trail on south side, Coon Butte crater + The craters Clavius, Longomontanus, Tycho, etc. + Western part of the Mare Serenitatis + Mare Tranquilitatis and surroundings + Lunar craters Theophilus and surrounding region + Mare Crisium + Schiaparelli’s chart of Mars, showing the so-called system of canals + + + + +PREFACE + + +What Froude says of history is true also of astronomy: it is the most +impressive where it transcends explanation. It is not the mathematics +of astronomy, but the wonder and the mystery that seize upon the +imagination. The calculation of an eclipse owes all its prestige to the +sublimity of its data; the operation, in itself, requires no more +mental effort than the preparation of a railway time-table. + +The dominion which astronomy has always held over the minds of men is +akin to that of poetry; when the former becomes merely instructive and +the latter purely didactic, both lose their power over the imagination. +Astronomy is known as the oldest of the sciences, and it will be the +longest-lived because it will always have arcana that have not been +penetrated. + +Some of the things described in this book are little known to the +average reader, while others are well known; but all possess the +fascination of whatever is strange, marvelous, obscure, or +mysterious—magnified, in this case, by the portentous scale of the +phenomena. + +The idea of the author is to tell about these things in plain language, +but with as much scientific accuracy as plain language will permit, +showing the wonder that is in them without getting away from the facts. +Most of them have hitherto been discussed only in technical form, and +in treatises that the general public seldom sees and never reads. + +Among the topics touched upon are: + +The strange unfixedness of the “fixed stars,” the vast migrations of +the suns and worlds constituting the universe. + +The slow passing out of existence of those collocations of stars which +for thousands of years have formed famous “constellations,” preserving +the memory of mythological heroes and heroines, and perhaps of +otherwise unrecorded history. + +The tendency of stars to assemble in immense clouds, swarms, and +clusters. + +The existence in some of the richest regions of the universe of +absolutely black, starless gaps, deeps, or holes, as if one were +looking out of a window into the murkiest night. + +The marvelous phenomena of new, or temporary, stars, which appear as +suddenly as conflagrations, and often turn into something else as +eccentric as themselves. + +The amazing forms of the “whirlpool,” “spiral,” “pinwheel,” and “lace,” +or “tress,” nebulæ. + +The strange surroundings of the sun, only seen in particular +circumstances, but evidently playing a constant part in the daily +phenomena of the solar system. + +The mystery of the Zodiacal Light and the Gegenschein. + +The extraordinary transformations undergone by comets and their tails. + +The prodigies of meteorites and masses of stone and metal fallen from +the sky. + +The cataclysms that have wrecked the moon. + +The problem of life and intelligence on the planet Mars. + +The problematical origin and fate of the asteroids. + +The strange phenomena of the auroral lights. + + +An attempt has been made to develop these topics in an orderly way, +showing their connection, so that the reader may obtain a broad general +view of the chief mysteries and problems of astronomy, and an idea of +the immense field of discovery which still lies, almost unexplored, +before it. + + + + +CURIOSITIES OF THE SKY + + + + +I +The Windows of Absolute Night + + +To most minds mystery is more fascinating than science. But when +science itself leads straight up to the borders of mystery and there +comes to a dead stop, saying, “At present I can no longer see my way,” +the force of the charm is redoubled. On the other hand, the illimitable +is no less potent in mystery than the invisible, whence the dramatic +effect of Keats’ “stout Cortez” staring at the boundless Pacific while +all his men look at each other with a wild surmise, “silent upon a peak +in Darien.” It is with similar feelings that the astronomer regards +certain places where from the peaks of the universe his vision seems to +range out into endless empty space. He sees there the shore of his +little isthmus, and, beyond, unexplored immensity. + +The name, “coal-sacks,” given to these strange voids is hardly +descriptive. Rather they produce upon the mind the effect of blank +windows in a lonely house on a pitch-dark night, which, when looked at +from the brilliant interior, become appalling in their rayless murk. +Infinity seems to acquire a new meaning in the presence of these black +openings in the sky, for as one continues to gaze it loses its purely +metaphysical quality and becomes a kind of entity, like the ocean. The +observer is conscious that he can actually _see_ the beginning of its +ebon depths, in which the visible universe appears to float like an +enchanted island, resplendent within with lights and life and gorgeous +spectacles, and encircled with screens of crowded stars, but with its +dazzling vistas ending at the fathomless sea of pure darkness which +encloses all. + +The Galaxy, or Milky Way, surrounds the borders of our island in space +like a stellar garland, and when openings appear in it they are, by +contrast, far more impressive than the general darkness of the +interstellar expanse seen in other directions. Yet even that expanse is +not everywhere equally dark, for it contains gloomy deeps discernable +with careful watching. Here, too, contrast plays an important part, +though less striking than within the galactic region. Some of Sir +William Herschel’s observations appear to indicate an association +between these tenebrious spots and neighboring star clouds and nebulæ. +It is an illuminating bit of astronomical history that when he was +sweeping the then virgin heavens with his great telescopes he was +accustomed to say to his sister who, note-book in hand, waited at his +side to take down his words, fresh with the inspiration of discovery: +“Prepare to write; the nebulæ are coming; here space is vacant.” + +The most famous of the “coal-sacks,” and the first to be brought to +general attention before astronomers had awakened to the significance +of such things, lies adjacent to the “Southern Cross,” and is truly an +amazing phenomenon. It is not alone the conspicuousness of this +celestial vacancy, opening suddenly in the midst of one of the richest +parts of the Galaxy, that has given it its fame, but quite as much the +superstitious awe with which it was regarded by the early explorers of +the South Seas. To them, as well as to those who listened in rapt +wonder to their tales, the “Coal-sack” seemed to possess some occult +connection with the mystic “Cross.” In the eyes of the sailors it was +not a vacancy so much as a sable reality in the sky, and as, +shuddering, they stared at it, they piously crossed themselves. It was +another of the magical wonders of the unknown South, and as such it +formed the basis of many a “wild surmise” and many a sea-dog’s yarn. +Scientific investigation has not diminished its prestige, and today no +traveler in the southern hemisphere is indifferent to its fascinating +strangeness, while some find it the most impressive spectacle of the +antarctic heavens. + +All around, up to the very edge of the yawning gap, the sheen of the +Milky Way is surpassingly glorious; but there, as if in obedience to an +almighty edict, everything vanishes. A single faint star is visible +within the opening, producing a curious effect upon the sensitive +spectator, like the sight of a tiny islet in the midst of a black, +motionless, waveless tarn. The dimensions of the lagoon of darkness, +which is oval or pear-shaped, are eight degrees by five, so that it +occupies a space in the sky about one hundred and thirty times greater +than the area of the full moon. It attracts attention as soon as the +eye is directed toward the quarter where it exists, and by virtue of +the rarity of such phenomena it appears a far greater wonder than the +drifts of stars that are heaped around it. Now that observatories are +multiplying in the southern hemisphere, the great austral “Coal-sack” +will, no doubt, receive attention proportioned to its importance as one +of the most significant features of the sky. Already at the Sydney +Observatory photographs have shown that the southern portion of this +Dead Sea of Space is not quite “bottomless,” although its northern part +defies the longest sounding lines of the astronomer. + +There is a similar, but less perfect, “coal-sack” in the northern +hemisphere, in the constellation of “The Swan,” which, strange to say, +also contains a well-marked figure of a cross outlined by stars. This +gap lies near the top of the cross-shaped figure. It is best seen by +averted vision, which brings out the contrast with the Milky Way, which +is quite brilliant around it. It does not, however, exercise the same +weird attraction upon the eye as the southern “Coal-sack,” for instead +of looking like an absolute void in the sky, it rather appears as if a +canopy of dark gauze had been drawn over the stars. We shall see the +possible significance of this appearance later. + + +[Illustration: The Milky Way. Region near M.S. Photographed by +Professor Barnard] + + +Just above the southern horizon of our northern middle latitudes, in +summer, where the Milky Way breaks up into vast sheets of nebulous +luminosity, lying over and between the constellations Scorpio and +Sagittarius, there is a remarkable assemblage of “coal-sacks,” though +none is of great size. One of them, near a conspicuous star-cluster in +Scorpio, M80, is interesting for having been the first of these strange +objects noted by Herschel. Probably it was its nearness to M80 which +suggested to his mind the apparent connection of such vacancies with +star-clusters which we have already mentioned. + +But the most marvelous of the “coal-sacks” are those that have been +found by photography in Sagittarius. One of Barnard’s earliest and most +excellent photographs includes two of them, both in the star-cluster +M8. The larger, which is roughly rectangular in outline, contains one +little star, and its smaller neighbor is lune-shaped—surely a most +singular form for such an object. Both are associated with curious dark +lanes running through the clustered stars like trails in the woods. +Along the borders of these lanes the stars are ranked in parallel rows, +and what may be called the bottoms of the lanes are not entirely dark, +but pebbled with faint stellar points. One of them which skirts the two +dark gaps and traverses the cluster along its greatest diameter is +edged with lines of stars, recalling the alignment of the trees +bordering a French highway. This _road of stars_ cannot be less than +many billions of miles in length! + +All about the cluster the bed of the Galaxy is strangely disturbed, and +in places nearly denuded, as if its contents had been raked away to +form the immense stack and the smaller accumulations of stars around +it. The well-known “Trifid Nebula” is also included in the field of the +photograph, which covers a truly marvelous region, so intricate in its +mingling of nebulæ, star-clusters, star-swarms, star-streams, and dark +vacancies that no description can do it justice. Yet, chaotic as it +appears, there is an unmistakable suggestion of unity about it, +impressing the beholder with the idea that all the different parts are +in some way connected, and have not been fortuitously thrown together. +Miss Agnes M. Clerke made the striking remark that the dusky lanes in +M8 are exemplified on the largest scale in the great rift dividing the +Milky Way, from Cygnus in the northern hemisphere all the way to the +“Cross” in the southern. Similar lanes are found in many other +clusters, and they are generally associated with flanking rows of +stars, resembling in their arrangement the thick-set houses and villas +along the roadways that traverse the approaches to a great city. + +But to return to the black gaps. Are they really windows in the +star-walls of the universe? Some of them look rather as if they had +been made by a shell fired through a luminous target, allowing the eye +to range through the hole into the void space beyond. If science is +discretely silent about these things, what can the more venturesome and +less responsible imagination suggest? Would a huge “runaway sun,” like +Arcturus, for instance, make such an opening if it should pass like a +projectile through the Milky Way? It is at least a stimulating inquiry. +Being probably many thousands of times more massive than the galactic +stars, such a stellar missile would not be stopped by them, though its +direction of flight might be altered. It would drag the small stars +lying close to its course out of their spheres, but the ultimate +tendency of its attraction would be to sweep them round in its wake, +thus producing rather a star-swarm than a vacancy. Those that were very +close to it might be swept away in its rush and become its satellites, +careering away with it in its flight into outer space; but those that +were farther off, and they would, of course, greatly outnumber the +nearer ones, would tend inward from all sides toward the line of +flight, as dust and leaves collect behind a speeding motor (though the +forces operating would be different), and would fill up the hole, if +hole it were. A swarm thus collected should be rounded in outline and +bordered with a relatively barren ring from which the stars had been +“sucked” away. In a general sense the M8 cluster answers to this +description, but even if we undertook to account for its existence by a +supposition like the above, the black gaps would remain unexplained, +unless one could make a further draft on the imagination and suggest +that the stars had been thrown into a vast eddy, or system of eddies, +whose vortices appear as dark holes. Only a maelstrom-like motion could +keep such a funnel open, for without regard to the impulse derived from +the projectile, the proper motions of the stars themselves would tend +to fill it. Perhaps some other cause of the whirling motion may be +found. As we shall see when we come to the spiral nebulæ, gyratory +movements are exceedingly prevalent throughout the universe, and the +structure of the Milky Way is everywhere suggestive of them. But this +is hazardous sport even for the imagination—to play with _suns_ as if +they were but thistle-down in the wind or corks in a mill-race. + +Another question arises: What is the thickness of the hedge of stars +through which the holes penetrate? Is the depth of the openings +proportionate to their width? In other words, is the Milky Way round in +section like a rope, or flat and thin like a ribbon? The answer is not +obvious, for we have little or no information concerning the relative +distances of the faint galactic stars. It would be easier, certainly, +to conceive of openings in a thin belt than in a massive ring, for in +the first case they would resemble mere rifts and breaks, while in the +second they would be like wells or bore-holes. Then, too, the fact that +the Milky Way is not a _continuous_ body but is made up of stars whose +actual distances apart is great, offers another quandary; persistent +and sharply bordered apertures in such an assemblage are _a priori_ as +improbable, if not impossible, as straight, narrow holes running +through a swarm of bees. + +The difficulty of these questions indicates one of the reasons why it +has been suggested that the seeming gaps, or many of them, are not +openings at all, but opaque screens cutting off the light from stars +behind them. That this is quite possible in some cases is shown by +Barnard’s later photographs, particularly those of the singular region +around the star Rho Ophiuchi. Here are to be seen somber lanes and +patches, apparently forming a connected system which covers an immense +space, and which their discoverer thinks may constitute a “dark +nebula.” This seems at first a startling suggestion; but, after all, +why should their not be dark nebulæ as well as visible ones? In truth, +it has troubled some astronomers to explain the luminosity of the +bright nebulæ, since it is not to be supposed that matter in so diffuse +a state can be incandescent through heat, and phosphorescent light is +in itself a mystery. The supposition is also in accord with what we +know of the existence of dark solid bodies in space. Many bright stars +are accompanied by obscure companions, sometimes as massive as +themselves; the planets are non-luminous; the same is true of meteors +before they plunge into the atmosphere and become heated by friction; +and many plausible reasons have been found for believing that space +contains as many obscure as shining bodies of great size. It is not so +difficult, after all, then, to believe that there are immense +collections of shadowy gases and meteoric dust whose presence is only +manifested when they intercept the light coming from shining bodies +behind them. + +This would account for the apparent extinguishment of light in open +space, which is indicated by the falling off in relative number of +telescopic stars below the tenth magnitude. Even as things are, the +amount of light coming to us from stars too faint to be seen with the +naked eye is so great that the statement of it generally surprises +persons who are unfamiliar with the inner facts of astronomy. It has +been calculated that on a clear night the total starlight from the +entire celestial sphere amounts to one-sixtieth of the light of the +full moon; but of this less than one-twenty-fifth is due to stars +separately distinguished by the eye. If there were no obscuring medium +in space, it is probable that the amount of starlight would be +noticeably and perhaps enormously increased. + +But while it seems certain that some of the obscure spots in the Milky +Way are due to the presence of “dark nebulæ,” or concealing veils of +one kind or another, it is equally certain that there are many which +are true apertures, however they may have been formed, and by whatever +forces they may be maintained. These, then, are veritable windows of +the Galaxy, and when looking out of them one is face to face with the +great mystery of infinite space. _There_ the known universe visibly +ends, but manifestly space itself does not end there. It is not within +the power of thought to conceive an end to space, for the instant we +think of a terminal point or line the mind leaps forward to the +_beyond._ There must be space outside as well as inside. Eternity of +time and infinity of space are ideas that the intellect cannot fully +grasp, but neither can it grasp the idea of a limitation to either +space or time. The metaphysical conceptions of hypergeometry, or +fourth-dimensional space, do not aid us. + +Having, then, discovered that the universe is a thing _contained_ in +something indefinitely greater than itself; having looked out of its +windows and found only the gloom of starless night outside—what +conclusions are we to draw concerning the beyond? It _seems_ as empty +as a vacuum, but is it really so? If it be, then our universe is a +single atom astray in the infinite; it is the only island in an ocean +without shores; it is the one oasis in an illimitable desert. Then the +Milky Way, with its wide-flung garland of stars, is afloat like a tiny +smoke-wreath amid a horror of immeasurable vacancy, or it is an +evanescent and solitary ring of sparkling froth cast up for a moment on +the viewless billows of immensity. From such conclusions the mind +instinctively shrinks. It prefers to think that there is _something_ +beyond, though we cannot see it. Even the universe could not bear to be +alone—a Crusoe lost in the Cosmos! As the inhabitants of the most +elegant château, with its gardens, parks, and crowds of attendants, +would die of loneliness if they did not know that they have neighbors, +though not seen, and that a living world of indefinite extent surrounds +them, so we, when we perceive that the universe has limits, wish to +feel that it is not solitary; that beyond the hedges and the hills +there are other centers of life and activity. Could anything be more +terrible than the thought of an _isolated universe?_ The greater the +being, the greater the aversion to seclusion. Only the infinite +satisfies; in that alone the mind finds rest. + +We are driven, then, to believe that the universal night which +envelopes us is not tenantless; that as we stare out of the star-framed +windows of the Galaxy and see nothing but uniform blackness, the fault +is with our eyes or is due to an obscuring medium. Since _our_ universe +is limited in extent, there must be _other_ universes beyond it on all +sides. Perhaps if we could carry our telescopes to the verge of the +great “Coal-sack” near the “Cross,” being then on the frontier of our +starry system, we could discern, sparkling afar off in the vast night, +some of the outer galaxies. They may be grander than ours, just as many +of the suns surrounding us are immensely greater than ours. If we could +take our stand somewhere in the midst of immensity and, with vision of +infinite reach, look about us, we should perhaps see a countless number +of stellar systems, amid which ours would be unnoticeable, like a +single star among the multitude glittering in the terrestial sky on a +clear night. Some might be in the form of a wreath, like our own; some +might be globular, like the great star-clusters in Hercules and +Centaurus; some might be glittering circles, or disks, or rings within +rings. If we could enter them we should probably find a vast variety of +composition, including elements unknown to terrestrial chemistry; for +while the _visible_ universe appears to contain few if any substances +not existing on the earth or in the sun, we have no warrant to assume +that others may not exist in infinite space. + +And how as to gravitation? We do not _know_ that gravitation acts +beyond the visible universe, but it is reasonable to suppose that it +does. At any rate, if we let go _its_ sustaining hand we are lost, and +can only wander hopelessly in our speculations, like children astray. +If the empire of gravitation is infinite, then the various outer +systems must have _some,_ though measuring by our standards an +imperceptible, attractive influence upon each other, for gravitation +never lets go its hold, however great the space over which it is +required to act. Just as the stars about us are all in motion, so the +starry systems beyond our sight may be in motion, and our system as a +whole may be moving in concert with them. If this be so, then after +interminable ages the aspect of the entire system of systems must +change, its various members assuming new positions with respect to one +another. In the course of time we may even suppose that our universe +will approach relatively close to one of the others; and then, if men +are yet living on the earth, they may glimpse through the openings +which reveal nothing to us now, the lights of another nearing star +system, like the signals of a strange squadron, bringing them the +assurance (which can be but an inference at present) that the ocean of +space has other argosies venturing on its limitless expanse. + +There remains the question of the luminiferous ether by whose agency +the waves of light are borne through space. The ether is as mysterious +as gravitation. With regard to ether we only infer its existence from +the effects which we ascribe to it. Evidently the ether must extend as +far as the most distant visible stars. But does it continue on +indefinitely in outer space? If it does, then the invisibility of the +other systems must be due to their distance diminishing the quantity of +light that comes from them below the limit of perceptibility, or to the +interposition of absorbing media; if it does not, then the reason why +we cannot see them is owing to the absence of a means of conveyance for +the light waves, as the lack of an interplanetary atmosphere prevents +us from hearing the thunder of sun-spots. (It is interesting to recall +that Mr Edison was once credited with the intention to construct a +gigantic microphone which should render the roar of sun-spots audible +by transforming the electric vibrations into sound-waves). On this +supposition each starry system would be enveloped in its own globule of +ether, and no light could cross from one to another. But the +probability is that both the ether and gravitation are ubiquitous, and +that all the stellar systems are immersed in the former like clouds of +phosphorescent organisms in the sea. + +So astronomy carries the mind from height to greater height. Men were +long in accepting the proofs of the relative insignificance of the +earth; they were more quickly convinced of the comparative littleness +of the solar system; and now the evidence assails their reason that +what they had regarded as _the_ universe is only one mote gleaming in +the sunbeams of Infinity. + + + + +II +Star-Clouds, Star-Clusters, and Star-Streams + + +In the preceding chapter we have seen something of the strangely +complicated structure of the Galaxy, or Milky Way. We now proceed to +study more comprehensively that garlanded “Pathway of the Gods.” + +Judged by the eye alone, the Milky Way is one of the most delicately +beautiful phenomena in the entire realm of nature—a shimmer of silvery +gauze stretched across the sky; but studied in the light of its +revelations, it is the most stupendous object presented to human ken. +Let us consider, first, its appearance to ordinary vision. Its apparent +position in the sky shifts according to the season. On a serene, +cloudless summer evening, in the absence of the moon, whose light +obscures it, one sees the Galaxy spanning the heavens from north to +southeast of the zenith like a phosphorescent arch. In early spring it +forms a similar but, upon the whole, less brilliant arch west of the +zenith. Between spring and summer it lies like a long, faint, twilight +band along the northern horizon. At the beginning of winter it again +forms an arch, this time spanning the sky from east to west, a little +north of the zenith. These are its positions as viewed from the mean +latitude of the United States. Even the beginner in star-gazing does +not have to watch it throughout the year in order to be convinced that +it is, in reality, a great circle, extending entirely around the +celestial sphere. We appear to be situated near its center, but its +periphery is evidently far away in the depths of space. + +Although to the casual observer it seems but a delicate scarf of light, +brighter in some places than in others, but hazy and indefinite at the +best, such is not its appearance to those who study it with care. They +perceive that it is an organic whole, though marvelously complex in +detail. The telescope shows that it consists of stars too faint and +small through excess of distance to be separately visible. Of the +hundred million suns which some estimates have fixed as the probable +population of the starry universe, the vast majority (at least thirty +to one) are included in this strange belt of misty light. But they are +not uniformly distributed in it; on the contrary, they are arrayed in +clusters, knots, bunches, clouds, and streams. The appearance is +somewhat as if the Galaxy consisted of innumerable swarms of +silver-winged bees, more or less intermixed, some massed together, some +crossing the paths of others, but all governed by a single purpose +which leads them to encircle the region of space in which we are +situated. + +From the beginning of the systematic study of the heavens, the fact has +been recognized that the form of the Milky Way denotes the scheme of +the sidereal system. At first it was thought that the shape of the +system was that of a vast round disk, flat like a cheese, and filled +with stars, our sun and his relatively few neighbors being placed near +the center. According to this view, the galactic belt was an effect of +perspective; for when looking in the direction of the plane of the +disk, the eye ranged through an immense extension of stars which +blended into a glimmering blur, surrounding us like a ring; while when +looking out from the sides of the disk we saw but few stars, and in +those directions the heavens appeared relatively blank. Finally it was +recognized that this theory did not correspond with the observed +appearances, and it became evident that the Milky Way was not a mere +effect of perspective, but an actual band of enormously distant stars, +forming a circle about the sphere, the central opening of the ring +(containing many scattered stars) being many times broader than the +width of the ring itself. Our sun is one of the scattered stars in the +central opening. + +As already remarked, the ring of the Galaxy is very irregular, and in +places it is partly broken. With its sinuous outline, its pendant +sprays, its graceful and accordant curves, its bunching of masses, its +occasional interstices, and the manifest order of a general plan +governing the jumble of its details, it bears a remarkable resemblance +to a garland—a fact which appears the more wonderful when we recall its +composition. That an elm-tree should trace the lines of beauty with its +leafy and pendulous branches does not surprise us; but we can only gaze +with growing amazement when we behold _a hundred million suns imitating +the form of a chaplet!_ And then we have to remember that this form +furnishes the ground-plan of the universe. + +As an indication of the extraordinary speculations to which the mystery +of the Milky Way has given rise, a theory recently (1909) proposed by +Prof. George C. Comstock may be mentioned. Starting with the data +(first) that the number of stars increases as the Milky Way is +approached, and reaches a maximum in its plane, while on the other hand +the number of nebulæ is greatest outside the Milky Way and increases +with distance from it, and (second) that the Milky Way, although a +complete ring, is broad and diffuse on one side through one-half its +course—that half alone containing nebulæ—and relatively narrow and well +defined on the opposite side, the author of this singular speculation +avers that these facts can best be explained by supposing that the +invisible universe consists of two interpenetrating parts, one of which +is a chaos of indefinite extent, strewn with stars and nebulous dust, +and the other a long, broad but comparatively thin cluster of stars, +including the sun as one of its central members. This flat star-cluster +is conceived to be moving edgewise through the chaos, and, according to +Professor Comstock, it acts after the manner of a snow-plough sweeping +away the cosmic dust and piling it on either hand above and below the +plane of the moving cluster. It thus forms a transparent rift, through +which we see farther and command a view of more stars than through the +intensified dust-clouds on either hand. This rift is the Milky Way. The +dust thrown aside toward the poles of the Milky Way is the substance of +the nebulæ which abound there. Ahead, where the front of the +star-plough is clearing the way, the chaos is nearer at hand, and +consequently there the rift subtends a broader angle, and is filled +with primordial dust, which, having been annexed by the vanguard of the +star-swarm, forms the nebulæ seen only in that part of the Milky Way. +But behind, the rift appears narrow because there we look farther away +between dust-clouds produced ages ago by the front of the plough, and +no scattered dust remains in that part of the rift. + +In quoting an outline of this strikingly original theory the present +writer should not be understood as assenting to it. That it appears +bizarre is not, in itself, a reason for rejecting it, when we are +dealing with so problematical and enigmatical a subject as the Milky +Way; but the serious objection is that the theory does not sufficiently +accord with the observed phenomena. There is too much evidence that the +Milky Way is an organic system, however fantastic its form, to permit +the belief that it can only be a rift in chaotic clouds. As with every +organism, we find that its parts are more or less clearly repeated in +its ensemble. Among all the strange things that the Milky Way contains +there is nothing so extraordinary as itself. Every astronomer must many +times have found himself marveling at it in those comparatively rare +nights when it shows all its beauty and all its strangeness. In its +great broken rifts, divisions, and spirals are found the gigantic +prototypes of similar forms in its star-clouds and clusters. As we have +said, it determines the general shape of the whole sidereal system. +Some of the brightest stars in the sky appear to hang like jewels +suspended at the ends of tassels dropped from the Galaxy. Among these +pendants are the Pleiades and the Hyades. Orion, too, the “Mighty +Hunter,” is caught in “a loop of light” thrown out from it. The +majority of the great first-magnitude stars seem related to it, as if +they formed an inner ring inclined at an angle of some twenty degrees +to its plane. Many of the long curves that set off from it on both +sides are accompanied by corresponding curves of lucid stars. In a +word, it offers every appearance of structural connection with the +entire starry system. That the universe should have assumed the form of +a wreath is certainly a matter for astonishment; but it would have been +still more astonishing if it had been a cube, a rhomboid, or a +dodecahedron, for then we should have had to suppose that something +resembling the forces that shape crystals had acted upon the stars, and +the difficulty of explaining the universe by the laws of gravitation +would have been increased. + +From the Milky Way as a whole we pass to the vast clouds, swarms, and +clusters of stars of which it is made up. It may be, as some +astronomers hold, that most of the galactic stars are much smaller than +the sun, so that their faintness is not due entirely to the effect of +distance. Still, their intrinsic brilliance attests their solar +character, and considering their remoteness, which has been estimated +at not less than ten thousand to twenty thousand light-years (a +light-year is equal to nearly six thousand thousand million miles) +their actual masses cannot be extremely small. The minutest of them are +entitled to be regarded as real suns, and they vary enormously in +magnitude. The effects of their attractions upon one another can only +be inferred from their clustering, because their relative movements are +not apparent on account of the brevity of the observations that we can +make. But imagine a being for whom a million years would be but as a +flitting moment; to him the Milky Way would appear in a state of +ceaseless agitation—swirling with “a fury of whirlpool motion.” + +The cloud-like aspect of large parts of the Galaxy must always have +attracted attention, even from naked-eye observers, but the true +star-clouds were first satisfactorily represented in Barnard’s +photographs. The resemblance to actual clouds is often startling. Some +are close-packed and dense, like cumuli; some are wispy or mottled, +like cirri. The rifts and modulations, as well as the general outlines, +are the same as those of clouds of vapor or dust, and one notices also +the characteristic thinning out at the edges. But we must beware of +supposing that the component suns are thickly crowded as the particles +forming an ordinary cloud. They _look,_ indeed, as if they were matted +together, because of the irradiation of light, but in reality millions +and billions of miles separate each star from its neighbors. +Nevertheless they form real assemblages, whose members are far more +closely related to one another than is our sun to the stars around him, +and if we were in the Milky Way the aspect of the nocturnal sky would +be marvelously different from its present appearance. + +Stellar clouds are characteristic of the Galaxy and are not found +beyond its borders, except in the “Magellanic Clouds” of the southern +hemisphere, which resemble detached portions of the Milky Way. These +singular objects form as striking a peculiarity of the austral heavens +as does the great “Coal-sack” described in Chapter 1. But it is their +isolation that makes them so remarkable, for their composition is +essentially galactic, and if they were included within its boundaries +they would not appear more wonderful than many other parts of the Milky +Way. Placed where they are, they look like masses fallen from the great +stellar arch. They are full of nebulæ and star-clusters, and show +striking evidences of spiral movement. + +Star-swarms, which are also characteristic features of the Galaxy, +differ from star-clouds very much in the way that their name would +imply—_i.e.,_ their component stars are so arranged, even when they are +countless in number, that the idea of an exceedingly numerous +assemblage rather than that of a cloud is impressed on the observer’s +mind. In a star-swarm the separate members are distinguishable because +they are either larger or nearer than the stars composing a “cloud.” A +splendid example of a true star-swarm is furnished by Chi Persei, in +that part of the Milky Way which runs between the constellations +Perseus and Cassiopeia. This swarm is much coarser than many others, +and can be seen by the naked eye. In a small telescope it appears +double, as if the suns composing it had divided into two parties which +keep on their way side by side, with some commingling of their members +where the skirts of the two companies come in contact. + +Smaller than either star-clouds or star-swarms, and differing from both +in their organization, are star-clusters. These, unlike the others, are +found outside as well as inside the Milky Way, although they are more +numerous inside its boundaries than elsewhere. The term star-cluster is +sometimes applied, though improperly, to assemblages which are rather +groups, such, for instance, as the Pleiades. In their most +characteristic aspect star-clusters are of a globular shape—globes of +suns! A famous example of a globular star-cluster, but one not included +in the Milky Way, is the “Great Cluster in Hercules.” This is barely +visible to the naked eye, but a small telescope shows its character, +and in a large one it presents a marvelous spectacle. Photographs of +such clusters are, perhaps, less effective than those of star-clouds, +because the central condensation of stars in them is so great that +their light becomes blended in an indistinguishable blur. The beautiful +effect of the incessant play of infinitesimal rays over the apparently +compact surface of the cluster, as if it were a globe of the finest +frosted silver shining in an electric beam, is also lost in a +photograph. Still, even to the eye looking directly at the cluster +through a powerful telescope, the central part of the wonderful +congregation seems almost a solid mass in which the stars are packed +like the ice crystals in a snowball. + +The same question rises to the lips of every observer: How can they +possibly have been brought into such a situation? The marvel does not +grow less when we know that, instead of being closely compacted, the +stars of the cluster are probably separated by millions of miles; for +we know that their distances apart are slight as compared with their +remoteness from the Earth. Sir William Herschel estimated their number +to be about fourteen thousand, but in fact they are uncountable. If we +could view them from a point just within the edge of the assemblage, +they would offer the appearance of a hollow hemisphere emblazoned with +stars of astonishing brilliancy; the near-by ones unparalleled in +splendor by any celestial object known to us, while the more distant +ones would resemble ordinary stars. An inhabitant of the cluster would +not know, except by a process of ratiocination, that he was dwelling in +a globular assemblage of suns; only from a point far outside would +their spherical arrangement become evident to the eye. Imagine +fourteen-thousand fire-balloons with an approach to regularity in a +spherical space—say, ten miles in diameter; there would be an average +of less than thirty in every cubic mile, and it would be necessary to +go to a considerable distance in order to see them as a globular +aggregation; yet from a point sufficiently far away they would blend +into a glowing ball. + + +[Illustration: Star-cluster in Hercules +(Photographed with a two-foot reflector)] + + +Photographs show even better than the best telescopic views that the +great cluster is surrounded with a multitude of dispersed stars, +suggestively arrayed in more or less curving lines, which radiate from +the principle mass, with which their connection is manifest. These +stars, situated outside the central sphere, look somewhat like vagrant +bees buzzing round a dense swarm where the queen bee is sitting. Yet +while there is so much to suggest the operation of central forces, +bringing and keeping the members of the cluster together, the attentive +observer is also impressed with the idea that the whole wonderful +phenomenon may be _the result of explosion._ As soon as this thought +seizes the mind, confirmation of it seems to be found in the appearance +of the outlying stars, which could be as readily explained by the +supposition that they have been blown apart as that they have flocked +together toward a center. The probable fact that the stars constituting +the cluster are very much smaller than our sun might be regarded as +favoring the hypothesis of an explosion. Of their real size we know +nothing, but, on the basis of an uncertain estimate of their parallax, +it has been calculated that they may average forty-five thousand miles +in diameter—something more than half the diameter of the planet +Jupiter. Assuming the same mean density, fourteen thousand such stars +might have been formed by the explosion of a body about twice the size +of the sun. This recalls the theory of Olbers, which has never been +altogether abandoned or disproved, that the Asteroids were formed by +the explosion of a planet circulating between the orbits of Mars and +Jupiter. The Asteroids, whatever their manner of origin, form a ring +around the sun; but, of course, the explosion of a great independent +body, not originally revolving about a superior center of gravitational +force, would not result in the formation of a ring of small bodies, but +rather of a dispersed mass of them. But back of any speculation of this +kind lies the problem, at present insoluble: How could the explosion be +produced? (See the question of explosions in Chapters 6 and 14). + +Then, on the other hand, we have the observation of Herschel, since +abundantly confirmed, that space is unusually vacant in the immediate +neighborhood of condensed star-clusters and nebulæ, which, as far as it +goes, might be taken as an indication that the assembled stars had been +drawn together by their mutual attractions, and that the tendency to +aggregation is still bringing new members toward the cluster. But in +that case there must have been an original condensation of stars at +that point in space. This could probably have been produced by the +coagulation of a great nebula into stellar nuclei, a process which +seems now to be taking place in the Orion Nebula. + + +[Illustration: Great southern star-cluster, Omega Centauri] + + +A yet more remarkable globular star-cluster exists in the southern +hemisphere, Omega Centauri. In this case the central condensation of +stars presents an almost uniform blaze of light. Like the Hercules +cluster, that in Centaurus is surrounded with stars scattered over a +broad field and showing an appearance of radial arrangement. In fact, +except for its greater richness, Omega Centauri is an exact duplicate +of its northern rival. Each appears to an imaginative spectator as a +veritable “city of suns.” Mathematics shrinks from the task of +disentangling the maze of motions in such an assemblage. It would seem +that the chance of collisions is not to be neglected, and this idea +finds a certain degree of confirmation in the appearance of “temporary +stars” which have more than once blazed out in, or close by, globular +star-clusters. + +This leads up to the notable fact, first established by Professor +Bailey a few years ago, that such clusters are populous with variable +stars. Omega Centauri and the Hercules cluster are especially +remarkable in this respect. The variables found in them are all of +short period and the changes of light show a noteworthy tendency to +uniformity. The first thought is that these phenomena must be due to +collisions among the crowded stars, but, if so, the encounters cannot +be between the stars themselves, but probably between stars and meteor +swarms revolving around them. Such periodic collisions might go on for +ages without the meteors being exhausted by incorporation with the +stars. This explanation appears all the more probable because one would +naturally expect that flocks of meteors would abound in a close +aggregation of stars. It is also consistent with Perrine’s +discovery—that the globular star clusters are powdered with minute +stars strewn thickly among the brighter ones. + +In speaking of Professor Comstock’s extraordinary theory of the Milky +Way, the fact was mentioned that, broadly speaking, the nebulæ are less +numerous in the galactic belt than in the comparatively open spaces on +either side of it, but that they are, nevertheless, abundant in the +broader half of the Milky Way which he designates as the front of the +gigantic “plough” supposed to be forcing its way through the enveloping +chaos. In and around the Sagittarius region the intermingling of nebulæ +and galactic star clouds and clusters is particularly remarkable. That +there is a causal connection no thoughtful person can doubt. We are +unable to get away from the evidence that a nebula is like a +seed-ground from which stars spring forth; or we may say that nebulæ +resemble clouds in whose bosom raindrops are forming. The wonderful +aspect of the admixtures of nebulæ and star-clusters in Sagittarius has +been described in Chapter 1. We now come to a still more extraordinary +phenomenon of this kind—the Pleiades nebulæ. + + +[Illustration: The Pleiades] + + +The group of the Pleiades, although lying outside the main course of +the Galaxy, is connected with it by a faint loop, and is the scene of +the most remarkable association of stars and nebulous matter known in +the visible universe. The naked eye is unaware of the existence of +nebulæ in the Pleiades, or, at the best, merely suspects that there is +something of the kind there; and even the most powerful telescopes are +far from revealing the full wonder of the spectacle; but in photographs +which have been exposed for many hours consecutively, in order to +accumulate the impression of the actinic rays, the revelation is +stunning. The principle stars are seen surrounded by, and, as it were, +_drowned in,_ dense nebulous clouds of an unparalleled kind. The forms +assumed by these clouds seem at first sight inexplicable. They look +like fleeces, or perhaps more like splashes and daubs of luminous paint +dashed carelessly from a brush. But closer inspection shows that they +are, to a large extent, _woven_ out of innumerable threads of filmy +texture, and there are many indications of spiral tendencies. Each of +the bright stars of the group—Alcyone, Merope, Maia, Electra, Taygeta, +Atlas—is the focus of a dense fog (totally invisible, remember, alike +to the naked eye and to the telescope), and these particular stars are +veiled from sight behind the strange mists. Running in all directions +across the relatively open spaces are nebulous wisps and streaks of the +most curious forms. On some of the nebular lines, which are either +straight throughout, or if they change direction do so at an angle, +little stars are strung like beads. In one case seven or eight stars +are thus aligned, and, as if to emphasize their dependence upon the +chain which connects them, when it makes a slight bend the file of +stars turns the same way. Many other star rows in the group suggest by +their arrangement that they, too, were once strung upon similar threads +which have now disappeared, leaving the stars spaced along their +ancient tracks. We seem forced to the conclusion that there was a time +when the Pleiades were embedded in a vast nebula resembling that of +Orion, and that the cloud has now become so rare by gradual +condensation into stars that the merest trace of it remains, and this +would probably have escaped detection but for the remarkable actinic +power of the radiant matter of which it consists. The richness of many +of these faint nebulous masses in ultra-violet radiations, which are +those that specifically affect the photographic plate, is the cause of +the marvelous revelatory power of celestial photography. So the +veritable unseen universe, as distinguished from the “unseen universe” +of metaphysical speculation, is shown to us. + +A different kind of association between stars and nebulæ is shown in +some surprising photographic objects in the constellation Cygnus, where +long, wispy nebulæ, billions of miles in length, some of them looking +like tresses streaming in a breeze, lie amid fields of stars which seem +related to them. But the relation is of a most singular kind, for +notwithstanding the delicate structure of the long nebulæ they appear +to act as barriers, causing the stars to heap themselves on one side. +The stars are two, three, or four times as numerous on one side of the +nebulæ as on the other. These nebulæ, as far as appearance goes, might +be likened to rail fences, or thin hedges, against which the wind is +driving drifts of powdery snow, which, while scattered plentifully all +around, tends to bank itself on the leeward side of the obstruction. +The imagination is at a loss to account for these extraordinary +phenomena; yet there they are, faithfully giving us their images +whenever the photographic plate is exposed to their radiations. + +Thus the more we see of the universe with improved methods of +observation, and the more we invent aids to human senses, each enabling +us to penetrate a little deeper into the unseen, the greater becomes +the mystery. The telescope carried us far, photography is carrying us +still farther; but what as yet unimagined instrument will take us to +the bottom, the top, and the end? And then, what hitherto untried power +of thought will enable us to comprehend the meaning of it all? + + + + +III +Stellar Migrations + + +To the untrained eye the stars and the planets are not distinguishable. +It is customary to call them all alike “stars.” But since the planets +more or less rapidly change their places in the sky, in consequence of +their revolution about the sun, while the stars proper seem to remain +always in the same relative positions, the latter are spoken of as +“fixed stars.” In the beginnings of astronomy it was not known that the +“fixed stars” had any motion independent of their apparent annual +revolution with the whole sky about the earth as a seeming center. Now, +however, we know that the term “fixed stars” is paradoxical, for there +is not a single really fixed object in the whole celestial sphere. The +apparent fixity in the positions of the stars is due to their immense +distance, combined with the shortness of the time during which we are +able to observe them. It is like viewing the plume of smoke issuing +from a steamer, hull down, at sea: if one does not continue to watch it +for a long time it appears to be motionless, although in reality it may +be traveling at great speed across the line of sight. Even the planets +seem fixed in position if one watches them for a single night only, and +the more distant ones do not sensibly change their places, except after +many nights of observation. Neptune, for instance, moves but little +more than two degrees in the course of an entire year, and in a month +its change of place is only about one-third of the diameter of the full +moon. + +Yet, fixed as they seem, the stars are actually moving with a speed in +comparison with which, in some cases, the planets might almost be said +to stand fast in their tracks. Jupiter’s speed in his orbit is about +eight miles per second, Neptune’s is less than three and one-half +miles, and the earth’s is about eighteen and one-half miles; while +there are “fixed stars” which move two hundred or three hundred miles +per second. They do not all, however, move with so great a velocity, +for some appear to travel no faster than the planets. But in all cases, +notwithstanding their real speed, long-continued and exceedingly +careful observations are required to demonstrate that they are moving +at all. No more overwhelming impression of the frightful depths of +space in which the stars are buried can be obtained than by reflecting +upon the fact that a star whose actual motion across the line of sight +amounts to two hundred miles per second does not change its apparent +place in the sky, in the course of a thousand years, sufficiently to be +noticed by the casual observer of the heavens! + +There is one vast difference between the motions of the stars and those +of the planets to which attention should be at once called: the +planets, being under the control of a central force emanating from +their immediate master, the sun, all move in the same direction and in +orbits concentric about the sun; the stars, on the other hand, move in +every conceivable direction and have no apparent center of motion, for +all efforts to discover such a center have failed. At one time, when +theology had finally to accept the facts of science, a grandiose +conception arose in some pious minds, according to which the Throne of +God was situated at the exact center of His Creation, and, seated +there, He watched the magnificent spectacle of the starry systems +obediently revolving around Him. Astronomical discoveries and +speculations seemed for a time to afford some warrant for this view, +which was, moreover, an acceptable substitute for the abandoned +geocentric theory in minds that could only conceive of God as a +superhuman artificer, constantly admiring his own work. No longer ago +than the middle of the nineteenth century a German astronomer, Maedler, +believed that he had actually found the location of the center about +which the stellar universe revolved. He placed it in the group of the +Pleiades, and upon his authority an extraordinary imaginative picture +was sometimes drawn of the star Alcyone, the brightest of the Pleiades, +as the very seat of the Almighty. This idea even seemed to gain a kind +of traditional support from the mystic significance, without known +historical origin, which has for many ages, and among widely separated +peoples, been attached to the remarkable group of which Alcyone is the +chief. But since Maedler’s time it has been demonstrated that the +Pleiades cannot be the center of revolution of the universe, and, as +already remarked, all attempts to find or fix such a center have proved +abortive. Yet so powerful was the hold that the theory took upon the +popular imagination, that even today astronomers are often asked if +Alcyone is not the probable site of “Jerusalem the Golden.” + +If there were a discoverable center of predominant gravitative power, +to which the motions of all the stars could be referred, those motions +would appear less mysterious, and we should then be able to conclude +that the universe was, as a whole, a prototype of the subsidiary +systems of which it is composed. We should look simply to the law of +gravitation for an explanation, and, naturally, the center would be +placed within the opening enclosed by the Milky Way. If it were there +the Milky Way itself should exhibit signs of revolution about it, like +a wheel turning upon its hub. No theory of the star motions as a whole +could stand which failed to take account of the Milky Way as the basis +of all. But the very form of that divided wreath of stars forbids the +assumption of its revolution about a center. Even if it could be +conceived as a wheel having no material center it would not have the +form which it actually presents. As was shown in Chapter 2, there is +abundant evidence of motion in the Milky Way; but it is not motion of +the system as a whole, but motion affecting its separate parts. Instead +of all moving one way, the galactic stars, as far as their movements +can be inferred, are governed by local influences and conditions. They +appear to travel crosswise and in contrary directions, and perhaps they +eddy around foci where great numbers have assembled; but of a universal +revolution involving the entire mass we have no evidence. + +Most of our knowledge of star motions, called “proper motions,” relates +to individual stars and to a few groups which happen to be so near that +the effects of their movements are measurable. In some cases the motion +is so rapid (not in appearance, but in reality) that the chief +difficulty is to imagine how it can have been imparted, and what will +eventually become of the “runaways.” Without a collision, or a series +of very close approaches to great gravitational centers, a star +traveling through space at the rate of two hundred or three hundred +miles per second could not be arrested or turned into an orbit which +would keep it forever flying within the limits of the visible universe. +A famous example of these speeding stars is “1830 Groombridge,” a star +of only the sixth magnitude, and consequently just visible to the naked +eye, whose motion across the line of sight is so rapid that it moves +upon the face of the sky a distance equal to the apparent diameter of +the moon every 280 years. The distance of this star is at least +200,000,000,000,000 miles, and may be two or three times greater, so +that its actual speed cannot be less than two hundred, and may be as +much as four hundred, miles per second. It could be turned into a new +course by a close approach to a great sun, but it could only be stopped +by collision, head-on, with a body of enormous mass. Barring such +accidents it must, as far as we can see, keep on until it has traversed +our stellar system, whence in may escape and pass out into space +beyond, to join, perhaps, one of those other universes of which we have +spoken. Arcturus, one of the greatest suns in the universe, is also a +runaway, whose speed of flight has been estimated all the way from +fifty to two hundred miles per second. Arcturus, we have every reason +to believe, possesses hundreds of times the mass of our sun—think, +then, of the prodigious momentum that its motion implies! Sirius moves +more moderately, its motion across the line of sight amounting to only +ten miles per second, but it is at the same time approaching the sun at +about the same speed, its actual velocity in space being the resultant +of the two displacements. + +What has been said about the motion of Sirius brings us to another +aspect of this subject. The fact is, that in every case of stellar +motion the displacement that we observe represents only a part of the +actual movement of the star concerned. There are stars whose motion +carries them straight toward or straight away from the earth, and such +stars, of course, show no cross motion. But the vast majority are +traveling in paths inclined from a perpendicular to our line of sight. +Taken as a whole, the stars may be said to be flying about like the +molecules in a mass of gas. The discovery of the radial component in +the movements of the stars is due to the spectroscope. If a star is +approaching, its spectral lines are shifted toward the violet end of +the spectrum by an amount depending upon the velocity of approach; if +it is receding, the lines are correspondingly shifted toward the red +end. Spectroscopic observation, then, combined with micrometric +measurements of the cross motion, enables us to detect the real +movement of the star in space. Sometimes it happens that a star’s +radial movement is periodically reversed; first it approaches, and then +it recedes. This indicates that it is revolving around a near-by +companion, which is often invisible, and superposed upon this motion is +that of the two stars concerned, which together may be approaching or +receding or traveling across the line of sight. Thus the complications +involved in the stellar motions are often exceedingly great and +puzzling. + +Yet another source of complication exists in the movement of our own +star, the sun. There is no more difficult problem in astronomy than +that of disentangling the effects of the solar motion from those of the +motions of the other stars. But the problem, difficult as it is, has +been solved, and upon its solution depends our knowledge of the speed +and direction of the movement of the solar system through space, for of +course the sun carries its planets with it. One element of the solution +is found in the fact that, as a result of perspective, the stars toward +which we are going appear to move apart toward all points of the +compass, while those behind appear to close up together. Then the +spectroscopic principle already mentioned is invoked for studying the +shift of the lines, which is toward the violet in the stars ahead of us +and toward the red in those that we are leaving behind. Of course the +effects of the independent motions of the stars must be carefully +excluded. The result of the studies devoted to this subject is to show +that we are traveling at a speed of twelve to fifteen miles per second +in a northerly direction, toward the border of the constellations +Hercules and Lyra. A curious fact is that the more recent estimates +show that the direction is not very much out of a straight line drawn +from the sun to the star Vega, one of the most magnificent suns in the +heavens. But it should not be inferred from this that Vega is drawing +us on; it is too distant for its gravitation to have such an effect. + +Many unaccustomed thoughts are suggested by this mighty voyage of the +solar system. Whence have we come, and whither do we go? Every year of +our lives we advance at least 375,000,000 miles. Since the traditional +time of Adam the sun has led his planets through the wastes of space no +less than 225,000,000,000 miles, or more than 2400 times the distance +that separates him from the earth. Go back in imagination to the +geologic ages, and try to comprehend the distance over which the earth +has flown. Where was our little planet when it emerged out of the +clouds of chaos? Where was the sun when his “thunder march” began? What +strange constellations shone down upon our globe when its masters of +life were the monstrous beasts of the “Age of Reptiles”? A million +years is not much of a span of time in geologic reckoning, yet a +million years ago the earth was farther from its present place in space +than any of the stars with a measurable parallax are now. It was more +than seven times as far as Sirius, nearly fourteen times as far as +Alpha Centauri, three times as far as Vega, and twice as far as +Arcturus. But some geologists demand two hundred, three hundred, even +one thousand million years to enable them to account for the +evolutionary development of the earth and its inhabitants. In a +thousand million years the earth would have traveled farther than from +the remotest conceivable depths of the Milky Way! + +Other curious reflections arise when we think of the form of the +earth’s track as it follows the lead of the sun, in a journey which has +neither known beginning nor conceivable end. There are probably many +minds which have found a kind of consolation in the thought that every +year the globe returns to the same place, on the same side of the sun. +This idea may have an occult connection with our traditional regard for +anniversaries. When that period of the year returns at which any great +event in our lives has occurred we have the feeling that the earth, in +its annual round, has, in a manner, brought us back to the scene of +that event. We think of the earth’s orbit as a well-worn path which we +traverse many times in the course of a lifetime. It seems familiar to +us, and we grow to have a sort of attachment to it. The sun we are +accustomed to regard as a fixed center in space, like the mill or pump +around which the harnessed patient mule makes his endless circuits. But +the real fact is that the earth never returns to the place in space +where it has once quitted. In consequence of the motion of the sun +carrying the earth and the other planets along, the track pursued by +our globe is a vast spiral in space continually developing and never +returning upon its course. It is probable that the tracks of the sun +and the others stars are also irregular, and possibly spiral, although, +as far as can be at present determined, they appear to be practically +straight. Every star, wherever it may be situated, is attracted by its +fellow-stars from many sides at once, and although the force is +minimized by distance, yet in the course of many ages its effects must +become manifest. + +Looked at from another side, is there not something immensely +stimulating and pleasing to the imagination in the idea of so +stupendous a journey, which makes all of us the greatest of travelers? +In the course of a long life a man is transported through space thirty +thousand million miles; Halley’s Comet does not travel one-quarter as +far in making one of its immense circuits. And there are adventures on +this voyage of which we are just beginning to learn to take account. +Space is full of strange things, and the earth must encounter some of +them as it advances through the unknown. Many singular speculations +have been indulged in by astronomers concerning the possible effects +upon the earth of the varying state of the space that it traverses. +Even the alternation of hot and glacial periods has sometimes been +ascribed to this source. When tropical life flourished around the +poles, as the remains in the rocks assure us, the needed high +temperature may, it has been thought, have been derived from the +presence of the earth in a warm region of space. Then, too, there is a +certain interest for us in the thought of what our familiar planet has +passed through. We cannot but admire it for its long journeying as we +admire the traveler who comes to us from remote and unexplored lands, +or as we gaze with a glow of interest upon the first locomotive that +has crossed a continent, or a ship that has visited the Arctic or +Antarctic regions. If we may trust the indications of the present +course, the earth, piloted by the sun, has come from the Milky Way in +the far south and may eventually rejoin that mighty band of stars in +the far north. + +While the stars in general appear to travel independently of one +another, except when they are combined in binary or trinary systems, +there are notable exceptions to this rule. In some quarters of the sky +we behold veritable migrations of entire groups of stars whose members +are too widely separated to show any indications of revolution about a +common center of gravity. This leads us back again to the wonderful +group of the Pleiades. All of the principle stars composing that group +are traveling in virtually parallel lines. Whatever force set them +going evidently acted upon all alike. This might be explained by the +assumption that when the original projective force acted upon them they +were more closely united than they are at present, and that in drifting +apart they have not lost the impulse of the primal motion. Or it may be +supposed that they are carried along by some current in space, although +it would be exceedingly difficult, in the present state of our +knowledge, to explain the nature of such a current. Yet the theory of a +current has been proposed. As to an attractive center around which they +might revolve, none has been found. Another instance of similar +“star-drift” is furnished by five of the seven stars constituting the +figure of the “Great Dipper.” In this case the stars concerned are +separated very widely, the two extreme ones by not less than fifteen +degrees, so that the idea of a common motion would never have been +suggested by their aspect in the sky; and the case becomes the more +remarkable from the fact that among and between them there are other +stars, some of the same magnitude, which do not share their motion, but +are traveling in other directions. Still other examples of the same +phenomenon are found in other parts of the sky. Of course, in the case +of compact star-clusters, it is assumed that all the members share a +like motion of translation through space, and the same is probably true +of dense star-swarms and star-clouds. + +The whole question of star-drift has lately assumed a new phase, in +consequence of the investigations of Kapteyn, Dyson, and Eddington on +the “systematic motions of the stars.” This research will, it is hoped, +lead to an understanding of the general law governing the movements of +the whole body of stars constituting the visible universe. Taking about +eleven hundred stars whose proper motions have been ascertained with an +approach to certainty, and which are distributed in all parts of the +sky, it has been shown that there exists an apparent double drift, in +two independent streams, moving in different and nearly opposed +directions. The apex of the motion of what is called “Stream I” is +situated, according to Professor Kapteyn, in right ascension 85°, +declination south 11°, which places it just south of the constellation +Orion; while the apex of “Stream II” is in right ascension 260°, +declination south 48°, placing it in the constellation Ara, south of +Scorpio. The two apices differ very nearly 180° in right ascension and +about 120° in declination. The discovery of these vast star-streams, if +they really exist, is one of the most extraordinary in modern +astronomy. It offers the correlation of stellar movements needed as the +basis of a theory of those movements, but it seems far from revealing a +physical cause for them. As projected against the celestial sphere the +stars forming the two opposite streams appear intermingled, some +obeying one tendency and some the other. As Professor Dyson has said, +the hypothesis of this double movement is of a revolutionary character, +and calls for further investigation. Indeed, it seems at first glance +not less surprising than would be the observation that in a snow-storm +the flakes over our heads were divided into two parties and driving +across each other’s course in nearly opposite directions, as if urged +by interpenetrating winds. + +But whatever explanation may eventually be found for the motions of the +stars, the knowledge of the existence of those motions must always +afford a new charm to the contemplative observer of the heavens, for +they impart a sense of life to the starry system that would otherwise +be lacking. A stagnant universe, with every star fixed immovably in its +place, would not content the imagination or satisfy our longing for +ceaseless activity. The majestic grandeur of the evolutions of the +celestial hosts, the inconceivable vastness of the fields of space in +which they are executed, the countless numbers, the immeasurable +distances, the involved convolutions, the flocking and the scattering, +the interpenetrating marches and countermarches, the strange community +of impulsion affecting stars that are wide apart in space and causing +them to traverse the general movement about them like aides and +despatch-bearers on a battle-field—all these arouse an intensity of +interest which is heightened by the mystery behind them. + + + + +IV +The Passing of the Constellations + + +From a historical and picturesque point of view, one of the most +striking results of the motions of the stars described in the last +chapter is their effect upon the forms of the constellations, which +have been watched and admired by mankind from a period so early that +the date of their invention is now unknown. The constellations are +formed by chance combinations of conspicuous stars, like figures in a +kaleidoscope, and if our lives were commensurate with the æons of +cosmic existence we should perceive that the kaleidoscope of the +heavens was ceaselessly turning and throwing the stars into new +symmetries. Even if the stars stood fast, the motion of the solar +system would gradually alter the configurations, as the elements of a +landscape dissolve and recombine in fresh groupings with the traveler’s +progress amid them. But with the stars themselves all in motion at +various speeds and in many directions, the changes occur more rapidly. +Of course, “rapid” is here understood in a relative sense; the wheel of +human history to an eye accustomed to the majestic progression of the +universe would appear to revolve with the velocity of a whirling +dynamo. Only the deliberation of geological movements can be contrasted +with the evolution and devolution of the constellations. + +And yet this secular fluctuation of the constellation figures is not +without keen interest for the meditative observer. It is another +reminder of the swift mutability of terrestial affairs. To the passing +glance, which is all that we can bestow upon these figures, they appear +so immutable that they have been called into service to form the most +lasting records of ancient thought and imagination that we possess. In +the forms of the constellations, the most beautiful, and, in +imaginative quality, the finest, mythology that the world has ever +known has been perpetuated. Yet, in a broad sense, this scroll of human +thought imprinted on the heavens is as evanescent as the summer clouds. +Although more enduring than parchment, tombs, pyramids, and temples, it +is as far as they from truly eternizing the memory of what man has +fancied and done. + +Before studying the effects that the motions of the stars have had and +will have upon the constellations, it is worth while to consider a +little further the importance of the stellar pictures as archives of +history. To emphasize the importance of these effects it is only +necessary to recall that the constellations register the oldest +traditions of our race. In the history of primeval religions they are +the most valuable of documents. Leaving out of account for the moment +the more familiar mythology of the Greeks, based on something older +yet, we may refer for illustration to that of the mysterious Maya race +of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group +of ruins perched, after the Mexican and Central-American plan, on the +summits of pyramidal mounds which mark the site of an ancient theogonic +center of the Mayas. Here the temples all evidently refer to a cult +based upon the constellations as symbols. The figures and the names, of +course, were not the same as those that we have derived from our Aryan +ancestors, but the star groups were the same or nearly so. For +instance, the loftiest of the temples at Izamal was connected with the +sign of the constellation known to us as Cancer, marking the place of +the sun at the summer solstice, at which period the sun was supposed to +descend at noon like a great bird of fire and consume the offerings +left upon the altar. Our Scorpio was known to the Mayas as a sign of +the “Death God.” Our Libra, the “Balance,” with which the idea of a +divine weighing out of justice has always been connected, seems to be +identical with the Mayan constellation Teoyaotlatohua, with which was +associated a temple where dwelt the priests whose special business it +was to administer justice and to foretell the future by means of +information obtained from the spirits of the dead. Orion, the “Hunter” +of our celestial mythology, was among the Mayas a “Warrior,” while +Sagittarius and others of our constellations were known to them (under +different names, of course), and all were endowed with a religious +symbolism. And the same star figures, having the same significance, +were familiar to the Peruvians, as shown by the temples at Cuzco. Thus +the imagination of ancient America sought in the constellations symbols +of the unchanging gods. + +But, in fact, there is no nation and no people that has not recognized +the constellations, and at one period or another in its history +employed them in some symbolic or representative capacity. As handled +by the Greeks from prehistoric times, the constellation myths became +the very soul of poetry. The imagination of that wonderful race +idealized the principal star groups so effectively that the figures and +traditions thus attached to them have, for civilized mankind, displaced +all others, just as Greek art in its highest forms stands without +parallel and eclipses every rival. The Romans translated no heroes and +heroines of the mythical period of their history to the sky, and the +deified Cæsars never entered that lofty company, but the heavens are +filled with the early myths of the Greeks. Herakles nightly resumes his +mighty labors in the stars; Zeus, in the form of the white “Bull,” +Taurus, bears the fair Europa on his back through the celestial waves; +Andromeda stretches forth her shackled arms in the star-gemmed ether, +beseeching aid; and Perseus, in a blaze of diamond armor, revives his +heroic deeds amid sparkling clouds of stellar dust. There, too, sits +Queen Cassiopeia in her dazzling chair, while the Great King, Cepheus, +towers gigantic over the pole. Professor Young has significantly +remarked that a great number of the constellations are connected in +some way or other with the Argonautic Expedition—that strangely +fascinating legend of earliest Greek story which has never lost its +charm for mankind. In view of all this, we may well congratulate +ourselves that the constellations will outlast our time and the time of +countless generations to follow us; and yet they are very far from +being eternal. Let us now study some of the effects of the stellar +motions upon them. + +We begin with the familiar figure of the “Great Dipper.” He who has not +drunk inspiration from its celestial bowl is not yet admitted to the +circle of Olympus. This figure is made up of seven conspicuous stars in +the constellation Ursa Major, the “Greater Bear.” The handle of the +“Dipper” corresponds to the tail of the imaginary “Bear,” and the bowl +lies upon his flank. In fact, the figure of a dipper is so evident and +that of a bear so unevident, that to most persons the “Great Dipper” is +the only part of the constellation that is recognizable. Of the seven +stars mentioned, six are of nearly equal brightness, ranking as of the +second magnitude, while the seventh is of only the third magnitude. The +difference is very striking, since every increase of one magnitude +involves an increase of two-and-a-half times in brightness. There +appears to be little doubt that the faint star, which is situated at +the junction of the bowl and the handle, is a variable of long period, +since three hundred years ago it was as bright as its companions. But +however that may be, its relative faintness at the present time +interferes but little with the perfection of the “Dipper’s” figure. In +order the more readily to understand the changes which are taking +place, it will be well to mention both the names and the Greek letters +which are attached to the seven stars. Beginning at the star in the +upper outer edge of the rim of the bowl and running in regular order +round the bottom and then out to the end of the handle, the names and +letters are as follows: Dubhe (α), Merak (β), Phaed (γ), Megrez (δ), +Alioth (ε), Mizar (ζ), and Benetnasch (η). Megrez is the faint star +already mentioned at the junction of the bowl and handle, and Mizar, in +the middle of the handle, has a close, naked-eye companion which is +named Alcor. The Arabs called this singular pair of stars “The Horse +and Rider.” Merak and Duhbe are called “The Pointers,” because an +imaginary line drawn northward through them indicates the Pole Star. + + +[Illustration: The “Great Dipper”] + + +Now it has been found that five of these stars—_viz.,_ Merak, Phaed, +Megrez, Alioth, and Mizar (with its comrade)—are moving with +practically the same speed in an easterly direction, while the other +two, Dubhe and Benetnasch, are simultaneously moving westward, the +motions of Benetnasch being apparently more rapid. The consequence of +these opposed motions is, of course, that the figure of the “Dipper” +cannot always have existed and will not continue to exist. In the +accompanying diagrams it has been thought interesting to show the +relative positions of these seven stars, as seen from the point which +the earth now occupies, both in the past and in the future. Arrows +attached to the stars in the figure representing the present appearance +of the “Dipper” indicate the directions of the motions and the +distances over which they will carry the stars in a period of about +five hundred centuries. The time, no doubt, seems long, but remember +the vast stretch of ages through which the earth has passed, and then +reflect that no reason is apparent why our globe should not continue to +be a scene of animation for ten thousand centuries yet to come. The +fact that the little star Alcor placed so close to Mizar should +accompany the latter in its flight is not surprising, but that two of +the principal stars of the group should be found moving in a direction +directly opposed to that pursued by the other five is surprising in the +highest degree; and it recalls the strange theory of a double drift +affecting all the stars, to which attention was called in the preceding +chapter. It would appear that Benetnasch and Dubhe belong to one +“current,” and Merak, Phaed, Megrez, Alioth, and Mizar to the other. As +far as is known, the motion of the seven stars are not shared by the +smaller stars scattered about them, but on the theory of currents there +should be such a community of motion, and further investigation may +reveal it. + + +[Illustration: Cassiopeia] + + +From the “Great Dipper” we turn to a constellation hardly less +conspicuous and situated at an equal distance from the pole on the +other side—Cassiopeia. This famous star-group commemorating the +romantic Queen of Ethiopia whose vain boasting of her beauty was +punished by the exposure of her daughter Andromeda to the “Sea +Monster,” is well-marked by five stars which form an irregular letter +“W” with its open side toward the pole. Three of these stars are +usually ranked as of the second magnitude, and two of the third; but to +ordinary observation they appear of nearly equal brightness, and +present a very striking picture. They mark out the chair and a part of +the figure of the beautiful queen. Beginning at the right-hand, or +western, end of the “W,” their Greek letter designations are: Beta (β), +Alpha (α), Gamma (γ), Delta (δ), and Epsilon (ε). Four of them, Beta, +Alpha, Delta, and Epsilon are traveling eastwardly at various speeds, +while the fifth, Gamma, moves in a westerly direction. The motion of +Beta is more rapid than that of any of the others. It should be said, +however, that no little uncertainty attaches to the estimates of the +rate of motion of stars which are not going very rapidly, and different +observers often vary considerably in their results. + +In the beautiful “Northern Crown,” one of the most perfect and charming +of all the figures to be found in the stars, the alternate combining +and scattering effects of the stellar motions are shown by comparing +the appearance which the constellation must have had five hundred +centuries ago with that which it has at present and that which it will +have in the future. The seven principle stars of the asterism, forming +a surprisingly perfect coronet, have movements in three directions at +right angles to one another. That in these circumstances they should +ever have arrived at positions giving them so striking an appearance of +definite association is certainly surprising; from its aspect one would +have expected to find a community of movement governing the brilliants +of the “Crown,” but instead of that we find evidence that they will +inevitably drift apart and the beautiful figure will dissolve. + +A similar fate awaits such asterisms as the “Northern Cross” in Cygnus; +the “Crow” (Corvus), which stands on the back of the great “Sea +Serpent,” Hydra, and pecks at his scales; “Job’s Coffin” (Delphinus); +the “Great Square of Pegasus”; the “Twins” (Gemini); the beautiful +“Sickle” in Leo; and the exquisite group of the Hyades in Taurus. In +the case of the Hyades, two controlling movements are manifest: one, +affecting five of the stars which form the well-known figure of a +letter “V,” is directed northerly; the other, which controls the +direction of two stars, has an easterly trend. The chief star of the +group, Aldebaran, one of the finest of all stars both for its +brilliance and its color, is the most affected by the easterly motion. +In time it will drift entirely out of connection with its present +neighbors. Although the Hyades do not form so compact a group as the +Pleiades in the same constellation, yet their appearance of +relationship is sufficient to awaken a feeling of surprise over the +fact that, as with the stars of the “Dipper,” their association is only +temporary or apparent. + + +[Illustration: The “Northern Crown”] + + +The great figure of Orion appears to be more lasting, not because its +stars are physically connected, but because of their great distance, +which renders their movements too deliberate to be exactly ascertained. +Two of the greatest of its stars, Betelgeuse and Rigel, possess, as far +as has been ascertained, no perceptible motion across the line of +sight, but there is a little movement perceptible in the “Belt.” At the +present time this consists of an almost perfect straight line, a row of +second-magnitude stars about equally spaced and of the most striking +beauty. In the course of time, however, the two right-hand stars, +Mintaka and Alnilam (how fine are these Arabic star names!) will +approach each other and form a naked-eye double, but the third, Alnita, +will drift away eastward, so that the “Belt” will no longer exist. + +For one more example, let us go to the southern hemisphere, whose most +celebrated constellation, the “Southern Cross,” has found a place in +all modern literatures, although it has no claim to consideration on +account of association with ancient legends. This most attractive +asterism, which has never ceased to fascinate the imagination of +Christendom since it was first devoutly described by the early +explorers of the South, is but a passing collocation of brilliant +stars. Yet even in its transfigurations it has been for hundreds of +centuries, and will continue to be for hundreds of centuries to come, a +most striking object in the sky. Our figures show its appearance in +three successive phases: first, as it was fifty thousand years ago +(viewed from the earth’s present location); second, as it is in our +day; and, third, as it will be an equal time in the future. The +nearness of these bright stars to one another—the length of the longer +beam of the “Cross” is only sixdegrees—makes this group very +noticeable, whatever the arrangement of its components may be. The +largest star, at the base of the “Cross,” is of the first magnitude, +two of the others are of the second magnitude, and the fourth is of the +third. Other stars, not represented in the figures, increase the effect +of a celestial blazonry, although they do not help the resemblance to a +cross. + + +[Illustration: The “Southern Cross”] + + +But since the motion of the solar system itself will, in the course of +so long a period as fifty thousand years, produce a great change in the +perspective of the heavens as seen from the earth, by carrying us +nearly nineteen trillion miles from our present place, why, it may be +asked, seek to represent future appearances of the constellations which +we could not hope to see, even if we could survive so long? The answer +is: Because these things aid the mind to form a picture of the effects +of the mobility of the starry universe. Only by showing the changes +from some definite point of view can we arrive at a due comprehension +of them. The constellations are more or less familiar to everybody, so +that impending changes of their forms must at once strike the eye and +the imagination, and make clearer the significance of the movements of +the stars. If the future history of mankind is to resemble its past and +if our race is destined to survive yet a million years, then our remote +descendents will see a “new heavens” if not a “new earth,” and will +have to invent novel constellations to perpetuate their legends and +mythologies. + +If our knowledge of the relative distances of the stars were more +complete, it would be an interesting exercise in celestial geometry to +project the constellations probably visible to the inhabitants of +worlds revolving around some of the other suns of space. Our sun is too +insignificant for us to think that he can make a conspicuous appearance +among them, except, perhaps, in a few cases. As seen, for instance, +from the nearest known star, Alpha Centauri, the sun would appear of +the average first magnitude, and consequently from that standpoint he +might be the gem of some little constellation which had no Sirius, or +Arcturus, or Vega to eclipse him with its superior splendor. But from +the distance of the vast majority of the stars the sun would probably +be invisible to the naked eye, and as seen from nearer systems could +only rank as a fifth or sixth magnitude star, unnoticed and unknown +except by the star-charting astronomer. + + + + +V +Conflagrations in the Heavens + + +Suppose it were possible for the world to take fire and burn up—as some +pessimists think that it will do when the Divine wrath shall have +sufficiently accumulated against it—nobody out of our own little corner +of space would ever be aware of the catastrophe! With all their +telescopes, the astronomers living in the golden light of Arcturus or +the diamond blaze of Canopus would be unable to detect the least +glimmer of the conflagration that had destroyed the seat of Adam and +his descendents, just as now they are totally ignorant of its +existence. + +But at least fifteen times in the course of recorded history men +looking out from the earth have beheld in the remote depths of space +great outbursts of fiery light, some of them more splendidly luminous +than anything else in the firmament except the sun! If _they_ were +conflagrations, how many million worlds like ours were required to feed +their blaze? + +It is probable that “temporary” or “new” stars, as these wonderful +apparitions are called, really are conflagrations; not in the sense of +a bonfire or a burning house or city, but in that of a sudden eruption +of inconceivable heat and light, such as would result from the +stripping off the shell of an encrusted sun or the crashing together of +two mighty orbs flying through space with a hundred times the velocity +of the swiftest cannon-shot. + +Temporary stars are the rarest and most erratic of astronomical +phenomena. The earliest records relating to them are not very clear, +and we cannot in every instance be certain that it was one of these +appearances that the ignorant and superstitious old chroniclers are +trying to describe. The first temporary star that we are absolutely +sure of appeared in 1572, and is known as “Tycho’s Star,” because the +celebrated Danish astronomer (whose remains, with his gold-and-silver +artificial nose—made necessary by a duel—still intact, were disinterred +and reburied in 1901) was the first to perceive it in the sky, and the +most assiduous and successful in his studies of it. As the first fully +accredited representative of its class, this new star made its entry +upon the scene with becoming _éclat._ It is characteristic of these +phenomena that they burst into view with amazing suddenness, and, of +course, entirely unexpectedly. Tycho’s star appeared in the +constellation Cassiopeia, near a now well-known and much-watched little +star named Kappa, on the evening of November 11, 1572. The story has +often been repeated, but it never loses interest, how Tycho, going home +that evening, saw people in the street pointing and staring at the sky +directly over their heads, and following the direction of their hands +and eyes he was astonished to see, near the zenith, an unknown star of +surpassing brilliance. It outshone the planet Jupiter, and was +therefore far brighter than the first magnitude. There was not another +star in the heavens that could be compared with it in splendor. Tycho +was not in all respects free from the superstitions of his time—and who +is?—but he had the true scientific instinct, and immediately he began +to study the stranger, and to record with the greatest care every +change in its aspect. First he determined as well as he could with the +imperfect instruments of his day, many of which he himself had +invented, the precise location of the phenomena in the sky. Then he +followed the changes that it underwent. At first it brightened until +its light equaled or exceeded that of the planet Venus at her +brightest, a statement which will be appreciated at its full value by +anyone who has ever watched Venus when she plays her dazzling rôle of +“Evening Star,” flaring like an arc light in the sunset sky. It even +became so brilliant as to be visible in full daylight, since, its +position being circumpolar, it never set in the latitude of Northern +Europe. Finally it began to fade, turning red as it did so, and in +March, 1574, it disappeared from Tycho’s searching gaze, and has never +been seen again from that day to this. None of the astronomers of the +time could make anything of it. They had not yet as many bases of +speculation as we possess today. + + +[Illustration: Chart showing location of Tycho’s star, 1572, and Nova +Persei of 1901] + + +Tycho’s star has achieved a romantic reputation by being fancifully +identified with the “Star of Bethlehem,” said to have led the wondering +Magi from their eastern deserts to the cradle-manger of the Savior in +Palestine. Many attempts have been made to connect this traditional +“star” with some known phenomenon of the heavens, and none seems more +idle than this. Yet it persistently survives, and no astronomer is free +from eager questions about it addressed by people whose imagination has +been excited by the legend. It is only necessary to say that the +supposition of a connection between the phenomenon of the Magi and +Tycho’s star is without any scientific foundation. It was originally +based on an unwarranted assumption that the star of Tycho was a +variable of long period, appearing once every three hundred and fifteen +years, or thereabout. If that were true there would have been an +apparition somewhere near the traditional date of the birth of Christ, +a date which is itself uncertain. But even the data on which the +assumption was based are inconsistent with the theory. Certain monkish +records speak of something wonderful appearing in the sky in the years +1264 and 945, and these were taken to have been outbursts of Tycho’s +star. Investigation shows that the records more probably refer to +comets, but even if the objects seen were temporary stars, their dates +do not suit the hypothesis; from 945 to 1264 there is a gap of 319 +years, and from 1264 to 1572 one of only 308 years; moreover 337 years +have now (1909) elapsed since Tycho saw the last glimmer of his star. +Upon a variability so irregular and uncertain as that, even if we felt +sure that it existed, no conclusion could be found concerning an +apparition occurring 2000 years ago. + +In the year 1600 (the year in which Giordano Bruno was burned at the +stake for teaching that there is more than one physical world), a +temporary star of the third magnitude broke out in the constellation +Cygnus, and curiously enough, considering the rarity of such phenomena, +only four years later another surprisingly brilliant one appeared in +the constellation Ophiuchus. This is often called “Kepler’s star,” +because the great German astronomer devoted to it the same attention +that Tycho had given to the earlier phenomenon. It, too, like Tycho’s, +was at first the brightest object in the stellar heavens, although it +seems never to have quite equaled its famous predecessor in splendor. +It disappeared after a year, also turning of a red color as it became +more faint. We shall see the significance of this as we go on. Some of +Kepler’s contemporaries suggested that the outburst of this star was +due to a meeting of atoms in space, and idea bearing a striking +resemblance to the modern theory of “astronomical collisions.” + +In 1670, 1848, and 1860 temporary stars made their appearance, but none +of them was of great brilliance. In 1866 one of the second magnitude +broke forth in the “Northern Crown” and awoke much interest, because by +that time the spectroscope had begun to be employed in studying the +composition of the stars, and Huggins demonstrated that the new star +consisted largely of incandescent hydrogen. But this star, apparently +unlike the others mentioned, was not absolutely new. Before its +outburst it had shown as a star of the ninth magnitude (entirely +invisible, of course, to the naked eye), and after about six weeks it +faded to its original condition in which it has ever since remained. In +1876 a temporary star appeared in the constellation Cygnus, and +attained at one time the brightness of the second magnitude. Its +spectrum and its behavior resembled those of its immediate predecessor. +In 1885, astronomers were surprised to see a sixth-magnitude star +glimmering in the midst of the hazy cloud of the great Andromeda +Nebula. It soon absolutely disappeared. Its spectrum was remarkable for +being “continuous,” like that of the nebula itself. A continuous +spectrum is supposed to represent a body, or a mass, which is either +solid or liquid, or composed of gas under great pressure. In January, +1892, a new star was suddenly seen in the constellation Auriga. It +never rose much above the fourth magnitude, but it showed a peculiar +spectrum containing both bright and dark lines of hydrogen. + +But a bewildering surprise was now in store; the world was to behold at +the opening of the twentieth century such a celestial spectacle as had +not been on view since the times of Tycho and Kepler. Before daylight +on the morning of February 22, 1901, the Rev. Doctor Anderson, of +Edinburgh, an amateur astronomer, who had also been the first to see +the new star in Auriga, beheld a strange object in the constellation +Perseus not far from the celebrated variable star Algol. He recognized +its character at once, and immediately telegraphed the news, which +awoke the startled attention of astronomers all over the world. When +first seen the new star was no brighter than Algol (less than the +second magnitude), but within twenty-four hours it was ablaze, +outshining even the brilliant Capella, and far surpassing the first +magnitude. At the spot in the sky where it appeared nothing whatever +was visible on the night before its coming. This is known with +certainty because a photograph had been made of that very region on +February 21, and this photograph showed everything down to the twelfth +magnitude, but not a trace of the stranger which burst into view +between the 21st and the 22nd like the explosion of a rocket. + +Upon one who knew the stars the apparition of this intruder in a +well-known constellation had the effect of a sudden invasion. The new +star was not far west of the zenith in the early evening, and in that +position showed to the best advantage. To see Capella, the hitherto +unchallenged ruler of that quarter of the sky, abased by comparison +with this stranger of alien aspect, for there was always an unfamiliar +look about the “nova,” was decidedly disconcerting. It seemed to +portend the beginning of a revolution in the heavens. One could +understand what the effect of such an apparition must have been in the +superstitious times of Tycho. The star of Tycho had burst forth on the +northern border of the Milky Way; this one was on its southern border, +some forty-five degrees farther east. + +Astronomers were well-prepared this time for the scientific study of +the new star, both astronomical photography and spectroscopy having +been perfected, and the results of their investigations were calculated +to increase the wonder with which the phenomenon was regarded. The star +remained at its brightest only a few days; then, like a veritable +conflagration, it began to languish; and, like the reflection of a +dying fire, as it sank it began to glow with the red color of embers. +But its changes were spasmodic; once about every three days it flared +up only to die away again. During these fluctuations its light varied +alternately in the ratio of one to six. Finally it took a permanent +downward course, and after a few months the naked eye could no longer +perceive it; but it remained visible with telescopes, gradually fading +until it had sunk to the ninth magnitude. Then another astonishing +change happened: in August photographs taken at the Yerkes Observatory +and at Heidelberg showed that the “nova” was _surrounded by a spiral +nebula!_ The nebula had not been there before, and no one could doubt +that it represented a phase of the same catastrophe that had produced +the outburst of the new star. At one time the star seemed virtually to +have disappeared, as if all its substance had been expanded into the +nebulous cloud, but always there remained a stellar nucleus about which +the misty spiral spread wider and ever wider, like a wave expanding +around a center of disturbance. The nebula too showed a variability of +brightness, and four condensations which formed in it seemed to have a +motion of revolution about the star. As time went on the nebula +continued to expand at a rate which was computed to be not less than +twenty thousand miles per second! And now the star itself, showing +indications of having turned into a nebula, behaved in a most erratic +manner, giving rise to the suspicion that it was about to burst out +again. But this did not occur, and at length it sunk into a state of +lethargy from which it has to the present time not recovered. But the +nebulous spiral has disappeared, and the entire phenomena as it now +(1909) exists consists of a faint nebulous star of less than the ninth +magnitude. + +The wonderful transformations just described had been forecast in +advance of the discovery of the nebulous spiral encircling the star by +the spectroscopic study of the latter. At first there was no suggestion +of a nebular constitution, but within a month or two characteristic +nebular lines began to appear, and in less than six months the whole +spectrum had been transformed to the nebular type. In the mean time the +shifting of the spectral lines indicated a complication of rapid +motions in several directions simultaneously. These motions were +estimated to amount to from one hundred to five hundred miles per +second. + +The human mind is so constituted that it feels forced to seek an +explanation of so marvelous a phenomenon as this, even in the absence +of the data needed for a sound conclusion. The most natural hypothesis, +perhaps, is that of a collision. Such a catastrophe could certainly +happen. It has been shown, for instance, that in infinity of time the +earth is sure to be hit by a comet; in the same way it may be asserted +that, if no time limit is fixed, the sun is certain to run against some +obstacle in space, either another star, or a dense meteor swarm, or one +of the dark bodies which there is every reason to believe abound around +us. The consequences of such a collision are easy to foretell, provided +that we know the masses and the velocities of the colliding bodies. In +a preceding chapter we have discussed the motions of the sun and stars, +and have seen that they are so swift that an encounter between any two +of them could not but be disastrous. But this is not all; for as soon +as two stars approached within a few million miles their speed would be +enormously increased by their reciprocal attractions and, if their +motion was directed radially with respect to their centers, they would +come together with a crash that would reduce them both to nebulous +clouds. It is true that the chances of such a “head-on” collision are +relatively very small; two stars approaching each other would most +probably fall into closed orbits around their common center of gravity. +If there were a collision it would most likely be a grazing one instead +of a direct front-to-front encounter. But even a close approach, +without any actual collision, would probably prove disastrous, owing to +the tidal influence of each of the bodies on the other. Suns, in +consequence of their enormous masses and dimensions and the +peculiarities of their constitution, are exceedingly dangerous to one +another at close quarters. Propinquity awakes in them a mutually +destructive tendency. Consisting of matter in the gaseous, or perhaps, +in some cases, liquid, state, their tidal pull upon each other if +brought close together might burst them asunder, and the photospheric +envelope being destroyed the internal incandescent mass would gush out, +bringing fiery death to any planets that were revolving near. Without +regard to the resulting disturbance of the earth’s orbit, the close +approach of a great star to the sun would be in the highest degree +perilous to us. But this is a danger which may properly be regarded as +indefinitely remote, since, at our present location in space, we are +certainly far from every star except the sun, and we may feel confident +that no great invisible body is near, for if there were one we should +be aware of its presence from the effects of its attraction. As to dark +nebulæ which may possibly lie in the track that the solar system is +pursuing at the rate of 375,000,000 miles per year, that is another +question—and they, too, could be dangerous! + + +[Illustration: Nova Persei, with its nebular rings] + + +This brings us directly back to “Nova Persei,” for among the many +suggestions offered to explain its outburst, as well as those of other +temporary stars, one of the most fruitful is that of a collision +between a star and a vast invisible nebula. Professor Seeliger, of +Munich, first proposed this theory, but it afterward underwent some +modifications from others. Stated in a general form, the idea is that a +huge dark body, perhaps an extinguished sun, encountered in its +progress through space a widespread flock of small meteors forming a +dark nebula. As it plunged into the swarm the friction of the +innumerable collisions with the meteors heated its surface to +incandescence, and being of vast size it then became visible to us as a +new star. Meanwhile the motion of the body through the nebula, and its +rotation upon itself, set up a gyration in the blazing atmosphere +formed around it by the vaporized meteors; and as this atmosphere +spread wider, under the laws of gyratory motion a rotation in the +opposite direction began in the inflamed meteoric cloud outside the +central part of the vortex. Thus the spectral lines were caused to show +motion in opposite directions, a part of the incandescent mass +approaching the earth simultaneously with the retreat of another part. +So the curious spectroscopic observations before mentioned were +explained. This theory might also account for the appearance of the +nebulous spiral first seen some six months after the original outburst. +The sequent changes in the spectrum of the “nova” are accounted for by +this theory on the assumption, reasonable enough in itself, that at +first the invading body would be enveloped in a vaporized atmosphere of +relatively slight depth, producing by its absorption the fine dark +lines first observed; but that as time went on and the incessant +collisions continued, the blazing atmosphere would become very deep and +extensive, whereupon the appearance of the spectral lines would change, +and bright lines due to the light of the incandescent meteors +surrounding the nucleus at a great distance would take the place of the +original dark ones. The vortex of meteors once formed would protect the +flying body within from further immediate collisions, the latter now +occurring mainly among the meteors themselves, and then the central +blaze would die down, and the original splendor of the phenomenon would +fade. + +But the theories about Nova Persei have been almost as numerous as the +astronomers who have speculated about it. One of the most startling of +them assumed that the outburst was caused by the running amuck of a +dark star which had encountered another star surrounded with planets, +the renewed outbreaks of light after the principal one had faded being +due to the successive running down of the unfortunate planets! Yet +another hypothesis is based on what we have already said of the tidal +influence that two close approaching suns would have upon each other. +Supposing two such bodies which had become encrusted, but remained +incandescent and fluid within, to approach within almost striking +distance; they would whirl each other about their common center of +gravity, and at the same time their shells would burst under the tidal +strain, and their glowing nuclei being disclosed would produce a great +outburst of light. Applying this theory to a “nova,” like that of 1866 +in the “Northern Crown,” which had been visible as a small star before +the outbreak, and which afterward resumed its former aspect, we should +have to assume that a yet shining sun had been approached by a dark +body whose attraction temporarily burst open its photosphere. It might +be supposed that in this case the dark body was too far advanced in +cooling to suffer the same fate from the tidal pull of its victim. But +a close approach of that kind would be expected to result in the +formation of a binary system, with orbits of great eccentricity, +perhaps, and after the lapse of a certain time the outburst should be +renewed by another approximation of the two bodies. A temporary star of +that kind would rather be ranked as a variable. + +The celebrated French astronomer, Janssen, had a different theory of +Nova Persei, and of temporary stars in general. According to his idea, +such phenomena might be the result of chemical changes taking place in +a sun without interference by, or collision with, another body. Janssen +was engaged for many years in trying to discover evidence of the +existence of oxygen in the sun, and he constructed his observatory on +the summit of Mount Blanc specially to pursue that research. He +believed that oxygen must surely exist in the sun since we find so many +other familiar elements included in the constitution of the solar +globe, and as he was unable to discover satisfactory evidence of its +presence he assumed that it existed in a form unknown on the earth. If +it were normally in the sun’s chromosphere, or coronal atmosphere, he +said, it would combine with the hydrogen which we know is there and +form an obscuring envelope of water vapor. It exists, then, in a +special state, uncombined with hydrogen; but let the temperature of the +sun sink to a critical point and the oxygen will assume its normal +properties and combine with the hydrogen, producing a mighty outburst +of light and heat. This, Janssen thought, might explain the phenomena +of the temporary stars. It would also, he suggested, account for their +brief career, because the combination of the elements would be quickly +accomplished, and then the resulting water vapor would form an +atmosphere cutting off the radiation from the star within. + +This theory may be said to have a livelier human interest than some of +the others, since, according to it, the sun may carry in its very +constitution a menace to mankind; one does not like to think of it +being suddenly transformed into a gigantic laboratory for the explosive +combination of oxygen and hydrogen! But while Janssen’s theory might do +for some temporary stars, it is inadequate to explain all the phenomena +of Nova Persei, and particularly the appearance of the great spiral +nebula that seemed to exhale from the heart of the star. Upon the +whole, the theory of an encounter between a star and a dark nebula +seems best to fit the observations. By that hypothesis the expanding +billow of light surrounding the core of the conflagration is very well +accounted for, and the spectroscopic peculiarities are also explained. + +Dr Gustov Le Bon offers a yet more alarming theory, suggesting that +temporary stars are the result of _atomic explosion;_ but we shall +touch upon this more fully in Chapter 14. + +Twice in the course of this discussion we have called attention to the +change of color invariably undergone by temporary stars in the later +stages of their career. This was conspicuous with Nova Persei which +glowed more and more redly as it faded, until the nebulous light began +to overpower that of the stellar nucleus. Nothing could be more +suggestive of the dying out of a great fire. Moreover, change of color +from white to red is characteristic of all variable stars of long +period, such as “Mira” in Cetus. It is also characteristic of stars +believed to be in the later stages of evolution, and consequently +approaching extinction, like Antares and Betelgeuse, and still more +notably certain small stars which “gleam like rubies in the field of +the telescope.” These last appear to be suns in the closing period of +existence as self-luminous bodies. Between the white stars, such as +Sirius and Rigel, and the red stars, such as Aldebaran and Alpha +Herculis, there is a progressive series of colors from golden yellow +through orange to deep red. The change is believed to be due to the +increase of absorbing vapors in the stellar atmosphere as the body +cools down. In the case of ordinary stars these changes no doubt occupy +many millions of years, which represent the average duration of solar +life; but the temporary stars run through similar changes in a few +months: they resemble ephemeral insects—born in the morning and doomed +to perish with the going down of the sun. + + + + +VI +Explosive and Whirling Nebulæ + + +One of the most surprising triumphs of celestial photography was +Professor Keeler’s discovery, in 1899, that the great majority of the +nebulæ have a distinctly spiral form. This form, previously known in +Lord Rosse’s great “Whirlpool Nebula,” had been supposed to be +exceptional; now the photographs, far excelling telescopic views in the +revelation of nebular forms, showed the spiral to be the typical shape. +Indeed, it is a question whether all nebulæ are not to some extent +spiral. The extreme importance of this discovery is shown in the effect +that it has had upon hitherto prevailing views of solar and planetary +evolution. For more than three-quarters of a century Laplace’s +celebrated hypothesis of the manner of origin of the solar system from +a rotating and contracting nebula surrounding the sun had guided +speculation on that subject, and had been tentatively extended to cover +the evolution of systems in general. The apparent forms of some of the +nebulæ which the telescope had revealed were regarded, and by some are +still regarded, as giving visual evidence in favor of this theory. +There is a “ring nebula” in Lyra with a central star, and a “planetary +nebula” in Gemini bearing no little resemblance to the planet Saturn +with its rings, both of which appear to be practical realizations of +Laplace’s idea, and the elliptical rings surrounding the central +condensation of the Andromeda Nebula may be cited for the same kind of +proof. + + +[Illustration: Lord Rosse’s nebula] + + +But since Keeler’s discovery there has been a decided turning away of +speculation another way. The form of the spiral nebulæ seems to be +entirely inconsistent with the theory of an originally globular or +disk-shaped nebula condensing around a sun and throwing or leaving off +rings, to be subsequently shaped into planets. Some astronomers, +indeed, now reject Laplace’s hypothesis _in toto,_ preferring to think +that even our solar system originated from a spiral nebula. Since the +spiral type prevails among the existing nebulæ, we must make any +mechanical theory of the development of stars and planetary systems +from them accord with the requirements which that form imposes. A +glance at the extraordinary variations upon the spiral which Professor +Keeler’s photographs reveal is sufficient to convince one of the +difficulty of the task of basing a general theory upon them. In truth, +it is much easier to criticize Laplace’s hypothesis than to invent a +satisfactory substitute for it. If the spiral nebulæ seem to oppose it +there are other nebulæ which appear to support it, and it may be that +no one fixed theory can account for all the forms of stellar evolution +in the universe. Our particular planetary system may have originated +very much as the great French mathematician supposed, while others have +undergone, or are now undergoing, a different process of development. +There is always a too strong tendency to regard an important new +discovery and the theories and speculations based upon it as +revolutionizing knowledge, and displacing or overthrowing everything +that went before. Upon the plea that “Laplace only made a guess” more +recent guesses have been driven to extremes and treated by injudicious +exponents as “the solid facts at last.” + + +[Illustration: Wonderful spiral in triangulum] + + +Before considering more recent theories than Laplace’s, let us see what +the nature of the photographic revelations is. The vast celestial +maelstrom discovered by Lord Rosse in the “Hunting Dogs” may be taken +as the leading type of the spiral nebulæ, although there are less +conspicuous objects of the kind which, perhaps, better illustrate some +of their peculiarities. Lord Rosse’s nebula appears far more wonderful +in the photographs than in his drawings made with the aid of his giant +reflecting telescope at Parsonstown, for the photographic plate records +details that no telescope is capable of showing. Suppose we look at the +photograph of this object as any person of common sense would look at +any great and strange natural phenomenon. What is the first thing that +strikes the mind? It is certainly the appearance of violent whirling +motion. One would say that the whole glowing mass had been spun about +with tremendous velocity, or that it had been set rotating so rapidly +that it had become the victim of “centrifugal force,” one huge fragment +having broken loose and started to gyrate off into space. Closer +inspection shows that in addition to the principal focus there are +various smaller condensations scattered through the mass. These are +conspicuous in the spirals. Some of them are stellar points, and but +for the significance of their location we might suppose them to be +stars which happen to lie in a line between us and the nebula. But when +we observe how many of them follow most faithfully the curves of the +spirals we cannot but conclude that they form an essential part of the +phenomenon; it is not possible to believe that their presence in such +situations is merely fortuitous. One of the outer spirals has at least +a dozen of these star-like points strung upon it; some of them sharp, +small, and distinct, others more blurred and nebulous, suggesting +different stages of condensation. Even the part which seems to have +been flung loose from the main mass has, in addition to its central +condensation, at least one stellar point gleaming in the half-vanished +spire attached to it. Some of the more distant stars scattered around +the “whirlpool” look as if they too had been shot out of the mighty +vortex, afterward condensing into unmistakable solar bodies. There are +at least two curved rows of minute stars a little beyond the periphery +of the luminous whirl which clearly follow lines concentric with those +of the nebulous spirals. Such facts are simply dumbfounding for anyone +who will bestow sufficient thought upon them, for these are _suns,_ +though they may be small ones; and what a birth is that for a sun! + +Look now again at the glowing spirals. We observe that hardly have they +left the central mass before they begin to coagulate. In some places +they have a “ropy” aspect; or they are like peascods filled with +growing seeds, which eventually will become stars. The great focus +itself shows a similar tendency, especially around its circumference. +The sense that it imparts of a tremendous shattering force at work is +overwhelming. There is probably more matter in that whirling and +bursting nebula than would suffice to make a hundred solar systems! It +must be confessed at once that there is no confirmation of the +Laplacean hypothesis here; but what hypothesis will fit the facts? +There is one which it has been claimed does so, but we shall come to +that later. In the meanwhile, as a preparation, fix in the memory the +appearance of that second spiral mass spinning beside its master which +seems to have spurned it away. + + +[Illustration: Spiral in Ursa Major] + + +For a second example of the spiral nebulæ look at the one in the +constellation Triangulum. _God, how hath the imagination of puny man +failed to comprehend Thee!_ Here is creation through destruction with a +vengeance! The spiral form of the nebula is unmistakable, but it is +half obliterated amid the turmoil of flying masses hurled away on all +sides with tornadic fury. The focus itself is splitting asunder under +the intolerable strain, and in a little while, as time is reckoned in +the Cosmos, it will be gyrating into stars. And then look at the +cyclonic rain of already finished stars whirling round the outskirts of +the storm. Observe how scores of them are yet involved in the fading +streams of the nebulous spirals; see how they have been thrown into +vast loops and curves, of a beauty that half redeems the terror of the +spectacle enclosed within their lines—like iridescent cirri hovering +about the edges of a hurricane. And so again are suns born! + +Let us turn to the exquisite spiral in Ursa Major; how different its +aspect from that of the other! One would say that if the terrific coil +in Triangulum has all but destroyed itself in its fury, this one on the +contrary has just begun its self-demolition. As one gazes one seems to +see in it the smooth, swift, accelerating motion that precedes +catastrophe. The central part is still intact, dense, and uniform in +texture. How graceful are the spirals that smoothly rise from its oval +rim and, gemmed with little stars, wind off into the darkness until +they have become as delicate as threads of gossamer! But at bottom the +story told here is the same—creation by gyration! + +Compare with the above the curious mass in Cetus. Here the plane of the +whirling nebula nearly coincides with our line of sight and we see the +object at a low angle. It is far advanced and torn to shreds, and if we +could look at it perpendicularly to its plane it is evident that it +would closely resemble the spectacle in Triangulum. + +Then take the famous Andromeda Nebula (see Frontispiece), which is so +vast that notwithstanding its immense distance even the naked eye +perceives it as an enigmatical wisp in the sky. Its image on the +sensitive plate is the masterpiece of astronomical photography; for +wild, incomprehensible beauty there is nothing that can be compared +with it. Here, if anywhere, we look upon the spectacle of creation in +one of its earliest stages. The Andromeda Nebula is apparently less +advanced toward transformation into stellar bodies than is that in +Triangulum. The immense crowd of stars sprinkled over it and its +neighborhood seem in the main to lie this side of the nebula, and +consequently to have no connection with it. But incipient stars (in +some places clusters of them) are seen in the nebulous rings, while one +or two huge masses seem to give promise of transformation into stellar +bodies of unusual magnitude. I say “rings” because although the loops +encompassing the Andromeda Nebula have been called spirals by those who +wish utterly to demolish Laplace’s hypothesis, yet they are not +manifestly such, as can be seen on comparing them with the undoubted +spirals of the Lord Rosse Nebula. They look quite as much like circles +or ellipses seen at an angle of, say, fifteen or twenty degrees to +their plane. If they are truly elliptical they accord fairly well with +Laplace’s idea, except that the scale of magnitude is stupendous, and +if the Andromeda Nebula is to become a solar system it will surpass +ours in grandeur beyond all possibility of comparison. + + +[Illustration: Nebula in Cetus] + + +There is one circumstance connected with the spiral nebulæ, and +conspicuous in the Andromeda Nebula on account of its brightness, which +makes the question of their origin still more puzzling; they all show +continuous spectra, which, as we have before remarked, indicate that +the mass from which the light comes is either solid or liquid, or a gas +under heavy pressure. Thus nebulæ fall into two classes: the “white” +nebulæ, giving a continuous spectrum; and the “green” nebulæ whose +spectra are distinctly gaseous. The Andromeda Nebula is the great +representative of the former class and the Orion Nebula of the latter. +The spectrum of the Andromeda Nebula has been interpreted to mean that +it consists not of luminous gas, but of a flock of stars so distant +that they are separately indistinguishable even with powerful +telescopes, just as the component stars of the Milky Way are +indistinguishable with the naked eye; and upon this has been based the +suggestion that what we see in Andromeda is an outer universe whose +stars form a series of elliptical garlands surrounding a central mass +of amazing richness. But this idea is unacceptable if for no other +reason than that, as just said, all the spiral nebulæ possess the same +kind of spectrum, and probably no one would be disposed to regard them +all as outer universes. As we shall see later, the peculiarity of the +spectra of the spiral nebulæ is appealed to in support of a modern +substitute for Laplace’s hypothesis. + +Finally, without having by any means exhausted the variety exhibited by +the spiral nebulæ, let us turn to the great representative of the other +species, the Orion Nebula. In some ways this is even more marvelous +than the others. The early drawings with the telescope failed to convey +an adequate conception either of its sublimity or of its complication +of structure. It exists in a nebulous region of space, since +photographs show that nearly the whole constellation is interwoven with +faintly luminous coils. To behold the entry of the great nebula into +the field even of a small telescope is a startling experience which +never loses its novelty. As shown by the photographs, it is an +inscrutable chaos of perfectly amazing extent, where spiral bands, +radiating streaks, dense masses, and dark yawning gaps are strangely +intermingled without apparent order. In one place four conspicuous +little stars, better seen in a telescope than in the photograph on +account of the blurring produced by over-exposure, are suggestively +situated in the midst of a dark opening, and no observer has ever felt +any doubt that these stars have been formed from the substance of the +surrounding nebula. There are many other stars scattered over its +expanse which manifestly owe their origin to the same source. But +compare the general appearance of this nebula with the others that we +have studied, and remark the difference. If the unmistakably spiral +nebulæ resemble bursting fly-wheels or grindstones from whose +perimeters torrents of sparks are flying, the Orion Nebula rather +recalls the aspect of a cloud of smoke and fragments produced by the +explosion of a shell. This idea is enforced by the look of the outer +portion farthest from the bright half of the nebula, where sharply +edged clouds with dark spaces behind seem to be billowing away as if +driven by a wind blowing from the center. + + +[Illustration: The Orion nebula] + + +Next let us consider what scientific speculation has done in the effort +to explain these mysteries. Laplace’s hypothesis can certainly find no +standing ground either in the Orion Nebula or in those of a spiral +configuration, whatever may be its situation with respect to the grand +Nebula of Andromeda, or the “ring” and “planetary” nebulæ. Some other +hypothesis more consonant with the appearances must be found. Among the +many that have been proposed the most elaborate is the “Planetesimal +Hypothesis” of Professors Chamberlin and Moulton. It is to be remarked +that it applies to the spiral nebulæ distinctively, and not to an +apparently chaotic mass of gas like the vast luminous cloud in Orion. +The gist of the theory is that these curious objects are probably the +result of close approaches to each other of two independent suns, +reminding us of what was said on this subject when we were dealing with +temporary stars. Of the previous history of these appulsing suns the +theory gives us no account; they are simply supposed to arrive within +what may be called an effective tide-producing distance, and then the +drama begins. Some of the probable consequences of such an approach +have been noticed in Chapter 5; let us now consider them a little more +in detail. + +Tides always go in couples; if there is a tide on one side of a globe +there will be a corresponding tide on the other side. The cause is to +be found in the law that the force of gravitation varies inversely as +the square of the distance; the attraction on the nearest surface of +the body exercised by another body is greater than on its center, and +greater yet than on its opposite surface. If two great globes attract +each other, each tends to draw the other out into an ellipsoidal +figure; they must be more rigid than steel to resist this—and even then +they cannot altogether resist. If they are liquid or gaseous they will +yield readily to the force of distortion, the amount of which will +depend upon their distance apart, for the nearer they are the greater +becomes the tidal strain. If they are encrusted without and liquid or +gaseous in the interior, the internal mass will strive to assume the +figure demanded by the tidal force, and will, if it can, burst the +restraining envelope. Now this is virtually the predicament of the body +we call a sun when in the immediate presence of another body of +similarly great mass. Such a body is presumably gaseous throughout, the +component gases being held in a state of rigidity by the compression +produced by the tremendous gravitational force of their own aggregate +mass. At the surface such a body is enveloped in a shell of relatively +cool matter. Now suppose a great attracting body, such as another sun, +to approach near enough for the difference in its attraction on the two +opposite sides of the body and on its center to become very great; the +consequence will be a tidal deformation of the whole body, and it will +lengthen out along the line of the gravitational pull and draw in at +the sides, and if its shell offers considerable resistance, but not +enough to exercise a complete restraint, it will be violently burst +apart, or blown to atoms, and the internal mass will leap out on the +two opposite sides in great fiery spouts. In the case of a sun further +advanced in cooling than ours the interior might be composed of molten +matter while the exterior crust had become rigid like the shell of an +egg; then the force of the “tidal explosion” produced by the appulse of +another sun would be more violent in consequence of the greater +resistance overcome. Such, then, is the mechanism of the first phase in +the history of a spiral nebula according to the Planetesimal +Hypothesis. Two suns, perhaps extinguished ones, have drawn near +together, and an explosive outburst has occured in one or both. The +second phase calls for a more agile exercise of the imagination. + +To simplify the case, let us suppose that only one of the tugging suns +is seriously affected by the strain. Its vast wings produced by the +outburst are twisted into spirals by their rotation and the contending +attractions exercised upon them, as the two suns, like battleships in +desperate conflict, curve round each other, concentrating their +destructive energies. Then immense quantities of débris are scattered +about in which eddies are created, and finally, as the sun that caused +the damage goes on its way, leaving its victim to repair its injuries +as it may, the dispersed matter cools, condenses, and turns into +streams of solid particles circling in elliptical paths about their +parent sun. These particles, or fragments, are the “planetesimals” of +the theory. In consequence of the inevitable intersection of the orbits +of the planetesimals, nodes are formed where the flying particles meet, +and at these nodes large masses are gradually accumulated. The larger +the mass the greater its attraction, and at last the nodal points +become the nuclei of great aggregations from which planets are shaped. + +This, in very brief form, is the Planetesimal Hypothesis which we are +asked to substitute for that based on Laplace’s suggestion as an +explanation of the mode of origin of the solar system; and the +phenomena of the spiral nebulæ are appealed to as offering evident +support to the new hypothesis. We are reminded that they are elliptical +in outline, which accords with the hypothesis; that their spectra are +not gaseous, which shows that they may be composed of solid particles +like the planetesimals; and that their central masses present an oval +form, which is what would result from the tidal effects, as just +described. We also remember that some of them, like the Lord Rosse and +the Andromeda nebulæ, are visually double, and in these cases we might +suppose that the two masses represent the tide-burst suns that ventured +into too close proximity. It may be added that the authors of the +theory do not insist upon the appulse of two suns as the _only_ way in +which the planetesimals may have originated, but it is the only +supposition that has been worked out. + +But serious questions remain. It needs, for instance, but a glance at +the Triangulum monster to convince the observer that it cannot be a +solar system which is being evolved there, but rather a swarm of stars. +Many of the detached masses are too vast to admit of the supposition +that they are to be transformed into planets, in our sense of planets, +and the distances of the stars which appear to have been originally +ejected from the focal masses are too great to allow us to liken the +assemblage that they form to a solar system. Then, too, no nodes such +as the hypothesis calls for are visible. Moreover, in most of the +spiral nebulæ the appearances favor the view that the supposititious +encountering suns have not separated and gone each rejoicing on its +way, after having inflicted the maximum possible damage on its +opponent, but that, on the contrary, they remain in close association +like two wrestlers who cannot escape from each other’s grasp. And this +is exactly what the law of gravitation demands; stars cannot approach +one another with impunity, with regard either to their physical make-up +or their future independence of movement. The theory undertakes to +avoid this difficulty by assuming that in the case of our system the +approach of the foreign body to the sun was not a close one—just close +enough to produce the tidal extrusion of the relatively insignificant +quantity of matter needed to form the planets. But even then the effect +of the appulse would be to change the direction of flight, both of the +sun and of its visitor, and there is no known star in the sky which can +be selected as the sun’s probable partner in their ancient _pas deux._ +That there are unconquered difficulties in Laplace’s hypothesis no one +would deny, but in simplicity of conception it is incomparably more +satisfactory, and with proper modifications could probably be made more +consonant with existing facts in our solar system than that which is +offered to replace it. Even as an explanation of the spiral nebulæ, not +as solar systems in process of formation, but as the birthplaces of +stellar clusters, the Planetesimal Hypothesis would be open to many +objections. Granting its assumptions, it has undoubtedly a strong +mathematical framework, but the trouble is not with the mathematics but +with the assumptions. Laplace was one of the ablest mathematicians that +ever lived, but he had never seen a spiral nebula; if he had, he might +have invented a hypothesis to suit its phenomena. His actual hypothesis +was intended only for our solar system, and he left it in the form of a +“note” for the consideration of his successors, with the hope that they +might be able to discover the full truth, which he confessed was hidden +from him. It cannot be said that that truth has yet been found, and +when it is found the chances are that intuition and not logic will have +led to it. + +The spiral nebulæ, then, remain among the greatest riddles of the +universe, while the gaseous nebulæ, like that of Orion, are no less +mysterious, although it seems impossible to doubt that both forms give +birth to stars. It is but natural to look to them for light on the +question of the origin of our planetary system; but we should not +forget that the scale of the phenomena in the two cases is vastly +different, and the forces in operation may be equally different. A hill +may have been built up by a glacier, while a mountain may be the +product of volcanic forces or of the upheaval of the strata of the +planet. + + + + +VII +The Banners of the Sun + + +As all the world knows, the sun, a blinding globe pouring forth an +inconceivable quantity of light and heat, whose daily passage through +the sky is caused by the earth’s rotation on its axis, constitutes the +most important phenomenon of terrestial existence. Viewed with a dark +glass to take off the glare, or with a telescope, its rim is seen to be +a sharp and smooth circle, and nothing but dark sky is visible around +it. Except for the interference of the moon, we should probably never +have known that there is any more of the sun than our eyes ordinarily +see. + +But when an eclipse of the sun occurs, caused by the interposition of +the opaque globe of the moon, we see its immediate surroundings, which +in some respects are more wonderful than the glowing central orb. These +surroundings, although not in the sense in which we apply the term to +the gaseous envelope of the earth, may be called the sun’s atmosphere. +They consist of two very different parts—first, the red “prominences,” +which resemble tongues of flame ascending thousands of miles above the +sun’s surface; and, second, the “corona,” which extends to distances of +millions of miles from the sun, and shines with a soft, glowing light. +The two combined, when well seen, make a spectacle without parallel +among the marvels of the sky. Although many attempts have been made to +render the corona visible when there is no eclipse, all have failed, +and it is to the moon alone that we owe its revelation. To cover the +sun’s disk with a circular screen will not answer the purpose because +of the illumination of the air all about the observer. When the moon +hides the sun, on the other hand, the sunlight is withdrawn from a +great cylinder of air extending to the top of the atmosphere and +spreading many miles around the observer. There is then no glare to +interfere with the spectacle, and the corona appears in all its +surprising beauty. The prominences, however, although they were +discovered during an eclipse, can now, with the aid of the +spectroscope, be seen at any time. But the prominences are rarely large +enough to be noticed by the naked eye, while the streamers of the +corona, stretching far away in space, like ghostly banners blown out +from the black circle of the obscuring moon, attract every eye, and to +this weird apparition much of the fear inspired by eclipses has been +due. But if the corona has been a cause of terror in the past it has +become a source of growing knowledge in our time. + + +[Illustration: The Corona] + + +The story of the first scientific observation of the corona and the +prominences is thrillingly interesting, and in fact dramatic. The +observation was made during the eclipse of 1842, which fortunately was +visible all over Central and Southern Europe so that scores of +astronomers saw it. The interest centers in what happened at Pavia in +Northern Italy, where the English astronomer Francis Baily had set up +his telescope. The eclipse had begun and Bailey was busy at his +telescope when, to quote his own words in the account which he wrote +for the _Memoirs of the Royal Astronomical Society:_ + + +I was astounded by a tremendous burst of applause from the streets +below, and at the same moment was electrified by the sight of one of +the most brilliant and splendid phenomena that can well be imagined; +for at that instant the dark body of the moon was suddenly surrounded +with a corona, or kind of bright glory, similar in shape and magnitude +to that which painters draw round the heads of saints... + +Pavia contains many thousand inhabitants, the major part of whom were +at this early hour walking about the streets and squares or looking out +of windows in order to witness this long-talked-of phenomenon; and when +the total obscuration took place, which was _instantaneous,_ there was +a universal shout from every observer which “made the welkin ring,” and +for the moment withdrew my attention from the object with which I was +immediately occupied. I had, indeed, expected the appearance of a +luminous circle round the moon during the time of total obscurity; but +I did not expect, from any of the accounts of preceding eclipses that I +had read, to witness so magnificent an exhibition as that which took +place... + +Splendid and astonishing, however, as this remarkable phenomenon really +was, and although it could not fail to call forth the admiration and +applause of every beholder, yet I must confess that there was at the +same time something in its singular and wonderful appearance that was +appalling... + +But the most remarkable circumstance attending the phenomenon was the +appearance of _three large protuberances_ apparently emanating from the +circumference of the moon, but evidently forming a portion of the +corona. They had the appearance of mountains of a prodigious elevation; +their color was red tinged with lilac or purple; perhaps the color of +the peach-blossom would more nearly represent it. They somewhat +resembled the tops of the snowy Alpine mountains when colored by the +rising or the setting sun. They resembled the Alpine mountains in +another respect, inasmuch as their light was perfectly steady, and had +none of that flickering or sparkling motion so visible in other parts +of the corona... + +The whole of these protuberances were visible even to the last moment +of total obscuration, and when the first ray of light was admitted from +the sun they vanished, with the corona, altogether, and daylight was +instantly restored. + + +I have quoted nearly all of this remarkable description not alone for +its intrinsic interest, but because it is the best depiction that can +be found of the general phenomena of a total solar eclipse. Still, not +every such eclipse offers an equally magnificent spectacle. The +eclipses of 1900 and 1905, for instance, which were seen by the writer, +the first in South Carolina and the second in Spain, fell far short of +that described by Bailey in splendor and impressiveness. Of course, +something must be allowed for the effect of surprise; Bailey had not +expected to see what was so suddenly disclosed to him. But both in 1900 +and 1905 the amount of scattered light in the sky was sufficient in +itself to make the corona appear faint, and there were no very +conspicuous prominences visible. Yet on both occasions there was +manifest among the spectators that mingling of admiration and awe of +which Bailey speaks. The South Carolinians gave a cheer and the ladies +waved their handkerchiefs when the corona, ineffably delicate of form +and texture, _melted_ into sight and then in two minutes melted away +again. The Spaniards, crowded on the citadel hill of Burgos, with their +king and his royal retinue in their midst, broke out with a great +clapping of hands as the awaited spectacle unfolded itself in the sky; +and on both occasions, before the applause began, after an awed silence +a low murmur ran through the crowds. At Burgos it is said many made the +sign of the cross. + +It was not long before Bailey’s idea that the prominences were a part +of the corona was abandoned, and it was perceived that the two +phenomena were to a great extent independent. At the eclipse of 1868, +which the astronomers, aroused by the wonderful scene of 1842, and +eager to test the powers of the newly invented spectroscope, flocked to +India to witness, Janssen conceived the idea of employing the +spectroscope to render the prominences visible when there was no +eclipse. He succeeded the very next day, and these phenomena have been +studied in that way ever since. + +There are recognized two kinds of prominences—the “erruptive” and the +“quiescent.” The latter, which are cloud-like in form, may be seen +almost anywhere along the edge of the sun; but the former, which often +shoot up as if hurled from mighty volcanoes, appear to be associated +with sun-spots, and appear only above the zones where spots abound. +Either of them, when seen in projection against the brilliant solar +disk, appears white, not red, as against a background of sky. The +quiescent prominences, whose elevation is often from forty thousand to +sixty thousand miles, consist, as the spectroscope shows, mainly of +hydrogen and helium. The latter, it will be remembered, is an element +which was known to be in the sun many years before the discovery that +it also exists in small quantities on the earth. A fact which may have +a significance which we cannot at present see is that the emanation +from radium gradually and spontaneously changes into helium, an +alchemistical feat of nature that has opened many curious vistas to +speculative thinkers. The eruptive prominences, which do not spread +horizontally like the others, but ascend with marvelous velocity to +elevations of half a million miles or more, are apparently composed +largely of metallic vapors—_i.e._ metals which are usually solid on the +earth, but which at solar temperatures are kept in a volatilized state. +The velocity of their ascent occasionally amounts to three hundred or +four hundred miles per second. It is known from mathematical +considerations that the gravitation of the sun would not be able to +bring back any body that started from its surface with a velocity +exceeding three hundred and eighty-three miles per second; so it is +evident that some of the matter hurled forth in eruptive prominences +may escape from solar control and go speeding out into space, cooling +and condensing into solid masses. There seems to be no reason why some +of the projectiles from the sun might not reach the planets. Here, +then, we have on a relatively small scale, _explosions_ recalling those +which it has been imagined may be the originating cause of some of the +sudden phenomena of the stellar heavens. + + +[Illustration: A solar “prominence.” Photographed May 21, 1907] + + +Of the sun-spots it is not our intention here specifically to speak, +but they evidently have an intimate connection with eruptive +prominences, as well as some relation, not yet fully understood, with +the corona. Of the real cause of sun-spots we know virtually nothing, +but recent studies by Professor Hale and others have revealed a strange +state of things in the clouds of metallic vapors floating above them +and their surroundings. Evidences of a cyclonic tendency have been +found, and Professor Hale has proved that sun-spots are strong magnetic +fields, and consist of columns of ionized vapors rotating in opposite +directions in the two hemispheres. A fact which may have the greatest +significance is that titanium and vanadium have been found both in +sun-spots and in the remarkable variable Mira Ceti, a star which every +eleven months, or thereabout, flames up with great brilliancy and then +sinks back to invisibility with the naked eye. It has been suggested +that sun-spots are indications of the beginning of a process in the sun +which will be intensified until it falls into the state of such a star +as Mira. Stars very far advanced in evolution, without showing +variability, also exhibit similar spectra; so that there is much reason +for regarding sunspots as emblems of advancing age. + +The association of the corona with sun-spots is less evident than that +of the eruptive prominences; still such an association exists, for the +form and extent of the corona vary with the sun-spot period of which we +shall presently speak. The constitution of the corona remains to be +discovered. It is evidently in part gaseous, but it also probably +contains matter in the form of dust and small meteors. It includes one +substance altogether mysterious—“coronium.” There are reasons for +thinking that this may be the lightest of all the elements, and +Professor Young, its discoverer, said that it was “absolutely unique in +nature; utterly distinct from any other known form of matter, +terrestial, solar, or cosmical.” The enormous extent of the corona is +one of its riddles. Since the development of the curious subject of the +“pressure of light” it has been proposed to account for the +sustentation of the corona by supposing that it is borne upon the +billows of light continually poured out from the sun. Experiment has +proved, what mathematical considerations had previously pointed out as +probable, that the waves of light exert a pressure or driving force, +which becomes evident in its effects if the body acted upon is +sufficiently small. In that case the light pressure will prevail over +the attraction of gravitation, and propel the attenuated matter away +from the sun in the teeth of its attraction. The earth itself would be +driven away if, instead of consisting of a solid globe of immense +aggregate mass, it were a cloud of microscopic particles. The reason is +that the pressure varies in proportion to the _surface_ of the body +acted upon, while the gravitational attraction is proportional to the +_volume,_ or the total amount of matter in the body. But the surface of +any body depends upon the _square_ of its diameter, while the volume +depends upon the _cube_ of the diameter. If, for instance, the diameter +is represented by 4, the surface will be proportional to 4 × 4, or 16, +and the volume to 4 × 4 × 4, or 64; but if the diameter is taken as 2, +the surface will be 2 × 2, or 4, and the volume 2 × 2 × 2, or 8. Now, +the ratio of 4 to 8 is twice as great as that of 16 to 64. If the +diameter is still further decreased, the ratio of the surface to the +volume will proportionally grow larger; in other words, the pressure +will gain upon the attraction, and whatever their original ratio may +have been, a time will come, if the diminution of size continues, when +the pressure will become more effective than the attraction, and the +body will be driven away. Supposing the particles of the corona to be +below the critical size for the attraction of a mass like that of the +sun to control them, they would be driven off into the surrounding +space and appear around the sun like the clouds of dust around a mill. +We shall return to this subject in connection with the Zodiacal Light, +the Aurora, and Comets. + +On the other hand, there are parts of the corona which suggest by their +forms the play of electric or magnetic forces. This is beautifully +shown in some of the photographs that have been made of the corona +during recent eclipses. Take, for instance, that of the eclipse of +1900. The sheaves of light emanating from the poles look precisely like +the “lines of force” surrounding the poles of a magnet. It will be +noticed in this photograph that the corona appears to consist of two +portions: one comprising the polar rays just spoken of, and the other +consisting of the broader, longer, and less-defined masses of light +extending out from the equatorial and middle-latitude zones. Yet even +in this more diffuse part of the phenomenon one can detect the presence +of submerged curves bearing more or less resemblance to those about the +poles. Just what part electricity or electro-magnetism plays in the +mechanism of the solar radiation it is impossible to say, but on the +assumption that it is a very important part is based the hypothesis +that there exists a direct solar influence not only upon the magnetism, +but upon the weather of the earth. This hypothesis has been under +discussion for half a century, and still we do not know just how much +truth it represents. It is certain that the outbreak of great +disturbances on the sun, accompanied by the formation of sun-spots and +the upshooting of eruptive prominences (phenomena which we should +naturally expect to be attended by action), have been instantly +followed by corresponding “magnetic storms” on the earth and brilliant +displays of the auroral lights. There have been occasions when the +influence has manifested itself in the most startling ways, a great +solar outburst being followed by a mysterious gripping of the cable and +telegraph systems of the world, as if an invisible and irresistible +hand had seized them. Messages are abruptly cut off, sparks leap from +the telegraph instruments, and the entire earth seems to have been +thrown into a magnetic flurry. These occurrences affect the mind with a +deep impression of the dependence of our planet on the sun, such as we +do not derive from the more familiar action of the sunlight on the +growth of plants and other phenomena of life depending on solar +influences. + +Perhaps the theory of solar magnetic influence upon the weather is best +known in connection with the “sun-spot cycle.” This, at any rate, is, +as already remarked, closely associated with the corona. Its existence +was discovered in 1843 by the German astronomer Schwabe. It is a period +of variable length, averaging about eleven years, during which the +number of spots visible on the sun first increases to a maximum, then +diminishes to a minimum, and finally increases again to a maximum. For +unknown reasons the period is sometimes two or three years longer than +the average and sometimes as much shorter. Nevertheless, the phenomena +always recur in the same order. Starting, for instance, with a time +when the observer can find few or no spots, they gradually increase in +number and size until a maximum, in both senses, is reached, during +which the spots are often of enormous size and exceedingly active. +After two or three years they begin to diminish in number, magnitude, +and activity until they almost or quite disappear. A strange fact is +that when a new period opens, the spots appear first in high northern +and southern latitudes, far from the solar equator, and as the period +advances they not only increase in number and size, but break out +nearer and nearer to the equator, the last spots of a vanishing period +sometimes lingering in the equatorial region after the advance-guard of +its successor has made its appearance in the high latitudes. Spots are +never seen on the equator nor near the poles. It was not very long +after the discovery of the sun-spot cycle that the curious observation +was made that a striking coincidence existed between the period of the +sun-spots and another period affecting the general magnetic condition +of the earth. When a curved line representing the varying number of +sun-spots was compared with another curve showing the variations in the +magnetic state of the earth the two were seen to be in almost exact +accord, a rise in one curve corresponding to a rise in the other, and a +fall to a fall. Continued observation has proved that this is a real +coincidence and not an accidental one, so that the connection, although +as yet unexplained, is accepted as established. But does the influence +extend further, and directly affect the weather and the seasons as well +as the magnetic elements of the earth? A final answer to this question +cannot yet be given, for the evidence is contradictory, and the +interpretations put upon it depend largely on the predilections of the +judges. + +But, in a broad sense, the sun-spots and the phenomena connected with +them _must_ have a relation to terrestial meteorology, for they prove +the sun to be a variable star. Reference was made, a few lines above, +to the resemblance of the spectra of sun-spots to those of certain +stars which seem to be failing through age. This in itself is extremely +suggestive; but if this resemblance had never been discovered, we +should have been justified in regarding the sun as variable in its +output of energy; and not only variable, but probably increasingly so. +The very inequalities in the sun-spot cycle are suspicious. When the +sun is most spotted its total light may be reduced by one-thousandth +part, although it is by no means certain that its outgiving of thermal +radiations is then reduced. A loss of one-thousandth of its luminosity +would correspond to a decrease of .0025 of a stellar magnitude, +considering the sun as a star viewed from distant space. So slight a +change would not be perceptible; but it is not alone sun-spots which +obscure the solar surface, its entire globe is enveloped with an +obscuring veil. When studied with a powerful telescope the sun’s +surface is seen to be thickly mottled with relatively obscure specks, +so numerous that it has been estimated that they cut off from one-tenth +to one-twentieth of the light that we should receive from it if the +whole surface were as brilliant as its brightest parts. The condition +of other stars warrants the conclusion that this obscuring envelope is +the product of a process of refrigeration which will gradually make the +sun more and more variable until its history ends in extinction. +Looking backward, we see a time when the sun must have been more +brilliant than it is now. At that time it probably shone with the +blinding white splendor of such stars as Sirius, Spica, and Vega; now +it resembles the relatively dull Procyon; in time it will turn ruddy +and fall into the closing cycle represented by Antares. Considering +that once it must have been more radiantly powerful than at present, +one is tempted to wonder if that could have been the time when tropical +life flourished within the earth’s polar circles, sustained by a +vivific energy in the sun which it has now lost. + +The corona, as we have said, varies with the sun-spot cycle. When the +spots are abundant and active the corona rises strong above the spotted +zones, forming immense beams or streamers, which on one occasion, at +least, had an observed length of _ten million miles._ At the time of a +spot minimum the corona is less brilliant and has a different outline. +It is then that the curved polar rays are most conspicuous. Thus the +vast banners of the sun, shaken out in the eclipse, are signals to tell +of its varying state, but it will probably be long before we can read +correctly their messages. + + + + +VIII +The Zodiacal Light Mystery + + +There is a singular phenomenon in the sky—one of the most puzzling of +all—which has long arrested the attention of astronomers, defying their +efforts at explanation, but which probably not one in a hundred, and +possibly not one in a thousand, of the readers of this book has ever +seen. Yet its name is often spoken, and it is a conspicuous object if +one knows when and where to look for it, and when well seen it exhibits +a mystical beauty which at the same time charms and awes the beholder. +It is called “The Zodiacal Light,” because it lies within the broad +circle of the Zodiac, marking the sun’s apparent annual path through +the stars. What it is nobody has yet been able to find out with +certainty, and books on astronomy usually speak of it with singular +reserve. But it has given rise to many remarkable theories, and a true +explanation of it would probably throw light on a great many other +celestial mysteries. The Milky Way is a more wonderful object to look +upon, but its nature can be comprehended, while there is a sort of +uncanniness about the Zodiacal Light which immediately impresses one +upon seeing it, for its part in the great scheme of extra-terrestrial +affairs is not evident. + +If you are out-of-doors soon after sunset—say, on an evening late in +the month of February—you may perceive, just after the angry flush of +the dying winter’s day has faded from the sky, a pale ghostly presence +rising above the place where the sun went down. The writer remembers +from boyhood the first time it was pointed out to him and the unearthly +impression that it made, so that he afterward avoided being out alone +at night, fearful of seeing the spectral thing again. The phenomenon +brightens slowly with the fading of the twilight, and soon distinctly +assumes the shape of an elongated pyramid of pearly light, leaning +toward the south if the place of observation is in the northern +hemisphere. It does not impress the observer at all in the same manner +as the Milky Way; that looks far off and is clearly among the stars, +but the Zodiacal Light seems closer at hand, as if it were something +more intimately concerning the earth. To all it immediately suggests a +connection, also, with the sunken sun. If the night is clear and the +moon absent (and if you are in the country, for city lights ruin the +spectacles of the sky), you will be able to watch the apparition for a +long time. You will observe that the light is brightest near the +horizon, gradually fading as the pyramidal beam mounts higher, but in +favorable circumstances it may be traced nearly to the meridian south +of the zenith, where its apex at last vanishes in the starlight. It +continues visible during the evenings of March and part of April, after +which, ordinarily, it is seen no more, or if seen is relatively faint +and unimpressive. But when autumn comes it appears again, this time not +like a wraith hovering above the westward tomb of the day-god, but +rather like a spirit of the morning announcing his reincarnation in the +east. + +The reason why the Zodiacal Light is best seen in our latitudes at the +periods just mentioned is because at those times the Zodiac is more +nearly perpendicular to the horizon, first in the west and then in the +east; and, since the phenomenon is confined within the borders of the +Zodiac, it cannot be favorably placed for observation when the zodiacal +plane is but slightly inclined to the horizon. Its faint light requires +the contrast of a background of dark sky in order to be readily +perceptible. But within the tropics, where the Zodiac is always at a +favorable angle, the mysterious light is more constantly visible. +Nearly all observant travelers in the equatorial regions have taken +particular note of this phenomenon, for being so much more conspicuous +there than in the temperate zones it at once catches the eye and holds +the attention as a novelty. Humboldt mentions it many times in his +works, for his genius was always attracted by things out of the +ordinary and difficult of explanation, and he made many careful +observations on its shape, its brilliancy, and its variations; for +there can be no doubt that it does vary, and sometimes to an +astonishing degree. It is said that it once remained practically +invisible in Europe for several years in succession. During a trip to +South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a +remarkable difference between the appearance of the Zodiacal Light on +his going and coming voyages. In fact, when crossing the equator going +south he did not see it at all; but on returning he had, on March 6th, +when one degree south of the equator, a memorable view of it. + +It was a bright, clear night, and the Zodiacal Light was +extraordinarily brilliant—brighter than he had ever seen it before. The +Milky Way was not to be compared with it. The brightest part extended +75° from the sun. There was a faint and much narrower extension which +they could just make out beyond the Pleiades along the ecliptic, but +the greater part of the Zodiacal Light showed as a broad truncated +column, and it did not appear nearly as conical as he had before seen +it. + + +When out of the brief twilight of intertropical lands, where the sun +drops vertically to the horizon and night rushes on like a wave of +darkness, the Zodiacal Light shoots to the very zenith, its color is +described as a golden tint, entirely different from the silvery sheen +of the Milky Way. If I may venture again to refer to personal +experiences and impressions, I will recall a view of the Zodiacal Light +from the summit of the cone of Mt Etna in the autumn of the year 1896 +(more briefly described in _Astronomy with the Naked Eye_). There are +few lofty mountains so favorably placed as Etna for observations of +this kind. It was once resorted to by Prof. George E. Hale, in an +attempt to see the solar corona without an eclipse. Rising directly +from sea-level to an elevation of nearly eleven thousand feet, the +observer on its summit at night finds himself, as it were, lost in the +midst of the sky. But for the black flanks of the great cone on which +he stands he might fancy himself to be in a balloon. On the occasion to +which I refer the world beneath was virtually invisible in the moonless +night. The blaze of the constellations overhead was astonishingly +brilliant, yet amid all their magnificence my attention was immediately +drawn to a great tapering light that sprang from the place on the +horizon where the sun would rise later, and that seemed to be blown out +over the stars like a long, luminous veil. It was the finest view of +the Zodiacal light that I had ever enjoyed—thrilling in its +strangeness—but I was almost disheartened by the indifference of my +guide, to whom it was only a light and nothing more. If he had no +science, he had less poetry—rather a remarkable thing, I thought, for a +child of his clime. The Light appeared to me to be distinctly brighter +than the visible part of the Milky Way which included the brilliant +stretches in Auriga and Perseus, and its color, if one may speak of +color in connection with such an object, seemed richer than that of the +galactic band; but I did not think of it as yellow, although Humboldt +has described it as resembling a golden curtain drawn over the stars, +and Du Chaillu in Equatorial Africa found it of a bright yellow color. +It may vary in color as in conspicuousness. The fascination of that +extraordinary sight has never faded from my memory. I turned to regard +it again and again, although I had never seen the stellar heavens so +brilliant, and it was one of the last things I looked for when the +morning glow began softly to mount in the east, and Sicily and the +Mediterranean slowly emerged from the profound shadow beneath us. + +The Zodiacal Light seems never to have attracted from astronomers in +general the amount of careful attention that it deserves; perhaps +because so little can really be made of it as far as explanation is +concerned. I have referred to the restraint that scientific writers +apparently feel in speaking of it. The grounds for speculation that it +affords may be too scanty to lead to long discussions, yet it piques +curiosity, and as we shall see in a moment has finally led to a most +interesting theory. Once it was the subject of an elaborate series of +studies which carried the observer all round the world. That was in +1845—46, during the United States Exploring Expedition that visited the +then little known Japan. The chaplain of the fleet, the Rev. Mr Jones, +went out prepared to study the mysterious light in all its phases. He +saw it from many latitudes on both sides of the equator, and the +imagination cannot but follow him with keen interest in his +world-circling tour, keeping his eyes every night fixed upon the +phantasm overhead, whose position shifted with that of the hidden sun. +He demonstrated that the flow extends at times completely across the +celestial dome, although it is relatively faint directly behind the +earth. On his return the government published a large volume of his +observations, in which he undertook to show that the phenomenon was due +to the reflection of sunlight from a ring of meteoric bodies encircling +the earth. But, after all, this elaborate investigation settled +nothing. + +Prof. E. E. Barnard has more recently devoted much attention to the +Zodiacal Light, as well as to a strange attendant phenomenon called the +“Gegenschein,” or Counterglow, because it always appears at that point +in the sky which is exactly opposite the sun. The Gegenschein is an +extremely elusive phenomenon, suitable only for eyes that have been +specially trained to see it. Professor Newcomb has cautiously remarked +that + +it is said that in that point of the heavens directly opposite the sun +there is an elliptical patch of light... This phenomenon is so +difficult to account for that its existence is sometimes doubted; yet +the testimony in its favor is difficult to set aside. + + +It certainly cannot be set aside at all since the observations of +Barnard. I recall an attempt to see it under his guidance during a +visit to Mount Hamilton, when he was occupied there with the Lick +telescope. Of course, both the Gegenschein and the Zodiacal Light are +too diffuse to be studied with telescopes, which, so to speak, magnify +them out of existence. They can only be successfully studied with the +naked eye, since every faintest glimmer that they afford must be +utilized. This is especially true of the Gegenschein. At Mount +Hamilton, Mr Barnard pointed out to me its location with reference to +certain stars, but with all my gazing I could not be sure that I saw +it. To him, on the contrary, it was obvious; he had studied it for +months, and was able to indicate its shape, its boundaries, its +diameter, and the declination of its center with regard to the +ecliptic. There is not, of course, the shadow of a doubt of the +existence of the Gegenschein, and yet I question if one person in a +million has ever seen or ever will see it. The Zodiacal Light, on the +other hand, is plain enough, provided that the time and the +circumstances of the observation are properly chosen. + +In the attempts to explain the Zodiacal Light, the favorite hypothesis +has been that it is an appendage of the sun—perhaps simply an extension +of the corona in the plane of the ecliptic, which is not very far from +coinciding with that of the sun’s equator. This idea is quite a natural +one, because of the evident relation of the light to the position of +the sun. The vast extension of the equatorial wings of the corona in +1878 gave apparent support to this hypothesis; if the substance of the +corona could extend ten million miles from the sun, why might it not +extend even one hundred million, gradually fading out beyond the orbit +of the earth? A variation of this hypothesis assumes that the +reflection is due to swarms of meteors circling about the sun, in the +plane of its equator, all the way from its immediate neighborhood to a +distance exceeding that of the earth. But in neither form is the +hypothesis satisfactory; there is nothing in the appearance of the +corona to indicate that it extends even as far as the planet Mercury, +while as to meteors, the orbits of the known swarms do not accord with +the hypothesis, and we have no reason to believe that clouds of others +exist traveling in the part of space where they would have to be in +order to answer the requirements of the theory. The extension of the +corona in 1878 did not resemble in its texture the Zodiacal Light. + +Now, it has so often happened in the history of science that an +important discovery in one branch has thrown unexpected but most +welcome light upon some pending problem in some other branch, that a +strong argument might be based upon that fact alone against the too +exclusive devotion of many investigators to the narrow lines of their +own particular specialty; and the Zodiacal Light affords a case in +point, when it is considered in connection with recent discoveries in +chemistry and physics. From the fact that atoms are compound bodies +made up of corpuscles at least a thousand times smaller than the +smallest known atom—a fact which astounded most men of science when it +was announced a few years ago—a new hypothesis has been developed +concerning the nature of the Zodiacal Light (as well as other +astronomical riddles), and this hypothesis comes not from an +astronomer, but from a chemist and physicist, the Swede, Svante +Arrhenius. In considering an outline of this new hypothesis we need +neither accept nor reject it; it is a case rather for suspension of +judgment. + +To begin with, it carries us back to the “pressure of light” mentioned +in the preceding chapter. The manner in which this pressure is believed +generally to act was there sufficiently explained, and it only remains +to see how it is theoretically extended to the particles of matter +supposed to constitute the Zodiacal Light. We know that corpuscles, or +“fragments of atoms” negatively electrified, are discharged from hot +bodies. Streams of these “ions” pour from many flames and from molten +metals; and the impact of the cathode and ultra-violet rays causes them +to gush even from cold bodies. In the vast laboratory of the sun it is +but reasonable to suppose that similar processes are taking place. “As +a very hot metal emits these corpuscles,” says Prof. J. J. Thomson, “it +does not seem an improbable hypothesis that they are emitted by that +very hot body, the sun.” Let it be assumed, then, that the sun does +emit them; what happens next? Negatively charged corpuscles, it is +known, serve as nuclei to which particles of matter in the ordinary +state are attracted, and it is probable that those emitted from the sun +immediately pick up loads in this manner and so grow in bulk. If they +grow large enough the gravitation of the sun draws them back, and they +produce a negative charge in the solar atmosphere. But it is probable +that many of the particles do not attain the critical size which, +according to the principles before explained, would enable the +gravitation of the sun to retain them in opposition to the pressure of +the waves of light, and with these particles the light pressure is +dominant. Clouds of them may be supposed to be continually swept away +from the sun into surrounding space, moving mostly in or near the plane +of the solar equator, where the greatest activity, as indicated by +sunspots and related phenomena, is taking place. As they pass outward +into space many of them encounter the earth. If the earth, like the +moon, had no atmosphere the particles would impinge directly on its +surface, giving it a negative electric charge. But the presence of the +atmosphere changes all that, for the first of the flying particles that +encounter it impart to it their negative electricity, and then, since +like electric charges repel like, the storm of particles following will +be sheered off from the earth, and will stream around it in a maze of +hyperbolic paths. Those that continue on into space beyond the earth +may be expected to continue picking up wandering particles of matter +until their bulk has become so great that the solar attraction prevails +again over the light pressure acting upon them, and they turn again +sunward. Passing the earth on their return they will increase the +amount of dust-clouds careering round it; and these will be further +increased by the action of the ultra-violet rays of the sunlight +causing particles to shoot radially away from the earth when the +negative charge of the upper atmosphere has reached a certain amount, +which particles, although starting sunward, will be swept back to the +earth with the oncoming streams. As the final result of all this +accumulation of flying and gyrating particles in the earth’s +neighborhood, we are told that the latter must be transformed into the +semblance of a gigantic solid-headed comet provided with streaming +tails, the longest of them stretching away from the direction of the +sun, while another shorter one extends toward the sun. This shorter +tail is due to the particles that we have just spoken of as being +driven sunward from the earth by the action of ultra-violet light. No +doubt this whole subject is too technical for popular statement; but at +any rate the general reader can understand the picturesque side of the +theory, for its advocates assure us that if we were on the moon we +would doubtless be able to see the comet-like tails of the earth, and +then we could appreciate the part that they play in producing the +phenomenon of the Zodiacal Light. + +That the Light as we see it could be produced by the reflection of +sunlight from swarms of particles careering round the earth in the +manner supposed by Arrhenius’ hypothesis is evident enough; and it will +be observed that the new theory, after all, is only another variant of +the older one which attributes the Zodiacal Light to an extension of +the solar corona. But it differs from the older theory in offering an +explanation of the manner in which the extension is effected, and it +differentiates between the corona proper and the streams of negative +particles shot away from the sun. In its details the hypothesis of +Arrhenius also affords an explanation of many peculiarities of the +Zodiacal Light, such as that it is confined to the neighborhood of the +ecliptic, and that it is stronger on the side of the earth which is +just turning away from a position under the sun than on the other side; +but it would carry us beyond our limits to go into these particulars. +The Gegenschein, according to this theory, is a part of the same +phenomenon as the Zodiacal Light, for by the laws of perspective it is +evident that the reflection from the streams of particles situated at a +point directly opposite to the sun would be at a maximum, and this is +the place which the Gegenschein occupies. Apart from its geometrical +relations to the position of the sun, the variability of the Zodiacal +Light appears to affirm its solar dependence, and this too would be +accounted for by Arrhenius’ hypothesis better than by the old theory of +coronal extension. The amount of corpuscular discharge from the sun +must naturally be governed by the state of relative activity or +inactivity of the latter, and this could not but be reflected in the +varying splendor of the Zodiacal Light. But much more extended study +than has yet been given to the subject will be required before we can +feel that we know with reasonable certainty what this mysterious +phenomenon really is. By the hypothesis of Arrhenius every planet that +has an atmosphere must have a Zodiacal Light attending it, but the +phenomenon is too faint for us to be able to see it in the case, for +instance, of Venus, whose atmosphere is very abundant. The moon has no +corresponding “comet’s tail” because, as already explained, of the lack +of a lunar atmosphere to repel the streams by becoming itself +electrified; but if there were a lunar Zodiacal Light, no doubt we +could see it because of the relative nearness of our satellite. + + + + +IX +Marvels of the Aurora + + +One of the most vivid recollections of my early boyhood is that of +seeing my father return hastily into the house one evening and call out +to the family: “Come outside and look at the sky!” Ours was a country +house situated on a commanding site, and as we all emerged from the +doorway we were dumbfounded to see the heavens filled with pale flames +which ran licking and quivering over the stars. Instantly there sprang +into my terrified mind the recollection of an awful description of “the +Day of Judgment” (the _Dies Iræ_), which I had heard with much +perturbation of spirit in the Dutch Reformed church from the lips of a +tall, dark-browed, dreadfully-in-earnest preacher of the old-fashioned +type. My heart literally sank at sight of the spectacle, for it +recalled the preacher’s very words; it was just as he had said it would +be, and it needed the assured bearing of my elders finally to convince +me that + +That Day of Wrath, O dreadful day, +When Heaven and Earth shall pass away, +As David and the Sibyl say + + +had not actually come upon us. And even the older members of the +household were not untouched with misgivings when menacing spots of +crimson appeared, breaking out now here, now there, in the shuddering +sky. Toward the north the spectacle was appalling. A huge arch spanned +an unnaturally dark segment resting on the horizon, and above this arch +sprang up beams and streamers in a state of incessant agitation, +sometimes shooting up to the zenith with a velocity that took one’s +breath, and sometimes suddenly falling into long ranks, and _marching, +marching, marching,_ like an endless phalanx of fiery specters, and +moving, as I remember, always from east to west. The absolute silence +with which these mysterious evolutions were performed and the quavering +reflections which were thrown upon the ground increased the awfulness +of the exhibition. Occasionally enormous curtains of lambent flame +rolled and unrolled with a majestic motion, or were shaken to and fro +as if by a mighty, noiseless wind. At times, too, a sudden billowing +rush would be made toward the zenith, and for a minute the sky overhead +would glow so brightly that the stars seemed to have been consumed. The +spectacle continued with varying intensity for hours. + + +[Illustration: Auroral beams seen in England] + + +This exhibition occurred in Central New York, a latitude in which the +Aurora Borealis is seldom seen with so much splendor. I remember +another similar one seen from the city of New York in November, 1882. +On this last occasion some observers saw a great upright beam of light +which majestically moved across the heavens, stalking like an +apparition in the midst of the auroral pageant, of whose general +movements it seemed to be independent, maintaining always its upright +posture, and following a magnetic parallel from east to west. This +mysterious beam was seen by no less than twenty-six observers in +different parts of the country, and a comparison of their observations +led to a curious calculation indicating that the apparition was about +_one hundred and thirty-three miles tall_ and moved at the speed of ten +miles per second! + + +[Illustration: Auroral arches of an eliptic form seen in the Arctic +regions] + + +But, as everybody knows, it is in the Arctic regions that the Aurora, +or the “Northern Lights,” can best be seen. There, in the long polar +night, when for months together the sun does not rise, the strange +coruscations in the sky often afford a kind of spectral daylight in +unison with the weird scenery of the world of ice. The pages in the +narratives of Arctic exploration that are devoted to descriptions of +the wonderful effects of the Northern Lights are second to none that +man has ever penned in their fascination. The lights, as I have already +intimated, display astonishing colors, particularly shades of red and +green, as they flit from place to place in the sky. The discovery that +the magnetic needle is affected by the Aurora, quivering and darting +about in a state of extraordinary excitement when the lights are +playing in the sky, only added to the mystery of the phenomenon until +its electro-magnetic nature had been established. This became evident +as soon as it was known that the focus of the displays was the magnetic +pole; and when the far South was visited the Aurora Australis was +found, having its center at the South Magnetic Pole. Then, if not +before, it was clear that the earth was a great globular magnet, having +its poles of opposite magnetism, and that the auroral lights, whatever +their precise cause might be, were manifestations of the magnetic +activity of our planet. After the invention of magnetic telegraphy it +was found that whenever a great Aurora occurred the telegraph lines +were interrupted in their operation, and the ocean cables ceased to +work. Such a phenomenon is called a “magnetic storm.” + +The interest excited by the Aurora in scientific circles was greatly +stimulated when, in the last half of the nineteenth century, it was +discovered that it is a phenomenon intimately associated with +disturbances on the sun. The ancient “Zurich Chronicles,” extending +from the year 1000 to the year 1800, in which both sun-spots visible to +the naked eye and great displays of the auroral lights were recorded, +first set Rudolf Wolf on the track of this discovery. The first notable +proof of the suspected connection was furnished with dramatic emphasis +by an occurrence which happened on September 1, 1859. Near noon on that +day two intensely brilliant points suddenly broke out in a group of +sun-spots which were under observation by Mr R. C. Carrington at his +observatory at Redhill, England. The points remained visible for not +more than five minutes, during which interval they moved _thirty-five +thousand miles_ across the solar disk. Mr R. Hodgson happened to see +the same phenomenon at his observatory at Highgate, and thus all +possibility of deception was removed. But neither of the startled +observers could have anticipated what was to follow, and, indeed, it +was an occurrence which has never been precisely duplicated. I quote +the eloquent account given by Miss Clerke in her _History of Astronomy +During the Nineteenth Century._ + +This unique phenomenon seemed as if specially designed to accentuate +the inference of a sympathetic relation between the earth and the sun. +From August 28 to September 4, 1859, a magnetic storm of unparalleled +intensity, extent, and duration was in progress over the entire globe. +Telegraphic communication was everywhere interrupted—except, indeed, +that it was in some cases found practicable to work the lines _without +batteries_ by the agency of the earth-currents alone; sparks issued +from the wires; gorgeous auroras draped the skies in solemn crimson +over both hemispheres, and even in the tropics; the magnetic needle +lost all trace of continuity in its movements and darted to and fro as +if stricken with inexplicable panic. The coincidence was even closer. +_At the very instant_ of the solar outburst witnessed by Carrington and +Hodgson the photographic apparatus at Kew registered a marked +disturbance of all the three magnetic elements; while shortly after the +ensuing midnight the electric agitation culminated, thrilling the whole +earth with subtle vibrations, and lighting up the atmosphere from pole +to pole with coruscating splendors which perhaps dimly recall the times +when our ancient planet itself shone as a star. + + +If this amazing occurrence stood alone, and as I have already said it +has never been exactly duplicated, doubt might be felt concerning some +of the inferences drawn from it; but in varying forms it has been +repeated many times, so that now hardly anyone questions the reality of +the assumed connection between solar outbursts and magnetic storms +accompanied by auroral displays on the earth. It is true that the late +Lord Kelvin raised difficulties in the way of the hypothesis of a +direct magnetic action of the sun upon the earth, because it seemed to +him that an inadmissible quantity of energy was demanded to account for +such action. But no calculation like that which he made is final, since +all calculations depend upon the validity of the data; and no authority +is unshakable in science, because no man can possess omniscience. It +was Lord Kelvin who, but a few years before the thing was actually +accomplished, declared that aerial navigation was an impracticable +dream, and demonstrated its impracticability by calculation. However +the connection may be brought about, it is as certain as evidence can +make it that solar outbursts are coincident with terrestial magnetic +disturbances, and coincident in such a way as to make the inference of +a causal connection irresistible. The sun is only a little more than a +hundred times its own diameter away from the earth. Why, then, with the +subtle connection between them afforded by the ether which conveys to +us the blinding solar light and the life-sustaining solar heat, should +it be so difficult to believe that the sun’s enormous electric energies +find a way to us also? No doubt the impulse coming from the sun acts +upon the earth after the manner of a touch upon a trigger, releasing +energies which are already stored up in our planet. + +But besides the evidence afforded by such occurrences as have been +related of an intimate connection between solar outbreaks and +terrestial magnetic flurries, attended by magnificent auroral displays, +there is another line of proof pointing in the same direction. Thus, it +is known that the sun-spot period, as remarked in a preceding chapter, +coincides in a most remarkable manner with the periodic fluctuations in +the magnetic state of the earth. This coincidence runs into the most +astonishing details. For instance, when the sun-spot period shortens, +the auroral period shortens to precisely the same extent; as the short +sun-spot periods usually bring the most intense outbreaks of solar +activity, so the corresponding short auroral periods are attended by +the most violent magnetic storms; a secular period of about two hundred +and twenty-two years affecting sun-spots is said to have its auroral +duplicate; a shorter period of fifty-five and a half years, which some +observers believe that they have discovered appears also to be common +to the two phenomena; and yet another “superposed” period of about +thirty-five years, which some investigators aver exists, affects +sun-spots and aurora alike. In short, the coincidences are so numerous +and significant that one would have to throw the doctrine of +probability to the winds in order to be able to reject the conclusion +to which they so plainly lead. + + +[Illustration: Auroral curtain seen in Scandinavia] + + +But still the question recurs: How is the influence transmitted? Here +Arrhenius comes once more with his hypothesis of negative corpuscles, +or ions, driven away from the sun by light-pressure—a hypothesis which +seems to explain so many things—and offers it also as an explanation of +the way in which the sun creates the Aurora. He would give the Aurora +the same lineage with the Zodiacal Light. To understand the application +of this theory we must first recall the fact that the earth is a great +magnet having its two opposite poles of magnetism, one near the Arctic +and the other near the Antarctic Circle. Like all magnets, the earth is +surrounded with “lines of force,” which, after the manner of the curved +rays we saw in the photograph of a solar eclipse, start from a pole, +rising at first nearly vertically, then bend gradually over, passing +high above the equator, and finally descending in converging sheaves to +the opposite pole. Now the axis of the earth is so placed in space that +it lies at nearly a right angle to the direction of the sun, and as the +streams of negatively charged particles come pouring on from the sun +(see the last preceding chapter), they arrive in the greatest numbers +over the earth’s equatorial regions. There they encounter the lines of +magnetic force at the place where the latter have their greatest +elevation above the earth, and where their direction is horizontal to +the earth’s surface. Obeying a law which has been demonstrated in the +laboratory, the particles then follow the lines of force toward the +poles. While they are above the equatorial regions they do not become +luminescent, because at the great elevation that they there occupy +there is virtually no atmosphere; but as they pass on toward the north +and the south they begin to descend with the lines of force, curving +down to meet at the poles; and, encountering a part of the atmosphere +comparable in density with what remains in an exhausted Crookes tube, +they produce a glow of cathode rays. This glow is conceived to +represent the Aurora, which may consequently be likened to a gigantic +exhibition of vacuum-tube lights. Anybody who recalls his student days +in the college laboratory and who has witnessed a display of Northern +Lights will at once recognize the resemblance between them in colors, +forms, and behavior. This resemblance had often been noted before +Arrhenius elaborated his hypothesis. + +Without intending to treat his interesting theory as more than a +possibly correct explanation of the phenomena of the Aurora, we may +call attention to some apparently confirmatory facts. One of the most +striking of these relates to a seasonal variation in the average number +of auroræ. It has been observed that there are more in March and +September than at any other time of the year, and fewer in June and +December; moreover (and this is a delicate test as applied to the +theory), they are slightly rarer in June than in December. Now all +these facts seem to find a ready explanation in the hypothesis of +Arrhenius, thus: (1) The particles issuing from the sun are supposed to +come principally from the regions whose excitement is indicated by the +presence of sun-spots (which accords with Hale’s observation that +sun-spots are columns of ionized vapors), and these regions have a +definite location on either side of the solar equator, seldom +approaching it nearer than within 5° or 10° north or south, and never +extending much beyond 35° toward either pole; (2) The equator of the +sun is inclined about 7° to the plane of the earth’s orbit, from which +it results that twice in a year—_viz.,_ in June and December—the earth +is directly over the solar equator, and twice a year—_viz.,_ in March +and September—when it is farthest north or south of the solar equator, +it is over the inner edge of the sun-spot belts. Since the corpuscles +must be supposed to be propelled radially from the sun, few will reach +the earth when the latter is over the solar equator in June and +December, but when it is over, or nearly over, the spot belts, in March +and September, it will be in the line of fire of the more active parts +of the solar surface, and relatively rich streams of particles will +reach it. This, as will be seen from what has been said above, is in +strict accord with the observed variations in the frequency of auroræ. +Even the fact that somewhat fewer auroræ are seen in June than in +December also finds its explanation in the known fact that the earth is +about three million miles nearer the sun in the winter than in the +summer, and the number of particles reaching it will vary, like the +intensity of light, inversely as the square of the distance. These +coincidences are certainly very striking, and they have a cumulative +force. If we accept the theory, it would appear that we ought to +congratulate ourselves that the inclination of the sun’s equator is so +slight, for as things stand the earth is never directly over the most +active regions of the sun-spots, and consequently never suffers from +the maximum bombardment of charged particles of which the sun is +capable. Incessant auroral displays, with their undulating draperies, +flitting colors, and marching columns might not be objectionable from +the point of view of picturesqueness, but one magnetic storm of extreme +intensity following closely upon the heels of another, for months on +end, crazing the magnetic needle and continually putting the telegraph +and cable lines out of commission, to say nothing of their effect upon +“wireless telegraphy”, would hardly add to the charms of terrestrial +existence. + + +[Illustration: Auroral arches seen in Scandinavia] + + +One or two other curious points in connection with Arrhenius’ +hypothesis may be mentioned. First, the number of auroræ, according to +his explanation, ought to be greatest in the daytime, when the face of +the earth on the sunward side is directly exposed to the atomic +bombardment. Of course visual observation can give us no information +about this, since the light of the Aurora is never sufficiently intense +to be visible in the presence of daylight, but the records of the +magnetic observatories can be, and have been, appealed to for +information, and they indicate that the facts actually accord with the +theory. Behind the veil of sunlight in the middle of the afternoon, +there is good reason to believe, auroral exhibitions often take place +which would eclipse in magnificence those seen at night if we could +behold them. Observation shows, too, that auroræ are more frequent +before than after midnight, which is just what we should expect if they +originate in the way that Arrhenius supposes. Second, the theory offers +an explanation of the alleged fact that the formation of clouds in the +upper air is more frequent in years when auroræ are most abundant, +because clouds are the result of the condensation of moisture upon +floating particles in the atmosphere (in an absolutely dustless +atmosphere there would be no clouds), and it has been proved that +negative ions like those supposed to come from the sun play a master +part in the phenomena of cloud formation. + +Yet another singular fact, almost mystical in its suggestions, may be +mentioned. It seems that the dance of the auroral lights occurs most +frequently during the absence of the moon from the hemisphere in which +they appear, and that they flee, in greater part, to the opposite +hemisphere when the moon’s revolution in an orbit considerably inclined +to the earth’s equator brings her into that where they have been +performing. Arrhenius himself discovered this curious relation of +auroral frequency to the position of the moon north or south of the +equator, and he explains it in this way. The moon, like the earth, is +exposed to the influx of the ions from the sun; but having no +atmosphere, or almost none, to interfere with them, they descend +directly upon her surface and charge her with an electric negative +potential to a very high degree. In consequence of this she affects the +electric state of the upper parts of the earth’s atmosphere where they +lie most directly beneath her, and thus prevents, to a large extent, +the negative discharges to which the appearance of the Aurora is due. +And so “the extravagant and erring spirit” of the Aurora avoids the +moon as Hamlet’s ghost fled at the voice of the cock announcing the +awakening of the god of day. + +There are even other apparent confirmations of the hypothesis, but we +need not go into them. We shall, however, find one more application of +it in the next chapter, for it appears to be a kind of cure-all for +astronomical troubles; at any rate it offers a conceivable solution of +the question, How does the sun manage to transmit its electric +influence to the earth? And this solution is so grandiose in +conception, and so novel in the mental pictures that it offers, that +its acceptance would not in the least detract from the impression that +the Aurora makes upon the imagination. + + + + +X +Strange Adventures of Comets + + +The fears and legends of ancient times before Science was born, and the +superstitions of the Dark Ages, sedulously cultivated for theological +purposes by monks and priests, have so colored our ideas of the +influence that comets have had upon the human mind that many readers +may be surprised to learn that it was the apparition of a wonderful +comet, that of 1843, which led to the foundation of our greatest +astronomical institution, the Harvard College Observatory. No doubt the +comet superstition existed half a century ago, as, indeed, it exists +yet today, but in this case the marvelous spectacle in the sky proved +less effective in inspiring terror than in awakening a desire for +knowledge. Even in the sixteenth century the views that enlightened +minds took of comets tended powerfully to inspire popular confidence in +science, and Halley’s prediction, after seeing and studying the motion +of the comet which appeared in 1682, that it would prove to be a +regular member of the sun’s family and would be seen returning after a +period of about seventy-six years, together with the fulfillment of +that prediction, produced a revulsion from the superstitious notions +which had so long prevailed. + + +[Illustration: Swift’s comet. Taken at Arequipa, March 30 1892] + + +Then the facts were made plain that comets are subject to the law of +gravitation equally with the planets; that there are many which +regularly return to the neighborhood of the sun (perihelion); and that +these travel in orbits differing from those of the planets only in +their greater eccentricity, although they have the peculiarity that +they do not, like the planets, all go round the sun in the same +direction, and do not keep within the general plane of the planetary +system, but traverse it sometimes from above and sometimes from below. +Other comets, including most of the “great” ones, appear to travel in +parabolic or, in a few cases, hyperbolic orbits, which, not being +closed curves, never bring them back again. But it is not certain that +these orbits may not be extremely eccentric ellipses, and that after +the lapse of hundreds, or thousands, of years the comets that follow +them may not reappear. The question is an interesting one, because if +all orbits are really ellipses, then all comets must be permanent +members of the solar system, while in the contrary case many of them +are simply visitors, seen once and never to be seen again. The +hypothesis that comets are originally interlopers might seem to derive +some support from the fact that the certainly periodic ones are +associated, in groups, with the great outer planets, whose attraction +appears to have served as a trap for them by turning them into +elliptical orbits and thus making them prisoners in the solar system. +Jupiter, owing to his great mass and his commanding situation in the +system, is the chief “comet-catcher;” but he catches them not for +himself, but for the sun. Yet if comets do come originally from without +the borders of the planetary system, it does not, by any means, follow +that they were wanderers at large in space before they yielded to the +overmastering attraction of the sun. Investigation of the known +cometary orbits, combined with theoretical considerations, has led some +astronomers to the conclusion that as the sun travels onward through +space he “picks up _en route_” cometary masses which, without belonging +strictly to his empire, are borne along in the same vast “cosmical +current” that carries the solar system. + +But while no intelligent person any longer thinks that the appearance +of a great comet is a token from the heavenly powers of the approaching +death of a mighty ruler, or the outbreak of a devastating war, or the +infliction of a terrible plague upon wicked mankind, science itself has +discovered mysteries about comets which are not less fascinating +because they are more intellectual than the irrational fancies that +they have displaced. To bring the subject properly before the mind, let +us see what the principal phenomena connected with a comet are. + +At the present day comets are ordinarily “picked up” with the telescope +or the photographic plate before any one except their discoverer is +aware of their existence, and usually they remain so insignificant in +appearance that only astronomers ever see them. Yet so great is the +prestige of the word “comet” that the discovery of one of these +inconspicuous wanderers, and its subsequent movements, become items of +the day’s news which everybody reads with the feeling, perhaps, that at +least he knows what is going on in the universe even if he doesn’t +understand it. But a truly great comet presents quite a different +proposition. It, too, is apt to be detected coming out of the depths of +space before the world at large can get a glimpse of it, but as it +approaches the sun its aspect undergoes a marvelous change. Agitated +apparently by solar influence, it throws out a long streaming tail of +nebulous light, directed away from the sun and looking as if blown out +like a pennon by a powerful wind. Whatever may be the position of the +comet with regard to the sun, as it circles round him it continually +keeps its tail on the off side. This, as we shall soon see, is a fact +of capital importance in relation to the probable nature of comets’ +tails. Almost at the same time that the formation of the tail is +observed a remarkable change takes place in the comet’s head, which, by +the way, is invariably and not merely occasionally its most important +part. On approaching the sun the head usually contracts. Coincidently +with this contraction a nucleus generally makes its appearance. This is +a bright, star-like point in the head, and it probably represents the +totality of solid matter that the comet possesses. But it is regarded +as extremely unlikely that even the nucleus consists of a uniformly +solid mass. If it were such, comets would be far more formidable +visitors when they pass near the planets than they have been found to +be. The diameter of the nucleus may vary from a few hundred up to +several thousand miles; the heads, on the average, are from twenty-five +thousand to one hundred thousand miles in diameter, although a few have +greatly exceeded these dimensions; that of the comet of 1811, one of +the most stupendous ever seen, was a million and a quarter miles in +diameter! As to the tails, not withstanding their enormous length—some +have been more than a hundred million miles long—there is reason to +believe that they are of extreme tenuity, “as rare as vacuum.” The +smallest stars have been seen shining through their most brilliant +portions with undiminished luster. + +After the nucleus has been formed it begins to throw out bright jets +directed toward the sun. A stream, and sometimes several streams, of +light also project sunward from the nucleus, occasionally appearing +like a stunted tail directed oppositely to the real tail. Symmetrical +envelopes which, seen in section, appear as half circles or parabolas, +rise sunward from the nucleus, forming a concentric series. The ends of +these stream backward into the tail, to which they seem to supply +material. Ordinarily the formation of these ejections and envelopes is +attended by intense agitation of the nucleus, which twists and turns, +swinging and gyrating with an appearance of the greatest violence. +Sometimes the nucleus is seen to break up into several parts. The +entire heads of some comets have been split asunder in passing close +around the sun; The comet of 1882 retreated into space after its +perihelion passage with _five heads_ instead of the one that it had +originally, and each of these heads had its own tail! + +The possession of the spectroscope has enabled astronomers during later +years to study the chemical composition of comets by analyzing their +light. At first the only substances thus discovered in them were +hydro-carbon compounds, due evidently to the gaseous envelopes in which +some combination of hydrogen with carbon existed. Behind this gaseous +spectrum was found a faint continuous spectrum ascribed to the nucleus, +which apparently both reflects the sunlight and gives forth the light +of a glowing solid or liquid. Subsequently sodium and iron lines were +found in cometary spectra. The presence of iron would seem to indicate +that some of these bodies may be much more massive than observations on +their attractive effects have indicated. In some recent comets, such as +Morehouse’s, in 1908, several lines have been found, the origin of +which is unknown. + +Without going back of the nineteenth century we may find records of +some of the most extraordinary comets that man has ever looked upon. In +1811, still spoken of as “the year of the comet,” because of the +wonderful vintage ascribed to the skyey visitor, a comet shaped like a +gigantic sword amazed the whole world, and, as it remained visible for +seventeen months, was regarded by superstitious persons as a symbol of +the fearful happenings of Napoleon’s Russian campaign. This comet, the +extraordinary size of whose head, greatly exceeding that of the sun +itself, has already been mentioned, was also remarkable for exhibiting +so great a brilliancy without approaching even to the earth’s distance +from the sun. But there was once a comet (and only once—in the year +1729) which never got nearer to the sun than four times the distance of +the earth and yet appeared as a formidable object in the sky. As +Professor Young has remarked, “it must have been an enormous comet to +be visible from such a distance.” And we are to remember that there +were no great telescopes in the year 1729. That comet affects the +imagination like a phantom of space peering into the solar system, +displaying its enormous train afar off (which, if it had approached as +near as other comets, would probably have become _the_ celestial wonder +of all human memory), and then turning away and vanishing in the depths +of immensity. + +In 1843 a comet appeared which was so brilliant that it could be seen +in broad day close beside the sun! This was the first authenticated +instance of that kind, but the occurrence was to be repeated, as we +shall see in a moment, less than forty years later. + +The splendid comet of 1858, usually called Donati’s, is remembered by +many persons yet living. It was, perhaps, both as seen by the naked eye +and with the telescope, the most beautiful comet of which we have any +record. It too marked a rich vintage year, still remembered in the +vineyards of France, where there is a popular belief that a great comet +ripens the grape and imparts to the wine a flavor not attainable by the +mere skill of the cultivator. There are “comet wines,” carefully +treasured in certain cellars, and brought forth only when their owner +wishes to treat his guests to a sip from paradise. + +The year 1861 saw another very remarkable comet, of an aspect strangely +vast and diffuse, which is believed to have swept the earth with its +immense tail when it passed between us and the sun on the night of June +30th, an event which produced no other known effect than the appearance +of an unwonted amount of scattered light in the sky. + +The next very notable comet was the “Great Southern Comet” of 1880, +which was not seen from the northern hemisphere. It mimicked the aspect +of the famous comet of 1843, and to the great surprise of astronomers +appeared to be traveling in the same path. This proved to be the rising +of the curtain for an astronomical sensation unparalleled in its kind; +for two years later another brilliant comet appeared, first in the +southern hemisphere, _and it too followed the same track._ The +startling suggestion was now made that this comet was identical with +those of 1843 and 1880, its return having been hastened by the +resistance experienced in passing twice through the coronal envelope, +and there were some who thought that it would now swing swiftly round +and then plunge straight into the sun, with consequences that might be +disastrous to us on account of the “flash of heat” that would be +produced by the impact. Nervous people were frightened, but observation +soon proved that the danger was imaginary, for although the comet +almost grazed the sun, and must have rushed through two or three +million miles of the coronal region, no retardation of its immense +velocity was perceptible, and it finally passed away in a damaged +condition, as before remarked, and has never since appeared. + +Then the probable truth was perceived—_viz.,_ that the three comets +(1843, 1880, and 1882) were not one identical body, but three separate +ones all traveling in the same orbit. It was found, too, that a comet +seen in 1668 bore similar insignia of relationship. The natural +inference was that these four bodies had once formed a single mass +which had been split apart by the disruptive action of the sun. +Strength was lent to this hypothesis by the fact that the comet of 1882 +was apparently torn asunder during its perihelion passage, retreating +into space in a dissevered state. But Prof. George Forbes has a theory +that the splitting of the original cometary mass was effected by an +unknown planet, probably greater than Jupiter, situated at a hundred +times the earth’s distance from the sun, and revolving in a period of a +thousand years. He supposes that the original comet was not that of +1668, but one seen in 1556, which has since been “missing,” and that +its disruption occurred from an encounter with the supposititious +planet about the year 1700. Truly from every point of view comets are +the most extraordinary of adventurers! + +The comet of 1882 was likewise remarkable for being visible, like its +predecessor of 1843, in full daylight in close proximity to the sun. +The story of its detection when almost in contact with the solar disk +is dramatic. It had been discovered in the southern hemisphere only a +couple of weeks before its perihelion, which occurred on September +17th, and on the forenoon of that day it was seen by Doctor Common in +England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope, +almost touching the sun. It looked like a dazzling white bird with +outspread wings. The southern observers watched it go _right into the +sun,_ when it instantly disappeared. What had happened was that the +comet in passing its perihelion point had swung exactly between the +earth and the sun. On the following morning it was seen from all parts +of the world close by the sun on the opposite side, and it remained +thus visible for three days, gradually receding from the solar disk. It +then became visible for northern observers in the morning sky before +sunrise, brandishing a portentous sword-shaped tail which, if it had +been in the evening sky, would have excited the wonder of hundreds of +millions, but situated where it was, comparatively few ever saw it. + + +[Illustration: Daniels’ comet. August 11, 1907] + + +The application of photography to the study of comets has revealed many +curious details which might otherwise have escaped detection, or at +best have remained subject to doubt. It has in particular shown not +only the precise form of the tails, but the remarkable vicissitudes +that they undergo. Professor Barnard’s photographs of Brooks’ comet in +1893 suggested, by the extraordinary changes in the form of the tail +which they revealed, that the comet was encountering a series of +obstructions in space which bent and twisted its tail into fantastic +shapes. The reader will observe the strange form into which the tail +was thrown on the night of October 21st. A cloud of meteors through +which the comet was passing might have produced such deformations of +its tail. In the photograph of Daniels’ comet of 1907, a curious +striping of the tail will be noticed. The short bright streaks seen in +the photograph, it may be explained, are the images of stars which are +drawn out into lines in consequence of the fact that the photographic +telescope was adjusted to follow the motion of the comet while the +stars remained at rest. + +But the adventures of comets are not confined to possible encounters +with unknown obstacles. We have referred to the fact that the great +planets, and especially Jupiter, frequently interfere with the motions +of comets. This interference is not limited to the original alteration +of their orbits from possible parabolas to ellipses, but is sometimes +exercised again and again, turning the bewildered comets into +elliptical paths of all degrees of eccentricity. A famous example of +this kind of planetary horse-play is furnished by the story of Lexell’s +missing comet. This comet was first seen in 1770. Investigation showed +that it was moving in an orbit which should bring it back to perihelion +every five and a half years; yet it had never been seen before and, +although often searched for, has never been seen since. Laplace and +Leverrier proved mathematically that in 1767 it had approached so close +to Jupiter as to be involved among the orbits of his satellites. What +its track had been before is not known, but on that occasion the giant +planet seized the interloper, threw it into a short elliptic orbit and +sent it, like an arrested vagrant, to receive sentence at the bar of +the sun. On this journey it passed within less than 1,500,000 miles of +the earth. The form of orbit which Jupiter had impressed required, as +we have said, its return in about five and a half years; but soon after +1770 it had the misfortune a second time to encounter Jupiter at close +range, and he, as if dissatisfied with the leniency of the sun, or +indignant at the stranger’s familiarity, seized the comet and hurled it +out of the system, or at any rate so far away that it has never since +been able to rejoin the family circle that basks in the immediate rays +of the solar hearth. Nor is this the only instance in which Jupiter has +dealt summarily with small comets that have approached him with too +little deference. + + +[Illustration: Brooks’ comet. Photographed by Barnard, October 21, +1893] + + +The function which Jupiter so conspicuously fulfills as master of the +hounds to the sun is worth considering a little more in detail. To +change the figure, imagine the sun in its voyage through space to be +like a majestic battleship surrounded by its scouts. Small vessels (the +comets, as they are overhauled by the squadron, are taken in charge by +the scouts, with Jupiter for their chief, and are forced to accompany +the fleet, but not all are impressed. If a strange comet undertakes to +run across Jupiter’s bows the latter brings it to, and makes prize of +it by throwing it into a relatively small ellipse with the sun for its +focus. Thenceforth, unless, as happened to the unhappy comet of Lexell, +it encounters Jupiter again in such a way as to be diverted by him into +a more distant orbit, it can never get away. About thirty comets are +now known to have thus been captured by the great planet, and they are +called “Jupiter’s Comet Family.” But, on the other hand, if a wandering +comet crosses the wake of the chief planetary scout the latter simply +drives it away by accelerating its motion and compels it to steer off +into open space. The transformation of comets into meteors will be +considered in the next chapter, but here, in passing, mention may be +made of the strange fate of one member of Jupiter’s family, Biela’s +comet, which, having become over bold in its advances to its captor, +was, after a few revolutions in is impressed orbit, torn to pieces and +turned into a flock of meteors. + +And now let us return to the mystery of comets’ tails. That we are +fully justified in speaking of the tails of comets as mysterious is +proved by the declaration of Sir John Herschel, who averred, in so many +words, that “there is some profound secret and mystery of nature +concerned in this phenomenon,” and this profound secret and mystery has +not yet been altogether cleared up. Nevertheless, the all-explaining +hypothesis of Arrhenius offers us once more a certain amount of aid. +Comets’ tails, Arrhenius assures us, are but another result of the +pressure of light. The reader will recall the applications of this +theory to the Zodiacal Light and the Aurora. In the form in which we +now have to deal with it, the supposition is made that as a comet +approaches the sun eruptions of vapor, due to the solar heat, occur in +its nucleus. These are naturally most active on the side which is +directly exposed to the sun, whence the appearance of the immense +glowing envelopes that surround the nucleus on the sunward side. Among +the particles of hydro-carbon, and perhaps solid carbon in the state of +fine dust, which are thus set free there will be many whose size is +within the critical limit which enables the light-waves from the sun to +drive them away. Clouds of such particles, then, will stream off behind +the advancing comet, producing the appearance of a tail. This accounts +for the fact that the tails of comets are always directed away from the +sun, and it also explains the varying forms of the tails and the +extraordinary changes that they undergo. The speed of the particles +driven before the light-waves must depend upon their size and weight, +the lightest of a given size traveling the most swiftly. By accretion +certain particles might grow, thus losing velocity and producing the +appearance of bunches in the tail, such as have been observed. The +hypothesis also falls in with the researches of Bredichin, who has +divided the tails of comets into three principal classes—_viz.:_ (1) +Those which appear as long, straight rays; (2) Those which have the +form of curved plumes or scimitars; (3) Those which are short, brushy, +and curved sharply backward along the comet’s path. In the first type +he calculates the repulsive force at from twelve to fifteen times the +force of gravity; in the second at from two to four times; and in the +third at about one and a half times. The straight tails he ascribes to +hydrogen because the hydrogen atom is the lightest known; the +sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized +iron. It will be seen that, if the force driving off the tails is that +which Arrhenius assumes it to be, the forms of those appendages would +accord with those that Bredichin’s theory calls for. At the same time +we have an explanation of the multiple tails with which some comets +have adorned themselves. The comet of 1744, for instance, had at one +time no less than seven tails spread in a wide curved brush behind it. +Donati’s comet of 1858 also had at least two tails, the principal one +sword-shaped and the other long, narrow, and as straight as a rule. +According to Bredichin, the straight tail must have been composed of +hydrogen, and the other of some form of hydro-carbon whose atoms are +heavier than those of hydrogen, and, consequently, when swept away by +the storm of light-waves, followed a curvature depending upon the +resultant of the forces operating upon them. The seven tails of the +comet of 1744 presented a kind of diagram graphically exhibiting its +complex composition, and, if we knew a little more about the +constituents of a comet, we might be able to say from the amount of +curvature of the different tails just what were the seven substances of +which that comet consisted. + +If these theories seem to the reader fantastic, at any rate they are no +more fantastic than the phenomena that they seek to explain. + + + + +XI +Meteors, Fire-Balls, and Meteorites + + +One of the most terrorizing spectacles with which the heavens have ever +caused the hearts of men to quake occurred on the night of November 13, +1833. On that night North America, which faced the storm, was under a +continual rain of fire from about ten o’clock in the evening until +daybreak. + +_The fragments of a comet had struck the earth._ + +But the meaning of what had happened was not discovered until long +afterward. To the astronomers who, with astonishment not less than that +of other people, watched the wonderful scene, it was an unparalleled +“shower of meteors.” They did not then suspect that those meteors had +once formed the head of a comet. Light dawned when, a year later, Prof. +Denison Olmsted, of Yale College, demonstrated that the meteors had all +moved in parallel orbits around the sun, and that these orbits +intersected that of the earth at the point where our planet happened to +be on the memorable night of November 13th. Professor Olmsted even went +so far as to suggest that the cloud of meteors that had encountered the +earth might form a diffuse comet; but full recognition of the fact that +they were cometary débris came later, as the result of further +investigation. The key to the secret was plainly displayed in the +spectacle itself, and was noticed without being understood by thousands +of the terror-stricken beholders. It was _an umbrella of fire_ that had +opened overhead and covered the heavens; in other words, the meteors +all radiated from a particular point in the constellation Leo, and, +being countless as the snowflakes in a winter tempest, they ribbed the +sky with fiery streaks. Professor Olmsted showed that the radiation of +the meteors from a fixed point was an effect of perspective, and in +itself a proof that they were moving in parallel paths when they +encountered the earth. The fact was noted that there had been a +similar, but incomparably less brilliant, display of meteors on the +same day of November, 1832, and it was rightly concluded that these had +belonged to the same stream, although the true relationship of the +phenomena was not immediately apprehended. Olmsted ascribed to the +meteors a revolution about the sun once in every six months, bringing +them to the intersection of their orbit with that of the earth every +November 13th; but later investigators found that the real period was +about thirty-three and one-quarter years, so that the great displays +were due three times in a century, and their return was confidently +predicted for the year 1866. The appearance of the meteors in 1832, a +year before the great display, was ascribed to the great length of the +stream which they formed in space—so great that they required more than +two years to cross the earth’s orbit. In 1832 the earth had encountered +a relatively rare part of the stream, but in 1833, on returning to the +crossing-place, it found there the richest part of the stream pouring +across its orbit. This explanation also proved to be correct, and the +predicted return in 1866 was duly witnessed, although the display was +much less brilliant than in 1833. It was followed by another in 1867. + + +[Illustration: Curious forms of meteorite trains +Nos. 1 to 6 show the changes undergone by a train left by a meteorite +which passed near the “Great Dipper”; 7 shows the changes and drift of +a train seen in the constellation Virgo; 8 is the singular train of the +meteorite of February 22, 1909, near the Pole Star. (From _La +Nature_.)] + + +In the mean time Olmsted’s idea of a cometary relationship of the +meteors was demonstrated to be correct by the researches of +Schiaparelli and others, who showed that not only the November meteors, +but those of August, which are seen more or less abundantly every year, +traveled in the tracks of well-known comets, and had undoubtedly an +identical origin with those comets. In other words the comets and the +meteor-swarms were both remnants of original masses which had probably +been split up by the action of the sun, or of some planet to which they +had made close approaches. The annual periodicity of the August meteors +was ascribed to the fact that the separation had taken place so long +ago that the meteors had become distributed all around the orbit, in +consequence of which the earth encountered some of them every year when +it arrived at the crossing-point. Then Leverrier showed that the +original comet associated with the November meteors was probably +brought into the system by the influence of the planet Uranus in the +year 126 of the Christian era. Afterward Alexander Herschel identified +the tracks of no less than seventy-six meteor-swarms (most of them +inconspicuous) with those of comets. The still more recent researches +of Mr W. F. Denning make it probable that there are no meteors which do +not belong to a flock or system probably formed by the disintegration +of a cometary mass; even the apparently sporadic ones which shoot +across the sky, “lost souls in the night,” being members of flocks +which have become so widely scattered that the earth sometimes takes +weeks to pass through the region of space where their paths lie. + +The November meteors should have exhibited another pair of spectacles +in 1899 and 1900, and their failure to do so caused at first much +disappointment, until it was made plain that a good reason existed for +their absence. It was found that after their last appearance, in 1867, +they had been disturbed in their movements by the planets Jupiter and +Saturn, whose attractions had so shifted the position of their orbit +that it no longer intersected that of the earth, as it did before. +Whether another planetary interference will sometime bring the +principal mass of the November meteors back to the former point of +intersection with the earth’s orbit is a question for the future to +decide. It would seem that there may be several parallel streams of the +November meteors, and that some of them, like those of August, are +distributed entirely around the orbit, so that every mid-November we +see a few of them. + +We come now to a very remarkable example of the disintegration of a +comet and the formation of a meteor-stream. In 1826 Biela, of +Josephstadt, Austria, discovered a comet to which his name was given. +Calculation showed that it had an orbital period of about six and a +half years, belonging to Jupiter’s “family.” On one of its returns, in +1846, it astonished its watchers by suddenly splitting in two. The two +comets thus formed out of one separated to a distance of about one +hundred and sixty thousand miles, and then raced side by side, +sometimes with a curious ligature connecting them, like Siamese twins, +until they disappeared together in interplanetary space. In 1852 they +came back, still nearly side by side, but now the distance between them +had increased to a million and a quarter of miles. After that, at every +recurrence of their period, astronomers looked for them in vain, until +1872, when an amazing thing happened. On the night of November 28th, +when the earth was crossing the plane of the orbit of the missing +comet, a brilliant shower of meteors burst from the northern sky, +traveling nearly in the track which the comet should have pursued. The +astronomers were electrified. Klinkerfues, of Göttingen, telegraphed to +Pogson, of Madras: _“Biela touched earth; search near Theta Centauri.”_ +Pogson searched in the place indicated and saw a cometary mass +retreating into the southern heavens, where it was soon swallowed from +sight! + + +[Illustration: Section of the atmosphere up to 100 kilometers. +Showing the mean elevation at which meteorites and meteors make their +appearance. Below are shown the elevation of Mount Everest, the highest +manned balloon ascent by M. Berson; the height of cirrus clouds; the +highest free balloon ascent; and the elevation attained by the clouds +of fire-dust ejected by the Krakatoa eruption in 1883. (From _La +Nature_)] + + +Since then the Biela meteors have been among the recognized periodic +spectacles of the sky, and few if any doubt that they represent a +portion of the missing comet whose disintegration began with the +separation into two parts in 1846. The comet itself has never since +been seen. The first display of these meteors, sometimes called the +“Andromedes,” because they radiate from the constellation Andromeda, +was remarkable for the great brilliancy of many of the fire-balls that +shot among the shower of smaller sparks, some of which were described +as equaling the full moon in size. None of them is known to have +reached the earth, but during the display of the same meteors in 1885 a +meteoric mass fell at Mazapil in Northern Mexico (it is now in the +Museum at Vienna), which many have thought may actually be a piece of +the original comet of Biela. This brings us to the second branch of our +subject. + +More rare than meteors or falling stars, and more startling, except +that they never appear in showers, are the huge balls of fire which +occasionally dart through the sky, lighting up the landscapes beneath +with their glare, leaving trains of sparks behind them, often producing +peals of thunder when they explode, and in many cases falling upon the +earth and burying themselves from a few inches to several feet in the +soil, from which, more than once, they have been picked up while yet +hot and fuming. These balls are sometimes called bolides. They are not +really round in shape, although they often look so while traversing the +sky, but their forms are fragmentary, and occasionally fantastic. It +has been supposed that their origin is different from that of the true +meteors; it has even been conjectured that they may have originated +from the giant volcanoes of the moon or have been shot out from the sun +during some of the tremendous explosions that accompany the formation +of eruptive prominences. By the same reasoning some of them might be +supposed to have come from some distant star. Others have conjectured +that they are wanderers in space, of unknown origin, which the earth +encounters as it journeys on, and Lord Kelvin made a suggestion which +has become classic because of its imaginative reach—_viz.,_ that the +first germs of life may have been brought to the earth by one of these +bodies, “a fragment of an exploded world.” + +It is a singular fact that astronomers and scientific men in general +were among the last to admit the possibility of solid masses falling +from the sky. The people had believed in the reality of such phenomena +from the earliest times, but the savants shook their heads and talked +of superstition. This was the less surprising because no scientifically +authenticated instance of such an occurrence was known, and the stones +popularly believed to have fallen from the sky had become the objects +of worship or superstitious reverence, a fact not calculated to +recommend them to scientific credence. The celebrated “black stone” +suspended in the Kaaba at Mecca is one of these reputed gifts from +heaven; the “Palladium” of ancient Troy was another; and a stone which +fell near Ensisheim, in Germany, was placed in a church as an object to +be religiously venerated. Many legends of falling stones existed in +antiquity, some of them curiously transfigured by the imagination, like +the “Lion of the Peloponnesus,” which was said to have sprung down from +the sky upon the Isthmus of Corinth. But near the beginning of the +nineteenth century, in 1803, a veritable shower of falling stones +occurred at L’Aigle, in Northern France, and this time astronomers took +note of the phenomenon and scientifically investigated it. Thousands of +the strange projectiles came from the sky on this occasion, and were +scattered over a wide area of country, and some buildings were hit. +Four years later another shower of stones occurred at Weston, Conn., +numbering thousands of individuals. The local alarm created in both +cases was great, as well it might be, for what could be more +intimidating than to find the blue vault of heaven suddenly hurling +solid missiles at the homes of men? After these occurrences it was +impossible for the most skeptical to doubt any longer, and the regular +study of “aerolites,” or “meteorites,” began. + +One of the first things recognized was the fact that fire-balls are +solid meteorites in flight, and not gaseous exhalations in the air, as +some had assumed. They burn in the air during their flight, and +sometimes, perhaps, are entirely consumed before reaching the ground. +Their velocity before entering the earth’s atmosphere is equal to that +of the planets in their orbits—_viz.,_ from twenty to thirty miles per +second—a fact which proves that the sun is the seat of the central +force governing them. Their burning in the air is not difficult to +explain; it is the heat of friction which so quickly brings them to +incandescence. Calculation shows that a body moving through the air at +a velocity of about a mile per second will be brought, superficially, +to the temperature of “red heat” by friction with the atmosphere. If +its velocity is twenty miles per second the temperature will become +thousands of degrees. This is the state of affairs with a meteorite +rushing into the earth’s atmosphere; its surface is liquefied within a +few seconds after the friction begins to act, and the melted and +vaporized portion of its mass is swept backward, forming the train of +sparks that follows every great fire-ball. However, there is one +phenomenon connected with the trains of meteorites which has never been +satisfactorily explained: they often persist for long periods of time, +drifting and turning with the wind, but not ceasing to glow with a +phosphorescent luminosity. The question is, Whence comes this light? It +must be light without heat, since the fine dust or vapor of which the +train can only consist would not retain sufficient heat to render it +luminous for so long a time. An extremely remarkable incident of this +kind occurred on February 22, 1909, when an immense fire-ball that +passed over southern England left a train that remained visible during +two hours, assuming many curious shapes as it was drifted about by +currents in the air. + + +[Illustration: A meteor photographed in flight] + + +But notwithstanding the enormous velocity with which meteorites enter +the air they are soon slowed down to comparatively moderate speed, so +that when they disappear they are usually traveling not faster than a +mile a second. The courses of many have been traced by observers +situated along their track at various points, and thus a knowledge has +been obtained of their height above the ground during their flight and +of the length of their visible courses. They generally appear at an +elevation of eighty or a hundred miles, and are seldom visible after +having descended to within five miles of the ground, unless the +observer happens to be near the striking-point, when he may actually +witness the fall. Frequently they burst while high in the air and their +fragments are scattered like shrapnel over the surface of the ground, +sometimes covering an area of several square miles, but of course not +thickly; different fragments of the same meteorite may reach the ground +at points several miles apart. The observed length of their courses in +the atmosphere varies from fifty to five hundred miles. If they +continued a long time in flight after entering the air, even the +largest of them would probably be consumed to the last scrap, but their +fiery career is so short on account of their great speed that the heat +does not have time to penetrate very deeply, and some that have been +picked up immediately after their fall have been found cold as ice +within. Their size after reaching the ground is variable within wide +limits; some are known which weigh several tons, but the great majority +weigh only a few pounds and many only a few ounces. + +Meteorites are of two kinds: _stony_ meteorites and _iron_ meteorites. +The former outnumber the latter twenty to one; but many stone +meteorites contain grains of iron. Nickel is commonly found in iron +meteorites, so that it might be said that that redoubtable alloy +nickel-steel is of cosmical invention. Some twenty-five chemical +elements have been found in meteorites, including carbon and the +“sun-metal,” helium. The presence of the latter is certainly highly +suggestive in connection with the question of the origin of meteorites. +The iron meteorites, besides metallic iron and nickel, of which they +are almost entirely composed, contain hydrogen, helium, and carbonic +oxide, and about the only imaginable way in which these gases could +have become absorbed in the iron would be through the immersion of the +latter while in a molten or vaporized state in a hot and dense +atmosphere composed of them, a condition which we know to exist only in +the envelopes of the sun and the stars. + +The existence of carbon in the Canyon Diablo iron meteorites is +attended by a circumstance of the most singular character—a very “fairy +tale of science.” In some cases _the carbon has become diamond!_ These +meteoric diamonds are very small; nevertheless, they are true diamonds, +resembling in many ways the little black gems produced by Moissan’s +method with the aid of the electric furnace. The fact that they are +found embedded in these iron meteorites is another argument in favor of +the hypothesis of the solar or stellar origin of the latter. To +appreciate this it is necessary to recall the way in which Moissan made +his diamonds. It was by a combination of the effects of great heat, +great pressure, and sudden or rapid superficial cooling on a mass of +iron containing carbon. When he finally broke open his iron he found it +a pudding stuffed with miniature black diamonds. When a fragment of the +Canyon Diablo meteoric iron was polished in Philadelphia over fifteen +years ago it cut the emery-wheel to pieces, and examination showed that +the damage had been effected by microscopic diamonds peppered through +the mass. How were those diamonds formed? If the sun or Sirius was the +laboratory that prepared them, we can get a glimpse at the process of +their formation. There is plenty of heat, plenty of pressure, and an +abundance of vaporized iron in the sun and the stars. When a great +solar eruption takes place, masses of iron which have absorbed carbon +may be shot out with a velocity which forbids their return. Plunged +into the frightful cold of space, their surfaces are quickly cooled, as +Moissan cooled his prepared iron by throwing it into water, and thus +the requisite stress is set up within, and, as the iron solidifies, the +included carbon crystallizes into diamonds. Whether this explanation +has a germ of truth in it or not, at any rate it is evident that iron +meteorites were not created in the form in which they come to us; they +must once have been parts of immeasurably more massive bodies than +themselves. + +The fall of meteorites offers an appreciable, though numerically +insignificant, peril to the inhabitants of the earth. Historical +records show perhaps three or four instances of people being killed by +these bodies. But for the protection afforded by the atmosphere, which +acts as a very effective shield, the danger would doubtless be very +much greater. In the absence of an atmosphere not only would more +meteorites reach the ground, but their striking force would be +incomparably greater, since, as we have seen, the larger part of their +original velocity is destroyed by the resistance of the air. A +meteorite weighing many tons and striking the earth with a velocity of +twenty or thirty miles per second, would probably cause frightful +havoc. + + +[Illustration: Looking across Coon Butte crater from northern rim] + + +It is a singular fact that recent investigations seem to have proved +that an event of this kind actually happened in North America—perhaps +not longer than a thousand or two thousand years ago. The scene of the +supposed catastrophe is in northern central Arizona, at Coon Butte, +where there is a nearly circular crater in the middle of a circular +elevation or small mountain. The crater is somewhat over four thousand +feet in diameter, and the surrounding rim, formed of upturned strata +and ejected rock fragments, rises at its highest point one hundred and +sixty feet above the plain. The crater is about six hundred feet in +depth—that is, from the rim to the visible floor or bottom of the +crater. There is no evidence that volcanic action has ever taken place +in the immediate neighborhood of Coon Butte. The rock in which the +crater has been made is composed of horizontal sandstone and limestone +strata. Between three hundred and four hundred million tons of rock +fragments have been detached, and a large portion hurled by some cause +out of the crater. These fragments lie concentrically distributed +around the crater, and in large measure form the elevation known as +Coon Butte. The region has been famous for nearly twenty years on +account of the masses of meteoric iron found scattered about and known +as the “Canyon Diablo” meteorites. It was one of these masses, which +consist of nickel-iron containing a small quantity of platinum, and of +which in all some ten tons have been recovered for sale to the various +collectors throughout the world, that as before mentioned destroyed the +grinding-tool at Philadelphia through the cutting power of its embedded +diamonds. These meteoric irons are scattered about the crater-hill, in +concentric distribution, to a maximum distance of about five miles. +When the suggestion was first made in 1896 that a monster meteorite +might have created by its fall this singular lone crater _in stratified +rocks,_ it was greeted with incredulous smiles; but since then the +matter has assumed a different aspect. The Standard Iron Company, +formed by Messrs. D. M. Barringer, B. C. Tilghman, E. J. Bennitt, and +S. J. Holsinger, having become, in 1903, the owner of this freak of +nature, sunk shafts and bored holes to a great depth in the interior of +the crater, and also trenched the slopes of the mountain, and the +result of their investigations has proved that the meteoric hypothesis +of origin is correct. (See the papers published in the _Proceedings of +the Academy of Natural Sciences of Philadelphia,_ December, 1905, +wherein it is proved that the United States Geological Survey was wrong +in believing this crater to have been due to a steam explosion. Since +that date there has been discovered a great amount of additional +confirmatory proof). Material of unmistakably meteoric origin was found +by means of the drills, mixed with crushed rock, to a depth of six +hundred to seven hundred feet below the floor of the crater, and a +great deal of it has been found admixed with the ejected rock fragments +on the outer slopes of the mountain, absolutely proving synchronism +between the two events, the formation of this great crater and the +falling of the meteoric iron out of the sky. The drill located in the +bottom of the crater was sent, in a number of cases, much deeper (over +one thousand feet) into unaltered horizontal red sandstone strata, but +no meteoric material was found below this depth (seven hundred feet, or +between eleven and twelve hundred feet below the level of the +surrounding plain), which has been assumed as being about the limit of +penetration. It is not possible to sink a shaft at present, owing to +the water which has drained into the crater, and which forms, with the +finely pulverized sandstone, a very troublesome quicksand encountered +at about two hundred feet below the visible floor of the crater. As +soon as this water is removed by pumping it will be easy to explore the +depths of the crater by means of shafts and drifts. The rock strata +(sandstone and limestone) of which the walls consist present every +appearance of having been violently upturned by a huge body penetrating +the earth like a cannon-ball. The general aspect of the crater +strikingly resembles the impression made by a steel projectile shot +into an armor-plate. Mr Tilghman has estimated that a meteorite about +five hundred feet in diameter and moving with a velocity of about five +miles per second would have made just such a perforation upon striking +rocks of the character of those found at this place. There was some +fusion of the colliding masses, and the heat produced some steam from +the small amount of water in the rocks. As a result there has been +found at depth a considerable amount of fused quartz (original +sandstone), and with it innumerable particles or sparks of fused +nickel-iron (original meteorite). A projectile of that size penetrating +eleven to twelve hundred feet into the rocky shell of the globe must +have produced a shock which was perceptible several hundred miles away. + + +[Illustration: Trail on south side, Coon Butte crater] + + +The great velocity ascribed to the supposed meteorite at the moment of +striking could be accounted for by the fact that it probably plunged +nearly vertically downward, for it formed a circular crater in the +rocky crust of the earth. In that case it would have been less retarded +by the resistance of the atmosphere than are meteorites which enter the +air at a lower angle and shoot ahead hundreds of miles until friction +has nearly destroyed their original motion when they drop upon the +earth. Some meteoric masses of great size, such as Peary’s iron +meteorite found at Cape York, Greenland, and the almost equally large +mass discovered at Bacubirito, Mexico, appear to have penetrated but +slightly on striking the earth. This may be explained by supposing that +they pursued a long, horizontal course through the air before falling. +The result would be that, their original velocity having been +practically destroyed, they would drop to the ground with a velocity +nearly corresponding to that which gravity would impart within the +perpendicular distance of their final fall. A +six-hundred-and-sixty-pound meteorite, which fell at Knyahinya, +Hungary, striking at an angle of 27° from the vertical, penetrated the +ground to a depth of eleven feet. + +It has been remarked that the Coon Butte meteorite may have fallen not +longer ago than a few thousand years. This is based upon the fact that +the geological indications favor the supposition that the event did not +occur more than five thousand years ago, while on the other hand the +rings of growth in the cedar-trees growing on the slopes of the crater +show that they have existed there about seven hundred years. Prof. +William H. Pickering has recently correlated this with an ancient +chronicle which states that at Cairo, Egypt, in the year 1029, “many +stars passed with a great noise.” He remarks that Cairo is about 100°, +by great circle, from Coon Butte, so that if the meteorite that made +the crater was a member of a flock of similar bodies which encountered +the earth moving in parallel lines, some of them might have traversed +the sky tangent to the earth’s surface at Cairo. That the spectacle +spoken of in the chronicle was caused by meteorites he deems +exceedingly probable because of what is said about “a great noise;” +meteorites are the only celestial phenomena attended with perceptible +sounds. Professor Pickering conjectures that this supposed flock of +great meteorites may have formed the nucleus of a comet which struck +the earth, and he finds confirmation of the idea in the fact that out +of the ten largest meteorites known, no less than seven were found +within nine hundred miles of Coon Butte. It would be interesting if we +could trace back the history of that comet, and find out what malicious +planet caught it up in its innocent wanderings and hurled it with so +true an aim at the earth! This remarkable crater is one of the most +interesting places in the world, for there is absolutely no record of +such a mass, possibly an iron-headed comet, from outer space having +come into collision with our earth. The results of the future +exploration of the depths of the crater will be awaited with much +interest. + + + + +XII +The Wrecking of the Moon + + +There are sympathetic moods under whose influence one gazes with a +certain poignant tenderness at the worn face of the moon; that little +“fossil world” (the child of our mother earth, too) bears such terrible +scars of its brief convulsive life that a sense of pity is awakened by +the sight. The moon is the wonder-land of the telescope. Those towering +mountains, whose “proud aspiring peaks” cast silhouettes of shadow that +seem drawn with india-ink; those vast plains, enchained with gentle +winding hills and bordered with giant ranges; those oval “oceans,” +where one looks expectant for the flash of wind-whipped waves; those +enchanting “bays” and recesses at the seaward feet of the Alps; those +broad straits passing between guardian heights incomparably mightier +than Gibraltar; those locket-like valleys as secluded among their +mountains as the Vale of Cashmere; those colossal craters that make us +smile at the pretensions of Vesuvius, Etna, and Cotopaxi; those strange +white ways which pass with the unconcern of Roman roads across +mountain, gorge, and valley—all these give the beholder an irresistible +impression that it is truly a world into which he is looking, a world +akin to ours, and yet no more like our world than Pompeii is like +Naples. Its air, its waters, its clouds, its life are gone, and only a +skeleton remains—a mute but eloquent witness to a cosmical tragedy +without parallel in the range of human knowledge. + +One cannot but regret that the moon, if it ever was the seat of +intelligent life, has not remained so until our time. Think what the +consequences would have been if this other world at our very door had +been found to be both habitable and inhabited! We talk rather airily of +communicating with Mars by signals; but Mars never approaches nearer +than 35,000,000 miles, while the moon when nearest is only a little +more than 220,000 miles away. Given an effective magnifying power of +five thousand diameters, which will perhaps be possible at the mountain +observatories as telescopes improve, and we should be able to bring the +moon within an apparent distance of about forty miles, while the +corresponding distance for Mars would be more than seven thousand +miles. But even with existing telescopic powers we can see details on +the moon no larger than some artificial constructions on the earth. St +Peter’s at Rome, with the Vatican palace and the great piazza, if +existing on the moon, would unquestionably be recognizable as something +else than a freak of nature. Large cities, with their radiating lines +of communication, would at once betray their real character. Cultivated +tracts, and the changes produced by the interference of intelligent +beings, would be clearly recognizable. The electric illumination of a +large town at night would probably be markedly visible. Gleams of +reflected sunlight would come to us from the surfaces of the lakes and +oceans, and a huge “liner” traversing a lunar sea could probably be +followed by its trail of smoke. As to communications by “wireless” +signals, which certain enthusiasts have thought of in connection with +Mars, in the case of the moon they should be a relatively simple +matter, and the feat might actually be accomplished. Think what a +literature would grow up about the moon if it were a living world! Its +very differences from the earth would only accentuate its interest for +us. Night and day on the moon are each two weeks in length; how +interesting it would be to watch the manner in which the lunarians +dealt with such a situation as that. Lunar and terrestrial history +would keep step with each other, and we should record them both. Truly +one might well wish to have a neighbor world to study; one would feel +so much the less alone in space. + +It is not impossible that the moon did at one time have inhabitants of +some kind. But, if so, they vanished with the disappearance of its +atmosphere and seas, or with the advent of its cataclysmic age. At the +best, its career as a living world must have been brief. If the water +and air were gradually absorbed, as some have conjectured, by its +cooling interior rocks, its surface might, nevertheless, have retained +them for long ages; but if, as others think, their disappearance was +due to the escape of their gaseous molecules in consequence of the +inability of the relatively small lunar gravitation to retain them, +then the final catastrophe must have been as swift as it was +inevitable. Accepting Darwin’s hypothesis, that the moon was separated +from the earth by tidal action while both were yet plastic or nebulous, +we may reasonably conclude that it began its career with a good supply +of both water and air, but did not possess sufficient mass to hold them +permanently. Yet it may have retained them long enough for life to +develop in many forms upon its surface; in fact, there are so many +indications that air and water have not always been lacking to the +lunar world that we are driven to invent theories to explain both their +former presence and their present absence. + + +[Illustration: The craters Clavius, Longomontanus, Tycho, etc.] + + +But whatever the former condition of the moon may have been, its +existing appearance gives it a resistless fascination, and it bears so +clearly the story of a vast catastrophe sculptured on its rocky face +that the thoughtful observer cannot look upon it without a feeling of +awe. The gigantic character of the lunar features impresses the +beholder not less than the universality of the play of destructive +forces which they attest. Let us make a few comparisons. Take the lunar +crater called “Tycho”, which is a typical example of its kind. In the +telescope Tycho appears as a perfect ring surrounding a circular +depression, in the center of which rises a group of mountains. Its +superficial resemblance to some terrestrial volcanic craters is very +striking. Vesuvius, seen from a point vertically above, would no doubt +look something like that (the resemblance would have been greater when +the Monte del Cavallo formed a more complete circuit about the crater +cone). But compare the dimensions. The remains of the outer crater ring +of Vesuvius are perhaps half a mile in diameter, while the active +crater itself is only two or three hundred feet across at the most; +Tycho has a diameter of fifty-four miles! The group of relatively +insignificant peaks in the center of the crater floor of Tycho is far +more massive than the entire mountain that we call Vesuvius. The +largest known volcanic crater on the earth, Aso San, in Japan, has a +diameter of seven miles; it would take _sixty_ craters like Aso San to +equal Tycho in area! And Tycho, though one of the most perfect, is by +no means the largest crater on the moon. Another, called “Theophilus,” +has a diameter of sixty-four miles, and is eighteen thousand feet deep. +There are hundreds from ten to forty miles in diameter, and thousands +from one to ten miles. They are so numerous in many places that they +break into one another, like the cells of a crushed honeycomb. + +The lunar craters differ from those of the earth more fundamentally +than in the matter of mere size; _they are not situated on the tops of +mountains._ If they were, and if all the proportions were the same, a +crater like Tycho might crown a conical peak fifty or one hundred miles +high! Instead of being cavities in the summits of mountains, the lunar +craters are rather gigantic sink-holes whose bottoms in many cases lie +two or three miles below the general surface of the lunar world. Around +their rims the rocks are piled up to a height of from a few hundred to +two or three thousand feet, with a comparatively gentle inclination, +but on the inner side they fall away in gigantic broken precipices +which make the dizzy cliffs of the Matterhorn seem but “lover’s leaps.” +Down they drop, ridge below ridge, crag under crag, tottering wall +beneath wall, until, in a crater named “Newton,” near the south lunar +pole, they attain a depth where the rays of the sun never reach. +Nothing more frightful than the spectacle which many of these terrible +chasms present can be pictured by the imagination. As the lazy lunar +day slowly advances, the sunshine, unmitigated by clouds or atmospheric +veil of any kind, creeps across their rims and begins to descend the +opposite walls. Presently it strikes the ragged crest of a ridge which +had lain hidden in such darkness as we never know on the earth, and +runs along it like a line of kindling fire. Rocky pinnacles and needles +shoot up into the sunlight out of the black depths. Down sinks the line +of light, mile after mile, and continually new precipices and cliffs +are brought into view, until at last the vast floor is attained and +begins to be illuminated. In the meanwhile the sun’s rays, darting +across the gulf, have touched the summits of the central peaks, twenty +or thirty miles from the crater’s inmost edge, and they immediately +kindle and blaze like huge stars amid the darkness. So profound are +some of these awful craters that days pass before the sun has risen +high enough above them to chase the last shadows from their depths. + +Although several long ranges of mountains resembling those of the earth +exist on the moon, the great majority of its elevations assume the +crateriform aspect. Sometimes, instead of a crater, we find an immense +mountain ring whose form and aspect hardly suggest volcanic action. But +everywhere the true craters are in evidence, even on the sea-beds, +although they attain their greatest number and size on those parts of +the moon—covering sixty per cent of its visible surface—which are +distinctly mountainous in character and which constitute its most +brilliant portions. Broadly speaking, the southwestern half of the moon +is the most mountainous and broken, and the northeastern half the least +so. Right down through the center, from pole to pole, runs a wonderful +line of craters and crateriform valleys of a magnitude stupendous even +for the moon. Another similar line follows the western edge. Three or +four “seas” are thrust between these mountainous belts. By the effects +of “libration” parts of the opposite hemisphere of the moon which is +turned away from the earth are from time to time brought into view, and +their aspect indicates that that hemisphere resembles in its surface +features the one which faces the earth. There are many things about the +craters which seem to give some warrant for the hypothesis which has +been particularly urged by Mr G. K. Gilbert, that they were formed by +the impact of meteors; but there are also many things which militate +against that idea, and, upon the whole, the volcanic theory of their +origin is to be preferred. + +The enormous size of the lunar volcanoes is not so difficult to account +for when we remember how slight is the force of lunar gravity as +compared with that of the earth. With equal size and density, bodies on +the moon weigh only one-sixth as much as on the earth. Impelled by the +same force, a projectile that would go ten miles on the earth would go +sixty miles on the moon. A lunar giant thirty-five feet tall would +weigh no more than an ordinary son of Adam weighs on his greater +planet. To shoot a body from the earth so that it would not drop back +again, we should have to start it with a velocity of seven miles per +second; a mile and a half per second would serve on the moon. It is by +no means difficult to believe, then, that a lunar volcano might form a +crater ring eight or ten times broader than the greatest to be found on +the earth, especially when we reflect that in addition to the +relatively slight force of gravity, the materials of the lunar crust +are probably lighter than those of our terrestrial rocks. + + +[Illustration: Western part of the Mare Serenitatis] + + +For similar reasons it seems not impossible that the theory mentioned +in a former chapter—that some of the meteorites that have fallen upon +the earth originated from the lunar volcanoes—is well founded. This +would apply especially to the stony meteorites, for it is hardly to be +supposed that the moon, at least in its superficial parts, contains +much iron. It is surely a scene most strange that is thus presented to +the mind’s eye—that little attendant of the earth’s (the moon has only +one-fiftieth of the volume, and only one-eightieth of the mass of the +earth) firing great stones back at its parent planet! And what can have +been the cause of this furious outbreak of volcanic forces on the moon? +Evidently it was but a passing stage in its history; it had enjoyed +more quiet times before. As it cooled down from the plastic state in +which it parted from the earth, it became incrusted after the normal +manner of a planet, and then oceans were formed, its atmosphere being +sufficiently dense to prevent the water from evaporating and the +would-be oceans from disappearing continually in mist. This, if any, +must have been the period of life in the lunar world. As we look upon +the vestiges of that ancient world buried in the wreck that now covers +so much of its surface, it is difficult to restrain the imagination +from picturing the scenes which were once presented there; and, in such +a case, should the imagination be fettered? We give it free rein in +terrestrial life, and it rewards us with some of our greatest +intellectual pleasures. The wonderful landscapes of the moon offer it +an ideal field with just enough half-hidden suggestions of facts to +stimulate its powers. + +The great plains of the _Mare Imbrium_ and the _Mare Serenitatis_ (the +“Sea of Showers” and the “Sea of Serenity”), bordered in part by lofty +mountain ranges precisely like terrestrial mountains, scalloped along +their shores with beautiful bays curving back into the adjoining +highlands, and united by a great strait passing between the nearly +abutting ends of the “Lunar Apennines” and the “Lunar Caucasus,” offer +the elements of a scene of world beauty such as it would be difficult +to match upon our planet. Look at the finely modulated bottom of the +ancient sea in Mr Ritchey’s exquisite photograph of the western part of +the _Mare Serenitatis,_ where one seems to see the play of the watery +currents heaping the ocean sands in waving lines, making shallows, +bars, and deeps for the mariner to avoid or seek, and affording a +playground for the creatures of the main. What geologist would not wish +to try his hammer on those rocks with their stony pages of fossilized +history? There is in us an instinct which forbids us to think that +there was never any life there. If we could visit the moon, there is +not among us a person so prosaic and unimaginative that he would not, +the very first thing, begin to search for traces of its inhabitants. We +would look for them in the deposits on the sea bottoms; we would +examine the shores wherever the configuration seemed favorable for +harbors and the sites of maritime cities—forgetting that it may be a +little ridiculous to ascribe to the ancient lunarians the same ideas +that have governed the development of our race; we would search through +the valleys and along the seeming courses of vanished streams; we would +explore the mountains, not the terrible craters, but the pinnacled +chains that recall our own Alps and Rockies; seeking everywhere some +vestige of the transforming presence of intelligent life. Perhaps we +should find such traces, and perhaps, with all our searching, we should +find nothing to suggest that life had ever existed amid that universal +ruin. + + +[Illustration: Mare Tranquilitatis and surroundings] + + +Look again at the border of the “Sea of Serenity”—what a name for such +a scene!—and observe how it has been rent with almost inconceivable +violence, the wall of the colossal crater Posidonius dropping +vertically upon the ancient shore and obliterating it, while its giant +neighbor, Le Monnier, opens a yawning mouth as if to swallow the sea +itself. A scene like this makes one question whether, after all, those +may not be right who have imagined that the so-called sea bottoms are +really vast plains of frozen lava which gushed up in floods so +extensive that even the mighty volcanoes were half drowned in the fiery +sea. This suggestion becomes even stronger when we turn to another of +the photographs of Mr Ritchey’s wonderful series, showing a part of the +_Mare Tranquilitatis_ (“Sea of Tranquility”!). Notice how near the +center of the picture the outline of a huge ring with radiating ridges +shows through the sea bottom; a fossil volcano submerged in a petrified +ocean! This is by no means the only instance in which a buried world +shows itself under the great lunar plains. Yet, as the newer craters in +the sea itself prove, the volcanic activity survived this other +catastrophe, or broke out again subsequently, bringing more ruin to +pile upon ruin. + +Yet notwithstanding the evidence which we have just been considering in +support of the hypothesis that the “seas” are lava floods, Messrs. +Loewy and Puiseux, the selenographers of the Paris Observatory, are +convinced that these great plains bear characteristic marks of the +former presence of immense bodies of water. In that case we should be +forced to conclude that the later oceans of the moon lay upon vast +sheets of solidified lava; and thus the catastrophe of the lunar world +assumes a double aspect, the earliest oceans being swallowed up in +molten floods issuing from the interior, while the lands were reduced +to chaos by a universal eruption of tremendous volcanoes; and then a +period of comparative quiet followed, during which new seas were +formed, and new life perhaps began to flourish in the lunar world, only +to end in another cataclysm, which finally put a term to the existence +of the moon as a life-supporting world. + +Suppose we examine two more of Mr Ritchey’s illuminating photographs, +and, first, the one showing the crater Theophilus and its surroundings. +We have spoken of Theophilus before, citing the facts that it is +sixty-four miles in diameter and eighteen thousand feet deep. It will +be noticed that it has two brother giants—Cyrillus the nearer, and +Catharina the more distant; but Theophilus is plainly the youngest of +the trio. Centuries, and perhaps thousands of years, must have elapsed +between the periods of their upheaval, for the two older craters are +partly filled with débris, while it is manifest at a glance that when +the south eastern wall of Theophilus was formed, it broke away and +destroyed a part of the more ancient ring of Cyrillus. There is no more +tremendous scene on the moon than this; viewed with a powerful +telescope, it is absolutely appalling. + + +[Illustration: Lunar craters Theophilus and surrounding region] + + +The next photograph shows, if possible, a still wilder region. It is +the part of the moon lying between Tycho and the south pole. Tycho is +seen in the lower left-hand part of the picture. To the right, at the +edge of the illuminated portion of the moon, are the crater-rings, +Longomontanus and Wilhelm I, the former being the larger. Between them +are to be seen the ruins of two or three more ancient craters which, +together with portions of the walls of Wilhelm I and Longomontanus, +have been honeycombed with smaller craters. The vast crateriform +depression above the center of the picture is Clavius, an unrivaled +wonder of lunar scenery, a hundred and forty-two miles in its greatest +length, while its whole immense floor has sunk two miles below the +general surface of the moon outside the ring. The monstrous +shadow-filled cavity above Clavius toward the right is Blancanus, whose +aspect here gives a good idea of the appearance of these chasms when +only their rims are in the sunlight. But observe the indescribable +savagery of the entire scene. It looks as though the spirit of +destruction had gone mad in this spot. The mighty craters have broken +forth one after another, each rending its predecessor; and when their +work was finished, a minor but yet tremendous outbreak occurred, and +the face of the moon was gored and punctured with thousands of smaller +craters. These relatively small craters (small, however, only in a +lunar sense, for many of them would appear gigantic on the earth) +recall once more the theory of meteoric impact. It does not seem +impossible that some of them may have been formed by such an agency. + +One would not wish for our planet such a fate as that which has +overtaken the moon, but we cannot be absolutely sure that something of +the kind may not be in store for it. We really know nothing of the +ultimate causes of volcanic activity, and some have suggested that the +internal energies of the earth may be accumulating instead of dying +out, and may never yet have exhibited their utmost destructive power. +Perhaps the best assurance that we can find that the earth will escape +the catastrophe that has overtaken its satellite is to be found in the +relatively great force of its gravitation. The moon has been the victim +of its weakness; given equal forces, and the earth would be the better +able to withstand them. It is significant, in connection with these +considerations, that the little planet Mercury, which seems also to +have parted with its air and water, shows to the telescope some +indications that it is pitted with craters resembling those that have +torn to pieces the face of the moon. + +Upon the whole, after studying the dreadful lunar landscapes, one +cannot feel a very enthusiastic sympathy with those who are seeking +indications of the continued existence of some kind of life on the +moon; such a world is better without inhabitants. It has met its fate; +let it go! Fortunately, it is not so near that it cannot hide its scars +and appear beautiful—except when curiosity impels us to look with the +penetrating eyes of the astronomer. + + +[Illustration: Mare Crisium] + + + + +XIII +The Great Mars Problem + + +Let any thoughtful person who is acquainted with the general facts of +astronomy look up at the heavens some night when they appear in their +greatest splendor, and ask himself what is the strongest impression +that they make upon his mind. He may not find it easy to frame an +answer, but when he has succeeded it will probably be to the effect +that the stars give him an impression of the universality of +intelligence; they make him feel, as the sun and the moon cannot do, +that his world is not alone; that all this was not made simply to form +a gorgeous canopy over the tents of men. If he is of a devout turn of +mind, he thinks, as he gazes into those fathomless deeps and among +those bewildering hosts, of the infinite multitude of created beings +that the Almighty has taken under his care. The narrow ideas of the old +geocentric theology, which made the earth God’s especial footstool, and +man his only rational creature, fall away from him like a veil that had +obscured his vision; they are impossible in the presence of what he +sees above. Thus the natural tendency, in the light of modern progress, +is to regard the universe as everywhere filled with life. + +But science, which is responsible for this broadening of men’s thoughts +concerning the universality of life, itself proceeds to set limits. Of +spiritual existences it pretends to know nothing, but as to physical +beings, it declares that it can only entertain the supposition of their +existence where it finds evidence of an environment suited to their +needs, and such environment may not everywhere exist. Science, though +repelled by the antiquated theological conception of the supreme +isolation of man among created beings, regards with complacency the +probability that there are regions in the universe where no organic +life exists, stars which shine upon no inhabited worlds, and planets +which nourish no animate creatures. The astronomical view of the +universe is that it consists of matter in every stage of evolution: +some nebulous and chaotic; some just condensing into stars (suns) of +every magnitude and order; some shaped into finished solar bodies +surrounded by dependent planets; some forming stars that perhaps have +no planets, and will have none; some constituting suns that are already +aging, and will soon lose their radiant energy and disappear; and some +aggregated into masses that long ago became inert, cold, and rayless, +and that can only be revivified by means about which we can form +conjectures, but of which we actually know nothing. + +As with the stars, so with the planets, which are the satellites of +stars. All investigations unite to tell us that the planets are not all +in the same state of development. As some are large and some small, so +some are, in an evolutionary sense, young, and some old. As they depend +upon the suns around which they revolve for their light, heat, and +other forms of radiant energy, so their condition varies with their +distance from those suns. Many may never arrive at a state suitable for +the maintenance of life upon their surfaces; some which are not at +present in such a state may attain it later; and the forms of life +themselves may vary with the peculiar environment that different +planets afford. Thus we see that we are not scientifically justified in +affirming that life is ubiquitous, although we are thus justified in +saying that it must be, in a general sense, universal. We might liken +the universe to a garden known to contain every variety of plant. If on +entering it we see no flowers, we examine the species before us and +find that they are not of those which bloom at this particular season, +or perhaps they are such as never bear flowers. Yet we feel no doubt +that we shall find flowers somewhere in the garden, because there _are_ +species which bloom at this season, and the garden contains _all_ +varieties. + +While it is tacitly assumed that there are planets revolving around +other stars than the sun, it would be impossible for us to see them +with any telescope yet invented, and no instrument now in the +possession of astronomers could assure us of their existence; so the +only planetary system of which we have visual knowledge is our own. +Excluding the asteroids, which could not from any point of view be +considered as habitable, we have in the solar system eight planets of +various sizes and situated at various distances from the sun. Of these +eight we know that one, the earth, is inhabited. The question, then, +arises: Are there any of the others which are inhabited or habitable? +Since it is our intention to discuss the habitability of only one of +the seven to which the question applies, the rest may be dismissed in a +few words. The smallest of them, and the nearest to the sun, is +Mercury, which is regarded as uninhabitable because it has no +perceptible supply of water and air, and because, owing to the +extraordinary eccentricity of its orbit, it is subjected to excessive +and very rapid alterations in the amount of solar heat and light poured +upon its surface, such alterations being inconsistent with the +supposition that it can support living beings. Even its average +temperature is more than six and a half times that prevailing on the +earth! Another circumstance which militates against its habitability is +that, according to the results of the best telescopic studies, it +always keeps the same face toward the sun, so that one half of the +planet is perpetually exposed to the fierce solar rays, and the other +half faces the unmitigated cold of open space. Venus, the next in +distance from the sun, is almost the exact twin of the earth in size, +and many arguments may be urged in favor of its habitability, although +it is suspected of possessing the same peculiarity as Mercury, in +always keeping the same side sunward. Unfortunately its atmosphere +appears to be so dense that no permanent markings on its surface are +certainly visible, and the question of its actual condition must, for +the present, be left in abeyance. Mars, the first planet more distant +from the sun than the earth, is the special subject of this chapter, +and will be described and discussed a few lines further on. Jupiter, +Saturn, Uranus, and Neptune, the four giant planets, all more distant +than Mars, and each more distant than the other in the order named, are +all regarded as uninhabitable because none of them appears to possess +any degree of solidity. They may have solid or liquid nuclei, but +exteriorly they seem to be mere balls of cloud. Of course, one can +imagine what he pleases about the existence of creatures suited to the +physical constitution of such planets as these, but they must be +excluded from the category of habitable worlds in the ordinary sense of +the term. We go back, then, to Mars. + +It will be best to begin with a description of the planet. Mars is 4230 +miles in diameter; its surface is not much more than one-quarter as +extensive as that of the earth (.285). Its mean distance from the sun +is 141,500,000 miles, 48,500,000 miles greater than that of the earth. +Since radiant energy varies inversely as the square of distance, Mars +receives less than half as much solar light and heat as the earth gets. +Mars’ year (period of revolution round the sun) is 687 days. Its mean +density is 71 per cent of the earth’s, and the force of gravity on its +surface is 38 per cent of that on the surface of the earth; _i.e.,_ a +body weighing one hundred pounds on the earth would, if transported to +Mars, weigh but thirty-eight pounds. The inclination of its equator to +the plane of its orbit differs very little from that of the earth’s +equator, and its axial rotation occupies 24 hours 37 minutes. so that +the length of day and night, and the extent of the seasonal changes on +Mars, are almost precisely the same as on the earth. But owing to the +greater length of its year, the seasons of Mars, while occurring in the +same order, are almost twice as long as ours. The surface of the planet +is manifestly solid, like that of our globe, and the telescope reveals +many permanent markings on it, recalling the appearance of a globe on +which geographical features have been represented in reddish and dusky +tints. Around the poles are plainly to be seen rounded white areas, +which vary in extent with the Martian seasons, nearly vanishing in +summer and extending widely in winter. The most recent spectroscopic +determinations indicate that Mars has an atmosphere perhaps as dense as +that to be found on our loftiest mountain peaks, and there is a +perceptible amount of watery vapor in this atmosphere. The surface of +the planet appears to be remarkably level, and it has no mountain +ranges. No evidences of volcanic action have been discovered on Mars. +The dusky and reddish areas were regarded by the early observers as +respectively seas and lands, but at present it is not believed that +there are any bodies of water on the planet. There has never been much +doubt expressed that the white areas about the poles represent snow. + +It will be seen from this brief description that many remarkable +resemblances exist between Mars and the earth, and there is nothing +wonderful in the fact that the question of the habitability of the +former has become one of extreme and wide-spread interest, giving rise +to the most diverse views, to many extraordinary speculations, and +sometimes to regrettably heated controversy. The first champion of the +habitability of Mars was Sir William Herschel, although even before his +time the idea had been suggested. He was convinced by the revelations +of his telescopes, continually increasing in power, that Mars was more +like the earth than any other planet. He could not resist the testimony +of the polar snows, whose suggestive conduct was in such striking +accord with what occurs upon the earth. Gradually, as telescopes +improved and observers increased in number, the principal features of +the planet were disclosed and charted, and “areography,” as the +geography of Mars was called, took its place among the recognized +branches of astronomical study. But it was not before 1877 that a +fundamentally new discovery in areography gave a truly sensational turn +to speculation about life on “the red planet.” In that year Mars made +one of its nearest approaches to the earth, and was so situated in its +orbit that it could be observed to great advantage from the northern +hemisphere of the earth. The celebrated Italian astronomer, +Schiaparelli, took advantage of this opportunity to make a +trigonometrical survey of the surface of Mars—as coolly and confidently +as if he were not taking his sights across a thirty-five-million-mile +gulf of empty space—and in the course of this survey he was astonished +to perceive that the reddish areas, then called continents, were +crossed in many directions by narrow, dusky lines, to which he gave the +suggestive name of “canals.” Thus a kind of firebrand was cast into the +field of astronomical speculation, which has ever since produced +disputes that have sometimes approached the violence of political +faction. At first the accuracy of Schiaparelli’s observations was +contested; it required a powerful telescope, and the most excellent +“seeing,” to render the enigmatical lines visible at all, and many +searchers were unable to detect them. But Schiaparelli continued his +studies in the serene sky of Italy, and produced charts of the +gridironed face of Mars containing so much astonishing detail that one +had either to reject them _in toto_ or to confess that Schiaparelli was +right. As subsequent favorable oppositions of Mars occurred, other +observers began to see the “canals” and to confirm the substantial +accuracy of the Italian astronomer’s work, and finally few were found +who would venture to affirm that the “canals” did not exist, whatever +their meaning might be. + + +[Illustration: Schiaparelli’s chart of Mars, showing the so-called +system of canals] + + +When Schiaparelli began his observations it was generally believed, as +we have said, that the dusky areas on Mars were seas, and since +Schiaparelli thought that the “canals” invariably began and ended at +the shores of the “seas,” the appropriateness of the title given to the +lines seemed apparent. Their artificial character was immediately +assumed by many, because they were too straight and too suggestively +geometrical in their arrangement to permit the conclusion that they +were natural watercourses. A most surprising circumstance noted by +Schiaparelli was that the “canals” made their appearance _after_ the +melting of the polar snow in the corresponding hemisphere had begun, +and that they grew darker, longer, and more numerous in proportion as +the polar liquidation proceeded; another very puzzling observation was +that many of them became double as the season advanced; close beside an +already existing “canal,” and in perfect parallelism with it, another +would gradually make its appearance. That these phenomena actually +existed and were not illusions was proved by later observations, and +today they are seen whenever Mars is favorably situated for +observation. + +In the closing decade of the nineteenth century, Mr Percival Lowell +took up the work where Schiaparelli had virtually dropped it, and soon +added a great number of “canals” to those previously known, so that in +his charts the surface of the wonderful little planet appears covered +as with a spider’s web, the dusky lines criss-crossing in every +direction, with conspicuous knots wherever a number of them come +together. Mr Lowell has demonstrated that the areas originally called +seas, and thus named on the earlier charts, are not bodies of water, +whatever else they may be. He has also found that the mysterious lines +do not, as Schiaparelli supposed, begin and end at the edges of the +dusky regions, but often continue on across them, reaching in some +cases far up into the polar regions. But Schiaparelli was right in his +observation that the appearance of the “canals” is synchronous with the +gradual disappearance of the polar snows, and this fact has become the +basis of the most extraordinary theory that the subject of life in +other worlds has ever given birth to. + +Now, the effect of such discoveries, as we have related, depends upon +the type of mind to whose attention they are called. Many are content +to accept them as strange and inexplicable at present, and to wait for +further light upon them; others insist upon an immediate inquiry +concerning their probable nature and meaning. Such an inquiry can only +be based upon inference proceeding from analogy. Mars, say Mr Lowell +and those who are of his opinion, is manifestly a solidly incrusted +planet like the earth; it has an atmosphere, though one of great +rarity; it has water vapor, as the snows in themselves prove; it has +the alternation of day and night, and a succession of seasons closely +resembling those of the earth; its surface is suggestively divided into +regions of contrasting colors and appearance, and upon that surface we +see an immense number of lines geometrically arranged, with a system of +symmetrical intersections where the lines expand into circular and oval +areas—and all connected with the annual melting of the polar snows in a +way which irresistibly suggests the interference of intelligence +directed to a definite end. Why, with so many concurrent circumstances +to support the hypothesis, should we not regard Mars as an inhabited +globe? + +But the differences between Mars and the earth are in many ways as +striking as their resemblances. Mars is relatively small; it gets less +than half as much light and heat as we receive; its atmosphere is so +rare that it would be distressing to us, even if we could survive in it +at all; it has no lakes, rivers, or seas; its surface is an endless +prairie. and its “canals” are phenomena utterly unlike anything on the +earth. Yet it is precisely upon these divergences between the earth and +Mars, this repudiation of terrestrial standards, that the theory of +“life on Mars,” for which Mr Lowell is mainly responsible, is based. +Because Mars is smaller than the earth, we are told it must necessarily +be more advanced in planetary evolution, the underlying cause of which +is the gradual cooling and contraction of the planet’s mass. Mars has +parted with its internal heat more rapidly than the earth; consequently +its waters and its atmosphere have been mostly withdrawn by chemical +combinations, but enough of both yet remain to render life still +possible on its surface. As the globe of Mars is evolutionally older +than that of the earth, so its forms of organic life may be +proportionally further advanced, and its inhabitants may have attained +a degree of cultivated intelligence much superior to what at present +exists upon the earth. Understanding the nature and the causes of the +desiccation of their planet, and possessing engineering science and +capabilities far in advance of ours, they may be conceived to have +grappled with the stupendous problem of keeping their world in a +habitable condition as long as possible. Supposing them to have become +accustomed to live in their rarefied atmosphere (a thing not +inconceivable, since men can live for a time at least in air hardly +less rare), the most pressing problem for them is that of a +water-supply, without which plant life cannot exist, while animal life +in turn depends for its existence upon vegetation. The only direction +in which they can seek water is that of the polar regions, where it is +alternately condensed into snow and released in the liquid form by the +effect of the seasonal changes. It is, then, to the annual melting of +the polar snow-fields that the Martian engineers are supposed to have +recourse in supplying the needs of their planet, and thus providing the +means of prolonging their own existence. It is imagined that they have +for this purpose constructed a stupendous system of irrigation +extending over the temperate and equatorial regions of the planet. The +“canals” represent the lines of irrigation, but the narrow streaks that +we see are not the canals themselves, but the irrigated bands covered +by them. Their dark hue, and their gradual appearance after the polar +melting has begun, are due to the growth of vegetation stimulated by +the water. The rounded areas visible where several “canals” meet and +cross are called by Mr Lowell “oases.” These are supposed to be the +principal centers of population and industry. It must be confessed that +some of them, with their complicated systems of radiating lines, appear +to answer very well to such a theory. No attempt to explain them by +analogy with natural phenomena on the earth has proved successful. + +But a great difficulty yet remains: How to explain the seemingly +miraculous powers of the supposed engineers? Here recourse is had once +more to the relative smallness of the planet. We have remarked that the +force of gravity on Mars is only thirty-eight per cent of that on the +earth. A steam-shovel driven by a certain horse-power would be nearly +three times as effective there as here. A man of our stature on Mars +would find his effective strength increased in the same proportion. But +just because of the slight force of gravity there, a Martian might +attain to the traditional stature of Goliath without finding his own +weight an encumbrance to his activity, while at the same time his huge +muscles would come into unimpeded play, enabling him single-handed to +perform labors that would be impossible to a whole gang of terrestrial +workmen. The effective powers of huge machines would be increased in +the same way; and to all this must be added the fact that the mean +density of the materials of which Mars is composed is much less than +that of the constituents of the earth. Combining all these +considerations, it becomes much less difficult to conceive that public +works might be successfully undertaken on Mars which would be +hopelessly beyond the limits of human accomplishment. + +Certain other difficulties have also to be met; as, for instance, the +relative coldness of the climate of Mars. At its distance it gets +considerably less than half as much light and heat as we receive. In +addition to this, the rarity of its atmosphere would naturally be +expected to decrease the effective temperature at the planet’s surface, +since an atmosphere acts somewhat like the glass cover of a hot-house +in retaining the solar heat which has penetrated it. It has been +calculated that, unless there are mitigating circumstances of which we +know nothing, the average temperature at the surface of Mars must be +far below the freezing-point of water. To this it is replied that the +possible mitigating circumstances spoken of evidently exist in fact, +because we can _see_ that the watery vapor condenses into snow around +the poles in winter, but melts again when summer comes. The mitigating +agent may be supposed to exist in the atmosphere where the presence of +certain gases would completely alter the temperature gradients. + +It might also be objected that it is inconceivable that the Martian +engineers, however great may be their physical powers, and however +gigantic the mechanical energies under their control, could force water +in large quantities from the poles to the equator. This is an +achievement that measures up to the cosmical standard. It is admitted +by the champions of the theory that the difficulty is a formidable one; +but they call attention to the singular fact that on Mars there can be +found no chains of mountains, and it is even doubtful if ranges of +hills exist there. The entire surface of the planet appears to be +almost “as smooth as a billiard ball,” and even the broad regions which +were once supposed to be seas apparently lie at practically the same +level as the other parts, since the “canals” in many cases run +uninterruptedly across them. Lowell’s idea is that these sombre areas +may be expanses of vegetation covering ground of a more or less marshy +character, for while the largest of them appear to be permanent, there +are some which vary coincidently with the variations of the canals. + +As to the kind of machinery employed to force the water from the poles, +it has been conjectured that it may have taken the form of a gigantic +system of pumps and conduits; and since the Martians are assumed to be +so far in advance of us in their mastery of scientific principles, the +hypothesis will at least not be harmed by supposing that they have +learned to harness forces of nature whose very existence in a +manageable form is yet unrecognized on the earth. If we wish to let the +imagination loose, we may conjecture that they have conquered the +secret of those intra-atomic forces whose resistless energy is +beginning to become evident to us, but the possibility of whose +utilization remains a dream, the fulfillment of which nobody dares to +predict. + +Such, in very brief form, is the celebrated theory of Mars as an +inhabited world. It certainly captivates the imagination, and if we +believe it to represent the facts, we cannot but watch with the deepest +sympathy this gallant struggle of an intellectual race to preserve its +planet from the effects of advancing age and death. We may, indeed, +wonder whether our own humanity, confronted by such a calamity, could +be counted on to meet the emergency with equal stoutness of heart and +inexhaustibleness of resource. Up to the present time we certainly have +shown no capacity to confront Nature toe to toe, and to seize her by +the shoulders and turn her round when she refuses to go our way. If we +could get into wireless telephonic communication with the Martians we +might learn from their own lips the secret of their more than “Roman +recovery.” + + + + +XIV +The Riddle of the Asteroids + + +Between the orbits of Mars and Jupiter revolves the most remarkable +system of little bodies with which we are acquainted—the Asteroids, or +Minor Planets. Some six hundred are now known, and they may actually +number thousands. They form virtually a ring about the sun. The most +striking general fact about them is that they occupy the place in the +sky which should be occupied, according to Bode’s Law, by a single +large planet. This fact, as we shall see, has led to the invention of +one of the most extraordinary theories in astronomy—_viz.,_ that of the +explosion of a world! + +Bode’s Law, so-called, is only an empiric formula, but until the +discovery of Neptune it accorded so well with the distances of the +planets that astronomers were disposed to look upon it as really +representing some underlying principle of planetary distribution. They +were puzzled by the absence of a planet in the space between Mars and +Jupiter, where the “law” demanded that there should be one, and an +association of astronomers was formed to search for it. There was a +decided sensation when, in 1801, Piazzi, of Palermo, announced that he +had found a little planet which apparently occupied the place in the +system which belonged to the missing body. He named it Ceres, and it +was the first of the Asteroids. The next year Olbers, of Bremen, while +looking for Ceres with his telescope, stumbled upon another small +planet which he named Pallas. Immediately he was inspired with the idea +that these two planets were fragments of a larger one which had +formerly occupied the vacant place in the planetary ranks, and he +predicted that others would be found by searching in the neighborhood +of the intersection of the orbits of the two already discovered. This +bold prediction was brilliantly fulfilled by the finding of two +more—Juno in 1804, and Vesta in 1807. Olbers would seem to have been +led to the invention of his hypothesis of a planetary explosion by the +faith which astronomers at that time had in Bode’s Law. They appear to +have thought that several planets revolving in the gap where the “law” +called for but one could only be accounted for upon the theory that the +original _one_ had been broken up to form the several. Gravitation +demanded that the remnants of a planet blown to pieces, no matter how +their orbits might otherwise differ, should all return at stated +periods to the point where the explosion had occurred; hence Olbers’ +prediction that any asteroids that might subsequently be discovered +would be found to have a common point of orbital intersection. And +curiously enough all of the first asteroids found practically answered +to this requirement. Olbers’ theory seemed to be established. + +After the first four, no more asteroids were found until 1845, when one +was discovered; then, in 1847, three more were added to the list; and +after that searchers began to pick them up with such rapidity that by +the close of the century hundreds were known, and it had become almost +impossible to keep track of them. The first four are by far the largest +members of the group, but their actual sizes remained unknown until +less than twenty years ago. It was long supposed that Vesta was the +largest, because it shines more brightly than any of the others; but +finally, in 1895, Barnard, with the Lick telescope, definitely measured +their diameters, and proved to everybody’s surprise that Ceres is +really the chief, and Vesta only the third in rank. His measures are as +follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, 239 miles; and +Juno, 120 miles. They differ greatly in the reflective power of their +surfaces, a fact of much significance in connection with the question +of their origin. Vesta is, surface for surface, rather more than three +times as brilliant as Ceres, whence the original mistake about its +magnitude. + +Nowadays new asteroids are found frequently by photography, but +physically they are most insignificant bodies, their average diameter +probably not exceeding twenty miles, and some are believed not to +exceed ten. On a planet only ten miles in diameter, assuming the same +mean density as the earth’s, which is undoubtedly too much, the force +of gravity would be so slight that an average man would not weigh more +than three ounces, and could jump off into space whenever he liked. + +Although the asteroids all revolve around the sun in the same direction +as that pursued by the major planets, their orbits are inclined at a +great variety of angles to the general plane of the planetary system, +and some of them are very eccentric—almost as much so as the orbits of +many of the periodic comets. It has even been conjectured that the two +tiny moons of Mars and the four smaller satellites of Jupiter may be +asteroids gone astray and captured by those planets. Two of the +asteroids are exceedingly remarkable for the shapes and positions of +their orbits; these are Eros, discovered in 1898, and T. G., 1906, +found eight years later. The latter has a mean distance from the sun +slightly greater than that of Jupiter, while the mean distance of Eros +is less than that of Mars. The orbit of Eros is so eccentric that at +times it approaches within 15,000,000 miles of the earth, nearer than +any other regular member of the solar system except the moon, thus +affording an unrivaled means of measuring the solar parallax. But for +our present purpose the chief interest of Eros lies in its +extraordinary changes of light. + +These changes, although irregular, have been observed and photographed +many times, and there seems to be no doubt of their reality. Their +significance consists in their possible connection with the form of the +little planet, whose diameter is generally estimated at not more than +twenty miles. Von Oppolzer found, in 1901, that Eros lost three-fourths +of its brilliancy once in every two hours and thirty-eight minutes. +Other observers have found slightly different periods of variability, +but none as long as three hours. The most interesting interpretation +that has been offered of this phenomenon is that it is due to a great +irregularity of figure, recalling at once Olbers’ hypothesis. According +to some, Eros may be double, the two bodies composing it revolving +around each other at very close quarters; but a more striking, and it +may be said probable, suggestion is that Eros has a form not unlike +that of a dumb-bell, or hour-glass, turning rapidly end over end so +that the area of illuminated surface presented to our eyes continually +changes, reaching at certain times a minimum when the amount of light +that it reflects toward the earth is reduced to a quarter of its +maximum value. Various other bizarre shapes have been ascribed to Eros, +such, for instance, as that of a flat stone revolving about one of its +longer axes, so that sometimes we see its face and sometimes its edge. + +All of these explanations proceed upon the assumption that Eros cannot +have a simple globular figure like that of a typical planet, a figure +which is prescribed by the law of gravitation, but that its shape is +what may be called accidental; in a word, it is a _fragment,_ for it +seems impossible to believe that a body formed in interplanetary space, +either through nebular condensation or through the aggregation of +particles drawn together by their mutual attractions, should not be +practically spherical in shape. Nor is Eros the only asteroid that +gives evidence by variations of brilliancy that there is something +abnormal in its constitution; several others present the same +phenomenon in varying degrees. Even Vesta was regarded by Olbers as +sufficiently variable in its light to warrant the conclusion that it +was an angular mass instead of a globe. Some of the smaller ones show +very notable variations, and all in short periods, of three or four +hours, suggesting that in turning about one of their axes they present +a surface of variable extent toward the sun and the earth. + +The theory which some have preferred—that the variability of light is +due to the differences of reflective power on different parts of the +surface—would, if accepted, be hardly less suggestive of the origin of +these little bodies by the breaking up of a larger one, because the +most natural explanation of such differences would seem to be that they +arose from variations in the roughness or smoothness of the reflecting +surface, which would be characteristic of fragmentary bodies. In the +case of a large planet alternating expanses of land and water, or of +vegetation and desert, would produce a notable variation in the amount +of reflection, but on bodies of the size of the asteroids neither water +nor vegetation could exist, and an atmosphere would be equally +impossible. + +One of the strongest objections to Olbers’ hypothesis is that only a +few of the first asteroids discovered travel in orbits which measurably +satisfy the requirement that they should all intersect at the point +where the explosion occurred. To this it was at first replied that the +perturbations of the asteroidal orbits, by the attractions of the major +planets, would soon displace them in such a manner that they would +cease to intersect. One of the first investigations undertaken by the +late Prof. Simon Newcomb was directed to the solution of this question, +and he arrived at the conclusion that the planetary perturbations could +not explain the actual situation of the asteroidal orbits. But +afterward it was pointed out that the difficulty could be avoided by +supposing that not one but a series of explosions had produced the +asteroids as they now are. After the primary disruption the fragments +themselves, according to this suggestion, may have exploded, and then +the resulting orbits would be as “tangled” as the heart could wish. +This has so far rehabilitated the explosion theory that it has never +been entirely abandoned, and the evidence which we have just cited of +the probably abnormal shapes of Eros and other asteroids has lately +given it renewed life. It is a subject that needs a thorough +rediscussion. + +We must not fail to mention, however, that there is a rival hypothesis +which commends itself to many astronomers—_viz.,_ that the asteroids +were formed out of a relatively scant ring of matter, situated between +Mars and Jupiter and resembling in composition the immensely more +massive rings from which, according to Laplace’s hypothesis, the +planets were born. It is held by the supporters of this theory that the +attraction of the giant Jupiter was sufficient to prevent the small, +nebulous ring that gave birth to the asteroids from condensing like the +others into a single planet. + +But if we accept the explosion theory, with its corollary that minor +explosions followed the principal one, we have still an unanswered +question before us: What caused the explosions? The idea of _a world +blowing up_ is too Titanic to be shocking; it rather amuses the +imagination than seriously impresses it; in a word, it seems +essentially chimerical. We can by no appeal to experience form a mental +picture of such an occurrence. Even the moon did not blow up when it +was wrecked by volcanoes. The explosive nebulæ and new stars are far +away in space, and suggest no connection with such a catastrophe as the +bursting of a planet into hundreds of pieces. We cannot conceive of a +great globe thousands of miles in diameter resembling a pellet of +gunpowder only awaiting the touch of a match to cause its sudden +disruption. Somehow the thought of human agency obtrudes itself in +connection with the word “explosion,” and we smile at the idea that +giant powder or nitro-glycerine could blow up a planet. Yet it would +only need _enough_ of them to do it. + +After all, we may deceive ourselves in thinking, as we are apt to do, +that explosive energies lock themselves up only in small masses of +matter. There are many causes producing explosions in nature, every +volcanic eruption manifests the activity of some of them. Think of the +giant power of confined steam; if enough steam could be suddenly +generated in the center of the earth by a downpour of all the waters of +the oceans, what might not the consequences be for our globe? In a +smaller globe, and it has never been estimated that the original +asteroid was even as large as the moon, such a catastrophe would, +perhaps, be more easily conceivable; but since we are compelled in this +case to assume that there was a series of successive explosions, steam +would hardly answer the purpose; it would be more reasonable to suppose +that the cause of the explosion was some kind of chemical reaction, or +something affecting the atoms composing the exploding body. Here Dr +Gustav Le Bon comes to our aid with a most startling suggestion, based +on his theory of the dissipation of intra-atomic energy. It will be +best to quote him at some length from his book on _The Evolution of +Forces._ + +“It does not seem at first sight,” says Doctor Le Bon, + + +very comprehensible that worlds which appear more and more stable as +they cool could become so unstable as to afterward dissociate entirely. +To explain this phenomenon, we will inquire whether astronomical +observations do not allow us to witness this dissociation. + +We know that the stability of a body in motion, such as a top or a +bicycle, ceases to be possible when its velocity of rotation descends +below a certain limit. Once this limit is reached it loses its +stability and falls to the ground. Prof. J. J. Thomson even interprets +radio-activity in this manner, and points out that when the speed of +the elements composing the atoms descends below a certain limit they +become unstable and tend to lose their equilibria. There would result +from this a commencement of dissociation, with diminution of their +potential energy and a corresponding increase of their kinetic energy +sufficient to launch into space the products of intra-atomic +disintegration. + +It must not be forgotten that the atom being an enormous reservoir of +energy is by this very fact comparable with explosive bodies. These +last remain inert so long as their internal equilibria are undisturbed. +So soon as some cause or other modifies these, they explode and smash +everything around them after being themselves broken to pieces. + +Atoms, therefore, which grow old in consequence of the diminution of a +part of their intra-atomic energy gradually lose their stability. A +moment, then, arrives when this stability is so weak that the matter +disappears by a sort of explosion more or less rapid. The bodies of the +radium group offer an image of this phenomenon—a rather faint image, +however, because the atoms of this body have only reached a period of +instability when the dissociation is rather slow. It probably precedes +another and more rapid period of dissociation capable of producing +their final explosion. Bodies such as radium, thorium, etc., represent, +no doubt, a state of old age at which all bodies must some day arrive, +and which they already begin to manifest in our universe, since all +matter is slightly radio-active. It would suffice for the dissociation +to be fairly general and fairly rapid for an explosion to occur in a +world where it was manifested. + +These theoretical considerations find a solid support in the sudden +appearances and disappearances of stars. The explosions of a world +which produce them reveal to us, perhaps, how the universes perish when +they become old. + +As astronomical observations show the relative frequency of these rapid +destructions, we may ask ourselves whether the end of a universe by a +sudden explosion after a long period of old age does not represent its +most general ending. + + +Here, perhaps, it will be well to stop, since, entrancing as the +subject may be, we know very little about it, and Doctor Le Bon’s +theory affords a limitless field for the reader’s imagination. + + + + +*** END OF THE PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY *** + +Updated editions will replace the previous one--the old editions will +be renamed. + +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the +United States without permission and without paying copyright +royalties. 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Serviss</div> +<div style='display:block; margin:1em 0'> +This eBook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online +at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you +are not located in the United States, you will have to check the laws of the +country where you are located before using this eBook. +</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: Curiosities of the Sky</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Garrett P. Serviss</div> +<div style='display:block; margin:1em 0'>Release Date: January 6, 2003 [eBook #6630]<br /> +[Most recently updated: December 28, 2022]</div> +<div style='display:block; margin:1em 0'>Language: English</div> +<div style='display:block; margin:1em 0'>Character set encoding: UTF-8</div> +<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: William McClain</div> +<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY ***</div> + +<div class="fig" style="width:55%;"> +<img src="images/cover.jpg" style="width:100%;" alt="[Illustration]" /> +</div> + +<h1>Curiosities of the Sky</h1> + +<h2 class="no-break">by Garrett P. Serviss</h2> + +<hr /> + +<div class="chapter"> + +<h2>Contents</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#pref01">PREFACE</a></td> +</tr> + +<tr> +<td> <a href="#chap00">CURIOSITIES OF THE SKY</a></td> +</tr> + +<tr> +<td> <a href="#chap01">I. The Windows of Absolute Night</a></td> +</tr> + +<tr> +<td> <a href="#chap02">II. Star-Clouds, Star-Clusters, and Star-Streams</a></td> +</tr> + +<tr> +<td> <a href="#chap03">III. Stellar Migrations</a></td> +</tr> + +<tr> +<td> <a href="#chap04">IV. The Passing of the Constellations</a></td> +</tr> + +<tr> +<td> <a href="#chap05">V. Conflagrations in the Heavens</a></td> +</tr> + +<tr> +<td> <a href="#chap06">VI. Explosive and Whirling Nebulæ</a></td> +</tr> + +<tr> +<td> <a href="#chap07">VII. The Banners of the Sun</a></td> +</tr> + +<tr> +<td> <a href="#chap08">VIII. The Zodiacal Light Mystery</a></td> +</tr> + +<tr> +<td> <a href="#chap09">IX. Marvels of the Aurora</a></td> +</tr> + +<tr> +<td> <a href="#chap10">X. Strange Adventures of Comets</a></td> +</tr> + +<tr> +<td> <a href="#chap11">XI. Meteors, Fire-Balls, and Meteorites</a></td> +</tr> + +<tr> +<td> <a href="#chap12">XII. The Wrecking of the Moon</a></td> +</tr> + +<tr> +<td> <a href="#chap13">XIII. The Great Mars Problem</a></td> +</tr> + +<tr> +<td> <a href="#chap14">XIV. The Riddle of the Asteroids</a></td> +</tr> + +</table> + +<div class="fig" style="width:100%;"> +<a name="illus01"></a> +<a href="images/image01.jpg"> +<img src="images/image01.jpg" width="376" height="600" alt="Illustration:" /></a> +<p class="caption">Great spiral nebula in Andromeda</p> +</div> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2>Illustrations</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#illus01">Great spiral nebula in Andromeda</a></td> +</tr> + +<tr> +<td> <a href="#illus02">The Milky Way</a></td> +</tr> + +<tr> +<td> <a href="#illus03">Star-cluster in Hercules</a></td> +</tr> + +<tr> +<td> <a href="#illus04">Great southern star-cluster, Omega Centauri</a></td> +</tr> + +<tr> +<td> <a href="#illus05">The Pleiades</a></td> +</tr> + +<tr> +<td> <a href="#illus06">The “Great Dipper”</a></td> +</tr> + +<tr> +<td> <a href="#illus07">Cassiopeia</a></td> +</tr> + +<tr> +<td> <a href="#illus08">The “Northern Crown”</a></td> +</tr> + +<tr> +<td> <a href="#illus09">The “Southern Cross”</a></td> +</tr> + +<tr> +<td> <a href="#illus10">Chart showing location of Tycho’s star, 1572, and Nova Persei of 1901</a></td> +</tr> + +<tr> +<td> <a href="#illus11">Nova Persei, with its nebular rings</a></td> +</tr> + +<tr> +<td> <a href="#illus12">Lord Rosse’s nebula</a></td> +</tr> + +<tr> +<td> <a href="#illus13">Wonderful spiral in triangulum</a></td> +</tr> + +<tr> +<td> <a href="#illus14">Spiral in Ursa Major</a></td> +</tr> + +<tr> +<td> <a href="#illus15">Nebula in Cetus</a></td> +</tr> + +<tr> +<td> <a href="#illus16">The Orion nebula</a></td> +</tr> + +<tr> +<td> <a href="#illus17">The Corona</a></td> +</tr> + +<tr> +<td> <a href="#illus18">A solar “prominence”</a></td> +</tr> + +<tr> +<td> <a href="#illus19">Auroral beams seen in England</a></td> +</tr> + +<tr> +<td> <a href="#illus20">Auroral arches of an eliptic form seen in the Arctic regions</a></td> +</tr> + +<tr> +<td> <a href="#illus21">Auroral curtain seen in Scandinavia</a></td> +</tr> + +<tr> +<td> <a href="#illus22">Auroral arches seen in Scandinavia</a></td> +</tr> + +<tr> +<td> <a href="#illus23">Swift’s comet</a></td> +</tr> + +<tr> +<td> <a href="#illus24">Daniels’ comet</a></td> +</tr> + +<tr> +<td> <a href="#illus25">Brooks’ comet</a></td> +</tr> + +<tr> +<td> <a href="#illus26">Curious forms of meteorite trains</a></td> +</tr> + +<tr> +<td> <a href="#illus27">Section of the atmosphere up to 100 kilometers</a></td> +</tr> + +<tr> +<td> <a href="#illus28">A meteor photographed in flight</a></td> +</tr> + +<tr> +<td> <a href="#illus29">Looking across Coon Butte crater from northern rim</a></td> +</tr> + +<tr> +<td> <a href="#illus30">Trail on south side, Coon Butte crater</a></td> +</tr> + +<tr> +<td> <a href="#illus31">The craters Clavius, Longomontanus, Tycho, etc.</a></td> +</tr> + +<tr> +<td> <a href="#illus32">Western part of the Mare Serenitatis</a></td> +</tr> + +<tr> +<td> <a href="#illus33">Mare Tranquilitatis and surroundings</a></td> +</tr> + +<tr> +<td> <a href="#illus34">Lunar craters Theophilus and surrounding region</a></td> +</tr> + +<tr> +<td> <a href="#illus35">Mare Crisium</a></td> +</tr> + +<tr> +<td> <a href="#illus36">Schiaparelli’s chart of Mars, showing the so-called system of canals</a></td> +</tr> + +</table> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="pref01"></a>PREFACE</h2> + +<p> +What Froude says of history is true also of astronomy: it is the most +impressive where it transcends explanation. It is not the mathematics of +astronomy, but the wonder and the mystery that seize upon the imagination. The +calculation of an eclipse owes all its prestige to the sublimity of its data; +the operation, in itself, requires no more mental effort than the preparation +of a railway time-table. +</p> + +<p> +The dominion which astronomy has always held over the minds of men is akin to +that of poetry; when the former becomes merely instructive and the latter +purely didactic, both lose their power over the imagination. Astronomy is known +as the oldest of the sciences, and it will be the longest-lived because it will +always have arcana that have not been penetrated. +</p> + +<p> +Some of the things described in this book are little known to the average +reader, while others are well known; but all possess the fascination of +whatever is strange, marvelous, obscure, or mysterious—magnified, in this case, +by the portentous scale of the phenomena. +</p> + +<p> +The idea of the author is to tell about these things in plain language, but +with as much scientific accuracy as plain language will permit, showing the +wonder that is in them without getting away from the facts. Most of them have +hitherto been discussed only in technical form, and in treatises that the +general public seldom sees and never reads. +</p> + +<p> +Among the topics touched upon are: +</p> + +<ul> <li>The strange unfixedness of the “fixed stars,” the vast migrations of +the suns and worlds constituting the universe.</li> + +<li>The slow passing out of existence of those collocations of stars which for +thousands of years have formed famous “constellations,” preserving the memory +of mythological heroes and heroines, and perhaps of otherwise unrecorded +history.</li> + +<li>The tendency of stars to assemble in immense clouds, swarms, and +clusters.</li> + +<li>The existence in some of the richest regions of the universe of absolutely +black, starless gaps, deeps, or holes, as if one were looking out of a window +into the murkiest night.</li> + +<li>The marvelous phenomena of new, or temporary, stars, which appear as +suddenly as conflagrations, and often turn into something else as eccentric as +themselves.</li> + +<li>The amazing forms of the “whirlpool,” “spiral,” “pinwheel,” and “lace,” or +“tress,” nebulæ.</li> + +<li>The strange surroundings of the sun, only seen in particular circumstances, +but evidently playing a constant part in the daily phenomena of the solar +system.</li> + +<li>The mystery of the Zodiacal Light and the Gegenschein.</li> + +<li>The extraordinary transformations undergone by comets and their tails.</li> + +<li>The prodigies of meteorites and masses of stone and metal fallen from the +sky.</li> + +<li>The cataclysms that have wrecked the moon.</li> + +<li>The problem of life and intelligence on the planet Mars.</li> + +<li>The problematical origin and fate of the asteroids.</li> + +<li>The strange phenomena of the auroral lights.</li> </ul> + +<p> +An attempt has been made to develop these topics in an orderly way, showing +their connection, so that the reader may obtain a broad general view of the +chief mysteries and problems of astronomy, and an idea of the immense field of +discovery which still lies, almost unexplored, before it. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap00"></a>CURIOSITIES OF THE SKY</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap01"></a>I<br/> +The Windows of Absolute Night</h2> + +<p> +To most minds mystery is more fascinating than science. But when science itself +leads straight up to the borders of mystery and there comes to a dead stop, +saying, “At present I can no longer see my way,” the force of the charm is +redoubled. On the other hand, the illimitable is no less potent in mystery than +the invisible, whence the dramatic effect of Keats’ “stout Cortez” staring at +the boundless Pacific while all his men look at each other with a wild surmise, +“silent upon a peak in Darien.” It is with similar feelings that the astronomer +regards certain places where from the peaks of the universe his vision seems to +range out into endless empty space. He sees there the shore of his little +isthmus, and, beyond, unexplored immensity. +</p> + +<p> +The name, “coal-sacks,” given to these strange voids is hardly descriptive. +Rather they produce upon the mind the effect of blank windows in a lonely house +on a pitch-dark night, which, when looked at from the brilliant interior, +become appalling in their rayless murk. Infinity seems to acquire a new meaning +in the presence of these black openings in the sky, for as one continues to +gaze it loses its purely metaphysical quality and becomes a kind of entity, +like the ocean. The observer is conscious that he can actually <i>see</i> the +beginning of its ebon depths, in which the visible universe appears to float +like an enchanted island, resplendent within with lights and life and gorgeous +spectacles, and encircled with screens of crowded stars, but with its dazzling +vistas ending at the fathomless sea of pure darkness which encloses all. +</p> + +<p> +The Galaxy, or Milky Way, surrounds the borders of our island in space like a +stellar garland, and when openings appear in it they are, by contrast, far more +impressive than the general darkness of the interstellar expanse seen in other +directions. Yet even that expanse is not everywhere equally dark, for it +contains gloomy deeps discernable with careful watching. Here, too, contrast +plays an important part, though less striking than within the galactic region. +Some of Sir William Herschel’s observations appear to indicate an association +between these tenebrious spots and neighboring star clouds and nebulæ. It is an +illuminating bit of astronomical history that when he was sweeping the then +virgin heavens with his great telescopes he was accustomed to say to his sister +who, note-book in hand, waited at his side to take down his words, fresh with +the inspiration of discovery: “Prepare to write; the nebulæ are coming; here +space is vacant.” +</p> + +<p> +The most famous of the “coal-sacks,” and the first to be brought to general +attention before astronomers had awakened to the significance of such things, +lies adjacent to the “Southern Cross,” and is truly an amazing phenomenon. It +is not alone the conspicuousness of this celestial vacancy, opening suddenly in +the midst of one of the richest parts of the Galaxy, that has given it its +fame, but quite as much the superstitious awe with which it was regarded by the +early explorers of the South Seas. To them, as well as to those who listened in +rapt wonder to their tales, the “Coal-sack” seemed to possess some occult +connection with the mystic “Cross.” In the eyes of the sailors it was not a +vacancy so much as a sable reality in the sky, and as, shuddering, they stared +at it, they piously crossed themselves. It was another of the magical wonders +of the unknown South, and as such it formed the basis of many a “wild surmise” +and many a sea-dog’s yarn. Scientific investigation has not diminished its +prestige, and today no traveler in the southern hemisphere is indifferent to +its fascinating strangeness, while some find it the most impressive spectacle +of the antarctic heavens. +</p> + +<p> +All around, up to the very edge of the yawning gap, the sheen of the Milky Way +is surpassingly glorious; but there, as if in obedience to an almighty edict, +everything vanishes. A single faint star is visible within the opening, +producing a curious effect upon the sensitive spectator, like the sight of a +tiny islet in the midst of a black, motionless, waveless tarn. The dimensions +of the lagoon of darkness, which is oval or pear-shaped, are eight degrees by +five, so that it occupies a space in the sky about one hundred and thirty times +greater than the area of the full moon. It attracts attention as soon as the +eye is directed toward the quarter where it exists, and by virtue of the rarity +of such phenomena it appears a far greater wonder than the drifts of stars that +are heaped around it. Now that observatories are multiplying in the southern +hemisphere, the great austral “Coal-sack” will, no doubt, receive attention +proportioned to its importance as one of the most significant features of the +sky. Already at the Sydney Observatory photographs have shown that the southern +portion of this Dead Sea of Space is not quite “bottomless,” although its +northern part defies the longest sounding lines of the astronomer. +</p> + +<p> +There is a similar, but less perfect, “coal-sack” in the northern hemisphere, +in the constellation of “The Swan,” which, strange to say, also contains a +well-marked figure of a cross outlined by stars. This gap lies near the top of +the cross-shaped figure. It is best seen by averted vision, which brings out +the contrast with the Milky Way, which is quite brilliant around it. It does +not, however, exercise the same weird attraction upon the eye as the southern +“Coal-sack,” for instead of looking like an absolute void in the sky, it rather +appears as if a canopy of dark gauze had been drawn over the stars. We shall +see the possible significance of this appearance later. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus02"></a> +<a href="images/image02.jpg"> +<img src="images/image02.jpg" width="481" height="600" alt="Illustration:" /></a> +<p class="caption">The Milky Way. Region near M.S. Photographed by Professor +Barnard</p> +</div> + +<p> +Just above the southern horizon of our northern middle latitudes, in summer, +where the Milky Way breaks up into vast sheets of nebulous luminosity, lying +over and between the constellations Scorpio and Sagittarius, there is a +remarkable assemblage of “coal-sacks,” though none is of great size. One of +them, near a conspicuous star-cluster in Scorpio, M80, is interesting for +having been the first of these strange objects noted by Herschel. Probably it +was its nearness to M80 which suggested to his mind the apparent connection of +such vacancies with star-clusters which we have already mentioned. +</p> + +<p> +But the most marvelous of the “coal-sacks” are those that have been found by +photography in Sagittarius. One of Barnard’s earliest and most excellent +photographs includes two of them, both in the star-cluster M8. The larger, +which is roughly rectangular in outline, contains one little star, and its +smaller neighbor is lune-shaped—surely a most singular form for such an object. +Both are associated with curious dark lanes running through the clustered stars +like trails in the woods. Along the borders of these lanes the stars are ranked +in parallel rows, and what may be called the bottoms of the lanes are not +entirely dark, but pebbled with faint stellar points. One of them which skirts +the two dark gaps and traverses the cluster along its greatest diameter is +edged with lines of stars, recalling the alignment of the trees bordering a +French highway. This <i>road of stars</i> cannot be less than many billions of +miles in length! +</p> + +<p> +All about the cluster the bed of the Galaxy is strangely disturbed, and in +places nearly denuded, as if its contents had been raked away to form the +immense stack and the smaller accumulations of stars around it. The well-known +“Trifid Nebula” is also included in the field of the photograph, which covers a +truly marvelous region, so intricate in its mingling of nebulæ, star-clusters, +star-swarms, star-streams, and dark vacancies that no description can do it +justice. Yet, chaotic as it appears, there is an unmistakable suggestion of +unity about it, impressing the beholder with the idea that all the different +parts are in some way connected, and have not been fortuitously thrown +together. Miss Agnes M. Clerke made the striking remark that the dusky lanes in +M8 are exemplified on the largest scale in the great rift dividing the Milky +Way, from Cygnus in the northern hemisphere all the way to the “Cross” in the +southern. Similar lanes are found in many other clusters, and they are +generally associated with flanking rows of stars, resembling in their +arrangement the thick-set houses and villas along the roadways that traverse +the approaches to a great city. +</p> + +<p> +But to return to the black gaps. Are they really windows in the star-walls of +the universe? Some of them look rather as if they had been made by a shell +fired through a luminous target, allowing the eye to range through the hole +into the void space beyond. If science is discretely silent about these things, +what can the more venturesome and less responsible imagination suggest? Would a +huge “runaway sun,” like Arcturus, for instance, make such an opening if it +should pass like a projectile through the Milky Way? It is at least a +stimulating inquiry. Being probably many thousands of times more massive than +the galactic stars, such a stellar missile would not be stopped by them, though +its direction of flight might be altered. It would drag the small stars lying +close to its course out of their spheres, but the ultimate tendency of its +attraction would be to sweep them round in its wake, thus producing rather a +star-swarm than a vacancy. Those that were very close to it might be swept away +in its rush and become its satellites, careering away with it in its flight +into outer space; but those that were farther off, and they would, of course, +greatly outnumber the nearer ones, would tend inward from all sides toward the +line of flight, as dust and leaves collect behind a speeding motor (though the +forces operating would be different), and would fill up the hole, if hole it +were. A swarm thus collected should be rounded in outline and bordered with a +relatively barren ring from which the stars had been “sucked” away. In a +general sense the M8 cluster answers to this description, but even if we +undertook to account for its existence by a supposition like the above, the +black gaps would remain unexplained, unless one could make a further draft on +the imagination and suggest that the stars had been thrown into a vast eddy, or +system of eddies, whose vortices appear as dark holes. Only a maelstrom-like +motion could keep such a funnel open, for without regard to the impulse derived +from the projectile, the proper motions of the stars themselves would tend to +fill it. Perhaps some other cause of the whirling motion may be found. As we +shall see when we come to the spiral nebulæ, gyratory movements are exceedingly +prevalent throughout the universe, and the structure of the Milky Way is +everywhere suggestive of them. But this is hazardous sport even for the +imagination—to play with <i>suns</i> as if they were but thistle-down in the +wind or corks in a mill-race. +</p> + +<p> +Another question arises: What is the thickness of the hedge of stars through +which the holes penetrate? Is the depth of the openings proportionate to their +width? In other words, is the Milky Way round in section like a rope, or flat +and thin like a ribbon? The answer is not obvious, for we have little or no +information concerning the relative distances of the faint galactic stars. It +would be easier, certainly, to conceive of openings in a thin belt than in a +massive ring, for in the first case they would resemble mere rifts and breaks, +while in the second they would be like wells or bore-holes. Then, too, the fact +that the Milky Way is not a <i>continuous</i> body but is made up of stars +whose actual distances apart is great, offers another quandary; persistent and +sharply bordered apertures in such an assemblage are <i>a priori</i> as +improbable, if not impossible, as straight, narrow holes running through a +swarm of bees. +</p> + +<p> +The difficulty of these questions indicates one of the reasons why it has been +suggested that the seeming gaps, or many of them, are not openings at all, but +opaque screens cutting off the light from stars behind them. That this is quite +possible in some cases is shown by Barnard’s later photographs, particularly +those of the singular region around the star Rho Ophiuchi. Here are to be seen +somber lanes and patches, apparently forming a connected system which covers an +immense space, and which their discoverer thinks may constitute a “dark +nebula.” This seems at first a startling suggestion; but, after all, why should +their not be dark nebulæ as well as visible ones? In truth, it has troubled +some astronomers to explain the luminosity of the bright nebulæ, since it is +not to be supposed that matter in so diffuse a state can be incandescent +through heat, and phosphorescent light is in itself a mystery. The supposition +is also in accord with what we know of the existence of dark solid bodies in +space. Many bright stars are accompanied by obscure companions, sometimes as +massive as themselves; the planets are non-luminous; the same is true of +meteors before they plunge into the atmosphere and become heated by friction; +and many plausible reasons have been found for believing that space contains as +many obscure as shining bodies of great size. It is not so difficult, after +all, then, to believe that there are immense collections of shadowy gases and +meteoric dust whose presence is only manifested when they intercept the light +coming from shining bodies behind them. +</p> + +<p> +This would account for the apparent extinguishment of light in open space, +which is indicated by the falling off in relative number of telescopic stars +below the tenth magnitude. Even as things are, the amount of light coming to us +from stars too faint to be seen with the naked eye is so great that the +statement of it generally surprises persons who are unfamiliar with the inner +facts of astronomy. It has been calculated that on a clear night the total +starlight from the entire celestial sphere amounts to one-sixtieth of the light +of the full moon; but of this less than one-twenty-fifth is due to stars +separately distinguished by the eye. If there were no obscuring medium in +space, it is probable that the amount of starlight would be noticeably and +perhaps enormously increased. +</p> + +<p> +But while it seems certain that some of the obscure spots in the Milky Way are +due to the presence of “dark nebulæ,” or concealing veils of one kind or +another, it is equally certain that there are many which are true apertures, +however they may have been formed, and by whatever forces they may be +maintained. These, then, are veritable windows of the Galaxy, and when looking +out of them one is face to face with the great mystery of infinite space. +<i>There</i> the known universe visibly ends, but manifestly space itself does +not end there. It is not within the power of thought to conceive an end to +space, for the instant we think of a terminal point or line the mind leaps +forward to the <i>beyond.</i> There must be space outside as well as inside. +Eternity of time and infinity of space are ideas that the intellect cannot +fully grasp, but neither can it grasp the idea of a limitation to either space +or time. The metaphysical conceptions of hypergeometry, or fourth-dimensional +space, do not aid us. +</p> + +<p> +Having, then, discovered that the universe is a thing <i>contained</i> in +something indefinitely greater than itself; having looked out of its windows +and found only the gloom of starless night outside—what conclusions are we to +draw concerning the beyond? It <i>seems</i> as empty as a vacuum, but is it +really so? If it be, then our universe is a single atom astray in the infinite; +it is the only island in an ocean without shores; it is the one oasis in an +illimitable desert. Then the Milky Way, with its wide-flung garland of stars, +is afloat like a tiny smoke-wreath amid a horror of immeasurable vacancy, or it +is an evanescent and solitary ring of sparkling froth cast up for a moment on +the viewless billows of immensity. From such conclusions the mind instinctively +shrinks. It prefers to think that there is <i>something</i> beyond, though we +cannot see it. Even the universe could not bear to be alone—a Crusoe lost in +the Cosmos! As the inhabitants of the most elegant château, with its gardens, +parks, and crowds of attendants, would die of loneliness if they did not know +that they have neighbors, though not seen, and that a living world of +indefinite extent surrounds them, so we, when we perceive that the universe has +limits, wish to feel that it is not solitary; that beyond the hedges and the +hills there are other centers of life and activity. Could anything be more +terrible than the thought of an <i>isolated universe?</i> The greater the +being, the greater the aversion to seclusion. Only the infinite satisfies; in +that alone the mind finds rest. +</p> + +<p> +We are driven, then, to believe that the universal night which envelopes us is +not tenantless; that as we stare out of the star-framed windows of the Galaxy +and see nothing but uniform blackness, the fault is with our eyes or is due to +an obscuring medium. Since <i>our</i> universe is limited in extent, there must +be <i>other</i> universes beyond it on all sides. Perhaps if we could carry our +telescopes to the verge of the great “Coal-sack” near the “Cross,” being then +on the frontier of our starry system, we could discern, sparkling afar off in +the vast night, some of the outer galaxies. They may be grander than ours, just +as many of the suns surrounding us are immensely greater than ours. If we could +take our stand somewhere in the midst of immensity and, with vision of infinite +reach, look about us, we should perhaps see a countless number of stellar +systems, amid which ours would be unnoticeable, like a single star among the +multitude glittering in the terrestial sky on a clear night. Some might be in +the form of a wreath, like our own; some might be globular, like the great +star-clusters in Hercules and Centaurus; some might be glittering circles, or +disks, or rings within rings. If we could enter them we should probably find a +vast variety of composition, including elements unknown to terrestrial +chemistry; for while the <i>visible</i> universe appears to contain few if any +substances not existing on the earth or in the sun, we have no warrant to +assume that others may not exist in infinite space. +</p> + +<p> +And how as to gravitation? We do not <i>know</i> that gravitation acts beyond +the visible universe, but it is reasonable to suppose that it does. At any +rate, if we let go <i>its</i> sustaining hand we are lost, and can only wander +hopelessly in our speculations, like children astray. If the empire of +gravitation is infinite, then the various outer systems must have <i>some,</i> +though measuring by our standards an imperceptible, attractive influence upon +each other, for gravitation never lets go its hold, however great the space +over which it is required to act. Just as the stars about us are all in motion, +so the starry systems beyond our sight may be in motion, and our system as a +whole may be moving in concert with them. If this be so, then after +interminable ages the aspect of the entire system of systems must change, its +various members assuming new positions with respect to one another. In the +course of time we may even suppose that our universe will approach relatively +close to one of the others; and then, if men are yet living on the earth, they +may glimpse through the openings which reveal nothing to us now, the lights of +another nearing star system, like the signals of a strange squadron, bringing +them the assurance (which can be but an inference at present) that the ocean of +space has other argosies venturing on its limitless expanse. +</p> + +<p> +There remains the question of the luminiferous ether by whose agency the waves +of light are borne through space. The ether is as mysterious as gravitation. +With regard to ether we only infer its existence from the effects which we +ascribe to it. Evidently the ether must extend as far as the most distant +visible stars. But does it continue on indefinitely in outer space? If it does, +then the invisibility of the other systems must be due to their distance +diminishing the quantity of light that comes from them below the limit of +perceptibility, or to the interposition of absorbing media; if it does not, +then the reason why we cannot see them is owing to the absence of a means of +conveyance for the light waves, as the lack of an interplanetary atmosphere +prevents us from hearing the thunder of sun-spots. (It is interesting to recall +that Mr Edison was once credited with the intention to construct a gigantic +microphone which should render the roar of sun-spots audible by transforming +the electric vibrations into sound-waves). On this supposition each starry +system would be enveloped in its own globule of ether, and no light could cross +from one to another. But the probability is that both the ether and gravitation +are ubiquitous, and that all the stellar systems are immersed in the former +like clouds of phosphorescent organisms in the sea. +</p> + +<p> +So astronomy carries the mind from height to greater height. Men were long in +accepting the proofs of the relative insignificance of the earth; they were +more quickly convinced of the comparative littleness of the solar system; and +now the evidence assails their reason that what they had regarded as <i>the</i> +universe is only one mote gleaming in the sunbeams of Infinity. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap02"></a>II<br/> +Star-Clouds, Star-Clusters, and Star-Streams</h2> + +<p> +In the preceding chapter we have seen something of the strangely complicated +structure of the Galaxy, or Milky Way. We now proceed to study more +comprehensively that garlanded “Pathway of the Gods.” +</p> + +<p> +Judged by the eye alone, the Milky Way is one of the most delicately beautiful +phenomena in the entire realm of nature—a shimmer of silvery gauze stretched +across the sky; but studied in the light of its revelations, it is the most +stupendous object presented to human ken. Let us consider, first, its +appearance to ordinary vision. Its apparent position in the sky shifts +according to the season. On a serene, cloudless summer evening, in the absence +of the moon, whose light obscures it, one sees the Galaxy spanning the heavens +from north to southeast of the zenith like a phosphorescent arch. In early +spring it forms a similar but, upon the whole, less brilliant arch west of the +zenith. Between spring and summer it lies like a long, faint, twilight band +along the northern horizon. At the beginning of winter it again forms an arch, +this time spanning the sky from east to west, a little north of the zenith. +These are its positions as viewed from the mean latitude of the United States. +Even the beginner in star-gazing does not have to watch it throughout the year +in order to be convinced that it is, in reality, a great circle, extending +entirely around the celestial sphere. We appear to be situated near its center, +but its periphery is evidently far away in the depths of space. +</p> + +<p> +Although to the casual observer it seems but a delicate scarf of light, +brighter in some places than in others, but hazy and indefinite at the best, +such is not its appearance to those who study it with care. They perceive that +it is an organic whole, though marvelously complex in detail. The telescope +shows that it consists of stars too faint and small through excess of distance +to be separately visible. Of the hundred million suns which some estimates have +fixed as the probable population of the starry universe, the vast majority (at +least thirty to one) are included in this strange belt of misty light. But they +are not uniformly distributed in it; on the contrary, they are arrayed in +clusters, knots, bunches, clouds, and streams. The appearance is somewhat as if +the Galaxy consisted of innumerable swarms of silver-winged bees, more or less +intermixed, some massed together, some crossing the paths of others, but all +governed by a single purpose which leads them to encircle the region of space +in which we are situated. +</p> + +<p> +From the beginning of the systematic study of the heavens, the fact has been +recognized that the form of the Milky Way denotes the scheme of the sidereal +system. At first it was thought that the shape of the system was that of a vast +round disk, flat like a cheese, and filled with stars, our sun and his +relatively few neighbors being placed near the center. According to this view, +the galactic belt was an effect of perspective; for when looking in the +direction of the plane of the disk, the eye ranged through an immense extension +of stars which blended into a glimmering blur, surrounding us like a ring; +while when looking out from the sides of the disk we saw but few stars, and in +those directions the heavens appeared relatively blank. Finally it was +recognized that this theory did not correspond with the observed appearances, +and it became evident that the Milky Way was not a mere effect of perspective, +but an actual band of enormously distant stars, forming a circle about the +sphere, the central opening of the ring (containing many scattered stars) being +many times broader than the width of the ring itself. Our sun is one of the +scattered stars in the central opening. +</p> + +<p> +As already remarked, the ring of the Galaxy is very irregular, and in places it +is partly broken. With its sinuous outline, its pendant sprays, its graceful +and accordant curves, its bunching of masses, its occasional interstices, and +the manifest order of a general plan governing the jumble of its details, it +bears a remarkable resemblance to a garland—a fact which appears the more +wonderful when we recall its composition. That an elm-tree should trace the +lines of beauty with its leafy and pendulous branches does not surprise us; but +we can only gaze with growing amazement when we behold <i>a hundred million +suns imitating the form of a chaplet!</i> And then we have to remember that +this form furnishes the ground-plan of the universe. +</p> + +<p> +As an indication of the extraordinary speculations to which the mystery of the +Milky Way has given rise, a theory recently (1909) proposed by Prof. George C. +Comstock may be mentioned. Starting with the data (first) that the number of +stars increases as the Milky Way is approached, and reaches a maximum in its +plane, while on the other hand the number of nebulæ is greatest outside the +Milky Way and increases with distance from it, and (second) that the Milky Way, +although a complete ring, is broad and diffuse on one side through one-half its +course—that half alone containing nebulæ—and relatively narrow and well defined +on the opposite side, the author of this singular speculation avers that these +facts can best be explained by supposing that the invisible universe consists +of two interpenetrating parts, one of which is a chaos of indefinite extent, +strewn with stars and nebulous dust, and the other a long, broad but +comparatively thin cluster of stars, including the sun as one of its central +members. This flat star-cluster is conceived to be moving edgewise through the +chaos, and, according to Professor Comstock, it acts after the manner of a +snow-plough sweeping away the cosmic dust and piling it on either hand above +and below the plane of the moving cluster. It thus forms a transparent rift, +through which we see farther and command a view of more stars than through the +intensified dust-clouds on either hand. This rift is the Milky Way. The dust +thrown aside toward the poles of the Milky Way is the substance of the nebulæ +which abound there. Ahead, where the front of the star-plough is clearing the +way, the chaos is nearer at hand, and consequently there the rift subtends a +broader angle, and is filled with primordial dust, which, having been annexed +by the vanguard of the star-swarm, forms the nebulæ seen only in that part of +the Milky Way. But behind, the rift appears narrow because there we look +farther away between dust-clouds produced ages ago by the front of the plough, +and no scattered dust remains in that part of the rift. +</p> + +<p> +In quoting an outline of this strikingly original theory the present writer +should not be understood as assenting to it. That it appears bizarre is not, in +itself, a reason for rejecting it, when we are dealing with so problematical +and enigmatical a subject as the Milky Way; but the serious objection is that +the theory does not sufficiently accord with the observed phenomena. There is +too much evidence that the Milky Way is an organic system, however fantastic +its form, to permit the belief that it can only be a rift in chaotic clouds. As +with every organism, we find that its parts are more or less clearly repeated +in its ensemble. Among all the strange things that the Milky Way contains there +is nothing so extraordinary as itself. Every astronomer must many times have +found himself marveling at it in those comparatively rare nights when it shows +all its beauty and all its strangeness. In its great broken rifts, divisions, +and spirals are found the gigantic prototypes of similar forms in its +star-clouds and clusters. As we have said, it determines the general shape of +the whole sidereal system. Some of the brightest stars in the sky appear to +hang like jewels suspended at the ends of tassels dropped from the Galaxy. +Among these pendants are the Pleiades and the Hyades. Orion, too, the “Mighty +Hunter,” is caught in “a loop of light” thrown out from it. The majority of the +great first-magnitude stars seem related to it, as if they formed an inner ring +inclined at an angle of some twenty degrees to its plane. Many of the long +curves that set off from it on both sides are accompanied by corresponding +curves of lucid stars. In a word, it offers every appearance of structural +connection with the entire starry system. That the universe should have assumed +the form of a wreath is certainly a matter for astonishment; but it would have +been still more astonishing if it had been a cube, a rhomboid, or a +dodecahedron, for then we should have had to suppose that something resembling +the forces that shape crystals had acted upon the stars, and the difficulty of +explaining the universe by the laws of gravitation would have been increased. +</p> + +<p> +From the Milky Way as a whole we pass to the vast clouds, swarms, and clusters +of stars of which it is made up. It may be, as some astronomers hold, that most +of the galactic stars are much smaller than the sun, so that their faintness is +not due entirely to the effect of distance. Still, their intrinsic brilliance +attests their solar character, and considering their remoteness, which has been +estimated at not less than ten thousand to twenty thousand light-years (a +light-year is equal to nearly six thousand thousand million miles) their actual +masses cannot be extremely small. The minutest of them are entitled to be +regarded as real suns, and they vary enormously in magnitude. The effects of +their attractions upon one another can only be inferred from their clustering, +because their relative movements are not apparent on account of the brevity of +the observations that we can make. But imagine a being for whom a million years +would be but as a flitting moment; to him the Milky Way would appear in a state +of ceaseless agitation—swirling with “a fury of whirlpool motion.” +</p> + +<p> +The cloud-like aspect of large parts of the Galaxy must always have attracted +attention, even from naked-eye observers, but the true star-clouds were first +satisfactorily represented in Barnard’s photographs. The resemblance to actual +clouds is often startling. Some are close-packed and dense, like cumuli; some +are wispy or mottled, like cirri. The rifts and modulations, as well as the +general outlines, are the same as those of clouds of vapor or dust, and one +notices also the characteristic thinning out at the edges. But we must beware +of supposing that the component suns are thickly crowded as the particles +forming an ordinary cloud. They <i>look,</i> indeed, as if they were matted +together, because of the irradiation of light, but in reality millions and +billions of miles separate each star from its neighbors. Nevertheless they form +real assemblages, whose members are far more closely related to one another +than is our sun to the stars around him, and if we were in the Milky Way the +aspect of the nocturnal sky would be marvelously different from its present +appearance. +</p> + +<p> +Stellar clouds are characteristic of the Galaxy and are not found beyond its +borders, except in the “Magellanic Clouds” of the southern hemisphere, which +resemble detached portions of the Milky Way. These singular objects form as +striking a peculiarity of the austral heavens as does the great “Coal-sack” +described in Chapter 1. But it is their isolation that makes them so +remarkable, for their composition is essentially galactic, and if they were +included within its boundaries they would not appear more wonderful than many +other parts of the Milky Way. Placed where they are, they look like masses +fallen from the great stellar arch. They are full of nebulæ and star-clusters, +and show striking evidences of spiral movement. +</p> + +<p> +Star-swarms, which are also characteristic features of the Galaxy, differ from +star-clouds very much in the way that their name would imply—<i>i.e.,</i> their +component stars are so arranged, even when they are countless in number, that +the idea of an exceedingly numerous assemblage rather than that of a cloud is +impressed on the observer’s mind. In a star-swarm the separate members are +distinguishable because they are either larger or nearer than the stars +composing a “cloud.” A splendid example of a true star-swarm is furnished by +Chi Persei, in that part of the Milky Way which runs between the constellations +Perseus and Cassiopeia. This swarm is much coarser than many others, and can be +seen by the naked eye. In a small telescope it appears double, as if the suns +composing it had divided into two parties which keep on their way side by side, +with some commingling of their members where the skirts of the two companies +come in contact. +</p> + +<p> +Smaller than either star-clouds or star-swarms, and differing from both in +their organization, are star-clusters. These, unlike the others, are found +outside as well as inside the Milky Way, although they are more numerous inside +its boundaries than elsewhere. The term star-cluster is sometimes applied, +though improperly, to assemblages which are rather groups, such, for instance, +as the Pleiades. In their most characteristic aspect star-clusters are of a +globular shape—globes of suns! A famous example of a globular star-cluster, but +one not included in the Milky Way, is the “Great Cluster in Hercules.” This is +barely visible to the naked eye, but a small telescope shows its character, and +in a large one it presents a marvelous spectacle. Photographs of such clusters +are, perhaps, less effective than those of star-clouds, because the central +condensation of stars in them is so great that their light becomes blended in +an indistinguishable blur. The beautiful effect of the incessant play of +infinitesimal rays over the apparently compact surface of the cluster, as if it +were a globe of the finest frosted silver shining in an electric beam, is also +lost in a photograph. Still, even to the eye looking directly at the cluster +through a powerful telescope, the central part of the wonderful congregation +seems almost a solid mass in which the stars are packed like the ice crystals +in a snowball. +</p> + +<p> +The same question rises to the lips of every observer: How can they possibly +have been brought into such a situation? The marvel does not grow less when we +know that, instead of being closely compacted, the stars of the cluster are +probably separated by millions of miles; for we know that their distances apart +are slight as compared with their remoteness from the Earth. Sir William +Herschel estimated their number to be about fourteen thousand, but in fact they +are uncountable. If we could view them from a point just within the edge of the +assemblage, they would offer the appearance of a hollow hemisphere emblazoned +with stars of astonishing brilliancy; the near-by ones unparalleled in splendor +by any celestial object known to us, while the more distant ones would resemble +ordinary stars. An inhabitant of the cluster would not know, except by a +process of ratiocination, that he was dwelling in a globular assemblage of +suns; only from a point far outside would their spherical arrangement become +evident to the eye. Imagine fourteen-thousand fire-balloons with an approach to +regularity in a spherical space—say, ten miles in diameter; there would be an +average of less than thirty in every cubic mile, and it would be necessary to +go to a considerable distance in order to see them as a globular aggregation; +yet from a point sufficiently far away they would blend into a glowing ball. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus03"></a> +<a href="images/image03.jpg"> +<img src="images/image03.jpg" width="409" height="600" alt="Illustration:" /></a> +<p class="caption">Star-cluster in Hercules<br/> +(Photographed with a two-foot reflector)</p> +</div> + +<p> +Photographs show even better than the best telescopic views that the great +cluster is surrounded with a multitude of dispersed stars, suggestively arrayed +in more or less curving lines, which radiate from the principle mass, with +which their connection is manifest. These stars, situated outside the central +sphere, look somewhat like vagrant bees buzzing round a dense swarm where the +queen bee is sitting. Yet while there is so much to suggest the operation of +central forces, bringing and keeping the members of the cluster together, the +attentive observer is also impressed with the idea that the whole wonderful +phenomenon may be <i>the result of explosion.</i> As soon as this thought +seizes the mind, confirmation of it seems to be found in the appearance of the +outlying stars, which could be as readily explained by the supposition that +they have been blown apart as that they have flocked together toward a center. +The probable fact that the stars constituting the cluster are very much smaller +than our sun might be regarded as favoring the hypothesis of an explosion. Of +their real size we know nothing, but, on the basis of an uncertain estimate of +their parallax, it has been calculated that they may average forty-five +thousand miles in diameter—something more than half the diameter of the planet +Jupiter. Assuming the same mean density, fourteen thousand such stars might +have been formed by the explosion of a body about twice the size of the sun. +This recalls the theory of Olbers, which has never been altogether abandoned or +disproved, that the Asteroids were formed by the explosion of a planet +circulating between the orbits of Mars and Jupiter. The Asteroids, whatever +their manner of origin, form a ring around the sun; but, of course, the +explosion of a great independent body, not originally revolving about a +superior center of gravitational force, would not result in the formation of a +ring of small bodies, but rather of a dispersed mass of them. But back of any +speculation of this kind lies the problem, at present insoluble: How could the +explosion be produced? (See the question of explosions in Chapters 6 and 14). +</p> + +<p> +Then, on the other hand, we have the observation of Herschel, since abundantly +confirmed, that space is unusually vacant in the immediate neighborhood of +condensed star-clusters and nebulæ, which, as far as it goes, might be taken as +an indication that the assembled stars had been drawn together by their mutual +attractions, and that the tendency to aggregation is still bringing new members +toward the cluster. But in that case there must have been an original +condensation of stars at that point in space. This could probably have been +produced by the coagulation of a great nebula into stellar nuclei, a process +which seems now to be taking place in the Orion Nebula. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus04"></a> +<a href="images/image04.jpg"> +<img src="images/image04.jpg" width="417" height="600" alt="Illustration:" /></a> +<p class="caption">Great southern star-cluster, Omega Centauri</p> +</div> + +<p> +A yet more remarkable globular star-cluster exists in the southern hemisphere, +Omega Centauri. In this case the central condensation of stars presents an +almost uniform blaze of light. Like the Hercules cluster, that in Centaurus is +surrounded with stars scattered over a broad field and showing an appearance of +radial arrangement. In fact, except for its greater richness, Omega Centauri is +an exact duplicate of its northern rival. Each appears to an imaginative +spectator as a veritable “city of suns.” Mathematics shrinks from the task of +disentangling the maze of motions in such an assemblage. It would seem that the +chance of collisions is not to be neglected, and this idea finds a certain +degree of confirmation in the appearance of “temporary stars” which have more +than once blazed out in, or close by, globular star-clusters. +</p> + +<p> +This leads up to the notable fact, first established by Professor Bailey a few +years ago, that such clusters are populous with variable stars. Omega Centauri +and the Hercules cluster are especially remarkable in this respect. The +variables found in them are all of short period and the changes of light show a +noteworthy tendency to uniformity. The first thought is that these phenomena +must be due to collisions among the crowded stars, but, if so, the encounters +cannot be between the stars themselves, but probably between stars and meteor +swarms revolving around them. Such periodic collisions might go on for ages +without the meteors being exhausted by incorporation with the stars. This +explanation appears all the more probable because one would naturally expect +that flocks of meteors would abound in a close aggregation of stars. It is also +consistent with Perrine’s discovery—that the globular star clusters are +powdered with minute stars strewn thickly among the brighter ones. +</p> + +<p> +In speaking of Professor Comstock’s extraordinary theory of the Milky Way, the +fact was mentioned that, broadly speaking, the nebulæ are less numerous in the +galactic belt than in the comparatively open spaces on either side of it, but +that they are, nevertheless, abundant in the broader half of the Milky Way +which he designates as the front of the gigantic “plough” supposed to be +forcing its way through the enveloping chaos. In and around the Sagittarius +region the intermingling of nebulæ and galactic star clouds and clusters is +particularly remarkable. That there is a causal connection no thoughtful person +can doubt. We are unable to get away from the evidence that a nebula is like a +seed-ground from which stars spring forth; or we may say that nebulæ resemble +clouds in whose bosom raindrops are forming. The wonderful aspect of the +admixtures of nebulæ and star-clusters in Sagittarius has been described in +Chapter 1. We now come to a still more extraordinary phenomenon of this +kind—the Pleiades nebulæ. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus05"></a> +<a href="images/image05.jpg"> +<img src="images/image05.jpg" width="600" height="436" alt="Illustration:" /></a> +<p class="caption">The Pleiades</p> +</div> + +<p> +The group of the Pleiades, although lying outside the main course of the +Galaxy, is connected with it by a faint loop, and is the scene of the most +remarkable association of stars and nebulous matter known in the visible +universe. The naked eye is unaware of the existence of nebulæ in the Pleiades, +or, at the best, merely suspects that there is something of the kind there; and +even the most powerful telescopes are far from revealing the full wonder of the +spectacle; but in photographs which have been exposed for many hours +consecutively, in order to accumulate the impression of the actinic rays, the +revelation is stunning. The principle stars are seen surrounded by, and, as it +were, <i>drowned in,</i> dense nebulous clouds of an unparalleled kind. The +forms assumed by these clouds seem at first sight inexplicable. They look like +fleeces, or perhaps more like splashes and daubs of luminous paint dashed +carelessly from a brush. But closer inspection shows that they are, to a large +extent, <i>woven</i> out of innumerable threads of filmy texture, and there are +many indications of spiral tendencies. Each of the bright stars of the +group—Alcyone, Merope, Maia, Electra, Taygeta, Atlas—is the focus of a dense +fog (totally invisible, remember, alike to the naked eye and to the telescope), +and these particular stars are veiled from sight behind the strange mists. +Running in all directions across the relatively open spaces are nebulous wisps +and streaks of the most curious forms. On some of the nebular lines, which are +either straight throughout, or if they change direction do so at an angle, +little stars are strung like beads. In one case seven or eight stars are thus +aligned, and, as if to emphasize their dependence upon the chain which connects +them, when it makes a slight bend the file of stars turns the same way. Many +other star rows in the group suggest by their arrangement that they, too, were +once strung upon similar threads which have now disappeared, leaving the stars +spaced along their ancient tracks. We seem forced to the conclusion that there +was a time when the Pleiades were embedded in a vast nebula resembling that of +Orion, and that the cloud has now become so rare by gradual condensation into +stars that the merest trace of it remains, and this would probably have escaped +detection but for the remarkable actinic power of the radiant matter of which +it consists. The richness of many of these faint nebulous masses in +ultra-violet radiations, which are those that specifically affect the +photographic plate, is the cause of the marvelous revelatory power of celestial +photography. So the veritable unseen universe, as distinguished from the +“unseen universe” of metaphysical speculation, is shown to us. +</p> + +<p> +A different kind of association between stars and nebulæ is shown in some +surprising photographic objects in the constellation Cygnus, where long, wispy +nebulæ, billions of miles in length, some of them looking like tresses +streaming in a breeze, lie amid fields of stars which seem related to them. But +the relation is of a most singular kind, for notwithstanding the delicate +structure of the long nebulæ they appear to act as barriers, causing the stars +to heap themselves on one side. The stars are two, three, or four times as +numerous on one side of the nebulæ as on the other. These nebulæ, as far as +appearance goes, might be likened to rail fences, or thin hedges, against which +the wind is driving drifts of powdery snow, which, while scattered plentifully +all around, tends to bank itself on the leeward side of the obstruction. The +imagination is at a loss to account for these extraordinary phenomena; yet +there they are, faithfully giving us their images whenever the photographic +plate is exposed to their radiations. +</p> + +<p> +Thus the more we see of the universe with improved methods of observation, and +the more we invent aids to human senses, each enabling us to penetrate a little +deeper into the unseen, the greater becomes the mystery. The telescope carried +us far, photography is carrying us still farther; but what as yet unimagined +instrument will take us to the bottom, the top, and the end? And then, what +hitherto untried power of thought will enable us to comprehend the meaning of +it all? +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap03"></a>III<br/> +Stellar Migrations</h2> + +<p> +To the untrained eye the stars and the planets are not distinguishable. It is +customary to call them all alike “stars.” But since the planets more or less +rapidly change their places in the sky, in consequence of their revolution +about the sun, while the stars proper seem to remain always in the same +relative positions, the latter are spoken of as “fixed stars.” In the +beginnings of astronomy it was not known that the “fixed stars” had any motion +independent of their apparent annual revolution with the whole sky about the +earth as a seeming center. Now, however, we know that the term “fixed stars” is +paradoxical, for there is not a single really fixed object in the whole +celestial sphere. The apparent fixity in the positions of the stars is due to +their immense distance, combined with the shortness of the time during which we +are able to observe them. It is like viewing the plume of smoke issuing from a +steamer, hull down, at sea: if one does not continue to watch it for a long +time it appears to be motionless, although in reality it may be traveling at +great speed across the line of sight. Even the planets seem fixed in position +if one watches them for a single night only, and the more distant ones do not +sensibly change their places, except after many nights of observation. Neptune, +for instance, moves but little more than two degrees in the course of an entire +year, and in a month its change of place is only about one-third of the +diameter of the full moon. +</p> + +<p> +Yet, fixed as they seem, the stars are actually moving with a speed in +comparison with which, in some cases, the planets might almost be said to stand +fast in their tracks. Jupiter’s speed in his orbit is about eight miles per +second, Neptune’s is less than three and one-half miles, and the earth’s is +about eighteen and one-half miles; while there are “fixed stars” which move two +hundred or three hundred miles per second. They do not all, however, move with +so great a velocity, for some appear to travel no faster than the planets. But +in all cases, notwithstanding their real speed, long-continued and exceedingly +careful observations are required to demonstrate that they are moving at all. +No more overwhelming impression of the frightful depths of space in which the +stars are buried can be obtained than by reflecting upon the fact that a star +whose actual motion across the line of sight amounts to two hundred miles per +second does not change its apparent place in the sky, in the course of a +thousand years, sufficiently to be noticed by the casual observer of the +heavens! +</p> + +<p> +There is one vast difference between the motions of the stars and those of the +planets to which attention should be at once called: the planets, being under +the control of a central force emanating from their immediate master, the sun, +all move in the same direction and in orbits concentric about the sun; the +stars, on the other hand, move in every conceivable direction and have no +apparent center of motion, for all efforts to discover such a center have +failed. At one time, when theology had finally to accept the facts of science, +a grandiose conception arose in some pious minds, according to which the Throne +of God was situated at the exact center of His Creation, and, seated there, He +watched the magnificent spectacle of the starry systems obediently revolving +around Him. Astronomical discoveries and speculations seemed for a time to +afford some warrant for this view, which was, moreover, an acceptable +substitute for the abandoned geocentric theory in minds that could only +conceive of God as a superhuman artificer, constantly admiring his own work. No +longer ago than the middle of the nineteenth century a German astronomer, +Maedler, believed that he had actually found the location of the center about +which the stellar universe revolved. He placed it in the group of the Pleiades, +and upon his authority an extraordinary imaginative picture was sometimes drawn +of the star Alcyone, the brightest of the Pleiades, as the very seat of the +Almighty. This idea even seemed to gain a kind of traditional support from the +mystic significance, without known historical origin, which has for many ages, +and among widely separated peoples, been attached to the remarkable group of +which Alcyone is the chief. But since Maedler’s time it has been demonstrated +that the Pleiades cannot be the center of revolution of the universe, and, as +already remarked, all attempts to find or fix such a center have proved +abortive. Yet so powerful was the hold that the theory took upon the popular +imagination, that even today astronomers are often asked if Alcyone is not the +probable site of “Jerusalem the Golden.” +</p> + +<p> +If there were a discoverable center of predominant gravitative power, to which +the motions of all the stars could be referred, those motions would appear less +mysterious, and we should then be able to conclude that the universe was, as a +whole, a prototype of the subsidiary systems of which it is composed. We should +look simply to the law of gravitation for an explanation, and, naturally, the +center would be placed within the opening enclosed by the Milky Way. If it were +there the Milky Way itself should exhibit signs of revolution about it, like a +wheel turning upon its hub. No theory of the star motions as a whole could +stand which failed to take account of the Milky Way as the basis of all. But +the very form of that divided wreath of stars forbids the assumption of its +revolution about a center. Even if it could be conceived as a wheel having no +material center it would not have the form which it actually presents. As was +shown in Chapter 2, there is abundant evidence of motion in the Milky Way; but +it is not motion of the system as a whole, but motion affecting its separate +parts. Instead of all moving one way, the galactic stars, as far as their +movements can be inferred, are governed by local influences and conditions. +They appear to travel crosswise and in contrary directions, and perhaps they +eddy around foci where great numbers have assembled; but of a universal +revolution involving the entire mass we have no evidence. +</p> + +<p> +Most of our knowledge of star motions, called “proper motions,” relates to +individual stars and to a few groups which happen to be so near that the +effects of their movements are measurable. In some cases the motion is so rapid +(not in appearance, but in reality) that the chief difficulty is to imagine how +it can have been imparted, and what will eventually become of the “runaways.” +Without a collision, or a series of very close approaches to great +gravitational centers, a star traveling through space at the rate of two +hundred or three hundred miles per second could not be arrested or turned into +an orbit which would keep it forever flying within the limits of the visible +universe. A famous example of these speeding stars is “1830 Groombridge,” a +star of only the sixth magnitude, and consequently just visible to the naked +eye, whose motion across the line of sight is so rapid that it moves upon the +face of the sky a distance equal to the apparent diameter of the moon every 280 +years. The distance of this star is at least 200,000,000,000,000 miles, and may +be two or three times greater, so that its actual speed cannot be less than two +hundred, and may be as much as four hundred, miles per second. It could be +turned into a new course by a close approach to a great sun, but it could only +be stopped by collision, head-on, with a body of enormous mass. Barring such +accidents it must, as far as we can see, keep on until it has traversed our +stellar system, whence in may escape and pass out into space beyond, to join, +perhaps, one of those other universes of which we have spoken. Arcturus, one of +the greatest suns in the universe, is also a runaway, whose speed of flight has +been estimated all the way from fifty to two hundred miles per second. +Arcturus, we have every reason to believe, possesses hundreds of times the mass +of our sun—think, then, of the prodigious momentum that its motion implies! +Sirius moves more moderately, its motion across the line of sight amounting to +only ten miles per second, but it is at the same time approaching the sun at +about the same speed, its actual velocity in space being the resultant of the +two displacements. +</p> + +<p> +What has been said about the motion of Sirius brings us to another aspect of +this subject. The fact is, that in every case of stellar motion the +displacement that we observe represents only a part of the actual movement of +the star concerned. There are stars whose motion carries them straight toward +or straight away from the earth, and such stars, of course, show no cross +motion. But the vast majority are traveling in paths inclined from a +perpendicular to our line of sight. Taken as a whole, the stars may be said to +be flying about like the molecules in a mass of gas. The discovery of the +radial component in the movements of the stars is due to the spectroscope. If a +star is approaching, its spectral lines are shifted toward the violet end of +the spectrum by an amount depending upon the velocity of approach; if it is +receding, the lines are correspondingly shifted toward the red end. +Spectroscopic observation, then, combined with micrometric measurements of the +cross motion, enables us to detect the real movement of the star in space. +Sometimes it happens that a star’s radial movement is periodically reversed; +first it approaches, and then it recedes. This indicates that it is revolving +around a near-by companion, which is often invisible, and superposed upon this +motion is that of the two stars concerned, which together may be approaching or +receding or traveling across the line of sight. Thus the complications involved +in the stellar motions are often exceedingly great and puzzling. +</p> + +<p> +Yet another source of complication exists in the movement of our own star, the +sun. There is no more difficult problem in astronomy than that of disentangling +the effects of the solar motion from those of the motions of the other stars. +But the problem, difficult as it is, has been solved, and upon its solution +depends our knowledge of the speed and direction of the movement of the solar +system through space, for of course the sun carries its planets with it. One +element of the solution is found in the fact that, as a result of perspective, +the stars toward which we are going appear to move apart toward all points of +the compass, while those behind appear to close up together. Then the +spectroscopic principle already mentioned is invoked for studying the shift of +the lines, which is toward the violet in the stars ahead of us and toward the +red in those that we are leaving behind. Of course the effects of the +independent motions of the stars must be carefully excluded. The result of the +studies devoted to this subject is to show that we are traveling at a speed of +twelve to fifteen miles per second in a northerly direction, toward the border +of the constellations Hercules and Lyra. A curious fact is that the more recent +estimates show that the direction is not very much out of a straight line drawn +from the sun to the star Vega, one of the most magnificent suns in the heavens. +But it should not be inferred from this that Vega is drawing us on; it is too +distant for its gravitation to have such an effect. +</p> + +<p> +Many unaccustomed thoughts are suggested by this mighty voyage of the solar +system. Whence have we come, and whither do we go? Every year of our lives we +advance at least 375,000,000 miles. Since the traditional time of Adam the sun +has led his planets through the wastes of space no less than 225,000,000,000 +miles, or more than 2400 times the distance that separates him from the earth. +Go back in imagination to the geologic ages, and try to comprehend the distance +over which the earth has flown. Where was our little planet when it emerged out +of the clouds of chaos? Where was the sun when his “thunder march” began? What +strange constellations shone down upon our globe when its masters of life were +the monstrous beasts of the “Age of Reptiles”? A million years is not much of a +span of time in geologic reckoning, yet a million years ago the earth was +farther from its present place in space than any of the stars with a measurable +parallax are now. It was more than seven times as far as Sirius, nearly +fourteen times as far as Alpha Centauri, three times as far as Vega, and twice +as far as Arcturus. But some geologists demand two hundred, three hundred, even +one thousand million years to enable them to account for the evolutionary +development of the earth and its inhabitants. In a thousand million years the +earth would have traveled farther than from the remotest conceivable depths of +the Milky Way! +</p> + +<p> +Other curious reflections arise when we think of the form of the earth’s track +as it follows the lead of the sun, in a journey which has neither known +beginning nor conceivable end. There are probably many minds which have found a +kind of consolation in the thought that every year the globe returns to the +same place, on the same side of the sun. This idea may have an occult +connection with our traditional regard for anniversaries. When that period of +the year returns at which any great event in our lives has occurred we have the +feeling that the earth, in its annual round, has, in a manner, brought us back +to the scene of that event. We think of the earth’s orbit as a well-worn path +which we traverse many times in the course of a lifetime. It seems familiar to +us, and we grow to have a sort of attachment to it. The sun we are accustomed +to regard as a fixed center in space, like the mill or pump around which the +harnessed patient mule makes his endless circuits. But the real fact is that +the earth never returns to the place in space where it has once quitted. In +consequence of the motion of the sun carrying the earth and the other planets +along, the track pursued by our globe is a vast spiral in space continually +developing and never returning upon its course. It is probable that the tracks +of the sun and the others stars are also irregular, and possibly spiral, +although, as far as can be at present determined, they appear to be practically +straight. Every star, wherever it may be situated, is attracted by its +fellow-stars from many sides at once, and although the force is minimized by +distance, yet in the course of many ages its effects must become manifest. +</p> + +<p> +Looked at from another side, is there not something immensely stimulating and +pleasing to the imagination in the idea of so stupendous a journey, which makes +all of us the greatest of travelers? In the course of a long life a man is +transported through space thirty thousand million miles; Halley’s Comet does +not travel one-quarter as far in making one of its immense circuits. And there +are adventures on this voyage of which we are just beginning to learn to take +account. Space is full of strange things, and the earth must encounter some of +them as it advances through the unknown. Many singular speculations have been +indulged in by astronomers concerning the possible effects upon the earth of +the varying state of the space that it traverses. Even the alternation of hot +and glacial periods has sometimes been ascribed to this source. When tropical +life flourished around the poles, as the remains in the rocks assure us, the +needed high temperature may, it has been thought, have been derived from the +presence of the earth in a warm region of space. Then, too, there is a certain +interest for us in the thought of what our familiar planet has passed through. +We cannot but admire it for its long journeying as we admire the traveler who +comes to us from remote and unexplored lands, or as we gaze with a glow of +interest upon the first locomotive that has crossed a continent, or a ship that +has visited the Arctic or Antarctic regions. If we may trust the indications of +the present course, the earth, piloted by the sun, has come from the Milky Way +in the far south and may eventually rejoin that mighty band of stars in the far +north. +</p> + +<p> +While the stars in general appear to travel independently of one another, +except when they are combined in binary or trinary systems, there are notable +exceptions to this rule. In some quarters of the sky we behold veritable +migrations of entire groups of stars whose members are too widely separated to +show any indications of revolution about a common center of gravity. This leads +us back again to the wonderful group of the Pleiades. All of the principle +stars composing that group are traveling in virtually parallel lines. Whatever +force set them going evidently acted upon all alike. This might be explained by +the assumption that when the original projective force acted upon them they +were more closely united than they are at present, and that in drifting apart +they have not lost the impulse of the primal motion. Or it may be supposed that +they are carried along by some current in space, although it would be +exceedingly difficult, in the present state of our knowledge, to explain the +nature of such a current. Yet the theory of a current has been proposed. As to +an attractive center around which they might revolve, none has been found. +Another instance of similar “star-drift” is furnished by five of the seven +stars constituting the figure of the “Great Dipper.” In this case the stars +concerned are separated very widely, the two extreme ones by not less than +fifteen degrees, so that the idea of a common motion would never have been +suggested by their aspect in the sky; and the case becomes the more remarkable +from the fact that among and between them there are other stars, some of the +same magnitude, which do not share their motion, but are traveling in other +directions. Still other examples of the same phenomenon are found in other +parts of the sky. Of course, in the case of compact star-clusters, it is +assumed that all the members share a like motion of translation through space, +and the same is probably true of dense star-swarms and star-clouds. +</p> + +<p> +The whole question of star-drift has lately assumed a new phase, in consequence +of the investigations of Kapteyn, Dyson, and Eddington on the “systematic +motions of the stars.” This research will, it is hoped, lead to an +understanding of the general law governing the movements of the whole body of +stars constituting the visible universe. Taking about eleven hundred stars +whose proper motions have been ascertained with an approach to certainty, and +which are distributed in all parts of the sky, it has been shown that there +exists an apparent double drift, in two independent streams, moving in +different and nearly opposed directions. The apex of the motion of what is +called “Stream I” is situated, according to Professor Kapteyn, in right +ascension 85°, declination south 11°, which places it just south of the +constellation Orion; while the apex of “Stream II” is in right ascension +260°, declination south 48°, placing it in the constellation Ara, south +of Scorpio. The two apices differ very nearly 180° in right ascension and +about 120° in declination. The discovery of these vast star-streams, if +they really exist, is one of the most extraordinary in modern astronomy. It +offers the correlation of stellar movements needed as the basis of a theory of +those movements, but it seems far from revealing a physical cause for them. As +projected against the celestial sphere the stars forming the two opposite +streams appear intermingled, some obeying one tendency and some the other. As +Professor Dyson has said, the hypothesis of this double movement is of a +revolutionary character, and calls for further investigation. Indeed, it seems +at first glance not less surprising than would be the observation that in a +snow-storm the flakes over our heads were divided into two parties and driving +across each other’s course in nearly opposite directions, as if urged by +interpenetrating winds. +</p> + +<p> +But whatever explanation may eventually be found for the motions of the stars, +the knowledge of the existence of those motions must always afford a new charm +to the contemplative observer of the heavens, for they impart a sense of life +to the starry system that would otherwise be lacking. A stagnant universe, with +every star fixed immovably in its place, would not content the imagination or +satisfy our longing for ceaseless activity. The majestic grandeur of the +evolutions of the celestial hosts, the inconceivable vastness of the fields of +space in which they are executed, the countless numbers, the immeasurable +distances, the involved convolutions, the flocking and the scattering, the +interpenetrating marches and countermarches, the strange community of impulsion +affecting stars that are wide apart in space and causing them to traverse the +general movement about them like aides and despatch-bearers on a +battle-field—all these arouse an intensity of interest which is heightened by +the mystery behind them. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap04"></a>IV<br/> +The Passing of the Constellations</h2> + +<p> +From a historical and picturesque point of view, one of the most striking +results of the motions of the stars described in the last chapter is their +effect upon the forms of the constellations, which have been watched and +admired by mankind from a period so early that the date of their invention is +now unknown. The constellations are formed by chance combinations of +conspicuous stars, like figures in a kaleidoscope, and if our lives were +commensurate with the æons of cosmic existence we should perceive that the +kaleidoscope of the heavens was ceaselessly turning and throwing the stars into +new symmetries. Even if the stars stood fast, the motion of the solar system +would gradually alter the configurations, as the elements of a landscape +dissolve and recombine in fresh groupings with the traveler’s progress amid +them. But with the stars themselves all in motion at various speeds and in many +directions, the changes occur more rapidly. Of course, “rapid” is here +understood in a relative sense; the wheel of human history to an eye accustomed +to the majestic progression of the universe would appear to revolve with the +velocity of a whirling dynamo. Only the deliberation of geological movements +can be contrasted with the evolution and devolution of the constellations. +</p> + +<p> +And yet this secular fluctuation of the constellation figures is not without +keen interest for the meditative observer. It is another reminder of the swift +mutability of terrestial affairs. To the passing glance, which is all that we +can bestow upon these figures, they appear so immutable that they have been +called into service to form the most lasting records of ancient thought and +imagination that we possess. In the forms of the constellations, the most +beautiful, and, in imaginative quality, the finest, mythology that the world +has ever known has been perpetuated. Yet, in a broad sense, this scroll of +human thought imprinted on the heavens is as evanescent as the summer clouds. +Although more enduring than parchment, tombs, pyramids, and temples, it is as +far as they from truly eternizing the memory of what man has fancied and done. +</p> + +<p> +Before studying the effects that the motions of the stars have had and will +have upon the constellations, it is worth while to consider a little further +the importance of the stellar pictures as archives of history. To emphasize the +importance of these effects it is only necessary to recall that the +constellations register the oldest traditions of our race. In the history of +primeval religions they are the most valuable of documents. Leaving out of +account for the moment the more familiar mythology of the Greeks, based on +something older yet, we may refer for illustration to that of the mysterious +Maya race of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a +group of ruins perched, after the Mexican and Central-American plan, on the +summits of pyramidal mounds which mark the site of an ancient theogonic center +of the Mayas. Here the temples all evidently refer to a cult based upon the +constellations as symbols. The figures and the names, of course, were not the +same as those that we have derived from our Aryan ancestors, but the star +groups were the same or nearly so. For instance, the loftiest of the temples at +Izamal was connected with the sign of the constellation known to us as Cancer, +marking the place of the sun at the summer solstice, at which period the sun +was supposed to descend at noon like a great bird of fire and consume the +offerings left upon the altar. Our Scorpio was known to the Mayas as a sign of +the “Death God.” Our Libra, the “Balance,” with which the idea of a divine +weighing out of justice has always been connected, seems to be identical with +the Mayan constellation Teoyaotlatohua, with which was associated a temple +where dwelt the priests whose special business it was to administer justice and +to foretell the future by means of information obtained from the spirits of the +dead. Orion, the “Hunter” of our celestial mythology, was among the Mayas a +“Warrior,” while Sagittarius and others of our constellations were known to +them (under different names, of course), and all were endowed with a religious +symbolism. And the same star figures, having the same significance, were +familiar to the Peruvians, as shown by the temples at Cuzco. Thus the +imagination of ancient America sought in the constellations symbols of the +unchanging gods. +</p> + +<p> +But, in fact, there is no nation and no people that has not recognized the +constellations, and at one period or another in its history employed them in +some symbolic or representative capacity. As handled by the Greeks from +prehistoric times, the constellation myths became the very soul of poetry. The +imagination of that wonderful race idealized the principal star groups so +effectively that the figures and traditions thus attached to them have, for +civilized mankind, displaced all others, just as Greek art in its highest forms +stands without parallel and eclipses every rival. The Romans translated no +heroes and heroines of the mythical period of their history to the sky, and the +deified Cæsars never entered that lofty company, but the heavens are filled +with the early myths of the Greeks. Herakles nightly resumes his mighty labors +in the stars; Zeus, in the form of the white “Bull,” Taurus, bears the fair +Europa on his back through the celestial waves; Andromeda stretches forth her +shackled arms in the star-gemmed ether, beseeching aid; and Perseus, in a blaze +of diamond armor, revives his heroic deeds amid sparkling clouds of stellar +dust. There, too, sits Queen Cassiopeia in her dazzling chair, while the Great +King, Cepheus, towers gigantic over the pole. Professor Young has significantly +remarked that a great number of the constellations are connected in some way or +other with the Argonautic Expedition—that strangely fascinating legend of +earliest Greek story which has never lost its charm for mankind. In view of all +this, we may well congratulate ourselves that the constellations will outlast +our time and the time of countless generations to follow us; and yet they are +very far from being eternal. Let us now study some of the effects of the +stellar motions upon them. +</p> + +<p> +We begin with the familiar figure of the “Great Dipper.” He who has not drunk +inspiration from its celestial bowl is not yet admitted to the circle of +Olympus. This figure is made up of seven conspicuous stars in the constellation +Ursa Major, the “Greater Bear.” The handle of the “Dipper” corresponds to the +tail of the imaginary “Bear,” and the bowl lies upon his flank. In fact, the +figure of a dipper is so evident and that of a bear so unevident, that to most +persons the “Great Dipper” is the only part of the constellation that is +recognizable. Of the seven stars mentioned, six are of nearly equal brightness, +ranking as of the second magnitude, while the seventh is of only the third +magnitude. The difference is very striking, since every increase of one +magnitude involves an increase of two-and-a-half times in brightness. There +appears to be little doubt that the faint star, which is situated at the +junction of the bowl and the handle, is a variable of long period, since three +hundred years ago it was as bright as its companions. But however that may be, +its relative faintness at the present time interferes but little with the +perfection of the “Dipper’s” figure. In order the more readily to understand +the changes which are taking place, it will be well to mention both the names +and the Greek letters which are attached to the seven stars. Beginning at the +star in the upper outer edge of the rim of the bowl and running in regular +order round the bottom and then out to the end of the handle, the names and +letters are as follows: Dubhe (α), Merak (β), Phaed (γ), Megrez +(δ), Alioth (ε), Mizar (ζ), and Benetnasch (η). Megrez is +the faint star already mentioned at the junction of the bowl and handle, and +Mizar, in the middle of the handle, has a close, naked-eye companion which is +named Alcor. The Arabs called this singular pair of stars “The Horse and +Rider.” Merak and Duhbe are called “The Pointers,” because an imaginary line +drawn northward through them indicates the Pole Star. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus06"></a> +<a href="images/image06.jpg"> +<img src="images/image06.jpg" width="515" height="600" alt="Illustration:" /></a> +<p class="caption">The “Great Dipper”</p> +</div> + +<p> +Now it has been found that five of these stars—<i>viz.,</i> Merak, Phaed, +Megrez, Alioth, and Mizar (with its comrade)—are moving with practically the +same speed in an easterly direction, while the other two, Dubhe and Benetnasch, +are simultaneously moving westward, the motions of Benetnasch being apparently +more rapid. The consequence of these opposed motions is, of course, that the +figure of the “Dipper” cannot always have existed and will not continue to +exist. In the accompanying diagrams it has been thought interesting to show the +relative positions of these seven stars, as seen from the point which the earth +now occupies, both in the past and in the future. Arrows attached to the stars +in the figure representing the present appearance of the “Dipper” indicate the +directions of the motions and the distances over which they will carry the +stars in a period of about five hundred centuries. The time, no doubt, seems +long, but remember the vast stretch of ages through which the earth has passed, +and then reflect that no reason is apparent why our globe should not continue +to be a scene of animation for ten thousand centuries yet to come. The fact +that the little star Alcor placed so close to Mizar should accompany the latter +in its flight is not surprising, but that two of the principal stars of the +group should be found moving in a direction directly opposed to that pursued by +the other five is surprising in the highest degree; and it recalls the strange +theory of a double drift affecting all the stars, to which attention was called +in the preceding chapter. It would appear that Benetnasch and Dubhe belong to +one “current,” and Merak, Phaed, Megrez, Alioth, and Mizar to the other. As far +as is known, the motion of the seven stars are not shared by the smaller stars +scattered about them, but on the theory of currents there should be such a +community of motion, and further investigation may reveal it. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus07"></a> +<a href="images/image07.jpg"> +<img src="images/image07.jpg" width="519" height="600" alt="Illustration:" /></a> +<p class="caption">Cassiopeia</p> +</div> + +<p> +From the “Great Dipper” we turn to a constellation hardly less conspicuous and +situated at an equal distance from the pole on the other side—Cassiopeia. This +famous star-group commemorating the romantic Queen of Ethiopia whose vain +boasting of her beauty was punished by the exposure of her daughter Andromeda +to the “Sea Monster,” is well-marked by five stars which form an irregular +letter “W” with its open side toward the pole. Three of these stars are usually +ranked as of the second magnitude, and two of the third; but to ordinary +observation they appear of nearly equal brightness, and present a very striking +picture. They mark out the chair and a part of the figure of the beautiful +queen. Beginning at the right-hand, or western, end of the “W,” their Greek +letter designations are: Beta (β), Alpha (α), Gamma (γ), Delta +(δ), and Epsilon (ε). Four of them, Beta, Alpha, Delta, and Epsilon +are traveling eastwardly at various speeds, while the fifth, Gamma, moves in a +westerly direction. The motion of Beta is more rapid than that of any of the +others. It should be said, however, that no little uncertainty attaches to the +estimates of the rate of motion of stars which are not going very rapidly, and +different observers often vary considerably in their results. +</p> + +<p> +In the beautiful “Northern Crown,” one of the most perfect and charming of all +the figures to be found in the stars, the alternate combining and scattering +effects of the stellar motions are shown by comparing the appearance which the +constellation must have had five hundred centuries ago with that which it has +at present and that which it will have in the future. The seven principle stars +of the asterism, forming a surprisingly perfect coronet, have movements in +three directions at right angles to one another. That in these circumstances +they should ever have arrived at positions giving them so striking an +appearance of definite association is certainly surprising; from its aspect one +would have expected to find a community of movement governing the brilliants of +the “Crown,” but instead of that we find evidence that they will inevitably +drift apart and the beautiful figure will dissolve. +</p> + +<p> +A similar fate awaits such asterisms as the “Northern Cross” in Cygnus; the +“Crow” (Corvus), which stands on the back of the great “Sea Serpent,” Hydra, +and pecks at his scales; “Job’s Coffin” (Delphinus); the “Great Square of +Pegasus”; the “Twins” (Gemini); the beautiful “Sickle” in Leo; and the +exquisite group of the Hyades in Taurus. In the case of the Hyades, two +controlling movements are manifest: one, affecting five of the stars which form +the well-known figure of a letter “V,” is directed northerly; the other, which +controls the direction of two stars, has an easterly trend. The chief star of +the group, Aldebaran, one of the finest of all stars both for its brilliance +and its color, is the most affected by the easterly motion. In time it will +drift entirely out of connection with its present neighbors. Although the +Hyades do not form so compact a group as the Pleiades in the same +constellation, yet their appearance of relationship is sufficient to awaken a +feeling of surprise over the fact that, as with the stars of the “Dipper,” +their association is only temporary or apparent. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus08"></a> +<a href="images/image08.jpg"> +<img src="images/image08.jpg" width="560" height="600" alt="Illustration:" /></a> +<p class="caption">The “Northern Crown”</p> +</div> + +<p> +The great figure of Orion appears to be more lasting, not because its stars are +physically connected, but because of their great distance, which renders their +movements too deliberate to be exactly ascertained. Two of the greatest of its +stars, Betelgeuse and Rigel, possess, as far as has been ascertained, no +perceptible motion across the line of sight, but there is a little movement +perceptible in the “Belt.” At the present time this consists of an almost +perfect straight line, a row of second-magnitude stars about equally spaced and +of the most striking beauty. In the course of time, however, the two right-hand +stars, Mintaka and Alnilam (how fine are these Arabic star names!) will +approach each other and form a naked-eye double, but the third, Alnita, will +drift away eastward, so that the “Belt” will no longer exist. +</p> + +<p> +For one more example, let us go to the southern hemisphere, whose most +celebrated constellation, the “Southern Cross,” has found a place in all modern +literatures, although it has no claim to consideration on account of +association with ancient legends. This most attractive asterism, which has +never ceased to fascinate the imagination of Christendom since it was first +devoutly described by the early explorers of the South, is but a passing +collocation of brilliant stars. Yet even in its transfigurations it has been +for hundreds of centuries, and will continue to be for hundreds of centuries to +come, a most striking object in the sky. Our figures show its appearance in +three successive phases: first, as it was fifty thousand years ago (viewed from +the earth’s present location); second, as it is in our day; and, third, as it +will be an equal time in the future. The nearness of these bright stars to one +another—the length of the longer beam of the “Cross” is only sixdegrees—makes +this group very noticeable, whatever the arrangement of its components may be. +The largest star, at the base of the “Cross,” is of the first magnitude, two of +the others are of the second magnitude, and the fourth is of the third. Other +stars, not represented in the figures, increase the effect of a celestial +blazonry, although they do not help the resemblance to a cross. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus09"></a> +<a href="images/image09.jpg"> +<img src="images/image09.jpg" width="500" height="444" alt="Illustration:" /></a> +<p class="caption">The “Southern Cross”</p> +</div> + +<p> +But since the motion of the solar system itself will, in the course of so long +a period as fifty thousand years, produce a great change in the perspective of +the heavens as seen from the earth, by carrying us nearly nineteen trillion +miles from our present place, why, it may be asked, seek to represent future +appearances of the constellations which we could not hope to see, even if we +could survive so long? The answer is: Because these things aid the mind to form +a picture of the effects of the mobility of the starry universe. Only by +showing the changes from some definite point of view can we arrive at a due +comprehension of them. The constellations are more or less familiar to +everybody, so that impending changes of their forms must at once strike the eye +and the imagination, and make clearer the significance of the movements of the +stars. If the future history of mankind is to resemble its past and if our race +is destined to survive yet a million years, then our remote descendents will +see a “new heavens” if not a “new earth,” and will have to invent novel +constellations to perpetuate their legends and mythologies. +</p> + +<p> +If our knowledge of the relative distances of the stars were more complete, it +would be an interesting exercise in celestial geometry to project the +constellations probably visible to the inhabitants of worlds revolving around +some of the other suns of space. Our sun is too insignificant for us to think +that he can make a conspicuous appearance among them, except, perhaps, in a few +cases. As seen, for instance, from the nearest known star, Alpha Centauri, the +sun would appear of the average first magnitude, and consequently from that +standpoint he might be the gem of some little constellation which had no +Sirius, or Arcturus, or Vega to eclipse him with its superior splendor. But +from the distance of the vast majority of the stars the sun would probably be +invisible to the naked eye, and as seen from nearer systems could only rank as +a fifth or sixth magnitude star, unnoticed and unknown except by the +star-charting astronomer. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap05"></a>V<br/> +Conflagrations in the Heavens</h2> + +<p> +Suppose it were possible for the world to take fire and burn up—as some +pessimists think that it will do when the Divine wrath shall have sufficiently +accumulated against it—nobody out of our own little corner of space would ever +be aware of the catastrophe! With all their telescopes, the astronomers living +in the golden light of Arcturus or the diamond blaze of Canopus would be unable +to detect the least glimmer of the conflagration that had destroyed the seat of +Adam and his descendents, just as now they are totally ignorant of its +existence. +</p> + +<p> +But at least fifteen times in the course of recorded history men looking out +from the earth have beheld in the remote depths of space great outbursts of +fiery light, some of them more splendidly luminous than anything else in the +firmament except the sun! If <i>they</i> were conflagrations, how many million +worlds like ours were required to feed their blaze? +</p> + +<p> +It is probable that “temporary” or “new” stars, as these wonderful apparitions +are called, really are conflagrations; not in the sense of a bonfire or a +burning house or city, but in that of a sudden eruption of inconceivable heat +and light, such as would result from the stripping off the shell of an +encrusted sun or the crashing together of two mighty orbs flying through space +with a hundred times the velocity of the swiftest cannon-shot. +</p> + +<p> +Temporary stars are the rarest and most erratic of astronomical phenomena. The +earliest records relating to them are not very clear, and we cannot in every +instance be certain that it was one of these appearances that the ignorant and +superstitious old chroniclers are trying to describe. The first temporary star +that we are absolutely sure of appeared in 1572, and is known as “Tycho’s +Star,” because the celebrated Danish astronomer (whose remains, with his +gold-and-silver artificial nose—made necessary by a duel—still intact, were +disinterred and reburied in 1901) was the first to perceive it in the sky, and +the most assiduous and successful in his studies of it. As the first fully +accredited representative of its class, this new star made its entry upon the +scene with becoming <i>éclat.</i> It is characteristic of these phenomena that +they burst into view with amazing suddenness, and, of course, entirely +unexpectedly. Tycho’s star appeared in the constellation Cassiopeia, near a now +well-known and much-watched little star named Kappa, on the evening of November +11, 1572. The story has often been repeated, but it never loses interest, how +Tycho, going home that evening, saw people in the street pointing and staring +at the sky directly over their heads, and following the direction of their +hands and eyes he was astonished to see, near the zenith, an unknown star of +surpassing brilliance. It outshone the planet Jupiter, and was therefore far +brighter than the first magnitude. There was not another star in the heavens +that could be compared with it in splendor. Tycho was not in all respects free +from the superstitions of his time—and who is?—but he had the true scientific +instinct, and immediately he began to study the stranger, and to record with +the greatest care every change in its aspect. First he determined as well as he +could with the imperfect instruments of his day, many of which he himself had +invented, the precise location of the phenomena in the sky. Then he followed +the changes that it underwent. At first it brightened until its light equaled +or exceeded that of the planet Venus at her brightest, a statement which will +be appreciated at its full value by anyone who has ever watched Venus when she +plays her dazzling rôle of “Evening Star,” flaring like an arc light in the +sunset sky. It even became so brilliant as to be visible in full daylight, +since, its position being circumpolar, it never set in the latitude of Northern +Europe. Finally it began to fade, turning red as it did so, and in March, 1574, +it disappeared from Tycho’s searching gaze, and has never been seen again from +that day to this. None of the astronomers of the time could make anything of +it. They had not yet as many bases of speculation as we possess today. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus10"></a> +<a href="images/image10.jpg"> +<img src="images/image10.jpg" width="500" height="408" alt="Illustration:" /></a> +<p class="caption">Chart showing location of Tycho’s star, 1572, and Nova +Persei of 1901</p> +</div> + +<p> +Tycho’s star has achieved a romantic reputation by being fancifully identified +with the “Star of Bethlehem,” said to have led the wondering Magi from their +eastern deserts to the cradle-manger of the Savior in Palestine. Many attempts +have been made to connect this traditional “star” with some known phenomenon of +the heavens, and none seems more idle than this. Yet it persistently survives, +and no astronomer is free from eager questions about it addressed by people +whose imagination has been excited by the legend. It is only necessary to say +that the supposition of a connection between the phenomenon of the Magi and +Tycho’s star is without any scientific foundation. It was originally based on +an unwarranted assumption that the star of Tycho was a variable of long period, +appearing once every three hundred and fifteen years, or thereabout. If that +were true there would have been an apparition somewhere near the traditional +date of the birth of Christ, a date which is itself uncertain. But even the +data on which the assumption was based are inconsistent with the theory. +Certain monkish records speak of something wonderful appearing in the sky in +the years 1264 and 945, and these were taken to have been outbursts of Tycho’s +star. Investigation shows that the records more probably refer to comets, but +even if the objects seen were temporary stars, their dates do not suit the +hypothesis; from 945 to 1264 there is a gap of 319 years, and from 1264 to 1572 +one of only 308 years; moreover 337 years have now (1909) elapsed since Tycho +saw the last glimmer of his star. Upon a variability so irregular and uncertain +as that, even if we felt sure that it existed, no conclusion could be found +concerning an apparition occurring 2000 years ago. +</p> + +<p> +In the year 1600 (the year in which Giordano Bruno was burned at the stake for +teaching that there is more than one physical world), a temporary star of the +third magnitude broke out in the constellation Cygnus, and curiously enough, +considering the rarity of such phenomena, only four years later another +surprisingly brilliant one appeared in the constellation Ophiuchus. This is +often called “Kepler’s star,” because the great German astronomer devoted to it +the same attention that Tycho had given to the earlier phenomenon. It, too, +like Tycho’s, was at first the brightest object in the stellar heavens, +although it seems never to have quite equaled its famous predecessor in +splendor. It disappeared after a year, also turning of a red color as it became +more faint. We shall see the significance of this as we go on. Some of Kepler’s +contemporaries suggested that the outburst of this star was due to a meeting of +atoms in space, and idea bearing a striking resemblance to the modern theory of +“astronomical collisions.” +</p> + +<p> +In 1670, 1848, and 1860 temporary stars made their appearance, but none of them +was of great brilliance. In 1866 one of the second magnitude broke forth in the +“Northern Crown” and awoke much interest, because by that time the spectroscope +had begun to be employed in studying the composition of the stars, and Huggins +demonstrated that the new star consisted largely of incandescent hydrogen. But +this star, apparently unlike the others mentioned, was not absolutely new. +Before its outburst it had shown as a star of the ninth magnitude (entirely +invisible, of course, to the naked eye), and after about six weeks it faded to +its original condition in which it has ever since remained. In 1876 a temporary +star appeared in the constellation Cygnus, and attained at one time the +brightness of the second magnitude. Its spectrum and its behavior resembled +those of its immediate predecessor. In 1885, astronomers were surprised to see +a sixth-magnitude star glimmering in the midst of the hazy cloud of the great +Andromeda Nebula. It soon absolutely disappeared. Its spectrum was remarkable +for being “continuous,” like that of the nebula itself. A continuous spectrum +is supposed to represent a body, or a mass, which is either solid or liquid, or +composed of gas under great pressure. In January, 1892, a new star was suddenly +seen in the constellation Auriga. It never rose much above the fourth +magnitude, but it showed a peculiar spectrum containing both bright and dark +lines of hydrogen. +</p> + +<p> +But a bewildering surprise was now in store; the world was to behold at the +opening of the twentieth century such a celestial spectacle as had not been on +view since the times of Tycho and Kepler. Before daylight on the morning of +February 22, 1901, the Rev. Doctor Anderson, of Edinburgh, an amateur +astronomer, who had also been the first to see the new star in Auriga, beheld a +strange object in the constellation Perseus not far from the celebrated +variable star Algol. He recognized its character at once, and immediately +telegraphed the news, which awoke the startled attention of astronomers all +over the world. When first seen the new star was no brighter than Algol (less +than the second magnitude), but within twenty-four hours it was ablaze, +outshining even the brilliant Capella, and far surpassing the first magnitude. +At the spot in the sky where it appeared nothing whatever was visible on the +night before its coming. This is known with certainty because a photograph had +been made of that very region on February 21, and this photograph showed +everything down to the twelfth magnitude, but not a trace of the stranger which +burst into view between the 21st and the 22nd like the explosion of a rocket. +</p> + +<p> +Upon one who knew the stars the apparition of this intruder in a well-known +constellation had the effect of a sudden invasion. The new star was not far +west of the zenith in the early evening, and in that position showed to the +best advantage. To see Capella, the hitherto unchallenged ruler of that quarter +of the sky, abased by comparison with this stranger of alien aspect, for there +was always an unfamiliar look about the “nova,” was decidedly disconcerting. It +seemed to portend the beginning of a revolution in the heavens. One could +understand what the effect of such an apparition must have been in the +superstitious times of Tycho. The star of Tycho had burst forth on the northern +border of the Milky Way; this one was on its southern border, some forty-five +degrees farther east. +</p> + +<p> +Astronomers were well-prepared this time for the scientific study of the new +star, both astronomical photography and spectroscopy having been perfected, and +the results of their investigations were calculated to increase the wonder with +which the phenomenon was regarded. The star remained at its brightest only a +few days; then, like a veritable conflagration, it began to languish; and, like +the reflection of a dying fire, as it sank it began to glow with the red color +of embers. But its changes were spasmodic; once about every three days it +flared up only to die away again. During these fluctuations its light varied +alternately in the ratio of one to six. Finally it took a permanent downward +course, and after a few months the naked eye could no longer perceive it; but +it remained visible with telescopes, gradually fading until it had sunk to the +ninth magnitude. Then another astonishing change happened: in August +photographs taken at the Yerkes Observatory and at Heidelberg showed that the +“nova” was <i>surrounded by a spiral nebula!</i> The nebula had not been there +before, and no one could doubt that it represented a phase of the same +catastrophe that had produced the outburst of the new star. At one time the +star seemed virtually to have disappeared, as if all its substance had been +expanded into the nebulous cloud, but always there remained a stellar nucleus +about which the misty spiral spread wider and ever wider, like a wave expanding +around a center of disturbance. The nebula too showed a variability of +brightness, and four condensations which formed in it seemed to have a motion +of revolution about the star. As time went on the nebula continued to expand at +a rate which was computed to be not less than twenty thousand miles per second! +And now the star itself, showing indications of having turned into a nebula, +behaved in a most erratic manner, giving rise to the suspicion that it was +about to burst out again. But this did not occur, and at length it sunk into a +state of lethargy from which it has to the present time not recovered. But the +nebulous spiral has disappeared, and the entire phenomena as it now (1909) +exists consists of a faint nebulous star of less than the ninth magnitude. +</p> + +<p> +The wonderful transformations just described had been forecast in advance of +the discovery of the nebulous spiral encircling the star by the spectroscopic +study of the latter. At first there was no suggestion of a nebular +constitution, but within a month or two characteristic nebular lines began to +appear, and in less than six months the whole spectrum had been transformed to +the nebular type. In the mean time the shifting of the spectral lines indicated +a complication of rapid motions in several directions simultaneously. These +motions were estimated to amount to from one hundred to five hundred miles per +second. +</p> + +<p> +The human mind is so constituted that it feels forced to seek an explanation of +so marvelous a phenomenon as this, even in the absence of the data needed for a +sound conclusion. The most natural hypothesis, perhaps, is that of a collision. +Such a catastrophe could certainly happen. It has been shown, for instance, +that in infinity of time the earth is sure to be hit by a comet; in the same +way it may be asserted that, if no time limit is fixed, the sun is certain to +run against some obstacle in space, either another star, or a dense meteor +swarm, or one of the dark bodies which there is every reason to believe abound +around us. The consequences of such a collision are easy to foretell, provided +that we know the masses and the velocities of the colliding bodies. In a +preceding chapter we have discussed the motions of the sun and stars, and have +seen that they are so swift that an encounter between any two of them could not +but be disastrous. But this is not all; for as soon as two stars approached +within a few million miles their speed would be enormously increased by their +reciprocal attractions and, if their motion was directed radially with respect +to their centers, they would come together with a crash that would reduce them +both to nebulous clouds. It is true that the chances of such a “head-on” +collision are relatively very small; two stars approaching each other would +most probably fall into closed orbits around their common center of gravity. If +there were a collision it would most likely be a grazing one instead of a +direct front-to-front encounter. But even a close approach, without any actual +collision, would probably prove disastrous, owing to the tidal influence of +each of the bodies on the other. Suns, in consequence of their enormous masses +and dimensions and the peculiarities of their constitution, are exceedingly +dangerous to one another at close quarters. Propinquity awakes in them a +mutually destructive tendency. Consisting of matter in the gaseous, or perhaps, +in some cases, liquid, state, their tidal pull upon each other if brought close +together might burst them asunder, and the photospheric envelope being +destroyed the internal incandescent mass would gush out, bringing fiery death +to any planets that were revolving near. Without regard to the resulting +disturbance of the earth’s orbit, the close approach of a great star to the sun +would be in the highest degree perilous to us. But this is a danger which may +properly be regarded as indefinitely remote, since, at our present location in +space, we are certainly far from every star except the sun, and we may feel +confident that no great invisible body is near, for if there were one we should +be aware of its presence from the effects of its attraction. As to dark nebulæ +which may possibly lie in the track that the solar system is pursuing at the +rate of 375,000,000 miles per year, that is another question—and they, too, +could be dangerous! +</p> + +<div class="fig" style="width:100%;"> +<a name="illus11"></a> +<a href="images/image11.jpg"> +<img src="images/image11.jpg" width="600" height="563" alt="Illustration:" /></a> +<p class="caption">Nova Persei, with its nebular rings</p> +</div> + +<p> +This brings us directly back to “Nova Persei,” for among the many suggestions +offered to explain its outburst, as well as those of other temporary stars, one +of the most fruitful is that of a collision between a star and a vast invisible +nebula. Professor Seeliger, of Munich, first proposed this theory, but it +afterward underwent some modifications from others. Stated in a general form, +the idea is that a huge dark body, perhaps an extinguished sun, encountered in +its progress through space a widespread flock of small meteors forming a dark +nebula. As it plunged into the swarm the friction of the innumerable collisions +with the meteors heated its surface to incandescence, and being of vast size it +then became visible to us as a new star. Meanwhile the motion of the body +through the nebula, and its rotation upon itself, set up a gyration in the +blazing atmosphere formed around it by the vaporized meteors; and as this +atmosphere spread wider, under the laws of gyratory motion a rotation in the +opposite direction began in the inflamed meteoric cloud outside the central +part of the vortex. Thus the spectral lines were caused to show motion in +opposite directions, a part of the incandescent mass approaching the earth +simultaneously with the retreat of another part. So the curious spectroscopic +observations before mentioned were explained. This theory might also account +for the appearance of the nebulous spiral first seen some six months after the +original outburst. The sequent changes in the spectrum of the “nova” are +accounted for by this theory on the assumption, reasonable enough in itself, +that at first the invading body would be enveloped in a vaporized atmosphere of +relatively slight depth, producing by its absorption the fine dark lines first +observed; but that as time went on and the incessant collisions continued, the +blazing atmosphere would become very deep and extensive, whereupon the +appearance of the spectral lines would change, and bright lines due to the +light of the incandescent meteors surrounding the nucleus at a great distance +would take the place of the original dark ones. The vortex of meteors once +formed would protect the flying body within from further immediate collisions, +the latter now occurring mainly among the meteors themselves, and then the +central blaze would die down, and the original splendor of the phenomenon would +fade. +</p> + +<p> +But the theories about Nova Persei have been almost as numerous as the +astronomers who have speculated about it. One of the most startling of them +assumed that the outburst was caused by the running amuck of a dark star which +had encountered another star surrounded with planets, the renewed outbreaks of +light after the principal one had faded being due to the successive running +down of the unfortunate planets! Yet another hypothesis is based on what we +have already said of the tidal influence that two close approaching suns would +have upon each other. Supposing two such bodies which had become encrusted, but +remained incandescent and fluid within, to approach within almost striking +distance; they would whirl each other about their common center of gravity, and +at the same time their shells would burst under the tidal strain, and their +glowing nuclei being disclosed would produce a great outburst of light. +Applying this theory to a “nova,” like that of 1866 in the “Northern Crown,” +which had been visible as a small star before the outbreak, and which afterward +resumed its former aspect, we should have to assume that a yet shining sun had +been approached by a dark body whose attraction temporarily burst open its +photosphere. It might be supposed that in this case the dark body was too far +advanced in cooling to suffer the same fate from the tidal pull of its victim. +But a close approach of that kind would be expected to result in the formation +of a binary system, with orbits of great eccentricity, perhaps, and after the +lapse of a certain time the outburst should be renewed by another approximation +of the two bodies. A temporary star of that kind would rather be ranked as a +variable. +</p> + +<p> +The celebrated French astronomer, Janssen, had a different theory of Nova +Persei, and of temporary stars in general. According to his idea, such +phenomena might be the result of chemical changes taking place in a sun without +interference by, or collision with, another body. Janssen was engaged for many +years in trying to discover evidence of the existence of oxygen in the sun, and +he constructed his observatory on the summit of Mount Blanc specially to pursue +that research. He believed that oxygen must surely exist in the sun since we +find so many other familiar elements included in the constitution of the solar +globe, and as he was unable to discover satisfactory evidence of its presence +he assumed that it existed in a form unknown on the earth. If it were normally +in the sun’s chromosphere, or coronal atmosphere, he said, it would combine +with the hydrogen which we know is there and form an obscuring envelope of +water vapor. It exists, then, in a special state, uncombined with hydrogen; but +let the temperature of the sun sink to a critical point and the oxygen will +assume its normal properties and combine with the hydrogen, producing a mighty +outburst of light and heat. This, Janssen thought, might explain the phenomena +of the temporary stars. It would also, he suggested, account for their brief +career, because the combination of the elements would be quickly accomplished, +and then the resulting water vapor would form an atmosphere cutting off the +radiation from the star within. +</p> + +<p> +This theory may be said to have a livelier human interest than some of the +others, since, according to it, the sun may carry in its very constitution a +menace to mankind; one does not like to think of it being suddenly transformed +into a gigantic laboratory for the explosive combination of oxygen and +hydrogen! But while Janssen’s theory might do for some temporary stars, it is +inadequate to explain all the phenomena of Nova Persei, and particularly the +appearance of the great spiral nebula that seemed to exhale from the heart of +the star. Upon the whole, the theory of an encounter between a star and a dark +nebula seems best to fit the observations. By that hypothesis the expanding +billow of light surrounding the core of the conflagration is very well +accounted for, and the spectroscopic peculiarities are also explained. +</p> + +<p> +Dr Gustov Le Bon offers a yet more alarming theory, suggesting that temporary +stars are the result of <i>atomic explosion;</i> but we shall touch upon this +more fully in Chapter 14. +</p> + +<p> +Twice in the course of this discussion we have called attention to the change +of color invariably undergone by temporary stars in the later stages of their +career. This was conspicuous with Nova Persei which glowed more and more redly +as it faded, until the nebulous light began to overpower that of the stellar +nucleus. Nothing could be more suggestive of the dying out of a great fire. +Moreover, change of color from white to red is characteristic of all variable +stars of long period, such as “Mira” in Cetus. It is also characteristic of +stars believed to be in the later stages of evolution, and consequently +approaching extinction, like Antares and Betelgeuse, and still more notably +certain small stars which “gleam like rubies in the field of the telescope.” +These last appear to be suns in the closing period of existence as +self-luminous bodies. Between the white stars, such as Sirius and Rigel, and +the red stars, such as Aldebaran and Alpha Herculis, there is a progressive +series of colors from golden yellow through orange to deep red. The change is +believed to be due to the increase of absorbing vapors in the stellar +atmosphere as the body cools down. In the case of ordinary stars these changes +no doubt occupy many millions of years, which represent the average duration of +solar life; but the temporary stars run through similar changes in a few +months: they resemble ephemeral insects—born in the morning and doomed to +perish with the going down of the sun. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap06"></a>VI<br/> +Explosive and Whirling Nebulæ</h2> + +<p> +One of the most surprising triumphs of celestial photography was Professor +Keeler’s discovery, in 1899, that the great majority of the nebulæ have a +distinctly spiral form. This form, previously known in Lord Rosse’s great +“Whirlpool Nebula,” had been supposed to be exceptional; now the photographs, +far excelling telescopic views in the revelation of nebular forms, showed the +spiral to be the typical shape. Indeed, it is a question whether all nebulæ are +not to some extent spiral. The extreme importance of this discovery is shown in +the effect that it has had upon hitherto prevailing views of solar and +planetary evolution. For more than three-quarters of a century Laplace’s +celebrated hypothesis of the manner of origin of the solar system from a +rotating and contracting nebula surrounding the sun had guided speculation on +that subject, and had been tentatively extended to cover the evolution of +systems in general. The apparent forms of some of the nebulæ which the +telescope had revealed were regarded, and by some are still regarded, as giving +visual evidence in favor of this theory. There is a “ring nebula” in Lyra with +a central star, and a “planetary nebula” in Gemini bearing no little +resemblance to the planet Saturn with its rings, both of which appear to be +practical realizations of Laplace’s idea, and the elliptical rings surrounding +the central condensation of the Andromeda Nebula may be cited for the same kind +of proof. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus12"></a> +<a href="images/image12.jpg"> +<img src="images/image12.jpg" width="425" height="600" alt="Illustration:" /></a> +<p class="caption">Lord Rosse’s nebula</p> +</div> + +<p> +But since Keeler’s discovery there has been a decided turning away of +speculation another way. The form of the spiral nebulæ seems to be entirely +inconsistent with the theory of an originally globular or disk-shaped nebula +condensing around a sun and throwing or leaving off rings, to be subsequently +shaped into planets. Some astronomers, indeed, now reject Laplace’s hypothesis +<i>in toto,</i> preferring to think that even our solar system originated from +a spiral nebula. Since the spiral type prevails among the existing nebulæ, we +must make any mechanical theory of the development of stars and planetary +systems from them accord with the requirements which that form imposes. A +glance at the extraordinary variations upon the spiral which Professor Keeler’s +photographs reveal is sufficient to convince one of the difficulty of the task +of basing a general theory upon them. In truth, it is much easier to criticize +Laplace’s hypothesis than to invent a satisfactory substitute for it. If the +spiral nebulæ seem to oppose it there are other nebulæ which appear to support +it, and it may be that no one fixed theory can account for all the forms of +stellar evolution in the universe. Our particular planetary system may have +originated very much as the great French mathematician supposed, while others +have undergone, or are now undergoing, a different process of development. +There is always a too strong tendency to regard an important new discovery and +the theories and speculations based upon it as revolutionizing knowledge, and +displacing or overthrowing everything that went before. Upon the plea that +“Laplace only made a guess” more recent guesses have been driven to extremes +and treated by injudicious exponents as “the solid facts at last.” +</p> + +<div class="fig" style="width:100%;"> +<a name="illus13"></a> +<a href="images/image13.jpg"> +<img src="images/image13.jpg" width="482" height="600" alt="Illustration:" /></a> +<p class="caption">Wonderful spiral in triangulum</p> +</div> + +<p> +Before considering more recent theories than Laplace’s, let us see what the +nature of the photographic revelations is. The vast celestial maelstrom +discovered by Lord Rosse in the “Hunting Dogs” may be taken as the leading type +of the spiral nebulæ, although there are less conspicuous objects of the kind +which, perhaps, better illustrate some of their peculiarities. Lord Rosse’s +nebula appears far more wonderful in the photographs than in his drawings made +with the aid of his giant reflecting telescope at Parsonstown, for the +photographic plate records details that no telescope is capable of showing. +Suppose we look at the photograph of this object as any person of common sense +would look at any great and strange natural phenomenon. What is the first thing +that strikes the mind? It is certainly the appearance of violent whirling +motion. One would say that the whole glowing mass had been spun about with +tremendous velocity, or that it had been set rotating so rapidly that it had +become the victim of “centrifugal force,” one huge fragment having broken loose +and started to gyrate off into space. Closer inspection shows that in addition +to the principal focus there are various smaller condensations scattered +through the mass. These are conspicuous in the spirals. Some of them are +stellar points, and but for the significance of their location we might suppose +them to be stars which happen to lie in a line between us and the nebula. But +when we observe how many of them follow most faithfully the curves of the +spirals we cannot but conclude that they form an essential part of the +phenomenon; it is not possible to believe that their presence in such +situations is merely fortuitous. One of the outer spirals has at least a dozen +of these star-like points strung upon it; some of them sharp, small, and +distinct, others more blurred and nebulous, suggesting different stages of +condensation. Even the part which seems to have been flung loose from the main +mass has, in addition to its central condensation, at least one stellar point +gleaming in the half-vanished spire attached to it. Some of the more distant +stars scattered around the “whirlpool” look as if they too had been shot out of +the mighty vortex, afterward condensing into unmistakable solar bodies. There +are at least two curved rows of minute stars a little beyond the periphery of +the luminous whirl which clearly follow lines concentric with those of the +nebulous spirals. Such facts are simply dumbfounding for anyone who will bestow +sufficient thought upon them, for these are <i>suns,</i> though they may be +small ones; and what a birth is that for a sun! +</p> + +<p> +Look now again at the glowing spirals. We observe that hardly have they left +the central mass before they begin to coagulate. In some places they have a +“ropy” aspect; or they are like peascods filled with growing seeds, which +eventually will become stars. The great focus itself shows a similar tendency, +especially around its circumference. The sense that it imparts of a tremendous +shattering force at work is overwhelming. There is probably more matter in that +whirling and bursting nebula than would suffice to make a hundred solar +systems! It must be confessed at once that there is no confirmation of the +Laplacean hypothesis here; but what hypothesis will fit the facts? There is one +which it has been claimed does so, but we shall come to that later. In the +meanwhile, as a preparation, fix in the memory the appearance of that second +spiral mass spinning beside its master which seems to have spurned it away. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus14"></a> +<a href="images/image14.jpg"> +<img src="images/image14.jpg" width="407" height="600" alt="Illustration:" /></a> +<p class="caption">Spiral in Ursa Major</p> +</div> + +<p> +For a second example of the spiral nebulæ look at the one in the constellation +Triangulum. <i>God, how hath the imagination of puny man failed to comprehend +Thee!</i> Here is creation through destruction with a vengeance! The spiral +form of the nebula is unmistakable, but it is half obliterated amid the turmoil +of flying masses hurled away on all sides with tornadic fury. The focus itself +is splitting asunder under the intolerable strain, and in a little while, as +time is reckoned in the Cosmos, it will be gyrating into stars. And then look +at the cyclonic rain of already finished stars whirling round the outskirts of +the storm. Observe how scores of them are yet involved in the fading streams of +the nebulous spirals; see how they have been thrown into vast loops and curves, +of a beauty that half redeems the terror of the spectacle enclosed within their +lines—like iridescent cirri hovering about the edges of a hurricane. And so +again are suns born! +</p> + +<p> +Let us turn to the exquisite spiral in Ursa Major; how different its aspect +from that of the other! One would say that if the terrific coil in Triangulum +has all but destroyed itself in its fury, this one on the contrary has just +begun its self-demolition. As one gazes one seems to see in it the smooth, +swift, accelerating motion that precedes catastrophe. The central part is still +intact, dense, and uniform in texture. How graceful are the spirals that +smoothly rise from its oval rim and, gemmed with little stars, wind off into +the darkness until they have become as delicate as threads of gossamer! But at +bottom the story told here is the same—creation by gyration! +</p> + +<p> +Compare with the above the curious mass in Cetus. Here the plane of the +whirling nebula nearly coincides with our line of sight and we see the object +at a low angle. It is far advanced and torn to shreds, and if we could look at +it perpendicularly to its plane it is evident that it would closely resemble +the spectacle in Triangulum. +</p> + +<p> +Then take the famous Andromeda Nebula (see Frontispiece), which is so vast that +notwithstanding its immense distance even the naked eye perceives it as an +enigmatical wisp in the sky. Its image on the sensitive plate is the +masterpiece of astronomical photography; for wild, incomprehensible beauty +there is nothing that can be compared with it. Here, if anywhere, we look upon +the spectacle of creation in one of its earliest stages. The Andromeda Nebula +is apparently less advanced toward transformation into stellar bodies than is +that in Triangulum. The immense crowd of stars sprinkled over it and its +neighborhood seem in the main to lie this side of the nebula, and consequently +to have no connection with it. But incipient stars (in some places clusters of +them) are seen in the nebulous rings, while one or two huge masses seem to give +promise of transformation into stellar bodies of unusual magnitude. I say +“rings” because although the loops encompassing the Andromeda Nebula have been +called spirals by those who wish utterly to demolish Laplace’s hypothesis, yet +they are not manifestly such, as can be seen on comparing them with the +undoubted spirals of the Lord Rosse Nebula. They look quite as much like +circles or ellipses seen at an angle of, say, fifteen or twenty degrees to +their plane. If they are truly elliptical they accord fairly well with +Laplace’s idea, except that the scale of magnitude is stupendous, and if the +Andromeda Nebula is to become a solar system it will surpass ours in grandeur +beyond all possibility of comparison. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus15"></a> +<a href="images/image15.jpg"> +<img src="images/image15.jpg" width="457" height="600" alt="Illustration:" /></a> +<p class="caption">Nebula in Cetus</p> +</div> + +<p> +There is one circumstance connected with the spiral nebulæ, and conspicuous in +the Andromeda Nebula on account of its brightness, which makes the question of +their origin still more puzzling; they all show continuous spectra, which, as +we have before remarked, indicate that the mass from which the light comes is +either solid or liquid, or a gas under heavy pressure. Thus nebulæ fall into +two classes: the “white” nebulæ, giving a continuous spectrum; and the “green” +nebulæ whose spectra are distinctly gaseous. The Andromeda Nebula is the great +representative of the former class and the Orion Nebula of the latter. The +spectrum of the Andromeda Nebula has been interpreted to mean that it consists +not of luminous gas, but of a flock of stars so distant that they are +separately indistinguishable even with powerful telescopes, just as the +component stars of the Milky Way are indistinguishable with the naked eye; and +upon this has been based the suggestion that what we see in Andromeda is an +outer universe whose stars form a series of elliptical garlands surrounding a +central mass of amazing richness. But this idea is unacceptable if for no other +reason than that, as just said, all the spiral nebulæ possess the same kind of +spectrum, and probably no one would be disposed to regard them all as outer +universes. As we shall see later, the peculiarity of the spectra of the spiral +nebulæ is appealed to in support of a modern substitute for Laplace’s +hypothesis. +</p> + +<p> +Finally, without having by any means exhausted the variety exhibited by the +spiral nebulæ, let us turn to the great representative of the other species, +the Orion Nebula. In some ways this is even more marvelous than the others. The +early drawings with the telescope failed to convey an adequate conception +either of its sublimity or of its complication of structure. It exists in a +nebulous region of space, since photographs show that nearly the whole +constellation is interwoven with faintly luminous coils. To behold the entry of +the great nebula into the field even of a small telescope is a startling +experience which never loses its novelty. As shown by the photographs, it is an +inscrutable chaos of perfectly amazing extent, where spiral bands, radiating +streaks, dense masses, and dark yawning gaps are strangely intermingled without +apparent order. In one place four conspicuous little stars, better seen in a +telescope than in the photograph on account of the blurring produced by +over-exposure, are suggestively situated in the midst of a dark opening, and no +observer has ever felt any doubt that these stars have been formed from the +substance of the surrounding nebula. There are many other stars scattered over +its expanse which manifestly owe their origin to the same source. But compare +the general appearance of this nebula with the others that we have studied, and +remark the difference. If the unmistakably spiral nebulæ resemble bursting +fly-wheels or grindstones from whose perimeters torrents of sparks are flying, +the Orion Nebula rather recalls the aspect of a cloud of smoke and fragments +produced by the explosion of a shell. This idea is enforced by the look of the +outer portion farthest from the bright half of the nebula, where sharply edged +clouds with dark spaces behind seem to be billowing away as if driven by a wind +blowing from the center. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus16"></a> +<a href="images/image16.jpg"> +<img src="images/image16.jpg" width="600" height="595" alt="Illustration:" /></a> +<p class="caption">The Orion nebula</p> +</div> + +<p> +Next let us consider what scientific speculation has done in the effort to +explain these mysteries. Laplace’s hypothesis can certainly find no standing +ground either in the Orion Nebula or in those of a spiral configuration, +whatever may be its situation with respect to the grand Nebula of Andromeda, or +the “ring” and “planetary” nebulæ. Some other hypothesis more consonant with +the appearances must be found. Among the many that have been proposed the most +elaborate is the “Planetesimal Hypothesis” of Professors Chamberlin and +Moulton. It is to be remarked that it applies to the spiral nebulæ +distinctively, and not to an apparently chaotic mass of gas like the vast +luminous cloud in Orion. The gist of the theory is that these curious objects +are probably the result of close approaches to each other of two independent +suns, reminding us of what was said on this subject when we were dealing with +temporary stars. Of the previous history of these appulsing suns the theory +gives us no account; they are simply supposed to arrive within what may be +called an effective tide-producing distance, and then the drama begins. Some of +the probable consequences of such an approach have been noticed in Chapter 5; +let us now consider them a little more in detail. +</p> + +<p> +Tides always go in couples; if there is a tide on one side of a globe there +will be a corresponding tide on the other side. The cause is to be found in the +law that the force of gravitation varies inversely as the square of the +distance; the attraction on the nearest surface of the body exercised by +another body is greater than on its center, and greater yet than on its +opposite surface. If two great globes attract each other, each tends to draw +the other out into an ellipsoidal figure; they must be more rigid than steel to +resist this—and even then they cannot altogether resist. If they are liquid or +gaseous they will yield readily to the force of distortion, the amount of which +will depend upon their distance apart, for the nearer they are the greater +becomes the tidal strain. If they are encrusted without and liquid or gaseous +in the interior, the internal mass will strive to assume the figure demanded by +the tidal force, and will, if it can, burst the restraining envelope. Now this +is virtually the predicament of the body we call a sun when in the immediate +presence of another body of similarly great mass. Such a body is presumably +gaseous throughout, the component gases being held in a state of rigidity by +the compression produced by the tremendous gravitational force of their own +aggregate mass. At the surface such a body is enveloped in a shell of +relatively cool matter. Now suppose a great attracting body, such as another +sun, to approach near enough for the difference in its attraction on the two +opposite sides of the body and on its center to become very great; the +consequence will be a tidal deformation of the whole body, and it will lengthen +out along the line of the gravitational pull and draw in at the sides, and if +its shell offers considerable resistance, but not enough to exercise a complete +restraint, it will be violently burst apart, or blown to atoms, and the +internal mass will leap out on the two opposite sides in great fiery spouts. In +the case of a sun further advanced in cooling than ours the interior might be +composed of molten matter while the exterior crust had become rigid like the +shell of an egg; then the force of the “tidal explosion” produced by the +appulse of another sun would be more violent in consequence of the greater +resistance overcome. Such, then, is the mechanism of the first phase in the +history of a spiral nebula according to the Planetesimal Hypothesis. Two suns, +perhaps extinguished ones, have drawn near together, and an explosive outburst +has occured in one or both. The second phase calls for a more agile exercise of +the imagination. +</p> + +<p> +To simplify the case, let us suppose that only one of the tugging suns is +seriously affected by the strain. Its vast wings produced by the outburst are +twisted into spirals by their rotation and the contending attractions exercised +upon them, as the two suns, like battleships in desperate conflict, curve round +each other, concentrating their destructive energies. Then immense quantities +of débris are scattered about in which eddies are created, and finally, as the +sun that caused the damage goes on its way, leaving its victim to repair its +injuries as it may, the dispersed matter cools, condenses, and turns into +streams of solid particles circling in elliptical paths about their parent sun. +These particles, or fragments, are the “planetesimals” of the theory. In +consequence of the inevitable intersection of the orbits of the planetesimals, +nodes are formed where the flying particles meet, and at these nodes large +masses are gradually accumulated. The larger the mass the greater its +attraction, and at last the nodal points become the nuclei of great +aggregations from which planets are shaped. +</p> + +<p> +This, in very brief form, is the Planetesimal Hypothesis which we are asked to +substitute for that based on Laplace’s suggestion as an explanation of the mode +of origin of the solar system; and the phenomena of the spiral nebulæ are +appealed to as offering evident support to the new hypothesis. We are reminded +that they are elliptical in outline, which accords with the hypothesis; that +their spectra are not gaseous, which shows that they may be composed of solid +particles like the planetesimals; and that their central masses present an oval +form, which is what would result from the tidal effects, as just described. We +also remember that some of them, like the Lord Rosse and the Andromeda nebulæ, +are visually double, and in these cases we might suppose that the two masses +represent the tide-burst suns that ventured into too close proximity. It may be +added that the authors of the theory do not insist upon the appulse of two suns +as the <i>only</i> way in which the planetesimals may have originated, but it +is the only supposition that has been worked out. +</p> + +<p> +But serious questions remain. It needs, for instance, but a glance at the +Triangulum monster to convince the observer that it cannot be a solar system +which is being evolved there, but rather a swarm of stars. Many of the detached +masses are too vast to admit of the supposition that they are to be transformed +into planets, in our sense of planets, and the distances of the stars which +appear to have been originally ejected from the focal masses are too great to +allow us to liken the assemblage that they form to a solar system. Then, too, +no nodes such as the hypothesis calls for are visible. Moreover, in most of the +spiral nebulæ the appearances favor the view that the supposititious +encountering suns have not separated and gone each rejoicing on its way, after +having inflicted the maximum possible damage on its opponent, but that, on the +contrary, they remain in close association like two wrestlers who cannot escape +from each other’s grasp. And this is exactly what the law of gravitation +demands; stars cannot approach one another with impunity, with regard either to +their physical make-up or their future independence of movement. The theory +undertakes to avoid this difficulty by assuming that in the case of our system +the approach of the foreign body to the sun was not a close one—just close +enough to produce the tidal extrusion of the relatively insignificant quantity +of matter needed to form the planets. But even then the effect of the appulse +would be to change the direction of flight, both of the sun and of its visitor, +and there is no known star in the sky which can be selected as the sun’s +probable partner in their ancient <i>pas deux.</i> That there are unconquered +difficulties in Laplace’s hypothesis no one would deny, but in simplicity of +conception it is incomparably more satisfactory, and with proper modifications +could probably be made more consonant with existing facts in our solar system +than that which is offered to replace it. Even as an explanation of the spiral +nebulæ, not as solar systems in process of formation, but as the birthplaces of +stellar clusters, the Planetesimal Hypothesis would be open to many objections. +Granting its assumptions, it has undoubtedly a strong mathematical framework, +but the trouble is not with the mathematics but with the assumptions. Laplace +was one of the ablest mathematicians that ever lived, but he had never seen a +spiral nebula; if he had, he might have invented a hypothesis to suit its +phenomena. His actual hypothesis was intended only for our solar system, and he +left it in the form of a “note” for the consideration of his successors, with +the hope that they might be able to discover the full truth, which he confessed +was hidden from him. It cannot be said that that truth has yet been found, and +when it is found the chances are that intuition and not logic will have led to +it. +</p> + +<p> +The spiral nebulæ, then, remain among the greatest riddles of the universe, +while the gaseous nebulæ, like that of Orion, are no less mysterious, although +it seems impossible to doubt that both forms give birth to stars. It is but +natural to look to them for light on the question of the origin of our +planetary system; but we should not forget that the scale of the phenomena in +the two cases is vastly different, and the forces in operation may be equally +different. A hill may have been built up by a glacier, while a mountain may be +the product of volcanic forces or of the upheaval of the strata of the planet. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap07"></a>VII<br/> +The Banners of the Sun</h2> + +<p> +As all the world knows, the sun, a blinding globe pouring forth an +inconceivable quantity of light and heat, whose daily passage through the sky +is caused by the earth’s rotation on its axis, constitutes the most important +phenomenon of terrestial existence. Viewed with a dark glass to take off the +glare, or with a telescope, its rim is seen to be a sharp and smooth circle, +and nothing but dark sky is visible around it. Except for the interference of +the moon, we should probably never have known that there is any more of the sun +than our eyes ordinarily see. +</p> + +<p> +But when an eclipse of the sun occurs, caused by the interposition of the +opaque globe of the moon, we see its immediate surroundings, which in some +respects are more wonderful than the glowing central orb. These surroundings, +although not in the sense in which we apply the term to the gaseous envelope of +the earth, may be called the sun’s atmosphere. They consist of two very +different parts—first, the red “prominences,” which resemble tongues of flame +ascending thousands of miles above the sun’s surface; and, second, the +“corona,” which extends to distances of millions of miles from the sun, and +shines with a soft, glowing light. The two combined, when well seen, make a +spectacle without parallel among the marvels of the sky. Although many attempts +have been made to render the corona visible when there is no eclipse, all have +failed, and it is to the moon alone that we owe its revelation. To cover the +sun’s disk with a circular screen will not answer the purpose because of the +illumination of the air all about the observer. When the moon hides the sun, on +the other hand, the sunlight is withdrawn from a great cylinder of air +extending to the top of the atmosphere and spreading many miles around the +observer. There is then no glare to interfere with the spectacle, and the +corona appears in all its surprising beauty. The prominences, however, although +they were discovered during an eclipse, can now, with the aid of the +spectroscope, be seen at any time. But the prominences are rarely large enough +to be noticed by the naked eye, while the streamers of the corona, stretching +far away in space, like ghostly banners blown out from the black circle of the +obscuring moon, attract every eye, and to this weird apparition much of the +fear inspired by eclipses has been due. But if the corona has been a cause of +terror in the past it has become a source of growing knowledge in our time. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus17"></a> +<a href="images/image17.jpg"> +<img src="images/image17.jpg" width="600" height="454" alt="Illustration:" /></a> +<p class="caption">The Corona</p> +</div> + +<p> +The story of the first scientific observation of the corona and the prominences +is thrillingly interesting, and in fact dramatic. The observation was made +during the eclipse of 1842, which fortunately was visible all over Central and +Southern Europe so that scores of astronomers saw it. The interest centers in +what happened at Pavia in Northern Italy, where the English astronomer Francis +Baily had set up his telescope. The eclipse had begun and Bailey was busy at +his telescope when, to quote his own words in the account which he wrote for +the <i>Memoirs of the Royal Astronomical Society:</i> +</p> + +<div class="letter"> +<p> +I was astounded by a tremendous burst of applause from the streets below, and +at the same moment was electrified by the sight of one of the most brilliant +and splendid phenomena that can well be imagined; for at that instant the dark +body of the moon was suddenly surrounded with a corona, or kind of bright +glory, similar in shape and magnitude to that which painters draw round the +heads of saints... +</p> + +<p> +Pavia contains many thousand inhabitants, the major part of whom were at this +early hour walking about the streets and squares or looking out of windows in +order to witness this long-talked-of phenomenon; and when the total obscuration +took place, which was <i>instantaneous,</i> there was a universal shout from +every observer which “made the welkin ring,” and for the moment withdrew my +attention from the object with which I was immediately occupied. I had, indeed, +expected the appearance of a luminous circle round the moon during the time of +total obscurity; but I did not expect, from any of the accounts of preceding +eclipses that I had read, to witness so magnificent an exhibition as that which +took place... +</p> + +<p> +Splendid and astonishing, however, as this remarkable phenomenon really was, +and although it could not fail to call forth the admiration and applause of +every beholder, yet I must confess that there was at the same time something in +its singular and wonderful appearance that was appalling... +</p> + +<p> +But the most remarkable circumstance attending the phenomenon was the +appearance of <i>three large protuberances</i> apparently emanating from the +circumference of the moon, but evidently forming a portion of the corona. They +had the appearance of mountains of a prodigious elevation; their color was red +tinged with lilac or purple; perhaps the color of the peach-blossom would more +nearly represent it. They somewhat resembled the tops of the snowy Alpine +mountains when colored by the rising or the setting sun. They resembled the +Alpine mountains in another respect, inasmuch as their light was perfectly +steady, and had none of that flickering or sparkling motion so visible in other +parts of the corona... +</p> + +<p> +The whole of these protuberances were visible even to the last moment of total +obscuration, and when the first ray of light was admitted from the sun they +vanished, with the corona, altogether, and daylight was instantly restored. +</p> +</div> + +<p> +I have quoted nearly all of this remarkable description not alone for its +intrinsic interest, but because it is the best depiction that can be found of +the general phenomena of a total solar eclipse. Still, not every such eclipse +offers an equally magnificent spectacle. The eclipses of 1900 and 1905, for +instance, which were seen by the writer, the first in South Carolina and the +second in Spain, fell far short of that described by Bailey in splendor and +impressiveness. Of course, something must be allowed for the effect of +surprise; Bailey had not expected to see what was so suddenly disclosed to him. +But both in 1900 and 1905 the amount of scattered light in the sky was +sufficient in itself to make the corona appear faint, and there were no very +conspicuous prominences visible. Yet on both occasions there was manifest among +the spectators that mingling of admiration and awe of which Bailey speaks. The +South Carolinians gave a cheer and the ladies waved their handkerchiefs when +the corona, ineffably delicate of form and texture, <i>melted</i> into sight +and then in two minutes melted away again. The Spaniards, crowded on the +citadel hill of Burgos, with their king and his royal retinue in their midst, +broke out with a great clapping of hands as the awaited spectacle unfolded +itself in the sky; and on both occasions, before the applause began, after an +awed silence a low murmur ran through the crowds. At Burgos it is said many +made the sign of the cross. +</p> + +<p> +It was not long before Bailey’s idea that the prominences were a part of the +corona was abandoned, and it was perceived that the two phenomena were to a +great extent independent. At the eclipse of 1868, which the astronomers, +aroused by the wonderful scene of 1842, and eager to test the powers of the +newly invented spectroscope, flocked to India to witness, Janssen conceived the +idea of employing the spectroscope to render the prominences visible when there +was no eclipse. He succeeded the very next day, and these phenomena have been +studied in that way ever since. +</p> + +<p> +There are recognized two kinds of prominences—the “erruptive” and the +“quiescent.” The latter, which are cloud-like in form, may be seen almost +anywhere along the edge of the sun; but the former, which often shoot up as if +hurled from mighty volcanoes, appear to be associated with sun-spots, and +appear only above the zones where spots abound. Either of them, when seen in +projection against the brilliant solar disk, appears white, not red, as against +a background of sky. The quiescent prominences, whose elevation is often from +forty thousand to sixty thousand miles, consist, as the spectroscope shows, +mainly of hydrogen and helium. The latter, it will be remembered, is an element +which was known to be in the sun many years before the discovery that it also +exists in small quantities on the earth. A fact which may have a significance +which we cannot at present see is that the emanation from radium gradually and +spontaneously changes into helium, an alchemistical feat of nature that has +opened many curious vistas to speculative thinkers. The eruptive prominences, +which do not spread horizontally like the others, but ascend with marvelous +velocity to elevations of half a million miles or more, are apparently composed +largely of metallic vapors—<i>i.e.</i> metals which are usually solid on the +earth, but which at solar temperatures are kept in a volatilized state. The +velocity of their ascent occasionally amounts to three hundred or four hundred +miles per second. It is known from mathematical considerations that the +gravitation of the sun would not be able to bring back any body that started +from its surface with a velocity exceeding three hundred and eighty-three miles +per second; so it is evident that some of the matter hurled forth in eruptive +prominences may escape from solar control and go speeding out into space, +cooling and condensing into solid masses. There seems to be no reason why some +of the projectiles from the sun might not reach the planets. Here, then, we +have on a relatively small scale, <i>explosions</i> recalling those which it +has been imagined may be the originating cause of some of the sudden phenomena +of the stellar heavens. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus18"></a> +<a href="images/image18.jpg"> +<img src="images/image18.jpg" width="478" height="600" alt="Illustration:" /></a> +<p class="caption">A solar “prominence.” Photographed May 21, 1907</p> +</div> + +<p> +Of the sun-spots it is not our intention here specifically to speak, but they +evidently have an intimate connection with eruptive prominences, as well as +some relation, not yet fully understood, with the corona. Of the real cause of +sun-spots we know virtually nothing, but recent studies by Professor Hale and +others have revealed a strange state of things in the clouds of metallic vapors +floating above them and their surroundings. Evidences of a cyclonic tendency +have been found, and Professor Hale has proved that sun-spots are strong +magnetic fields, and consist of columns of ionized vapors rotating in opposite +directions in the two hemispheres. A fact which may have the greatest +significance is that titanium and vanadium have been found both in sun-spots +and in the remarkable variable Mira Ceti, a star which every eleven months, or +thereabout, flames up with great brilliancy and then sinks back to invisibility +with the naked eye. It has been suggested that sun-spots are indications of the +beginning of a process in the sun which will be intensified until it falls into +the state of such a star as Mira. Stars very far advanced in evolution, without +showing variability, also exhibit similar spectra; so that there is much reason +for regarding sunspots as emblems of advancing age. +</p> + +<p> +The association of the corona with sun-spots is less evident than that of the +eruptive prominences; still such an association exists, for the form and extent +of the corona vary with the sun-spot period of which we shall presently speak. +The constitution of the corona remains to be discovered. It is evidently in +part gaseous, but it also probably contains matter in the form of dust and +small meteors. It includes one substance altogether mysterious—“coronium.” +There are reasons for thinking that this may be the lightest of all the +elements, and Professor Young, its discoverer, said that it was “absolutely +unique in nature; utterly distinct from any other known form of matter, +terrestial, solar, or cosmical.” The enormous extent of the corona is one of +its riddles. Since the development of the curious subject of the “pressure of +light” it has been proposed to account for the sustentation of the corona by +supposing that it is borne upon the billows of light continually poured out +from the sun. Experiment has proved, what mathematical considerations had +previously pointed out as probable, that the waves of light exert a pressure or +driving force, which becomes evident in its effects if the body acted upon is +sufficiently small. In that case the light pressure will prevail over the +attraction of gravitation, and propel the attenuated matter away from the sun +in the teeth of its attraction. The earth itself would be driven away if, +instead of consisting of a solid globe of immense aggregate mass, it were a +cloud of microscopic particles. The reason is that the pressure varies in +proportion to the <i>surface</i> of the body acted upon, while the +gravitational attraction is proportional to the <i>volume,</i> or the total +amount of matter in the body. But the surface of any body depends upon the +<i>square</i> of its diameter, while the volume depends upon the <i>cube</i> of +the diameter. If, for instance, the diameter is represented by 4, the surface +will be proportional to 4 × 4, or 16, and the volume to 4 × 4 +× 4, or 64; but if the diameter is taken as 2, the surface will be 2 +× 2, or 4, and the volume 2 × 2 × 2, or 8. Now, the ratio of +4 to 8 is twice as great as that of 16 to 64. If the diameter is still further +decreased, the ratio of the surface to the volume will proportionally grow +larger; in other words, the pressure will gain upon the attraction, and +whatever their original ratio may have been, a time will come, if the +diminution of size continues, when the pressure will become more effective than +the attraction, and the body will be driven away. Supposing the particles of +the corona to be below the critical size for the attraction of a mass like that +of the sun to control them, they would be driven off into the surrounding space +and appear around the sun like the clouds of dust around a mill. We shall +return to this subject in connection with the Zodiacal Light, the Aurora, and +Comets. +</p> + +<p> +On the other hand, there are parts of the corona which suggest by their forms +the play of electric or magnetic forces. This is beautifully shown in some of +the photographs that have been made of the corona during recent eclipses. Take, +for instance, that of the eclipse of 1900. The sheaves of light emanating from +the poles look precisely like the “lines of force” surrounding the poles of a +magnet. It will be noticed in this photograph that the corona appears to +consist of two portions: one comprising the polar rays just spoken of, and the +other consisting of the broader, longer, and less-defined masses of light +extending out from the equatorial and middle-latitude zones. Yet even in this +more diffuse part of the phenomenon one can detect the presence of submerged +curves bearing more or less resemblance to those about the poles. Just what +part electricity or electro-magnetism plays in the mechanism of the solar +radiation it is impossible to say, but on the assumption that it is a very +important part is based the hypothesis that there exists a direct solar +influence not only upon the magnetism, but upon the weather of the earth. This +hypothesis has been under discussion for half a century, and still we do not +know just how much truth it represents. It is certain that the outbreak of +great disturbances on the sun, accompanied by the formation of sun-spots and +the upshooting of eruptive prominences (phenomena which we should naturally +expect to be attended by action), have been instantly followed by corresponding +“magnetic storms” on the earth and brilliant displays of the auroral lights. +There have been occasions when the influence has manifested itself in the most +startling ways, a great solar outburst being followed by a mysterious gripping +of the cable and telegraph systems of the world, as if an invisible and +irresistible hand had seized them. Messages are abruptly cut off, sparks leap +from the telegraph instruments, and the entire earth seems to have been thrown +into a magnetic flurry. These occurrences affect the mind with a deep +impression of the dependence of our planet on the sun, such as we do not derive +from the more familiar action of the sunlight on the growth of plants and other +phenomena of life depending on solar influences. +</p> + +<p> +Perhaps the theory of solar magnetic influence upon the weather is best known +in connection with the “sun-spot cycle.” This, at any rate, is, as already +remarked, closely associated with the corona. Its existence was discovered in +1843 by the German astronomer Schwabe. It is a period of variable length, +averaging about eleven years, during which the number of spots visible on the +sun first increases to a maximum, then diminishes to a minimum, and finally +increases again to a maximum. For unknown reasons the period is sometimes two +or three years longer than the average and sometimes as much shorter. +Nevertheless, the phenomena always recur in the same order. Starting, for +instance, with a time when the observer can find few or no spots, they +gradually increase in number and size until a maximum, in both senses, is +reached, during which the spots are often of enormous size and exceedingly +active. After two or three years they begin to diminish in number, magnitude, +and activity until they almost or quite disappear. A strange fact is that when +a new period opens, the spots appear first in high northern and southern +latitudes, far from the solar equator, and as the period advances they not only +increase in number and size, but break out nearer and nearer to the equator, +the last spots of a vanishing period sometimes lingering in the equatorial +region after the advance-guard of its successor has made its appearance in the +high latitudes. Spots are never seen on the equator nor near the poles. It was +not very long after the discovery of the sun-spot cycle that the curious +observation was made that a striking coincidence existed between the period of +the sun-spots and another period affecting the general magnetic condition of +the earth. When a curved line representing the varying number of sun-spots was +compared with another curve showing the variations in the magnetic state of the +earth the two were seen to be in almost exact accord, a rise in one curve +corresponding to a rise in the other, and a fall to a fall. Continued +observation has proved that this is a real coincidence and not an accidental +one, so that the connection, although as yet unexplained, is accepted as +established. But does the influence extend further, and directly affect the +weather and the seasons as well as the magnetic elements of the earth? A final +answer to this question cannot yet be given, for the evidence is contradictory, +and the interpretations put upon it depend largely on the predilections of the +judges. +</p> + +<p> +But, in a broad sense, the sun-spots and the phenomena connected with them +<i>must</i> have a relation to terrestial meteorology, for they prove the sun +to be a variable star. Reference was made, a few lines above, to the +resemblance of the spectra of sun-spots to those of certain stars which seem to +be failing through age. This in itself is extremely suggestive; but if this +resemblance had never been discovered, we should have been justified in +regarding the sun as variable in its output of energy; and not only variable, +but probably increasingly so. The very inequalities in the sun-spot cycle are +suspicious. When the sun is most spotted its total light may be reduced by +one-thousandth part, although it is by no means certain that its outgiving of +thermal radiations is then reduced. A loss of one-thousandth of its luminosity +would correspond to a decrease of .0025 of a stellar magnitude, considering the +sun as a star viewed from distant space. So slight a change would not be +perceptible; but it is not alone sun-spots which obscure the solar surface, its +entire globe is enveloped with an obscuring veil. When studied with a powerful +telescope the sun’s surface is seen to be thickly mottled with relatively +obscure specks, so numerous that it has been estimated that they cut off from +one-tenth to one-twentieth of the light that we should receive from it if the +whole surface were as brilliant as its brightest parts. The condition of other +stars warrants the conclusion that this obscuring envelope is the product of a +process of refrigeration which will gradually make the sun more and more +variable until its history ends in extinction. Looking backward, we see a time +when the sun must have been more brilliant than it is now. At that time it +probably shone with the blinding white splendor of such stars as Sirius, Spica, +and Vega; now it resembles the relatively dull Procyon; in time it will turn +ruddy and fall into the closing cycle represented by Antares. Considering that +once it must have been more radiantly powerful than at present, one is tempted +to wonder if that could have been the time when tropical life flourished within +the earth’s polar circles, sustained by a vivific energy in the sun which it +has now lost. +</p> + +<p> +The corona, as we have said, varies with the sun-spot cycle. When the spots are +abundant and active the corona rises strong above the spotted zones, forming +immense beams or streamers, which on one occasion, at least, had an observed +length of <i>ten million miles.</i> At the time of a spot minimum the corona is +less brilliant and has a different outline. It is then that the curved polar +rays are most conspicuous. Thus the vast banners of the sun, shaken out in the +eclipse, are signals to tell of its varying state, but it will probably be long +before we can read correctly their messages. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap08"></a>VIII<br/> +The Zodiacal Light Mystery</h2> + +<p> +There is a singular phenomenon in the sky—one of the most puzzling of all—which +has long arrested the attention of astronomers, defying their efforts at +explanation, but which probably not one in a hundred, and possibly not one in a +thousand, of the readers of this book has ever seen. Yet its name is often +spoken, and it is a conspicuous object if one knows when and where to look for +it, and when well seen it exhibits a mystical beauty which at the same time +charms and awes the beholder. It is called “The Zodiacal Light,” because it +lies within the broad circle of the Zodiac, marking the sun’s apparent annual +path through the stars. What it is nobody has yet been able to find out with +certainty, and books on astronomy usually speak of it with singular reserve. +But it has given rise to many remarkable theories, and a true explanation of it +would probably throw light on a great many other celestial mysteries. The Milky +Way is a more wonderful object to look upon, but its nature can be +comprehended, while there is a sort of uncanniness about the Zodiacal Light +which immediately impresses one upon seeing it, for its part in the great +scheme of extra-terrestrial affairs is not evident. +</p> + +<p> +If you are out-of-doors soon after sunset—say, on an evening late in the month +of February—you may perceive, just after the angry flush of the dying winter’s +day has faded from the sky, a pale ghostly presence rising above the place +where the sun went down. The writer remembers from boyhood the first time it +was pointed out to him and the unearthly impression that it made, so that he +afterward avoided being out alone at night, fearful of seeing the spectral +thing again. The phenomenon brightens slowly with the fading of the twilight, +and soon distinctly assumes the shape of an elongated pyramid of pearly light, +leaning toward the south if the place of observation is in the northern +hemisphere. It does not impress the observer at all in the same manner as the +Milky Way; that looks far off and is clearly among the stars, but the Zodiacal +Light seems closer at hand, as if it were something more intimately concerning +the earth. To all it immediately suggests a connection, also, with the sunken +sun. If the night is clear and the moon absent (and if you are in the country, +for city lights ruin the spectacles of the sky), you will be able to watch the +apparition for a long time. You will observe that the light is brightest near +the horizon, gradually fading as the pyramidal beam mounts higher, but in +favorable circumstances it may be traced nearly to the meridian south of the +zenith, where its apex at last vanishes in the starlight. It continues visible +during the evenings of March and part of April, after which, ordinarily, it is +seen no more, or if seen is relatively faint and unimpressive. But when autumn +comes it appears again, this time not like a wraith hovering above the westward +tomb of the day-god, but rather like a spirit of the morning announcing his +reincarnation in the east. +</p> + +<p> +The reason why the Zodiacal Light is best seen in our latitudes at the periods +just mentioned is because at those times the Zodiac is more nearly +perpendicular to the horizon, first in the west and then in the east; and, +since the phenomenon is confined within the borders of the Zodiac, it cannot be +favorably placed for observation when the zodiacal plane is but slightly +inclined to the horizon. Its faint light requires the contrast of a background +of dark sky in order to be readily perceptible. But within the tropics, where +the Zodiac is always at a favorable angle, the mysterious light is more +constantly visible. Nearly all observant travelers in the equatorial regions +have taken particular note of this phenomenon, for being so much more +conspicuous there than in the temperate zones it at once catches the eye and +holds the attention as a novelty. Humboldt mentions it many times in his works, +for his genius was always attracted by things out of the ordinary and difficult +of explanation, and he made many careful observations on its shape, its +brilliancy, and its variations; for there can be no doubt that it does vary, +and sometimes to an astonishing degree. It is said that it once remained +practically invisible in Europe for several years in succession. During a trip +to South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a +remarkable difference between the appearance of the Zodiacal Light on his going +and coming voyages. In fact, when crossing the equator going south he did not +see it at all; but on returning he had, on March 6th, when one degree south of +the equator, a memorable view of it. +</p> + +<p class="letter"> +It was a bright, clear night, and the Zodiacal Light was extraordinarily +brilliant—brighter than he had ever seen it before. The Milky Way was not to be +compared with it. The brightest part extended 75° from the sun. There was a +faint and much narrower extension which they could just make out beyond the +Pleiades along the ecliptic, but the greater part of the Zodiacal Light showed +as a broad truncated column, and it did not appear nearly as conical as he had +before seen it. +</p> + +<p> +When out of the brief twilight of intertropical lands, where the sun drops +vertically to the horizon and night rushes on like a wave of darkness, the +Zodiacal Light shoots to the very zenith, its color is described as a golden +tint, entirely different from the silvery sheen of the Milky Way. If I may +venture again to refer to personal experiences and impressions, I will recall a +view of the Zodiacal Light from the summit of the cone of Mt Etna in the autumn +of the year 1896 (more briefly described in <i>Astronomy with the Naked +Eye</i>). There are few lofty mountains so favorably placed as Etna for +observations of this kind. It was once resorted to by Prof. George E. Hale, in +an attempt to see the solar corona without an eclipse. Rising directly from +sea-level to an elevation of nearly eleven thousand feet, the observer on its +summit at night finds himself, as it were, lost in the midst of the sky. But +for the black flanks of the great cone on which he stands he might fancy +himself to be in a balloon. On the occasion to which I refer the world beneath +was virtually invisible in the moonless night. The blaze of the constellations +overhead was astonishingly brilliant, yet amid all their magnificence my +attention was immediately drawn to a great tapering light that sprang from the +place on the horizon where the sun would rise later, and that seemed to be +blown out over the stars like a long, luminous veil. It was the finest view of +the Zodiacal light that I had ever enjoyed—thrilling in its strangeness—but I +was almost disheartened by the indifference of my guide, to whom it was only a +light and nothing more. If he had no science, he had less poetry—rather a +remarkable thing, I thought, for a child of his clime. The Light appeared to me +to be distinctly brighter than the visible part of the Milky Way which included +the brilliant stretches in Auriga and Perseus, and its color, if one may speak +of color in connection with such an object, seemed richer than that of the +galactic band; but I did not think of it as yellow, although Humboldt has +described it as resembling a golden curtain drawn over the stars, and Du +Chaillu in Equatorial Africa found it of a bright yellow color. It may vary in +color as in conspicuousness. The fascination of that extraordinary sight has +never faded from my memory. I turned to regard it again and again, although I +had never seen the stellar heavens so brilliant, and it was one of the last +things I looked for when the morning glow began softly to mount in the east, +and Sicily and the Mediterranean slowly emerged from the profound shadow +beneath us. +</p> + +<p> +The Zodiacal Light seems never to have attracted from astronomers in general +the amount of careful attention that it deserves; perhaps because so little can +really be made of it as far as explanation is concerned. I have referred to the +restraint that scientific writers apparently feel in speaking of it. The +grounds for speculation that it affords may be too scanty to lead to long +discussions, yet it piques curiosity, and as we shall see in a moment has +finally led to a most interesting theory. Once it was the subject of an +elaborate series of studies which carried the observer all round the world. +That was in 1845—46, during the United States Exploring Expedition that visited +the then little known Japan. The chaplain of the fleet, the Rev. Mr Jones, went +out prepared to study the mysterious light in all its phases. He saw it from +many latitudes on both sides of the equator, and the imagination cannot but +follow him with keen interest in his world-circling tour, keeping his eyes +every night fixed upon the phantasm overhead, whose position shifted with that +of the hidden sun. He demonstrated that the flow extends at times completely +across the celestial dome, although it is relatively faint directly behind the +earth. On his return the government published a large volume of his +observations, in which he undertook to show that the phenomenon was due to the +reflection of sunlight from a ring of meteoric bodies encircling the earth. +But, after all, this elaborate investigation settled nothing. +</p> + +<p> +Prof. E. E. Barnard has more recently devoted much attention to the Zodiacal +Light, as well as to a strange attendant phenomenon called the “Gegenschein,” +or Counterglow, because it always appears at that point in the sky which is +exactly opposite the sun. The Gegenschein is an extremely elusive phenomenon, +suitable only for eyes that have been specially trained to see it. Professor +Newcomb has cautiously remarked that +</p> + +<p class="letter"> +it is said that in that point of the heavens directly opposite the sun there is +an elliptical patch of light... This phenomenon is so difficult to account for +that its existence is sometimes doubted; yet the testimony in its favor is +difficult to set aside. +</p> + +<p> +It certainly cannot be set aside at all since the observations of Barnard. I +recall an attempt to see it under his guidance during a visit to Mount +Hamilton, when he was occupied there with the Lick telescope. Of course, both +the Gegenschein and the Zodiacal Light are too diffuse to be studied with +telescopes, which, so to speak, magnify them out of existence. They can only be +successfully studied with the naked eye, since every faintest glimmer that they +afford must be utilized. This is especially true of the Gegenschein. At Mount +Hamilton, Mr Barnard pointed out to me its location with reference to certain +stars, but with all my gazing I could not be sure that I saw it. To him, on the +contrary, it was obvious; he had studied it for months, and was able to +indicate its shape, its boundaries, its diameter, and the declination of its +center with regard to the ecliptic. There is not, of course, the shadow of a +doubt of the existence of the Gegenschein, and yet I question if one person in +a million has ever seen or ever will see it. The Zodiacal Light, on the other +hand, is plain enough, provided that the time and the circumstances of the +observation are properly chosen. +</p> + +<p> +In the attempts to explain the Zodiacal Light, the favorite hypothesis has been +that it is an appendage of the sun—perhaps simply an extension of the corona in +the plane of the ecliptic, which is not very far from coinciding with that of +the sun’s equator. This idea is quite a natural one, because of the evident +relation of the light to the position of the sun. The vast extension of the +equatorial wings of the corona in 1878 gave apparent support to this +hypothesis; if the substance of the corona could extend ten million miles from +the sun, why might it not extend even one hundred million, gradually fading out +beyond the orbit of the earth? A variation of this hypothesis assumes that the +reflection is due to swarms of meteors circling about the sun, in the plane of +its equator, all the way from its immediate neighborhood to a distance +exceeding that of the earth. But in neither form is the hypothesis +satisfactory; there is nothing in the appearance of the corona to indicate that +it extends even as far as the planet Mercury, while as to meteors, the orbits +of the known swarms do not accord with the hypothesis, and we have no reason to +believe that clouds of others exist traveling in the part of space where they +would have to be in order to answer the requirements of the theory. The +extension of the corona in 1878 did not resemble in its texture the Zodiacal +Light. +</p> + +<p> +Now, it has so often happened in the history of science that an important +discovery in one branch has thrown unexpected but most welcome light upon some +pending problem in some other branch, that a strong argument might be based +upon that fact alone against the too exclusive devotion of many investigators +to the narrow lines of their own particular specialty; and the Zodiacal Light +affords a case in point, when it is considered in connection with recent +discoveries in chemistry and physics. From the fact that atoms are compound +bodies made up of corpuscles at least a thousand times smaller than the +smallest known atom—a fact which astounded most men of science when it was +announced a few years ago—a new hypothesis has been developed concerning the +nature of the Zodiacal Light (as well as other astronomical riddles), and this +hypothesis comes not from an astronomer, but from a chemist and physicist, the +Swede, Svante Arrhenius. In considering an outline of this new hypothesis we +need neither accept nor reject it; it is a case rather for suspension of +judgment. +</p> + +<p> +To begin with, it carries us back to the “pressure of light” mentioned in the +preceding chapter. The manner in which this pressure is believed generally to +act was there sufficiently explained, and it only remains to see how it is +theoretically extended to the particles of matter supposed to constitute the +Zodiacal Light. We know that corpuscles, or “fragments of atoms” negatively +electrified, are discharged from hot bodies. Streams of these “ions” pour from +many flames and from molten metals; and the impact of the cathode and +ultra-violet rays causes them to gush even from cold bodies. In the vast +laboratory of the sun it is but reasonable to suppose that similar processes +are taking place. “As a very hot metal emits these corpuscles,” says Prof. J. +J. Thomson, “it does not seem an improbable hypothesis that they are emitted by +that very hot body, the sun.” Let it be assumed, then, that the sun does emit +them; what happens next? Negatively charged corpuscles, it is known, serve as +nuclei to which particles of matter in the ordinary state are attracted, and it +is probable that those emitted from the sun immediately pick up loads in this +manner and so grow in bulk. If they grow large enough the gravitation of the +sun draws them back, and they produce a negative charge in the solar +atmosphere. But it is probable that many of the particles do not attain the +critical size which, according to the principles before explained, would enable +the gravitation of the sun to retain them in opposition to the pressure of the +waves of light, and with these particles the light pressure is dominant. Clouds +of them may be supposed to be continually swept away from the sun into +surrounding space, moving mostly in or near the plane of the solar equator, +where the greatest activity, as indicated by sunspots and related phenomena, is +taking place. As they pass outward into space many of them encounter the earth. +If the earth, like the moon, had no atmosphere the particles would impinge +directly on its surface, giving it a negative electric charge. But the presence +of the atmosphere changes all that, for the first of the flying particles that +encounter it impart to it their negative electricity, and then, since like +electric charges repel like, the storm of particles following will be sheered +off from the earth, and will stream around it in a maze of hyperbolic paths. +Those that continue on into space beyond the earth may be expected to continue +picking up wandering particles of matter until their bulk has become so great +that the solar attraction prevails again over the light pressure acting upon +them, and they turn again sunward. Passing the earth on their return they will +increase the amount of dust-clouds careering round it; and these will be +further increased by the action of the ultra-violet rays of the sunlight +causing particles to shoot radially away from the earth when the negative +charge of the upper atmosphere has reached a certain amount, which particles, +although starting sunward, will be swept back to the earth with the oncoming +streams. As the final result of all this accumulation of flying and gyrating +particles in the earth’s neighborhood, we are told that the latter must be +transformed into the semblance of a gigantic solid-headed comet provided with +streaming tails, the longest of them stretching away from the direction of the +sun, while another shorter one extends toward the sun. This shorter tail is due +to the particles that we have just spoken of as being driven sunward from the +earth by the action of ultra-violet light. No doubt this whole subject is too +technical for popular statement; but at any rate the general reader can +understand the picturesque side of the theory, for its advocates assure us that +if we were on the moon we would doubtless be able to see the comet-like tails +of the earth, and then we could appreciate the part that they play in producing +the phenomenon of the Zodiacal Light. +</p> + +<p> +That the Light as we see it could be produced by the reflection of sunlight +from swarms of particles careering round the earth in the manner supposed by +Arrhenius’ hypothesis is evident enough; and it will be observed that the new +theory, after all, is only another variant of the older one which attributes +the Zodiacal Light to an extension of the solar corona. But it differs from the +older theory in offering an explanation of the manner in which the extension is +effected, and it differentiates between the corona proper and the streams of +negative particles shot away from the sun. In its details the hypothesis of +Arrhenius also affords an explanation of many peculiarities of the Zodiacal +Light, such as that it is confined to the neighborhood of the ecliptic, and +that it is stronger on the side of the earth which is just turning away from a +position under the sun than on the other side; but it would carry us beyond our +limits to go into these particulars. The Gegenschein, according to this theory, +is a part of the same phenomenon as the Zodiacal Light, for by the laws of +perspective it is evident that the reflection from the streams of particles +situated at a point directly opposite to the sun would be at a maximum, and +this is the place which the Gegenschein occupies. Apart from its geometrical +relations to the position of the sun, the variability of the Zodiacal Light +appears to affirm its solar dependence, and this too would be accounted for by +Arrhenius’ hypothesis better than by the old theory of coronal extension. The +amount of corpuscular discharge from the sun must naturally be governed by the +state of relative activity or inactivity of the latter, and this could not but +be reflected in the varying splendor of the Zodiacal Light. But much more +extended study than has yet been given to the subject will be required before +we can feel that we know with reasonable certainty what this mysterious +phenomenon really is. By the hypothesis of Arrhenius every planet that has an +atmosphere must have a Zodiacal Light attending it, but the phenomenon is too +faint for us to be able to see it in the case, for instance, of Venus, whose +atmosphere is very abundant. The moon has no corresponding “comet’s tail” +because, as already explained, of the lack of a lunar atmosphere to repel the +streams by becoming itself electrified; but if there were a lunar Zodiacal +Light, no doubt we could see it because of the relative nearness of our +satellite. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap09"></a>IX<br/> +Marvels of the Aurora</h2> + +<p> +One of the most vivid recollections of my early boyhood is that of seeing my +father return hastily into the house one evening and call out to the family: +“Come outside and look at the sky!” Ours was a country house situated on a +commanding site, and as we all emerged from the doorway we were dumbfounded to +see the heavens filled with pale flames which ran licking and quivering over +the stars. Instantly there sprang into my terrified mind the recollection of an +awful description of “the Day of Judgment” (the <i>Dies Iræ</i>), which I had +heard with much perturbation of spirit in the Dutch Reformed church from the +lips of a tall, dark-browed, dreadfully-in-earnest preacher of the +old-fashioned type. My heart literally sank at sight of the spectacle, for it +recalled the preacher’s very words; it was just as he had said it would be, and +it needed the assured bearing of my elders finally to convince me that +</p> + +<p class="poem"> +That Day of Wrath, O dreadful day,<br/> +When Heaven and Earth shall pass away,<br/> +As David and the Sibyl say +</p> + +<p class="noindent"> +had not actually come upon us. And even the older members of the household were +not untouched with misgivings when menacing spots of crimson appeared, breaking +out now here, now there, in the shuddering sky. Toward the north the spectacle +was appalling. A huge arch spanned an unnaturally dark segment resting on the +horizon, and above this arch sprang up beams and streamers in a state of +incessant agitation, sometimes shooting up to the zenith with a velocity that +took one’s breath, and sometimes suddenly falling into long ranks, and +<i>marching, marching, marching,</i> like an endless phalanx of fiery specters, +and moving, as I remember, always from east to west. The absolute silence with +which these mysterious evolutions were performed and the quavering reflections +which were thrown upon the ground increased the awfulness of the exhibition. +Occasionally enormous curtains of lambent flame rolled and unrolled with a +majestic motion, or were shaken to and fro as if by a mighty, noiseless wind. +At times, too, a sudden billowing rush would be made toward the zenith, and for +a minute the sky overhead would glow so brightly that the stars seemed to have +been consumed. The spectacle continued with varying intensity for hours. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus19"></a> +<a href="images/image19.jpg"> +<img src="images/image19.jpg" width="600" height="459" alt="Illustration:" /></a> +<p class="caption">Auroral beams seen in England</p> +</div> + +<p> +This exhibition occurred in Central New York, a latitude in which the Aurora +Borealis is seldom seen with so much splendor. I remember another similar one +seen from the city of New York in November, 1882. On this last occasion some +observers saw a great upright beam of light which majestically moved across the +heavens, stalking like an apparition in the midst of the auroral pageant, of +whose general movements it seemed to be independent, maintaining always its +upright posture, and following a magnetic parallel from east to west. This +mysterious beam was seen by no less than twenty-six observers in different +parts of the country, and a comparison of their observations led to a curious +calculation indicating that the apparition was about <i>one hundred and +thirty-three miles tall</i> and moved at the speed of ten miles per second! +</p> + +<div class="fig" style="width:100%;"> +<a name="illus20"></a> +<a href="images/image20.jpg"> +<img src="images/image20.jpg" width="600" height="403" alt="Illustration:" /></a> +<p class="caption">Auroral arches of an eliptic form seen in the Arctic regions</p> +</div> + +<p> +But, as everybody knows, it is in the Arctic regions that the Aurora, or the +“Northern Lights,” can best be seen. There, in the long polar night, when for +months together the sun does not rise, the strange coruscations in the sky +often afford a kind of spectral daylight in unison with the weird scenery of +the world of ice. The pages in the narratives of Arctic exploration that are +devoted to descriptions of the wonderful effects of the Northern Lights are +second to none that man has ever penned in their fascination. The lights, as I +have already intimated, display astonishing colors, particularly shades of red +and green, as they flit from place to place in the sky. The discovery that the +magnetic needle is affected by the Aurora, quivering and darting about in a +state of extraordinary excitement when the lights are playing in the sky, only +added to the mystery of the phenomenon until its electro-magnetic nature had +been established. This became evident as soon as it was known that the focus of +the displays was the magnetic pole; and when the far South was visited the +Aurora Australis was found, having its center at the South Magnetic Pole. Then, +if not before, it was clear that the earth was a great globular magnet, having +its poles of opposite magnetism, and that the auroral lights, whatever their +precise cause might be, were manifestations of the magnetic activity of our +planet. After the invention of magnetic telegraphy it was found that whenever a +great Aurora occurred the telegraph lines were interrupted in their operation, +and the ocean cables ceased to work. Such a phenomenon is called a “magnetic +storm.” +</p> + +<p> +The interest excited by the Aurora in scientific circles was greatly stimulated +when, in the last half of the nineteenth century, it was discovered that it is +a phenomenon intimately associated with disturbances on the sun. The ancient +“Zurich Chronicles,” extending from the year 1000 to the year 1800, in which +both sun-spots visible to the naked eye and great displays of the auroral +lights were recorded, first set Rudolf Wolf on the track of this discovery. The +first notable proof of the suspected connection was furnished with dramatic +emphasis by an occurrence which happened on September 1, 1859. Near noon on +that day two intensely brilliant points suddenly broke out in a group of +sun-spots which were under observation by Mr R. C. Carrington at his +observatory at Redhill, England. The points remained visible for not more than +five minutes, during which interval they moved <i>thirty-five thousand +miles</i> across the solar disk. Mr R. Hodgson happened to see the same +phenomenon at his observatory at Highgate, and thus all possibility of +deception was removed. But neither of the startled observers could have +anticipated what was to follow, and, indeed, it was an occurrence which has +never been precisely duplicated. I quote the eloquent account given by Miss +Clerke in her <i>History of Astronomy During the Nineteenth Century.</i> +</p> + +<p class="letter"> +This unique phenomenon seemed as if specially designed to accentuate the +inference of a sympathetic relation between the earth and the sun. From August +28 to September 4, 1859, a magnetic storm of unparalleled intensity, extent, +and duration was in progress over the entire globe. Telegraphic communication +was everywhere interrupted—except, indeed, that it was in some cases found +practicable to work the lines <i>without batteries</i> by the agency of the +earth-currents alone; sparks issued from the wires; gorgeous auroras draped the +skies in solemn crimson over both hemispheres, and even in the tropics; the +magnetic needle lost all trace of continuity in its movements and darted to and +fro as if stricken with inexplicable panic. The coincidence was even closer. +<i>At the very instant</i> of the solar outburst witnessed by Carrington and +Hodgson the photographic apparatus at Kew registered a marked disturbance of +all the three magnetic elements; while shortly after the ensuing midnight the +electric agitation culminated, thrilling the whole earth with subtle +vibrations, and lighting up the atmosphere from pole to pole with coruscating +splendors which perhaps dimly recall the times when our ancient planet itself +shone as a star. +</p> + +<p> +If this amazing occurrence stood alone, and as I have already said it has never +been exactly duplicated, doubt might be felt concerning some of the inferences +drawn from it; but in varying forms it has been repeated many times, so that +now hardly anyone questions the reality of the assumed connection between solar +outbursts and magnetic storms accompanied by auroral displays on the earth. It +is true that the late Lord Kelvin raised difficulties in the way of the +hypothesis of a direct magnetic action of the sun upon the earth, because it +seemed to him that an inadmissible quantity of energy was demanded to account +for such action. But no calculation like that which he made is final, since all +calculations depend upon the validity of the data; and no authority is +unshakable in science, because no man can possess omniscience. It was Lord +Kelvin who, but a few years before the thing was actually accomplished, +declared that aerial navigation was an impracticable dream, and demonstrated +its impracticability by calculation. However the connection may be brought +about, it is as certain as evidence can make it that solar outbursts are +coincident with terrestial magnetic disturbances, and coincident in such a way +as to make the inference of a causal connection irresistible. The sun is only a +little more than a hundred times its own diameter away from the earth. Why, +then, with the subtle connection between them afforded by the ether which +conveys to us the blinding solar light and the life-sustaining solar heat, +should it be so difficult to believe that the sun’s enormous electric energies +find a way to us also? No doubt the impulse coming from the sun acts upon the +earth after the manner of a touch upon a trigger, releasing energies which are +already stored up in our planet. +</p> + +<p> +But besides the evidence afforded by such occurrences as have been related of +an intimate connection between solar outbreaks and terrestial magnetic +flurries, attended by magnificent auroral displays, there is another line of +proof pointing in the same direction. Thus, it is known that the sun-spot +period, as remarked in a preceding chapter, coincides in a most remarkable +manner with the periodic fluctuations in the magnetic state of the earth. This +coincidence runs into the most astonishing details. For instance, when the +sun-spot period shortens, the auroral period shortens to precisely the same +extent; as the short sun-spot periods usually bring the most intense outbreaks +of solar activity, so the corresponding short auroral periods are attended by +the most violent magnetic storms; a secular period of about two hundred and +twenty-two years affecting sun-spots is said to have its auroral duplicate; a +shorter period of fifty-five and a half years, which some observers believe +that they have discovered appears also to be common to the two phenomena; and +yet another “superposed” period of about thirty-five years, which some +investigators aver exists, affects sun-spots and aurora alike. In short, the +coincidences are so numerous and significant that one would have to throw the +doctrine of probability to the winds in order to be able to reject the +conclusion to which they so plainly lead. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus21"></a> +<a href="images/image21.jpg"> +<img src="images/image21.jpg" width="600" height="395" alt="Illustration:" /></a> +<p class="caption">Auroral curtain seen in Scandinavia</p> +</div> + +<p> +But still the question recurs: How is the influence transmitted? Here Arrhenius +comes once more with his hypothesis of negative corpuscles, or ions, driven +away from the sun by light-pressure—a hypothesis which seems to explain so many +things—and offers it also as an explanation of the way in which the sun creates +the Aurora. He would give the Aurora the same lineage with the Zodiacal Light. +To understand the application of this theory we must first recall the fact that +the earth is a great magnet having its two opposite poles of magnetism, one +near the Arctic and the other near the Antarctic Circle. Like all magnets, the +earth is surrounded with “lines of force,” which, after the manner of the +curved rays we saw in the photograph of a solar eclipse, start from a pole, +rising at first nearly vertically, then bend gradually over, passing high above +the equator, and finally descending in converging sheaves to the opposite pole. +Now the axis of the earth is so placed in space that it lies at nearly a right +angle to the direction of the sun, and as the streams of negatively charged +particles come pouring on from the sun (see the last preceding chapter), they +arrive in the greatest numbers over the earth’s equatorial regions. There they +encounter the lines of magnetic force at the place where the latter have their +greatest elevation above the earth, and where their direction is horizontal to +the earth’s surface. Obeying a law which has been demonstrated in the +laboratory, the particles then follow the lines of force toward the poles. +While they are above the equatorial regions they do not become luminescent, +because at the great elevation that they there occupy there is virtually no +atmosphere; but as they pass on toward the north and the south they begin to +descend with the lines of force, curving down to meet at the poles; and, +encountering a part of the atmosphere comparable in density with what remains +in an exhausted Crookes tube, they produce a glow of cathode rays. This glow is +conceived to represent the Aurora, which may consequently be likened to a +gigantic exhibition of vacuum-tube lights. Anybody who recalls his student days +in the college laboratory and who has witnessed a display of Northern Lights +will at once recognize the resemblance between them in colors, forms, and +behavior. This resemblance had often been noted before Arrhenius elaborated his +hypothesis. +</p> + +<p> +Without intending to treat his interesting theory as more than a possibly +correct explanation of the phenomena of the Aurora, we may call attention to +some apparently confirmatory facts. One of the most striking of these relates +to a seasonal variation in the average number of auroræ. It has been observed +that there are more in March and September than at any other time of the year, +and fewer in June and December; moreover (and this is a delicate test as +applied to the theory), they are slightly rarer in June than in December. Now +all these facts seem to find a ready explanation in the hypothesis of +Arrhenius, thus: (1) The particles issuing from the sun are supposed to come +principally from the regions whose excitement is indicated by the presence of +sun-spots (which accords with Hale’s observation that sun-spots are columns of +ionized vapors), and these regions have a definite location on either side of +the solar equator, seldom approaching it nearer than within 5° or 10° +north or south, and never extending much beyond 35° toward either pole; (2) +The equator of the sun is inclined about 7° to the plane of the earth’s +orbit, from which it results that twice in a year—<i>viz.,</i> in June and +December—the earth is directly over the solar equator, and twice a +year—<i>viz.,</i> in March and September—when it is farthest north or south of +the solar equator, it is over the inner edge of the sun-spot belts. Since the +corpuscles must be supposed to be propelled radially from the sun, few will +reach the earth when the latter is over the solar equator in June and December, +but when it is over, or nearly over, the spot belts, in March and September, it +will be in the line of fire of the more active parts of the solar surface, and +relatively rich streams of particles will reach it. This, as will be seen from +what has been said above, is in strict accord with the observed variations in +the frequency of auroræ. Even the fact that somewhat fewer auroræ are seen in +June than in December also finds its explanation in the known fact that the +earth is about three million miles nearer the sun in the winter than in the +summer, and the number of particles reaching it will vary, like the intensity +of light, inversely as the square of the distance. These coincidences are +certainly very striking, and they have a cumulative force. If we accept the +theory, it would appear that we ought to congratulate ourselves that the +inclination of the sun’s equator is so slight, for as things stand the earth is +never directly over the most active regions of the sun-spots, and consequently +never suffers from the maximum bombardment of charged particles of which the +sun is capable. Incessant auroral displays, with their undulating draperies, +flitting colors, and marching columns might not be objectionable from the point +of view of picturesqueness, but one magnetic storm of extreme intensity +following closely upon the heels of another, for months on end, crazing the +magnetic needle and continually putting the telegraph and cable lines out of +commission, to say nothing of their effect upon “wireless telegraphy”, would +hardly add to the charms of terrestrial existence. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus22"></a> +<a href="images/image22.jpg"> +<img src="images/image22.jpg" width="600" height="462" alt="Illustration:" /></a> +<p class="caption">Auroral arches seen in Scandinavia</p> +</div> + +<p> +One or two other curious points in connection with Arrhenius’ hypothesis may be +mentioned. First, the number of auroræ, according to his explanation, ought to +be greatest in the daytime, when the face of the earth on the sunward side is +directly exposed to the atomic bombardment. Of course visual observation can +give us no information about this, since the light of the Aurora is never +sufficiently intense to be visible in the presence of daylight, but the records +of the magnetic observatories can be, and have been, appealed to for +information, and they indicate that the facts actually accord with the theory. +Behind the veil of sunlight in the middle of the afternoon, there is good +reason to believe, auroral exhibitions often take place which would eclipse in +magnificence those seen at night if we could behold them. Observation shows, +too, that auroræ are more frequent before than after midnight, which is just +what we should expect if they originate in the way that Arrhenius supposes. +Second, the theory offers an explanation of the alleged fact that the formation +of clouds in the upper air is more frequent in years when auroræ are most +abundant, because clouds are the result of the condensation of moisture upon +floating particles in the atmosphere (in an absolutely dustless atmosphere +there would be no clouds), and it has been proved that negative ions like those +supposed to come from the sun play a master part in the phenomena of cloud +formation. +</p> + +<p> +Yet another singular fact, almost mystical in its suggestions, may be +mentioned. It seems that the dance of the auroral lights occurs most frequently +during the absence of the moon from the hemisphere in which they appear, and +that they flee, in greater part, to the opposite hemisphere when the moon’s +revolution in an orbit considerably inclined to the earth’s equator brings her +into that where they have been performing. Arrhenius himself discovered this +curious relation of auroral frequency to the position of the moon north or +south of the equator, and he explains it in this way. The moon, like the earth, +is exposed to the influx of the ions from the sun; but having no atmosphere, or +almost none, to interfere with them, they descend directly upon her surface and +charge her with an electric negative potential to a very high degree. In +consequence of this she affects the electric state of the upper parts of the +earth’s atmosphere where they lie most directly beneath her, and thus prevents, +to a large extent, the negative discharges to which the appearance of the +Aurora is due. And so “the extravagant and erring spirit” of the Aurora avoids +the moon as Hamlet’s ghost fled at the voice of the cock announcing the +awakening of the god of day. +</p> + +<p> +There are even other apparent confirmations of the hypothesis, but we need not +go into them. We shall, however, find one more application of it in the next +chapter, for it appears to be a kind of cure-all for astronomical troubles; at +any rate it offers a conceivable solution of the question, How does the sun +manage to transmit its electric influence to the earth? And this solution is so +grandiose in conception, and so novel in the mental pictures that it offers, +that its acceptance would not in the least detract from the impression that the +Aurora makes upon the imagination. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap10"></a>X<br/> +Strange Adventures of Comets</h2> + +<p> +The fears and legends of ancient times before Science was born, and the +superstitions of the Dark Ages, sedulously cultivated for theological purposes +by monks and priests, have so colored our ideas of the influence that comets +have had upon the human mind that many readers may be surprised to learn that +it was the apparition of a wonderful comet, that of 1843, which led to the +foundation of our greatest astronomical institution, the Harvard College +Observatory. No doubt the comet superstition existed half a century ago, as, +indeed, it exists yet today, but in this case the marvelous spectacle in the +sky proved less effective in inspiring terror than in awakening a desire for +knowledge. Even in the sixteenth century the views that enlightened minds took +of comets tended powerfully to inspire popular confidence in science, and +Halley’s prediction, after seeing and studying the motion of the comet which +appeared in 1682, that it would prove to be a regular member of the sun’s +family and would be seen returning after a period of about seventy-six years, +together with the fulfillment of that prediction, produced a revulsion from the +superstitious notions which had so long prevailed. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus23"></a> +<a href="images/image23.jpg"> +<img src="images/image23.jpg" width="508" height="600" alt="Illustration:" /></a> +<p class="caption">Swift’s comet. Taken at Arequipa, March 30 1892</p> +</div> + +<p> +Then the facts were made plain that comets are subject to the law of +gravitation equally with the planets; that there are many which regularly +return to the neighborhood of the sun (perihelion); and that these travel in +orbits differing from those of the planets only in their greater eccentricity, +although they have the peculiarity that they do not, like the planets, all go +round the sun in the same direction, and do not keep within the general plane +of the planetary system, but traverse it sometimes from above and sometimes +from below. Other comets, including most of the “great” ones, appear to travel +in parabolic or, in a few cases, hyperbolic orbits, which, not being closed +curves, never bring them back again. But it is not certain that these orbits +may not be extremely eccentric ellipses, and that after the lapse of hundreds, +or thousands, of years the comets that follow them may not reappear. The +question is an interesting one, because if all orbits are really ellipses, then +all comets must be permanent members of the solar system, while in the contrary +case many of them are simply visitors, seen once and never to be seen again. +The hypothesis that comets are originally interlopers might seem to derive some +support from the fact that the certainly periodic ones are associated, in +groups, with the great outer planets, whose attraction appears to have served +as a trap for them by turning them into elliptical orbits and thus making them +prisoners in the solar system. Jupiter, owing to his great mass and his +commanding situation in the system, is the chief “comet-catcher;” but he +catches them not for himself, but for the sun. Yet if comets do come originally +from without the borders of the planetary system, it does not, by any means, +follow that they were wanderers at large in space before they yielded to the +overmastering attraction of the sun. Investigation of the known cometary +orbits, combined with theoretical considerations, has led some astronomers to +the conclusion that as the sun travels onward through space he “picks up <i>en +route</i>” cometary masses which, without belonging strictly to his empire, are +borne along in the same vast “cosmical current” that carries the solar system. +</p> + +<p> +But while no intelligent person any longer thinks that the appearance of a +great comet is a token from the heavenly powers of the approaching death of a +mighty ruler, or the outbreak of a devastating war, or the infliction of a +terrible plague upon wicked mankind, science itself has discovered mysteries +about comets which are not less fascinating because they are more intellectual +than the irrational fancies that they have displaced. To bring the subject +properly before the mind, let us see what the principal phenomena connected +with a comet are. +</p> + +<p> +At the present day comets are ordinarily “picked up” with the telescope or the +photographic plate before any one except their discoverer is aware of their +existence, and usually they remain so insignificant in appearance that only +astronomers ever see them. Yet so great is the prestige of the word “comet” +that the discovery of one of these inconspicuous wanderers, and its subsequent +movements, become items of the day’s news which everybody reads with the +feeling, perhaps, that at least he knows what is going on in the universe even +if he doesn’t understand it. But a truly great comet presents quite a different +proposition. It, too, is apt to be detected coming out of the depths of space +before the world at large can get a glimpse of it, but as it approaches the sun +its aspect undergoes a marvelous change. Agitated apparently by solar +influence, it throws out a long streaming tail of nebulous light, directed away +from the sun and looking as if blown out like a pennon by a powerful wind. +Whatever may be the position of the comet with regard to the sun, as it circles +round him it continually keeps its tail on the off side. This, as we shall soon +see, is a fact of capital importance in relation to the probable nature of +comets’ tails. Almost at the same time that the formation of the tail is +observed a remarkable change takes place in the comet’s head, which, by the +way, is invariably and not merely occasionally its most important part. On +approaching the sun the head usually contracts. Coincidently with this +contraction a nucleus generally makes its appearance. This is a bright, +star-like point in the head, and it probably represents the totality of solid +matter that the comet possesses. But it is regarded as extremely unlikely that +even the nucleus consists of a uniformly solid mass. If it were such, comets +would be far more formidable visitors when they pass near the planets than they +have been found to be. The diameter of the nucleus may vary from a few hundred +up to several thousand miles; the heads, on the average, are from twenty-five +thousand to one hundred thousand miles in diameter, although a few have greatly +exceeded these dimensions; that of the comet of 1811, one of the most +stupendous ever seen, was a million and a quarter miles in diameter! As to the +tails, not withstanding their enormous length—some have been more than a +hundred million miles long—there is reason to believe that they are of extreme +tenuity, “as rare as vacuum.” The smallest stars have been seen shining through +their most brilliant portions with undiminished luster. +</p> + +<p> +After the nucleus has been formed it begins to throw out bright jets directed +toward the sun. A stream, and sometimes several streams, of light also project +sunward from the nucleus, occasionally appearing like a stunted tail directed +oppositely to the real tail. Symmetrical envelopes which, seen in section, +appear as half circles or parabolas, rise sunward from the nucleus, forming a +concentric series. The ends of these stream backward into the tail, to which +they seem to supply material. Ordinarily the formation of these ejections and +envelopes is attended by intense agitation of the nucleus, which twists and +turns, swinging and gyrating with an appearance of the greatest violence. +Sometimes the nucleus is seen to break up into several parts. The entire heads +of some comets have been split asunder in passing close around the sun; The +comet of 1882 retreated into space after its perihelion passage with <i>five +heads</i> instead of the one that it had originally, and each of these heads +had its own tail! +</p> + +<p> +The possession of the spectroscope has enabled astronomers during later years +to study the chemical composition of comets by analyzing their light. At first +the only substances thus discovered in them were hydro-carbon compounds, due +evidently to the gaseous envelopes in which some combination of hydrogen with +carbon existed. Behind this gaseous spectrum was found a faint continuous +spectrum ascribed to the nucleus, which apparently both reflects the sunlight +and gives forth the light of a glowing solid or liquid. Subsequently sodium and +iron lines were found in cometary spectra. The presence of iron would seem to +indicate that some of these bodies may be much more massive than observations +on their attractive effects have indicated. In some recent comets, such as +Morehouse’s, in 1908, several lines have been found, the origin of which is +unknown. +</p> + +<p> +Without going back of the nineteenth century we may find records of some of the +most extraordinary comets that man has ever looked upon. In 1811, still spoken +of as “the year of the comet,” because of the wonderful vintage ascribed to the +skyey visitor, a comet shaped like a gigantic sword amazed the whole world, +and, as it remained visible for seventeen months, was regarded by superstitious +persons as a symbol of the fearful happenings of Napoleon’s Russian campaign. +This comet, the extraordinary size of whose head, greatly exceeding that of the +sun itself, has already been mentioned, was also remarkable for exhibiting so +great a brilliancy without approaching even to the earth’s distance from the +sun. But there was once a comet (and only once—in the year 1729) which never +got nearer to the sun than four times the distance of the earth and yet +appeared as a formidable object in the sky. As Professor Young has remarked, +“it must have been an enormous comet to be visible from such a distance.” And +we are to remember that there were no great telescopes in the year 1729. That +comet affects the imagination like a phantom of space peering into the solar +system, displaying its enormous train afar off (which, if it had approached as +near as other comets, would probably have become <i>the</i> celestial wonder of +all human memory), and then turning away and vanishing in the depths of +immensity. +</p> + +<p> +In 1843 a comet appeared which was so brilliant that it could be seen in broad +day close beside the sun! This was the first authenticated instance of that +kind, but the occurrence was to be repeated, as we shall see in a moment, less +than forty years later. +</p> + +<p> +The splendid comet of 1858, usually called Donati’s, is remembered by many +persons yet living. It was, perhaps, both as seen by the naked eye and with the +telescope, the most beautiful comet of which we have any record. It too marked +a rich vintage year, still remembered in the vineyards of France, where there +is a popular belief that a great comet ripens the grape and imparts to the wine +a flavor not attainable by the mere skill of the cultivator. There are “comet +wines,” carefully treasured in certain cellars, and brought forth only when +their owner wishes to treat his guests to a sip from paradise. +</p> + +<p> +The year 1861 saw another very remarkable comet, of an aspect strangely vast +and diffuse, which is believed to have swept the earth with its immense tail +when it passed between us and the sun on the night of June 30th, an event which +produced no other known effect than the appearance of an unwonted amount of +scattered light in the sky. +</p> + +<p> +The next very notable comet was the “Great Southern Comet” of 1880, which was +not seen from the northern hemisphere. It mimicked the aspect of the famous +comet of 1843, and to the great surprise of astronomers appeared to be +traveling in the same path. This proved to be the rising of the curtain for an +astronomical sensation unparalleled in its kind; for two years later another +brilliant comet appeared, first in the southern hemisphere, <i>and it too +followed the same track.</i> The startling suggestion was now made that this +comet was identical with those of 1843 and 1880, its return having been +hastened by the resistance experienced in passing twice through the coronal +envelope, and there were some who thought that it would now swing swiftly round +and then plunge straight into the sun, with consequences that might be +disastrous to us on account of the “flash of heat” that would be produced by +the impact. Nervous people were frightened, but observation soon proved that +the danger was imaginary, for although the comet almost grazed the sun, and +must have rushed through two or three million miles of the coronal region, no +retardation of its immense velocity was perceptible, and it finally passed away +in a damaged condition, as before remarked, and has never since appeared. +</p> + +<p> +Then the probable truth was perceived—<i>viz.,</i> that the three comets (1843, +1880, and 1882) were not one identical body, but three separate ones all +traveling in the same orbit. It was found, too, that a comet seen in 1668 bore +similar insignia of relationship. The natural inference was that these four +bodies had once formed a single mass which had been split apart by the +disruptive action of the sun. Strength was lent to this hypothesis by the fact +that the comet of 1882 was apparently torn asunder during its perihelion +passage, retreating into space in a dissevered state. But Prof. George Forbes +has a theory that the splitting of the original cometary mass was effected by +an unknown planet, probably greater than Jupiter, situated at a hundred times +the earth’s distance from the sun, and revolving in a period of a thousand +years. He supposes that the original comet was not that of 1668, but one seen +in 1556, which has since been “missing,” and that its disruption occurred from +an encounter with the supposititious planet about the year 1700. Truly from +every point of view comets are the most extraordinary of adventurers! +</p> + +<p> +The comet of 1882 was likewise remarkable for being visible, like its +predecessor of 1843, in full daylight in close proximity to the sun. The story +of its detection when almost in contact with the solar disk is dramatic. It had +been discovered in the southern hemisphere only a couple of weeks before its +perihelion, which occurred on September 17th, and on the forenoon of that day +it was seen by Doctor Common in England, and by Doctor Elkin and Mr Finlay at +the Cape of Good Hope, almost touching the sun. It looked like a dazzling white +bird with outspread wings. The southern observers watched it go <i>right into +the sun,</i> when it instantly disappeared. What had happened was that the +comet in passing its perihelion point had swung exactly between the earth and +the sun. On the following morning it was seen from all parts of the world close +by the sun on the opposite side, and it remained thus visible for three days, +gradually receding from the solar disk. It then became visible for northern +observers in the morning sky before sunrise, brandishing a portentous +sword-shaped tail which, if it had been in the evening sky, would have excited +the wonder of hundreds of millions, but situated where it was, comparatively +few ever saw it. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus24"></a> +<a href="images/image24.jpg"> +<img src="images/image24.jpg" width="285" height="650" alt="Illustration:" /></a> +<p class="caption">Daniels’ comet. August 11, 1907</p> +</div> + +<p> +The application of photography to the study of comets has revealed many curious +details which might otherwise have escaped detection, or at best have remained +subject to doubt. It has in particular shown not only the precise form of the +tails, but the remarkable vicissitudes that they undergo. Professor Barnard’s +photographs of Brooks’ comet in 1893 suggested, by the extraordinary changes in +the form of the tail which they revealed, that the comet was encountering a +series of obstructions in space which bent and twisted its tail into fantastic +shapes. The reader will observe the strange form into which the tail was thrown +on the night of October 21st. A cloud of meteors through which the comet was +passing might have produced such deformations of its tail. In the photograph of +Daniels’ comet of 1907, a curious striping of the tail will be noticed. The +short bright streaks seen in the photograph, it may be explained, are the +images of stars which are drawn out into lines in consequence of the fact that +the photographic telescope was adjusted to follow the motion of the comet while +the stars remained at rest. +</p> + +<p> +But the adventures of comets are not confined to possible encounters with +unknown obstacles. We have referred to the fact that the great planets, and +especially Jupiter, frequently interfere with the motions of comets. This +interference is not limited to the original alteration of their orbits from +possible parabolas to ellipses, but is sometimes exercised again and again, +turning the bewildered comets into elliptical paths of all degrees of +eccentricity. A famous example of this kind of planetary horse-play is +furnished by the story of Lexell’s missing comet. This comet was first seen in +1770. Investigation showed that it was moving in an orbit which should bring it +back to perihelion every five and a half years; yet it had never been seen +before and, although often searched for, has never been seen since. Laplace and +Leverrier proved mathematically that in 1767 it had approached so close to +Jupiter as to be involved among the orbits of his satellites. What its track +had been before is not known, but on that occasion the giant planet seized the +interloper, threw it into a short elliptic orbit and sent it, like an arrested +vagrant, to receive sentence at the bar of the sun. On this journey it passed +within less than 1,500,000 miles of the earth. The form of orbit which Jupiter +had impressed required, as we have said, its return in about five and a half +years; but soon after 1770 it had the misfortune a second time to encounter +Jupiter at close range, and he, as if dissatisfied with the leniency of the +sun, or indignant at the stranger’s familiarity, seized the comet and hurled it +out of the system, or at any rate so far away that it has never since been able +to rejoin the family circle that basks in the immediate rays of the solar +hearth. Nor is this the only instance in which Jupiter has dealt summarily with +small comets that have approached him with too little deference. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus25"></a> +<a href="images/image25.jpg"> +<img src="images/image25.jpg" width="600" height="463" alt="Illustration:" /></a> +<p class="caption">Brooks’ comet. Photographed by Barnard, October 21, 1893</p> +</div> + +<p> +The function which Jupiter so conspicuously fulfills as master of the hounds to +the sun is worth considering a little more in detail. To change the figure, +imagine the sun in its voyage through space to be like a majestic battleship +surrounded by its scouts. Small vessels (the comets, as they are overhauled by +the squadron, are taken in charge by the scouts, with Jupiter for their chief, +and are forced to accompany the fleet, but not all are impressed. If a strange +comet undertakes to run across Jupiter’s bows the latter brings it to, and +makes prize of it by throwing it into a relatively small ellipse with the sun +for its focus. Thenceforth, unless, as happened to the unhappy comet of Lexell, +it encounters Jupiter again in such a way as to be diverted by him into a more +distant orbit, it can never get away. About thirty comets are now known to have +thus been captured by the great planet, and they are called “Jupiter’s Comet +Family.” But, on the other hand, if a wandering comet crosses the wake of the +chief planetary scout the latter simply drives it away by accelerating its +motion and compels it to steer off into open space. The transformation of +comets into meteors will be considered in the next chapter, but here, in +passing, mention may be made of the strange fate of one member of Jupiter’s +family, Biela’s comet, which, having become over bold in its advances to its +captor, was, after a few revolutions in is impressed orbit, torn to pieces and +turned into a flock of meteors. +</p> + +<p> +And now let us return to the mystery of comets’ tails. That we are fully +justified in speaking of the tails of comets as mysterious is proved by the +declaration of Sir John Herschel, who averred, in so many words, that “there is +some profound secret and mystery of nature concerned in this phenomenon,” and +this profound secret and mystery has not yet been altogether cleared up. +Nevertheless, the all-explaining hypothesis of Arrhenius offers us once more a +certain amount of aid. Comets’ tails, Arrhenius assures us, are but another +result of the pressure of light. The reader will recall the applications of +this theory to the Zodiacal Light and the Aurora. In the form in which we now +have to deal with it, the supposition is made that as a comet approaches the +sun eruptions of vapor, due to the solar heat, occur in its nucleus. These are +naturally most active on the side which is directly exposed to the sun, whence +the appearance of the immense glowing envelopes that surround the nucleus on +the sunward side. Among the particles of hydro-carbon, and perhaps solid carbon +in the state of fine dust, which are thus set free there will be many whose +size is within the critical limit which enables the light-waves from the sun to +drive them away. Clouds of such particles, then, will stream off behind the +advancing comet, producing the appearance of a tail. This accounts for the fact +that the tails of comets are always directed away from the sun, and it also +explains the varying forms of the tails and the extraordinary changes that they +undergo. The speed of the particles driven before the light-waves must depend +upon their size and weight, the lightest of a given size traveling the most +swiftly. By accretion certain particles might grow, thus losing velocity and +producing the appearance of bunches in the tail, such as have been observed. +The hypothesis also falls in with the researches of Bredichin, who has divided +the tails of comets into three principal classes—<i>viz.:</i> (1) Those which +appear as long, straight rays; (2) Those which have the form of curved plumes +or scimitars; (3) Those which are short, brushy, and curved sharply backward +along the comet’s path. In the first type he calculates the repulsive force at +from twelve to fifteen times the force of gravity; in the second at from two to +four times; and in the third at about one and a half times. The straight tails +he ascribes to hydrogen because the hydrogen atom is the lightest known; the +sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized iron. It +will be seen that, if the force driving off the tails is that which Arrhenius +assumes it to be, the forms of those appendages would accord with those that +Bredichin’s theory calls for. At the same time we have an explanation of the +multiple tails with which some comets have adorned themselves. The comet of +1744, for instance, had at one time no less than seven tails spread in a wide +curved brush behind it. Donati’s comet of 1858 also had at least two tails, the +principal one sword-shaped and the other long, narrow, and as straight as a +rule. According to Bredichin, the straight tail must have been composed of +hydrogen, and the other of some form of hydro-carbon whose atoms are heavier +than those of hydrogen, and, consequently, when swept away by the storm of +light-waves, followed a curvature depending upon the resultant of the forces +operating upon them. The seven tails of the comet of 1744 presented a kind of +diagram graphically exhibiting its complex composition, and, if we knew a +little more about the constituents of a comet, we might be able to say from the +amount of curvature of the different tails just what were the seven substances +of which that comet consisted. +</p> + +<p> +If these theories seem to the reader fantastic, at any rate they are no more +fantastic than the phenomena that they seek to explain. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap11"></a>XI<br/> +Meteors, Fire-Balls, and Meteorites</h2> + +<p> +One of the most terrorizing spectacles with which the heavens have ever caused +the hearts of men to quake occurred on the night of November 13, 1833. On that +night North America, which faced the storm, was under a continual rain of fire +from about ten o’clock in the evening until daybreak. +</p> + +<p> +<i>The fragments of a comet had struck the earth.</i> +</p> + +<p> +But the meaning of what had happened was not discovered until long afterward. +To the astronomers who, with astonishment not less than that of other people, +watched the wonderful scene, it was an unparalleled “shower of meteors.” They +did not then suspect that those meteors had once formed the head of a comet. +Light dawned when, a year later, Prof. Denison Olmsted, of Yale College, +demonstrated that the meteors had all moved in parallel orbits around the sun, +and that these orbits intersected that of the earth at the point where our +planet happened to be on the memorable night of November 13th. Professor +Olmsted even went so far as to suggest that the cloud of meteors that had +encountered the earth might form a diffuse comet; but full recognition of the +fact that they were cometary débris came later, as the result of further +investigation. The key to the secret was plainly displayed in the spectacle +itself, and was noticed without being understood by thousands of the +terror-stricken beholders. It was <i>an umbrella of fire</i> that had opened +overhead and covered the heavens; in other words, the meteors all radiated from +a particular point in the constellation Leo, and, being countless as the +snowflakes in a winter tempest, they ribbed the sky with fiery streaks. +Professor Olmsted showed that the radiation of the meteors from a fixed point +was an effect of perspective, and in itself a proof that they were moving in +parallel paths when they encountered the earth. The fact was noted that there +had been a similar, but incomparably less brilliant, display of meteors on the +same day of November, 1832, and it was rightly concluded that these had +belonged to the same stream, although the true relationship of the phenomena +was not immediately apprehended. Olmsted ascribed to the meteors a revolution +about the sun once in every six months, bringing them to the intersection of +their orbit with that of the earth every November 13th; but later investigators +found that the real period was about thirty-three and one-quarter years, so +that the great displays were due three times in a century, and their return was +confidently predicted for the year 1866. The appearance of the meteors in 1832, +a year before the great display, was ascribed to the great length of the stream +which they formed in space—so great that they required more than two years to +cross the earth’s orbit. In 1832 the earth had encountered a relatively rare +part of the stream, but in 1833, on returning to the crossing-place, it found +there the richest part of the stream pouring across its orbit. This explanation +also proved to be correct, and the predicted return in 1866 was duly witnessed, +although the display was much less brilliant than in 1833. It was followed by +another in 1867. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus26"></a> +<a href="images/image26.jpg"> +<img src="images/image26.jpg" width="600" height="459" alt="Illustration:" /></a> +<p class="caption">Curious forms of meteorite trains<br/> +Nos. 1 to 6 show the changes undergone by a train left by a meteorite which passed near the “Great Dipper”; +7 shows the changes and drift of a train seen in the constellation Virgo; 8 is the singular train +of the meteorite of February 22, 1909, near the Pole Star. (From <i>La Nature</i>.)</p> +</div> + +<p> +In the mean time Olmsted’s idea of a cometary relationship of the meteors was +demonstrated to be correct by the researches of Schiaparelli and others, who +showed that not only the November meteors, but those of August, which are seen +more or less abundantly every year, traveled in the tracks of well-known +comets, and had undoubtedly an identical origin with those comets. In other +words the comets and the meteor-swarms were both remnants of original masses +which had probably been split up by the action of the sun, or of some planet to +which they had made close approaches. The annual periodicity of the August +meteors was ascribed to the fact that the separation had taken place so long +ago that the meteors had become distributed all around the orbit, in +consequence of which the earth encountered some of them every year when it +arrived at the crossing-point. Then Leverrier showed that the original comet +associated with the November meteors was probably brought into the system by +the influence of the planet Uranus in the year 126 of the Christian era. +Afterward Alexander Herschel identified the tracks of no less than seventy-six +meteor-swarms (most of them inconspicuous) with those of comets. The still more +recent researches of Mr W. F. Denning make it probable that there are no +meteors which do not belong to a flock or system probably formed by the +disintegration of a cometary mass; even the apparently sporadic ones which +shoot across the sky, “lost souls in the night,” being members of flocks which +have become so widely scattered that the earth sometimes takes weeks to pass +through the region of space where their paths lie. +</p> + +<p> +The November meteors should have exhibited another pair of spectacles in 1899 +and 1900, and their failure to do so caused at first much disappointment, until +it was made plain that a good reason existed for their absence. It was found +that after their last appearance, in 1867, they had been disturbed in their +movements by the planets Jupiter and Saturn, whose attractions had so shifted +the position of their orbit that it no longer intersected that of the earth, as +it did before. Whether another planetary interference will sometime bring the +principal mass of the November meteors back to the former point of intersection +with the earth’s orbit is a question for the future to decide. It would seem +that there may be several parallel streams of the November meteors, and that +some of them, like those of August, are distributed entirely around the orbit, +so that every mid-November we see a few of them. +</p> + +<p> +We come now to a very remarkable example of the disintegration of a comet and +the formation of a meteor-stream. In 1826 Biela, of Josephstadt, Austria, +discovered a comet to which his name was given. Calculation showed that it had +an orbital period of about six and a half years, belonging to Jupiter’s +“family.” On one of its returns, in 1846, it astonished its watchers by +suddenly splitting in two. The two comets thus formed out of one separated to a +distance of about one hundred and sixty thousand miles, and then raced side by +side, sometimes with a curious ligature connecting them, like Siamese twins, +until they disappeared together in interplanetary space. In 1852 they came +back, still nearly side by side, but now the distance between them had +increased to a million and a quarter of miles. After that, at every recurrence +of their period, astronomers looked for them in vain, until 1872, when an +amazing thing happened. On the night of November 28th, when the earth was +crossing the plane of the orbit of the missing comet, a brilliant shower of +meteors burst from the northern sky, traveling nearly in the track which the +comet should have pursued. The astronomers were electrified. Klinkerfues, of +Göttingen, telegraphed to Pogson, of Madras: <i>“Biela touched earth; search +near Theta Centauri.”</i> Pogson searched in the place indicated and saw a +cometary mass retreating into the southern heavens, where it was soon swallowed +from sight! +</p> + +<div class="fig" style="width:100%;"> +<a name="illus27"></a> +<a href="images/image27.jpg"> +<img src="images/image27.jpg" width="151" height="700" alt="Illustration:" /></a> +<p class="caption">Section of the atmosphere up to 100 kilometers.<br/> +Showing the mean elevation at which meteorites and meteors make their +appearance. Below are shown the elevation of Mount Everest, the highest manned +balloon ascent by M. Berson; the height of cirrus clouds; the highest free +balloon ascent; and the elevation attained by the clouds of fire-dust ejected +by the Krakatoa eruption in 1883. (From <i>La Nature</i>) +</p> +</div> + +<p> +Since then the Biela meteors have been among the recognized periodic spectacles +of the sky, and few if any doubt that they represent a portion of the missing +comet whose disintegration began with the separation into two parts in 1846. +The comet itself has never since been seen. The first display of these meteors, +sometimes called the “Andromedes,” because they radiate from the constellation +Andromeda, was remarkable for the great brilliancy of many of the fire-balls +that shot among the shower of smaller sparks, some of which were described as +equaling the full moon in size. None of them is known to have reached the +earth, but during the display of the same meteors in 1885 a meteoric mass fell +at Mazapil in Northern Mexico (it is now in the Museum at Vienna), which many +have thought may actually be a piece of the original comet of Biela. This +brings us to the second branch of our subject. +</p> + +<p> +More rare than meteors or falling stars, and more startling, except that they +never appear in showers, are the huge balls of fire which occasionally dart +through the sky, lighting up the landscapes beneath with their glare, leaving +trains of sparks behind them, often producing peals of thunder when they +explode, and in many cases falling upon the earth and burying themselves from a +few inches to several feet in the soil, from which, more than once, they have +been picked up while yet hot and fuming. These balls are sometimes called +bolides. They are not really round in shape, although they often look so while +traversing the sky, but their forms are fragmentary, and occasionally +fantastic. It has been supposed that their origin is different from that of the +true meteors; it has even been conjectured that they may have originated from +the giant volcanoes of the moon or have been shot out from the sun during some +of the tremendous explosions that accompany the formation of eruptive +prominences. By the same reasoning some of them might be supposed to have come +from some distant star. Others have conjectured that they are wanderers in +space, of unknown origin, which the earth encounters as it journeys on, and +Lord Kelvin made a suggestion which has become classic because of its +imaginative reach—<i>viz.,</i> that the first germs of life may have been +brought to the earth by one of these bodies, “a fragment of an exploded world.” +</p> + +<p> +It is a singular fact that astronomers and scientific men in general were among +the last to admit the possibility of solid masses falling from the sky. The +people had believed in the reality of such phenomena from the earliest times, +but the savants shook their heads and talked of superstition. This was the less +surprising because no scientifically authenticated instance of such an +occurrence was known, and the stones popularly believed to have fallen from the +sky had become the objects of worship or superstitious reverence, a fact not +calculated to recommend them to scientific credence. The celebrated “black +stone” suspended in the Kaaba at Mecca is one of these reputed gifts from +heaven; the “Palladium” of ancient Troy was another; and a stone which fell +near Ensisheim, in Germany, was placed in a church as an object to be +religiously venerated. Many legends of falling stones existed in antiquity, +some of them curiously transfigured by the imagination, like the “Lion of the +Peloponnesus,” which was said to have sprung down from the sky upon the Isthmus +of Corinth. But near the beginning of the nineteenth century, in 1803, a +veritable shower of falling stones occurred at L’Aigle, in Northern France, and +this time astronomers took note of the phenomenon and scientifically +investigated it. Thousands of the strange projectiles came from the sky on this +occasion, and were scattered over a wide area of country, and some buildings +were hit. Four years later another shower of stones occurred at Weston, Conn., +numbering thousands of individuals. The local alarm created in both cases was +great, as well it might be, for what could be more intimidating than to find +the blue vault of heaven suddenly hurling solid missiles at the homes of men? +After these occurrences it was impossible for the most skeptical to doubt any +longer, and the regular study of “aerolites,” or “meteorites,” began. +</p> + +<p> +One of the first things recognized was the fact that fire-balls are solid +meteorites in flight, and not gaseous exhalations in the air, as some had +assumed. They burn in the air during their flight, and sometimes, perhaps, are +entirely consumed before reaching the ground. Their velocity before entering +the earth’s atmosphere is equal to that of the planets in their +orbits—<i>viz.,</i> from twenty to thirty miles per second—a fact which proves +that the sun is the seat of the central force governing them. Their burning in +the air is not difficult to explain; it is the heat of friction which so +quickly brings them to incandescence. Calculation shows that a body moving +through the air at a velocity of about a mile per second will be brought, +superficially, to the temperature of “red heat” by friction with the +atmosphere. If its velocity is twenty miles per second the temperature will +become thousands of degrees. This is the state of affairs with a meteorite +rushing into the earth’s atmosphere; its surface is liquefied within a few +seconds after the friction begins to act, and the melted and vaporized portion +of its mass is swept backward, forming the train of sparks that follows every +great fire-ball. However, there is one phenomenon connected with the trains of +meteorites which has never been satisfactorily explained: they often persist +for long periods of time, drifting and turning with the wind, but not ceasing +to glow with a phosphorescent luminosity. The question is, Whence comes this +light? It must be light without heat, since the fine dust or vapor of which the +train can only consist would not retain sufficient heat to render it luminous +for so long a time. An extremely remarkable incident of this kind occurred on +February 22, 1909, when an immense fire-ball that passed over southern England +left a train that remained visible during two hours, assuming many curious +shapes as it was drifted about by currents in the air. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus28"></a> +<a href="images/image28.jpg"> +<img src="images/image28.jpg" width="474" height="600" alt="Illustration:" /></a> +<p class="caption">A meteor photographed in flight</p> +</div> + +<p> +But notwithstanding the enormous velocity with which meteorites enter the air +they are soon slowed down to comparatively moderate speed, so that when they +disappear they are usually traveling not faster than a mile a second. The +courses of many have been traced by observers situated along their track at +various points, and thus a knowledge has been obtained of their height above +the ground during their flight and of the length of their visible courses. They +generally appear at an elevation of eighty or a hundred miles, and are seldom +visible after having descended to within five miles of the ground, unless the +observer happens to be near the striking-point, when he may actually witness +the fall. Frequently they burst while high in the air and their fragments are +scattered like shrapnel over the surface of the ground, sometimes covering an +area of several square miles, but of course not thickly; different fragments of +the same meteorite may reach the ground at points several miles apart. The +observed length of their courses in the atmosphere varies from fifty to five +hundred miles. If they continued a long time in flight after entering the air, +even the largest of them would probably be consumed to the last scrap, but +their fiery career is so short on account of their great speed that the heat +does not have time to penetrate very deeply, and some that have been picked up +immediately after their fall have been found cold as ice within. Their size +after reaching the ground is variable within wide limits; some are known which +weigh several tons, but the great majority weigh only a few pounds and many +only a few ounces. +</p> + +<p> +Meteorites are of two kinds: <i>stony</i> meteorites and <i>iron</i> +meteorites. The former outnumber the latter twenty to one; but many stone +meteorites contain grains of iron. Nickel is commonly found in iron meteorites, +so that it might be said that that redoubtable alloy nickel-steel is of +cosmical invention. Some twenty-five chemical elements have been found in +meteorites, including carbon and the “sun-metal,” helium. The presence of the +latter is certainly highly suggestive in connection with the question of the +origin of meteorites. The iron meteorites, besides metallic iron and nickel, of +which they are almost entirely composed, contain hydrogen, helium, and carbonic +oxide, and about the only imaginable way in which these gases could have become +absorbed in the iron would be through the immersion of the latter while in a +molten or vaporized state in a hot and dense atmosphere composed of them, a +condition which we know to exist only in the envelopes of the sun and the +stars. +</p> + +<p> +The existence of carbon in the Canyon Diablo iron meteorites is attended by a +circumstance of the most singular character—a very “fairy tale of science.” In +some cases <i>the carbon has become diamond!</i> These meteoric diamonds are +very small; nevertheless, they are true diamonds, resembling in many ways the +little black gems produced by Moissan’s method with the aid of the electric +furnace. The fact that they are found embedded in these iron meteorites is +another argument in favor of the hypothesis of the solar or stellar origin of +the latter. To appreciate this it is necessary to recall the way in which +Moissan made his diamonds. It was by a combination of the effects of great +heat, great pressure, and sudden or rapid superficial cooling on a mass of iron +containing carbon. When he finally broke open his iron he found it a pudding +stuffed with miniature black diamonds. When a fragment of the Canyon Diablo +meteoric iron was polished in Philadelphia over fifteen years ago it cut the +emery-wheel to pieces, and examination showed that the damage had been effected +by microscopic diamonds peppered through the mass. How were those diamonds +formed? If the sun or Sirius was the laboratory that prepared them, we can get +a glimpse at the process of their formation. There is plenty of heat, plenty of +pressure, and an abundance of vaporized iron in the sun and the stars. When a +great solar eruption takes place, masses of iron which have absorbed carbon may +be shot out with a velocity which forbids their return. Plunged into the +frightful cold of space, their surfaces are quickly cooled, as Moissan cooled +his prepared iron by throwing it into water, and thus the requisite stress is +set up within, and, as the iron solidifies, the included carbon crystallizes +into diamonds. Whether this explanation has a germ of truth in it or not, at +any rate it is evident that iron meteorites were not created in the form in +which they come to us; they must once have been parts of immeasurably more +massive bodies than themselves. +</p> + +<p> +The fall of meteorites offers an appreciable, though numerically insignificant, +peril to the inhabitants of the earth. Historical records show perhaps three or +four instances of people being killed by these bodies. But for the protection +afforded by the atmosphere, which acts as a very effective shield, the danger +would doubtless be very much greater. In the absence of an atmosphere not only +would more meteorites reach the ground, but their striking force would be +incomparably greater, since, as we have seen, the larger part of their original +velocity is destroyed by the resistance of the air. A meteorite weighing many +tons and striking the earth with a velocity of twenty or thirty miles per +second, would probably cause frightful havoc. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus29"></a> +<a href="images/image29.jpg"> +<img src="images/image29.jpg" width="600" height="406" alt="Illustration:" /></a> +<p class="caption">Looking across Coon Butte crater from northern rim</p> +</div> + +<p> +It is a singular fact that recent investigations seem to have proved that an +event of this kind actually happened in North America—perhaps not longer than a +thousand or two thousand years ago. The scene of the supposed catastrophe is in +northern central Arizona, at Coon Butte, where there is a nearly circular +crater in the middle of a circular elevation or small mountain. The crater is +somewhat over four thousand feet in diameter, and the surrounding rim, formed +of upturned strata and ejected rock fragments, rises at its highest point one +hundred and sixty feet above the plain. The crater is about six hundred feet in +depth—that is, from the rim to the visible floor or bottom of the crater. There +is no evidence that volcanic action has ever taken place in the immediate +neighborhood of Coon Butte. The rock in which the crater has been made is +composed of horizontal sandstone and limestone strata. Between three hundred +and four hundred million tons of rock fragments have been detached, and a large +portion hurled by some cause out of the crater. These fragments lie +concentrically distributed around the crater, and in large measure form the +elevation known as Coon Butte. The region has been famous for nearly twenty +years on account of the masses of meteoric iron found scattered about and known +as the “Canyon Diablo” meteorites. It was one of these masses, which consist of +nickel-iron containing a small quantity of platinum, and of which in all some +ten tons have been recovered for sale to the various collectors throughout the +world, that as before mentioned destroyed the grinding-tool at Philadelphia +through the cutting power of its embedded diamonds. These meteoric irons are +scattered about the crater-hill, in concentric distribution, to a maximum +distance of about five miles. When the suggestion was first made in 1896 that a +monster meteorite might have created by its fall this singular lone crater +<i>in stratified rocks,</i> it was greeted with incredulous smiles; but since +then the matter has assumed a different aspect. The Standard Iron Company, +formed by Messrs. D. M. Barringer, B. C. Tilghman, E. J. Bennitt, and S. J. +Holsinger, having become, in 1903, the owner of this freak of nature, sunk +shafts and bored holes to a great depth in the interior of the crater, and also +trenched the slopes of the mountain, and the result of their investigations has +proved that the meteoric hypothesis of origin is correct. (See the papers +published in the <i>Proceedings of the Academy of Natural Sciences of +Philadelphia,</i> December, 1905, wherein it is proved that the United States +Geological Survey was wrong in believing this crater to have been due to a +steam explosion. Since that date there has been discovered a great amount of +additional confirmatory proof). Material of unmistakably meteoric origin was +found by means of the drills, mixed with crushed rock, to a depth of six +hundred to seven hundred feet below the floor of the crater, and a great deal +of it has been found admixed with the ejected rock fragments on the outer +slopes of the mountain, absolutely proving synchronism between the two events, +the formation of this great crater and the falling of the meteoric iron out of +the sky. The drill located in the bottom of the crater was sent, in a number of +cases, much deeper (over one thousand feet) into unaltered horizontal red +sandstone strata, but no meteoric material was found below this depth (seven +hundred feet, or between eleven and twelve hundred feet below the level of the +surrounding plain), which has been assumed as being about the limit of +penetration. It is not possible to sink a shaft at present, owing to the water +which has drained into the crater, and which forms, with the finely pulverized +sandstone, a very troublesome quicksand encountered at about two hundred feet +below the visible floor of the crater. As soon as this water is removed by +pumping it will be easy to explore the depths of the crater by means of shafts +and drifts. The rock strata (sandstone and limestone) of which the walls +consist present every appearance of having been violently upturned by a huge +body penetrating the earth like a cannon-ball. The general aspect of the crater +strikingly resembles the impression made by a steel projectile shot into an +armor-plate. Mr Tilghman has estimated that a meteorite about five hundred feet +in diameter and moving with a velocity of about five miles per second would +have made just such a perforation upon striking rocks of the character of those +found at this place. There was some fusion of the colliding masses, and the +heat produced some steam from the small amount of water in the rocks. As a +result there has been found at depth a considerable amount of fused quartz +(original sandstone), and with it innumerable particles or sparks of fused +nickel-iron (original meteorite). A projectile of that size penetrating eleven +to twelve hundred feet into the rocky shell of the globe must have produced a +shock which was perceptible several hundred miles away. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus30"></a> +<a href="images/image30.jpg"> +<img src="images/image30.jpg" width="600" height="405" alt="Illustration:" /></a> +<p class="caption">Trail on south side, Coon Butte crater</p> +</div> + +<p> +The great velocity ascribed to the supposed meteorite at the moment of striking +could be accounted for by the fact that it probably plunged nearly vertically +downward, for it formed a circular crater in the rocky crust of the earth. In +that case it would have been less retarded by the resistance of the atmosphere +than are meteorites which enter the air at a lower angle and shoot ahead +hundreds of miles until friction has nearly destroyed their original motion +when they drop upon the earth. Some meteoric masses of great size, such as +Peary’s iron meteorite found at Cape York, Greenland, and the almost equally +large mass discovered at Bacubirito, Mexico, appear to have penetrated but +slightly on striking the earth. This may be explained by supposing that they +pursued a long, horizontal course through the air before falling. The result +would be that, their original velocity having been practically destroyed, they +would drop to the ground with a velocity nearly corresponding to that which +gravity would impart within the perpendicular distance of their final fall. A +six-hundred-and-sixty-pound meteorite, which fell at Knyahinya, Hungary, +striking at an angle of 27° from the vertical, penetrated the ground to a +depth of eleven feet. +</p> + +<p> +It has been remarked that the Coon Butte meteorite may have fallen not longer +ago than a few thousand years. This is based upon the fact that the geological +indications favor the supposition that the event did not occur more than five +thousand years ago, while on the other hand the rings of growth in the +cedar-trees growing on the slopes of the crater show that they have existed +there about seven hundred years. Prof. William H. Pickering has recently +correlated this with an ancient chronicle which states that at Cairo, Egypt, in +the year 1029, “many stars passed with a great noise.” He remarks that Cairo is +about 100°, by great circle, from Coon Butte, so that if the meteorite that +made the crater was a member of a flock of similar bodies which encountered the +earth moving in parallel lines, some of them might have traversed the sky +tangent to the earth’s surface at Cairo. That the spectacle spoken of in the +chronicle was caused by meteorites he deems exceedingly probable because of +what is said about “a great noise;” meteorites are the only celestial phenomena +attended with perceptible sounds. Professor Pickering conjectures that this +supposed flock of great meteorites may have formed the nucleus of a comet which +struck the earth, and he finds confirmation of the idea in the fact that out of +the ten largest meteorites known, no less than seven were found within nine +hundred miles of Coon Butte. It would be interesting if we could trace back the +history of that comet, and find out what malicious planet caught it up in its +innocent wanderings and hurled it with so true an aim at the earth! This +remarkable crater is one of the most interesting places in the world, for there +is absolutely no record of such a mass, possibly an iron-headed comet, from +outer space having come into collision with our earth. The results of the +future exploration of the depths of the crater will be awaited with much +interest. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap12"></a>XII<br/> +The Wrecking of the Moon</h2> + +<p> +There are sympathetic moods under whose influence one gazes with a certain +poignant tenderness at the worn face of the moon; that little “fossil world” +(the child of our mother earth, too) bears such terrible scars of its brief +convulsive life that a sense of pity is awakened by the sight. The moon is the +wonder-land of the telescope. Those towering mountains, whose “proud aspiring +peaks” cast silhouettes of shadow that seem drawn with india-ink; those vast +plains, enchained with gentle winding hills and bordered with giant ranges; +those oval “oceans,” where one looks expectant for the flash of wind-whipped +waves; those enchanting “bays” and recesses at the seaward feet of the Alps; +those broad straits passing between guardian heights incomparably mightier than +Gibraltar; those locket-like valleys as secluded among their mountains as the +Vale of Cashmere; those colossal craters that make us smile at the pretensions +of Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with the +unconcern of Roman roads across mountain, gorge, and valley—all these give the +beholder an irresistible impression that it is truly a world into which he is +looking, a world akin to ours, and yet no more like our world than Pompeii is +like Naples. Its air, its waters, its clouds, its life are gone, and only a +skeleton remains—a mute but eloquent witness to a cosmical tragedy without +parallel in the range of human knowledge. +</p> + +<p> +One cannot but regret that the moon, if it ever was the seat of intelligent +life, has not remained so until our time. Think what the consequences would +have been if this other world at our very door had been found to be both +habitable and inhabited! We talk rather airily of communicating with Mars by +signals; but Mars never approaches nearer than 35,000,000 miles, while the moon +when nearest is only a little more than 220,000 miles away. Given an effective +magnifying power of five thousand diameters, which will perhaps be possible at +the mountain observatories as telescopes improve, and we should be able to +bring the moon within an apparent distance of about forty miles, while the +corresponding distance for Mars would be more than seven thousand miles. But +even with existing telescopic powers we can see details on the moon no larger +than some artificial constructions on the earth. St Peter’s at Rome, with the +Vatican palace and the great piazza, if existing on the moon, would +unquestionably be recognizable as something else than a freak of nature. Large +cities, with their radiating lines of communication, would at once betray their +real character. Cultivated tracts, and the changes produced by the interference +of intelligent beings, would be clearly recognizable. The electric illumination +of a large town at night would probably be markedly visible. Gleams of +reflected sunlight would come to us from the surfaces of the lakes and oceans, +and a huge “liner” traversing a lunar sea could probably be followed by its +trail of smoke. As to communications by “wireless” signals, which certain +enthusiasts have thought of in connection with Mars, in the case of the moon +they should be a relatively simple matter, and the feat might actually be +accomplished. Think what a literature would grow up about the moon if it were a +living world! Its very differences from the earth would only accentuate its +interest for us. Night and day on the moon are each two weeks in length; how +interesting it would be to watch the manner in which the lunarians dealt with +such a situation as that. Lunar and terrestrial history would keep step with +each other, and we should record them both. Truly one might well wish to have a +neighbor world to study; one would feel so much the less alone in space. +</p> + +<p> +It is not impossible that the moon did at one time have inhabitants of some +kind. But, if so, they vanished with the disappearance of its atmosphere and +seas, or with the advent of its cataclysmic age. At the best, its career as a +living world must have been brief. If the water and air were gradually +absorbed, as some have conjectured, by its cooling interior rocks, its surface +might, nevertheless, have retained them for long ages; but if, as others think, +their disappearance was due to the escape of their gaseous molecules in +consequence of the inability of the relatively small lunar gravitation to +retain them, then the final catastrophe must have been as swift as it was +inevitable. Accepting Darwin’s hypothesis, that the moon was separated from the +earth by tidal action while both were yet plastic or nebulous, we may +reasonably conclude that it began its career with a good supply of both water +and air, but did not possess sufficient mass to hold them permanently. Yet it +may have retained them long enough for life to develop in many forms upon its +surface; in fact, there are so many indications that air and water have not +always been lacking to the lunar world that we are driven to invent theories to +explain both their former presence and their present absence. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus31"></a> +<a href="images/image31.jpg"> +<img src="images/image31.jpg" width="405" height="600" alt="Illustration:" /></a> +<p class="caption">The craters Clavius, Longomontanus, Tycho, etc.</p> +</div> + +<p> +But whatever the former condition of the moon may have been, its existing +appearance gives it a resistless fascination, and it bears so clearly the story +of a vast catastrophe sculptured on its rocky face that the thoughtful observer +cannot look upon it without a feeling of awe. The gigantic character of the +lunar features impresses the beholder not less than the universality of the +play of destructive forces which they attest. Let us make a few comparisons. +Take the lunar crater called “Tycho”, which is a typical example of its kind. +In the telescope Tycho appears as a perfect ring surrounding a circular +depression, in the center of which rises a group of mountains. Its superficial +resemblance to some terrestrial volcanic craters is very striking. Vesuvius, +seen from a point vertically above, would no doubt look something like that +(the resemblance would have been greater when the Monte del Cavallo formed a +more complete circuit about the crater cone). But compare the dimensions. The +remains of the outer crater ring of Vesuvius are perhaps half a mile in +diameter, while the active crater itself is only two or three hundred feet +across at the most; Tycho has a diameter of fifty-four miles! The group of +relatively insignificant peaks in the center of the crater floor of Tycho is +far more massive than the entire mountain that we call Vesuvius. The largest +known volcanic crater on the earth, Aso San, in Japan, has a diameter of seven +miles; it would take <i>sixty</i> craters like Aso San to equal Tycho in area! +And Tycho, though one of the most perfect, is by no means the largest crater on +the moon. Another, called “Theophilus,” has a diameter of sixty-four miles, and +is eighteen thousand feet deep. There are hundreds from ten to forty miles in +diameter, and thousands from one to ten miles. They are so numerous in many +places that they break into one another, like the cells of a crushed honeycomb. +</p> + +<p> +The lunar craters differ from those of the earth more fundamentally than in the +matter of mere size; <i>they are not situated on the tops of mountains.</i> If +they were, and if all the proportions were the same, a crater like Tycho might +crown a conical peak fifty or one hundred miles high! Instead of being cavities +in the summits of mountains, the lunar craters are rather gigantic sink-holes +whose bottoms in many cases lie two or three miles below the general surface of +the lunar world. Around their rims the rocks are piled up to a height of from a +few hundred to two or three thousand feet, with a comparatively gentle +inclination, but on the inner side they fall away in gigantic broken precipices +which make the dizzy cliffs of the Matterhorn seem but “lover’s leaps.” Down +they drop, ridge below ridge, crag under crag, tottering wall beneath wall, +until, in a crater named “Newton,” near the south lunar pole, they attain a +depth where the rays of the sun never reach. Nothing more frightful than the +spectacle which many of these terrible chasms present can be pictured by the +imagination. As the lazy lunar day slowly advances, the sunshine, unmitigated +by clouds or atmospheric veil of any kind, creeps across their rims and begins +to descend the opposite walls. Presently it strikes the ragged crest of a ridge +which had lain hidden in such darkness as we never know on the earth, and runs +along it like a line of kindling fire. Rocky pinnacles and needles shoot up +into the sunlight out of the black depths. Down sinks the line of light, mile +after mile, and continually new precipices and cliffs are brought into view, +until at last the vast floor is attained and begins to be illuminated. In the +meanwhile the sun’s rays, darting across the gulf, have touched the summits of +the central peaks, twenty or thirty miles from the crater’s inmost edge, and +they immediately kindle and blaze like huge stars amid the darkness. So +profound are some of these awful craters that days pass before the sun has +risen high enough above them to chase the last shadows from their depths. +</p> + +<p> +Although several long ranges of mountains resembling those of the earth exist +on the moon, the great majority of its elevations assume the crateriform +aspect. Sometimes, instead of a crater, we find an immense mountain ring whose +form and aspect hardly suggest volcanic action. But everywhere the true craters +are in evidence, even on the sea-beds, although they attain their greatest +number and size on those parts of the moon—covering sixty per cent of its +visible surface—which are distinctly mountainous in character and which +constitute its most brilliant portions. Broadly speaking, the southwestern half +of the moon is the most mountainous and broken, and the northeastern half the +least so. Right down through the center, from pole to pole, runs a wonderful +line of craters and crateriform valleys of a magnitude stupendous even for the +moon. Another similar line follows the western edge. Three or four “seas” are +thrust between these mountainous belts. By the effects of “libration” parts of +the opposite hemisphere of the moon which is turned away from the earth are +from time to time brought into view, and their aspect indicates that that +hemisphere resembles in its surface features the one which faces the earth. +There are many things about the craters which seem to give some warrant for the +hypothesis which has been particularly urged by Mr G. K. Gilbert, that they +were formed by the impact of meteors; but there are also many things which +militate against that idea, and, upon the whole, the volcanic theory of their +origin is to be preferred. +</p> + +<p> +The enormous size of the lunar volcanoes is not so difficult to account for +when we remember how slight is the force of lunar gravity as compared with that +of the earth. With equal size and density, bodies on the moon weigh only +one-sixth as much as on the earth. Impelled by the same force, a projectile +that would go ten miles on the earth would go sixty miles on the moon. A lunar +giant thirty-five feet tall would weigh no more than an ordinary son of Adam +weighs on his greater planet. To shoot a body from the earth so that it would +not drop back again, we should have to start it with a velocity of seven miles +per second; a mile and a half per second would serve on the moon. It is by no +means difficult to believe, then, that a lunar volcano might form a crater ring +eight or ten times broader than the greatest to be found on the earth, +especially when we reflect that in addition to the relatively slight force of +gravity, the materials of the lunar crust are probably lighter than those of +our terrestrial rocks. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus32"></a> +<a href="images/image32.jpg"> +<img src="images/image32.jpg" width="409" height="600" alt="Illustration:" /></a> +<p class="caption">Western part of the Mare Serenitatis</p> +</div> + +<p> +For similar reasons it seems not impossible that the theory mentioned in a +former chapter—that some of the meteorites that have fallen upon the earth +originated from the lunar volcanoes—is well founded. This would apply +especially to the stony meteorites, for it is hardly to be supposed that the +moon, at least in its superficial parts, contains much iron. It is surely a +scene most strange that is thus presented to the mind’s eye—that little +attendant of the earth’s (the moon has only one-fiftieth of the volume, and +only one-eightieth of the mass of the earth) firing great stones back at its +parent planet! And what can have been the cause of this furious outbreak of +volcanic forces on the moon? Evidently it was but a passing stage in its +history; it had enjoyed more quiet times before. As it cooled down from the +plastic state in which it parted from the earth, it became incrusted after the +normal manner of a planet, and then oceans were formed, its atmosphere being +sufficiently dense to prevent the water from evaporating and the would-be +oceans from disappearing continually in mist. This, if any, must have been the +period of life in the lunar world. As we look upon the vestiges of that ancient +world buried in the wreck that now covers so much of its surface, it is +difficult to restrain the imagination from picturing the scenes which were once +presented there; and, in such a case, should the imagination be fettered? We +give it free rein in terrestrial life, and it rewards us with some of our +greatest intellectual pleasures. The wonderful landscapes of the moon offer it +an ideal field with just enough half-hidden suggestions of facts to stimulate +its powers. +</p> + +<p> +The great plains of the <i>Mare Imbrium</i> and the <i>Mare Serenitatis</i> +(the “Sea of Showers” and the “Sea of Serenity”), bordered in part by lofty +mountain ranges precisely like terrestrial mountains, scalloped along their +shores with beautiful bays curving back into the adjoining highlands, and +united by a great strait passing between the nearly abutting ends of the “Lunar +Apennines” and the “Lunar Caucasus,” offer the elements of a scene of world +beauty such as it would be difficult to match upon our planet. Look at the +finely modulated bottom of the ancient sea in Mr Ritchey’s exquisite photograph +of the western part of the <i>Mare Serenitatis,</i> where one seems to see the +play of the watery currents heaping the ocean sands in waving lines, making +shallows, bars, and deeps for the mariner to avoid or seek, and affording a +playground for the creatures of the main. What geologist would not wish to try +his hammer on those rocks with their stony pages of fossilized history? There +is in us an instinct which forbids us to think that there was never any life +there. If we could visit the moon, there is not among us a person so prosaic +and unimaginative that he would not, the very first thing, begin to search for +traces of its inhabitants. We would look for them in the deposits on the sea +bottoms; we would examine the shores wherever the configuration seemed +favorable for harbors and the sites of maritime cities—forgetting that it may +be a little ridiculous to ascribe to the ancient lunarians the same ideas that +have governed the development of our race; we would search through the valleys +and along the seeming courses of vanished streams; we would explore the +mountains, not the terrible craters, but the pinnacled chains that recall our +own Alps and Rockies; seeking everywhere some vestige of the transforming +presence of intelligent life. Perhaps we should find such traces, and perhaps, +with all our searching, we should find nothing to suggest that life had ever +existed amid that universal ruin. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus33"></a> +<a href="images/image33.jpg"> +<img src="images/image33.jpg" width="421" height="600" alt="Illustration:" /></a> +<p class="caption">Mare Tranquilitatis and surroundings</p> +</div> + +<p> +Look again at the border of the “Sea of Serenity”—what a name for such a +scene!—and observe how it has been rent with almost inconceivable violence, the +wall of the colossal crater Posidonius dropping vertically upon the ancient +shore and obliterating it, while its giant neighbor, Le Monnier, opens a +yawning mouth as if to swallow the sea itself. A scene like this makes one +question whether, after all, those may not be right who have imagined that the +so-called sea bottoms are really vast plains of frozen lava which gushed up in +floods so extensive that even the mighty volcanoes were half drowned in the +fiery sea. This suggestion becomes even stronger when we turn to another of the +photographs of Mr Ritchey’s wonderful series, showing a part of the <i>Mare +Tranquilitatis</i> (“Sea of Tranquility”!). Notice how near the center of the +picture the outline of a huge ring with radiating ridges shows through the sea +bottom; a fossil volcano submerged in a petrified ocean! This is by no means +the only instance in which a buried world shows itself under the great lunar +plains. Yet, as the newer craters in the sea itself prove, the volcanic +activity survived this other catastrophe, or broke out again subsequently, +bringing more ruin to pile upon ruin. +</p> + +<p> +Yet notwithstanding the evidence which we have just been considering in support +of the hypothesis that the “seas” are lava floods, Messrs. Loewy and Puiseux, +the selenographers of the Paris Observatory, are convinced that these great +plains bear characteristic marks of the former presence of immense bodies of +water. In that case we should be forced to conclude that the later oceans of +the moon lay upon vast sheets of solidified lava; and thus the catastrophe of +the lunar world assumes a double aspect, the earliest oceans being swallowed up +in molten floods issuing from the interior, while the lands were reduced to +chaos by a universal eruption of tremendous volcanoes; and then a period of +comparative quiet followed, during which new seas were formed, and new life +perhaps began to flourish in the lunar world, only to end in another cataclysm, +which finally put a term to the existence of the moon as a life-supporting +world. +</p> + +<p> +Suppose we examine two more of Mr Ritchey’s illuminating photographs, and, +first, the one showing the crater Theophilus and its surroundings. We have +spoken of Theophilus before, citing the facts that it is sixty-four miles in +diameter and eighteen thousand feet deep. It will be noticed that it has two +brother giants—Cyrillus the nearer, and Catharina the more distant; but +Theophilus is plainly the youngest of the trio. Centuries, and perhaps +thousands of years, must have elapsed between the periods of their upheaval, +for the two older craters are partly filled with débris, while it is manifest +at a glance that when the south eastern wall of Theophilus was formed, it broke +away and destroyed a part of the more ancient ring of Cyrillus. There is no +more tremendous scene on the moon than this; viewed with a powerful telescope, +it is absolutely appalling. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus34"></a> +<a href="images/image34.jpg"> +<img src="images/image34.jpg" width="425" height="600" alt="Illustration:" /></a> +<p class="caption">Lunar craters Theophilus and surrounding region</p> +</div> + +<p> +The next photograph shows, if possible, a still wilder region. It is the part +of the moon lying between Tycho and the south pole. Tycho is seen in the lower +left-hand part of the picture. To the right, at the edge of the illuminated +portion of the moon, are the crater-rings, Longomontanus and Wilhelm I, the +former being the larger. Between them are to be seen the ruins of two or three +more ancient craters which, together with portions of the walls of Wilhelm I +and Longomontanus, have been honeycombed with smaller craters. The vast +crateriform depression above the center of the picture is Clavius, an unrivaled +wonder of lunar scenery, a hundred and forty-two miles in its greatest length, +while its whole immense floor has sunk two miles below the general surface of +the moon outside the ring. The monstrous shadow-filled cavity above Clavius +toward the right is Blancanus, whose aspect here gives a good idea of the +appearance of these chasms when only their rims are in the sunlight. But +observe the indescribable savagery of the entire scene. It looks as though the +spirit of destruction had gone mad in this spot. The mighty craters have broken +forth one after another, each rending its predecessor; and when their work was +finished, a minor but yet tremendous outbreak occurred, and the face of the +moon was gored and punctured with thousands of smaller craters. These +relatively small craters (small, however, only in a lunar sense, for many of +them would appear gigantic on the earth) recall once more the theory of +meteoric impact. It does not seem impossible that some of them may have been +formed by such an agency. +</p> + +<p> +One would not wish for our planet such a fate as that which has overtaken the +moon, but we cannot be absolutely sure that something of the kind may not be in +store for it. We really know nothing of the ultimate causes of volcanic +activity, and some have suggested that the internal energies of the earth may +be accumulating instead of dying out, and may never yet have exhibited their +utmost destructive power. Perhaps the best assurance that we can find that the +earth will escape the catastrophe that has overtaken its satellite is to be +found in the relatively great force of its gravitation. The moon has been the +victim of its weakness; given equal forces, and the earth would be the better +able to withstand them. It is significant, in connection with these +considerations, that the little planet Mercury, which seems also to have parted +with its air and water, shows to the telescope some indications that it is +pitted with craters resembling those that have torn to pieces the face of the +moon. +</p> + +<p> +Upon the whole, after studying the dreadful lunar landscapes, one cannot feel a +very enthusiastic sympathy with those who are seeking indications of the +continued existence of some kind of life on the moon; such a world is better +without inhabitants. It has met its fate; let it go! Fortunately, it is not so +near that it cannot hide its scars and appear beautiful—except when curiosity +impels us to look with the penetrating eyes of the astronomer. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus35"></a> +<a href="images/image35.jpg"> +<img src="images/image35.jpg" width="484" height="600" alt="Illustration:" /></a> +<p class="caption">Mare Crisium</p> +</div> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap13"></a>XIII<br/> +The Great Mars Problem</h2> + +<p> +Let any thoughtful person who is acquainted with the general facts of astronomy +look up at the heavens some night when they appear in their greatest splendor, +and ask himself what is the strongest impression that they make upon his mind. +He may not find it easy to frame an answer, but when he has succeeded it will +probably be to the effect that the stars give him an impression of the +universality of intelligence; they make him feel, as the sun and the moon +cannot do, that his world is not alone; that all this was not made simply to +form a gorgeous canopy over the tents of men. If he is of a devout turn of +mind, he thinks, as he gazes into those fathomless deeps and among those +bewildering hosts, of the infinite multitude of created beings that the +Almighty has taken under his care. The narrow ideas of the old geocentric +theology, which made the earth God’s especial footstool, and man his only +rational creature, fall away from him like a veil that had obscured his vision; +they are impossible in the presence of what he sees above. Thus the natural +tendency, in the light of modern progress, is to regard the universe as +everywhere filled with life. +</p> + +<p> +But science, which is responsible for this broadening of men’s thoughts +concerning the universality of life, itself proceeds to set limits. Of +spiritual existences it pretends to know nothing, but as to physical beings, it +declares that it can only entertain the supposition of their existence where it +finds evidence of an environment suited to their needs, and such environment +may not everywhere exist. Science, though repelled by the antiquated +theological conception of the supreme isolation of man among created beings, +regards with complacency the probability that there are regions in the universe +where no organic life exists, stars which shine upon no inhabited worlds, and +planets which nourish no animate creatures. The astronomical view of the +universe is that it consists of matter in every stage of evolution: some +nebulous and chaotic; some just condensing into stars (suns) of every magnitude +and order; some shaped into finished solar bodies surrounded by dependent +planets; some forming stars that perhaps have no planets, and will have none; +some constituting suns that are already aging, and will soon lose their radiant +energy and disappear; and some aggregated into masses that long ago became +inert, cold, and rayless, and that can only be revivified by means about which +we can form conjectures, but of which we actually know nothing. +</p> + +<p> +As with the stars, so with the planets, which are the satellites of stars. All +investigations unite to tell us that the planets are not all in the same state +of development. As some are large and some small, so some are, in an +evolutionary sense, young, and some old. As they depend upon the suns around +which they revolve for their light, heat, and other forms of radiant energy, so +their condition varies with their distance from those suns. Many may never +arrive at a state suitable for the maintenance of life upon their surfaces; +some which are not at present in such a state may attain it later; and the +forms of life themselves may vary with the peculiar environment that different +planets afford. Thus we see that we are not scientifically justified in +affirming that life is ubiquitous, although we are thus justified in saying +that it must be, in a general sense, universal. We might liken the universe to +a garden known to contain every variety of plant. If on entering it we see no +flowers, we examine the species before us and find that they are not of those +which bloom at this particular season, or perhaps they are such as never bear +flowers. Yet we feel no doubt that we shall find flowers somewhere in the +garden, because there <i>are</i> species which bloom at this season, and the +garden contains <i>all</i> varieties. +</p> + +<p> +While it is tacitly assumed that there are planets revolving around other stars +than the sun, it would be impossible for us to see them with any telescope yet +invented, and no instrument now in the possession of astronomers could assure +us of their existence; so the only planetary system of which we have visual +knowledge is our own. Excluding the asteroids, which could not from any point +of view be considered as habitable, we have in the solar system eight planets +of various sizes and situated at various distances from the sun. Of these eight +we know that one, the earth, is inhabited. The question, then, arises: Are +there any of the others which are inhabited or habitable? Since it is our +intention to discuss the habitability of only one of the seven to which the +question applies, the rest may be dismissed in a few words. The smallest of +them, and the nearest to the sun, is Mercury, which is regarded as +uninhabitable because it has no perceptible supply of water and air, and +because, owing to the extraordinary eccentricity of its orbit, it is subjected +to excessive and very rapid alterations in the amount of solar heat and light +poured upon its surface, such alterations being inconsistent with the +supposition that it can support living beings. Even its average temperature is +more than six and a half times that prevailing on the earth! Another +circumstance which militates against its habitability is that, according to the +results of the best telescopic studies, it always keeps the same face toward +the sun, so that one half of the planet is perpetually exposed to the fierce +solar rays, and the other half faces the unmitigated cold of open space. Venus, +the next in distance from the sun, is almost the exact twin of the earth in +size, and many arguments may be urged in favor of its habitability, although it +is suspected of possessing the same peculiarity as Mercury, in always keeping +the same side sunward. Unfortunately its atmosphere appears to be so dense that +no permanent markings on its surface are certainly visible, and the question of +its actual condition must, for the present, be left in abeyance. Mars, the +first planet more distant from the sun than the earth, is the special subject +of this chapter, and will be described and discussed a few lines further on. +Jupiter, Saturn, Uranus, and Neptune, the four giant planets, all more distant +than Mars, and each more distant than the other in the order named, are all +regarded as uninhabitable because none of them appears to possess any degree of +solidity. They may have solid or liquid nuclei, but exteriorly they seem to be +mere balls of cloud. Of course, one can imagine what he pleases about the +existence of creatures suited to the physical constitution of such planets as +these, but they must be excluded from the category of habitable worlds in the +ordinary sense of the term. We go back, then, to Mars. +</p> + +<p> +It will be best to begin with a description of the planet. Mars is 4230 miles +in diameter; its surface is not much more than one-quarter as extensive as that +of the earth (.285). Its mean distance from the sun is 141,500,000 miles, +48,500,000 miles greater than that of the earth. Since radiant energy varies +inversely as the square of distance, Mars receives less than half as much solar +light and heat as the earth gets. Mars’ year (period of revolution round the +sun) is 687 days. Its mean density is 71 per cent of the earth’s, and the force +of gravity on its surface is 38 per cent of that on the surface of the earth; +<i>i.e.,</i> a body weighing one hundred pounds on the earth would, if +transported to Mars, weigh but thirty-eight pounds. The inclination of its +equator to the plane of its orbit differs very little from that of the earth’s +equator, and its axial rotation occupies 24 hours 37 minutes. so that the +length of day and night, and the extent of the seasonal changes on Mars, are +almost precisely the same as on the earth. But owing to the greater length of +its year, the seasons of Mars, while occurring in the same order, are almost +twice as long as ours. The surface of the planet is manifestly solid, like that +of our globe, and the telescope reveals many permanent markings on it, +recalling the appearance of a globe on which geographical features have been +represented in reddish and dusky tints. Around the poles are plainly to be seen +rounded white areas, which vary in extent with the Martian seasons, nearly +vanishing in summer and extending widely in winter. The most recent +spectroscopic determinations indicate that Mars has an atmosphere perhaps as +dense as that to be found on our loftiest mountain peaks, and there is a +perceptible amount of watery vapor in this atmosphere. The surface of the +planet appears to be remarkably level, and it has no mountain ranges. No +evidences of volcanic action have been discovered on Mars. The dusky and +reddish areas were regarded by the early observers as respectively seas and +lands, but at present it is not believed that there are any bodies of water on +the planet. There has never been much doubt expressed that the white areas +about the poles represent snow. +</p> + +<p> +It will be seen from this brief description that many remarkable resemblances +exist between Mars and the earth, and there is nothing wonderful in the fact +that the question of the habitability of the former has become one of extreme +and wide-spread interest, giving rise to the most diverse views, to many +extraordinary speculations, and sometimes to regrettably heated controversy. +The first champion of the habitability of Mars was Sir William Herschel, +although even before his time the idea had been suggested. He was convinced by +the revelations of his telescopes, continually increasing in power, that Mars +was more like the earth than any other planet. He could not resist the +testimony of the polar snows, whose suggestive conduct was in such striking +accord with what occurs upon the earth. Gradually, as telescopes improved and +observers increased in number, the principal features of the planet were +disclosed and charted, and “areography,” as the geography of Mars was called, +took its place among the recognized branches of astronomical study. But it was +not before 1877 that a fundamentally new discovery in areography gave a truly +sensational turn to speculation about life on “the red planet.” In that year +Mars made one of its nearest approaches to the earth, and was so situated in +its orbit that it could be observed to great advantage from the northern +hemisphere of the earth. The celebrated Italian astronomer, Schiaparelli, took +advantage of this opportunity to make a trigonometrical survey of the surface +of Mars—as coolly and confidently as if he were not taking his sights across a +thirty-five-million-mile gulf of empty space—and in the course of this survey +he was astonished to perceive that the reddish areas, then called continents, +were crossed in many directions by narrow, dusky lines, to which he gave the +suggestive name of “canals.” Thus a kind of firebrand was cast into the field +of astronomical speculation, which has ever since produced disputes that have +sometimes approached the violence of political faction. At first the accuracy +of Schiaparelli’s observations was contested; it required a powerful telescope, +and the most excellent “seeing,” to render the enigmatical lines visible at +all, and many searchers were unable to detect them. But Schiaparelli continued +his studies in the serene sky of Italy, and produced charts of the gridironed +face of Mars containing so much astonishing detail that one had either to +reject them <i>in toto</i> or to confess that Schiaparelli was right. As +subsequent favorable oppositions of Mars occurred, other observers began to see +the “canals” and to confirm the substantial accuracy of the Italian +astronomer’s work, and finally few were found who would venture to affirm that +the “canals” did not exist, whatever their meaning might be. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus36"></a> +<a href="images/image36.jpg"> +<img src="images/image36.jpg" width="750" height="419" alt="Illustration:" /></a> +<p class="caption">Schiaparelli’s chart of Mars, showing the so-called system +of canals</p> +</div> + +<p> +When Schiaparelli began his observations it was generally believed, as we have +said, that the dusky areas on Mars were seas, and since Schiaparelli thought +that the “canals” invariably began and ended at the shores of the “seas,” the +appropriateness of the title given to the lines seemed apparent. Their +artificial character was immediately assumed by many, because they were too +straight and too suggestively geometrical in their arrangement to permit the +conclusion that they were natural watercourses. A most surprising circumstance +noted by Schiaparelli was that the “canals” made their appearance <i>after</i> +the melting of the polar snow in the corresponding hemisphere had begun, and +that they grew darker, longer, and more numerous in proportion as the polar +liquidation proceeded; another very puzzling observation was that many of them +became double as the season advanced; close beside an already existing “canal,” +and in perfect parallelism with it, another would gradually make its +appearance. That these phenomena actually existed and were not illusions was +proved by later observations, and today they are seen whenever Mars is +favorably situated for observation. +</p> + +<p> +In the closing decade of the nineteenth century, Mr Percival Lowell took up the +work where Schiaparelli had virtually dropped it, and soon added a great number +of “canals” to those previously known, so that in his charts the surface of the +wonderful little planet appears covered as with a spider’s web, the dusky lines +criss-crossing in every direction, with conspicuous knots wherever a number of +them come together. Mr Lowell has demonstrated that the areas originally called +seas, and thus named on the earlier charts, are not bodies of water, whatever +else they may be. He has also found that the mysterious lines do not, as +Schiaparelli supposed, begin and end at the edges of the dusky regions, but +often continue on across them, reaching in some cases far up into the polar +regions. But Schiaparelli was right in his observation that the appearance of +the “canals” is synchronous with the gradual disappearance of the polar snows, +and this fact has become the basis of the most extraordinary theory that the +subject of life in other worlds has ever given birth to. +</p> + +<p> +Now, the effect of such discoveries, as we have related, depends upon the type +of mind to whose attention they are called. Many are content to accept them as +strange and inexplicable at present, and to wait for further light upon them; +others insist upon an immediate inquiry concerning their probable nature and +meaning. Such an inquiry can only be based upon inference proceeding from +analogy. Mars, say Mr Lowell and those who are of his opinion, is manifestly a +solidly incrusted planet like the earth; it has an atmosphere, though one of +great rarity; it has water vapor, as the snows in themselves prove; it has the +alternation of day and night, and a succession of seasons closely resembling +those of the earth; its surface is suggestively divided into regions of +contrasting colors and appearance, and upon that surface we see an immense +number of lines geometrically arranged, with a system of symmetrical +intersections where the lines expand into circular and oval areas—and all +connected with the annual melting of the polar snows in a way which +irresistibly suggests the interference of intelligence directed to a definite +end. Why, with so many concurrent circumstances to support the hypothesis, +should we not regard Mars as an inhabited globe? +</p> + +<p> +But the differences between Mars and the earth are in many ways as striking as +their resemblances. Mars is relatively small; it gets less than half as much +light and heat as we receive; its atmosphere is so rare that it would be +distressing to us, even if we could survive in it at all; it has no lakes, +rivers, or seas; its surface is an endless prairie. and its “canals” are +phenomena utterly unlike anything on the earth. Yet it is precisely upon these +divergences between the earth and Mars, this repudiation of terrestrial +standards, that the theory of “life on Mars,” for which Mr Lowell is mainly +responsible, is based. Because Mars is smaller than the earth, we are told it +must necessarily be more advanced in planetary evolution, the underlying cause +of which is the gradual cooling and contraction of the planet’s mass. Mars has +parted with its internal heat more rapidly than the earth; consequently its +waters and its atmosphere have been mostly withdrawn by chemical combinations, +but enough of both yet remain to render life still possible on its surface. As +the globe of Mars is evolutionally older than that of the earth, so its forms +of organic life may be proportionally further advanced, and its inhabitants may +have attained a degree of cultivated intelligence much superior to what at +present exists upon the earth. Understanding the nature and the causes of the +desiccation of their planet, and possessing engineering science and +capabilities far in advance of ours, they may be conceived to have grappled +with the stupendous problem of keeping their world in a habitable condition as +long as possible. Supposing them to have become accustomed to live in their +rarefied atmosphere (a thing not inconceivable, since men can live for a time +at least in air hardly less rare), the most pressing problem for them is that +of a water-supply, without which plant life cannot exist, while animal life in +turn depends for its existence upon vegetation. The only direction in which +they can seek water is that of the polar regions, where it is alternately +condensed into snow and released in the liquid form by the effect of the +seasonal changes. It is, then, to the annual melting of the polar snow-fields +that the Martian engineers are supposed to have recourse in supplying the needs +of their planet, and thus providing the means of prolonging their own +existence. It is imagined that they have for this purpose constructed a +stupendous system of irrigation extending over the temperate and equatorial +regions of the planet. The “canals” represent the lines of irrigation, but the +narrow streaks that we see are not the canals themselves, but the irrigated +bands covered by them. Their dark hue, and their gradual appearance after the +polar melting has begun, are due to the growth of vegetation stimulated by the +water. The rounded areas visible where several “canals” meet and cross are +called by Mr Lowell “oases.” These are supposed to be the principal centers of +population and industry. It must be confessed that some of them, with their +complicated systems of radiating lines, appear to answer very well to such a +theory. No attempt to explain them by analogy with natural phenomena on the +earth has proved successful. +</p> + +<p> +But a great difficulty yet remains: How to explain the seemingly miraculous +powers of the supposed engineers? Here recourse is had once more to the +relative smallness of the planet. We have remarked that the force of gravity on +Mars is only thirty-eight per cent of that on the earth. A steam-shovel driven +by a certain horse-power would be nearly three times as effective there as +here. A man of our stature on Mars would find his effective strength increased +in the same proportion. But just because of the slight force of gravity there, +a Martian might attain to the traditional stature of Goliath without finding +his own weight an encumbrance to his activity, while at the same time his huge +muscles would come into unimpeded play, enabling him single-handed to perform +labors that would be impossible to a whole gang of terrestrial workmen. The +effective powers of huge machines would be increased in the same way; and to +all this must be added the fact that the mean density of the materials of which +Mars is composed is much less than that of the constituents of the earth. +Combining all these considerations, it becomes much less difficult to conceive +that public works might be successfully undertaken on Mars which would be +hopelessly beyond the limits of human accomplishment. +</p> + +<p> +Certain other difficulties have also to be met; as, for instance, the relative +coldness of the climate of Mars. At its distance it gets considerably less than +half as much light and heat as we receive. In addition to this, the rarity of +its atmosphere would naturally be expected to decrease the effective +temperature at the planet’s surface, since an atmosphere acts somewhat like the +glass cover of a hot-house in retaining the solar heat which has penetrated it. +It has been calculated that, unless there are mitigating circumstances of which +we know nothing, the average temperature at the surface of Mars must be far +below the freezing-point of water. To this it is replied that the possible +mitigating circumstances spoken of evidently exist in fact, because we can +<i>see</i> that the watery vapor condenses into snow around the poles in +winter, but melts again when summer comes. The mitigating agent may be supposed +to exist in the atmosphere where the presence of certain gases would completely +alter the temperature gradients. +</p> + +<p> +It might also be objected that it is inconceivable that the Martian engineers, +however great may be their physical powers, and however gigantic the mechanical +energies under their control, could force water in large quantities from the +poles to the equator. This is an achievement that measures up to the cosmical +standard. It is admitted by the champions of the theory that the difficulty is +a formidable one; but they call attention to the singular fact that on Mars +there can be found no chains of mountains, and it is even doubtful if ranges of +hills exist there. The entire surface of the planet appears to be almost “as +smooth as a billiard ball,” and even the broad regions which were once supposed +to be seas apparently lie at practically the same level as the other parts, +since the “canals” in many cases run uninterruptedly across them. Lowell’s idea +is that these sombre areas may be expanses of vegetation covering ground of a +more or less marshy character, for while the largest of them appear to be +permanent, there are some which vary coincidently with the variations of the +canals. +</p> + +<p> +As to the kind of machinery employed to force the water from the poles, it has +been conjectured that it may have taken the form of a gigantic system of pumps +and conduits; and since the Martians are assumed to be so far in advance of us +in their mastery of scientific principles, the hypothesis will at least not be +harmed by supposing that they have learned to harness forces of nature whose +very existence in a manageable form is yet unrecognized on the earth. If we +wish to let the imagination loose, we may conjecture that they have conquered +the secret of those intra-atomic forces whose resistless energy is beginning to +become evident to us, but the possibility of whose utilization remains a dream, +the fulfillment of which nobody dares to predict. +</p> + +<p> +Such, in very brief form, is the celebrated theory of Mars as an inhabited +world. It certainly captivates the imagination, and if we believe it to +represent the facts, we cannot but watch with the deepest sympathy this gallant +struggle of an intellectual race to preserve its planet from the effects of +advancing age and death. We may, indeed, wonder whether our own humanity, +confronted by such a calamity, could be counted on to meet the emergency with +equal stoutness of heart and inexhaustibleness of resource. Up to the present +time we certainly have shown no capacity to confront Nature toe to toe, and to +seize her by the shoulders and turn her round when she refuses to go our way. +If we could get into wireless telephonic communication with the Martians we +might learn from their own lips the secret of their more than “Roman recovery.” +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap14"></a>XIV<br/> +The Riddle of the Asteroids</h2> + +<p> +Between the orbits of Mars and Jupiter revolves the most remarkable system of +little bodies with which we are acquainted—the Asteroids, or Minor Planets. +Some six hundred are now known, and they may actually number thousands. They +form virtually a ring about the sun. The most striking general fact about them +is that they occupy the place in the sky which should be occupied, according to +Bode’s Law, by a single large planet. This fact, as we shall see, has led to +the invention of one of the most extraordinary theories in +astronomy—<i>viz.,</i> that of the explosion of a world! +</p> + +<p> +Bode’s Law, so-called, is only an empiric formula, but until the discovery of +Neptune it accorded so well with the distances of the planets that astronomers +were disposed to look upon it as really representing some underlying principle +of planetary distribution. They were puzzled by the absence of a planet in the +space between Mars and Jupiter, where the “law” demanded that there should be +one, and an association of astronomers was formed to search for it. There was a +decided sensation when, in 1801, Piazzi, of Palermo, announced that he had +found a little planet which apparently occupied the place in the system which +belonged to the missing body. He named it Ceres, and it was the first of the +Asteroids. The next year Olbers, of Bremen, while looking for Ceres with his +telescope, stumbled upon another small planet which he named Pallas. +Immediately he was inspired with the idea that these two planets were fragments +of a larger one which had formerly occupied the vacant place in the planetary +ranks, and he predicted that others would be found by searching in the +neighborhood of the intersection of the orbits of the two already discovered. +This bold prediction was brilliantly fulfilled by the finding of two more—Juno +in 1804, and Vesta in 1807. Olbers would seem to have been led to the invention +of his hypothesis of a planetary explosion by the faith which astronomers at +that time had in Bode’s Law. They appear to have thought that several planets +revolving in the gap where the “law” called for but one could only be accounted +for upon the theory that the original <i>one</i> had been broken up to form the +several. Gravitation demanded that the remnants of a planet blown to pieces, no +matter how their orbits might otherwise differ, should all return at stated +periods to the point where the explosion had occurred; hence Olbers’ prediction +that any asteroids that might subsequently be discovered would be found to have +a common point of orbital intersection. And curiously enough all of the first +asteroids found practically answered to this requirement. Olbers’ theory seemed +to be established. +</p> + +<p> +After the first four, no more asteroids were found until 1845, when one was +discovered; then, in 1847, three more were added to the list; and after that +searchers began to pick them up with such rapidity that by the close of the +century hundreds were known, and it had become almost impossible to keep track +of them. The first four are by far the largest members of the group, but their +actual sizes remained unknown until less than twenty years ago. It was long +supposed that Vesta was the largest, because it shines more brightly than any +of the others; but finally, in 1895, Barnard, with the Lick telescope, +definitely measured their diameters, and proved to everybody’s surprise that +Ceres is really the chief, and Vesta only the third in rank. His measures are +as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, 239 miles; and Juno, +120 miles. They differ greatly in the reflective power of their surfaces, a +fact of much significance in connection with the question of their origin. +Vesta is, surface for surface, rather more than three times as brilliant as +Ceres, whence the original mistake about its magnitude. +</p> + +<p> +Nowadays new asteroids are found frequently by photography, but physically they +are most insignificant bodies, their average diameter probably not exceeding +twenty miles, and some are believed not to exceed ten. On a planet only ten +miles in diameter, assuming the same mean density as the earth’s, which is +undoubtedly too much, the force of gravity would be so slight that an average +man would not weigh more than three ounces, and could jump off into space +whenever he liked. +</p> + +<p> +Although the asteroids all revolve around the sun in the same direction as that +pursued by the major planets, their orbits are inclined at a great variety of +angles to the general plane of the planetary system, and some of them are very +eccentric—almost as much so as the orbits of many of the periodic comets. It +has even been conjectured that the two tiny moons of Mars and the four smaller +satellites of Jupiter may be asteroids gone astray and captured by those +planets. Two of the asteroids are exceedingly remarkable for the shapes and +positions of their orbits; these are Eros, discovered in 1898, and T. G., 1906, +found eight years later. The latter has a mean distance from the sun slightly +greater than that of Jupiter, while the mean distance of Eros is less than that +of Mars. The orbit of Eros is so eccentric that at times it approaches within +15,000,000 miles of the earth, nearer than any other regular member of the +solar system except the moon, thus affording an unrivaled means of measuring +the solar parallax. But for our present purpose the chief interest of Eros lies +in its extraordinary changes of light. +</p> + +<p> +These changes, although irregular, have been observed and photographed many +times, and there seems to be no doubt of their reality. Their significance +consists in their possible connection with the form of the little planet, whose +diameter is generally estimated at not more than twenty miles. Von Oppolzer +found, in 1901, that Eros lost three-fourths of its brilliancy once in every +two hours and thirty-eight minutes. Other observers have found slightly +different periods of variability, but none as long as three hours. The most +interesting interpretation that has been offered of this phenomenon is that it +is due to a great irregularity of figure, recalling at once Olbers’ hypothesis. +According to some, Eros may be double, the two bodies composing it revolving +around each other at very close quarters; but a more striking, and it may be +said probable, suggestion is that Eros has a form not unlike that of a +dumb-bell, or hour-glass, turning rapidly end over end so that the area of +illuminated surface presented to our eyes continually changes, reaching at +certain times a minimum when the amount of light that it reflects toward the +earth is reduced to a quarter of its maximum value. Various other bizarre +shapes have been ascribed to Eros, such, for instance, as that of a flat stone +revolving about one of its longer axes, so that sometimes we see its face and +sometimes its edge. +</p> + +<p> +All of these explanations proceed upon the assumption that Eros cannot have a +simple globular figure like that of a typical planet, a figure which is +prescribed by the law of gravitation, but that its shape is what may be called +accidental; in a word, it is a <i>fragment,</i> for it seems impossible to +believe that a body formed in interplanetary space, either through nebular +condensation or through the aggregation of particles drawn together by their +mutual attractions, should not be practically spherical in shape. Nor is Eros +the only asteroid that gives evidence by variations of brilliancy that there is +something abnormal in its constitution; several others present the same +phenomenon in varying degrees. Even Vesta was regarded by Olbers as +sufficiently variable in its light to warrant the conclusion that it was an +angular mass instead of a globe. Some of the smaller ones show very notable +variations, and all in short periods, of three or four hours, suggesting that +in turning about one of their axes they present a surface of variable extent +toward the sun and the earth. +</p> + +<p> +The theory which some have preferred—that the variability of light is due to +the differences of reflective power on different parts of the surface—would, if +accepted, be hardly less suggestive of the origin of these little bodies by the +breaking up of a larger one, because the most natural explanation of such +differences would seem to be that they arose from variations in the roughness +or smoothness of the reflecting surface, which would be characteristic of +fragmentary bodies. In the case of a large planet alternating expanses of land +and water, or of vegetation and desert, would produce a notable variation in +the amount of reflection, but on bodies of the size of the asteroids neither +water nor vegetation could exist, and an atmosphere would be equally +impossible. +</p> + +<p> +One of the strongest objections to Olbers’ hypothesis is that only a few of the +first asteroids discovered travel in orbits which measurably satisfy the +requirement that they should all intersect at the point where the explosion +occurred. To this it was at first replied that the perturbations of the +asteroidal orbits, by the attractions of the major planets, would soon displace +them in such a manner that they would cease to intersect. One of the first +investigations undertaken by the late Prof. Simon Newcomb was directed to the +solution of this question, and he arrived at the conclusion that the planetary +perturbations could not explain the actual situation of the asteroidal orbits. +But afterward it was pointed out that the difficulty could be avoided by +supposing that not one but a series of explosions had produced the asteroids as +they now are. After the primary disruption the fragments themselves, according +to this suggestion, may have exploded, and then the resulting orbits would be +as “tangled” as the heart could wish. This has so far rehabilitated the +explosion theory that it has never been entirely abandoned, and the evidence +which we have just cited of the probably abnormal shapes of Eros and other +asteroids has lately given it renewed life. It is a subject that needs a +thorough rediscussion. +</p> + +<p> +We must not fail to mention, however, that there is a rival hypothesis which +commends itself to many astronomers—<i>viz.,</i> that the asteroids were formed +out of a relatively scant ring of matter, situated between Mars and Jupiter and +resembling in composition the immensely more massive rings from which, +according to Laplace’s hypothesis, the planets were born. It is held by the +supporters of this theory that the attraction of the giant Jupiter was +sufficient to prevent the small, nebulous ring that gave birth to the asteroids +from condensing like the others into a single planet. +</p> + +<p> +But if we accept the explosion theory, with its corollary that minor explosions +followed the principal one, we have still an unanswered question before us: +What caused the explosions? The idea of <i>a world blowing up</i> is too +Titanic to be shocking; it rather amuses the imagination than seriously +impresses it; in a word, it seems essentially chimerical. We can by no appeal +to experience form a mental picture of such an occurrence. Even the moon did +not blow up when it was wrecked by volcanoes. The explosive nebulæ and new +stars are far away in space, and suggest no connection with such a catastrophe +as the bursting of a planet into hundreds of pieces. We cannot conceive of a +great globe thousands of miles in diameter resembling a pellet of gunpowder +only awaiting the touch of a match to cause its sudden disruption. Somehow the +thought of human agency obtrudes itself in connection with the word +“explosion,” and we smile at the idea that giant powder or nitro-glycerine +could blow up a planet. Yet it would only need <i>enough</i> of them to do it. +</p> + +<p> +After all, we may deceive ourselves in thinking, as we are apt to do, that +explosive energies lock themselves up only in small masses of matter. There are +many causes producing explosions in nature, every volcanic eruption manifests +the activity of some of them. Think of the giant power of confined steam; if +enough steam could be suddenly generated in the center of the earth by a +downpour of all the waters of the oceans, what might not the consequences be +for our globe? In a smaller globe, and it has never been estimated that the +original asteroid was even as large as the moon, such a catastrophe would, +perhaps, be more easily conceivable; but since we are compelled in this case to +assume that there was a series of successive explosions, steam would hardly +answer the purpose; it would be more reasonable to suppose that the cause of +the explosion was some kind of chemical reaction, or something affecting the +atoms composing the exploding body. Here Dr Gustav Le Bon comes to our aid with +a most startling suggestion, based on his theory of the dissipation of +intra-atomic energy. It will be best to quote him at some length from his book +on <i>The Evolution of Forces.</i> +</p> + +<p> +“It does not seem at first sight,” says Doctor Le Bon, +</p> + +<div class="letter"> +<p> +very comprehensible that worlds which appear more and more stable as they cool +could become so unstable as to afterward dissociate entirely. To explain this +phenomenon, we will inquire whether astronomical observations do not allow us +to witness this dissociation. +</p> + +<p> +We know that the stability of a body in motion, such as a top or a bicycle, +ceases to be possible when its velocity of rotation descends below a certain +limit. Once this limit is reached it loses its stability and falls to the +ground. Prof. J. J. Thomson even interprets radio-activity in this manner, and +points out that when the speed of the elements composing the atoms descends +below a certain limit they become unstable and tend to lose their equilibria. +There would result from this a commencement of dissociation, with diminution of +their potential energy and a corresponding increase of their kinetic energy +sufficient to launch into space the products of intra-atomic disintegration. +</p> + +<p> +It must not be forgotten that the atom being an enormous reservoir of energy is +by this very fact comparable with explosive bodies. These last remain inert so +long as their internal equilibria are undisturbed. So soon as some cause or +other modifies these, they explode and smash everything around them after being +themselves broken to pieces. +</p> + +<p> +Atoms, therefore, which grow old in consequence of the diminution of a part of +their intra-atomic energy gradually lose their stability. A moment, then, +arrives when this stability is so weak that the matter disappears by a sort of +explosion more or less rapid. The bodies of the radium group offer an image of +this phenomenon—a rather faint image, however, because the atoms of this body +have only reached a period of instability when the dissociation is rather slow. +It probably precedes another and more rapid period of dissociation capable of +producing their final explosion. Bodies such as radium, thorium, etc., +represent, no doubt, a state of old age at which all bodies must some day +arrive, and which they already begin to manifest in our universe, since all +matter is slightly radio-active. It would suffice for the dissociation to be +fairly general and fairly rapid for an explosion to occur in a world where it +was manifested. +</p> + +<p> +These theoretical considerations find a solid support in the sudden appearances +and disappearances of stars. The explosions of a world which produce them +reveal to us, perhaps, how the universes perish when they become old. +</p> + +<p> +As astronomical observations show the relative frequency of these rapid +destructions, we may ask ourselves whether the end of a universe by a sudden +explosion after a long period of old age does not represent its most general +ending. +</p> +</div> + +<p> +Here, perhaps, it will be well to stop, since, entrancing as the subject may +be, we know very little about it, and Doctor Le Bon’s theory affords a +limitless field for the reader’s imagination. +</p> + +</div><!--end chapter--> + +<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY ***</div> +<div style='text-align:left'> + +<div style='display:block; margin:1em 0'> +Updated editions will replace the previous one—the old editions will +be renamed. +</div> + +<div style='display:block; margin:1em 0'> +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the United +States without permission and without paying copyright +royalties. 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Anyone seeking to utilize +this eBook outside of the United States should confirm copyright +status under the laws that apply to them. diff --git a/README.md b/README.md new file mode 100644 index 0000000..3b87267 --- /dev/null +++ b/README.md @@ -0,0 +1,2 @@ +Project Gutenberg (https://www.gutenberg.org) public repository for +eBook #6630 (https://www.gutenberg.org/ebooks/6630) diff --git a/old/6630-8.txt b/old/6630-8.txt new file mode 100644 index 0000000..eb33078 --- /dev/null +++ b/old/6630-8.txt @@ -0,0 +1,5339 @@ +The Project Gutenberg EBook of Curiosities of the Sky, by Garrett Serviss + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Curiosities of the Sky + +Author: Garrett Serviss + +Posting Date: August 25, 2012 [EBook #6630] +Release Date: October, 2004 +First Posted: January 6, 2003 + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY *** + + + + +Produced by William McClain + + + + + + + + + + + Curiosities of the Sky + +by Garrett Serviss + +Curiosities of the Sky was first published in 1909 and the text is in +the public domain. The transcription was done by William McClain +(info@sattre-press.com), 2002. + +A printed version of this book is available from Sattre Press +(http://csky.sattre-press.com). It includes extensive annotations, a +new introduction and all the original photographs and diagrams. +_________________________________________________________________ + +Preface + +What Froude says of history is true also of astronomy: it is the most +impressive where it transcends explanation. It is not the mathematics +of astronomy, but the wonder and the mystery that seize upon the +imagination. The calculation of an eclipse owes all its prestige to +the sublimity of its data; the operation, in itself, requires no more +mental effort than the preparation of a railway time-table. + +The dominion which astronomy has always held over the minds of men is +akin to that of poetry; when the former becomes merely instructive and +the latter purely didactic, both lose their power over the +imagination. Astronomy is known as the oldest of the sciences, and it +will be the longest-lived because it will always have arcana that have +not been penetrated. + +Some of the things described in this book are little known to the +average reader, while others are well known; but all possess the +fascination of whatever is strange, marvelous, obscure, or mysterious +-- magnified, in this case, by the portentous scale of the phenomena. + +The idea of the author is to tell about these things in plain +language, but with as much scientific accuracy as plain language will +permit, showing the wonder that is in them without getting away from +the facts. Most of them have hitherto been discussed only in technical +form, and in treatises that the general public seldom sees and never +reads. + +Among the topics touched upon are: + * The strange unfixedness of the ``fixed stars,'' the vast + migrations of the suns and worlds constituting the universe. + * The slow passing out of existence of those collocations of stars + which for thousands of years have formed famous + ``constellations,'' preserving the memory of mythological heroes + and heroines, and perhaps of otherwise unrecorded history. + * The tendency of stars to assemble in immense clouds, swarms, and + clusters. + * The existence in some of the richest regions of the universe of + absolutely black, starless gaps, deeps, or holes, as if one were + looking out of a window into the murkiest night. + * The marvelous phenomena of new, or temporary, stars, which appear + as suddenly as conflagrations, and often turn into something else + as eccentric as themselves. + * The amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,'' + and ``lace,'' or ``tress,'' nebul. + * The strange surroundings of the sun, only seen in particular + circumstances, but evidently playing a constant part in the daily + phenomena of the solar system. + * The mystery of the Zodiacal Light and the Gegenschein. + * The extraordinary transformations undergone by comets and their + tails. + * The prodigies of meteorites and masses of stone and metal fallen + from the sky. + * The cataclysms that have wrecked the moon. + * The problem of life and intelligence on the planet Mars. + * The problematical origin and fate of the asteroids. + * The strange phenomena of the auroral lights. + +An attempt has been made to develop these topics in an orderly way, +showing their connection, so that the reader may obtain a broad +general view of the chief mysteries and problems of astronomy, and an +idea of the immense field of discovery which still lies, almost +unexplored, before it. + +The Windows of Absolute Night + +To most minds mystery is more fascinating than science. But when +science itself leads straight up to the borders of mystery and there +comes to a dead stop, saying, ``At present I can no longer see my +way,'' the force of the charm is redoubled. On the other hand, the +illimitable is no less potent in mystery than the invisible, whence +the dramatic effect of Keats' ``stout Cortez'' staring at the +boundless Pacific while all his men look at each other with a wild +surmise, ``silent upon a peak in Darien.'' It is with similar feelings +that the astronomer regards certain places where from the peaks of the +universe his vision seems to range out into endless empty space. He +sees there the shore of his little isthmus, and, beyond, unexplored +immensity. + +The name, ``coal-sacks,'' given to these strange voids is hardly +descriptive. Rather they produce upon the mind the effect of blank +windows in a lonely house on a pitch-dark night, which, when looked at +from the brilliant interior, become appalling in their rayless murk. +Infinity seems to acquire a new meaning in the presence of these black +openings in the sky, for as one continues to gaze it loses its purely +metaphysical quality and becomes a kind of entity, like the ocean. The +observer is conscious that he can actually see the beginning of its +ebon depths, in which the visible universe appears to float like an +enchanted island, resplendent within with lights and life and gorgeous +spectacles, and encircled with screens of crowded stars, but with its +dazzling vistas ending at the fathomless sea of pure darkness which +encloses all. + +The Galaxy, or Milky Way, surrounds the borders of our island in space +like a stellar garland, and when openings appear in it they are, by +contrast, far more impressive than the general darkness of the +interstellar expanse seen in other directions. Yet even that expanse +is not everywhere equally dark, for it contains gloomy deeps +discernable with careful watching. Here, too, contrast plays an +important part, though less striking than within the galactic region. +Some of Sir William Herschel's observations appear to indicate an +association between these tenebrious spots and neighboring star clouds +and nebul. It is an illuminating bit of astronomical history that +when he was sweeping the then virgin heavens with his great telescopes +he was accustomed to say to his sister who, note-book in hand, waited +at his side to take down his words, fresh with the inspiration of +discovery: ``Prepare to write; the nebul are coming; here space is +vacant.'' + +The most famous of the ``coal-sacks,'' and the first to be brought to +general attention before astronomers had awakened to the significance +of such things, lies adjacent to the ``Southern Cross,'' and is truly +an amazing phenomenon. It is not alone the conspicuousness of this +celestial vacancy, opening suddenly in the midst of one of the richest +parts of the Galaxy, that has given it its fame, but quite as much the +superstitious awe with which it was regarded by the early explorers of +the South Seas. To them, as well as to those who listened in rapt +wonder to their tales, the ``Coal-sack'' seemed to possess some occult +connection with the mystic ``Cross.'' In the eyes of the sailors it +was not a vacancy so much as a sable reality in the sky, and as, +shuddering, they stared at it, they piously crossed themselves. It was +another of the magical wonders of the unknown South, and as such it +formed the basis of many a ``wild surmise'' and many a sea-dog's yarn. +Scientific investigation has not diminished its prestige, and today no +traveler in the southern hemisphere is indifferent to its fascinating +strangeness, while some find it the most impressive spectacle of the +antarctic heavens. + +All around, up to the very edge of the yawning gap, the sheen of the +Milky Way is surpassingly glorious; but there, as if in obedience to +an almighty edict, everything vanishes. A single faint star is visible +within the opening, producing a curious effect upon the sensitive +spectator, like the sight of a tiny islet in the midst of a black, +motionless, waveless tarn. The dimensions of the lagoon of darkness, +which is oval or pear-shaped, are eight degrees by five, so that it +occupies a space in the sky about one hundred and thirty times greater +than the area of the full moon. It attracts attention as soon as the +eye is directed toward the quarter where it exists, and by virtue of +the rarity of such phenomena it appears a far greater wonder than the +drifts of stars that are heaped around it. Now that observatories are +multiplying in the southern hemisphere, the great austral +``Coal-sack'' will, no doubt, receive attention proportioned to its +importance as one of the most significant features of the sky. Already +at the Sydney Observatory photographs have shown that the southern +portion of this Dead Sea of Space is not quite ``bottomless,'' +although its northern part defies the longest sounding lines of the +astronomer. + +There is a similar, but less perfect, ``coal-sack'' in the northern +hemisphere, in the constellation of ``The Swan,'' which, strange to +say, also contains a well-marked figure of a cross outlined by stars. +This gap lies near the top of the cross-shaped figure. It is best seen +by averted vision, which brings out the contrast with the Milky Way, +which is quite brilliant around it. It does not, however, exercise the +same weird attraction upon the eye as the southern ``Coal-sack,'' for +instead of looking like an absolute void in the sky, it rather appears +as if a canopy of dark gauze had been drawn over the stars. We shall +see the possible significance of this appearance later. + +Just above the southern horizon of our northern middle latitudes, in +summer, where the Milky Way breaks up into vast sheets of nebulous +luminosity, lying over and between the constellations Scorpio and +Sagittarius, there is a remarkable assemblage of ``coal-sacks,'' +though none is of great size. One of them, near a conspicuous +star-cluster in Scorpio, M80, is interesting for having been the first +of these strange objects noted by Herschel. Probably it was its +nearness to M80 which suggested to his mind the apparent connection of +such vacancies with star-clusters which we have already mentioned. + +But the most marvelous of the ``coal-sacks'' are those that have been +found by photography in Sagittarius. One of Barnard's earliest and +most excellent photographs includes two of them, both in the +star-cluster M8. The larger, which is roughly rectangular in outline, +contains one little star, and its smaller neighbor is lune-shaped -- +surely a most singular form for such an object. Both are associated +with curious dark lanes running through the clustered stars like +trails in the woods. Along the borders of these lanes the stars are +ranked in parallel rows, and what may be called the bottoms of the +lanes are not entirely dark, but pebbled with faint stellar points. +One of them which skirts the two dark gaps and traverses the cluster +along its greatest diameter is edged with lines of stars, recalling +the alignment of the trees bordering a French highway. This road of +stars cannot be less than many billions of miles in length! + +All about the cluster the bed of the Galaxy is strangely disturbed, +and in places nearly denuded, as if its contents had been raked away +to form the immense stack and the smaller accumulations of stars +around it. The well-known ``Trifid Nebula'' is also included in the +field of the photograph, which covers a truly marvelous region, so +intricate in its mingling of nebul, star-clusters, star-swarms, +star-streams, and dark vacancies that no description can do it +justice. Yet, chaotic as it appears, there is an unmistakable +suggestion of unity about it, impressing the beholder with the idea +that all the different parts are in some way connected, and have not +been fortuitously thrown together. Miss Agnes M. Clerke made the +striking remark that the dusky lanes in M8 are exemplified on the +largest scale in the great rift dividing the Milky Way, from Cygnus in +the northern hemisphere all the way to the ``Cross'' in the southern. +Similar lanes are found in many other clusters, and they are generally +associated with flanking rows of stars, resembling in their +arrangement the thick-set houses and villas along the roadways that +traverse the approaches to a great city. + +But to return to the black gaps. Are they really windows in the +star-walls of the universe? Some of them look rather as if they had +been made by a shell fired through a luminous target, allowing the eye +to range through the hole into the void space beyond. If science is +discretely silent about these things, what can the more venturesome +and less responsible imagination suggest? Would a huge ``runaway +sun,'' like Arcturus, for instance, make such an opening if it should +pass like a projectile through the Milky Way? It is at least a +stimulating inquiry. Being probably many thousands of times more +massive than the galactic stars, such a stellar missile would not be +stopped by them, though its direction of flight might be altered. It +would drag the small stars lying close to its course out of their +spheres, but the ultimate tendency of its attraction would be to sweep +them round in its wake, thus producing rather a star-swarm than a +vacancy. Those that were very close to it might be swept away in its +rush and become its satellites, careering away with it in its flight +into outer space; but those that were farther off, and they would, of +course, greatly outnumber the nearer ones, would tend inward from all +sides toward the line of flight, as dust and leaves collect behind a +speeding motor (though the forces operating would be different), and +would fill up the hole, if hole it were. A swarm thus collected should +be rounded in outline and bordered with a relatively barren ring from +which the stars had been ``sucked'' away. In a general sense the M8 +cluster answers to this description, but even if we undertook to +account for its existence by a supposition like the above, the black +gaps would remain unexplained, unless one could make a further draft +on the imagination and suggest that the stars had been thrown into a +vast eddy, or system of eddies, whose vortices appear as dark holes. +Only a maelstrom-like motion could keep such a funnel open, for +without regard to the impulse derived from the projectile, the proper +motions of the stars themselves would tend to fill it. Perhaps some +other cause of the whirling motion may be found. As we shall see when +we come to the spiral nebul, gyratory movements are exceedingly +prevalent throughout the universe, and the structure of the Milky Way +is everywhere suggestive of them. But this is hazardous sport even for +the imagination -- to play with suns as if they were but thistle-down +in the wind or corks in a mill-race. + +Another question arises: What is the thickness of the hedge of stars +through which the holes penetrate? Is the depth of the openings +proportionate to their width? In other words, is the Milky Way round +in section like a rope, or flat and thin like a ribbon? The answer is +not obvious, for we have little or no information concerning the +relative distances of the faint galactic stars. It would be easier, +certainly, to conceive of openings in a thin belt than in a massive +ring, for in the first case they would resemble mere rifts and breaks, +while in the second they would be like wells or bore-holes. Then, too, +the fact that the Milky Way is not a continuous body but is made up of +stars whose actual distances apart is great, offers another quandary; +persistent and sharply bordered apertures in such an assemblage are a +priori as improbable, if not impossible, as straight, narrow holes +running through a swarm of bees. + +The difficulty of these questions indicates one of the reasons why it +has been suggested that the seeming gaps, or many of them, are not +openings at all, but opaque screens cutting off the light from stars +behind them. That this is quite possible in some cases is shown by +Barnard's later photographs, particularly those of the singular region +around the star Rho Ophiuchi. Here are to be seen somber lanes and +patches, apparently forming a connected system which covers an immense +space, and which their discoverer thinks may constitute a ``dark +nebula.'' This seems at first a startling suggestion; but, after all, +why should their not be dark nebul as well as visible ones? In truth, +it has troubled some astronomers to explain the luminosity of the +bright nebul, since it is not to be supposed that matter in so +diffuse a state can be incandescent through heat, and phosphorescent +light is in itself a mystery. The supposition is also in accord with +what we know of the existence of dark solid bodies in space. Many +bright stars are accompanied by obscure companions, sometimes as +massive as themselves; the planets are non-luminous; the same is true +of meteors before they plunge into the atmosphere and become heated by +friction; and many plausible reasons have been found for believing +that space contains as many obscure as shining bodies of great size. +It is not so difficult, after all, then, to believe that there are +immense collections of shadowy gases and meteoric dust whose presence +is only manifested when they intercept the light coming from shining +bodies behind them. + +This would account for the apparent extinguishment of light in open +space, which is indicated by the falling off in relative number of +telescopic stars below the tenth magnitude. Even as things are, the +amount of light coming to us from stars too faint to be seen with the +naked eye is so great that the statement of it generally surprises +persons who are unfamiliar with the inner facts of astronomy. It has +been calculated that on a clear night the total starlight from the +entire celestial sphere amounts to one-sixtieth of the light of the +full moon; but of this less than one-twenty-fifth is due to stars +separately distinguished by the eye. If there were no obscuring medium +in space, it is probable that the amount of starlight would be +noticeably and perhaps enormously increased. + +But while it seems certain that some of the obscure spots in the Milky +Way are due to the presence of ``dark nebul,'' or concealing veils of +one kind or another, it is equally certain that there are many which +are true apertures, however they may have been formed, and by whatever +forces they may be maintained. These, then, are veritable windows of +the Galaxy, and when looking out of them one is face to face with the +great mystery of infinite space. There the known universe visibly +ends, but manifestly space itself does not end there. It is not within +the power of thought to conceive an end to space, for the instant we +think of a terminal point or line the mind leaps forward to the +beyond. There must be space outside as well as inside. Eternity of +time and infinity of space are ideas that the intellect cannot fully +grasp, but neither can it grasp the idea of a limitation to either +space or time. The metaphysical conceptions of hypergeometry, or +fourth-dimensional space, do not aid us. + +Having, then, discovered that the universe is a thing contained in +something indefinitely greater than itself; having looked out of its +windows and found only the gloom of starless night outside -- what +conclusions are we to draw concerning the beyond? It seems as empty as +a vacuum, but is it really so? If it be, then our universe is a single +atom astray in the infinite; it is the only island in an ocean without +shores; it is the one oasis in an illimitable desert. Then the Milky +Way, with its wide-flung garland of stars, is afloat like a tiny +smoke-wreath amid a horror of immeasurable vacancy, or it is an +evanescent and solitary ring of sparkling froth cast up for a moment +on the viewless billows of immensity. From such conclusions the mind +instinctively shrinks. It prefers to think that there is something +beyond, though we cannot see it. Even the universe could not bear to +be alone -- a Crusoe lost in the Cosmos! As the inhabitants of the +most elegant chteau, with its gardens, parks, and crowds of +attendants, would die of loneliness if they did not know that they +have neighbors, though not seen, and that a living world of indefinite +extent surrounds them, so we, when we perceive that the universe has +limits, wish to feel that it is not solitary; that beyond the hedges +and the hills there are other centers of life and activity. Could +anything be more terrible than the thought of an isolated universe? +The greater the being, the greater the aversion to seclusion. Only the +infinite satisfies; in that alone the mind finds rest. + +We are driven, then, to believe that the universal night which +envelopes us is not tenantless; that as we stare out of the +star-framed windows of the Galaxy and see nothing but uniform +blackness, the fault is with our eyes or is due to an obscuring +medium. Since our universe is limited in extent, there must be other +universes beyond it on all sides. Perhaps if we could carry our +telescopes to the verge of the great ``Coal-sack'' near the ``Cross,'' +being then on the frontier of our starry system, we could discern, +sparkling afar off in the vast night, some of the outer galaxies. They +may be grander than ours, just as many of the suns surrounding us are +immensely greater than ours. If we could take our stand somewhere in +the midst of immensity and, with vision of infinite reach, look about +us, we should perhaps see a countless number of stellar systems, amid +which ours would be unnoticeable, like a single star among the +multitude glittering in the terrestial sky on a clear night. Some +might be in the form of a wreath, like our own; some might be +globular, like the great star-clusters in Hercules and Centaurus; some +might be glittering circles, or disks, or rings within rings. If we +could enter them we should probably find a vast variety of +composition, including elements unknown to terrestrial chemistry; for +while the visible universe appears to contain few if any substances +not existing on the earth or in the sun, we have no warrant to assume +that others may not exist in infinite space. + +And how as to gravitation? We do not know that gravitation acts beyond +the visible universe, but it is reasonable to suppose that it does. At +any rate, if we let go its sustaining hand we are lost, and can only +wander hopelessly in our speculations, like children astray. If the +empire of gravitation is infinite, then the various outer systems must +have some, though measuring by our standards an imperceptible, +attractive influence upon each other, for gravitation never lets go +its hold, however great the space over which it is required to act. +Just as the stars about us are all in motion, so the starry systems +beyond our sight may be in motion, and our system as a whole may be +moving in concert with them. If this be so, then after interminable +ages the aspect of the entire system of systems must change, its +various members assuming new positions with respect to one another. In +the course of time we may even suppose that our universe will approach +relatively close to one of the others; and then, if men are yet living +on the earth, they may glimpse through the openings which reveal +nothing to us now, the lights of another nearing star system, like the +signals of a strange squadron, bringing them the assurance (which can +be but an inference at present) that the ocean of space has other +argosies venturing on its limitless expanse. + +There remains the question of the luminiferous ether by whose agency +the waves of light are borne through space. The ether is as mysterious +as gravitation. With regard to ether we only infer its existence from +the effects which we ascribe to it. Evidently the ether must extend as +far as the most distant visible stars. But does it continue on +indefinitely in outer space? If it does, then the invisibility of the +other systems must be due to their distance diminishing the quantity +of light that comes from them below the limit of perceptibility, or to +the interposition of absorbing media; if it does not, then the reason +why we cannot see them is owing to the absence of a means of +conveyance for the light waves, as the lack of an interplanetary +atmosphere prevents us from hearing the thunder of sun-spots. (It is +interesting to recall that Mr Edison was once credited with the +intention to construct a gigantic microphone which should render the +roar of sun-spots audible by transforming the electric vibrations into +sound-waves). On this supposition each starry system would be +enveloped in its own globule of ether, and no light could cross from +one to another. But the probability is that both the ether and +gravitation are ubiquitous, and that all the stellar systems are +immersed in the former like clouds of phosphorescent organisms in the +sea. + +So astronomy carries the mind from height to greater height. Men were +long in accepting the proofs of the relative insignificance of the +earth; they were more quickly convinced of the comparative littleness +of the solar system; and now the evidence assails their reason that +what they had regarded as the universe is only one mote gleaming in +the sunbeams of Infinity. + +Star-Clouds, Star-Clusters, and Star-Streams + +In the preceding chapter we have seen something of the strangely +complicated structure of the Galaxy, or Milky Way. We now proceed to +study more comprehensively that garlanded ``Pathway of the Gods.'' + +Judged by the eye alone, the Milky Way is one of the most delicately +beautiful phenomena in the entire realm of nature -- a shimmer of +silvery gauze stretched across the sky; but studied in the light of +its revelations, it is the most stupendous object presented to human +ken. Let us consider, first, its appearance to ordinary vision. Its +apparent position in the sky shifts according to the season. On a +serene, cloudless summer evening, in the absence of the moon, whose +light obscures it, one sees the Galaxy spanning the heavens from north +to southeast of the zenith like a phosphorescent arch. In early spring +it forms a similar but, upon the whole, less brilliant arch west of +the zenith. Between spring and summer it lies like a long, faint, +twilight band along the northern horizon. At the beginning of winter +it again forms an arch, this time spanning the sky from east to west, +a little north of the zenith. These are its positions as viewed from +the mean latitude of the United States. Even the beginner in +star-gazing does not have to watch it throughout the year in order to +be convinced that it is, in reality, a great circle, extending +entirely around the celestial sphere. We appear to be situated near +its center, but its periphery is evidently far away in the depths of +space. + +Although to the casual observer it seems but a delicate scarf of +light, brighter in some places than in others, but hazy and indefinite +at the best, such is not its appearance to those who study it with +care. They perceive that it is an organic whole, though marvelously +complex in detail. The telescope shows that it consists of stars too +faint and small through excess of distance to be separately visible. +Of the hundred million suns which some estimates have fixed as the +probable population of the starry universe, the vast majority (at +least thirty to one) are included in this strange belt of misty light. +But they are not uniformly distributed in it; on the contrary, they +are arrayed in clusters, knots, bunches, clouds, and streams. The +appearance is somewhat as if the Galaxy consisted of innumerable +swarms of silver-winged bees, more or less intermixed, some massed +together, some crossing the paths of others, but all governed by a +single purpose which leads them to encircle the region of space in +which we are situated. + +From the beginning of the systematic study of the heavens, the fact +has been recognized that the form of the Milky Way denotes the scheme +of the sidereal system. At first it was thought that the shape of the +system was that of a vast round disk, flat like a cheese, and filled +with stars, our sun and his relatively few neighbors being placed near +the center. According to this view, the galactic belt was an effect of +perspective; for when looking in the direction of the plane of the +disk, the eye ranged through an immense extension of stars which +blended into a glimmering blur, surrounding us like a ring; while when +looking out from the sides of the disk we saw but few stars, and in +those directions the heavens appeared relatively blank. Finally it was +recognized that this theory did not correspond with the observed +appearances, and it became evident that the Milky Way was not a mere +effect of perspective, but an actual band of enormously distant stars, +forming a circle about the sphere, the central opening of the ring +(containing many scattered stars) being many times broader than the +width of the ring itself. Our sun is one of the scattered stars in the +central opening. + +As already remarked, the ring of the Galaxy is very irregular, and in +places it is partly broken. With its sinuous outline, its pendant +sprays, its graceful and accordant curves, its bunching of masses, its +occasional interstices, and the manifest order of a general plan +governing the jumble of its details, it bears a remarkable resemblance +to a garland -- a fact which appears the more wonderful when we recall +its composition. That an elm-tree should trace the lines of beauty +with its leafy and pendulous branches does not surprise us; but we can +only gaze with growing amazement when we behold a hundred million suns +imitating the form of a chaplet! And then we have to remember that +this form furnishes the ground-plan of the universe. + +As an indication of the extraordinary speculations to which the +mystery of the Milky Way has given rise, a theory recently (1909) +proposed by Prof. George C. Comstock may be mentioned. Starting with +the data (first) that the number of stars increases as the Milky Way +is approached, and reaches a maximum in its plane, while on the other +hand the number of nebul is greatest outside the Milky Way and +increases with distance from it, and (second) that the Milky Way, +although a complete ring, is broad and diffuse on one side through +one-half its course -- that half alone containing nebul -- and +relatively narrow and well defined on the opposite side, the author of +this singular speculation avers that these facts can best be explained +by supposing that the invisible universe consists of two +interpenetrating parts, one of which is a chaos of indefinite extent, +strewn with stars and nebulous dust, and the other a long, broad but +comparatively thin cluster of stars, including the sun as one of its +central members. This flat star-cluster is conceived to be moving +edgewise through the chaos, and, according to Professor Comstock, it +acts after the manner of a snow-plough sweeping away the cosmic dust +and piling it on either hand above and below the plane of the moving +cluster. It thus forms a transparent rift, through which we see +farther and command a view of more stars than through the intensified +dust-clouds on either hand. This rift is the Milky Way. The dust +thrown aside toward the poles of the Milky Way is the substance of the +nebul which abound there. Ahead, where the front of the star-plough +is clearing the way, the chaos is nearer at hand, and consequently +there the rift subtends a broader angle, and is filled with primordial +dust, which, having been annexed by the vanguard of the star-swarm, +forms the nebul seen only in that part of the Milky Way. But behind, +the rift appears narrow because there we look farther away between +dust-clouds produced ages ago by the front of the plough, and no +scattered dust remains in that part of the rift. + +In quoting an outline of this strikingly original theory the present +writer should not be understood as assenting to it. That it appears +bizarre is not, in itself, a reason for rejecting it, when we are +dealing with so problematical and enigmatical a subject as the Milky +Way; but the serious objection is that the theory does not +sufficiently accord with the observed phenomena. There is too much +evidence that the Milky Way is an organic system, however fantastic +its form, to permit the belief that it can only be a rift in chaotic +clouds. As with every organism, we find that its parts are more or +less clearly repeated in its ensemble. Among all the strange things +that the Milky Way contains there is nothing so extraordinary as +itself. Every astronomer must many times have found himself marveling +at it in those comparatively rare nights when it shows all its beauty +and all its strangeness. In its great broken rifts, divisions, and +spirals are found the gigantic prototypes of similar forms in its +star-clouds and clusters. As we have said, it determines the general +shape of the whole sidereal system. Some of the brightest stars in the +sky appear to hang like jewels suspended at the ends of tassels +dropped from the Galaxy. Among these pendants are the Pleiades and the +Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a loop of +light'' thrown out from it. The majority of the great first-magnitude +stars seem related to it, as if they formed an inner ring inclined at +an angle of some twenty degrees to its plane. Many of the long curves +that set off from it on both sides are accompanied by corresponding +curves of lucid stars. In a word, it offers every appearance of +structural connection with the entire starry system. That the universe +should have assumed the form of a wreath is certainly a matter for +astonishment; but it would have been still more astonishing if it had +been a cube, a rhomboid, or a dodecahedron, for then we should have +had to suppose that something resembling the forces that shape +crystals had acted upon the stars, and the difficulty of explaining +the universe by the laws of gravitation would have been increased. + +From the Milky Way as a whole we pass to the vast clouds, swarms, and +clusters of stars of which it is made up. It may be, as some +astronomers hold, that most of the galactic stars are much smaller +than the sun, so that their faintness is not due entirely to the +effect of distance. Still, their intrinsic brilliance attests their +solar character, and considering their remoteness, which has been +estimated at not less than ten thousand to twenty thousand light-years +(a light-year is equal to nearly six thousand thousand million miles) +their actual masses cannot be extremely small. The minutest of them +are entitled to be regarded as real suns, and they vary enormously in +magnitude. The effects of their attractions upon one another can only +be inferred from their clustering, because their relative movements +are not apparent on account of the brevity of the observations that we +can make. But imagine a being for whom a million years would be but as +a flitting moment; to him the Milky Way would appear in a state of +ceaseless agitation -- swirling with ``a fury of whirlpool motion.'' + +The cloud-like aspect of large parts of the Galaxy must always have +attracted attention, even from naked-eye observers, but the true +star-clouds were first satisfactorily represented in Barnard's +photographs. The resemblance to actual clouds is often startling. Some +are close-packed and dense, like cumuli; some are wispy or mottled, +like cirri. The rifts and modulations, as well as the general +outlines, are the same as those of clouds of vapor or dust, and one +notices also the characteristic thinning out at the edges. But we must +beware of supposing that the component suns are thickly crowded as the +particles forming an ordinary cloud. They look, indeed, as if they +were matted together, because of the irradiation of light, but in +reality millions and billions of miles separate each star from its +neighbors. Nevertheless they form real assemblages, whose members are +far more closely related to one another than is our sun to the stars +around him, and if we were in the Milky Way the aspect of the +nocturnal sky would be marvelously different from its present +appearance. + +Stellar clouds are characteristic of the Galaxy and are not found +beyond its borders, except in the ``Magellanic Clouds'' of the +southern hemisphere, which resemble detached portions of the Milky +Way. These singular objects form as striking a peculiarity of the +austral heavens as does the great ``Coal-sack'' described in Chapter +1. But it is their isolation that makes them so remarkable, for their +composition is essentially galactic, and if they were included within +its boundaries they would not appear more wonderful than many other +parts of the Milky Way. Placed where they are, they look like masses +fallen from the great stellar arch. They are full of nebul and +star-clusters, and show striking evidences of spiral movement. + +Star-swarms, which are also characteristic features of the Galaxy, +differ from star-clouds very much in the way that their name would +imply -- i.e., their component stars are so arranged, even when they +are countless in number, that the idea of an exceedingly numerous +assemblage rather than that of a cloud is impressed on the observer's +mind. In a star-swarm the separate members are distinguishable because +they are either larger or nearer than the stars composing a ``cloud.'' +A splendid example of a true star-swarm is furnished by Chi Persei, in +that part of the Milky Way which runs between the constellations +Perseus and Cassiopeia. This swarm is much coarser than many others, +and can be seen by the naked eye. In a small telescope it appears +double, as if the suns composing it had divided into two parties which +keep on their way side by side, with some commingling of their members +where the skirts of the two companies come in contact. + +Smaller than either star-clouds or star-swarms, and differing from +both in their organization, are star-clusters. These, unlike the +others, are found outside as well as inside the Milky Way, although +they are more numerous inside its boundaries than elsewhere. The term +star-cluster is sometimes applied, though improperly, to assemblages +which are rather groups, such, for instance, as the Pleiades. In their +most characteristic aspect star-clusters are of a globular shape -- +globes of suns! A famous example of a globular star-cluster, but one +not included in the Milky Way, is the ``Great Cluster in Hercules.'' +This is barely visible to the naked eye, but a small telescope shows +its character, and in a large one it presents a marvelous spectacle. +Photographs of such clusters are, perhaps, less effective than those +of star-clouds, because the central condensation of stars in them is +so great that their light becomes blended in an indistinguishable +blur. The beautiful effect of the incessant play of infinitesimal rays +over the apparently compact surface of the cluster, as if it were a +globe of the finest frosted silver shining in an electric beam, is +also lost in a photograph. Still, even to the eye looking directly at +the cluster through a powerful telescope, the central part of the +wonderful congregation seems almost a solid mass in which the stars +are packed like the ice crystals in a snowball. + +The same question rises to the lips of every observer: How can they +possibly have been brought into such a situation? The marvel does not +grow less when we know that, instead of being closely compacted, the +stars of the cluster are probably separated by millions of miles; for +we know that their distances apart are slight as compared with their +remoteness from the Earth. Sir William Herschel estimated their number +to be about fourteen thousand, but in fact they are uncountable. If we +could view them from a point just within the edge of the assemblage, +they would offer the appearance of a hollow hemisphere emblazoned with +stars of astonishing brilliancy; the near-by ones unparalleled in +splendor by any celestial object known to us, while the more distant +ones would resemble ordinary stars. An inhabitant of the cluster would +not know, except by a process of ratiocination, that he was dwelling +in a globular assemblage of suns; only from a point far outside would +their spherical arrangement become evident to the eye. Imagine +fourteen-thousand fire-balloons with an approach to regularity in a +spherical space -- say, ten miles in diameter; there would be an +average of less than thirty in every cubic mile, and it would be +necessary to go to a considerable distance in order to see them as a +globular aggregation; yet from a point sufficiently far away they +would blend into a glowing ball. + +Photographs show even better than the best telescopic views that the +great cluster is surrounded with a multitude of dispersed stars, +suggestively arrayed in more or less curving lines, which radiate from +the principle mass, with which their connection is manifest. These +stars, situated outside the central sphere, look somewhat like vagrant +bees buzzing round a dense swarm where the queen bee is sitting. Yet +while there is so much to suggest the operation of central forces, +bringing and keeping the members of the cluster together, the +attentive observer is also impressed with the idea that the whole +wonderful phenomenon may be the result of explosion. As soon as this +thought seizes the mind, confirmation of it seems to be found in the +appearance of the outlying stars, which could be as readily explained +by the supposition that they have been blown apart as that they have +flocked together toward a center. The probable fact that the stars +constituting the cluster are very much smaller than our sun might be +regarded as favoring the hypothesis of an explosion. Of their real +size we know nothing, but, on the basis of an uncertain estimate of +their parallax, it has been calculated that they may average +forty-five thousand miles in diameter -- something more than half the +diameter of the planet Jupiter. Assuming the same mean density, +fourteen thousand such stars might have been formed by the explosion +of a body about twice the size of the sun. This recalls the theory of +Olbers, which has never been altogether abandoned or disproved, that +the Asteroids were formed by the explosion of a planet circulating +between the orbits of Mars and Jupiter. The Asteroids, whatever their +manner of origin, form a ring around the sun; but, of course, the +explosion of a great independent body, not originally revolving about +a superior center of gravitational force, would not result in the +formation of a ring of small bodies, but rather of a dispersed mass of +them. But back of any speculation of this kind lies the problem, at +present insoluble: How could the explosion be produced? (See the +question of explosions in Chapters 6 and 14). + +Then, on the other hand, we have the observation of Herschel, since +abundantly confirmed, that space is unusually vacant in the immediate +neighborhood of condensed star-clusters and nebul, which, as far as +it goes, might be taken as an indication that the assembled stars had +been drawn together by their mutual attractions, and that the tendency +to aggregation is still bringing new members toward the cluster. But +in that case there must have been an original condensation of stars at +that point in space. This could probably have been produced by the +coagulation of a great nebula into stellar nuclei, a process which +seems now to be taking place in the Orion Nebula. + +A yet more remarkable globular star-cluster exists in the southern +hemisphere, Omega Centauri. In this case the central condensation of +stars presents an almost uniform blaze of light. Like the Hercules +cluster, that in Centaurus is surrounded with stars scattered over a +broad field and showing an appearance of radial arrangement. In fact, +except for its greater richness, Omega Centauri is an exact duplicate +of its northern rival. Each appears to an imaginative spectator as a +veritable ``city of suns.'' Mathematics shrinks from the task of +disentangling the maze of motions in such an assemblage. It would seem +that the chance of collisions is not to be neglected, and this idea +finds a certain degree of confirmation in the appearance of +``temporary stars'' which have more than once blazed out in, or close +by, globular star-clusters. + +This leads up to the notable fact, first established by Professor +Bailey a few years ago, that such clusters are populous with variable +stars. Omega Centauri and the Hercules cluster are especially +remarkable in this respect. The variables found in them are all of +short period and the changes of light show a noteworthy tendency to +uniformity. The first thought is that these phenomena must be due to +collisions among the crowded stars, but, if so, the encounters cannot +be between the stars themselves, but probably between stars and meteor +swarms revolving around them. Such periodic collisions might go on for +ages without the meteors being exhausted by incorporation with the +stars. This explanation appears all the more probable because one +would naturally expect that flocks of meteors would abound in a close +aggregation of stars. It is also consistent with Perrine's discovery +-- that the globular star clusters are powdered with minute stars +strewn thickly among the brighter ones. + +In speaking of Professor Comstock's extraordinary theory of the Milky +Way, the fact was mentioned that, broadly speaking, the nebul are +less numerous in the galactic belt than in the comparatively open +spaces on either side of it, but that they are, nevertheless, abundant +in the broader half of the Milky Way which he designates as the front +of the gigantic ``plough'' supposed to be forcing its way through the +enveloping chaos. In and around the Sagittarius region the +intermingling of nebul and galactic star clouds and clusters is +particularly remarkable. That there is a causal connection no +thoughtful person can doubt. We are unable to get away from the +evidence that a nebula is like a seed-ground from which stars spring +forth; or we may say that nebul resemble clouds in whose bosom +raindrops are forming. The wonderful aspect of the admixtures of +nebul and star-clusters in Sagittarius has been described in Chapter +1. We now come to a still more extraordinary phenomenon of this kind +-- the Pleiades nebul. + +The group of the Pleiades, although lying outside the main course of +the Galaxy, is connected with it by a faint loop, and is the scene of +the most remarkable association of stars and nebulous matter known in +the visible universe. The naked eye is unaware of the existence of +nebul in the Pleiades, or, at the best, merely suspects that there is +something of the kind there; and even the most powerful telescopes are +far from revealing the full wonder of the spectacle; but in +photographs which have been exposed for many hours consecutively, in +order to accumulate the impression of the actinic rays, the revelation +is stunning. The principle stars are seen surrounded by, and, as it +were, drowned in, dense nebulous clouds of an unparalleled kind. The +forms assumed by these clouds seem at first sight inexplicable. They +look like fleeces, or perhaps more like splashes and daubs of luminous +paint dashed carelessly from a brush. But closer inspection shows that +they are, to a large extent, woven out of innumerable threads of filmy +texture, and there are many indications of spiral tendencies. Each of +the bright stars of the group -- Alcyone, Merope, Maia, Electra, +Taygeta, Atlas -- is the focus of a dense fog (totally invisible, +remember, alike to the naked eye and to the telescope), and these +particular stars are veiled from sight behind the strange mists. +Running in all directions across the relatively open spaces are +nebulous wisps and streaks of the most curious forms. On some of the +nebular lines, which are either straight throughout, or if they change +direction do so at an angle, little stars are strung like beads. In +one case seven or eight stars are thus aligned, and, as if to +emphasize their dependence upon the chain which connects them, when it +makes a slight bend the file of stars turns the same way. Many other +star rows in the group suggest by their arrangement that they, too, +were once strung upon similar threads which have now disappeared, +leaving the stars spaced along their ancient tracks. We seem forced to +the conclusion that there was a time when the Pleiades were embedded +in a vast nebula resembling that of Orion, and that the cloud has now +become so rare by gradual condensation into stars that the merest +trace of it remains, and this would probably have escaped detection +but for the remarkable actinic power of the radiant matter of which it +consists. The richness of many of these faint nebulous masses in +ultra-violet radiations, which are those that specifically affect the +photographic plate, is the cause of the marvelous revelatory power of +celestial photography. So the veritable unseen universe, as +distinguished from the ``unseen universe'' of metaphysical +speculation, is shown to us. + +A different kind of association between stars and nebul is shown in +some surprising photographic objects in the constellation Cygnus, +where long, wispy nebul, billions of miles in length, some of them +looking like tresses streaming in a breeze, lie amid fields of stars +which seem related to them. But the relation is of a most singular +kind, for notwithstanding the delicate structure of the long nebul +they appear to act as barriers, causing the stars to heap themselves +on one side. The stars are two, three, or four times as numerous on +one side of the nebul as on the other. These nebul, as far as +appearance goes, might be likened to rail fences, or thin hedges, +against which the wind is driving drifts of powdery snow, which, while +scattered plentifully all around, tends to bank itself on the leeward +side of the obstruction. The imagination is at a loss to account for +these extraordinary phenomena; yet there they are, faithfully giving +us their images whenever the photographic plate is exposed to their +radiations. + +Thus the more we see of the universe with improved methods of +observation, and the more we invent aids to human senses, each +enabling us to penetrate a little deeper into the unseen, the greater +becomes the mystery. The telescope carried us far, photography is +carrying us still farther; but what as yet unimagined instrument will +take us to the bottom, the top, and the end? And then, what hitherto +untried power of thought will enable us to comprehend the meaning of +it all? + +Stellar Migrations + +To the untrained eye the stars and the planets are not +distinguishable. It is customary to call them all alike ``stars.'' But +since the planets more or less rapidly change their places in the sky, +in consequence of their revolution about the sun, while the stars +proper seem to remain always in the same relative positions, the +latter are spoken of as ``fixed stars.'' In the beginnings of +astronomy it was not known that the ``fixed stars'' had any motion +independent of their apparent annual revolution with the whole sky +about the earth as a seeming center. Now, however, we know that the +term ``fixed stars'' is paradoxical, for there is not a single really +fixed object in the whole celestial sphere. The apparent fixity in the +positions of the stars is due to their immense distance, combined with +the shortness of the time during which we are able to observe them. It +is like viewing the plume of smoke issuing from a steamer, hull down, +at sea: if one does not continue to watch it for a long time it +appears to be motionless, although in reality it may be traveling at +great speed across the line of sight. Even the planets seem fixed in +position if one watches them for a single night only, and the more +distant ones do not sensibly change their places, except after many +nights of observation. Neptune, for instance, moves but little more +than two degrees in the course of an entire year, and in a month its +change of place is only about one-third of the diameter of the full +moon. + +Yet, fixed as they seem, the stars are actually moving with a speed in +comparison with which, in some cases, the planets might almost be said +to stand fast in their tracks. Jupiter's speed in his orbit is about +eight miles per second, Neptune's is less than three and one-half +miles, and the earth's is about eighteen and one-half miles; while +there are ``fixed stars'' which move two hundred or three hundred +miles per second. They do not all, however, move with so great a +velocity, for some appear to travel no faster than the planets. But in +all cases, notwithstanding their real speed, long-continued and +exceedingly careful observations are required to demonstrate that they +are moving at all. No more overwhelming impression of the frightful +depths of space in which the stars are buried can be obtained than by +reflecting upon the fact that a star whose actual motion across the +line of sight amounts to two hundred miles per second does not change +its apparent place in the sky, in the course of a thousand years, +sufficiently to be noticed by the casual observer of the heavens! + +There is one vast difference between the motions of the stars and +those of the planets to which attention should be at once called: the +planets, being under the control of a central force emanating from +their immediate master, the sun, all move in the same direction and in +orbits concentric about the sun; the stars, on the other hand, move in +every conceivable direction and have no apparent center of motion, for +all efforts to discover such a center have failed. At one time, when +theology had finally to accept the facts of science, a grandiose +conception arose in some pious minds, according to which the Throne of +God was situated at the exact center of His Creation, and, seated +there, He watched the magnificent spectacle of the starry systems +obediently revolving around Him. Astronomical discoveries and +speculations seemed for a time to afford some warrant for this view, +which was, moreover, an acceptable substitute for the abandoned +geocentric theory in minds that could only conceive of God as a +superhuman artificer, constantly admiring his own work. No longer ago +than the middle of the nineteenth century a German astronomer, +Maedler, believed that he had actually found the location of the +center about which the stellar universe revolved. He placed it in the +group of the Pleiades, and upon his authority an extraordinary +imaginative picture was sometimes drawn of the star Alcyone, the +brightest of the Pleiades, as the very seat of the Almighty. This idea +even seemed to gain a kind of traditional support from the mystic +significance, without known historical origin, which has for many +ages, and among widely separated peoples, been attached to the +remarkable group of which Alcyone is the chief. But since Maedler's +time it has been demonstrated that the Pleiades cannot be the center +of revolution of the universe, and, as already remarked, all attempts +to find or fix such a center have proved abortive. Yet so powerful was +the hold that the theory took upon the popular imagination, that even +today astronomers are often asked if Alcyone is not the probable site +of ``Jerusalem the Golden.'' + +If there were a discoverable center of predominant gravitative power, +to which the motions of all the stars could be referred, those motions +would appear less mysterious, and we should then be able to conclude +that the universe was, as a whole, a prototype of the subsidiary +systems of which it is composed. We should look simply to the law of +gravitation for an explanation, and, naturally, the center would be +placed within the opening enclosed by the Milky Way. If it were there +the Milky Way itself should exhibit signs of revolution about it, like +a wheel turning upon its hub. No theory of the star motions as a whole +could stand which failed to take account of the Milky Way as the basis +of all. But the very form of that divided wreath of stars forbids the +assumption of its revolution about a center. Even if it could be +conceived as a wheel having no material center it would not have the +form which it actually presents. As was shown in Chapter 2, there is +abundant evidence of motion in the Milky Way; but it is not motion of +the system as a whole, but motion affecting its separate parts. +Instead of all moving one way, the galactic stars, as far as their +movements can be inferred, are governed by local influences and +conditions. They appear to travel crosswise and in contrary +directions, and perhaps they eddy around foci where great numbers have +assembled; but of a universal revolution involving the entire mass we +have no evidence. + +Most of our knowledge of star motions, called ``proper motions,'' +relates to individual stars and to a few groups which happen to be so +near that the effects of their movements are measurable. In some cases +the motion is so rapid (not in appearance, but in reality) that the +chief difficulty is to imagine how it can have been imparted, and what +will eventually become of the ``runaways.'' Without a collision, or a +series of very close approaches to great gravitational centers, a star +traveling through space at the rate of two hundred or three hundred +miles per second could not be arrested or turned into an orbit which +would keep it forever flying within the limits of the visible +universe. A famous example of these speeding stars is ``1830 +Groombridge,'' a star of only the sixth magnitude, and consequently +just visible to the naked eye, whose motion across the line of sight +is so rapid that it moves upon the face of the sky a distance equal to +the apparent diameter of the moon every 280 years. The distance of +this star is at least 200,000,000,000,000 miles, and may be two or +three times greater, so that its actual speed cannot be less than two +hundred, and may be as much as four hundred, miles per second. It +could be turned into a new course by a close approach to a great sun, +but it could only be stopped by collision, head-on, with a body of +enormous mass. Barring such accidents it must, as far as we can see, +keep on until it has traversed our stellar system, whence in may +escape and pass out into space beyond, to join, perhaps, one of those +other universes of which we have spoken. Arcturus, one of the greatest +suns in the universe, is also a runaway, whose speed of flight has +been estimated all the way from fifty to two hundred miles per second. +Arcturus, we have every reason to believe, possesses hundreds of times +the mass of our sun -- think, then, of the prodigious momentum that +its motion implies! Sirius moves more moderately, its motion across +the line of sight amounting to only ten miles per second, but it is at +the same time approaching the sun at about the same speed, its actual +velocity in space being the resultant of the two displacements. + +What has been said about the motion of Sirius brings us to another +aspect of this subject. The fact is, that in every case of stellar +motion the displacement that we observe represents only a part of the +actual movement of the star concerned. There are stars whose motion +carries them straight toward or straight away from the earth, and such +stars, of course, show no cross motion. But the vast majority are +traveling in paths inclined from a perpendicular to our line of sight. +Taken as a whole, the stars may be said to be flying about like the +molecules in a mass of gas. The discovery of the radial component in +the movements of the stars is due to the spectroscope. If a star is +approaching, its spectral lines are shifted toward the violet end of +the spectrum by an amount depending upon the velocity of approach; if +it is receding, the lines are correspondingly shifted toward the red +end. Spectroscopic observation, then, combined with micrometric +measurements of the cross motion, enables us to detect the real +movement of the star in space. Sometimes it happens that a star's +radial movement is periodically reversed; first it approaches, and +then it recedes. This indicates that it is revolving around a near-by +companion, which is often invisible, and superposed upon this motion +is that of the two stars concerned, which together may be approaching +or receding or traveling across the line of sight. Thus the +complications involved in the stellar motions are often exceedingly +great and puzzling. + +Yet another source of complication exists in the movement of our own +star, the sun. There is no more difficult problem in astronomy than +that of disentangling the effects of the solar motion from those of +the motions of the other stars. But the problem, difficult as it is, +has been solved, and upon its solution depends our knowledge of the +speed and direction of the movement of the solar system through space, +for of course the sun carries its planets with it. One element of the +solution is found in the fact that, as a result of perspective, the +stars toward which we are going appear to move apart toward all points +of the compass, while those behind appear to close up together. Then +the spectroscopic principle already mentioned is invoked for studying +the shift of the lines, which is toward the violet in the stars ahead +of us and toward the red in those that we are leaving behind. Of +course the effects of the independent motions of the stars must be +carefully excluded. The result of the studies devoted to this subject +is to show that we are traveling at a speed of twelve to fifteen miles +per second in a northerly direction, toward the border of the +constellations Hercules and Lyra. A curious fact is that the more +recent estimates show that the direction is not very much out of a +straight line drawn from the sun to the star Vega, one of the most +magnificent suns in the heavens. But it should not be inferred from +this that Vega is drawing us on; it is too distant for its gravitation +to have such an effect. + +Many unaccustomed thoughts are suggested by this mighty voyage of the +solar system. Whence have we come, and whither do we go? Every year of +our lives we advance at least 375,000,000 miles. Since the traditional +time of Adam the sun has led his planets through the wastes of space +no less than 225,000,000,000 miles, or more than 2400 times the +distance that separates him from the earth. Go back in imagination to +the geologic ages, and try to comprehend the distance over which the +earth has flown. Where was our little planet when it emerged out of +the clouds of chaos? Where was the sun when his ``thunder march'' +began? What strange constellations shone down upon our globe when its +masters of life were the monstrous beasts of the ``Age of Reptiles''? +A million years is not much of a span of time in geologic reckoning, +yet a million years ago the earth was farther from its present place +in space than any of the stars with a measurable parallax are now. It +was more than seven times as far as Sirius, nearly fourteen times as +far as Alpha Centauri, three times as far as Vega, and twice as far as +Arcturus. But some geologists demand two hundred, three hundred, even +one thousand million years to enable them to account for the +evolutionary development of the earth and its inhabitants. In a +thousand million years the earth would have traveled farther than from +the remotest conceivable depths of the Milky Way! + +Other curious reflections arise when we think of the form of the +earth's track as it follows the lead of the sun, in a journey which +has neither known beginning nor conceivable end. There are probably +many minds which have found a kind of consolation in the thought that +every year the globe returns to the same place, on the same side of +the sun. This idea may have an occult connection with our traditional +regard for anniversaries. When that period of the year returns at +which any great event in our lives has occurred we have the feeling +that the earth, in its annual round, has, in a manner, brought us back +to the scene of that event. We think of the earth's orbit as a +well-worn path which we traverse many times in the course of a +lifetime. It seems familiar to us, and we grow to have a sort of +attachment to it. The sun we are accustomed to regard as a fixed +center in space, like the mill or pump around which the harnessed +patient mule makes his endless circuits. But the real fact is that the +earth never returns to the place in space where it has once quitted. +In consequence of the motion of the sun carrying the earth and the +other planets along, the track pursued by our globe is a vast spiral +in space continually developing and never returning upon its course. +It is probable that the tracks of the sun and the others stars are +also irregular, and possibly spiral, although, as far as can be at +present determined, they appear to be practically straight. Every +star, wherever it may be situated, is attracted by its fellow-stars +from many sides at once, and although the force is minimized by +distance, yet in the course of many ages its effects must become +manifest. + +Looked at from another side, is there not something immensely +stimulating and pleasing to the imagination in the idea of so +stupendous a journey, which makes all of us the greatest of travelers? +In the course of a long life a man is transported through space thirty +thousand million miles; Halley's Comet does not travel one-quarter as +far in making one of its immense circuits. And there are adventures on +this voyage of which we are just beginning to learn to take account. +Space is full of strange things, and the earth must encounter some of +them as it advances through the unknown. Many singular speculations +have been indulged in by astronomers concerning the possible effects +upon the earth of the varying state of the space that it traverses. +Even the alternation of hot and glacial periods has sometimes been +ascribed to this source. When tropical life flourished around the +poles, as the remains in the rocks assure us, the needed high +temperature may, it has been thought, have been derived from the +presence of the earth in a warm region of space. Then, too, there is a +certain interest for us in the thought of what our familiar planet has +passed through. We cannot but admire it for its long journeying as we +admire the traveler who comes to us from remote and unexplored lands, +or as we gaze with a glow of interest upon the first locomotive that +has crossed a continent, or a ship that has visited the Arctic or +Antarctic regions. If we may trust the indications of the present +course, the earth, piloted by the sun, has come from the Milky Way in +the far south and may eventually rejoin that mighty band of stars in +the far north. + +While the stars in general appear to travel independently of one +another, except when they are combined in binary or trinary systems, +there are notable exceptions to this rule. In some quarters of the sky +we behold veritable migrations of entire groups of stars whose members +are too widely separated to show any indications of revolution about a +common center of gravity. This leads us back again to the wonderful +group of the Pleiades. All of the principle stars composing that group +are traveling in virtually parallel lines. Whatever force set them +going evidently acted upon all alike. This might be explained by the +assumption that when the original projective force acted upon them +they were more closely united than they are at present, and that in +drifting apart they have not lost the impulse of the primal motion. Or +it may be supposed that they are carried along by some current in +space, although it would be exceedingly difficult, in the present +state of our knowledge, to explain the nature of such a current. Yet +the theory of a current has been proposed. As to an attractive center +around which they might revolve, none has been found. Another instance +of similar ``star-drift'' is furnished by five of the seven stars +constituting the figure of the ``Great Dipper.'' In this case the +stars concerned are separated very widely, the two extreme ones by not +less than fifteen degrees, so that the idea of a common motion would +never have been suggested by their aspect in the sky; and the case +becomes the more remarkable from the fact that among and between them +there are other stars, some of the same magnitude, which do not share +their motion, but are traveling in other directions. Still other +examples of the same phenomenon are found in other parts of the sky. +Of course, in the case of compact star-clusters, it is assumed that +all the members share a like motion of translation through space, and +the same is probably true of dense star-swarms and star-clouds. + +The whole question of star-drift has lately assumed a new phase, in +consequence of the investigations of Kapteyn, Dyson, and Eddington on +the ``systematic motions of the stars.'' This research will, it is +hoped, lead to an understanding of the general law governing the +movements of the whole body of stars constituting the visible +universe. Taking about eleven hundred stars whose proper motions have +been ascertained with an approach to certainty, and which are +distributed in all parts of the sky, it has been shown that there +exists an apparent double drift, in two independent streams, moving in +different and nearly opposed directions. The apex of the motion of +what is called ``Stream I'' is situated, according to Professor +Kapteyn, in right ascension 85, declination south 11, which places +it just south of the constellation Orion; while the apex of ``Stream +II'' is in right ascension 260, declination south 48, placing it in +the constellation Ara, south of Scorpio. The two apices differ very +nearly 180 in right ascension and about 120 in declination. The +discovery of these vast star-streams, if they really exist, is one of +the most extraordinary in modern astronomy. It offers the correlation +of stellar movements needed as the basis of a theory of those +movements, but it seems far from revealing a physical cause for them. +As projected against the celestial sphere the stars forming the two +opposite streams appear intermingled, some obeying one tendency and +some the other. As Professor Dyson has said, the hypothesis of this +double movement is of a revolutionary character, and calls for further +investigation. Indeed, it seems at first glance not less surprising +than would be the observation that in a snow-storm the flakes over our +heads were divided into two parties and driving across each other's +course in nearly opposite directions, as if urged by interpenetrating +winds. + +But whatever explanation may eventually be found for the motions of +the stars, the knowledge of the existence of those motions must always +afford a new charm to the contemplative observer of the heavens, for +they impart a sense of life to the starry system that would otherwise +be lacking. A stagnant universe, with every star fixed immovably in +its place, would not content the imagination or satisfy our longing +for ceaseless activity. The majestic grandeur of the evolutions of the +celestial hosts, the inconceivable vastness of the fields of space in +which they are executed, the countless numbers, the immeasurable +distances, the involved convolutions, the flocking and the scattering, +the interpenetrating marches and countermarches, the strange community +of impulsion affecting stars that are wide apart in space and causing +them to traverse the general movement about them like aides and +despatch-bearers on a battle-field -- all these arouse an intensity of +interest which is heightened by the mystery behind them. + +The Passing of the Constellations + +From a historical and picturesque point of view, one of the most +striking results of the motions of the stars described in the last +chapter is their effect upon the forms of the constellations, which +have been watched and admired by mankind from a period so early that +the date of their invention is now unknown. The constellations are +formed by chance combinations of conspicuous stars, like figures in a +kaleidoscope, and if our lives were commensurate with the ons of +cosmic existence we should perceive that the kaleidoscope of the +heavens was ceaselessly turning and throwing the stars into new +symmetries. Even if the stars stood fast, the motion of the solar +system would gradually alter the configurations, as the elements of a +landscape dissolve and recombine in fresh groupings with the +traveler's progress amid them. But with the stars themselves all in +motion at various speeds and in many directions, the changes occur +more rapidly. Of course, ``rapid'' is here understood in a relative +sense; the wheel of human history to an eye accustomed to the majestic +progression of the universe would appear to revolve with the velocity +of a whirling dynamo. Only the deliberation of geological movements +can be contrasted with the evolution and devolution of the +constellations. + +And yet this secular fluctuation of the constellation figures is not +without keen interest for the meditative observer. It is another +reminder of the swift mutability of terrestial affairs. To the passing +glance, which is all that we can bestow upon these figures, they +appear so immutable that they have been called into service to form +the most lasting records of ancient thought and imagination that we +possess. In the forms of the constellations, the most beautiful, and, +in imaginative quality, the finest, mythology that the world has ever +known has been perpetuated. Yet, in a broad sense, this scroll of +human thought imprinted on the heavens is as evanescent as the summer +clouds. Although more enduring than parchment, tombs, pyramids, and +temples, it is as far as they from truly eternizing the memory of what +man has fancied and done. + +Before studying the effects that the motions of the stars have had and +will have upon the constellations, it is worth while to consider a +little further the importance of the stellar pictures as archives of +history. To emphasize the importance of these effects it is only +necessary to recall that the constellations register the oldest +traditions of our race. In the history of primeval religions they are +the most valuable of documents. Leaving out of account for the moment +the more familiar mythology of the Greeks, based on something older +yet, we may refer for illustration to that of the mysterious Maya race +of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group +of ruins perched, after the Mexican and Central-American plan, on the +summits of pyramidal mounds which mark the site of an ancient +theogonic center of the Mayas. Here the temples all evidently refer to +a cult based upon the constellations as symbols. The figures and the +names, of course, were not the same as those that we have derived from +our Aryan ancestors, but the star groups were the same or nearly so. +For instance, the loftiest of the temples at Izamal was connected with +the sign of the constellation known to us as Cancer, marking the place +of the sun at the summer solstice, at which period the sun was +supposed to descend at noon like a great bird of fire and consume the +offerings left upon the altar. Our Scorpio was known to the Mayas as a +sign of the ``Death God.'' Our Libra, the ``Balance,'' with which the +idea of a divine weighing out of justice has always been connected, +seems to be identical with the Mayan constellation Teoyaotlatohua, +with which was associated a temple where dwelt the priests whose +special business it was to administer justice and to foretell the +future by means of information obtained from the spirits of the dead. +Orion, the ``Hunter'' of our celestial mythology, was among the Mayas +a ``Warrior,'' while Sagittarius and others of our constellations were +known to them (under different names, of course), and all were endowed +with a religious symbolism. And the same star figures, having the same +significance, were familiar to the Peruvians, as shown by the temples +at Cuzco. Thus the imagination of ancient America sought in the +constellations symbols of the unchanging gods. + +But, in fact, there is no nation and no people that has not recognized +the constellations, and at one period or another in its history +employed them in some symbolic or representative capacity. As handled +by the Greeks from prehistoric times, the constellation myths became +the very soul of poetry. The imagination of that wonderful race +idealized the principal star groups so effectively that the figures +and traditions thus attached to them have, for civilized mankind, +displaced all others, just as Greek art in its highest forms stands +without parallel and eclipses every rival. The Romans translated no +heroes and heroines of the mythical period of their history to the +sky, and the deified Csars never entered that lofty company, but the +heavens are filled with the early myths of the Greeks. Herakles +nightly resumes his mighty labors in the stars; Zeus, in the form of +the white ``Bull,'' Taurus, bears the fair Europa on his back through +the celestial waves; Andromeda stretches forth her shackled arms in +the star-gemmed ether, beseeching aid; and Perseus, in a blaze of +diamond armor, revives his heroic deeds amid sparkling clouds of +stellar dust. There, too, sits Queen Cassiopeia in her dazzling chair, +while the Great King, Cepheus, towers gigantic over the pole. +Professor Young has significantly remarked that a great number of the +constellations are connected in some way or other with the Argonautic +Expedition -- that strangely fascinating legend of earliest Greek +story which has never lost its charm for mankind. In view of all this, +we may well congratulate ourselves that the constellations will +outlast our time and the time of countless generations to follow us; +and yet they are very far from being eternal. Let us now study some of +the effects of the stellar motions upon them. + +We begin with the familiar figure of the ``Great Dipper.'' He who has +not drunk inspiration from its celestial bowl is not yet admitted to +the circle of Olympus. This figure is made up of seven conspicuous +stars in the constellation Ursa Major, the ``Greater Bear.'' The +handle of the ``Dipper'' corresponds to the tail of the imaginary +``Bear,'' and the bowl lies upon his flank. In fact, the figure of a +dipper is so evident and that of a bear so unevident, that to most +persons the ``Great Dipper'' is the only part of the constellation +that is recognizable. Of the seven stars mentioned, six are of nearly +equal brightness, ranking as of the second magnitude, while the +seventh is of only the third magnitude. The difference is very +striking, since every increase of one magnitude involves an increase +of two-and-a-half times in brightness. There appears to be little +doubt that the faint star, which is situated at the junction of the +bowl and the handle, is a variable of long period, since three hundred +years ago it was as bright as its companions. But however that may be, +its relative faintness at the present time interferes but little with +the perfection of the ``Dipper's'' figure. In order the more readily +to understand the changes which are taking place, it will be well to +mention both the names and the Greek letters which are attached to the +seven stars. Beginning at the star in the upper outer edge of the rim +of the bowl and running in regular order round the bottom and then out +to the end of the handle, the names and letters are as follows: Dubhe +({\alpha}), Merak ({\beta}), Phaed ({\gamma}), Megrez ({\delta}), +Alioth ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez +is the faint star already mentioned at the junction of the bowl and +handle, and Mizar, in the middle of the handle, has a close, naked-eye +companion which is named Alcor. The Arabs called this singular pair of +stars ``The Horse and Rider.'' Merak and Duhbe are called ``The +Pointers,'' because an imaginary line drawn northward through them +indicates the Pole Star. + +Now it has been found that five of these stars -- viz., Merak, Phaed, +Megrez, Alioth, and Mizar (with its comrade) -- are moving with +practically the same speed in an easterly direction, while the other +two, Dubhe and Benetnasch, are simultaneously moving westward, the +motions of Benetnasch being apparently more rapid. The consequence of +these opposed motions is, of course, that the figure of the ``Dipper'' +cannot always have existed and will not continue to exist. In the +accompanying diagrams it has been thought interesting to show the +relative positions of these seven stars, as seen from the point which +the earth now occupies, both in the past and in the future. Arrows +attached to the stars in the figure representing the present +appearance of the ``Dipper'' indicate the directions of the motions +and the distances over which they will carry the stars in a period of +about five hundred centuries. The time, no doubt, seems long, but +remember the vast stretch of ages through which the earth has passed, +and then reflect that no reason is apparent why our globe should not +continue to be a scene of animation for ten thousand centuries yet to +come. The fact that the little star Alcor placed so close to Mizar +should accompany the latter in its flight is not surprising, but that +two of the principal stars of the group should be found moving in a +direction directly opposed to that pursued by the other five is +surprising in the highest degree; and it recalls the strange theory of +a double drift affecting all the stars, to which attention was called +in the preceding chapter. It would appear that Benetnasch and Dubhe +belong to one ``current,'' and Merak, Phaed, Megrez, Alioth, and Mizar +to the other. As far as is known, the motion of the seven stars are +not shared by the smaller stars scattered about them, but on the +theory of currents there should be such a community of motion, and +further investigation may reveal it. + +From the ``Great Dipper'' we turn to a constellation hardly less +conspicuous and situated at an equal distance from the pole on the +other side -- Cassiopeia. This famous star-group commemorating the +romantic Queen of Ethiopia whose vain boasting of her beauty was +punished by the exposure of her daughter Andromeda to the ``Sea +Monster,'' is well-marked by five stars which form an irregular letter +``W'' with its open side toward the pole. Three of these stars are +usually ranked as of the second magnitude, and two of the third; but +to ordinary observation they appear of nearly equal brightness, and +present a very striking picture. They mark out the chair and a part of +the figure of the beautiful queen. Beginning at the right-hand, or +western, end of the ``W,'' their Greek letter designations are: Beta +({\beta}), Alpha ({\alpha}), Gamma ({\gamma}), Delta ({\delta}), and +Epsilon ({\epsilon}). Four of them, Beta, Alpha, Delta, and Epsilon +are traveling eastwardly at various speeds, while the fifth, Gamma, +moves in a westerly direction. The motion of Beta is more rapid than +that of any of the others. It should be said, however, that no little +uncertainty attaches to the estimates of the rate of motion of stars +which are not going very rapidly, and different observers often vary +considerably in their results. + +In the beautiful ``Northern Crown,'' one of the most perfect and +charming of all the figures to be found in the stars, the alternate +combining and scattering effects of the stellar motions are shown by +comparing the appearance which the constellation must have had five +hundred centuries ago with that which it has at present and that which +it will have in the future. The seven principle stars of the asterism, +forming a surprisingly perfect coronet, have movements in three +directions at right angles to one another. That in these circumstances +they should ever have arrived at positions giving them so striking an +appearance of definite association is certainly surprising; from its +aspect one would have expected to find a community of movement +governing the brilliants of the ``Crown,'' but instead of that we find +evidence that they will inevitably drift apart and the beautiful +figure will dissolve. + +A similar fate awaits such asterisms as the ``Northern Cross'' in +Cygnus; the ``Crow'' (Corvus), which stands on the back of the great +``Sea Serpent,'' Hydra, and pecks at his scales; ``Job's Coffin'' +(Delphinus); the ``Great Square of Pegasus''; the ``Twins'' (Gemini); +the beautiful ``Sickle'' in Leo; and the exquisite group of the Hyades +in Taurus. In the case of the Hyades, two controlling movements are +manifest: one, affecting five of the stars which form the well-known +figure of a letter ``V,'' is directed northerly; the other, which +controls the direction of two stars, has an easterly trend. The chief +star of the group, Aldebaran, one of the finest of all stars both for +its brilliance and its color, is the most affected by the easterly +motion. In time it will drift entirely out of connection with its +present neighbors. Although the Hyades do not form so compact a group +as the Pleiades in the same constellation, yet their appearance of +relationship is sufficient to awaken a feeling of surprise over the +fact that, as with the stars of the ``Dipper,'' their association is +only temporary or apparent. + +The great figure of Orion appears to be more lasting, not because its +stars are physically connected, but because of their great distance, +which renders their movements too deliberate to be exactly +ascertained. Two of the greatest of its stars, Betelgeuse and Rigel, +possess, as far as has been ascertained, no perceptible motion across +the line of sight, but there is a little movement perceptible in the +``Belt.'' At the present time this consists of an almost perfect +straight line, a row of second-magnitude stars about equally spaced +and of the most striking beauty. In the course of time, however, the +two right-hand stars, Mintaka and Alnilam (how fine are these Arabic +star names!) will approach each other and form a naked-eye double, but +the third, Alnita, will drift away eastward, so that the ``Belt'' will +no longer exist. + +For one more example, let us go to the southern hemisphere, whose most +celebrated constellation, the ``Southern Cross,'' has found a place in +all modern literatures, although it has no claim to consideration on +account of association with ancient legends. This most attractive +asterism, which has never ceased to fascinate the imagination of +Christendom since it was first devoutly described by the early +explorers of the South, is but a passing collocation of brilliant +stars. Yet even in its transfigurations it has been for hundreds of +centuries, and will continue to be for hundreds of centuries to come, +a most striking object in the sky. Our figures show its appearance in +three successive phases: first, as it was fifty thousand years ago +(viewed from the earth's present location); second, as it is in our +day; and, third, as it will be an equal time in the future. The +nearness of these bright stars to one another -- the length of the +longer beam of the ``Cross'' is only six degrees -- makes this group +very noticeable, whatever the arrangement of its components may be. +The largest star, at the base of the ``Cross,'' is of the first +magnitude, two of the others are of the second magnitude, and the +fourth is of the third. Other stars, not represented in the figures, +increase the effect of a celestial blazonry, although they do not help +the resemblance to a cross. + +But since the motion of the solar system itself will, in the course of +so long a period as fifty thousand years, produce a great change in +the perspective of the heavens as seen from the earth, by carrying us +nearly nineteen trillion miles from our present place, why, it may be +asked, seek to represent future appearances of the constellations +which we could not hope to see, even if we could survive so long? The +answer is: Because these things aid the mind to form a picture of the +effects of the mobility of the starry universe. Only by showing the +changes from some definite point of view can we arrive at a due +comprehension of them. The constellations are more or less familiar to +everybody, so that impending changes of their forms must at once +strike the eye and the imagination, and make clearer the significance +of the movements of the stars. If the future history of mankind is to +resemble its past and if our race is destined to survive yet a million +years, then our remote descendents will see a ``new heavens'' if not a +``new earth,'' and will have to invent novel constellations to +perpetuate their legends and mythologies. + +If our knowledge of the relative distances of the stars were more +complete, it would be an interesting exercise in celestial geometry to +project the constellations probably visible to the inhabitants of +worlds revolving around some of the other suns of space. Our sun is +too insignificant for us to think that he can make a conspicuous +appearance among them, except, perhaps, in a few cases. As seen, for +instance, from the nearest known star, Alpha Centauri, the sun would +appear of the average first magnitude, and consequently from that +standpoint he might be the gem of some little constellation which had +no Sirius, or Arcturus, or Vega to eclipse him with its superior +splendor. But from the distance of the vast majority of the stars the +sun would probably be invisible to the naked eye, and as seen from +nearer systems could only rank as a fifth or sixth magnitude star, +unnoticed and unknown except by the star-charting astronomer. + +Conflagrations in the Heavens + +Suppose it were possible for the world to take fire and burn up -- as +some pessimists think that it will do when the Divine wrath shall have +sufficiently accumulated against it -- nobody out of our own little +corner of space would ever be aware of the catastrophe! With all their +telescopes, the astronomers living in the golden light of Arcturus or +the diamond blaze of Canopus would be unable to detect the least +glimmer of the conflagration that had destroyed the seat of Adam and +his descendents, just as now they are totally ignorant of its +existence. + +But at least fifteen times in the course of recorded history men +looking out from the earth have beheld in the remote depths of space +great outbursts of fiery light, some of them more splendidly luminous +than anything else in the firmament except the sun! If they were +conflagrations, how many million worlds like ours were required to +feed their blaze? + +It is probable that ``temporary'' or ``new'' stars, as these wonderful +apparitions are called, really are conflagrations; not in the sense of +a bonfire or a burning house or city, but in that of a sudden eruption +of inconceivable heat and light, such as would result from the +stripping off the shell of an encrusted sun or the crashing together +of two mighty orbs flying through space with a hundred times the +velocity of the swiftest cannon-shot. + +Temporary stars are the rarest and most erratic of astronomical +phenomena. The earliest records relating to them are not very clear, +and we cannot in every instance be certain that it was one of these +appearances that the ignorant and superstitious old chroniclers are +trying to describe. The first temporary star that we are absolutely +sure of appeared in 1572, and is known as ``Tycho's Star,'' because +the celebrated Danish astronomer (whose remains, with his +gold-and-silver artificial nose -- made necessary by a duel -- still +intact, were disinterred and reburied in 1901) was the first to +perceive it in the sky, and the most assiduous and successful in his +studies of it. As the first fully accredited representative of its +class, this new star made its entry upon the scene with becoming +clat. It is characteristic of these phenomena that they burst into +view with amazing suddenness, and, of course, entirely unexpectedly. +Tycho's star appeared in the constellation Cassiopeia, near a now +well-known and much-watched little star named Kappa, on the evening of +November 11, 1572. The story has often been repeated, but it never +loses interest, how Tycho, going home that evening, saw people in the +street pointing and staring at the sky directly over their heads, and +following the direction of their hands and eyes he was astonished to +see, near the zenith, an unknown star of surpassing brilliance. It +outshone the planet Jupiter, and was therefore far brighter than the +first magnitude. There was not another star in the heavens that could +be compared with it in splendor. Tycho was not in all respects free +from the superstitions of his time -- and who is? -- but he had the +true scientific instinct, and immediately he began to study the +stranger, and to record with the greatest care every change in its +aspect. First he determined as well as he could with the imperfect +instruments of his day, many of which he himself had invented, the +precise location of the phenomena in the sky. Then he followed the +changes that it underwent. At first it brightened until its light +equaled or exceeded that of the planet Venus at her brightest, a +statement which will be appreciated at its full value by anyone who +has ever watched Venus when she plays her dazzling rle of ``Evening +Star,'' flaring like an arc light in the sunset sky. It even became so +brilliant as to be visible in full daylight, since, its position being +circumpolar, it never set in the latitude of Northern Europe. Finally +it began to fade, turning red as it did so, and in March, 1574, it +disappeared from Tycho's searching gaze, and has never been seen again +from that day to this. None of the astronomers of the time could make +anything of it. They had not yet as many bases of speculation as we +possess today. + +Tycho's star has achieved a romantic reputation by being fancifully +identified with the ``Star of Bethlehem,'' said to have led the +wondering Magi from their eastern deserts to the cradle-manger of the +Savior in Palestine. Many attempts have been made to connect this +traditional ``star'' with some known phenomenon of the heavens, and +none seems more idle than this. Yet it persistently survives, and no +astronomer is free from eager questions about it addressed by people +whose imagination has been excited by the legend. It is only necessary +to say that the supposition of a connection between the phenomenon of +the Magi and Tycho's star is without any scientific foundation. It was +originally based on an unwarranted assumption that the star of Tycho +was a variable of long period, appearing once every three hundred and +fifteen years, or thereabout. If that were true there would have been +an apparition somewhere near the traditional date of the birth of +Christ, a date which is itself uncertain. But even the data on which +the assumption was based are inconsistent with the theory. Certain +monkish records speak of something wonderful appearing in the sky in +the years 1264 and 945, and these were taken to have been outbursts of +Tycho's star. Investigation shows that the records more probably refer +to comets, but even if the objects seen were temporary stars, their +dates do not suit the hypothesis; from 945 to 1264 there is a gap of +319 years, and from 1264 to 1572 one of only 308 years; moreover 337 +years have now (1909) elapsed since Tycho saw the last glimmer of his +star. Upon a variability so irregular and uncertain as that, even if +we felt sure that it existed, no conclusion could be found concerning +an apparition occurring 2000 years ago. + +In the year 1600 (the year in which Giordano Bruno was burned at the +stake for teaching that there is more than one physical world), a +temporary star of the third magnitude broke out in the constellation +Cygnus, and curiously enough, considering the rarity of such +phenomena, only four years later another surprisingly brilliant one +appeared in the constellation Ophiuchus. This is often called +``Kepler's star,'' because the great German astronomer devoted to it +the same attention that Tycho had given to the earlier phenomenon. It, +too, like Tycho's, was at first the brightest object in the stellar +heavens, although it seems never to have quite equaled its famous +predecessor in splendor. It disappeared after a year, also turning of +a red color as it became more faint. We shall see the significance of +this as we go on. Some of Kepler's contemporaries suggested that the +outburst of this star was due to a meeting of atoms in space, and idea +bearing a striking resemblance to the modern theory of ``astronomical +collisions.'' + +In 1670, 1848, and 1860 temporary stars made their appearance, but +none of them was of great brilliance. In 1866 one of the second +magnitude broke forth in the ``Northern Crown'' and awoke much +interest, because by that time the spectroscope had begun to be +employed in studying the composition of the stars, and Huggins +demonstrated that the new star consisted largely of incandescent +hydrogen. But this star, apparently unlike the others mentioned, was +not absolutely new. Before its outburst it had shown as a star of the +ninth magnitude (entirely invisible, of course, to the naked eye), and +after about six weeks it faded to its original condition in which it +has ever since remained. In 1876 a temporary star appeared in the +constellation Cygnus, and attained at one time the brightness of the +second magnitude. Its spectrum and its behavior resembled those of its +immediate predecessor. In 1885, astronomers were surprised to see a +sixth-magnitude star glimmering in the midst of the hazy cloud of the +great Andromeda Nebula. It soon absolutely disappeared. Its spectrum +was remarkable for being ``continuous,'' like that of the nebula +itself. A continuous spectrum is supposed to represent a body, or a +mass, which is either solid or liquid, or composed of gas under great +pressure. In January, 1892, a new star was suddenly seen in the +constellation Auriga. It never rose much above the fourth magnitude, +but it showed a peculiar spectrum containing both bright and dark +lines of hydrogen. + +But a bewildering surprise was now in store; the world was to behold +at the opening of the twentieth century such a celestial spectacle as +had not been on view since the times of Tycho and Kepler. Before +daylight on the morning of February 22, 1901, the Rev. Doctor +Anderson, of Edinburgh, an amateur astronomer, who had also been the +first to see the new star in Auriga, beheld a strange object in the +constellation Perseus not far from the celebrated variable star Algol. +He recognized its character at once, and immediately telegraphed the +news, which awoke the startled attention of astronomers all over the +world. When first seen the new star was no brighter than Algol (less +than the second magnitude), but within twenty-four hours it was +ablaze, outshining even the brilliant Capella, and far surpassing the +first magnitude. At the spot in the sky where it appeared nothing +whatever was visible on the night before its coming. This is known +with certainty because a photograph had been made of that very region +on February 21, and this photograph showed everything down to the +twelfth magnitude, but not a trace of the stranger which burst into +view between the 21st and the 22nd like the explosion of a rocket. + +Upon one who knew the stars the apparition of this intruder in a +well-known constellation had the effect of a sudden invasion. The new +star was not far west of the zenith in the early evening, and in that +position showed to the best advantage. To see Capella, the hitherto +unchallenged ruler of that quarter of the sky, abased by comparison +with this stranger of alien aspect, for there was always an unfamiliar +look about the ``nova,'' was decidedly disconcerting. It seemed to +portend the beginning of a revolution in the heavens. One could +understand what the effect of such an apparition must have been in the +superstitious times of Tycho. The star of Tycho had burst forth on the +northern border of the Milky Way; this one was on its southern border, +some forty-five degrees farther east. + +Astronomers were well-prepared this time for the scientific study of +the new star, both astronomical photography and spectroscopy having +been perfected, and the results of their investigations were +calculated to increase the wonder with which the phenomenon was +regarded. The star remained at its brightest only a few days; then, +like a veritable conflagration, it began to languish; and, like the +reflection of a dying fire, as it sank it began to glow with the red +color of embers. But its changes were spasmodic; once about every +three days it flared up only to die away again. During these +fluctuations its light varied alternately in the ratio of one to six. +Finally it took a permanent downward course, and after a few months +the naked eye could no longer perceive it; but it remained visible +with telescopes, gradually fading until it had sunk to the ninth +magnitude. Then another astonishing change happened: in August +photographs taken at the Yerkes Observatory and at Heidelberg showed +that the ``nova'' was surrounded by a spiral nebula! The nebula had +not been there before, and no one could doubt that it represented a +phase of the same catastrophe that had produced the outburst of the +new star. At one time the star seemed virtually to have disappeared, +as if all its substance had been expanded into the nebulous cloud, but +always there remained a stellar nucleus about which the misty spiral +spread wider and ever wider, like a wave expanding around a center of +disturbance. The nebula too showed a variability of brightness, and +four condensations which formed in it seemed to have a motion of +revolution about the star. As time went on the nebula continued to +expand at a rate which was computed to be not less than twenty +thousand miles per second! And now the star itself, showing +indications of having turned into a nebula, behaved in a most erratic +manner, giving rise to the suspicion that it was about to burst out +again. But this did not occur, and at length it sunk into a state of +lethargy from which it has to the present time not recovered. But the +nebulous spiral has disappeared, and the entire phenomena as it now +(1909) exists consists of a faint nebulous star of less than the ninth +magnitude. + +The wonderful transformations just described had been forecast in +advance of the discovery of the nebulous spiral encircling the star by +the spectroscopic study of the latter. At first there was no +suggestion of a nebular constitution, but within a month or two +characteristic nebular lines began to appear, and in less than six +months the whole spectrum had been transformed to the nebular type. In +the mean time the shifting of the spectral lines indicated a +complication of rapid motions in several directions simultaneously. +These motions were estimated to amount to from one hundred to five +hundred miles per second. + +The human mind is so constituted that it feels forced to seek an +explanation of so marvelous a phenomenon as this, even in the absence +of the data needed for a sound conclusion. The most natural +hypothesis, perhaps, is that of a collision. Such a catastrophe could +certainly happen. It has been shown, for instance, that in infinity of +time the earth is sure to be hit by a comet; in the same way it may be +asserted that, if no time limit is fixed, the sun is certain to run +against some obstacle in space, either another star, or a dense meteor +swarm, or one of the dark bodies which there is every reason to +believe abound around us. The consequences of such a collision are +easy to foretell, provided that we know the masses and the velocities +of the colliding bodies. In a preceding chapter we have discussed the +motions of the sun and stars, and have seen that they are so swift +that an encounter between any two of them could not but be disastrous. +But this is not all; for as soon as two stars approached within a few +million miles their speed would be enormously increased by their +reciprocal attractions and, if their motion was directed radially with +respect to their centers, they would come together with a crash that +would reduce them both to nebulous clouds. It is true that the chances +of such a ``head-on'' collision are relatively very small; two stars +approaching each other would most probably fall into closed orbits +around their common center of gravity. If there were a collision it +would most likely be a grazing one instead of a direct front-to-front +encounter. But even a close approach, without any actual collision, +would probably prove disastrous, owing to the tidal influence of each +of the bodies on the other. Suns, in consequence of their enormous +masses and dimensions and the peculiarities of their constitution, are +exceedingly dangerous to one another at close quarters. Propinquity +awakes in them a mutually destructive tendency. Consisting of matter +in the gaseous, or perhaps, in some cases, liquid, state, their tidal +pull upon each other if brought close together might burst them +asunder, and the photospheric envelope being destroyed the internal +incandescent mass would gush out, bringing fiery death to any planets +that were revolving near. Without regard to the resulting disturbance +of the earth's orbit, the close approach of a great star to the sun +would be in the highest degree perilous to us. But this is a danger +which may properly be regarded as indefinitely remote, since, at our +present location in space, we are certainly far from every star except +the sun, and we may feel confident that no great invisible body is +near, for if there were one we should be aware of its presence from +the effects of its attraction. As to dark nebul which may possibly +lie in the track that the solar system is pursuing at the rate of +375,000,000 miles per year, that is another question -- and they, too, +could be dangerous! + +This brings us directly back to ``Nova Persei,'' for among the many +suggestions offered to explain its outburst, as well as those of other +temporary stars, one of the most fruitful is that of a collision +between a star and a vast invisible nebula. Professor Seeliger, of +Munich, first proposed this theory, but it afterward underwent some +modifications from others. Stated in a general form, the idea is that +a huge dark body, perhaps an extinguished sun, encountered in its +progress through space a widespread flock of small meteors forming a +dark nebula. As it plunged into the swarm the friction of the +innumerable collisions with the meteors heated its surface to +incandescence, and being of vast size it then became visible to us as +a new star. Meanwhile the motion of the body through the nebula, and +its rotation upon itself, set up a gyration in the blazing atmosphere +formed around it by the vaporized meteors; and as this atmosphere +spread wider, under the laws of gyratory motion a rotation in the +opposite direction began in the inflamed meteoric cloud outside the +central part of the vortex. Thus the spectral lines were caused to +show motion in opposite directions, a part of the incandescent mass +approaching the earth simultaneously with the retreat of another part. +So the curious spectroscopic observations before mentioned were +explained. This theory might also account for the appearance of the +nebulous spiral first seen some six months after the original +outburst. The sequent changes in the spectrum of the ``nova'' are +accounted for by this theory on the assumption, reasonable enough in +itself, that at first the invading body would be enveloped in a +vaporized atmosphere of relatively slight depth, producing by its +absorption the fine dark lines first observed; but that as time went +on and the incessant collisions continued, the blazing atmosphere +would become very deep and extensive, whereupon the appearance of the +spectral lines would change, and bright lines due to the light of the +incandescent meteors surrounding the nucleus at a great distance would +take the place of the original dark ones. The vortex of meteors once +formed would protect the flying body within from further immediate +collisions, the latter now occurring mainly among the meteors +themselves, and then the central blaze would die down, and the +original splendor of the phenomenon would fade. + +But the theories about Nova Persei have been almost as numerous as the +astronomers who have speculated about it. One of the most startling of +them assumed that the outburst was caused by the running amuck of a +dark star which had encountered another star surrounded with planets, +the renewed outbreaks of light after the principal one had faded being +due to the successive running down of the unfortunate planets! Yet +another hypothesis is based on what we have already said of the tidal +influence that two close approaching suns would have upon each other. +Supposing two such bodies which had become encrusted, but remained +incandescent and fluid within, to approach within almost striking +distance; they would whirl each other about their common center of +gravity, and at the same time their shells would burst under the tidal +strain, and their glowing nuclei being disclosed would produce a great +outburst of light. Applying this theory to a ``nova,'' like that of +1866 in the ``Northern Crown,'' which had been visible as a small star +before the outbreak, and which afterward resumed its former aspect, we +should have to assume that a yet shining sun had been approached by a +dark body whose attraction temporarily burst open its photosphere. It +might be supposed that in this case the dark body was too far advanced +in cooling to suffer the same fate from the tidal pull of its victim. +But a close approach of that kind would be expected to result in the +formation of a binary system, with orbits of great eccentricity, +perhaps, and after the lapse of a certain time the outburst should be +renewed by another approximation of the two bodies. A temporary star +of that kind would rather be ranked as a variable. + +The celebrated French astronomer, Janssen, had a different theory of +Nova Persei, and of temporary stars in general. According to his idea, +such phenomena might be the result of chemical changes taking place in +a sun without interference by, or collision with, another body. +Janssen was engaged for many years in trying to discover evidence of +the existence of oxygen in the sun, and he constructed his observatory +on the summit of Mount Blanc specially to pursue that research. He +believed that oxygen must surely exist in the sun since we find so +many other familiar elements included in the constitution of the solar +globe, and as he was unable to discover satisfactory evidence of its +presence he assumed that it existed in a form unknown on the earth. If +it were normally in the sun's chromosphere, or coronal atmosphere, he +said, it would combine with the hydrogen which we know is there and +form an obscuring envelope of water vapor. It exists, then, in a +special state, uncombined with hydrogen; but let the temperature of +the sun sink to a critical point and the oxygen will assume its normal +properties and combine with the hydrogen, producing a mighty outburst +of light and heat. This, Janssen thought, might explain the phenomena +of the temporary stars. It would also, he suggested, account for their +brief career, because the combination of the elements would be quickly +accomplished, and then the resulting water vapor would form an +atmosphere cutting off the radiation from the star within. + +This theory may be said to have a livelier human interest than some of +the others, since, according to it, the sun may carry in its very +constitution a menace to mankind; one does not like to think of it +being suddenly transformed into a gigantic laboratory for the +explosive combination of oxygen and hydrogen! But while Janssen's +theory might do for some temporary stars, it is inadequate to explain +all the phenomena of Nova Persei, and particularly the appearance of +the great spiral nebula that seemed to exhale from the heart of the +star. Upon the whole, the theory of an encounter between a star and a +dark nebula seems best to fit the observations. By that hypothesis the +expanding billow of light surrounding the core of the conflagration is +very well accounted for, and the spectroscopic peculiarities are also +explained. + +Dr Gustov Le Bon offers a yet more alarming theory, suggesting that +temporary stars are the result of atomic explosion; but we shall touch +upon this more fully in Chapter 14. + +Twice in the course of this discussion we have called attention to the +change of color invariably undergone by temporary stars in the later +stages of their career. This was conspicuous with Nova Persei which +glowed more and more redly as it faded, until the nebulous light began +to overpower that of the stellar nucleus. Nothing could be more +suggestive of the dying out of a great fire. Moreover, change of color +from white to red is characteristic of all variable stars of long +period, such as ``Mira'' in Cetus. It is also characteristic of stars +believed to be in the later stages of evolution, and consequently +approaching extinction, like Antares and Betelgeuse, and still more +notably certain small stars which ``gleam like rubies in the field of +the telescope.'' These last appear to be suns in the closing period of +existence as self-luminous bodies. Between the white stars, such as +Sirius and Rigel, and the red stars, such as Aldebaran and Alpha +Herculis, there is a progressive series of colors from golden yellow +through orange to deep red. The change is believed to be due to the +increase of absorbing vapors in the stellar atmosphere as the body +cools down. In the case of ordinary stars these changes no doubt +occupy many millions of years, which represent the average duration of +solar life; but the temporary stars run through similar changes in a +few months: they resemble ephemeral insects -- born in the morning and +doomed to perish with the going down of the sun. + +Explosive and Whirling Nebul + +One of the most surprising triumphs of celestial photography was +Professor Keeler's discovery, in 1899, that the great majority of the +nebul have a distinctly spiral form. This form, previously known in +Lord Rosse's great ``Whirlpool Nebula,'' had been supposed to be +exceptional; now the photographs, far excelling telescopic views in +the revelation of nebular forms, showed the spiral to be the typical +shape. Indeed, it is a question whether all nebul are not to some +extent spiral. The extreme importance of this discovery is shown in +the effect that it has had upon hitherto prevailing views of solar and +planetary evolution. For more than three-quarters of a century +Laplace's celebrated hypothesis of the manner of origin of the solar +system from a rotating and contracting nebula surrounding the sun had +guided speculation on that subject, and had been tentatively extended +to cover the evolution of systems in general. The apparent forms of +some of the nebul which the telescope had revealed were regarded, and +by some are still regarded, as giving visual evidence in favor of this +theory. There is a ``ring nebula'' in Lyra with a central star, and a +``planetary nebula'' in Gemini bearing no little resemblance to the +planet Saturn with its rings, both of which appear to be practical +realizations of Laplace's idea, and the elliptical rings surrounding +the central condensation of the Andromeda Nebula may be cited for the +same kind of proof. + +But since Keeler's discovery there has been a decided turning away of +speculation another way. The form of the spiral nebul seems to be +entirely inconsistent with the theory of an originally globular or +disk-shaped nebula condensing around a sun and throwing or leaving off +rings, to be subsequently shaped into planets. Some astronomers, +indeed, now reject Laplace's hypothesis in toto, preferring to think +that even our solar system originated from a spiral nebula. Since the +spiral type prevails among the existing nebul, we must make any +mechanical theory of the development of stars and planetary systems +from them accord with the requirements which that form imposes. A +glance at the extraordinary variations upon the spiral which Professor +Keeler's photographs reveal is sufficient to convince one of the +difficulty of the task of basing a general theory upon them. In truth, +it is much easier to criticize Laplace's hypothesis than to invent a +satisfactory substitute for it. If the spiral nebul seem to oppose it +there are other nebul which appear to support it, and it may be that +no one fixed theory can account for all the forms of stellar evolution +in the universe. Our particular planetary system may have originated +very much as the great French mathematician supposed, while others +have undergone, or are now undergoing, a different process of +development. There is always a too strong tendency to regard an +important new discovery and the theories and speculations based upon +it as revolutionizing knowledge, and displacing or overthrowing +everything that went before. Upon the plea that ``Laplace only made a +guess'' more recent guesses have been driven to extremes and treated +by injudicious exponents as ``the solid facts at last.'' + +Before considering more recent theories than Laplace's, let us see +what the nature of the photographic revelations is. The vast celestial +maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be +taken as the leading type of the spiral nebul, although there are +less conspicuous objects of the kind which, perhaps, better illustrate +some of their peculiarities. Lord Rosse's nebula appears far more +wonderful in the photographs than in his drawings made with the aid of +his giant reflecting telescope at Parsonstown, for the photographic +plate records details that no telescope is capable of showing. Suppose +we look at the photograph of this object as any person of common sense +would look at any great and strange natural phenomenon. What is the +first thing that strikes the mind? It is certainly the appearance of +violent whirling motion. One would say that the whole glowing mass had +been spun about with tremendous velocity, or that it had been set +rotating so rapidly that it had become the victim of ``centrifugal +force,'' one huge fragment having broken loose and started to gyrate +off into space. Closer inspection shows that in addition to the +principal focus there are various smaller condensations scattered +through the mass. These are conspicuous in the spirals. Some of them +are stellar points, and but for the significance of their location we +might suppose them to be stars which happen to lie in a line between +us and the nebula. But when we observe how many of them follow most +faithfully the curves of the spirals we cannot but conclude that they +form an essential part of the phenomenon; it is not possible to +believe that their presence in such situations is merely fortuitous. +One of the outer spirals has at least a dozen of these star-like +points strung upon it; some of them sharp, small, and distinct, others +more blurred and nebulous, suggesting different stages of +condensation. Even the part which seems to have been flung loose from +the main mass has, in addition to its central condensation, at least +one stellar point gleaming in the half-vanished spire attached to it. +Some of the more distant stars scattered around the ``whirlpool'' look +as if they too had been shot out of the mighty vortex, afterward +condensing into unmistakable solar bodies. There are at least two +curved rows of minute stars a little beyond the periphery of the +luminous whirl which clearly follow lines concentric with those of the +nebulous spirals. Such facts are simply dumbfounding for anyone who +will bestow sufficient thought upon them, for these are suns, though +they may be small ones; and what a birth is that for a sun! + +Look now again at the glowing spirals. We observe that hardly have +they left the central mass before they begin to coagulate. In some +places they have a ``ropy'' aspect; or they are like peascods filled +with growing seeds, which eventually will become stars. The great +focus itself shows a similar tendency, especially around its +circumference. The sense that it imparts of a tremendous shattering +force at work is overwhelming. There is probably more matter in that +whirling and bursting nebula than would suffice to make a hundred +solar systems! It must be confessed at once that there is no +confirmation of the Laplacean hypothesis here; but what hypothesis +will fit the facts? There is one which it has been claimed does so, +but we shall come to that later. In the meanwhile, as a preparation, +fix in the memory the appearance of that second spiral mass spinning +beside its master which seems to have spurned it away. + +For a second example of the spiral nebul look at the one in the +constellation Triangulum. God, how hath the imagination of puny man +failed to comprehend Thee! Here is creation through destruction with a +vengeance! The spiral form of the nebula is unmistakable, but it is +half obliterated amid the turmoil of flying masses hurled away on all +sides with tornadic fury. The focus itself is splitting asunder under +the intolerable strain, and in a little while, as time is reckoned in +the Cosmos, it will be gyrating into stars. And then look at the +cyclonic rain of already finished stars whirling round the outskirts +of the storm. Observe how scores of them are yet involved in the +fading streams of the nebulous spirals; see how they have been thrown +into vast loops and curves, of a beauty that half redeems the terror +of the spectacle enclosed within their lines -- like iridescent cirri +hovering about the edges of a hurricane. And so again are suns born! + +Let us turn to the exquisite spiral in Ursa Major; how different its +aspect from that of the other! One would say that if the terrific coil +in Triangulum has all but destroyed itself in its fury, this one on +the contrary has just begun its self-demolition. As one gazes one +seems to see in it the smooth, swift, accelerating motion that +precedes catastrophe. The central part is still intact, dense, and +uniform in texture. How graceful are the spirals that smoothly rise +from its oval rim and, gemmed with little stars, wind off into the +darkness until they have become as delicate as threads of gossamer! +But at bottom the story told here is the same -- creation by gyration! + +Compare with the above the curious mass in Cetus. Here the plane of +the whirling nebula nearly coincides with our line of sight and we see +the object at a low angle. It is far advanced and torn to shreds, and +if we could look at it perpendicularly to its plane it is evident that +it would closely resemble the spectacle in Triangulum. + +Then take the famous Andromeda Nebula (see Frontispiece), which is so +vast that notwithstanding its immense distance even the naked eye +perceives it as an enigmatical wisp in the sky. Its image on the +sensitive plate is the masterpiece of astronomical photography; for +wild, incomprehensible beauty there is nothing that can be compared +with it. Here, if anywhere, we look upon the spectacle of creation in +one of its earliest stages. The Andromeda Nebula is apparently less +advanced toward transformation into stellar bodies than is that in +Triangulum. The immense crowd of stars sprinkled over it and its +neighborhood seem in the main to lie this side of the nebula, and +consequently to have no connection with it. But incipient stars (in +some places clusters of them) are seen in the nebulous rings, while +one or two huge masses seem to give promise of transformation into +stellar bodies of unusual magnitude. I say ``rings'' because although +the loops encompassing the Andromeda Nebula have been called spirals +by those who wish utterly to demolish Laplace's hypothesis, yet they +are not manifestly such, as can be seen on comparing them with the +undoubted spirals of the Lord Rosse Nebula. They look quite as much +like circles or ellipses seen at an angle of, say, fifteen or twenty +degrees to their plane. If they are truly elliptical they accord +fairly well with Laplace's idea, except that the scale of magnitude is +stupendous, and if the Andromeda Nebula is to become a solar system it +will surpass ours in grandeur beyond all possibility of comparison. + +There is one circumstance connected with the spiral nebul, and +conspicuous in the Andromeda Nebula on account of its brightness, +which makes the question of their origin still more puzzling; they all +show continuous spectra, which, as we have before remarked, indicate +that the mass from which the light comes is either solid or liquid, or +a gas under heavy pressure. Thus nebul fall into two classes: the +``white'' nebul, giving a continuous spectrum; and the ``green'' +nebul whose spectra are distinctly gaseous. The Andromeda Nebula is +the great representative of the former class and the Orion Nebula of +the latter. The spectrum of the Andromeda Nebula has been interpreted +to mean that it consists not of luminous gas, but of a flock of stars +so distant that they are separately indistinguishable even with +powerful telescopes, just as the component stars of the Milky Way are +indistinguishable with the naked eye; and upon this has been based the +suggestion that what we see in Andromeda is an outer universe whose +stars form a series of elliptical garlands surrounding a central mass +of amazing richness. But this idea is unacceptable if for no other +reason than that, as just said, all the spiral nebul possess the same +kind of spectrum, and probably no one would be disposed to regard them +all as outer universes. As we shall see later, the peculiarity of the +spectra of the spiral nebul is appealed to in support of a modern +substitute for Laplace's hypothesis. + +Finally, without having by any means exhausted the variety exhibited +by the spiral nebul, let us turn to the great representative of the +other species, the Orion Nebula. In some ways this is even more +marvelous than the others. The early drawings with the telescope +failed to convey an adequate conception either of its sublimity or of +its complication of structure. It exists in a nebulous region of +space, since photographs show that nearly the whole constellation is +interwoven with faintly luminous coils. To behold the entry of the +great nebula into the field even of a small telescope is a startling +experience which never loses its novelty. As shown by the photographs, +it is an inscrutable chaos of perfectly amazing extent, where spiral +bands, radiating streaks, dense masses, and dark yawning gaps are +strangely intermingled without apparent order. In one place four +conspicuous little stars, better seen in a telescope than in the +photograph on account of the blurring produced by over-exposure, are +suggestively situated in the midst of a dark opening, and no observer +has ever felt any doubt that these stars have been formed from the +substance of the surrounding nebula. There are many other stars +scattered over its expanse which manifestly owe their origin to the +same source. But compare the general appearance of this nebula with +the others that we have studied, and remark the difference. If the +unmistakably spiral nebul resemble bursting fly-wheels or grindstones +from whose perimeters torrents of sparks are flying, the Orion Nebula +rather recalls the aspect of a cloud of smoke and fragments produced +by the explosion of a shell. This idea is enforced by the look of the +outer portion farthest from the bright half of the nebula, where +sharply edged clouds with dark spaces behind seem to be billowing away +as if driven by a wind blowing from the center. + +Next let us consider what scientific speculation has done in the +effort to explain these mysteries. Laplace's hypothesis can certainly +find no standing ground either in the Orion Nebula or in those of a +spiral configuration, whatever may be its situation with respect to +the grand Nebula of Andromeda, or the ``ring'' and ``planetary'' +nebul. Some other hypothesis more consonant with the appearances must +be found. Among the many that have been proposed the most elaborate is +the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton. +It is to be remarked that it applies to the spiral nebul +distinctively, and not to an apparently chaotic mass of gas like the +vast luminous cloud in Orion. The gist of the theory is that these +curious objects are probably the result of close approaches to each +other of two independent suns, reminding us of what was said on this +subject when we were dealing with temporary stars. Of the previous +history of these appulsing suns the theory gives us no account; they +are simply supposed to arrive within what may be called an effective +tide-producing distance, and then the drama begins. Some of the +probable consequences of such an approach have been noticed in Chapter +5; let us now consider them a little more in detail. + +Tides always go in couples; if there is a tide on one side of a globe +there will be a corresponding tide on the other side. The cause is to +be found in the law that the force of gravitation varies inversely as +the square of the distance; the attraction on the nearest surface of +the body exercised by another body is greater than on its center, and +greater yet than on its opposite surface. If two great globes attract +each other, each tends to draw the other out into an ellipsoidal +figure; they must be more rigid than steel to resist this -- and even +then they cannot altogether resist. If they are liquid or gaseous they +will yield readily to the force of distortion, the amount of which +will depend upon their distance apart, for the nearer they are the +greater becomes the tidal strain. If they are encrusted without and +liquid or gaseous in the interior, the internal mass will strive to +assume the figure demanded by the tidal force, and will, if it can, +burst the restraining envelope. Now this is virtually the predicament +of the body we call a sun when in the immediate presence of another +body of similarly great mass. Such a body is presumably gaseous +throughout, the component gases being held in a state of rigidity by +the compression produced by the tremendous gravitational force of +their own aggregate mass. At the surface such a body is enveloped in a +shell of relatively cool matter. Now suppose a great attracting body, +such as another sun, to approach near enough for the difference in its +attraction on the two opposite sides of the body and on its center to +become very great; the consequence will be a tidal deformation of the +whole body, and it will lengthen out along the line of the +gravitational pull and draw in at the sides, and if its shell offers +considerable resistance, but not enough to exercise a complete +restraint, it will be violently burst apart, or blown to atoms, and +the internal mass will leap out on the two opposite sides in great +fiery spouts. In the case of a sun further advanced in cooling than +ours the interior might be composed of molten matter while the +exterior crust had become rigid like the shell of an egg; then the +force of the ``tidal explosion'' produced by the appulse of another +sun would be more violent in consequence of the greater resistance +overcome. Such, then, is the mechanism of the first phase in the +history of a spiral nebula according to the Planetesimal Hypothesis. +Two suns, perhaps extinguished ones, have drawn near together, and an +explosive outburst has occured in one or both. The second phase calls +for a more agile exercise of the imagination. + +To simplify the case, let us suppose that only one of the tugging suns +is seriously affected by the strain. Its vast wings produced by the +outburst are twisted into spirals by their rotation and the contending +attractions exercised upon them, as the two suns, like battleships in +desperate conflict, curve round each other, concentrating their +destructive energies. Then immense quantities of dbris are scattered +about in which eddies are created, and finally, as the sun that caused +the damage goes on its way, leaving its victim to repair its injuries +as it may, the dispersed matter cools, condenses, and turns into +streams of solid particles circling in elliptical paths about their +parent sun. These particles, or fragments, are the ``planetesimals'' +of the theory. In consequence of the inevitable intersection of the +orbits of the planetesimals, nodes are formed where the flying +particles meet, and at these nodes large masses are gradually +accumulated. The larger the mass the greater its attraction, and at +last the nodal points become the nuclei of great aggregations from +which planets are shaped. + +This, in very brief form, is the Planetesimal Hypothesis which we are +asked to substitute for that based on Laplace's suggestion as an +explanation of the mode of origin of the solar system; and the +phenomena of the spiral nebul are appealed to as offering evident +support to the new hypothesis. We are reminded that they are +elliptical in outline, which accords with the hypothesis; that their +spectra are not gaseous, which shows that they may be composed of +solid particles like the planetesimals; and that their central masses +present an oval form, which is what would result from the tidal +effects, as just described. We also remember that some of them, like +the Lord Rosse and the Andromeda nebul, are visually double, and in +these cases we might suppose that the two masses represent the +tide-burst suns that ventured into too close proximity. It may be +added that the authors of the theory do not insist upon the appulse of +two suns as the only way in which the planetesimals may have +originated, but it is the only supposition that has been worked out. + +But serious questions remain. It needs, for instance, but a glance at +the Triangulum monster to convince the observer that it cannot be a +solar system which is being evolved there, but rather a swarm of +stars. Many of the detached masses are too vast to admit of the +supposition that they are to be transformed into planets, in our sense +of planets, and the distances of the stars which appear to have been +originally ejected from the focal masses are too great to allow us to +liken the assemblage that they form to a solar system. Then, too, no +nodes such as the hypothesis calls for are visible. Moreover, in most +of the spiral nebul the appearances favor the view that the +supposititious encountering suns have not separated and gone each +rejoicing on its way, after having inflicted the maximum possible +damage on its opponent, but that, on the contrary, they remain in +close association like two wrestlers who cannot escape from each +other's grasp. And this is exactly what the law of gravitation +demands; stars cannot approach one another with impunity, with regard +either to their physical make-up or their future independence of +movement. The theory undertakes to avoid this difficulty by assuming +that in the case of our system the approach of the foreign body to the +sun was not a close one -- just close enough to produce the tidal +extrusion of the relatively insignificant quantity of matter needed to +form the planets. But even then the effect of the appulse would be to +change the direction of flight, both of the sun and of its visitor, +and there is no known star in the sky which can be selected as the +sun's probable partner in their ancient pas deux. That there are +unconquered difficulties in Laplace's hypothesis no one would deny, +but in simplicity of conception it is incomparably more satisfactory, +and with proper modifications could probably be made more consonant +with existing facts in our solar system than that which is offered to +replace it. Even as an explanation of the spiral nebul, not as solar +systems in process of formation, but as the birthplaces of stellar +clusters, the Planetesimal Hypothesis would be open to many +objections. Granting its assumptions, it has undoubtedly a strong +mathematical framework, but the trouble is not with the mathematics +but with the assumptions. Laplace was one of the ablest mathematicians +that ever lived, but he had never seen a spiral nebula; if he had, he +might have invented a hypothesis to suit its phenomena. His actual +hypothesis was intended only for our solar system, and he left it in +the form of a ``note'' for the consideration of his successors, with +the hope that they might be able to discover the full truth, which he +confessed was hidden from him. It cannot be said that that truth has +yet been found, and when it is found the chances are that intuition +and not logic will have led to it. + +The spiral nebul, then, remain among the greatest riddles of the +universe, while the gaseous nebul, like that of Orion, are no less +mysterious, although it seems impossible to doubt that both forms give +birth to stars. It is but natural to look to them for light on the +question of the origin of our planetary system; but we should not +forget that the scale of the phenomena in the two cases is vastly +different, and the forces in operation may be equally different. A +hill may have been built up by a glacier, while a mountain may be the +product of volcanic forces or of the upheaval of the strata of the +planet. + +The Banners of the Sun + +As all the world knows, the sun, a blinding globe pouring forth an +inconceivable quantity of light and heat, whose daily passage through +the sky is caused by the earth's rotation on its axis, constitutes the +most important phenomenon of terrestial existence. Viewed with a dark +glass to take off the glare, or with a telescope, its rim is seen to +be a sharp and smooth circle, and nothing but dark sky is visible +around it. Except for the interference of the moon, we should probably +never have known that there is any more of the sun than our eyes +ordinarily see. + +But when an eclipse of the sun occurs, caused by the interposition of +the opaque globe of the moon, we see its immediate surroundings, which +in some respects are more wonderful than the glowing central orb. +These surroundings, although not in the sense in which we apply the +term to the gaseous envelope of the earth, may be called the sun's +atmosphere. They consist of two very different parts -- first, the red +``prominences,'' which resemble tongues of flame ascending thousands +of miles above the sun's surface; and, second, the ``corona,'' which +extends to distances of millions of miles from the sun, and shines +with a soft, glowing light. The two combined, when well seen, make a +spectacle without parallel among the marvels of the sky. Although many +attempts have been made to render the corona visible when there is no +eclipse, all have failed, and it is to the moon alone that we owe its +revelation. To cover the sun's disk with a circular screen will not +answer the purpose because of the illumination of the air all about +the observer. When the moon hides the sun, on the other hand, the +sunlight is withdrawn from a great cylinder of air extending to the +top of the atmosphere and spreading many miles around the observer. +There is then no glare to interfere with the spectacle, and the corona +appears in all its surprising beauty. The prominences, however, +although they were discovered during an eclipse, can now, with the aid +of the spectroscope, be seen at any time. But the prominences are +rarely large enough to be noticed by the naked eye, while the +streamers of the corona, stretching far away in space, like ghostly +banners blown out from the black circle of the obscuring moon, attract +every eye, and to this weird apparition much of the fear inspired by +eclipses has been due. But if the corona has been a cause of terror in +the past it has become a source of growing knowledge in our time. + +The story of the first scientific observation of the corona and the +prominences is thrillingly interesting, and in fact dramatic. The +observation was made during the eclipse of 1842, which fortunately was +visible all over Central and Southern Europe so that scores of +astronomers saw it. The interest centers in what happened at Pavia in +Northern Italy, where the English astronomer Francis Baily had set up +his telescope. The eclipse had begun and Bailey was busy at his +telescope when, to quote his own words in the account which he wrote +for the Memoirs of the Royal Astronomical Society: + + I was astounded by a tremendous burst of applause from the streets + below, and at the same moment was electrified by the sight of one + of the most brilliant and splendid phenomena that can well be + imagined; for at that instant the dark body of the moon was + suddenly surrounded with a corona, or kind of bright glory, similar + in shape and magnitude to that which painters draw round the heads + of saints... + + Pavia contains many thousand inhabitants, the major part of whom + were at this early hour walking about the streets and squares or + looking out of windows in order to witness this long-talked-of + phenomenon; and when the total obscuration took place, which was + instantaneous, there was a universal shout from every observer + which ``made the welkin ring,'' and for the moment withdrew my + attention from the object with which I was immediately occupied. I + had, indeed, expected the appearance of a luminous circle round the + moon during the time of total obscurity; but I did not expect, from + any of the accounts of preceding eclipses that I had read, to + witness so magnificent an exhibition as that which took place... + + Splendid and astonishing, however, as this remarkable phenomenon + really was, and although it could not fail to call forth the + admiration and applause of every beholder, yet I must confess that + there was at the same time something in its singular and wonderful + appearance that was appalling... + + But the most remarkable circumstance attending the phenomenon was + the appearance of three large protuberances apparently emanating + from the circumference of the moon, but evidently forming a portion + of the corona. They had the appearance of mountains of a prodigious + elevation; their color was red tinged with lilac or purple; perhaps + the color of the peach-blossom would more nearly represent it. They + somewhat resembled the tops of the snowy Alpine mountains when + colored by the rising or the setting sun. They resembled the Alpine + mountains in another respect, inasmuch as their light was perfectly + steady, and had none of that flickering or sparkling motion so + visible in other parts of the corona... + + The whole of these protuberances were visible even to the last + moment of total obscuration, and when the first ray of light was + admitted from the sun they vanished, with the corona, altogether, + and daylight was instantly restored. + +I have quoted nearly all of this remarkable description not alone for +its intrinsic interest, but because it is the best depiction that can +be found of the general phenomena of a total solar eclipse. Still, not +every such eclipse offers an equally magnificent spectacle. The +eclipses of 1900 and 1905, for instance, which were seen by the +writer, the first in South Carolina and the second in Spain, fell far +short of that described by Bailey in splendor and impressiveness. Of +course, something must be allowed for the effect of surprise; Bailey +had not expected to see what was so suddenly disclosed to him. But +both in 1900 and 1905 the amount of scattered light in the sky was +sufficient in itself to make the corona appear faint, and there were +no very conspicuous prominences visible. Yet on both occasions there +was manifest among the spectators that mingling of admiration and awe +of which Bailey speaks. The South Carolinians gave a cheer and the +ladies waved their handkerchiefs when the corona, ineffably delicate +of form and texture, melted into sight and then in two minutes melted +away again. The Spaniards, crowded on the citadel hill of Burgos, with +their king and his royal retinue in their midst, broke out with a +great clapping of hands as the awaited spectacle unfolded itself in +the sky; and on both occasions, before the applause began, after an +awed silence a low murmur ran through the crowds. At Burgos it is said +many made the sign of the cross. + +It was not long before Bailey's idea that the prominences were a part +of the corona was abandoned, and it was perceived that the two +phenomena were to a great extent independent. At the eclipse of 1868, +which the astronomers, aroused by the wonderful scene of 1842, and +eager to test the powers of the newly invented spectroscope, flocked +to India to witness, Janssen conceived the idea of employing the +spectroscope to render the prominences visible when there was no +eclipse. He succeeded the very next day, and these phenomena have been +studied in that way ever since. + +There are recognized two kinds of prominences -- the ``erruptive'' and +the ``quiescent.'' The latter, which are cloud-like in form, may be +seen almost anywhere along the edge of the sun; but the former, which +often shoot up as if hurled from mighty volcanoes, appear to be +associated with sun-spots, and appear only above the zones where spots +abound. Either of them, when seen in projection against the brilliant +solar disk, appears white, not red, as against a background of sky. +The quiescent prominences, whose elevation is often from forty +thousand to sixty thousand miles, consist, as the spectroscope shows, +mainly of hydrogen and helium. The latter, it will be remembered, is +an element which was known to be in the sun many years before the +discovery that it also exists in small quantities on the earth. A fact +which may have a significance which we cannot at present see is that +the emanation from radium gradually and spontaneously changes into +helium, an alchemistical feat of nature that has opened many curious +vistas to speculative thinkers. The eruptive prominences, which do not +spread horizontally like the others, but ascend with marvelous +velocity to elevations of half a million miles or more, are apparently +composed largely of metallic vapors -- i.e. metals which are usually +solid on the earth, but which at solar temperatures are kept in a +volatilized state. The velocity of their ascent occasionally amounts +to three hundred or four hundred miles per second. It is known from +mathematical considerations that the gravitation of the sun would not +be able to bring back any body that started from its surface with a +velocity exceeding three hundred and eighty-three miles per second; so +it is evident that some of the matter hurled forth in eruptive +prominences may escape from solar control and go speeding out into +space, cooling and condensing into solid masses. There seems to be no +reason why some of the projectiles from the sun might not reach the +planets. Here, then, we have on a relatively small scale, explosions +recalling those which it has been imagined may be the originating +cause of some of the sudden phenomena of the stellar heavens. + +Of the sun-spots it is not our intention here specifically to speak, +but they evidently have an intimate connection with eruptive +prominences, as well as some relation, not yet fully understood, with +the corona. Of the real cause of sun-spots we know virtually nothing, +but recent studies by Professor Hale and others have revealed a +strange state of things in the clouds of metallic vapors floating +above them and their surroundings. Evidences of a cyclonic tendency +have been found, and Professor Hale has proved that sun-spots are +strong magnetic fields, and consist of columns of ionized vapors +rotating in opposite directions in the two hemispheres. A fact which +may have the greatest significance is that titanium and vanadium have +been found both in sun-spots and in the remarkable variable Mira Ceti, +a star which every eleven months, or thereabout, flames up with great +brilliancy and then sinks back to invisibility with the naked eye. It +has been suggested that sun-spots are indications of the beginning of +a process in the sun which will be intensified until it falls into the +state of such a star as Mira. Stars very far advanced in evolution, +without showing variability, also exhibit similar spectra; so that +there is much reason for regarding sunspots as emblems of advancing +age. + +The association of the corona with sun-spots is less evident than that +of the eruptive prominences; still such an association exists, for the +form and extent of the corona vary with the sun-spot period of which +we shall presently speak. The constitution of the corona remains to be +discovered. It is evidently in part gaseous, but it also probably +contains matter in the form of dust and small meteors. It includes one +substance altogether mysterious -- ``coronium.'' There are reasons for +thinking that this may be the lightest of all the elements, and +Professor Young, its discoverer, said that it was ``absolutely unique +in nature; utterly distinct from any other known form of matter, +terrestial, solar, or cosmical.'' The enormous extent of the corona is +one of its riddles. Since the development of the curious subject of +the ``pressure of light'' it has been proposed to account for the +sustentation of the corona by supposing that it is borne upon the +billows of light continually poured out from the sun. Experiment has +proved, what mathematical considerations had previously pointed out as +probable, that the waves of light exert a pressure or driving force, +which becomes evident in its effects if the body acted upon is +sufficiently small. In that case the light pressure will prevail over +the attraction of gravitation, and propel the attenuated matter away +from the sun in the teeth of its attraction. The earth itself would be +driven away if, instead of consisting of a solid globe of immense +aggregate mass, it were a cloud of microscopic particles. The reason +is that the pressure varies in proportion to the surface of the body +acted upon, while the gravitational attraction is proportional to the +volume, or the total amount of matter in the body. But the surface of +any body depends upon the square of its diameter, while the volume +depends upon the cube of the diameter. If, for instance, the diameter +is represented by 4, the surface will be proportional to 4 4, or 16, +and the volume to 4 4 4, or 64; but if the diameter is taken as 2, +the surface will be 2 2, or 4, and the volume 2 2 2, or 8. Now, +the ratio of 4 to 8 is twice as great as that of 16 to 64. If the +diameter is still further decreased, the ratio of the surface to the +volume will proportionally grow larger; in other words, the pressure +will gain upon the attraction, and whatever their original ratio may +have been, a time will come, if the diminution of size continues, when +the pressure will become more effective than the attraction, and the +body will be driven away. Supposing the particles of the corona to be +below the critical size for the attraction of a mass like that of the +sun to control them, they would be driven off into the surrounding +space and appear around the sun like the clouds of dust around a mill. +We shall return to this subject in connection with the Zodiacal Light, +the Aurora, and Comets. + +On the other hand, there are parts of the corona which suggest by +their forms the play of electric or magnetic forces. This is +beautifully shown in some of the photographs that have been made of +the corona during recent eclipses. Take, for instance, that of the +eclipse of 1900. The sheaves of light emanating from the poles look +precisely like the ``lines of force'' surrounding the poles of a +magnet. It will be noticed in this photograph that the corona appears +to consist of two portions: one comprising the polar rays just spoken +of, and the other consisting of the broader, longer, and less-defined +masses of light extending out from the equatorial and middle-latitude +zones. Yet even in this more diffuse part of the phenomenon one can +detect the presence of submerged curves bearing more or less +resemblance to those about the poles. Just what part electricity or +electro-magnetism plays in the mechanism of the solar radiation it is +impossible to say, but on the assumption that it is a very important +part is based the hypothesis that there exists a direct solar +influence not only upon the magnetism, but upon the weather of the +earth. This hypothesis has been under discussion for half a century, +and still we do not know just how much truth it represents. It is +certain that the outbreak of great disturbances on the sun, +accompanied by the formation of sun-spots and the upshooting of +eruptive prominences (phenomena which we should naturally expect to be +attended by action), have been instantly followed by corresponding +``magnetic storms'' on the earth and brilliant displays of the auroral +lights. There have been occasions when the influence has manifested +itself in the most startling ways, a great solar outburst being +followed by a mysterious gripping of the cable and telegraph systems +of the world, as if an invisible and irresistible hand had seized +them. Messages are abruptly cut off, sparks leap from the telegraph +instruments, and the entire earth seems to have been thrown into a +magnetic flurry. These occurrences affect the mind with a deep +impression of the dependence of our planet on the sun, such as we do +not derive from the more familiar action of the sunlight on the growth +of plants and other phenomena of life depending on solar influences. + +Perhaps the theory of solar magnetic influence upon the weather is +best known in connection with the ``sun-spot cycle.'' This, at any +rate, is, as already remarked, closely associated with the corona. Its +existence was discovered in 1843 by the German astronomer Schwabe. It +is a period of variable length, averaging about eleven years, during +which the number of spots visible on the sun first increases to a +maximum, then diminishes to a minimum, and finally increases again to +a maximum. For unknown reasons the period is sometimes two or three +years longer than the average and sometimes as much shorter. +Nevertheless, the phenomena always recur in the same order. Starting, +for instance, with a time when the observer can find few or no spots, +they gradually increase in number and size until a maximum, in both +senses, is reached, during which the spots are often of enormous size +and exceedingly active. After two or three years they begin to +diminish in number, magnitude, and activity until they almost or quite +disappear. A strange fact is that when a new period opens, the spots +appear first in high northern and southern latitudes, far from the +solar equator, and as the period advances they not only increase in +number and size, but break out nearer and nearer to the equator, the +last spots of a vanishing period sometimes lingering in the equatorial +region after the advance-guard of its successor has made its +appearance in the high latitudes. Spots are never seen on the equator +nor near the poles. It was not very long after the discovery of the +sun-spot cycle that the curious observation was made that a striking +coincidence existed between the period of the sun-spots and another +period affecting the general magnetic condition of the earth. When a +curved line representing the varying number of sun-spots was compared +with another curve showing the variations in the magnetic state of the +earth the two were seen to be in almost exact accord, a rise in one +curve corresponding to a rise in the other, and a fall to a fall. +Continued observation has proved that this is a real coincidence and +not an accidental one, so that the connection, although as yet +unexplained, is accepted as established. But does the influence extend +further, and directly affect the weather and the seasons as well as +the magnetic elements of the earth? A final answer to this question +cannot yet be given, for the evidence is contradictory, and the +interpretations put upon it depend largely on the predilections of the +judges. + +But, in a broad sense, the sun-spots and the phenomena connected with +them must have a relation to terrestial meteorology, for they prove +the sun to be a variable star. Reference was made, a few lines above, +to the resemblance of the spectra of sun-spots to those of certain +stars which seem to be failing through age. This in itself is +extremely suggestive; but if this resemblance had never been +discovered, we should have been justified in regarding the sun as +variable in its output of energy; and not only variable, but probably +increasingly so. The very inequalities in the sun-spot cycle are +suspicious. When the sun is most spotted its total light may be +reduced by one-thousandth part, although it is by no means certain +that its outgiving of thermal radiations is then reduced. A loss of +one-thousandth of its luminosity would correspond to a decrease of +.0025 of a stellar magnitude, considering the sun as a star viewed +from distant space. So slight a change would not be perceptible; but +it is not alone sun-spots which obscure the solar surface, its entire +globe is enveloped with an obscuring veil. When studied with a +powerful telescope the sun's surface is seen to be thickly mottled +with relatively obscure specks, so numerous that it has been estimated +that they cut off from one-tenth to one-twentieth of the light that we +should receive from it if the whole surface were as brilliant as its +brightest parts. The condition of other stars warrants the conclusion +that this obscuring envelope is the product of a process of +refrigeration which will gradually make the sun more and more variable +until its history ends in extinction. Looking backward, we see a time +when the sun must have been more brilliant than it is now. At that +time it probably shone with the blinding white splendor of such stars +as Sirius, Spica, and Vega; now it resembles the relatively dull +Procyon; in time it will turn ruddy and fall into the closing cycle +represented by Antares. Considering that once it must have been more +radiantly powerful than at present, one is tempted to wonder if that +could have been the time when tropical life flourished within the +earth's polar circles, sustained by a vivific energy in the sun which +it has now lost. + +The corona, as we have said, varies with the sun-spot cycle. When the +spots are abundant and active the corona rises strong above the +spotted zones, forming immense beams or streamers, which on one +occasion, at least, had an observed length of ten million miles. At +the time of a spot minimum the corona is less brilliant and has a +different outline. It is then that the curved polar rays are most +conspicuous. Thus the vast banners of the sun, shaken out in the +eclipse, are signals to tell of its varying state, but it will +probably be long before we can read correctly their messages. + +The Zodiacal Light Mystery + +There is a singular phenomenon in the sky -- one of the most puzzling +of all -- which has long arrested the attention of astronomers, +defying their efforts at explanation, but which probably not one in a +hundred, and possibly not one in a thousand, of the readers of this +book has ever seen. Yet its name is often spoken, and it is a +conspicuous object if one knows when and where to look for it, and +when well seen it exhibits a mystical beauty which at the same time +charms and awes the beholder. It is called ``The Zodiacal Light,'' +because it lies within the broad circle of the Zodiac, marking the +sun's apparent annual path through the stars. What it is nobody has +yet been able to find out with certainty, and books on astronomy +usually speak of it with singular reserve. But it has given rise to +many remarkable theories, and a true explanation of it would probably +throw light on a great many other celestial mysteries. The Milky Way +is a more wonderful object to look upon, but its nature can be +comprehended, while there is a sort of uncanniness about the Zodiacal +Light which immediately impresses one upon seeing it, for its part in +the great scheme of extra-terrestrial affairs is not evident. + +If you are out-of-doors soon after sunset -- say, on an evening late +in the month of February -- you may perceive, just after the angry +flush of the dying winter's day has faded from the sky, a pale ghostly +presence rising above the place where the sun went down. The writer +remembers from boyhood the first time it was pointed out to him and +the unearthly impression that it made, so that he afterward avoided +being out alone at night, fearful of seeing the spectral thing again. +The phenomenon brightens slowly with the fading of the twilight, and +soon distinctly assumes the shape of an elongated pyramid of pearly +light, leaning toward the south if the place of observation is in the +northern hemisphere. It does not impress the observer at all in the +same manner as the Milky Way; that looks far off and is clearly among +the stars, but the Zodiacal Light seems closer at hand, as if it were +something more intimately concerning the earth. To all it immediately +suggests a connection, also, with the sunken sun. If the night is +clear and the moon absent (and if you are in the country, for city +lights ruin the spectacles of the sky), you will be able to watch the +apparition for a long time. You will observe that the light is +brightest near the horizon, gradually fading as the pyramidal beam +mounts higher, but in favorable circumstances it may be traced nearly +to the meridian south of the zenith, where its apex at last vanishes +in the starlight. It continues visible during the evenings of March +and part of April, after which, ordinarily, it is seen no more, or if +seen is relatively faint and unimpressive. But when autumn comes it +appears again, this time not like a wraith hovering above the westward +tomb of the day-god, but rather like a spirit of the morning +announcing his reincarnation in the east. + +The reason why the Zodiacal Light is best seen in our latitudes at the +periods just mentioned is because at those times the Zodiac is more +nearly perpendicular to the horizon, first in the west and then in the +east; and, since the phenomenon is confined within the borders of the +Zodiac, it cannot be favorably placed for observation when the +zodiacal plane is but slightly inclined to the horizon. Its faint +light requires the contrast of a background of dark sky in order to be +readily perceptible. But within the tropics, where the Zodiac is +always at a favorable angle, the mysterious light is more constantly +visible. Nearly all observant travelers in the equatorial regions have +taken particular note of this phenomenon, for being so much more +conspicuous there than in the temperate zones it at once catches the +eye and holds the attention as a novelty. Humboldt mentions it many +times in his works, for his genius was always attracted by things out +of the ordinary and difficult of explanation, and he made many careful +observations on its shape, its brilliancy, and its variations; for +there can be no doubt that it does vary, and sometimes to an +astonishing degree. It is said that it once remained practically +invisible in Europe for several years in succession. During a trip to +South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a +remarkable difference between the appearance of the Zodiacal Light on +his going and coming voyages. In fact, when crossing the equator going +south he did not see it at all; but on returning he had, on March 6th, +when one degree south of the equator, a memorable view of it. + + It was a bright, clear night, and the Zodiacal Light was + extraordinarily brilliant -- brighter than he had ever seen it + before. The Milky Way was not to be compared with it. The brightest + part extended 75 from the sun. There was a faint and much narrower + extension which they could just make out beyond the Pleiades along + the ecliptic, but the greater part of the Zodiacal Light showed as + a broad truncated column, and it did not appear nearly as conical + as he had before seen it. + +When out of the brief twilight of intertropical lands, where the sun +drops vertically to the horizon and night rushes on like a wave of +darkness, the Zodiacal Light shoots to the very zenith, its color is +described as a golden tint, entirely different from the silvery sheen +of the Milky Way. If I may venture again to refer to personal +experiences and impressions, I will recall a view of the Zodiacal +Light from the summit of the cone of Mt Etna in the autumn of the year +1896 (more briefly described in Astronomy with the Naked Eye). There +are few lofty mountains so favorably placed as Etna for observations +of this kind. It was once resorted to by Prof. George E. Hale, in an +attempt to see the solar corona without an eclipse. Rising directly +from sea-level to an elevation of nearly eleven thousand feet, the +observer on its summit at night finds himself, as it were, lost in the +midst of the sky. But for the black flanks of the great cone on which +he stands he might fancy himself to be in a balloon. On the occasion +to which I refer the world beneath was virtually invisible in the +moonless night. The blaze of the constellations overhead was +astonishingly brilliant, yet amid all their magnificence my attention +was immediately drawn to a great tapering light that sprang from the +place on the horizon where the sun would rise later, and that seemed +to be blown out over the stars like a long, luminous veil. It was the +finest view of the Zodiacal light that I had ever enjoyed -- thrilling +in its strangeness -- but I was almost disheartened by the +indifference of my guide, to whom it was only a light and nothing +more. If he had no science, he had less poetry -- rather a remarkable +thing, I thought, for a child of his clime. The Light appeared to me +to be distinctly brighter than the visible part of the Milky Way which +included the brilliant stretches in Auriga and Perseus, and its color, +if one may speak of color in connection with such an object, seemed +richer than that of the galactic band; but I did not think of it as +yellow, although Humboldt has described it as resembling a golden +curtain drawn over the stars, and Du Chaillu in Equatorial Africa +found it of a bright yellow color. It may vary in color as in +conspicuousness. The fascination of that extraordinary sight has never +faded from my memory. I turned to regard it again and again, although +I had never seen the stellar heavens so brilliant, and it was one of +the last things I looked for when the morning glow began softly to +mount in the east, and Sicily and the Mediterranean slowly emerged +from the profound shadow beneath us. + +The Zodiacal Light seems never to have attracted from astronomers in +general the amount of careful attention that it deserves; perhaps +because so little can really be made of it as far as explanation is +concerned. I have referred to the restraint that scientific writers +apparently feel in speaking of it. The grounds for speculation that it +affords may be too scanty to lead to long discussions, yet it piques +curiosity, and as we shall see in a moment has finally led to a most +interesting theory. Once it was the subject of an elaborate series of +studies which carried the observer all round the world. That was in +1845--46, during the United States Exploring Expedition that visited +the then little known Japan. The chaplain of the fleet, the Rev. Mr +Jones, went out prepared to study the mysterious light in all its +phases. He saw it from many latitudes on both sides of the equator, +and the imagination cannot but follow him with keen interest in his +world-circling tour, keeping his eyes every night fixed upon the +phantasm overhead, whose position shifted with that of the hidden sun. +He demonstrated that the flow extends at times completely across the +celestial dome, although it is relatively faint directly behind the +earth. On his return the government published a large volume of his +observations, in which he undertook to show that the phenomenon was +due to the reflection of sunlight from a ring of meteoric bodies +encircling the earth. But, after all, this elaborate investigation +settled nothing. + +Prof. E. E. Barnard has more recently devoted much attention to the +Zodiacal Light, as well as to a strange attendant phenomenon called +the ``Gegenschein,'' or Counterglow, because it always appears at that +point in the sky which is exactly opposite the sun. The Gegenschein is +an extremely elusive phenomenon, suitable only for eyes that have been +specially trained to see it. Professor Newcomb has cautiously remarked +that + + it is said that in that point of the heavens directly opposite the + sun there is an elliptical patch of light... This phenomenon is so + difficult to account for that its existence is sometimes doubted; + yet the testimony in its favor is difficult to set aside. + +It certainly cannot be set aside at all since the observations of +Barnard. I recall an attempt to see it under his guidance during a +visit to Mount Hamilton, when he was occupied there with the Lick +telescope. Of course, both the Gegenschein and the Zodiacal Light are +too diffuse to be studied with telescopes, which, so to speak, magnify +them out of existence. They can only be successfully studied with the +naked eye, since every faintest glimmer that they afford must be +utilized. This is especially true of the Gegenschein. At Mount +Hamilton, Mr Barnard pointed out to me its location with reference to +certain stars, but with all my gazing I could not be sure that I saw +it. To him, on the contrary, it was obvious; he had studied it for +months, and was able to indicate its shape, its boundaries, its +diameter, and the declination of its center with regard to the +ecliptic. There is not, of course, the shadow of a doubt of the +existence of the Gegenschein, and yet I question if one person in a +million has ever seen or ever will see it. The Zodiacal Light, on the +other hand, is plain enough, provided that the time and the +circumstances of the observation are properly chosen. + +In the attempts to explain the Zodiacal Light, the favorite hypothesis +has been that it is an appendage of the sun -- perhaps simply an +extension of the corona in the plane of the ecliptic, which is not +very far from coinciding with that of the sun's equator. This idea is +quite a natural one, because of the evident relation of the light to +the position of the sun. The vast extension of the equatorial wings of +the corona in 1878 gave apparent support to this hypothesis; if the +substance of the corona could extend ten million miles from the sun, +why might it not extend even one hundred million, gradually fading out +beyond the orbit of the earth? A variation of this hypothesis assumes +that the reflection is due to swarms of meteors circling about the +sun, in the plane of its equator, all the way from its immediate +neighborhood to a distance exceeding that of the earth. But in neither +form is the hypothesis satisfactory; there is nothing in the +appearance of the corona to indicate that it extends even as far as +the planet Mercury, while as to meteors, the orbits of the known +swarms do not accord with the hypothesis, and we have no reason to +believe that clouds of others exist traveling in the part of space +where they would have to be in order to answer the requirements of the +theory. The extension of the corona in 1878 did not resemble in its +texture the Zodiacal Light. + +Now, it has so often happened in the history of science that an +important discovery in one branch has thrown unexpected but most +welcome light upon some pending problem in some other branch, that a +strong argument might be based upon that fact alone against the too +exclusive devotion of many investigators to the narrow lines of their +own particular specialty; and the Zodiacal Light affords a case in +point, when it is considered in connection with recent discoveries in +chemistry and physics. From the fact that atoms are compound bodies +made up of corpuscles at least a thousand times smaller than the +smallest known atom -- a fact which astounded most men of science when +it was announced a few years ago -- a new hypothesis has been +developed concerning the nature of the Zodiacal Light (as well as +other astronomical riddles), and this hypothesis comes not from an +astronomer, but from a chemist and physicist, the Swede, Svante +Arrhenius. In considering an outline of this new hypothesis we need +neither accept nor reject it; it is a case rather for suspension of +judgment. + +To begin with, it carries us back to the ``pressure of light'' +mentioned in the preceding chapter. The manner in which this pressure +is believed generally to act was there sufficiently explained, and it +only remains to see how it is theoretically extended to the particles +of matter supposed to constitute the Zodiacal Light. We know that +corpuscles, or ``fragments of atoms'' negatively electrified, are +discharged from hot bodies. Streams of these ``ions'' pour from many +flames and from molten metals; and the impact of the cathode and +ultra-violet rays causes them to gush even from cold bodies. In the +vast laboratory of the sun it is but reasonable to suppose that +similar processes are taking place. ``As a very hot metal emits these +corpuscles,'' says Prof. J. J. Thomson, ``it does not seem an +improbable hypothesis that they are emitted by that very hot body, the +sun.'' Let it be assumed, then, that the sun does emit them; what +happens next? Negatively charged corpuscles, it is known, serve as +nuclei to which particles of matter in the ordinary state are +attracted, and it is probable that those emitted from the sun +immediately pick up loads in this manner and so grow in bulk. If they +grow large enough the gravitation of the sun draws them back, and they +produce a negative charge in the solar atmosphere. But it is probable +that many of the particles do not attain the critical size which, +according to the principles before explained, would enable the +gravitation of the sun to retain them in opposition to the pressure of +the waves of light, and with these particles the light pressure is +dominant. Clouds of them may be supposed to be continually swept away +from the sun into surrounding space, moving mostly in or near the +plane of the solar equator, where the greatest activity, as indicated +by sunspots and related phenomena, is taking place. As they pass +outward into space many of them encounter the earth. If the earth, +like the moon, had no atmosphere the particles would impinge directly +on its surface, giving it a negative electric charge. But the presence +of the atmosphere changes all that, for the first of the flying +particles that encounter it impart to it their negative electricity, +and then, since like electric charges repel like, the storm of +particles following will be sheered off from the earth, and will +stream around it in a maze of hyperbolic paths. Those that continue on +into space beyond the earth may be expected to continue picking up +wandering particles of matter until their bulk has become so great +that the solar attraction prevails again over the light pressure +acting upon them, and they turn again sunward. Passing the earth on +their return they will increase the amount of dust-clouds careering +round it; and these will be further increased by the action of the +ultra-violet rays of the sunlight causing particles to shoot radially +away from the earth when the negative charge of the upper atmosphere +has reached a certain amount, which particles, although starting +sunward, will be swept back to the earth with the oncoming streams. As +the final result of all this accumulation of flying and gyrating +particles in the earth's neighborhood, we are told that the latter +must be transformed into the semblance of a gigantic solid-headed +comet provided with streaming tails, the longest of them stretching +away from the direction of the sun, while another shorter one extends +toward the sun. This shorter tail is due to the particles that we have +just spoken of as being driven sunward from the earth by the action of +ultra-violet light. No doubt this whole subject is too technical for +popular statement; but at any rate the general reader can understand +the picturesque side of the theory, for its advocates assure us that +if we were on the moon we would doubtless be able to see the +comet-like tails of the earth, and then we could appreciate the part +that they play in producing the phenomenon of the Zodiacal Light. + +That the Light as we see it could be produced by the reflection of +sunlight from swarms of particles careering round the earth in the +manner supposed by Arrhenius' hypothesis is evident enough; and it +will be observed that the new theory, after all, is only another +variant of the older one which attributes the Zodiacal Light to an +extension of the solar corona. But it differs from the older theory in +offering an explanation of the manner in which the extension is +effected, and it differentiates between the corona proper and the +streams of negative particles shot away from the sun. In its details +the hypothesis of Arrhenius also affords an explanation of many +peculiarities of the Zodiacal Light, such as that it is confined to +the neighborhood of the ecliptic, and that it is stronger on the side +of the earth which is just turning away from a position under the sun +than on the other side; but it would carry us beyond our limits to go +into these particulars. The Gegenschein, according to this theory, is +a part of the same phenomenon as the Zodiacal Light, for by the laws +of perspective it is evident that the reflection from the streams of +particles situated at a point directly opposite to the sun would be at +a maximum, and this is the place which the Gegenschein occupies. Apart +from its geometrical relations to the position of the sun, the +variability of the Zodiacal Light appears to affirm its solar +dependence, and this too would be accounted for by Arrhenius' +hypothesis better than by the old theory of coronal extension. The +amount of corpuscular discharge from the sun must naturally be +governed by the state of relative activity or inactivity of the +latter, and this could not but be reflected in the varying splendor of +the Zodiacal Light. But much more extended study than has yet been +given to the subject will be required before we can feel that we know +with reasonable certainty what this mysterious phenomenon really is. +By the hypothesis of Arrhenius every planet that has an atmosphere +must have a Zodiacal Light attending it, but the phenomenon is too +faint for us to be able to see it in the case, for instance, of Venus, +whose atmosphere is very abundant. The moon has no corresponding +``comet's tail'' because, as already explained, of the lack of a lunar +atmosphere to repel the streams by becoming itself electrified; but if +there were a lunar Zodiacal Light, no doubt we could see it because of +the relative nearness of our satellite. + +Marvels of the Aurora + +One of the most vivid recollections of my early boyhood is that of +seeing my father return hastily into the house one evening and call +out to the family: ``Come outside and look at the sky!'' Ours was a +country house situated on a commanding site, and as we all emerged +from the doorway we were dumbfounded to see the heavens filled with +pale flames which ran licking and quivering over the stars. Instantly +there sprang into my terrified mind the recollection of an awful +description of ``the Day of Judgment'' (the Dies Ir), which I had +heard with much perturbation of spirit in the Dutch Reformed church +from the lips of a tall, dark-browed, dreadfully-in-earnest preacher +of the old-fashioned type. My heart literally sank at sight of the +spectacle, for it recalled the preacher's very words; it was just as +he had said it would be, and it needed the assured bearing of my +elders finally to convince me that + + That Day of Wrath, O dreadful day, + When Heaven and Earth shall pass away, + As David and the Sibyl say + +had not actually come upon us. And even the older members of the +household were not untouched with misgivings when menacing spots of +crimson appeared, breaking out now here, now there, in the shuddering +sky. Toward the north the spectacle was appalling. A huge arch spanned +an unnaturally dark segment resting on the horizon, and above this +arch sprang up beams and streamers in a state of incessant agitation, +sometimes shooting up to the zenith with a velocity that took one's +breath, and sometimes suddenly falling into long ranks, and marching, +marching, marching, like an endless phalanx of fiery specters, and +moving, as I remember, always from east to west. The absolute silence +with which these mysterious evolutions were performed and the +quavering reflections which were thrown upon the ground increased the +awfulness of the exhibition. Occasionally enormous curtains of lambent +flame rolled and unrolled with a majestic motion, or were shaken to +and fro as if by a mighty, noiseless wind. At times, too, a sudden +billowing rush would be made toward the zenith, and for a minute the +sky overhead would glow so brightly that the stars seemed to have been +consumed. The spectacle continued with varying intensity for hours. + +This exhibition occurred in Central New York, a latitude in which the +Aurora Borealis is seldom seen with so much splendor. I remember +another similar one seen from the city of New York in November, 1882. +On this last occasion some observers saw a great upright beam of light +which majestically moved across the heavens, stalking like an +apparition in the midst of the auroral pageant, of whose general +movements it seemed to be independent, maintaining always its upright +posture, and following a magnetic parallel from east to west. This +mysterious beam was seen by no less than twenty-six observers in +different parts of the country, and a comparison of their observations +led to a curious calculation indicating that the apparition was about +one hundred and thirty-three miles tall and moved at the speed of ten +miles per second! + +But, as everybody knows, it is in the Arctic regions that the Aurora, +or the ``Northern Lights,'' can best be seen. There, in the long polar +night, when for months together the sun does not rise, the strange +coruscations in the sky often afford a kind of spectral daylight in +unison with the weird scenery of the world of ice. The pages in the +narratives of Arctic exploration that are devoted to descriptions of +the wonderful effects of the Northern Lights are second to none that +man has ever penned in their fascination. The lights, as I have +already intimated, display astonishing colors, particularly shades of +red and green, as they flit from place to place in the sky. The +discovery that the magnetic needle is affected by the Aurora, +quivering and darting about in a state of extraordinary excitement +when the lights are playing in the sky, only added to the mystery of +the phenomenon until its electro-magnetic nature had been established. +This became evident as soon as it was known that the focus of the +displays was the magnetic pole; and when the far South was visited the +Aurora Australis was found, having its center at the South Magnetic +Pole. Then, if not before, it was clear that the earth was a great +globular magnet, having its poles of opposite magnetism, and that the +auroral lights, whatever their precise cause might be, were +manifestations of the magnetic activity of our planet. After the +invention of magnetic telegraphy it was found that whenever a great +Aurora occurred the telegraph lines were interrupted in their +operation, and the ocean cables ceased to work. Such a phenomenon is +called a ``magnetic storm.'' + +The interest excited by the Aurora in scientific circles was greatly +stimulated when, in the last half of the nineteenth century, it was +discovered that it is a phenomenon intimately associated with +disturbances on the sun. The ancient ``Zurich Chronicles,'' extending +from the year 1000 to the year 1800, in which both sun-spots visible +to the naked eye and great displays of the auroral lights were +recorded, first set Rudolf Wolf on the track of this discovery. The +first notable proof of the suspected connection was furnished with +dramatic emphasis by an occurrence which happened on September 1, +1859. Near noon on that day two intensely brilliant points suddenly +broke out in a group of sun-spots which were under observation by Mr +R. C. Carrington at his observatory at Redhill, England. The points +remained visible for not more than five minutes, during which interval +they moved thirty-five thousand miles across the solar disk. Mr R. +Hodgson happened to see the same phenomenon at his observatory at +Highgate, and thus all possibility of deception was removed. But +neither of the startled observers could have anticipated what was to +follow, and, indeed, it was an occurrence which has never been +precisely duplicated. I quote the eloquent account given by Miss +Clerke in her History of Astronomy During the Nineteenth Century. + + This unique phenomenon seemed as if specially designed to + accentuate the inference of a sympathetic relation between the + earth and the sun. From August 28 to September 4, 1859, a magnetic + storm of unparalleled intensity, extent, and duration was in + progress over the entire globe. Telegraphic communication was + everywhere interrupted -- except, indeed, that it was in some cases + found practicable to work the lines without batteries by the agency + of the earth-currents alone; sparks issued from the wires; gorgeous + auroras draped the skies in solemn crimson over both hemispheres, + and even in the tropics; the magnetic needle lost all trace of + continuity in its movements and darted to and fro as if stricken + with inexplicable panic. The coincidence was even closer. At the + very instant of the solar outburst witnessed by Carrington and + Hodgson the photographic apparatus at Kew registered a marked + disturbance of all the three magnetic elements; while shortly after + the ensuing midnight the electric agitation culminated, thrilling + the whole earth with subtle vibrations, and lighting up the + atmosphere from pole to pole with coruscating splendors which + perhaps dimly recall the times when our ancient planet itself shone + as a star. + +If this amazing occurrence stood alone, and as I have already said it +has never been exactly duplicated, doubt might be felt concerning some +of the inferences drawn from it; but in varying forms it has been +repeated many times, so that now hardly anyone questions the reality +of the assumed connection between solar outbursts and magnetic storms +accompanied by auroral displays on the earth. It is true that the late +Lord Kelvin raised difficulties in the way of the hypothesis of a +direct magnetic action of the sun upon the earth, because it seemed to +him that an inadmissible quantity of energy was demanded to account +for such action. But no calculation like that which he made is final, +since all calculations depend upon the validity of the data; and no +authority is unshakable in science, because no man can possess +omniscience. It was Lord Kelvin who, but a few years before the thing +was actually accomplished, declared that aerial navigation was an +impracticable dream, and demonstrated its impracticability by +calculation. However the connection may be brought about, it is as +certain as evidence can make it that solar outbursts are coincident +with terrestial magnetic disturbances, and coincident in such a way as +to make the inference of a causal connection irresistible. The sun is +only a little more than a hundred times its own diameter away from the +earth. Why, then, with the subtle connection between them afforded by +the ether which conveys to us the blinding solar light and the +life-sustaining solar heat, should it be so difficult to believe that +the sun's enormous electric energies find a way to us also? No doubt +the impulse coming from the sun acts upon the earth after the manner +of a touch upon a trigger, releasing energies which are already stored +up in our planet. + +But besides the evidence afforded by such occurrences as have been +related of an intimate connection between solar outbreaks and +terrestial magnetic flurries, attended by magnificent auroral +displays, there is another line of proof pointing in the same +direction. Thus, it is known that the sun-spot period, as remarked in +a preceding chapter, coincides in a most remarkable manner with the +periodic fluctuations in the magnetic state of the earth. This +coincidence runs into the most astonishing details. For instance, when +the sun-spot period shortens, the auroral period shortens to precisely +the same extent; as the short sun-spot periods usually bring the most +intense outbreaks of solar activity, so the corresponding short +auroral periods are attended by the most violent magnetic storms; a +secular period of about two hundred and twenty-two years affecting +sun-spots is said to have its auroral duplicate; a shorter period of +fifty-five and a half years, which some observers believe that they +have discovered appears also to be common to the two phenomena; and +yet another ``superposed'' period of about thirty-five years, which +some investigators aver exists, affects sun-spots and aurora alike. In +short, the coincidences are so numerous and significant that one would +have to throw the doctrine of probability to the winds in order to be +able to reject the conclusion to which they so plainly lead. + +But still the question recurs: How is the influence transmitted? Here +Arrhenius comes once more with his hypothesis of negative corpuscles, +or ions, driven away from the sun by light-pressure -- a hypothesis +which seems to explain so many things -- and offers it also as an +explanation of the way in which the sun creates the Aurora. He would +give the Aurora the same lineage with the Zodiacal Light. To +understand the application of this theory we must first recall the +fact that the earth is a great magnet having its two opposite poles of +magnetism, one near the Arctic and the other near the Antarctic +Circle. Like all magnets, the earth is surrounded with ``lines of +force,'' which, after the manner of the curved rays we saw in the +photograph of a solar eclipse, start from a pole, rising at first +nearly vertically, then bend gradually over, passing high above the +equator, and finally descending in converging sheaves to the opposite +pole. Now the axis of the earth is so placed in space that it lies at +nearly a right angle to the direction of the sun, and as the streams +of negatively charged particles come pouring on from the sun (see the +last preceding chapter), they arrive in the greatest numbers over the +earth's equatorial regions. There they encounter the lines of magnetic +force at the place where the latter have their greatest elevation +above the earth, and where their direction is horizontal to the +earth's surface. Obeying a law which has been demonstrated in the +laboratory, the particles then follow the lines of force toward the +poles. While they are above the equatorial regions they do not become +luminescent, because at the great elevation that they there occupy +there is virtually no atmosphere; but as they pass on toward the north +and the south they begin to descend with the lines of force, curving +down to meet at the poles; and, encountering a part of the atmosphere +comparable in density with what remains in an exhausted Crookes tube, +they produce a glow of cathode rays. This glow is conceived to +represent the Aurora, which may consequently be likened to a gigantic +exhibition of vacuum-tube lights. Anybody who recalls his student days +in the college laboratory and who has witnessed a display of Northern +Lights will at once recognize the resemblance between them in colors, +forms, and behavior. This resemblance had often been noted before +Arrhenius elaborated his hypothesis. + +Without intending to treat his interesting theory as more than a +possibly correct explanation of the phenomena of the Aurora, we may +call attention to some apparently confirmatory facts. One of the most +striking of these relates to a seasonal variation in the average +number of auror. It has been observed that there are more in March +and September than at any other time of the year, and fewer in June +and December; moreover (and this is a delicate test as applied to the +theory), they are slightly rarer in June than in December. Now all +these facts seem to find a ready explanation in the hypothesis of +Arrhenius, thus: (1) The particles issuing from the sun are supposed +to come principally from the regions whose excitement is indicated by +the presence of sun-spots (which accords with Hale's observation that +sun-spots are columns of ionized vapors), and these regions have a +definite location on either side of the solar equator, seldom +approaching it nearer than within 5 or 10 north or south, and never +extending much beyond 35 toward either pole; (2) The equator of the +sun is inclined about 7 to the plane of the earth's orbit, from which +it results that twice in a year -- viz., in June and December -- the +earth is directly over the solar equator, and twice a year -- viz., in +March and September -- when it is farthest north or south of the solar +equator, it is over the inner edge of the sun-spot belts. Since the +corpuscles must be supposed to be propelled radially from the sun, few +will reach the earth when the latter is over the solar equator in June +and December, but when it is over, or nearly over, the spot belts, in +March and September, it will be in the line of fire of the more active +parts of the solar surface, and relatively rich streams of particles +will reach it. This, as will be seen from what has been said above, is +in strict accord with the observed variations in the frequency of +auror. Even the fact that somewhat fewer auror are seen in June than +in December also finds its explanation in the known fact that the +earth is about three million miles nearer the sun in the winter than +in the summer, and the number of particles reaching it will vary, like +the intensity of light, inversely as the square of the distance. These +coincidences are certainly very striking, and they have a cumulative +force. If we accept the theory, it would appear that we ought to +congratulate ourselves that the inclination of the sun's equator is so +slight, for as things stand the earth is never directly over the most +active regions of the sun-spots, and consequently never suffers from +the maximum bombardment of charged particles of which the sun is +capable. Incessant auroral displays, with their undulating draperies, +flitting colors, and marching columns might not be objectionable from +the point of view of picturesqueness, but one magnetic storm of +extreme intensity following closely upon the heels of another, for +months on end, crazing the magnetic needle and continually putting the +telegraph and cable lines out of commission, to say nothing of their +effect upon ``wireless telegraphy'', would hardly add to the charms of +terrestrial existence. + +One or two other curious points in connection with Arrhenius' +hypothesis may be mentioned. First, the number of auror, according to +his explanation, ought to be greatest in the daytime, when the face of +the earth on the sunward side is directly exposed to the atomic +bombardment. Of course visual observation can give us no information +about this, since the light of the Aurora is never sufficiently +intense to be visible in the presence of daylight, but the records of +the magnetic observatories can be, and have been, appealed to for +information, and they indicate that the facts actually accord with the +theory. Behind the veil of sunlight in the middle of the afternoon, +there is good reason to believe, auroral exhibitions often take place +which would eclipse in magnificence those seen at night if we could +behold them. Observation shows, too, that auror are more frequent +before than after midnight, which is just what we should expect if +they originate in the way that Arrhenius supposes. Second, the theory +offers an explanation of the alleged fact that the formation of clouds +in the upper air is more frequent in years when auror are most +abundant, because clouds are the result of the condensation of +moisture upon floating particles in the atmosphere (in an absolutely +dustless atmosphere there would be no clouds), and it has been proved +that negative ions like those supposed to come from the sun play a +master part in the phenomena of cloud formation. + +Yet another singular fact, almost mystical in its suggestions, may be +mentioned. It seems that the dance of the auroral lights occurs most +frequently during the absence of the moon from the hemisphere in which +they appear, and that they flee, in greater part, to the opposite +hemisphere when the moon's revolution in an orbit considerably +inclined to the earth's equator brings her into that where they have +been performing. Arrhenius himself discovered this curious relation of +auroral frequency to the position of the moon north or south of the +equator, and he explains it in this way. The moon, like the earth, is +exposed to the influx of the ions from the sun; but having no +atmosphere, or almost none, to interfere with them, they descend +directly upon her surface and charge her with an electric negative +potential to a very high degree. In consequence of this she affects +the electric state of the upper parts of the earth's atmosphere where +they lie most directly beneath her, and thus prevents, to a large +extent, the negative discharges to which the appearance of the Aurora +is due. And so ``the extravagant and erring spirit'' of the Aurora +avoids the moon as Hamlet's ghost fled at the voice of the cock +announcing the awakening of the god of day. + +There are even other apparent confirmations of the hypothesis, but we +need not go into them. We shall, however, find one more application of +it in the next chapter, for it appears to be a kind of cure-all for +astronomical troubles; at any rate it offers a conceivable solution of +the question, How does the sun manage to transmit its electric +influence to the earth? And this solution is so grandiose in +conception, and so novel in the mental pictures that it offers, that +its acceptance would not in the least detract from the impression that +the Aurora makes upon the imagination. + +Strange Adventures of Comets + +The fears and legends of ancient times before Science was born, and +the superstitions of the Dark Ages, sedulously cultivated for +theological purposes by monks and priests, have so colored our ideas +of the influence that comets have had upon the human mind that many +readers may be surprised to learn that it was the apparition of a +wonderful comet, that of 1843, which led to the foundation of our +greatest astronomical institution, the Harvard College Observatory. No +doubt the comet superstition existed half a century ago, as, indeed, +it exists yet today, but in this case the marvelous spectacle in the +sky proved less effective in inspiring terror than in awakening a +desire for knowledge. Even in the sixteenth century the views that +enlightened minds took of comets tended powerfully to inspire popular +confidence in science, and Halley's prediction, after seeing and +studying the motion of the comet which appeared in 1682, that it would +prove to be a regular member of the sun's family and would be seen +returning after a period of about seventy-six years, together with the +fulfillment of that prediction, produced a revulsion from the +superstitious notions which had so long prevailed. + +Then the facts were made plain that comets are subject to the law of +gravitation equally with the planets; that there are many which +regularly return to the neighborhood of the sun (perihelion); and that +these travel in orbits differing from those of the planets only in +their greater eccentricity, although they have the peculiarity that +they do not, like the planets, all go round the sun in the same +direction, and do not keep within the general plane of the planetary +system, but traverse it sometimes from above and sometimes from below. +Other comets, including most of the ``great'' ones, appear to travel +in parabolic or, in a few cases, hyperbolic orbits, which, not being +closed curves, never bring them back again. But it is not certain that +these orbits may not be extremely eccentric ellipses, and that after +the lapse of hundreds, or thousands, of years the comets that follow +them may not reappear. The question is an interesting one, because if +all orbits are really ellipses, then all comets must be permanent +members of the solar system, while in the contrary case many of them +are simply visitors, seen once and never to be seen again. The +hypothesis that comets are originally interlopers might seem to derive +some support from the fact that the certainly periodic ones are +associated, in groups, with the great outer planets, whose attraction +appears to have served as a trap for them by turning them into +elliptical orbits and thus making them prisoners in the solar system. +Jupiter, owing to his great mass and his commanding situation in the +system, is the chief ``comet-catcher;'' but he catches them not for +himself, but for the sun. Yet if comets do come originally from +without the borders of the planetary system, it does not, by any +means, follow that they were wanderers at large in space before they +yielded to the overmastering attraction of the sun. Investigation of +the known cometary orbits, combined with theoretical considerations, +has led some astronomers to the conclusion that as the sun travels +onward through space he ``picks up en route'' cometary masses which, +without belonging strictly to his empire, are borne along in the same +vast ``cosmical current'' that carries the solar system. + +But while no intelligent person any longer thinks that the appearance +of a great comet is a token from the heavenly powers of the +approaching death of a mighty ruler, or the outbreak of a devastating +war, or the infliction of a terrible plague upon wicked mankind, +science itself has discovered mysteries about comets which are not +less fascinating because they are more intellectual than the +irrational fancies that they have displaced. To bring the subject +properly before the mind, let us see what the principal phenomena +connected with a comet are. + +At the present day comets are ordinarily ``picked up'' with the +telescope or the photographic plate before any one except their +discoverer is aware of their existence, and usually they remain so +insignificant in appearance that only astronomers ever see them. Yet +so great is the prestige of the word ``comet'' that the discovery of +one of these inconspicuous wanderers, and its subsequent movements, +become items of the day's news which everybody reads with the feeling, +perhaps, that at least he knows what is going on in the universe even +if he doesn't understand it. But a truly great comet presents quite a +different proposition. It, too, is apt to be detected coming out of +the depths of space before the world at large can get a glimpse of it, +but as it approaches the sun its aspect undergoes a marvelous change. +Agitated apparently by solar influence, it throws out a long streaming +tail of nebulous light, directed away from the sun and looking as if +blown out like a pennon by a powerful wind. Whatever may be the +position of the comet with regard to the sun, as it circles round him +it continually keeps its tail on the off side. This, as we shall soon +see, is a fact of capital importance in relation to the probable +nature of comets' tails. Almost at the same time that the formation of +the tail is observed a remarkable change takes place in the comet's +head, which, by the way, is invariably and not merely occasionally its +most important part. On approaching the sun the head usually +contracts. Coincidently with this contraction a nucleus generally +makes its appearance. This is a bright, star-like point in the head, +and it probably represents the totality of solid matter that the comet +possesses. But it is regarded as extremely unlikely that even the +nucleus consists of a uniformly solid mass. If it were such, comets +would be far more formidable visitors when they pass near the planets +than they have been found to be. The diameter of the nucleus may vary +from a few hundred up to several thousand miles; the heads, on the +average, are from twenty-five thousand to one hundred thousand miles +in diameter, although a few have greatly exceeded these dimensions; +that of the comet of 1811, one of the most stupendous ever seen, was a +million and a quarter miles in diameter! As to the tails, not +withstanding their enormous length -- some have been more than a +hundred million miles long -- there is reason to believe that they are +of extreme tenuity, ``as rare as vacuum.'' The smallest stars have +been seen shining through their most brilliant portions with +undiminished luster. + +After the nucleus has been formed it begins to throw out bright jets +directed toward the sun. A stream, and sometimes several streams, of +light also project sunward from the nucleus, occasionally appearing +like a stunted tail directed oppositely to the real tail. Symmetrical +envelopes which, seen in section, appear as half circles or parabolas, +rise sunward from the nucleus, forming a concentric series. The ends +of these stream backward into the tail, to which they seem to supply +material. Ordinarily the formation of these ejections and envelopes is +attended by intense agitation of the nucleus, which twists and turns, +swinging and gyrating with an appearance of the greatest violence. +Sometimes the nucleus is seen to break up into several parts. The +entire heads of some comets have been split asunder in passing close +around the sun; The comet of 1882 retreated into space after its +perihelion passage with five heads instead of the one that it had +originally, and each of these heads had its own tail! + +The possession of the spectroscope has enabled astronomers during +later years to study the chemical composition of comets by analyzing +their light. At first the only substances thus discovered in them were +hydro-carbon compounds, due evidently to the gaseous envelopes in +which some combination of hydrogen with carbon existed. Behind this +gaseous spectrum was found a faint continuous spectrum ascribed to the +nucleus, which apparently both reflects the sunlight and gives forth +the light of a glowing solid or liquid. Subsequently sodium and iron +lines were found in cometary spectra. The presence of iron would seem +to indicate that some of these bodies may be much more massive than +observations on their attractive effects have indicated. In some +recent comets, such as Morehouse's, in 1908, several lines have been +found, the origin of which is unknown. + +Without going back of the nineteenth century we may find records of +some of the most extraordinary comets that man has ever looked upon. +In 1811, still spoken of as ``the year of the comet,'' because of the +wonderful vintage ascribed to the skyey visitor, a comet shaped like a +gigantic sword amazed the whole world, and, as it remained visible for +seventeen months, was regarded by superstitious persons as a symbol of +the fearful happenings of Napoleon's Russian campaign. This comet, the +extraordinary size of whose head, greatly exceeding that of the sun +itself, has already been mentioned, was also remarkable for exhibiting +so great a brilliancy without approaching even to the earth's distance +from the sun. But there was once a comet (and only once -- in the year +1729) which never got nearer to the sun than four times the distance +of the earth and yet appeared as a formidable object in the sky. As +Professor Young has remarked, ``it must have been an enormous comet to +be visible from such a distance.'' And we are to remember that there +were no great telescopes in the year 1729. That comet affects the +imagination like a phantom of space peering into the solar system, +displaying its enormous train afar off (which, if it had approached as +near as other comets, would probably have become the celestial wonder +of all human memory), and then turning away and vanishing in the +depths of immensity. + +In 1843 a comet appeared which was so brilliant that it could be seen +in broad day close beside the sun! This was the first authenticated +instance of that kind, but the occurrence was to be repeated, as we +shall see in a moment, less than forty years later. + +The splendid comet of 1858, usually called Donati's, is remembered by +many persons yet living. It was, perhaps, both as seen by the naked +eye and with the telescope, the most beautiful comet of which we have +any record. It too marked a rich vintage year, still remembered in the +vineyards of France, where there is a popular belief that a great +comet ripens the grape and imparts to the wine a flavor not attainable +by the mere skill of the cultivator. There are ``comet wines,'' +carefully treasured in certain cellars, and brought forth only when +their owner wishes to treat his guests to a sip from paradise. + +The year 1861 saw another very remarkable comet, of an aspect +strangely vast and diffuse, which is believed to have swept the earth +with its immense tail when it passed between us and the sun on the +night of June 30th, an event which produced no other known effect than +the appearance of an unwonted amount of scattered light in the sky. + +The next very notable comet was the ``Great Southern Comet'' of 1880, +which was not seen from the northern hemisphere. It mimicked the +aspect of the famous comet of 1843, and to the great surprise of +astronomers appeared to be traveling in the same path. This proved to +be the rising of the curtain for an astronomical sensation +unparalleled in its kind; for two years later another brilliant comet +appeared, first in the southern hemisphere, and it too followed the +same track. The startling suggestion was now made that this comet was +identical with those of 1843 and 1880, its return having been hastened +by the resistance experienced in passing twice through the coronal +envelope, and there were some who thought that it would now swing +swiftly round and then plunge straight into the sun, with consequences +that might be disastrous to us on account of the ``flash of heat'' +that would be produced by the impact. Nervous people were frightened, +but observation soon proved that the danger was imaginary, for +although the comet almost grazed the sun, and must have rushed through +two or three million miles of the coronal region, no retardation of +its immense velocity was perceptible, and it finally passed away in a +damaged condition, as before remarked, and has never since appeared. + +Then the probable truth was perceived -- viz., that the three comets +(1843, 1880, and 1882) were not one identical body, but three separate +ones all traveling in the same orbit. It was found, too, that a comet +seen in 1668 bore similar insignia of relationship. The natural +inference was that these four bodies had once formed a single mass +which had been split apart by the disruptive action of the sun. +Strength was lent to this hypothesis by the fact that the comet of +1882 was apparently torn asunder during its perihelion passage, +retreating into space in a dissevered state. But Prof. George Forbes +has a theory that the splitting of the original cometary mass was +effected by an unknown planet, probably greater than Jupiter, situated +at a hundred times the earth's distance from the sun, and revolving in +a period of a thousand years. He supposes that the original comet was +not that of 1668, but one seen in 1556, which has since been +``missing,'' and that its disruption occurred from an encounter with +the supposititious planet about the year 1700. Truly from every point +of view comets are the most extraordinary of adventurers! + +The comet of 1882 was likewise remarkable for being visible, like its +predecessor of 1843, in full daylight in close proximity to the sun. +The story of its detection when almost in contact with the solar disk +is dramatic. It had been discovered in the southern hemisphere only a +couple of weeks before its perihelion, which occurred on September +17th, and on the forenoon of that day it was seen by Doctor Common in +England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope, +almost touching the sun. It looked like a dazzling white bird with +outspread wings. The southern observers watched it go right into the +sun, when it instantly disappeared. What had happened was that the +comet in passing its perihelion point had swung exactly between the +earth and the sun. On the following morning it was seen from all parts +of the world close by the sun on the opposite side, and it remained +thus visible for three days, gradually receding from the solar disk. +It then became visible for northern observers in the morning sky +before sunrise, brandishing a portentous sword-shaped tail which, if +it had been in the evening sky, would have excited the wonder of +hundreds of millions, but situated where it was, comparatively few +ever saw it. + +The application of photography to the study of comets has revealed +many curious details which might otherwise have escaped detection, or +at best have remained subject to doubt. It has in particular shown not +only the precise form of the tails, but the remarkable vicissitudes +that they undergo. Professor Barnard's photographs of Brooks' comet in +1893 suggested, by the extraordinary changes in the form of the tail +which they revealed, that the comet was encountering a series of +obstructions in space which bent and twisted its tail into fantastic +shapes. The reader will observe the strange form into which the tail +was thrown on the night of October 21st. A cloud of meteors through +which the comet was passing might have produced such deformations of +its tail. In the photograph of Daniels' comet of 1907, a curious +striping of the tail will be noticed. The short bright streaks seen in +the photograph, it may be explained, are the images of stars which are +drawn out into lines in consequence of the fact that the photographic +telescope was adjusted to follow the motion of the comet while the +stars remained at rest. + +But the adventures of comets are not confined to possible encounters +with unknown obstacles. We have referred to the fact that the great +planets, and especially Jupiter, frequently interfere with the motions +of comets. This interference is not limited to the original alteration +of their orbits from possible parabolas to ellipses, but is sometimes +exercised again and again, turning the bewildered comets into +elliptical paths of all degrees of eccentricity. A famous example of +this kind of planetary horse-play is furnished by the story of +Lexell's missing comet. This comet was first seen in 1770. +Investigation showed that it was moving in an orbit which should bring +it back to perihelion every five and a half years; yet it had never +been seen before and, although often searched for, has never been seen +since. Laplace and Leverrier proved mathematically that in 1767 it had +approached so close to Jupiter as to be involved among the orbits of +his satellites. What its track had been before is not known, but on +that occasion the giant planet seized the interloper, threw it into a +short elliptic orbit and sent it, like an arrested vagrant, to receive +sentence at the bar of the sun. On this journey it passed within less +than 1,500,000 miles of the earth. The form of orbit which Jupiter had +impressed required, as we have said, its return in about five and a +half years; but soon after 1770 it had the misfortune a second time to +encounter Jupiter at close range, and he, as if dissatisfied with the +leniency of the sun, or indignant at the stranger's familiarity, +seized the comet and hurled it out of the system, or at any rate so +far away that it has never since been able to rejoin the family circle +that basks in the immediate rays of the solar hearth. Nor is this the +only instance in which Jupiter has dealt summarily with small comets +that have approached him with too little deference. + +The function which Jupiter so conspicuously fulfills as master of the +hounds to the sun is worth considering a little more in detail. To +change the figure, imagine the sun in its voyage through space to be +like a majestic battleship surrounded by its scouts. Small vessels +(the comets, as they are overhauled by the squadron, are taken in +charge by the scouts, with Jupiter for their chief, and are forced to +accompany the fleet, but not all are impressed. If a strange comet +undertakes to run across Jupiter's bows the latter brings it to, and +makes prize of it by throwing it into a relatively small ellipse with +the sun for its focus. Thenceforth, unless, as happened to the unhappy +comet of Lexell, it encounters Jupiter again in such a way as to be +diverted by him into a more distant orbit, it can never get away. +About thirty comets are now known to have thus been captured by the +great planet, and they are called ``Jupiter's Comet Family.'' But, on +the other hand, if a wandering comet crosses the wake of the chief +planetary scout the latter simply drives it away by accelerating its +motion and compels it to steer off into open space. The transformation +of comets into meteors will be considered in the next chapter, but +here, in passing, mention may be made of the strange fate of one +member of Jupiter's family, Biela's comet, which, having become over +bold in its advances to its captor, was, after a few revolutions in is +impressed orbit, torn to pieces and turned into a flock of meteors. + +And now let us return to the mystery of comets' tails. That we are +fully justified in speaking of the tails of comets as mysterious is +proved by the declaration of Sir John Herschel, who averred, in so +many words, that ``there is some profound secret and mystery of nature +concerned in this phenomenon,'' and this profound secret and mystery +has not yet been altogether cleared up. Nevertheless, the +all-explaining hypothesis of Arrhenius offers us once more a certain +amount of aid. Comets' tails, Arrhenius assures us, are but another +result of the pressure of light. The reader will recall the +applications of this theory to the Zodiacal Light and the Aurora. In +the form in which we now have to deal with it, the supposition is made +that as a comet approaches the sun eruptions of vapor, due to the +solar heat, occur in its nucleus. These are naturally most active on +the side which is directly exposed to the sun, whence the appearance +of the immense glowing envelopes that surround the nucleus on the +sunward side. Among the particles of hydro-carbon, and perhaps solid +carbon in the state of fine dust, which are thus set free there will +be many whose size is within the critical limit which enables the +light-waves from the sun to drive them away. Clouds of such particles, +then, will stream off behind the advancing comet, producing the +appearance of a tail. This accounts for the fact that the tails of +comets are always directed away from the sun, and it also explains the +varying forms of the tails and the extraordinary changes that they +undergo. The speed of the particles driven before the light-waves must +depend upon their size and weight, the lightest of a given size +traveling the most swiftly. By accretion certain particles might grow, +thus losing velocity and producing the appearance of bunches in the +tail, such as have been observed. The hypothesis also falls in with +the researches of Bredichin, who has divided the tails of comets into +three principal classes -- viz.: (1) Those which appear as long, +straight rays; (2) Those which have the form of curved plumes or +scimitars; (3) Those which are short, brushy, and curved sharply +backward along the comet's path. In the first type he calculates the +repulsive force at from twelve to fifteen times the force of gravity; +in the second at from two to four times; and in the third at about one +and a half times. The straight tails he ascribes to hydrogen because +the hydrogen atom is the lightest known; the sword-shaped tails to +hydro-carbons; and the stumpy tails to vaporized iron. It will be seen +that, if the force driving off the tails is that which Arrhenius +assumes it to be, the forms of those appendages would accord with +those that Bredichin's theory calls for. At the same time we have an +explanation of the multiple tails with which some comets have adorned +themselves. The comet of 1744, for instance, had at one time no less +than seven tails spread in a wide curved brush behind it. Donati's +comet of 1858 also had at least two tails, the principal one +sword-shaped and the other long, narrow, and as straight as a rule. +According to Bredichin, the straight tail must have been composed of +hydrogen, and the other of some form of hydro-carbon whose atoms are +heavier than those of hydrogen, and, consequently, when swept away by +the storm of light-waves, followed a curvature depending upon the +resultant of the forces operating upon them. The seven tails of the +comet of 1744 presented a kind of diagram graphically exhibiting its +complex composition, and, if we knew a little more about the +constituents of a comet, we might be able to say from the amount of +curvature of the different tails just what were the seven substances +of which that comet consisted. + +If these theories seem to the reader fantastic, at any rate they are +no more fantastic than the phenomena that they seek to explain. + +Meteors, Fire-Balls, and Meteorites + +One of the most terrorizing spectacles with which the heavens have +ever caused the hearts of men to quake occurred on the night of +November 13, 1833. On that night North America, which faced the storm, +was under a continual rain of fire from about ten o'clock in the +evening until daybreak. + +The fragments of a comet had struck the earth. + +But the meaning of what had happened was not discovered until long +afterward. To the astronomers who, with astonishment not less than +that of other people, watched the wonderful scene, it was an +unparalleled ``shower of meteors.'' They did not then suspect that +those meteors had once formed the head of a comet. Light dawned when, +a year later, Prof. Denison Olmsted, of Yale College, demonstrated +that the meteors had all moved in parallel orbits around the sun, and +that these orbits intersected that of the earth at the point where our +planet happened to be on the memorable night of November 13th. +Professor Olmsted even went so far as to suggest that the cloud of +meteors that had encountered the earth might form a diffuse comet; but +full recognition of the fact that they were cometary dbris came +later, as the result of further investigation. The key to the secret +was plainly displayed in the spectacle itself, and was noticed without +being understood by thousands of the terror-stricken beholders. It was +an umbrella of fire that had opened overhead and covered the heavens; +in other words, the meteors all radiated from a particular point in +the constellation Leo, and, being countless as the snowflakes in a +winter tempest, they ribbed the sky with fiery streaks. Professor +Olmsted showed that the radiation of the meteors from a fixed point +was an effect of perspective, and in itself a proof that they were +moving in parallel paths when they encountered the earth. The fact was +noted that there had been a similar, but incomparably less brilliant, +display of meteors on the same day of November, 1832, and it was +rightly concluded that these had belonged to the same stream, although +the true relationship of the phenomena was not immediately +apprehended. Olmsted ascribed to the meteors a revolution about the +sun once in every six months, bringing them to the intersection of +their orbit with that of the earth every November 13th; but later +investigators found that the real period was about thirty-three and +one-quarter years, so that the great displays were due three times in +a century, and their return was confidently predicted for the year +1866. The appearance of the meteors in 1832, a year before the great +display, was ascribed to the great length of the stream which they +formed in space -- so great that they required more than two years to +cross the earth's orbit. In 1832 the earth had encountered a +relatively rare part of the stream, but in 1833, on returning to the +crossing-place, it found there the richest part of the stream pouring +across its orbit. This explanation also proved to be correct, and the +predicted return in 1866 was duly witnessed, although the display was +much less brilliant than in 1833. It was followed by another in 1867. + +In the mean time Olmsted's idea of a cometary relationship of the +meteors was demonstrated to be correct by the researches of +Schiaparelli and others, who showed that not only the November +meteors, but those of August, which are seen more or less abundantly +every year, traveled in the tracks of well-known comets, and had +undoubtedly an identical origin with those comets. In other words the +comets and the meteor-swarms were both remnants of original masses +which had probably been split up by the action of the sun, or of some +planet to which they had made close approaches. The annual periodicity +of the August meteors was ascribed to the fact that the separation had +taken place so long ago that the meteors had become distributed all +around the orbit, in consequence of which the earth encountered some +of them every year when it arrived at the crossing-point. Then +Leverrier showed that the original comet associated with the November +meteors was probably brought into the system by the influence of the +planet Uranus in the year 126 of the Christian era. Afterward +Alexander Herschel identified the tracks of no less than seventy-six +meteor-swarms (most of them inconspicuous) with those of comets. The +still more recent researches of Mr W. F. Denning make it probable that +there are no meteors which do not belong to a flock or system probably +formed by the disintegration of a cometary mass; even the apparently +sporadic ones which shoot across the sky, ``lost souls in the night,'' +being members of flocks which have become so widely scattered that the +earth sometimes takes weeks to pass through the region of space where +their paths lie. + +The November meteors should have exhibited another pair of spectacles +in 1899 and 1900, and their failure to do so caused at first much +disappointment, until it was made plain that a good reason existed for +their absence. It was found that after their last appearance, in 1867, +they had been disturbed in their movements by the planets Jupiter and +Saturn, whose attractions had so shifted the position of their orbit +that it no longer intersected that of the earth, as it did before. +Whether another planetary interference will sometime bring the +principal mass of the November meteors back to the former point of +intersection with the earth's orbit is a question for the future to +decide. It would seem that there may be several parallel streams of +the November meteors, and that some of them, like those of August, are +distributed entirely around the orbit, so that every mid-November we +see a few of them. + +We come now to a very remarkable example of the disintegration of a +comet and the formation of a meteor-stream. In 1826 Biela, of +Josephstadt, Austria, discovered a comet to which his name was given. +Calculation showed that it had an orbital period of about six and a +half years, belonging to Jupiter's ``family.'' On one of its returns, +in 1846, it astonished its watchers by suddenly splitting in two. The +two comets thus formed out of one separated to a distance of about one +hundred and sixty thousand miles, and then raced side by side, +sometimes with a curious ligature connecting them, like Siamese twins, +until they disappeared together in interplanetary space. In 1852 they +came back, still nearly side by side, but now the distance between +them had increased to a million and a quarter of miles. After that, at +every recurrence of their period, astronomers looked for them in vain, +until 1872, when an amazing thing happened. On the night of November +28th, when the earth was crossing the plane of the orbit of the +missing comet, a brilliant shower of meteors burst from the northern +sky, traveling nearly in the track which the comet should have +pursued. The astronomers were electrified. Klinkerfues, of Gttingen, +telegraphed to Pogson, of Madras: ``Biela touched earth; search near +Theta Centauri.'' Pogson searched in the place indicated and saw a +cometary mass retreating into the southern heavens, where it was soon +swallowed from sight! + +Since then the Biela meteors have been among the recognized periodic +spectacles of the sky, and few if any doubt that they represent a +portion of the missing comet whose disintegration began with the +separation into two parts in 1846. The comet itself has never since +been seen. The first display of these meteors, sometimes called the +``Andromedes,'' because they radiate from the constellation Andromeda, +was remarkable for the great brilliancy of many of the fire-balls that +shot among the shower of smaller sparks, some of which were described +as equaling the full moon in size. None of them is known to have +reached the earth, but during the display of the same meteors in 1885 +a meteoric mass fell at Mazapil in Northern Mexico (it is now in the +Museum at Vienna), which many have thought may actually be a piece of +the original comet of Biela. This brings us to the second branch of +our subject. + +More rare than meteors or falling stars, and more startling, except +that they never appear in showers, are the huge balls of fire which +occasionally dart through the sky, lighting up the landscapes beneath +with their glare, leaving trains of sparks behind them, often +producing peals of thunder when they explode, and in many cases +falling upon the earth and burying themselves from a few inches to +several feet in the soil, from which, more than once, they have been +picked up while yet hot and fuming. These balls are sometimes called +bolides. They are not really round in shape, although they often look +so while traversing the sky, but their forms are fragmentary, and +occasionally fantastic. It has been supposed that their origin is +different from that of the true meteors; it has even been conjectured +that they may have originated from the giant volcanoes of the moon or +have been shot out from the sun during some of the tremendous +explosions that accompany the formation of eruptive prominences. By +the same reasoning some of them might be supposed to have come from +some distant star. Others have conjectured that they are wanderers in +space, of unknown origin, which the earth encounters as it journeys +on, and Lord Kelvin made a suggestion which has become classic because +of its imaginative reach -- viz., that the first germs of life may +have been brought to the earth by one of these bodies, ``a fragment of +an exploded world.'' + +It is a singular fact that astronomers and scientific men in general +were among the last to admit the possibility of solid masses falling +from the sky. The people had believed in the reality of such phenomena +from the earliest times, but the savants shook their heads and talked +of superstition. This was the less surprising because no +scientifically authenticated instance of such an occurrence was known, +and the stones popularly believed to have fallen from the sky had +become the objects of worship or superstitious reverence, a fact not +calculated to recommend them to scientific credence. The celebrated +``black stone'' suspended in the Kaaba at Mecca is one of these +reputed gifts from heaven; the ``Palladium'' of ancient Troy was +another; and a stone which fell near Ensisheim, in Germany, was placed +in a church as an object to be religiously venerated. Many legends of +falling stones existed in antiquity, some of them curiously +transfigured by the imagination, like the ``Lion of the +Peloponnesus,'' which was said to have sprung down from the sky upon +the Isthmus of Corinth. But near the beginning of the nineteenth +century, in 1803, a veritable shower of falling stones occurred at +L'Aigle, in Northern France, and this time astronomers took note of +the phenomenon and scientifically investigated it. Thousands of the +strange projectiles came from the sky on this occasion, and were +scattered over a wide area of country, and some buildings were hit. +Four years later another shower of stones occurred at Weston, Conn., +numbering thousands of individuals. The local alarm created in both +cases was great, as well it might be, for what could be more +intimidating than to find the blue vault of heaven suddenly hurling +solid missiles at the homes of men? After these occurrences it was +impossible for the most skeptical to doubt any longer, and the regular +study of ``aerolites,'' or ``meteorites,'' began. + +One of the first things recognized was the fact that fire-balls are +solid meteorites in flight, and not gaseous exhalations in the air, as +some had assumed. They burn in the air during their flight, and +sometimes, perhaps, are entirely consumed before reaching the ground. +Their velocity before entering the earth's atmosphere is equal to that +of the planets in their orbits -- viz., from twenty to thirty miles +per second -- a fact which proves that the sun is the seat of the +central force governing them. Their burning in the air is not +difficult to explain; it is the heat of friction which so quickly +brings them to incandescence. Calculation shows that a body moving +through the air at a velocity of about a mile per second will be +brought, superficially, to the temperature of ``red heat'' by friction +with the atmosphere. If its velocity is twenty miles per second the +temperature will become thousands of degrees. This is the state of +affairs with a meteorite rushing into the earth's atmosphere; its +surface is liquefied within a few seconds after the friction begins to +act, and the melted and vaporized portion of its mass is swept +backward, forming the train of sparks that follows every great +fire-ball. However, there is one phenomenon connected with the trains +of meteorites which has never been satisfactorily explained: they +often persist for long periods of time, drifting and turning with the +wind, but not ceasing to glow with a phosphorescent luminosity. The +question is, Whence comes this light? It must be light without heat, +since the fine dust or vapor of which the train can only consist would +not retain sufficient heat to render it luminous for so long a time. +An extremely remarkable incident of this kind occurred on February 22, +1909, when an immense fire-ball that passed over southern England left +a train that remained visible during two hours, assuming many curious +shapes as it was drifted about by currents in the air. + +But notwithstanding the enormous velocity with which meteorites enter +the air they are soon slowed down to comparatively moderate speed, so +that when they disappear they are usually traveling not faster than a +mile a second. The courses of many have been traced by observers +situated along their track at various points, and thus a knowledge has +been obtained of their height above the ground during their flight and +of the length of their visible courses. They generally appear at an +elevation of eighty or a hundred miles, and are seldom visible after +having descended to within five miles of the ground, unless the +observer happens to be near the striking-point, when he may actually +witness the fall. Frequently they burst while high in the air and +their fragments are scattered like shrapnel over the surface of the +ground, sometimes covering an area of several square miles, but of +course not thickly; different fragments of the same meteorite may +reach the ground at points several miles apart. The observed length of +their courses in the atmosphere varies from fifty to five hundred +miles. If they continued a long time in flight after entering the air, +even the largest of them would probably be consumed to the last scrap, +but their fiery career is so short on account of their great speed +that the heat does not have time to penetrate very deeply, and some +that have been picked up immediately after their fall have been found +cold as ice within. Their size after reaching the ground is variable +within wide limits; some are known which weigh several tons, but the +great majority weigh only a few pounds and many only a few ounces. + +Meteorites are of two kinds: stony meteorites and iron meteorites. The +former outnumber the latter twenty to one; but many stone meteorites +contain grains of iron. Nickel is commonly found in iron meteorites, +so that it might be said that that redoubtable alloy nickel-steel is +of cosmical invention. Some twenty-five chemical elements have been +found in meteorites, including carbon and the ``sun-metal,'' helium. +The presence of the latter is certainly highly suggestive in +connection with the question of the origin of meteorites. The iron +meteorites, besides metallic iron and nickel, of which they are almost +entirely composed, contain hydrogen, helium, and carbonic oxide, and +about the only imaginable way in which these gases could have become +absorbed in the iron would be through the immersion of the latter +while in a molten or vaporized state in a hot and dense atmosphere +composed of them, a condition which we know to exist only in the +envelopes of the sun and the stars. + +The existence of carbon in the Canyon Diablo iron meteorites is +attended by a circumstance of the most singular character -- a very +``fairy tale of science.'' In some cases the carbon has become +diamond! These meteoric diamonds are very small; nevertheless, they +are true diamonds, resembling in many ways the little black gems +produced by Moissan's method with the aid of the electric furnace. The +fact that they are found embedded in these iron meteorites is another +argument in favor of the hypothesis of the solar or stellar origin of +the latter. To appreciate this it is necessary to recall the way in +which Moissan made his diamonds. It was by a combination of the +effects of great heat, great pressure, and sudden or rapid superficial +cooling on a mass of iron containing carbon. When he finally broke +open his iron he found it a pudding stuffed with miniature black +diamonds. When a fragment of the Canyon Diablo meteoric iron was +polished in Philadelphia over fifteen years ago it cut the emery-wheel +to pieces, and examination showed that the damage had been effected by +microscopic diamonds peppered through the mass. How were those +diamonds formed? If the sun or Sirius was the laboratory that prepared +them, we can get a glimpse at the process of their formation. There is +plenty of heat, plenty of pressure, and an abundance of vaporized iron +in the sun and the stars. When a great solar eruption takes place, +masses of iron which have absorbed carbon may be shot out with a +velocity which forbids their return. Plunged into the frightful cold +of space, their surfaces are quickly cooled, as Moissan cooled his +prepared iron by throwing it into water, and thus the requisite stress +is set up within, and, as the iron solidifies, the included carbon +crystallizes into diamonds. Whether this explanation has a germ of +truth in it or not, at any rate it is evident that iron meteorites +were not created in the form in which they come to us; they must once +have been parts of immeasurably more massive bodies than themselves. + +The fall of meteorites offers an appreciable, though numerically +insignificant, peril to the inhabitants of the earth. Historical +records show perhaps three or four instances of people being killed by +these bodies. But for the protection afforded by the atmosphere, which +acts as a very effective shield, the danger would doubtless be very +much greater. In the absence of an atmosphere not only would more +meteorites reach the ground, but their striking force would be +incomparably greater, since, as we have seen, the larger part of their +original velocity is destroyed by the resistance of the air. A +meteorite weighing many tons and striking the earth with a velocity of +twenty or thirty miles per second, would probably cause frightful +havoc. + +It is a singular fact that recent investigations seem to have proved +that an event of this kind actually happened in North America -- +perhaps not longer than a thousand or two thousand years ago. The +scene of the supposed catastrophe is in northern central Arizona, at +Coon Butte, where there is a nearly circular crater in the middle of a +circular elevation or small mountain. The crater is somewhat over four +thousand feet in diameter, and the surrounding rim, formed of upturned +strata and ejected rock fragments, rises at its highest point one +hundred and sixty feet above the plain. The crater is about six +hundred feet in depth -- that is, from the rim to the visible floor or +bottom of the crater. There is no evidence that volcanic action has +ever taken place in the immediate neighborhood of Coon Butte. The rock +in which the crater has been made is composed of horizontal sandstone +and limestone strata. Between three hundred and four hundred million +tons of rock fragments have been detached, and a large portion hurled +by some cause out of the crater. These fragments lie concentrically +distributed around the crater, and in large measure form the elevation +known as Coon Butte. The region has been famous for nearly twenty +years on account of the masses of meteoric iron found scattered about +and known as the ``Canyon Diablo'' meteorites. It was one of these +masses, which consist of nickel-iron containing a small quantity of +platinum, and of which in all some ten tons have been recovered for +sale to the various collectors throughout the world, that as before +mentioned destroyed the grinding-tool at Philadelphia through the +cutting power of its embedded diamonds. These meteoric irons are +scattered about the crater-hill, in concentric distribution, to a +maximum distance of about five miles. When the suggestion was first +made in 1896 that a monster meteorite might have created by its fall +this singular lone crater in stratified rocks, it was greeted with +incredulous smiles; but since then the matter has assumed a different +aspect. The Standard Iron Company, formed by Messrs. D. M. Barringer, +B. C. Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in +1903, the owner of this freak of nature, sunk shafts and bored holes +to a great depth in the interior of the crater, and also trenched the +slopes of the mountain, and the result of their investigations has +proved that the meteoric hypothesis of origin is correct. (See the +papers published in the Proceedings of the Academy of Natural Sciences +of Philadelphia, December, 1905, wherein it is proved that the United +States Geological Survey was wrong in believing this crater to have +been due to a steam explosion. Since that date there has been +discovered a great amount of additional confirmatory proof). Material +of unmistakably meteoric origin was found by means of the drills, +mixed with crushed rock, to a depth of six hundred to seven hundred +feet below the floor of the crater, and a great deal of it has been +found admixed with the ejected rock fragments on the outer slopes of +the mountain, absolutely proving synchronism between the two events, +the formation of this great crater and the falling of the meteoric +iron out of the sky. The drill located in the bottom of the crater was +sent, in a number of cases, much deeper (over one thousand feet) into +unaltered horizontal red sandstone strata, but no meteoric material +was found below this depth (seven hundred feet, or between eleven and +twelve hundred feet below the level of the surrounding plain), which +has been assumed as being about the limit of penetration. It is not +possible to sink a shaft at present, owing to the water which has +drained into the crater, and which forms, with the finely pulverized +sandstone, a very troublesome quicksand encountered at about two +hundred feet below the visible floor of the crater. As soon as this +water is removed by pumping it will be easy to explore the depths of +the crater by means of shafts and drifts. The rock strata (sandstone +and limestone) of which the walls consist present every appearance of +having been violently upturned by a huge body penetrating the earth +like a cannon-ball. The general aspect of the crater strikingly +resembles the impression made by a steel projectile shot into an +armor-plate. Mr Tilghman has estimated that a meteorite about five +hundred feet in diameter and moving with a velocity of about five +miles per second would have made just such a perforation upon striking +rocks of the character of those found at this place. There was some +fusion of the colliding masses, and the heat produced some steam from +the small amount of water in the rocks. As a result there has been +found at depth a considerable amount of fused quartz (original +sandstone), and with it innumerable particles or sparks of fused +nickel-iron (original meteorite). A projectile of that size +penetrating eleven to twelve hundred feet into the rocky shell of the +globe must have produced a shock which was perceptible several hundred +miles away. + +The great velocity ascribed to the supposed meteorite at the moment of +striking could be accounted for by the fact that it probably plunged +nearly vertically downward, for it formed a circular crater in the +rocky crust of the earth. In that case it would have been less +retarded by the resistance of the atmosphere than are meteorites which +enter the air at a lower angle and shoot ahead hundreds of miles until +friction has nearly destroyed their original motion when they drop +upon the earth. Some meteoric masses of great size, such as Peary's +iron meteorite found at Cape York, Greenland, and the almost equally +large mass discovered at Bacubirito, Mexico, appear to have penetrated +but slightly on striking the earth. This may be explained by supposing +that they pursued a long, horizontal course through the air before +falling. The result would be that, their original velocity having been +practically destroyed, they would drop to the ground with a velocity +nearly corresponding to that which gravity would impart within the +perpendicular distance of their final fall. A +six-hundred-and-sixty-pound meteorite, which fell at Knyahinya, +Hungary, striking at an angle of 27 from the vertical, penetrated the +ground to a depth of eleven feet. + +It has been remarked that the Coon Butte meteorite may have fallen not +longer ago than a few thousand years. This is based upon the fact that +the geological indications favor the supposition that the event did +not occur more than five thousand years ago, while on the other hand +the rings of growth in the cedar-trees growing on the slopes of the +crater show that they have existed there about seven hundred years. +Prof. William H. Pickering has recently correlated this with an +ancient chronicle which states that at Cairo, Egypt, in the year 1029, +``many stars passed with a great noise.'' He remarks that Cairo is +about 100, by great circle, from Coon Butte, so that if the meteorite +that made the crater was a member of a flock of similar bodies which +encountered the earth moving in parallel lines, some of them might +have traversed the sky tangent to the earth's surface at Cairo. That +the spectacle spoken of in the chronicle was caused by meteorites he +deems exceedingly probable because of what is said about ``a great +noise;'' meteorites are the only celestial phenomena attended with +perceptible sounds. Professor Pickering conjectures that this supposed +flock of great meteorites may have formed the nucleus of a comet which +struck the earth, and he finds confirmation of the idea in the fact +that out of the ten largest meteorites known, no less than seven were +found within nine hundred miles of Coon Butte. It would be interesting +if we could trace back the history of that comet, and find out what +malicious planet caught it up in its innocent wanderings and hurled it +with so true an aim at the earth! This remarkable crater is one of the +most interesting places in the world, for there is absolutely no +record of such a mass, possibly an iron-headed comet, from outer space +having come into collision with our earth. The results of the future +exploration of the depths of the crater will be awaited with much +interest. + +The Wrecking of the Moon + +There are sympathetic moods under whose influence one gazes with a +certain poignant tenderness at the worn face of the moon; that little +``fossil world'' (the child of our mother earth, too) bears such +terrible scars of its brief convulsive life that a sense of pity is +awakened by the sight. The moon is the wonder-land of the telescope. +Those towering mountains, whose ``proud aspiring peaks'' cast +silhouettes of shadow that seem drawn with india-ink; those vast +plains, enchained with gentle winding hills and bordered with giant +ranges; those oval ``oceans,'' where one looks expectant for the flash +of wind-whipped waves; those enchanting ``bays'' and recesses at the +seaward feet of the Alps; those broad straits passing between guardian +heights incomparably mightier than Gibraltar; those locket-like +valleys as secluded among their mountains as the Vale of Cashmere; +those colossal craters that make us smile at the pretensions of +Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with +the unconcern of Roman roads across mountain, gorge, and valley -- all +these give the beholder an irresistible impression that it is truly a +world into which he is looking, a world akin to ours, and yet no more +like our world than Pompeii is like Naples. Its air, its waters, its +clouds, its life are gone, and only a skeleton remains -- a mute but +eloquent witness to a cosmical tragedy without parallel in the range +of human knowledge. + +One cannot but regret that the moon, if it ever was the seat of +intelligent life, has not remained so until our time. Think what the +consequences would have been if this other world at our very door had +been found to be both habitable and inhabited! We talk rather airily +of communicating with Mars by signals; but Mars never approaches +nearer than 35,000,000 miles, while the moon when nearest is only a +little more than 220,000 miles away. Given an effective magnifying +power of five thousand diameters, which will perhaps be possible at +the mountain observatories as telescopes improve, and we should be +able to bring the moon within an apparent distance of about forty +miles, while the corresponding distance for Mars would be more than +seven thousand miles. But even with existing telescopic powers we can +see details on the moon no larger than some artificial constructions +on the earth. St Peter's at Rome, with the Vatican palace and the +great piazza, if existing on the moon, would unquestionably be +recognizable as something else than a freak of nature. Large cities, +with their radiating lines of communication, would at once betray +their real character. Cultivated tracts, and the changes produced by +the interference of intelligent beings, would be clearly recognizable. +The electric illumination of a large town at night would probably be +markedly visible. Gleams of reflected sunlight would come to us from +the surfaces of the lakes and oceans, and a huge ``liner'' traversing +a lunar sea could probably be followed by its trail of smoke. As to +communications by ``wireless'' signals, which certain enthusiasts have +thought of in connection with Mars, in the case of the moon they +should be a relatively simple matter, and the feat might actually be +accomplished. Think what a literature would grow up about the moon if +it were a living world! Its very differences from the earth would only +accentuate its interest for us. Night and day on the moon are each two +weeks in length; how interesting it would be to watch the manner in +which the lunarians dealt with such a situation as that. Lunar and +terrestrial history would keep step with each other, and we should +record them both. Truly one might well wish to have a neighbor world +to study; one would feel so much the less alone in space. + +It is not impossible that the moon did at one time have inhabitants of +some kind. But, if so, they vanished with the disappearance of its +atmosphere and seas, or with the advent of its cataclysmic age. At the +best, its career as a living world must have been brief. If the water +and air were gradually absorbed, as some have conjectured, by its +cooling interior rocks, its surface might, nevertheless, have retained +them for long ages; but if, as others think, their disappearance was +due to the escape of their gaseous molecules in consequence of the +inability of the relatively small lunar gravitation to retain them, +then the final catastrophe must have been as swift as it was +inevitable. Accepting Darwin's hypothesis, that the moon was separated +from the earth by tidal action while both were yet plastic or +nebulous, we may reasonably conclude that it began its career with a +good supply of both water and air, but did not possess sufficient mass +to hold them permanently. Yet it may have retained them long enough +for life to develop in many forms upon its surface; in fact, there are +so many indications that air and water have not always been lacking to +the lunar world that we are driven to invent theories to explain both +their former presence and their present absence. + +But whatever the former condition of the moon may have been, its +existing appearance gives it a resistless fascination, and it bears so +clearly the story of a vast catastrophe sculptured on its rocky face +that the thoughtful observer cannot look upon it without a feeling of +awe. The gigantic character of the lunar features impresses the +beholder not less than the universality of the play of destructive +forces which they attest. Let us make a few comparisons. Take the +lunar crater called ``Tycho'', which is a typical example of its kind. +In the telescope Tycho appears as a perfect ring surrounding a +circular depression, in the center of which rises a group of +mountains. Its superficial resemblance to some terrestrial volcanic +craters is very striking. Vesuvius, seen from a point vertically +above, would no doubt look something like that (the resemblance would +have been greater when the Monte del Cavallo formed a more complete +circuit about the crater cone). But compare the dimensions. The +remains of the outer crater ring of Vesuvius are perhaps half a mile +in diameter, while the active crater itself is only two or three +hundred feet across at the most; Tycho has a diameter of fifty-four +miles! The group of relatively insignificant peaks in the center of +the crater floor of Tycho is far more massive than the entire mountain +that we call Vesuvius. The largest known volcanic crater on the earth, +Aso San, in Japan, has a diameter of seven miles; it would take sixty +craters like Aso San to equal Tycho in area! And Tycho, though one of +the most perfect, is by no means the largest crater on the moon. +Another, called ``Theophilus,'' has a diameter of sixty-four miles, +and is eighteen thousand feet deep. There are hundreds from ten to +forty miles in diameter, and thousands from one to ten miles. They are +so numerous in many places that they break into one another, like the +cells of a crushed honeycomb. + +The lunar craters differ from those of the earth more fundamentally +than in the matter of mere size; they are not situated on the tops of +mountains. If they were, and if all the proportions were the same, a +crater like Tycho might crown a conical peak fifty or one hundred +miles high! Instead of being cavities in the summits of mountains, the +lunar craters are rather gigantic sink-holes whose bottoms in many +cases lie two or three miles below the general surface of the lunar +world. Around their rims the rocks are piled up to a height of from a +few hundred to two or three thousand feet, with a comparatively gentle +inclination, but on the inner side they fall away in gigantic broken +precipices which make the dizzy cliffs of the Matterhorn seem but +``lover's leaps.'' Down they drop, ridge below ridge, crag under crag, +tottering wall beneath wall, until, in a crater named ``Newton,'' near +the south lunar pole, they attain a depth where the rays of the sun +never reach. Nothing more frightful than the spectacle which many of +these terrible chasms present can be pictured by the imagination. As +the lazy lunar day slowly advances, the sunshine, unmitigated by +clouds or atmospheric veil of any kind, creeps across their rims and +begins to descend the opposite walls. Presently it strikes the ragged +crest of a ridge which had lain hidden in such darkness as we never +know on the earth, and runs along it like a line of kindling fire. +Rocky pinnacles and needles shoot up into the sunlight out of the +black depths. Down sinks the line of light, mile after mile, and +continually new precipices and cliffs are brought into view, until at +last the vast floor is attained and begins to be illuminated. In the +meanwhile the sun's rays, darting across the gulf, have touched the +summits of the central peaks, twenty or thirty miles from the crater's +inmost edge, and they immediately kindle and blaze like huge stars +amid the darkness. So profound are some of these awful craters that +days pass before the sun has risen high enough above them to chase the +last shadows from their depths. + +Although several long ranges of mountains resembling those of the +earth exist on the moon, the great majority of its elevations assume +the crateriform aspect. Sometimes, instead of a crater, we find an +immense mountain ring whose form and aspect hardly suggest volcanic +action. But everywhere the true craters are in evidence, even on the +sea-beds, although they attain their greatest number and size on those +parts of the moon -- covering sixty per cent of its visible surface -- +which are distinctly mountainous in character and which constitute its +most brilliant portions. Broadly speaking, the southwestern half of +the moon is the most mountainous and broken, and the northeastern half +the least so. Right down through the center, from pole to pole, runs a +wonderful line of craters and crateriform valleys of a magnitude +stupendous even for the moon. Another similar line follows the western +edge. Three or four ``seas'' are thrust between these mountainous +belts. By the effects of ``libration'' parts of the opposite +hemisphere of the moon which is turned away from the earth are from +time to time brought into view, and their aspect indicates that that +hemisphere resembles in its surface features the one which faces the +earth. There are many things about the craters which seem to give some +warrant for the hypothesis which has been particularly urged by Mr G. +K. Gilbert, that they were formed by the impact of meteors; but there +are also many things which militate against that idea, and, upon the +whole, the volcanic theory of their origin is to be preferred. + +The enormous size of the lunar volcanoes is not so difficult to +account for when we remember how slight is the force of lunar gravity +as compared with that of the earth. With equal size and density, +bodies on the moon weigh only one-sixth as much as on the earth. +Impelled by the same force, a projectile that would go ten miles on +the earth would go sixty miles on the moon. A lunar giant thirty-five +feet tall would weigh no more than an ordinary son of Adam weighs on +his greater planet. To shoot a body from the earth so that it would +not drop back again, we should have to start it with a velocity of +seven miles per second; a mile and a half per second would serve on +the moon. It is by no means difficult to believe, then, that a lunar +volcano might form a crater ring eight or ten times broader than the +greatest to be found on the earth, especially when we reflect that in +addition to the relatively slight force of gravity, the materials of +the lunar crust are probably lighter than those of our terrestrial +rocks. + +For similar reasons it seems not impossible that the theory mentioned +in a former chapter -- that some of the meteorites that have fallen +upon the earth originated from the lunar volcanoes -- is well founded. +This would apply especially to the stony meteorites, for it is hardly +to be supposed that the moon, at least in its superficial parts, +contains much iron. It is surely a scene most strange that is thus +presented to the mind's eye -- that little attendant of the earth's +(the moon has only one-fiftieth of the volume, and only one-eightieth +of the mass of the earth) firing great stones back at its parent +planet! And what can have been the cause of this furious outbreak of +volcanic forces on the moon? Evidently it was but a passing stage in +its history; it had enjoyed more quiet times before. As it cooled down +from the plastic state in which it parted from the earth, it became +incrusted after the normal manner of a planet, and then oceans were +formed, its atmosphere being sufficiently dense to prevent the water +from evaporating and the would-be oceans from disappearing continually +in mist. This, if any, must have been the period of life in the lunar +world. As we look upon the vestiges of that ancient world buried in +the wreck that now covers so much of its surface, it is difficult to +restrain the imagination from picturing the scenes which were once +presented there; and, in such a case, should the imagination be +fettered? We give it free rein in terrestrial life, and it rewards us +with some of our greatest intellectual pleasures. The wonderful +landscapes of the moon offer it an ideal field with just enough +half-hidden suggestions of facts to stimulate its powers. + +The great plains of the Mare Imbrium and the Mare Serenitatis (the +``Sea of Showers'' and the ``Sea of Serenity''), bordered in part by +lofty mountain ranges precisely like terrestrial mountains, scalloped +along their shores with beautiful bays curving back into the adjoining +highlands, and united by a great strait passing between the nearly +abutting ends of the ``Lunar Apennines'' and the ``Lunar Caucasus,'' +offer the elements of a scene of world beauty such as it would be +difficult to match upon our planet. Look at the finely modulated +bottom of the ancient sea in Mr Ritchey's exquisite photograph of the +western part of the Mare Serenitatis, where one seems to see the play +of the watery currents heaping the ocean sands in waving lines, making +shallows, bars, and deeps for the mariner to avoid or seek, and +affording a playground for the creatures of the main. What geologist +would not wish to try his hammer on those rocks with their stony pages +of fossilized history? There is in us an instinct which forbids us to +think that there was never any life there. If we could visit the moon, +there is not among us a person so prosaic and unimaginative that he +would not, the very first thing, begin to search for traces of its +inhabitants. We would look for them in the deposits on the sea +bottoms; we would examine the shores wherever the configuration seemed +favorable for harbors and the sites of maritime cities -- forgetting +that it may be a little ridiculous to ascribe to the ancient lunarians +the same ideas that have governed the development of our race; we +would search through the valleys and along the seeming courses of +vanished streams; we would explore the mountains, not the terrible +craters, but the pinnacled chains that recall our own Alps and +Rockies; seeking everywhere some vestige of the transforming presence +of intelligent life. Perhaps we should find such traces, and perhaps, +with all our searching, we should find nothing to suggest that life +had ever existed amid that universal ruin. + +Look again at the border of the ``Sea of Serenity'' -- what a name for +such a scene! -- and observe how it has been rent with almost +inconceivable violence, the wall of the colossal crater Posidonius +dropping vertically upon the ancient shore and obliterating it, while +its giant neighbor, Le Monnier, opens a yawning mouth as if to swallow +the sea itself. A scene like this makes one question whether, after +all, those may not be right who have imagined that the so-called sea +bottoms are really vast plains of frozen lava which gushed up in +floods so extensive that even the mighty volcanoes were half drowned +in the fiery sea. This suggestion becomes even stronger when we turn +to another of the photographs of Mr Ritchey's wonderful series, +showing a part of the Mare Tranquilitatis (``Sea of Tranquility''!). +Notice how near the center of the picture the outline of a huge ring +with radiating ridges shows through the sea bottom; a fossil volcano +submerged in a petrified ocean! This is by no means the only instance +in which a buried world shows itself under the great lunar plains. +Yet, as the newer craters in the sea itself prove, the volcanic +activity survived this other catastrophe, or broke out again +subsequently, bringing more ruin to pile upon ruin. + +Yet notwithstanding the evidence which we have just been considering +in support of the hypothesis that the ``seas'' are lava floods, +Messrs. Loewy and Puiseux, the selenographers of the Paris +Observatory, are convinced that these great plains bear characteristic +marks of the former presence of immense bodies of water. In that case +we should be forced to conclude that the later oceans of the moon lay +upon vast sheets of solidified lava; and thus the catastrophe of the +lunar world assumes a double aspect, the earliest oceans being +swallowed up in molten floods issuing from the interior, while the +lands were reduced to chaos by a universal eruption of tremendous +volcanoes; and then a period of comparative quiet followed, during +which new seas were formed, and new life perhaps began to flourish in +the lunar world, only to end in another cataclysm, which finally put a +term to the existence of the moon as a life-supporting world. + +Suppose we examine two more of Mr Ritchey's illuminating photographs, +and, first, the one showing the crater Theophilus and its +surroundings. We have spoken of Theophilus before, citing the facts +that it is sixty-four miles in diameter and eighteen thousand feet +deep. It will be noticed that it has two brother giants -- Cyrillus +the nearer, and Catharina the more distant; but Theophilus is plainly +the youngest of the trio. Centuries, and perhaps thousands of years, +must have elapsed between the periods of their upheaval, for the two +older craters are partly filled with dbris, while it is manifest at a +glance that when the south eastern wall of Theophilus was formed, it +broke away and destroyed a part of the more ancient ring of Cyrillus. +There is no more tremendous scene on the moon than this; viewed with a +powerful telescope, it is absolutely appalling. + +The next photograph shows, if possible, a still wilder region. It is +the part of the moon lying between Tycho and the south pole. Tycho is +seen in the lower left-hand part of the picture. To the right, at the +edge of the illuminated portion of the moon, are the crater-rings, +Longomontanus and Wilhelm I, the former being the larger. Between them +are to be seen the ruins of two or three more ancient craters which, +together with portions of the walls of Wilhelm I and Longomontanus, +have been honeycombed with smaller craters. The vast crateriform +depression above the center of the picture is Clavius, an unrivaled +wonder of lunar scenery, a hundred and forty-two miles in its greatest +length, while its whole immense floor has sunk two miles below the +general surface of the moon outside the ring. The monstrous +shadow-filled cavity above Clavius toward the right is Blancanus, +whose aspect here gives a good idea of the appearance of these chasms +when only their rims are in the sunlight. But observe the +indescribable savagery of the entire scene. It looks as though the +spirit of destruction had gone mad in this spot. The mighty craters +have broken forth one after another, each rending its predecessor; and +when their work was finished, a minor but yet tremendous outbreak +occurred, and the face of the moon was gored and punctured with +thousands of smaller craters. These relatively small craters (small, +however, only in a lunar sense, for many of them would appear gigantic +on the earth) recall once more the theory of meteoric impact. It does +not seem impossible that some of them may have been formed by such an +agency. + +One would not wish for our planet such a fate as that which has +overtaken the moon, but we cannot be absolutely sure that something of +the kind may not be in store for it. We really know nothing of the +ultimate causes of volcanic activity, and some have suggested that the +internal energies of the earth may be accumulating instead of dying +out, and may never yet have exhibited their utmost destructive power. +Perhaps the best assurance that we can find that the earth will escape +the catastrophe that has overtaken its satellite is to be found in the +relatively great force of its gravitation. The moon has been the +victim of its weakness; given equal forces, and the earth would be the +better able to withstand them. It is significant, in connection with +these considerations, that the little planet Mercury, which seems also +to have parted with its air and water, shows to the telescope some +indications that it is pitted with craters resembling those that have +torn to pieces the face of the moon. + +Upon the whole, after studying the dreadful lunar landscapes, one +cannot feel a very enthusiastic sympathy with those who are seeking +indications of the continued existence of some kind of life on the +moon; such a world is better without inhabitants. It has met its fate; +let it go! Fortunately, it is not so near that it cannot hide its +scars and appear beautiful -- except when curiosity impels us to look +with the penetrating eyes of the astronomer. + +The Great Mars Problem + +Let any thoughtful person who is acquainted with the general facts of +astronomy look up at the heavens some night when they appear in their +greatest splendor, and ask himself what is the strongest impression +that they make upon his mind. He may not find it easy to frame an +answer, but when he has succeeded it will probably be to the effect +that the stars give him an impression of the universality of +intelligence; they make him feel, as the sun and the moon cannot do, +that his world is not alone; that all this was not made simply to form +a gorgeous canopy over the tents of men. If he is of a devout turn of +mind, he thinks, as he gazes into those fathomless deeps and among +those bewildering hosts, of the infinite multitude of created beings +that the Almighty has taken under his care. The narrow ideas of the +old geocentric theology, which made the earth God's especial +footstool, and man his only rational creature, fall away from him like +a veil that had obscured his vision; they are impossible in the +presence of what he sees above. Thus the natural tendency, in the +light of modern progress, is to regard the universe as everywhere +filled with life. + +But science, which is responsible for this broadening of men's +thoughts concerning the universality of life, itself proceeds to set +limits. Of spiritual existences it pretends to know nothing, but as to +physical beings, it declares that it can only entertain the +supposition of their existence where it finds evidence of an +environment suited to their needs, and such environment may not +everywhere exist. Science, though repelled by the antiquated +theological conception of the supreme isolation of man among created +beings, regards with complacency the probability that there are +regions in the universe where no organic life exists, stars which +shine upon no inhabited worlds, and planets which nourish no animate +creatures. The astronomical view of the universe is that it consists +of matter in every stage of evolution: some nebulous and chaotic; some +just condensing into stars (suns) of every magnitude and order; some +shaped into finished solar bodies surrounded by dependent planets; +some forming stars that perhaps have no planets, and will have none; +some constituting suns that are already aging, and will soon lose +their radiant energy and disappear; and some aggregated into masses +that long ago became inert, cold, and rayless, and that can only be +revivified by means about which we can form conjectures, but of which +we actually know nothing. + +As with the stars, so with the planets, which are the satellites of +stars. All investigations unite to tell us that the planets are not +all in the same state of development. As some are large and some +small, so some are, in an evolutionary sense, young, and some old. As +they depend upon the suns around which they revolve for their light, +heat, and other forms of radiant energy, so their condition varies +with their distance from those suns. Many may never arrive at a state +suitable for the maintenance of life upon their surfaces; some which +are not at present in such a state may attain it later; and the forms +of life themselves may vary with the peculiar environment that +different planets afford. Thus we see that we are not scientifically +justified in affirming that life is ubiquitous, although we are thus +justified in saying that it must be, in a general sense, universal. We +might liken the universe to a garden known to contain every variety of +plant. If on entering it we see no flowers, we examine the species +before us and find that they are not of those which bloom at this +particular season, or perhaps they are such as never bear flowers. Yet +we feel no doubt that we shall find flowers somewhere in the garden, +because there are species which bloom at this season, and the garden +contains all varieties. + +While it is tacitly assumed that there are planets revolving around +other stars than the sun, it would be impossible for us to see them +with any telescope yet invented, and no instrument now in the +possession of astronomers could assure us of their existence; so the +only planetary system of which we have visual knowledge is our own. +Excluding the asteroids, which could not from any point of view be +considered as habitable, we have in the solar system eight planets of +various sizes and situated at various distances from the sun. Of these +eight we know that one, the earth, is inhabited. The question, then, +arises: Are there any of the others which are inhabited or habitable? +Since it is our intention to discuss the habitability of only one of +the seven to which the question applies, the rest may be dismissed in +a few words. The smallest of them, and the nearest to the sun, is +Mercury, which is regarded as uninhabitable because it has no +perceptible supply of water and air, and because, owing to the +extraordinary eccentricity of its orbit, it is subjected to excessive +and very rapid alterations in the amount of solar heat and light +poured upon its surface, such alterations being inconsistent with the +supposition that it can support living beings. Even its average +temperature is more than six and a half times that prevailing on the +earth! Another circumstance which militates against its habitability +is that, according to the results of the best telescopic studies, it +always keeps the same face toward the sun, so that one half of the +planet is perpetually exposed to the fierce solar rays, and the other +half faces the unmitigated cold of open space. Venus, the next in +distance from the sun, is almost the exact twin of the earth in size, +and many arguments may be urged in favor of its habitability, although +it is suspected of possessing the same peculiarity as Mercury, in +always keeping the same side sunward. Unfortunately its atmosphere +appears to be so dense that no permanent markings on its surface are +certainly visible, and the question of its actual condition must, for +the present, be left in abeyance. Mars, the first planet more distant +from the sun than the earth, is the special subject of this chapter, +and will be described and discussed a few lines further on. Jupiter, +Saturn, Uranus, and Neptune, the four giant planets, all more distant +than Mars, and each more distant than the other in the order named, +are all regarded as uninhabitable because none of them appears to +possess any degree of solidity. They may have solid or liquid nuclei, +but exteriorly they seem to be mere balls of cloud. Of course, one can +imagine what he pleases about the existence of creatures suited to the +physical constitution of such planets as these, but they must be +excluded from the category of habitable worlds in the ordinary sense +of the term. We go back, then, to Mars. + +It will be best to begin with a description of the planet. Mars is +4230 miles in diameter; its surface is not much more than one-quarter +as extensive as that of the earth (.285). Its mean distance from the +sun is 141,500,000 miles, 48,500,000 miles greater than that of the +earth. Since radiant energy varies inversely as the square of +distance, Mars receives less than half as much solar light and heat as +the earth gets. Mars' year (period of revolution round the sun) is 687 +days. Its mean density is 71 per cent of the earth's, and the force of +gravity on its surface is 38 per cent of that on the surface of the +earth; i.e., a body weighing one hundred pounds on the earth would, if +transported to Mars, weigh but thirty-eight pounds. The inclination of +its equator to the plane of its orbit differs very little from that of +the earth's equator, and its axial rotation occupies 24 hours 37 +minutes. so that the length of day and night, and the extent of the +seasonal changes on Mars, are almost precisely the same as on the +earth. But owing to the greater length of its year, the seasons of +Mars, while occurring in the same order, are almost twice as long as +ours. The surface of the planet is manifestly solid, like that of our +globe, and the telescope reveals many permanent markings on it, +recalling the appearance of a globe on which geographical features +have been represented in reddish and dusky tints. Around the poles are +plainly to be seen rounded white areas, which vary in extent with the +Martian seasons, nearly vanishing in summer and extending widely in +winter. The most recent spectroscopic determinations indicate that +Mars has an atmosphere perhaps as dense as that to be found on our +loftiest mountain peaks, and there is a perceptible amount of watery +vapor in this atmosphere. The surface of the planet appears to be +remarkably level, and it has no mountain ranges. No evidences of +volcanic action have been discovered on Mars. The dusky and reddish +areas were regarded by the early observers as respectively seas and +lands, but at present it is not believed that there are any bodies of +water on the planet. There has never been much doubt expressed that +the white areas about the poles represent snow. + +It will be seen from this brief description that many remarkable +resemblances exist between Mars and the earth, and there is nothing +wonderful in the fact that the question of the habitability of the +former has become one of extreme and wide-spread interest, giving rise +to the most diverse views, to many extraordinary speculations, and +sometimes to regrettably heated controversy. The first champion of the +habitability of Mars was Sir William Herschel, although even before +his time the idea had been suggested. He was convinced by the +revelations of his telescopes, continually increasing in power, that +Mars was more like the earth than any other planet. He could not +resist the testimony of the polar snows, whose suggestive conduct was +in such striking accord with what occurs upon the earth. Gradually, as +telescopes improved and observers increased in number, the principal +features of the planet were disclosed and charted, and ``areography,'' +as the geography of Mars was called, took its place among the +recognized branches of astronomical study. But it was not before 1877 +that a fundamentally new discovery in areography gave a truly +sensational turn to speculation about life on ``the red planet.'' In +that year Mars made one of its nearest approaches to the earth, and +was so situated in its orbit that it could be observed to great +advantage from the northern hemisphere of the earth. The celebrated +Italian astronomer, Schiaparelli, took advantage of this opportunity +to make a trigonometrical survey of the surface of Mars -- as coolly +and confidently as if he were not taking his sights across a +thirty-five-million-mile gulf of empty space -- and in the course of +this survey he was astonished to perceive that the reddish areas, then +called continents, were crossed in many directions by narrow, dusky +lines, to which he gave the suggestive name of ``canals.'' Thus a kind +of firebrand was cast into the field of astronomical speculation, +which has ever since produced disputes that have sometimes approached +the violence of political faction. At first the accuracy of +Schiaparelli's observations was contested; it required a powerful +telescope, and the most excellent ``seeing,'' to render the +enigmatical lines visible at all, and many searchers were unable to +detect them. But Schiaparelli continued his studies in the serene sky +of Italy, and produced charts of the gridironed face of Mars +containing so much astonishing detail that one had either to reject +them in toto or to confess that Schiaparelli was right. As subsequent +favorable oppositions of Mars occurred, other observers began to see +the ``canals'' and to confirm the substantial accuracy of the Italian +astronomer's work, and finally few were found who would venture to +affirm that the ``canals'' did not exist, whatever their meaning might +be. + +When Schiaparelli began his observations it was generally believed, as +we have said, that the dusky areas on Mars were seas, and since +Schiaparelli thought that the ``canals'' invariably began and ended at +the shores of the ``seas,'' the appropriateness of the title given to +the lines seemed apparent. Their artificial character was immediately +assumed by many, because they were too straight and too suggestively +geometrical in their arrangement to permit the conclusion that they +were natural watercourses. A most surprising circumstance noted by +Schiaparelli was that the ``canals'' made their appearance after the +melting of the polar snow in the corresponding hemisphere had begun, +and that they grew darker, longer, and more numerous in proportion as +the polar liquidation proceeded; another very puzzling observation was +that many of them became double as the season advanced; close beside +an already existing ``canal,'' and in perfect parallelism with it, +another would gradually make its appearance. That these phenomena +actually existed and were not illusions was proved by later +observations, and today they are seen whenever Mars is favorably +situated for observation. + +In the closing decade of the nineteenth century, Mr Percival Lowell +took up the work where Schiaparelli had virtually dropped it, and soon +added a great number of ``canals'' to those previously known, so that +in his charts the surface of the wonderful little planet appears +covered as with a spider's web, the dusky lines criss-crossing in +every direction, with conspicuous knots wherever a number of them come +together. Mr Lowell has demonstrated that the areas originally called +seas, and thus named on the earlier charts, are not bodies of water, +whatever else they may be. He has also found that the mysterious lines +do not, as Schiaparelli supposed, begin and end at the edges of the +dusky regions, but often continue on across them, reaching in some +cases far up into the polar regions. But Schiaparelli was right in his +observation that the appearance of the ``canals'' is synchronous with +the gradual disappearance of the polar snows, and this fact has become +the basis of the most extraordinary theory that the subject of life in +other worlds has ever given birth to. + +Now, the effect of such discoveries, as we have related, depends upon +the type of mind to whose attention they are called. Many are content +to accept them as strange and inexplicable at present, and to wait for +further light upon them; others insist upon an immediate inquiry +concerning their probable nature and meaning. Such an inquiry can only +be based upon inference proceeding from analogy. Mars, say Mr Lowell +and those who are of his opinion, is manifestly a solidly incrusted +planet like the earth; it has an atmosphere, though one of great +rarity; it has water vapor, as the snows in themselves prove; it has +the alternation of day and night, and a succession of seasons closely +resembling those of the earth; its surface is suggestively divided +into regions of contrasting colors and appearance, and upon that +surface we see an immense number of lines geometrically arranged, with +a system of symmetrical intersections where the lines expand into +circular and oval areas -- and all connected with the annual melting +of the polar snows in a way which irresistibly suggests the +interference of intelligence directed to a definite end. Why, with so +many concurrent circumstances to support the hypothesis, should we not +regard Mars as an inhabited globe? + +But the differences between Mars and the earth are in many ways as +striking as their resemblances. Mars is relatively small; it gets less +than half as much light and heat as we receive; its atmosphere is so +rare that it would be distressing to us, even if we could survive in +it at all; it has no lakes, rivers, or seas; its surface is an endless +prairie. and its ``canals'' are phenomena utterly unlike anything on +the earth. Yet it is precisely upon these divergences between the +earth and Mars, this repudiation of terrestrial standards, that the +theory of ``life on Mars,'' for which Mr Lowell is mainly responsible, +is based. Because Mars is smaller than the earth, we are told it must +necessarily be more advanced in planetary evolution, the underlying +cause of which is the gradual cooling and contraction of the planet's +mass. Mars has parted with its internal heat more rapidly than the +earth; consequently its waters and its atmosphere have been mostly +withdrawn by chemical combinations, but enough of both yet remain to +render life still possible on its surface. As the globe of Mars is +evolutionally older than that of the earth, so its forms of organic +life may be proportionally further advanced, and its inhabitants may +have attained a degree of cultivated intelligence much superior to +what at present exists upon the earth. Understanding the nature and +the causes of the desiccation of their planet, and possessing +engineering science and capabilities far in advance of ours, they may +be conceived to have grappled with the stupendous problem of keeping +their world in a habitable condition as long as possible. Supposing +them to have become accustomed to live in their rarefied atmosphere (a +thing not inconceivable, since men can live for a time at least in air +hardly less rare), the most pressing problem for them is that of a +water-supply, without which plant life cannot exist, while animal life +in turn depends for its existence upon vegetation. The only direction +in which they can seek water is that of the polar regions, where it is +alternately condensed into snow and released in the liquid form by the +effect of the seasonal changes. It is, then, to the annual melting of +the polar snow-fields that the Martian engineers are supposed to have +recourse in supplying the needs of their planet, and thus providing +the means of prolonging their own existence. It is imagined that they +have for this purpose constructed a stupendous system of irrigation +extending over the temperate and equatorial regions of the planet. The +``canals'' represent the lines of irrigation, but the narrow streaks +that we see are not the canals themselves, but the irrigated bands +covered by them. Their dark hue, and their gradual appearance after +the polar melting has begun, are due to the growth of vegetation +stimulated by the water. The rounded areas visible where several +``canals'' meet and cross are called by Mr Lowell ``oases.'' These are +supposed to be the principal centers of population and industry. It +must be confessed that some of them, with their complicated systems of +radiating lines, appear to answer very well to such a theory. No +attempt to explain them by analogy with natural phenomena on the earth +has proved successful. + +But a great difficulty yet remains: How to explain the seemingly +miraculous powers of the supposed engineers? Here recourse is had once +more to the relative smallness of the planet. We have remarked that +the force of gravity on Mars is only thirty-eight per cent of that on +the earth. A steam-shovel driven by a certain horse-power would be +nearly three times as effective there as here. A man of our stature on +Mars would find his effective strength increased in the same +proportion. But just because of the slight force of gravity there, a +Martian might attain to the traditional stature of Goliath without +finding his own weight an encumbrance to his activity, while at the +same time his huge muscles would come into unimpeded play, enabling +him single-handed to perform labors that would be impossible to a +whole gang of terrestrial workmen. The effective powers of huge +machines would be increased in the same way; and to all this must be +added the fact that the mean density of the materials of which Mars is +composed is much less than that of the constituents of the earth. +Combining all these considerations, it becomes much less difficult to +conceive that public works might be successfully undertaken on Mars +which would be hopelessly beyond the limits of human accomplishment. + +Certain other difficulties have also to be met; as, for instance, the +relative coldness of the climate of Mars. At its distance it gets +considerably less than half as much light and heat as we receive. In +addition to this, the rarity of its atmosphere would naturally be +expected to decrease the effective temperature at the planet's +surface, since an atmosphere acts somewhat like the glass cover of a +hot-house in retaining the solar heat which has penetrated it. It has +been calculated that, unless there are mitigating circumstances of +which we know nothing, the average temperature at the surface of Mars +must be far below the freezing-point of water. To this it is replied +that the possible mitigating circumstances spoken of evidently exist +in fact, because we can see that the watery vapor condenses into snow +around the poles in winter, but melts again when summer comes. The +mitigating agent may be supposed to exist in the atmosphere where the +presence of certain gases would completely alter the temperature +gradients. + +It might also be objected that it is inconceivable that the Martian +engineers, however great may be their physical powers, and however +gigantic the mechanical energies under their control, could force +water in large quantities from the poles to the equator. This is an +achievement that measures up to the cosmical standard. It is admitted +by the champions of the theory that the difficulty is a formidable +one; but they call attention to the singular fact that on Mars there +can be found no chains of mountains, and it is even doubtful if ranges +of hills exist there. The entire surface of the planet appears to be +almost ``as smooth as a billiard ball,'' and even the broad regions +which were once supposed to be seas apparently lie at practically the +same level as the other parts, since the ``canals'' in many cases run +uninterruptedly across them. Lowell's idea is that these sombre areas +may be expanses of vegetation covering ground of a more or less marshy +character, for while the largest of them appear to be permanent, there +are some which vary coincidently with the variations of the canals. + +As to the kind of machinery employed to force the water from the +poles, it has been conjectured that it may have taken the form of a +gigantic system of pumps and conduits; and since the Martians are +assumed to be so far in advance of us in their mastery of scientific +principles, the hypothesis will at least not be harmed by supposing +that they have learned to harness forces of nature whose very +existence in a manageable form is yet unrecognized on the earth. If we +wish to let the imagination loose, we may conjecture that they have +conquered the secret of those intra-atomic forces whose resistless +energy is beginning to become evident to us, but the possibility of +whose utilization remains a dream, the fulfillment of which nobody +dares to predict. + +Such, in very brief form, is the celebrated theory of Mars as an +inhabited world. It certainly captivates the imagination, and if we +believe it to represent the facts, we cannot but watch with the +deepest sympathy this gallant struggle of an intellectual race to +preserve its planet from the effects of advancing age and death. We +may, indeed, wonder whether our own humanity, confronted by such a +calamity, could be counted on to meet the emergency with equal +stoutness of heart and inexhaustibleness of resource. Up to the +present time we certainly have shown no capacity to confront Nature +toe to toe, and to seize her by the shoulders and turn her round when +she refuses to go our way. If we could get into wireless telephonic +communication with the Martians we might learn from their own lips the +secret of their more than ``Roman recovery.'' + +The Riddle of the Asteroids + +Between the orbits of Mars and Jupiter revolves the most remarkable +system of little bodies with which we are acquainted -- the Asteroids, +or Minor Planets. Some six hundred are now known, and they may +actually number thousands. They form virtually a ring about the sun. +The most striking general fact about them is that they occupy the +place in the sky which should be occupied, according to Bode's Law, by +a single large planet. This fact, as we shall see, has led to the +invention of one of the most extraordinary theories in astronomy -- +viz., that of the explosion of a world! + +Bode's Law, so-called, is only an empiric formula, but until the +discovery of Neptune it accorded so well with the distances of the +planets that astronomers were disposed to look upon it as really +representing some underlying principle of planetary distribution. They +were puzzled by the absence of a planet in the space between Mars and +Jupiter, where the ``law'' demanded that there should be one, and an +association of astronomers was formed to search for it. There was a +decided sensation when, in 1801, Piazzi, of Palermo, announced that he +had found a little planet which apparently occupied the place in the +system which belonged to the missing body. He named it Ceres, and it +was the first of the Asteroids. The next year Olbers, of Bremen, while +looking for Ceres with his telescope, stumbled upon another small +planet which he named Pallas. Immediately he was inspired with the +idea that these two planets were fragments of a larger one which had +formerly occupied the vacant place in the planetary ranks, and he +predicted that others would be found by searching in the neighborhood +of the intersection of the orbits of the two already discovered. This +bold prediction was brilliantly fulfilled by the finding of two more +-- Juno in 1804, and Vesta in 1807. Olbers would seem to have been led +to the invention of his hypothesis of a planetary explosion by the +faith which astronomers at that time had in Bode's Law. They appear to +have thought that several planets revolving in the gap where the +``law'' called for but one could only be accounted for upon the theory +that the original one had been broken up to form the several. +Gravitation demanded that the remnants of a planet blown to pieces, no +matter how their orbits might otherwise differ, should all return at +stated periods to the point where the explosion had occurred; hence +Olbers' prediction that any asteroids that might subsequently be +discovered would be found to have a common point of orbital +intersection. And curiously enough all of the first asteroids found +practically answered to this requirement. Olbers' theory seemed to be +established. + +After the first four, no more asteroids were found until 1845, when +one was discovered; then, in 1847, three more were added to the list; +and after that searchers began to pick them up with such rapidity that +by the close of the century hundreds were known, and it had become +almost impossible to keep track of them. The first four are by far the +largest members of the group, but their actual sizes remained unknown +until less than twenty years ago. It was long supposed that Vesta was +the largest, because it shines more brightly than any of the others; +but finally, in 1895, Barnard, with the Lick telescope, definitely +measured their diameters, and proved to everybody's surprise that +Ceres is really the chief, and Vesta only the third in rank. His +measures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, +239 miles; and Juno, 120 miles. They differ greatly in the reflective +power of their surfaces, a fact of much significance in connection +with the question of their origin. Vesta is, surface for surface, +rather more than three times as brilliant as Ceres, whence the +original mistake about its magnitude. + +Nowadays new asteroids are found frequently by photography, but +physically they are most insignificant bodies, their average diameter +probably not exceeding twenty miles, and some are believed not to +exceed ten. On a planet only ten miles in diameter, assuming the same +mean density as the earth's, which is undoubtedly too much, the force +of gravity would be so slight that an average man would not weigh more +than three ounces, and could jump off into space whenever he liked. + +Although the asteroids all revolve around the sun in the same +direction as that pursued by the major planets, their orbits are +inclined at a great variety of angles to the general plane of the +planetary system, and some of them are very eccentric -- almost as +much so as the orbits of many of the periodic comets. It has even been +conjectured that the two tiny moons of Mars and the four smaller +satellites of Jupiter may be asteroids gone astray and captured by +those planets. Two of the asteroids are exceedingly remarkable for the +shapes and positions of their orbits; these are Eros, discovered in +1898, and T. G., 1906, found eight years later. The latter has a mean +distance from the sun slightly greater than that of Jupiter, while the +mean distance of Eros is less than that of Mars. The orbit of Eros is +so eccentric that at times it approaches within 15,000,000 miles of +the earth, nearer than any other regular member of the solar system +except the moon, thus affording an unrivaled means of measuring the +solar parallax. But for our present purpose the chief interest of Eros +lies in its extraordinary changes of light. + +These changes, although irregular, have been observed and photographed +many times, and there seems to be no doubt of their reality. Their +significance consists in their possible connection with the form of +the little planet, whose diameter is generally estimated at not more +than twenty miles. Von Oppolzer found, in 1901, that Eros lost +three-fourths of its brilliancy once in every two hours and +thirty-eight minutes. Other observers have found slightly different +periods of variability, but none as long as three hours. The most +interesting interpretation that has been offered of this phenomenon is +that it is due to a great irregularity of figure, recalling at once +Olbers' hypothesis. According to some, Eros may be double, the two +bodies composing it revolving around each other at very close +quarters; but a more striking, and it may be said probable, suggestion +is that Eros has a form not unlike that of a dumb-bell, or hour-glass, +turning rapidly end over end so that the area of illuminated surface +presented to our eyes continually changes, reaching at certain times a +minimum when the amount of light that it reflects toward the earth is +reduced to a quarter of its maximum value. Various other bizarre +shapes have been ascribed to Eros, such, for instance, as that of a +flat stone revolving about one of its longer axes, so that sometimes +we see its face and sometimes its edge. + +All of these explanations proceed upon the assumption that Eros cannot +have a simple globular figure like that of a typical planet, a figure +which is prescribed by the law of gravitation, but that its shape is +what may be called accidental; in a word, it is a fragment, for it +seems impossible to believe that a body formed in interplanetary +space, either through nebular condensation or through the aggregation +of particles drawn together by their mutual attractions, should not be +practically spherical in shape. Nor is Eros the only asteroid that +gives evidence by variations of brilliancy that there is something +abnormal in its constitution; several others present the same +phenomenon in varying degrees. Even Vesta was regarded by Olbers as +sufficiently variable in its light to warrant the conclusion that it +was an angular mass instead of a globe. Some of the smaller ones show +very notable variations, and all in short periods, of three or four +hours, suggesting that in turning about one of their axes they present +a surface of variable extent toward the sun and the earth. + +The theory which some have preferred -- that the variability of light +is due to the differences of reflective power on different parts of +the surface -- would, if accepted, be hardly less suggestive of the +origin of these little bodies by the breaking up of a larger one, +because the most natural explanation of such differences would seem to +be that they arose from variations in the roughness or smoothness of +the reflecting surface, which would be characteristic of fragmentary +bodies. In the case of a large planet alternating expanses of land and +water, or of vegetation and desert, would produce a notable variation +in the amount of reflection, but on bodies of the size of the +asteroids neither water nor vegetation could exist, and an atmosphere +would be equally impossible. + +One of the strongest objections to Olbers' hypothesis is that only a +few of the first asteroids discovered travel in orbits which +measurably satisfy the requirement that they should all intersect at +the point where the explosion occurred. To this it was at first +replied that the perturbations of the asteroidal orbits, by the +attractions of the major planets, would soon displace them in such a +manner that they would cease to intersect. One of the first +investigations undertaken by the late Prof. Simon Newcomb was directed +to the solution of this question, and he arrived at the conclusion +that the planetary perturbations could not explain the actual +situation of the asteroidal orbits. But afterward it was pointed out +that the difficulty could be avoided by supposing that not one but a +series of explosions had produced the asteroids as they now are. After +the primary disruption the fragments themselves, according to this +suggestion, may have exploded, and then the resulting orbits would be +as ``tangled'' as the heart could wish. This has so far rehabilitated +the explosion theory that it has never been entirely abandoned, and +the evidence which we have just cited of the probably abnormal shapes +of Eros and other asteroids has lately given it renewed life. It is a +subject that needs a thorough rediscussion. + +We must not fail to mention, however, that there is a rival hypothesis +which commends itself to many astronomers -- viz., that the asteroids +were formed out of a relatively scant ring of matter, situated between +Mars and Jupiter and resembling in composition the immensely more +massive rings from which, according to Laplace's hypothesis, the +planets were born. It is held by the supporters of this theory that +the attraction of the giant Jupiter was sufficient to prevent the +small, nebulous ring that gave birth to the asteroids from condensing +like the others into a single planet. + +But if we accept the explosion theory, with its corollary that minor +explosions followed the principal one, we have still an unanswered +question before us: What caused the explosions? The idea of a world +blowing up is too Titanic to be shocking; it rather amuses the +imagination than seriously impresses it; in a word, it seems +essentially chimerical. We can by no appeal to experience form a +mental picture of such an occurrence. Even the moon did not blow up +when it was wrecked by volcanoes. The explosive nebul and new stars +are far away in space, and suggest no connection with such a +catastrophe as the bursting of a planet into hundreds of pieces. We +cannot conceive of a great globe thousands of miles in diameter +resembling a pellet of gunpowder only awaiting the touch of a match to +cause its sudden disruption. Somehow the thought of human agency +obtrudes itself in connection with the word ``explosion,'' and we +smile at the idea that giant powder or nitro-glycerine could blow up a +planet. Yet it would only need enough of them to do it. + +After all, we may deceive ourselves in thinking, as we are apt to do, +that explosive energies lock themselves up only in small masses of +matter. There are many causes producing explosions in nature, every +volcanic eruption manifests the activity of some of them. Think of the +giant power of confined steam; if enough steam could be suddenly +generated in the center of the earth by a downpour of all the waters +of the oceans, what might not the consequences be for our globe? In a +smaller globe, and it has never been estimated that the original +asteroid was even as large as the moon, such a catastrophe would, +perhaps, be more easily conceivable; but since we are compelled in +this case to assume that there was a series of successive explosions, +steam would hardly answer the purpose; it would be more reasonable to +suppose that the cause of the explosion was some kind of chemical +reaction, or something affecting the atoms composing the exploding +body. Here Dr Gustav Le Bon comes to our aid with a most startling +suggestion, based on his theory of the dissipation of intra-atomic +energy. It will be best to quote him at some length from his book on +The Evolution of Forces. + +``It does not seem at first sight,'' says Doctor Le Bon, + + very comprehensible that worlds which appear more and more stable + as they cool could become so unstable as to afterward dissociate + entirely. To explain this phenomenon, we will inquire whether + astronomical observations do not allow us to witness this + dissociation. + + We know that the stability of a body in motion, such as a top or a + bicycle, ceases to be possible when its velocity of rotation + descends below a certain limit. Once this limit is reached it loses + its stability and falls to the ground. Prof. J. J. Thomson even + interprets radio-activity in this manner, and points out that when + the speed of the elements composing the atoms descends below a + certain limit they become unstable and tend to lose their + equilibria. There would result from this a commencement of + dissociation, with diminution of their potential energy and a + corresponding increase of their kinetic energy sufficient to launch + into space the products of intra-atomic disintegration. + + It must not be forgotten that the atom being an enormous reservoir + of energy is by this very fact comparable with explosive bodies. + These last remain inert so long as their internal equilibria are + undisturbed. So soon as some cause or other modifies these, they + explode and smash everything around them after being themselves + broken to pieces. + + Atoms, therefore, which grow old in consequence of the diminution + of a part of their intra-atomic energy gradually lose their + stability. A moment, then, arrives when this stability is so weak + that the matter disappears by a sort of explosion more or less + rapid. The bodies of the radium group offer an image of this + phenomenon -- a rather faint image, however, because the atoms of + this body have only reached a period of instability when the + dissociation is rather slow. It probably precedes another and more + rapid period of dissociation capable of producing their final + explosion. Bodies such as radium, thorium, etc., represent, no + doubt, a state of old age at which all bodies must some day arrive, + and which they already begin to manifest in our universe, since all + matter is slightly radio-active. It would suffice for the + dissociation to be fairly general and fairly rapid for an explosion + to occur in a world where it was manifested. + + These theoretical considerations find a solid support in the sudden + appearances and disappearances of stars. The explosions of a world + which produce them reveal to us, perhaps, how the universes perish + when they become old. + + As astronomical observations show the relative frequency of these + rapid destructions, we may ask ourselves whether the end of a + universe by a sudden explosion after a long period of old age does + not represent its most general ending. + +Here, perhaps, it will be well to stop, since, entrancing as the +subject may be, we know very little about it, and Doctor Le Bon's +theory affords a limitless field for the reader's imagination. + _________________________________________________________________ + +A printed version of this book is available from Sattre Press +(http://csky.sattre-press.com). 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Thus, we do not necessarily +keep eBooks in compliance with any particular paper edition. + +Most people start at our Web site which has the main PG search facility: + + www.gutenberg.org + +This Web site includes information about Project Gutenberg-tm, +including how to make donations to the Project Gutenberg Literary +Archive Foundation, how to help produce our new eBooks, and how to +subscribe to our email newsletter to hear about new eBooks. diff --git a/old/6630-8.zip b/old/6630-8.zip Binary files differnew file mode 100644 index 0000000..acffd8b --- /dev/null +++ b/old/6630-8.zip diff --git a/old/6630.txt b/old/6630.txt new file mode 100644 index 0000000..5cc0903 --- /dev/null +++ b/old/6630.txt @@ -0,0 +1,5339 @@ +The Project Gutenberg EBook of Curiosities of the Sky, by Garrett Serviss + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Curiosities of the Sky + +Author: Garrett Serviss + +Posting Date: August 25, 2012 [EBook #6630] +Release Date: October, 2004 +First Posted: January 6, 2003 + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY *** + + + + +Produced by William McClain + + + + + + + + + + + Curiosities of the Sky + +by Garrett Serviss + +Curiosities of the Sky was first published in 1909 and the text is in +the public domain. The transcription was done by William McClain +(info@sattre-press.com), 2002. + +A printed version of this book is available from Sattre Press +(http://csky.sattre-press.com). It includes extensive annotations, a +new introduction and all the original photographs and diagrams. +_________________________________________________________________ + +Preface + +What Froude says of history is true also of astronomy: it is the most +impressive where it transcends explanation. It is not the mathematics +of astronomy, but the wonder and the mystery that seize upon the +imagination. The calculation of an eclipse owes all its prestige to +the sublimity of its data; the operation, in itself, requires no more +mental effort than the preparation of a railway time-table. + +The dominion which astronomy has always held over the minds of men is +akin to that of poetry; when the former becomes merely instructive and +the latter purely didactic, both lose their power over the +imagination. Astronomy is known as the oldest of the sciences, and it +will be the longest-lived because it will always have arcana that have +not been penetrated. + +Some of the things described in this book are little known to the +average reader, while others are well known; but all possess the +fascination of whatever is strange, marvelous, obscure, or mysterious +-- magnified, in this case, by the portentous scale of the phenomena. + +The idea of the author is to tell about these things in plain +language, but with as much scientific accuracy as plain language will +permit, showing the wonder that is in them without getting away from +the facts. Most of them have hitherto been discussed only in technical +form, and in treatises that the general public seldom sees and never +reads. + +Among the topics touched upon are: + * The strange unfixedness of the ``fixed stars,'' the vast + migrations of the suns and worlds constituting the universe. + * The slow passing out of existence of those collocations of stars + which for thousands of years have formed famous + ``constellations,'' preserving the memory of mythological heroes + and heroines, and perhaps of otherwise unrecorded history. + * The tendency of stars to assemble in immense clouds, swarms, and + clusters. + * The existence in some of the richest regions of the universe of + absolutely black, starless gaps, deeps, or holes, as if one were + looking out of a window into the murkiest night. + * The marvelous phenomena of new, or temporary, stars, which appear + as suddenly as conflagrations, and often turn into something else + as eccentric as themselves. + * The amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,'' + and ``lace,'' or ``tress,'' nebulae. + * The strange surroundings of the sun, only seen in particular + circumstances, but evidently playing a constant part in the daily + phenomena of the solar system. + * The mystery of the Zodiacal Light and the Gegenschein. + * The extraordinary transformations undergone by comets and their + tails. + * The prodigies of meteorites and masses of stone and metal fallen + from the sky. + * The cataclysms that have wrecked the moon. + * The problem of life and intelligence on the planet Mars. + * The problematical origin and fate of the asteroids. + * The strange phenomena of the auroral lights. + +An attempt has been made to develop these topics in an orderly way, +showing their connection, so that the reader may obtain a broad +general view of the chief mysteries and problems of astronomy, and an +idea of the immense field of discovery which still lies, almost +unexplored, before it. + +The Windows of Absolute Night + +To most minds mystery is more fascinating than science. But when +science itself leads straight up to the borders of mystery and there +comes to a dead stop, saying, ``At present I can no longer see my +way,'' the force of the charm is redoubled. On the other hand, the +illimitable is no less potent in mystery than the invisible, whence +the dramatic effect of Keats' ``stout Cortez'' staring at the +boundless Pacific while all his men look at each other with a wild +surmise, ``silent upon a peak in Darien.'' It is with similar feelings +that the astronomer regards certain places where from the peaks of the +universe his vision seems to range out into endless empty space. He +sees there the shore of his little isthmus, and, beyond, unexplored +immensity. + +The name, ``coal-sacks,'' given to these strange voids is hardly +descriptive. Rather they produce upon the mind the effect of blank +windows in a lonely house on a pitch-dark night, which, when looked at +from the brilliant interior, become appalling in their rayless murk. +Infinity seems to acquire a new meaning in the presence of these black +openings in the sky, for as one continues to gaze it loses its purely +metaphysical quality and becomes a kind of entity, like the ocean. The +observer is conscious that he can actually see the beginning of its +ebon depths, in which the visible universe appears to float like an +enchanted island, resplendent within with lights and life and gorgeous +spectacles, and encircled with screens of crowded stars, but with its +dazzling vistas ending at the fathomless sea of pure darkness which +encloses all. + +The Galaxy, or Milky Way, surrounds the borders of our island in space +like a stellar garland, and when openings appear in it they are, by +contrast, far more impressive than the general darkness of the +interstellar expanse seen in other directions. Yet even that expanse +is not everywhere equally dark, for it contains gloomy deeps +discernable with careful watching. Here, too, contrast plays an +important part, though less striking than within the galactic region. +Some of Sir William Herschel's observations appear to indicate an +association between these tenebrious spots and neighboring star clouds +and nebulae. It is an illuminating bit of astronomical history that +when he was sweeping the then virgin heavens with his great telescopes +he was accustomed to say to his sister who, note-book in hand, waited +at his side to take down his words, fresh with the inspiration of +discovery: ``Prepare to write; the nebulae are coming; here space is +vacant.'' + +The most famous of the ``coal-sacks,'' and the first to be brought to +general attention before astronomers had awakened to the significance +of such things, lies adjacent to the ``Southern Cross,'' and is truly +an amazing phenomenon. It is not alone the conspicuousness of this +celestial vacancy, opening suddenly in the midst of one of the richest +parts of the Galaxy, that has given it its fame, but quite as much the +superstitious awe with which it was regarded by the early explorers of +the South Seas. To them, as well as to those who listened in rapt +wonder to their tales, the ``Coal-sack'' seemed to possess some occult +connection with the mystic ``Cross.'' In the eyes of the sailors it +was not a vacancy so much as a sable reality in the sky, and as, +shuddering, they stared at it, they piously crossed themselves. It was +another of the magical wonders of the unknown South, and as such it +formed the basis of many a ``wild surmise'' and many a sea-dog's yarn. +Scientific investigation has not diminished its prestige, and today no +traveler in the southern hemisphere is indifferent to its fascinating +strangeness, while some find it the most impressive spectacle of the +antarctic heavens. + +All around, up to the very edge of the yawning gap, the sheen of the +Milky Way is surpassingly glorious; but there, as if in obedience to +an almighty edict, everything vanishes. A single faint star is visible +within the opening, producing a curious effect upon the sensitive +spectator, like the sight of a tiny islet in the midst of a black, +motionless, waveless tarn. The dimensions of the lagoon of darkness, +which is oval or pear-shaped, are eight degrees by five, so that it +occupies a space in the sky about one hundred and thirty times greater +than the area of the full moon. It attracts attention as soon as the +eye is directed toward the quarter where it exists, and by virtue of +the rarity of such phenomena it appears a far greater wonder than the +drifts of stars that are heaped around it. Now that observatories are +multiplying in the southern hemisphere, the great austral +``Coal-sack'' will, no doubt, receive attention proportioned to its +importance as one of the most significant features of the sky. Already +at the Sydney Observatory photographs have shown that the southern +portion of this Dead Sea of Space is not quite ``bottomless,'' +although its northern part defies the longest sounding lines of the +astronomer. + +There is a similar, but less perfect, ``coal-sack'' in the northern +hemisphere, in the constellation of ``The Swan,'' which, strange to +say, also contains a well-marked figure of a cross outlined by stars. +This gap lies near the top of the cross-shaped figure. It is best seen +by averted vision, which brings out the contrast with the Milky Way, +which is quite brilliant around it. It does not, however, exercise the +same weird attraction upon the eye as the southern ``Coal-sack,'' for +instead of looking like an absolute void in the sky, it rather appears +as if a canopy of dark gauze had been drawn over the stars. We shall +see the possible significance of this appearance later. + +Just above the southern horizon of our northern middle latitudes, in +summer, where the Milky Way breaks up into vast sheets of nebulous +luminosity, lying over and between the constellations Scorpio and +Sagittarius, there is a remarkable assemblage of ``coal-sacks,'' +though none is of great size. One of them, near a conspicuous +star-cluster in Scorpio, M80, is interesting for having been the first +of these strange objects noted by Herschel. Probably it was its +nearness to M80 which suggested to his mind the apparent connection of +such vacancies with star-clusters which we have already mentioned. + +But the most marvelous of the ``coal-sacks'' are those that have been +found by photography in Sagittarius. One of Barnard's earliest and +most excellent photographs includes two of them, both in the +star-cluster M8. The larger, which is roughly rectangular in outline, +contains one little star, and its smaller neighbor is lune-shaped -- +surely a most singular form for such an object. Both are associated +with curious dark lanes running through the clustered stars like +trails in the woods. Along the borders of these lanes the stars are +ranked in parallel rows, and what may be called the bottoms of the +lanes are not entirely dark, but pebbled with faint stellar points. +One of them which skirts the two dark gaps and traverses the cluster +along its greatest diameter is edged with lines of stars, recalling +the alignment of the trees bordering a French highway. This road of +stars cannot be less than many billions of miles in length! + +All about the cluster the bed of the Galaxy is strangely disturbed, +and in places nearly denuded, as if its contents had been raked away +to form the immense stack and the smaller accumulations of stars +around it. The well-known ``Trifid Nebula'' is also included in the +field of the photograph, which covers a truly marvelous region, so +intricate in its mingling of nebulae, star-clusters, star-swarms, +star-streams, and dark vacancies that no description can do it +justice. Yet, chaotic as it appears, there is an unmistakable +suggestion of unity about it, impressing the beholder with the idea +that all the different parts are in some way connected, and have not +been fortuitously thrown together. Miss Agnes M. Clerke made the +striking remark that the dusky lanes in M8 are exemplified on the +largest scale in the great rift dividing the Milky Way, from Cygnus in +the northern hemisphere all the way to the ``Cross'' in the southern. +Similar lanes are found in many other clusters, and they are generally +associated with flanking rows of stars, resembling in their +arrangement the thick-set houses and villas along the roadways that +traverse the approaches to a great city. + +But to return to the black gaps. Are they really windows in the +star-walls of the universe? Some of them look rather as if they had +been made by a shell fired through a luminous target, allowing the eye +to range through the hole into the void space beyond. If science is +discretely silent about these things, what can the more venturesome +and less responsible imagination suggest? Would a huge ``runaway +sun,'' like Arcturus, for instance, make such an opening if it should +pass like a projectile through the Milky Way? It is at least a +stimulating inquiry. Being probably many thousands of times more +massive than the galactic stars, such a stellar missile would not be +stopped by them, though its direction of flight might be altered. It +would drag the small stars lying close to its course out of their +spheres, but the ultimate tendency of its attraction would be to sweep +them round in its wake, thus producing rather a star-swarm than a +vacancy. Those that were very close to it might be swept away in its +rush and become its satellites, careering away with it in its flight +into outer space; but those that were farther off, and they would, of +course, greatly outnumber the nearer ones, would tend inward from all +sides toward the line of flight, as dust and leaves collect behind a +speeding motor (though the forces operating would be different), and +would fill up the hole, if hole it were. A swarm thus collected should +be rounded in outline and bordered with a relatively barren ring from +which the stars had been ``sucked'' away. In a general sense the M8 +cluster answers to this description, but even if we undertook to +account for its existence by a supposition like the above, the black +gaps would remain unexplained, unless one could make a further draft +on the imagination and suggest that the stars had been thrown into a +vast eddy, or system of eddies, whose vortices appear as dark holes. +Only a maelstrom-like motion could keep such a funnel open, for +without regard to the impulse derived from the projectile, the proper +motions of the stars themselves would tend to fill it. Perhaps some +other cause of the whirling motion may be found. As we shall see when +we come to the spiral nebulae, gyratory movements are exceedingly +prevalent throughout the universe, and the structure of the Milky Way +is everywhere suggestive of them. But this is hazardous sport even for +the imagination -- to play with suns as if they were but thistle-down +in the wind or corks in a mill-race. + +Another question arises: What is the thickness of the hedge of stars +through which the holes penetrate? Is the depth of the openings +proportionate to their width? In other words, is the Milky Way round +in section like a rope, or flat and thin like a ribbon? The answer is +not obvious, for we have little or no information concerning the +relative distances of the faint galactic stars. It would be easier, +certainly, to conceive of openings in a thin belt than in a massive +ring, for in the first case they would resemble mere rifts and breaks, +while in the second they would be like wells or bore-holes. Then, too, +the fact that the Milky Way is not a continuous body but is made up of +stars whose actual distances apart is great, offers another quandary; +persistent and sharply bordered apertures in such an assemblage are a +priori as improbable, if not impossible, as straight, narrow holes +running through a swarm of bees. + +The difficulty of these questions indicates one of the reasons why it +has been suggested that the seeming gaps, or many of them, are not +openings at all, but opaque screens cutting off the light from stars +behind them. That this is quite possible in some cases is shown by +Barnard's later photographs, particularly those of the singular region +around the star Rho Ophiuchi. Here are to be seen somber lanes and +patches, apparently forming a connected system which covers an immense +space, and which their discoverer thinks may constitute a ``dark +nebula.'' This seems at first a startling suggestion; but, after all, +why should their not be dark nebulae as well as visible ones? In truth, +it has troubled some astronomers to explain the luminosity of the +bright nebulae, since it is not to be supposed that matter in so +diffuse a state can be incandescent through heat, and phosphorescent +light is in itself a mystery. The supposition is also in accord with +what we know of the existence of dark solid bodies in space. Many +bright stars are accompanied by obscure companions, sometimes as +massive as themselves; the planets are non-luminous; the same is true +of meteors before they plunge into the atmosphere and become heated by +friction; and many plausible reasons have been found for believing +that space contains as many obscure as shining bodies of great size. +It is not so difficult, after all, then, to believe that there are +immense collections of shadowy gases and meteoric dust whose presence +is only manifested when they intercept the light coming from shining +bodies behind them. + +This would account for the apparent extinguishment of light in open +space, which is indicated by the falling off in relative number of +telescopic stars below the tenth magnitude. Even as things are, the +amount of light coming to us from stars too faint to be seen with the +naked eye is so great that the statement of it generally surprises +persons who are unfamiliar with the inner facts of astronomy. It has +been calculated that on a clear night the total starlight from the +entire celestial sphere amounts to one-sixtieth of the light of the +full moon; but of this less than one-twenty-fifth is due to stars +separately distinguished by the eye. If there were no obscuring medium +in space, it is probable that the amount of starlight would be +noticeably and perhaps enormously increased. + +But while it seems certain that some of the obscure spots in the Milky +Way are due to the presence of ``dark nebulae,'' or concealing veils of +one kind or another, it is equally certain that there are many which +are true apertures, however they may have been formed, and by whatever +forces they may be maintained. These, then, are veritable windows of +the Galaxy, and when looking out of them one is face to face with the +great mystery of infinite space. There the known universe visibly +ends, but manifestly space itself does not end there. It is not within +the power of thought to conceive an end to space, for the instant we +think of a terminal point or line the mind leaps forward to the +beyond. There must be space outside as well as inside. Eternity of +time and infinity of space are ideas that the intellect cannot fully +grasp, but neither can it grasp the idea of a limitation to either +space or time. The metaphysical conceptions of hypergeometry, or +fourth-dimensional space, do not aid us. + +Having, then, discovered that the universe is a thing contained in +something indefinitely greater than itself; having looked out of its +windows and found only the gloom of starless night outside -- what +conclusions are we to draw concerning the beyond? It seems as empty as +a vacuum, but is it really so? If it be, then our universe is a single +atom astray in the infinite; it is the only island in an ocean without +shores; it is the one oasis in an illimitable desert. Then the Milky +Way, with its wide-flung garland of stars, is afloat like a tiny +smoke-wreath amid a horror of immeasurable vacancy, or it is an +evanescent and solitary ring of sparkling froth cast up for a moment +on the viewless billows of immensity. From such conclusions the mind +instinctively shrinks. It prefers to think that there is something +beyond, though we cannot see it. Even the universe could not bear to +be alone -- a Crusoe lost in the Cosmos! As the inhabitants of the +most elegant chateau, with its gardens, parks, and crowds of +attendants, would die of loneliness if they did not know that they +have neighbors, though not seen, and that a living world of indefinite +extent surrounds them, so we, when we perceive that the universe has +limits, wish to feel that it is not solitary; that beyond the hedges +and the hills there are other centers of life and activity. Could +anything be more terrible than the thought of an isolated universe? +The greater the being, the greater the aversion to seclusion. Only the +infinite satisfies; in that alone the mind finds rest. + +We are driven, then, to believe that the universal night which +envelopes us is not tenantless; that as we stare out of the +star-framed windows of the Galaxy and see nothing but uniform +blackness, the fault is with our eyes or is due to an obscuring +medium. Since our universe is limited in extent, there must be other +universes beyond it on all sides. Perhaps if we could carry our +telescopes to the verge of the great ``Coal-sack'' near the ``Cross,'' +being then on the frontier of our starry system, we could discern, +sparkling afar off in the vast night, some of the outer galaxies. They +may be grander than ours, just as many of the suns surrounding us are +immensely greater than ours. If we could take our stand somewhere in +the midst of immensity and, with vision of infinite reach, look about +us, we should perhaps see a countless number of stellar systems, amid +which ours would be unnoticeable, like a single star among the +multitude glittering in the terrestial sky on a clear night. Some +might be in the form of a wreath, like our own; some might be +globular, like the great star-clusters in Hercules and Centaurus; some +might be glittering circles, or disks, or rings within rings. If we +could enter them we should probably find a vast variety of +composition, including elements unknown to terrestrial chemistry; for +while the visible universe appears to contain few if any substances +not existing on the earth or in the sun, we have no warrant to assume +that others may not exist in infinite space. + +And how as to gravitation? We do not know that gravitation acts beyond +the visible universe, but it is reasonable to suppose that it does. At +any rate, if we let go its sustaining hand we are lost, and can only +wander hopelessly in our speculations, like children astray. If the +empire of gravitation is infinite, then the various outer systems must +have some, though measuring by our standards an imperceptible, +attractive influence upon each other, for gravitation never lets go +its hold, however great the space over which it is required to act. +Just as the stars about us are all in motion, so the starry systems +beyond our sight may be in motion, and our system as a whole may be +moving in concert with them. If this be so, then after interminable +ages the aspect of the entire system of systems must change, its +various members assuming new positions with respect to one another. In +the course of time we may even suppose that our universe will approach +relatively close to one of the others; and then, if men are yet living +on the earth, they may glimpse through the openings which reveal +nothing to us now, the lights of another nearing star system, like the +signals of a strange squadron, bringing them the assurance (which can +be but an inference at present) that the ocean of space has other +argosies venturing on its limitless expanse. + +There remains the question of the luminiferous ether by whose agency +the waves of light are borne through space. The ether is as mysterious +as gravitation. With regard to ether we only infer its existence from +the effects which we ascribe to it. Evidently the ether must extend as +far as the most distant visible stars. But does it continue on +indefinitely in outer space? If it does, then the invisibility of the +other systems must be due to their distance diminishing the quantity +of light that comes from them below the limit of perceptibility, or to +the interposition of absorbing media; if it does not, then the reason +why we cannot see them is owing to the absence of a means of +conveyance for the light waves, as the lack of an interplanetary +atmosphere prevents us from hearing the thunder of sun-spots. (It is +interesting to recall that Mr Edison was once credited with the +intention to construct a gigantic microphone which should render the +roar of sun-spots audible by transforming the electric vibrations into +sound-waves). On this supposition each starry system would be +enveloped in its own globule of ether, and no light could cross from +one to another. But the probability is that both the ether and +gravitation are ubiquitous, and that all the stellar systems are +immersed in the former like clouds of phosphorescent organisms in the +sea. + +So astronomy carries the mind from height to greater height. Men were +long in accepting the proofs of the relative insignificance of the +earth; they were more quickly convinced of the comparative littleness +of the solar system; and now the evidence assails their reason that +what they had regarded as the universe is only one mote gleaming in +the sunbeams of Infinity. + +Star-Clouds, Star-Clusters, and Star-Streams + +In the preceding chapter we have seen something of the strangely +complicated structure of the Galaxy, or Milky Way. We now proceed to +study more comprehensively that garlanded ``Pathway of the Gods.'' + +Judged by the eye alone, the Milky Way is one of the most delicately +beautiful phenomena in the entire realm of nature -- a shimmer of +silvery gauze stretched across the sky; but studied in the light of +its revelations, it is the most stupendous object presented to human +ken. Let us consider, first, its appearance to ordinary vision. Its +apparent position in the sky shifts according to the season. On a +serene, cloudless summer evening, in the absence of the moon, whose +light obscures it, one sees the Galaxy spanning the heavens from north +to southeast of the zenith like a phosphorescent arch. In early spring +it forms a similar but, upon the whole, less brilliant arch west of +the zenith. Between spring and summer it lies like a long, faint, +twilight band along the northern horizon. At the beginning of winter +it again forms an arch, this time spanning the sky from east to west, +a little north of the zenith. These are its positions as viewed from +the mean latitude of the United States. Even the beginner in +star-gazing does not have to watch it throughout the year in order to +be convinced that it is, in reality, a great circle, extending +entirely around the celestial sphere. We appear to be situated near +its center, but its periphery is evidently far away in the depths of +space. + +Although to the casual observer it seems but a delicate scarf of +light, brighter in some places than in others, but hazy and indefinite +at the best, such is not its appearance to those who study it with +care. They perceive that it is an organic whole, though marvelously +complex in detail. The telescope shows that it consists of stars too +faint and small through excess of distance to be separately visible. +Of the hundred million suns which some estimates have fixed as the +probable population of the starry universe, the vast majority (at +least thirty to one) are included in this strange belt of misty light. +But they are not uniformly distributed in it; on the contrary, they +are arrayed in clusters, knots, bunches, clouds, and streams. The +appearance is somewhat as if the Galaxy consisted of innumerable +swarms of silver-winged bees, more or less intermixed, some massed +together, some crossing the paths of others, but all governed by a +single purpose which leads them to encircle the region of space in +which we are situated. + +From the beginning of the systematic study of the heavens, the fact +has been recognized that the form of the Milky Way denotes the scheme +of the sidereal system. At first it was thought that the shape of the +system was that of a vast round disk, flat like a cheese, and filled +with stars, our sun and his relatively few neighbors being placed near +the center. According to this view, the galactic belt was an effect of +perspective; for when looking in the direction of the plane of the +disk, the eye ranged through an immense extension of stars which +blended into a glimmering blur, surrounding us like a ring; while when +looking out from the sides of the disk we saw but few stars, and in +those directions the heavens appeared relatively blank. Finally it was +recognized that this theory did not correspond with the observed +appearances, and it became evident that the Milky Way was not a mere +effect of perspective, but an actual band of enormously distant stars, +forming a circle about the sphere, the central opening of the ring +(containing many scattered stars) being many times broader than the +width of the ring itself. Our sun is one of the scattered stars in the +central opening. + +As already remarked, the ring of the Galaxy is very irregular, and in +places it is partly broken. With its sinuous outline, its pendant +sprays, its graceful and accordant curves, its bunching of masses, its +occasional interstices, and the manifest order of a general plan +governing the jumble of its details, it bears a remarkable resemblance +to a garland -- a fact which appears the more wonderful when we recall +its composition. That an elm-tree should trace the lines of beauty +with its leafy and pendulous branches does not surprise us; but we can +only gaze with growing amazement when we behold a hundred million suns +imitating the form of a chaplet! And then we have to remember that +this form furnishes the ground-plan of the universe. + +As an indication of the extraordinary speculations to which the +mystery of the Milky Way has given rise, a theory recently (1909) +proposed by Prof. George C. Comstock may be mentioned. Starting with +the data (first) that the number of stars increases as the Milky Way +is approached, and reaches a maximum in its plane, while on the other +hand the number of nebulae is greatest outside the Milky Way and +increases with distance from it, and (second) that the Milky Way, +although a complete ring, is broad and diffuse on one side through +one-half its course -- that half alone containing nebulae -- and +relatively narrow and well defined on the opposite side, the author of +this singular speculation avers that these facts can best be explained +by supposing that the invisible universe consists of two +interpenetrating parts, one of which is a chaos of indefinite extent, +strewn with stars and nebulous dust, and the other a long, broad but +comparatively thin cluster of stars, including the sun as one of its +central members. This flat star-cluster is conceived to be moving +edgewise through the chaos, and, according to Professor Comstock, it +acts after the manner of a snow-plough sweeping away the cosmic dust +and piling it on either hand above and below the plane of the moving +cluster. It thus forms a transparent rift, through which we see +farther and command a view of more stars than through the intensified +dust-clouds on either hand. This rift is the Milky Way. The dust +thrown aside toward the poles of the Milky Way is the substance of the +nebulae which abound there. Ahead, where the front of the star-plough +is clearing the way, the chaos is nearer at hand, and consequently +there the rift subtends a broader angle, and is filled with primordial +dust, which, having been annexed by the vanguard of the star-swarm, +forms the nebulae seen only in that part of the Milky Way. But behind, +the rift appears narrow because there we look farther away between +dust-clouds produced ages ago by the front of the plough, and no +scattered dust remains in that part of the rift. + +In quoting an outline of this strikingly original theory the present +writer should not be understood as assenting to it. That it appears +bizarre is not, in itself, a reason for rejecting it, when we are +dealing with so problematical and enigmatical a subject as the Milky +Way; but the serious objection is that the theory does not +sufficiently accord with the observed phenomena. There is too much +evidence that the Milky Way is an organic system, however fantastic +its form, to permit the belief that it can only be a rift in chaotic +clouds. As with every organism, we find that its parts are more or +less clearly repeated in its ensemble. Among all the strange things +that the Milky Way contains there is nothing so extraordinary as +itself. Every astronomer must many times have found himself marveling +at it in those comparatively rare nights when it shows all its beauty +and all its strangeness. In its great broken rifts, divisions, and +spirals are found the gigantic prototypes of similar forms in its +star-clouds and clusters. As we have said, it determines the general +shape of the whole sidereal system. Some of the brightest stars in the +sky appear to hang like jewels suspended at the ends of tassels +dropped from the Galaxy. Among these pendants are the Pleiades and the +Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a loop of +light'' thrown out from it. The majority of the great first-magnitude +stars seem related to it, as if they formed an inner ring inclined at +an angle of some twenty degrees to its plane. Many of the long curves +that set off from it on both sides are accompanied by corresponding +curves of lucid stars. In a word, it offers every appearance of +structural connection with the entire starry system. That the universe +should have assumed the form of a wreath is certainly a matter for +astonishment; but it would have been still more astonishing if it had +been a cube, a rhomboid, or a dodecahedron, for then we should have +had to suppose that something resembling the forces that shape +crystals had acted upon the stars, and the difficulty of explaining +the universe by the laws of gravitation would have been increased. + +From the Milky Way as a whole we pass to the vast clouds, swarms, and +clusters of stars of which it is made up. It may be, as some +astronomers hold, that most of the galactic stars are much smaller +than the sun, so that their faintness is not due entirely to the +effect of distance. Still, their intrinsic brilliance attests their +solar character, and considering their remoteness, which has been +estimated at not less than ten thousand to twenty thousand light-years +(a light-year is equal to nearly six thousand thousand million miles) +their actual masses cannot be extremely small. The minutest of them +are entitled to be regarded as real suns, and they vary enormously in +magnitude. The effects of their attractions upon one another can only +be inferred from their clustering, because their relative movements +are not apparent on account of the brevity of the observations that we +can make. But imagine a being for whom a million years would be but as +a flitting moment; to him the Milky Way would appear in a state of +ceaseless agitation -- swirling with ``a fury of whirlpool motion.'' + +The cloud-like aspect of large parts of the Galaxy must always have +attracted attention, even from naked-eye observers, but the true +star-clouds were first satisfactorily represented in Barnard's +photographs. The resemblance to actual clouds is often startling. Some +are close-packed and dense, like cumuli; some are wispy or mottled, +like cirri. The rifts and modulations, as well as the general +outlines, are the same as those of clouds of vapor or dust, and one +notices also the characteristic thinning out at the edges. But we must +beware of supposing that the component suns are thickly crowded as the +particles forming an ordinary cloud. They look, indeed, as if they +were matted together, because of the irradiation of light, but in +reality millions and billions of miles separate each star from its +neighbors. Nevertheless they form real assemblages, whose members are +far more closely related to one another than is our sun to the stars +around him, and if we were in the Milky Way the aspect of the +nocturnal sky would be marvelously different from its present +appearance. + +Stellar clouds are characteristic of the Galaxy and are not found +beyond its borders, except in the ``Magellanic Clouds'' of the +southern hemisphere, which resemble detached portions of the Milky +Way. These singular objects form as striking a peculiarity of the +austral heavens as does the great ``Coal-sack'' described in Chapter +1. But it is their isolation that makes them so remarkable, for their +composition is essentially galactic, and if they were included within +its boundaries they would not appear more wonderful than many other +parts of the Milky Way. Placed where they are, they look like masses +fallen from the great stellar arch. They are full of nebulae and +star-clusters, and show striking evidences of spiral movement. + +Star-swarms, which are also characteristic features of the Galaxy, +differ from star-clouds very much in the way that their name would +imply -- i.e., their component stars are so arranged, even when they +are countless in number, that the idea of an exceedingly numerous +assemblage rather than that of a cloud is impressed on the observer's +mind. In a star-swarm the separate members are distinguishable because +they are either larger or nearer than the stars composing a ``cloud.'' +A splendid example of a true star-swarm is furnished by Chi Persei, in +that part of the Milky Way which runs between the constellations +Perseus and Cassiopeia. This swarm is much coarser than many others, +and can be seen by the naked eye. In a small telescope it appears +double, as if the suns composing it had divided into two parties which +keep on their way side by side, with some commingling of their members +where the skirts of the two companies come in contact. + +Smaller than either star-clouds or star-swarms, and differing from +both in their organization, are star-clusters. These, unlike the +others, are found outside as well as inside the Milky Way, although +they are more numerous inside its boundaries than elsewhere. The term +star-cluster is sometimes applied, though improperly, to assemblages +which are rather groups, such, for instance, as the Pleiades. In their +most characteristic aspect star-clusters are of a globular shape -- +globes of suns! A famous example of a globular star-cluster, but one +not included in the Milky Way, is the ``Great Cluster in Hercules.'' +This is barely visible to the naked eye, but a small telescope shows +its character, and in a large one it presents a marvelous spectacle. +Photographs of such clusters are, perhaps, less effective than those +of star-clouds, because the central condensation of stars in them is +so great that their light becomes blended in an indistinguishable +blur. The beautiful effect of the incessant play of infinitesimal rays +over the apparently compact surface of the cluster, as if it were a +globe of the finest frosted silver shining in an electric beam, is +also lost in a photograph. Still, even to the eye looking directly at +the cluster through a powerful telescope, the central part of the +wonderful congregation seems almost a solid mass in which the stars +are packed like the ice crystals in a snowball. + +The same question rises to the lips of every observer: How can they +possibly have been brought into such a situation? The marvel does not +grow less when we know that, instead of being closely compacted, the +stars of the cluster are probably separated by millions of miles; for +we know that their distances apart are slight as compared with their +remoteness from the Earth. Sir William Herschel estimated their number +to be about fourteen thousand, but in fact they are uncountable. If we +could view them from a point just within the edge of the assemblage, +they would offer the appearance of a hollow hemisphere emblazoned with +stars of astonishing brilliancy; the near-by ones unparalleled in +splendor by any celestial object known to us, while the more distant +ones would resemble ordinary stars. An inhabitant of the cluster would +not know, except by a process of ratiocination, that he was dwelling +in a globular assemblage of suns; only from a point far outside would +their spherical arrangement become evident to the eye. Imagine +fourteen-thousand fire-balloons with an approach to regularity in a +spherical space -- say, ten miles in diameter; there would be an +average of less than thirty in every cubic mile, and it would be +necessary to go to a considerable distance in order to see them as a +globular aggregation; yet from a point sufficiently far away they +would blend into a glowing ball. + +Photographs show even better than the best telescopic views that the +great cluster is surrounded with a multitude of dispersed stars, +suggestively arrayed in more or less curving lines, which radiate from +the principle mass, with which their connection is manifest. These +stars, situated outside the central sphere, look somewhat like vagrant +bees buzzing round a dense swarm where the queen bee is sitting. Yet +while there is so much to suggest the operation of central forces, +bringing and keeping the members of the cluster together, the +attentive observer is also impressed with the idea that the whole +wonderful phenomenon may be the result of explosion. As soon as this +thought seizes the mind, confirmation of it seems to be found in the +appearance of the outlying stars, which could be as readily explained +by the supposition that they have been blown apart as that they have +flocked together toward a center. The probable fact that the stars +constituting the cluster are very much smaller than our sun might be +regarded as favoring the hypothesis of an explosion. Of their real +size we know nothing, but, on the basis of an uncertain estimate of +their parallax, it has been calculated that they may average +forty-five thousand miles in diameter -- something more than half the +diameter of the planet Jupiter. Assuming the same mean density, +fourteen thousand such stars might have been formed by the explosion +of a body about twice the size of the sun. This recalls the theory of +Olbers, which has never been altogether abandoned or disproved, that +the Asteroids were formed by the explosion of a planet circulating +between the orbits of Mars and Jupiter. The Asteroids, whatever their +manner of origin, form a ring around the sun; but, of course, the +explosion of a great independent body, not originally revolving about +a superior center of gravitational force, would not result in the +formation of a ring of small bodies, but rather of a dispersed mass of +them. But back of any speculation of this kind lies the problem, at +present insoluble: How could the explosion be produced? (See the +question of explosions in Chapters 6 and 14). + +Then, on the other hand, we have the observation of Herschel, since +abundantly confirmed, that space is unusually vacant in the immediate +neighborhood of condensed star-clusters and nebulae, which, as far as +it goes, might be taken as an indication that the assembled stars had +been drawn together by their mutual attractions, and that the tendency +to aggregation is still bringing new members toward the cluster. But +in that case there must have been an original condensation of stars at +that point in space. This could probably have been produced by the +coagulation of a great nebula into stellar nuclei, a process which +seems now to be taking place in the Orion Nebula. + +A yet more remarkable globular star-cluster exists in the southern +hemisphere, Omega Centauri. In this case the central condensation of +stars presents an almost uniform blaze of light. Like the Hercules +cluster, that in Centaurus is surrounded with stars scattered over a +broad field and showing an appearance of radial arrangement. In fact, +except for its greater richness, Omega Centauri is an exact duplicate +of its northern rival. Each appears to an imaginative spectator as a +veritable ``city of suns.'' Mathematics shrinks from the task of +disentangling the maze of motions in such an assemblage. It would seem +that the chance of collisions is not to be neglected, and this idea +finds a certain degree of confirmation in the appearance of +``temporary stars'' which have more than once blazed out in, or close +by, globular star-clusters. + +This leads up to the notable fact, first established by Professor +Bailey a few years ago, that such clusters are populous with variable +stars. Omega Centauri and the Hercules cluster are especially +remarkable in this respect. The variables found in them are all of +short period and the changes of light show a noteworthy tendency to +uniformity. The first thought is that these phenomena must be due to +collisions among the crowded stars, but, if so, the encounters cannot +be between the stars themselves, but probably between stars and meteor +swarms revolving around them. Such periodic collisions might go on for +ages without the meteors being exhausted by incorporation with the +stars. This explanation appears all the more probable because one +would naturally expect that flocks of meteors would abound in a close +aggregation of stars. It is also consistent with Perrine's discovery +-- that the globular star clusters are powdered with minute stars +strewn thickly among the brighter ones. + +In speaking of Professor Comstock's extraordinary theory of the Milky +Way, the fact was mentioned that, broadly speaking, the nebulae are +less numerous in the galactic belt than in the comparatively open +spaces on either side of it, but that they are, nevertheless, abundant +in the broader half of the Milky Way which he designates as the front +of the gigantic ``plough'' supposed to be forcing its way through the +enveloping chaos. In and around the Sagittarius region the +intermingling of nebulae and galactic star clouds and clusters is +particularly remarkable. That there is a causal connection no +thoughtful person can doubt. We are unable to get away from the +evidence that a nebula is like a seed-ground from which stars spring +forth; or we may say that nebulae resemble clouds in whose bosom +raindrops are forming. The wonderful aspect of the admixtures of +nebulae and star-clusters in Sagittarius has been described in Chapter +1. We now come to a still more extraordinary phenomenon of this kind +-- the Pleiades nebulae. + +The group of the Pleiades, although lying outside the main course of +the Galaxy, is connected with it by a faint loop, and is the scene of +the most remarkable association of stars and nebulous matter known in +the visible universe. The naked eye is unaware of the existence of +nebulae in the Pleiades, or, at the best, merely suspects that there is +something of the kind there; and even the most powerful telescopes are +far from revealing the full wonder of the spectacle; but in +photographs which have been exposed for many hours consecutively, in +order to accumulate the impression of the actinic rays, the revelation +is stunning. The principle stars are seen surrounded by, and, as it +were, drowned in, dense nebulous clouds of an unparalleled kind. The +forms assumed by these clouds seem at first sight inexplicable. They +look like fleeces, or perhaps more like splashes and daubs of luminous +paint dashed carelessly from a brush. But closer inspection shows that +they are, to a large extent, woven out of innumerable threads of filmy +texture, and there are many indications of spiral tendencies. Each of +the bright stars of the group -- Alcyone, Merope, Maia, Electra, +Taygeta, Atlas -- is the focus of a dense fog (totally invisible, +remember, alike to the naked eye and to the telescope), and these +particular stars are veiled from sight behind the strange mists. +Running in all directions across the relatively open spaces are +nebulous wisps and streaks of the most curious forms. On some of the +nebular lines, which are either straight throughout, or if they change +direction do so at an angle, little stars are strung like beads. In +one case seven or eight stars are thus aligned, and, as if to +emphasize their dependence upon the chain which connects them, when it +makes a slight bend the file of stars turns the same way. Many other +star rows in the group suggest by their arrangement that they, too, +were once strung upon similar threads which have now disappeared, +leaving the stars spaced along their ancient tracks. We seem forced to +the conclusion that there was a time when the Pleiades were embedded +in a vast nebula resembling that of Orion, and that the cloud has now +become so rare by gradual condensation into stars that the merest +trace of it remains, and this would probably have escaped detection +but for the remarkable actinic power of the radiant matter of which it +consists. The richness of many of these faint nebulous masses in +ultra-violet radiations, which are those that specifically affect the +photographic plate, is the cause of the marvelous revelatory power of +celestial photography. So the veritable unseen universe, as +distinguished from the ``unseen universe'' of metaphysical +speculation, is shown to us. + +A different kind of association between stars and nebulae is shown in +some surprising photographic objects in the constellation Cygnus, +where long, wispy nebulae, billions of miles in length, some of them +looking like tresses streaming in a breeze, lie amid fields of stars +which seem related to them. But the relation is of a most singular +kind, for notwithstanding the delicate structure of the long nebulae +they appear to act as barriers, causing the stars to heap themselves +on one side. The stars are two, three, or four times as numerous on +one side of the nebulae as on the other. These nebulae, as far as +appearance goes, might be likened to rail fences, or thin hedges, +against which the wind is driving drifts of powdery snow, which, while +scattered plentifully all around, tends to bank itself on the leeward +side of the obstruction. The imagination is at a loss to account for +these extraordinary phenomena; yet there they are, faithfully giving +us their images whenever the photographic plate is exposed to their +radiations. + +Thus the more we see of the universe with improved methods of +observation, and the more we invent aids to human senses, each +enabling us to penetrate a little deeper into the unseen, the greater +becomes the mystery. The telescope carried us far, photography is +carrying us still farther; but what as yet unimagined instrument will +take us to the bottom, the top, and the end? And then, what hitherto +untried power of thought will enable us to comprehend the meaning of +it all? + +Stellar Migrations + +To the untrained eye the stars and the planets are not +distinguishable. It is customary to call them all alike ``stars.'' But +since the planets more or less rapidly change their places in the sky, +in consequence of their revolution about the sun, while the stars +proper seem to remain always in the same relative positions, the +latter are spoken of as ``fixed stars.'' In the beginnings of +astronomy it was not known that the ``fixed stars'' had any motion +independent of their apparent annual revolution with the whole sky +about the earth as a seeming center. Now, however, we know that the +term ``fixed stars'' is paradoxical, for there is not a single really +fixed object in the whole celestial sphere. The apparent fixity in the +positions of the stars is due to their immense distance, combined with +the shortness of the time during which we are able to observe them. It +is like viewing the plume of smoke issuing from a steamer, hull down, +at sea: if one does not continue to watch it for a long time it +appears to be motionless, although in reality it may be traveling at +great speed across the line of sight. Even the planets seem fixed in +position if one watches them for a single night only, and the more +distant ones do not sensibly change their places, except after many +nights of observation. Neptune, for instance, moves but little more +than two degrees in the course of an entire year, and in a month its +change of place is only about one-third of the diameter of the full +moon. + +Yet, fixed as they seem, the stars are actually moving with a speed in +comparison with which, in some cases, the planets might almost be said +to stand fast in their tracks. Jupiter's speed in his orbit is about +eight miles per second, Neptune's is less than three and one-half +miles, and the earth's is about eighteen and one-half miles; while +there are ``fixed stars'' which move two hundred or three hundred +miles per second. They do not all, however, move with so great a +velocity, for some appear to travel no faster than the planets. But in +all cases, notwithstanding their real speed, long-continued and +exceedingly careful observations are required to demonstrate that they +are moving at all. No more overwhelming impression of the frightful +depths of space in which the stars are buried can be obtained than by +reflecting upon the fact that a star whose actual motion across the +line of sight amounts to two hundred miles per second does not change +its apparent place in the sky, in the course of a thousand years, +sufficiently to be noticed by the casual observer of the heavens! + +There is one vast difference between the motions of the stars and +those of the planets to which attention should be at once called: the +planets, being under the control of a central force emanating from +their immediate master, the sun, all move in the same direction and in +orbits concentric about the sun; the stars, on the other hand, move in +every conceivable direction and have no apparent center of motion, for +all efforts to discover such a center have failed. At one time, when +theology had finally to accept the facts of science, a grandiose +conception arose in some pious minds, according to which the Throne of +God was situated at the exact center of His Creation, and, seated +there, He watched the magnificent spectacle of the starry systems +obediently revolving around Him. Astronomical discoveries and +speculations seemed for a time to afford some warrant for this view, +which was, moreover, an acceptable substitute for the abandoned +geocentric theory in minds that could only conceive of God as a +superhuman artificer, constantly admiring his own work. No longer ago +than the middle of the nineteenth century a German astronomer, +Maedler, believed that he had actually found the location of the +center about which the stellar universe revolved. He placed it in the +group of the Pleiades, and upon his authority an extraordinary +imaginative picture was sometimes drawn of the star Alcyone, the +brightest of the Pleiades, as the very seat of the Almighty. This idea +even seemed to gain a kind of traditional support from the mystic +significance, without known historical origin, which has for many +ages, and among widely separated peoples, been attached to the +remarkable group of which Alcyone is the chief. But since Maedler's +time it has been demonstrated that the Pleiades cannot be the center +of revolution of the universe, and, as already remarked, all attempts +to find or fix such a center have proved abortive. Yet so powerful was +the hold that the theory took upon the popular imagination, that even +today astronomers are often asked if Alcyone is not the probable site +of ``Jerusalem the Golden.'' + +If there were a discoverable center of predominant gravitative power, +to which the motions of all the stars could be referred, those motions +would appear less mysterious, and we should then be able to conclude +that the universe was, as a whole, a prototype of the subsidiary +systems of which it is composed. We should look simply to the law of +gravitation for an explanation, and, naturally, the center would be +placed within the opening enclosed by the Milky Way. If it were there +the Milky Way itself should exhibit signs of revolution about it, like +a wheel turning upon its hub. No theory of the star motions as a whole +could stand which failed to take account of the Milky Way as the basis +of all. But the very form of that divided wreath of stars forbids the +assumption of its revolution about a center. Even if it could be +conceived as a wheel having no material center it would not have the +form which it actually presents. As was shown in Chapter 2, there is +abundant evidence of motion in the Milky Way; but it is not motion of +the system as a whole, but motion affecting its separate parts. +Instead of all moving one way, the galactic stars, as far as their +movements can be inferred, are governed by local influences and +conditions. They appear to travel crosswise and in contrary +directions, and perhaps they eddy around foci where great numbers have +assembled; but of a universal revolution involving the entire mass we +have no evidence. + +Most of our knowledge of star motions, called ``proper motions,'' +relates to individual stars and to a few groups which happen to be so +near that the effects of their movements are measurable. In some cases +the motion is so rapid (not in appearance, but in reality) that the +chief difficulty is to imagine how it can have been imparted, and what +will eventually become of the ``runaways.'' Without a collision, or a +series of very close approaches to great gravitational centers, a star +traveling through space at the rate of two hundred or three hundred +miles per second could not be arrested or turned into an orbit which +would keep it forever flying within the limits of the visible +universe. A famous example of these speeding stars is ``1830 +Groombridge,'' a star of only the sixth magnitude, and consequently +just visible to the naked eye, whose motion across the line of sight +is so rapid that it moves upon the face of the sky a distance equal to +the apparent diameter of the moon every 280 years. The distance of +this star is at least 200,000,000,000,000 miles, and may be two or +three times greater, so that its actual speed cannot be less than two +hundred, and may be as much as four hundred, miles per second. It +could be turned into a new course by a close approach to a great sun, +but it could only be stopped by collision, head-on, with a body of +enormous mass. Barring such accidents it must, as far as we can see, +keep on until it has traversed our stellar system, whence in may +escape and pass out into space beyond, to join, perhaps, one of those +other universes of which we have spoken. Arcturus, one of the greatest +suns in the universe, is also a runaway, whose speed of flight has +been estimated all the way from fifty to two hundred miles per second. +Arcturus, we have every reason to believe, possesses hundreds of times +the mass of our sun -- think, then, of the prodigious momentum that +its motion implies! Sirius moves more moderately, its motion across +the line of sight amounting to only ten miles per second, but it is at +the same time approaching the sun at about the same speed, its actual +velocity in space being the resultant of the two displacements. + +What has been said about the motion of Sirius brings us to another +aspect of this subject. The fact is, that in every case of stellar +motion the displacement that we observe represents only a part of the +actual movement of the star concerned. There are stars whose motion +carries them straight toward or straight away from the earth, and such +stars, of course, show no cross motion. But the vast majority are +traveling in paths inclined from a perpendicular to our line of sight. +Taken as a whole, the stars may be said to be flying about like the +molecules in a mass of gas. The discovery of the radial component in +the movements of the stars is due to the spectroscope. If a star is +approaching, its spectral lines are shifted toward the violet end of +the spectrum by an amount depending upon the velocity of approach; if +it is receding, the lines are correspondingly shifted toward the red +end. Spectroscopic observation, then, combined with micrometric +measurements of the cross motion, enables us to detect the real +movement of the star in space. Sometimes it happens that a star's +radial movement is periodically reversed; first it approaches, and +then it recedes. This indicates that it is revolving around a near-by +companion, which is often invisible, and superposed upon this motion +is that of the two stars concerned, which together may be approaching +or receding or traveling across the line of sight. Thus the +complications involved in the stellar motions are often exceedingly +great and puzzling. + +Yet another source of complication exists in the movement of our own +star, the sun. There is no more difficult problem in astronomy than +that of disentangling the effects of the solar motion from those of +the motions of the other stars. But the problem, difficult as it is, +has been solved, and upon its solution depends our knowledge of the +speed and direction of the movement of the solar system through space, +for of course the sun carries its planets with it. One element of the +solution is found in the fact that, as a result of perspective, the +stars toward which we are going appear to move apart toward all points +of the compass, while those behind appear to close up together. Then +the spectroscopic principle already mentioned is invoked for studying +the shift of the lines, which is toward the violet in the stars ahead +of us and toward the red in those that we are leaving behind. Of +course the effects of the independent motions of the stars must be +carefully excluded. The result of the studies devoted to this subject +is to show that we are traveling at a speed of twelve to fifteen miles +per second in a northerly direction, toward the border of the +constellations Hercules and Lyra. A curious fact is that the more +recent estimates show that the direction is not very much out of a +straight line drawn from the sun to the star Vega, one of the most +magnificent suns in the heavens. But it should not be inferred from +this that Vega is drawing us on; it is too distant for its gravitation +to have such an effect. + +Many unaccustomed thoughts are suggested by this mighty voyage of the +solar system. Whence have we come, and whither do we go? Every year of +our lives we advance at least 375,000,000 miles. Since the traditional +time of Adam the sun has led his planets through the wastes of space +no less than 225,000,000,000 miles, or more than 2400 times the +distance that separates him from the earth. Go back in imagination to +the geologic ages, and try to comprehend the distance over which the +earth has flown. Where was our little planet when it emerged out of +the clouds of chaos? Where was the sun when his ``thunder march'' +began? What strange constellations shone down upon our globe when its +masters of life were the monstrous beasts of the ``Age of Reptiles''? +A million years is not much of a span of time in geologic reckoning, +yet a million years ago the earth was farther from its present place +in space than any of the stars with a measurable parallax are now. It +was more than seven times as far as Sirius, nearly fourteen times as +far as Alpha Centauri, three times as far as Vega, and twice as far as +Arcturus. But some geologists demand two hundred, three hundred, even +one thousand million years to enable them to account for the +evolutionary development of the earth and its inhabitants. In a +thousand million years the earth would have traveled farther than from +the remotest conceivable depths of the Milky Way! + +Other curious reflections arise when we think of the form of the +earth's track as it follows the lead of the sun, in a journey which +has neither known beginning nor conceivable end. There are probably +many minds which have found a kind of consolation in the thought that +every year the globe returns to the same place, on the same side of +the sun. This idea may have an occult connection with our traditional +regard for anniversaries. When that period of the year returns at +which any great event in our lives has occurred we have the feeling +that the earth, in its annual round, has, in a manner, brought us back +to the scene of that event. We think of the earth's orbit as a +well-worn path which we traverse many times in the course of a +lifetime. It seems familiar to us, and we grow to have a sort of +attachment to it. The sun we are accustomed to regard as a fixed +center in space, like the mill or pump around which the harnessed +patient mule makes his endless circuits. But the real fact is that the +earth never returns to the place in space where it has once quitted. +In consequence of the motion of the sun carrying the earth and the +other planets along, the track pursued by our globe is a vast spiral +in space continually developing and never returning upon its course. +It is probable that the tracks of the sun and the others stars are +also irregular, and possibly spiral, although, as far as can be at +present determined, they appear to be practically straight. Every +star, wherever it may be situated, is attracted by its fellow-stars +from many sides at once, and although the force is minimized by +distance, yet in the course of many ages its effects must become +manifest. + +Looked at from another side, is there not something immensely +stimulating and pleasing to the imagination in the idea of so +stupendous a journey, which makes all of us the greatest of travelers? +In the course of a long life a man is transported through space thirty +thousand million miles; Halley's Comet does not travel one-quarter as +far in making one of its immense circuits. And there are adventures on +this voyage of which we are just beginning to learn to take account. +Space is full of strange things, and the earth must encounter some of +them as it advances through the unknown. Many singular speculations +have been indulged in by astronomers concerning the possible effects +upon the earth of the varying state of the space that it traverses. +Even the alternation of hot and glacial periods has sometimes been +ascribed to this source. When tropical life flourished around the +poles, as the remains in the rocks assure us, the needed high +temperature may, it has been thought, have been derived from the +presence of the earth in a warm region of space. Then, too, there is a +certain interest for us in the thought of what our familiar planet has +passed through. We cannot but admire it for its long journeying as we +admire the traveler who comes to us from remote and unexplored lands, +or as we gaze with a glow of interest upon the first locomotive that +has crossed a continent, or a ship that has visited the Arctic or +Antarctic regions. If we may trust the indications of the present +course, the earth, piloted by the sun, has come from the Milky Way in +the far south and may eventually rejoin that mighty band of stars in +the far north. + +While the stars in general appear to travel independently of one +another, except when they are combined in binary or trinary systems, +there are notable exceptions to this rule. In some quarters of the sky +we behold veritable migrations of entire groups of stars whose members +are too widely separated to show any indications of revolution about a +common center of gravity. This leads us back again to the wonderful +group of the Pleiades. All of the principle stars composing that group +are traveling in virtually parallel lines. Whatever force set them +going evidently acted upon all alike. This might be explained by the +assumption that when the original projective force acted upon them +they were more closely united than they are at present, and that in +drifting apart they have not lost the impulse of the primal motion. Or +it may be supposed that they are carried along by some current in +space, although it would be exceedingly difficult, in the present +state of our knowledge, to explain the nature of such a current. Yet +the theory of a current has been proposed. As to an attractive center +around which they might revolve, none has been found. Another instance +of similar ``star-drift'' is furnished by five of the seven stars +constituting the figure of the ``Great Dipper.'' In this case the +stars concerned are separated very widely, the two extreme ones by not +less than fifteen degrees, so that the idea of a common motion would +never have been suggested by their aspect in the sky; and the case +becomes the more remarkable from the fact that among and between them +there are other stars, some of the same magnitude, which do not share +their motion, but are traveling in other directions. Still other +examples of the same phenomenon are found in other parts of the sky. +Of course, in the case of compact star-clusters, it is assumed that +all the members share a like motion of translation through space, and +the same is probably true of dense star-swarms and star-clouds. + +The whole question of star-drift has lately assumed a new phase, in +consequence of the investigations of Kapteyn, Dyson, and Eddington on +the ``systematic motions of the stars.'' This research will, it is +hoped, lead to an understanding of the general law governing the +movements of the whole body of stars constituting the visible +universe. Taking about eleven hundred stars whose proper motions have +been ascertained with an approach to certainty, and which are +distributed in all parts of the sky, it has been shown that there +exists an apparent double drift, in two independent streams, moving in +different and nearly opposed directions. The apex of the motion of +what is called ``Stream I'' is situated, according to Professor +Kapteyn, in right ascension 85 deg., declination south 11 deg., which places +it just south of the constellation Orion; while the apex of ``Stream +II'' is in right ascension 260 deg., declination south 48 deg., placing it in +the constellation Ara, south of Scorpio. The two apices differ very +nearly 180 deg. in right ascension and about 120 deg. in declination. The +discovery of these vast star-streams, if they really exist, is one of +the most extraordinary in modern astronomy. It offers the correlation +of stellar movements needed as the basis of a theory of those +movements, but it seems far from revealing a physical cause for them. +As projected against the celestial sphere the stars forming the two +opposite streams appear intermingled, some obeying one tendency and +some the other. As Professor Dyson has said, the hypothesis of this +double movement is of a revolutionary character, and calls for further +investigation. Indeed, it seems at first glance not less surprising +than would be the observation that in a snow-storm the flakes over our +heads were divided into two parties and driving across each other's +course in nearly opposite directions, as if urged by interpenetrating +winds. + +But whatever explanation may eventually be found for the motions of +the stars, the knowledge of the existence of those motions must always +afford a new charm to the contemplative observer of the heavens, for +they impart a sense of life to the starry system that would otherwise +be lacking. A stagnant universe, with every star fixed immovably in +its place, would not content the imagination or satisfy our longing +for ceaseless activity. The majestic grandeur of the evolutions of the +celestial hosts, the inconceivable vastness of the fields of space in +which they are executed, the countless numbers, the immeasurable +distances, the involved convolutions, the flocking and the scattering, +the interpenetrating marches and countermarches, the strange community +of impulsion affecting stars that are wide apart in space and causing +them to traverse the general movement about them like aides and +despatch-bearers on a battle-field -- all these arouse an intensity of +interest which is heightened by the mystery behind them. + +The Passing of the Constellations + +From a historical and picturesque point of view, one of the most +striking results of the motions of the stars described in the last +chapter is their effect upon the forms of the constellations, which +have been watched and admired by mankind from a period so early that +the date of their invention is now unknown. The constellations are +formed by chance combinations of conspicuous stars, like figures in a +kaleidoscope, and if our lives were commensurate with the aeons of +cosmic existence we should perceive that the kaleidoscope of the +heavens was ceaselessly turning and throwing the stars into new +symmetries. Even if the stars stood fast, the motion of the solar +system would gradually alter the configurations, as the elements of a +landscape dissolve and recombine in fresh groupings with the +traveler's progress amid them. But with the stars themselves all in +motion at various speeds and in many directions, the changes occur +more rapidly. Of course, ``rapid'' is here understood in a relative +sense; the wheel of human history to an eye accustomed to the majestic +progression of the universe would appear to revolve with the velocity +of a whirling dynamo. Only the deliberation of geological movements +can be contrasted with the evolution and devolution of the +constellations. + +And yet this secular fluctuation of the constellation figures is not +without keen interest for the meditative observer. It is another +reminder of the swift mutability of terrestial affairs. To the passing +glance, which is all that we can bestow upon these figures, they +appear so immutable that they have been called into service to form +the most lasting records of ancient thought and imagination that we +possess. In the forms of the constellations, the most beautiful, and, +in imaginative quality, the finest, mythology that the world has ever +known has been perpetuated. Yet, in a broad sense, this scroll of +human thought imprinted on the heavens is as evanescent as the summer +clouds. Although more enduring than parchment, tombs, pyramids, and +temples, it is as far as they from truly eternizing the memory of what +man has fancied and done. + +Before studying the effects that the motions of the stars have had and +will have upon the constellations, it is worth while to consider a +little further the importance of the stellar pictures as archives of +history. To emphasize the importance of these effects it is only +necessary to recall that the constellations register the oldest +traditions of our race. In the history of primeval religions they are +the most valuable of documents. Leaving out of account for the moment +the more familiar mythology of the Greeks, based on something older +yet, we may refer for illustration to that of the mysterious Maya race +of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group +of ruins perched, after the Mexican and Central-American plan, on the +summits of pyramidal mounds which mark the site of an ancient +theogonic center of the Mayas. Here the temples all evidently refer to +a cult based upon the constellations as symbols. The figures and the +names, of course, were not the same as those that we have derived from +our Aryan ancestors, but the star groups were the same or nearly so. +For instance, the loftiest of the temples at Izamal was connected with +the sign of the constellation known to us as Cancer, marking the place +of the sun at the summer solstice, at which period the sun was +supposed to descend at noon like a great bird of fire and consume the +offerings left upon the altar. Our Scorpio was known to the Mayas as a +sign of the ``Death God.'' Our Libra, the ``Balance,'' with which the +idea of a divine weighing out of justice has always been connected, +seems to be identical with the Mayan constellation Teoyaotlatohua, +with which was associated a temple where dwelt the priests whose +special business it was to administer justice and to foretell the +future by means of information obtained from the spirits of the dead. +Orion, the ``Hunter'' of our celestial mythology, was among the Mayas +a ``Warrior,'' while Sagittarius and others of our constellations were +known to them (under different names, of course), and all were endowed +with a religious symbolism. And the same star figures, having the same +significance, were familiar to the Peruvians, as shown by the temples +at Cuzco. Thus the imagination of ancient America sought in the +constellations symbols of the unchanging gods. + +But, in fact, there is no nation and no people that has not recognized +the constellations, and at one period or another in its history +employed them in some symbolic or representative capacity. As handled +by the Greeks from prehistoric times, the constellation myths became +the very soul of poetry. The imagination of that wonderful race +idealized the principal star groups so effectively that the figures +and traditions thus attached to them have, for civilized mankind, +displaced all others, just as Greek art in its highest forms stands +without parallel and eclipses every rival. The Romans translated no +heroes and heroines of the mythical period of their history to the +sky, and the deified Caesars never entered that lofty company, but the +heavens are filled with the early myths of the Greeks. Herakles +nightly resumes his mighty labors in the stars; Zeus, in the form of +the white ``Bull,'' Taurus, bears the fair Europa on his back through +the celestial waves; Andromeda stretches forth her shackled arms in +the star-gemmed ether, beseeching aid; and Perseus, in a blaze of +diamond armor, revives his heroic deeds amid sparkling clouds of +stellar dust. There, too, sits Queen Cassiopeia in her dazzling chair, +while the Great King, Cepheus, towers gigantic over the pole. +Professor Young has significantly remarked that a great number of the +constellations are connected in some way or other with the Argonautic +Expedition -- that strangely fascinating legend of earliest Greek +story which has never lost its charm for mankind. In view of all this, +we may well congratulate ourselves that the constellations will +outlast our time and the time of countless generations to follow us; +and yet they are very far from being eternal. Let us now study some of +the effects of the stellar motions upon them. + +We begin with the familiar figure of the ``Great Dipper.'' He who has +not drunk inspiration from its celestial bowl is not yet admitted to +the circle of Olympus. This figure is made up of seven conspicuous +stars in the constellation Ursa Major, the ``Greater Bear.'' The +handle of the ``Dipper'' corresponds to the tail of the imaginary +``Bear,'' and the bowl lies upon his flank. In fact, the figure of a +dipper is so evident and that of a bear so unevident, that to most +persons the ``Great Dipper'' is the only part of the constellation +that is recognizable. Of the seven stars mentioned, six are of nearly +equal brightness, ranking as of the second magnitude, while the +seventh is of only the third magnitude. The difference is very +striking, since every increase of one magnitude involves an increase +of two-and-a-half times in brightness. There appears to be little +doubt that the faint star, which is situated at the junction of the +bowl and the handle, is a variable of long period, since three hundred +years ago it was as bright as its companions. But however that may be, +its relative faintness at the present time interferes but little with +the perfection of the ``Dipper's'' figure. In order the more readily +to understand the changes which are taking place, it will be well to +mention both the names and the Greek letters which are attached to the +seven stars. Beginning at the star in the upper outer edge of the rim +of the bowl and running in regular order round the bottom and then out +to the end of the handle, the names and letters are as follows: Dubhe +({\alpha}), Merak ({\beta}), Phaed ({\gamma}), Megrez ({\delta}), +Alioth ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez +is the faint star already mentioned at the junction of the bowl and +handle, and Mizar, in the middle of the handle, has a close, naked-eye +companion which is named Alcor. The Arabs called this singular pair of +stars ``The Horse and Rider.'' Merak and Duhbe are called ``The +Pointers,'' because an imaginary line drawn northward through them +indicates the Pole Star. + +Now it has been found that five of these stars -- viz., Merak, Phaed, +Megrez, Alioth, and Mizar (with its comrade) -- are moving with +practically the same speed in an easterly direction, while the other +two, Dubhe and Benetnasch, are simultaneously moving westward, the +motions of Benetnasch being apparently more rapid. The consequence of +these opposed motions is, of course, that the figure of the ``Dipper'' +cannot always have existed and will not continue to exist. In the +accompanying diagrams it has been thought interesting to show the +relative positions of these seven stars, as seen from the point which +the earth now occupies, both in the past and in the future. Arrows +attached to the stars in the figure representing the present +appearance of the ``Dipper'' indicate the directions of the motions +and the distances over which they will carry the stars in a period of +about five hundred centuries. The time, no doubt, seems long, but +remember the vast stretch of ages through which the earth has passed, +and then reflect that no reason is apparent why our globe should not +continue to be a scene of animation for ten thousand centuries yet to +come. The fact that the little star Alcor placed so close to Mizar +should accompany the latter in its flight is not surprising, but that +two of the principal stars of the group should be found moving in a +direction directly opposed to that pursued by the other five is +surprising in the highest degree; and it recalls the strange theory of +a double drift affecting all the stars, to which attention was called +in the preceding chapter. It would appear that Benetnasch and Dubhe +belong to one ``current,'' and Merak, Phaed, Megrez, Alioth, and Mizar +to the other. As far as is known, the motion of the seven stars are +not shared by the smaller stars scattered about them, but on the +theory of currents there should be such a community of motion, and +further investigation may reveal it. + +From the ``Great Dipper'' we turn to a constellation hardly less +conspicuous and situated at an equal distance from the pole on the +other side -- Cassiopeia. This famous star-group commemorating the +romantic Queen of Ethiopia whose vain boasting of her beauty was +punished by the exposure of her daughter Andromeda to the ``Sea +Monster,'' is well-marked by five stars which form an irregular letter +``W'' with its open side toward the pole. Three of these stars are +usually ranked as of the second magnitude, and two of the third; but +to ordinary observation they appear of nearly equal brightness, and +present a very striking picture. They mark out the chair and a part of +the figure of the beautiful queen. Beginning at the right-hand, or +western, end of the ``W,'' their Greek letter designations are: Beta +({\beta}), Alpha ({\alpha}), Gamma ({\gamma}), Delta ({\delta}), and +Epsilon ({\epsilon}). Four of them, Beta, Alpha, Delta, and Epsilon +are traveling eastwardly at various speeds, while the fifth, Gamma, +moves in a westerly direction. The motion of Beta is more rapid than +that of any of the others. It should be said, however, that no little +uncertainty attaches to the estimates of the rate of motion of stars +which are not going very rapidly, and different observers often vary +considerably in their results. + +In the beautiful ``Northern Crown,'' one of the most perfect and +charming of all the figures to be found in the stars, the alternate +combining and scattering effects of the stellar motions are shown by +comparing the appearance which the constellation must have had five +hundred centuries ago with that which it has at present and that which +it will have in the future. The seven principle stars of the asterism, +forming a surprisingly perfect coronet, have movements in three +directions at right angles to one another. That in these circumstances +they should ever have arrived at positions giving them so striking an +appearance of definite association is certainly surprising; from its +aspect one would have expected to find a community of movement +governing the brilliants of the ``Crown,'' but instead of that we find +evidence that they will inevitably drift apart and the beautiful +figure will dissolve. + +A similar fate awaits such asterisms as the ``Northern Cross'' in +Cygnus; the ``Crow'' (Corvus), which stands on the back of the great +``Sea Serpent,'' Hydra, and pecks at his scales; ``Job's Coffin'' +(Delphinus); the ``Great Square of Pegasus''; the ``Twins'' (Gemini); +the beautiful ``Sickle'' in Leo; and the exquisite group of the Hyades +in Taurus. In the case of the Hyades, two controlling movements are +manifest: one, affecting five of the stars which form the well-known +figure of a letter ``V,'' is directed northerly; the other, which +controls the direction of two stars, has an easterly trend. The chief +star of the group, Aldebaran, one of the finest of all stars both for +its brilliance and its color, is the most affected by the easterly +motion. In time it will drift entirely out of connection with its +present neighbors. Although the Hyades do not form so compact a group +as the Pleiades in the same constellation, yet their appearance of +relationship is sufficient to awaken a feeling of surprise over the +fact that, as with the stars of the ``Dipper,'' their association is +only temporary or apparent. + +The great figure of Orion appears to be more lasting, not because its +stars are physically connected, but because of their great distance, +which renders their movements too deliberate to be exactly +ascertained. Two of the greatest of its stars, Betelgeuse and Rigel, +possess, as far as has been ascertained, no perceptible motion across +the line of sight, but there is a little movement perceptible in the +``Belt.'' At the present time this consists of an almost perfect +straight line, a row of second-magnitude stars about equally spaced +and of the most striking beauty. In the course of time, however, the +two right-hand stars, Mintaka and Alnilam (how fine are these Arabic +star names!) will approach each other and form a naked-eye double, but +the third, Alnita, will drift away eastward, so that the ``Belt'' will +no longer exist. + +For one more example, let us go to the southern hemisphere, whose most +celebrated constellation, the ``Southern Cross,'' has found a place in +all modern literatures, although it has no claim to consideration on +account of association with ancient legends. This most attractive +asterism, which has never ceased to fascinate the imagination of +Christendom since it was first devoutly described by the early +explorers of the South, is but a passing collocation of brilliant +stars. Yet even in its transfigurations it has been for hundreds of +centuries, and will continue to be for hundreds of centuries to come, +a most striking object in the sky. Our figures show its appearance in +three successive phases: first, as it was fifty thousand years ago +(viewed from the earth's present location); second, as it is in our +day; and, third, as it will be an equal time in the future. The +nearness of these bright stars to one another -- the length of the +longer beam of the ``Cross'' is only six degrees -- makes this group +very noticeable, whatever the arrangement of its components may be. +The largest star, at the base of the ``Cross,'' is of the first +magnitude, two of the others are of the second magnitude, and the +fourth is of the third. Other stars, not represented in the figures, +increase the effect of a celestial blazonry, although they do not help +the resemblance to a cross. + +But since the motion of the solar system itself will, in the course of +so long a period as fifty thousand years, produce a great change in +the perspective of the heavens as seen from the earth, by carrying us +nearly nineteen trillion miles from our present place, why, it may be +asked, seek to represent future appearances of the constellations +which we could not hope to see, even if we could survive so long? The +answer is: Because these things aid the mind to form a picture of the +effects of the mobility of the starry universe. Only by showing the +changes from some definite point of view can we arrive at a due +comprehension of them. The constellations are more or less familiar to +everybody, so that impending changes of their forms must at once +strike the eye and the imagination, and make clearer the significance +of the movements of the stars. If the future history of mankind is to +resemble its past and if our race is destined to survive yet a million +years, then our remote descendents will see a ``new heavens'' if not a +``new earth,'' and will have to invent novel constellations to +perpetuate their legends and mythologies. + +If our knowledge of the relative distances of the stars were more +complete, it would be an interesting exercise in celestial geometry to +project the constellations probably visible to the inhabitants of +worlds revolving around some of the other suns of space. Our sun is +too insignificant for us to think that he can make a conspicuous +appearance among them, except, perhaps, in a few cases. As seen, for +instance, from the nearest known star, Alpha Centauri, the sun would +appear of the average first magnitude, and consequently from that +standpoint he might be the gem of some little constellation which had +no Sirius, or Arcturus, or Vega to eclipse him with its superior +splendor. But from the distance of the vast majority of the stars the +sun would probably be invisible to the naked eye, and as seen from +nearer systems could only rank as a fifth or sixth magnitude star, +unnoticed and unknown except by the star-charting astronomer. + +Conflagrations in the Heavens + +Suppose it were possible for the world to take fire and burn up -- as +some pessimists think that it will do when the Divine wrath shall have +sufficiently accumulated against it -- nobody out of our own little +corner of space would ever be aware of the catastrophe! With all their +telescopes, the astronomers living in the golden light of Arcturus or +the diamond blaze of Canopus would be unable to detect the least +glimmer of the conflagration that had destroyed the seat of Adam and +his descendents, just as now they are totally ignorant of its +existence. + +But at least fifteen times in the course of recorded history men +looking out from the earth have beheld in the remote depths of space +great outbursts of fiery light, some of them more splendidly luminous +than anything else in the firmament except the sun! If they were +conflagrations, how many million worlds like ours were required to +feed their blaze? + +It is probable that ``temporary'' or ``new'' stars, as these wonderful +apparitions are called, really are conflagrations; not in the sense of +a bonfire or a burning house or city, but in that of a sudden eruption +of inconceivable heat and light, such as would result from the +stripping off the shell of an encrusted sun or the crashing together +of two mighty orbs flying through space with a hundred times the +velocity of the swiftest cannon-shot. + +Temporary stars are the rarest and most erratic of astronomical +phenomena. The earliest records relating to them are not very clear, +and we cannot in every instance be certain that it was one of these +appearances that the ignorant and superstitious old chroniclers are +trying to describe. The first temporary star that we are absolutely +sure of appeared in 1572, and is known as ``Tycho's Star,'' because +the celebrated Danish astronomer (whose remains, with his +gold-and-silver artificial nose -- made necessary by a duel -- still +intact, were disinterred and reburied in 1901) was the first to +perceive it in the sky, and the most assiduous and successful in his +studies of it. As the first fully accredited representative of its +class, this new star made its entry upon the scene with becoming +eclat. It is characteristic of these phenomena that they burst into +view with amazing suddenness, and, of course, entirely unexpectedly. +Tycho's star appeared in the constellation Cassiopeia, near a now +well-known and much-watched little star named Kappa, on the evening of +November 11, 1572. The story has often been repeated, but it never +loses interest, how Tycho, going home that evening, saw people in the +street pointing and staring at the sky directly over their heads, and +following the direction of their hands and eyes he was astonished to +see, near the zenith, an unknown star of surpassing brilliance. It +outshone the planet Jupiter, and was therefore far brighter than the +first magnitude. There was not another star in the heavens that could +be compared with it in splendor. Tycho was not in all respects free +from the superstitions of his time -- and who is? -- but he had the +true scientific instinct, and immediately he began to study the +stranger, and to record with the greatest care every change in its +aspect. First he determined as well as he could with the imperfect +instruments of his day, many of which he himself had invented, the +precise location of the phenomena in the sky. Then he followed the +changes that it underwent. At first it brightened until its light +equaled or exceeded that of the planet Venus at her brightest, a +statement which will be appreciated at its full value by anyone who +has ever watched Venus when she plays her dazzling role of ``Evening +Star,'' flaring like an arc light in the sunset sky. It even became so +brilliant as to be visible in full daylight, since, its position being +circumpolar, it never set in the latitude of Northern Europe. Finally +it began to fade, turning red as it did so, and in March, 1574, it +disappeared from Tycho's searching gaze, and has never been seen again +from that day to this. None of the astronomers of the time could make +anything of it. They had not yet as many bases of speculation as we +possess today. + +Tycho's star has achieved a romantic reputation by being fancifully +identified with the ``Star of Bethlehem,'' said to have led the +wondering Magi from their eastern deserts to the cradle-manger of the +Savior in Palestine. Many attempts have been made to connect this +traditional ``star'' with some known phenomenon of the heavens, and +none seems more idle than this. Yet it persistently survives, and no +astronomer is free from eager questions about it addressed by people +whose imagination has been excited by the legend. It is only necessary +to say that the supposition of a connection between the phenomenon of +the Magi and Tycho's star is without any scientific foundation. It was +originally based on an unwarranted assumption that the star of Tycho +was a variable of long period, appearing once every three hundred and +fifteen years, or thereabout. If that were true there would have been +an apparition somewhere near the traditional date of the birth of +Christ, a date which is itself uncertain. But even the data on which +the assumption was based are inconsistent with the theory. Certain +monkish records speak of something wonderful appearing in the sky in +the years 1264 and 945, and these were taken to have been outbursts of +Tycho's star. Investigation shows that the records more probably refer +to comets, but even if the objects seen were temporary stars, their +dates do not suit the hypothesis; from 945 to 1264 there is a gap of +319 years, and from 1264 to 1572 one of only 308 years; moreover 337 +years have now (1909) elapsed since Tycho saw the last glimmer of his +star. Upon a variability so irregular and uncertain as that, even if +we felt sure that it existed, no conclusion could be found concerning +an apparition occurring 2000 years ago. + +In the year 1600 (the year in which Giordano Bruno was burned at the +stake for teaching that there is more than one physical world), a +temporary star of the third magnitude broke out in the constellation +Cygnus, and curiously enough, considering the rarity of such +phenomena, only four years later another surprisingly brilliant one +appeared in the constellation Ophiuchus. This is often called +``Kepler's star,'' because the great German astronomer devoted to it +the same attention that Tycho had given to the earlier phenomenon. It, +too, like Tycho's, was at first the brightest object in the stellar +heavens, although it seems never to have quite equaled its famous +predecessor in splendor. It disappeared after a year, also turning of +a red color as it became more faint. We shall see the significance of +this as we go on. Some of Kepler's contemporaries suggested that the +outburst of this star was due to a meeting of atoms in space, and idea +bearing a striking resemblance to the modern theory of ``astronomical +collisions.'' + +In 1670, 1848, and 1860 temporary stars made their appearance, but +none of them was of great brilliance. In 1866 one of the second +magnitude broke forth in the ``Northern Crown'' and awoke much +interest, because by that time the spectroscope had begun to be +employed in studying the composition of the stars, and Huggins +demonstrated that the new star consisted largely of incandescent +hydrogen. But this star, apparently unlike the others mentioned, was +not absolutely new. Before its outburst it had shown as a star of the +ninth magnitude (entirely invisible, of course, to the naked eye), and +after about six weeks it faded to its original condition in which it +has ever since remained. In 1876 a temporary star appeared in the +constellation Cygnus, and attained at one time the brightness of the +second magnitude. Its spectrum and its behavior resembled those of its +immediate predecessor. In 1885, astronomers were surprised to see a +sixth-magnitude star glimmering in the midst of the hazy cloud of the +great Andromeda Nebula. It soon absolutely disappeared. Its spectrum +was remarkable for being ``continuous,'' like that of the nebula +itself. A continuous spectrum is supposed to represent a body, or a +mass, which is either solid or liquid, or composed of gas under great +pressure. In January, 1892, a new star was suddenly seen in the +constellation Auriga. It never rose much above the fourth magnitude, +but it showed a peculiar spectrum containing both bright and dark +lines of hydrogen. + +But a bewildering surprise was now in store; the world was to behold +at the opening of the twentieth century such a celestial spectacle as +had not been on view since the times of Tycho and Kepler. Before +daylight on the morning of February 22, 1901, the Rev. Doctor +Anderson, of Edinburgh, an amateur astronomer, who had also been the +first to see the new star in Auriga, beheld a strange object in the +constellation Perseus not far from the celebrated variable star Algol. +He recognized its character at once, and immediately telegraphed the +news, which awoke the startled attention of astronomers all over the +world. When first seen the new star was no brighter than Algol (less +than the second magnitude), but within twenty-four hours it was +ablaze, outshining even the brilliant Capella, and far surpassing the +first magnitude. At the spot in the sky where it appeared nothing +whatever was visible on the night before its coming. This is known +with certainty because a photograph had been made of that very region +on February 21, and this photograph showed everything down to the +twelfth magnitude, but not a trace of the stranger which burst into +view between the 21st and the 22nd like the explosion of a rocket. + +Upon one who knew the stars the apparition of this intruder in a +well-known constellation had the effect of a sudden invasion. The new +star was not far west of the zenith in the early evening, and in that +position showed to the best advantage. To see Capella, the hitherto +unchallenged ruler of that quarter of the sky, abased by comparison +with this stranger of alien aspect, for there was always an unfamiliar +look about the ``nova,'' was decidedly disconcerting. It seemed to +portend the beginning of a revolution in the heavens. One could +understand what the effect of such an apparition must have been in the +superstitious times of Tycho. The star of Tycho had burst forth on the +northern border of the Milky Way; this one was on its southern border, +some forty-five degrees farther east. + +Astronomers were well-prepared this time for the scientific study of +the new star, both astronomical photography and spectroscopy having +been perfected, and the results of their investigations were +calculated to increase the wonder with which the phenomenon was +regarded. The star remained at its brightest only a few days; then, +like a veritable conflagration, it began to languish; and, like the +reflection of a dying fire, as it sank it began to glow with the red +color of embers. But its changes were spasmodic; once about every +three days it flared up only to die away again. During these +fluctuations its light varied alternately in the ratio of one to six. +Finally it took a permanent downward course, and after a few months +the naked eye could no longer perceive it; but it remained visible +with telescopes, gradually fading until it had sunk to the ninth +magnitude. Then another astonishing change happened: in August +photographs taken at the Yerkes Observatory and at Heidelberg showed +that the ``nova'' was surrounded by a spiral nebula! The nebula had +not been there before, and no one could doubt that it represented a +phase of the same catastrophe that had produced the outburst of the +new star. At one time the star seemed virtually to have disappeared, +as if all its substance had been expanded into the nebulous cloud, but +always there remained a stellar nucleus about which the misty spiral +spread wider and ever wider, like a wave expanding around a center of +disturbance. The nebula too showed a variability of brightness, and +four condensations which formed in it seemed to have a motion of +revolution about the star. As time went on the nebula continued to +expand at a rate which was computed to be not less than twenty +thousand miles per second! And now the star itself, showing +indications of having turned into a nebula, behaved in a most erratic +manner, giving rise to the suspicion that it was about to burst out +again. But this did not occur, and at length it sunk into a state of +lethargy from which it has to the present time not recovered. But the +nebulous spiral has disappeared, and the entire phenomena as it now +(1909) exists consists of a faint nebulous star of less than the ninth +magnitude. + +The wonderful transformations just described had been forecast in +advance of the discovery of the nebulous spiral encircling the star by +the spectroscopic study of the latter. At first there was no +suggestion of a nebular constitution, but within a month or two +characteristic nebular lines began to appear, and in less than six +months the whole spectrum had been transformed to the nebular type. In +the mean time the shifting of the spectral lines indicated a +complication of rapid motions in several directions simultaneously. +These motions were estimated to amount to from one hundred to five +hundred miles per second. + +The human mind is so constituted that it feels forced to seek an +explanation of so marvelous a phenomenon as this, even in the absence +of the data needed for a sound conclusion. The most natural +hypothesis, perhaps, is that of a collision. Such a catastrophe could +certainly happen. It has been shown, for instance, that in infinity of +time the earth is sure to be hit by a comet; in the same way it may be +asserted that, if no time limit is fixed, the sun is certain to run +against some obstacle in space, either another star, or a dense meteor +swarm, or one of the dark bodies which there is every reason to +believe abound around us. The consequences of such a collision are +easy to foretell, provided that we know the masses and the velocities +of the colliding bodies. In a preceding chapter we have discussed the +motions of the sun and stars, and have seen that they are so swift +that an encounter between any two of them could not but be disastrous. +But this is not all; for as soon as two stars approached within a few +million miles their speed would be enormously increased by their +reciprocal attractions and, if their motion was directed radially with +respect to their centers, they would come together with a crash that +would reduce them both to nebulous clouds. It is true that the chances +of such a ``head-on'' collision are relatively very small; two stars +approaching each other would most probably fall into closed orbits +around their common center of gravity. If there were a collision it +would most likely be a grazing one instead of a direct front-to-front +encounter. But even a close approach, without any actual collision, +would probably prove disastrous, owing to the tidal influence of each +of the bodies on the other. Suns, in consequence of their enormous +masses and dimensions and the peculiarities of their constitution, are +exceedingly dangerous to one another at close quarters. Propinquity +awakes in them a mutually destructive tendency. Consisting of matter +in the gaseous, or perhaps, in some cases, liquid, state, their tidal +pull upon each other if brought close together might burst them +asunder, and the photospheric envelope being destroyed the internal +incandescent mass would gush out, bringing fiery death to any planets +that were revolving near. Without regard to the resulting disturbance +of the earth's orbit, the close approach of a great star to the sun +would be in the highest degree perilous to us. But this is a danger +which may properly be regarded as indefinitely remote, since, at our +present location in space, we are certainly far from every star except +the sun, and we may feel confident that no great invisible body is +near, for if there were one we should be aware of its presence from +the effects of its attraction. As to dark nebulae which may possibly +lie in the track that the solar system is pursuing at the rate of +375,000,000 miles per year, that is another question -- and they, too, +could be dangerous! + +This brings us directly back to ``Nova Persei,'' for among the many +suggestions offered to explain its outburst, as well as those of other +temporary stars, one of the most fruitful is that of a collision +between a star and a vast invisible nebula. Professor Seeliger, of +Munich, first proposed this theory, but it afterward underwent some +modifications from others. Stated in a general form, the idea is that +a huge dark body, perhaps an extinguished sun, encountered in its +progress through space a widespread flock of small meteors forming a +dark nebula. As it plunged into the swarm the friction of the +innumerable collisions with the meteors heated its surface to +incandescence, and being of vast size it then became visible to us as +a new star. Meanwhile the motion of the body through the nebula, and +its rotation upon itself, set up a gyration in the blazing atmosphere +formed around it by the vaporized meteors; and as this atmosphere +spread wider, under the laws of gyratory motion a rotation in the +opposite direction began in the inflamed meteoric cloud outside the +central part of the vortex. Thus the spectral lines were caused to +show motion in opposite directions, a part of the incandescent mass +approaching the earth simultaneously with the retreat of another part. +So the curious spectroscopic observations before mentioned were +explained. This theory might also account for the appearance of the +nebulous spiral first seen some six months after the original +outburst. The sequent changes in the spectrum of the ``nova'' are +accounted for by this theory on the assumption, reasonable enough in +itself, that at first the invading body would be enveloped in a +vaporized atmosphere of relatively slight depth, producing by its +absorption the fine dark lines first observed; but that as time went +on and the incessant collisions continued, the blazing atmosphere +would become very deep and extensive, whereupon the appearance of the +spectral lines would change, and bright lines due to the light of the +incandescent meteors surrounding the nucleus at a great distance would +take the place of the original dark ones. The vortex of meteors once +formed would protect the flying body within from further immediate +collisions, the latter now occurring mainly among the meteors +themselves, and then the central blaze would die down, and the +original splendor of the phenomenon would fade. + +But the theories about Nova Persei have been almost as numerous as the +astronomers who have speculated about it. One of the most startling of +them assumed that the outburst was caused by the running amuck of a +dark star which had encountered another star surrounded with planets, +the renewed outbreaks of light after the principal one had faded being +due to the successive running down of the unfortunate planets! Yet +another hypothesis is based on what we have already said of the tidal +influence that two close approaching suns would have upon each other. +Supposing two such bodies which had become encrusted, but remained +incandescent and fluid within, to approach within almost striking +distance; they would whirl each other about their common center of +gravity, and at the same time their shells would burst under the tidal +strain, and their glowing nuclei being disclosed would produce a great +outburst of light. Applying this theory to a ``nova,'' like that of +1866 in the ``Northern Crown,'' which had been visible as a small star +before the outbreak, and which afterward resumed its former aspect, we +should have to assume that a yet shining sun had been approached by a +dark body whose attraction temporarily burst open its photosphere. It +might be supposed that in this case the dark body was too far advanced +in cooling to suffer the same fate from the tidal pull of its victim. +But a close approach of that kind would be expected to result in the +formation of a binary system, with orbits of great eccentricity, +perhaps, and after the lapse of a certain time the outburst should be +renewed by another approximation of the two bodies. A temporary star +of that kind would rather be ranked as a variable. + +The celebrated French astronomer, Janssen, had a different theory of +Nova Persei, and of temporary stars in general. According to his idea, +such phenomena might be the result of chemical changes taking place in +a sun without interference by, or collision with, another body. +Janssen was engaged for many years in trying to discover evidence of +the existence of oxygen in the sun, and he constructed his observatory +on the summit of Mount Blanc specially to pursue that research. He +believed that oxygen must surely exist in the sun since we find so +many other familiar elements included in the constitution of the solar +globe, and as he was unable to discover satisfactory evidence of its +presence he assumed that it existed in a form unknown on the earth. If +it were normally in the sun's chromosphere, or coronal atmosphere, he +said, it would combine with the hydrogen which we know is there and +form an obscuring envelope of water vapor. It exists, then, in a +special state, uncombined with hydrogen; but let the temperature of +the sun sink to a critical point and the oxygen will assume its normal +properties and combine with the hydrogen, producing a mighty outburst +of light and heat. This, Janssen thought, might explain the phenomena +of the temporary stars. It would also, he suggested, account for their +brief career, because the combination of the elements would be quickly +accomplished, and then the resulting water vapor would form an +atmosphere cutting off the radiation from the star within. + +This theory may be said to have a livelier human interest than some of +the others, since, according to it, the sun may carry in its very +constitution a menace to mankind; one does not like to think of it +being suddenly transformed into a gigantic laboratory for the +explosive combination of oxygen and hydrogen! But while Janssen's +theory might do for some temporary stars, it is inadequate to explain +all the phenomena of Nova Persei, and particularly the appearance of +the great spiral nebula that seemed to exhale from the heart of the +star. Upon the whole, the theory of an encounter between a star and a +dark nebula seems best to fit the observations. By that hypothesis the +expanding billow of light surrounding the core of the conflagration is +very well accounted for, and the spectroscopic peculiarities are also +explained. + +Dr Gustov Le Bon offers a yet more alarming theory, suggesting that +temporary stars are the result of atomic explosion; but we shall touch +upon this more fully in Chapter 14. + +Twice in the course of this discussion we have called attention to the +change of color invariably undergone by temporary stars in the later +stages of their career. This was conspicuous with Nova Persei which +glowed more and more redly as it faded, until the nebulous light began +to overpower that of the stellar nucleus. Nothing could be more +suggestive of the dying out of a great fire. Moreover, change of color +from white to red is characteristic of all variable stars of long +period, such as ``Mira'' in Cetus. It is also characteristic of stars +believed to be in the later stages of evolution, and consequently +approaching extinction, like Antares and Betelgeuse, and still more +notably certain small stars which ``gleam like rubies in the field of +the telescope.'' These last appear to be suns in the closing period of +existence as self-luminous bodies. Between the white stars, such as +Sirius and Rigel, and the red stars, such as Aldebaran and Alpha +Herculis, there is a progressive series of colors from golden yellow +through orange to deep red. The change is believed to be due to the +increase of absorbing vapors in the stellar atmosphere as the body +cools down. In the case of ordinary stars these changes no doubt +occupy many millions of years, which represent the average duration of +solar life; but the temporary stars run through similar changes in a +few months: they resemble ephemeral insects -- born in the morning and +doomed to perish with the going down of the sun. + +Explosive and Whirling Nebulae + +One of the most surprising triumphs of celestial photography was +Professor Keeler's discovery, in 1899, that the great majority of the +nebulae have a distinctly spiral form. This form, previously known in +Lord Rosse's great ``Whirlpool Nebula,'' had been supposed to be +exceptional; now the photographs, far excelling telescopic views in +the revelation of nebular forms, showed the spiral to be the typical +shape. Indeed, it is a question whether all nebulae are not to some +extent spiral. The extreme importance of this discovery is shown in +the effect that it has had upon hitherto prevailing views of solar and +planetary evolution. For more than three-quarters of a century +Laplace's celebrated hypothesis of the manner of origin of the solar +system from a rotating and contracting nebula surrounding the sun had +guided speculation on that subject, and had been tentatively extended +to cover the evolution of systems in general. The apparent forms of +some of the nebulae which the telescope had revealed were regarded, and +by some are still regarded, as giving visual evidence in favor of this +theory. There is a ``ring nebula'' in Lyra with a central star, and a +``planetary nebula'' in Gemini bearing no little resemblance to the +planet Saturn with its rings, both of which appear to be practical +realizations of Laplace's idea, and the elliptical rings surrounding +the central condensation of the Andromeda Nebula may be cited for the +same kind of proof. + +But since Keeler's discovery there has been a decided turning away of +speculation another way. The form of the spiral nebulae seems to be +entirely inconsistent with the theory of an originally globular or +disk-shaped nebula condensing around a sun and throwing or leaving off +rings, to be subsequently shaped into planets. Some astronomers, +indeed, now reject Laplace's hypothesis in toto, preferring to think +that even our solar system originated from a spiral nebula. Since the +spiral type prevails among the existing nebulae, we must make any +mechanical theory of the development of stars and planetary systems +from them accord with the requirements which that form imposes. A +glance at the extraordinary variations upon the spiral which Professor +Keeler's photographs reveal is sufficient to convince one of the +difficulty of the task of basing a general theory upon them. In truth, +it is much easier to criticize Laplace's hypothesis than to invent a +satisfactory substitute for it. If the spiral nebulae seem to oppose it +there are other nebulae which appear to support it, and it may be that +no one fixed theory can account for all the forms of stellar evolution +in the universe. Our particular planetary system may have originated +very much as the great French mathematician supposed, while others +have undergone, or are now undergoing, a different process of +development. There is always a too strong tendency to regard an +important new discovery and the theories and speculations based upon +it as revolutionizing knowledge, and displacing or overthrowing +everything that went before. Upon the plea that ``Laplace only made a +guess'' more recent guesses have been driven to extremes and treated +by injudicious exponents as ``the solid facts at last.'' + +Before considering more recent theories than Laplace's, let us see +what the nature of the photographic revelations is. The vast celestial +maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be +taken as the leading type of the spiral nebulae, although there are +less conspicuous objects of the kind which, perhaps, better illustrate +some of their peculiarities. Lord Rosse's nebula appears far more +wonderful in the photographs than in his drawings made with the aid of +his giant reflecting telescope at Parsonstown, for the photographic +plate records details that no telescope is capable of showing. Suppose +we look at the photograph of this object as any person of common sense +would look at any great and strange natural phenomenon. What is the +first thing that strikes the mind? It is certainly the appearance of +violent whirling motion. One would say that the whole glowing mass had +been spun about with tremendous velocity, or that it had been set +rotating so rapidly that it had become the victim of ``centrifugal +force,'' one huge fragment having broken loose and started to gyrate +off into space. Closer inspection shows that in addition to the +principal focus there are various smaller condensations scattered +through the mass. These are conspicuous in the spirals. Some of them +are stellar points, and but for the significance of their location we +might suppose them to be stars which happen to lie in a line between +us and the nebula. But when we observe how many of them follow most +faithfully the curves of the spirals we cannot but conclude that they +form an essential part of the phenomenon; it is not possible to +believe that their presence in such situations is merely fortuitous. +One of the outer spirals has at least a dozen of these star-like +points strung upon it; some of them sharp, small, and distinct, others +more blurred and nebulous, suggesting different stages of +condensation. Even the part which seems to have been flung loose from +the main mass has, in addition to its central condensation, at least +one stellar point gleaming in the half-vanished spire attached to it. +Some of the more distant stars scattered around the ``whirlpool'' look +as if they too had been shot out of the mighty vortex, afterward +condensing into unmistakable solar bodies. There are at least two +curved rows of minute stars a little beyond the periphery of the +luminous whirl which clearly follow lines concentric with those of the +nebulous spirals. Such facts are simply dumbfounding for anyone who +will bestow sufficient thought upon them, for these are suns, though +they may be small ones; and what a birth is that for a sun! + +Look now again at the glowing spirals. We observe that hardly have +they left the central mass before they begin to coagulate. In some +places they have a ``ropy'' aspect; or they are like peascods filled +with growing seeds, which eventually will become stars. The great +focus itself shows a similar tendency, especially around its +circumference. The sense that it imparts of a tremendous shattering +force at work is overwhelming. There is probably more matter in that +whirling and bursting nebula than would suffice to make a hundred +solar systems! It must be confessed at once that there is no +confirmation of the Laplacean hypothesis here; but what hypothesis +will fit the facts? There is one which it has been claimed does so, +but we shall come to that later. In the meanwhile, as a preparation, +fix in the memory the appearance of that second spiral mass spinning +beside its master which seems to have spurned it away. + +For a second example of the spiral nebulae look at the one in the +constellation Triangulum. God, how hath the imagination of puny man +failed to comprehend Thee! Here is creation through destruction with a +vengeance! The spiral form of the nebula is unmistakable, but it is +half obliterated amid the turmoil of flying masses hurled away on all +sides with tornadic fury. The focus itself is splitting asunder under +the intolerable strain, and in a little while, as time is reckoned in +the Cosmos, it will be gyrating into stars. And then look at the +cyclonic rain of already finished stars whirling round the outskirts +of the storm. Observe how scores of them are yet involved in the +fading streams of the nebulous spirals; see how they have been thrown +into vast loops and curves, of a beauty that half redeems the terror +of the spectacle enclosed within their lines -- like iridescent cirri +hovering about the edges of a hurricane. And so again are suns born! + +Let us turn to the exquisite spiral in Ursa Major; how different its +aspect from that of the other! One would say that if the terrific coil +in Triangulum has all but destroyed itself in its fury, this one on +the contrary has just begun its self-demolition. As one gazes one +seems to see in it the smooth, swift, accelerating motion that +precedes catastrophe. The central part is still intact, dense, and +uniform in texture. How graceful are the spirals that smoothly rise +from its oval rim and, gemmed with little stars, wind off into the +darkness until they have become as delicate as threads of gossamer! +But at bottom the story told here is the same -- creation by gyration! + +Compare with the above the curious mass in Cetus. Here the plane of +the whirling nebula nearly coincides with our line of sight and we see +the object at a low angle. It is far advanced and torn to shreds, and +if we could look at it perpendicularly to its plane it is evident that +it would closely resemble the spectacle in Triangulum. + +Then take the famous Andromeda Nebula (see Frontispiece), which is so +vast that notwithstanding its immense distance even the naked eye +perceives it as an enigmatical wisp in the sky. Its image on the +sensitive plate is the masterpiece of astronomical photography; for +wild, incomprehensible beauty there is nothing that can be compared +with it. Here, if anywhere, we look upon the spectacle of creation in +one of its earliest stages. The Andromeda Nebula is apparently less +advanced toward transformation into stellar bodies than is that in +Triangulum. The immense crowd of stars sprinkled over it and its +neighborhood seem in the main to lie this side of the nebula, and +consequently to have no connection with it. But incipient stars (in +some places clusters of them) are seen in the nebulous rings, while +one or two huge masses seem to give promise of transformation into +stellar bodies of unusual magnitude. I say ``rings'' because although +the loops encompassing the Andromeda Nebula have been called spirals +by those who wish utterly to demolish Laplace's hypothesis, yet they +are not manifestly such, as can be seen on comparing them with the +undoubted spirals of the Lord Rosse Nebula. They look quite as much +like circles or ellipses seen at an angle of, say, fifteen or twenty +degrees to their plane. If they are truly elliptical they accord +fairly well with Laplace's idea, except that the scale of magnitude is +stupendous, and if the Andromeda Nebula is to become a solar system it +will surpass ours in grandeur beyond all possibility of comparison. + +There is one circumstance connected with the spiral nebulae, and +conspicuous in the Andromeda Nebula on account of its brightness, +which makes the question of their origin still more puzzling; they all +show continuous spectra, which, as we have before remarked, indicate +that the mass from which the light comes is either solid or liquid, or +a gas under heavy pressure. Thus nebulae fall into two classes: the +``white'' nebulae, giving a continuous spectrum; and the ``green'' +nebulae whose spectra are distinctly gaseous. The Andromeda Nebula is +the great representative of the former class and the Orion Nebula of +the latter. The spectrum of the Andromeda Nebula has been interpreted +to mean that it consists not of luminous gas, but of a flock of stars +so distant that they are separately indistinguishable even with +powerful telescopes, just as the component stars of the Milky Way are +indistinguishable with the naked eye; and upon this has been based the +suggestion that what we see in Andromeda is an outer universe whose +stars form a series of elliptical garlands surrounding a central mass +of amazing richness. But this idea is unacceptable if for no other +reason than that, as just said, all the spiral nebulae possess the same +kind of spectrum, and probably no one would be disposed to regard them +all as outer universes. As we shall see later, the peculiarity of the +spectra of the spiral nebulae is appealed to in support of a modern +substitute for Laplace's hypothesis. + +Finally, without having by any means exhausted the variety exhibited +by the spiral nebulae, let us turn to the great representative of the +other species, the Orion Nebula. In some ways this is even more +marvelous than the others. The early drawings with the telescope +failed to convey an adequate conception either of its sublimity or of +its complication of structure. It exists in a nebulous region of +space, since photographs show that nearly the whole constellation is +interwoven with faintly luminous coils. To behold the entry of the +great nebula into the field even of a small telescope is a startling +experience which never loses its novelty. As shown by the photographs, +it is an inscrutable chaos of perfectly amazing extent, where spiral +bands, radiating streaks, dense masses, and dark yawning gaps are +strangely intermingled without apparent order. In one place four +conspicuous little stars, better seen in a telescope than in the +photograph on account of the blurring produced by over-exposure, are +suggestively situated in the midst of a dark opening, and no observer +has ever felt any doubt that these stars have been formed from the +substance of the surrounding nebula. There are many other stars +scattered over its expanse which manifestly owe their origin to the +same source. But compare the general appearance of this nebula with +the others that we have studied, and remark the difference. If the +unmistakably spiral nebulae resemble bursting fly-wheels or grindstones +from whose perimeters torrents of sparks are flying, the Orion Nebula +rather recalls the aspect of a cloud of smoke and fragments produced +by the explosion of a shell. This idea is enforced by the look of the +outer portion farthest from the bright half of the nebula, where +sharply edged clouds with dark spaces behind seem to be billowing away +as if driven by a wind blowing from the center. + +Next let us consider what scientific speculation has done in the +effort to explain these mysteries. Laplace's hypothesis can certainly +find no standing ground either in the Orion Nebula or in those of a +spiral configuration, whatever may be its situation with respect to +the grand Nebula of Andromeda, or the ``ring'' and ``planetary'' +nebulae. Some other hypothesis more consonant with the appearances must +be found. Among the many that have been proposed the most elaborate is +the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton. +It is to be remarked that it applies to the spiral nebulae +distinctively, and not to an apparently chaotic mass of gas like the +vast luminous cloud in Orion. The gist of the theory is that these +curious objects are probably the result of close approaches to each +other of two independent suns, reminding us of what was said on this +subject when we were dealing with temporary stars. Of the previous +history of these appulsing suns the theory gives us no account; they +are simply supposed to arrive within what may be called an effective +tide-producing distance, and then the drama begins. Some of the +probable consequences of such an approach have been noticed in Chapter +5; let us now consider them a little more in detail. + +Tides always go in couples; if there is a tide on one side of a globe +there will be a corresponding tide on the other side. The cause is to +be found in the law that the force of gravitation varies inversely as +the square of the distance; the attraction on the nearest surface of +the body exercised by another body is greater than on its center, and +greater yet than on its opposite surface. If two great globes attract +each other, each tends to draw the other out into an ellipsoidal +figure; they must be more rigid than steel to resist this -- and even +then they cannot altogether resist. If they are liquid or gaseous they +will yield readily to the force of distortion, the amount of which +will depend upon their distance apart, for the nearer they are the +greater becomes the tidal strain. If they are encrusted without and +liquid or gaseous in the interior, the internal mass will strive to +assume the figure demanded by the tidal force, and will, if it can, +burst the restraining envelope. Now this is virtually the predicament +of the body we call a sun when in the immediate presence of another +body of similarly great mass. Such a body is presumably gaseous +throughout, the component gases being held in a state of rigidity by +the compression produced by the tremendous gravitational force of +their own aggregate mass. At the surface such a body is enveloped in a +shell of relatively cool matter. Now suppose a great attracting body, +such as another sun, to approach near enough for the difference in its +attraction on the two opposite sides of the body and on its center to +become very great; the consequence will be a tidal deformation of the +whole body, and it will lengthen out along the line of the +gravitational pull and draw in at the sides, and if its shell offers +considerable resistance, but not enough to exercise a complete +restraint, it will be violently burst apart, or blown to atoms, and +the internal mass will leap out on the two opposite sides in great +fiery spouts. In the case of a sun further advanced in cooling than +ours the interior might be composed of molten matter while the +exterior crust had become rigid like the shell of an egg; then the +force of the ``tidal explosion'' produced by the appulse of another +sun would be more violent in consequence of the greater resistance +overcome. Such, then, is the mechanism of the first phase in the +history of a spiral nebula according to the Planetesimal Hypothesis. +Two suns, perhaps extinguished ones, have drawn near together, and an +explosive outburst has occured in one or both. The second phase calls +for a more agile exercise of the imagination. + +To simplify the case, let us suppose that only one of the tugging suns +is seriously affected by the strain. Its vast wings produced by the +outburst are twisted into spirals by their rotation and the contending +attractions exercised upon them, as the two suns, like battleships in +desperate conflict, curve round each other, concentrating their +destructive energies. Then immense quantities of debris are scattered +about in which eddies are created, and finally, as the sun that caused +the damage goes on its way, leaving its victim to repair its injuries +as it may, the dispersed matter cools, condenses, and turns into +streams of solid particles circling in elliptical paths about their +parent sun. These particles, or fragments, are the ``planetesimals'' +of the theory. In consequence of the inevitable intersection of the +orbits of the planetesimals, nodes are formed where the flying +particles meet, and at these nodes large masses are gradually +accumulated. The larger the mass the greater its attraction, and at +last the nodal points become the nuclei of great aggregations from +which planets are shaped. + +This, in very brief form, is the Planetesimal Hypothesis which we are +asked to substitute for that based on Laplace's suggestion as an +explanation of the mode of origin of the solar system; and the +phenomena of the spiral nebulae are appealed to as offering evident +support to the new hypothesis. We are reminded that they are +elliptical in outline, which accords with the hypothesis; that their +spectra are not gaseous, which shows that they may be composed of +solid particles like the planetesimals; and that their central masses +present an oval form, which is what would result from the tidal +effects, as just described. We also remember that some of them, like +the Lord Rosse and the Andromeda nebulae, are visually double, and in +these cases we might suppose that the two masses represent the +tide-burst suns that ventured into too close proximity. It may be +added that the authors of the theory do not insist upon the appulse of +two suns as the only way in which the planetesimals may have +originated, but it is the only supposition that has been worked out. + +But serious questions remain. It needs, for instance, but a glance at +the Triangulum monster to convince the observer that it cannot be a +solar system which is being evolved there, but rather a swarm of +stars. Many of the detached masses are too vast to admit of the +supposition that they are to be transformed into planets, in our sense +of planets, and the distances of the stars which appear to have been +originally ejected from the focal masses are too great to allow us to +liken the assemblage that they form to a solar system. Then, too, no +nodes such as the hypothesis calls for are visible. Moreover, in most +of the spiral nebulae the appearances favor the view that the +supposititious encountering suns have not separated and gone each +rejoicing on its way, after having inflicted the maximum possible +damage on its opponent, but that, on the contrary, they remain in +close association like two wrestlers who cannot escape from each +other's grasp. And this is exactly what the law of gravitation +demands; stars cannot approach one another with impunity, with regard +either to their physical make-up or their future independence of +movement. The theory undertakes to avoid this difficulty by assuming +that in the case of our system the approach of the foreign body to the +sun was not a close one -- just close enough to produce the tidal +extrusion of the relatively insignificant quantity of matter needed to +form the planets. But even then the effect of the appulse would be to +change the direction of flight, both of the sun and of its visitor, +and there is no known star in the sky which can be selected as the +sun's probable partner in their ancient pas deux. That there are +unconquered difficulties in Laplace's hypothesis no one would deny, +but in simplicity of conception it is incomparably more satisfactory, +and with proper modifications could probably be made more consonant +with existing facts in our solar system than that which is offered to +replace it. Even as an explanation of the spiral nebulae, not as solar +systems in process of formation, but as the birthplaces of stellar +clusters, the Planetesimal Hypothesis would be open to many +objections. Granting its assumptions, it has undoubtedly a strong +mathematical framework, but the trouble is not with the mathematics +but with the assumptions. Laplace was one of the ablest mathematicians +that ever lived, but he had never seen a spiral nebula; if he had, he +might have invented a hypothesis to suit its phenomena. His actual +hypothesis was intended only for our solar system, and he left it in +the form of a ``note'' for the consideration of his successors, with +the hope that they might be able to discover the full truth, which he +confessed was hidden from him. It cannot be said that that truth has +yet been found, and when it is found the chances are that intuition +and not logic will have led to it. + +The spiral nebulae, then, remain among the greatest riddles of the +universe, while the gaseous nebulae, like that of Orion, are no less +mysterious, although it seems impossible to doubt that both forms give +birth to stars. It is but natural to look to them for light on the +question of the origin of our planetary system; but we should not +forget that the scale of the phenomena in the two cases is vastly +different, and the forces in operation may be equally different. A +hill may have been built up by a glacier, while a mountain may be the +product of volcanic forces or of the upheaval of the strata of the +planet. + +The Banners of the Sun + +As all the world knows, the sun, a blinding globe pouring forth an +inconceivable quantity of light and heat, whose daily passage through +the sky is caused by the earth's rotation on its axis, constitutes the +most important phenomenon of terrestial existence. Viewed with a dark +glass to take off the glare, or with a telescope, its rim is seen to +be a sharp and smooth circle, and nothing but dark sky is visible +around it. Except for the interference of the moon, we should probably +never have known that there is any more of the sun than our eyes +ordinarily see. + +But when an eclipse of the sun occurs, caused by the interposition of +the opaque globe of the moon, we see its immediate surroundings, which +in some respects are more wonderful than the glowing central orb. +These surroundings, although not in the sense in which we apply the +term to the gaseous envelope of the earth, may be called the sun's +atmosphere. They consist of two very different parts -- first, the red +``prominences,'' which resemble tongues of flame ascending thousands +of miles above the sun's surface; and, second, the ``corona,'' which +extends to distances of millions of miles from the sun, and shines +with a soft, glowing light. The two combined, when well seen, make a +spectacle without parallel among the marvels of the sky. Although many +attempts have been made to render the corona visible when there is no +eclipse, all have failed, and it is to the moon alone that we owe its +revelation. To cover the sun's disk with a circular screen will not +answer the purpose because of the illumination of the air all about +the observer. When the moon hides the sun, on the other hand, the +sunlight is withdrawn from a great cylinder of air extending to the +top of the atmosphere and spreading many miles around the observer. +There is then no glare to interfere with the spectacle, and the corona +appears in all its surprising beauty. The prominences, however, +although they were discovered during an eclipse, can now, with the aid +of the spectroscope, be seen at any time. But the prominences are +rarely large enough to be noticed by the naked eye, while the +streamers of the corona, stretching far away in space, like ghostly +banners blown out from the black circle of the obscuring moon, attract +every eye, and to this weird apparition much of the fear inspired by +eclipses has been due. But if the corona has been a cause of terror in +the past it has become a source of growing knowledge in our time. + +The story of the first scientific observation of the corona and the +prominences is thrillingly interesting, and in fact dramatic. The +observation was made during the eclipse of 1842, which fortunately was +visible all over Central and Southern Europe so that scores of +astronomers saw it. The interest centers in what happened at Pavia in +Northern Italy, where the English astronomer Francis Baily had set up +his telescope. The eclipse had begun and Bailey was busy at his +telescope when, to quote his own words in the account which he wrote +for the Memoirs of the Royal Astronomical Society: + + I was astounded by a tremendous burst of applause from the streets + below, and at the same moment was electrified by the sight of one + of the most brilliant and splendid phenomena that can well be + imagined; for at that instant the dark body of the moon was + suddenly surrounded with a corona, or kind of bright glory, similar + in shape and magnitude to that which painters draw round the heads + of saints... + + Pavia contains many thousand inhabitants, the major part of whom + were at this early hour walking about the streets and squares or + looking out of windows in order to witness this long-talked-of + phenomenon; and when the total obscuration took place, which was + instantaneous, there was a universal shout from every observer + which ``made the welkin ring,'' and for the moment withdrew my + attention from the object with which I was immediately occupied. I + had, indeed, expected the appearance of a luminous circle round the + moon during the time of total obscurity; but I did not expect, from + any of the accounts of preceding eclipses that I had read, to + witness so magnificent an exhibition as that which took place... + + Splendid and astonishing, however, as this remarkable phenomenon + really was, and although it could not fail to call forth the + admiration and applause of every beholder, yet I must confess that + there was at the same time something in its singular and wonderful + appearance that was appalling... + + But the most remarkable circumstance attending the phenomenon was + the appearance of three large protuberances apparently emanating + from the circumference of the moon, but evidently forming a portion + of the corona. They had the appearance of mountains of a prodigious + elevation; their color was red tinged with lilac or purple; perhaps + the color of the peach-blossom would more nearly represent it. They + somewhat resembled the tops of the snowy Alpine mountains when + colored by the rising or the setting sun. They resembled the Alpine + mountains in another respect, inasmuch as their light was perfectly + steady, and had none of that flickering or sparkling motion so + visible in other parts of the corona... + + The whole of these protuberances were visible even to the last + moment of total obscuration, and when the first ray of light was + admitted from the sun they vanished, with the corona, altogether, + and daylight was instantly restored. + +I have quoted nearly all of this remarkable description not alone for +its intrinsic interest, but because it is the best depiction that can +be found of the general phenomena of a total solar eclipse. Still, not +every such eclipse offers an equally magnificent spectacle. The +eclipses of 1900 and 1905, for instance, which were seen by the +writer, the first in South Carolina and the second in Spain, fell far +short of that described by Bailey in splendor and impressiveness. Of +course, something must be allowed for the effect of surprise; Bailey +had not expected to see what was so suddenly disclosed to him. But +both in 1900 and 1905 the amount of scattered light in the sky was +sufficient in itself to make the corona appear faint, and there were +no very conspicuous prominences visible. Yet on both occasions there +was manifest among the spectators that mingling of admiration and awe +of which Bailey speaks. The South Carolinians gave a cheer and the +ladies waved their handkerchiefs when the corona, ineffably delicate +of form and texture, melted into sight and then in two minutes melted +away again. The Spaniards, crowded on the citadel hill of Burgos, with +their king and his royal retinue in their midst, broke out with a +great clapping of hands as the awaited spectacle unfolded itself in +the sky; and on both occasions, before the applause began, after an +awed silence a low murmur ran through the crowds. At Burgos it is said +many made the sign of the cross. + +It was not long before Bailey's idea that the prominences were a part +of the corona was abandoned, and it was perceived that the two +phenomena were to a great extent independent. At the eclipse of 1868, +which the astronomers, aroused by the wonderful scene of 1842, and +eager to test the powers of the newly invented spectroscope, flocked +to India to witness, Janssen conceived the idea of employing the +spectroscope to render the prominences visible when there was no +eclipse. He succeeded the very next day, and these phenomena have been +studied in that way ever since. + +There are recognized two kinds of prominences -- the ``erruptive'' and +the ``quiescent.'' The latter, which are cloud-like in form, may be +seen almost anywhere along the edge of the sun; but the former, which +often shoot up as if hurled from mighty volcanoes, appear to be +associated with sun-spots, and appear only above the zones where spots +abound. Either of them, when seen in projection against the brilliant +solar disk, appears white, not red, as against a background of sky. +The quiescent prominences, whose elevation is often from forty +thousand to sixty thousand miles, consist, as the spectroscope shows, +mainly of hydrogen and helium. The latter, it will be remembered, is +an element which was known to be in the sun many years before the +discovery that it also exists in small quantities on the earth. A fact +which may have a significance which we cannot at present see is that +the emanation from radium gradually and spontaneously changes into +helium, an alchemistical feat of nature that has opened many curious +vistas to speculative thinkers. The eruptive prominences, which do not +spread horizontally like the others, but ascend with marvelous +velocity to elevations of half a million miles or more, are apparently +composed largely of metallic vapors -- i.e. metals which are usually +solid on the earth, but which at solar temperatures are kept in a +volatilized state. The velocity of their ascent occasionally amounts +to three hundred or four hundred miles per second. It is known from +mathematical considerations that the gravitation of the sun would not +be able to bring back any body that started from its surface with a +velocity exceeding three hundred and eighty-three miles per second; so +it is evident that some of the matter hurled forth in eruptive +prominences may escape from solar control and go speeding out into +space, cooling and condensing into solid masses. There seems to be no +reason why some of the projectiles from the sun might not reach the +planets. Here, then, we have on a relatively small scale, explosions +recalling those which it has been imagined may be the originating +cause of some of the sudden phenomena of the stellar heavens. + +Of the sun-spots it is not our intention here specifically to speak, +but they evidently have an intimate connection with eruptive +prominences, as well as some relation, not yet fully understood, with +the corona. Of the real cause of sun-spots we know virtually nothing, +but recent studies by Professor Hale and others have revealed a +strange state of things in the clouds of metallic vapors floating +above them and their surroundings. Evidences of a cyclonic tendency +have been found, and Professor Hale has proved that sun-spots are +strong magnetic fields, and consist of columns of ionized vapors +rotating in opposite directions in the two hemispheres. A fact which +may have the greatest significance is that titanium and vanadium have +been found both in sun-spots and in the remarkable variable Mira Ceti, +a star which every eleven months, or thereabout, flames up with great +brilliancy and then sinks back to invisibility with the naked eye. It +has been suggested that sun-spots are indications of the beginning of +a process in the sun which will be intensified until it falls into the +state of such a star as Mira. Stars very far advanced in evolution, +without showing variability, also exhibit similar spectra; so that +there is much reason for regarding sunspots as emblems of advancing +age. + +The association of the corona with sun-spots is less evident than that +of the eruptive prominences; still such an association exists, for the +form and extent of the corona vary with the sun-spot period of which +we shall presently speak. The constitution of the corona remains to be +discovered. It is evidently in part gaseous, but it also probably +contains matter in the form of dust and small meteors. It includes one +substance altogether mysterious -- ``coronium.'' There are reasons for +thinking that this may be the lightest of all the elements, and +Professor Young, its discoverer, said that it was ``absolutely unique +in nature; utterly distinct from any other known form of matter, +terrestial, solar, or cosmical.'' The enormous extent of the corona is +one of its riddles. Since the development of the curious subject of +the ``pressure of light'' it has been proposed to account for the +sustentation of the corona by supposing that it is borne upon the +billows of light continually poured out from the sun. Experiment has +proved, what mathematical considerations had previously pointed out as +probable, that the waves of light exert a pressure or driving force, +which becomes evident in its effects if the body acted upon is +sufficiently small. In that case the light pressure will prevail over +the attraction of gravitation, and propel the attenuated matter away +from the sun in the teeth of its attraction. The earth itself would be +driven away if, instead of consisting of a solid globe of immense +aggregate mass, it were a cloud of microscopic particles. The reason +is that the pressure varies in proportion to the surface of the body +acted upon, while the gravitational attraction is proportional to the +volume, or the total amount of matter in the body. But the surface of +any body depends upon the square of its diameter, while the volume +depends upon the cube of the diameter. If, for instance, the diameter +is represented by 4, the surface will be proportional to 4 x 4, or 16, +and the volume to 4 x 4 x 4, or 64; but if the diameter is taken as 2, +the surface will be 2 x 2, or 4, and the volume 2 x 2 x 2, or 8. Now, +the ratio of 4 to 8 is twice as great as that of 16 to 64. If the +diameter is still further decreased, the ratio of the surface to the +volume will proportionally grow larger; in other words, the pressure +will gain upon the attraction, and whatever their original ratio may +have been, a time will come, if the diminution of size continues, when +the pressure will become more effective than the attraction, and the +body will be driven away. Supposing the particles of the corona to be +below the critical size for the attraction of a mass like that of the +sun to control them, they would be driven off into the surrounding +space and appear around the sun like the clouds of dust around a mill. +We shall return to this subject in connection with the Zodiacal Light, +the Aurora, and Comets. + +On the other hand, there are parts of the corona which suggest by +their forms the play of electric or magnetic forces. This is +beautifully shown in some of the photographs that have been made of +the corona during recent eclipses. Take, for instance, that of the +eclipse of 1900. The sheaves of light emanating from the poles look +precisely like the ``lines of force'' surrounding the poles of a +magnet. It will be noticed in this photograph that the corona appears +to consist of two portions: one comprising the polar rays just spoken +of, and the other consisting of the broader, longer, and less-defined +masses of light extending out from the equatorial and middle-latitude +zones. Yet even in this more diffuse part of the phenomenon one can +detect the presence of submerged curves bearing more or less +resemblance to those about the poles. Just what part electricity or +electro-magnetism plays in the mechanism of the solar radiation it is +impossible to say, but on the assumption that it is a very important +part is based the hypothesis that there exists a direct solar +influence not only upon the magnetism, but upon the weather of the +earth. This hypothesis has been under discussion for half a century, +and still we do not know just how much truth it represents. It is +certain that the outbreak of great disturbances on the sun, +accompanied by the formation of sun-spots and the upshooting of +eruptive prominences (phenomena which we should naturally expect to be +attended by action), have been instantly followed by corresponding +``magnetic storms'' on the earth and brilliant displays of the auroral +lights. There have been occasions when the influence has manifested +itself in the most startling ways, a great solar outburst being +followed by a mysterious gripping of the cable and telegraph systems +of the world, as if an invisible and irresistible hand had seized +them. Messages are abruptly cut off, sparks leap from the telegraph +instruments, and the entire earth seems to have been thrown into a +magnetic flurry. These occurrences affect the mind with a deep +impression of the dependence of our planet on the sun, such as we do +not derive from the more familiar action of the sunlight on the growth +of plants and other phenomena of life depending on solar influences. + +Perhaps the theory of solar magnetic influence upon the weather is +best known in connection with the ``sun-spot cycle.'' This, at any +rate, is, as already remarked, closely associated with the corona. Its +existence was discovered in 1843 by the German astronomer Schwabe. It +is a period of variable length, averaging about eleven years, during +which the number of spots visible on the sun first increases to a +maximum, then diminishes to a minimum, and finally increases again to +a maximum. For unknown reasons the period is sometimes two or three +years longer than the average and sometimes as much shorter. +Nevertheless, the phenomena always recur in the same order. Starting, +for instance, with a time when the observer can find few or no spots, +they gradually increase in number and size until a maximum, in both +senses, is reached, during which the spots are often of enormous size +and exceedingly active. After two or three years they begin to +diminish in number, magnitude, and activity until they almost or quite +disappear. A strange fact is that when a new period opens, the spots +appear first in high northern and southern latitudes, far from the +solar equator, and as the period advances they not only increase in +number and size, but break out nearer and nearer to the equator, the +last spots of a vanishing period sometimes lingering in the equatorial +region after the advance-guard of its successor has made its +appearance in the high latitudes. Spots are never seen on the equator +nor near the poles. It was not very long after the discovery of the +sun-spot cycle that the curious observation was made that a striking +coincidence existed between the period of the sun-spots and another +period affecting the general magnetic condition of the earth. When a +curved line representing the varying number of sun-spots was compared +with another curve showing the variations in the magnetic state of the +earth the two were seen to be in almost exact accord, a rise in one +curve corresponding to a rise in the other, and a fall to a fall. +Continued observation has proved that this is a real coincidence and +not an accidental one, so that the connection, although as yet +unexplained, is accepted as established. But does the influence extend +further, and directly affect the weather and the seasons as well as +the magnetic elements of the earth? A final answer to this question +cannot yet be given, for the evidence is contradictory, and the +interpretations put upon it depend largely on the predilections of the +judges. + +But, in a broad sense, the sun-spots and the phenomena connected with +them must have a relation to terrestial meteorology, for they prove +the sun to be a variable star. Reference was made, a few lines above, +to the resemblance of the spectra of sun-spots to those of certain +stars which seem to be failing through age. This in itself is +extremely suggestive; but if this resemblance had never been +discovered, we should have been justified in regarding the sun as +variable in its output of energy; and not only variable, but probably +increasingly so. The very inequalities in the sun-spot cycle are +suspicious. When the sun is most spotted its total light may be +reduced by one-thousandth part, although it is by no means certain +that its outgiving of thermal radiations is then reduced. A loss of +one-thousandth of its luminosity would correspond to a decrease of +.0025 of a stellar magnitude, considering the sun as a star viewed +from distant space. So slight a change would not be perceptible; but +it is not alone sun-spots which obscure the solar surface, its entire +globe is enveloped with an obscuring veil. When studied with a +powerful telescope the sun's surface is seen to be thickly mottled +with relatively obscure specks, so numerous that it has been estimated +that they cut off from one-tenth to one-twentieth of the light that we +should receive from it if the whole surface were as brilliant as its +brightest parts. The condition of other stars warrants the conclusion +that this obscuring envelope is the product of a process of +refrigeration which will gradually make the sun more and more variable +until its history ends in extinction. Looking backward, we see a time +when the sun must have been more brilliant than it is now. At that +time it probably shone with the blinding white splendor of such stars +as Sirius, Spica, and Vega; now it resembles the relatively dull +Procyon; in time it will turn ruddy and fall into the closing cycle +represented by Antares. Considering that once it must have been more +radiantly powerful than at present, one is tempted to wonder if that +could have been the time when tropical life flourished within the +earth's polar circles, sustained by a vivific energy in the sun which +it has now lost. + +The corona, as we have said, varies with the sun-spot cycle. When the +spots are abundant and active the corona rises strong above the +spotted zones, forming immense beams or streamers, which on one +occasion, at least, had an observed length of ten million miles. At +the time of a spot minimum the corona is less brilliant and has a +different outline. It is then that the curved polar rays are most +conspicuous. Thus the vast banners of the sun, shaken out in the +eclipse, are signals to tell of its varying state, but it will +probably be long before we can read correctly their messages. + +The Zodiacal Light Mystery + +There is a singular phenomenon in the sky -- one of the most puzzling +of all -- which has long arrested the attention of astronomers, +defying their efforts at explanation, but which probably not one in a +hundred, and possibly not one in a thousand, of the readers of this +book has ever seen. Yet its name is often spoken, and it is a +conspicuous object if one knows when and where to look for it, and +when well seen it exhibits a mystical beauty which at the same time +charms and awes the beholder. It is called ``The Zodiacal Light,'' +because it lies within the broad circle of the Zodiac, marking the +sun's apparent annual path through the stars. What it is nobody has +yet been able to find out with certainty, and books on astronomy +usually speak of it with singular reserve. But it has given rise to +many remarkable theories, and a true explanation of it would probably +throw light on a great many other celestial mysteries. The Milky Way +is a more wonderful object to look upon, but its nature can be +comprehended, while there is a sort of uncanniness about the Zodiacal +Light which immediately impresses one upon seeing it, for its part in +the great scheme of extra-terrestrial affairs is not evident. + +If you are out-of-doors soon after sunset -- say, on an evening late +in the month of February -- you may perceive, just after the angry +flush of the dying winter's day has faded from the sky, a pale ghostly +presence rising above the place where the sun went down. The writer +remembers from boyhood the first time it was pointed out to him and +the unearthly impression that it made, so that he afterward avoided +being out alone at night, fearful of seeing the spectral thing again. +The phenomenon brightens slowly with the fading of the twilight, and +soon distinctly assumes the shape of an elongated pyramid of pearly +light, leaning toward the south if the place of observation is in the +northern hemisphere. It does not impress the observer at all in the +same manner as the Milky Way; that looks far off and is clearly among +the stars, but the Zodiacal Light seems closer at hand, as if it were +something more intimately concerning the earth. To all it immediately +suggests a connection, also, with the sunken sun. If the night is +clear and the moon absent (and if you are in the country, for city +lights ruin the spectacles of the sky), you will be able to watch the +apparition for a long time. You will observe that the light is +brightest near the horizon, gradually fading as the pyramidal beam +mounts higher, but in favorable circumstances it may be traced nearly +to the meridian south of the zenith, where its apex at last vanishes +in the starlight. It continues visible during the evenings of March +and part of April, after which, ordinarily, it is seen no more, or if +seen is relatively faint and unimpressive. But when autumn comes it +appears again, this time not like a wraith hovering above the westward +tomb of the day-god, but rather like a spirit of the morning +announcing his reincarnation in the east. + +The reason why the Zodiacal Light is best seen in our latitudes at the +periods just mentioned is because at those times the Zodiac is more +nearly perpendicular to the horizon, first in the west and then in the +east; and, since the phenomenon is confined within the borders of the +Zodiac, it cannot be favorably placed for observation when the +zodiacal plane is but slightly inclined to the horizon. Its faint +light requires the contrast of a background of dark sky in order to be +readily perceptible. But within the tropics, where the Zodiac is +always at a favorable angle, the mysterious light is more constantly +visible. Nearly all observant travelers in the equatorial regions have +taken particular note of this phenomenon, for being so much more +conspicuous there than in the temperate zones it at once catches the +eye and holds the attention as a novelty. Humboldt mentions it many +times in his works, for his genius was always attracted by things out +of the ordinary and difficult of explanation, and he made many careful +observations on its shape, its brilliancy, and its variations; for +there can be no doubt that it does vary, and sometimes to an +astonishing degree. It is said that it once remained practically +invisible in Europe for several years in succession. During a trip to +South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a +remarkable difference between the appearance of the Zodiacal Light on +his going and coming voyages. In fact, when crossing the equator going +south he did not see it at all; but on returning he had, on March 6th, +when one degree south of the equator, a memorable view of it. + + It was a bright, clear night, and the Zodiacal Light was + extraordinarily brilliant -- brighter than he had ever seen it + before. The Milky Way was not to be compared with it. The brightest + part extended 75 deg. from the sun. There was a faint and much narrower + extension which they could just make out beyond the Pleiades along + the ecliptic, but the greater part of the Zodiacal Light showed as + a broad truncated column, and it did not appear nearly as conical + as he had before seen it. + +When out of the brief twilight of intertropical lands, where the sun +drops vertically to the horizon and night rushes on like a wave of +darkness, the Zodiacal Light shoots to the very zenith, its color is +described as a golden tint, entirely different from the silvery sheen +of the Milky Way. If I may venture again to refer to personal +experiences and impressions, I will recall a view of the Zodiacal +Light from the summit of the cone of Mt Etna in the autumn of the year +1896 (more briefly described in Astronomy with the Naked Eye). There +are few lofty mountains so favorably placed as Etna for observations +of this kind. It was once resorted to by Prof. George E. Hale, in an +attempt to see the solar corona without an eclipse. Rising directly +from sea-level to an elevation of nearly eleven thousand feet, the +observer on its summit at night finds himself, as it were, lost in the +midst of the sky. But for the black flanks of the great cone on which +he stands he might fancy himself to be in a balloon. On the occasion +to which I refer the world beneath was virtually invisible in the +moonless night. The blaze of the constellations overhead was +astonishingly brilliant, yet amid all their magnificence my attention +was immediately drawn to a great tapering light that sprang from the +place on the horizon where the sun would rise later, and that seemed +to be blown out over the stars like a long, luminous veil. It was the +finest view of the Zodiacal light that I had ever enjoyed -- thrilling +in its strangeness -- but I was almost disheartened by the +indifference of my guide, to whom it was only a light and nothing +more. If he had no science, he had less poetry -- rather a remarkable +thing, I thought, for a child of his clime. The Light appeared to me +to be distinctly brighter than the visible part of the Milky Way which +included the brilliant stretches in Auriga and Perseus, and its color, +if one may speak of color in connection with such an object, seemed +richer than that of the galactic band; but I did not think of it as +yellow, although Humboldt has described it as resembling a golden +curtain drawn over the stars, and Du Chaillu in Equatorial Africa +found it of a bright yellow color. It may vary in color as in +conspicuousness. The fascination of that extraordinary sight has never +faded from my memory. I turned to regard it again and again, although +I had never seen the stellar heavens so brilliant, and it was one of +the last things I looked for when the morning glow began softly to +mount in the east, and Sicily and the Mediterranean slowly emerged +from the profound shadow beneath us. + +The Zodiacal Light seems never to have attracted from astronomers in +general the amount of careful attention that it deserves; perhaps +because so little can really be made of it as far as explanation is +concerned. I have referred to the restraint that scientific writers +apparently feel in speaking of it. The grounds for speculation that it +affords may be too scanty to lead to long discussions, yet it piques +curiosity, and as we shall see in a moment has finally led to a most +interesting theory. Once it was the subject of an elaborate series of +studies which carried the observer all round the world. That was in +1845--46, during the United States Exploring Expedition that visited +the then little known Japan. The chaplain of the fleet, the Rev. Mr +Jones, went out prepared to study the mysterious light in all its +phases. He saw it from many latitudes on both sides of the equator, +and the imagination cannot but follow him with keen interest in his +world-circling tour, keeping his eyes every night fixed upon the +phantasm overhead, whose position shifted with that of the hidden sun. +He demonstrated that the flow extends at times completely across the +celestial dome, although it is relatively faint directly behind the +earth. On his return the government published a large volume of his +observations, in which he undertook to show that the phenomenon was +due to the reflection of sunlight from a ring of meteoric bodies +encircling the earth. But, after all, this elaborate investigation +settled nothing. + +Prof. E. E. Barnard has more recently devoted much attention to the +Zodiacal Light, as well as to a strange attendant phenomenon called +the ``Gegenschein,'' or Counterglow, because it always appears at that +point in the sky which is exactly opposite the sun. The Gegenschein is +an extremely elusive phenomenon, suitable only for eyes that have been +specially trained to see it. Professor Newcomb has cautiously remarked +that + + it is said that in that point of the heavens directly opposite the + sun there is an elliptical patch of light... This phenomenon is so + difficult to account for that its existence is sometimes doubted; + yet the testimony in its favor is difficult to set aside. + +It certainly cannot be set aside at all since the observations of +Barnard. I recall an attempt to see it under his guidance during a +visit to Mount Hamilton, when he was occupied there with the Lick +telescope. Of course, both the Gegenschein and the Zodiacal Light are +too diffuse to be studied with telescopes, which, so to speak, magnify +them out of existence. They can only be successfully studied with the +naked eye, since every faintest glimmer that they afford must be +utilized. This is especially true of the Gegenschein. At Mount +Hamilton, Mr Barnard pointed out to me its location with reference to +certain stars, but with all my gazing I could not be sure that I saw +it. To him, on the contrary, it was obvious; he had studied it for +months, and was able to indicate its shape, its boundaries, its +diameter, and the declination of its center with regard to the +ecliptic. There is not, of course, the shadow of a doubt of the +existence of the Gegenschein, and yet I question if one person in a +million has ever seen or ever will see it. The Zodiacal Light, on the +other hand, is plain enough, provided that the time and the +circumstances of the observation are properly chosen. + +In the attempts to explain the Zodiacal Light, the favorite hypothesis +has been that it is an appendage of the sun -- perhaps simply an +extension of the corona in the plane of the ecliptic, which is not +very far from coinciding with that of the sun's equator. This idea is +quite a natural one, because of the evident relation of the light to +the position of the sun. The vast extension of the equatorial wings of +the corona in 1878 gave apparent support to this hypothesis; if the +substance of the corona could extend ten million miles from the sun, +why might it not extend even one hundred million, gradually fading out +beyond the orbit of the earth? A variation of this hypothesis assumes +that the reflection is due to swarms of meteors circling about the +sun, in the plane of its equator, all the way from its immediate +neighborhood to a distance exceeding that of the earth. But in neither +form is the hypothesis satisfactory; there is nothing in the +appearance of the corona to indicate that it extends even as far as +the planet Mercury, while as to meteors, the orbits of the known +swarms do not accord with the hypothesis, and we have no reason to +believe that clouds of others exist traveling in the part of space +where they would have to be in order to answer the requirements of the +theory. The extension of the corona in 1878 did not resemble in its +texture the Zodiacal Light. + +Now, it has so often happened in the history of science that an +important discovery in one branch has thrown unexpected but most +welcome light upon some pending problem in some other branch, that a +strong argument might be based upon that fact alone against the too +exclusive devotion of many investigators to the narrow lines of their +own particular specialty; and the Zodiacal Light affords a case in +point, when it is considered in connection with recent discoveries in +chemistry and physics. From the fact that atoms are compound bodies +made up of corpuscles at least a thousand times smaller than the +smallest known atom -- a fact which astounded most men of science when +it was announced a few years ago -- a new hypothesis has been +developed concerning the nature of the Zodiacal Light (as well as +other astronomical riddles), and this hypothesis comes not from an +astronomer, but from a chemist and physicist, the Swede, Svante +Arrhenius. In considering an outline of this new hypothesis we need +neither accept nor reject it; it is a case rather for suspension of +judgment. + +To begin with, it carries us back to the ``pressure of light'' +mentioned in the preceding chapter. The manner in which this pressure +is believed generally to act was there sufficiently explained, and it +only remains to see how it is theoretically extended to the particles +of matter supposed to constitute the Zodiacal Light. We know that +corpuscles, or ``fragments of atoms'' negatively electrified, are +discharged from hot bodies. Streams of these ``ions'' pour from many +flames and from molten metals; and the impact of the cathode and +ultra-violet rays causes them to gush even from cold bodies. In the +vast laboratory of the sun it is but reasonable to suppose that +similar processes are taking place. ``As a very hot metal emits these +corpuscles,'' says Prof. J. J. Thomson, ``it does not seem an +improbable hypothesis that they are emitted by that very hot body, the +sun.'' Let it be assumed, then, that the sun does emit them; what +happens next? Negatively charged corpuscles, it is known, serve as +nuclei to which particles of matter in the ordinary state are +attracted, and it is probable that those emitted from the sun +immediately pick up loads in this manner and so grow in bulk. If they +grow large enough the gravitation of the sun draws them back, and they +produce a negative charge in the solar atmosphere. But it is probable +that many of the particles do not attain the critical size which, +according to the principles before explained, would enable the +gravitation of the sun to retain them in opposition to the pressure of +the waves of light, and with these particles the light pressure is +dominant. Clouds of them may be supposed to be continually swept away +from the sun into surrounding space, moving mostly in or near the +plane of the solar equator, where the greatest activity, as indicated +by sunspots and related phenomena, is taking place. As they pass +outward into space many of them encounter the earth. If the earth, +like the moon, had no atmosphere the particles would impinge directly +on its surface, giving it a negative electric charge. But the presence +of the atmosphere changes all that, for the first of the flying +particles that encounter it impart to it their negative electricity, +and then, since like electric charges repel like, the storm of +particles following will be sheered off from the earth, and will +stream around it in a maze of hyperbolic paths. Those that continue on +into space beyond the earth may be expected to continue picking up +wandering particles of matter until their bulk has become so great +that the solar attraction prevails again over the light pressure +acting upon them, and they turn again sunward. Passing the earth on +their return they will increase the amount of dust-clouds careering +round it; and these will be further increased by the action of the +ultra-violet rays of the sunlight causing particles to shoot radially +away from the earth when the negative charge of the upper atmosphere +has reached a certain amount, which particles, although starting +sunward, will be swept back to the earth with the oncoming streams. As +the final result of all this accumulation of flying and gyrating +particles in the earth's neighborhood, we are told that the latter +must be transformed into the semblance of a gigantic solid-headed +comet provided with streaming tails, the longest of them stretching +away from the direction of the sun, while another shorter one extends +toward the sun. This shorter tail is due to the particles that we have +just spoken of as being driven sunward from the earth by the action of +ultra-violet light. No doubt this whole subject is too technical for +popular statement; but at any rate the general reader can understand +the picturesque side of the theory, for its advocates assure us that +if we were on the moon we would doubtless be able to see the +comet-like tails of the earth, and then we could appreciate the part +that they play in producing the phenomenon of the Zodiacal Light. + +That the Light as we see it could be produced by the reflection of +sunlight from swarms of particles careering round the earth in the +manner supposed by Arrhenius' hypothesis is evident enough; and it +will be observed that the new theory, after all, is only another +variant of the older one which attributes the Zodiacal Light to an +extension of the solar corona. But it differs from the older theory in +offering an explanation of the manner in which the extension is +effected, and it differentiates between the corona proper and the +streams of negative particles shot away from the sun. In its details +the hypothesis of Arrhenius also affords an explanation of many +peculiarities of the Zodiacal Light, such as that it is confined to +the neighborhood of the ecliptic, and that it is stronger on the side +of the earth which is just turning away from a position under the sun +than on the other side; but it would carry us beyond our limits to go +into these particulars. The Gegenschein, according to this theory, is +a part of the same phenomenon as the Zodiacal Light, for by the laws +of perspective it is evident that the reflection from the streams of +particles situated at a point directly opposite to the sun would be at +a maximum, and this is the place which the Gegenschein occupies. Apart +from its geometrical relations to the position of the sun, the +variability of the Zodiacal Light appears to affirm its solar +dependence, and this too would be accounted for by Arrhenius' +hypothesis better than by the old theory of coronal extension. The +amount of corpuscular discharge from the sun must naturally be +governed by the state of relative activity or inactivity of the +latter, and this could not but be reflected in the varying splendor of +the Zodiacal Light. But much more extended study than has yet been +given to the subject will be required before we can feel that we know +with reasonable certainty what this mysterious phenomenon really is. +By the hypothesis of Arrhenius every planet that has an atmosphere +must have a Zodiacal Light attending it, but the phenomenon is too +faint for us to be able to see it in the case, for instance, of Venus, +whose atmosphere is very abundant. The moon has no corresponding +``comet's tail'' because, as already explained, of the lack of a lunar +atmosphere to repel the streams by becoming itself electrified; but if +there were a lunar Zodiacal Light, no doubt we could see it because of +the relative nearness of our satellite. + +Marvels of the Aurora + +One of the most vivid recollections of my early boyhood is that of +seeing my father return hastily into the house one evening and call +out to the family: ``Come outside and look at the sky!'' Ours was a +country house situated on a commanding site, and as we all emerged +from the doorway we were dumbfounded to see the heavens filled with +pale flames which ran licking and quivering over the stars. Instantly +there sprang into my terrified mind the recollection of an awful +description of ``the Day of Judgment'' (the Dies Irae), which I had +heard with much perturbation of spirit in the Dutch Reformed church +from the lips of a tall, dark-browed, dreadfully-in-earnest preacher +of the old-fashioned type. My heart literally sank at sight of the +spectacle, for it recalled the preacher's very words; it was just as +he had said it would be, and it needed the assured bearing of my +elders finally to convince me that + + That Day of Wrath, O dreadful day, + When Heaven and Earth shall pass away, + As David and the Sibyl say + +had not actually come upon us. And even the older members of the +household were not untouched with misgivings when menacing spots of +crimson appeared, breaking out now here, now there, in the shuddering +sky. Toward the north the spectacle was appalling. A huge arch spanned +an unnaturally dark segment resting on the horizon, and above this +arch sprang up beams and streamers in a state of incessant agitation, +sometimes shooting up to the zenith with a velocity that took one's +breath, and sometimes suddenly falling into long ranks, and marching, +marching, marching, like an endless phalanx of fiery specters, and +moving, as I remember, always from east to west. The absolute silence +with which these mysterious evolutions were performed and the +quavering reflections which were thrown upon the ground increased the +awfulness of the exhibition. Occasionally enormous curtains of lambent +flame rolled and unrolled with a majestic motion, or were shaken to +and fro as if by a mighty, noiseless wind. At times, too, a sudden +billowing rush would be made toward the zenith, and for a minute the +sky overhead would glow so brightly that the stars seemed to have been +consumed. The spectacle continued with varying intensity for hours. + +This exhibition occurred in Central New York, a latitude in which the +Aurora Borealis is seldom seen with so much splendor. I remember +another similar one seen from the city of New York in November, 1882. +On this last occasion some observers saw a great upright beam of light +which majestically moved across the heavens, stalking like an +apparition in the midst of the auroral pageant, of whose general +movements it seemed to be independent, maintaining always its upright +posture, and following a magnetic parallel from east to west. This +mysterious beam was seen by no less than twenty-six observers in +different parts of the country, and a comparison of their observations +led to a curious calculation indicating that the apparition was about +one hundred and thirty-three miles tall and moved at the speed of ten +miles per second! + +But, as everybody knows, it is in the Arctic regions that the Aurora, +or the ``Northern Lights,'' can best be seen. There, in the long polar +night, when for months together the sun does not rise, the strange +coruscations in the sky often afford a kind of spectral daylight in +unison with the weird scenery of the world of ice. The pages in the +narratives of Arctic exploration that are devoted to descriptions of +the wonderful effects of the Northern Lights are second to none that +man has ever penned in their fascination. The lights, as I have +already intimated, display astonishing colors, particularly shades of +red and green, as they flit from place to place in the sky. The +discovery that the magnetic needle is affected by the Aurora, +quivering and darting about in a state of extraordinary excitement +when the lights are playing in the sky, only added to the mystery of +the phenomenon until its electro-magnetic nature had been established. +This became evident as soon as it was known that the focus of the +displays was the magnetic pole; and when the far South was visited the +Aurora Australis was found, having its center at the South Magnetic +Pole. Then, if not before, it was clear that the earth was a great +globular magnet, having its poles of opposite magnetism, and that the +auroral lights, whatever their precise cause might be, were +manifestations of the magnetic activity of our planet. After the +invention of magnetic telegraphy it was found that whenever a great +Aurora occurred the telegraph lines were interrupted in their +operation, and the ocean cables ceased to work. Such a phenomenon is +called a ``magnetic storm.'' + +The interest excited by the Aurora in scientific circles was greatly +stimulated when, in the last half of the nineteenth century, it was +discovered that it is a phenomenon intimately associated with +disturbances on the sun. The ancient ``Zurich Chronicles,'' extending +from the year 1000 to the year 1800, in which both sun-spots visible +to the naked eye and great displays of the auroral lights were +recorded, first set Rudolf Wolf on the track of this discovery. The +first notable proof of the suspected connection was furnished with +dramatic emphasis by an occurrence which happened on September 1, +1859. Near noon on that day two intensely brilliant points suddenly +broke out in a group of sun-spots which were under observation by Mr +R. C. Carrington at his observatory at Redhill, England. The points +remained visible for not more than five minutes, during which interval +they moved thirty-five thousand miles across the solar disk. Mr R. +Hodgson happened to see the same phenomenon at his observatory at +Highgate, and thus all possibility of deception was removed. But +neither of the startled observers could have anticipated what was to +follow, and, indeed, it was an occurrence which has never been +precisely duplicated. I quote the eloquent account given by Miss +Clerke in her History of Astronomy During the Nineteenth Century. + + This unique phenomenon seemed as if specially designed to + accentuate the inference of a sympathetic relation between the + earth and the sun. From August 28 to September 4, 1859, a magnetic + storm of unparalleled intensity, extent, and duration was in + progress over the entire globe. Telegraphic communication was + everywhere interrupted -- except, indeed, that it was in some cases + found practicable to work the lines without batteries by the agency + of the earth-currents alone; sparks issued from the wires; gorgeous + auroras draped the skies in solemn crimson over both hemispheres, + and even in the tropics; the magnetic needle lost all trace of + continuity in its movements and darted to and fro as if stricken + with inexplicable panic. The coincidence was even closer. At the + very instant of the solar outburst witnessed by Carrington and + Hodgson the photographic apparatus at Kew registered a marked + disturbance of all the three magnetic elements; while shortly after + the ensuing midnight the electric agitation culminated, thrilling + the whole earth with subtle vibrations, and lighting up the + atmosphere from pole to pole with coruscating splendors which + perhaps dimly recall the times when our ancient planet itself shone + as a star. + +If this amazing occurrence stood alone, and as I have already said it +has never been exactly duplicated, doubt might be felt concerning some +of the inferences drawn from it; but in varying forms it has been +repeated many times, so that now hardly anyone questions the reality +of the assumed connection between solar outbursts and magnetic storms +accompanied by auroral displays on the earth. It is true that the late +Lord Kelvin raised difficulties in the way of the hypothesis of a +direct magnetic action of the sun upon the earth, because it seemed to +him that an inadmissible quantity of energy was demanded to account +for such action. But no calculation like that which he made is final, +since all calculations depend upon the validity of the data; and no +authority is unshakable in science, because no man can possess +omniscience. It was Lord Kelvin who, but a few years before the thing +was actually accomplished, declared that aerial navigation was an +impracticable dream, and demonstrated its impracticability by +calculation. However the connection may be brought about, it is as +certain as evidence can make it that solar outbursts are coincident +with terrestial magnetic disturbances, and coincident in such a way as +to make the inference of a causal connection irresistible. The sun is +only a little more than a hundred times its own diameter away from the +earth. Why, then, with the subtle connection between them afforded by +the ether which conveys to us the blinding solar light and the +life-sustaining solar heat, should it be so difficult to believe that +the sun's enormous electric energies find a way to us also? No doubt +the impulse coming from the sun acts upon the earth after the manner +of a touch upon a trigger, releasing energies which are already stored +up in our planet. + +But besides the evidence afforded by such occurrences as have been +related of an intimate connection between solar outbreaks and +terrestial magnetic flurries, attended by magnificent auroral +displays, there is another line of proof pointing in the same +direction. Thus, it is known that the sun-spot period, as remarked in +a preceding chapter, coincides in a most remarkable manner with the +periodic fluctuations in the magnetic state of the earth. This +coincidence runs into the most astonishing details. For instance, when +the sun-spot period shortens, the auroral period shortens to precisely +the same extent; as the short sun-spot periods usually bring the most +intense outbreaks of solar activity, so the corresponding short +auroral periods are attended by the most violent magnetic storms; a +secular period of about two hundred and twenty-two years affecting +sun-spots is said to have its auroral duplicate; a shorter period of +fifty-five and a half years, which some observers believe that they +have discovered appears also to be common to the two phenomena; and +yet another ``superposed'' period of about thirty-five years, which +some investigators aver exists, affects sun-spots and aurora alike. In +short, the coincidences are so numerous and significant that one would +have to throw the doctrine of probability to the winds in order to be +able to reject the conclusion to which they so plainly lead. + +But still the question recurs: How is the influence transmitted? Here +Arrhenius comes once more with his hypothesis of negative corpuscles, +or ions, driven away from the sun by light-pressure -- a hypothesis +which seems to explain so many things -- and offers it also as an +explanation of the way in which the sun creates the Aurora. He would +give the Aurora the same lineage with the Zodiacal Light. To +understand the application of this theory we must first recall the +fact that the earth is a great magnet having its two opposite poles of +magnetism, one near the Arctic and the other near the Antarctic +Circle. Like all magnets, the earth is surrounded with ``lines of +force,'' which, after the manner of the curved rays we saw in the +photograph of a solar eclipse, start from a pole, rising at first +nearly vertically, then bend gradually over, passing high above the +equator, and finally descending in converging sheaves to the opposite +pole. Now the axis of the earth is so placed in space that it lies at +nearly a right angle to the direction of the sun, and as the streams +of negatively charged particles come pouring on from the sun (see the +last preceding chapter), they arrive in the greatest numbers over the +earth's equatorial regions. There they encounter the lines of magnetic +force at the place where the latter have their greatest elevation +above the earth, and where their direction is horizontal to the +earth's surface. Obeying a law which has been demonstrated in the +laboratory, the particles then follow the lines of force toward the +poles. While they are above the equatorial regions they do not become +luminescent, because at the great elevation that they there occupy +there is virtually no atmosphere; but as they pass on toward the north +and the south they begin to descend with the lines of force, curving +down to meet at the poles; and, encountering a part of the atmosphere +comparable in density with what remains in an exhausted Crookes tube, +they produce a glow of cathode rays. This glow is conceived to +represent the Aurora, which may consequently be likened to a gigantic +exhibition of vacuum-tube lights. Anybody who recalls his student days +in the college laboratory and who has witnessed a display of Northern +Lights will at once recognize the resemblance between them in colors, +forms, and behavior. This resemblance had often been noted before +Arrhenius elaborated his hypothesis. + +Without intending to treat his interesting theory as more than a +possibly correct explanation of the phenomena of the Aurora, we may +call attention to some apparently confirmatory facts. One of the most +striking of these relates to a seasonal variation in the average +number of aurorae. It has been observed that there are more in March +and September than at any other time of the year, and fewer in June +and December; moreover (and this is a delicate test as applied to the +theory), they are slightly rarer in June than in December. Now all +these facts seem to find a ready explanation in the hypothesis of +Arrhenius, thus: (1) The particles issuing from the sun are supposed +to come principally from the regions whose excitement is indicated by +the presence of sun-spots (which accords with Hale's observation that +sun-spots are columns of ionized vapors), and these regions have a +definite location on either side of the solar equator, seldom +approaching it nearer than within 5 deg. or 10 deg. north or south, and never +extending much beyond 35 deg. toward either pole; (2) The equator of the +sun is inclined about 7 deg. to the plane of the earth's orbit, from which +it results that twice in a year -- viz., in June and December -- the +earth is directly over the solar equator, and twice a year -- viz., in +March and September -- when it is farthest north or south of the solar +equator, it is over the inner edge of the sun-spot belts. Since the +corpuscles must be supposed to be propelled radially from the sun, few +will reach the earth when the latter is over the solar equator in June +and December, but when it is over, or nearly over, the spot belts, in +March and September, it will be in the line of fire of the more active +parts of the solar surface, and relatively rich streams of particles +will reach it. This, as will be seen from what has been said above, is +in strict accord with the observed variations in the frequency of +aurorae. Even the fact that somewhat fewer aurorae are seen in June than +in December also finds its explanation in the known fact that the +earth is about three million miles nearer the sun in the winter than +in the summer, and the number of particles reaching it will vary, like +the intensity of light, inversely as the square of the distance. These +coincidences are certainly very striking, and they have a cumulative +force. If we accept the theory, it would appear that we ought to +congratulate ourselves that the inclination of the sun's equator is so +slight, for as things stand the earth is never directly over the most +active regions of the sun-spots, and consequently never suffers from +the maximum bombardment of charged particles of which the sun is +capable. Incessant auroral displays, with their undulating draperies, +flitting colors, and marching columns might not be objectionable from +the point of view of picturesqueness, but one magnetic storm of +extreme intensity following closely upon the heels of another, for +months on end, crazing the magnetic needle and continually putting the +telegraph and cable lines out of commission, to say nothing of their +effect upon ``wireless telegraphy'', would hardly add to the charms of +terrestrial existence. + +One or two other curious points in connection with Arrhenius' +hypothesis may be mentioned. First, the number of aurorae, according to +his explanation, ought to be greatest in the daytime, when the face of +the earth on the sunward side is directly exposed to the atomic +bombardment. Of course visual observation can give us no information +about this, since the light of the Aurora is never sufficiently +intense to be visible in the presence of daylight, but the records of +the magnetic observatories can be, and have been, appealed to for +information, and they indicate that the facts actually accord with the +theory. Behind the veil of sunlight in the middle of the afternoon, +there is good reason to believe, auroral exhibitions often take place +which would eclipse in magnificence those seen at night if we could +behold them. Observation shows, too, that aurorae are more frequent +before than after midnight, which is just what we should expect if +they originate in the way that Arrhenius supposes. Second, the theory +offers an explanation of the alleged fact that the formation of clouds +in the upper air is more frequent in years when aurorae are most +abundant, because clouds are the result of the condensation of +moisture upon floating particles in the atmosphere (in an absolutely +dustless atmosphere there would be no clouds), and it has been proved +that negative ions like those supposed to come from the sun play a +master part in the phenomena of cloud formation. + +Yet another singular fact, almost mystical in its suggestions, may be +mentioned. It seems that the dance of the auroral lights occurs most +frequently during the absence of the moon from the hemisphere in which +they appear, and that they flee, in greater part, to the opposite +hemisphere when the moon's revolution in an orbit considerably +inclined to the earth's equator brings her into that where they have +been performing. Arrhenius himself discovered this curious relation of +auroral frequency to the position of the moon north or south of the +equator, and he explains it in this way. The moon, like the earth, is +exposed to the influx of the ions from the sun; but having no +atmosphere, or almost none, to interfere with them, they descend +directly upon her surface and charge her with an electric negative +potential to a very high degree. In consequence of this she affects +the electric state of the upper parts of the earth's atmosphere where +they lie most directly beneath her, and thus prevents, to a large +extent, the negative discharges to which the appearance of the Aurora +is due. And so ``the extravagant and erring spirit'' of the Aurora +avoids the moon as Hamlet's ghost fled at the voice of the cock +announcing the awakening of the god of day. + +There are even other apparent confirmations of the hypothesis, but we +need not go into them. We shall, however, find one more application of +it in the next chapter, for it appears to be a kind of cure-all for +astronomical troubles; at any rate it offers a conceivable solution of +the question, How does the sun manage to transmit its electric +influence to the earth? And this solution is so grandiose in +conception, and so novel in the mental pictures that it offers, that +its acceptance would not in the least detract from the impression that +the Aurora makes upon the imagination. + +Strange Adventures of Comets + +The fears and legends of ancient times before Science was born, and +the superstitions of the Dark Ages, sedulously cultivated for +theological purposes by monks and priests, have so colored our ideas +of the influence that comets have had upon the human mind that many +readers may be surprised to learn that it was the apparition of a +wonderful comet, that of 1843, which led to the foundation of our +greatest astronomical institution, the Harvard College Observatory. No +doubt the comet superstition existed half a century ago, as, indeed, +it exists yet today, but in this case the marvelous spectacle in the +sky proved less effective in inspiring terror than in awakening a +desire for knowledge. Even in the sixteenth century the views that +enlightened minds took of comets tended powerfully to inspire popular +confidence in science, and Halley's prediction, after seeing and +studying the motion of the comet which appeared in 1682, that it would +prove to be a regular member of the sun's family and would be seen +returning after a period of about seventy-six years, together with the +fulfillment of that prediction, produced a revulsion from the +superstitious notions which had so long prevailed. + +Then the facts were made plain that comets are subject to the law of +gravitation equally with the planets; that there are many which +regularly return to the neighborhood of the sun (perihelion); and that +these travel in orbits differing from those of the planets only in +their greater eccentricity, although they have the peculiarity that +they do not, like the planets, all go round the sun in the same +direction, and do not keep within the general plane of the planetary +system, but traverse it sometimes from above and sometimes from below. +Other comets, including most of the ``great'' ones, appear to travel +in parabolic or, in a few cases, hyperbolic orbits, which, not being +closed curves, never bring them back again. But it is not certain that +these orbits may not be extremely eccentric ellipses, and that after +the lapse of hundreds, or thousands, of years the comets that follow +them may not reappear. The question is an interesting one, because if +all orbits are really ellipses, then all comets must be permanent +members of the solar system, while in the contrary case many of them +are simply visitors, seen once and never to be seen again. The +hypothesis that comets are originally interlopers might seem to derive +some support from the fact that the certainly periodic ones are +associated, in groups, with the great outer planets, whose attraction +appears to have served as a trap for them by turning them into +elliptical orbits and thus making them prisoners in the solar system. +Jupiter, owing to his great mass and his commanding situation in the +system, is the chief ``comet-catcher;'' but he catches them not for +himself, but for the sun. Yet if comets do come originally from +without the borders of the planetary system, it does not, by any +means, follow that they were wanderers at large in space before they +yielded to the overmastering attraction of the sun. Investigation of +the known cometary orbits, combined with theoretical considerations, +has led some astronomers to the conclusion that as the sun travels +onward through space he ``picks up en route'' cometary masses which, +without belonging strictly to his empire, are borne along in the same +vast ``cosmical current'' that carries the solar system. + +But while no intelligent person any longer thinks that the appearance +of a great comet is a token from the heavenly powers of the +approaching death of a mighty ruler, or the outbreak of a devastating +war, or the infliction of a terrible plague upon wicked mankind, +science itself has discovered mysteries about comets which are not +less fascinating because they are more intellectual than the +irrational fancies that they have displaced. To bring the subject +properly before the mind, let us see what the principal phenomena +connected with a comet are. + +At the present day comets are ordinarily ``picked up'' with the +telescope or the photographic plate before any one except their +discoverer is aware of their existence, and usually they remain so +insignificant in appearance that only astronomers ever see them. Yet +so great is the prestige of the word ``comet'' that the discovery of +one of these inconspicuous wanderers, and its subsequent movements, +become items of the day's news which everybody reads with the feeling, +perhaps, that at least he knows what is going on in the universe even +if he doesn't understand it. But a truly great comet presents quite a +different proposition. It, too, is apt to be detected coming out of +the depths of space before the world at large can get a glimpse of it, +but as it approaches the sun its aspect undergoes a marvelous change. +Agitated apparently by solar influence, it throws out a long streaming +tail of nebulous light, directed away from the sun and looking as if +blown out like a pennon by a powerful wind. Whatever may be the +position of the comet with regard to the sun, as it circles round him +it continually keeps its tail on the off side. This, as we shall soon +see, is a fact of capital importance in relation to the probable +nature of comets' tails. Almost at the same time that the formation of +the tail is observed a remarkable change takes place in the comet's +head, which, by the way, is invariably and not merely occasionally its +most important part. On approaching the sun the head usually +contracts. Coincidently with this contraction a nucleus generally +makes its appearance. This is a bright, star-like point in the head, +and it probably represents the totality of solid matter that the comet +possesses. But it is regarded as extremely unlikely that even the +nucleus consists of a uniformly solid mass. If it were such, comets +would be far more formidable visitors when they pass near the planets +than they have been found to be. The diameter of the nucleus may vary +from a few hundred up to several thousand miles; the heads, on the +average, are from twenty-five thousand to one hundred thousand miles +in diameter, although a few have greatly exceeded these dimensions; +that of the comet of 1811, one of the most stupendous ever seen, was a +million and a quarter miles in diameter! As to the tails, not +withstanding their enormous length -- some have been more than a +hundred million miles long -- there is reason to believe that they are +of extreme tenuity, ``as rare as vacuum.'' The smallest stars have +been seen shining through their most brilliant portions with +undiminished luster. + +After the nucleus has been formed it begins to throw out bright jets +directed toward the sun. A stream, and sometimes several streams, of +light also project sunward from the nucleus, occasionally appearing +like a stunted tail directed oppositely to the real tail. Symmetrical +envelopes which, seen in section, appear as half circles or parabolas, +rise sunward from the nucleus, forming a concentric series. The ends +of these stream backward into the tail, to which they seem to supply +material. Ordinarily the formation of these ejections and envelopes is +attended by intense agitation of the nucleus, which twists and turns, +swinging and gyrating with an appearance of the greatest violence. +Sometimes the nucleus is seen to break up into several parts. The +entire heads of some comets have been split asunder in passing close +around the sun; The comet of 1882 retreated into space after its +perihelion passage with five heads instead of the one that it had +originally, and each of these heads had its own tail! + +The possession of the spectroscope has enabled astronomers during +later years to study the chemical composition of comets by analyzing +their light. At first the only substances thus discovered in them were +hydro-carbon compounds, due evidently to the gaseous envelopes in +which some combination of hydrogen with carbon existed. Behind this +gaseous spectrum was found a faint continuous spectrum ascribed to the +nucleus, which apparently both reflects the sunlight and gives forth +the light of a glowing solid or liquid. Subsequently sodium and iron +lines were found in cometary spectra. The presence of iron would seem +to indicate that some of these bodies may be much more massive than +observations on their attractive effects have indicated. In some +recent comets, such as Morehouse's, in 1908, several lines have been +found, the origin of which is unknown. + +Without going back of the nineteenth century we may find records of +some of the most extraordinary comets that man has ever looked upon. +In 1811, still spoken of as ``the year of the comet,'' because of the +wonderful vintage ascribed to the skyey visitor, a comet shaped like a +gigantic sword amazed the whole world, and, as it remained visible for +seventeen months, was regarded by superstitious persons as a symbol of +the fearful happenings of Napoleon's Russian campaign. This comet, the +extraordinary size of whose head, greatly exceeding that of the sun +itself, has already been mentioned, was also remarkable for exhibiting +so great a brilliancy without approaching even to the earth's distance +from the sun. But there was once a comet (and only once -- in the year +1729) which never got nearer to the sun than four times the distance +of the earth and yet appeared as a formidable object in the sky. As +Professor Young has remarked, ``it must have been an enormous comet to +be visible from such a distance.'' And we are to remember that there +were no great telescopes in the year 1729. That comet affects the +imagination like a phantom of space peering into the solar system, +displaying its enormous train afar off (which, if it had approached as +near as other comets, would probably have become the celestial wonder +of all human memory), and then turning away and vanishing in the +depths of immensity. + +In 1843 a comet appeared which was so brilliant that it could be seen +in broad day close beside the sun! This was the first authenticated +instance of that kind, but the occurrence was to be repeated, as we +shall see in a moment, less than forty years later. + +The splendid comet of 1858, usually called Donati's, is remembered by +many persons yet living. It was, perhaps, both as seen by the naked +eye and with the telescope, the most beautiful comet of which we have +any record. It too marked a rich vintage year, still remembered in the +vineyards of France, where there is a popular belief that a great +comet ripens the grape and imparts to the wine a flavor not attainable +by the mere skill of the cultivator. There are ``comet wines,'' +carefully treasured in certain cellars, and brought forth only when +their owner wishes to treat his guests to a sip from paradise. + +The year 1861 saw another very remarkable comet, of an aspect +strangely vast and diffuse, which is believed to have swept the earth +with its immense tail when it passed between us and the sun on the +night of June 30th, an event which produced no other known effect than +the appearance of an unwonted amount of scattered light in the sky. + +The next very notable comet was the ``Great Southern Comet'' of 1880, +which was not seen from the northern hemisphere. It mimicked the +aspect of the famous comet of 1843, and to the great surprise of +astronomers appeared to be traveling in the same path. This proved to +be the rising of the curtain for an astronomical sensation +unparalleled in its kind; for two years later another brilliant comet +appeared, first in the southern hemisphere, and it too followed the +same track. The startling suggestion was now made that this comet was +identical with those of 1843 and 1880, its return having been hastened +by the resistance experienced in passing twice through the coronal +envelope, and there were some who thought that it would now swing +swiftly round and then plunge straight into the sun, with consequences +that might be disastrous to us on account of the ``flash of heat'' +that would be produced by the impact. Nervous people were frightened, +but observation soon proved that the danger was imaginary, for +although the comet almost grazed the sun, and must have rushed through +two or three million miles of the coronal region, no retardation of +its immense velocity was perceptible, and it finally passed away in a +damaged condition, as before remarked, and has never since appeared. + +Then the probable truth was perceived -- viz., that the three comets +(1843, 1880, and 1882) were not one identical body, but three separate +ones all traveling in the same orbit. It was found, too, that a comet +seen in 1668 bore similar insignia of relationship. The natural +inference was that these four bodies had once formed a single mass +which had been split apart by the disruptive action of the sun. +Strength was lent to this hypothesis by the fact that the comet of +1882 was apparently torn asunder during its perihelion passage, +retreating into space in a dissevered state. But Prof. George Forbes +has a theory that the splitting of the original cometary mass was +effected by an unknown planet, probably greater than Jupiter, situated +at a hundred times the earth's distance from the sun, and revolving in +a period of a thousand years. He supposes that the original comet was +not that of 1668, but one seen in 1556, which has since been +``missing,'' and that its disruption occurred from an encounter with +the supposititious planet about the year 1700. Truly from every point +of view comets are the most extraordinary of adventurers! + +The comet of 1882 was likewise remarkable for being visible, like its +predecessor of 1843, in full daylight in close proximity to the sun. +The story of its detection when almost in contact with the solar disk +is dramatic. It had been discovered in the southern hemisphere only a +couple of weeks before its perihelion, which occurred on September +17th, and on the forenoon of that day it was seen by Doctor Common in +England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope, +almost touching the sun. It looked like a dazzling white bird with +outspread wings. The southern observers watched it go right into the +sun, when it instantly disappeared. What had happened was that the +comet in passing its perihelion point had swung exactly between the +earth and the sun. On the following morning it was seen from all parts +of the world close by the sun on the opposite side, and it remained +thus visible for three days, gradually receding from the solar disk. +It then became visible for northern observers in the morning sky +before sunrise, brandishing a portentous sword-shaped tail which, if +it had been in the evening sky, would have excited the wonder of +hundreds of millions, but situated where it was, comparatively few +ever saw it. + +The application of photography to the study of comets has revealed +many curious details which might otherwise have escaped detection, or +at best have remained subject to doubt. It has in particular shown not +only the precise form of the tails, but the remarkable vicissitudes +that they undergo. Professor Barnard's photographs of Brooks' comet in +1893 suggested, by the extraordinary changes in the form of the tail +which they revealed, that the comet was encountering a series of +obstructions in space which bent and twisted its tail into fantastic +shapes. The reader will observe the strange form into which the tail +was thrown on the night of October 21st. A cloud of meteors through +which the comet was passing might have produced such deformations of +its tail. In the photograph of Daniels' comet of 1907, a curious +striping of the tail will be noticed. The short bright streaks seen in +the photograph, it may be explained, are the images of stars which are +drawn out into lines in consequence of the fact that the photographic +telescope was adjusted to follow the motion of the comet while the +stars remained at rest. + +But the adventures of comets are not confined to possible encounters +with unknown obstacles. We have referred to the fact that the great +planets, and especially Jupiter, frequently interfere with the motions +of comets. This interference is not limited to the original alteration +of their orbits from possible parabolas to ellipses, but is sometimes +exercised again and again, turning the bewildered comets into +elliptical paths of all degrees of eccentricity. A famous example of +this kind of planetary horse-play is furnished by the story of +Lexell's missing comet. This comet was first seen in 1770. +Investigation showed that it was moving in an orbit which should bring +it back to perihelion every five and a half years; yet it had never +been seen before and, although often searched for, has never been seen +since. Laplace and Leverrier proved mathematically that in 1767 it had +approached so close to Jupiter as to be involved among the orbits of +his satellites. What its track had been before is not known, but on +that occasion the giant planet seized the interloper, threw it into a +short elliptic orbit and sent it, like an arrested vagrant, to receive +sentence at the bar of the sun. On this journey it passed within less +than 1,500,000 miles of the earth. The form of orbit which Jupiter had +impressed required, as we have said, its return in about five and a +half years; but soon after 1770 it had the misfortune a second time to +encounter Jupiter at close range, and he, as if dissatisfied with the +leniency of the sun, or indignant at the stranger's familiarity, +seized the comet and hurled it out of the system, or at any rate so +far away that it has never since been able to rejoin the family circle +that basks in the immediate rays of the solar hearth. Nor is this the +only instance in which Jupiter has dealt summarily with small comets +that have approached him with too little deference. + +The function which Jupiter so conspicuously fulfills as master of the +hounds to the sun is worth considering a little more in detail. To +change the figure, imagine the sun in its voyage through space to be +like a majestic battleship surrounded by its scouts. Small vessels +(the comets, as they are overhauled by the squadron, are taken in +charge by the scouts, with Jupiter for their chief, and are forced to +accompany the fleet, but not all are impressed. If a strange comet +undertakes to run across Jupiter's bows the latter brings it to, and +makes prize of it by throwing it into a relatively small ellipse with +the sun for its focus. Thenceforth, unless, as happened to the unhappy +comet of Lexell, it encounters Jupiter again in such a way as to be +diverted by him into a more distant orbit, it can never get away. +About thirty comets are now known to have thus been captured by the +great planet, and they are called ``Jupiter's Comet Family.'' But, on +the other hand, if a wandering comet crosses the wake of the chief +planetary scout the latter simply drives it away by accelerating its +motion and compels it to steer off into open space. The transformation +of comets into meteors will be considered in the next chapter, but +here, in passing, mention may be made of the strange fate of one +member of Jupiter's family, Biela's comet, which, having become over +bold in its advances to its captor, was, after a few revolutions in is +impressed orbit, torn to pieces and turned into a flock of meteors. + +And now let us return to the mystery of comets' tails. That we are +fully justified in speaking of the tails of comets as mysterious is +proved by the declaration of Sir John Herschel, who averred, in so +many words, that ``there is some profound secret and mystery of nature +concerned in this phenomenon,'' and this profound secret and mystery +has not yet been altogether cleared up. Nevertheless, the +all-explaining hypothesis of Arrhenius offers us once more a certain +amount of aid. Comets' tails, Arrhenius assures us, are but another +result of the pressure of light. The reader will recall the +applications of this theory to the Zodiacal Light and the Aurora. In +the form in which we now have to deal with it, the supposition is made +that as a comet approaches the sun eruptions of vapor, due to the +solar heat, occur in its nucleus. These are naturally most active on +the side which is directly exposed to the sun, whence the appearance +of the immense glowing envelopes that surround the nucleus on the +sunward side. Among the particles of hydro-carbon, and perhaps solid +carbon in the state of fine dust, which are thus set free there will +be many whose size is within the critical limit which enables the +light-waves from the sun to drive them away. Clouds of such particles, +then, will stream off behind the advancing comet, producing the +appearance of a tail. This accounts for the fact that the tails of +comets are always directed away from the sun, and it also explains the +varying forms of the tails and the extraordinary changes that they +undergo. The speed of the particles driven before the light-waves must +depend upon their size and weight, the lightest of a given size +traveling the most swiftly. By accretion certain particles might grow, +thus losing velocity and producing the appearance of bunches in the +tail, such as have been observed. The hypothesis also falls in with +the researches of Bredichin, who has divided the tails of comets into +three principal classes -- viz.: (1) Those which appear as long, +straight rays; (2) Those which have the form of curved plumes or +scimitars; (3) Those which are short, brushy, and curved sharply +backward along the comet's path. In the first type he calculates the +repulsive force at from twelve to fifteen times the force of gravity; +in the second at from two to four times; and in the third at about one +and a half times. The straight tails he ascribes to hydrogen because +the hydrogen atom is the lightest known; the sword-shaped tails to +hydro-carbons; and the stumpy tails to vaporized iron. It will be seen +that, if the force driving off the tails is that which Arrhenius +assumes it to be, the forms of those appendages would accord with +those that Bredichin's theory calls for. At the same time we have an +explanation of the multiple tails with which some comets have adorned +themselves. The comet of 1744, for instance, had at one time no less +than seven tails spread in a wide curved brush behind it. Donati's +comet of 1858 also had at least two tails, the principal one +sword-shaped and the other long, narrow, and as straight as a rule. +According to Bredichin, the straight tail must have been composed of +hydrogen, and the other of some form of hydro-carbon whose atoms are +heavier than those of hydrogen, and, consequently, when swept away by +the storm of light-waves, followed a curvature depending upon the +resultant of the forces operating upon them. The seven tails of the +comet of 1744 presented a kind of diagram graphically exhibiting its +complex composition, and, if we knew a little more about the +constituents of a comet, we might be able to say from the amount of +curvature of the different tails just what were the seven substances +of which that comet consisted. + +If these theories seem to the reader fantastic, at any rate they are +no more fantastic than the phenomena that they seek to explain. + +Meteors, Fire-Balls, and Meteorites + +One of the most terrorizing spectacles with which the heavens have +ever caused the hearts of men to quake occurred on the night of +November 13, 1833. On that night North America, which faced the storm, +was under a continual rain of fire from about ten o'clock in the +evening until daybreak. + +The fragments of a comet had struck the earth. + +But the meaning of what had happened was not discovered until long +afterward. To the astronomers who, with astonishment not less than +that of other people, watched the wonderful scene, it was an +unparalleled ``shower of meteors.'' They did not then suspect that +those meteors had once formed the head of a comet. Light dawned when, +a year later, Prof. Denison Olmsted, of Yale College, demonstrated +that the meteors had all moved in parallel orbits around the sun, and +that these orbits intersected that of the earth at the point where our +planet happened to be on the memorable night of November 13th. +Professor Olmsted even went so far as to suggest that the cloud of +meteors that had encountered the earth might form a diffuse comet; but +full recognition of the fact that they were cometary debris came +later, as the result of further investigation. The key to the secret +was plainly displayed in the spectacle itself, and was noticed without +being understood by thousands of the terror-stricken beholders. It was +an umbrella of fire that had opened overhead and covered the heavens; +in other words, the meteors all radiated from a particular point in +the constellation Leo, and, being countless as the snowflakes in a +winter tempest, they ribbed the sky with fiery streaks. Professor +Olmsted showed that the radiation of the meteors from a fixed point +was an effect of perspective, and in itself a proof that they were +moving in parallel paths when they encountered the earth. The fact was +noted that there had been a similar, but incomparably less brilliant, +display of meteors on the same day of November, 1832, and it was +rightly concluded that these had belonged to the same stream, although +the true relationship of the phenomena was not immediately +apprehended. Olmsted ascribed to the meteors a revolution about the +sun once in every six months, bringing them to the intersection of +their orbit with that of the earth every November 13th; but later +investigators found that the real period was about thirty-three and +one-quarter years, so that the great displays were due three times in +a century, and their return was confidently predicted for the year +1866. The appearance of the meteors in 1832, a year before the great +display, was ascribed to the great length of the stream which they +formed in space -- so great that they required more than two years to +cross the earth's orbit. In 1832 the earth had encountered a +relatively rare part of the stream, but in 1833, on returning to the +crossing-place, it found there the richest part of the stream pouring +across its orbit. This explanation also proved to be correct, and the +predicted return in 1866 was duly witnessed, although the display was +much less brilliant than in 1833. It was followed by another in 1867. + +In the mean time Olmsted's idea of a cometary relationship of the +meteors was demonstrated to be correct by the researches of +Schiaparelli and others, who showed that not only the November +meteors, but those of August, which are seen more or less abundantly +every year, traveled in the tracks of well-known comets, and had +undoubtedly an identical origin with those comets. In other words the +comets and the meteor-swarms were both remnants of original masses +which had probably been split up by the action of the sun, or of some +planet to which they had made close approaches. The annual periodicity +of the August meteors was ascribed to the fact that the separation had +taken place so long ago that the meteors had become distributed all +around the orbit, in consequence of which the earth encountered some +of them every year when it arrived at the crossing-point. Then +Leverrier showed that the original comet associated with the November +meteors was probably brought into the system by the influence of the +planet Uranus in the year 126 of the Christian era. Afterward +Alexander Herschel identified the tracks of no less than seventy-six +meteor-swarms (most of them inconspicuous) with those of comets. The +still more recent researches of Mr W. F. Denning make it probable that +there are no meteors which do not belong to a flock or system probably +formed by the disintegration of a cometary mass; even the apparently +sporadic ones which shoot across the sky, ``lost souls in the night,'' +being members of flocks which have become so widely scattered that the +earth sometimes takes weeks to pass through the region of space where +their paths lie. + +The November meteors should have exhibited another pair of spectacles +in 1899 and 1900, and their failure to do so caused at first much +disappointment, until it was made plain that a good reason existed for +their absence. It was found that after their last appearance, in 1867, +they had been disturbed in their movements by the planets Jupiter and +Saturn, whose attractions had so shifted the position of their orbit +that it no longer intersected that of the earth, as it did before. +Whether another planetary interference will sometime bring the +principal mass of the November meteors back to the former point of +intersection with the earth's orbit is a question for the future to +decide. It would seem that there may be several parallel streams of +the November meteors, and that some of them, like those of August, are +distributed entirely around the orbit, so that every mid-November we +see a few of them. + +We come now to a very remarkable example of the disintegration of a +comet and the formation of a meteor-stream. In 1826 Biela, of +Josephstadt, Austria, discovered a comet to which his name was given. +Calculation showed that it had an orbital period of about six and a +half years, belonging to Jupiter's ``family.'' On one of its returns, +in 1846, it astonished its watchers by suddenly splitting in two. The +two comets thus formed out of one separated to a distance of about one +hundred and sixty thousand miles, and then raced side by side, +sometimes with a curious ligature connecting them, like Siamese twins, +until they disappeared together in interplanetary space. In 1852 they +came back, still nearly side by side, but now the distance between +them had increased to a million and a quarter of miles. After that, at +every recurrence of their period, astronomers looked for them in vain, +until 1872, when an amazing thing happened. On the night of November +28th, when the earth was crossing the plane of the orbit of the +missing comet, a brilliant shower of meteors burst from the northern +sky, traveling nearly in the track which the comet should have +pursued. The astronomers were electrified. Klinkerfues, of Goettingen, +telegraphed to Pogson, of Madras: ``Biela touched earth; search near +Theta Centauri.'' Pogson searched in the place indicated and saw a +cometary mass retreating into the southern heavens, where it was soon +swallowed from sight! + +Since then the Biela meteors have been among the recognized periodic +spectacles of the sky, and few if any doubt that they represent a +portion of the missing comet whose disintegration began with the +separation into two parts in 1846. The comet itself has never since +been seen. The first display of these meteors, sometimes called the +``Andromedes,'' because they radiate from the constellation Andromeda, +was remarkable for the great brilliancy of many of the fire-balls that +shot among the shower of smaller sparks, some of which were described +as equaling the full moon in size. None of them is known to have +reached the earth, but during the display of the same meteors in 1885 +a meteoric mass fell at Mazapil in Northern Mexico (it is now in the +Museum at Vienna), which many have thought may actually be a piece of +the original comet of Biela. This brings us to the second branch of +our subject. + +More rare than meteors or falling stars, and more startling, except +that they never appear in showers, are the huge balls of fire which +occasionally dart through the sky, lighting up the landscapes beneath +with their glare, leaving trains of sparks behind them, often +producing peals of thunder when they explode, and in many cases +falling upon the earth and burying themselves from a few inches to +several feet in the soil, from which, more than once, they have been +picked up while yet hot and fuming. These balls are sometimes called +bolides. They are not really round in shape, although they often look +so while traversing the sky, but their forms are fragmentary, and +occasionally fantastic. It has been supposed that their origin is +different from that of the true meteors; it has even been conjectured +that they may have originated from the giant volcanoes of the moon or +have been shot out from the sun during some of the tremendous +explosions that accompany the formation of eruptive prominences. By +the same reasoning some of them might be supposed to have come from +some distant star. Others have conjectured that they are wanderers in +space, of unknown origin, which the earth encounters as it journeys +on, and Lord Kelvin made a suggestion which has become classic because +of its imaginative reach -- viz., that the first germs of life may +have been brought to the earth by one of these bodies, ``a fragment of +an exploded world.'' + +It is a singular fact that astronomers and scientific men in general +were among the last to admit the possibility of solid masses falling +from the sky. The people had believed in the reality of such phenomena +from the earliest times, but the savants shook their heads and talked +of superstition. This was the less surprising because no +scientifically authenticated instance of such an occurrence was known, +and the stones popularly believed to have fallen from the sky had +become the objects of worship or superstitious reverence, a fact not +calculated to recommend them to scientific credence. The celebrated +``black stone'' suspended in the Kaaba at Mecca is one of these +reputed gifts from heaven; the ``Palladium'' of ancient Troy was +another; and a stone which fell near Ensisheim, in Germany, was placed +in a church as an object to be religiously venerated. Many legends of +falling stones existed in antiquity, some of them curiously +transfigured by the imagination, like the ``Lion of the +Peloponnesus,'' which was said to have sprung down from the sky upon +the Isthmus of Corinth. But near the beginning of the nineteenth +century, in 1803, a veritable shower of falling stones occurred at +L'Aigle, in Northern France, and this time astronomers took note of +the phenomenon and scientifically investigated it. Thousands of the +strange projectiles came from the sky on this occasion, and were +scattered over a wide area of country, and some buildings were hit. +Four years later another shower of stones occurred at Weston, Conn., +numbering thousands of individuals. The local alarm created in both +cases was great, as well it might be, for what could be more +intimidating than to find the blue vault of heaven suddenly hurling +solid missiles at the homes of men? After these occurrences it was +impossible for the most skeptical to doubt any longer, and the regular +study of ``aerolites,'' or ``meteorites,'' began. + +One of the first things recognized was the fact that fire-balls are +solid meteorites in flight, and not gaseous exhalations in the air, as +some had assumed. They burn in the air during their flight, and +sometimes, perhaps, are entirely consumed before reaching the ground. +Their velocity before entering the earth's atmosphere is equal to that +of the planets in their orbits -- viz., from twenty to thirty miles +per second -- a fact which proves that the sun is the seat of the +central force governing them. Their burning in the air is not +difficult to explain; it is the heat of friction which so quickly +brings them to incandescence. Calculation shows that a body moving +through the air at a velocity of about a mile per second will be +brought, superficially, to the temperature of ``red heat'' by friction +with the atmosphere. If its velocity is twenty miles per second the +temperature will become thousands of degrees. This is the state of +affairs with a meteorite rushing into the earth's atmosphere; its +surface is liquefied within a few seconds after the friction begins to +act, and the melted and vaporized portion of its mass is swept +backward, forming the train of sparks that follows every great +fire-ball. However, there is one phenomenon connected with the trains +of meteorites which has never been satisfactorily explained: they +often persist for long periods of time, drifting and turning with the +wind, but not ceasing to glow with a phosphorescent luminosity. The +question is, Whence comes this light? It must be light without heat, +since the fine dust or vapor of which the train can only consist would +not retain sufficient heat to render it luminous for so long a time. +An extremely remarkable incident of this kind occurred on February 22, +1909, when an immense fire-ball that passed over southern England left +a train that remained visible during two hours, assuming many curious +shapes as it was drifted about by currents in the air. + +But notwithstanding the enormous velocity with which meteorites enter +the air they are soon slowed down to comparatively moderate speed, so +that when they disappear they are usually traveling not faster than a +mile a second. The courses of many have been traced by observers +situated along their track at various points, and thus a knowledge has +been obtained of their height above the ground during their flight and +of the length of their visible courses. They generally appear at an +elevation of eighty or a hundred miles, and are seldom visible after +having descended to within five miles of the ground, unless the +observer happens to be near the striking-point, when he may actually +witness the fall. Frequently they burst while high in the air and +their fragments are scattered like shrapnel over the surface of the +ground, sometimes covering an area of several square miles, but of +course not thickly; different fragments of the same meteorite may +reach the ground at points several miles apart. The observed length of +their courses in the atmosphere varies from fifty to five hundred +miles. If they continued a long time in flight after entering the air, +even the largest of them would probably be consumed to the last scrap, +but their fiery career is so short on account of their great speed +that the heat does not have time to penetrate very deeply, and some +that have been picked up immediately after their fall have been found +cold as ice within. Their size after reaching the ground is variable +within wide limits; some are known which weigh several tons, but the +great majority weigh only a few pounds and many only a few ounces. + +Meteorites are of two kinds: stony meteorites and iron meteorites. The +former outnumber the latter twenty to one; but many stone meteorites +contain grains of iron. Nickel is commonly found in iron meteorites, +so that it might be said that that redoubtable alloy nickel-steel is +of cosmical invention. Some twenty-five chemical elements have been +found in meteorites, including carbon and the ``sun-metal,'' helium. +The presence of the latter is certainly highly suggestive in +connection with the question of the origin of meteorites. The iron +meteorites, besides metallic iron and nickel, of which they are almost +entirely composed, contain hydrogen, helium, and carbonic oxide, and +about the only imaginable way in which these gases could have become +absorbed in the iron would be through the immersion of the latter +while in a molten or vaporized state in a hot and dense atmosphere +composed of them, a condition which we know to exist only in the +envelopes of the sun and the stars. + +The existence of carbon in the Canyon Diablo iron meteorites is +attended by a circumstance of the most singular character -- a very +``fairy tale of science.'' In some cases the carbon has become +diamond! These meteoric diamonds are very small; nevertheless, they +are true diamonds, resembling in many ways the little black gems +produced by Moissan's method with the aid of the electric furnace. The +fact that they are found embedded in these iron meteorites is another +argument in favor of the hypothesis of the solar or stellar origin of +the latter. To appreciate this it is necessary to recall the way in +which Moissan made his diamonds. It was by a combination of the +effects of great heat, great pressure, and sudden or rapid superficial +cooling on a mass of iron containing carbon. When he finally broke +open his iron he found it a pudding stuffed with miniature black +diamonds. When a fragment of the Canyon Diablo meteoric iron was +polished in Philadelphia over fifteen years ago it cut the emery-wheel +to pieces, and examination showed that the damage had been effected by +microscopic diamonds peppered through the mass. How were those +diamonds formed? If the sun or Sirius was the laboratory that prepared +them, we can get a glimpse at the process of their formation. There is +plenty of heat, plenty of pressure, and an abundance of vaporized iron +in the sun and the stars. When a great solar eruption takes place, +masses of iron which have absorbed carbon may be shot out with a +velocity which forbids their return. Plunged into the frightful cold +of space, their surfaces are quickly cooled, as Moissan cooled his +prepared iron by throwing it into water, and thus the requisite stress +is set up within, and, as the iron solidifies, the included carbon +crystallizes into diamonds. Whether this explanation has a germ of +truth in it or not, at any rate it is evident that iron meteorites +were not created in the form in which they come to us; they must once +have been parts of immeasurably more massive bodies than themselves. + +The fall of meteorites offers an appreciable, though numerically +insignificant, peril to the inhabitants of the earth. Historical +records show perhaps three or four instances of people being killed by +these bodies. But for the protection afforded by the atmosphere, which +acts as a very effective shield, the danger would doubtless be very +much greater. In the absence of an atmosphere not only would more +meteorites reach the ground, but their striking force would be +incomparably greater, since, as we have seen, the larger part of their +original velocity is destroyed by the resistance of the air. A +meteorite weighing many tons and striking the earth with a velocity of +twenty or thirty miles per second, would probably cause frightful +havoc. + +It is a singular fact that recent investigations seem to have proved +that an event of this kind actually happened in North America -- +perhaps not longer than a thousand or two thousand years ago. The +scene of the supposed catastrophe is in northern central Arizona, at +Coon Butte, where there is a nearly circular crater in the middle of a +circular elevation or small mountain. The crater is somewhat over four +thousand feet in diameter, and the surrounding rim, formed of upturned +strata and ejected rock fragments, rises at its highest point one +hundred and sixty feet above the plain. The crater is about six +hundred feet in depth -- that is, from the rim to the visible floor or +bottom of the crater. There is no evidence that volcanic action has +ever taken place in the immediate neighborhood of Coon Butte. The rock +in which the crater has been made is composed of horizontal sandstone +and limestone strata. Between three hundred and four hundred million +tons of rock fragments have been detached, and a large portion hurled +by some cause out of the crater. These fragments lie concentrically +distributed around the crater, and in large measure form the elevation +known as Coon Butte. The region has been famous for nearly twenty +years on account of the masses of meteoric iron found scattered about +and known as the ``Canyon Diablo'' meteorites. It was one of these +masses, which consist of nickel-iron containing a small quantity of +platinum, and of which in all some ten tons have been recovered for +sale to the various collectors throughout the world, that as before +mentioned destroyed the grinding-tool at Philadelphia through the +cutting power of its embedded diamonds. These meteoric irons are +scattered about the crater-hill, in concentric distribution, to a +maximum distance of about five miles. When the suggestion was first +made in 1896 that a monster meteorite might have created by its fall +this singular lone crater in stratified rocks, it was greeted with +incredulous smiles; but since then the matter has assumed a different +aspect. The Standard Iron Company, formed by Messrs. D. M. Barringer, +B. C. Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in +1903, the owner of this freak of nature, sunk shafts and bored holes +to a great depth in the interior of the crater, and also trenched the +slopes of the mountain, and the result of their investigations has +proved that the meteoric hypothesis of origin is correct. (See the +papers published in the Proceedings of the Academy of Natural Sciences +of Philadelphia, December, 1905, wherein it is proved that the United +States Geological Survey was wrong in believing this crater to have +been due to a steam explosion. Since that date there has been +discovered a great amount of additional confirmatory proof). Material +of unmistakably meteoric origin was found by means of the drills, +mixed with crushed rock, to a depth of six hundred to seven hundred +feet below the floor of the crater, and a great deal of it has been +found admixed with the ejected rock fragments on the outer slopes of +the mountain, absolutely proving synchronism between the two events, +the formation of this great crater and the falling of the meteoric +iron out of the sky. The drill located in the bottom of the crater was +sent, in a number of cases, much deeper (over one thousand feet) into +unaltered horizontal red sandstone strata, but no meteoric material +was found below this depth (seven hundred feet, or between eleven and +twelve hundred feet below the level of the surrounding plain), which +has been assumed as being about the limit of penetration. It is not +possible to sink a shaft at present, owing to the water which has +drained into the crater, and which forms, with the finely pulverized +sandstone, a very troublesome quicksand encountered at about two +hundred feet below the visible floor of the crater. As soon as this +water is removed by pumping it will be easy to explore the depths of +the crater by means of shafts and drifts. The rock strata (sandstone +and limestone) of which the walls consist present every appearance of +having been violently upturned by a huge body penetrating the earth +like a cannon-ball. The general aspect of the crater strikingly +resembles the impression made by a steel projectile shot into an +armor-plate. Mr Tilghman has estimated that a meteorite about five +hundred feet in diameter and moving with a velocity of about five +miles per second would have made just such a perforation upon striking +rocks of the character of those found at this place. There was some +fusion of the colliding masses, and the heat produced some steam from +the small amount of water in the rocks. As a result there has been +found at depth a considerable amount of fused quartz (original +sandstone), and with it innumerable particles or sparks of fused +nickel-iron (original meteorite). A projectile of that size +penetrating eleven to twelve hundred feet into the rocky shell of the +globe must have produced a shock which was perceptible several hundred +miles away. + +The great velocity ascribed to the supposed meteorite at the moment of +striking could be accounted for by the fact that it probably plunged +nearly vertically downward, for it formed a circular crater in the +rocky crust of the earth. In that case it would have been less +retarded by the resistance of the atmosphere than are meteorites which +enter the air at a lower angle and shoot ahead hundreds of miles until +friction has nearly destroyed their original motion when they drop +upon the earth. Some meteoric masses of great size, such as Peary's +iron meteorite found at Cape York, Greenland, and the almost equally +large mass discovered at Bacubirito, Mexico, appear to have penetrated +but slightly on striking the earth. This may be explained by supposing +that they pursued a long, horizontal course through the air before +falling. The result would be that, their original velocity having been +practically destroyed, they would drop to the ground with a velocity +nearly corresponding to that which gravity would impart within the +perpendicular distance of their final fall. A +six-hundred-and-sixty-pound meteorite, which fell at Knyahinya, +Hungary, striking at an angle of 27 deg. from the vertical, penetrated the +ground to a depth of eleven feet. + +It has been remarked that the Coon Butte meteorite may have fallen not +longer ago than a few thousand years. This is based upon the fact that +the geological indications favor the supposition that the event did +not occur more than five thousand years ago, while on the other hand +the rings of growth in the cedar-trees growing on the slopes of the +crater show that they have existed there about seven hundred years. +Prof. William H. Pickering has recently correlated this with an +ancient chronicle which states that at Cairo, Egypt, in the year 1029, +``many stars passed with a great noise.'' He remarks that Cairo is +about 100 deg., by great circle, from Coon Butte, so that if the meteorite +that made the crater was a member of a flock of similar bodies which +encountered the earth moving in parallel lines, some of them might +have traversed the sky tangent to the earth's surface at Cairo. That +the spectacle spoken of in the chronicle was caused by meteorites he +deems exceedingly probable because of what is said about ``a great +noise;'' meteorites are the only celestial phenomena attended with +perceptible sounds. Professor Pickering conjectures that this supposed +flock of great meteorites may have formed the nucleus of a comet which +struck the earth, and he finds confirmation of the idea in the fact +that out of the ten largest meteorites known, no less than seven were +found within nine hundred miles of Coon Butte. It would be interesting +if we could trace back the history of that comet, and find out what +malicious planet caught it up in its innocent wanderings and hurled it +with so true an aim at the earth! This remarkable crater is one of the +most interesting places in the world, for there is absolutely no +record of such a mass, possibly an iron-headed comet, from outer space +having come into collision with our earth. The results of the future +exploration of the depths of the crater will be awaited with much +interest. + +The Wrecking of the Moon + +There are sympathetic moods under whose influence one gazes with a +certain poignant tenderness at the worn face of the moon; that little +``fossil world'' (the child of our mother earth, too) bears such +terrible scars of its brief convulsive life that a sense of pity is +awakened by the sight. The moon is the wonder-land of the telescope. +Those towering mountains, whose ``proud aspiring peaks'' cast +silhouettes of shadow that seem drawn with india-ink; those vast +plains, enchained with gentle winding hills and bordered with giant +ranges; those oval ``oceans,'' where one looks expectant for the flash +of wind-whipped waves; those enchanting ``bays'' and recesses at the +seaward feet of the Alps; those broad straits passing between guardian +heights incomparably mightier than Gibraltar; those locket-like +valleys as secluded among their mountains as the Vale of Cashmere; +those colossal craters that make us smile at the pretensions of +Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with +the unconcern of Roman roads across mountain, gorge, and valley -- all +these give the beholder an irresistible impression that it is truly a +world into which he is looking, a world akin to ours, and yet no more +like our world than Pompeii is like Naples. Its air, its waters, its +clouds, its life are gone, and only a skeleton remains -- a mute but +eloquent witness to a cosmical tragedy without parallel in the range +of human knowledge. + +One cannot but regret that the moon, if it ever was the seat of +intelligent life, has not remained so until our time. Think what the +consequences would have been if this other world at our very door had +been found to be both habitable and inhabited! We talk rather airily +of communicating with Mars by signals; but Mars never approaches +nearer than 35,000,000 miles, while the moon when nearest is only a +little more than 220,000 miles away. Given an effective magnifying +power of five thousand diameters, which will perhaps be possible at +the mountain observatories as telescopes improve, and we should be +able to bring the moon within an apparent distance of about forty +miles, while the corresponding distance for Mars would be more than +seven thousand miles. But even with existing telescopic powers we can +see details on the moon no larger than some artificial constructions +on the earth. St Peter's at Rome, with the Vatican palace and the +great piazza, if existing on the moon, would unquestionably be +recognizable as something else than a freak of nature. Large cities, +with their radiating lines of communication, would at once betray +their real character. Cultivated tracts, and the changes produced by +the interference of intelligent beings, would be clearly recognizable. +The electric illumination of a large town at night would probably be +markedly visible. Gleams of reflected sunlight would come to us from +the surfaces of the lakes and oceans, and a huge ``liner'' traversing +a lunar sea could probably be followed by its trail of smoke. As to +communications by ``wireless'' signals, which certain enthusiasts have +thought of in connection with Mars, in the case of the moon they +should be a relatively simple matter, and the feat might actually be +accomplished. Think what a literature would grow up about the moon if +it were a living world! Its very differences from the earth would only +accentuate its interest for us. Night and day on the moon are each two +weeks in length; how interesting it would be to watch the manner in +which the lunarians dealt with such a situation as that. Lunar and +terrestrial history would keep step with each other, and we should +record them both. Truly one might well wish to have a neighbor world +to study; one would feel so much the less alone in space. + +It is not impossible that the moon did at one time have inhabitants of +some kind. But, if so, they vanished with the disappearance of its +atmosphere and seas, or with the advent of its cataclysmic age. At the +best, its career as a living world must have been brief. If the water +and air were gradually absorbed, as some have conjectured, by its +cooling interior rocks, its surface might, nevertheless, have retained +them for long ages; but if, as others think, their disappearance was +due to the escape of their gaseous molecules in consequence of the +inability of the relatively small lunar gravitation to retain them, +then the final catastrophe must have been as swift as it was +inevitable. Accepting Darwin's hypothesis, that the moon was separated +from the earth by tidal action while both were yet plastic or +nebulous, we may reasonably conclude that it began its career with a +good supply of both water and air, but did not possess sufficient mass +to hold them permanently. Yet it may have retained them long enough +for life to develop in many forms upon its surface; in fact, there are +so many indications that air and water have not always been lacking to +the lunar world that we are driven to invent theories to explain both +their former presence and their present absence. + +But whatever the former condition of the moon may have been, its +existing appearance gives it a resistless fascination, and it bears so +clearly the story of a vast catastrophe sculptured on its rocky face +that the thoughtful observer cannot look upon it without a feeling of +awe. The gigantic character of the lunar features impresses the +beholder not less than the universality of the play of destructive +forces which they attest. Let us make a few comparisons. Take the +lunar crater called ``Tycho'', which is a typical example of its kind. +In the telescope Tycho appears as a perfect ring surrounding a +circular depression, in the center of which rises a group of +mountains. Its superficial resemblance to some terrestrial volcanic +craters is very striking. Vesuvius, seen from a point vertically +above, would no doubt look something like that (the resemblance would +have been greater when the Monte del Cavallo formed a more complete +circuit about the crater cone). But compare the dimensions. The +remains of the outer crater ring of Vesuvius are perhaps half a mile +in diameter, while the active crater itself is only two or three +hundred feet across at the most; Tycho has a diameter of fifty-four +miles! The group of relatively insignificant peaks in the center of +the crater floor of Tycho is far more massive than the entire mountain +that we call Vesuvius. The largest known volcanic crater on the earth, +Aso San, in Japan, has a diameter of seven miles; it would take sixty +craters like Aso San to equal Tycho in area! And Tycho, though one of +the most perfect, is by no means the largest crater on the moon. +Another, called ``Theophilus,'' has a diameter of sixty-four miles, +and is eighteen thousand feet deep. There are hundreds from ten to +forty miles in diameter, and thousands from one to ten miles. They are +so numerous in many places that they break into one another, like the +cells of a crushed honeycomb. + +The lunar craters differ from those of the earth more fundamentally +than in the matter of mere size; they are not situated on the tops of +mountains. If they were, and if all the proportions were the same, a +crater like Tycho might crown a conical peak fifty or one hundred +miles high! Instead of being cavities in the summits of mountains, the +lunar craters are rather gigantic sink-holes whose bottoms in many +cases lie two or three miles below the general surface of the lunar +world. Around their rims the rocks are piled up to a height of from a +few hundred to two or three thousand feet, with a comparatively gentle +inclination, but on the inner side they fall away in gigantic broken +precipices which make the dizzy cliffs of the Matterhorn seem but +``lover's leaps.'' Down they drop, ridge below ridge, crag under crag, +tottering wall beneath wall, until, in a crater named ``Newton,'' near +the south lunar pole, they attain a depth where the rays of the sun +never reach. Nothing more frightful than the spectacle which many of +these terrible chasms present can be pictured by the imagination. As +the lazy lunar day slowly advances, the sunshine, unmitigated by +clouds or atmospheric veil of any kind, creeps across their rims and +begins to descend the opposite walls. Presently it strikes the ragged +crest of a ridge which had lain hidden in such darkness as we never +know on the earth, and runs along it like a line of kindling fire. +Rocky pinnacles and needles shoot up into the sunlight out of the +black depths. Down sinks the line of light, mile after mile, and +continually new precipices and cliffs are brought into view, until at +last the vast floor is attained and begins to be illuminated. In the +meanwhile the sun's rays, darting across the gulf, have touched the +summits of the central peaks, twenty or thirty miles from the crater's +inmost edge, and they immediately kindle and blaze like huge stars +amid the darkness. So profound are some of these awful craters that +days pass before the sun has risen high enough above them to chase the +last shadows from their depths. + +Although several long ranges of mountains resembling those of the +earth exist on the moon, the great majority of its elevations assume +the crateriform aspect. Sometimes, instead of a crater, we find an +immense mountain ring whose form and aspect hardly suggest volcanic +action. But everywhere the true craters are in evidence, even on the +sea-beds, although they attain their greatest number and size on those +parts of the moon -- covering sixty per cent of its visible surface -- +which are distinctly mountainous in character and which constitute its +most brilliant portions. Broadly speaking, the southwestern half of +the moon is the most mountainous and broken, and the northeastern half +the least so. Right down through the center, from pole to pole, runs a +wonderful line of craters and crateriform valleys of a magnitude +stupendous even for the moon. Another similar line follows the western +edge. Three or four ``seas'' are thrust between these mountainous +belts. By the effects of ``libration'' parts of the opposite +hemisphere of the moon which is turned away from the earth are from +time to time brought into view, and their aspect indicates that that +hemisphere resembles in its surface features the one which faces the +earth. There are many things about the craters which seem to give some +warrant for the hypothesis which has been particularly urged by Mr G. +K. Gilbert, that they were formed by the impact of meteors; but there +are also many things which militate against that idea, and, upon the +whole, the volcanic theory of their origin is to be preferred. + +The enormous size of the lunar volcanoes is not so difficult to +account for when we remember how slight is the force of lunar gravity +as compared with that of the earth. With equal size and density, +bodies on the moon weigh only one-sixth as much as on the earth. +Impelled by the same force, a projectile that would go ten miles on +the earth would go sixty miles on the moon. A lunar giant thirty-five +feet tall would weigh no more than an ordinary son of Adam weighs on +his greater planet. To shoot a body from the earth so that it would +not drop back again, we should have to start it with a velocity of +seven miles per second; a mile and a half per second would serve on +the moon. It is by no means difficult to believe, then, that a lunar +volcano might form a crater ring eight or ten times broader than the +greatest to be found on the earth, especially when we reflect that in +addition to the relatively slight force of gravity, the materials of +the lunar crust are probably lighter than those of our terrestrial +rocks. + +For similar reasons it seems not impossible that the theory mentioned +in a former chapter -- that some of the meteorites that have fallen +upon the earth originated from the lunar volcanoes -- is well founded. +This would apply especially to the stony meteorites, for it is hardly +to be supposed that the moon, at least in its superficial parts, +contains much iron. It is surely a scene most strange that is thus +presented to the mind's eye -- that little attendant of the earth's +(the moon has only one-fiftieth of the volume, and only one-eightieth +of the mass of the earth) firing great stones back at its parent +planet! And what can have been the cause of this furious outbreak of +volcanic forces on the moon? Evidently it was but a passing stage in +its history; it had enjoyed more quiet times before. As it cooled down +from the plastic state in which it parted from the earth, it became +incrusted after the normal manner of a planet, and then oceans were +formed, its atmosphere being sufficiently dense to prevent the water +from evaporating and the would-be oceans from disappearing continually +in mist. This, if any, must have been the period of life in the lunar +world. As we look upon the vestiges of that ancient world buried in +the wreck that now covers so much of its surface, it is difficult to +restrain the imagination from picturing the scenes which were once +presented there; and, in such a case, should the imagination be +fettered? We give it free rein in terrestrial life, and it rewards us +with some of our greatest intellectual pleasures. The wonderful +landscapes of the moon offer it an ideal field with just enough +half-hidden suggestions of facts to stimulate its powers. + +The great plains of the Mare Imbrium and the Mare Serenitatis (the +``Sea of Showers'' and the ``Sea of Serenity''), bordered in part by +lofty mountain ranges precisely like terrestrial mountains, scalloped +along their shores with beautiful bays curving back into the adjoining +highlands, and united by a great strait passing between the nearly +abutting ends of the ``Lunar Apennines'' and the ``Lunar Caucasus,'' +offer the elements of a scene of world beauty such as it would be +difficult to match upon our planet. Look at the finely modulated +bottom of the ancient sea in Mr Ritchey's exquisite photograph of the +western part of the Mare Serenitatis, where one seems to see the play +of the watery currents heaping the ocean sands in waving lines, making +shallows, bars, and deeps for the mariner to avoid or seek, and +affording a playground for the creatures of the main. What geologist +would not wish to try his hammer on those rocks with their stony pages +of fossilized history? There is in us an instinct which forbids us to +think that there was never any life there. If we could visit the moon, +there is not among us a person so prosaic and unimaginative that he +would not, the very first thing, begin to search for traces of its +inhabitants. We would look for them in the deposits on the sea +bottoms; we would examine the shores wherever the configuration seemed +favorable for harbors and the sites of maritime cities -- forgetting +that it may be a little ridiculous to ascribe to the ancient lunarians +the same ideas that have governed the development of our race; we +would search through the valleys and along the seeming courses of +vanished streams; we would explore the mountains, not the terrible +craters, but the pinnacled chains that recall our own Alps and +Rockies; seeking everywhere some vestige of the transforming presence +of intelligent life. Perhaps we should find such traces, and perhaps, +with all our searching, we should find nothing to suggest that life +had ever existed amid that universal ruin. + +Look again at the border of the ``Sea of Serenity'' -- what a name for +such a scene! -- and observe how it has been rent with almost +inconceivable violence, the wall of the colossal crater Posidonius +dropping vertically upon the ancient shore and obliterating it, while +its giant neighbor, Le Monnier, opens a yawning mouth as if to swallow +the sea itself. A scene like this makes one question whether, after +all, those may not be right who have imagined that the so-called sea +bottoms are really vast plains of frozen lava which gushed up in +floods so extensive that even the mighty volcanoes were half drowned +in the fiery sea. This suggestion becomes even stronger when we turn +to another of the photographs of Mr Ritchey's wonderful series, +showing a part of the Mare Tranquilitatis (``Sea of Tranquility''!). +Notice how near the center of the picture the outline of a huge ring +with radiating ridges shows through the sea bottom; a fossil volcano +submerged in a petrified ocean! This is by no means the only instance +in which a buried world shows itself under the great lunar plains. +Yet, as the newer craters in the sea itself prove, the volcanic +activity survived this other catastrophe, or broke out again +subsequently, bringing more ruin to pile upon ruin. + +Yet notwithstanding the evidence which we have just been considering +in support of the hypothesis that the ``seas'' are lava floods, +Messrs. Loewy and Puiseux, the selenographers of the Paris +Observatory, are convinced that these great plains bear characteristic +marks of the former presence of immense bodies of water. In that case +we should be forced to conclude that the later oceans of the moon lay +upon vast sheets of solidified lava; and thus the catastrophe of the +lunar world assumes a double aspect, the earliest oceans being +swallowed up in molten floods issuing from the interior, while the +lands were reduced to chaos by a universal eruption of tremendous +volcanoes; and then a period of comparative quiet followed, during +which new seas were formed, and new life perhaps began to flourish in +the lunar world, only to end in another cataclysm, which finally put a +term to the existence of the moon as a life-supporting world. + +Suppose we examine two more of Mr Ritchey's illuminating photographs, +and, first, the one showing the crater Theophilus and its +surroundings. We have spoken of Theophilus before, citing the facts +that it is sixty-four miles in diameter and eighteen thousand feet +deep. It will be noticed that it has two brother giants -- Cyrillus +the nearer, and Catharina the more distant; but Theophilus is plainly +the youngest of the trio. Centuries, and perhaps thousands of years, +must have elapsed between the periods of their upheaval, for the two +older craters are partly filled with debris, while it is manifest at a +glance that when the south eastern wall of Theophilus was formed, it +broke away and destroyed a part of the more ancient ring of Cyrillus. +There is no more tremendous scene on the moon than this; viewed with a +powerful telescope, it is absolutely appalling. + +The next photograph shows, if possible, a still wilder region. It is +the part of the moon lying between Tycho and the south pole. Tycho is +seen in the lower left-hand part of the picture. To the right, at the +edge of the illuminated portion of the moon, are the crater-rings, +Longomontanus and Wilhelm I, the former being the larger. Between them +are to be seen the ruins of two or three more ancient craters which, +together with portions of the walls of Wilhelm I and Longomontanus, +have been honeycombed with smaller craters. The vast crateriform +depression above the center of the picture is Clavius, an unrivaled +wonder of lunar scenery, a hundred and forty-two miles in its greatest +length, while its whole immense floor has sunk two miles below the +general surface of the moon outside the ring. The monstrous +shadow-filled cavity above Clavius toward the right is Blancanus, +whose aspect here gives a good idea of the appearance of these chasms +when only their rims are in the sunlight. But observe the +indescribable savagery of the entire scene. It looks as though the +spirit of destruction had gone mad in this spot. The mighty craters +have broken forth one after another, each rending its predecessor; and +when their work was finished, a minor but yet tremendous outbreak +occurred, and the face of the moon was gored and punctured with +thousands of smaller craters. These relatively small craters (small, +however, only in a lunar sense, for many of them would appear gigantic +on the earth) recall once more the theory of meteoric impact. It does +not seem impossible that some of them may have been formed by such an +agency. + +One would not wish for our planet such a fate as that which has +overtaken the moon, but we cannot be absolutely sure that something of +the kind may not be in store for it. We really know nothing of the +ultimate causes of volcanic activity, and some have suggested that the +internal energies of the earth may be accumulating instead of dying +out, and may never yet have exhibited their utmost destructive power. +Perhaps the best assurance that we can find that the earth will escape +the catastrophe that has overtaken its satellite is to be found in the +relatively great force of its gravitation. The moon has been the +victim of its weakness; given equal forces, and the earth would be the +better able to withstand them. It is significant, in connection with +these considerations, that the little planet Mercury, which seems also +to have parted with its air and water, shows to the telescope some +indications that it is pitted with craters resembling those that have +torn to pieces the face of the moon. + +Upon the whole, after studying the dreadful lunar landscapes, one +cannot feel a very enthusiastic sympathy with those who are seeking +indications of the continued existence of some kind of life on the +moon; such a world is better without inhabitants. It has met its fate; +let it go! Fortunately, it is not so near that it cannot hide its +scars and appear beautiful -- except when curiosity impels us to look +with the penetrating eyes of the astronomer. + +The Great Mars Problem + +Let any thoughtful person who is acquainted with the general facts of +astronomy look up at the heavens some night when they appear in their +greatest splendor, and ask himself what is the strongest impression +that they make upon his mind. He may not find it easy to frame an +answer, but when he has succeeded it will probably be to the effect +that the stars give him an impression of the universality of +intelligence; they make him feel, as the sun and the moon cannot do, +that his world is not alone; that all this was not made simply to form +a gorgeous canopy over the tents of men. If he is of a devout turn of +mind, he thinks, as he gazes into those fathomless deeps and among +those bewildering hosts, of the infinite multitude of created beings +that the Almighty has taken under his care. The narrow ideas of the +old geocentric theology, which made the earth God's especial +footstool, and man his only rational creature, fall away from him like +a veil that had obscured his vision; they are impossible in the +presence of what he sees above. Thus the natural tendency, in the +light of modern progress, is to regard the universe as everywhere +filled with life. + +But science, which is responsible for this broadening of men's +thoughts concerning the universality of life, itself proceeds to set +limits. Of spiritual existences it pretends to know nothing, but as to +physical beings, it declares that it can only entertain the +supposition of their existence where it finds evidence of an +environment suited to their needs, and such environment may not +everywhere exist. Science, though repelled by the antiquated +theological conception of the supreme isolation of man among created +beings, regards with complacency the probability that there are +regions in the universe where no organic life exists, stars which +shine upon no inhabited worlds, and planets which nourish no animate +creatures. The astronomical view of the universe is that it consists +of matter in every stage of evolution: some nebulous and chaotic; some +just condensing into stars (suns) of every magnitude and order; some +shaped into finished solar bodies surrounded by dependent planets; +some forming stars that perhaps have no planets, and will have none; +some constituting suns that are already aging, and will soon lose +their radiant energy and disappear; and some aggregated into masses +that long ago became inert, cold, and rayless, and that can only be +revivified by means about which we can form conjectures, but of which +we actually know nothing. + +As with the stars, so with the planets, which are the satellites of +stars. All investigations unite to tell us that the planets are not +all in the same state of development. As some are large and some +small, so some are, in an evolutionary sense, young, and some old. As +they depend upon the suns around which they revolve for their light, +heat, and other forms of radiant energy, so their condition varies +with their distance from those suns. Many may never arrive at a state +suitable for the maintenance of life upon their surfaces; some which +are not at present in such a state may attain it later; and the forms +of life themselves may vary with the peculiar environment that +different planets afford. Thus we see that we are not scientifically +justified in affirming that life is ubiquitous, although we are thus +justified in saying that it must be, in a general sense, universal. We +might liken the universe to a garden known to contain every variety of +plant. If on entering it we see no flowers, we examine the species +before us and find that they are not of those which bloom at this +particular season, or perhaps they are such as never bear flowers. Yet +we feel no doubt that we shall find flowers somewhere in the garden, +because there are species which bloom at this season, and the garden +contains all varieties. + +While it is tacitly assumed that there are planets revolving around +other stars than the sun, it would be impossible for us to see them +with any telescope yet invented, and no instrument now in the +possession of astronomers could assure us of their existence; so the +only planetary system of which we have visual knowledge is our own. +Excluding the asteroids, which could not from any point of view be +considered as habitable, we have in the solar system eight planets of +various sizes and situated at various distances from the sun. Of these +eight we know that one, the earth, is inhabited. The question, then, +arises: Are there any of the others which are inhabited or habitable? +Since it is our intention to discuss the habitability of only one of +the seven to which the question applies, the rest may be dismissed in +a few words. The smallest of them, and the nearest to the sun, is +Mercury, which is regarded as uninhabitable because it has no +perceptible supply of water and air, and because, owing to the +extraordinary eccentricity of its orbit, it is subjected to excessive +and very rapid alterations in the amount of solar heat and light +poured upon its surface, such alterations being inconsistent with the +supposition that it can support living beings. Even its average +temperature is more than six and a half times that prevailing on the +earth! Another circumstance which militates against its habitability +is that, according to the results of the best telescopic studies, it +always keeps the same face toward the sun, so that one half of the +planet is perpetually exposed to the fierce solar rays, and the other +half faces the unmitigated cold of open space. Venus, the next in +distance from the sun, is almost the exact twin of the earth in size, +and many arguments may be urged in favor of its habitability, although +it is suspected of possessing the same peculiarity as Mercury, in +always keeping the same side sunward. Unfortunately its atmosphere +appears to be so dense that no permanent markings on its surface are +certainly visible, and the question of its actual condition must, for +the present, be left in abeyance. Mars, the first planet more distant +from the sun than the earth, is the special subject of this chapter, +and will be described and discussed a few lines further on. Jupiter, +Saturn, Uranus, and Neptune, the four giant planets, all more distant +than Mars, and each more distant than the other in the order named, +are all regarded as uninhabitable because none of them appears to +possess any degree of solidity. They may have solid or liquid nuclei, +but exteriorly they seem to be mere balls of cloud. Of course, one can +imagine what he pleases about the existence of creatures suited to the +physical constitution of such planets as these, but they must be +excluded from the category of habitable worlds in the ordinary sense +of the term. We go back, then, to Mars. + +It will be best to begin with a description of the planet. Mars is +4230 miles in diameter; its surface is not much more than one-quarter +as extensive as that of the earth (.285). Its mean distance from the +sun is 141,500,000 miles, 48,500,000 miles greater than that of the +earth. Since radiant energy varies inversely as the square of +distance, Mars receives less than half as much solar light and heat as +the earth gets. Mars' year (period of revolution round the sun) is 687 +days. Its mean density is 71 per cent of the earth's, and the force of +gravity on its surface is 38 per cent of that on the surface of the +earth; i.e., a body weighing one hundred pounds on the earth would, if +transported to Mars, weigh but thirty-eight pounds. The inclination of +its equator to the plane of its orbit differs very little from that of +the earth's equator, and its axial rotation occupies 24 hours 37 +minutes. so that the length of day and night, and the extent of the +seasonal changes on Mars, are almost precisely the same as on the +earth. But owing to the greater length of its year, the seasons of +Mars, while occurring in the same order, are almost twice as long as +ours. The surface of the planet is manifestly solid, like that of our +globe, and the telescope reveals many permanent markings on it, +recalling the appearance of a globe on which geographical features +have been represented in reddish and dusky tints. Around the poles are +plainly to be seen rounded white areas, which vary in extent with the +Martian seasons, nearly vanishing in summer and extending widely in +winter. The most recent spectroscopic determinations indicate that +Mars has an atmosphere perhaps as dense as that to be found on our +loftiest mountain peaks, and there is a perceptible amount of watery +vapor in this atmosphere. The surface of the planet appears to be +remarkably level, and it has no mountain ranges. No evidences of +volcanic action have been discovered on Mars. The dusky and reddish +areas were regarded by the early observers as respectively seas and +lands, but at present it is not believed that there are any bodies of +water on the planet. There has never been much doubt expressed that +the white areas about the poles represent snow. + +It will be seen from this brief description that many remarkable +resemblances exist between Mars and the earth, and there is nothing +wonderful in the fact that the question of the habitability of the +former has become one of extreme and wide-spread interest, giving rise +to the most diverse views, to many extraordinary speculations, and +sometimes to regrettably heated controversy. The first champion of the +habitability of Mars was Sir William Herschel, although even before +his time the idea had been suggested. He was convinced by the +revelations of his telescopes, continually increasing in power, that +Mars was more like the earth than any other planet. He could not +resist the testimony of the polar snows, whose suggestive conduct was +in such striking accord with what occurs upon the earth. Gradually, as +telescopes improved and observers increased in number, the principal +features of the planet were disclosed and charted, and ``areography,'' +as the geography of Mars was called, took its place among the +recognized branches of astronomical study. But it was not before 1877 +that a fundamentally new discovery in areography gave a truly +sensational turn to speculation about life on ``the red planet.'' In +that year Mars made one of its nearest approaches to the earth, and +was so situated in its orbit that it could be observed to great +advantage from the northern hemisphere of the earth. The celebrated +Italian astronomer, Schiaparelli, took advantage of this opportunity +to make a trigonometrical survey of the surface of Mars -- as coolly +and confidently as if he were not taking his sights across a +thirty-five-million-mile gulf of empty space -- and in the course of +this survey he was astonished to perceive that the reddish areas, then +called continents, were crossed in many directions by narrow, dusky +lines, to which he gave the suggestive name of ``canals.'' Thus a kind +of firebrand was cast into the field of astronomical speculation, +which has ever since produced disputes that have sometimes approached +the violence of political faction. At first the accuracy of +Schiaparelli's observations was contested; it required a powerful +telescope, and the most excellent ``seeing,'' to render the +enigmatical lines visible at all, and many searchers were unable to +detect them. But Schiaparelli continued his studies in the serene sky +of Italy, and produced charts of the gridironed face of Mars +containing so much astonishing detail that one had either to reject +them in toto or to confess that Schiaparelli was right. As subsequent +favorable oppositions of Mars occurred, other observers began to see +the ``canals'' and to confirm the substantial accuracy of the Italian +astronomer's work, and finally few were found who would venture to +affirm that the ``canals'' did not exist, whatever their meaning might +be. + +When Schiaparelli began his observations it was generally believed, as +we have said, that the dusky areas on Mars were seas, and since +Schiaparelli thought that the ``canals'' invariably began and ended at +the shores of the ``seas,'' the appropriateness of the title given to +the lines seemed apparent. Their artificial character was immediately +assumed by many, because they were too straight and too suggestively +geometrical in their arrangement to permit the conclusion that they +were natural watercourses. A most surprising circumstance noted by +Schiaparelli was that the ``canals'' made their appearance after the +melting of the polar snow in the corresponding hemisphere had begun, +and that they grew darker, longer, and more numerous in proportion as +the polar liquidation proceeded; another very puzzling observation was +that many of them became double as the season advanced; close beside +an already existing ``canal,'' and in perfect parallelism with it, +another would gradually make its appearance. That these phenomena +actually existed and were not illusions was proved by later +observations, and today they are seen whenever Mars is favorably +situated for observation. + +In the closing decade of the nineteenth century, Mr Percival Lowell +took up the work where Schiaparelli had virtually dropped it, and soon +added a great number of ``canals'' to those previously known, so that +in his charts the surface of the wonderful little planet appears +covered as with a spider's web, the dusky lines criss-crossing in +every direction, with conspicuous knots wherever a number of them come +together. Mr Lowell has demonstrated that the areas originally called +seas, and thus named on the earlier charts, are not bodies of water, +whatever else they may be. He has also found that the mysterious lines +do not, as Schiaparelli supposed, begin and end at the edges of the +dusky regions, but often continue on across them, reaching in some +cases far up into the polar regions. But Schiaparelli was right in his +observation that the appearance of the ``canals'' is synchronous with +the gradual disappearance of the polar snows, and this fact has become +the basis of the most extraordinary theory that the subject of life in +other worlds has ever given birth to. + +Now, the effect of such discoveries, as we have related, depends upon +the type of mind to whose attention they are called. Many are content +to accept them as strange and inexplicable at present, and to wait for +further light upon them; others insist upon an immediate inquiry +concerning their probable nature and meaning. Such an inquiry can only +be based upon inference proceeding from analogy. Mars, say Mr Lowell +and those who are of his opinion, is manifestly a solidly incrusted +planet like the earth; it has an atmosphere, though one of great +rarity; it has water vapor, as the snows in themselves prove; it has +the alternation of day and night, and a succession of seasons closely +resembling those of the earth; its surface is suggestively divided +into regions of contrasting colors and appearance, and upon that +surface we see an immense number of lines geometrically arranged, with +a system of symmetrical intersections where the lines expand into +circular and oval areas -- and all connected with the annual melting +of the polar snows in a way which irresistibly suggests the +interference of intelligence directed to a definite end. Why, with so +many concurrent circumstances to support the hypothesis, should we not +regard Mars as an inhabited globe? + +But the differences between Mars and the earth are in many ways as +striking as their resemblances. Mars is relatively small; it gets less +than half as much light and heat as we receive; its atmosphere is so +rare that it would be distressing to us, even if we could survive in +it at all; it has no lakes, rivers, or seas; its surface is an endless +prairie. and its ``canals'' are phenomena utterly unlike anything on +the earth. Yet it is precisely upon these divergences between the +earth and Mars, this repudiation of terrestrial standards, that the +theory of ``life on Mars,'' for which Mr Lowell is mainly responsible, +is based. Because Mars is smaller than the earth, we are told it must +necessarily be more advanced in planetary evolution, the underlying +cause of which is the gradual cooling and contraction of the planet's +mass. Mars has parted with its internal heat more rapidly than the +earth; consequently its waters and its atmosphere have been mostly +withdrawn by chemical combinations, but enough of both yet remain to +render life still possible on its surface. As the globe of Mars is +evolutionally older than that of the earth, so its forms of organic +life may be proportionally further advanced, and its inhabitants may +have attained a degree of cultivated intelligence much superior to +what at present exists upon the earth. Understanding the nature and +the causes of the desiccation of their planet, and possessing +engineering science and capabilities far in advance of ours, they may +be conceived to have grappled with the stupendous problem of keeping +their world in a habitable condition as long as possible. Supposing +them to have become accustomed to live in their rarefied atmosphere (a +thing not inconceivable, since men can live for a time at least in air +hardly less rare), the most pressing problem for them is that of a +water-supply, without which plant life cannot exist, while animal life +in turn depends for its existence upon vegetation. The only direction +in which they can seek water is that of the polar regions, where it is +alternately condensed into snow and released in the liquid form by the +effect of the seasonal changes. It is, then, to the annual melting of +the polar snow-fields that the Martian engineers are supposed to have +recourse in supplying the needs of their planet, and thus providing +the means of prolonging their own existence. It is imagined that they +have for this purpose constructed a stupendous system of irrigation +extending over the temperate and equatorial regions of the planet. The +``canals'' represent the lines of irrigation, but the narrow streaks +that we see are not the canals themselves, but the irrigated bands +covered by them. Their dark hue, and their gradual appearance after +the polar melting has begun, are due to the growth of vegetation +stimulated by the water. The rounded areas visible where several +``canals'' meet and cross are called by Mr Lowell ``oases.'' These are +supposed to be the principal centers of population and industry. It +must be confessed that some of them, with their complicated systems of +radiating lines, appear to answer very well to such a theory. No +attempt to explain them by analogy with natural phenomena on the earth +has proved successful. + +But a great difficulty yet remains: How to explain the seemingly +miraculous powers of the supposed engineers? Here recourse is had once +more to the relative smallness of the planet. We have remarked that +the force of gravity on Mars is only thirty-eight per cent of that on +the earth. A steam-shovel driven by a certain horse-power would be +nearly three times as effective there as here. A man of our stature on +Mars would find his effective strength increased in the same +proportion. But just because of the slight force of gravity there, a +Martian might attain to the traditional stature of Goliath without +finding his own weight an encumbrance to his activity, while at the +same time his huge muscles would come into unimpeded play, enabling +him single-handed to perform labors that would be impossible to a +whole gang of terrestrial workmen. The effective powers of huge +machines would be increased in the same way; and to all this must be +added the fact that the mean density of the materials of which Mars is +composed is much less than that of the constituents of the earth. +Combining all these considerations, it becomes much less difficult to +conceive that public works might be successfully undertaken on Mars +which would be hopelessly beyond the limits of human accomplishment. + +Certain other difficulties have also to be met; as, for instance, the +relative coldness of the climate of Mars. At its distance it gets +considerably less than half as much light and heat as we receive. In +addition to this, the rarity of its atmosphere would naturally be +expected to decrease the effective temperature at the planet's +surface, since an atmosphere acts somewhat like the glass cover of a +hot-house in retaining the solar heat which has penetrated it. It has +been calculated that, unless there are mitigating circumstances of +which we know nothing, the average temperature at the surface of Mars +must be far below the freezing-point of water. To this it is replied +that the possible mitigating circumstances spoken of evidently exist +in fact, because we can see that the watery vapor condenses into snow +around the poles in winter, but melts again when summer comes. The +mitigating agent may be supposed to exist in the atmosphere where the +presence of certain gases would completely alter the temperature +gradients. + +It might also be objected that it is inconceivable that the Martian +engineers, however great may be their physical powers, and however +gigantic the mechanical energies under their control, could force +water in large quantities from the poles to the equator. This is an +achievement that measures up to the cosmical standard. It is admitted +by the champions of the theory that the difficulty is a formidable +one; but they call attention to the singular fact that on Mars there +can be found no chains of mountains, and it is even doubtful if ranges +of hills exist there. The entire surface of the planet appears to be +almost ``as smooth as a billiard ball,'' and even the broad regions +which were once supposed to be seas apparently lie at practically the +same level as the other parts, since the ``canals'' in many cases run +uninterruptedly across them. Lowell's idea is that these sombre areas +may be expanses of vegetation covering ground of a more or less marshy +character, for while the largest of them appear to be permanent, there +are some which vary coincidently with the variations of the canals. + +As to the kind of machinery employed to force the water from the +poles, it has been conjectured that it may have taken the form of a +gigantic system of pumps and conduits; and since the Martians are +assumed to be so far in advance of us in their mastery of scientific +principles, the hypothesis will at least not be harmed by supposing +that they have learned to harness forces of nature whose very +existence in a manageable form is yet unrecognized on the earth. If we +wish to let the imagination loose, we may conjecture that they have +conquered the secret of those intra-atomic forces whose resistless +energy is beginning to become evident to us, but the possibility of +whose utilization remains a dream, the fulfillment of which nobody +dares to predict. + +Such, in very brief form, is the celebrated theory of Mars as an +inhabited world. It certainly captivates the imagination, and if we +believe it to represent the facts, we cannot but watch with the +deepest sympathy this gallant struggle of an intellectual race to +preserve its planet from the effects of advancing age and death. We +may, indeed, wonder whether our own humanity, confronted by such a +calamity, could be counted on to meet the emergency with equal +stoutness of heart and inexhaustibleness of resource. Up to the +present time we certainly have shown no capacity to confront Nature +toe to toe, and to seize her by the shoulders and turn her round when +she refuses to go our way. If we could get into wireless telephonic +communication with the Martians we might learn from their own lips the +secret of their more than ``Roman recovery.'' + +The Riddle of the Asteroids + +Between the orbits of Mars and Jupiter revolves the most remarkable +system of little bodies with which we are acquainted -- the Asteroids, +or Minor Planets. Some six hundred are now known, and they may +actually number thousands. They form virtually a ring about the sun. +The most striking general fact about them is that they occupy the +place in the sky which should be occupied, according to Bode's Law, by +a single large planet. This fact, as we shall see, has led to the +invention of one of the most extraordinary theories in astronomy -- +viz., that of the explosion of a world! + +Bode's Law, so-called, is only an empiric formula, but until the +discovery of Neptune it accorded so well with the distances of the +planets that astronomers were disposed to look upon it as really +representing some underlying principle of planetary distribution. They +were puzzled by the absence of a planet in the space between Mars and +Jupiter, where the ``law'' demanded that there should be one, and an +association of astronomers was formed to search for it. There was a +decided sensation when, in 1801, Piazzi, of Palermo, announced that he +had found a little planet which apparently occupied the place in the +system which belonged to the missing body. He named it Ceres, and it +was the first of the Asteroids. The next year Olbers, of Bremen, while +looking for Ceres with his telescope, stumbled upon another small +planet which he named Pallas. Immediately he was inspired with the +idea that these two planets were fragments of a larger one which had +formerly occupied the vacant place in the planetary ranks, and he +predicted that others would be found by searching in the neighborhood +of the intersection of the orbits of the two already discovered. This +bold prediction was brilliantly fulfilled by the finding of two more +-- Juno in 1804, and Vesta in 1807. Olbers would seem to have been led +to the invention of his hypothesis of a planetary explosion by the +faith which astronomers at that time had in Bode's Law. They appear to +have thought that several planets revolving in the gap where the +``law'' called for but one could only be accounted for upon the theory +that the original one had been broken up to form the several. +Gravitation demanded that the remnants of a planet blown to pieces, no +matter how their orbits might otherwise differ, should all return at +stated periods to the point where the explosion had occurred; hence +Olbers' prediction that any asteroids that might subsequently be +discovered would be found to have a common point of orbital +intersection. And curiously enough all of the first asteroids found +practically answered to this requirement. Olbers' theory seemed to be +established. + +After the first four, no more asteroids were found until 1845, when +one was discovered; then, in 1847, three more were added to the list; +and after that searchers began to pick them up with such rapidity that +by the close of the century hundreds were known, and it had become +almost impossible to keep track of them. The first four are by far the +largest members of the group, but their actual sizes remained unknown +until less than twenty years ago. It was long supposed that Vesta was +the largest, because it shines more brightly than any of the others; +but finally, in 1895, Barnard, with the Lick telescope, definitely +measured their diameters, and proved to everybody's surprise that +Ceres is really the chief, and Vesta only the third in rank. His +measures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, +239 miles; and Juno, 120 miles. They differ greatly in the reflective +power of their surfaces, a fact of much significance in connection +with the question of their origin. Vesta is, surface for surface, +rather more than three times as brilliant as Ceres, whence the +original mistake about its magnitude. + +Nowadays new asteroids are found frequently by photography, but +physically they are most insignificant bodies, their average diameter +probably not exceeding twenty miles, and some are believed not to +exceed ten. On a planet only ten miles in diameter, assuming the same +mean density as the earth's, which is undoubtedly too much, the force +of gravity would be so slight that an average man would not weigh more +than three ounces, and could jump off into space whenever he liked. + +Although the asteroids all revolve around the sun in the same +direction as that pursued by the major planets, their orbits are +inclined at a great variety of angles to the general plane of the +planetary system, and some of them are very eccentric -- almost as +much so as the orbits of many of the periodic comets. It has even been +conjectured that the two tiny moons of Mars and the four smaller +satellites of Jupiter may be asteroids gone astray and captured by +those planets. Two of the asteroids are exceedingly remarkable for the +shapes and positions of their orbits; these are Eros, discovered in +1898, and T. G., 1906, found eight years later. The latter has a mean +distance from the sun slightly greater than that of Jupiter, while the +mean distance of Eros is less than that of Mars. The orbit of Eros is +so eccentric that at times it approaches within 15,000,000 miles of +the earth, nearer than any other regular member of the solar system +except the moon, thus affording an unrivaled means of measuring the +solar parallax. But for our present purpose the chief interest of Eros +lies in its extraordinary changes of light. + +These changes, although irregular, have been observed and photographed +many times, and there seems to be no doubt of their reality. Their +significance consists in their possible connection with the form of +the little planet, whose diameter is generally estimated at not more +than twenty miles. Von Oppolzer found, in 1901, that Eros lost +three-fourths of its brilliancy once in every two hours and +thirty-eight minutes. Other observers have found slightly different +periods of variability, but none as long as three hours. The most +interesting interpretation that has been offered of this phenomenon is +that it is due to a great irregularity of figure, recalling at once +Olbers' hypothesis. According to some, Eros may be double, the two +bodies composing it revolving around each other at very close +quarters; but a more striking, and it may be said probable, suggestion +is that Eros has a form not unlike that of a dumb-bell, or hour-glass, +turning rapidly end over end so that the area of illuminated surface +presented to our eyes continually changes, reaching at certain times a +minimum when the amount of light that it reflects toward the earth is +reduced to a quarter of its maximum value. Various other bizarre +shapes have been ascribed to Eros, such, for instance, as that of a +flat stone revolving about one of its longer axes, so that sometimes +we see its face and sometimes its edge. + +All of these explanations proceed upon the assumption that Eros cannot +have a simple globular figure like that of a typical planet, a figure +which is prescribed by the law of gravitation, but that its shape is +what may be called accidental; in a word, it is a fragment, for it +seems impossible to believe that a body formed in interplanetary +space, either through nebular condensation or through the aggregation +of particles drawn together by their mutual attractions, should not be +practically spherical in shape. Nor is Eros the only asteroid that +gives evidence by variations of brilliancy that there is something +abnormal in its constitution; several others present the same +phenomenon in varying degrees. Even Vesta was regarded by Olbers as +sufficiently variable in its light to warrant the conclusion that it +was an angular mass instead of a globe. Some of the smaller ones show +very notable variations, and all in short periods, of three or four +hours, suggesting that in turning about one of their axes they present +a surface of variable extent toward the sun and the earth. + +The theory which some have preferred -- that the variability of light +is due to the differences of reflective power on different parts of +the surface -- would, if accepted, be hardly less suggestive of the +origin of these little bodies by the breaking up of a larger one, +because the most natural explanation of such differences would seem to +be that they arose from variations in the roughness or smoothness of +the reflecting surface, which would be characteristic of fragmentary +bodies. In the case of a large planet alternating expanses of land and +water, or of vegetation and desert, would produce a notable variation +in the amount of reflection, but on bodies of the size of the +asteroids neither water nor vegetation could exist, and an atmosphere +would be equally impossible. + +One of the strongest objections to Olbers' hypothesis is that only a +few of the first asteroids discovered travel in orbits which +measurably satisfy the requirement that they should all intersect at +the point where the explosion occurred. To this it was at first +replied that the perturbations of the asteroidal orbits, by the +attractions of the major planets, would soon displace them in such a +manner that they would cease to intersect. One of the first +investigations undertaken by the late Prof. Simon Newcomb was directed +to the solution of this question, and he arrived at the conclusion +that the planetary perturbations could not explain the actual +situation of the asteroidal orbits. But afterward it was pointed out +that the difficulty could be avoided by supposing that not one but a +series of explosions had produced the asteroids as they now are. After +the primary disruption the fragments themselves, according to this +suggestion, may have exploded, and then the resulting orbits would be +as ``tangled'' as the heart could wish. This has so far rehabilitated +the explosion theory that it has never been entirely abandoned, and +the evidence which we have just cited of the probably abnormal shapes +of Eros and other asteroids has lately given it renewed life. It is a +subject that needs a thorough rediscussion. + +We must not fail to mention, however, that there is a rival hypothesis +which commends itself to many astronomers -- viz., that the asteroids +were formed out of a relatively scant ring of matter, situated between +Mars and Jupiter and resembling in composition the immensely more +massive rings from which, according to Laplace's hypothesis, the +planets were born. It is held by the supporters of this theory that +the attraction of the giant Jupiter was sufficient to prevent the +small, nebulous ring that gave birth to the asteroids from condensing +like the others into a single planet. + +But if we accept the explosion theory, with its corollary that minor +explosions followed the principal one, we have still an unanswered +question before us: What caused the explosions? The idea of a world +blowing up is too Titanic to be shocking; it rather amuses the +imagination than seriously impresses it; in a word, it seems +essentially chimerical. We can by no appeal to experience form a +mental picture of such an occurrence. Even the moon did not blow up +when it was wrecked by volcanoes. The explosive nebulae and new stars +are far away in space, and suggest no connection with such a +catastrophe as the bursting of a planet into hundreds of pieces. We +cannot conceive of a great globe thousands of miles in diameter +resembling a pellet of gunpowder only awaiting the touch of a match to +cause its sudden disruption. Somehow the thought of human agency +obtrudes itself in connection with the word ``explosion,'' and we +smile at the idea that giant powder or nitro-glycerine could blow up a +planet. Yet it would only need enough of them to do it. + +After all, we may deceive ourselves in thinking, as we are apt to do, +that explosive energies lock themselves up only in small masses of +matter. There are many causes producing explosions in nature, every +volcanic eruption manifests the activity of some of them. Think of the +giant power of confined steam; if enough steam could be suddenly +generated in the center of the earth by a downpour of all the waters +of the oceans, what might not the consequences be for our globe? In a +smaller globe, and it has never been estimated that the original +asteroid was even as large as the moon, such a catastrophe would, +perhaps, be more easily conceivable; but since we are compelled in +this case to assume that there was a series of successive explosions, +steam would hardly answer the purpose; it would be more reasonable to +suppose that the cause of the explosion was some kind of chemical +reaction, or something affecting the atoms composing the exploding +body. Here Dr Gustav Le Bon comes to our aid with a most startling +suggestion, based on his theory of the dissipation of intra-atomic +energy. It will be best to quote him at some length from his book on +The Evolution of Forces. + +``It does not seem at first sight,'' says Doctor Le Bon, + + very comprehensible that worlds which appear more and more stable + as they cool could become so unstable as to afterward dissociate + entirely. To explain this phenomenon, we will inquire whether + astronomical observations do not allow us to witness this + dissociation. + + We know that the stability of a body in motion, such as a top or a + bicycle, ceases to be possible when its velocity of rotation + descends below a certain limit. Once this limit is reached it loses + its stability and falls to the ground. Prof. J. J. Thomson even + interprets radio-activity in this manner, and points out that when + the speed of the elements composing the atoms descends below a + certain limit they become unstable and tend to lose their + equilibria. There would result from this a commencement of + dissociation, with diminution of their potential energy and a + corresponding increase of their kinetic energy sufficient to launch + into space the products of intra-atomic disintegration. + + It must not be forgotten that the atom being an enormous reservoir + of energy is by this very fact comparable with explosive bodies. + These last remain inert so long as their internal equilibria are + undisturbed. So soon as some cause or other modifies these, they + explode and smash everything around them after being themselves + broken to pieces. + + Atoms, therefore, which grow old in consequence of the diminution + of a part of their intra-atomic energy gradually lose their + stability. A moment, then, arrives when this stability is so weak + that the matter disappears by a sort of explosion more or less + rapid. The bodies of the radium group offer an image of this + phenomenon -- a rather faint image, however, because the atoms of + this body have only reached a period of instability when the + dissociation is rather slow. It probably precedes another and more + rapid period of dissociation capable of producing their final + explosion. Bodies such as radium, thorium, etc., represent, no + doubt, a state of old age at which all bodies must some day arrive, + and which they already begin to manifest in our universe, since all + matter is slightly radio-active. It would suffice for the + dissociation to be fairly general and fairly rapid for an explosion + to occur in a world where it was manifested. + + These theoretical considerations find a solid support in the sudden + appearances and disappearances of stars. The explosions of a world + which produce them reveal to us, perhaps, how the universes perish + when they become old. + + As astronomical observations show the relative frequency of these + rapid destructions, we may ask ourselves whether the end of a + universe by a sudden explosion after a long period of old age does + not represent its most general ending. + +Here, perhaps, it will be well to stop, since, entrancing as the +subject may be, we know very little about it, and Doctor Le Bon's +theory affords a limitless field for the reader's imagination. + _________________________________________________________________ + +A printed version of this book is available from Sattre Press +(http://csky.sattre-press.com). 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Be sure to check the +copyright laws for your country before downloading or redistributing +this or any other Project Gutenberg eBook. + +This header should be the first thing seen when viewing this Project +Gutenberg file. Please do not remove it. Do not change or edit the +header without written permission. + +Please read the "legal small print," and other information about the +eBook and Project Gutenberg at the bottom of this file. Included is +important information about your specific rights and restrictions in +how the file may be used. You can also find out about how to make a +donation to Project Gutenberg, and how to get involved. + + +**Welcome To The World of Free Plain Vanilla Electronic Texts** + +**eBooks Readable By Both Humans and By Computers, Since 1971** + +*****These eBooks Were Prepared By Thousands of Volunteers!***** + + +Title: Curiosities of the Sky + +Author: Garrett Serviss + +Release Date: October, 2004 [EBook #6630] +[Yes, we are more than one year ahead of schedule] +[This file was first posted on January 6, 2003] + +Edition: 10 + +Language: English + +Character set encoding: ASCII + +*** START OF THE PROJECT GUTENBERG EBOOK, CURIOSITIES OF THE SKY *** + + + + + + Curiosities of the Sky + +by Garrett Serviss + +Curiosities of the Sky was first published in 1909 and the text is in +the public domain. The transcription was done by William McClain +(info@sattre-press.com), 2002. + +A printed version of this book is available from Sattre Press +(http://csky.sattre-press.com). It includes extensive annotations, a +new introduction and all the original photographs and diagrams. +_________________________________________________________________ + +Preface + +What Froude says of history is true also of astronomy: it is the most +impressive where it transcends explanation. It is not the mathematics +of astronomy, but the wonder and the mystery that seize upon the +imagination. The calculation of an eclipse owes all its prestige to +the sublimity of its data; the operation, in itself, requires no more +mental effort than the preparation of a railway time-table. + +The dominion which astronomy has always held over the minds of men is +akin to that of poetry; when the former becomes merely instructive and +the latter purely didactic, both lose their power over the +imagination. Astronomy is known as the oldest of the sciences, and it +will be the longest-lived because it will always have arcana that have +not been penetrated. + +Some of the things described in this book are little known to the +average reader, while others are well known; but all possess the +fascination of whatever is strange, marvelous, obscure, or mysterious +-- magnified, in this case, by the portentous scale of the phenomena. + +The idea of the author is to tell about these things in plain +language, but with as much scientific accuracy as plain language will +permit, showing the wonder that is in them without getting away from +the facts. Most of them have hitherto been discussed only in technical +form, and in treatises that the general public seldom sees and never +reads. + +Among the topics touched upon are: + * The strange unfixedness of the ``fixed stars,'' the vast + migrations of the suns and worlds constituting the universe. + * The slow passing out of existence of those collocations of stars + which for thousands of years have formed famous + ``constellations,'' preserving the memory of mythological heroes + and heroines, and perhaps of otherwise unrecorded history. + * The tendency of stars to assemble in immense clouds, swarms, and + clusters. + * The existence in some of the richest regions of the universe of + absolutely black, starless gaps, deeps, or holes, as if one were + looking out of a window into the murkiest night. + * The marvelous phenomena of new, or temporary, stars, which appear + as suddenly as conflagrations, and often turn into something else + as eccentric as themselves. + * The amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,'' + and ``lace,'' or ``tress,'' nebul. + * The strange surroundings of the sun, only seen in particular + circumstances, but evidently playing a constant part in the daily + phenomena of the solar system. + * The mystery of the Zodiacal Light and the Gegenschein. + * The extraordinary transformations undergone by comets and their + tails. + * The prodigies of meteorites and masses of stone and metal fallen + from the sky. + * The cataclysms that have wrecked the moon. + * The problem of life and intelligence on the planet Mars. + * The problematical origin and fate of the asteroids. + * The strange phenomena of the auroral lights. + +An attempt has been made to develop these topics in an orderly way, +showing their connection, so that the reader may obtain a broad +general view of the chief mysteries and problems of astronomy, and an +idea of the immense field of discovery which still lies, almost +unexplored, before it. + +The Windows of Absolute Night + +To most minds mystery is more fascinating than science. But when +science itself leads straight up to the borders of mystery and there +comes to a dead stop, saying, ``At present I can no longer see my +way,'' the force of the charm is redoubled. On the other hand, the +illimitable is no less potent in mystery than the invisible, whence +the dramatic effect of Keats' ``stout Cortez'' staring at the +boundless Pacific while all his men look at each other with a wild +surmise, ``silent upon a peak in Darien.'' It is with similar feelings +that the astronomer regards certain places where from the peaks of the +universe his vision seems to range out into endless empty space. He +sees there the shore of his little isthmus, and, beyond, unexplored +immensity. + +The name, ``coal-sacks,'' given to these strange voids is hardly +descriptive. Rather they produce upon the mind the effect of blank +windows in a lonely house on a pitch-dark night, which, when looked at +from the brilliant interior, become appalling in their rayless murk. +Infinity seems to acquire a new meaning in the presence of these black +openings in the sky, for as one continues to gaze it loses its purely +metaphysical quality and becomes a kind of entity, like the ocean. The +observer is conscious that he can actually see the beginning of its +ebon depths, in which the visible universe appears to float like an +enchanted island, resplendent within with lights and life and gorgeous +spectacles, and encircled with screens of crowded stars, but with its +dazzling vistas ending at the fathomless sea of pure darkness which +encloses all. + +The Galaxy, or Milky Way, surrounds the borders of our island in space +like a stellar garland, and when openings appear in it they are, by +contrast, far more impressive than the general darkness of the +interstellar expanse seen in other directions. Yet even that expanse +is not everywhere equally dark, for it contains gloomy deeps +discernable with careful watching. Here, too, contrast plays an +important part, though less striking than within the galactic region. +Some of Sir William Herschel's observations appear to indicate an +association between these tenebrious spots and neighboring star clouds +and nebul. It is an illuminating bit of astronomical history that +when he was sweeping the then virgin heavens with his great telescopes +he was accustomed to say to his sister who, note-book in hand, waited +at his side to take down his words, fresh with the inspiration of +discovery: ``Prepare to write; the nebul are coming; here space is +vacant.'' + +The most famous of the ``coal-sacks,'' and the first to be brought to +general attention before astronomers had awakened to the significance +of such things, lies adjacent to the ``Southern Cross,'' and is truly +an amazing phenomenon. It is not alone the conspicuousness of this +celestial vacancy, opening suddenly in the midst of one of the richest +parts of the Galaxy, that has given it its fame, but quite as much the +superstitious awe with which it was regarded by the early explorers of +the South Seas. To them, as well as to those who listened in rapt +wonder to their tales, the ``Coal-sack'' seemed to possess some occult +connection with the mystic ``Cross.'' In the eyes of the sailors it +was not a vacancy so much as a sable reality in the sky, and as, +shuddering, they stared at it, they piously crossed themselves. It was +another of the magical wonders of the unknown South, and as such it +formed the basis of many a ``wild surmise'' and many a sea-dog's yarn. +Scientific investigation has not diminished its prestige, and today no +traveler in the southern hemisphere is indifferent to its fascinating +strangeness, while some find it the most impressive spectacle of the +antarctic heavens. + +All around, up to the very edge of the yawning gap, the sheen of the +Milky Way is surpassingly glorious; but there, as if in obedience to +an almighty edict, everything vanishes. A single faint star is visible +within the opening, producing a curious effect upon the sensitive +spectator, like the sight of a tiny islet in the midst of a black, +motionless, waveless tarn. The dimensions of the lagoon of darkness, +which is oval or pear-shaped, are eight degrees by five, so that it +occupies a space in the sky about one hundred and thirty times greater +than the area of the full moon. It attracts attention as soon as the +eye is directed toward the quarter where it exists, and by virtue of +the rarity of such phenomena it appears a far greater wonder than the +drifts of stars that are heaped around it. Now that observatories are +multiplying in the southern hemisphere, the great austral +``Coal-sack'' will, no doubt, receive attention proportioned to its +importance as one of the most significant features of the sky. Already +at the Sydney Observatory photographs have shown that the southern +portion of this Dead Sea of Space is not quite ``bottomless,'' +although its northern part defies the longest sounding lines of the +astronomer. + +There is a similar, but less perfect, ``coal-sack'' in the northern +hemisphere, in the constellation of ``The Swan,'' which, strange to +say, also contains a well-marked figure of a cross outlined by stars. +This gap lies near the top of the cross-shaped figure. It is best seen +by averted vision, which brings out the contrast with the Milky Way, +which is quite brilliant around it. It does not, however, exercise the +same weird attraction upon the eye as the southern ``Coal-sack,'' for +instead of looking like an absolute void in the sky, it rather appears +as if a canopy of dark gauze had been drawn over the stars. We shall +see the possible significance of this appearance later. + +Just above the southern horizon of our northern middle latitudes, in +summer, where the Milky Way breaks up into vast sheets of nebulous +luminosity, lying over and between the constellations Scorpio and +Sagittarius, there is a remarkable assemblage of ``coal-sacks,'' +though none is of great size. One of them, near a conspicuous +star-cluster in Scorpio, M80, is interesting for having been the first +of these strange objects noted by Herschel. Probably it was its +nearness to M80 which suggested to his mind the apparent connection of +such vacancies with star-clusters which we have already mentioned. + +But the most marvelous of the ``coal-sacks'' are those that have been +found by photography in Sagittarius. One of Barnard's earliest and +most excellent photographs includes two of them, both in the +star-cluster M8. The larger, which is roughly rectangular in outline, +contains one little star, and its smaller neighbor is lune-shaped -- +surely a most singular form for such an object. Both are associated +with curious dark lanes running through the clustered stars like +trails in the woods. Along the borders of these lanes the stars are +ranked in parallel rows, and what may be called the bottoms of the +lanes are not entirely dark, but pebbled with faint stellar points. +One of them which skirts the two dark gaps and traverses the cluster +along its greatest diameter is edged with lines of stars, recalling +the alignment of the trees bordering a French highway. This road of +stars cannot be less than many billions of miles in length! + +All about the cluster the bed of the Galaxy is strangely disturbed, +and in places nearly denuded, as if its contents had been raked away +to form the immense stack and the smaller accumulations of stars +around it. The well-known ``Trifid Nebula'' is also included in the +field of the photograph, which covers a truly marvelous region, so +intricate in its mingling of nebul, star-clusters, star-swarms, +star-streams, and dark vacancies that no description can do it +justice. Yet, chaotic as it appears, there is an unmistakable +suggestion of unity about it, impressing the beholder with the idea +that all the different parts are in some way connected, and have not +been fortuitously thrown together. Miss Agnes M. Clerke made the +striking remark that the dusky lanes in M8 are exemplified on the +largest scale in the great rift dividing the Milky Way, from Cygnus in +the northern hemisphere all the way to the ``Cross'' in the southern. +Similar lanes are found in many other clusters, and they are generally +associated with flanking rows of stars, resembling in their +arrangement the thick-set houses and villas along the roadways that +traverse the approaches to a great city. + +But to return to the black gaps. Are they really windows in the +star-walls of the universe? Some of them look rather as if they had +been made by a shell fired through a luminous target, allowing the eye +to range through the hole into the void space beyond. If science is +discretely silent about these things, what can the more venturesome +and less responsible imagination suggest? Would a huge ``runaway +sun,'' like Arcturus, for instance, make such an opening if it should +pass like a projectile through the Milky Way? It is at least a +stimulating inquiry. Being probably many thousands of times more +massive than the galactic stars, such a stellar missile would not be +stopped by them, though its direction of flight might be altered. It +would drag the small stars lying close to its course out of their +spheres, but the ultimate tendency of its attraction would be to sweep +them round in its wake, thus producing rather a star-swarm than a +vacancy. Those that were very close to it might be swept away in its +rush and become its satellites, careering away with it in its flight +into outer space; but those that were farther off, and they would, of +course, greatly outnumber the nearer ones, would tend inward from all +sides toward the line of flight, as dust and leaves collect behind a +speeding motor (though the forces operating would be different), and +would fill up the hole, if hole it were. A swarm thus collected should +be rounded in outline and bordered with a relatively barren ring from +which the stars had been ``sucked'' away. In a general sense the M8 +cluster answers to this description, but even if we undertook to +account for its existence by a supposition like the above, the black +gaps would remain unexplained, unless one could make a further draft +on the imagination and suggest that the stars had been thrown into a +vast eddy, or system of eddies, whose vortices appear as dark holes. +Only a maelstrom-like motion could keep such a funnel open, for +without regard to the impulse derived from the projectile, the proper +motions of the stars themselves would tend to fill it. Perhaps some +other cause of the whirling motion may be found. As we shall see when +we come to the spiral nebul, gyratory movements are exceedingly +prevalent throughout the universe, and the structure of the Milky Way +is everywhere suggestive of them. But this is hazardous sport even for +the imagination -- to play with suns as if they were but thistle-down +in the wind or corks in a mill-race. + +Another question arises: What is the thickness of the hedge of stars +through which the holes penetrate? Is the depth of the openings +proportionate to their width? In other words, is the Milky Way round +in section like a rope, or flat and thin like a ribbon? The answer is +not obvious, for we have little or no information concerning the +relative distances of the faint galactic stars. It would be easier, +certainly, to conceive of openings in a thin belt than in a massive +ring, for in the first case they would resemble mere rifts and breaks, +while in the second they would be like wells or bore-holes. Then, too, +the fact that the Milky Way is not a continuous body but is made up of +stars whose actual distances apart is great, offers another quandary; +persistent and sharply bordered apertures in such an assemblage are a +priori as improbable, if not impossible, as straight, narrow holes +running through a swarm of bees. + +The difficulty of these questions indicates one of the reasons why it +has been suggested that the seeming gaps, or many of them, are not +openings at all, but opaque screens cutting off the light from stars +behind them. That this is quite possible in some cases is shown by +Barnard's later photographs, particularly those of the singular region +around the star Rho Ophiuchi. Here are to be seen somber lanes and +patches, apparently forming a connected system which covers an immense +space, and which their discoverer thinks may constitute a ``dark +nebula.'' This seems at first a startling suggestion; but, after all, +why should their not be dark nebul as well as visible ones? In truth, +it has troubled some astronomers to explain the luminosity of the +bright nebul, since it is not to be supposed that matter in so +diffuse a state can be incandescent through heat, and phosphorescent +light is in itself a mystery. The supposition is also in accord with +what we know of the existence of dark solid bodies in space. Many +bright stars are accompanied by obscure companions, sometimes as +massive as themselves; the planets are non-luminous; the same is true +of meteors before they plunge into the atmosphere and become heated by +friction; and many plausible reasons have been found for believing +that space contains as many obscure as shining bodies of great size. +It is not so difficult, after all, then, to believe that there are +immense collections of shadowy gases and meteoric dust whose presence +is only manifested when they intercept the light coming from shining +bodies behind them. + +This would account for the apparent extinguishment of light in open +space, which is indicated by the falling off in relative number of +telescopic stars below the tenth magnitude. Even as things are, the +amount of light coming to us from stars too faint to be seen with the +naked eye is so great that the statement of it generally surprises +persons who are unfamiliar with the inner facts of astronomy. It has +been calculated that on a clear night the total starlight from the +entire celestial sphere amounts to one-sixtieth of the light of the +full moon; but of this less than one-twenty-fifth is due to stars +separately distinguished by the eye. If there were no obscuring medium +in space, it is probable that the amount of starlight would be +noticeably and perhaps enormously increased. + +But while it seems certain that some of the obscure spots in the Milky +Way are due to the presence of ``dark nebul,'' or concealing veils of +one kind or another, it is equally certain that there are many which +are true apertures, however they may have been formed, and by whatever +forces they may be maintained. These, then, are veritable windows of +the Galaxy, and when looking out of them one is face to face with the +great mystery of infinite space. There the known universe visibly +ends, but manifestly space itself does not end there. It is not within +the power of thought to conceive an end to space, for the instant we +think of a terminal point or line the mind leaps forward to the +beyond. There must be space outside as well as inside. Eternity of +time and infinity of space are ideas that the intellect cannot fully +grasp, but neither can it grasp the idea of a limitation to either +space or time. The metaphysical conceptions of hypergeometry, or +fourth-dimensional space, do not aid us. + +Having, then, discovered that the universe is a thing contained in +something indefinitely greater than itself; having looked out of its +windows and found only the gloom of starless night outside -- what +conclusions are we to draw concerning the beyond? It seems as empty as +a vacuum, but is it really so? If it be, then our universe is a single +atom astray in the infinite; it is the only island in an ocean without +shores; it is the one oasis in an illimitable desert. Then the Milky +Way, with its wide-flung garland of stars, is afloat like a tiny +smoke-wreath amid a horror of immeasurable vacancy, or it is an +evanescent and solitary ring of sparkling froth cast up for a moment +on the viewless billows of immensity. From such conclusions the mind +instinctively shrinks. It prefers to think that there is something +beyond, though we cannot see it. Even the universe could not bear to +be alone -- a Crusoe lost in the Cosmos! As the inhabitants of the +most elegant chteau, with its gardens, parks, and crowds of +attendants, would die of loneliness if they did not know that they +have neighbors, though not seen, and that a living world of indefinite +extent surrounds them, so we, when we perceive that the universe has +limits, wish to feel that it is not solitary; that beyond the hedges +and the hills there are other centers of life and activity. Could +anything be more terrible than the thought of an isolated universe? +The greater the being, the greater the aversion to seclusion. Only the +infinite satisfies; in that alone the mind finds rest. + +We are driven, then, to believe that the universal night which +envelopes us is not tenantless; that as we stare out of the +star-framed windows of the Galaxy and see nothing but uniform +blackness, the fault is with our eyes or is due to an obscuring +medium. Since our universe is limited in extent, there must be other +universes beyond it on all sides. Perhaps if we could carry our +telescopes to the verge of the great ``Coal-sack'' near the ``Cross,'' +being then on the frontier of our starry system, we could discern, +sparkling afar off in the vast night, some of the outer galaxies. They +may be grander than ours, just as many of the suns surrounding us are +immensely greater than ours. If we could take our stand somewhere in +the midst of immensity and, with vision of infinite reach, look about +us, we should perhaps see a countless number of stellar systems, amid +which ours would be unnoticeable, like a single star among the +multitude glittering in the terrestial sky on a clear night. Some +might be in the form of a wreath, like our own; some might be +globular, like the great star-clusters in Hercules and Centaurus; some +might be glittering circles, or disks, or rings within rings. If we +could enter them we should probably find a vast variety of +composition, including elements unknown to terrestrial chemistry; for +while the visible universe appears to contain few if any substances +not existing on the earth or in the sun, we have no warrant to assume +that others may not exist in infinite space. + +And how as to gravitation? We do not know that gravitation acts beyond +the visible universe, but it is reasonable to suppose that it does. At +any rate, if we let go its sustaining hand we are lost, and can only +wander hopelessly in our speculations, like children astray. If the +empire of gravitation is infinite, then the various outer systems must +have some, though measuring by our standards an imperceptible, +attractive influence upon each other, for gravitation never lets go +its hold, however great the space over which it is required to act. +Just as the stars about us are all in motion, so the starry systems +beyond our sight may be in motion, and our system as a whole may be +moving in concert with them. If this be so, then after interminable +ages the aspect of the entire system of systems must change, its +various members assuming new positions with respect to one another. In +the course of time we may even suppose that our universe will approach +relatively close to one of the others; and then, if men are yet living +on the earth, they may glimpse through the openings which reveal +nothing to us now, the lights of another nearing star system, like the +signals of a strange squadron, bringing them the assurance (which can +be but an inference at present) that the ocean of space has other +argosies venturing on its limitless expanse. + +There remains the question of the luminiferous ether by whose agency +the waves of light are borne through space. The ether is as mysterious +as gravitation. With regard to ether we only infer its existence from +the effects which we ascribe to it. Evidently the ether must extend as +far as the most distant visible stars. But does it continue on +indefinitely in outer space? If it does, then the invisibility of the +other systems must be due to their distance diminishing the quantity +of light that comes from them below the limit of perceptibility, or to +the interposition of absorbing media; if it does not, then the reason +why we cannot see them is owing to the absence of a means of +conveyance for the light waves, as the lack of an interplanetary +atmosphere prevents us from hearing the thunder of sun-spots. (It is +interesting to recall that Mr Edison was once credited with the +intention to construct a gigantic microphone which should render the +roar of sun-spots audible by transforming the electric vibrations into +sound-waves). On this supposition each starry system would be +enveloped in its own globule of ether, and no light could cross from +one to another. But the probability is that both the ether and +gravitation are ubiquitous, and that all the stellar systems are +immersed in the former like clouds of phosphorescent organisms in the +sea. + +So astronomy carries the mind from height to greater height. Men were +long in accepting the proofs of the relative insignificance of the +earth; they were more quickly convinced of the comparative littleness +of the solar system; and now the evidence assails their reason that +what they had regarded as the universe is only one mote gleaming in +the sunbeams of Infinity. + +Star-Clouds, Star-Clusters, and Star-Streams + +In the preceding chapter we have seen something of the strangely +complicated structure of the Galaxy, or Milky Way. We now proceed to +study more comprehensively that garlanded ``Pathway of the Gods.'' + +Judged by the eye alone, the Milky Way is one of the most delicately +beautiful phenomena in the entire realm of nature -- a shimmer of +silvery gauze stretched across the sky; but studied in the light of +its revelations, it is the most stupendous object presented to human +ken. Let us consider, first, its appearance to ordinary vision. Its +apparent position in the sky shifts according to the season. On a +serene, cloudless summer evening, in the absence of the moon, whose +light obscures it, one sees the Galaxy spanning the heavens from north +to southeast of the zenith like a phosphorescent arch. In early spring +it forms a similar but, upon the whole, less brilliant arch west of +the zenith. Between spring and summer it lies like a long, faint, +twilight band along the northern horizon. At the beginning of winter +it again forms an arch, this time spanning the sky from east to west, +a little north of the zenith. These are its positions as viewed from +the mean latitude of the United States. Even the beginner in +star-gazing does not have to watch it throughout the year in order to +be convinced that it is, in reality, a great circle, extending +entirely around the celestial sphere. We appear to be situated near +its center, but its periphery is evidently far away in the depths of +space. + +Although to the casual observer it seems but a delicate scarf of +light, brighter in some places than in others, but hazy and indefinite +at the best, such is not its appearance to those who study it with +care. They perceive that it is an organic whole, though marvelously +complex in detail. The telescope shows that it consists of stars too +faint and small through excess of distance to be separately visible. +Of the hundred million suns which some estimates have fixed as the +probable population of the starry universe, the vast majority (at +least thirty to one) are included in this strange belt of misty light. +But they are not uniformly distributed in it; on the contrary, they +are arrayed in clusters, knots, bunches, clouds, and streams. The +appearance is somewhat as if the Galaxy consisted of innumerable +swarms of silver-winged bees, more or less intermixed, some massed +together, some crossing the paths of others, but all governed by a +single purpose which leads them to encircle the region of space in +which we are situated. + +From the beginning of the systematic study of the heavens, the fact +has been recognized that the form of the Milky Way denotes the scheme +of the sidereal system. At first it was thought that the shape of the +system was that of a vast round disk, flat like a cheese, and filled +with stars, our sun and his relatively few neighbors being placed near +the center. According to this view, the galactic belt was an effect of +perspective; for when looking in the direction of the plane of the +disk, the eye ranged through an immense extension of stars which +blended into a glimmering blur, surrounding us like a ring; while when +looking out from the sides of the disk we saw but few stars, and in +those directions the heavens appeared relatively blank. Finally it was +recognized that this theory did not correspond with the observed +appearances, and it became evident that the Milky Way was not a mere +effect of perspective, but an actual band of enormously distant stars, +forming a circle about the sphere, the central opening of the ring +(containing many scattered stars) being many times broader than the +width of the ring itself. Our sun is one of the scattered stars in the +central opening. + +As already remarked, the ring of the Galaxy is very irregular, and in +places it is partly broken. With its sinuous outline, its pendant +sprays, its graceful and accordant curves, its bunching of masses, its +occasional interstices, and the manifest order of a general plan +governing the jumble of its details, it bears a remarkable resemblance +to a garland -- a fact which appears the more wonderful when we recall +its composition. That an elm-tree should trace the lines of beauty +with its leafy and pendulous branches does not surprise us; but we can +only gaze with growing amazement when we behold a hundred million suns +imitating the form of a chaplet! And then we have to remember that +this form furnishes the ground-plan of the universe. + +As an indication of the extraordinary speculations to which the +mystery of the Milky Way has given rise, a theory recently (1909) +proposed by Prof. George C. Comstock may be mentioned. Starting with +the data (first) that the number of stars increases as the Milky Way +is approached, and reaches a maximum in its plane, while on the other +hand the number of nebul is greatest outside the Milky Way and +increases with distance from it, and (second) that the Milky Way, +although a complete ring, is broad and diffuse on one side through +one-half its course -- that half alone containing nebul -- and +relatively narrow and well defined on the opposite side, the author of +this singular speculation avers that these facts can best be explained +by supposing that the invisible universe consists of two +interpenetrating parts, one of which is a chaos of indefinite extent, +strewn with stars and nebulous dust, and the other a long, broad but +comparatively thin cluster of stars, including the sun as one of its +central members. This flat star-cluster is conceived to be moving +edgewise through the chaos, and, according to Professor Comstock, it +acts after the manner of a snow-plough sweeping away the cosmic dust +and piling it on either hand above and below the plane of the moving +cluster. It thus forms a transparent rift, through which we see +farther and command a view of more stars than through the intensified +dust-clouds on either hand. This rift is the Milky Way. The dust +thrown aside toward the poles of the Milky Way is the substance of the +nebul which abound there. Ahead, where the front of the star-plough +is clearing the way, the chaos is nearer at hand, and consequently +there the rift subtends a broader angle, and is filled with primordial +dust, which, having been annexed by the vanguard of the star-swarm, +forms the nebul seen only in that part of the Milky Way. But behind, +the rift appears narrow because there we look farther away between +dust-clouds produced ages ago by the front of the plough, and no +scattered dust remains in that part of the rift. + +In quoting an outline of this strikingly original theory the present +writer should not be understood as assenting to it. That it appears +bizarre is not, in itself, a reason for rejecting it, when we are +dealing with so problematical and enigmatical a subject as the Milky +Way; but the serious objection is that the theory does not +sufficiently accord with the observed phenomena. There is too much +evidence that the Milky Way is an organic system, however fantastic +its form, to permit the belief that it can only be a rift in chaotic +clouds. As with every organism, we find that its parts are more or +less clearly repeated in its ensemble. Among all the strange things +that the Milky Way contains there is nothing so extraordinary as +itself. Every astronomer must many times have found himself marveling +at it in those comparatively rare nights when it shows all its beauty +and all its strangeness. In its great broken rifts, divisions, and +spirals are found the gigantic prototypes of similar forms in its +star-clouds and clusters. As we have said, it determines the general +shape of the whole sidereal system. Some of the brightest stars in the +sky appear to hang like jewels suspended at the ends of tassels +dropped from the Galaxy. Among these pendants are the Pleiades and the +Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a loop of +light'' thrown out from it. The majority of the great first-magnitude +stars seem related to it, as if they formed an inner ring inclined at +an angle of some twenty degrees to its plane. Many of the long curves +that set off from it on both sides are accompanied by corresponding +curves of lucid stars. In a word, it offers every appearance of +structural connection with the entire starry system. That the universe +should have assumed the form of a wreath is certainly a matter for +astonishment; but it would have been still more astonishing if it had +been a cube, a rhomboid, or a dodecahedron, for then we should have +had to suppose that something resembling the forces that shape +crystals had acted upon the stars, and the difficulty of explaining +the universe by the laws of gravitation would have been increased. + +From the Milky Way as a whole we pass to the vast clouds, swarms, and +clusters of stars of which it is made up. It may be, as some +astronomers hold, that most of the galactic stars are much smaller +than the sun, so that their faintness is not due entirely to the +effect of distance. Still, their intrinsic brilliance attests their +solar character, and considering their remoteness, which has been +estimated at not less than ten thousand to twenty thousand light-years +(a light-year is equal to nearly six thousand thousand million miles) +their actual masses cannot be extremely small. The minutest of them +are entitled to be regarded as real suns, and they vary enormously in +magnitude. The effects of their attractions upon one another can only +be inferred from their clustering, because their relative movements +are not apparent on account of the brevity of the observations that we +can make. But imagine a being for whom a million years would be but as +a flitting moment; to him the Milky Way would appear in a state of +ceaseless agitation -- swirling with ``a fury of whirlpool motion.'' + +The cloud-like aspect of large parts of the Galaxy must always have +attracted attention, even from naked-eye observers, but the true +star-clouds were first satisfactorily represented in Barnard's +photographs. The resemblance to actual clouds is often startling. Some +are close-packed and dense, like cumuli; some are wispy or mottled, +like cirri. The rifts and modulations, as well as the general +outlines, are the same as those of clouds of vapor or dust, and one +notices also the characteristic thinning out at the edges. But we must +beware of supposing that the component suns are thickly crowded as the +particles forming an ordinary cloud. They look, indeed, as if they +were matted together, because of the irradiation of light, but in +reality millions and billions of miles separate each star from its +neighbors. Nevertheless they form real assemblages, whose members are +far more closely related to one another than is our sun to the stars +around him, and if we were in the Milky Way the aspect of the +nocturnal sky would be marvelously different from its present +appearance. + +Stellar clouds are characteristic of the Galaxy and are not found +beyond its borders, except in the ``Magellanic Clouds'' of the +southern hemisphere, which resemble detached portions of the Milky +Way. These singular objects form as striking a peculiarity of the +austral heavens as does the great ``Coal-sack'' described in Chapter +1. But it is their isolation that makes them so remarkable, for their +composition is essentially galactic, and if they were included within +its boundaries they would not appear more wonderful than many other +parts of the Milky Way. Placed where they are, they look like masses +fallen from the great stellar arch. They are full of nebul and +star-clusters, and show striking evidences of spiral movement. + +Star-swarms, which are also characteristic features of the Galaxy, +differ from star-clouds very much in the way that their name would +imply -- i.e., their component stars are so arranged, even when they +are countless in number, that the idea of an exceedingly numerous +assemblage rather than that of a cloud is impressed on the observer's +mind. In a star-swarm the separate members are distinguishable because +they are either larger or nearer than the stars composing a ``cloud.'' +A splendid example of a true star-swarm is furnished by Chi Persei, in +that part of the Milky Way which runs between the constellations +Perseus and Cassiopeia. This swarm is much coarser than many others, +and can be seen by the naked eye. In a small telescope it appears +double, as if the suns composing it had divided into two parties which +keep on their way side by side, with some commingling of their members +where the skirts of the two companies come in contact. + +Smaller than either star-clouds or star-swarms, and differing from +both in their organization, are star-clusters. These, unlike the +others, are found outside as well as inside the Milky Way, although +they are more numerous inside its boundaries than elsewhere. The term +star-cluster is sometimes applied, though improperly, to assemblages +which are rather groups, such, for instance, as the Pleiades. In their +most characteristic aspect star-clusters are of a globular shape -- +globes of suns! A famous example of a globular star-cluster, but one +not included in the Milky Way, is the ``Great Cluster in Hercules.'' +This is barely visible to the naked eye, but a small telescope shows +its character, and in a large one it presents a marvelous spectacle. +Photographs of such clusters are, perhaps, less effective than those +of star-clouds, because the central condensation of stars in them is +so great that their light becomes blended in an indistinguishable +blur. The beautiful effect of the incessant play of infinitesimal rays +over the apparently compact surface of the cluster, as if it were a +globe of the finest frosted silver shining in an electric beam, is +also lost in a photograph. Still, even to the eye looking directly at +the cluster through a powerful telescope, the central part of the +wonderful congregation seems almost a solid mass in which the stars +are packed like the ice crystals in a snowball. + +The same question rises to the lips of every observer: How can they +possibly have been brought into such a situation? The marvel does not +grow less when we know that, instead of being closely compacted, the +stars of the cluster are probably separated by millions of miles; for +we know that their distances apart are slight as compared with their +remoteness from the Earth. Sir William Herschel estimated their number +to be about fourteen thousand, but in fact they are uncountable. If we +could view them from a point just within the edge of the assemblage, +they would offer the appearance of a hollow hemisphere emblazoned with +stars of astonishing brilliancy; the near-by ones unparalleled in +splendor by any celestial object known to us, while the more distant +ones would resemble ordinary stars. An inhabitant of the cluster would +not know, except by a process of ratiocination, that he was dwelling +in a globular assemblage of suns; only from a point far outside would +their spherical arrangement become evident to the eye. Imagine +fourteen-thousand fire-balloons with an approach to regularity in a +spherical space -- say, ten miles in diameter; there would be an +average of less than thirty in every cubic mile, and it would be +necessary to go to a considerable distance in order to see them as a +globular aggregation; yet from a point sufficiently far away they +would blend into a glowing ball. + +Photographs show even better than the best telescopic views that the +great cluster is surrounded with a multitude of dispersed stars, +suggestively arrayed in more or less curving lines, which radiate from +the principle mass, with which their connection is manifest. These +stars, situated outside the central sphere, look somewhat like vagrant +bees buzzing round a dense swarm where the queen bee is sitting. Yet +while there is so much to suggest the operation of central forces, +bringing and keeping the members of the cluster together, the +attentive observer is also impressed with the idea that the whole +wonderful phenomenon may be the result of explosion. As soon as this +thought seizes the mind, confirmation of it seems to be found in the +appearance of the outlying stars, which could be as readily explained +by the supposition that they have been blown apart as that they have +flocked together toward a center. The probable fact that the stars +constituting the cluster are very much smaller than our sun might be +regarded as favoring the hypothesis of an explosion. Of their real +size we know nothing, but, on the basis of an uncertain estimate of +their parallax, it has been calculated that they may average +forty-five thousand miles in diameter -- something more than half the +diameter of the planet Jupiter. Assuming the same mean density, +fourteen thousand such stars might have been formed by the explosion +of a body about twice the size of the sun. This recalls the theory of +Olbers, which has never been altogether abandoned or disproved, that +the Asteroids were formed by the explosion of a planet circulating +between the orbits of Mars and Jupiter. The Asteroids, whatever their +manner of origin, form a ring around the sun; but, of course, the +explosion of a great independent body, not originally revolving about +a superior center of gravitational force, would not result in the +formation of a ring of small bodies, but rather of a dispersed mass of +them. But back of any speculation of this kind lies the problem, at +present insoluble: How could the explosion be produced? (See the +question of explosions in Chapters 6 and 14). + +Then, on the other hand, we have the observation of Herschel, since +abundantly confirmed, that space is unusually vacant in the immediate +neighborhood of condensed star-clusters and nebul, which, as far as +it goes, might be taken as an indication that the assembled stars had +been drawn together by their mutual attractions, and that the tendency +to aggregation is still bringing new members toward the cluster. But +in that case there must have been an original condensation of stars at +that point in space. This could probably have been produced by the +coagulation of a great nebula into stellar nuclei, a process which +seems now to be taking place in the Orion Nebula. + +A yet more remarkable globular star-cluster exists in the southern +hemisphere, Omega Centauri. In this case the central condensation of +stars presents an almost uniform blaze of light. Like the Hercules +cluster, that in Centaurus is surrounded with stars scattered over a +broad field and showing an appearance of radial arrangement. In fact, +except for its greater richness, Omega Centauri is an exact duplicate +of its northern rival. Each appears to an imaginative spectator as a +veritable ``city of suns.'' Mathematics shrinks from the task of +disentangling the maze of motions in such an assemblage. It would seem +that the chance of collisions is not to be neglected, and this idea +finds a certain degree of confirmation in the appearance of +``temporary stars'' which have more than once blazed out in, or close +by, globular star-clusters. + +This leads up to the notable fact, first established by Professor +Bailey a few years ago, that such clusters are populous with variable +stars. Omega Centauri and the Hercules cluster are especially +remarkable in this respect. The variables found in them are all of +short period and the changes of light show a noteworthy tendency to +uniformity. The first thought is that these phenomena must be due to +collisions among the crowded stars, but, if so, the encounters cannot +be between the stars themselves, but probably between stars and meteor +swarms revolving around them. Such periodic collisions might go on for +ages without the meteors being exhausted by incorporation with the +stars. This explanation appears all the more probable because one +would naturally expect that flocks of meteors would abound in a close +aggregation of stars. It is also consistent with Perrine's discovery +-- that the globular star clusters are powdered with minute stars +strewn thickly among the brighter ones. + +In speaking of Professor Comstock's extraordinary theory of the Milky +Way, the fact was mentioned that, broadly speaking, the nebul are +less numerous in the galactic belt than in the comparatively open +spaces on either side of it, but that they are, nevertheless, abundant +in the broader half of the Milky Way which he designates as the front +of the gigantic ``plough'' supposed to be forcing its way through the +enveloping chaos. In and around the Sagittarius region the +intermingling of nebul and galactic star clouds and clusters is +particularly remarkable. That there is a causal connection no +thoughtful person can doubt. We are unable to get away from the +evidence that a nebula is like a seed-ground from which stars spring +forth; or we may say that nebul resemble clouds in whose bosom +raindrops are forming. The wonderful aspect of the admixtures of +nebul and star-clusters in Sagittarius has been described in Chapter +1. We now come to a still more extraordinary phenomenon of this kind +-- the Pleiades nebul. + +The group of the Pleiades, although lying outside the main course of +the Galaxy, is connected with it by a faint loop, and is the scene of +the most remarkable association of stars and nebulous matter known in +the visible universe. The naked eye is unaware of the existence of +nebul in the Pleiades, or, at the best, merely suspects that there is +something of the kind there; and even the most powerful telescopes are +far from revealing the full wonder of the spectacle; but in +photographs which have been exposed for many hours consecutively, in +order to accumulate the impression of the actinic rays, the revelation +is stunning. The principle stars are seen surrounded by, and, as it +were, drowned in, dense nebulous clouds of an unparalleled kind. The +forms assumed by these clouds seem at first sight inexplicable. They +look like fleeces, or perhaps more like splashes and daubs of luminous +paint dashed carelessly from a brush. But closer inspection shows that +they are, to a large extent, woven out of innumerable threads of filmy +texture, and there are many indications of spiral tendencies. Each of +the bright stars of the group -- Alcyone, Merope, Maia, Electra, +Taygeta, Atlas -- is the focus of a dense fog (totally invisible, +remember, alike to the naked eye and to the telescope), and these +particular stars are veiled from sight behind the strange mists. +Running in all directions across the relatively open spaces are +nebulous wisps and streaks of the most curious forms. On some of the +nebular lines, which are either straight throughout, or if they change +direction do so at an angle, little stars are strung like beads. In +one case seven or eight stars are thus aligned, and, as if to +emphasize their dependence upon the chain which connects them, when it +makes a slight bend the file of stars turns the same way. Many other +star rows in the group suggest by their arrangement that they, too, +were once strung upon similar threads which have now disappeared, +leaving the stars spaced along their ancient tracks. We seem forced to +the conclusion that there was a time when the Pleiades were embedded +in a vast nebula resembling that of Orion, and that the cloud has now +become so rare by gradual condensation into stars that the merest +trace of it remains, and this would probably have escaped detection +but for the remarkable actinic power of the radiant matter of which it +consists. The richness of many of these faint nebulous masses in +ultra-violet radiations, which are those that specifically affect the +photographic plate, is the cause of the marvelous revelatory power of +celestial photography. So the veritable unseen universe, as +distinguished from the ``unseen universe'' of metaphysical +speculation, is shown to us. + +A different kind of association between stars and nebul is shown in +some surprising photographic objects in the constellation Cygnus, +where long, wispy nebul, billions of miles in length, some of them +looking like tresses streaming in a breeze, lie amid fields of stars +which seem related to them. But the relation is of a most singular +kind, for notwithstanding the delicate structure of the long nebul +they appear to act as barriers, causing the stars to heap themselves +on one side. The stars are two, three, or four times as numerous on +one side of the nebul as on the other. These nebul, as far as +appearance goes, might be likened to rail fences, or thin hedges, +against which the wind is driving drifts of powdery snow, which, while +scattered plentifully all around, tends to bank itself on the leeward +side of the obstruction. The imagination is at a loss to account for +these extraordinary phenomena; yet there they are, faithfully giving +us their images whenever the photographic plate is exposed to their +radiations. + +Thus the more we see of the universe with improved methods of +observation, and the more we invent aids to human senses, each +enabling us to penetrate a little deeper into the unseen, the greater +becomes the mystery. The telescope carried us far, photography is +carrying us still farther; but what as yet unimagined instrument will +take us to the bottom, the top, and the end? And then, what hitherto +untried power of thought will enable us to comprehend the meaning of +it all? + +Stellar Migrations + +To the untrained eye the stars and the planets are not +distinguishable. It is customary to call them all alike ``stars.'' But +since the planets more or less rapidly change their places in the sky, +in consequence of their revolution about the sun, while the stars +proper seem to remain always in the same relative positions, the +latter are spoken of as ``fixed stars.'' In the beginnings of +astronomy it was not known that the ``fixed stars'' had any motion +independent of their apparent annual revolution with the whole sky +about the earth as a seeming center. Now, however, we know that the +term ``fixed stars'' is paradoxical, for there is not a single really +fixed object in the whole celestial sphere. The apparent fixity in the +positions of the stars is due to their immense distance, combined with +the shortness of the time during which we are able to observe them. It +is like viewing the plume of smoke issuing from a steamer, hull down, +at sea: if one does not continue to watch it for a long time it +appears to be motionless, although in reality it may be traveling at +great speed across the line of sight. Even the planets seem fixed in +position if one watches them for a single night only, and the more +distant ones do not sensibly change their places, except after many +nights of observation. Neptune, for instance, moves but little more +than two degrees in the course of an entire year, and in a month its +change of place is only about one-third of the diameter of the full +moon. + +Yet, fixed as they seem, the stars are actually moving with a speed in +comparison with which, in some cases, the planets might almost be said +to stand fast in their tracks. Jupiter's speed in his orbit is about +eight miles per second, Neptune's is less than three and one-half +miles, and the earth's is about eighteen and one-half miles; while +there are ``fixed stars'' which move two hundred or three hundred +miles per second. They do not all, however, move with so great a +velocity, for some appear to travel no faster than the planets. But in +all cases, notwithstanding their real speed, long-continued and +exceedingly careful observations are required to demonstrate that they +are moving at all. No more overwhelming impression of the frightful +depths of space in which the stars are buried can be obtained than by +reflecting upon the fact that a star whose actual motion across the +line of sight amounts to two hundred miles per second does not change +its apparent place in the sky, in the course of a thousand years, +sufficiently to be noticed by the casual observer of the heavens! + +There is one vast difference between the motions of the stars and +those of the planets to which attention should be at once called: the +planets, being under the control of a central force emanating from +their immediate master, the sun, all move in the same direction and in +orbits concentric about the sun; the stars, on the other hand, move in +every conceivable direction and have no apparent center of motion, for +all efforts to discover such a center have failed. At one time, when +theology had finally to accept the facts of science, a grandiose +conception arose in some pious minds, according to which the Throne of +God was situated at the exact center of His Creation, and, seated +there, He watched the magnificent spectacle of the starry systems +obediently revolving around Him. Astronomical discoveries and +speculations seemed for a time to afford some warrant for this view, +which was, moreover, an acceptable substitute for the abandoned +geocentric theory in minds that could only conceive of God as a +superhuman artificer, constantly admiring his own work. No longer ago +than the middle of the nineteenth century a German astronomer, +Maedler, believed that he had actually found the location of the +center about which the stellar universe revolved. He placed it in the +group of the Pleiades, and upon his authority an extraordinary +imaginative picture was sometimes drawn of the star Alcyone, the +brightest of the Pleiades, as the very seat of the Almighty. This idea +even seemed to gain a kind of traditional support from the mystic +significance, without known historical origin, which has for many +ages, and among widely separated peoples, been attached to the +remarkable group of which Alcyone is the chief. But since Maedler's +time it has been demonstrated that the Pleiades cannot be the center +of revolution of the universe, and, as already remarked, all attempts +to find or fix such a center have proved abortive. Yet so powerful was +the hold that the theory took upon the popular imagination, that even +today astronomers are often asked if Alcyone is not the probable site +of ``Jerusalem the Golden.'' + +If there were a discoverable center of predominant gravitative power, +to which the motions of all the stars could be referred, those motions +would appear less mysterious, and we should then be able to conclude +that the universe was, as a whole, a prototype of the subsidiary +systems of which it is composed. We should look simply to the law of +gravitation for an explanation, and, naturally, the center would be +placed within the opening enclosed by the Milky Way. If it were there +the Milky Way itself should exhibit signs of revolution about it, like +a wheel turning upon its hub. No theory of the star motions as a whole +could stand which failed to take account of the Milky Way as the basis +of all. But the very form of that divided wreath of stars forbids the +assumption of its revolution about a center. Even if it could be +conceived as a wheel having no material center it would not have the +form which it actually presents. As was shown in Chapter 2, there is +abundant evidence of motion in the Milky Way; but it is not motion of +the system as a whole, but motion affecting its separate parts. +Instead of all moving one way, the galactic stars, as far as their +movements can be inferred, are governed by local influences and +conditions. They appear to travel crosswise and in contrary +directions, and perhaps they eddy around foci where great numbers have +assembled; but of a universal revolution involving the entire mass we +have no evidence. + +Most of our knowledge of star motions, called ``proper motions,'' +relates to individual stars and to a few groups which happen to be so +near that the effects of their movements are measurable. In some cases +the motion is so rapid (not in appearance, but in reality) that the +chief difficulty is to imagine how it can have been imparted, and what +will eventually become of the ``runaways.'' Without a collision, or a +series of very close approaches to great gravitational centers, a star +traveling through space at the rate of two hundred or three hundred +miles per second could not be arrested or turned into an orbit which +would keep it forever flying within the limits of the visible +universe. A famous example of these speeding stars is ``1830 +Groombridge,'' a star of only the sixth magnitude, and consequently +just visible to the naked eye, whose motion across the line of sight +is so rapid that it moves upon the face of the sky a distance equal to +the apparent diameter of the moon every 280 years. The distance of +this star is at least 200,000,000,000,000 miles, and may be two or +three times greater, so that its actual speed cannot be less than two +hundred, and may be as much as four hundred, miles per second. It +could be turned into a new course by a close approach to a great sun, +but it could only be stopped by collision, head-on, with a body of +enormous mass. Barring such accidents it must, as far as we can see, +keep on until it has traversed our stellar system, whence in may +escape and pass out into space beyond, to join, perhaps, one of those +other universes of which we have spoken. Arcturus, one of the greatest +suns in the universe, is also a runaway, whose speed of flight has +been estimated all the way from fifty to two hundred miles per second. +Arcturus, we have every reason to believe, possesses hundreds of times +the mass of our sun -- think, then, of the prodigious momentum that +its motion implies! Sirius moves more moderately, its motion across +the line of sight amounting to only ten miles per second, but it is at +the same time approaching the sun at about the same speed, its actual +velocity in space being the resultant of the two displacements. + +What has been said about the motion of Sirius brings us to another +aspect of this subject. The fact is, that in every case of stellar +motion the displacement that we observe represents only a part of the +actual movement of the star concerned. There are stars whose motion +carries them straight toward or straight away from the earth, and such +stars, of course, show no cross motion. But the vast majority are +traveling in paths inclined from a perpendicular to our line of sight. +Taken as a whole, the stars may be said to be flying about like the +molecules in a mass of gas. The discovery of the radial component in +the movements of the stars is due to the spectroscope. If a star is +approaching, its spectral lines are shifted toward the violet end of +the spectrum by an amount depending upon the velocity of approach; if +it is receding, the lines are correspondingly shifted toward the red +end. Spectroscopic observation, then, combined with micrometric +measurements of the cross motion, enables us to detect the real +movement of the star in space. Sometimes it happens that a star's +radial movement is periodically reversed; first it approaches, and +then it recedes. This indicates that it is revolving around a near-by +companion, which is often invisible, and superposed upon this motion +is that of the two stars concerned, which together may be approaching +or receding or traveling across the line of sight. Thus the +complications involved in the stellar motions are often exceedingly +great and puzzling. + +Yet another source of complication exists in the movement of our own +star, the sun. There is no more difficult problem in astronomy than +that of disentangling the effects of the solar motion from those of +the motions of the other stars. But the problem, difficult as it is, +has been solved, and upon its solution depends our knowledge of the +speed and direction of the movement of the solar system through space, +for of course the sun carries its planets with it. One element of the +solution is found in the fact that, as a result of perspective, the +stars toward which we are going appear to move apart toward all points +of the compass, while those behind appear to close up together. Then +the spectroscopic principle already mentioned is invoked for studying +the shift of the lines, which is toward the violet in the stars ahead +of us and toward the red in those that we are leaving behind. Of +course the effects of the independent motions of the stars must be +carefully excluded. The result of the studies devoted to this subject +is to show that we are traveling at a speed of twelve to fifteen miles +per second in a northerly direction, toward the border of the +constellations Hercules and Lyra. A curious fact is that the more +recent estimates show that the direction is not very much out of a +straight line drawn from the sun to the star Vega, one of the most +magnificent suns in the heavens. But it should not be inferred from +this that Vega is drawing us on; it is too distant for its gravitation +to have such an effect. + +Many unaccustomed thoughts are suggested by this mighty voyage of the +solar system. Whence have we come, and whither do we go? Every year of +our lives we advance at least 375,000,000 miles. Since the traditional +time of Adam the sun has led his planets through the wastes of space +no less than 225,000,000,000 miles, or more than 2400 times the +distance that separates him from the earth. Go back in imagination to +the geologic ages, and try to comprehend the distance over which the +earth has flown. Where was our little planet when it emerged out of +the clouds of chaos? Where was the sun when his ``thunder march'' +began? What strange constellations shone down upon our globe when its +masters of life were the monstrous beasts of the ``Age of Reptiles''? +A million years is not much of a span of time in geologic reckoning, +yet a million years ago the earth was farther from its present place +in space than any of the stars with a measurable parallax are now. It +was more than seven times as far as Sirius, nearly fourteen times as +far as Alpha Centauri, three times as far as Vega, and twice as far as +Arcturus. But some geologists demand two hundred, three hundred, even +one thousand million years to enable them to account for the +evolutionary development of the earth and its inhabitants. In a +thousand million years the earth would have traveled farther than from +the remotest conceivable depths of the Milky Way! + +Other curious reflections arise when we think of the form of the +earth's track as it follows the lead of the sun, in a journey which +has neither known beginning nor conceivable end. There are probably +many minds which have found a kind of consolation in the thought that +every year the globe returns to the same place, on the same side of +the sun. This idea may have an occult connection with our traditional +regard for anniversaries. When that period of the year returns at +which any great event in our lives has occurred we have the feeling +that the earth, in its annual round, has, in a manner, brought us back +to the scene of that event. We think of the earth's orbit as a +well-worn path which we traverse many times in the course of a +lifetime. It seems familiar to us, and we grow to have a sort of +attachment to it. The sun we are accustomed to regard as a fixed +center in space, like the mill or pump around which the harnessed +patient mule makes his endless circuits. But the real fact is that the +earth never returns to the place in space where it has once quitted. +In consequence of the motion of the sun carrying the earth and the +other planets along, the track pursued by our globe is a vast spiral +in space continually developing and never returning upon its course. +It is probable that the tracks of the sun and the others stars are +also irregular, and possibly spiral, although, as far as can be at +present determined, they appear to be practically straight. Every +star, wherever it may be situated, is attracted by its fellow-stars +from many sides at once, and although the force is minimized by +distance, yet in the course of many ages its effects must become +manifest. + +Looked at from another side, is there not something immensely +stimulating and pleasing to the imagination in the idea of so +stupendous a journey, which makes all of us the greatest of travelers? +In the course of a long life a man is transported through space thirty +thousand million miles; Halley's Comet does not travel one-quarter as +far in making one of its immense circuits. And there are adventures on +this voyage of which we are just beginning to learn to take account. +Space is full of strange things, and the earth must encounter some of +them as it advances through the unknown. Many singular speculations +have been indulged in by astronomers concerning the possible effects +upon the earth of the varying state of the space that it traverses. +Even the alternation of hot and glacial periods has sometimes been +ascribed to this source. When tropical life flourished around the +poles, as the remains in the rocks assure us, the needed high +temperature may, it has been thought, have been derived from the +presence of the earth in a warm region of space. Then, too, there is a +certain interest for us in the thought of what our familiar planet has +passed through. We cannot but admire it for its long journeying as we +admire the traveler who comes to us from remote and unexplored lands, +or as we gaze with a glow of interest upon the first locomotive that +has crossed a continent, or a ship that has visited the Arctic or +Antarctic regions. If we may trust the indications of the present +course, the earth, piloted by the sun, has come from the Milky Way in +the far south and may eventually rejoin that mighty band of stars in +the far north. + +While the stars in general appear to travel independently of one +another, except when they are combined in binary or trinary systems, +there are notable exceptions to this rule. In some quarters of the sky +we behold veritable migrations of entire groups of stars whose members +are too widely separated to show any indications of revolution about a +common center of gravity. This leads us back again to the wonderful +group of the Pleiades. All of the principle stars composing that group +are traveling in virtually parallel lines. Whatever force set them +going evidently acted upon all alike. This might be explained by the +assumption that when the original projective force acted upon them +they were more closely united than they are at present, and that in +drifting apart they have not lost the impulse of the primal motion. Or +it may be supposed that they are carried along by some current in +space, although it would be exceedingly difficult, in the present +state of our knowledge, to explain the nature of such a current. Yet +the theory of a current has been proposed. As to an attractive center +around which they might revolve, none has been found. Another instance +of similar ``star-drift'' is furnished by five of the seven stars +constituting the figure of the ``Great Dipper.'' In this case the +stars concerned are separated very widely, the two extreme ones by not +less than fifteen degrees, so that the idea of a common motion would +never have been suggested by their aspect in the sky; and the case +becomes the more remarkable from the fact that among and between them +there are other stars, some of the same magnitude, which do not share +their motion, but are traveling in other directions. Still other +examples of the same phenomenon are found in other parts of the sky. +Of course, in the case of compact star-clusters, it is assumed that +all the members share a like motion of translation through space, and +the same is probably true of dense star-swarms and star-clouds. + +The whole question of star-drift has lately assumed a new phase, in +consequence of the investigations of Kapteyn, Dyson, and Eddington on +the ``systematic motions of the stars.'' This research will, it is +hoped, lead to an understanding of the general law governing the +movements of the whole body of stars constituting the visible +universe. Taking about eleven hundred stars whose proper motions have +been ascertained with an approach to certainty, and which are +distributed in all parts of the sky, it has been shown that there +exists an apparent double drift, in two independent streams, moving in +different and nearly opposed directions. The apex of the motion of +what is called ``Stream I'' is situated, according to Professor +Kapteyn, in right ascension 85, declination south 11, which places +it just south of the constellation Orion; while the apex of ``Stream +II'' is in right ascension 260, declination south 48, placing it in +the constellation Ara, south of Scorpio. The two apices differ very +nearly 180 in right ascension and about 120 in declination. The +discovery of these vast star-streams, if they really exist, is one of +the most extraordinary in modern astronomy. It offers the correlation +of stellar movements needed as the basis of a theory of those +movements, but it seems far from revealing a physical cause for them. +As projected against the celestial sphere the stars forming the two +opposite streams appear intermingled, some obeying one tendency and +some the other. As Professor Dyson has said, the hypothesis of this +double movement is of a revolutionary character, and calls for further +investigation. Indeed, it seems at first glance not less surprising +than would be the observation that in a snow-storm the flakes over our +heads were divided into two parties and driving across each other's +course in nearly opposite directions, as if urged by interpenetrating +winds. + +But whatever explanation may eventually be found for the motions of +the stars, the knowledge of the existence of those motions must always +afford a new charm to the contemplative observer of the heavens, for +they impart a sense of life to the starry system that would otherwise +be lacking. A stagnant universe, with every star fixed immovably in +its place, would not content the imagination or satisfy our longing +for ceaseless activity. The majestic grandeur of the evolutions of the +celestial hosts, the inconceivable vastness of the fields of space in +which they are executed, the countless numbers, the immeasurable +distances, the involved convolutions, the flocking and the scattering, +the interpenetrating marches and countermarches, the strange community +of impulsion affecting stars that are wide apart in space and causing +them to traverse the general movement about them like aides and +despatch-bearers on a battle-field -- all these arouse an intensity of +interest which is heightened by the mystery behind them. + +The Passing of the Constellations + +From a historical and picturesque point of view, one of the most +striking results of the motions of the stars described in the last +chapter is their effect upon the forms of the constellations, which +have been watched and admired by mankind from a period so early that +the date of their invention is now unknown. The constellations are +formed by chance combinations of conspicuous stars, like figures in a +kaleidoscope, and if our lives were commensurate with the ons of +cosmic existence we should perceive that the kaleidoscope of the +heavens was ceaselessly turning and throwing the stars into new +symmetries. Even if the stars stood fast, the motion of the solar +system would gradually alter the configurations, as the elements of a +landscape dissolve and recombine in fresh groupings with the +traveler's progress amid them. But with the stars themselves all in +motion at various speeds and in many directions, the changes occur +more rapidly. Of course, ``rapid'' is here understood in a relative +sense; the wheel of human history to an eye accustomed to the majestic +progression of the universe would appear to revolve with the velocity +of a whirling dynamo. Only the deliberation of geological movements +can be contrasted with the evolution and devolution of the +constellations. + +And yet this secular fluctuation of the constellation figures is not +without keen interest for the meditative observer. It is another +reminder of the swift mutability of terrestial affairs. To the passing +glance, which is all that we can bestow upon these figures, they +appear so immutable that they have been called into service to form +the most lasting records of ancient thought and imagination that we +possess. In the forms of the constellations, the most beautiful, and, +in imaginative quality, the finest, mythology that the world has ever +known has been perpetuated. Yet, in a broad sense, this scroll of +human thought imprinted on the heavens is as evanescent as the summer +clouds. Although more enduring than parchment, tombs, pyramids, and +temples, it is as far as they from truly eternizing the memory of what +man has fancied and done. + +Before studying the effects that the motions of the stars have had and +will have upon the constellations, it is worth while to consider a +little further the importance of the stellar pictures as archives of +history. To emphasize the importance of these effects it is only +necessary to recall that the constellations register the oldest +traditions of our race. In the history of primeval religions they are +the most valuable of documents. Leaving out of account for the moment +the more familiar mythology of the Greeks, based on something older +yet, we may refer for illustration to that of the mysterious Maya race +of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group +of ruins perched, after the Mexican and Central-American plan, on the +summits of pyramidal mounds which mark the site of an ancient +theogonic center of the Mayas. Here the temples all evidently refer to +a cult based upon the constellations as symbols. The figures and the +names, of course, were not the same as those that we have derived from +our Aryan ancestors, but the star groups were the same or nearly so. +For instance, the loftiest of the temples at Izamal was connected with +the sign of the constellation known to us as Cancer, marking the place +of the sun at the summer solstice, at which period the sun was +supposed to descend at noon like a great bird of fire and consume the +offerings left upon the altar. Our Scorpio was known to the Mayas as a +sign of the ``Death God.'' Our Libra, the ``Balance,'' with which the +idea of a divine weighing out of justice has always been connected, +seems to be identical with the Mayan constellation Teoyaotlatohua, +with which was associated a temple where dwelt the priests whose +special business it was to administer justice and to foretell the +future by means of information obtained from the spirits of the dead. +Orion, the ``Hunter'' of our celestial mythology, was among the Mayas +a ``Warrior,'' while Sagittarius and others of our constellations were +known to them (under different names, of course), and all were endowed +with a religious symbolism. And the same star figures, having the same +significance, were familiar to the Peruvians, as shown by the temples +at Cuzco. Thus the imagination of ancient America sought in the +constellations symbols of the unchanging gods. + +But, in fact, there is no nation and no people that has not recognized +the constellations, and at one period or another in its history +employed them in some symbolic or representative capacity. As handled +by the Greeks from prehistoric times, the constellation myths became +the very soul of poetry. The imagination of that wonderful race +idealized the principal star groups so effectively that the figures +and traditions thus attached to them have, for civilized mankind, +displaced all others, just as Greek art in its highest forms stands +without parallel and eclipses every rival. The Romans translated no +heroes and heroines of the mythical period of their history to the +sky, and the deified Csars never entered that lofty company, but the +heavens are filled with the early myths of the Greeks. Herakles +nightly resumes his mighty labors in the stars; Zeus, in the form of +the white ``Bull,'' Taurus, bears the fair Europa on his back through +the celestial waves; Andromeda stretches forth her shackled arms in +the star-gemmed ether, beseeching aid; and Perseus, in a blaze of +diamond armor, revives his heroic deeds amid sparkling clouds of +stellar dust. There, too, sits Queen Cassiopeia in her dazzling chair, +while the Great King, Cepheus, towers gigantic over the pole. +Professor Young has significantly remarked that a great number of the +constellations are connected in some way or other with the Argonautic +Expedition -- that strangely fascinating legend of earliest Greek +story which has never lost its charm for mankind. In view of all this, +we may well congratulate ourselves that the constellations will +outlast our time and the time of countless generations to follow us; +and yet they are very far from being eternal. Let us now study some of +the effects of the stellar motions upon them. + +We begin with the familiar figure of the ``Great Dipper.'' He who has +not drunk inspiration from its celestial bowl is not yet admitted to +the circle of Olympus. This figure is made up of seven conspicuous +stars in the constellation Ursa Major, the ``Greater Bear.'' The +handle of the ``Dipper'' corresponds to the tail of the imaginary +``Bear,'' and the bowl lies upon his flank. In fact, the figure of a +dipper is so evident and that of a bear so unevident, that to most +persons the ``Great Dipper'' is the only part of the constellation +that is recognizable. Of the seven stars mentioned, six are of nearly +equal brightness, ranking as of the second magnitude, while the +seventh is of only the third magnitude. The difference is very +striking, since every increase of one magnitude involves an increase +of two-and-a-half times in brightness. There appears to be little +doubt that the faint star, which is situated at the junction of the +bowl and the handle, is a variable of long period, since three hundred +years ago it was as bright as its companions. But however that may be, +its relative faintness at the present time interferes but little with +the perfection of the ``Dipper's'' figure. In order the more readily +to understand the changes which are taking place, it will be well to +mention both the names and the Greek letters which are attached to the +seven stars. Beginning at the star in the upper outer edge of the rim +of the bowl and running in regular order round the bottom and then out +to the end of the handle, the names and letters are as follows: Dubhe +({\alpha}), Merak ({\beta}), Phaed ({\gamma}), Megrez ({\delta}), +Alioth ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez +is the faint star already mentioned at the junction of the bowl and +handle, and Mizar, in the middle of the handle, has a close, naked-eye +companion which is named Alcor. The Arabs called this singular pair of +stars ``The Horse and Rider.'' Merak and Duhbe are called ``The +Pointers,'' because an imaginary line drawn northward through them +indicates the Pole Star. + +Now it has been found that five of these stars -- viz., Merak, Phaed, +Megrez, Alioth, and Mizar (with its comrade) -- are moving with +practically the same speed in an easterly direction, while the other +two, Dubhe and Benetnasch, are simultaneously moving westward, the +motions of Benetnasch being apparently more rapid. The consequence of +these opposed motions is, of course, that the figure of the ``Dipper'' +cannot always have existed and will not continue to exist. In the +accompanying diagrams it has been thought interesting to show the +relative positions of these seven stars, as seen from the point which +the earth now occupies, both in the past and in the future. Arrows +attached to the stars in the figure representing the present +appearance of the ``Dipper'' indicate the directions of the motions +and the distances over which they will carry the stars in a period of +about five hundred centuries. The time, no doubt, seems long, but +remember the vast stretch of ages through which the earth has passed, +and then reflect that no reason is apparent why our globe should not +continue to be a scene of animation for ten thousand centuries yet to +come. The fact that the little star Alcor placed so close to Mizar +should accompany the latter in its flight is not surprising, but that +two of the principal stars of the group should be found moving in a +direction directly opposed to that pursued by the other five is +surprising in the highest degree; and it recalls the strange theory of +a double drift affecting all the stars, to which attention was called +in the preceding chapter. It would appear that Benetnasch and Dubhe +belong to one ``current,'' and Merak, Phaed, Megrez, Alioth, and Mizar +to the other. As far as is known, the motion of the seven stars are +not shared by the smaller stars scattered about them, but on the +theory of currents there should be such a community of motion, and +further investigation may reveal it. + +From the ``Great Dipper'' we turn to a constellation hardly less +conspicuous and situated at an equal distance from the pole on the +other side -- Cassiopeia. This famous star-group commemorating the +romantic Queen of Ethiopia whose vain boasting of her beauty was +punished by the exposure of her daughter Andromeda to the ``Sea +Monster,'' is well-marked by five stars which form an irregular letter +``W'' with its open side toward the pole. Three of these stars are +usually ranked as of the second magnitude, and two of the third; but +to ordinary observation they appear of nearly equal brightness, and +present a very striking picture. They mark out the chair and a part of +the figure of the beautiful queen. Beginning at the right-hand, or +western, end of the ``W,'' their Greek letter designations are: Beta +({\beta}), Alpha ({\alpha}), Gamma ({\gamma}), Delta ({\delta}), and +Epsilon ({\epsilon}). Four of them, Beta, Alpha, Delta, and Epsilon +are traveling eastwardly at various speeds, while the fifth, Gamma, +moves in a westerly direction. The motion of Beta is more rapid than +that of any of the others. It should be said, however, that no little +uncertainty attaches to the estimates of the rate of motion of stars +which are not going very rapidly, and different observers often vary +considerably in their results. + +In the beautiful ``Northern Crown,'' one of the most perfect and +charming of all the figures to be found in the stars, the alternate +combining and scattering effects of the stellar motions are shown by +comparing the appearance which the constellation must have had five +hundred centuries ago with that which it has at present and that which +it will have in the future. The seven principle stars of the asterism, +forming a surprisingly perfect coronet, have movements in three +directions at right angles to one another. That in these circumstances +they should ever have arrived at positions giving them so striking an +appearance of definite association is certainly surprising; from its +aspect one would have expected to find a community of movement +governing the brilliants of the ``Crown,'' but instead of that we find +evidence that they will inevitably drift apart and the beautiful +figure will dissolve. + +A similar fate awaits such asterisms as the ``Northern Cross'' in +Cygnus; the ``Crow'' (Corvus), which stands on the back of the great +``Sea Serpent,'' Hydra, and pecks at his scales; ``Job's Coffin'' +(Delphinus); the ``Great Square of Pegasus''; the ``Twins'' (Gemini); +the beautiful ``Sickle'' in Leo; and the exquisite group of the Hyades +in Taurus. In the case of the Hyades, two controlling movements are +manifest: one, affecting five of the stars which form the well-known +figure of a letter ``V,'' is directed northerly; the other, which +controls the direction of two stars, has an easterly trend. The chief +star of the group, Aldebaran, one of the finest of all stars both for +its brilliance and its color, is the most affected by the easterly +motion. In time it will drift entirely out of connection with its +present neighbors. Although the Hyades do not form so compact a group +as the Pleiades in the same constellation, yet their appearance of +relationship is sufficient to awaken a feeling of surprise over the +fact that, as with the stars of the ``Dipper,'' their association is +only temporary or apparent. + +The great figure of Orion appears to be more lasting, not because its +stars are physically connected, but because of their great distance, +which renders their movements too deliberate to be exactly +ascertained. Two of the greatest of its stars, Betelgeuse and Rigel, +possess, as far as has been ascertained, no perceptible motion across +the line of sight, but there is a little movement perceptible in the +``Belt.'' At the present time this consists of an almost perfect +straight line, a row of second-magnitude stars about equally spaced +and of the most striking beauty. In the course of time, however, the +two right-hand stars, Mintaka and Alnilam (how fine are these Arabic +star names!) will approach each other and form a naked-eye double, but +the third, Alnita, will drift away eastward, so that the ``Belt'' will +no longer exist. + +For one more example, let us go to the southern hemisphere, whose most +celebrated constellation, the ``Southern Cross,'' has found a place in +all modern literatures, although it has no claim to consideration on +account of association with ancient legends. This most attractive +asterism, which has never ceased to fascinate the imagination of +Christendom since it was first devoutly described by the early +explorers of the South, is but a passing collocation of brilliant +stars. Yet even in its transfigurations it has been for hundreds of +centuries, and will continue to be for hundreds of centuries to come, +a most striking object in the sky. Our figures show its appearance in +three successive phases: first, as it was fifty thousand years ago +(viewed from the earth's present location); second, as it is in our +day; and, third, as it will be an equal time in the future. The +nearness of these bright stars to one another -- the length of the +longer beam of the ``Cross'' is only six degrees -- makes this group +very noticeable, whatever the arrangement of its components may be. +The largest star, at the base of the ``Cross,'' is of the first +magnitude, two of the others are of the second magnitude, and the +fourth is of the third. Other stars, not represented in the figures, +increase the effect of a celestial blazonry, although they do not help +the resemblance to a cross. + +But since the motion of the solar system itself will, in the course of +so long a period as fifty thousand years, produce a great change in +the perspective of the heavens as seen from the earth, by carrying us +nearly nineteen trillion miles from our present place, why, it may be +asked, seek to represent future appearances of the constellations +which we could not hope to see, even if we could survive so long? The +answer is: Because these things aid the mind to form a picture of the +effects of the mobility of the starry universe. Only by showing the +changes from some definite point of view can we arrive at a due +comprehension of them. The constellations are more or less familiar to +everybody, so that impending changes of their forms must at once +strike the eye and the imagination, and make clearer the significance +of the movements of the stars. If the future history of mankind is to +resemble its past and if our race is destined to survive yet a million +years, then our remote descendents will see a ``new heavens'' if not a +``new earth,'' and will have to invent novel constellations to +perpetuate their legends and mythologies. + +If our knowledge of the relative distances of the stars were more +complete, it would be an interesting exercise in celestial geometry to +project the constellations probably visible to the inhabitants of +worlds revolving around some of the other suns of space. Our sun is +too insignificant for us to think that he can make a conspicuous +appearance among them, except, perhaps, in a few cases. As seen, for +instance, from the nearest known star, Alpha Centauri, the sun would +appear of the average first magnitude, and consequently from that +standpoint he might be the gem of some little constellation which had +no Sirius, or Arcturus, or Vega to eclipse him with its superior +splendor. But from the distance of the vast majority of the stars the +sun would probably be invisible to the naked eye, and as seen from +nearer systems could only rank as a fifth or sixth magnitude star, +unnoticed and unknown except by the star-charting astronomer. + +Conflagrations in the Heavens + +Suppose it were possible for the world to take fire and burn up -- as +some pessimists think that it will do when the Divine wrath shall have +sufficiently accumulated against it -- nobody out of our own little +corner of space would ever be aware of the catastrophe! With all their +telescopes, the astronomers living in the golden light of Arcturus or +the diamond blaze of Canopus would be unable to detect the least +glimmer of the conflagration that had destroyed the seat of Adam and +his descendents, just as now they are totally ignorant of its +existence. + +But at least fifteen times in the course of recorded history men +looking out from the earth have beheld in the remote depths of space +great outbursts of fiery light, some of them more splendidly luminous +than anything else in the firmament except the sun! If they were +conflagrations, how many million worlds like ours were required to +feed their blaze? + +It is probable that ``temporary'' or ``new'' stars, as these wonderful +apparitions are called, really are conflagrations; not in the sense of +a bonfire or a burning house or city, but in that of a sudden eruption +of inconceivable heat and light, such as would result from the +stripping off the shell of an encrusted sun or the crashing together +of two mighty orbs flying through space with a hundred times the +velocity of the swiftest cannon-shot. + +Temporary stars are the rarest and most erratic of astronomical +phenomena. The earliest records relating to them are not very clear, +and we cannot in every instance be certain that it was one of these +appearances that the ignorant and superstitious old chroniclers are +trying to describe. The first temporary star that we are absolutely +sure of appeared in 1572, and is known as ``Tycho's Star,'' because +the celebrated Danish astronomer (whose remains, with his +gold-and-silver artificial nose -- made necessary by a duel -- still +intact, were disinterred and reburied in 1901) was the first to +perceive it in the sky, and the most assiduous and successful in his +studies of it. As the first fully accredited representative of its +class, this new star made its entry upon the scene with becoming +clat. It is characteristic of these phenomena that they burst into +view with amazing suddenness, and, of course, entirely unexpectedly. +Tycho's star appeared in the constellation Cassiopeia, near a now +well-known and much-watched little star named Kappa, on the evening of +November 11, 1572. The story has often been repeated, but it never +loses interest, how Tycho, going home that evening, saw people in the +street pointing and staring at the sky directly over their heads, and +following the direction of their hands and eyes he was astonished to +see, near the zenith, an unknown star of surpassing brilliance. It +outshone the planet Jupiter, and was therefore far brighter than the +first magnitude. There was not another star in the heavens that could +be compared with it in splendor. Tycho was not in all respects free +from the superstitions of his time -- and who is? -- but he had the +true scientific instinct, and immediately he began to study the +stranger, and to record with the greatest care every change in its +aspect. First he determined as well as he could with the imperfect +instruments of his day, many of which he himself had invented, the +precise location of the phenomena in the sky. Then he followed the +changes that it underwent. At first it brightened until its light +equaled or exceeded that of the planet Venus at her brightest, a +statement which will be appreciated at its full value by anyone who +has ever watched Venus when she plays her dazzling rle of ``Evening +Star,'' flaring like an arc light in the sunset sky. It even became so +brilliant as to be visible in full daylight, since, its position being +circumpolar, it never set in the latitude of Northern Europe. Finally +it began to fade, turning red as it did so, and in March, 1574, it +disappeared from Tycho's searching gaze, and has never been seen again +from that day to this. None of the astronomers of the time could make +anything of it. They had not yet as many bases of speculation as we +possess today. + +Tycho's star has achieved a romantic reputation by being fancifully +identified with the ``Star of Bethlehem,'' said to have led the +wondering Magi from their eastern deserts to the cradle-manger of the +Savior in Palestine. Many attempts have been made to connect this +traditional ``star'' with some known phenomenon of the heavens, and +none seems more idle than this. Yet it persistently survives, and no +astronomer is free from eager questions about it addressed by people +whose imagination has been excited by the legend. It is only necessary +to say that the supposition of a connection between the phenomenon of +the Magi and Tycho's star is without any scientific foundation. It was +originally based on an unwarranted assumption that the star of Tycho +was a variable of long period, appearing once every three hundred and +fifteen years, or thereabout. If that were true there would have been +an apparition somewhere near the traditional date of the birth of +Christ, a date which is itself uncertain. But even the data on which +the assumption was based are inconsistent with the theory. Certain +monkish records speak of something wonderful appearing in the sky in +the years 1264 and 945, and these were taken to have been outbursts of +Tycho's star. Investigation shows that the records more probably refer +to comets, but even if the objects seen were temporary stars, their +dates do not suit the hypothesis; from 945 to 1264 there is a gap of +319 years, and from 1264 to 1572 one of only 308 years; moreover 337 +years have now (1909) elapsed since Tycho saw the last glimmer of his +star. Upon a variability so irregular and uncertain as that, even if +we felt sure that it existed, no conclusion could be found concerning +an apparition occurring 2000 years ago. + +In the year 1600 (the year in which Giordano Bruno was burned at the +stake for teaching that there is more than one physical world), a +temporary star of the third magnitude broke out in the constellation +Cygnus, and curiously enough, considering the rarity of such +phenomena, only four years later another surprisingly brilliant one +appeared in the constellation Ophiuchus. This is often called +``Kepler's star,'' because the great German astronomer devoted to it +the same attention that Tycho had given to the earlier phenomenon. It, +too, like Tycho's, was at first the brightest object in the stellar +heavens, although it seems never to have quite equaled its famous +predecessor in splendor. It disappeared after a year, also turning of +a red color as it became more faint. We shall see the significance of +this as we go on. Some of Kepler's contemporaries suggested that the +outburst of this star was due to a meeting of atoms in space, and idea +bearing a striking resemblance to the modern theory of ``astronomical +collisions.'' + +In 1670, 1848, and 1860 temporary stars made their appearance, but +none of them was of great brilliance. In 1866 one of the second +magnitude broke forth in the ``Northern Crown'' and awoke much +interest, because by that time the spectroscope had begun to be +employed in studying the composition of the stars, and Huggins +demonstrated that the new star consisted largely of incandescent +hydrogen. But this star, apparently unlike the others mentioned, was +not absolutely new. Before its outburst it had shown as a star of the +ninth magnitude (entirely invisible, of course, to the naked eye), and +after about six weeks it faded to its original condition in which it +has ever since remained. In 1876 a temporary star appeared in the +constellation Cygnus, and attained at one time the brightness of the +second magnitude. Its spectrum and its behavior resembled those of its +immediate predecessor. In 1885, astronomers were surprised to see a +sixth-magnitude star glimmering in the midst of the hazy cloud of the +great Andromeda Nebula. It soon absolutely disappeared. Its spectrum +was remarkable for being ``continuous,'' like that of the nebula +itself. A continuous spectrum is supposed to represent a body, or a +mass, which is either solid or liquid, or composed of gas under great +pressure. In January, 1892, a new star was suddenly seen in the +constellation Auriga. It never rose much above the fourth magnitude, +but it showed a peculiar spectrum containing both bright and dark +lines of hydrogen. + +But a bewildering surprise was now in store; the world was to behold +at the opening of the twentieth century such a celestial spectacle as +had not been on view since the times of Tycho and Kepler. Before +daylight on the morning of February 22, 1901, the Rev. Doctor +Anderson, of Edinburgh, an amateur astronomer, who had also been the +first to see the new star in Auriga, beheld a strange object in the +constellation Perseus not far from the celebrated variable star Algol. +He recognized its character at once, and immediately telegraphed the +news, which awoke the startled attention of astronomers all over the +world. When first seen the new star was no brighter than Algol (less +than the second magnitude), but within twenty-four hours it was +ablaze, outshining even the brilliant Capella, and far surpassing the +first magnitude. At the spot in the sky where it appeared nothing +whatever was visible on the night before its coming. This is known +with certainty because a photograph had been made of that very region +on February 21, and this photograph showed everything down to the +twelfth magnitude, but not a trace of the stranger which burst into +view between the 21st and the 22nd like the explosion of a rocket. + +Upon one who knew the stars the apparition of this intruder in a +well-known constellation had the effect of a sudden invasion. The new +star was not far west of the zenith in the early evening, and in that +position showed to the best advantage. To see Capella, the hitherto +unchallenged ruler of that quarter of the sky, abased by comparison +with this stranger of alien aspect, for there was always an unfamiliar +look about the ``nova,'' was decidedly disconcerting. It seemed to +portend the beginning of a revolution in the heavens. One could +understand what the effect of such an apparition must have been in the +superstitious times of Tycho. The star of Tycho had burst forth on the +northern border of the Milky Way; this one was on its southern border, +some forty-five degrees farther east. + +Astronomers were well-prepared this time for the scientific study of +the new star, both astronomical photography and spectroscopy having +been perfected, and the results of their investigations were +calculated to increase the wonder with which the phenomenon was +regarded. The star remained at its brightest only a few days; then, +like a veritable conflagration, it began to languish; and, like the +reflection of a dying fire, as it sank it began to glow with the red +color of embers. But its changes were spasmodic; once about every +three days it flared up only to die away again. During these +fluctuations its light varied alternately in the ratio of one to six. +Finally it took a permanent downward course, and after a few months +the naked eye could no longer perceive it; but it remained visible +with telescopes, gradually fading until it had sunk to the ninth +magnitude. Then another astonishing change happened: in August +photographs taken at the Yerkes Observatory and at Heidelberg showed +that the ``nova'' was surrounded by a spiral nebula! The nebula had +not been there before, and no one could doubt that it represented a +phase of the same catastrophe that had produced the outburst of the +new star. At one time the star seemed virtually to have disappeared, +as if all its substance had been expanded into the nebulous cloud, but +always there remained a stellar nucleus about which the misty spiral +spread wider and ever wider, like a wave expanding around a center of +disturbance. The nebula too showed a variability of brightness, and +four condensations which formed in it seemed to have a motion of +revolution about the star. As time went on the nebula continued to +expand at a rate which was computed to be not less than twenty +thousand miles per second! And now the star itself, showing +indications of having turned into a nebula, behaved in a most erratic +manner, giving rise to the suspicion that it was about to burst out +again. But this did not occur, and at length it sunk into a state of +lethargy from which it has to the present time not recovered. But the +nebulous spiral has disappeared, and the entire phenomena as it now +(1909) exists consists of a faint nebulous star of less than the ninth +magnitude. + +The wonderful transformations just described had been forecast in +advance of the discovery of the nebulous spiral encircling the star by +the spectroscopic study of the latter. At first there was no +suggestion of a nebular constitution, but within a month or two +characteristic nebular lines began to appear, and in less than six +months the whole spectrum had been transformed to the nebular type. In +the mean time the shifting of the spectral lines indicated a +complication of rapid motions in several directions simultaneously. +These motions were estimated to amount to from one hundred to five +hundred miles per second. + +The human mind is so constituted that it feels forced to seek an +explanation of so marvelous a phenomenon as this, even in the absence +of the data needed for a sound conclusion. The most natural +hypothesis, perhaps, is that of a collision. Such a catastrophe could +certainly happen. It has been shown, for instance, that in infinity of +time the earth is sure to be hit by a comet; in the same way it may be +asserted that, if no time limit is fixed, the sun is certain to run +against some obstacle in space, either another star, or a dense meteor +swarm, or one of the dark bodies which there is every reason to +believe abound around us. The consequences of such a collision are +easy to foretell, provided that we know the masses and the velocities +of the colliding bodies. In a preceding chapter we have discussed the +motions of the sun and stars, and have seen that they are so swift +that an encounter between any two of them could not but be disastrous. +But this is not all; for as soon as two stars approached within a few +million miles their speed would be enormously increased by their +reciprocal attractions and, if their motion was directed radially with +respect to their centers, they would come together with a crash that +would reduce them both to nebulous clouds. It is true that the chances +of such a ``head-on'' collision are relatively very small; two stars +approaching each other would most probably fall into closed orbits +around their common center of gravity. If there were a collision it +would most likely be a grazing one instead of a direct front-to-front +encounter. But even a close approach, without any actual collision, +would probably prove disastrous, owing to the tidal influence of each +of the bodies on the other. Suns, in consequence of their enormous +masses and dimensions and the peculiarities of their constitution, are +exceedingly dangerous to one another at close quarters. Propinquity +awakes in them a mutually destructive tendency. Consisting of matter +in the gaseous, or perhaps, in some cases, liquid, state, their tidal +pull upon each other if brought close together might burst them +asunder, and the photospheric envelope being destroyed the internal +incandescent mass would gush out, bringing fiery death to any planets +that were revolving near. Without regard to the resulting disturbance +of the earth's orbit, the close approach of a great star to the sun +would be in the highest degree perilous to us. But this is a danger +which may properly be regarded as indefinitely remote, since, at our +present location in space, we are certainly far from every star except +the sun, and we may feel confident that no great invisible body is +near, for if there were one we should be aware of its presence from +the effects of its attraction. As to dark nebul which may possibly +lie in the track that the solar system is pursuing at the rate of +375,000,000 miles per year, that is another question -- and they, too, +could be dangerous! + +This brings us directly back to ``Nova Persei,'' for among the many +suggestions offered to explain its outburst, as well as those of other +temporary stars, one of the most fruitful is that of a collision +between a star and a vast invisible nebula. Professor Seeliger, of +Munich, first proposed this theory, but it afterward underwent some +modifications from others. Stated in a general form, the idea is that +a huge dark body, perhaps an extinguished sun, encountered in its +progress through space a widespread flock of small meteors forming a +dark nebula. As it plunged into the swarm the friction of the +innumerable collisions with the meteors heated its surface to +incandescence, and being of vast size it then became visible to us as +a new star. Meanwhile the motion of the body through the nebula, and +its rotation upon itself, set up a gyration in the blazing atmosphere +formed around it by the vaporized meteors; and as this atmosphere +spread wider, under the laws of gyratory motion a rotation in the +opposite direction began in the inflamed meteoric cloud outside the +central part of the vortex. Thus the spectral lines were caused to +show motion in opposite directions, a part of the incandescent mass +approaching the earth simultaneously with the retreat of another part. +So the curious spectroscopic observations before mentioned were +explained. This theory might also account for the appearance of the +nebulous spiral first seen some six months after the original +outburst. The sequent changes in the spectrum of the ``nova'' are +accounted for by this theory on the assumption, reasonable enough in +itself, that at first the invading body would be enveloped in a +vaporized atmosphere of relatively slight depth, producing by its +absorption the fine dark lines first observed; but that as time went +on and the incessant collisions continued, the blazing atmosphere +would become very deep and extensive, whereupon the appearance of the +spectral lines would change, and bright lines due to the light of the +incandescent meteors surrounding the nucleus at a great distance would +take the place of the original dark ones. The vortex of meteors once +formed would protect the flying body within from further immediate +collisions, the latter now occurring mainly among the meteors +themselves, and then the central blaze would die down, and the +original splendor of the phenomenon would fade. + +But the theories about Nova Persei have been almost as numerous as the +astronomers who have speculated about it. One of the most startling of +them assumed that the outburst was caused by the running amuck of a +dark star which had encountered another star surrounded with planets, +the renewed outbreaks of light after the principal one had faded being +due to the successive running down of the unfortunate planets! Yet +another hypothesis is based on what we have already said of the tidal +influence that two close approaching suns would have upon each other. +Supposing two such bodies which had become encrusted, but remained +incandescent and fluid within, to approach within almost striking +distance; they would whirl each other about their common center of +gravity, and at the same time their shells would burst under the tidal +strain, and their glowing nuclei being disclosed would produce a great +outburst of light. Applying this theory to a ``nova,'' like that of +1866 in the ``Northern Crown,'' which had been visible as a small star +before the outbreak, and which afterward resumed its former aspect, we +should have to assume that a yet shining sun had been approached by a +dark body whose attraction temporarily burst open its photosphere. It +might be supposed that in this case the dark body was too far advanced +in cooling to suffer the same fate from the tidal pull of its victim. +But a close approach of that kind would be expected to result in the +formation of a binary system, with orbits of great eccentricity, +perhaps, and after the lapse of a certain time the outburst should be +renewed by another approximation of the two bodies. A temporary star +of that kind would rather be ranked as a variable. + +The celebrated French astronomer, Janssen, had a different theory of +Nova Persei, and of temporary stars in general. According to his idea, +such phenomena might be the result of chemical changes taking place in +a sun without interference by, or collision with, another body. +Janssen was engaged for many years in trying to discover evidence of +the existence of oxygen in the sun, and he constructed his observatory +on the summit of Mount Blanc specially to pursue that research. He +believed that oxygen must surely exist in the sun since we find so +many other familiar elements included in the constitution of the solar +globe, and as he was unable to discover satisfactory evidence of its +presence he assumed that it existed in a form unknown on the earth. If +it were normally in the sun's chromosphere, or coronal atmosphere, he +said, it would combine with the hydrogen which we know is there and +form an obscuring envelope of water vapor. It exists, then, in a +special state, uncombined with hydrogen; but let the temperature of +the sun sink to a critical point and the oxygen will assume its normal +properties and combine with the hydrogen, producing a mighty outburst +of light and heat. This, Janssen thought, might explain the phenomena +of the temporary stars. It would also, he suggested, account for their +brief career, because the combination of the elements would be quickly +accomplished, and then the resulting water vapor would form an +atmosphere cutting off the radiation from the star within. + +This theory may be said to have a livelier human interest than some of +the others, since, according to it, the sun may carry in its very +constitution a menace to mankind; one does not like to think of it +being suddenly transformed into a gigantic laboratory for the +explosive combination of oxygen and hydrogen! But while Janssen's +theory might do for some temporary stars, it is inadequate to explain +all the phenomena of Nova Persei, and particularly the appearance of +the great spiral nebula that seemed to exhale from the heart of the +star. Upon the whole, the theory of an encounter between a star and a +dark nebula seems best to fit the observations. By that hypothesis the +expanding billow of light surrounding the core of the conflagration is +very well accounted for, and the spectroscopic peculiarities are also +explained. + +Dr Gustov Le Bon offers a yet more alarming theory, suggesting that +temporary stars are the result of atomic explosion; but we shall touch +upon this more fully in Chapter 14. + +Twice in the course of this discussion we have called attention to the +change of color invariably undergone by temporary stars in the later +stages of their career. This was conspicuous with Nova Persei which +glowed more and more redly as it faded, until the nebulous light began +to overpower that of the stellar nucleus. Nothing could be more +suggestive of the dying out of a great fire. Moreover, change of color +from white to red is characteristic of all variable stars of long +period, such as ``Mira'' in Cetus. It is also characteristic of stars +believed to be in the later stages of evolution, and consequently +approaching extinction, like Antares and Betelgeuse, and still more +notably certain small stars which ``gleam like rubies in the field of +the telescope.'' These last appear to be suns in the closing period of +existence as self-luminous bodies. Between the white stars, such as +Sirius and Rigel, and the red stars, such as Aldebaran and Alpha +Herculis, there is a progressive series of colors from golden yellow +through orange to deep red. The change is believed to be due to the +increase of absorbing vapors in the stellar atmosphere as the body +cools down. In the case of ordinary stars these changes no doubt +occupy many millions of years, which represent the average duration of +solar life; but the temporary stars run through similar changes in a +few months: they resemble ephemeral insects -- born in the morning and +doomed to perish with the going down of the sun. + +Explosive and Whirling Nebul + +One of the most surprising triumphs of celestial photography was +Professor Keeler's discovery, in 1899, that the great majority of the +nebul have a distinctly spiral form. This form, previously known in +Lord Rosse's great ``Whirlpool Nebula,'' had been supposed to be +exceptional; now the photographs, far excelling telescopic views in +the revelation of nebular forms, showed the spiral to be the typical +shape. Indeed, it is a question whether all nebul are not to some +extent spiral. The extreme importance of this discovery is shown in +the effect that it has had upon hitherto prevailing views of solar and +planetary evolution. For more than three-quarters of a century +Laplace's celebrated hypothesis of the manner of origin of the solar +system from a rotating and contracting nebula surrounding the sun had +guided speculation on that subject, and had been tentatively extended +to cover the evolution of systems in general. The apparent forms of +some of the nebul which the telescope had revealed were regarded, and +by some are still regarded, as giving visual evidence in favor of this +theory. There is a ``ring nebula'' in Lyra with a central star, and a +``planetary nebula'' in Gemini bearing no little resemblance to the +planet Saturn with its rings, both of which appear to be practical +realizations of Laplace's idea, and the elliptical rings surrounding +the central condensation of the Andromeda Nebula may be cited for the +same kind of proof. + +But since Keeler's discovery there has been a decided turning away of +speculation another way. The form of the spiral nebul seems to be +entirely inconsistent with the theory of an originally globular or +disk-shaped nebula condensing around a sun and throwing or leaving off +rings, to be subsequently shaped into planets. Some astronomers, +indeed, now reject Laplace's hypothesis in toto, preferring to think +that even our solar system originated from a spiral nebula. Since the +spiral type prevails among the existing nebul, we must make any +mechanical theory of the development of stars and planetary systems +from them accord with the requirements which that form imposes. A +glance at the extraordinary variations upon the spiral which Professor +Keeler's photographs reveal is sufficient to convince one of the +difficulty of the task of basing a general theory upon them. In truth, +it is much easier to criticize Laplace's hypothesis than to invent a +satisfactory substitute for it. If the spiral nebul seem to oppose it +there are other nebul which appear to support it, and it may be that +no one fixed theory can account for all the forms of stellar evolution +in the universe. Our particular planetary system may have originated +very much as the great French mathematician supposed, while others +have undergone, or are now undergoing, a different process of +development. There is always a too strong tendency to regard an +important new discovery and the theories and speculations based upon +it as revolutionizing knowledge, and displacing or overthrowing +everything that went before. Upon the plea that ``Laplace only made a +guess'' more recent guesses have been driven to extremes and treated +by injudicious exponents as ``the solid facts at last.'' + +Before considering more recent theories than Laplace's, let us see +what the nature of the photographic revelations is. The vast celestial +maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be +taken as the leading type of the spiral nebul, although there are +less conspicuous objects of the kind which, perhaps, better illustrate +some of their peculiarities. Lord Rosse's nebula appears far more +wonderful in the photographs than in his drawings made with the aid of +his giant reflecting telescope at Parsonstown, for the photographic +plate records details that no telescope is capable of showing. Suppose +we look at the photograph of this object as any person of common sense +would look at any great and strange natural phenomenon. What is the +first thing that strikes the mind? It is certainly the appearance of +violent whirling motion. One would say that the whole glowing mass had +been spun about with tremendous velocity, or that it had been set +rotating so rapidly that it had become the victim of ``centrifugal +force,'' one huge fragment having broken loose and started to gyrate +off into space. Closer inspection shows that in addition to the +principal focus there are various smaller condensations scattered +through the mass. These are conspicuous in the spirals. Some of them +are stellar points, and but for the significance of their location we +might suppose them to be stars which happen to lie in a line between +us and the nebula. But when we observe how many of them follow most +faithfully the curves of the spirals we cannot but conclude that they +form an essential part of the phenomenon; it is not possible to +believe that their presence in such situations is merely fortuitous. +One of the outer spirals has at least a dozen of these star-like +points strung upon it; some of them sharp, small, and distinct, others +more blurred and nebulous, suggesting different stages of +condensation. Even the part which seems to have been flung loose from +the main mass has, in addition to its central condensation, at least +one stellar point gleaming in the half-vanished spire attached to it. +Some of the more distant stars scattered around the ``whirlpool'' look +as if they too had been shot out of the mighty vortex, afterward +condensing into unmistakable solar bodies. There are at least two +curved rows of minute stars a little beyond the periphery of the +luminous whirl which clearly follow lines concentric with those of the +nebulous spirals. Such facts are simply dumbfounding for anyone who +will bestow sufficient thought upon them, for these are suns, though +they may be small ones; and what a birth is that for a sun! + +Look now again at the glowing spirals. We observe that hardly have +they left the central mass before they begin to coagulate. In some +places they have a ``ropy'' aspect; or they are like peascods filled +with growing seeds, which eventually will become stars. The great +focus itself shows a similar tendency, especially around its +circumference. The sense that it imparts of a tremendous shattering +force at work is overwhelming. There is probably more matter in that +whirling and bursting nebula than would suffice to make a hundred +solar systems! It must be confessed at once that there is no +confirmation of the Laplacean hypothesis here; but what hypothesis +will fit the facts? There is one which it has been claimed does so, +but we shall come to that later. In the meanwhile, as a preparation, +fix in the memory the appearance of that second spiral mass spinning +beside its master which seems to have spurned it away. + +For a second example of the spiral nebul look at the one in the +constellation Triangulum. God, how hath the imagination of puny man +failed to comprehend Thee! Here is creation through destruction with a +vengeance! The spiral form of the nebula is unmistakable, but it is +half obliterated amid the turmoil of flying masses hurled away on all +sides with tornadic fury. The focus itself is splitting asunder under +the intolerable strain, and in a little while, as time is reckoned in +the Cosmos, it will be gyrating into stars. And then look at the +cyclonic rain of already finished stars whirling round the outskirts +of the storm. Observe how scores of them are yet involved in the +fading streams of the nebulous spirals; see how they have been thrown +into vast loops and curves, of a beauty that half redeems the terror +of the spectacle enclosed within their lines -- like iridescent cirri +hovering about the edges of a hurricane. And so again are suns born! + +Let us turn to the exquisite spiral in Ursa Major; how different its +aspect from that of the other! One would say that if the terrific coil +in Triangulum has all but destroyed itself in its fury, this one on +the contrary has just begun its self-demolition. As one gazes one +seems to see in it the smooth, swift, accelerating motion that +precedes catastrophe. The central part is still intact, dense, and +uniform in texture. How graceful are the spirals that smoothly rise +from its oval rim and, gemmed with little stars, wind off into the +darkness until they have become as delicate as threads of gossamer! +But at bottom the story told here is the same -- creation by gyration! + +Compare with the above the curious mass in Cetus. Here the plane of +the whirling nebula nearly coincides with our line of sight and we see +the object at a low angle. It is far advanced and torn to shreds, and +if we could look at it perpendicularly to its plane it is evident that +it would closely resemble the spectacle in Triangulum. + +Then take the famous Andromeda Nebula (see Frontispiece), which is so +vast that notwithstanding its immense distance even the naked eye +perceives it as an enigmatical wisp in the sky. Its image on the +sensitive plate is the masterpiece of astronomical photography; for +wild, incomprehensible beauty there is nothing that can be compared +with it. Here, if anywhere, we look upon the spectacle of creation in +one of its earliest stages. The Andromeda Nebula is apparently less +advanced toward transformation into stellar bodies than is that in +Triangulum. The immense crowd of stars sprinkled over it and its +neighborhood seem in the main to lie this side of the nebula, and +consequently to have no connection with it. But incipient stars (in +some places clusters of them) are seen in the nebulous rings, while +one or two huge masses seem to give promise of transformation into +stellar bodies of unusual magnitude. I say ``rings'' because although +the loops encompassing the Andromeda Nebula have been called spirals +by those who wish utterly to demolish Laplace's hypothesis, yet they +are not manifestly such, as can be seen on comparing them with the +undoubted spirals of the Lord Rosse Nebula. They look quite as much +like circles or ellipses seen at an angle of, say, fifteen or twenty +degrees to their plane. If they are truly elliptical they accord +fairly well with Laplace's idea, except that the scale of magnitude is +stupendous, and if the Andromeda Nebula is to become a solar system it +will surpass ours in grandeur beyond all possibility of comparison. + +There is one circumstance connected with the spiral nebul, and +conspicuous in the Andromeda Nebula on account of its brightness, +which makes the question of their origin still more puzzling; they all +show continuous spectra, which, as we have before remarked, indicate +that the mass from which the light comes is either solid or liquid, or +a gas under heavy pressure. Thus nebul fall into two classes: the +``white'' nebul, giving a continuous spectrum; and the ``green'' +nebul whose spectra are distinctly gaseous. The Andromeda Nebula is +the great representative of the former class and the Orion Nebula of +the latter. The spectrum of the Andromeda Nebula has been interpreted +to mean that it consists not of luminous gas, but of a flock of stars +so distant that they are separately indistinguishable even with +powerful telescopes, just as the component stars of the Milky Way are +indistinguishable with the naked eye; and upon this has been based the +suggestion that what we see in Andromeda is an outer universe whose +stars form a series of elliptical garlands surrounding a central mass +of amazing richness. But this idea is unacceptable if for no other +reason than that, as just said, all the spiral nebul possess the same +kind of spectrum, and probably no one would be disposed to regard them +all as outer universes. As we shall see later, the peculiarity of the +spectra of the spiral nebul is appealed to in support of a modern +substitute for Laplace's hypothesis. + +Finally, without having by any means exhausted the variety exhibited +by the spiral nebul, let us turn to the great representative of the +other species, the Orion Nebula. In some ways this is even more +marvelous than the others. The early drawings with the telescope +failed to convey an adequate conception either of its sublimity or of +its complication of structure. It exists in a nebulous region of +space, since photographs show that nearly the whole constellation is +interwoven with faintly luminous coils. To behold the entry of the +great nebula into the field even of a small telescope is a startling +experience which never loses its novelty. As shown by the photographs, +it is an inscrutable chaos of perfectly amazing extent, where spiral +bands, radiating streaks, dense masses, and dark yawning gaps are +strangely intermingled without apparent order. In one place four +conspicuous little stars, better seen in a telescope than in the +photograph on account of the blurring produced by over-exposure, are +suggestively situated in the midst of a dark opening, and no observer +has ever felt any doubt that these stars have been formed from the +substance of the surrounding nebula. There are many other stars +scattered over its expanse which manifestly owe their origin to the +same source. But compare the general appearance of this nebula with +the others that we have studied, and remark the difference. If the +unmistakably spiral nebul resemble bursting fly-wheels or grindstones +from whose perimeters torrents of sparks are flying, the Orion Nebula +rather recalls the aspect of a cloud of smoke and fragments produced +by the explosion of a shell. This idea is enforced by the look of the +outer portion farthest from the bright half of the nebula, where +sharply edged clouds with dark spaces behind seem to be billowing away +as if driven by a wind blowing from the center. + +Next let us consider what scientific speculation has done in the +effort to explain these mysteries. Laplace's hypothesis can certainly +find no standing ground either in the Orion Nebula or in those of a +spiral configuration, whatever may be its situation with respect to +the grand Nebula of Andromeda, or the ``ring'' and ``planetary'' +nebul. Some other hypothesis more consonant with the appearances must +be found. Among the many that have been proposed the most elaborate is +the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton. +It is to be remarked that it applies to the spiral nebul +distinctively, and not to an apparently chaotic mass of gas like the +vast luminous cloud in Orion. The gist of the theory is that these +curious objects are probably the result of close approaches to each +other of two independent suns, reminding us of what was said on this +subject when we were dealing with temporary stars. Of the previous +history of these appulsing suns the theory gives us no account; they +are simply supposed to arrive within what may be called an effective +tide-producing distance, and then the drama begins. Some of the +probable consequences of such an approach have been noticed in Chapter +5; let us now consider them a little more in detail. + +Tides always go in couples; if there is a tide on one side of a globe +there will be a corresponding tide on the other side. The cause is to +be found in the law that the force of gravitation varies inversely as +the square of the distance; the attraction on the nearest surface of +the body exercised by another body is greater than on its center, and +greater yet than on its opposite surface. If two great globes attract +each other, each tends to draw the other out into an ellipsoidal +figure; they must be more rigid than steel to resist this -- and even +then they cannot altogether resist. If they are liquid or gaseous they +will yield readily to the force of distortion, the amount of which +will depend upon their distance apart, for the nearer they are the +greater becomes the tidal strain. If they are encrusted without and +liquid or gaseous in the interior, the internal mass will strive to +assume the figure demanded by the tidal force, and will, if it can, +burst the restraining envelope. Now this is virtually the predicament +of the body we call a sun when in the immediate presence of another +body of similarly great mass. Such a body is presumably gaseous +throughout, the component gases being held in a state of rigidity by +the compression produced by the tremendous gravitational force of +their own aggregate mass. At the surface such a body is enveloped in a +shell of relatively cool matter. Now suppose a great attracting body, +such as another sun, to approach near enough for the difference in its +attraction on the two opposite sides of the body and on its center to +become very great; the consequence will be a tidal deformation of the +whole body, and it will lengthen out along the line of the +gravitational pull and draw in at the sides, and if its shell offers +considerable resistance, but not enough to exercise a complete +restraint, it will be violently burst apart, or blown to atoms, and +the internal mass will leap out on the two opposite sides in great +fiery spouts. In the case of a sun further advanced in cooling than +ours the interior might be composed of molten matter while the +exterior crust had become rigid like the shell of an egg; then the +force of the ``tidal explosion'' produced by the appulse of another +sun would be more violent in consequence of the greater resistance +overcome. Such, then, is the mechanism of the first phase in the +history of a spiral nebula according to the Planetesimal Hypothesis. +Two suns, perhaps extinguished ones, have drawn near together, and an +explosive outburst has occured in one or both. The second phase calls +for a more agile exercise of the imagination. + +To simplify the case, let us suppose that only one of the tugging suns +is seriously affected by the strain. Its vast wings produced by the +outburst are twisted into spirals by their rotation and the contending +attractions exercised upon them, as the two suns, like battleships in +desperate conflict, curve round each other, concentrating their +destructive energies. Then immense quantities of dbris are scattered +about in which eddies are created, and finally, as the sun that caused +the damage goes on its way, leaving its victim to repair its injuries +as it may, the dispersed matter cools, condenses, and turns into +streams of solid particles circling in elliptical paths about their +parent sun. These particles, or fragments, are the ``planetesimals'' +of the theory. In consequence of the inevitable intersection of the +orbits of the planetesimals, nodes are formed where the flying +particles meet, and at these nodes large masses are gradually +accumulated. The larger the mass the greater its attraction, and at +last the nodal points become the nuclei of great aggregations from +which planets are shaped. + +This, in very brief form, is the Planetesimal Hypothesis which we are +asked to substitute for that based on Laplace's suggestion as an +explanation of the mode of origin of the solar system; and the +phenomena of the spiral nebul are appealed to as offering evident +support to the new hypothesis. We are reminded that they are +elliptical in outline, which accords with the hypothesis; that their +spectra are not gaseous, which shows that they may be composed of +solid particles like the planetesimals; and that their central masses +present an oval form, which is what would result from the tidal +effects, as just described. We also remember that some of them, like +the Lord Rosse and the Andromeda nebul, are visually double, and in +these cases we might suppose that the two masses represent the +tide-burst suns that ventured into too close proximity. It may be +added that the authors of the theory do not insist upon the appulse of +two suns as the only way in which the planetesimals may have +originated, but it is the only supposition that has been worked out. + +But serious questions remain. It needs, for instance, but a glance at +the Triangulum monster to convince the observer that it cannot be a +solar system which is being evolved there, but rather a swarm of +stars. Many of the detached masses are too vast to admit of the +supposition that they are to be transformed into planets, in our sense +of planets, and the distances of the stars which appear to have been +originally ejected from the focal masses are too great to allow us to +liken the assemblage that they form to a solar system. Then, too, no +nodes such as the hypothesis calls for are visible. Moreover, in most +of the spiral nebul the appearances favor the view that the +supposititious encountering suns have not separated and gone each +rejoicing on its way, after having inflicted the maximum possible +damage on its opponent, but that, on the contrary, they remain in +close association like two wrestlers who cannot escape from each +other's grasp. And this is exactly what the law of gravitation +demands; stars cannot approach one another with impunity, with regard +either to their physical make-up or their future independence of +movement. The theory undertakes to avoid this difficulty by assuming +that in the case of our system the approach of the foreign body to the +sun was not a close one -- just close enough to produce the tidal +extrusion of the relatively insignificant quantity of matter needed to +form the planets. But even then the effect of the appulse would be to +change the direction of flight, both of the sun and of its visitor, +and there is no known star in the sky which can be selected as the +sun's probable partner in their ancient pas deux. That there are +unconquered difficulties in Laplace's hypothesis no one would deny, +but in simplicity of conception it is incomparably more satisfactory, +and with proper modifications could probably be made more consonant +with existing facts in our solar system than that which is offered to +replace it. Even as an explanation of the spiral nebul, not as solar +systems in process of formation, but as the birthplaces of stellar +clusters, the Planetesimal Hypothesis would be open to many +objections. Granting its assumptions, it has undoubtedly a strong +mathematical framework, but the trouble is not with the mathematics +but with the assumptions. Laplace was one of the ablest mathematicians +that ever lived, but he had never seen a spiral nebula; if he had, he +might have invented a hypothesis to suit its phenomena. His actual +hypothesis was intended only for our solar system, and he left it in +the form of a ``note'' for the consideration of his successors, with +the hope that they might be able to discover the full truth, which he +confessed was hidden from him. It cannot be said that that truth has +yet been found, and when it is found the chances are that intuition +and not logic will have led to it. + +The spiral nebul, then, remain among the greatest riddles of the +universe, while the gaseous nebul, like that of Orion, are no less +mysterious, although it seems impossible to doubt that both forms give +birth to stars. It is but natural to look to them for light on the +question of the origin of our planetary system; but we should not +forget that the scale of the phenomena in the two cases is vastly +different, and the forces in operation may be equally different. A +hill may have been built up by a glacier, while a mountain may be the +product of volcanic forces or of the upheaval of the strata of the +planet. + +The Banners of the Sun + +As all the world knows, the sun, a blinding globe pouring forth an +inconceivable quantity of light and heat, whose daily passage through +the sky is caused by the earth's rotation on its axis, constitutes the +most important phenomenon of terrestial existence. Viewed with a dark +glass to take off the glare, or with a telescope, its rim is seen to +be a sharp and smooth circle, and nothing but dark sky is visible +around it. Except for the interference of the moon, we should probably +never have known that there is any more of the sun than our eyes +ordinarily see. + +But when an eclipse of the sun occurs, caused by the interposition of +the opaque globe of the moon, we see its immediate surroundings, which +in some respects are more wonderful than the glowing central orb. +These surroundings, although not in the sense in which we apply the +term to the gaseous envelope of the earth, may be called the sun's +atmosphere. They consist of two very different parts -- first, the red +``prominences,'' which resemble tongues of flame ascending thousands +of miles above the sun's surface; and, second, the ``corona,'' which +extends to distances of millions of miles from the sun, and shines +with a soft, glowing light. The two combined, when well seen, make a +spectacle without parallel among the marvels of the sky. Although many +attempts have been made to render the corona visible when there is no +eclipse, all have failed, and it is to the moon alone that we owe its +revelation. To cover the sun's disk with a circular screen will not +answer the purpose because of the illumination of the air all about +the observer. When the moon hides the sun, on the other hand, the +sunlight is withdrawn from a great cylinder of air extending to the +top of the atmosphere and spreading many miles around the observer. +There is then no glare to interfere with the spectacle, and the corona +appears in all its surprising beauty. The prominences, however, +although they were discovered during an eclipse, can now, with the aid +of the spectroscope, be seen at any time. But the prominences are +rarely large enough to be noticed by the naked eye, while the +streamers of the corona, stretching far away in space, like ghostly +banners blown out from the black circle of the obscuring moon, attract +every eye, and to this weird apparition much of the fear inspired by +eclipses has been due. But if the corona has been a cause of terror in +the past it has become a source of growing knowledge in our time. + +The story of the first scientific observation of the corona and the +prominences is thrillingly interesting, and in fact dramatic. The +observation was made during the eclipse of 1842, which fortunately was +visible all over Central and Southern Europe so that scores of +astronomers saw it. The interest centers in what happened at Pavia in +Northern Italy, where the English astronomer Francis Baily had set up +his telescope. The eclipse had begun and Bailey was busy at his +telescope when, to quote his own words in the account which he wrote +for the Memoirs of the Royal Astronomical Society: + + I was astounded by a tremendous burst of applause from the streets + below, and at the same moment was electrified by the sight of one + of the most brilliant and splendid phenomena that can well be + imagined; for at that instant the dark body of the moon was + suddenly surrounded with a corona, or kind of bright glory, similar + in shape and magnitude to that which painters draw round the heads + of saints... + + Pavia contains many thousand inhabitants, the major part of whom + were at this early hour walking about the streets and squares or + looking out of windows in order to witness this long-talked-of + phenomenon; and when the total obscuration took place, which was + instantaneous, there was a universal shout from every observer + which ``made the welkin ring,'' and for the moment withdrew my + attention from the object with which I was immediately occupied. I + had, indeed, expected the appearance of a luminous circle round the + moon during the time of total obscurity; but I did not expect, from + any of the accounts of preceding eclipses that I had read, to + witness so magnificent an exhibition as that which took place... + + Splendid and astonishing, however, as this remarkable phenomenon + really was, and although it could not fail to call forth the + admiration and applause of every beholder, yet I must confess that + there was at the same time something in its singular and wonderful + appearance that was appalling... + + But the most remarkable circumstance attending the phenomenon was + the appearance of three large protuberances apparently emanating + from the circumference of the moon, but evidently forming a portion + of the corona. They had the appearance of mountains of a prodigious + elevation; their color was red tinged with lilac or purple; perhaps + the color of the peach-blossom would more nearly represent it. They + somewhat resembled the tops of the snowy Alpine mountains when + colored by the rising or the setting sun. They resembled the Alpine + mountains in another respect, inasmuch as their light was perfectly + steady, and had none of that flickering or sparkling motion so + visible in other parts of the corona... + + The whole of these protuberances were visible even to the last + moment of total obscuration, and when the first ray of light was + admitted from the sun they vanished, with the corona, altogether, + and daylight was instantly restored. + +I have quoted nearly all of this remarkable description not alone for +its intrinsic interest, but because it is the best depiction that can +be found of the general phenomena of a total solar eclipse. Still, not +every such eclipse offers an equally magnificent spectacle. The +eclipses of 1900 and 1905, for instance, which were seen by the +writer, the first in South Carolina and the second in Spain, fell far +short of that described by Bailey in splendor and impressiveness. Of +course, something must be allowed for the effect of surprise; Bailey +had not expected to see what was so suddenly disclosed to him. But +both in 1900 and 1905 the amount of scattered light in the sky was +sufficient in itself to make the corona appear faint, and there were +no very conspicuous prominences visible. Yet on both occasions there +was manifest among the spectators that mingling of admiration and awe +of which Bailey speaks. The South Carolinians gave a cheer and the +ladies waved their handkerchiefs when the corona, ineffably delicate +of form and texture, melted into sight and then in two minutes melted +away again. The Spaniards, crowded on the citadel hill of Burgos, with +their king and his royal retinue in their midst, broke out with a +great clapping of hands as the awaited spectacle unfolded itself in +the sky; and on both occasions, before the applause began, after an +awed silence a low murmur ran through the crowds. At Burgos it is said +many made the sign of the cross. + +It was not long before Bailey's idea that the prominences were a part +of the corona was abandoned, and it was perceived that the two +phenomena were to a great extent independent. At the eclipse of 1868, +which the astronomers, aroused by the wonderful scene of 1842, and +eager to test the powers of the newly invented spectroscope, flocked +to India to witness, Janssen conceived the idea of employing the +spectroscope to render the prominences visible when there was no +eclipse. He succeeded the very next day, and these phenomena have been +studied in that way ever since. + +There are recognized two kinds of prominences -- the ``erruptive'' and +the ``quiescent.'' The latter, which are cloud-like in form, may be +seen almost anywhere along the edge of the sun; but the former, which +often shoot up as if hurled from mighty volcanoes, appear to be +associated with sun-spots, and appear only above the zones where spots +abound. Either of them, when seen in projection against the brilliant +solar disk, appears white, not red, as against a background of sky. +The quiescent prominences, whose elevation is often from forty +thousand to sixty thousand miles, consist, as the spectroscope shows, +mainly of hydrogen and helium. The latter, it will be remembered, is +an element which was known to be in the sun many years before the +discovery that it also exists in small quantities on the earth. A fact +which may have a significance which we cannot at present see is that +the emanation from radium gradually and spontaneously changes into +helium, an alchemistical feat of nature that has opened many curious +vistas to speculative thinkers. The eruptive prominences, which do not +spread horizontally like the others, but ascend with marvelous +velocity to elevations of half a million miles or more, are apparently +composed largely of metallic vapors -- i.e. metals which are usually +solid on the earth, but which at solar temperatures are kept in a +volatilized state. The velocity of their ascent occasionally amounts +to three hundred or four hundred miles per second. It is known from +mathematical considerations that the gravitation of the sun would not +be able to bring back any body that started from its surface with a +velocity exceeding three hundred and eighty-three miles per second; so +it is evident that some of the matter hurled forth in eruptive +prominences may escape from solar control and go speeding out into +space, cooling and condensing into solid masses. There seems to be no +reason why some of the projectiles from the sun might not reach the +planets. Here, then, we have on a relatively small scale, explosions +recalling those which it has been imagined may be the originating +cause of some of the sudden phenomena of the stellar heavens. + +Of the sun-spots it is not our intention here specifically to speak, +but they evidently have an intimate connection with eruptive +prominences, as well as some relation, not yet fully understood, with +the corona. Of the real cause of sun-spots we know virtually nothing, +but recent studies by Professor Hale and others have revealed a +strange state of things in the clouds of metallic vapors floating +above them and their surroundings. Evidences of a cyclonic tendency +have been found, and Professor Hale has proved that sun-spots are +strong magnetic fields, and consist of columns of ionized vapors +rotating in opposite directions in the two hemispheres. A fact which +may have the greatest significance is that titanium and vanadium have +been found both in sun-spots and in the remarkable variable Mira Ceti, +a star which every eleven months, or thereabout, flames up with great +brilliancy and then sinks back to invisibility with the naked eye. It +has been suggested that sun-spots are indications of the beginning of +a process in the sun which will be intensified until it falls into the +state of such a star as Mira. Stars very far advanced in evolution, +without showing variability, also exhibit similar spectra; so that +there is much reason for regarding sunspots as emblems of advancing +age. + +The association of the corona with sun-spots is less evident than that +of the eruptive prominences; still such an association exists, for the +form and extent of the corona vary with the sun-spot period of which +we shall presently speak. The constitution of the corona remains to be +discovered. It is evidently in part gaseous, but it also probably +contains matter in the form of dust and small meteors. It includes one +substance altogether mysterious -- ``coronium.'' There are reasons for +thinking that this may be the lightest of all the elements, and +Professor Young, its discoverer, said that it was ``absolutely unique +in nature; utterly distinct from any other known form of matter, +terrestial, solar, or cosmical.'' The enormous extent of the corona is +one of its riddles. Since the development of the curious subject of +the ``pressure of light'' it has been proposed to account for the +sustentation of the corona by supposing that it is borne upon the +billows of light continually poured out from the sun. Experiment has +proved, what mathematical considerations had previously pointed out as +probable, that the waves of light exert a pressure or driving force, +which becomes evident in its effects if the body acted upon is +sufficiently small. In that case the light pressure will prevail over +the attraction of gravitation, and propel the attenuated matter away +from the sun in the teeth of its attraction. The earth itself would be +driven away if, instead of consisting of a solid globe of immense +aggregate mass, it were a cloud of microscopic particles. The reason +is that the pressure varies in proportion to the surface of the body +acted upon, while the gravitational attraction is proportional to the +volume, or the total amount of matter in the body. But the surface of +any body depends upon the square of its diameter, while the volume +depends upon the cube of the diameter. If, for instance, the diameter +is represented by 4, the surface will be proportional to 4 4, or 16, +and the volume to 4 4 4, or 64; but if the diameter is taken as 2, +the surface will be 2 2, or 4, and the volume 2 2 2, or 8. Now, +the ratio of 4 to 8 is twice as great as that of 16 to 64. If the +diameter is still further decreased, the ratio of the surface to the +volume will proportionally grow larger; in other words, the pressure +will gain upon the attraction, and whatever their original ratio may +have been, a time will come, if the diminution of size continues, when +the pressure will become more effective than the attraction, and the +body will be driven away. Supposing the particles of the corona to be +below the critical size for the attraction of a mass like that of the +sun to control them, they would be driven off into the surrounding +space and appear around the sun like the clouds of dust around a mill. +We shall return to this subject in connection with the Zodiacal Light, +the Aurora, and Comets. + +On the other hand, there are parts of the corona which suggest by +their forms the play of electric or magnetic forces. This is +beautifully shown in some of the photographs that have been made of +the corona during recent eclipses. Take, for instance, that of the +eclipse of 1900. The sheaves of light emanating from the poles look +precisely like the ``lines of force'' surrounding the poles of a +magnet. It will be noticed in this photograph that the corona appears +to consist of two portions: one comprising the polar rays just spoken +of, and the other consisting of the broader, longer, and less-defined +masses of light extending out from the equatorial and middle-latitude +zones. Yet even in this more diffuse part of the phenomenon one can +detect the presence of submerged curves bearing more or less +resemblance to those about the poles. Just what part electricity or +electro-magnetism plays in the mechanism of the solar radiation it is +impossible to say, but on the assumption that it is a very important +part is based the hypothesis that there exists a direct solar +influence not only upon the magnetism, but upon the weather of the +earth. This hypothesis has been under discussion for half a century, +and still we do not know just how much truth it represents. It is +certain that the outbreak of great disturbances on the sun, +accompanied by the formation of sun-spots and the upshooting of +eruptive prominences (phenomena which we should naturally expect to be +attended by action), have been instantly followed by corresponding +``magnetic storms'' on the earth and brilliant displays of the auroral +lights. There have been occasions when the influence has manifested +itself in the most startling ways, a great solar outburst being +followed by a mysterious gripping of the cable and telegraph systems +of the world, as if an invisible and irresistible hand had seized +them. Messages are abruptly cut off, sparks leap from the telegraph +instruments, and the entire earth seems to have been thrown into a +magnetic flurry. These occurrences affect the mind with a deep +impression of the dependence of our planet on the sun, such as we do +not derive from the more familiar action of the sunlight on the growth +of plants and other phenomena of life depending on solar influences. + +Perhaps the theory of solar magnetic influence upon the weather is +best known in connection with the ``sun-spot cycle.'' This, at any +rate, is, as already remarked, closely associated with the corona. Its +existence was discovered in 1843 by the German astronomer Schwabe. It +is a period of variable length, averaging about eleven years, during +which the number of spots visible on the sun first increases to a +maximum, then diminishes to a minimum, and finally increases again to +a maximum. For unknown reasons the period is sometimes two or three +years longer than the average and sometimes as much shorter. +Nevertheless, the phenomena always recur in the same order. Starting, +for instance, with a time when the observer can find few or no spots, +they gradually increase in number and size until a maximum, in both +senses, is reached, during which the spots are often of enormous size +and exceedingly active. After two or three years they begin to +diminish in number, magnitude, and activity until they almost or quite +disappear. A strange fact is that when a new period opens, the spots +appear first in high northern and southern latitudes, far from the +solar equator, and as the period advances they not only increase in +number and size, but break out nearer and nearer to the equator, the +last spots of a vanishing period sometimes lingering in the equatorial +region after the advance-guard of its successor has made its +appearance in the high latitudes. Spots are never seen on the equator +nor near the poles. It was not very long after the discovery of the +sun-spot cycle that the curious observation was made that a striking +coincidence existed between the period of the sun-spots and another +period affecting the general magnetic condition of the earth. When a +curved line representing the varying number of sun-spots was compared +with another curve showing the variations in the magnetic state of the +earth the two were seen to be in almost exact accord, a rise in one +curve corresponding to a rise in the other, and a fall to a fall. +Continued observation has proved that this is a real coincidence and +not an accidental one, so that the connection, although as yet +unexplained, is accepted as established. But does the influence extend +further, and directly affect the weather and the seasons as well as +the magnetic elements of the earth? A final answer to this question +cannot yet be given, for the evidence is contradictory, and the +interpretations put upon it depend largely on the predilections of the +judges. + +But, in a broad sense, the sun-spots and the phenomena connected with +them must have a relation to terrestial meteorology, for they prove +the sun to be a variable star. Reference was made, a few lines above, +to the resemblance of the spectra of sun-spots to those of certain +stars which seem to be failing through age. This in itself is +extremely suggestive; but if this resemblance had never been +discovered, we should have been justified in regarding the sun as +variable in its output of energy; and not only variable, but probably +increasingly so. The very inequalities in the sun-spot cycle are +suspicious. When the sun is most spotted its total light may be +reduced by one-thousandth part, although it is by no means certain +that its outgiving of thermal radiations is then reduced. A loss of +one-thousandth of its luminosity would correspond to a decrease of +.0025 of a stellar magnitude, considering the sun as a star viewed +from distant space. So slight a change would not be perceptible; but +it is not alone sun-spots which obscure the solar surface, its entire +globe is enveloped with an obscuring veil. When studied with a +powerful telescope the sun's surface is seen to be thickly mottled +with relatively obscure specks, so numerous that it has been estimated +that they cut off from one-tenth to one-twentieth of the light that we +should receive from it if the whole surface were as brilliant as its +brightest parts. The condition of other stars warrants the conclusion +that this obscuring envelope is the product of a process of +refrigeration which will gradually make the sun more and more variable +until its history ends in extinction. Looking backward, we see a time +when the sun must have been more brilliant than it is now. At that +time it probably shone with the blinding white splendor of such stars +as Sirius, Spica, and Vega; now it resembles the relatively dull +Procyon; in time it will turn ruddy and fall into the closing cycle +represented by Antares. Considering that once it must have been more +radiantly powerful than at present, one is tempted to wonder if that +could have been the time when tropical life flourished within the +earth's polar circles, sustained by a vivific energy in the sun which +it has now lost. + +The corona, as we have said, varies with the sun-spot cycle. When the +spots are abundant and active the corona rises strong above the +spotted zones, forming immense beams or streamers, which on one +occasion, at least, had an observed length of ten million miles. At +the time of a spot minimum the corona is less brilliant and has a +different outline. It is then that the curved polar rays are most +conspicuous. Thus the vast banners of the sun, shaken out in the +eclipse, are signals to tell of its varying state, but it will +probably be long before we can read correctly their messages. + +The Zodiacal Light Mystery + +There is a singular phenomenon in the sky -- one of the most puzzling +of all -- which has long arrested the attention of astronomers, +defying their efforts at explanation, but which probably not one in a +hundred, and possibly not one in a thousand, of the readers of this +book has ever seen. Yet its name is often spoken, and it is a +conspicuous object if one knows when and where to look for it, and +when well seen it exhibits a mystical beauty which at the same time +charms and awes the beholder. It is called ``The Zodiacal Light,'' +because it lies within the broad circle of the Zodiac, marking the +sun's apparent annual path through the stars. What it is nobody has +yet been able to find out with certainty, and books on astronomy +usually speak of it with singular reserve. But it has given rise to +many remarkable theories, and a true explanation of it would probably +throw light on a great many other celestial mysteries. The Milky Way +is a more wonderful object to look upon, but its nature can be +comprehended, while there is a sort of uncanniness about the Zodiacal +Light which immediately impresses one upon seeing it, for its part in +the great scheme of extra-terrestrial affairs is not evident. + +If you are out-of-doors soon after sunset -- say, on an evening late +in the month of February -- you may perceive, just after the angry +flush of the dying winter's day has faded from the sky, a pale ghostly +presence rising above the place where the sun went down. The writer +remembers from boyhood the first time it was pointed out to him and +the unearthly impression that it made, so that he afterward avoided +being out alone at night, fearful of seeing the spectral thing again. +The phenomenon brightens slowly with the fading of the twilight, and +soon distinctly assumes the shape of an elongated pyramid of pearly +light, leaning toward the south if the place of observation is in the +northern hemisphere. It does not impress the observer at all in the +same manner as the Milky Way; that looks far off and is clearly among +the stars, but the Zodiacal Light seems closer at hand, as if it were +something more intimately concerning the earth. To all it immediately +suggests a connection, also, with the sunken sun. If the night is +clear and the moon absent (and if you are in the country, for city +lights ruin the spectacles of the sky), you will be able to watch the +apparition for a long time. You will observe that the light is +brightest near the horizon, gradually fading as the pyramidal beam +mounts higher, but in favorable circumstances it may be traced nearly +to the meridian south of the zenith, where its apex at last vanishes +in the starlight. It continues visible during the evenings of March +and part of April, after which, ordinarily, it is seen no more, or if +seen is relatively faint and unimpressive. But when autumn comes it +appears again, this time not like a wraith hovering above the westward +tomb of the day-god, but rather like a spirit of the morning +announcing his reincarnation in the east. + +The reason why the Zodiacal Light is best seen in our latitudes at the +periods just mentioned is because at those times the Zodiac is more +nearly perpendicular to the horizon, first in the west and then in the +east; and, since the phenomenon is confined within the borders of the +Zodiac, it cannot be favorably placed for observation when the +zodiacal plane is but slightly inclined to the horizon. Its faint +light requires the contrast of a background of dark sky in order to be +readily perceptible. But within the tropics, where the Zodiac is +always at a favorable angle, the mysterious light is more constantly +visible. Nearly all observant travelers in the equatorial regions have +taken particular note of this phenomenon, for being so much more +conspicuous there than in the temperate zones it at once catches the +eye and holds the attention as a novelty. Humboldt mentions it many +times in his works, for his genius was always attracted by things out +of the ordinary and difficult of explanation, and he made many careful +observations on its shape, its brilliancy, and its variations; for +there can be no doubt that it does vary, and sometimes to an +astonishing degree. It is said that it once remained practically +invisible in Europe for several years in succession. During a trip to +South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a +remarkable difference between the appearance of the Zodiacal Light on +his going and coming voyages. In fact, when crossing the equator going +south he did not see it at all; but on returning he had, on March 6th, +when one degree south of the equator, a memorable view of it. + + It was a bright, clear night, and the Zodiacal Light was + extraordinarily brilliant -- brighter than he had ever seen it + before. The Milky Way was not to be compared with it. The brightest + part extended 75 from the sun. There was a faint and much narrower + extension which they could just make out beyond the Pleiades along + the ecliptic, but the greater part of the Zodiacal Light showed as + a broad truncated column, and it did not appear nearly as conical + as he had before seen it. + +When out of the brief twilight of intertropical lands, where the sun +drops vertically to the horizon and night rushes on like a wave of +darkness, the Zodiacal Light shoots to the very zenith, its color is +described as a golden tint, entirely different from the silvery sheen +of the Milky Way. If I may venture again to refer to personal +experiences and impressions, I will recall a view of the Zodiacal +Light from the summit of the cone of Mt Etna in the autumn of the year +1896 (more briefly described in Astronomy with the Naked Eye). There +are few lofty mountains so favorably placed as Etna for observations +of this kind. It was once resorted to by Prof. George E. Hale, in an +attempt to see the solar corona without an eclipse. Rising directly +from sea-level to an elevation of nearly eleven thousand feet, the +observer on its summit at night finds himself, as it were, lost in the +midst of the sky. But for the black flanks of the great cone on which +he stands he might fancy himself to be in a balloon. On the occasion +to which I refer the world beneath was virtually invisible in the +moonless night. The blaze of the constellations overhead was +astonishingly brilliant, yet amid all their magnificence my attention +was immediately drawn to a great tapering light that sprang from the +place on the horizon where the sun would rise later, and that seemed +to be blown out over the stars like a long, luminous veil. It was the +finest view of the Zodiacal light that I had ever enjoyed -- thrilling +in its strangeness -- but I was almost disheartened by the +indifference of my guide, to whom it was only a light and nothing +more. If he had no science, he had less poetry -- rather a remarkable +thing, I thought, for a child of his clime. The Light appeared to me +to be distinctly brighter than the visible part of the Milky Way which +included the brilliant stretches in Auriga and Perseus, and its color, +if one may speak of color in connection with such an object, seemed +richer than that of the galactic band; but I did not think of it as +yellow, although Humboldt has described it as resembling a golden +curtain drawn over the stars, and Du Chaillu in Equatorial Africa +found it of a bright yellow color. It may vary in color as in +conspicuousness. The fascination of that extraordinary sight has never +faded from my memory. I turned to regard it again and again, although +I had never seen the stellar heavens so brilliant, and it was one of +the last things I looked for when the morning glow began softly to +mount in the east, and Sicily and the Mediterranean slowly emerged +from the profound shadow beneath us. + +The Zodiacal Light seems never to have attracted from astronomers in +general the amount of careful attention that it deserves; perhaps +because so little can really be made of it as far as explanation is +concerned. I have referred to the restraint that scientific writers +apparently feel in speaking of it. The grounds for speculation that it +affords may be too scanty to lead to long discussions, yet it piques +curiosity, and as we shall see in a moment has finally led to a most +interesting theory. Once it was the subject of an elaborate series of +studies which carried the observer all round the world. That was in +1845--46, during the United States Exploring Expedition that visited +the then little known Japan. The chaplain of the fleet, the Rev. Mr +Jones, went out prepared to study the mysterious light in all its +phases. He saw it from many latitudes on both sides of the equator, +and the imagination cannot but follow him with keen interest in his +world-circling tour, keeping his eyes every night fixed upon the +phantasm overhead, whose position shifted with that of the hidden sun. +He demonstrated that the flow extends at times completely across the +celestial dome, although it is relatively faint directly behind the +earth. On his return the government published a large volume of his +observations, in which he undertook to show that the phenomenon was +due to the reflection of sunlight from a ring of meteoric bodies +encircling the earth. But, after all, this elaborate investigation +settled nothing. + +Prof. E. E. Barnard has more recently devoted much attention to the +Zodiacal Light, as well as to a strange attendant phenomenon called +the ``Gegenschein,'' or Counterglow, because it always appears at that +point in the sky which is exactly opposite the sun. The Gegenschein is +an extremely elusive phenomenon, suitable only for eyes that have been +specially trained to see it. Professor Newcomb has cautiously remarked +that + + it is said that in that point of the heavens directly opposite the + sun there is an elliptical patch of light... This phenomenon is so + difficult to account for that its existence is sometimes doubted; + yet the testimony in its favor is difficult to set aside. + +It certainly cannot be set aside at all since the observations of +Barnard. I recall an attempt to see it under his guidance during a +visit to Mount Hamilton, when he was occupied there with the Lick +telescope. Of course, both the Gegenschein and the Zodiacal Light are +too diffuse to be studied with telescopes, which, so to speak, magnify +them out of existence. They can only be successfully studied with the +naked eye, since every faintest glimmer that they afford must be +utilized. This is especially true of the Gegenschein. At Mount +Hamilton, Mr Barnard pointed out to me its location with reference to +certain stars, but with all my gazing I could not be sure that I saw +it. To him, on the contrary, it was obvious; he had studied it for +months, and was able to indicate its shape, its boundaries, its +diameter, and the declination of its center with regard to the +ecliptic. There is not, of course, the shadow of a doubt of the +existence of the Gegenschein, and yet I question if one person in a +million has ever seen or ever will see it. The Zodiacal Light, on the +other hand, is plain enough, provided that the time and the +circumstances of the observation are properly chosen. + +In the attempts to explain the Zodiacal Light, the favorite hypothesis +has been that it is an appendage of the sun -- perhaps simply an +extension of the corona in the plane of the ecliptic, which is not +very far from coinciding with that of the sun's equator. This idea is +quite a natural one, because of the evident relation of the light to +the position of the sun. The vast extension of the equatorial wings of +the corona in 1878 gave apparent support to this hypothesis; if the +substance of the corona could extend ten million miles from the sun, +why might it not extend even one hundred million, gradually fading out +beyond the orbit of the earth? A variation of this hypothesis assumes +that the reflection is due to swarms of meteors circling about the +sun, in the plane of its equator, all the way from its immediate +neighborhood to a distance exceeding that of the earth. But in neither +form is the hypothesis satisfactory; there is nothing in the +appearance of the corona to indicate that it extends even as far as +the planet Mercury, while as to meteors, the orbits of the known +swarms do not accord with the hypothesis, and we have no reason to +believe that clouds of others exist traveling in the part of space +where they would have to be in order to answer the requirements of the +theory. The extension of the corona in 1878 did not resemble in its +texture the Zodiacal Light. + +Now, it has so often happened in the history of science that an +important discovery in one branch has thrown unexpected but most +welcome light upon some pending problem in some other branch, that a +strong argument might be based upon that fact alone against the too +exclusive devotion of many investigators to the narrow lines of their +own particular specialty; and the Zodiacal Light affords a case in +point, when it is considered in connection with recent discoveries in +chemistry and physics. From the fact that atoms are compound bodies +made up of corpuscles at least a thousand times smaller than the +smallest known atom -- a fact which astounded most men of science when +it was announced a few years ago -- a new hypothesis has been +developed concerning the nature of the Zodiacal Light (as well as +other astronomical riddles), and this hypothesis comes not from an +astronomer, but from a chemist and physicist, the Swede, Svante +Arrhenius. In considering an outline of this new hypothesis we need +neither accept nor reject it; it is a case rather for suspension of +judgment. + +To begin with, it carries us back to the ``pressure of light'' +mentioned in the preceding chapter. The manner in which this pressure +is believed generally to act was there sufficiently explained, and it +only remains to see how it is theoretically extended to the particles +of matter supposed to constitute the Zodiacal Light. We know that +corpuscles, or ``fragments of atoms'' negatively electrified, are +discharged from hot bodies. Streams of these ``ions'' pour from many +flames and from molten metals; and the impact of the cathode and +ultra-violet rays causes them to gush even from cold bodies. In the +vast laboratory of the sun it is but reasonable to suppose that +similar processes are taking place. ``As a very hot metal emits these +corpuscles,'' says Prof. J. J. Thomson, ``it does not seem an +improbable hypothesis that they are emitted by that very hot body, the +sun.'' Let it be assumed, then, that the sun does emit them; what +happens next? Negatively charged corpuscles, it is known, serve as +nuclei to which particles of matter in the ordinary state are +attracted, and it is probable that those emitted from the sun +immediately pick up loads in this manner and so grow in bulk. If they +grow large enough the gravitation of the sun draws them back, and they +produce a negative charge in the solar atmosphere. But it is probable +that many of the particles do not attain the critical size which, +according to the principles before explained, would enable the +gravitation of the sun to retain them in opposition to the pressure of +the waves of light, and with these particles the light pressure is +dominant. Clouds of them may be supposed to be continually swept away +from the sun into surrounding space, moving mostly in or near the +plane of the solar equator, where the greatest activity, as indicated +by sunspots and related phenomena, is taking place. As they pass +outward into space many of them encounter the earth. If the earth, +like the moon, had no atmosphere the particles would impinge directly +on its surface, giving it a negative electric charge. But the presence +of the atmosphere changes all that, for the first of the flying +particles that encounter it impart to it their negative electricity, +and then, since like electric charges repel like, the storm of +particles following will be sheered off from the earth, and will +stream around it in a maze of hyperbolic paths. Those that continue on +into space beyond the earth may be expected to continue picking up +wandering particles of matter until their bulk has become so great +that the solar attraction prevails again over the light pressure +acting upon them, and they turn again sunward. Passing the earth on +their return they will increase the amount of dust-clouds careering +round it; and these will be further increased by the action of the +ultra-violet rays of the sunlight causing particles to shoot radially +away from the earth when the negative charge of the upper atmosphere +has reached a certain amount, which particles, although starting +sunward, will be swept back to the earth with the oncoming streams. As +the final result of all this accumulation of flying and gyrating +particles in the earth's neighborhood, we are told that the latter +must be transformed into the semblance of a gigantic solid-headed +comet provided with streaming tails, the longest of them stretching +away from the direction of the sun, while another shorter one extends +toward the sun. This shorter tail is due to the particles that we have +just spoken of as being driven sunward from the earth by the action of +ultra-violet light. No doubt this whole subject is too technical for +popular statement; but at any rate the general reader can understand +the picturesque side of the theory, for its advocates assure us that +if we were on the moon we would doubtless be able to see the +comet-like tails of the earth, and then we could appreciate the part +that they play in producing the phenomenon of the Zodiacal Light. + +That the Light as we see it could be produced by the reflection of +sunlight from swarms of particles careering round the earth in the +manner supposed by Arrhenius' hypothesis is evident enough; and it +will be observed that the new theory, after all, is only another +variant of the older one which attributes the Zodiacal Light to an +extension of the solar corona. But it differs from the older theory in +offering an explanation of the manner in which the extension is +effected, and it differentiates between the corona proper and the +streams of negative particles shot away from the sun. In its details +the hypothesis of Arrhenius also affords an explanation of many +peculiarities of the Zodiacal Light, such as that it is confined to +the neighborhood of the ecliptic, and that it is stronger on the side +of the earth which is just turning away from a position under the sun +than on the other side; but it would carry us beyond our limits to go +into these particulars. The Gegenschein, according to this theory, is +a part of the same phenomenon as the Zodiacal Light, for by the laws +of perspective it is evident that the reflection from the streams of +particles situated at a point directly opposite to the sun would be at +a maximum, and this is the place which the Gegenschein occupies. Apart +from its geometrical relations to the position of the sun, the +variability of the Zodiacal Light appears to affirm its solar +dependence, and this too would be accounted for by Arrhenius' +hypothesis better than by the old theory of coronal extension. The +amount of corpuscular discharge from the sun must naturally be +governed by the state of relative activity or inactivity of the +latter, and this could not but be reflected in the varying splendor of +the Zodiacal Light. But much more extended study than has yet been +given to the subject will be required before we can feel that we know +with reasonable certainty what this mysterious phenomenon really is. +By the hypothesis of Arrhenius every planet that has an atmosphere +must have a Zodiacal Light attending it, but the phenomenon is too +faint for us to be able to see it in the case, for instance, of Venus, +whose atmosphere is very abundant. The moon has no corresponding +``comet's tail'' because, as already explained, of the lack of a lunar +atmosphere to repel the streams by becoming itself electrified; but if +there were a lunar Zodiacal Light, no doubt we could see it because of +the relative nearness of our satellite. + +Marvels of the Aurora + +One of the most vivid recollections of my early boyhood is that of +seeing my father return hastily into the house one evening and call +out to the family: ``Come outside and look at the sky!'' Ours was a +country house situated on a commanding site, and as we all emerged +from the doorway we were dumbfounded to see the heavens filled with +pale flames which ran licking and quivering over the stars. Instantly +there sprang into my terrified mind the recollection of an awful +description of ``the Day of Judgment'' (the Dies Ir), which I had +heard with much perturbation of spirit in the Dutch Reformed church +from the lips of a tall, dark-browed, dreadfully-in-earnest preacher +of the old-fashioned type. My heart literally sank at sight of the +spectacle, for it recalled the preacher's very words; it was just as +he had said it would be, and it needed the assured bearing of my +elders finally to convince me that + + That Day of Wrath, O dreadful day, + When Heaven and Earth shall pass away, + As David and the Sibyl say + +had not actually come upon us. And even the older members of the +household were not untouched with misgivings when menacing spots of +crimson appeared, breaking out now here, now there, in the shuddering +sky. Toward the north the spectacle was appalling. A huge arch spanned +an unnaturally dark segment resting on the horizon, and above this +arch sprang up beams and streamers in a state of incessant agitation, +sometimes shooting up to the zenith with a velocity that took one's +breath, and sometimes suddenly falling into long ranks, and marching, +marching, marching, like an endless phalanx of fiery specters, and +moving, as I remember, always from east to west. The absolute silence +with which these mysterious evolutions were performed and the +quavering reflections which were thrown upon the ground increased the +awfulness of the exhibition. Occasionally enormous curtains of lambent +flame rolled and unrolled with a majestic motion, or were shaken to +and fro as if by a mighty, noiseless wind. At times, too, a sudden +billowing rush would be made toward the zenith, and for a minute the +sky overhead would glow so brightly that the stars seemed to have been +consumed. The spectacle continued with varying intensity for hours. + +This exhibition occurred in Central New York, a latitude in which the +Aurora Borealis is seldom seen with so much splendor. I remember +another similar one seen from the city of New York in November, 1882. +On this last occasion some observers saw a great upright beam of light +which majestically moved across the heavens, stalking like an +apparition in the midst of the auroral pageant, of whose general +movements it seemed to be independent, maintaining always its upright +posture, and following a magnetic parallel from east to west. This +mysterious beam was seen by no less than twenty-six observers in +different parts of the country, and a comparison of their observations +led to a curious calculation indicating that the apparition was about +one hundred and thirty-three miles tall and moved at the speed of ten +miles per second! + +But, as everybody knows, it is in the Arctic regions that the Aurora, +or the ``Northern Lights,'' can best be seen. There, in the long polar +night, when for months together the sun does not rise, the strange +coruscations in the sky often afford a kind of spectral daylight in +unison with the weird scenery of the world of ice. The pages in the +narratives of Arctic exploration that are devoted to descriptions of +the wonderful effects of the Northern Lights are second to none that +man has ever penned in their fascination. The lights, as I have +already intimated, display astonishing colors, particularly shades of +red and green, as they flit from place to place in the sky. The +discovery that the magnetic needle is affected by the Aurora, +quivering and darting about in a state of extraordinary excitement +when the lights are playing in the sky, only added to the mystery of +the phenomenon until its electro-magnetic nature had been established. +This became evident as soon as it was known that the focus of the +displays was the magnetic pole; and when the far South was visited the +Aurora Australis was found, having its center at the South Magnetic +Pole. Then, if not before, it was clear that the earth was a great +globular magnet, having its poles of opposite magnetism, and that the +auroral lights, whatever their precise cause might be, were +manifestations of the magnetic activity of our planet. After the +invention of magnetic telegraphy it was found that whenever a great +Aurora occurred the telegraph lines were interrupted in their +operation, and the ocean cables ceased to work. Such a phenomenon is +called a ``magnetic storm.'' + +The interest excited by the Aurora in scientific circles was greatly +stimulated when, in the last half of the nineteenth century, it was +discovered that it is a phenomenon intimately associated with +disturbances on the sun. The ancient ``Zurich Chronicles,'' extending +from the year 1000 to the year 1800, in which both sun-spots visible +to the naked eye and great displays of the auroral lights were +recorded, first set Rudolf Wolf on the track of this discovery. The +first notable proof of the suspected connection was furnished with +dramatic emphasis by an occurrence which happened on September 1, +1859. Near noon on that day two intensely brilliant points suddenly +broke out in a group of sun-spots which were under observation by Mr +R. C. Carrington at his observatory at Redhill, England. The points +remained visible for not more than five minutes, during which interval +they moved thirty-five thousand miles across the solar disk. Mr R. +Hodgson happened to see the same phenomenon at his observatory at +Highgate, and thus all possibility of deception was removed. But +neither of the startled observers could have anticipated what was to +follow, and, indeed, it was an occurrence which has never been +precisely duplicated. I quote the eloquent account given by Miss +Clerke in her History of Astronomy During the Nineteenth Century. + + This unique phenomenon seemed as if specially designed to + accentuate the inference of a sympathetic relation between the + earth and the sun. From August 28 to September 4, 1859, a magnetic + storm of unparalleled intensity, extent, and duration was in + progress over the entire globe. Telegraphic communication was + everywhere interrupted -- except, indeed, that it was in some cases + found practicable to work the lines without batteries by the agency + of the earth-currents alone; sparks issued from the wires; gorgeous + auroras draped the skies in solemn crimson over both hemispheres, + and even in the tropics; the magnetic needle lost all trace of + continuity in its movements and darted to and fro as if stricken + with inexplicable panic. The coincidence was even closer. At the + very instant of the solar outburst witnessed by Carrington and + Hodgson the photographic apparatus at Kew registered a marked + disturbance of all the three magnetic elements; while shortly after + the ensuing midnight the electric agitation culminated, thrilling + the whole earth with subtle vibrations, and lighting up the + atmosphere from pole to pole with coruscating splendors which + perhaps dimly recall the times when our ancient planet itself shone + as a star. + +If this amazing occurrence stood alone, and as I have already said it +has never been exactly duplicated, doubt might be felt concerning some +of the inferences drawn from it; but in varying forms it has been +repeated many times, so that now hardly anyone questions the reality +of the assumed connection between solar outbursts and magnetic storms +accompanied by auroral displays on the earth. It is true that the late +Lord Kelvin raised difficulties in the way of the hypothesis of a +direct magnetic action of the sun upon the earth, because it seemed to +him that an inadmissible quantity of energy was demanded to account +for such action. But no calculation like that which he made is final, +since all calculations depend upon the validity of the data; and no +authority is unshakable in science, because no man can possess +omniscience. It was Lord Kelvin who, but a few years before the thing +was actually accomplished, declared that aerial navigation was an +impracticable dream, and demonstrated its impracticability by +calculation. However the connection may be brought about, it is as +certain as evidence can make it that solar outbursts are coincident +with terrestial magnetic disturbances, and coincident in such a way as +to make the inference of a causal connection irresistible. The sun is +only a little more than a hundred times its own diameter away from the +earth. Why, then, with the subtle connection between them afforded by +the ether which conveys to us the blinding solar light and the +life-sustaining solar heat, should it be so difficult to believe that +the sun's enormous electric energies find a way to us also? No doubt +the impulse coming from the sun acts upon the earth after the manner +of a touch upon a trigger, releasing energies which are already stored +up in our planet. + +But besides the evidence afforded by such occurrences as have been +related of an intimate connection between solar outbreaks and +terrestial magnetic flurries, attended by magnificent auroral +displays, there is another line of proof pointing in the same +direction. Thus, it is known that the sun-spot period, as remarked in +a preceding chapter, coincides in a most remarkable manner with the +periodic fluctuations in the magnetic state of the earth. This +coincidence runs into the most astonishing details. For instance, when +the sun-spot period shortens, the auroral period shortens to precisely +the same extent; as the short sun-spot periods usually bring the most +intense outbreaks of solar activity, so the corresponding short +auroral periods are attended by the most violent magnetic storms; a +secular period of about two hundred and twenty-two years affecting +sun-spots is said to have its auroral duplicate; a shorter period of +fifty-five and a half years, which some observers believe that they +have discovered appears also to be common to the two phenomena; and +yet another ``superposed'' period of about thirty-five years, which +some investigators aver exists, affects sun-spots and aurora alike. In +short, the coincidences are so numerous and significant that one would +have to throw the doctrine of probability to the winds in order to be +able to reject the conclusion to which they so plainly lead. + +But still the question recurs: How is the influence transmitted? Here +Arrhenius comes once more with his hypothesis of negative corpuscles, +or ions, driven away from the sun by light-pressure -- a hypothesis +which seems to explain so many things -- and offers it also as an +explanation of the way in which the sun creates the Aurora. He would +give the Aurora the same lineage with the Zodiacal Light. To +understand the application of this theory we must first recall the +fact that the earth is a great magnet having its two opposite poles of +magnetism, one near the Arctic and the other near the Antarctic +Circle. Like all magnets, the earth is surrounded with ``lines of +force,'' which, after the manner of the curved rays we saw in the +photograph of a solar eclipse, start from a pole, rising at first +nearly vertically, then bend gradually over, passing high above the +equator, and finally descending in converging sheaves to the opposite +pole. Now the axis of the earth is so placed in space that it lies at +nearly a right angle to the direction of the sun, and as the streams +of negatively charged particles come pouring on from the sun (see the +last preceding chapter), they arrive in the greatest numbers over the +earth's equatorial regions. There they encounter the lines of magnetic +force at the place where the latter have their greatest elevation +above the earth, and where their direction is horizontal to the +earth's surface. Obeying a law which has been demonstrated in the +laboratory, the particles then follow the lines of force toward the +poles. While they are above the equatorial regions they do not become +luminescent, because at the great elevation that they there occupy +there is virtually no atmosphere; but as they pass on toward the north +and the south they begin to descend with the lines of force, curving +down to meet at the poles; and, encountering a part of the atmosphere +comparable in density with what remains in an exhausted Crookes tube, +they produce a glow of cathode rays. This glow is conceived to +represent the Aurora, which may consequently be likened to a gigantic +exhibition of vacuum-tube lights. Anybody who recalls his student days +in the college laboratory and who has witnessed a display of Northern +Lights will at once recognize the resemblance between them in colors, +forms, and behavior. This resemblance had often been noted before +Arrhenius elaborated his hypothesis. + +Without intending to treat his interesting theory as more than a +possibly correct explanation of the phenomena of the Aurora, we may +call attention to some apparently confirmatory facts. One of the most +striking of these relates to a seasonal variation in the average +number of auror. It has been observed that there are more in March +and September than at any other time of the year, and fewer in June +and December; moreover (and this is a delicate test as applied to the +theory), they are slightly rarer in June than in December. Now all +these facts seem to find a ready explanation in the hypothesis of +Arrhenius, thus: (1) The particles issuing from the sun are supposed +to come principally from the regions whose excitement is indicated by +the presence of sun-spots (which accords with Hale's observation that +sun-spots are columns of ionized vapors), and these regions have a +definite location on either side of the solar equator, seldom +approaching it nearer than within 5 or 10 north or south, and never +extending much beyond 35 toward either pole; (2) The equator of the +sun is inclined about 7 to the plane of the earth's orbit, from which +it results that twice in a year -- viz., in June and December -- the +earth is directly over the solar equator, and twice a year -- viz., in +March and September -- when it is farthest north or south of the solar +equator, it is over the inner edge of the sun-spot belts. Since the +corpuscles must be supposed to be propelled radially from the sun, few +will reach the earth when the latter is over the solar equator in June +and December, but when it is over, or nearly over, the spot belts, in +March and September, it will be in the line of fire of the more active +parts of the solar surface, and relatively rich streams of particles +will reach it. This, as will be seen from what has been said above, is +in strict accord with the observed variations in the frequency of +auror. Even the fact that somewhat fewer auror are seen in June than +in December also finds its explanation in the known fact that the +earth is about three million miles nearer the sun in the winter than +in the summer, and the number of particles reaching it will vary, like +the intensity of light, inversely as the square of the distance. These +coincidences are certainly very striking, and they have a cumulative +force. If we accept the theory, it would appear that we ought to +congratulate ourselves that the inclination of the sun's equator is so +slight, for as things stand the earth is never directly over the most +active regions of the sun-spots, and consequently never suffers from +the maximum bombardment of charged particles of which the sun is +capable. Incessant auroral displays, with their undulating draperies, +flitting colors, and marching columns might not be objectionable from +the point of view of picturesqueness, but one magnetic storm of +extreme intensity following closely upon the heels of another, for +months on end, crazing the magnetic needle and continually putting the +telegraph and cable lines out of commission, to say nothing of their +effect upon ``wireless telegraphy'', would hardly add to the charms of +terrestrial existence. + +One or two other curious points in connection with Arrhenius' +hypothesis may be mentioned. First, the number of auror, according to +his explanation, ought to be greatest in the daytime, when the face of +the earth on the sunward side is directly exposed to the atomic +bombardment. Of course visual observation can give us no information +about this, since the light of the Aurora is never sufficiently +intense to be visible in the presence of daylight, but the records of +the magnetic observatories can be, and have been, appealed to for +information, and they indicate that the facts actually accord with the +theory. Behind the veil of sunlight in the middle of the afternoon, +there is good reason to believe, auroral exhibitions often take place +which would eclipse in magnificence those seen at night if we could +behold them. Observation shows, too, that auror are more frequent +before than after midnight, which is just what we should expect if +they originate in the way that Arrhenius supposes. Second, the theory +offers an explanation of the alleged fact that the formation of clouds +in the upper air is more frequent in years when auror are most +abundant, because clouds are the result of the condensation of +moisture upon floating particles in the atmosphere (in an absolutely +dustless atmosphere there would be no clouds), and it has been proved +that negative ions like those supposed to come from the sun play a +master part in the phenomena of cloud formation. + +Yet another singular fact, almost mystical in its suggestions, may be +mentioned. It seems that the dance of the auroral lights occurs most +frequently during the absence of the moon from the hemisphere in which +they appear, and that they flee, in greater part, to the opposite +hemisphere when the moon's revolution in an orbit considerably +inclined to the earth's equator brings her into that where they have +been performing. Arrhenius himself discovered this curious relation of +auroral frequency to the position of the moon north or south of the +equator, and he explains it in this way. The moon, like the earth, is +exposed to the influx of the ions from the sun; but having no +atmosphere, or almost none, to interfere with them, they descend +directly upon her surface and charge her with an electric negative +potential to a very high degree. In consequence of this she affects +the electric state of the upper parts of the earth's atmosphere where +they lie most directly beneath her, and thus prevents, to a large +extent, the negative discharges to which the appearance of the Aurora +is due. And so ``the extravagant and erring spirit'' of the Aurora +avoids the moon as Hamlet's ghost fled at the voice of the cock +announcing the awakening of the god of day. + +There are even other apparent confirmations of the hypothesis, but we +need not go into them. We shall, however, find one more application of +it in the next chapter, for it appears to be a kind of cure-all for +astronomical troubles; at any rate it offers a conceivable solution of +the question, How does the sun manage to transmit its electric +influence to the earth? And this solution is so grandiose in +conception, and so novel in the mental pictures that it offers, that +its acceptance would not in the least detract from the impression that +the Aurora makes upon the imagination. + +Strange Adventures of Comets + +The fears and legends of ancient times before Science was born, and +the superstitions of the Dark Ages, sedulously cultivated for +theological purposes by monks and priests, have so colored our ideas +of the influence that comets have had upon the human mind that many +readers may be surprised to learn that it was the apparition of a +wonderful comet, that of 1843, which led to the foundation of our +greatest astronomical institution, the Harvard College Observatory. No +doubt the comet superstition existed half a century ago, as, indeed, +it exists yet today, but in this case the marvelous spectacle in the +sky proved less effective in inspiring terror than in awakening a +desire for knowledge. Even in the sixteenth century the views that +enlightened minds took of comets tended powerfully to inspire popular +confidence in science, and Halley's prediction, after seeing and +studying the motion of the comet which appeared in 1682, that it would +prove to be a regular member of the sun's family and would be seen +returning after a period of about seventy-six years, together with the +fulfillment of that prediction, produced a revulsion from the +superstitious notions which had so long prevailed. + +Then the facts were made plain that comets are subject to the law of +gravitation equally with the planets; that there are many which +regularly return to the neighborhood of the sun (perihelion); and that +these travel in orbits differing from those of the planets only in +their greater eccentricity, although they have the peculiarity that +they do not, like the planets, all go round the sun in the same +direction, and do not keep within the general plane of the planetary +system, but traverse it sometimes from above and sometimes from below. +Other comets, including most of the ``great'' ones, appear to travel +in parabolic or, in a few cases, hyperbolic orbits, which, not being +closed curves, never bring them back again. But it is not certain that +these orbits may not be extremely eccentric ellipses, and that after +the lapse of hundreds, or thousands, of years the comets that follow +them may not reappear. The question is an interesting one, because if +all orbits are really ellipses, then all comets must be permanent +members of the solar system, while in the contrary case many of them +are simply visitors, seen once and never to be seen again. The +hypothesis that comets are originally interlopers might seem to derive +some support from the fact that the certainly periodic ones are +associated, in groups, with the great outer planets, whose attraction +appears to have served as a trap for them by turning them into +elliptical orbits and thus making them prisoners in the solar system. +Jupiter, owing to his great mass and his commanding situation in the +system, is the chief ``comet-catcher;'' but he catches them not for +himself, but for the sun. Yet if comets do come originally from +without the borders of the planetary system, it does not, by any +means, follow that they were wanderers at large in space before they +yielded to the overmastering attraction of the sun. Investigation of +the known cometary orbits, combined with theoretical considerations, +has led some astronomers to the conclusion that as the sun travels +onward through space he ``picks up en route'' cometary masses which, +without belonging strictly to his empire, are borne along in the same +vast ``cosmical current'' that carries the solar system. + +But while no intelligent person any longer thinks that the appearance +of a great comet is a token from the heavenly powers of the +approaching death of a mighty ruler, or the outbreak of a devastating +war, or the infliction of a terrible plague upon wicked mankind, +science itself has discovered mysteries about comets which are not +less fascinating because they are more intellectual than the +irrational fancies that they have displaced. To bring the subject +properly before the mind, let us see what the principal phenomena +connected with a comet are. + +At the present day comets are ordinarily ``picked up'' with the +telescope or the photographic plate before any one except their +discoverer is aware of their existence, and usually they remain so +insignificant in appearance that only astronomers ever see them. Yet +so great is the prestige of the word ``comet'' that the discovery of +one of these inconspicuous wanderers, and its subsequent movements, +become items of the day's news which everybody reads with the feeling, +perhaps, that at least he knows what is going on in the universe even +if he doesn't understand it. But a truly great comet presents quite a +different proposition. It, too, is apt to be detected coming out of +the depths of space before the world at large can get a glimpse of it, +but as it approaches the sun its aspect undergoes a marvelous change. +Agitated apparently by solar influence, it throws out a long streaming +tail of nebulous light, directed away from the sun and looking as if +blown out like a pennon by a powerful wind. Whatever may be the +position of the comet with regard to the sun, as it circles round him +it continually keeps its tail on the off side. This, as we shall soon +see, is a fact of capital importance in relation to the probable +nature of comets' tails. Almost at the same time that the formation of +the tail is observed a remarkable change takes place in the comet's +head, which, by the way, is invariably and not merely occasionally its +most important part. On approaching the sun the head usually +contracts. Coincidently with this contraction a nucleus generally +makes its appearance. This is a bright, star-like point in the head, +and it probably represents the totality of solid matter that the comet +possesses. But it is regarded as extremely unlikely that even the +nucleus consists of a uniformly solid mass. If it were such, comets +would be far more formidable visitors when they pass near the planets +than they have been found to be. The diameter of the nucleus may vary +from a few hundred up to several thousand miles; the heads, on the +average, are from twenty-five thousand to one hundred thousand miles +in diameter, although a few have greatly exceeded these dimensions; +that of the comet of 1811, one of the most stupendous ever seen, was a +million and a quarter miles in diameter! As to the tails, not +withstanding their enormous length -- some have been more than a +hundred million miles long -- there is reason to believe that they are +of extreme tenuity, ``as rare as vacuum.'' The smallest stars have +been seen shining through their most brilliant portions with +undiminished luster. + +After the nucleus has been formed it begins to throw out bright jets +directed toward the sun. A stream, and sometimes several streams, of +light also project sunward from the nucleus, occasionally appearing +like a stunted tail directed oppositely to the real tail. Symmetrical +envelopes which, seen in section, appear as half circles or parabolas, +rise sunward from the nucleus, forming a concentric series. The ends +of these stream backward into the tail, to which they seem to supply +material. Ordinarily the formation of these ejections and envelopes is +attended by intense agitation of the nucleus, which twists and turns, +swinging and gyrating with an appearance of the greatest violence. +Sometimes the nucleus is seen to break up into several parts. The +entire heads of some comets have been split asunder in passing close +around the sun; The comet of 1882 retreated into space after its +perihelion passage with five heads instead of the one that it had +originally, and each of these heads had its own tail! + +The possession of the spectroscope has enabled astronomers during +later years to study the chemical composition of comets by analyzing +their light. At first the only substances thus discovered in them were +hydro-carbon compounds, due evidently to the gaseous envelopes in +which some combination of hydrogen with carbon existed. Behind this +gaseous spectrum was found a faint continuous spectrum ascribed to the +nucleus, which apparently both reflects the sunlight and gives forth +the light of a glowing solid or liquid. Subsequently sodium and iron +lines were found in cometary spectra. The presence of iron would seem +to indicate that some of these bodies may be much more massive than +observations on their attractive effects have indicated. In some +recent comets, such as Morehouse's, in 1908, several lines have been +found, the origin of which is unknown. + +Without going back of the nineteenth century we may find records of +some of the most extraordinary comets that man has ever looked upon. +In 1811, still spoken of as ``the year of the comet,'' because of the +wonderful vintage ascribed to the skyey visitor, a comet shaped like a +gigantic sword amazed the whole world, and, as it remained visible for +seventeen months, was regarded by superstitious persons as a symbol of +the fearful happenings of Napoleon's Russian campaign. This comet, the +extraordinary size of whose head, greatly exceeding that of the sun +itself, has already been mentioned, was also remarkable for exhibiting +so great a brilliancy without approaching even to the earth's distance +from the sun. But there was once a comet (and only once -- in the year +1729) which never got nearer to the sun than four times the distance +of the earth and yet appeared as a formidable object in the sky. As +Professor Young has remarked, ``it must have been an enormous comet to +be visible from such a distance.'' And we are to remember that there +were no great telescopes in the year 1729. That comet affects the +imagination like a phantom of space peering into the solar system, +displaying its enormous train afar off (which, if it had approached as +near as other comets, would probably have become the celestial wonder +of all human memory), and then turning away and vanishing in the +depths of immensity. + +In 1843 a comet appeared which was so brilliant that it could be seen +in broad day close beside the sun! This was the first authenticated +instance of that kind, but the occurrence was to be repeated, as we +shall see in a moment, less than forty years later. + +The splendid comet of 1858, usually called Donati's, is remembered by +many persons yet living. It was, perhaps, both as seen by the naked +eye and with the telescope, the most beautiful comet of which we have +any record. It too marked a rich vintage year, still remembered in the +vineyards of France, where there is a popular belief that a great +comet ripens the grape and imparts to the wine a flavor not attainable +by the mere skill of the cultivator. There are ``comet wines,'' +carefully treasured in certain cellars, and brought forth only when +their owner wishes to treat his guests to a sip from paradise. + +The year 1861 saw another very remarkable comet, of an aspect +strangely vast and diffuse, which is believed to have swept the earth +with its immense tail when it passed between us and the sun on the +night of June 30th, an event which produced no other known effect than +the appearance of an unwonted amount of scattered light in the sky. + +The next very notable comet was the ``Great Southern Comet'' of 1880, +which was not seen from the northern hemisphere. It mimicked the +aspect of the famous comet of 1843, and to the great surprise of +astronomers appeared to be traveling in the same path. This proved to +be the rising of the curtain for an astronomical sensation +unparalleled in its kind; for two years later another brilliant comet +appeared, first in the southern hemisphere, and it too followed the +same track. The startling suggestion was now made that this comet was +identical with those of 1843 and 1880, its return having been hastened +by the resistance experienced in passing twice through the coronal +envelope, and there were some who thought that it would now swing +swiftly round and then plunge straight into the sun, with consequences +that might be disastrous to us on account of the ``flash of heat'' +that would be produced by the impact. Nervous people were frightened, +but observation soon proved that the danger was imaginary, for +although the comet almost grazed the sun, and must have rushed through +two or three million miles of the coronal region, no retardation of +its immense velocity was perceptible, and it finally passed away in a +damaged condition, as before remarked, and has never since appeared. + +Then the probable truth was perceived -- viz., that the three comets +(1843, 1880, and 1882) were not one identical body, but three separate +ones all traveling in the same orbit. It was found, too, that a comet +seen in 1668 bore similar insignia of relationship. The natural +inference was that these four bodies had once formed a single mass +which had been split apart by the disruptive action of the sun. +Strength was lent to this hypothesis by the fact that the comet of +1882 was apparently torn asunder during its perihelion passage, +retreating into space in a dissevered state. But Prof. George Forbes +has a theory that the splitting of the original cometary mass was +effected by an unknown planet, probably greater than Jupiter, situated +at a hundred times the earth's distance from the sun, and revolving in +a period of a thousand years. He supposes that the original comet was +not that of 1668, but one seen in 1556, which has since been +``missing,'' and that its disruption occurred from an encounter with +the supposititious planet about the year 1700. Truly from every point +of view comets are the most extraordinary of adventurers! + +The comet of 1882 was likewise remarkable for being visible, like its +predecessor of 1843, in full daylight in close proximity to the sun. +The story of its detection when almost in contact with the solar disk +is dramatic. It had been discovered in the southern hemisphere only a +couple of weeks before its perihelion, which occurred on September +17th, and on the forenoon of that day it was seen by Doctor Common in +England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope, +almost touching the sun. It looked like a dazzling white bird with +outspread wings. The southern observers watched it go right into the +sun, when it instantly disappeared. What had happened was that the +comet in passing its perihelion point had swung exactly between the +earth and the sun. On the following morning it was seen from all parts +of the world close by the sun on the opposite side, and it remained +thus visible for three days, gradually receding from the solar disk. +It then became visible for northern observers in the morning sky +before sunrise, brandishing a portentous sword-shaped tail which, if +it had been in the evening sky, would have excited the wonder of +hundreds of millions, but situated where it was, comparatively few +ever saw it. + +The application of photography to the study of comets has revealed +many curious details which might otherwise have escaped detection, or +at best have remained subject to doubt. It has in particular shown not +only the precise form of the tails, but the remarkable vicissitudes +that they undergo. Professor Barnard's photographs of Brooks' comet in +1893 suggested, by the extraordinary changes in the form of the tail +which they revealed, that the comet was encountering a series of +obstructions in space which bent and twisted its tail into fantastic +shapes. The reader will observe the strange form into which the tail +was thrown on the night of October 21st. A cloud of meteors through +which the comet was passing might have produced such deformations of +its tail. In the photograph of Daniels' comet of 1907, a curious +striping of the tail will be noticed. The short bright streaks seen in +the photograph, it may be explained, are the images of stars which are +drawn out into lines in consequence of the fact that the photographic +telescope was adjusted to follow the motion of the comet while the +stars remained at rest. + +But the adventures of comets are not confined to possible encounters +with unknown obstacles. We have referred to the fact that the great +planets, and especially Jupiter, frequently interfere with the motions +of comets. This interference is not limited to the original alteration +of their orbits from possible parabolas to ellipses, but is sometimes +exercised again and again, turning the bewildered comets into +elliptical paths of all degrees of eccentricity. A famous example of +this kind of planetary horse-play is furnished by the story of +Lexell's missing comet. This comet was first seen in 1770. +Investigation showed that it was moving in an orbit which should bring +it back to perihelion every five and a half years; yet it had never +been seen before and, although often searched for, has never been seen +since. Laplace and Leverrier proved mathematically that in 1767 it had +approached so close to Jupiter as to be involved among the orbits of +his satellites. What its track had been before is not known, but on +that occasion the giant planet seized the interloper, threw it into a +short elliptic orbit and sent it, like an arrested vagrant, to receive +sentence at the bar of the sun. On this journey it passed within less +than 1,500,000 miles of the earth. The form of orbit which Jupiter had +impressed required, as we have said, its return in about five and a +half years; but soon after 1770 it had the misfortune a second time to +encounter Jupiter at close range, and he, as if dissatisfied with the +leniency of the sun, or indignant at the stranger's familiarity, +seized the comet and hurled it out of the system, or at any rate so +far away that it has never since been able to rejoin the family circle +that basks in the immediate rays of the solar hearth. Nor is this the +only instance in which Jupiter has dealt summarily with small comets +that have approached him with too little deference. + +The function which Jupiter so conspicuously fulfills as master of the +hounds to the sun is worth considering a little more in detail. To +change the figure, imagine the sun in its voyage through space to be +like a majestic battleship surrounded by its scouts. Small vessels +(the comets, as they are overhauled by the squadron, are taken in +charge by the scouts, with Jupiter for their chief, and are forced to +accompany the fleet, but not all are impressed. If a strange comet +undertakes to run across Jupiter's bows the latter brings it to, and +makes prize of it by throwing it into a relatively small ellipse with +the sun for its focus. Thenceforth, unless, as happened to the unhappy +comet of Lexell, it encounters Jupiter again in such a way as to be +diverted by him into a more distant orbit, it can never get away. +About thirty comets are now known to have thus been captured by the +great planet, and they are called ``Jupiter's Comet Family.'' But, on +the other hand, if a wandering comet crosses the wake of the chief +planetary scout the latter simply drives it away by accelerating its +motion and compels it to steer off into open space. The transformation +of comets into meteors will be considered in the next chapter, but +here, in passing, mention may be made of the strange fate of one +member of Jupiter's family, Biela's comet, which, having become over +bold in its advances to its captor, was, after a few revolutions in is +impressed orbit, torn to pieces and turned into a flock of meteors. + +And now let us return to the mystery of comets' tails. That we are +fully justified in speaking of the tails of comets as mysterious is +proved by the declaration of Sir John Herschel, who averred, in so +many words, that ``there is some profound secret and mystery of nature +concerned in this phenomenon,'' and this profound secret and mystery +has not yet been altogether cleared up. Nevertheless, the +all-explaining hypothesis of Arrhenius offers us once more a certain +amount of aid. Comets' tails, Arrhenius assures us, are but another +result of the pressure of light. The reader will recall the +applications of this theory to the Zodiacal Light and the Aurora. In +the form in which we now have to deal with it, the supposition is made +that as a comet approaches the sun eruptions of vapor, due to the +solar heat, occur in its nucleus. These are naturally most active on +the side which is directly exposed to the sun, whence the appearance +of the immense glowing envelopes that surround the nucleus on the +sunward side. Among the particles of hydro-carbon, and perhaps solid +carbon in the state of fine dust, which are thus set free there will +be many whose size is within the critical limit which enables the +light-waves from the sun to drive them away. Clouds of such particles, +then, will stream off behind the advancing comet, producing the +appearance of a tail. This accounts for the fact that the tails of +comets are always directed away from the sun, and it also explains the +varying forms of the tails and the extraordinary changes that they +undergo. The speed of the particles driven before the light-waves must +depend upon their size and weight, the lightest of a given size +traveling the most swiftly. By accretion certain particles might grow, +thus losing velocity and producing the appearance of bunches in the +tail, such as have been observed. The hypothesis also falls in with +the researches of Bredichin, who has divided the tails of comets into +three principal classes -- viz.: (1) Those which appear as long, +straight rays; (2) Those which have the form of curved plumes or +scimitars; (3) Those which are short, brushy, and curved sharply +backward along the comet's path. In the first type he calculates the +repulsive force at from twelve to fifteen times the force of gravity; +in the second at from two to four times; and in the third at about one +and a half times. The straight tails he ascribes to hydrogen because +the hydrogen atom is the lightest known; the sword-shaped tails to +hydro-carbons; and the stumpy tails to vaporized iron. It will be seen +that, if the force driving off the tails is that which Arrhenius +assumes it to be, the forms of those appendages would accord with +those that Bredichin's theory calls for. At the same time we have an +explanation of the multiple tails with which some comets have adorned +themselves. The comet of 1744, for instance, had at one time no less +than seven tails spread in a wide curved brush behind it. Donati's +comet of 1858 also had at least two tails, the principal one +sword-shaped and the other long, narrow, and as straight as a rule. +According to Bredichin, the straight tail must have been composed of +hydrogen, and the other of some form of hydro-carbon whose atoms are +heavier than those of hydrogen, and, consequently, when swept away by +the storm of light-waves, followed a curvature depending upon the +resultant of the forces operating upon them. The seven tails of the +comet of 1744 presented a kind of diagram graphically exhibiting its +complex composition, and, if we knew a little more about the +constituents of a comet, we might be able to say from the amount of +curvature of the different tails just what were the seven substances +of which that comet consisted. + +If these theories seem to the reader fantastic, at any rate they are +no more fantastic than the phenomena that they seek to explain. + +Meteors, Fire-Balls, and Meteorites + +One of the most terrorizing spectacles with which the heavens have +ever caused the hearts of men to quake occurred on the night of +November 13, 1833. On that night North America, which faced the storm, +was under a continual rain of fire from about ten o'clock in the +evening until daybreak. + +The fragments of a comet had struck the earth. + +But the meaning of what had happened was not discovered until long +afterward. To the astronomers who, with astonishment not less than +that of other people, watched the wonderful scene, it was an +unparalleled ``shower of meteors.'' They did not then suspect that +those meteors had once formed the head of a comet. Light dawned when, +a year later, Prof. Denison Olmsted, of Yale College, demonstrated +that the meteors had all moved in parallel orbits around the sun, and +that these orbits intersected that of the earth at the point where our +planet happened to be on the memorable night of November 13th. +Professor Olmsted even went so far as to suggest that the cloud of +meteors that had encountered the earth might form a diffuse comet; but +full recognition of the fact that they were cometary dbris came +later, as the result of further investigation. The key to the secret +was plainly displayed in the spectacle itself, and was noticed without +being understood by thousands of the terror-stricken beholders. It was +an umbrella of fire that had opened overhead and covered the heavens; +in other words, the meteors all radiated from a particular point in +the constellation Leo, and, being countless as the snowflakes in a +winter tempest, they ribbed the sky with fiery streaks. Professor +Olmsted showed that the radiation of the meteors from a fixed point +was an effect of perspective, and in itself a proof that they were +moving in parallel paths when they encountered the earth. The fact was +noted that there had been a similar, but incomparably less brilliant, +display of meteors on the same day of November, 1832, and it was +rightly concluded that these had belonged to the same stream, although +the true relationship of the phenomena was not immediately +apprehended. Olmsted ascribed to the meteors a revolution about the +sun once in every six months, bringing them to the intersection of +their orbit with that of the earth every November 13th; but later +investigators found that the real period was about thirty-three and +one-quarter years, so that the great displays were due three times in +a century, and their return was confidently predicted for the year +1866. The appearance of the meteors in 1832, a year before the great +display, was ascribed to the great length of the stream which they +formed in space -- so great that they required more than two years to +cross the earth's orbit. In 1832 the earth had encountered a +relatively rare part of the stream, but in 1833, on returning to the +crossing-place, it found there the richest part of the stream pouring +across its orbit. This explanation also proved to be correct, and the +predicted return in 1866 was duly witnessed, although the display was +much less brilliant than in 1833. It was followed by another in 1867. + +In the mean time Olmsted's idea of a cometary relationship of the +meteors was demonstrated to be correct by the researches of +Schiaparelli and others, who showed that not only the November +meteors, but those of August, which are seen more or less abundantly +every year, traveled in the tracks of well-known comets, and had +undoubtedly an identical origin with those comets. In other words the +comets and the meteor-swarms were both remnants of original masses +which had probably been split up by the action of the sun, or of some +planet to which they had made close approaches. The annual periodicity +of the August meteors was ascribed to the fact that the separation had +taken place so long ago that the meteors had become distributed all +around the orbit, in consequence of which the earth encountered some +of them every year when it arrived at the crossing-point. Then +Leverrier showed that the original comet associated with the November +meteors was probably brought into the system by the influence of the +planet Uranus in the year 126 of the Christian era. Afterward +Alexander Herschel identified the tracks of no less than seventy-six +meteor-swarms (most of them inconspicuous) with those of comets. The +still more recent researches of Mr W. F. Denning make it probable that +there are no meteors which do not belong to a flock or system probably +formed by the disintegration of a cometary mass; even the apparently +sporadic ones which shoot across the sky, ``lost souls in the night,'' +being members of flocks which have become so widely scattered that the +earth sometimes takes weeks to pass through the region of space where +their paths lie. + +The November meteors should have exhibited another pair of spectacles +in 1899 and 1900, and their failure to do so caused at first much +disappointment, until it was made plain that a good reason existed for +their absence. It was found that after their last appearance, in 1867, +they had been disturbed in their movements by the planets Jupiter and +Saturn, whose attractions had so shifted the position of their orbit +that it no longer intersected that of the earth, as it did before. +Whether another planetary interference will sometime bring the +principal mass of the November meteors back to the former point of +intersection with the earth's orbit is a question for the future to +decide. It would seem that there may be several parallel streams of +the November meteors, and that some of them, like those of August, are +distributed entirely around the orbit, so that every mid-November we +see a few of them. + +We come now to a very remarkable example of the disintegration of a +comet and the formation of a meteor-stream. In 1826 Biela, of +Josephstadt, Austria, discovered a comet to which his name was given. +Calculation showed that it had an orbital period of about six and a +half years, belonging to Jupiter's ``family.'' On one of its returns, +in 1846, it astonished its watchers by suddenly splitting in two. The +two comets thus formed out of one separated to a distance of about one +hundred and sixty thousand miles, and then raced side by side, +sometimes with a curious ligature connecting them, like Siamese twins, +until they disappeared together in interplanetary space. In 1852 they +came back, still nearly side by side, but now the distance between +them had increased to a million and a quarter of miles. After that, at +every recurrence of their period, astronomers looked for them in vain, +until 1872, when an amazing thing happened. On the night of November +28th, when the earth was crossing the plane of the orbit of the +missing comet, a brilliant shower of meteors burst from the northern +sky, traveling nearly in the track which the comet should have +pursued. The astronomers were electrified. Klinkerfues, of Gttingen, +telegraphed to Pogson, of Madras: ``Biela touched earth; search near +Theta Centauri.'' Pogson searched in the place indicated and saw a +cometary mass retreating into the southern heavens, where it was soon +swallowed from sight! + +Since then the Biela meteors have been among the recognized periodic +spectacles of the sky, and few if any doubt that they represent a +portion of the missing comet whose disintegration began with the +separation into two parts in 1846. The comet itself has never since +been seen. The first display of these meteors, sometimes called the +``Andromedes,'' because they radiate from the constellation Andromeda, +was remarkable for the great brilliancy of many of the fire-balls that +shot among the shower of smaller sparks, some of which were described +as equaling the full moon in size. None of them is known to have +reached the earth, but during the display of the same meteors in 1885 +a meteoric mass fell at Mazapil in Northern Mexico (it is now in the +Museum at Vienna), which many have thought may actually be a piece of +the original comet of Biela. This brings us to the second branch of +our subject. + +More rare than meteors or falling stars, and more startling, except +that they never appear in showers, are the huge balls of fire which +occasionally dart through the sky, lighting up the landscapes beneath +with their glare, leaving trains of sparks behind them, often +producing peals of thunder when they explode, and in many cases +falling upon the earth and burying themselves from a few inches to +several feet in the soil, from which, more than once, they have been +picked up while yet hot and fuming. These balls are sometimes called +bolides. They are not really round in shape, although they often look +so while traversing the sky, but their forms are fragmentary, and +occasionally fantastic. It has been supposed that their origin is +different from that of the true meteors; it has even been conjectured +that they may have originated from the giant volcanoes of the moon or +have been shot out from the sun during some of the tremendous +explosions that accompany the formation of eruptive prominences. By +the same reasoning some of them might be supposed to have come from +some distant star. Others have conjectured that they are wanderers in +space, of unknown origin, which the earth encounters as it journeys +on, and Lord Kelvin made a suggestion which has become classic because +of its imaginative reach -- viz., that the first germs of life may +have been brought to the earth by one of these bodies, ``a fragment of +an exploded world.'' + +It is a singular fact that astronomers and scientific men in general +were among the last to admit the possibility of solid masses falling +from the sky. The people had believed in the reality of such phenomena +from the earliest times, but the savants shook their heads and talked +of superstition. This was the less surprising because no +scientifically authenticated instance of such an occurrence was known, +and the stones popularly believed to have fallen from the sky had +become the objects of worship or superstitious reverence, a fact not +calculated to recommend them to scientific credence. The celebrated +``black stone'' suspended in the Kaaba at Mecca is one of these +reputed gifts from heaven; the ``Palladium'' of ancient Troy was +another; and a stone which fell near Ensisheim, in Germany, was placed +in a church as an object to be religiously venerated. Many legends of +falling stones existed in antiquity, some of them curiously +transfigured by the imagination, like the ``Lion of the +Peloponnesus,'' which was said to have sprung down from the sky upon +the Isthmus of Corinth. But near the beginning of the nineteenth +century, in 1803, a veritable shower of falling stones occurred at +L'Aigle, in Northern France, and this time astronomers took note of +the phenomenon and scientifically investigated it. Thousands of the +strange projectiles came from the sky on this occasion, and were +scattered over a wide area of country, and some buildings were hit. +Four years later another shower of stones occurred at Weston, Conn., +numbering thousands of individuals. The local alarm created in both +cases was great, as well it might be, for what could be more +intimidating than to find the blue vault of heaven suddenly hurling +solid missiles at the homes of men? After these occurrences it was +impossible for the most skeptical to doubt any longer, and the regular +study of ``aerolites,'' or ``meteorites,'' began. + +One of the first things recognized was the fact that fire-balls are +solid meteorites in flight, and not gaseous exhalations in the air, as +some had assumed. They burn in the air during their flight, and +sometimes, perhaps, are entirely consumed before reaching the ground. +Their velocity before entering the earth's atmosphere is equal to that +of the planets in their orbits -- viz., from twenty to thirty miles +per second -- a fact which proves that the sun is the seat of the +central force governing them. Their burning in the air is not +difficult to explain; it is the heat of friction which so quickly +brings them to incandescence. Calculation shows that a body moving +through the air at a velocity of about a mile per second will be +brought, superficially, to the temperature of ``red heat'' by friction +with the atmosphere. If its velocity is twenty miles per second the +temperature will become thousands of degrees. This is the state of +affairs with a meteorite rushing into the earth's atmosphere; its +surface is liquefied within a few seconds after the friction begins to +act, and the melted and vaporized portion of its mass is swept +backward, forming the train of sparks that follows every great +fire-ball. However, there is one phenomenon connected with the trains +of meteorites which has never been satisfactorily explained: they +often persist for long periods of time, drifting and turning with the +wind, but not ceasing to glow with a phosphorescent luminosity. The +question is, Whence comes this light? It must be light without heat, +since the fine dust or vapor of which the train can only consist would +not retain sufficient heat to render it luminous for so long a time. +An extremely remarkable incident of this kind occurred on February 22, +1909, when an immense fire-ball that passed over southern England left +a train that remained visible during two hours, assuming many curious +shapes as it was drifted about by currents in the air. + +But notwithstanding the enormous velocity with which meteorites enter +the air they are soon slowed down to comparatively moderate speed, so +that when they disappear they are usually traveling not faster than a +mile a second. The courses of many have been traced by observers +situated along their track at various points, and thus a knowledge has +been obtained of their height above the ground during their flight and +of the length of their visible courses. They generally appear at an +elevation of eighty or a hundred miles, and are seldom visible after +having descended to within five miles of the ground, unless the +observer happens to be near the striking-point, when he may actually +witness the fall. Frequently they burst while high in the air and +their fragments are scattered like shrapnel over the surface of the +ground, sometimes covering an area of several square miles, but of +course not thickly; different fragments of the same meteorite may +reach the ground at points several miles apart. The observed length of +their courses in the atmosphere varies from fifty to five hundred +miles. If they continued a long time in flight after entering the air, +even the largest of them would probably be consumed to the last scrap, +but their fiery career is so short on account of their great speed +that the heat does not have time to penetrate very deeply, and some +that have been picked up immediately after their fall have been found +cold as ice within. Their size after reaching the ground is variable +within wide limits; some are known which weigh several tons, but the +great majority weigh only a few pounds and many only a few ounces. + +Meteorites are of two kinds: stony meteorites and iron meteorites. The +former outnumber the latter twenty to one; but many stone meteorites +contain grains of iron. Nickel is commonly found in iron meteorites, +so that it might be said that that redoubtable alloy nickel-steel is +of cosmical invention. Some twenty-five chemical elements have been +found in meteorites, including carbon and the ``sun-metal,'' helium. +The presence of the latter is certainly highly suggestive in +connection with the question of the origin of meteorites. The iron +meteorites, besides metallic iron and nickel, of which they are almost +entirely composed, contain hydrogen, helium, and carbonic oxide, and +about the only imaginable way in which these gases could have become +absorbed in the iron would be through the immersion of the latter +while in a molten or vaporized state in a hot and dense atmosphere +composed of them, a condition which we know to exist only in the +envelopes of the sun and the stars. + +The existence of carbon in the Canyon Diablo iron meteorites is +attended by a circumstance of the most singular character -- a very +``fairy tale of science.'' In some cases the carbon has become +diamond! These meteoric diamonds are very small; nevertheless, they +are true diamonds, resembling in many ways the little black gems +produced by Moissan's method with the aid of the electric furnace. The +fact that they are found embedded in these iron meteorites is another +argument in favor of the hypothesis of the solar or stellar origin of +the latter. To appreciate this it is necessary to recall the way in +which Moissan made his diamonds. It was by a combination of the +effects of great heat, great pressure, and sudden or rapid superficial +cooling on a mass of iron containing carbon. When he finally broke +open his iron he found it a pudding stuffed with miniature black +diamonds. When a fragment of the Canyon Diablo meteoric iron was +polished in Philadelphia over fifteen years ago it cut the emery-wheel +to pieces, and examination showed that the damage had been effected by +microscopic diamonds peppered through the mass. How were those +diamonds formed? If the sun or Sirius was the laboratory that prepared +them, we can get a glimpse at the process of their formation. There is +plenty of heat, plenty of pressure, and an abundance of vaporized iron +in the sun and the stars. When a great solar eruption takes place, +masses of iron which have absorbed carbon may be shot out with a +velocity which forbids their return. Plunged into the frightful cold +of space, their surfaces are quickly cooled, as Moissan cooled his +prepared iron by throwing it into water, and thus the requisite stress +is set up within, and, as the iron solidifies, the included carbon +crystallizes into diamonds. Whether this explanation has a germ of +truth in it or not, at any rate it is evident that iron meteorites +were not created in the form in which they come to us; they must once +have been parts of immeasurably more massive bodies than themselves. + +The fall of meteorites offers an appreciable, though numerically +insignificant, peril to the inhabitants of the earth. Historical +records show perhaps three or four instances of people being killed by +these bodies. But for the protection afforded by the atmosphere, which +acts as a very effective shield, the danger would doubtless be very +much greater. In the absence of an atmosphere not only would more +meteorites reach the ground, but their striking force would be +incomparably greater, since, as we have seen, the larger part of their +original velocity is destroyed by the resistance of the air. A +meteorite weighing many tons and striking the earth with a velocity of +twenty or thirty miles per second, would probably cause frightful +havoc. + +It is a singular fact that recent investigations seem to have proved +that an event of this kind actually happened in North America -- +perhaps not longer than a thousand or two thousand years ago. The +scene of the supposed catastrophe is in northern central Arizona, at +Coon Butte, where there is a nearly circular crater in the middle of a +circular elevation or small mountain. The crater is somewhat over four +thousand feet in diameter, and the surrounding rim, formed of upturned +strata and ejected rock fragments, rises at its highest point one +hundred and sixty feet above the plain. The crater is about six +hundred feet in depth -- that is, from the rim to the visible floor or +bottom of the crater. There is no evidence that volcanic action has +ever taken place in the immediate neighborhood of Coon Butte. The rock +in which the crater has been made is composed of horizontal sandstone +and limestone strata. Between three hundred and four hundred million +tons of rock fragments have been detached, and a large portion hurled +by some cause out of the crater. These fragments lie concentrically +distributed around the crater, and in large measure form the elevation +known as Coon Butte. The region has been famous for nearly twenty +years on account of the masses of meteoric iron found scattered about +and known as the ``Canyon Diablo'' meteorites. It was one of these +masses, which consist of nickel-iron containing a small quantity of +platinum, and of which in all some ten tons have been recovered for +sale to the various collectors throughout the world, that as before +mentioned destroyed the grinding-tool at Philadelphia through the +cutting power of its embedded diamonds. These meteoric irons are +scattered about the crater-hill, in concentric distribution, to a +maximum distance of about five miles. When the suggestion was first +made in 1896 that a monster meteorite might have created by its fall +this singular lone crater in stratified rocks, it was greeted with +incredulous smiles; but since then the matter has assumed a different +aspect. The Standard Iron Company, formed by Messrs. D. M. Barringer, +B. C. Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in +1903, the owner of this freak of nature, sunk shafts and bored holes +to a great depth in the interior of the crater, and also trenched the +slopes of the mountain, and the result of their investigations has +proved that the meteoric hypothesis of origin is correct. (See the +papers published in the Proceedings of the Academy of Natural Sciences +of Philadelphia, December, 1905, wherein it is proved that the United +States Geological Survey was wrong in believing this crater to have +been due to a steam explosion. Since that date there has been +discovered a great amount of additional confirmatory proof). Material +of unmistakably meteoric origin was found by means of the drills, +mixed with crushed rock, to a depth of six hundred to seven hundred +feet below the floor of the crater, and a great deal of it has been +found admixed with the ejected rock fragments on the outer slopes of +the mountain, absolutely proving synchronism between the two events, +the formation of this great crater and the falling of the meteoric +iron out of the sky. The drill located in the bottom of the crater was +sent, in a number of cases, much deeper (over one thousand feet) into +unaltered horizontal red sandstone strata, but no meteoric material +was found below this depth (seven hundred feet, or between eleven and +twelve hundred feet below the level of the surrounding plain), which +has been assumed as being about the limit of penetration. It is not +possible to sink a shaft at present, owing to the water which has +drained into the crater, and which forms, with the finely pulverized +sandstone, a very troublesome quicksand encountered at about two +hundred feet below the visible floor of the crater. As soon as this +water is removed by pumping it will be easy to explore the depths of +the crater by means of shafts and drifts. The rock strata (sandstone +and limestone) of which the walls consist present every appearance of +having been violently upturned by a huge body penetrating the earth +like a cannon-ball. The general aspect of the crater strikingly +resembles the impression made by a steel projectile shot into an +armor-plate. Mr Tilghman has estimated that a meteorite about five +hundred feet in diameter and moving with a velocity of about five +miles per second would have made just such a perforation upon striking +rocks of the character of those found at this place. There was some +fusion of the colliding masses, and the heat produced some steam from +the small amount of water in the rocks. As a result there has been +found at depth a considerable amount of fused quartz (original +sandstone), and with it innumerable particles or sparks of fused +nickel-iron (original meteorite). A projectile of that size +penetrating eleven to twelve hundred feet into the rocky shell of the +globe must have produced a shock which was perceptible several hundred +miles away. + +The great velocity ascribed to the supposed meteorite at the moment of +striking could be accounted for by the fact that it probably plunged +nearly vertically downward, for it formed a circular crater in the +rocky crust of the earth. In that case it would have been less +retarded by the resistance of the atmosphere than are meteorites which +enter the air at a lower angle and shoot ahead hundreds of miles until +friction has nearly destroyed their original motion when they drop +upon the earth. Some meteoric masses of great size, such as Peary's +iron meteorite found at Cape York, Greenland, and the almost equally +large mass discovered at Bacubirito, Mexico, appear to have penetrated +but slightly on striking the earth. This may be explained by supposing +that they pursued a long, horizontal course through the air before +falling. The result would be that, their original velocity having been +practically destroyed, they would drop to the ground with a velocity +nearly corresponding to that which gravity would impart within the +perpendicular distance of their final fall. A +six-hundred-and-sixty-pound meteorite, which fell at Knyahinya, +Hungary, striking at an angle of 27 from the vertical, penetrated the +ground to a depth of eleven feet. + +It has been remarked that the Coon Butte meteorite may have fallen not +longer ago than a few thousand years. This is based upon the fact that +the geological indications favor the supposition that the event did +not occur more than five thousand years ago, while on the other hand +the rings of growth in the cedar-trees growing on the slopes of the +crater show that they have existed there about seven hundred years. +Prof. William H. Pickering has recently correlated this with an +ancient chronicle which states that at Cairo, Egypt, in the year 1029, +``many stars passed with a great noise.'' He remarks that Cairo is +about 100, by great circle, from Coon Butte, so that if the meteorite +that made the crater was a member of a flock of similar bodies which +encountered the earth moving in parallel lines, some of them might +have traversed the sky tangent to the earth's surface at Cairo. That +the spectacle spoken of in the chronicle was caused by meteorites he +deems exceedingly probable because of what is said about ``a great +noise;'' meteorites are the only celestial phenomena attended with +perceptible sounds. Professor Pickering conjectures that this supposed +flock of great meteorites may have formed the nucleus of a comet which +struck the earth, and he finds confirmation of the idea in the fact +that out of the ten largest meteorites known, no less than seven were +found within nine hundred miles of Coon Butte. It would be interesting +if we could trace back the history of that comet, and find out what +malicious planet caught it up in its innocent wanderings and hurled it +with so true an aim at the earth! This remarkable crater is one of the +most interesting places in the world, for there is absolutely no +record of such a mass, possibly an iron-headed comet, from outer space +having come into collision with our earth. The results of the future +exploration of the depths of the crater will be awaited with much +interest. + +The Wrecking of the Moon + +There are sympathetic moods under whose influence one gazes with a +certain poignant tenderness at the worn face of the moon; that little +``fossil world'' (the child of our mother earth, too) bears such +terrible scars of its brief convulsive life that a sense of pity is +awakened by the sight. The moon is the wonder-land of the telescope. +Those towering mountains, whose ``proud aspiring peaks'' cast +silhouettes of shadow that seem drawn with india-ink; those vast +plains, enchained with gentle winding hills and bordered with giant +ranges; those oval ``oceans,'' where one looks expectant for the flash +of wind-whipped waves; those enchanting ``bays'' and recesses at the +seaward feet of the Alps; those broad straits passing between guardian +heights incomparably mightier than Gibraltar; those locket-like +valleys as secluded among their mountains as the Vale of Cashmere; +those colossal craters that make us smile at the pretensions of +Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with +the unconcern of Roman roads across mountain, gorge, and valley -- all +these give the beholder an irresistible impression that it is truly a +world into which he is looking, a world akin to ours, and yet no more +like our world than Pompeii is like Naples. Its air, its waters, its +clouds, its life are gone, and only a skeleton remains -- a mute but +eloquent witness to a cosmical tragedy without parallel in the range +of human knowledge. + +One cannot but regret that the moon, if it ever was the seat of +intelligent life, has not remained so until our time. Think what the +consequences would have been if this other world at our very door had +been found to be both habitable and inhabited! We talk rather airily +of communicating with Mars by signals; but Mars never approaches +nearer than 35,000,000 miles, while the moon when nearest is only a +little more than 220,000 miles away. Given an effective magnifying +power of five thousand diameters, which will perhaps be possible at +the mountain observatories as telescopes improve, and we should be +able to bring the moon within an apparent distance of about forty +miles, while the corresponding distance for Mars would be more than +seven thousand miles. But even with existing telescopic powers we can +see details on the moon no larger than some artificial constructions +on the earth. St Peter's at Rome, with the Vatican palace and the +great piazza, if existing on the moon, would unquestionably be +recognizable as something else than a freak of nature. Large cities, +with their radiating lines of communication, would at once betray +their real character. Cultivated tracts, and the changes produced by +the interference of intelligent beings, would be clearly recognizable. +The electric illumination of a large town at night would probably be +markedly visible. Gleams of reflected sunlight would come to us from +the surfaces of the lakes and oceans, and a huge ``liner'' traversing +a lunar sea could probably be followed by its trail of smoke. As to +communications by ``wireless'' signals, which certain enthusiasts have +thought of in connection with Mars, in the case of the moon they +should be a relatively simple matter, and the feat might actually be +accomplished. Think what a literature would grow up about the moon if +it were a living world! Its very differences from the earth would only +accentuate its interest for us. Night and day on the moon are each two +weeks in length; how interesting it would be to watch the manner in +which the lunarians dealt with such a situation as that. Lunar and +terrestrial history would keep step with each other, and we should +record them both. Truly one might well wish to have a neighbor world +to study; one would feel so much the less alone in space. + +It is not impossible that the moon did at one time have inhabitants of +some kind. But, if so, they vanished with the disappearance of its +atmosphere and seas, or with the advent of its cataclysmic age. At the +best, its career as a living world must have been brief. If the water +and air were gradually absorbed, as some have conjectured, by its +cooling interior rocks, its surface might, nevertheless, have retained +them for long ages; but if, as others think, their disappearance was +due to the escape of their gaseous molecules in consequence of the +inability of the relatively small lunar gravitation to retain them, +then the final catastrophe must have been as swift as it was +inevitable. Accepting Darwin's hypothesis, that the moon was separated +from the earth by tidal action while both were yet plastic or +nebulous, we may reasonably conclude that it began its career with a +good supply of both water and air, but did not possess sufficient mass +to hold them permanently. Yet it may have retained them long enough +for life to develop in many forms upon its surface; in fact, there are +so many indications that air and water have not always been lacking to +the lunar world that we are driven to invent theories to explain both +their former presence and their present absence. + +But whatever the former condition of the moon may have been, its +existing appearance gives it a resistless fascination, and it bears so +clearly the story of a vast catastrophe sculptured on its rocky face +that the thoughtful observer cannot look upon it without a feeling of +awe. The gigantic character of the lunar features impresses the +beholder not less than the universality of the play of destructive +forces which they attest. Let us make a few comparisons. Take the +lunar crater called ``Tycho'', which is a typical example of its kind. +In the telescope Tycho appears as a perfect ring surrounding a +circular depression, in the center of which rises a group of +mountains. Its superficial resemblance to some terrestrial volcanic +craters is very striking. Vesuvius, seen from a point vertically +above, would no doubt look something like that (the resemblance would +have been greater when the Monte del Cavallo formed a more complete +circuit about the crater cone). But compare the dimensions. The +remains of the outer crater ring of Vesuvius are perhaps half a mile +in diameter, while the active crater itself is only two or three +hundred feet across at the most; Tycho has a diameter of fifty-four +miles! The group of relatively insignificant peaks in the center of +the crater floor of Tycho is far more massive than the entire mountain +that we call Vesuvius. The largest known volcanic crater on the earth, +Aso San, in Japan, has a diameter of seven miles; it would take sixty +craters like Aso San to equal Tycho in area! And Tycho, though one of +the most perfect, is by no means the largest crater on the moon. +Another, called ``Theophilus,'' has a diameter of sixty-four miles, +and is eighteen thousand feet deep. There are hundreds from ten to +forty miles in diameter, and thousands from one to ten miles. They are +so numerous in many places that they break into one another, like the +cells of a crushed honeycomb. + +The lunar craters differ from those of the earth more fundamentally +than in the matter of mere size; they are not situated on the tops of +mountains. If they were, and if all the proportions were the same, a +crater like Tycho might crown a conical peak fifty or one hundred +miles high! Instead of being cavities in the summits of mountains, the +lunar craters are rather gigantic sink-holes whose bottoms in many +cases lie two or three miles below the general surface of the lunar +world. Around their rims the rocks are piled up to a height of from a +few hundred to two or three thousand feet, with a comparatively gentle +inclination, but on the inner side they fall away in gigantic broken +precipices which make the dizzy cliffs of the Matterhorn seem but +``lover's leaps.'' Down they drop, ridge below ridge, crag under crag, +tottering wall beneath wall, until, in a crater named ``Newton,'' near +the south lunar pole, they attain a depth where the rays of the sun +never reach. Nothing more frightful than the spectacle which many of +these terrible chasms present can be pictured by the imagination. As +the lazy lunar day slowly advances, the sunshine, unmitigated by +clouds or atmospheric veil of any kind, creeps across their rims and +begins to descend the opposite walls. Presently it strikes the ragged +crest of a ridge which had lain hidden in such darkness as we never +know on the earth, and runs along it like a line of kindling fire. +Rocky pinnacles and needles shoot up into the sunlight out of the +black depths. Down sinks the line of light, mile after mile, and +continually new precipices and cliffs are brought into view, until at +last the vast floor is attained and begins to be illuminated. In the +meanwhile the sun's rays, darting across the gulf, have touched the +summits of the central peaks, twenty or thirty miles from the crater's +inmost edge, and they immediately kindle and blaze like huge stars +amid the darkness. So profound are some of these awful craters that +days pass before the sun has risen high enough above them to chase the +last shadows from their depths. + +Although several long ranges of mountains resembling those of the +earth exist on the moon, the great majority of its elevations assume +the crateriform aspect. Sometimes, instead of a crater, we find an +immense mountain ring whose form and aspect hardly suggest volcanic +action. But everywhere the true craters are in evidence, even on the +sea-beds, although they attain their greatest number and size on those +parts of the moon -- covering sixty per cent of its visible surface -- +which are distinctly mountainous in character and which constitute its +most brilliant portions. Broadly speaking, the southwestern half of +the moon is the most mountainous and broken, and the northeastern half +the least so. Right down through the center, from pole to pole, runs a +wonderful line of craters and crateriform valleys of a magnitude +stupendous even for the moon. Another similar line follows the western +edge. Three or four ``seas'' are thrust between these mountainous +belts. By the effects of ``libration'' parts of the opposite +hemisphere of the moon which is turned away from the earth are from +time to time brought into view, and their aspect indicates that that +hemisphere resembles in its surface features the one which faces the +earth. There are many things about the craters which seem to give some +warrant for the hypothesis which has been particularly urged by Mr G. +K. Gilbert, that they were formed by the impact of meteors; but there +are also many things which militate against that idea, and, upon the +whole, the volcanic theory of their origin is to be preferred. + +The enormous size of the lunar volcanoes is not so difficult to +account for when we remember how slight is the force of lunar gravity +as compared with that of the earth. With equal size and density, +bodies on the moon weigh only one-sixth as much as on the earth. +Impelled by the same force, a projectile that would go ten miles on +the earth would go sixty miles on the moon. A lunar giant thirty-five +feet tall would weigh no more than an ordinary son of Adam weighs on +his greater planet. To shoot a body from the earth so that it would +not drop back again, we should have to start it with a velocity of +seven miles per second; a mile and a half per second would serve on +the moon. It is by no means difficult to believe, then, that a lunar +volcano might form a crater ring eight or ten times broader than the +greatest to be found on the earth, especially when we reflect that in +addition to the relatively slight force of gravity, the materials of +the lunar crust are probably lighter than those of our terrestrial +rocks. + +For similar reasons it seems not impossible that the theory mentioned +in a former chapter -- that some of the meteorites that have fallen +upon the earth originated from the lunar volcanoes -- is well founded. +This would apply especially to the stony meteorites, for it is hardly +to be supposed that the moon, at least in its superficial parts, +contains much iron. It is surely a scene most strange that is thus +presented to the mind's eye -- that little attendant of the earth's +(the moon has only one-fiftieth of the volume, and only one-eightieth +of the mass of the earth) firing great stones back at its parent +planet! And what can have been the cause of this furious outbreak of +volcanic forces on the moon? Evidently it was but a passing stage in +its history; it had enjoyed more quiet times before. As it cooled down +from the plastic state in which it parted from the earth, it became +incrusted after the normal manner of a planet, and then oceans were +formed, its atmosphere being sufficiently dense to prevent the water +from evaporating and the would-be oceans from disappearing continually +in mist. This, if any, must have been the period of life in the lunar +world. As we look upon the vestiges of that ancient world buried in +the wreck that now covers so much of its surface, it is difficult to +restrain the imagination from picturing the scenes which were once +presented there; and, in such a case, should the imagination be +fettered? We give it free rein in terrestrial life, and it rewards us +with some of our greatest intellectual pleasures. The wonderful +landscapes of the moon offer it an ideal field with just enough +half-hidden suggestions of facts to stimulate its powers. + +The great plains of the Mare Imbrium and the Mare Serenitatis (the +``Sea of Showers'' and the ``Sea of Serenity''), bordered in part by +lofty mountain ranges precisely like terrestrial mountains, scalloped +along their shores with beautiful bays curving back into the adjoining +highlands, and united by a great strait passing between the nearly +abutting ends of the ``Lunar Apennines'' and the ``Lunar Caucasus,'' +offer the elements of a scene of world beauty such as it would be +difficult to match upon our planet. Look at the finely modulated +bottom of the ancient sea in Mr Ritchey's exquisite photograph of the +western part of the Mare Serenitatis, where one seems to see the play +of the watery currents heaping the ocean sands in waving lines, making +shallows, bars, and deeps for the mariner to avoid or seek, and +affording a playground for the creatures of the main. What geologist +would not wish to try his hammer on those rocks with their stony pages +of fossilized history? There is in us an instinct which forbids us to +think that there was never any life there. If we could visit the moon, +there is not among us a person so prosaic and unimaginative that he +would not, the very first thing, begin to search for traces of its +inhabitants. We would look for them in the deposits on the sea +bottoms; we would examine the shores wherever the configuration seemed +favorable for harbors and the sites of maritime cities -- forgetting +that it may be a little ridiculous to ascribe to the ancient lunarians +the same ideas that have governed the development of our race; we +would search through the valleys and along the seeming courses of +vanished streams; we would explore the mountains, not the terrible +craters, but the pinnacled chains that recall our own Alps and +Rockies; seeking everywhere some vestige of the transforming presence +of intelligent life. Perhaps we should find such traces, and perhaps, +with all our searching, we should find nothing to suggest that life +had ever existed amid that universal ruin. + +Look again at the border of the ``Sea of Serenity'' -- what a name for +such a scene! -- and observe how it has been rent with almost +inconceivable violence, the wall of the colossal crater Posidonius +dropping vertically upon the ancient shore and obliterating it, while +its giant neighbor, Le Monnier, opens a yawning mouth as if to swallow +the sea itself. A scene like this makes one question whether, after +all, those may not be right who have imagined that the so-called sea +bottoms are really vast plains of frozen lava which gushed up in +floods so extensive that even the mighty volcanoes were half drowned +in the fiery sea. This suggestion becomes even stronger when we turn +to another of the photographs of Mr Ritchey's wonderful series, +showing a part of the Mare Tranquilitatis (``Sea of Tranquility''!). +Notice how near the center of the picture the outline of a huge ring +with radiating ridges shows through the sea bottom; a fossil volcano +submerged in a petrified ocean! This is by no means the only instance +in which a buried world shows itself under the great lunar plains. +Yet, as the newer craters in the sea itself prove, the volcanic +activity survived this other catastrophe, or broke out again +subsequently, bringing more ruin to pile upon ruin. + +Yet notwithstanding the evidence which we have just been considering +in support of the hypothesis that the ``seas'' are lava floods, +Messrs. Loewy and Puiseux, the selenographers of the Paris +Observatory, are convinced that these great plains bear characteristic +marks of the former presence of immense bodies of water. In that case +we should be forced to conclude that the later oceans of the moon lay +upon vast sheets of solidified lava; and thus the catastrophe of the +lunar world assumes a double aspect, the earliest oceans being +swallowed up in molten floods issuing from the interior, while the +lands were reduced to chaos by a universal eruption of tremendous +volcanoes; and then a period of comparative quiet followed, during +which new seas were formed, and new life perhaps began to flourish in +the lunar world, only to end in another cataclysm, which finally put a +term to the existence of the moon as a life-supporting world. + +Suppose we examine two more of Mr Ritchey's illuminating photographs, +and, first, the one showing the crater Theophilus and its +surroundings. We have spoken of Theophilus before, citing the facts +that it is sixty-four miles in diameter and eighteen thousand feet +deep. It will be noticed that it has two brother giants -- Cyrillus +the nearer, and Catharina the more distant; but Theophilus is plainly +the youngest of the trio. Centuries, and perhaps thousands of years, +must have elapsed between the periods of their upheaval, for the two +older craters are partly filled with dbris, while it is manifest at a +glance that when the south eastern wall of Theophilus was formed, it +broke away and destroyed a part of the more ancient ring of Cyrillus. +There is no more tremendous scene on the moon than this; viewed with a +powerful telescope, it is absolutely appalling. + +The next photograph shows, if possible, a still wilder region. It is +the part of the moon lying between Tycho and the south pole. Tycho is +seen in the lower left-hand part of the picture. To the right, at the +edge of the illuminated portion of the moon, are the crater-rings, +Longomontanus and Wilhelm I, the former being the larger. Between them +are to be seen the ruins of two or three more ancient craters which, +together with portions of the walls of Wilhelm I and Longomontanus, +have been honeycombed with smaller craters. The vast crateriform +depression above the center of the picture is Clavius, an unrivaled +wonder of lunar scenery, a hundred and forty-two miles in its greatest +length, while its whole immense floor has sunk two miles below the +general surface of the moon outside the ring. The monstrous +shadow-filled cavity above Clavius toward the right is Blancanus, +whose aspect here gives a good idea of the appearance of these chasms +when only their rims are in the sunlight. But observe the +indescribable savagery of the entire scene. It looks as though the +spirit of destruction had gone mad in this spot. The mighty craters +have broken forth one after another, each rending its predecessor; and +when their work was finished, a minor but yet tremendous outbreak +occurred, and the face of the moon was gored and punctured with +thousands of smaller craters. These relatively small craters (small, +however, only in a lunar sense, for many of them would appear gigantic +on the earth) recall once more the theory of meteoric impact. It does +not seem impossible that some of them may have been formed by such an +agency. + +One would not wish for our planet such a fate as that which has +overtaken the moon, but we cannot be absolutely sure that something of +the kind may not be in store for it. We really know nothing of the +ultimate causes of volcanic activity, and some have suggested that the +internal energies of the earth may be accumulating instead of dying +out, and may never yet have exhibited their utmost destructive power. +Perhaps the best assurance that we can find that the earth will escape +the catastrophe that has overtaken its satellite is to be found in the +relatively great force of its gravitation. The moon has been the +victim of its weakness; given equal forces, and the earth would be the +better able to withstand them. It is significant, in connection with +these considerations, that the little planet Mercury, which seems also +to have parted with its air and water, shows to the telescope some +indications that it is pitted with craters resembling those that have +torn to pieces the face of the moon. + +Upon the whole, after studying the dreadful lunar landscapes, one +cannot feel a very enthusiastic sympathy with those who are seeking +indications of the continued existence of some kind of life on the +moon; such a world is better without inhabitants. It has met its fate; +let it go! Fortunately, it is not so near that it cannot hide its +scars and appear beautiful -- except when curiosity impels us to look +with the penetrating eyes of the astronomer. + +The Great Mars Problem + +Let any thoughtful person who is acquainted with the general facts of +astronomy look up at the heavens some night when they appear in their +greatest splendor, and ask himself what is the strongest impression +that they make upon his mind. He may not find it easy to frame an +answer, but when he has succeeded it will probably be to the effect +that the stars give him an impression of the universality of +intelligence; they make him feel, as the sun and the moon cannot do, +that his world is not alone; that all this was not made simply to form +a gorgeous canopy over the tents of men. If he is of a devout turn of +mind, he thinks, as he gazes into those fathomless deeps and among +those bewildering hosts, of the infinite multitude of created beings +that the Almighty has taken under his care. The narrow ideas of the +old geocentric theology, which made the earth God's especial +footstool, and man his only rational creature, fall away from him like +a veil that had obscured his vision; they are impossible in the +presence of what he sees above. Thus the natural tendency, in the +light of modern progress, is to regard the universe as everywhere +filled with life. + +But science, which is responsible for this broadening of men's +thoughts concerning the universality of life, itself proceeds to set +limits. Of spiritual existences it pretends to know nothing, but as to +physical beings, it declares that it can only entertain the +supposition of their existence where it finds evidence of an +environment suited to their needs, and such environment may not +everywhere exist. Science, though repelled by the antiquated +theological conception of the supreme isolation of man among created +beings, regards with complacency the probability that there are +regions in the universe where no organic life exists, stars which +shine upon no inhabited worlds, and planets which nourish no animate +creatures. The astronomical view of the universe is that it consists +of matter in every stage of evolution: some nebulous and chaotic; some +just condensing into stars (suns) of every magnitude and order; some +shaped into finished solar bodies surrounded by dependent planets; +some forming stars that perhaps have no planets, and will have none; +some constituting suns that are already aging, and will soon lose +their radiant energy and disappear; and some aggregated into masses +that long ago became inert, cold, and rayless, and that can only be +revivified by means about which we can form conjectures, but of which +we actually know nothing. + +As with the stars, so with the planets, which are the satellites of +stars. All investigations unite to tell us that the planets are not +all in the same state of development. As some are large and some +small, so some are, in an evolutionary sense, young, and some old. As +they depend upon the suns around which they revolve for their light, +heat, and other forms of radiant energy, so their condition varies +with their distance from those suns. Many may never arrive at a state +suitable for the maintenance of life upon their surfaces; some which +are not at present in such a state may attain it later; and the forms +of life themselves may vary with the peculiar environment that +different planets afford. Thus we see that we are not scientifically +justified in affirming that life is ubiquitous, although we are thus +justified in saying that it must be, in a general sense, universal. We +might liken the universe to a garden known to contain every variety of +plant. If on entering it we see no flowers, we examine the species +before us and find that they are not of those which bloom at this +particular season, or perhaps they are such as never bear flowers. Yet +we feel no doubt that we shall find flowers somewhere in the garden, +because there are species which bloom at this season, and the garden +contains all varieties. + +While it is tacitly assumed that there are planets revolving around +other stars than the sun, it would be impossible for us to see them +with any telescope yet invented, and no instrument now in the +possession of astronomers could assure us of their existence; so the +only planetary system of which we have visual knowledge is our own. +Excluding the asteroids, which could not from any point of view be +considered as habitable, we have in the solar system eight planets of +various sizes and situated at various distances from the sun. Of these +eight we know that one, the earth, is inhabited. The question, then, +arises: Are there any of the others which are inhabited or habitable? +Since it is our intention to discuss the habitability of only one of +the seven to which the question applies, the rest may be dismissed in +a few words. The smallest of them, and the nearest to the sun, is +Mercury, which is regarded as uninhabitable because it has no +perceptible supply of water and air, and because, owing to the +extraordinary eccentricity of its orbit, it is subjected to excessive +and very rapid alterations in the amount of solar heat and light +poured upon its surface, such alterations being inconsistent with the +supposition that it can support living beings. Even its average +temperature is more than six and a half times that prevailing on the +earth! Another circumstance which militates against its habitability +is that, according to the results of the best telescopic studies, it +always keeps the same face toward the sun, so that one half of the +planet is perpetually exposed to the fierce solar rays, and the other +half faces the unmitigated cold of open space. Venus, the next in +distance from the sun, is almost the exact twin of the earth in size, +and many arguments may be urged in favor of its habitability, although +it is suspected of possessing the same peculiarity as Mercury, in +always keeping the same side sunward. Unfortunately its atmosphere +appears to be so dense that no permanent markings on its surface are +certainly visible, and the question of its actual condition must, for +the present, be left in abeyance. Mars, the first planet more distant +from the sun than the earth, is the special subject of this chapter, +and will be described and discussed a few lines further on. Jupiter, +Saturn, Uranus, and Neptune, the four giant planets, all more distant +than Mars, and each more distant than the other in the order named, +are all regarded as uninhabitable because none of them appears to +possess any degree of solidity. They may have solid or liquid nuclei, +but exteriorly they seem to be mere balls of cloud. Of course, one can +imagine what he pleases about the existence of creatures suited to the +physical constitution of such planets as these, but they must be +excluded from the category of habitable worlds in the ordinary sense +of the term. We go back, then, to Mars. + +It will be best to begin with a description of the planet. Mars is +4230 miles in diameter; its surface is not much more than one-quarter +as extensive as that of the earth (.285). Its mean distance from the +sun is 141,500,000 miles, 48,500,000 miles greater than that of the +earth. Since radiant energy varies inversely as the square of +distance, Mars receives less than half as much solar light and heat as +the earth gets. Mars' year (period of revolution round the sun) is 687 +days. Its mean density is 71 per cent of the earth's, and the force of +gravity on its surface is 38 per cent of that on the surface of the +earth; i.e., a body weighing one hundred pounds on the earth would, if +transported to Mars, weigh but thirty-eight pounds. The inclination of +its equator to the plane of its orbit differs very little from that of +the earth's equator, and its axial rotation occupies 24 hours 37 +minutes. so that the length of day and night, and the extent of the +seasonal changes on Mars, are almost precisely the same as on the +earth. But owing to the greater length of its year, the seasons of +Mars, while occurring in the same order, are almost twice as long as +ours. The surface of the planet is manifestly solid, like that of our +globe, and the telescope reveals many permanent markings on it, +recalling the appearance of a globe on which geographical features +have been represented in reddish and dusky tints. Around the poles are +plainly to be seen rounded white areas, which vary in extent with the +Martian seasons, nearly vanishing in summer and extending widely in +winter. The most recent spectroscopic determinations indicate that +Mars has an atmosphere perhaps as dense as that to be found on our +loftiest mountain peaks, and there is a perceptible amount of watery +vapor in this atmosphere. The surface of the planet appears to be +remarkably level, and it has no mountain ranges. No evidences of +volcanic action have been discovered on Mars. The dusky and reddish +areas were regarded by the early observers as respectively seas and +lands, but at present it is not believed that there are any bodies of +water on the planet. There has never been much doubt expressed that +the white areas about the poles represent snow. + +It will be seen from this brief description that many remarkable +resemblances exist between Mars and the earth, and there is nothing +wonderful in the fact that the question of the habitability of the +former has become one of extreme and wide-spread interest, giving rise +to the most diverse views, to many extraordinary speculations, and +sometimes to regrettably heated controversy. The first champion of the +habitability of Mars was Sir William Herschel, although even before +his time the idea had been suggested. He was convinced by the +revelations of his telescopes, continually increasing in power, that +Mars was more like the earth than any other planet. He could not +resist the testimony of the polar snows, whose suggestive conduct was +in such striking accord with what occurs upon the earth. Gradually, as +telescopes improved and observers increased in number, the principal +features of the planet were disclosed and charted, and ``areography,'' +as the geography of Mars was called, took its place among the +recognized branches of astronomical study. But it was not before 1877 +that a fundamentally new discovery in areography gave a truly +sensational turn to speculation about life on ``the red planet.'' In +that year Mars made one of its nearest approaches to the earth, and +was so situated in its orbit that it could be observed to great +advantage from the northern hemisphere of the earth. The celebrated +Italian astronomer, Schiaparelli, took advantage of this opportunity +to make a trigonometrical survey of the surface of Mars -- as coolly +and confidently as if he were not taking his sights across a +thirty-five-million-mile gulf of empty space -- and in the course of +this survey he was astonished to perceive that the reddish areas, then +called continents, were crossed in many directions by narrow, dusky +lines, to which he gave the suggestive name of ``canals.'' Thus a kind +of firebrand was cast into the field of astronomical speculation, +which has ever since produced disputes that have sometimes approached +the violence of political faction. At first the accuracy of +Schiaparelli's observations was contested; it required a powerful +telescope, and the most excellent ``seeing,'' to render the +enigmatical lines visible at all, and many searchers were unable to +detect them. But Schiaparelli continued his studies in the serene sky +of Italy, and produced charts of the gridironed face of Mars +containing so much astonishing detail that one had either to reject +them in toto or to confess that Schiaparelli was right. As subsequent +favorable oppositions of Mars occurred, other observers began to see +the ``canals'' and to confirm the substantial accuracy of the Italian +astronomer's work, and finally few were found who would venture to +affirm that the ``canals'' did not exist, whatever their meaning might +be. + +When Schiaparelli began his observations it was generally believed, as +we have said, that the dusky areas on Mars were seas, and since +Schiaparelli thought that the ``canals'' invariably began and ended at +the shores of the ``seas,'' the appropriateness of the title given to +the lines seemed apparent. Their artificial character was immediately +assumed by many, because they were too straight and too suggestively +geometrical in their arrangement to permit the conclusion that they +were natural watercourses. A most surprising circumstance noted by +Schiaparelli was that the ``canals'' made their appearance after the +melting of the polar snow in the corresponding hemisphere had begun, +and that they grew darker, longer, and more numerous in proportion as +the polar liquidation proceeded; another very puzzling observation was +that many of them became double as the season advanced; close beside +an already existing ``canal,'' and in perfect parallelism with it, +another would gradually make its appearance. That these phenomena +actually existed and were not illusions was proved by later +observations, and today they are seen whenever Mars is favorably +situated for observation. + +In the closing decade of the nineteenth century, Mr Percival Lowell +took up the work where Schiaparelli had virtually dropped it, and soon +added a great number of ``canals'' to those previously known, so that +in his charts the surface of the wonderful little planet appears +covered as with a spider's web, the dusky lines criss-crossing in +every direction, with conspicuous knots wherever a number of them come +together. Mr Lowell has demonstrated that the areas originally called +seas, and thus named on the earlier charts, are not bodies of water, +whatever else they may be. He has also found that the mysterious lines +do not, as Schiaparelli supposed, begin and end at the edges of the +dusky regions, but often continue on across them, reaching in some +cases far up into the polar regions. But Schiaparelli was right in his +observation that the appearance of the ``canals'' is synchronous with +the gradual disappearance of the polar snows, and this fact has become +the basis of the most extraordinary theory that the subject of life in +other worlds has ever given birth to. + +Now, the effect of such discoveries, as we have related, depends upon +the type of mind to whose attention they are called. Many are content +to accept them as strange and inexplicable at present, and to wait for +further light upon them; others insist upon an immediate inquiry +concerning their probable nature and meaning. Such an inquiry can only +be based upon inference proceeding from analogy. Mars, say Mr Lowell +and those who are of his opinion, is manifestly a solidly incrusted +planet like the earth; it has an atmosphere, though one of great +rarity; it has water vapor, as the snows in themselves prove; it has +the alternation of day and night, and a succession of seasons closely +resembling those of the earth; its surface is suggestively divided +into regions of contrasting colors and appearance, and upon that +surface we see an immense number of lines geometrically arranged, with +a system of symmetrical intersections where the lines expand into +circular and oval areas -- and all connected with the annual melting +of the polar snows in a way which irresistibly suggests the +interference of intelligence directed to a definite end. Why, with so +many concurrent circumstances to support the hypothesis, should we not +regard Mars as an inhabited globe? + +But the differences between Mars and the earth are in many ways as +striking as their resemblances. Mars is relatively small; it gets less +than half as much light and heat as we receive; its atmosphere is so +rare that it would be distressing to us, even if we could survive in +it at all; it has no lakes, rivers, or seas; its surface is an endless +prairie. and its ``canals'' are phenomena utterly unlike anything on +the earth. Yet it is precisely upon these divergences between the +earth and Mars, this repudiation of terrestrial standards, that the +theory of ``life on Mars,'' for which Mr Lowell is mainly responsible, +is based. Because Mars is smaller than the earth, we are told it must +necessarily be more advanced in planetary evolution, the underlying +cause of which is the gradual cooling and contraction of the planet's +mass. Mars has parted with its internal heat more rapidly than the +earth; consequently its waters and its atmosphere have been mostly +withdrawn by chemical combinations, but enough of both yet remain to +render life still possible on its surface. As the globe of Mars is +evolutionally older than that of the earth, so its forms of organic +life may be proportionally further advanced, and its inhabitants may +have attained a degree of cultivated intelligence much superior to +what at present exists upon the earth. Understanding the nature and +the causes of the desiccation of their planet, and possessing +engineering science and capabilities far in advance of ours, they may +be conceived to have grappled with the stupendous problem of keeping +their world in a habitable condition as long as possible. Supposing +them to have become accustomed to live in their rarefied atmosphere (a +thing not inconceivable, since men can live for a time at least in air +hardly less rare), the most pressing problem for them is that of a +water-supply, without which plant life cannot exist, while animal life +in turn depends for its existence upon vegetation. The only direction +in which they can seek water is that of the polar regions, where it is +alternately condensed into snow and released in the liquid form by the +effect of the seasonal changes. It is, then, to the annual melting of +the polar snow-fields that the Martian engineers are supposed to have +recourse in supplying the needs of their planet, and thus providing +the means of prolonging their own existence. It is imagined that they +have for this purpose constructed a stupendous system of irrigation +extending over the temperate and equatorial regions of the planet. The +``canals'' represent the lines of irrigation, but the narrow streaks +that we see are not the canals themselves, but the irrigated bands +covered by them. Their dark hue, and their gradual appearance after +the polar melting has begun, are due to the growth of vegetation +stimulated by the water. The rounded areas visible where several +``canals'' meet and cross are called by Mr Lowell ``oases.'' These are +supposed to be the principal centers of population and industry. It +must be confessed that some of them, with their complicated systems of +radiating lines, appear to answer very well to such a theory. No +attempt to explain them by analogy with natural phenomena on the earth +has proved successful. + +But a great difficulty yet remains: How to explain the seemingly +miraculous powers of the supposed engineers? Here recourse is had once +more to the relative smallness of the planet. We have remarked that +the force of gravity on Mars is only thirty-eight per cent of that on +the earth. A steam-shovel driven by a certain horse-power would be +nearly three times as effective there as here. A man of our stature on +Mars would find his effective strength increased in the same +proportion. But just because of the slight force of gravity there, a +Martian might attain to the traditional stature of Goliath without +finding his own weight an encumbrance to his activity, while at the +same time his huge muscles would come into unimpeded play, enabling +him single-handed to perform labors that would be impossible to a +whole gang of terrestrial workmen. The effective powers of huge +machines would be increased in the same way; and to all this must be +added the fact that the mean density of the materials of which Mars is +composed is much less than that of the constituents of the earth. +Combining all these considerations, it becomes much less difficult to +conceive that public works might be successfully undertaken on Mars +which would be hopelessly beyond the limits of human accomplishment. + +Certain other difficulties have also to be met; as, for instance, the +relative coldness of the climate of Mars. At its distance it gets +considerably less than half as much light and heat as we receive. In +addition to this, the rarity of its atmosphere would naturally be +expected to decrease the effective temperature at the planet's +surface, since an atmosphere acts somewhat like the glass cover of a +hot-house in retaining the solar heat which has penetrated it. It has +been calculated that, unless there are mitigating circumstances of +which we know nothing, the average temperature at the surface of Mars +must be far below the freezing-point of water. To this it is replied +that the possible mitigating circumstances spoken of evidently exist +in fact, because we can see that the watery vapor condenses into snow +around the poles in winter, but melts again when summer comes. The +mitigating agent may be supposed to exist in the atmosphere where the +presence of certain gases would completely alter the temperature +gradients. + +It might also be objected that it is inconceivable that the Martian +engineers, however great may be their physical powers, and however +gigantic the mechanical energies under their control, could force +water in large quantities from the poles to the equator. This is an +achievement that measures up to the cosmical standard. It is admitted +by the champions of the theory that the difficulty is a formidable +one; but they call attention to the singular fact that on Mars there +can be found no chains of mountains, and it is even doubtful if ranges +of hills exist there. The entire surface of the planet appears to be +almost ``as smooth as a billiard ball,'' and even the broad regions +which were once supposed to be seas apparently lie at practically the +same level as the other parts, since the ``canals'' in many cases run +uninterruptedly across them. Lowell's idea is that these sombre areas +may be expanses of vegetation covering ground of a more or less marshy +character, for while the largest of them appear to be permanent, there +are some which vary coincidently with the variations of the canals. + +As to the kind of machinery employed to force the water from the +poles, it has been conjectured that it may have taken the form of a +gigantic system of pumps and conduits; and since the Martians are +assumed to be so far in advance of us in their mastery of scientific +principles, the hypothesis will at least not be harmed by supposing +that they have learned to harness forces of nature whose very +existence in a manageable form is yet unrecognized on the earth. If we +wish to let the imagination loose, we may conjecture that they have +conquered the secret of those intra-atomic forces whose resistless +energy is beginning to become evident to us, but the possibility of +whose utilization remains a dream, the fulfillment of which nobody +dares to predict. + +Such, in very brief form, is the celebrated theory of Mars as an +inhabited world. It certainly captivates the imagination, and if we +believe it to represent the facts, we cannot but watch with the +deepest sympathy this gallant struggle of an intellectual race to +preserve its planet from the effects of advancing age and death. We +may, indeed, wonder whether our own humanity, confronted by such a +calamity, could be counted on to meet the emergency with equal +stoutness of heart and inexhaustibleness of resource. Up to the +present time we certainly have shown no capacity to confront Nature +toe to toe, and to seize her by the shoulders and turn her round when +she refuses to go our way. If we could get into wireless telephonic +communication with the Martians we might learn from their own lips the +secret of their more than ``Roman recovery.'' + +The Riddle of the Asteroids + +Between the orbits of Mars and Jupiter revolves the most remarkable +system of little bodies with which we are acquainted -- the Asteroids, +or Minor Planets. Some six hundred are now known, and they may +actually number thousands. They form virtually a ring about the sun. +The most striking general fact about them is that they occupy the +place in the sky which should be occupied, according to Bode's Law, by +a single large planet. This fact, as we shall see, has led to the +invention of one of the most extraordinary theories in astronomy -- +viz., that of the explosion of a world! + +Bode's Law, so-called, is only an empiric formula, but until the +discovery of Neptune it accorded so well with the distances of the +planets that astronomers were disposed to look upon it as really +representing some underlying principle of planetary distribution. They +were puzzled by the absence of a planet in the space between Mars and +Jupiter, where the ``law'' demanded that there should be one, and an +association of astronomers was formed to search for it. There was a +decided sensation when, in 1801, Piazzi, of Palermo, announced that he +had found a little planet which apparently occupied the place in the +system which belonged to the missing body. He named it Ceres, and it +was the first of the Asteroids. The next year Olbers, of Bremen, while +looking for Ceres with his telescope, stumbled upon another small +planet which he named Pallas. Immediately he was inspired with the +idea that these two planets were fragments of a larger one which had +formerly occupied the vacant place in the planetary ranks, and he +predicted that others would be found by searching in the neighborhood +of the intersection of the orbits of the two already discovered. This +bold prediction was brilliantly fulfilled by the finding of two more +-- Juno in 1804, and Vesta in 1807. Olbers would seem to have been led +to the invention of his hypothesis of a planetary explosion by the +faith which astronomers at that time had in Bode's Law. They appear to +have thought that several planets revolving in the gap where the +``law'' called for but one could only be accounted for upon the theory +that the original one had been broken up to form the several. +Gravitation demanded that the remnants of a planet blown to pieces, no +matter how their orbits might otherwise differ, should all return at +stated periods to the point where the explosion had occurred; hence +Olbers' prediction that any asteroids that might subsequently be +discovered would be found to have a common point of orbital +intersection. And curiously enough all of the first asteroids found +practically answered to this requirement. Olbers' theory seemed to be +established. + +After the first four, no more asteroids were found until 1845, when +one was discovered; then, in 1847, three more were added to the list; +and after that searchers began to pick them up with such rapidity that +by the close of the century hundreds were known, and it had become +almost impossible to keep track of them. The first four are by far the +largest members of the group, but their actual sizes remained unknown +until less than twenty years ago. It was long supposed that Vesta was +the largest, because it shines more brightly than any of the others; +but finally, in 1895, Barnard, with the Lick telescope, definitely +measured their diameters, and proved to everybody's surprise that +Ceres is really the chief, and Vesta only the third in rank. His +measures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, +239 miles; and Juno, 120 miles. They differ greatly in the reflective +power of their surfaces, a fact of much significance in connection +with the question of their origin. Vesta is, surface for surface, +rather more than three times as brilliant as Ceres, whence the +original mistake about its magnitude. + +Nowadays new asteroids are found frequently by photography, but +physically they are most insignificant bodies, their average diameter +probably not exceeding twenty miles, and some are believed not to +exceed ten. On a planet only ten miles in diameter, assuming the same +mean density as the earth's, which is undoubtedly too much, the force +of gravity would be so slight that an average man would not weigh more +than three ounces, and could jump off into space whenever he liked. + +Although the asteroids all revolve around the sun in the same +direction as that pursued by the major planets, their orbits are +inclined at a great variety of angles to the general plane of the +planetary system, and some of them are very eccentric -- almost as +much so as the orbits of many of the periodic comets. It has even been +conjectured that the two tiny moons of Mars and the four smaller +satellites of Jupiter may be asteroids gone astray and captured by +those planets. Two of the asteroids are exceedingly remarkable for the +shapes and positions of their orbits; these are Eros, discovered in +1898, and T. G., 1906, found eight years later. The latter has a mean +distance from the sun slightly greater than that of Jupiter, while the +mean distance of Eros is less than that of Mars. The orbit of Eros is +so eccentric that at times it approaches within 15,000,000 miles of +the earth, nearer than any other regular member of the solar system +except the moon, thus affording an unrivaled means of measuring the +solar parallax. But for our present purpose the chief interest of Eros +lies in its extraordinary changes of light. + +These changes, although irregular, have been observed and photographed +many times, and there seems to be no doubt of their reality. Their +significance consists in their possible connection with the form of +the little planet, whose diameter is generally estimated at not more +than twenty miles. Von Oppolzer found, in 1901, that Eros lost +three-fourths of its brilliancy once in every two hours and +thirty-eight minutes. Other observers have found slightly different +periods of variability, but none as long as three hours. The most +interesting interpretation that has been offered of this phenomenon is +that it is due to a great irregularity of figure, recalling at once +Olbers' hypothesis. According to some, Eros may be double, the two +bodies composing it revolving around each other at very close +quarters; but a more striking, and it may be said probable, suggestion +is that Eros has a form not unlike that of a dumb-bell, or hour-glass, +turning rapidly end over end so that the area of illuminated surface +presented to our eyes continually changes, reaching at certain times a +minimum when the amount of light that it reflects toward the earth is +reduced to a quarter of its maximum value. Various other bizarre +shapes have been ascribed to Eros, such, for instance, as that of a +flat stone revolving about one of its longer axes, so that sometimes +we see its face and sometimes its edge. + +All of these explanations proceed upon the assumption that Eros cannot +have a simple globular figure like that of a typical planet, a figure +which is prescribed by the law of gravitation, but that its shape is +what may be called accidental; in a word, it is a fragment, for it +seems impossible to believe that a body formed in interplanetary +space, either through nebular condensation or through the aggregation +of particles drawn together by their mutual attractions, should not be +practically spherical in shape. Nor is Eros the only asteroid that +gives evidence by variations of brilliancy that there is something +abnormal in its constitution; several others present the same +phenomenon in varying degrees. Even Vesta was regarded by Olbers as +sufficiently variable in its light to warrant the conclusion that it +was an angular mass instead of a globe. Some of the smaller ones show +very notable variations, and all in short periods, of three or four +hours, suggesting that in turning about one of their axes they present +a surface of variable extent toward the sun and the earth. + +The theory which some have preferred -- that the variability of light +is due to the differences of reflective power on different parts of +the surface -- would, if accepted, be hardly less suggestive of the +origin of these little bodies by the breaking up of a larger one, +because the most natural explanation of such differences would seem to +be that they arose from variations in the roughness or smoothness of +the reflecting surface, which would be characteristic of fragmentary +bodies. In the case of a large planet alternating expanses of land and +water, or of vegetation and desert, would produce a notable variation +in the amount of reflection, but on bodies of the size of the +asteroids neither water nor vegetation could exist, and an atmosphere +would be equally impossible. + +One of the strongest objections to Olbers' hypothesis is that only a +few of the first asteroids discovered travel in orbits which +measurably satisfy the requirement that they should all intersect at +the point where the explosion occurred. To this it was at first +replied that the perturbations of the asteroidal orbits, by the +attractions of the major planets, would soon displace them in such a +manner that they would cease to intersect. One of the first +investigations undertaken by the late Prof. Simon Newcomb was directed +to the solution of this question, and he arrived at the conclusion +that the planetary perturbations could not explain the actual +situation of the asteroidal orbits. But afterward it was pointed out +that the difficulty could be avoided by supposing that not one but a +series of explosions had produced the asteroids as they now are. After +the primary disruption the fragments themselves, according to this +suggestion, may have exploded, and then the resulting orbits would be +as ``tangled'' as the heart could wish. This has so far rehabilitated +the explosion theory that it has never been entirely abandoned, and +the evidence which we have just cited of the probably abnormal shapes +of Eros and other asteroids has lately given it renewed life. It is a +subject that needs a thorough rediscussion. + +We must not fail to mention, however, that there is a rival hypothesis +which commends itself to many astronomers -- viz., that the asteroids +were formed out of a relatively scant ring of matter, situated between +Mars and Jupiter and resembling in composition the immensely more +massive rings from which, according to Laplace's hypothesis, the +planets were born. It is held by the supporters of this theory that +the attraction of the giant Jupiter was sufficient to prevent the +small, nebulous ring that gave birth to the asteroids from condensing +like the others into a single planet. + +But if we accept the explosion theory, with its corollary that minor +explosions followed the principal one, we have still an unanswered +question before us: What caused the explosions? The idea of a world +blowing up is too Titanic to be shocking; it rather amuses the +imagination than seriously impresses it; in a word, it seems +essentially chimerical. We can by no appeal to experience form a +mental picture of such an occurrence. Even the moon did not blow up +when it was wrecked by volcanoes. The explosive nebul and new stars +are far away in space, and suggest no connection with such a +catastrophe as the bursting of a planet into hundreds of pieces. We +cannot conceive of a great globe thousands of miles in diameter +resembling a pellet of gunpowder only awaiting the touch of a match to +cause its sudden disruption. Somehow the thought of human agency +obtrudes itself in connection with the word ``explosion,'' and we +smile at the idea that giant powder or nitro-glycerine could blow up a +planet. Yet it would only need enough of them to do it. + +After all, we may deceive ourselves in thinking, as we are apt to do, +that explosive energies lock themselves up only in small masses of +matter. There are many causes producing explosions in nature, every +volcanic eruption manifests the activity of some of them. Think of the +giant power of confined steam; if enough steam could be suddenly +generated in the center of the earth by a downpour of all the waters +of the oceans, what might not the consequences be for our globe? In a +smaller globe, and it has never been estimated that the original +asteroid was even as large as the moon, such a catastrophe would, +perhaps, be more easily conceivable; but since we are compelled in +this case to assume that there was a series of successive explosions, +steam would hardly answer the purpose; it would be more reasonable to +suppose that the cause of the explosion was some kind of chemical +reaction, or something affecting the atoms composing the exploding +body. Here Dr Gustav Le Bon comes to our aid with a most startling +suggestion, based on his theory of the dissipation of intra-atomic +energy. It will be best to quote him at some length from his book on +The Evolution of Forces. + +``It does not seem at first sight,'' says Doctor Le Bon, + + very comprehensible that worlds which appear more and more stable + as they cool could become so unstable as to afterward dissociate + entirely. To explain this phenomenon, we will inquire whether + astronomical observations do not allow us to witness this + dissociation. + + We know that the stability of a body in motion, such as a top or a + bicycle, ceases to be possible when its velocity of rotation + descends below a certain limit. Once this limit is reached it loses + its stability and falls to the ground. Prof. J. J. Thomson even + interprets radio-activity in this manner, and points out that when + the speed of the elements composing the atoms descends below a + certain limit they become unstable and tend to lose their + equilibria. There would result from this a commencement of + dissociation, with diminution of their potential energy and a + corresponding increase of their kinetic energy sufficient to launch + into space the products of intra-atomic disintegration. + + It must not be forgotten that the atom being an enormous reservoir + of energy is by this very fact comparable with explosive bodies. + These last remain inert so long as their internal equilibria are + undisturbed. So soon as some cause or other modifies these, they + explode and smash everything around them after being themselves + broken to pieces. + + Atoms, therefore, which grow old in consequence of the diminution + of a part of their intra-atomic energy gradually lose their + stability. A moment, then, arrives when this stability is so weak + that the matter disappears by a sort of explosion more or less + rapid. The bodies of the radium group offer an image of this + phenomenon -- a rather faint image, however, because the atoms of + this body have only reached a period of instability when the + dissociation is rather slow. It probably precedes another and more + rapid period of dissociation capable of producing their final + explosion. Bodies such as radium, thorium, etc., represent, no + doubt, a state of old age at which all bodies must some day arrive, + and which they already begin to manifest in our universe, since all + matter is slightly radio-active. It would suffice for the + dissociation to be fairly general and fairly rapid for an explosion + to occur in a world where it was manifested. + + These theoretical considerations find a solid support in the sudden + appearances and disappearances of stars. The explosions of a world + which produce them reveal to us, perhaps, how the universes perish + when they become old. + + As astronomical observations show the relative frequency of these + rapid destructions, we may ask ourselves whether the end of a + universe by a sudden explosion after a long period of old age does + not represent its most general ending. + +Here, perhaps, it will be well to stop, since, entrancing as the +subject may be, we know very little about it, and Doctor Le Bon's +theory affords a limitless field for the reader's imagination. + _________________________________________________________________ + +A printed version of this book is available from Sattre Press +(http://csky.sattre-press.com). 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FOR PUBLIC DOMAIN EBOOKS*Ver.02/11/02*END* + diff --git a/old/cusky10.zip b/old/cusky10.zip Binary files differnew file mode 100644 index 0000000..2fb52d3 --- /dev/null +++ b/old/cusky10.zip diff --git a/old/cusky10h.htm b/old/cusky10h.htm new file mode 100644 index 0000000..ce2cd53 --- /dev/null +++ b/old/cusky10h.htm @@ -0,0 +1,5925 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> +<html> +<head> +<title>New File</title> +<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> +<style type="text/css"> +<!-- +body {margin:10%; text-align:justify} +blockquote {font-size:14pt} +P {font-size:14pt} +--> +</style> +</head> +<body> + + + +<h1>The Project Gutenberg EBook of +<br>Curiosities of the Sky, by Garrett Serviss</h1> +<pre> +Copyright laws are changing all over the world. Be sure to check the +copyright laws for your country before downloading or redistributing +this or any other Project Gutenberg eBook. + +This header should be the first thing seen when viewing this Project +Gutenberg file. Please do not remove it. Do not change or edit the +header without written permission. + +Please read the "legal small print," and other information about the +eBook and Project Gutenberg at the bottom of this file. Included is +important information about your specific rights and restrictions in +how the file may be used. You can also find out about how to make a +donation to Project Gutenberg, and how to get involved. + + +**Welcome To The World of Free Plain Vanilla Electronic Texts** + +**eBooks Readable By Both Humans and By Computers, Since 1971** + +*****These eBooks Were Prepared By Thousands of Volunteers!***** + + +Title: Curiosities of the Sky + +Author: Garrett Serviss + +Release Date: October, 2004 [EBook #6630] +[Yes, we are more than one year ahead of schedule] +[This file was first posted on January 6, 2003] + +Edition: 10 + +Language: English + +Character set encoding: ASCII + +*** START OF THE PROJECT GUTENBERG EBOOK, CURIOSITIES OF THE SKY *** +</pre> + + <h1>Curiosities of the Sky</h1> + + <h2>by Garrett Serviss</h2> + <cite>Curiosities of the Sky</cite> was first published in 1909 + and the text is in the public domain. The transcription was + done by <a href="mailto:info@sattre-press.com">William + McClain</a>, 2002. + + <p>A printed version of this book is available from <a href= + "http://csky.sattre-press.com">Sattre Press</a>. It includes + extensive annotations, a new introduction and all the original + photographs and diagrams.</p> + <hr> + <strong>Preface</strong> + + <p>What Froude says of history is true also of astronomy: it is + the most impressive where it transcends explanation. It is not + the mathematics of astronomy, but the wonder and the mystery + that seize upon the imagination. The calculation of an eclipse + owes all its prestige to the sublimity of its data; the + operation, in itself, requires no more mental effort than the + preparation of a railway time-table.</p> + + <p>The dominion which astronomy has always held over the minds + of men is akin to that of poetry; when the former becomes + merely instructive and the latter purely didactic, both lose + their power over the imagination. Astronomy is known as the + oldest of the sciences, and it will be the longest-lived + because it will always have arcana that have not been + penetrated.</p> + + <p>Some of the things described in this book are little known + to the average reader, while others are well known; but all + possess the fascination of whatever is strange, marvelous, + obscure, or mysterious -- magnified, in this case, by the + portentous scale of the phenomena.</p> + + <p>The idea of the author is to tell about these things in + plain language, but with as much scientific accuracy as plain + language will permit, showing the wonder that is in them + without getting away from the facts. Most of them have hitherto + been discussed only in technical form, and in treatises that + the general public seldom sees and never reads.</p> + + <p>Among the topics touched upon are:</p> + + <ul> + <li>The strange unfixedness of the ``fixed stars,'' the vast + migrations of the suns and worlds constituting the + universe.</li> + + <li>The slow passing out of existence of those collocations + of stars which for thousands of years have formed famous + ``constellations,'' preserving the memory of mythological + heroes and heroines, and perhaps of otherwise unrecorded + history.</li> + + <li>The tendency of stars to assemble in immense clouds, + swarms, and clusters.</li> + + <li>The existence in some of the richest regions of the + universe of absolutely black, starless gaps, deeps, or holes, + as if one were looking out of a window into the murkiest + night.</li> + + <li>The marvelous phenomena of new, or temporary, stars, + which appear as suddenly as conflagrations, and often turn + into something else as eccentric as themselves.</li> + + <li>The amazing forms of the ``whirlpool,'' ``spiral,'' + ``pinwheel,'' and ``lace,'' or ``tress,'' nebulæ.</li> + + <li>The strange surroundings of the sun, only seen in + particular circumstances, but evidently playing a constant + part in the daily phenomena of the solar system.</li> + + <li>The mystery of the Zodiacal Light and the + Gegenschein.</li> + + <li>The extraordinary transformations undergone by comets and + their tails.</li> + + <li>The prodigies of meteorites and masses of stone and metal + fallen from the sky.</li> + + <li>The cataclysms that have wrecked the moon.</li> + + <li>The problem of life and intelligence on the planet + Mars.</li> + + <li>The problematical origin and fate of the asteroids.</li> + + <li>The strange phenomena of the auroral lights.</li> + </ul> + + <p>An attempt has been made to develop these topics in an + orderly way, showing their connection, so that the reader may + obtain a broad general view of the chief mysteries and problems + of astronomy, and an idea of the immense field of discovery + which still lies, almost unexplored, before it.</p> + + <p><strong>The Windows of Absolute Night</strong></p> + + <p>To most minds mystery is more fascinating than science. But + when science itself leads straight up to the borders of mystery + and there comes to a dead stop, saying, ``At present I can no + longer see my way,'' the force of the charm is redoubled. On + the other hand, the illimitable is no less potent in mystery + than the invisible, whence the dramatic effect of Keats' + ``stout Cortez'' staring at the boundless Pacific while all his + men look at each other with a wild surmise, ``silent upon a + peak in Darien.'' It is with similar feelings that the + astronomer regards certain places where from the peaks of the + universe his vision seems to range out into endless empty + space. He sees there the shore of his little isthmus, and, + beyond, unexplored immensity.</p> + + <p>The name, ``coal-sacks,'' given to these strange voids is + hardly descriptive. Rather they produce upon the mind the + effect of blank windows in a lonely house on a pitch-dark + night, which, when looked at from the brilliant interior, + become appalling in their rayless murk. Infinity seems to + acquire a new meaning in the presence of these black openings + in the sky, for as one continues to gaze it loses its purely + metaphysical quality and becomes a kind of entity, like the + ocean. The observer is conscious that he can actually + <em>see</em> the beginning of its ebon depths, in which the + visible universe appears to float like an enchanted island, + resplendent within with lights and life and gorgeous + spectacles, and encircled with screens of crowded stars, but + with its dazzling vistas ending at the fathomless sea of pure + darkness which encloses all.</p> + + <p>The Galaxy, or Milky Way, surrounds the borders of our + island in space like a stellar garland, and when openings + appear in it they are, by contrast, far more impressive than + the general darkness of the interstellar expanse seen in other + directions. Yet even that expanse is not everywhere equally + dark, for it contains gloomy deeps discernable with careful + watching. Here, too, contrast plays an important part, though + less striking than within the galactic region. Some of Sir + William Herschel's observations appear to indicate an + association between these tenebrious spots and neighboring star + clouds and nebulæ. It is an illuminating bit of + astronomical history that when he was sweeping the then virgin + heavens with his great telescopes he was accustomed to say to + his sister who, note-book in hand, waited at his side to take + down his words, fresh with the inspiration of discovery: + ``Prepare to write; the nebulæ are coming; here space is + vacant.''</p> + + <p>The most famous of the ``coal-sacks,'' and the first to be + brought to general attention before astronomers had awakened to + the significance of such things, lies adjacent to the + ``Southern Cross,'' and is truly an amazing phenomenon. It is + not alone the conspicuousness of this celestial vacancy, + opening suddenly in the midst of one of the richest parts of + the Galaxy, that has given it its fame, but quite as much the + superstitious awe with which it was regarded by the early + explorers of the South Seas. To them, as well as to those who + listened in rapt wonder to their tales, the ``Coal-sack'' + seemed to possess some occult connection with the mystic + ``Cross.'' In the eyes of the sailors it was not a vacancy so + much as a sable reality in the sky, and as, shuddering, they + stared at it, they piously crossed themselves. It was another + of the magical wonders of the unknown South, and as such it + formed the basis of many a ``wild surmise'' and many a + sea-dog's yarn. Scientific investigation has not diminished its + prestige, and today no traveler in the southern hemisphere is + indifferent to its fascinating strangeness, while some find it + the most impressive spectacle of the antarctic heavens.</p> + + <p>All around, up to the very edge of the yawning gap, the + sheen of the Milky Way is surpassingly glorious; but there, as + if in obedience to an almighty edict, everything vanishes. A + single faint star is visible within the opening, producing a + curious effect upon the sensitive spectator, like the sight of + a tiny islet in the midst of a black, motionless, waveless + tarn. The dimensions of the lagoon of darkness, which is oval + or pear-shaped, are eight degrees by five, so that it occupies + a space in the sky about one hundred and thirty times greater + than the area of the full moon. It attracts attention as soon + as the eye is directed toward the quarter where it exists, and + by virtue of the rarity of such phenomena it appears a far + greater wonder than the drifts of stars that are heaped around + it. Now that observatories are multiplying in the southern + hemisphere, the great austral ``Coal-sack'' will, no doubt, + receive attention proportioned to its importance as one of the + most significant features of the sky. Already at the Sydney + Observatory photographs have shown that the southern portion of + this Dead Sea of Space is not quite ``bottomless,'' although + its northern part defies the longest sounding lines of the + astronomer.</p> + + <p>There is a similar, but less perfect, ``coal-sack'' in the + northern hemisphere, in the constellation of ``The Swan,'' + which, strange to say, also contains a well-marked figure of a + cross outlined by stars. This gap lies near the top of the + cross-shaped figure. It is best seen by averted vision, which + brings out the contrast with the Milky Way, which is quite + brilliant around it. It does not, however, exercise the same + weird attraction upon the eye as the southern ``Coal-sack,'' + for instead of looking like an absolute void in the sky, it + rather appears as if a canopy of dark gauze had been drawn over + the stars. We shall see the possible significance of this + appearance later.</p> + + <p>Just above the southern horizon of our northern middle + latitudes, in summer, where the Milky Way breaks up into vast + sheets of nebulous luminosity, lying over and between the + constellations Scorpio and Sagittarius, there is a remarkable + assemblage of ``coal-sacks,'' though none is of great size. One + of them, near a conspicuous star-cluster in Scorpio, M80, is + interesting for having been the first of these strange objects + noted by Herschel. Probably it was its nearness to M80 which + suggested to his mind the apparent connection of such vacancies + with star-clusters which we have already mentioned.</p> + + <p>But the most marvelous of the ``coal-sacks'' are those that + have been found by photography in Sagittarius. One of Barnard's + earliest and most excellent photographs includes two of them, + both in the star-cluster M8. The larger, which is roughly + rectangular in outline, contains one little star, and its + smaller neighbor is lune-shaped -- surely a most singular form + for such an object. Both are associated with curious dark lanes + running through the clustered stars like trails in the woods. + Along the borders of these lanes the stars are ranked in + parallel rows, and what may be called the bottoms of the lanes + are not entirely dark, but pebbled with faint stellar points. + One of them which skirts the two dark gaps and traverses the + cluster along its greatest diameter is edged with lines of + stars, recalling the alignment of the trees bordering a French + highway. This <em>road of stars</em> cannot be less than many + billions of miles in length!</p> + + <p>All about the cluster the bed of the Galaxy is strangely + disturbed, and in places nearly denuded, as if its contents had + been raked away to form the immense stack and the smaller + accumulations of stars around it. The well-known ``Trifid + Nebula'' is also included in the field of the photograph, which + covers a truly marvelous region, so intricate in its mingling + of nebulæ, star-clusters, star-swarms, star-streams, and + dark vacancies that no description can do it justice. Yet, + chaotic as it appears, there is an unmistakable suggestion of + unity about it, impressing the beholder with the idea that all + the different parts are in some way connected, and have not + been fortuitously thrown together. Miss Agnes M. Clerke made + the striking remark that the dusky lanes in M8 are exemplified + on the largest scale in the great rift dividing the Milky Way, + from Cygnus in the northern hemisphere all the way to the + ``Cross'' in the southern. Similar lanes are found in many + other clusters, and they are generally associated with flanking + rows of stars, resembling in their arrangement the thick-set + houses and villas along the roadways that traverse the + approaches to a great city.</p> + + <p>But to return to the black gaps. Are they really windows in + the star-walls of the universe? Some of them look rather as if + they had been made by a shell fired through a luminous target, + allowing the eye to range through the hole into the void space + beyond. If science is discretely silent about these things, + what can the more venturesome and less responsible imagination + suggest? Would a huge ``runaway sun,'' like Arcturus, for + instance, make such an opening if it should pass like a + projectile through the Milky Way? It is at least a stimulating + inquiry. Being probably many thousands of times more massive + than the galactic stars, such a stellar missile would not be + stopped by them, though its direction of flight might be + altered. It would drag the small stars lying close to its + course out of their spheres, but the ultimate tendency of its + attraction would be to sweep them round in its wake, thus + producing rather a star-swarm than a vacancy. Those that were + very close to it might be swept away in its rush and become its + satellites, careering away with it in its flight into outer + space; but those that were farther off, and they would, of + course, greatly outnumber the nearer ones, would tend inward + from all sides toward the line of flight, as dust and leaves + collect behind a speeding motor (though the forces operating + would be different), and would fill up the hole, if hole it + were. A swarm thus collected should be rounded in outline and + bordered with a relatively barren ring from which the stars had + been ``sucked'' away. In a general sense the M8 cluster answers + to this description, but even if we undertook to account for + its existence by a supposition like the above, the black gaps + would remain unexplained, unless one could make a further draft + on the imagination and suggest that the stars had been thrown + into a vast eddy, or system of eddies, whose vortices appear as + dark holes. Only a maelstrom-like motion could keep such a + funnel open, for without regard to the impulse derived from the + projectile, the proper motions of the stars themselves would + tend to fill it. Perhaps some other cause of the whirling + motion may be found. As we shall see when we come to the spiral + nebulæ, gyratory movements are exceedingly prevalent + throughout the universe, and the structure of the Milky Way is + everywhere suggestive of them. But this is hazardous sport even + for the imagination -- to play with <em>suns</em> as if they + were but thistle-down in the wind or corks in a mill-race.</p> + + <p>Another question arises: What is the thickness of the hedge + of stars through which the holes penetrate? Is the depth of the + openings proportionate to their width? In other words, is the + Milky Way round in section like a rope, or flat and thin like a + ribbon? The answer is not obvious, for we have little or no + information concerning the relative distances of the faint + galactic stars. It would be easier, certainly, to conceive of + openings in a thin belt than in a massive ring, for in the + first case they would resemble mere rifts and breaks, while in + the second they would be like wells or bore-holes. Then, too, + the fact that the Milky Way is not a <em>continuous</em> body + but is made up of stars whose actual distances apart is great, + offers another quandary; persistent and sharply bordered + apertures in such an assemblage are <em>a priori</em> as + improbable, if not impossible, as straight, narrow holes + running through a swarm of bees.</p> + + <p>The difficulty of these questions indicates one of the + reasons why it has been suggested that the seeming gaps, or + many of them, are not openings at all, but opaque screens + cutting off the light from stars behind them. That this is + quite possible in some cases is shown by Barnard's later + photographs, particularly those of the singular region around + the star Rho Ophiuchi. Here are to be seen somber lanes and + patches, apparently forming a connected system which covers an + immense space, and which their discoverer thinks may constitute + a ``dark nebula.'' This seems at first a startling suggestion; + but, after all, why should their not be dark nebulæ as + well as visible ones? In truth, it has troubled some + astronomers to explain the luminosity of the bright + nebulæ, since it is not to be supposed that matter in so + diffuse a state can be incandescent through heat, and + phosphorescent light is in itself a mystery. The supposition is + also in accord with what we know of the existence of dark solid + bodies in space. Many bright stars are accompanied by obscure + companions, sometimes as massive as themselves; the planets are + non-luminous; the same is true of meteors before they plunge + into the atmosphere and become heated by friction; and many + plausible reasons have been found for believing that space + contains as many obscure as shining bodies of great size. It is + not so difficult, after all, then, to believe that there are + immense collections of shadowy gases and meteoric dust whose + presence is only manifested when they intercept the light + coming from shining bodies behind them.</p> + + <p>This would account for the apparent extinguishment of light + in open space, which is indicated by the falling off in + relative number of telescopic stars below the tenth magnitude. + Even as things are, the amount of light coming to us from stars + too faint to be seen with the naked eye is so great that the + statement of it generally surprises persons who are unfamiliar + with the inner facts of astronomy. It has been calculated that + on a clear night the total starlight from the entire celestial + sphere amounts to one-sixtieth of the light of the full moon; + but of this less than one-twenty-fifth is due to stars + separately distinguished by the eye. If there were no obscuring + medium in space, it is probable that the amount of starlight + would be noticeably and perhaps enormously increased.</p> + + <p>But while it seems certain that some of the obscure spots in + the Milky Way are due to the presence of ``dark nebulæ,'' + or concealing veils of one kind or another, it is equally + certain that there are many which are true apertures, however + they may have been formed, and by whatever forces they may be + maintained. These, then, are veritable windows of the Galaxy, + and when looking out of them one is face to face with the great + mystery of infinite space. <em>There</em> the known universe + visibly ends, but manifestly space itself does not end there. + It is not within the power of thought to conceive an end to + space, for the instant we think of a terminal point or line the + mind leaps forward to the <em>beyond.</em> There must be space + outside as well as inside. Eternity of time and infinity of + space are ideas that the intellect cannot fully grasp, but + neither can it grasp the idea of a limitation to either space + or time. The metaphysical conceptions of hypergeometry, or + fourth-dimensional space, do not aid us.</p> + + <p>Having, then, discovered that the universe is a thing + <em>contained</em> in something indefinitely greater than + itself; having looked out of its windows and found only the + gloom of starless night outside -- what conclusions are we to + draw concerning the beyond? It <em>seems</em> as empty as a + vacuum, but is it really so? If it be, then our universe is a + single atom astray in the infinite; it is the only island in an + ocean without shores; it is the one oasis in an illimitable + desert. Then the Milky Way, with its wide-flung garland of + stars, is afloat like a tiny smoke-wreath amid a horror of + immeasurable vacancy, or it is an evanescent and solitary ring + of sparkling froth cast up for a moment on the viewless billows + of immensity. From such conclusions the mind instinctively + shrinks. It prefers to think that there is <em>something</em> + beyond, though we cannot see it. Even the universe could not + bear to be alone -- a Crusoe lost in the Cosmos! As the + inhabitants of the most elegant château, with its + gardens, parks, and crowds of attendants, would die of + loneliness if they did not know that they have neighbors, + though not seen, and that a living world of indefinite extent + surrounds them, so we, when we perceive that the universe has + limits, wish to feel that it is not solitary; that beyond the + hedges and the hills there are other centers of life and + activity. Could anything be more terrible than the thought of + an <em>isolated universe?</em> The greater the being, the + greater the aversion to seclusion. Only the infinite satisfies; + in that alone the mind finds rest.</p> + + <p>We are driven, then, to believe that the universal night + which envelopes us is not tenantless; that as we stare out of + the star-framed windows of the Galaxy and see nothing but + uniform blackness, the fault is with our eyes or is due to an + obscuring medium. Since <em>our</em> universe is limited in + extent, there must be <em>other</em> universes beyond it on all + sides. Perhaps if we could carry our telescopes to the verge of + the great ``Coal-sack'' near the ``Cross,'' being then on the + frontier of our starry system, we could discern, sparkling afar + off in the vast night, some of the outer galaxies. They may be + grander than ours, just as many of the suns surrounding us are + immensely greater than ours. If we could take our stand + somewhere in the midst of immensity and, with vision of + infinite reach, look about us, we should perhaps see a + countless number of stellar systems, amid which ours would be + unnoticeable, like a single star among the multitude glittering + in the terrestial sky on a clear night. Some might be in the + form of a wreath, like our own; some might be globular, like + the great star-clusters in Hercules and Centaurus; some might + be glittering circles, or disks, or rings within rings. If we + could enter them we should probably find a vast variety of + composition, including elements unknown to terrestrial + chemistry; for while the <em>visible</em> universe appears to + contain few if any substances not existing on the earth or in + the sun, we have no warrant to assume that others may not exist + in infinite space.</p> + + <p>And how as to gravitation? We do not <em>know</em> that + gravitation acts beyond the visible universe, but it is + reasonable to suppose that it does. At any rate, if we let go + <em>its</em> sustaining hand we are lost, and can only wander + hopelessly in our speculations, like children astray. If the + empire of gravitation is infinite, then the various outer + systems must have <em>some,</em> though measuring by our + standards an imperceptible, attractive influence upon each + other, for gravitation never lets go its hold, however great + the space over which it is required to act. Just as the stars + about us are all in motion, so the starry systems beyond our + sight may be in motion, and our system as a whole may be moving + in concert with them. If this be so, then after interminable + ages the aspect of the entire system of systems must change, + its various members assuming new positions with respect to one + another. In the course of time we may even suppose that our + universe will approach relatively close to one of the others; + and then, if men are yet living on the earth, they may glimpse + through the openings which reveal nothing to us now, the lights + of another nearing star system, like the signals of a strange + squadron, bringing them the assurance (which can be but an + inference at present) that the ocean of space has other + argosies venturing on its limitless expanse.</p> + + <p>There remains the question of the luminiferous ether by + whose agency the waves of light are borne through space. The + ether is as mysterious as gravitation. With regard to ether we + only infer its existence from the effects which we ascribe to + it. Evidently the ether must extend as far as the most distant + visible stars. But does it continue on indefinitely in outer + space? If it does, then the invisibility of the other systems + must be due to their distance diminishing the quantity of light + that comes from them below the limit of perceptibility, or to + the interposition of absorbing media; if it does not, then the + reason why we cannot see them is owing to the absence of a + means of conveyance for the light waves, as the lack of an + interplanetary atmosphere prevents us from hearing the thunder + of sun-spots. (It is interesting to recall that Mr Edison was + once credited with the intention to construct a gigantic + microphone which should render the roar of sun-spots audible by + transforming the electric vibrations into sound-waves). On this + supposition each starry system would be enveloped in its own + globule of ether, and no light could cross from one to another. + But the probability is that both the ether and gravitation are + ubiquitous, and that all the stellar systems are immersed in + the former like clouds of phosphorescent organisms in the + sea.</p> + + <p>So astronomy carries the mind from height to greater height. + Men were long in accepting the proofs of the relative + insignificance of the earth; they were more quickly convinced + of the comparative littleness of the solar system; and now the + evidence assails their reason that what they had regarded as + <em>the</em> universe is only one mote gleaming in the sunbeams + of Infinity.</p> + + <p><strong>Star-Clouds, Star-Clusters, and + Star-Streams</strong></p> + + <p>In the preceding chapter we have seen something of the + strangely complicated structure of the Galaxy, or Milky Way. We + now proceed to study more comprehensively that garlanded + ``Pathway of the Gods.''</p> + + <p>Judged by the eye alone, the Milky Way is one of the most + delicately beautiful phenomena in the entire realm of nature -- + a shimmer of silvery gauze stretched across the sky; but + studied in the light of its revelations, it is the most + stupendous object presented to human ken. Let us consider, + first, its appearance to ordinary vision. Its apparent position + in the sky shifts according to the season. On a serene, + cloudless summer evening, in the absence of the moon, whose + light obscures it, one sees the Galaxy spanning the heavens + from north to southeast of the zenith like a phosphorescent + arch. In early spring it forms a similar but, upon the whole, + less brilliant arch west of the zenith. Between spring and + summer it lies like a long, faint, twilight band along the + northern horizon. At the beginning of winter it again forms an + arch, this time spanning the sky from east to west, a little + north of the zenith. These are its positions as viewed from the + mean latitude of the United States. Even the beginner in + star-gazing does not have to watch it throughout the year in + order to be convinced that it is, in reality, a great circle, + extending entirely around the celestial sphere. We appear to be + situated near its center, but its periphery is evidently far + away in the depths of space.</p> + + <p>Although to the casual observer it seems but a delicate + scarf of light, brighter in some places than in others, but + hazy and indefinite at the best, such is not its appearance to + those who study it with care. They perceive that it is an + organic whole, though marvelously complex in detail. The + telescope shows that it consists of stars too faint and small + through excess of distance to be separately visible. Of the + hundred million suns which some estimates have fixed as the + probable population of the starry universe, the vast majority + (at least thirty to one) are included in this strange belt of + misty light. But they are not uniformly distributed in it; on + the contrary, they are arrayed in clusters, knots, bunches, + clouds, and streams. The appearance is somewhat as if the + Galaxy consisted of innumerable swarms of silver-winged bees, + more or less intermixed, some massed together, some crossing + the paths of others, but all governed by a single purpose which + leads them to encircle the region of space in which we are + situated.</p> + + <p>From the beginning of the systematic study of the heavens, + the fact has been recognized that the form of the Milky Way + denotes the scheme of the sidereal system. At first it was + thought that the shape of the system was that of a vast round + disk, flat like a cheese, and filled with stars, our sun and + his relatively few neighbors being placed near the center. + According to this view, the galactic belt was an effect of + perspective; for when looking in the direction of the plane of + the disk, the eye ranged through an immense extension of stars + which blended into a glimmering blur, surrounding us like a + ring; while when looking out from the sides of the disk we saw + but few stars, and in those directions the heavens appeared + relatively blank. Finally it was recognized that this theory + did not correspond with the observed appearances, and it became + evident that the Milky Way was not a mere effect of + perspective, but an actual band of enormously distant stars, + forming a circle about the sphere, the central opening of the + ring (containing many scattered stars) being many times broader + than the width of the ring itself. Our sun is one of the + scattered stars in the central opening.</p> + + <p>As already remarked, the ring of the Galaxy is very + irregular, and in places it is partly broken. With its sinuous + outline, its pendant sprays, its graceful and accordant curves, + its bunching of masses, its occasional interstices, and the + manifest order of a general plan governing the jumble of its + details, it bears a remarkable resemblance to a garland -- a + fact which appears the more wonderful when we recall its + composition. That an elm-tree should trace the lines of beauty + with its leafy and pendulous branches does not surprise us; but + we can only gaze with growing amazement when we behold <em>a + hundred million suns imitating the form of a chaplet!</em> And + then we have to remember that this form furnishes the + ground-plan of the universe.</p> + + <p>As an indication of the extraordinary speculations to which + the mystery of the Milky Way has given rise, a theory recently + (1909) proposed by Prof. George C. Comstock may be mentioned. + Starting with the data (first) that the number of stars + increases as the Milky Way is approached, and reaches a maximum + in its plane, while on the other hand the number of + nebulæ is greatest outside the Milky Way and increases + with distance from it, and (second) that the Milky Way, + although a complete ring, is broad and diffuse on one side + through one-half its course -- that half alone containing + nebulæ -- and relatively narrow and well defined on the + opposite side, the author of this singular speculation avers + that these facts can best be explained by supposing that the + invisible universe consists of two interpenetrating parts, one + of which is a chaos of indefinite extent, strewn with stars and + nebulous dust, and the other a long, broad but comparatively + thin cluster of stars, including the sun as one of its central + members. This flat star-cluster is conceived to be moving + edgewise through the chaos, and, according to Professor + Comstock, it acts after the manner of a snow-plough sweeping + away the cosmic dust and piling it on either hand above and + below the plane of the moving cluster. It thus forms a + transparent rift, through which we see farther and command a + view of more stars than through the intensified dust-clouds on + either hand. This rift is the Milky Way. The dust thrown aside + toward the poles of the Milky Way is the substance of the + nebulæ which abound there. Ahead, where the front of the + star-plough is clearing the way, the chaos is nearer at hand, + and consequently there the rift subtends a broader angle, and + is filled with primordial dust, which, having been annexed by + the vanguard of the star-swarm, forms the nebulæ seen + only in that part of the Milky Way. But behind, the rift + appears narrow because there we look farther away between + dust-clouds produced ages ago by the front of the plough, and + no scattered dust remains in that part of the rift.</p> + + <p>In quoting an outline of this strikingly original theory the + present writer should not be understood as assenting to it. + That it appears bizarre is not, in itself, a reason for + rejecting it, when we are dealing with so problematical and + enigmatical a subject as the Milky Way; but the serious + objection is that the theory does not sufficiently accord with + the observed phenomena. There is too much evidence that the + Milky Way is an organic system, however fantastic its form, to + permit the belief that it can only be a rift in chaotic clouds. + As with every organism, we find that its parts are more or less + clearly repeated in its ensemble. Among all the strange things + that the Milky Way contains there is nothing so extraordinary + as itself. Every astronomer must many times have found himself + marveling at it in those comparatively rare nights when it + shows all its beauty and all its strangeness. In its great + broken rifts, divisions, and spirals are found the gigantic + prototypes of similar forms in its star-clouds and clusters. As + we have said, it determines the general shape of the whole + sidereal system. Some of the brightest stars in the sky appear + to hang like jewels suspended at the ends of tassels dropped + from the Galaxy. Among these pendants are the Pleiades and the + Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a + loop of light'' thrown out from it. The majority of the great + first-magnitude stars seem related to it, as if they formed an + inner ring inclined at an angle of some twenty degrees to its + plane. Many of the long curves that set off from it on both + sides are accompanied by corresponding curves of lucid stars. + In a word, it offers every appearance of structural connection + with the entire starry system. That the universe should have + assumed the form of a wreath is certainly a matter for + astonishment; but it would have been still more astonishing if + it had been a cube, a rhomboid, or a dodecahedron, for then we + should have had to suppose that something resembling the forces + that shape crystals had acted upon the stars, and the + difficulty of explaining the universe by the laws of + gravitation would have been increased.</p> + + <p>From the Milky Way as a whole we pass to the vast clouds, + swarms, and clusters of stars of which it is made up. It may + be, as some astronomers hold, that most of the galactic stars + are much smaller than the sun, so that their faintness is not + due entirely to the effect of distance. Still, their intrinsic + brilliance attests their solar character, and considering their + remoteness, which has been estimated at not less than ten + thousand to twenty thousand light-years (a light-year is equal + to nearly six thousand thousand million miles) their actual + masses cannot be extremely small. The minutest of them are + entitled to be regarded as real suns, and they vary enormously + in magnitude. The effects of their attractions upon one another + can only be inferred from their clustering, because their + relative movements are not apparent on account of the brevity + of the observations that we can make. But imagine a being for + whom a million years would be but as a flitting moment; to him + the Milky Way would appear in a state of ceaseless agitation -- + swirling with ``a fury of whirlpool motion.''</p> + + <p>The cloud-like aspect of large parts of the Galaxy must + always have attracted attention, even from naked-eye observers, + but the true star-clouds were first satisfactorily represented + in Barnard's photographs. The resemblance to actual clouds is + often startling. Some are close-packed and dense, like cumuli; + some are wispy or mottled, like cirri. The rifts and + modulations, as well as the general outlines, are the same as + those of clouds of vapor or dust, and one notices also the + characteristic thinning out at the edges. But we must beware of + supposing that the component suns are thickly crowded as the + particles forming an ordinary cloud. They <em>look,</em> + indeed, as if they were matted together, because of the + irradiation of light, but in reality millions and billions of + miles separate each star from its neighbors. Nevertheless they + form real assemblages, whose members are far more closely + related to one another than is our sun to the stars around him, + and if we were in the Milky Way the aspect of the nocturnal sky + would be marvelously different from its present appearance.</p> + + <p>Stellar clouds are characteristic of the Galaxy and are not + found beyond its borders, except in the ``Magellanic Clouds'' + of the southern hemisphere, which resemble detached portions of + the Milky Way. These singular objects form as striking a + peculiarity of the austral heavens as does the great + ``Coal-sack'' described in Chapter 1. But it is their isolation + that makes them so remarkable, for their composition is + essentially galactic, and if they were included within its + boundaries they would not appear more wonderful than many other + parts of the Milky Way. Placed where they are, they look like + masses fallen from the great stellar arch. They are full of + nebulæ and star-clusters, and show striking evidences of + spiral movement.</p> + + <p>Star-swarms, which are also characteristic features of the + Galaxy, differ from star-clouds very much in the way that their + name would imply -- <em>i.e.,</em> their component stars are so + arranged, even when they are countless in number, that the idea + of an exceedingly numerous assemblage rather than that of a + cloud is impressed on the observer's mind. In a star-swarm the + separate members are distinguishable because they are either + larger or nearer than the stars composing a ``cloud.'' A + splendid example of a true star-swarm is furnished by Chi + Persei, in that part of the Milky Way which runs between the + constellations Perseus and Cassiopeia. This swarm is much + coarser than many others, and can be seen by the naked eye. In + a small telescope it appears double, as if the suns composing + it had divided into two parties which keep on their way side by + side, with some commingling of their members where the skirts + of the two companies come in contact.</p> + + <p>Smaller than either star-clouds or star-swarms, and + differing from both in their organization, are star-clusters. + These, unlike the others, are found outside as well as inside + the Milky Way, although they are more numerous inside its + boundaries than elsewhere. The term star-cluster is sometimes + applied, though improperly, to assemblages which are rather + groups, such, for instance, as the Pleiades. In their most + characteristic aspect star-clusters are of a globular shape -- + globes of suns! A famous example of a globular star-cluster, + but one not included in the Milky Way, is the ``Great Cluster + in Hercules.'' This is barely visible to the naked eye, but a + small telescope shows its character, and in a large one it + presents a marvelous spectacle. Photographs of such clusters + are, perhaps, less effective than those of star-clouds, because + the central condensation of stars in them is so great that + their light becomes blended in an indistinguishable blur. The + beautiful effect of the incessant play of infinitesimal rays + over the apparently compact surface of the cluster, as if it + were a globe of the finest frosted silver shining in an + electric beam, is also lost in a photograph. Still, even to the + eye looking directly at the cluster through a powerful + telescope, the central part of the wonderful congregation seems + almost a solid mass in which the stars are packed like the ice + crystals in a snowball.</p> + + <p>The same question rises to the lips of every observer: How + can they possibly have been brought into such a situation? The + marvel does not grow less when we know that, instead of being + closely compacted, the stars of the cluster are probably + separated by millions of miles; for we know that their + distances apart are slight as compared with their remoteness + from the Earth. Sir William Herschel estimated their number to + be about fourteen thousand, but in fact they are uncountable. + If we could view them from a point just within the edge of the + assemblage, they would offer the appearance of a hollow + hemisphere emblazoned with stars of astonishing brilliancy; the + near-by ones unparalleled in splendor by any celestial object + known to us, while the more distant ones would resemble + ordinary stars. An inhabitant of the cluster would not know, + except by a process of ratiocination, that he was dwelling in a + globular assemblage of suns; only from a point far outside + would their spherical arrangement become evident to the eye. + Imagine fourteen-thousand fire-balloons with an approach to + regularity in a spherical space -- say, ten miles in diameter; + there would be an average of less than thirty in every cubic + mile, and it would be necessary to go to a considerable + distance in order to see them as a globular aggregation; yet + from a point sufficiently far away they would blend into a + glowing ball.</p> + + <p>Photographs show even better than the best telescopic views + that the great cluster is surrounded with a multitude of + dispersed stars, suggestively arrayed in more or less curving + lines, which radiate from the principle mass, with which their + connection is manifest. These stars, situated outside the + central sphere, look somewhat like vagrant bees buzzing round a + dense swarm where the queen bee is sitting. Yet while there is + so much to suggest the operation of central forces, bringing + and keeping the members of the cluster together, the attentive + observer is also impressed with the idea that the whole + wonderful phenomenon may be <em>the result of explosion.</em> + As soon as this thought seizes the mind, confirmation of it + seems to be found in the appearance of the outlying stars, + which could be as readily explained by the supposition that + they have been blown apart as that they have flocked together + toward a center. The probable fact that the stars constituting + the cluster are very much smaller than our sun might be + regarded as favoring the hypothesis of an explosion. Of their + real size we know nothing, but, on the basis of an uncertain + estimate of their parallax, it has been calculated that they + may average forty-five thousand miles in diameter -- something + more than half the diameter of the planet Jupiter. Assuming the + same mean density, fourteen thousand such stars might have been + formed by the explosion of a body about twice the size of the + sun. This recalls the theory of Olbers, which has never been + altogether abandoned or disproved, that the Asteroids were + formed by the explosion of a planet circulating between the + orbits of Mars and Jupiter. The Asteroids, whatever their + manner of origin, form a ring around the sun; but, of course, + the explosion of a great independent body, not originally + revolving about a superior center of gravitational force, would + not result in the formation of a ring of small bodies, but + rather of a dispersed mass of them. But back of any speculation + of this kind lies the problem, at present insoluble: How could + the explosion be produced? (See the question of explosions in + Chapters 6 and 14).</p> + + <p>Then, on the other hand, we have the observation of + Herschel, since abundantly confirmed, that space is unusually + vacant in the immediate neighborhood of condensed star-clusters + and nebulæ, which, as far as it goes, might be taken as + an indication that the assembled stars had been drawn together + by their mutual attractions, and that the tendency to + aggregation is still bringing new members toward the cluster. + But in that case there must have been an original condensation + of stars at that point in space. This could probably have been + produced by the coagulation of a great nebula into stellar + nuclei, a process which seems now to be taking place in the + Orion Nebula.</p> + + <p>A yet more remarkable globular star-cluster exists in the + southern hemisphere, Omega Centauri. In this case the central + condensation of stars presents an almost uniform blaze of + light. Like the Hercules cluster, that in Centaurus is + surrounded with stars scattered over a broad field and showing + an appearance of radial arrangement. In fact, except for its + greater richness, Omega Centauri is an exact duplicate of its + northern rival. Each appears to an imaginative spectator as a + veritable ``city of suns.'' Mathematics shrinks from the task + of disentangling the maze of motions in such an assemblage. It + would seem that the chance of collisions is not to be + neglected, and this idea finds a certain degree of confirmation + in the appearance of ``temporary stars'' which have more than + once blazed out in, or close by, globular star-clusters.</p> + + <p>This leads up to the notable fact, first established by + Professor Bailey a few years ago, that such clusters are + populous with variable stars. Omega Centauri and the Hercules + cluster are especially remarkable in this respect. The + variables found in them are all of short period and the changes + of light show a noteworthy tendency to uniformity. The first + thought is that these phenomena must be due to collisions among + the crowded stars, but, if so, the encounters cannot be between + the stars themselves, but probably between stars and meteor + swarms revolving around them. Such periodic collisions might go + on for ages without the meteors being exhausted by + incorporation with the stars. This explanation appears all the + more probable because one would naturally expect that flocks of + meteors would abound in a close aggregation of stars. It is + also consistent with Perrine's discovery -- that the globular + star clusters are powdered with minute stars strewn thickly + among the brighter ones.</p> + + <p>In speaking of Professor Comstock's extraordinary theory of + the Milky Way, the fact was mentioned that, broadly speaking, + the nebulæ are less numerous in the galactic belt than in + the comparatively open spaces on either side of it, but that + they are, nevertheless, abundant in the broader half of the + Milky Way which he designates as the front of the gigantic + ``plough'' supposed to be forcing its way through the + enveloping chaos. In and around the Sagittarius region the + intermingling of nebulæ and galactic star clouds and + clusters is particularly remarkable. That there is a causal + connection no thoughtful person can doubt. We are unable to get + away from the evidence that a nebula is like a seed-ground from + which stars spring forth; or we may say that nebulæ + resemble clouds in whose bosom raindrops are forming. The + wonderful aspect of the admixtures of nebulæ and + star-clusters in Sagittarius has been described in Chapter 1. + We now come to a still more extraordinary phenomenon of this + kind -- the Pleiades nebulæ.</p> + + <p>The group of the Pleiades, although lying outside the main + course of the Galaxy, is connected with it by a faint loop, and + is the scene of the most remarkable association of stars and + nebulous matter known in the visible universe. The naked eye is + unaware of the existence of nebulæ in the Pleiades, or, + at the best, merely suspects that there is something of the + kind there; and even the most powerful telescopes are far from + revealing the full wonder of the spectacle; but in photographs + which have been exposed for many hours consecutively, in order + to accumulate the impression of the actinic rays, the + revelation is stunning. The principle stars are seen surrounded + by, and, as it were, <em>drowned in,</em> dense nebulous clouds + of an unparalleled kind. The forms assumed by these clouds seem + at first sight inexplicable. They look like fleeces, or perhaps + more like splashes and daubs of luminous paint dashed + carelessly from a brush. But closer inspection shows that they + are, to a large extent, <em>woven</em> out of innumerable + threads of filmy texture, and there are many indications of + spiral tendencies. Each of the bright stars of the group -- + Alcyone, Merope, Maia, Electra, Taygeta, Atlas -- is the focus + of a dense fog (totally invisible, remember, alike to the naked + eye and to the telescope), and these particular stars are + veiled from sight behind the strange mists. Running in all + directions across the relatively open spaces are nebulous wisps + and streaks of the most curious forms. On some of the nebular + lines, which are either straight throughout, or if they change + direction do so at an angle, little stars are strung like + beads. In one case seven or eight stars are thus aligned, and, + as if to emphasize their dependence upon the chain which + connects them, when it makes a slight bend the file of stars + turns the same way. Many other star rows in the group suggest + by their arrangement that they, too, were once strung upon + similar threads which have now disappeared, leaving the stars + spaced along their ancient tracks. We seem forced to the + conclusion that there was a time when the Pleiades were + embedded in a vast nebula resembling that of Orion, and that + the cloud has now become so rare by gradual condensation into + stars that the merest trace of it remains, and this would + probably have escaped detection but for the remarkable actinic + power of the radiant matter of which it consists. The richness + of many of these faint nebulous masses in ultra-violet + radiations, which are those that specifically affect the + photographic plate, is the cause of the marvelous revelatory + power of celestial photography. So the veritable unseen + universe, as distinguished from the ``unseen universe'' of + metaphysical speculation, is shown to us.</p> + + <p>A different kind of association between stars and + nebulæ is shown in some surprising photographic objects + in the constellation Cygnus, where long, wispy nebulæ, + billions of miles in length, some of them looking like tresses + streaming in a breeze, lie amid fields of stars which seem + related to them. But the relation is of a most singular kind, + for notwithstanding the delicate structure of the long + nebulæ they appear to act as barriers, causing the stars + to heap themselves on one side. The stars are two, three, or + four times as numerous on one side of the nebulæ as on + the other. These nebulæ, as far as appearance goes, might + be likened to rail fences, or thin hedges, against which the + wind is driving drifts of powdery snow, which, while scattered + plentifully all around, tends to bank itself on the leeward + side of the obstruction. The imagination is at a loss to + account for these extraordinary phenomena; yet there they are, + faithfully giving us their images whenever the photographic + plate is exposed to their radiations.</p> + + <p>Thus the more we see of the universe with improved methods + of observation, and the more we invent aids to human senses, + each enabling us to penetrate a little deeper into the unseen, + the greater becomes the mystery. The telescope carried us far, + photography is carrying us still farther; but what as yet + unimagined instrument will take us to the bottom, the top, and + the end? And then, what hitherto untried power of thought will + enable us to comprehend the meaning of it all?</p> + + <p><strong>Stellar Migrations</strong></p> + + <p>To the untrained eye the stars and the planets are not + distinguishable. It is customary to call them all alike + ``stars.'' But since the planets more or less rapidly change + their places in the sky, in consequence of their revolution + about the sun, while the stars proper seem to remain always in + the same relative positions, the latter are spoken of as + ``fixed stars.'' In the beginnings of astronomy it was not + known that the ``fixed stars'' had any motion independent of + their apparent annual revolution with the whole sky about the + earth as a seeming center. Now, however, we know that the term + ``fixed stars'' is paradoxical, for there is not a single + really fixed object in the whole celestial sphere. The apparent + fixity in the positions of the stars is due to their immense + distance, combined with the shortness of the time during which + we are able to observe them. It is like viewing the plume of + smoke issuing from a steamer, hull down, at sea: if one does + not continue to watch it for a long time it appears to be + motionless, although in reality it may be traveling at great + speed across the line of sight. Even the planets seem fixed in + position if one watches them for a single night only, and the + more distant ones do not sensibly change their places, except + after many nights of observation. Neptune, for instance, moves + but little more than two degrees in the course of an entire + year, and in a month its change of place is only about + one-third of the diameter of the full moon.</p> + + <p>Yet, fixed as they seem, the stars are actually moving with + a speed in comparison with which, in some cases, the planets + might almost be said to stand fast in their tracks. Jupiter's + speed in his orbit is about eight miles per second, Neptune's + is less than three and one-half miles, and the earth's is about + eighteen and one-half miles; while there are ``fixed stars'' + which move two hundred or three hundred miles per second. They + do not all, however, move with so great a velocity, for some + appear to travel no faster than the planets. But in all cases, + notwithstanding their real speed, long-continued and + exceedingly careful observations are required to demonstrate + that they are moving at all. No more overwhelming impression of + the frightful depths of space in which the stars are buried can + be obtained than by reflecting upon the fact that a star whose + actual motion across the line of sight amounts to two hundred + miles per second does not change its apparent place in the sky, + in the course of a thousand years, sufficiently to be noticed + by the casual observer of the heavens!</p> + + <p>There is one vast difference between the motions of the + stars and those of the planets to which attention should be at + once called: the planets, being under the control of a central + force emanating from their immediate master, the sun, all move + in the same direction and in orbits concentric about the sun; + the stars, on the other hand, move in every conceivable + direction and have no apparent center of motion, for all + efforts to discover such a center have failed. At one time, + when theology had finally to accept the facts of science, a + grandiose conception arose in some pious minds, according to + which the Throne of God was situated at the exact center of His + Creation, and, seated there, He watched the magnificent + spectacle of the starry systems obediently revolving around + Him. Astronomical discoveries and speculations seemed for a + time to afford some warrant for this view, which was, moreover, + an acceptable substitute for the abandoned geocentric theory in + minds that could only conceive of God as a superhuman + artificer, constantly admiring his own work. No longer ago than + the middle of the nineteenth century a German astronomer, + Maedler, believed that he had actually found the location of + the center about which the stellar universe revolved. He placed + it in the group of the Pleiades, and upon his authority an + extraordinary imaginative picture was sometimes drawn of the + star Alcyone, the brightest of the Pleiades, as the very seat + of the Almighty. This idea even seemed to gain a kind of + traditional support from the mystic significance, without known + historical origin, which has for many ages, and among widely + separated peoples, been attached to the remarkable group of + which Alcyone is the chief. But since Maedler's time it has + been demonstrated that the Pleiades cannot be the center of + revolution of the universe, and, as already remarked, all + attempts to find or fix such a center have proved abortive. Yet + so powerful was the hold that the theory took upon the popular + imagination, that even today astronomers are often asked if + Alcyone is not the probable site of ``Jerusalem the + Golden.''</p> + + <p>If there were a discoverable center of predominant + gravitative power, to which the motions of all the stars could + be referred, those motions would appear less mysterious, and we + should then be able to conclude that the universe was, as a + whole, a prototype of the subsidiary systems of which it is + composed. We should look simply to the law of gravitation for + an explanation, and, naturally, the center would be placed + within the opening enclosed by the Milky Way. If it were there + the Milky Way itself should exhibit signs of revolution about + it, like a wheel turning upon its hub. No theory of the star + motions as a whole could stand which failed to take account of + the Milky Way as the basis of all. But the very form of that + divided wreath of stars forbids the assumption of its + revolution about a center. Even if it could be conceived as a + wheel having no material center it would not have the form + which it actually presents. As was shown in Chapter 2, there is + abundant evidence of motion in the Milky Way; but it is not + motion of the system as a whole, but motion affecting its + separate parts. Instead of all moving one way, the galactic + stars, as far as their movements can be inferred, are governed + by local influences and conditions. They appear to travel + crosswise and in contrary directions, and perhaps they eddy + around foci where great numbers have assembled; but of a + universal revolution involving the entire mass we have no + evidence.</p> + + <p>Most of our knowledge of star motions, called ``proper + motions,'' relates to individual stars and to a few groups + which happen to be so near that the effects of their movements + are measurable. In some cases the motion is so rapid (not in + appearance, but in reality) that the chief difficulty is to + imagine how it can have been imparted, and what will eventually + become of the ``runaways.'' Without a collision, or a series of + very close approaches to great gravitational centers, a star + traveling through space at the rate of two hundred or three + hundred miles per second could not be arrested or turned into + an orbit which would keep it forever flying within the limits + of the visible universe. A famous example of these speeding + stars is ``1830 Groombridge,'' a star of only the sixth + magnitude, and consequently just visible to the naked eye, + whose motion across the line of sight is so rapid that it moves + upon the face of the sky a distance equal to the apparent + diameter of the moon every 280 years. The distance of this star + is at least 200,000,000,000,000 miles, and may be two or three + times greater, so that its actual speed cannot be less than two + hundred, and may be as much as four hundred, miles per second. + It could be turned into a new course by a close approach to a + great sun, but it could only be stopped by collision, head-on, + with a body of enormous mass. Barring such accidents it must, + as far as we can see, keep on until it has traversed our + stellar system, whence in may escape and pass out into space + beyond, to join, perhaps, one of those other universes of which + we have spoken. Arcturus, one of the greatest suns in the + universe, is also a runaway, whose speed of flight has been + estimated all the way from fifty to two hundred miles per + second. Arcturus, we have every reason to believe, possesses + hundreds of times the mass of our sun -- think, then, of the + prodigious momentum that its motion implies! Sirius moves more + moderately, its motion across the line of sight amounting to + only ten miles per second, but it is at the same time + approaching the sun at about the same speed, its actual + velocity in space being the resultant of the two + displacements.</p> + + <p>What has been said about the motion of Sirius brings us to + another aspect of this subject. The fact is, that in every case + of stellar motion the displacement that we observe represents + only a part of the actual movement of the star concerned. There + are stars whose motion carries them straight toward or straight + away from the earth, and such stars, of course, show no cross + motion. But the vast majority are traveling in paths inclined + from a perpendicular to our line of sight. Taken as a whole, + the stars may be said to be flying about like the molecules in + a mass of gas. The discovery of the radial component in the + movements of the stars is due to the spectroscope. If a star is + approaching, its spectral lines are shifted toward the violet + end of the spectrum by an amount depending upon the velocity of + approach; if it is receding, the lines are correspondingly + shifted toward the red end. Spectroscopic observation, then, + combined with micrometric measurements of the cross motion, + enables us to detect the real movement of the star in space. + Sometimes it happens that a star's radial movement is + periodically reversed; first it approaches, and then it + recedes. This indicates that it is revolving around a near-by + companion, which is often invisible, and superposed upon this + motion is that of the two stars concerned, which together may + be approaching or receding or traveling across the line of + sight. Thus the complications involved in the stellar motions + are often exceedingly great and puzzling.</p> + + <p>Yet another source of complication exists in the movement of + our own star, the sun. There is no more difficult problem in + astronomy than that of disentangling the effects of the solar + motion from those of the motions of the other stars. But the + problem, difficult as it is, has been solved, and upon its + solution depends our knowledge of the speed and direction of + the movement of the solar system through space, for of course + the sun carries its planets with it. One element of the + solution is found in the fact that, as a result of perspective, + the stars toward which we are going appear to move apart toward + all points of the compass, while those behind appear to close + up together. Then the spectroscopic principle already mentioned + is invoked for studying the shift of the lines, which is toward + the violet in the stars ahead of us and toward the red in those + that we are leaving behind. Of course the effects of the + independent motions of the stars must be carefully excluded. + The result of the studies devoted to this subject is to show + that we are traveling at a speed of twelve to fifteen miles per + second in a northerly direction, toward the border of the + constellations Hercules and Lyra. A curious fact is that the + more recent estimates show that the direction is not very much + out of a straight line drawn from the sun to the star Vega, one + of the most magnificent suns in the heavens. But it should not + be inferred from this that Vega is drawing us on; it is too + distant for its gravitation to have such an effect.</p> + + <p>Many unaccustomed thoughts are suggested by this mighty + voyage of the solar system. Whence have we come, and whither do + we go? Every year of our lives we advance at least 375,000,000 + miles. Since the traditional time of Adam the sun has led his + planets through the wastes of space no less than + 225,000,000,000 miles, or more than 2400 times the distance + that separates him from the earth. Go back in imagination to + the geologic ages, and try to comprehend the distance over + which the earth has flown. Where was our little planet when it + emerged out of the clouds of chaos? Where was the sun when his + ``thunder march'' began? What strange constellations shone down + upon our globe when its masters of life were the monstrous + beasts of the ``Age of Reptiles''? A million years is not much + of a span of time in geologic reckoning, yet a million years + ago the earth was farther from its present place in space than + any of the stars with a measurable parallax are now. It was + more than seven times as far as Sirius, nearly fourteen times + as far as Alpha Centauri, three times as far as Vega, and twice + as far as Arcturus. But some geologists demand two hundred, + three hundred, even one thousand million years to enable them + to account for the evolutionary development of the earth and + its inhabitants. In a thousand million years the earth would + have traveled farther than from the remotest conceivable depths + of the Milky Way!</p> + + <p>Other curious reflections arise when we think of the form of + the earth's track as it follows the lead of the sun, in a + journey which has neither known beginning nor conceivable end. + There are probably many minds which have found a kind of + consolation in the thought that every year the globe returns to + the same place, on the same side of the sun. This idea may have + an occult connection with our traditional regard for + anniversaries. When that period of the year returns at which + any great event in our lives has occurred we have the feeling + that the earth, in its annual round, has, in a manner, brought + us back to the scene of that event. We think of the earth's + orbit as a well-worn path which we traverse many times in the + course of a lifetime. It seems familiar to us, and we grow to + have a sort of attachment to it. The sun we are accustomed to + regard as a fixed center in space, like the mill or pump around + which the harnessed patient mule makes his endless circuits. + But the real fact is that the earth never returns to the place + in space where it has once quitted. In consequence of the + motion of the sun carrying the earth and the other planets + along, the track pursued by our globe is a vast spiral in space + continually developing and never returning upon its course. It + is probable that the tracks of the sun and the others stars are + also irregular, and possibly spiral, although, as far as can be + at present determined, they appear to be practically straight. + Every star, wherever it may be situated, is attracted by its + fellow-stars from many sides at once, and although the force is + minimized by distance, yet in the course of many ages its + effects must become manifest.</p> + + <p>Looked at from another side, is there not something + immensely stimulating and pleasing to the imagination in the + idea of so stupendous a journey, which makes all of us the + greatest of travelers? In the course of a long life a man is + transported through space thirty thousand million miles; + Halley's Comet does not travel one-quarter as far in making one + of its immense circuits. And there are adventures on this + voyage of which we are just beginning to learn to take account. + Space is full of strange things, and the earth must encounter + some of them as it advances through the unknown. Many singular + speculations have been indulged in by astronomers concerning + the possible effects upon the earth of the varying state of the + space that it traverses. Even the alternation of hot and + glacial periods has sometimes been ascribed to this source. + When tropical life flourished around the poles, as the remains + in the rocks assure us, the needed high temperature may, it has + been thought, have been derived from the presence of the earth + in a warm region of space. Then, too, there is a certain + interest for us in the thought of what our familiar planet has + passed through. We cannot but admire it for its long journeying + as we admire the traveler who comes to us from remote and + unexplored lands, or as we gaze with a glow of interest upon + the first locomotive that has crossed a continent, or a ship + that has visited the Arctic or Antarctic regions. If we may + trust the indications of the present course, the earth, piloted + by the sun, has come from the Milky Way in the far south and + may eventually rejoin that mighty band of stars in the far + north.</p> + + <p>While the stars in general appear to travel independently of + one another, except when they are combined in binary or trinary + systems, there are notable exceptions to this rule. In some + quarters of the sky we behold veritable migrations of entire + groups of stars whose members are too widely separated to show + any indications of revolution about a common center of gravity. + This leads us back again to the wonderful group of the + Pleiades. All of the principle stars composing that group are + traveling in virtually parallel lines. Whatever force set them + going evidently acted upon all alike. This might be explained + by the assumption that when the original projective force acted + upon them they were more closely united than they are at + present, and that in drifting apart they have not lost the + impulse of the primal motion. Or it may be supposed that they + are carried along by some current in space, although it would + be exceedingly difficult, in the present state of our + knowledge, to explain the nature of such a current. Yet the + theory of a current has been proposed. As to an attractive + center around which they might revolve, none has been found. + Another instance of similar ``star-drift'' is furnished by five + of the seven stars constituting the figure of the ``Great + Dipper.'' In this case the stars concerned are separated very + widely, the two extreme ones by not less than fifteen degrees, + so that the idea of a common motion would never have been + suggested by their aspect in the sky; and the case becomes the + more remarkable from the fact that among and between them there + are other stars, some of the same magnitude, which do not share + their motion, but are traveling in other directions. Still + other examples of the same phenomenon are found in other parts + of the sky. Of course, in the case of compact star-clusters, it + is assumed that all the members share a like motion of + translation through space, and the same is probably true of + dense star-swarms and star-clouds.</p> + + <p>The whole question of star-drift has lately assumed a new + phase, in consequence of the investigations of Kapteyn, Dyson, + and Eddington on the ``systematic motions of the stars.'' This + research will, it is hoped, lead to an understanding of the + general law governing the movements of the whole body of stars + constituting the visible universe. Taking about eleven hundred + stars whose proper motions have been ascertained with an + approach to certainty, and which are distributed in all parts + of the sky, it has been shown that there exists an apparent + double drift, in two independent streams, moving in different + and nearly opposed directions. The apex of the motion of what + is called ``Stream I'' is situated, according to Professor + Kapteyn, in right ascension 85°, declination south 11°, + which places it just south of the constellation Orion; while + the apex of ``Stream II'' is in right ascension 260°, + declination south 48°, placing it in the constellation Ara, + south of Scorpio. The two apices differ very nearly 180° in + right ascension and about 120° in declination. The + discovery of these vast star-streams, if they really exist, is + one of the most extraordinary in modern astronomy. It offers + the correlation of stellar movements needed as the basis of a + theory of those movements, but it seems far from revealing a + physical cause for them. As projected against the celestial + sphere the stars forming the two opposite streams appear + intermingled, some obeying one tendency and some the other. As + Professor Dyson has said, the hypothesis of this double + movement is of a revolutionary character, and calls for further + investigation. Indeed, it seems at first glance not less + surprising than would be the observation that in a snow-storm + the flakes over our heads were divided into two parties and + driving across each other's course in nearly opposite + directions, as if urged by interpenetrating winds.</p> + + <p>But whatever explanation may eventually be found for the + motions of the stars, the knowledge of the existence of those + motions must always afford a new charm to the contemplative + observer of the heavens, for they impart a sense of life to the + starry system that would otherwise be lacking. A stagnant + universe, with every star fixed immovably in its place, would + not content the imagination or satisfy our longing for + ceaseless activity. The majestic grandeur of the evolutions of + the celestial hosts, the inconceivable vastness of the fields + of space in which they are executed, the countless numbers, the + immeasurable distances, the involved convolutions, the flocking + and the scattering, the interpenetrating marches and + countermarches, the strange community of impulsion affecting + stars that are wide apart in space and causing them to traverse + the general movement about them like aides and despatch-bearers + on a battle-field -- all these arouse an intensity of interest + which is heightened by the mystery behind them.</p> + + <p><strong>The Passing of the Constellations</strong></p> + + <p>From a historical and picturesque point of view, one of the + most striking results of the motions of the stars described in + the last chapter is their effect upon the forms of the + constellations, which have been watched and admired by mankind + from a period so early that the date of their invention is now + unknown. The constellations are formed by chance combinations + of conspicuous stars, like figures in a kaleidoscope, and if + our lives were commensurate with the æons of cosmic + existence we should perceive that the kaleidoscope of the + heavens was ceaselessly turning and throwing the stars into new + symmetries. Even if the stars stood fast, the motion of the + solar system would gradually alter the configurations, as the + elements of a landscape dissolve and recombine in fresh + groupings with the traveler's progress amid them. But with the + stars themselves all in motion at various speeds and in many + directions, the changes occur more rapidly. Of course, + ``rapid'' is here understood in a relative sense; the wheel of + human history to an eye accustomed to the majestic progression + of the universe would appear to revolve with the velocity of a + whirling dynamo. Only the deliberation of geological movements + can be contrasted with the evolution and devolution of the + constellations.</p> + + <p>And yet this secular fluctuation of the constellation + figures is not without keen interest for the meditative + observer. It is another reminder of the swift mutability of + terrestial affairs. To the passing glance, which is all that we + can bestow upon these figures, they appear so immutable that + they have been called into service to form the most lasting + records of ancient thought and imagination that we possess. In + the forms of the constellations, the most beautiful, and, in + imaginative quality, the finest, mythology that the world has + ever known has been perpetuated. Yet, in a broad sense, this + scroll of human thought imprinted on the heavens is as + evanescent as the summer clouds. Although more enduring than + parchment, tombs, pyramids, and temples, it is as far as they + from truly eternizing the memory of what man has fancied and + done.</p> + + <p>Before studying the effects that the motions of the stars + have had and will have upon the constellations, it is worth + while to consider a little further the importance of the + stellar pictures as archives of history. To emphasize the + importance of these effects it is only necessary to recall that + the constellations register the oldest traditions of our race. + In the history of primeval religions they are the most valuable + of documents. Leaving out of account for the moment the more + familiar mythology of the Greeks, based on something older yet, + we may refer for illustration to that of the mysterious Maya + race of America. At Izamal, in Yucatan, says Mr Stansbury + Hagar, is a group of ruins perched, after the Mexican and + Central-American plan, on the summits of pyramidal mounds which + mark the site of an ancient theogonic center of the Mayas. Here + the temples all evidently refer to a cult based upon the + constellations as symbols. The figures and the names, of + course, were not the same as those that we have derived from + our Aryan ancestors, but the star groups were the same or + nearly so. For instance, the loftiest of the temples at Izamal + was connected with the sign of the constellation known to us as + Cancer, marking the place of the sun at the summer solstice, at + which period the sun was supposed to descend at noon like a + great bird of fire and consume the offerings left upon the + altar. Our Scorpio was known to the Mayas as a sign of the + ``Death God.'' Our Libra, the ``Balance,'' with which the idea + of a divine weighing out of justice has always been connected, + seems to be identical with the Mayan constellation + Teoyaotlatohua, with which was associated a temple where dwelt + the priests whose special business it was to administer justice + and to foretell the future by means of information obtained + from the spirits of the dead. Orion, the ``Hunter'' of our + celestial mythology, was among the Mayas a ``Warrior,'' while + Sagittarius and others of our constellations were known to them + (under different names, of course), and all were endowed with a + religious symbolism. And the same star figures, having the same + significance, were familiar to the Peruvians, as shown by the + temples at Cuzco. Thus the imagination of ancient America + sought in the constellations symbols of the unchanging + gods.</p> + + <p>But, in fact, there is no nation and no people that has not + recognized the constellations, and at one period or another in + its history employed them in some symbolic or representative + capacity. As handled by the Greeks from prehistoric times, the + constellation myths became the very soul of poetry. The + imagination of that wonderful race idealized the principal star + groups so effectively that the figures and traditions thus + attached to them have, for civilized mankind, displaced all + others, just as Greek art in its highest forms stands without + parallel and eclipses every rival. The Romans translated no + heroes and heroines of the mythical period of their history to + the sky, and the deified Cæsars never entered that lofty + company, but the heavens are filled with the early myths of the + Greeks. Herakles nightly resumes his mighty labors in the + stars; Zeus, in the form of the white ``Bull,'' Taurus, bears + the fair Europa on his back through the celestial waves; + Andromeda stretches forth her shackled arms in the star-gemmed + ether, beseeching aid; and Perseus, in a blaze of diamond + armor, revives his heroic deeds amid sparkling clouds of + stellar dust. There, too, sits Queen Cassiopeia in her dazzling + chair, while the Great King, Cepheus, towers gigantic over the + pole. Professor Young has significantly remarked that a great + number of the constellations are connected in some way or other + with the Argonautic Expedition -- that strangely fascinating + legend of earliest Greek story which has never lost its charm + for mankind. In view of all this, we may well congratulate + ourselves that the constellations will outlast our time and the + time of countless generations to follow us; and yet they are + very far from being eternal. Let us now study some of the + effects of the stellar motions upon them.</p> + + <p>We begin with the familiar figure of the ``Great Dipper.'' + He who has not drunk inspiration from its celestial bowl is not + yet admitted to the circle of Olympus. This figure is made up + of seven conspicuous stars in the constellation Ursa Major, the + ``Greater Bear.'' The handle of the ``Dipper'' corresponds to + the tail of the imaginary ``Bear,'' and the bowl lies upon his + flank. In fact, the figure of a dipper is so evident and that + of a bear so unevident, that to most persons the ``Great + Dipper'' is the only part of the constellation that is + recognizable. Of the seven stars mentioned, six are of nearly + equal brightness, ranking as of the second magnitude, while the + seventh is of only the third magnitude. The difference is very + striking, since every increase of one magnitude involves an + increase of two-and-a-half times in brightness. There appears + to be little doubt that the faint star, which is situated at + the junction of the bowl and the handle, is a variable of long + period, since three hundred years ago it was as bright as its + companions. But however that may be, its relative faintness at + the present time interferes but little with the perfection of + the ``Dipper's'' figure. In order the more readily to + understand the changes which are taking place, it will be well + to mention both the names and the Greek letters which are + attached to the seven stars. Beginning at the star in the upper + outer edge of the rim of the bowl and running in regular order + round the bottom and then out to the end of the handle, the + names and letters are as follows: Dubhe ({\alpha}), Merak + ({\beta}), Phaed ({\gamma}), Megrez ({\delta}), Alioth + ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez + is the faint star already mentioned at the junction of the bowl + and handle, and Mizar, in the middle of the handle, has a + close, naked-eye companion which is named Alcor. The Arabs + called this singular pair of stars ``The Horse and Rider.'' + Merak and Duhbe are called ``The Pointers,'' because an + imaginary line drawn northward through them indicates the Pole + Star.</p> + + <p>Now it has been found that five of these stars -- + <em>viz.,</em> Merak, Phaed, Megrez, Alioth, and Mizar (with + its comrade) -- are moving with practically the same speed in + an easterly direction, while the other two, Dubhe and + Benetnasch, are simultaneously moving westward, the motions of + Benetnasch being apparently more rapid. The consequence of + these opposed motions is, of course, that the figure of the + ``Dipper'' cannot always have existed and will not continue to + exist. In the accompanying diagrams it has been thought + interesting to show the relative positions of these seven + stars, as seen from the point which the earth now occupies, + both in the past and in the future. Arrows attached to the + stars in the figure representing the present appearance of the + ``Dipper'' indicate the directions of the motions and the + distances over which they will carry the stars in a period of + about five hundred centuries. The time, no doubt, seems long, + but remember the vast stretch of ages through which the earth + has passed, and then reflect that no reason is apparent why our + globe should not continue to be a scene of animation for ten + thousand centuries yet to come. The fact that the little star + Alcor placed so close to Mizar should accompany the latter in + its flight is not surprising, but that two of the principal + stars of the group should be found moving in a direction + directly opposed to that pursued by the other five is + surprising in the highest degree; and it recalls the strange + theory of a double drift affecting all the stars, to which + attention was called in the preceding chapter. It would appear + that Benetnasch and Dubhe belong to one ``current,'' and Merak, + Phaed, Megrez, Alioth, and Mizar to the other. As far as is + known, the motion of the seven stars are not shared by the + smaller stars scattered about them, but on the theory of + currents there should be such a community of motion, and + further investigation may reveal it.</p> + + <p>From the ``Great Dipper'' we turn to a constellation hardly + less conspicuous and situated at an equal distance from the + pole on the other side -- Cassiopeia. This famous star-group + commemorating the romantic Queen of Ethiopia whose vain + boasting of her beauty was punished by the exposure of her + daughter Andromeda to the ``Sea Monster,'' is well-marked by + five stars which form an irregular letter ``W'' with its open + side toward the pole. Three of these stars are usually ranked + as of the second magnitude, and two of the third; but to + ordinary observation they appear of nearly equal brightness, + and present a very striking picture. They mark out the chair + and a part of the figure of the beautiful queen. Beginning at + the right-hand, or western, end of the ``W,'' their Greek + letter designations are: Beta ({\beta}), Alpha ({\alpha}), + Gamma ({\gamma}), Delta ({\delta}), and Epsilon ({\epsilon}). + Four of them, Beta, Alpha, Delta, and Epsilon are traveling + eastwardly at various speeds, while the fifth, Gamma, moves in + a westerly direction. The motion of Beta is more rapid than + that of any of the others. It should be said, however, that no + little uncertainty attaches to the estimates of the rate of + motion of stars which are not going very rapidly, and different + observers often vary considerably in their results.</p> + + <p>In the beautiful ``Northern Crown,'' one of the most perfect + and charming of all the figures to be found in the stars, the + alternate combining and scattering effects of the stellar + motions are shown by comparing the appearance which the + constellation must have had five hundred centuries ago with + that which it has at present and that which it will have in the + future. The seven principle stars of the asterism, forming a + surprisingly perfect coronet, have movements in three + directions at right angles to one another. That in these + circumstances they should ever have arrived at positions giving + them so striking an appearance of definite association is + certainly surprising; from its aspect one would have expected + to find a community of movement governing the brilliants of the + ``Crown,'' but instead of that we find evidence that they will + inevitably drift apart and the beautiful figure will + dissolve.</p> + + <p>A similar fate awaits such asterisms as the ``Northern + Cross'' in Cygnus; the ``Crow'' (Corvus), which stands on the + back of the great ``Sea Serpent,'' Hydra, and pecks at his + scales; ``Job's Coffin'' (Delphinus); the ``Great Square of + Pegasus''; the ``Twins'' (Gemini); the beautiful ``Sickle'' in + Leo; and the exquisite group of the Hyades in Taurus. In the + case of the Hyades, two controlling movements are manifest: + one, affecting five of the stars which form the well-known + figure of a letter ``V,'' is directed northerly; the other, + which controls the direction of two stars, has an easterly + trend. The chief star of the group, Aldebaran, one of the + finest of all stars both for its brilliance and its color, is + the most affected by the easterly motion. In time it will drift + entirely out of connection with its present neighbors. Although + the Hyades do not form so compact a group as the Pleiades in + the same constellation, yet their appearance of relationship is + sufficient to awaken a feeling of surprise over the fact that, + as with the stars of the ``Dipper,'' their association is only + temporary or apparent.</p> + + <p>The great figure of Orion appears to be more lasting, not + because its stars are physically connected, but because of + their great distance, which renders their movements too + deliberate to be exactly ascertained. Two of the greatest of + its stars, Betelgeuse and Rigel, possess, as far as has been + ascertained, no perceptible motion across the line of sight, + but there is a little movement perceptible in the ``Belt.'' At + the present time this consists of an almost perfect straight + line, a row of second-magnitude stars about equally spaced and + of the most striking beauty. In the course of time, however, + the two right-hand stars, Mintaka and Alnilam (how fine are + these Arabic star names!) will approach each other and form a + naked-eye double, but the third, Alnita, will drift away + eastward, so that the ``Belt'' will no longer exist.</p> + + <p>For one more example, let us go to the southern hemisphere, + whose most celebrated constellation, the ``Southern Cross,'' + has found a place in all modern literatures, although it has no + claim to consideration on account of association with ancient + legends. This most attractive asterism, which has never ceased + to fascinate the imagination of Christendom since it was first + devoutly described by the early explorers of the South, is but + a passing collocation of brilliant stars. Yet even in its + transfigurations it has been for hundreds of centuries, and + will continue to be for hundreds of centuries to come, a most + striking object in the sky. Our figures show its appearance in + three successive phases: first, as it was fifty thousand years + ago (viewed from the earth's present location); second, as it + is in our day; and, third, as it will be an equal time in the + future. The nearness of these bright stars to one another -- + the length of the longer beam of the ``Cross'' is only six + degrees -- makes this group very noticeable, whatever the + arrangement of its components may be. The largest star, at the + base of the ``Cross,'' is of the first magnitude, two of the + others are of the second magnitude, and the fourth is of the + third. Other stars, not represented in the figures, increase + the effect of a celestial blazonry, although they do not help + the resemblance to a cross.</p> + + <p>But since the motion of the solar system itself will, in the + course of so long a period as fifty thousand years, produce a + great change in the perspective of the heavens as seen from the + earth, by carrying us nearly nineteen trillion miles from our + present place, why, it may be asked, seek to represent future + appearances of the constellations which we could not hope to + see, even if we could survive so long? The answer is: Because + these things aid the mind to form a picture of the effects of + the mobility of the starry universe. Only by showing the + changes from some definite point of view can we arrive at a due + comprehension of them. The constellations are more or less + familiar to everybody, so that impending changes of their forms + must at once strike the eye and the imagination, and make + clearer the significance of the movements of the stars. If the + future history of mankind is to resemble its past and if our + race is destined to survive yet a million years, then our + remote descendents will see a ``new heavens'' if not a ``new + earth,'' and will have to invent novel constellations to + perpetuate their legends and mythologies.</p> + + <p>If our knowledge of the relative distances of the stars were + more complete, it would be an interesting exercise in celestial + geometry to project the constellations probably visible to the + inhabitants of worlds revolving around some of the other suns + of space. Our sun is too insignificant for us to think that he + can make a conspicuous appearance among them, except, perhaps, + in a few cases. As seen, for instance, from the nearest known + star, Alpha Centauri, the sun would appear of the average first + magnitude, and consequently from that standpoint he might be + the gem of some little constellation which had no Sirius, or + Arcturus, or Vega to eclipse him with its superior splendor. + But from the distance of the vast majority of the stars the sun + would probably be invisible to the naked eye, and as seen from + nearer systems could only rank as a fifth or sixth magnitude + star, unnoticed and unknown except by the star-charting + astronomer.</p> + + <p><strong>Conflagrations in the Heavens</strong></p> + + <p>Suppose it were possible for the world to take fire and burn + up -- as some pessimists think that it will do when the Divine + wrath shall have sufficiently accumulated against it -- nobody + out of our own little corner of space would ever be aware of + the catastrophe! With all their telescopes, the astronomers + living in the golden light of Arcturus or the diamond blaze of + Canopus would be unable to detect the least glimmer of the + conflagration that had destroyed the seat of Adam and his + descendents, just as now they are totally ignorant of its + existence.</p> + + <p>But at least fifteen times in the course of recorded history + men looking out from the earth have beheld in the remote depths + of space great outbursts of fiery light, some of them more + splendidly luminous than anything else in the firmament except + the sun! If <em>they</em> were conflagrations, how many million + worlds like ours were required to feed their blaze?</p> + + <p>It is probable that ``temporary'' or ``new'' stars, as these + wonderful apparitions are called, really are conflagrations; + not in the sense of a bonfire or a burning house or city, but + in that of a sudden eruption of inconceivable heat and light, + such as would result from the stripping off the shell of an + encrusted sun or the crashing together of two mighty orbs + flying through space with a hundred times the velocity of the + swiftest cannon-shot.</p> + + <p>Temporary stars are the rarest and most erratic of + astronomical phenomena. The earliest records relating to them + are not very clear, and we cannot in every instance be certain + that it was one of these appearances that the ignorant and + superstitious old chroniclers are trying to describe. The first + temporary star that we are absolutely sure of appeared in 1572, + and is known as ``Tycho's Star,'' because the celebrated Danish + astronomer (whose remains, with his gold-and-silver artificial + nose -- made necessary by a duel -- still intact, were + disinterred and reburied in 1901) was the first to perceive it + in the sky, and the most assiduous and successful in his + studies of it. As the first fully accredited representative of + its class, this new star made its entry upon the scene with + becoming <em>éclat.</em> It is characteristic of these + phenomena that they burst into view with amazing suddenness, + and, of course, entirely unexpectedly. Tycho's star appeared in + the constellation Cassiopeia, near a now well-known and + much-watched little star named Kappa, on the evening of + November 11, 1572. The story has often been repeated, but it + never loses interest, how Tycho, going home that evening, saw + people in the street pointing and staring at the sky directly + over their heads, and following the direction of their hands + and eyes he was astonished to see, near the zenith, an unknown + star of surpassing brilliance. It outshone the planet Jupiter, + and was therefore far brighter than the first magnitude. There + was not another star in the heavens that could be compared with + it in splendor. Tycho was not in all respects free from the + superstitions of his time -- and who is? -- but he had the true + scientific instinct, and immediately he began to study the + stranger, and to record with the greatest care every change in + its aspect. First he determined as well as he could with the + imperfect instruments of his day, many of which he himself had + invented, the precise location of the phenomena in the sky. + Then he followed the changes that it underwent. At first it + brightened until its light equaled or exceeded that of the + planet Venus at her brightest, a statement which will be + appreciated at its full value by anyone who has ever watched + Venus when she plays her dazzling rôle of ``Evening + Star,'' flaring like an arc light in the sunset sky. It even + became so brilliant as to be visible in full daylight, since, + its position being circumpolar, it never set in the latitude of + Northern Europe. Finally it began to fade, turning red as it + did so, and in March, 1574, it disappeared from Tycho's + searching gaze, and has never been seen again from that day to + this. None of the astronomers of the time could make anything + of it. They had not yet as many bases of speculation as we + possess today.</p> + + <p>Tycho's star has achieved a romantic reputation by being + fancifully identified with the ``Star of Bethlehem,'' said to + have led the wondering Magi from their eastern deserts to the + cradle-manger of the Savior in Palestine. Many attempts have + been made to connect this traditional ``star'' with some known + phenomenon of the heavens, and none seems more idle than this. + Yet it persistently survives, and no astronomer is free from + eager questions about it addressed by people whose imagination + has been excited by the legend. It is only necessary to say + that the supposition of a connection between the phenomenon of + the Magi and Tycho's star is without any scientific foundation. + It was originally based on an unwarranted assumption that the + star of Tycho was a variable of long period, appearing once + every three hundred and fifteen years, or thereabout. If that + were true there would have been an apparition somewhere near + the traditional date of the birth of Christ, a date which is + itself uncertain. But even the data on which the assumption was + based are inconsistent with the theory. Certain monkish records + speak of something wonderful appearing in the sky in the years + 1264 and 945, and these were taken to have been outbursts of + Tycho's star. Investigation shows that the records more + probably refer to comets, but even if the objects seen were + temporary stars, their dates do not suit the hypothesis; from + 945 to 1264 there is a gap of 319 years, and from 1264 to 1572 + one of only 308 years; moreover 337 years have now (1909) + elapsed since Tycho saw the last glimmer of his star. Upon a + variability so irregular and uncertain as that, even if we felt + sure that it existed, no conclusion could be found concerning + an apparition occurring 2000 years ago.</p> + + <p>In the year 1600 (the year in which Giordano Bruno was + burned at the stake for teaching that there is more than one + physical world), a temporary star of the third magnitude broke + out in the constellation Cygnus, and curiously enough, + considering the rarity of such phenomena, only four years later + another surprisingly brilliant one appeared in the + constellation Ophiuchus. This is often called ``Kepler's + star,'' because the great German astronomer devoted to it the + same attention that Tycho had given to the earlier phenomenon. + It, too, like Tycho's, was at first the brightest object in the + stellar heavens, although it seems never to have quite equaled + its famous predecessor in splendor. It disappeared after a + year, also turning of a red color as it became more faint. We + shall see the significance of this as we go on. Some of + Kepler's contemporaries suggested that the outburst of this + star was due to a meeting of atoms in space, and idea bearing a + striking resemblance to the modern theory of ``astronomical + collisions.''</p> + + <p>In 1670, 1848, and 1860 temporary stars made their + appearance, but none of them was of great brilliance. In 1866 + one of the second magnitude broke forth in the ``Northern + Crown'' and awoke much interest, because by that time the + spectroscope had begun to be employed in studying the + composition of the stars, and Huggins demonstrated that the new + star consisted largely of incandescent hydrogen. But this star, + apparently unlike the others mentioned, was not absolutely new. + Before its outburst it had shown as a star of the ninth + magnitude (entirely invisible, of course, to the naked eye), + and after about six weeks it faded to its original condition in + which it has ever since remained. In 1876 a temporary star + appeared in the constellation Cygnus, and attained at one time + the brightness of the second magnitude. Its spectrum and its + behavior resembled those of its immediate predecessor. In 1885, + astronomers were surprised to see a sixth-magnitude star + glimmering in the midst of the hazy cloud of the great + Andromeda Nebula. It soon absolutely disappeared. Its spectrum + was remarkable for being ``continuous,'' like that of the + nebula itself. A continuous spectrum is supposed to represent a + body, or a mass, which is either solid or liquid, or composed + of gas under great pressure. In January, 1892, a new star was + suddenly seen in the constellation Auriga. It never rose much + above the fourth magnitude, but it showed a peculiar spectrum + containing both bright and dark lines of hydrogen.</p> + + <p>But a bewildering surprise was now in store; the world was + to behold at the opening of the twentieth century such a + celestial spectacle as had not been on view since the times of + Tycho and Kepler. Before daylight on the morning of February + 22, 1901, the Rev. Doctor Anderson, of Edinburgh, an amateur + astronomer, who had also been the first to see the new star in + Auriga, beheld a strange object in the constellation Perseus + not far from the celebrated variable star Algol. He recognized + its character at once, and immediately telegraphed the news, + which awoke the startled attention of astronomers all over the + world. When first seen the new star was no brighter than Algol + (less than the second magnitude), but within twenty-four hours + it was ablaze, outshining even the brilliant Capella, and far + surpassing the first magnitude. At the spot in the sky where it + appeared nothing whatever was visible on the night before its + coming. This is known with certainty because a photograph had + been made of that very region on February 21, and this + photograph showed everything down to the twelfth magnitude, but + not a trace of the stranger which burst into view between the + 21st and the 22nd like the explosion of a rocket.</p> + + <p>Upon one who knew the stars the apparition of this intruder + in a well-known constellation had the effect of a sudden + invasion. The new star was not far west of the zenith in the + early evening, and in that position showed to the best + advantage. To see Capella, the hitherto unchallenged ruler of + that quarter of the sky, abased by comparison with this + stranger of alien aspect, for there was always an unfamiliar + look about the ``nova,'' was decidedly disconcerting. It seemed + to portend the beginning of a revolution in the heavens. One + could understand what the effect of such an apparition must + have been in the superstitious times of Tycho. The star of + Tycho had burst forth on the northern border of the Milky Way; + this one was on its southern border, some forty-five degrees + farther east.</p> + + <p>Astronomers were well-prepared this time for the scientific + study of the new star, both astronomical photography and + spectroscopy having been perfected, and the results of their + investigations were calculated to increase the wonder with + which the phenomenon was regarded. The star remained at its + brightest only a few days; then, like a veritable + conflagration, it began to languish; and, like the reflection + of a dying fire, as it sank it began to glow with the red color + of embers. But its changes were spasmodic; once about every + three days it flared up only to die away again. During these + fluctuations its light varied alternately in the ratio of one + to six. Finally it took a permanent downward course, and after + a few months the naked eye could no longer perceive it; but it + remained visible with telescopes, gradually fading until it had + sunk to the ninth magnitude. Then another astonishing change + happened: in August photographs taken at the Yerkes Observatory + and at Heidelberg showed that the ``nova'' was <em>surrounded + by a spiral nebula!</em> The nebula had not been there before, + and no one could doubt that it represented a phase of the same + catastrophe that had produced the outburst of the new star. At + one time the star seemed virtually to have disappeared, as if + all its substance had been expanded into the nebulous cloud, + but always there remained a stellar nucleus about which the + misty spiral spread wider and ever wider, like a wave expanding + around a center of disturbance. The nebula too showed a + variability of brightness, and four condensations which formed + in it seemed to have a motion of revolution about the star. As + time went on the nebula continued to expand at a rate which was + computed to be not less than twenty thousand miles per second! + And now the star itself, showing indications of having turned + into a nebula, behaved in a most erratic manner, giving rise to + the suspicion that it was about to burst out again. But this + did not occur, and at length it sunk into a state of lethargy + from which it has to the present time not recovered. But the + nebulous spiral has disappeared, and the entire phenomena as it + now (1909) exists consists of a faint nebulous star of less + than the ninth magnitude.</p> + + <p>The wonderful transformations just described had been + forecast in advance of the discovery of the nebulous spiral + encircling the star by the spectroscopic study of the latter. + At first there was no suggestion of a nebular constitution, but + within a month or two characteristic nebular lines began to + appear, and in less than six months the whole spectrum had been + transformed to the nebular type. In the mean time the shifting + of the spectral lines indicated a complication of rapid motions + in several directions simultaneously. These motions were + estimated to amount to from one hundred to five hundred miles + per second.</p> + + <p>The human mind is so constituted that it feels forced to + seek an explanation of so marvelous a phenomenon as this, even + in the absence of the data needed for a sound conclusion. The + most natural hypothesis, perhaps, is that of a collision. Such + a catastrophe could certainly happen. It has been shown, for + instance, that in infinity of time the earth is sure to be hit + by a comet; in the same way it may be asserted that, if no time + limit is fixed, the sun is certain to run against some obstacle + in space, either another star, or a dense meteor swarm, or one + of the dark bodies which there is every reason to believe + abound around us. The consequences of such a collision are easy + to foretell, provided that we know the masses and the + velocities of the colliding bodies. In a preceding chapter we + have discussed the motions of the sun and stars, and have seen + that they are so swift that an encounter between any two of + them could not but be disastrous. But this is not all; for as + soon as two stars approached within a few million miles their + speed would be enormously increased by their reciprocal + attractions and, if their motion was directed radially with + respect to their centers, they would come together with a crash + that would reduce them both to nebulous clouds. It is true that + the chances of such a ``head-on'' collision are relatively very + small; two stars approaching each other would most probably + fall into closed orbits around their common center of gravity. + If there were a collision it would most likely be a grazing one + instead of a direct front-to-front encounter. But even a close + approach, without any actual collision, would probably prove + disastrous, owing to the tidal influence of each of the bodies + on the other. Suns, in consequence of their enormous masses and + dimensions and the peculiarities of their constitution, are + exceedingly dangerous to one another at close quarters. + Propinquity awakes in them a mutually destructive tendency. + Consisting of matter in the gaseous, or perhaps, in some cases, + liquid, state, their tidal pull upon each other if brought + close together might burst them asunder, and the photospheric + envelope being destroyed the internal incandescent mass would + gush out, bringing fiery death to any planets that were + revolving near. Without regard to the resulting disturbance of + the earth's orbit, the close approach of a great star to the + sun would be in the highest degree perilous to us. But this is + a danger which may properly be regarded as indefinitely remote, + since, at our present location in space, we are certainly far + from every star except the sun, and we may feel confident that + no great invisible body is near, for if there were one we + should be aware of its presence from the effects of its + attraction. As to dark nebulæ which may possibly lie in + the track that the solar system is pursuing at the rate of + 375,000,000 miles per year, that is another question -- and + they, too, could be dangerous!</p> + + <p>This brings us directly back to ``Nova Persei,'' for among + the many suggestions offered to explain its outburst, as well + as those of other temporary stars, one of the most fruitful is + that of a collision between a star and a vast invisible nebula. + Professor Seeliger, of Munich, first proposed this theory, but + it afterward underwent some modifications from others. Stated + in a general form, the idea is that a huge dark body, perhaps + an extinguished sun, encountered in its progress through space + a widespread flock of small meteors forming a dark nebula. As + it plunged into the swarm the friction of the innumerable + collisions with the meteors heated its surface to + incandescence, and being of vast size it then became visible to + us as a new star. Meanwhile the motion of the body through the + nebula, and its rotation upon itself, set up a gyration in the + blazing atmosphere formed around it by the vaporized meteors; + and as this atmosphere spread wider, under the laws of gyratory + motion a rotation in the opposite direction began in the + inflamed meteoric cloud outside the central part of the vortex. + Thus the spectral lines were caused to show motion in opposite + directions, a part of the incandescent mass approaching the + earth simultaneously with the retreat of another part. So the + curious spectroscopic observations before mentioned were + explained. This theory might also account for the appearance of + the nebulous spiral first seen some six months after the + original outburst. The sequent changes in the spectrum of the + ``nova'' are accounted for by this theory on the assumption, + reasonable enough in itself, that at first the invading body + would be enveloped in a vaporized atmosphere of relatively + slight depth, producing by its absorption the fine dark lines + first observed; but that as time went on and the incessant + collisions continued, the blazing atmosphere would become very + deep and extensive, whereupon the appearance of the spectral + lines would change, and bright lines due to the light of the + incandescent meteors surrounding the nucleus at a great + distance would take the place of the original dark ones. The + vortex of meteors once formed would protect the flying body + within from further immediate collisions, the latter now + occurring mainly among the meteors themselves, and then the + central blaze would die down, and the original splendor of the + phenomenon would fade.</p> + + <p>But the theories about Nova Persei have been almost as + numerous as the astronomers who have speculated about it. One + of the most startling of them assumed that the outburst was + caused by the running amuck of a dark star which had + encountered another star surrounded with planets, the renewed + outbreaks of light after the principal one had faded being due + to the successive running down of the unfortunate planets! Yet + another hypothesis is based on what we have already said of the + tidal influence that two close approaching suns would have upon + each other. Supposing two such bodies which had become + encrusted, but remained incandescent and fluid within, to + approach within almost striking distance; they would whirl each + other about their common center of gravity, and at the same + time their shells would burst under the tidal strain, and their + glowing nuclei being disclosed would produce a great outburst + of light. Applying this theory to a ``nova,'' like that of 1866 + in the ``Northern Crown,'' which had been visible as a small + star before the outbreak, and which afterward resumed its + former aspect, we should have to assume that a yet shining sun + had been approached by a dark body whose attraction temporarily + burst open its photosphere. It might be supposed that in this + case the dark body was too far advanced in cooling to suffer + the same fate from the tidal pull of its victim. But a close + approach of that kind would be expected to result in the + formation of a binary system, with orbits of great + eccentricity, perhaps, and after the lapse of a certain time + the outburst should be renewed by another approximation of the + two bodies. A temporary star of that kind would rather be + ranked as a variable.</p> + + <p>The celebrated French astronomer, Janssen, had a different + theory of Nova Persei, and of temporary stars in general. + According to his idea, such phenomena might be the result of + chemical changes taking place in a sun without interference by, + or collision with, another body. Janssen was engaged for many + years in trying to discover evidence of the existence of oxygen + in the sun, and he constructed his observatory on the summit of + Mount Blanc specially to pursue that research. He believed that + oxygen must surely exist in the sun since we find so many other + familiar elements included in the constitution of the solar + globe, and as he was unable to discover satisfactory evidence + of its presence he assumed that it existed in a form unknown on + the earth. If it were normally in the sun's chromosphere, or + coronal atmosphere, he said, it would combine with the hydrogen + which we know is there and form an obscuring envelope of water + vapor. It exists, then, in a special state, uncombined with + hydrogen; but let the temperature of the sun sink to a critical + point and the oxygen will assume its normal properties and + combine with the hydrogen, producing a mighty outburst of light + and heat. This, Janssen thought, might explain the phenomena of + the temporary stars. It would also, he suggested, account for + their brief career, because the combination of the elements + would be quickly accomplished, and then the resulting water + vapor would form an atmosphere cutting off the radiation from + the star within.</p> + + <p>This theory may be said to have a livelier human interest + than some of the others, since, according to it, the sun may + carry in its very constitution a menace to mankind; one does + not like to think of it being suddenly transformed into a + gigantic laboratory for the explosive combination of oxygen and + hydrogen! But while Janssen's theory might do for some + temporary stars, it is inadequate to explain all the phenomena + of Nova Persei, and particularly the appearance of the great + spiral nebula that seemed to exhale from the heart of the star. + Upon the whole, the theory of an encounter between a star and a + dark nebula seems best to fit the observations. By that + hypothesis the expanding billow of light surrounding the core + of the conflagration is very well accounted for, and the + spectroscopic peculiarities are also explained.</p> + + <p>Dr Gustov Le Bon offers a yet more alarming theory, + suggesting that temporary stars are the result of <em>atomic + explosion;</em> but we shall touch upon this more fully in + Chapter 14.</p> + + <p>Twice in the course of this discussion we have called + attention to the change of color invariably undergone by + temporary stars in the later stages of their career. This was + conspicuous with Nova Persei which glowed more and more redly + as it faded, until the nebulous light began to overpower that + of the stellar nucleus. Nothing could be more suggestive of the + dying out of a great fire. Moreover, change of color from white + to red is characteristic of all variable stars of long period, + such as ``Mira'' in Cetus. It is also characteristic of stars + believed to be in the later stages of evolution, and + consequently approaching extinction, like Antares and + Betelgeuse, and still more notably certain small stars which + ``gleam like rubies in the field of the telescope.'' These last + appear to be suns in the closing period of existence as + self-luminous bodies. Between the white stars, such as Sirius + and Rigel, and the red stars, such as Aldebaran and Alpha + Herculis, there is a progressive series of colors from golden + yellow through orange to deep red. The change is believed to be + due to the increase of absorbing vapors in the stellar + atmosphere as the body cools down. In the case of ordinary + stars these changes no doubt occupy many millions of years, + which represent the average duration of solar life; but the + temporary stars run through similar changes in a few months: + they resemble ephemeral insects -- born in the morning and + doomed to perish with the going down of the sun.</p> + + <p><strong>Explosive and Whirling Nebulæ</strong></p> + + <p>One of the most surprising triumphs of celestial photography + was Professor Keeler's discovery, in 1899, that the great + majority of the nebulæ have a distinctly spiral form. + This form, previously known in Lord Rosse's great ``Whirlpool + Nebula,'' had been supposed to be exceptional; now the + photographs, far excelling telescopic views in the revelation + of nebular forms, showed the spiral to be the typical shape. + Indeed, it is a question whether all nebulæ are not to + some extent spiral. The extreme importance of this discovery is + shown in the effect that it has had upon hitherto prevailing + views of solar and planetary evolution. For more than + three-quarters of a century Laplace's celebrated hypothesis of + the manner of origin of the solar system from a rotating and + contracting nebula surrounding the sun had guided speculation + on that subject, and had been tentatively extended to cover the + evolution of systems in general. The apparent forms of some of + the nebulæ which the telescope had revealed were + regarded, and by some are still regarded, as giving visual + evidence in favor of this theory. There is a ``ring nebula'' in + Lyra with a central star, and a ``planetary nebula'' in Gemini + bearing no little resemblance to the planet Saturn with its + rings, both of which appear to be practical realizations of + Laplace's idea, and the elliptical rings surrounding the + central condensation of the Andromeda Nebula may be cited for + the same kind of proof.</p> + + <p>But since Keeler's discovery there has been a decided + turning away of speculation another way. The form of the spiral + nebulæ seems to be entirely inconsistent with the theory + of an originally globular or disk-shaped nebula condensing + around a sun and throwing or leaving off rings, to be + subsequently shaped into planets. Some astronomers, indeed, now + reject Laplace's hypothesis <em>in toto,</em> preferring to + think that even our solar system originated from a spiral + nebula. Since the spiral type prevails among the existing + nebulæ, we must make any mechanical theory of the + development of stars and planetary systems from them accord + with the requirements which that form imposes. A glance at the + extraordinary variations upon the spiral which Professor + Keeler's photographs reveal is sufficient to convince one of + the difficulty of the task of basing a general theory upon + them. In truth, it is much easier to criticize Laplace's + hypothesis than to invent a satisfactory substitute for it. If + the spiral nebulæ seem to oppose it there are other + nebulæ which appear to support it, and it may be that no + one fixed theory can account for all the forms of stellar + evolution in the universe. Our particular planetary system may + have originated very much as the great French mathematician + supposed, while others have undergone, or are now undergoing, a + different process of development. There is always a too strong + tendency to regard an important new discovery and the theories + and speculations based upon it as revolutionizing knowledge, + and displacing or overthrowing everything that went before. + Upon the plea that ``Laplace only made a guess'' more recent + guesses have been driven to extremes and treated by injudicious + exponents as ``the solid facts at last.''</p> + + <p>Before considering more recent theories than Laplace's, let + us see what the nature of the photographic revelations is. The + vast celestial maelstrom discovered by Lord Rosse in the + ``Hunting Dogs'' may be taken as the leading type of the spiral + nebulæ, although there are less conspicuous objects of + the kind which, perhaps, better illustrate some of their + peculiarities. Lord Rosse's nebula appears far more wonderful + in the photographs than in his drawings made with the aid of + his giant reflecting telescope at Parsonstown, for the + photographic plate records details that no telescope is capable + of showing. Suppose we look at the photograph of this object as + any person of common sense would look at any great and strange + natural phenomenon. What is the first thing that strikes the + mind? It is certainly the appearance of violent whirling + motion. One would say that the whole glowing mass had been spun + about with tremendous velocity, or that it had been set + rotating so rapidly that it had become the victim of + ``centrifugal force,'' one huge fragment having broken loose + and started to gyrate off into space. Closer inspection shows + that in addition to the principal focus there are various + smaller condensations scattered through the mass. These are + conspicuous in the spirals. Some of them are stellar points, + and but for the significance of their location we might suppose + them to be stars which happen to lie in a line between us and + the nebula. But when we observe how many of them follow most + faithfully the curves of the spirals we cannot but conclude + that they form an essential part of the phenomenon; it is not + possible to believe that their presence in such situations is + merely fortuitous. One of the outer spirals has at least a + dozen of these star-like points strung upon it; some of them + sharp, small, and distinct, others more blurred and nebulous, + suggesting different stages of condensation. Even the part + which seems to have been flung loose from the main mass has, in + addition to its central condensation, at least one stellar + point gleaming in the half-vanished spire attached to it. Some + of the more distant stars scattered around the ``whirlpool'' + look as if they too had been shot out of the mighty vortex, + afterward condensing into unmistakable solar bodies. There are + at least two curved rows of minute stars a little beyond the + periphery of the luminous whirl which clearly follow lines + concentric with those of the nebulous spirals. Such facts are + simply dumbfounding for anyone who will bestow sufficient + thought upon them, for these are <em>suns,</em> though they may + be small ones; and what a birth is that for a sun!</p> + + <p>Look now again at the glowing spirals. We observe that + hardly have they left the central mass before they begin to + coagulate. In some places they have a ``ropy'' aspect; or they + are like peascods filled with growing seeds, which eventually + will become stars. The great focus itself shows a similar + tendency, especially around its circumference. The sense that + it imparts of a tremendous shattering force at work is + overwhelming. There is probably more matter in that whirling + and bursting nebula than would suffice to make a hundred solar + systems! It must be confessed at once that there is no + confirmation of the Laplacean hypothesis here; but what + hypothesis will fit the facts? There is one which it has been + claimed does so, but we shall come to that later. In the + meanwhile, as a preparation, fix in the memory the appearance + of that second spiral mass spinning beside its master which + seems to have spurned it away.</p> + + <p>For a second example of the spiral nebulæ look at the + one in the constellation Triangulum. <em>God, how hath the + imagination of puny man failed to comprehend Thee!</em> Here is + creation through destruction with a vengeance! The spiral form + of the nebula is unmistakable, but it is half obliterated amid + the turmoil of flying masses hurled away on all sides with + tornadic fury. The focus itself is splitting asunder under the + intolerable strain, and in a little while, as time is reckoned + in the Cosmos, it will be gyrating into stars. And then look at + the cyclonic rain of already finished stars whirling round the + outskirts of the storm. Observe how scores of them are yet + involved in the fading streams of the nebulous spirals; see how + they have been thrown into vast loops and curves, of a beauty + that half redeems the terror of the spectacle enclosed within + their lines -- like iridescent cirri hovering about the edges + of a hurricane. And so again are suns born!</p> + + <p>Let us turn to the exquisite spiral in Ursa Major; how + different its aspect from that of the other! One would say that + if the terrific coil in Triangulum has all but destroyed itself + in its fury, this one on the contrary has just begun its + self-demolition. As one gazes one seems to see in it the + smooth, swift, accelerating motion that precedes catastrophe. + The central part is still intact, dense, and uniform in + texture. How graceful are the spirals that smoothly rise from + its oval rim and, gemmed with little stars, wind off into the + darkness until they have become as delicate as threads of + gossamer! But at bottom the story told here is the same -- + creation by gyration!</p> + + <p>Compare with the above the curious mass in Cetus. Here the + plane of the whirling nebula nearly coincides with our line of + sight and we see the object at a low angle. It is far advanced + and torn to shreds, and if we could look at it perpendicularly + to its plane it is evident that it would closely resemble the + spectacle in Triangulum.</p> + + <p>Then take the famous Andromeda Nebula (see Frontispiece), + which is so vast that notwithstanding its immense distance even + the naked eye perceives it as an enigmatical wisp in the sky. + Its image on the sensitive plate is the masterpiece of + astronomical photography; for wild, incomprehensible beauty + there is nothing that can be compared with it. Here, if + anywhere, we look upon the spectacle of creation in one of its + earliest stages. The Andromeda Nebula is apparently less + advanced toward transformation into stellar bodies than is that + in Triangulum. The immense crowd of stars sprinkled over it and + its neighborhood seem in the main to lie this side of the + nebula, and consequently to have no connection with it. But + incipient stars (in some places clusters of them) are seen in + the nebulous rings, while one or two huge masses seem to give + promise of transformation into stellar bodies of unusual + magnitude. I say ``rings'' because although the loops + encompassing the Andromeda Nebula have been called spirals by + those who wish utterly to demolish Laplace's hypothesis, yet + they are not manifestly such, as can be seen on comparing them + with the undoubted spirals of the Lord Rosse Nebula. They look + quite as much like circles or ellipses seen at an angle of, + say, fifteen or twenty degrees to their plane. If they are + truly elliptical they accord fairly well with Laplace's idea, + except that the scale of magnitude is stupendous, and if the + Andromeda Nebula is to become a solar system it will surpass + ours in grandeur beyond all possibility of comparison.</p> + + <p>There is one circumstance connected with the spiral + nebulæ, and conspicuous in the Andromeda Nebula on + account of its brightness, which makes the question of their + origin still more puzzling; they all show continuous spectra, + which, as we have before remarked, indicate that the mass from + which the light comes is either solid or liquid, or a gas under + heavy pressure. Thus nebulæ fall into two classes: the + ``white'' nebulæ, giving a continuous spectrum; and the + ``green'' nebulæ whose spectra are distinctly gaseous. + The Andromeda Nebula is the great representative of the former + class and the Orion Nebula of the latter. The spectrum of the + Andromeda Nebula has been interpreted to mean that it consists + not of luminous gas, but of a flock of stars so distant that + they are separately indistinguishable even with powerful + telescopes, just as the component stars of the Milky Way are + indistinguishable with the naked eye; and upon this has been + based the suggestion that what we see in Andromeda is an outer + universe whose stars form a series of elliptical garlands + surrounding a central mass of amazing richness. But this idea + is unacceptable if for no other reason than that, as just said, + all the spiral nebulæ possess the same kind of spectrum, + and probably no one would be disposed to regard them all as + outer universes. As we shall see later, the peculiarity of the + spectra of the spiral nebulæ is appealed to in support of + a modern substitute for Laplace's hypothesis.</p> + + <p>Finally, without having by any means exhausted the variety + exhibited by the spiral nebulæ, let us turn to the great + representative of the other species, the Orion Nebula. In some + ways this is even more marvelous than the others. The early + drawings with the telescope failed to convey an adequate + conception either of its sublimity or of its complication of + structure. It exists in a nebulous region of space, since + photographs show that nearly the whole constellation is + interwoven with faintly luminous coils. To behold the entry of + the great nebula into the field even of a small telescope is a + startling experience which never loses its novelty. As shown by + the photographs, it is an inscrutable chaos of perfectly + amazing extent, where spiral bands, radiating streaks, dense + masses, and dark yawning gaps are strangely intermingled + without apparent order. In one place four conspicuous little + stars, better seen in a telescope than in the photograph on + account of the blurring produced by over-exposure, are + suggestively situated in the midst of a dark opening, and no + observer has ever felt any doubt that these stars have been + formed from the substance of the surrounding nebula. There are + many other stars scattered over its expanse which manifestly + owe their origin to the same source. But compare the general + appearance of this nebula with the others that we have studied, + and remark the difference. If the unmistakably spiral + nebulæ resemble bursting fly-wheels or grindstones from + whose perimeters torrents of sparks are flying, the Orion + Nebula rather recalls the aspect of a cloud of smoke and + fragments produced by the explosion of a shell. This idea is + enforced by the look of the outer portion farthest from the + bright half of the nebula, where sharply edged clouds with dark + spaces behind seem to be billowing away as if driven by a wind + blowing from the center.</p> + + <p>Next let us consider what scientific speculation has done in + the effort to explain these mysteries. Laplace's hypothesis can + certainly find no standing ground either in the Orion Nebula or + in those of a spiral configuration, whatever may be its + situation with respect to the grand Nebula of Andromeda, or the + ``ring'' and ``planetary'' nebulæ. Some other hypothesis + more consonant with the appearances must be found. Among the + many that have been proposed the most elaborate is the + ``Planetesimal Hypothesis'' of Professors Chamberlin and + Moulton. It is to be remarked that it applies to the spiral + nebulæ distinctively, and not to an apparently chaotic + mass of gas like the vast luminous cloud in Orion. The gist of + the theory is that these curious objects are probably the + result of close approaches to each other of two independent + suns, reminding us of what was said on this subject when we + were dealing with temporary stars. Of the previous history of + these appulsing suns the theory gives us no account; they are + simply supposed to arrive within what may be called an + effective tide-producing distance, and then the drama begins. + Some of the probable consequences of such an approach have been + noticed in Chapter 5; let us now consider them a little more in + detail.</p> + + <p>Tides always go in couples; if there is a tide on one side + of a globe there will be a corresponding tide on the other + side. The cause is to be found in the law that the force of + gravitation varies inversely as the square of the distance; the + attraction on the nearest surface of the body exercised by + another body is greater than on its center, and greater yet + than on its opposite surface. If two great globes attract each + other, each tends to draw the other out into an ellipsoidal + figure; they must be more rigid than steel to resist this -- + and even then they cannot altogether resist. If they are liquid + or gaseous they will yield readily to the force of distortion, + the amount of which will depend upon their distance apart, for + the nearer they are the greater becomes the tidal strain. If + they are encrusted without and liquid or gaseous in the + interior, the internal mass will strive to assume the figure + demanded by the tidal force, and will, if it can, burst the + restraining envelope. Now this is virtually the predicament of + the body we call a sun when in the immediate presence of + another body of similarly great mass. Such a body is presumably + gaseous throughout, the component gases being held in a state + of rigidity by the compression produced by the tremendous + gravitational force of their own aggregate mass. At the surface + such a body is enveloped in a shell of relatively cool matter. + Now suppose a great attracting body, such as another sun, to + approach near enough for the difference in its attraction on + the two opposite sides of the body and on its center to become + very great; the consequence will be a tidal deformation of the + whole body, and it will lengthen out along the line of the + gravitational pull and draw in at the sides, and if its shell + offers considerable resistance, but not enough to exercise a + complete restraint, it will be violently burst apart, or blown + to atoms, and the internal mass will leap out on the two + opposite sides in great fiery spouts. In the case of a sun + further advanced in cooling than ours the interior might be + composed of molten matter while the exterior crust had become + rigid like the shell of an egg; then the force of the ``tidal + explosion'' produced by the appulse of another sun would be + more violent in consequence of the greater resistance overcome. + Such, then, is the mechanism of the first phase in the history + of a spiral nebula according to the Planetesimal Hypothesis. + Two suns, perhaps extinguished ones, have drawn near together, + and an explosive outburst has occured in one or both. The + second phase calls for a more agile exercise of the + imagination.</p> + + <p>To simplify the case, let us suppose that only one of the + tugging suns is seriously affected by the strain. Its vast + wings produced by the outburst are twisted into spirals by + their rotation and the contending attractions exercised upon + them, as the two suns, like battleships in desperate conflict, + curve round each other, concentrating their destructive + energies. Then immense quantities of débris are + scattered about in which eddies are created, and finally, as + the sun that caused the damage goes on its way, leaving its + victim to repair its injuries as it may, the dispersed matter + cools, condenses, and turns into streams of solid particles + circling in elliptical paths about their parent sun. These + particles, or fragments, are the ``planetesimals'' of the + theory. In consequence of the inevitable intersection of the + orbits of the planetesimals, nodes are formed where the flying + particles meet, and at these nodes large masses are gradually + accumulated. The larger the mass the greater its attraction, + and at last the nodal points become the nuclei of great + aggregations from which planets are shaped.</p> + + <p>This, in very brief form, is the Planetesimal Hypothesis + which we are asked to substitute for that based on Laplace's + suggestion as an explanation of the mode of origin of the solar + system; and the phenomena of the spiral nebulæ are + appealed to as offering evident support to the new hypothesis. + We are reminded that they are elliptical in outline, which + accords with the hypothesis; that their spectra are not + gaseous, which shows that they may be composed of solid + particles like the planetesimals; and that their central masses + present an oval form, which is what would result from the tidal + effects, as just described. We also remember that some of them, + like the Lord Rosse and the Andromeda nebulæ, are + visually double, and in these cases we might suppose that the + two masses represent the tide-burst suns that ventured into too + close proximity. It may be added that the authors of the theory + do not insist upon the appulse of two suns as the <em>only</em> + way in which the planetesimals may have originated, but it is + the only supposition that has been worked out.</p> + + <p>But serious questions remain. It needs, for instance, but a + glance at the Triangulum monster to convince the observer that + it cannot be a solar system which is being evolved there, but + rather a swarm of stars. Many of the detached masses are too + vast to admit of the supposition that they are to be + transformed into planets, in our sense of planets, and the + distances of the stars which appear to have been originally + ejected from the focal masses are too great to allow us to + liken the assemblage that they form to a solar system. Then, + too, no nodes such as the hypothesis calls for are visible. + Moreover, in most of the spiral nebulæ the appearances + favor the view that the supposititious encountering suns have + not separated and gone each rejoicing on its way, after having + inflicted the maximum possible damage on its opponent, but + that, on the contrary, they remain in close association like + two wrestlers who cannot escape from each other's grasp. And + this is exactly what the law of gravitation demands; stars + cannot approach one another with impunity, with regard either + to their physical make-up or their future independence of + movement. The theory undertakes to avoid this difficulty by + assuming that in the case of our system the approach of the + foreign body to the sun was not a close one -- just close + enough to produce the tidal extrusion of the relatively + insignificant quantity of matter needed to form the planets. + But even then the effect of the appulse would be to change the + direction of flight, both of the sun and of its visitor, and + there is no known star in the sky which can be selected as the + sun's probable partner in their ancient <em>pas deux.</em> That + there are unconquered difficulties in Laplace's hypothesis no + one would deny, but in simplicity of conception it is + incomparably more satisfactory, and with proper modifications + could probably be made more consonant with existing facts in + our solar system than that which is offered to replace it. Even + as an explanation of the spiral nebulæ, not as solar + systems in process of formation, but as the birthplaces of + stellar clusters, the Planetesimal Hypothesis would be open to + many objections. Granting its assumptions, it has undoubtedly a + strong mathematical framework, but the trouble is not with the + mathematics but with the assumptions. Laplace was one of the + ablest mathematicians that ever lived, but he had never seen a + spiral nebula; if he had, he might have invented a hypothesis + to suit its phenomena. His actual hypothesis was intended only + for our solar system, and he left it in the form of a ``note'' + for the consideration of his successors, with the hope that + they might be able to discover the full truth, which he + confessed was hidden from him. It cannot be said that that + truth has yet been found, and when it is found the chances are + that intuition and not logic will have led to it.</p> + + <p>The spiral nebulæ, then, remain among the greatest + riddles of the universe, while the gaseous nebulæ, like + that of Orion, are no less mysterious, although it seems + impossible to doubt that both forms give birth to stars. It is + but natural to look to them for light on the question of the + origin of our planetary system; but we should not forget that + the scale of the phenomena in the two cases is vastly + different, and the forces in operation may be equally + different. A hill may have been built up by a glacier, while a + mountain may be the product of volcanic forces or of the + upheaval of the strata of the planet.</p> + + <p><strong>The Banners of the Sun</strong></p> + + <p>As all the world knows, the sun, a blinding globe pouring + forth an inconceivable quantity of light and heat, whose daily + passage through the sky is caused by the earth's rotation on + its axis, constitutes the most important phenomenon of + terrestial existence. Viewed with a dark glass to take off the + glare, or with a telescope, its rim is seen to be a sharp and + smooth circle, and nothing but dark sky is visible around it. + Except for the interference of the moon, we should probably + never have known that there is any more of the sun than our + eyes ordinarily see.</p> + + <p>But when an eclipse of the sun occurs, caused by the + interposition of the opaque globe of the moon, we see its + immediate surroundings, which in some respects are more + wonderful than the glowing central orb. These surroundings, + although not in the sense in which we apply the term to the + gaseous envelope of the earth, may be called the sun's + atmosphere. They consist of two very different parts -- first, + the red ``prominences,'' which resemble tongues of flame + ascending thousands of miles above the sun's surface; and, + second, the ``corona,'' which extends to distances of millions + of miles from the sun, and shines with a soft, glowing light. + The two combined, when well seen, make a spectacle without + parallel among the marvels of the sky. Although many attempts + have been made to render the corona visible when there is no + eclipse, all have failed, and it is to the moon alone that we + owe its revelation. To cover the sun's disk with a circular + screen will not answer the purpose because of the illumination + of the air all about the observer. When the moon hides the sun, + on the other hand, the sunlight is withdrawn from a great + cylinder of air extending to the top of the atmosphere and + spreading many miles around the observer. There is then no + glare to interfere with the spectacle, and the corona appears + in all its surprising beauty. The prominences, however, + although they were discovered during an eclipse, can now, with + the aid of the spectroscope, be seen at any time. But the + prominences are rarely large enough to be noticed by the naked + eye, while the streamers of the corona, stretching far away in + space, like ghostly banners blown out from the black circle of + the obscuring moon, attract every eye, and to this weird + apparition much of the fear inspired by eclipses has been due. + But if the corona has been a cause of terror in the past it has + become a source of growing knowledge in our time.</p> + + <p>The story of the first scientific observation of the corona + and the prominences is thrillingly interesting, and in fact + dramatic. The observation was made during the eclipse of 1842, + which fortunately was visible all over Central and Southern + Europe so that scores of astronomers saw it. The interest + centers in what happened at Pavia in Northern Italy, where the + English astronomer Francis Baily had set up his telescope. The + eclipse had begun and Bailey was busy at his telescope when, to + quote his own words in the account which he wrote for the + <em>Memoirs of the Royal Astronomical Society:</em></p> + + <blockquote> + I was astounded by a tremendous burst of applause from the + streets below, and at the same moment was electrified by the + sight of one of the most brilliant and splendid phenomena + that can well be imagined; for at that instant the dark body + of the moon was suddenly surrounded with a corona, or kind of + bright glory, similar in shape and magnitude to that which + painters draw round the heads of saints... + + <p>Pavia contains many thousand inhabitants, the major part + of whom were at this early hour walking about the streets and + squares or looking out of windows in order to witness this + long-talked-of phenomenon; and when the total obscuration + took place, which was <em>instantaneous,</em> there was a + universal shout from every observer which ``made the welkin + ring,'' and for the moment withdrew my attention from the + object with which I was immediately occupied. I had, indeed, + expected the appearance of a luminous circle round the moon + during the time of total obscurity; but I did not expect, + from any of the accounts of preceding eclipses that I had + read, to witness so magnificent an exhibition as that which + took place...</p> + + <p>Splendid and astonishing, however, as this remarkable + phenomenon really was, and although it could not fail to call + forth the admiration and applause of every beholder, yet I + must confess that there was at the same time something in its + singular and wonderful appearance that was appalling...</p> + + <p>But the most remarkable circumstance attending the + phenomenon was the appearance of <em>three large + protuberances</em> apparently emanating from the + circumference of the moon, but evidently forming a portion of + the corona. They had the appearance of mountains of a + prodigious elevation; their color was red tinged with lilac + or purple; perhaps the color of the peach-blossom would more + nearly represent it. They somewhat resembled the tops of the + snowy Alpine mountains when colored by the rising or the + setting sun. They resembled the Alpine mountains in another + respect, inasmuch as their light was perfectly steady, and + had none of that flickering or sparkling motion so visible in + other parts of the corona...</p> + + <p>The whole of these protuberances were visible even to the + last moment of total obscuration, and when the first ray of + light was admitted from the sun they vanished, with the + corona, altogether, and daylight was instantly restored.</p> + </blockquote> + + <p>I have quoted nearly all of this remarkable description not + alone for its intrinsic interest, but because it is the best + depiction that can be found of the general phenomena of a total + solar eclipse. Still, not every such eclipse offers an equally + magnificent spectacle. The eclipses of 1900 and 1905, for + instance, which were seen by the writer, the first in South + Carolina and the second in Spain, fell far short of that + described by Bailey in splendor and impressiveness. Of course, + something must be allowed for the effect of surprise; Bailey + had not expected to see what was so suddenly disclosed to him. + But both in 1900 and 1905 the amount of scattered light in the + sky was sufficient in itself to make the corona appear faint, + and there were no very conspicuous prominences visible. Yet on + both occasions there was manifest among the spectators that + mingling of admiration and awe of which Bailey speaks. The + South Carolinians gave a cheer and the ladies waved their + handkerchiefs when the corona, ineffably delicate of form and + texture, <em>melted</em> into sight and then in two minutes + melted away again. The Spaniards, crowded on the citadel hill + of Burgos, with their king and his royal retinue in their + midst, broke out with a great clapping of hands as the awaited + spectacle unfolded itself in the sky; and on both occasions, + before the applause began, after an awed silence a low murmur + ran through the crowds. At Burgos it is said many made the sign + of the cross.</p> + + <p>It was not long before Bailey's idea that the prominences + were a part of the corona was abandoned, and it was perceived + that the two phenomena were to a great extent independent. At + the eclipse of 1868, which the astronomers, aroused by the + wonderful scene of 1842, and eager to test the powers of the + newly invented spectroscope, flocked to India to witness, + Janssen conceived the idea of employing the spectroscope to + render the prominences visible when there was no eclipse. He + succeeded the very next day, and these phenomena have been + studied in that way ever since.</p> + + <p>There are recognized two kinds of prominences -- the + ``erruptive'' and the ``quiescent.'' The latter, which are + cloud-like in form, may be seen almost anywhere along the edge + of the sun; but the former, which often shoot up as if hurled + from mighty volcanoes, appear to be associated with sun-spots, + and appear only above the zones where spots abound. Either of + them, when seen in projection against the brilliant solar disk, + appears white, not red, as against a background of sky. The + quiescent prominences, whose elevation is often from forty + thousand to sixty thousand miles, consist, as the spectroscope + shows, mainly of hydrogen and helium. The latter, it will be + remembered, is an element which was known to be in the sun many + years before the discovery that it also exists in small + quantities on the earth. A fact which may have a significance + which we cannot at present see is that the emanation from + radium gradually and spontaneously changes into helium, an + alchemistical feat of nature that has opened many curious + vistas to speculative thinkers. The eruptive prominences, which + do not spread horizontally like the others, but ascend with + marvelous velocity to elevations of half a million miles or + more, are apparently composed largely of metallic vapors -- + <em>i.e.</em> metals which are usually solid on the earth, but + which at solar temperatures are kept in a volatilized state. + The velocity of their ascent occasionally amounts to three + hundred or four hundred miles per second. It is known from + mathematical considerations that the gravitation of the sun + would not be able to bring back any body that started from its + surface with a velocity exceeding three hundred and + eighty-three miles per second; so it is evident that some of + the matter hurled forth in eruptive prominences may escape from + solar control and go speeding out into space, cooling and + condensing into solid masses. There seems to be no reason why + some of the projectiles from the sun might not reach the + planets. Here, then, we have on a relatively small scale, + <em>explosions</em> recalling those which it has been imagined + may be the originating cause of some of the sudden phenomena of + the stellar heavens.</p> + + <p>Of the sun-spots it is not our intention here specifically + to speak, but they evidently have an intimate connection with + eruptive prominences, as well as some relation, not yet fully + understood, with the corona. Of the real cause of sun-spots we + know virtually nothing, but recent studies by Professor Hale + and others have revealed a strange state of things in the + clouds of metallic vapors floating above them and their + surroundings. Evidences of a cyclonic tendency have been found, + and Professor Hale has proved that sun-spots are strong + magnetic fields, and consist of columns of ionized vapors + rotating in opposite directions in the two hemispheres. A fact + which may have the greatest significance is that titanium and + vanadium have been found both in sun-spots and in the + remarkable variable Mira Ceti, a star which every eleven + months, or thereabout, flames up with great brilliancy and then + sinks back to invisibility with the naked eye. It has been + suggested that sun-spots are indications of the beginning of a + process in the sun which will be intensified until it falls + into the state of such a star as Mira. Stars very far advanced + in evolution, without showing variability, also exhibit similar + spectra; so that there is much reason for regarding sunspots as + emblems of advancing age.</p> + + <p>The association of the corona with sun-spots is less evident + than that of the eruptive prominences; still such an + association exists, for the form and extent of the corona vary + with the sun-spot period of which we shall presently speak. The + constitution of the corona remains to be discovered. It is + evidently in part gaseous, but it also probably contains matter + in the form of dust and small meteors. It includes one + substance altogether mysterious -- ``coronium.'' There are + reasons for thinking that this may be the lightest of all the + elements, and Professor Young, its discoverer, said that it was + ``absolutely unique in nature; utterly distinct from any other + known form of matter, terrestial, solar, or cosmical.'' The + enormous extent of the corona is one of its riddles. Since the + development of the curious subject of the ``pressure of light'' + it has been proposed to account for the sustentation of the + corona by supposing that it is borne upon the billows of light + continually poured out from the sun. Experiment has proved, + what mathematical considerations had previously pointed out as + probable, that the waves of light exert a pressure or driving + force, which becomes evident in its effects if the body acted + upon is sufficiently small. In that case the light pressure + will prevail over the attraction of gravitation, and propel the + attenuated matter away from the sun in the teeth of its + attraction. The earth itself would be driven away if, instead + of consisting of a solid globe of immense aggregate mass, it + were a cloud of microscopic particles. The reason is that the + pressure varies in proportion to the <em>surface</em> of the + body acted upon, while the gravitational attraction is + proportional to the <em>volume,</em> or the total amount of + matter in the body. But the surface of any body depends upon + the <em>square</em> of its diameter, while the volume depends + upon the <em>cube</em> of the diameter. If, for instance, the + diameter is represented by 4, the surface will be proportional + to 4 × 4, or 16, and the volume to 4 × 4 × 4, + or 64; but if the diameter is taken as 2, the surface will be 2 + × 2, or 4, and the volume 2 × 2 × 2, or 8. + Now, the ratio of 4 to 8 is twice as great as that of 16 to 64. + If the diameter is still further decreased, the ratio of the + surface to the volume will proportionally grow larger; in other + words, the pressure will gain upon the attraction, and whatever + their original ratio may have been, a time will come, if the + diminution of size continues, when the pressure will become + more effective than the attraction, and the body will be driven + away. Supposing the particles of the corona to be below the + critical size for the attraction of a mass like that of the sun + to control them, they would be driven off into the surrounding + space and appear around the sun like the clouds of dust around + a mill. We shall return to this subject in connection with the + Zodiacal Light, the Aurora, and Comets.</p> + + <p>On the other hand, there are parts of the corona which + suggest by their forms the play of electric or magnetic forces. + This is beautifully shown in some of the photographs that have + been made of the corona during recent eclipses. Take, for + instance, that of the eclipse of 1900. The sheaves of light + emanating from the poles look precisely like the ``lines of + force'' surrounding the poles of a magnet. It will be noticed + in this photograph that the corona appears to consist of two + portions: one comprising the polar rays just spoken of, and the + other consisting of the broader, longer, and less-defined + masses of light extending out from the equatorial and + middle-latitude zones. Yet even in this more diffuse part of + the phenomenon one can detect the presence of submerged curves + bearing more or less resemblance to those about the poles. Just + what part electricity or electro-magnetism plays in the + mechanism of the solar radiation it is impossible to say, but + on the assumption that it is a very important part is based the + hypothesis that there exists a direct solar influence not only + upon the magnetism, but upon the weather of the earth. This + hypothesis has been under discussion for half a century, and + still we do not know just how much truth it represents. It is + certain that the outbreak of great disturbances on the sun, + accompanied by the formation of sun-spots and the upshooting of + eruptive prominences (phenomena which we should naturally + expect to be attended by action), have been instantly followed + by corresponding ``magnetic storms'' on the earth and brilliant + displays of the auroral lights. There have been occasions when + the influence has manifested itself in the most startling ways, + a great solar outburst being followed by a mysterious gripping + of the cable and telegraph systems of the world, as if an + invisible and irresistible hand had seized them. Messages are + abruptly cut off, sparks leap from the telegraph instruments, + and the entire earth seems to have been thrown into a magnetic + flurry. These occurrences affect the mind with a deep + impression of the dependence of our planet on the sun, such as + we do not derive from the more familiar action of the sunlight + on the growth of plants and other phenomena of life depending + on solar influences.</p> + + <p>Perhaps the theory of solar magnetic influence upon the + weather is best known in connection with the ``sun-spot + cycle.'' This, at any rate, is, as already remarked, closely + associated with the corona. Its existence was discovered in + 1843 by the German astronomer Schwabe. It is a period of + variable length, averaging about eleven years, during which the + number of spots visible on the sun first increases to a + maximum, then diminishes to a minimum, and finally increases + again to a maximum. For unknown reasons the period is sometimes + two or three years longer than the average and sometimes as + much shorter. Nevertheless, the phenomena always recur in the + same order. Starting, for instance, with a time when the + observer can find few or no spots, they gradually increase in + number and size until a maximum, in both senses, is reached, + during which the spots are often of enormous size and + exceedingly active. After two or three years they begin to + diminish in number, magnitude, and activity until they almost + or quite disappear. A strange fact is that when a new period + opens, the spots appear first in high northern and southern + latitudes, far from the solar equator, and as the period + advances they not only increase in number and size, but break + out nearer and nearer to the equator, the last spots of a + vanishing period sometimes lingering in the equatorial region + after the advance-guard of its successor has made its + appearance in the high latitudes. Spots are never seen on the + equator nor near the poles. It was not very long after the + discovery of the sun-spot cycle that the curious observation + was made that a striking coincidence existed between the period + of the sun-spots and another period affecting the general + magnetic condition of the earth. When a curved line + representing the varying number of sun-spots was compared with + another curve showing the variations in the magnetic state of + the earth the two were seen to be in almost exact accord, a + rise in one curve corresponding to a rise in the other, and a + fall to a fall. Continued observation has proved that this is a + real coincidence and not an accidental one, so that the + connection, although as yet unexplained, is accepted as + established. But does the influence extend further, and + directly affect the weather and the seasons as well as the + magnetic elements of the earth? A final answer to this question + cannot yet be given, for the evidence is contradictory, and the + interpretations put upon it depend largely on the predilections + of the judges.</p> + + <p>But, in a broad sense, the sun-spots and the phenomena + connected with them <em>must</em> have a relation to terrestial + meteorology, for they prove the sun to be a variable star. + Reference was made, a few lines above, to the resemblance of + the spectra of sun-spots to those of certain stars which seem + to be failing through age. This in itself is extremely + suggestive; but if this resemblance had never been discovered, + we should have been justified in regarding the sun as variable + in its output of energy; and not only variable, but probably + increasingly so. The very inequalities in the sun-spot cycle + are suspicious. When the sun is most spotted its total light + may be reduced by one-thousandth part, although it is by no + means certain that its outgiving of thermal radiations is then + reduced. A loss of one-thousandth of its luminosity would + correspond to a decrease of .0025 of a stellar magnitude, + considering the sun as a star viewed from distant space. So + slight a change would not be perceptible; but it is not alone + sun-spots which obscure the solar surface, its entire globe is + enveloped with an obscuring veil. When studied with a powerful + telescope the sun's surface is seen to be thickly mottled with + relatively obscure specks, so numerous that it has been + estimated that they cut off from one-tenth to one-twentieth of + the light that we should receive from it if the whole surface + were as brilliant as its brightest parts. The condition of + other stars warrants the conclusion that this obscuring + envelope is the product of a process of refrigeration which + will gradually make the sun more and more variable until its + history ends in extinction. Looking backward, we see a time + when the sun must have been more brilliant than it is now. At + that time it probably shone with the blinding white splendor of + such stars as Sirius, Spica, and Vega; now it resembles the + relatively dull Procyon; in time it will turn ruddy and fall + into the closing cycle represented by Antares. Considering that + once it must have been more radiantly powerful than at present, + one is tempted to wonder if that could have been the time when + tropical life flourished within the earth's polar circles, + sustained by a vivific energy in the sun which it has now + lost.</p> + + <p>The corona, as we have said, varies with the sun-spot cycle. + When the spots are abundant and active the corona rises strong + above the spotted zones, forming immense beams or streamers, + which on one occasion, at least, had an observed length of + <em>ten million miles.</em> At the time of a spot minimum the + corona is less brilliant and has a different outline. It is + then that the curved polar rays are most conspicuous. Thus the + vast banners of the sun, shaken out in the eclipse, are signals + to tell of its varying state, but it will probably be long + before we can read correctly their messages.</p> + + <p><strong>The Zodiacal Light Mystery</strong></p> + + <p>There is a singular phenomenon in the sky -- one of the most + puzzling of all -- which has long arrested the attention of + astronomers, defying their efforts at explanation, but which + probably not one in a hundred, and possibly not one in a + thousand, of the readers of this book has ever seen. Yet its + name is often spoken, and it is a conspicuous object if one + knows when and where to look for it, and when well seen it + exhibits a mystical beauty which at the same time charms and + awes the beholder. It is called ``The Zodiacal Light,'' because + it lies within the broad circle of the Zodiac, marking the + sun's apparent annual path through the stars. What it is nobody + has yet been able to find out with certainty, and books on + astronomy usually speak of it with singular reserve. But it has + given rise to many remarkable theories, and a true explanation + of it would probably throw light on a great many other + celestial mysteries. The Milky Way is a more wonderful object + to look upon, but its nature can be comprehended, while there + is a sort of uncanniness about the Zodiacal Light which + immediately impresses one upon seeing it, for its part in the + great scheme of extra-terrestrial affairs is not evident.</p> + + <p>If you are out-of-doors soon after sunset -- say, on an + evening late in the month of February -- you may perceive, just + after the angry flush of the dying winter's day has faded from + the sky, a pale ghostly presence rising above the place where + the sun went down. The writer remembers from boyhood the first + time it was pointed out to him and the unearthly impression + that it made, so that he afterward avoided being out alone at + night, fearful of seeing the spectral thing again. The + phenomenon brightens slowly with the fading of the twilight, + and soon distinctly assumes the shape of an elongated pyramid + of pearly light, leaning toward the south if the place of + observation is in the northern hemisphere. It does not impress + the observer at all in the same manner as the Milky Way; that + looks far off and is clearly among the stars, but the Zodiacal + Light seems closer at hand, as if it were something more + intimately concerning the earth. To all it immediately suggests + a connection, also, with the sunken sun. If the night is clear + and the moon absent (and if you are in the country, for city + lights ruin the spectacles of the sky), you will be able to + watch the apparition for a long time. You will observe that the + light is brightest near the horizon, gradually fading as the + pyramidal beam mounts higher, but in favorable circumstances it + may be traced nearly to the meridian south of the zenith, where + its apex at last vanishes in the starlight. It continues + visible during the evenings of March and part of April, after + which, ordinarily, it is seen no more, or if seen is relatively + faint and unimpressive. But when autumn comes it appears again, + this time not like a wraith hovering above the westward tomb of + the day-god, but rather like a spirit of the morning announcing + his reincarnation in the east.</p> + + <p>The reason why the Zodiacal Light is best seen in our + latitudes at the periods just mentioned is because at those + times the Zodiac is more nearly perpendicular to the horizon, + first in the west and then in the east; and, since the + phenomenon is confined within the borders of the Zodiac, it + cannot be favorably placed for observation when the zodiacal + plane is but slightly inclined to the horizon. Its faint light + requires the contrast of a background of dark sky in order to + be readily perceptible. But within the tropics, where the + Zodiac is always at a favorable angle, the mysterious light is + more constantly visible. Nearly all observant travelers in the + equatorial regions have taken particular note of this + phenomenon, for being so much more conspicuous there than in + the temperate zones it at once catches the eye and holds the + attention as a novelty. Humboldt mentions it many times in his + works, for his genius was always attracted by things out of the + ordinary and difficult of explanation, and he made many careful + observations on its shape, its brilliancy, and its variations; + for there can be no doubt that it does vary, and sometimes to + an astonishing degree. It is said that it once remained + practically invisible in Europe for several years in + succession. During a trip to South Africa in 1909 an English + astronomer, Mr E. W. Maunder, found a remarkable difference + between the appearance of the Zodiacal Light on his going and + coming voyages. In fact, when crossing the equator going south + he did not see it at all; but on returning he had, on March + 6th, when one degree south of the equator, a memorable view of + it.</p> + + <blockquote> + It was a bright, clear night, and the Zodiacal Light was + extraordinarily brilliant -- brighter than he had ever seen + it before. The Milky Way was not to be compared with it. The + brightest part extended 75° from the sun. There was a + faint and much narrower extension which they could just make + out beyond the Pleiades along the ecliptic, but the greater + part of the Zodiacal Light showed as a broad truncated + column, and it did not appear nearly as conical as he had + before seen it. + </blockquote> + + <p>When out of the brief twilight of intertropical lands, where + the sun drops vertically to the horizon and night rushes on + like a wave of darkness, the Zodiacal Light shoots to the very + zenith, its color is described as a golden tint, entirely + different from the silvery sheen of the Milky Way. If I may + venture again to refer to personal experiences and impressions, + I will recall a view of the Zodiacal Light from the summit of + the cone of Mt Etna in the autumn of the year 1896 (more + briefly described in <em>Astronomy with the Naked Eye</em>). + There are few lofty mountains so favorably placed as Etna for + observations of this kind. It was once resorted to by Prof. + George E. Hale, in an attempt to see the solar corona without + an eclipse. Rising directly from sea-level to an elevation of + nearly eleven thousand feet, the observer on its summit at + night finds himself, as it were, lost in the midst of the sky. + But for the black flanks of the great cone on which he stands + he might fancy himself to be in a balloon. On the occasion to + which I refer the world beneath was virtually invisible in the + moonless night. The blaze of the constellations overhead was + astonishingly brilliant, yet amid all their magnificence my + attention was immediately drawn to a great tapering light that + sprang from the place on the horizon where the sun would rise + later, and that seemed to be blown out over the stars like a + long, luminous veil. It was the finest view of the Zodiacal + light that I had ever enjoyed -- thrilling in its strangeness + -- but I was almost disheartened by the indifference of my + guide, to whom it was only a light and nothing more. If he had + no science, he had less poetry -- rather a remarkable thing, I + thought, for a child of his clime. The Light appeared to me to + be distinctly brighter than the visible part of the Milky Way + which included the brilliant stretches in Auriga and Perseus, + and its color, if one may speak of color in connection with + such an object, seemed richer than that of the galactic band; + but I did not think of it as yellow, although Humboldt has + described it as resembling a golden curtain drawn over the + stars, and Du Chaillu in Equatorial Africa found it of a bright + yellow color. It may vary in color as in conspicuousness. The + fascination of that extraordinary sight has never faded from my + memory. I turned to regard it again and again, although I had + never seen the stellar heavens so brilliant, and it was one of + the last things I looked for when the morning glow began softly + to mount in the east, and Sicily and the Mediterranean slowly + emerged from the profound shadow beneath us.</p> + + <p>The Zodiacal Light seems never to have attracted from + astronomers in general the amount of careful attention that it + deserves; perhaps because so little can really be made of it as + far as explanation is concerned. I have referred to the + restraint that scientific writers apparently feel in speaking + of it. The grounds for speculation that it affords may be too + scanty to lead to long discussions, yet it piques curiosity, + and as we shall see in a moment has finally led to a most + interesting theory. Once it was the subject of an elaborate + series of studies which carried the observer all round the + world. That was in 1845--46, during the United States Exploring + Expedition that visited the then little known Japan. The + chaplain of the fleet, the Rev. Mr Jones, went out prepared to + study the mysterious light in all its phases. He saw it from + many latitudes on both sides of the equator, and the + imagination cannot but follow him with keen interest in his + world-circling tour, keeping his eyes every night fixed upon + the phantasm overhead, whose position shifted with that of the + hidden sun. He demonstrated that the flow extends at times + completely across the celestial dome, although it is relatively + faint directly behind the earth. On his return the government + published a large volume of his observations, in which he + undertook to show that the phenomenon was due to the reflection + of sunlight from a ring of meteoric bodies encircling the + earth. But, after all, this elaborate investigation settled + nothing.</p> + + <p>Prof. E. E. Barnard has more recently devoted much attention + to the Zodiacal Light, as well as to a strange attendant + phenomenon called the ``Gegenschein,'' or Counterglow, because + it always appears at that point in the sky which is exactly + opposite the sun. The Gegenschein is an extremely elusive + phenomenon, suitable only for eyes that have been specially + trained to see it. Professor Newcomb has cautiously remarked + that</p> + + <blockquote> + it is said that in that point of the heavens directly + opposite the sun there is an elliptical patch of light... + This phenomenon is so difficult to account for that its + existence is sometimes doubted; yet the testimony in its + favor is difficult to set aside. + </blockquote> + + <p>It certainly cannot be set aside at all since the + observations of Barnard. I recall an attempt to see it under + his guidance during a visit to Mount Hamilton, when he was + occupied there with the Lick telescope. Of course, both the + Gegenschein and the Zodiacal Light are too diffuse to be + studied with telescopes, which, so to speak, magnify them out + of existence. They can only be successfully studied with the + naked eye, since every faintest glimmer that they afford must + be utilized. This is especially true of the Gegenschein. At + Mount Hamilton, Mr Barnard pointed out to me its location with + reference to certain stars, but with all my gazing I could not + be sure that I saw it. To him, on the contrary, it was obvious; + he had studied it for months, and was able to indicate its + shape, its boundaries, its diameter, and the declination of its + center with regard to the ecliptic. There is not, of course, + the shadow of a doubt of the existence of the Gegenschein, and + yet I question if one person in a million has ever seen or ever + will see it. The Zodiacal Light, on the other hand, is plain + enough, provided that the time and the circumstances of the + observation are properly chosen.</p> + + <p>In the attempts to explain the Zodiacal Light, the favorite + hypothesis has been that it is an appendage of the sun -- + perhaps simply an extension of the corona in the plane of the + ecliptic, which is not very far from coinciding with that of + the sun's equator. This idea is quite a natural one, because of + the evident relation of the light to the position of the sun. + The vast extension of the equatorial wings of the corona in + 1878 gave apparent support to this hypothesis; if the substance + of the corona could extend ten million miles from the sun, why + might it not extend even one hundred million, gradually fading + out beyond the orbit of the earth? A variation of this + hypothesis assumes that the reflection is due to swarms of + meteors circling about the sun, in the plane of its equator, + all the way from its immediate neighborhood to a distance + exceeding that of the earth. But in neither form is the + hypothesis satisfactory; there is nothing in the appearance of + the corona to indicate that it extends even as far as the + planet Mercury, while as to meteors, the orbits of the known + swarms do not accord with the hypothesis, and we have no reason + to believe that clouds of others exist traveling in the part of + space where they would have to be in order to answer the + requirements of the theory. The extension of the corona in 1878 + did not resemble in its texture the Zodiacal Light.</p> + + <p>Now, it has so often happened in the history of science that + an important discovery in one branch has thrown unexpected but + most welcome light upon some pending problem in some other + branch, that a strong argument might be based upon that fact + alone against the too exclusive devotion of many investigators + to the narrow lines of their own particular specialty; and the + Zodiacal Light affords a case in point, when it is considered + in connection with recent discoveries in chemistry and physics. + From the fact that atoms are compound bodies made up of + corpuscles at least a thousand times smaller than the smallest + known atom -- a fact which astounded most men of science when + it was announced a few years ago -- a new hypothesis has been + developed concerning the nature of the Zodiacal Light (as well + as other astronomical riddles), and this hypothesis comes not + from an astronomer, but from a chemist and physicist, the + Swede, Svante Arrhenius. In considering an outline of this new + hypothesis we need neither accept nor reject it; it is a case + rather for suspension of judgment.</p> + + <p>To begin with, it carries us back to the ``pressure of + light'' mentioned in the preceding chapter. The manner in which + this pressure is believed generally to act was there + sufficiently explained, and it only remains to see how it is + theoretically extended to the particles of matter supposed to + constitute the Zodiacal Light. We know that corpuscles, or + ``fragments of atoms'' negatively electrified, are discharged + from hot bodies. Streams of these ``ions'' pour from many + flames and from molten metals; and the impact of the cathode + and ultra-violet rays causes them to gush even from cold + bodies. In the vast laboratory of the sun it is but reasonable + to suppose that similar processes are taking place. ``As a very + hot metal emits these corpuscles,'' says Prof. J. J. Thomson, + ``it does not seem an improbable hypothesis that they are + emitted by that very hot body, the sun.'' Let it be assumed, + then, that the sun does emit them; what happens next? + Negatively charged corpuscles, it is known, serve as nuclei to + which particles of matter in the ordinary state are attracted, + and it is probable that those emitted from the sun immediately + pick up loads in this manner and so grow in bulk. If they grow + large enough the gravitation of the sun draws them back, and + they produce a negative charge in the solar atmosphere. But it + is probable that many of the particles do not attain the + critical size which, according to the principles before + explained, would enable the gravitation of the sun to retain + them in opposition to the pressure of the waves of light, and + with these particles the light pressure is dominant. Clouds of + them may be supposed to be continually swept away from the sun + into surrounding space, moving mostly in or near the plane of + the solar equator, where the greatest activity, as indicated by + sunspots and related phenomena, is taking place. As they pass + outward into space many of them encounter the earth. If the + earth, like the moon, had no atmosphere the particles would + impinge directly on its surface, giving it a negative electric + charge. But the presence of the atmosphere changes all that, + for the first of the flying particles that encounter it impart + to it their negative electricity, and then, since like electric + charges repel like, the storm of particles following will be + sheered off from the earth, and will stream around it in a maze + of hyperbolic paths. Those that continue on into space beyond + the earth may be expected to continue picking up wandering + particles of matter until their bulk has become so great that + the solar attraction prevails again over the light pressure + acting upon them, and they turn again sunward. Passing the + earth on their return they will increase the amount of + dust-clouds careering round it; and these will be further + increased by the action of the ultra-violet rays of the + sunlight causing particles to shoot radially away from the + earth when the negative charge of the upper atmosphere has + reached a certain amount, which particles, although starting + sunward, will be swept back to the earth with the oncoming + streams. As the final result of all this accumulation of flying + and gyrating particles in the earth's neighborhood, we are told + that the latter must be transformed into the semblance of a + gigantic solid-headed comet provided with streaming tails, the + longest of them stretching away from the direction of the sun, + while another shorter one extends toward the sun. This shorter + tail is due to the particles that we have just spoken of as + being driven sunward from the earth by the action of + ultra-violet light. No doubt this whole subject is too + technical for popular statement; but at any rate the general + reader can understand the picturesque side of the theory, for + its advocates assure us that if we were on the moon we would + doubtless be able to see the comet-like tails of the earth, and + then we could appreciate the part that they play in producing + the phenomenon of the Zodiacal Light.</p> + + <p>That the Light as we see it could be produced by the + reflection of sunlight from swarms of particles careering round + the earth in the manner supposed by Arrhenius' hypothesis is + evident enough; and it will be observed that the new theory, + after all, is only another variant of the older one which + attributes the Zodiacal Light to an extension of the solar + corona. But it differs from the older theory in offering an + explanation of the manner in which the extension is effected, + and it differentiates between the corona proper and the streams + of negative particles shot away from the sun. In its details + the hypothesis of Arrhenius also affords an explanation of many + peculiarities of the Zodiacal Light, such as that it is + confined to the neighborhood of the ecliptic, and that it is + stronger on the side of the earth which is just turning away + from a position under the sun than on the other side; but it + would carry us beyond our limits to go into these particulars. + The Gegenschein, according to this theory, is a part of the + same phenomenon as the Zodiacal Light, for by the laws of + perspective it is evident that the reflection from the streams + of particles situated at a point directly opposite to the sun + would be at a maximum, and this is the place which the + Gegenschein occupies. Apart from its geometrical relations to + the position of the sun, the variability of the Zodiacal Light + appears to affirm its solar dependence, and this too would be + accounted for by Arrhenius' hypothesis better than by the old + theory of coronal extension. The amount of corpuscular + discharge from the sun must naturally be governed by the state + of relative activity or inactivity of the latter, and this + could not but be reflected in the varying splendor of the + Zodiacal Light. But much more extended study than has yet been + given to the subject will be required before we can feel that + we know with reasonable certainty what this mysterious + phenomenon really is. By the hypothesis of Arrhenius every + planet that has an atmosphere must have a Zodiacal Light + attending it, but the phenomenon is too faint for us to be able + to see it in the case, for instance, of Venus, whose atmosphere + is very abundant. The moon has no corresponding ``comet's + tail'' because, as already explained, of the lack of a lunar + atmosphere to repel the streams by becoming itself electrified; + but if there were a lunar Zodiacal Light, no doubt we could see + it because of the relative nearness of our satellite.</p> + + <p><strong>Marvels of the Aurora</strong></p> + + <p>One of the most vivid recollections of my early boyhood is + that of seeing my father return hastily into the house one + evening and call out to the family: ``Come outside and look at + the sky!'' Ours was a country house situated on a commanding + site, and as we all emerged from the doorway we were + dumbfounded to see the heavens filled with pale flames which + ran licking and quivering over the stars. Instantly there + sprang into my terrified mind the recollection of an awful + description of ``the Day of Judgment'' (the <em>Dies + Iræ</em>), which I had heard with much perturbation of + spirit in the Dutch Reformed church from the lips of a tall, + dark-browed, dreadfully-in-earnest preacher of the + old-fashioned type. My heart literally sank at sight of the + spectacle, for it recalled the preacher's very words; it was + just as he had said it would be, and it needed the assured + bearing of my elders finally to convince me that</p> + + <blockquote> + <br> + That Day of Wrath, O dreadful day,<br> + When Heaven and Earth shall pass away,<br> + As David and the Sibyl say<br> + <br> + </blockquote> + + <p>had not actually come upon us. And even the older members of + the household were not untouched with misgivings when menacing + spots of crimson appeared, breaking out now here, now there, in + the shuddering sky. Toward the north the spectacle was + appalling. A huge arch spanned an unnaturally dark segment + resting on the horizon, and above this arch sprang up beams and + streamers in a state of incessant agitation, sometimes shooting + up to the zenith with a velocity that took one's breath, and + sometimes suddenly falling into long ranks, and <em>marching, + marching, marching,</em> like an endless phalanx of fiery + specters, and moving, as I remember, always from east to west. + The absolute silence with which these mysterious evolutions + were performed and the quavering reflections which were thrown + upon the ground increased the awfulness of the exhibition. + Occasionally enormous curtains of lambent flame rolled and + unrolled with a majestic motion, or were shaken to and fro as + if by a mighty, noiseless wind. At times, too, a sudden + billowing rush would be made toward the zenith, and for a + minute the sky overhead would glow so brightly that the stars + seemed to have been consumed. The spectacle continued with + varying intensity for hours.</p> + + <p>This exhibition occurred in Central New York, a latitude in + which the Aurora Borealis is seldom seen with so much splendor. + I remember another similar one seen from the city of New York + in November, 1882. On this last occasion some observers saw a + great upright beam of light which majestically moved across the + heavens, stalking like an apparition in the midst of the + auroral pageant, of whose general movements it seemed to be + independent, maintaining always its upright posture, and + following a magnetic parallel from east to west. This + mysterious beam was seen by no less than twenty-six observers + in different parts of the country, and a comparison of their + observations led to a curious calculation indicating that the + apparition was about <em>one hundred and thirty-three miles + tall</em> and moved at the speed of ten miles per second!</p> + + <p>But, as everybody knows, it is in the Arctic regions that + the Aurora, or the ``Northern Lights,'' can best be seen. + There, in the long polar night, when for months together the + sun does not rise, the strange coruscations in the sky often + afford a kind of spectral daylight in unison with the weird + scenery of the world of ice. The pages in the narratives of + Arctic exploration that are devoted to descriptions of the + wonderful effects of the Northern Lights are second to none + that man has ever penned in their fascination. The lights, as I + have already intimated, display astonishing colors, + particularly shades of red and green, as they flit from place + to place in the sky. The discovery that the magnetic needle is + affected by the Aurora, quivering and darting about in a state + of extraordinary excitement when the lights are playing in the + sky, only added to the mystery of the phenomenon until its + electro-magnetic nature had been established. This became + evident as soon as it was known that the focus of the displays + was the magnetic pole; and when the far South was visited the + Aurora Australis was found, having its center at the South + Magnetic Pole. Then, if not before, it was clear that the earth + was a great globular magnet, having its poles of opposite + magnetism, and that the auroral lights, whatever their precise + cause might be, were manifestations of the magnetic activity of + our planet. After the invention of magnetic telegraphy it was + found that whenever a great Aurora occurred the telegraph lines + were interrupted in their operation, and the ocean cables + ceased to work. Such a phenomenon is called a ``magnetic + storm.''</p> + + <p>The interest excited by the Aurora in scientific circles was + greatly stimulated when, in the last half of the nineteenth + century, it was discovered that it is a phenomenon intimately + associated with disturbances on the sun. The ancient ``Zurich + Chronicles,'' extending from the year 1000 to the year 1800, in + which both sun-spots visible to the naked eye and great + displays of the auroral lights were recorded, first set Rudolf + Wolf on the track of this discovery. The first notable proof of + the suspected connection was furnished with dramatic emphasis + by an occurrence which happened on September 1, 1859. Near noon + on that day two intensely brilliant points suddenly broke out + in a group of sun-spots which were under observation by Mr R. + C. Carrington at his observatory at Redhill, England. The + points remained visible for not more than five minutes, during + which interval they moved <em>thirty-five thousand miles</em> + across the solar disk. Mr R. Hodgson happened to see the same + phenomenon at his observatory at Highgate, and thus all + possibility of deception was removed. But neither of the + startled observers could have anticipated what was to follow, + and, indeed, it was an occurrence which has never been + precisely duplicated. I quote the eloquent account given by + Miss Clerke in her <em>History of Astronomy During the + Nineteenth Century.</em></p> + + <blockquote> + <p>This unique phenomenon seemed as if specially designed to + accentuate the inference of a sympathetic relation between + the earth and the sun. From August 28 to September 4, 1859, a + magnetic storm of unparalleled intensity, extent, and + duration was in progress over the entire globe. Telegraphic + communication was everywhere interrupted -- except, indeed, + that it was in some cases found practicable to work the lines + <em>without batteries</em> by the agency of the + earth-currents alone; sparks issued from the wires; gorgeous + auroras draped the skies in solemn crimson over both + hemispheres, and even in the tropics; the magnetic needle + lost all trace of continuity in its movements and darted to + and fro as if stricken with inexplicable panic. The + coincidence was even closer. <em>At the very instant</em> of + the solar outburst witnessed by Carrington and Hodgson the + photographic apparatus at Kew registered a marked disturbance + of all the three magnetic elements; while shortly after the + ensuing midnight the electric agitation culminated, thrilling + the whole earth with subtle vibrations, and lighting up the + atmosphere from pole to pole with coruscating splendors which + perhaps dimly recall the times when our ancient planet itself + shone as a star.</p> + </blockquote> + + <p>If this amazing occurrence stood alone, and as I have + already said it has never been exactly duplicated, doubt might + be felt concerning some of the inferences drawn from it; but in + varying forms it has been repeated many times, so that now + hardly anyone questions the reality of the assumed connection + between solar outbursts and magnetic storms accompanied by + auroral displays on the earth. It is true that the late Lord + Kelvin raised difficulties in the way of the hypothesis of a + direct magnetic action of the sun upon the earth, because it + seemed to him that an inadmissible quantity of energy was + demanded to account for such action. But no calculation like + that which he made is final, since all calculations depend upon + the validity of the data; and no authority is unshakable in + science, because no man can possess omniscience. It was Lord + Kelvin who, but a few years before the thing was actually + accomplished, declared that aerial navigation was an + impracticable dream, and demonstrated its impracticability by + calculation. However the connection may be brought about, it is + as certain as evidence can make it that solar outbursts are + coincident with terrestial magnetic disturbances, and + coincident in such a way as to make the inference of a causal + connection irresistible. The sun is only a little more than a + hundred times its own diameter away from the earth. Why, then, + with the subtle connection between them afforded by the ether + which conveys to us the blinding solar light and the + life-sustaining solar heat, should it be so difficult to + believe that the sun's enormous electric energies find a way to + us also? No doubt the impulse coming from the sun acts upon the + earth after the manner of a touch upon a trigger, releasing + energies which are already stored up in our planet.</p> + + <p>But besides the evidence afforded by such occurrences as + have been related of an intimate connection between solar + outbreaks and terrestial magnetic flurries, attended by + magnificent auroral displays, there is another line of proof + pointing in the same direction. Thus, it is known that the + sun-spot period, as remarked in a preceding chapter, coincides + in a most remarkable manner with the periodic fluctuations in + the magnetic state of the earth. This coincidence runs into the + most astonishing details. For instance, when the sun-spot + period shortens, the auroral period shortens to precisely the + same extent; as the short sun-spot periods usually bring the + most intense outbreaks of solar activity, so the corresponding + short auroral periods are attended by the most violent magnetic + storms; a secular period of about two hundred and twenty-two + years affecting sun-spots is said to have its auroral + duplicate; a shorter period of fifty-five and a half years, + which some observers believe that they have discovered appears + also to be common to the two phenomena; and yet another + ``superposed'' period of about thirty-five years, which some + investigators aver exists, affects sun-spots and aurora alike. + In short, the coincidences are so numerous and significant that + one would have to throw the doctrine of probability to the + winds in order to be able to reject the conclusion to which + they so plainly lead.</p> + + <p>But still the question recurs: How is the influence + transmitted? Here Arrhenius comes once more with his hypothesis + of negative corpuscles, or ions, driven away from the sun by + light-pressure -- a hypothesis which seems to explain so many + things -- and offers it also as an explanation of the way in + which the sun creates the Aurora. He would give the Aurora the + same lineage with the Zodiacal Light. To understand the + application of this theory we must first recall the fact that + the earth is a great magnet having its two opposite poles of + magnetism, one near the Arctic and the other near the Antarctic + Circle. Like all magnets, the earth is surrounded with ``lines + of force,'' which, after the manner of the curved rays we saw + in the photograph of a solar eclipse, start from a pole, rising + at first nearly vertically, then bend gradually over, passing + high above the equator, and finally descending in converging + sheaves to the opposite pole. Now the axis of the earth is so + placed in space that it lies at nearly a right angle to the + direction of the sun, and as the streams of negatively charged + particles come pouring on from the sun (see the last preceding + chapter), they arrive in the greatest numbers over the earth's + equatorial regions. There they encounter the lines of magnetic + force at the place where the latter have their greatest + elevation above the earth, and where their direction is + horizontal to the earth's surface. Obeying a law which has been + demonstrated in the laboratory, the particles then follow the + lines of force toward the poles. While they are above the + equatorial regions they do not become luminescent, because at + the great elevation that they there occupy there is virtually + no atmosphere; but as they pass on toward the north and the + south they begin to descend with the lines of force, curving + down to meet at the poles; and, encountering a part of the + atmosphere comparable in density with what remains in an + exhausted Crookes tube, they produce a glow of cathode rays. + This glow is conceived to represent the Aurora, which may + consequently be likened to a gigantic exhibition of vacuum-tube + lights. Anybody who recalls his student days in the college + laboratory and who has witnessed a display of Northern Lights + will at once recognize the resemblance between them in colors, + forms, and behavior. This resemblance had often been noted + before Arrhenius elaborated his hypothesis.</p> + + <p>Without intending to treat his interesting theory as more + than a possibly correct explanation of the phenomena of the + Aurora, we may call attention to some apparently confirmatory + facts. One of the most striking of these relates to a seasonal + variation in the average number of auroræ. It has been + observed that there are more in March and September than at any + other time of the year, and fewer in June and December; + moreover (and this is a delicate test as applied to the + theory), they are slightly rarer in June than in December. Now + all these facts seem to find a ready explanation in the + hypothesis of Arrhenius, thus: (1) The particles issuing from + the sun are supposed to come principally from the regions whose + excitement is indicated by the presence of sun-spots (which + accords with Hale's observation that sun-spots are columns of + ionized vapors), and these regions have a definite location on + either side of the solar equator, seldom approaching it nearer + than within 5° or 10° north or south, and never + extending much beyond 35° toward either pole; (2) The + equator of the sun is inclined about 7° to the plane of the + earth's orbit, from which it results that twice in a year -- + <em>viz.,</em> in June and December -- the earth is directly + over the solar equator, and twice a year -- <em>viz.,</em> in + March and September -- when it is farthest north or south of + the solar equator, it is over the inner edge of the sun-spot + belts. Since the corpuscles must be supposed to be propelled + radially from the sun, few will reach the earth when the latter + is over the solar equator in June and December, but when it is + over, or nearly over, the spot belts, in March and September, + it will be in the line of fire of the more active parts of the + solar surface, and relatively rich streams of particles will + reach it. This, as will be seen from what has been said above, + is in strict accord with the observed variations in the + frequency of auroræ. Even the fact that somewhat fewer + auroræ are seen in June than in December also finds its + explanation in the known fact that the earth is about three + million miles nearer the sun in the winter than in the summer, + and the number of particles reaching it will vary, like the + intensity of light, inversely as the square of the distance. + These coincidences are certainly very striking, and they have a + cumulative force. If we accept the theory, it would appear that + we ought to congratulate ourselves that the inclination of the + sun's equator is so slight, for as things stand the earth is + never directly over the most active regions of the sun-spots, + and consequently never suffers from the maximum bombardment of + charged particles of which the sun is capable. Incessant + auroral displays, with their undulating draperies, flitting + colors, and marching columns might not be objectionable from + the point of view of picturesqueness, but one magnetic storm of + extreme intensity following closely upon the heels of another, + for months on end, crazing the magnetic needle and continually + putting the telegraph and cable lines out of commission, to say + nothing of their effect upon ``wireless telegraphy'', would + hardly add to the charms of terrestrial existence.</p> + + <p>One or two other curious points in connection with + Arrhenius' hypothesis may be mentioned. First, the number of + auroræ, according to his explanation, ought to be + greatest in the daytime, when the face of the earth on the + sunward side is directly exposed to the atomic bombardment. Of + course visual observation can give us no information about + this, since the light of the Aurora is never sufficiently + intense to be visible in the presence of daylight, but the + records of the magnetic observatories can be, and have been, + appealed to for information, and they indicate that the facts + actually accord with the theory. Behind the veil of sunlight in + the middle of the afternoon, there is good reason to believe, + auroral exhibitions often take place which would eclipse in + magnificence those seen at night if we could behold them. + Observation shows, too, that auroræ are more frequent + before than after midnight, which is just what we should expect + if they originate in the way that Arrhenius supposes. Second, + the theory offers an explanation of the alleged fact that the + formation of clouds in the upper air is more frequent in years + when auroræ are most abundant, because clouds are the + result of the condensation of moisture upon floating particles + in the atmosphere (in an absolutely dustless atmosphere there + would be no clouds), and it has been proved that negative ions + like those supposed to come from the sun play a master part in + the phenomena of cloud formation.</p> + + <p>Yet another singular fact, almost mystical in its + suggestions, may be mentioned. It seems that the dance of the + auroral lights occurs most frequently during the absence of the + moon from the hemisphere in which they appear, and that they + flee, in greater part, to the opposite hemisphere when the + moon's revolution in an orbit considerably inclined to the + earth's equator brings her into that where they have been + performing. Arrhenius himself discovered this curious relation + of auroral frequency to the position of the moon north or south + of the equator, and he explains it in this way. The moon, like + the earth, is exposed to the influx of the ions from the sun; + but having no atmosphere, or almost none, to interfere with + them, they descend directly upon her surface and charge her + with an electric negative potential to a very high degree. In + consequence of this she affects the electric state of the upper + parts of the earth's atmosphere where they lie most directly + beneath her, and thus prevents, to a large extent, the negative + discharges to which the appearance of the Aurora is due. And so + ``the extravagant and erring spirit'' of the Aurora avoids the + moon as Hamlet's ghost fled at the voice of the cock announcing + the awakening of the god of day.</p> + + <p>There are even other apparent confirmations of the + hypothesis, but we need not go into them. We shall, however, + find one more application of it in the next chapter, for it + appears to be a kind of cure-all for astronomical troubles; at + any rate it offers a conceivable solution of the question, How + does the sun manage to transmit its electric influence to the + earth? And this solution is so grandiose in conception, and so + novel in the mental pictures that it offers, that its + acceptance would not in the least detract from the impression + that the Aurora makes upon the imagination.</p> + + <p><strong>Strange Adventures of Comets</strong></p> + + <p>The fears and legends of ancient times before Science was + born, and the superstitions of the Dark Ages, sedulously + cultivated for theological purposes by monks and priests, have + so colored our ideas of the influence that comets have had upon + the human mind that many readers may be surprised to learn that + it was the apparition of a wonderful comet, that of 1843, which + led to the foundation of our greatest astronomical institution, + the Harvard College Observatory. No doubt the comet + superstition existed half a century ago, as, indeed, it exists + yet today, but in this case the marvelous spectacle in the sky + proved less effective in inspiring terror than in awakening a + desire for knowledge. Even in the sixteenth century the views + that enlightened minds took of comets tended powerfully to + inspire popular confidence in science, and Halley's prediction, + after seeing and studying the motion of the comet which + appeared in 1682, that it would prove to be a regular member of + the sun's family and would be seen returning after a period of + about seventy-six years, together with the fulfillment of that + prediction, produced a revulsion from the superstitious notions + which had so long prevailed.</p> + + <p>Then the facts were made plain that comets are subject to + the law of gravitation equally with the planets; that there are + many which regularly return to the neighborhood of the sun + (perihelion); and that these travel in orbits differing from + those of the planets only in their greater eccentricity, + although they have the peculiarity that they do not, like the + planets, all go round the sun in the same direction, and do not + keep within the general plane of the planetary system, but + traverse it sometimes from above and sometimes from below. + Other comets, including most of the ``great'' ones, appear to + travel in parabolic or, in a few cases, hyperbolic orbits, + which, not being closed curves, never bring them back again. + But it is not certain that these orbits may not be extremely + eccentric ellipses, and that after the lapse of hundreds, or + thousands, of years the comets that follow them may not + reappear. The question is an interesting one, because if all + orbits are really ellipses, then all comets must be permanent + members of the solar system, while in the contrary case many of + them are simply visitors, seen once and never to be seen again. + The hypothesis that comets are originally interlopers might + seem to derive some support from the fact that the certainly + periodic ones are associated, in groups, with the great outer + planets, whose attraction appears to have served as a trap for + them by turning them into elliptical orbits and thus making + them prisoners in the solar system. Jupiter, owing to his great + mass and his commanding situation in the system, is the chief + ``comet-catcher;'' but he catches them not for himself, but for + the sun. Yet if comets do come originally from without the + borders of the planetary system, it does not, by any means, + follow that they were wanderers at large in space before they + yielded to the overmastering attraction of the sun. + Investigation of the known cometary orbits, combined with + theoretical considerations, has led some astronomers to the + conclusion that as the sun travels onward through space he + ``picks up <em>en route</em>'' cometary masses which, without + belonging strictly to his empire, are borne along in the same + vast ``cosmical current'' that carries the solar system.</p> + + <p>But while no intelligent person any longer thinks that the + appearance of a great comet is a token from the heavenly powers + of the approaching death of a mighty ruler, or the outbreak of + a devastating war, or the infliction of a terrible plague upon + wicked mankind, science itself has discovered mysteries about + comets which are not less fascinating because they are more + intellectual than the irrational fancies that they have + displaced. To bring the subject properly before the mind, let + us see what the principal phenomena connected with a comet + are.</p> + + <p>At the present day comets are ordinarily ``picked up'' with + the telescope or the photographic plate before any one except + their discoverer is aware of their existence, and usually they + remain so insignificant in appearance that only astronomers + ever see them. Yet so great is the prestige of the word + ``comet'' that the discovery of one of these inconspicuous + wanderers, and its subsequent movements, become items of the + day's news which everybody reads with the feeling, perhaps, + that at least he knows what is going on in the universe even if + he doesn't understand it. But a truly great comet presents + quite a different proposition. It, too, is apt to be detected + coming out of the depths of space before the world at large can + get a glimpse of it, but as it approaches the sun its aspect + undergoes a marvelous change. Agitated apparently by solar + influence, it throws out a long streaming tail of nebulous + light, directed away from the sun and looking as if blown out + like a pennon by a powerful wind. Whatever may be the position + of the comet with regard to the sun, as it circles round him it + continually keeps its tail on the off side. This, as we shall + soon see, is a fact of capital importance in relation to the + probable nature of comets' tails. Almost at the same time that + the formation of the tail is observed a remarkable change takes + place in the comet's head, which, by the way, is invariably and + not merely occasionally its most important part. On approaching + the sun the head usually contracts. Coincidently with this + contraction a nucleus generally makes its appearance. This is a + bright, star-like point in the head, and it probably represents + the totality of solid matter that the comet possesses. But it + is regarded as extremely unlikely that even the nucleus + consists of a uniformly solid mass. If it were such, comets + would be far more formidable visitors when they pass near the + planets than they have been found to be. The diameter of the + nucleus may vary from a few hundred up to several thousand + miles; the heads, on the average, are from twenty-five thousand + to one hundred thousand miles in diameter, although a few have + greatly exceeded these dimensions; that of the comet of 1811, + one of the most stupendous ever seen, was a million and a + quarter miles in diameter! As to the tails, not withstanding + their enormous length -- some have been more than a hundred + million miles long -- there is reason to believe that they are + of extreme tenuity, ``as rare as vacuum.'' The smallest stars + have been seen shining through their most brilliant portions + with undiminished luster.</p> + + <p>After the nucleus has been formed it begins to throw out + bright jets directed toward the sun. A stream, and sometimes + several streams, of light also project sunward from the + nucleus, occasionally appearing like a stunted tail directed + oppositely to the real tail. Symmetrical envelopes which, seen + in section, appear as half circles or parabolas, rise sunward + from the nucleus, forming a concentric series. The ends of + these stream backward into the tail, to which they seem to + supply material. Ordinarily the formation of these ejections + and envelopes is attended by intense agitation of the nucleus, + which twists and turns, swinging and gyrating with an + appearance of the greatest violence. Sometimes the nucleus is + seen to break up into several parts. The entire heads of some + comets have been split asunder in passing close around the sun; + The comet of 1882 retreated into space after its perihelion + passage with <em>five heads</em> instead of the one that it had + originally, and each of these heads had its own tail!</p> + + <p>The possession of the spectroscope has enabled astronomers + during later years to study the chemical composition of comets + by analyzing their light. At first the only substances thus + discovered in them were hydro-carbon compounds, due evidently + to the gaseous envelopes in which some combination of hydrogen + with carbon existed. Behind this gaseous spectrum was found a + faint continuous spectrum ascribed to the nucleus, which + apparently both reflects the sunlight and gives forth the light + of a glowing solid or liquid. Subsequently sodium and iron + lines were found in cometary spectra. The presence of iron + would seem to indicate that some of these bodies may be much + more massive than observations on their attractive effects have + indicated. In some recent comets, such as Morehouse's, in 1908, + several lines have been found, the origin of which is + unknown.</p> + + <p>Without going back of the nineteenth century we may find + records of some of the most extraordinary comets that man has + ever looked upon. In 1811, still spoken of as ``the year of the + comet,'' because of the wonderful vintage ascribed to the skyey + visitor, a comet shaped like a gigantic sword amazed the whole + world, and, as it remained visible for seventeen months, was + regarded by superstitious persons as a symbol of the fearful + happenings of Napoleon's Russian campaign. This comet, the + extraordinary size of whose head, greatly exceeding that of the + sun itself, has already been mentioned, was also remarkable for + exhibiting so great a brilliancy without approaching even to + the earth's distance from the sun. But there was once a comet + (and only once -- in the year 1729) which never got nearer to + the sun than four times the distance of the earth and yet + appeared as a formidable object in the sky. As Professor Young + has remarked, ``it must have been an enormous comet to be + visible from such a distance.'' And we are to remember that + there were no great telescopes in the year 1729. That comet + affects the imagination like a phantom of space peering into + the solar system, displaying its enormous train afar off + (which, if it had approached as near as other comets, would + probably have become <em>the</em> celestial wonder of all human + memory), and then turning away and vanishing in the depths of + immensity.</p> + + <p>In 1843 a comet appeared which was so brilliant that it + could be seen in broad day close beside the sun! This was the + first authenticated instance of that kind, but the occurrence + was to be repeated, as we shall see in a moment, less than + forty years later.</p> + + <p>The splendid comet of 1858, usually called Donati's, is + remembered by many persons yet living. It was, perhaps, both as + seen by the naked eye and with the telescope, the most + beautiful comet of which we have any record. It too marked a + rich vintage year, still remembered in the vineyards of France, + where there is a popular belief that a great comet ripens the + grape and imparts to the wine a flavor not attainable by the + mere skill of the cultivator. There are ``comet wines,'' + carefully treasured in certain cellars, and brought forth only + when their owner wishes to treat his guests to a sip from + paradise.</p> + + <p>The year 1861 saw another very remarkable comet, of an + aspect strangely vast and diffuse, which is believed to have + swept the earth with its immense tail when it passed between us + and the sun on the night of June 30th, an event which produced + no other known effect than the appearance of an unwonted amount + of scattered light in the sky.</p> + + <p>The next very notable comet was the ``Great Southern Comet'' + of 1880, which was not seen from the northern hemisphere. It + mimicked the aspect of the famous comet of 1843, and to the + great surprise of astronomers appeared to be traveling in the + same path. This proved to be the rising of the curtain for an + astronomical sensation unparalleled in its kind; for two years + later another brilliant comet appeared, first in the southern + hemisphere, <em>and it too followed the same track.</em> The + startling suggestion was now made that this comet was identical + with those of 1843 and 1880, its return having been hastened by + the resistance experienced in passing twice through the coronal + envelope, and there were some who thought that it would now + swing swiftly round and then plunge straight into the sun, with + consequences that might be disastrous to us on account of the + ``flash of heat'' that would be produced by the impact. Nervous + people were frightened, but observation soon proved that the + danger was imaginary, for although the comet almost grazed the + sun, and must have rushed through two or three million miles of + the coronal region, no retardation of its immense velocity was + perceptible, and it finally passed away in a damaged condition, + as before remarked, and has never since appeared.</p> + + <p>Then the probable truth was perceived -- <em>viz.,</em> that + the three comets (1843, 1880, and 1882) were not one identical + body, but three separate ones all traveling in the same orbit. + It was found, too, that a comet seen in 1668 bore similar + insignia of relationship. The natural inference was that these + four bodies had once formed a single mass which had been split + apart by the disruptive action of the sun. Strength was lent to + this hypothesis by the fact that the comet of 1882 was + apparently torn asunder during its perihelion passage, + retreating into space in a dissevered state. But Prof. George + Forbes has a theory that the splitting of the original cometary + mass was effected by an unknown planet, probably greater than + Jupiter, situated at a hundred times the earth's distance from + the sun, and revolving in a period of a thousand years. He + supposes that the original comet was not that of 1668, but one + seen in 1556, which has since been ``missing,'' and that its + disruption occurred from an encounter with the supposititious + planet about the year 1700. Truly from every point of view + comets are the most extraordinary of adventurers!</p> + + <p>The comet of 1882 was likewise remarkable for being visible, + like its predecessor of 1843, in full daylight in close + proximity to the sun. The story of its detection when almost in + contact with the solar disk is dramatic. It had been discovered + in the southern hemisphere only a couple of weeks before its + perihelion, which occurred on September 17th, and on the + forenoon of that day it was seen by Doctor Common in England, + and by Doctor Elkin and Mr Finlay at the Cape of Good Hope, + almost touching the sun. It looked like a dazzling white bird + with outspread wings. The southern observers watched it go + <em>right into the sun,</em> when it instantly disappeared. + What had happened was that the comet in passing its perihelion + point had swung exactly between the earth and the sun. On the + following morning it was seen from all parts of the world close + by the sun on the opposite side, and it remained thus visible + for three days, gradually receding from the solar disk. It then + became visible for northern observers in the morning sky before + sunrise, brandishing a portentous sword-shaped tail which, if + it had been in the evening sky, would have excited the wonder + of hundreds of millions, but situated where it was, + comparatively few ever saw it.</p> + + <p>The application of photography to the study of comets has + revealed many curious details which might otherwise have + escaped detection, or at best have remained subject to doubt. + It has in particular shown not only the precise form of the + tails, but the remarkable vicissitudes that they undergo. + Professor Barnard's photographs of Brooks' comet in 1893 + suggested, by the extraordinary changes in the form of the tail + which they revealed, that the comet was encountering a series + of obstructions in space which bent and twisted its tail into + fantastic shapes. The reader will observe the strange form into + which the tail was thrown on the night of October 21st. A cloud + of meteors through which the comet was passing might have + produced such deformations of its tail. In the photograph of + Daniels' comet of 1907, a curious striping of the tail will be + noticed. The short bright streaks seen in the photograph, it + may be explained, are the images of stars which are drawn out + into lines in consequence of the fact that the photographic + telescope was adjusted to follow the motion of the comet while + the stars remained at rest.</p> + + <p>But the adventures of comets are not confined to possible + encounters with unknown obstacles. We have referred to the fact + that the great planets, and especially Jupiter, frequently + interfere with the motions of comets. This interference is not + limited to the original alteration of their orbits from + possible parabolas to ellipses, but is sometimes exercised + again and again, turning the bewildered comets into elliptical + paths of all degrees of eccentricity. A famous example of this + kind of planetary horse-play is furnished by the story of + Lexell's missing comet. This comet was first seen in 1770. + Investigation showed that it was moving in an orbit which + should bring it back to perihelion every five and a half years; + yet it had never been seen before and, although often searched + for, has never been seen since. Laplace and Leverrier proved + mathematically that in 1767 it had approached so close to + Jupiter as to be involved among the orbits of his satellites. + What its track had been before is not known, but on that + occasion the giant planet seized the interloper, threw it into + a short elliptic orbit and sent it, like an arrested vagrant, + to receive sentence at the bar of the sun. On this journey it + passed within less than 1,500,000 miles of the earth. The form + of orbit which Jupiter had impressed required, as we have said, + its return in about five and a half years; but soon after 1770 + it had the misfortune a second time to encounter Jupiter at + close range, and he, as if dissatisfied with the leniency of + the sun, or indignant at the stranger's familiarity, seized the + comet and hurled it out of the system, or at any rate so far + away that it has never since been able to rejoin the family + circle that basks in the immediate rays of the solar hearth. + Nor is this the only instance in which Jupiter has dealt + summarily with small comets that have approached him with too + little deference.</p> + + <p>The function which Jupiter so conspicuously fulfills as + master of the hounds to the sun is worth considering a little + more in detail. To change the figure, imagine the sun in its + voyage through space to be like a majestic battleship + surrounded by its scouts. Small vessels (the comets, as they + are overhauled by the squadron, are taken in charge by the + scouts, with Jupiter for their chief, and are forced to + accompany the fleet, but not all are impressed. If a strange + comet undertakes to run across Jupiter's bows the latter brings + it to, and makes prize of it by throwing it into a relatively + small ellipse with the sun for its focus. Thenceforth, unless, + as happened to the unhappy comet of Lexell, it encounters + Jupiter again in such a way as to be diverted by him into a + more distant orbit, it can never get away. About thirty comets + are now known to have thus been captured by the great planet, + and they are called ``Jupiter's Comet Family.'' But, on the + other hand, if a wandering comet crosses the wake of the chief + planetary scout the latter simply drives it away by + accelerating its motion and compels it to steer off into open + space. The transformation of comets into meteors will be + considered in the next chapter, but here, in passing, mention + may be made of the strange fate of one member of Jupiter's + family, Biela's comet, which, having become over bold in its + advances to its captor, was, after a few revolutions in is + impressed orbit, torn to pieces and turned into a flock of + meteors.</p> + + <p>And now let us return to the mystery of comets' tails. That + we are fully justified in speaking of the tails of comets as + mysterious is proved by the declaration of Sir John Herschel, + who averred, in so many words, that ``there is some profound + secret and mystery of nature concerned in this phenomenon,'' + and this profound secret and mystery has not yet been + altogether cleared up. Nevertheless, the all-explaining + hypothesis of Arrhenius offers us once more a certain amount of + aid. Comets' tails, Arrhenius assures us, are but another + result of the pressure of light. The reader will recall the + applications of this theory to the Zodiacal Light and the + Aurora. In the form in which we now have to deal with it, the + supposition is made that as a comet approaches the sun + eruptions of vapor, due to the solar heat, occur in its + nucleus. These are naturally most active on the side which is + directly exposed to the sun, whence the appearance of the + immense glowing envelopes that surround the nucleus on the + sunward side. Among the particles of hydro-carbon, and perhaps + solid carbon in the state of fine dust, which are thus set free + there will be many whose size is within the critical limit + which enables the light-waves from the sun to drive them away. + Clouds of such particles, then, will stream off behind the + advancing comet, producing the appearance of a tail. This + accounts for the fact that the tails of comets are always + directed away from the sun, and it also explains the varying + forms of the tails and the extraordinary changes that they + undergo. The speed of the particles driven before the + light-waves must depend upon their size and weight, the + lightest of a given size traveling the most swiftly. By + accretion certain particles might grow, thus losing velocity + and producing the appearance of bunches in the tail, such as + have been observed. The hypothesis also falls in with the + researches of Bredichin, who has divided the tails of comets + into three principal classes -- <em>viz.:</em> (1) Those which + appear as long, straight rays; (2) Those which have the form of + curved plumes or scimitars; (3) Those which are short, brushy, + and curved sharply backward along the comet's path. In the + first type he calculates the repulsive force at from twelve to + fifteen times the force of gravity; in the second at from two + to four times; and in the third at about one and a half times. + The straight tails he ascribes to hydrogen because the hydrogen + atom is the lightest known; the sword-shaped tails to + hydro-carbons; and the stumpy tails to vaporized iron. It will + be seen that, if the force driving off the tails is that which + Arrhenius assumes it to be, the forms of those appendages would + accord with those that Bredichin's theory calls for. At the + same time we have an explanation of the multiple tails with + which some comets have adorned themselves. The comet of 1744, + for instance, had at one time no less than seven tails spread + in a wide curved brush behind it. Donati's comet of 1858 also + had at least two tails, the principal one sword-shaped and the + other long, narrow, and as straight as a rule. According to + Bredichin, the straight tail must have been composed of + hydrogen, and the other of some form of hydro-carbon whose + atoms are heavier than those of hydrogen, and, consequently, + when swept away by the storm of light-waves, followed a + curvature depending upon the resultant of the forces operating + upon them. The seven tails of the comet of 1744 presented a + kind of diagram graphically exhibiting its complex composition, + and, if we knew a little more about the constituents of a + comet, we might be able to say from the amount of curvature of + the different tails just what were the seven substances of + which that comet consisted.</p> + + <p>If these theories seem to the reader fantastic, at any rate + they are no more fantastic than the phenomena that they seek to + explain.</p> + + <p><strong>Meteors, Fire-Balls, and Meteorites</strong></p> + + <p>One of the most terrorizing spectacles with which the + heavens have ever caused the hearts of men to quake occurred on + the night of November 13, 1833. On that night North America, + which faced the storm, was under a continual rain of fire from + about ten o'clock in the evening until daybreak.</p> + + <p><em>The fragments of a comet had struck the earth.</em></p> + + <p>But the meaning of what had happened was not discovered + until long afterward. To the astronomers who, with astonishment + not less than that of other people, watched the wonderful + scene, it was an unparalleled ``shower of meteors.'' They did + not then suspect that those meteors had once formed the head of + a comet. Light dawned when, a year later, Prof. Denison + Olmsted, of Yale College, demonstrated that the meteors had all + moved in parallel orbits around the sun, and that these orbits + intersected that of the earth at the point where our planet + happened to be on the memorable night of November 13th. + Professor Olmsted even went so far as to suggest that the cloud + of meteors that had encountered the earth might form a diffuse + comet; but full recognition of the fact that they were cometary + débris came later, as the result of further + investigation. The key to the secret was plainly displayed in + the spectacle itself, and was noticed without being understood + by thousands of the terror-stricken beholders. It was <em>an + umbrella of fire</em> that had opened overhead and covered the + heavens; in other words, the meteors all radiated from a + particular point in the constellation Leo, and, being countless + as the snowflakes in a winter tempest, they ribbed the sky with + fiery streaks. Professor Olmsted showed that the radiation of + the meteors from a fixed point was an effect of perspective, + and in itself a proof that they were moving in parallel paths + when they encountered the earth. The fact was noted that there + had been a similar, but incomparably less brilliant, display of + meteors on the same day of November, 1832, and it was rightly + concluded that these had belonged to the same stream, although + the true relationship of the phenomena was not immediately + apprehended. Olmsted ascribed to the meteors a revolution about + the sun once in every six months, bringing them to the + intersection of their orbit with that of the earth every + November 13th; but later investigators found that the real + period was about thirty-three and one-quarter years, so that + the great displays were due three times in a century, and their + return was confidently predicted for the year 1866. The + appearance of the meteors in 1832, a year before the great + display, was ascribed to the great length of the stream which + they formed in space -- so great that they required more than + two years to cross the earth's orbit. In 1832 the earth had + encountered a relatively rare part of the stream, but in 1833, + on returning to the crossing-place, it found there the richest + part of the stream pouring across its orbit. This explanation + also proved to be correct, and the predicted return in 1866 was + duly witnessed, although the display was much less brilliant + than in 1833. It was followed by another in 1867.</p> + + <p>In the mean time Olmsted's idea of a cometary relationship + of the meteors was demonstrated to be correct by the researches + of Schiaparelli and others, who showed that not only the + November meteors, but those of August, which are seen more or + less abundantly every year, traveled in the tracks of + well-known comets, and had undoubtedly an identical origin with + those comets. In other words the comets and the meteor-swarms + were both remnants of original masses which had probably been + split up by the action of the sun, or of some planet to which + they had made close approaches. The annual periodicity of the + August meteors was ascribed to the fact that the separation had + taken place so long ago that the meteors had become distributed + all around the orbit, in consequence of which the earth + encountered some of them every year when it arrived at the + crossing-point. Then Leverrier showed that the original comet + associated with the November meteors was probably brought into + the system by the influence of the planet Uranus in the year + 126 of the Christian era. Afterward Alexander Herschel + identified the tracks of no less than seventy-six meteor-swarms + (most of them inconspicuous) with those of comets. The still + more recent researches of Mr W. F. Denning make it probable + that there are no meteors which do not belong to a flock or + system probably formed by the disintegration of a cometary + mass; even the apparently sporadic ones which shoot across the + sky, ``lost souls in the night,'' being members of flocks which + have become so widely scattered that the earth sometimes takes + weeks to pass through the region of space where their paths + lie.</p> + + <p>The November meteors should have exhibited another pair of + spectacles in 1899 and 1900, and their failure to do so caused + at first much disappointment, until it was made plain that a + good reason existed for their absence. It was found that after + their last appearance, in 1867, they had been disturbed in + their movements by the planets Jupiter and Saturn, whose + attractions had so shifted the position of their orbit that it + no longer intersected that of the earth, as it did before. + Whether another planetary interference will sometime bring the + principal mass of the November meteors back to the former point + of intersection with the earth's orbit is a question for the + future to decide. It would seem that there may be several + parallel streams of the November meteors, and that some of + them, like those of August, are distributed entirely around the + orbit, so that every mid-November we see a few of them.</p> + + <p>We come now to a very remarkable example of the + disintegration of a comet and the formation of a meteor-stream. + In 1826 Biela, of Josephstadt, Austria, discovered a comet to + which his name was given. Calculation showed that it had an + orbital period of about six and a half years, belonging to + Jupiter's ``family.'' On one of its returns, in 1846, it + astonished its watchers by suddenly splitting in two. The two + comets thus formed out of one separated to a distance of about + one hundred and sixty thousand miles, and then raced side by + side, sometimes with a curious ligature connecting them, like + Siamese twins, until they disappeared together in + interplanetary space. In 1852 they came back, still nearly side + by side, but now the distance between them had increased to a + million and a quarter of miles. After that, at every recurrence + of their period, astronomers looked for them in vain, until + 1872, when an amazing thing happened. On the night of November + 28th, when the earth was crossing the plane of the orbit of the + missing comet, a brilliant shower of meteors burst from the + northern sky, traveling nearly in the track which the comet + should have pursued. The astronomers were electrified. + Klinkerfues, of Göttingen, telegraphed to Pogson, of + Madras: <em>``Biela touched earth; search near Theta + Centauri.''</em> Pogson searched in the place indicated and saw + a cometary mass retreating into the southern heavens, where it + was soon swallowed from sight!</p> + + <p>Since then the Biela meteors have been among the recognized + periodic spectacles of the sky, and few if any doubt that they + represent a portion of the missing comet whose disintegration + began with the separation into two parts in 1846. The comet + itself has never since been seen. The first display of these + meteors, sometimes called the ``Andromedes,'' because they + radiate from the constellation Andromeda, was remarkable for + the great brilliancy of many of the fire-balls that shot among + the shower of smaller sparks, some of which were described as + equaling the full moon in size. None of them is known to have + reached the earth, but during the display of the same meteors + in 1885 a meteoric mass fell at Mazapil in Northern Mexico (it + is now in the Museum at Vienna), which many have thought may + actually be a piece of the original comet of Biela. This brings + us to the second branch of our subject.</p> + + <p>More rare than meteors or falling stars, and more startling, + except that they never appear in showers, are the huge balls of + fire which occasionally dart through the sky, lighting up the + landscapes beneath with their glare, leaving trains of sparks + behind them, often producing peals of thunder when they + explode, and in many cases falling upon the earth and burying + themselves from a few inches to several feet in the soil, from + which, more than once, they have been picked up while yet hot + and fuming. These balls are sometimes called bolides. They are + not really round in shape, although they often look so while + traversing the sky, but their forms are fragmentary, and + occasionally fantastic. It has been supposed that their origin + is different from that of the true meteors; it has even been + conjectured that they may have originated from the giant + volcanoes of the moon or have been shot out from the sun during + some of the tremendous explosions that accompany the formation + of eruptive prominences. By the same reasoning some of them + might be supposed to have come from some distant star. Others + have conjectured that they are wanderers in space, of unknown + origin, which the earth encounters as it journeys on, and Lord + Kelvin made a suggestion which has become classic because of + its imaginative reach -- <em>viz.,</em> that the first germs of + life may have been brought to the earth by one of these bodies, + ``a fragment of an exploded world.''</p> + + <p>It is a singular fact that astronomers and scientific men in + general were among the last to admit the possibility of solid + masses falling from the sky. The people had believed in the + reality of such phenomena from the earliest times, but the + savants shook their heads and talked of superstition. This was + the less surprising because no scientifically authenticated + instance of such an occurrence was known, and the stones + popularly believed to have fallen from the sky had become the + objects of worship or superstitious reverence, a fact not + calculated to recommend them to scientific credence. The + celebrated ``black stone'' suspended in the Kaaba at Mecca is + one of these reputed gifts from heaven; the ``Palladium'' of + ancient Troy was another; and a stone which fell near + Ensisheim, in Germany, was placed in a church as an object to + be religiously venerated. Many legends of falling stones + existed in antiquity, some of them curiously transfigured by + the imagination, like the ``Lion of the Peloponnesus,'' which + was said to have sprung down from the sky upon the Isthmus of + Corinth. But near the beginning of the nineteenth century, in + 1803, a veritable shower of falling stones occurred at L'Aigle, + in Northern France, and this time astronomers took note of the + phenomenon and scientifically investigated it. Thousands of the + strange projectiles came from the sky on this occasion, and + were scattered over a wide area of country, and some buildings + were hit. Four years later another shower of stones occurred at + Weston, Conn., numbering thousands of individuals. The local + alarm created in both cases was great, as well it might be, for + what could be more intimidating than to find the blue vault of + heaven suddenly hurling solid missiles at the homes of men? + After these occurrences it was impossible for the most + skeptical to doubt any longer, and the regular study of + ``aerolites,'' or ``meteorites,'' began.</p> + + <p>One of the first things recognized was the fact that + fire-balls are solid meteorites in flight, and not gaseous + exhalations in the air, as some had assumed. They burn in the + air during their flight, and sometimes, perhaps, are entirely + consumed before reaching the ground. Their velocity before + entering the earth's atmosphere is equal to that of the planets + in their orbits -- <em>viz.,</em> from twenty to thirty miles + per second -- a fact which proves that the sun is the seat of + the central force governing them. Their burning in the air is + not difficult to explain; it is the heat of friction which so + quickly brings them to incandescence. Calculation shows that a + body moving through the air at a velocity of about a mile per + second will be brought, superficially, to the temperature of + ``red heat'' by friction with the atmosphere. If its velocity + is twenty miles per second the temperature will become + thousands of degrees. This is the state of affairs with a + meteorite rushing into the earth's atmosphere; its surface is + liquefied within a few seconds after the friction begins to + act, and the melted and vaporized portion of its mass is swept + backward, forming the train of sparks that follows every great + fire-ball. However, there is one phenomenon connected with the + trains of meteorites which has never been satisfactorily + explained: they often persist for long periods of time, + drifting and turning with the wind, but not ceasing to glow + with a phosphorescent luminosity. The question is, Whence comes + this light? It must be light without heat, since the fine dust + or vapor of which the train can only consist would not retain + sufficient heat to render it luminous for so long a time. An + extremely remarkable incident of this kind occurred on February + 22, 1909, when an immense fire-ball that passed over southern + England left a train that remained visible during two hours, + assuming many curious shapes as it was drifted about by + currents in the air.</p> + + <p>But notwithstanding the enormous velocity with which + meteorites enter the air they are soon slowed down to + comparatively moderate speed, so that when they disappear they + are usually traveling not faster than a mile a second. The + courses of many have been traced by observers situated along + their track at various points, and thus a knowledge has been + obtained of their height above the ground during their flight + and of the length of their visible courses. They generally + appear at an elevation of eighty or a hundred miles, and are + seldom visible after having descended to within five miles of + the ground, unless the observer happens to be near the + striking-point, when he may actually witness the fall. + Frequently they burst while high in the air and their fragments + are scattered like shrapnel over the surface of the ground, + sometimes covering an area of several square miles, but of + course not thickly; different fragments of the same meteorite + may reach the ground at points several miles apart. The + observed length of their courses in the atmosphere varies from + fifty to five hundred miles. If they continued a long time in + flight after entering the air, even the largest of them would + probably be consumed to the last scrap, but their fiery career + is so short on account of their great speed that the heat does + not have time to penetrate very deeply, and some that have been + picked up immediately after their fall have been found cold as + ice within. Their size after reaching the ground is variable + within wide limits; some are known which weigh several tons, + but the great majority weigh only a few pounds and many only a + few ounces.</p> + + <p>Meteorites are of two kinds: <em>stony</em> meteorites and + <em>iron</em> meteorites. The former outnumber the latter + twenty to one; but many stone meteorites contain grains of + iron. Nickel is commonly found in iron meteorites, so that it + might be said that that redoubtable alloy nickel-steel is of + cosmical invention. Some twenty-five chemical elements have + been found in meteorites, including carbon and the + ``sun-metal,'' helium. The presence of the latter is certainly + highly suggestive in connection with the question of the origin + of meteorites. The iron meteorites, besides metallic iron and + nickel, of which they are almost entirely composed, contain + hydrogen, helium, and carbonic oxide, and about the only + imaginable way in which these gases could have become absorbed + in the iron would be through the immersion of the latter while + in a molten or vaporized state in a hot and dense atmosphere + composed of them, a condition which we know to exist only in + the envelopes of the sun and the stars.</p> + + <p>The existence of carbon in the Canyon Diablo iron meteorites + is attended by a circumstance of the most singular character -- + a very ``fairy tale of science.'' In some cases <em>the carbon + has become diamond!</em> These meteoric diamonds are very + small; nevertheless, they are true diamonds, resembling in many + ways the little black gems produced by Moissan's method with + the aid of the electric furnace. The fact that they are found + embedded in these iron meteorites is another argument in favor + of the hypothesis of the solar or stellar origin of the latter. + To appreciate this it is necessary to recall the way in which + Moissan made his diamonds. It was by a combination of the + effects of great heat, great pressure, and sudden or rapid + superficial cooling on a mass of iron containing carbon. When + he finally broke open his iron he found it a pudding stuffed + with miniature black diamonds. When a fragment of the Canyon + Diablo meteoric iron was polished in Philadelphia over fifteen + years ago it cut the emery-wheel to pieces, and examination + showed that the damage had been effected by microscopic + diamonds peppered through the mass. How were those diamonds + formed? If the sun or Sirius was the laboratory that prepared + them, we can get a glimpse at the process of their formation. + There is plenty of heat, plenty of pressure, and an abundance + of vaporized iron in the sun and the stars. When a great solar + eruption takes place, masses of iron which have absorbed carbon + may be shot out with a velocity which forbids their return. + Plunged into the frightful cold of space, their surfaces are + quickly cooled, as Moissan cooled his prepared iron by throwing + it into water, and thus the requisite stress is set up within, + and, as the iron solidifies, the included carbon crystallizes + into diamonds. Whether this explanation has a germ of truth in + it or not, at any rate it is evident that iron meteorites were + not created in the form in which they come to us; they must + once have been parts of immeasurably more massive bodies than + themselves.</p> + + <p>The fall of meteorites offers an appreciable, though + numerically insignificant, peril to the inhabitants of the + earth. Historical records show perhaps three or four instances + of people being killed by these bodies. But for the protection + afforded by the atmosphere, which acts as a very effective + shield, the danger would doubtless be very much greater. In the + absence of an atmosphere not only would more meteorites reach + the ground, but their striking force would be incomparably + greater, since, as we have seen, the larger part of their + original velocity is destroyed by the resistance of the air. A + meteorite weighing many tons and striking the earth with a + velocity of twenty or thirty miles per second, would probably + cause frightful havoc.</p> + + <p>It is a singular fact that recent investigations seem to + have proved that an event of this kind actually happened in + North America -- perhaps not longer than a thousand or two + thousand years ago. The scene of the supposed catastrophe is in + northern central Arizona, at Coon Butte, where there is a + nearly circular crater in the middle of a circular elevation or + small mountain. The crater is somewhat over four thousand feet + in diameter, and the surrounding rim, formed of upturned strata + and ejected rock fragments, rises at its highest point one + hundred and sixty feet above the plain. The crater is about six + hundred feet in depth -- that is, from the rim to the visible + floor or bottom of the crater. There is no evidence that + volcanic action has ever taken place in the immediate + neighborhood of Coon Butte. The rock in which the crater has + been made is composed of horizontal sandstone and limestone + strata. Between three hundred and four hundred million tons of + rock fragments have been detached, and a large portion hurled + by some cause out of the crater. These fragments lie + concentrically distributed around the crater, and in large + measure form the elevation known as Coon Butte. The region has + been famous for nearly twenty years on account of the masses of + meteoric iron found scattered about and known as the ``Canyon + Diablo'' meteorites. It was one of these masses, which consist + of nickel-iron containing a small quantity of platinum, and of + which in all some ten tons have been recovered for sale to the + various collectors throughout the world, that as before + mentioned destroyed the grinding-tool at Philadelphia through + the cutting power of its embedded diamonds. These meteoric + irons are scattered about the crater-hill, in concentric + distribution, to a maximum distance of about five miles. When + the suggestion was first made in 1896 that a monster meteorite + might have created by its fall this singular lone crater <em>in + stratified rocks,</em> it was greeted with incredulous smiles; + but since then the matter has assumed a different aspect. The + Standard Iron Company, formed by Messrs. D. M. Barringer, B. C. + Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in + 1903, the owner of this freak of nature, sunk shafts and bored + holes to a great depth in the interior of the crater, and also + trenched the slopes of the mountain, and the result of their + investigations has proved that the meteoric hypothesis of + origin is correct. (See the papers published in the + <em>Proceedings of the Academy of Natural Sciences of + Philadelphia,</em> December, 1905, wherein it is proved that + the United States Geological Survey was wrong in believing this + crater to have been due to a steam explosion. Since that date + there has been discovered a great amount of additional + confirmatory proof). Material of unmistakably meteoric origin + was found by means of the drills, mixed with crushed rock, to a + depth of six hundred to seven hundred feet below the floor of + the crater, and a great deal of it has been found admixed with + the ejected rock fragments on the outer slopes of the mountain, + absolutely proving synchronism between the two events, the + formation of this great crater and the falling of the meteoric + iron out of the sky. The drill located in the bottom of the + crater was sent, in a number of cases, much deeper (over one + thousand feet) into unaltered horizontal red sandstone strata, + but no meteoric material was found below this depth (seven + hundred feet, or between eleven and twelve hundred feet below + the level of the surrounding plain), which has been assumed as + being about the limit of penetration. It is not possible to + sink a shaft at present, owing to the water which has drained + into the crater, and which forms, with the finely pulverized + sandstone, a very troublesome quicksand encountered at about + two hundred feet below the visible floor of the crater. As soon + as this water is removed by pumping it will be easy to explore + the depths of the crater by means of shafts and drifts. The + rock strata (sandstone and limestone) of which the walls + consist present every appearance of having been violently + upturned by a huge body penetrating the earth like a + cannon-ball. The general aspect of the crater strikingly + resembles the impression made by a steel projectile shot into + an armor-plate. Mr Tilghman has estimated that a meteorite + about five hundred feet in diameter and moving with a velocity + of about five miles per second would have made just such a + perforation upon striking rocks of the character of those found + at this place. There was some fusion of the colliding masses, + and the heat produced some steam from the small amount of water + in the rocks. As a result there has been found at depth a + considerable amount of fused quartz (original sandstone), and + with it innumerable particles or sparks of fused nickel-iron + (original meteorite). A projectile of that size penetrating + eleven to twelve hundred feet into the rocky shell of the globe + must have produced a shock which was perceptible several + hundred miles away.</p> + + <p>The great velocity ascribed to the supposed meteorite at the + moment of striking could be accounted for by the fact that it + probably plunged nearly vertically downward, for it formed a + circular crater in the rocky crust of the earth. In that case + it would have been less retarded by the resistance of the + atmosphere than are meteorites which enter the air at a lower + angle and shoot ahead hundreds of miles until friction has + nearly destroyed their original motion when they drop upon the + earth. Some meteoric masses of great size, such as Peary's iron + meteorite found at Cape York, Greenland, and the almost equally + large mass discovered at Bacubirito, Mexico, appear to have + penetrated but slightly on striking the earth. This may be + explained by supposing that they pursued a long, horizontal + course through the air before falling. The result would be + that, their original velocity having been practically + destroyed, they would drop to the ground with a velocity nearly + corresponding to that which gravity would impart within the + perpendicular distance of their final fall. A + six-hundred-and-sixty-pound meteorite, which fell at Knyahinya, + Hungary, striking at an angle of 27° from the vertical, + penetrated the ground to a depth of eleven feet.</p> + + <p>It has been remarked that the Coon Butte meteorite may have + fallen not longer ago than a few thousand years. This is based + upon the fact that the geological indications favor the + supposition that the event did not occur more than five + thousand years ago, while on the other hand the rings of growth + in the cedar-trees growing on the slopes of the crater show + that they have existed there about seven hundred years. Prof. + William H. Pickering has recently correlated this with an + ancient chronicle which states that at Cairo, Egypt, in the + year 1029, ``many stars passed with a great noise.'' He remarks + that Cairo is about 100°, by great circle, from Coon Butte, + so that if the meteorite that made the crater was a member of a + flock of similar bodies which encountered the earth moving in + parallel lines, some of them might have traversed the sky + tangent to the earth's surface at Cairo. That the spectacle + spoken of in the chronicle was caused by meteorites he deems + exceedingly probable because of what is said about ``a great + noise;'' meteorites are the only celestial phenomena attended + with perceptible sounds. Professor Pickering conjectures that + this supposed flock of great meteorites may have formed the + nucleus of a comet which struck the earth, and he finds + confirmation of the idea in the fact that out of the ten + largest meteorites known, no less than seven were found within + nine hundred miles of Coon Butte. It would be interesting if we + could trace back the history of that comet, and find out what + malicious planet caught it up in its innocent wanderings and + hurled it with so true an aim at the earth! This remarkable + crater is one of the most interesting places in the world, for + there is absolutely no record of such a mass, possibly an + iron-headed comet, from outer space having come into collision + with our earth. The results of the future exploration of the + depths of the crater will be awaited with much interest.</p> + + <p><strong>The Wrecking of the Moon</strong></p> + + <p>There are sympathetic moods under whose influence one gazes + with a certain poignant tenderness at the worn face of the + moon; that little ``fossil world'' (the child of our mother + earth, too) bears such terrible scars of its brief convulsive + life that a sense of pity is awakened by the sight. The moon is + the wonder-land of the telescope. Those towering mountains, + whose ``proud aspiring peaks'' cast silhouettes of shadow that + seem drawn with india-ink; those vast plains, enchained with + gentle winding hills and bordered with giant ranges; those oval + ``oceans,'' where one looks expectant for the flash of + wind-whipped waves; those enchanting ``bays'' and recesses at + the seaward feet of the Alps; those broad straits passing + between guardian heights incomparably mightier than Gibraltar; + those locket-like valleys as secluded among their mountains as + the Vale of Cashmere; those colossal craters that make us smile + at the pretensions of Vesuvius, Etna, and Cotopaxi; those + strange white ways which pass with the unconcern of Roman roads + across mountain, gorge, and valley -- all these give the + beholder an irresistible impression that it is truly a world + into which he is looking, a world akin to ours, and yet no more + like our world than Pompeii is like Naples. Its air, its + waters, its clouds, its life are gone, and only a skeleton + remains -- a mute but eloquent witness to a cosmical tragedy + without parallel in the range of human knowledge.</p> + + <p>One cannot but regret that the moon, if it ever was the seat + of intelligent life, has not remained so until our time. Think + what the consequences would have been if this other world at + our very door had been found to be both habitable and + inhabited! We talk rather airily of communicating with Mars by + signals; but Mars never approaches nearer than 35,000,000 + miles, while the moon when nearest is only a little more than + 220,000 miles away. Given an effective magnifying power of five + thousand diameters, which will perhaps be possible at the + mountain observatories as telescopes improve, and we should be + able to bring the moon within an apparent distance of about + forty miles, while the corresponding distance for Mars would be + more than seven thousand miles. But even with existing + telescopic powers we can see details on the moon no larger than + some artificial constructions on the earth. St Peter's at Rome, + with the Vatican palace and the great piazza, if existing on + the moon, would unquestionably be recognizable as something + else than a freak of nature. Large cities, with their radiating + lines of communication, would at once betray their real + character. Cultivated tracts, and the changes produced by the + interference of intelligent beings, would be clearly + recognizable. The electric illumination of a large town at + night would probably be markedly visible. Gleams of reflected + sunlight would come to us from the surfaces of the lakes and + oceans, and a huge ``liner'' traversing a lunar sea could + probably be followed by its trail of smoke. As to + communications by ``wireless'' signals, which certain + enthusiasts have thought of in connection with Mars, in the + case of the moon they should be a relatively simple matter, and + the feat might actually be accomplished. Think what a + literature would grow up about the moon if it were a living + world! Its very differences from the earth would only + accentuate its interest for us. Night and day on the moon are + each two weeks in length; how interesting it would be to watch + the manner in which the lunarians dealt with such a situation + as that. Lunar and terrestrial history would keep step with + each other, and we should record them both. Truly one might + well wish to have a neighbor world to study; one would feel so + much the less alone in space.</p> + + <p>It is not impossible that the moon did at one time have + inhabitants of some kind. But, if so, they vanished with the + disappearance of its atmosphere and seas, or with the advent of + its cataclysmic age. At the best, its career as a living world + must have been brief. If the water and air were gradually + absorbed, as some have conjectured, by its cooling interior + rocks, its surface might, nevertheless, have retained them for + long ages; but if, as others think, their disappearance was due + to the escape of their gaseous molecules in consequence of the + inability of the relatively small lunar gravitation to retain + them, then the final catastrophe must have been as swift as it + was inevitable. Accepting Darwin's hypothesis, that the moon + was separated from the earth by tidal action while both were + yet plastic or nebulous, we may reasonably conclude that it + began its career with a good supply of both water and air, but + did not possess sufficient mass to hold them permanently. Yet + it may have retained them long enough for life to develop in + many forms upon its surface; in fact, there are so many + indications that air and water have not always been lacking to + the lunar world that we are driven to invent theories to + explain both their former presence and their present + absence.</p> + + <p>But whatever the former condition of the moon may have been, + its existing appearance gives it a resistless fascination, and + it bears so clearly the story of a vast catastrophe sculptured + on its rocky face that the thoughtful observer cannot look upon + it without a feeling of awe. The gigantic character of the + lunar features impresses the beholder not less than the + universality of the play of destructive forces which they + attest. Let us make a few comparisons. Take the lunar crater + called ``Tycho'', which is a typical example of its kind. In + the telescope Tycho appears as a perfect ring surrounding a + circular depression, in the center of which rises a group of + mountains. Its superficial resemblance to some terrestrial + volcanic craters is very striking. Vesuvius, seen from a point + vertically above, would no doubt look something like that (the + resemblance would have been greater when the Monte del Cavallo + formed a more complete circuit about the crater cone). But + compare the dimensions. The remains of the outer crater ring of + Vesuvius are perhaps half a mile in diameter, while the active + crater itself is only two or three hundred feet across at the + most; Tycho has a diameter of fifty-four miles! The group of + relatively insignificant peaks in the center of the crater + floor of Tycho is far more massive than the entire mountain + that we call Vesuvius. The largest known volcanic crater on the + earth, Aso San, in Japan, has a diameter of seven miles; it + would take <em>sixty</em> craters like Aso San to equal Tycho + in area! And Tycho, though one of the most perfect, is by no + means the largest crater on the moon. Another, called + ``Theophilus,'' has a diameter of sixty-four miles, and is + eighteen thousand feet deep. There are hundreds from ten to + forty miles in diameter, and thousands from one to ten miles. + They are so numerous in many places that they break into one + another, like the cells of a crushed honeycomb.</p> + + <p>The lunar craters differ from those of the earth more + fundamentally than in the matter of mere size; <em>they are not + situated on the tops of mountains.</em> If they were, and if + all the proportions were the same, a crater like Tycho might + crown a conical peak fifty or one hundred miles high! Instead + of being cavities in the summits of mountains, the lunar + craters are rather gigantic sink-holes whose bottoms in many + cases lie two or three miles below the general surface of the + lunar world. Around their rims the rocks are piled up to a + height of from a few hundred to two or three thousand feet, + with a comparatively gentle inclination, but on the inner side + they fall away in gigantic broken precipices which make the + dizzy cliffs of the Matterhorn seem but ``lover's leaps.'' Down + they drop, ridge below ridge, crag under crag, tottering wall + beneath wall, until, in a crater named ``Newton,'' near the + south lunar pole, they attain a depth where the rays of the sun + never reach. Nothing more frightful than the spectacle which + many of these terrible chasms present can be pictured by the + imagination. As the lazy lunar day slowly advances, the + sunshine, unmitigated by clouds or atmospheric veil of any + kind, creeps across their rims and begins to descend the + opposite walls. Presently it strikes the ragged crest of a + ridge which had lain hidden in such darkness as we never know + on the earth, and runs along it like a line of kindling fire. + Rocky pinnacles and needles shoot up into the sunlight out of + the black depths. Down sinks the line of light, mile after + mile, and continually new precipices and cliffs are brought + into view, until at last the vast floor is attained and begins + to be illuminated. In the meanwhile the sun's rays, darting + across the gulf, have touched the summits of the central peaks, + twenty or thirty miles from the crater's inmost edge, and they + immediately kindle and blaze like huge stars amid the darkness. + So profound are some of these awful craters that days pass + before the sun has risen high enough above them to chase the + last shadows from their depths.</p> + + <p>Although several long ranges of mountains resembling those + of the earth exist on the moon, the great majority of its + elevations assume the crateriform aspect. Sometimes, instead of + a crater, we find an immense mountain ring whose form and + aspect hardly suggest volcanic action. But everywhere the true + craters are in evidence, even on the sea-beds, although they + attain their greatest number and size on those parts of the + moon -- covering sixty per cent of its visible surface -- which + are distinctly mountainous in character and which constitute + its most brilliant portions. Broadly speaking, the southwestern + half of the moon is the most mountainous and broken, and the + northeastern half the least so. Right down through the center, + from pole to pole, runs a wonderful line of craters and + crateriform valleys of a magnitude stupendous even for the + moon. Another similar line follows the western edge. Three or + four ``seas'' are thrust between these mountainous belts. By + the effects of ``libration'' parts of the opposite hemisphere + of the moon which is turned away from the earth are from time + to time brought into view, and their aspect indicates that that + hemisphere resembles in its surface features the one which + faces the earth. There are many things about the craters which + seem to give some warrant for the hypothesis which has been + particularly urged by Mr G. K. Gilbert, that they were formed + by the impact of meteors; but there are also many things which + militate against that idea, and, upon the whole, the volcanic + theory of their origin is to be preferred.</p> + + <p>The enormous size of the lunar volcanoes is not so difficult + to account for when we remember how slight is the force of + lunar gravity as compared with that of the earth. With equal + size and density, bodies on the moon weigh only one-sixth as + much as on the earth. Impelled by the same force, a projectile + that would go ten miles on the earth would go sixty miles on + the moon. A lunar giant thirty-five feet tall would weigh no + more than an ordinary son of Adam weighs on his greater planet. + To shoot a body from the earth so that it would not drop back + again, we should have to start it with a velocity of seven + miles per second; a mile and a half per second would serve on + the moon. It is by no means difficult to believe, then, that a + lunar volcano might form a crater ring eight or ten times + broader than the greatest to be found on the earth, especially + when we reflect that in addition to the relatively slight force + of gravity, the materials of the lunar crust are probably + lighter than those of our terrestrial rocks.</p> + + <p>For similar reasons it seems not impossible that the theory + mentioned in a former chapter -- that some of the meteorites + that have fallen upon the earth originated from the lunar + volcanoes -- is well founded. This would apply especially to + the stony meteorites, for it is hardly to be supposed that the + moon, at least in its superficial parts, contains much iron. It + is surely a scene most strange that is thus presented to the + mind's eye -- that little attendant of the earth's (the moon + has only one-fiftieth of the volume, and only one-eightieth of + the mass of the earth) firing great stones back at its parent + planet! And what can have been the cause of this furious + outbreak of volcanic forces on the moon? Evidently it was but a + passing stage in its history; it had enjoyed more quiet times + before. As it cooled down from the plastic state in which it + parted from the earth, it became incrusted after the normal + manner of a planet, and then oceans were formed, its atmosphere + being sufficiently dense to prevent the water from evaporating + and the would-be oceans from disappearing continually in mist. + This, if any, must have been the period of life in the lunar + world. As we look upon the vestiges of that ancient world + buried in the wreck that now covers so much of its surface, it + is difficult to restrain the imagination from picturing the + scenes which were once presented there; and, in such a case, + should the imagination be fettered? We give it free rein in + terrestrial life, and it rewards us with some of our greatest + intellectual pleasures. The wonderful landscapes of the moon + offer it an ideal field with just enough half-hidden + suggestions of facts to stimulate its powers.</p> + + <p>The great plains of the <em>Mare Imbrium</em> and the + <em>Mare Serenitatis</em> (the ``Sea of Showers'' and the ``Sea + of Serenity''), bordered in part by lofty mountain ranges + precisely like terrestrial mountains, scalloped along their + shores with beautiful bays curving back into the adjoining + highlands, and united by a great strait passing between the + nearly abutting ends of the ``Lunar Apennines'' and the ``Lunar + Caucasus,'' offer the elements of a scene of world beauty such + as it would be difficult to match upon our planet. Look at the + finely modulated bottom of the ancient sea in Mr Ritchey's + exquisite photograph of the western part of the <em>Mare + Serenitatis,</em> where one seems to see the play of the watery + currents heaping the ocean sands in waving lines, making + shallows, bars, and deeps for the mariner to avoid or seek, and + affording a playground for the creatures of the main. What + geologist would not wish to try his hammer on those rocks with + their stony pages of fossilized history? There is in us an + instinct which forbids us to think that there was never any + life there. If we could visit the moon, there is not among us a + person so prosaic and unimaginative that he would not, the very + first thing, begin to search for traces of its inhabitants. We + would look for them in the deposits on the sea bottoms; we + would examine the shores wherever the configuration seemed + favorable for harbors and the sites of maritime cities -- + forgetting that it may be a little ridiculous to ascribe to the + ancient lunarians the same ideas that have governed the + development of our race; we would search through the valleys + and along the seeming courses of vanished streams; we would + explore the mountains, not the terrible craters, but the + pinnacled chains that recall our own Alps and Rockies; seeking + everywhere some vestige of the transforming presence of + intelligent life. Perhaps we should find such traces, and + perhaps, with all our searching, we should find nothing to + suggest that life had ever existed amid that universal + ruin.</p> + + <p>Look again at the border of the ``Sea of Serenity'' -- what + a name for such a scene! -- and observe how it has been rent + with almost inconceivable violence, the wall of the colossal + crater Posidonius dropping vertically upon the ancient shore + and obliterating it, while its giant neighbor, Le Monnier, + opens a yawning mouth as if to swallow the sea itself. A scene + like this makes one question whether, after all, those may not + be right who have imagined that the so-called sea bottoms are + really vast plains of frozen lava which gushed up in floods so + extensive that even the mighty volcanoes were half drowned in + the fiery sea. This suggestion becomes even stronger when we + turn to another of the photographs of Mr Ritchey's wonderful + series, showing a part of the <em>Mare Tranquilitatis</em> + (``Sea of Tranquility''!). Notice how near the center of the + picture the outline of a huge ring with radiating ridges shows + through the sea bottom; a fossil volcano submerged in a + petrified ocean! This is by no means the only instance in which + a buried world shows itself under the great lunar plains. Yet, + as the newer craters in the sea itself prove, the volcanic + activity survived this other catastrophe, or broke out again + subsequently, bringing more ruin to pile upon ruin.</p> + + <p>Yet notwithstanding the evidence which we have just been + considering in support of the hypothesis that the ``seas'' are + lava floods, Messrs. Loewy and Puiseux, the selenographers of + the Paris Observatory, are convinced that these great plains + bear characteristic marks of the former presence of immense + bodies of water. In that case we should be forced to conclude + that the later oceans of the moon lay upon vast sheets of + solidified lava; and thus the catastrophe of the lunar world + assumes a double aspect, the earliest oceans being swallowed up + in molten floods issuing from the interior, while the lands + were reduced to chaos by a universal eruption of tremendous + volcanoes; and then a period of comparative quiet followed, + during which new seas were formed, and new life perhaps began + to flourish in the lunar world, only to end in another + cataclysm, which finally put a term to the existence of the + moon as a life-supporting world.</p> + + <p>Suppose we examine two more of Mr Ritchey's illuminating + photographs, and, first, the one showing the crater Theophilus + and its surroundings. We have spoken of Theophilus before, + citing the facts that it is sixty-four miles in diameter and + eighteen thousand feet deep. It will be noticed that it has two + brother giants -- Cyrillus the nearer, and Catharina the more + distant; but Theophilus is plainly the youngest of the trio. + Centuries, and perhaps thousands of years, must have elapsed + between the periods of their upheaval, for the two older + craters are partly filled with débris, while it is + manifest at a glance that when the south eastern wall of + Theophilus was formed, it broke away and destroyed a part of + the more ancient ring of Cyrillus. There is no more tremendous + scene on the moon than this; viewed with a powerful telescope, + it is absolutely appalling.</p> + + <p>The next photograph shows, if possible, a still wilder + region. It is the part of the moon lying between Tycho and the + south pole. Tycho is seen in the lower left-hand part of the + picture. To the right, at the edge of the illuminated portion + of the moon, are the crater-rings, Longomontanus and Wilhelm I, + the former being the larger. Between them are to be seen the + ruins of two or three more ancient craters which, together with + portions of the walls of Wilhelm I and Longomontanus, have been + honeycombed with smaller craters. The vast crateriform + depression above the center of the picture is Clavius, an + unrivaled wonder of lunar scenery, a hundred and forty-two + miles in its greatest length, while its whole immense floor has + sunk two miles below the general surface of the moon outside + the ring. The monstrous shadow-filled cavity above Clavius + toward the right is Blancanus, whose aspect here gives a good + idea of the appearance of these chasms when only their rims are + in the sunlight. But observe the indescribable savagery of the + entire scene. It looks as though the spirit of destruction had + gone mad in this spot. The mighty craters have broken forth one + after another, each rending its predecessor; and when their + work was finished, a minor but yet tremendous outbreak + occurred, and the face of the moon was gored and punctured with + thousands of smaller craters. These relatively small craters + (small, however, only in a lunar sense, for many of them would + appear gigantic on the earth) recall once more the theory of + meteoric impact. It does not seem impossible that some of them + may have been formed by such an agency.</p> + + <p>One would not wish for our planet such a fate as that which + has overtaken the moon, but we cannot be absolutely sure that + something of the kind may not be in store for it. We really + know nothing of the ultimate causes of volcanic activity, and + some have suggested that the internal energies of the earth may + be accumulating instead of dying out, and may never yet have + exhibited their utmost destructive power. Perhaps the best + assurance that we can find that the earth will escape the + catastrophe that has overtaken its satellite is to be found in + the relatively great force of its gravitation. The moon has + been the victim of its weakness; given equal forces, and the + earth would be the better able to withstand them. It is + significant, in connection with these considerations, that the + little planet Mercury, which seems also to have parted with its + air and water, shows to the telescope some indications that it + is pitted with craters resembling those that have torn to + pieces the face of the moon.</p> + + <p>Upon the whole, after studying the dreadful lunar + landscapes, one cannot feel a very enthusiastic sympathy with + those who are seeking indications of the continued existence of + some kind of life on the moon; such a world is better without + inhabitants. It has met its fate; let it go! Fortunately, it is + not so near that it cannot hide its scars and appear beautiful + -- except when curiosity impels us to look with the penetrating + eyes of the astronomer.</p> + + <p><strong>The Great Mars Problem</strong></p> + + <p>Let any thoughtful person who is acquainted with the general + facts of astronomy look up at the heavens some night when they + appear in their greatest splendor, and ask himself what is the + strongest impression that they make upon his mind. He may not + find it easy to frame an answer, but when he has succeeded it + will probably be to the effect that the stars give him an + impression of the universality of intelligence; they make him + feel, as the sun and the moon cannot do, that his world is not + alone; that all this was not made simply to form a gorgeous + canopy over the tents of men. If he is of a devout turn of + mind, he thinks, as he gazes into those fathomless deeps and + among those bewildering hosts, of the infinite multitude of + created beings that the Almighty has taken under his care. The + narrow ideas of the old geocentric theology, which made the + earth God's especial footstool, and man his only rational + creature, fall away from him like a veil that had obscured his + vision; they are impossible in the presence of what he sees + above. Thus the natural tendency, in the light of modern + progress, is to regard the universe as everywhere filled with + life.</p> + + <p>But science, which is responsible for this broadening of + men's thoughts concerning the universality of life, itself + proceeds to set limits. Of spiritual existences it pretends to + know nothing, but as to physical beings, it declares that it + can only entertain the supposition of their existence where it + finds evidence of an environment suited to their needs, and + such environment may not everywhere exist. Science, though + repelled by the antiquated theological conception of the + supreme isolation of man among created beings, regards with + complacency the probability that there are regions in the + universe where no organic life exists, stars which shine upon + no inhabited worlds, and planets which nourish no animate + creatures. The astronomical view of the universe is that it + consists of matter in every stage of evolution: some nebulous + and chaotic; some just condensing into stars (suns) of every + magnitude and order; some shaped into finished solar bodies + surrounded by dependent planets; some forming stars that + perhaps have no planets, and will have none; some constituting + suns that are already aging, and will soon lose their radiant + energy and disappear; and some aggregated into masses that long + ago became inert, cold, and rayless, and that can only be + revivified by means about which we can form conjectures, but of + which we actually know nothing.</p> + + <p>As with the stars, so with the planets, which are the + satellites of stars. All investigations unite to tell us that + the planets are not all in the same state of development. As + some are large and some small, so some are, in an evolutionary + sense, young, and some old. As they depend upon the suns around + which they revolve for their light, heat, and other forms of + radiant energy, so their condition varies with their distance + from those suns. Many may never arrive at a state suitable for + the maintenance of life upon their surfaces; some which are not + at present in such a state may attain it later; and the forms + of life themselves may vary with the peculiar environment that + different planets afford. Thus we see that we are not + scientifically justified in affirming that life is ubiquitous, + although we are thus justified in saying that it must be, in a + general sense, universal. We might liken the universe to a + garden known to contain every variety of plant. If on entering + it we see no flowers, we examine the species before us and find + that they are not of those which bloom at this particular + season, or perhaps they are such as never bear flowers. Yet we + feel no doubt that we shall find flowers somewhere in the + garden, because there <em>are</em> species which bloom at this + season, and the garden contains <em>all</em> varieties.</p> + + <p>While it is tacitly assumed that there are planets revolving + around other stars than the sun, it would be impossible for us + to see them with any telescope yet invented, and no instrument + now in the possession of astronomers could assure us of their + existence; so the only planetary system of which we have visual + knowledge is our own. Excluding the asteroids, which could not + from any point of view be considered as habitable, we have in + the solar system eight planets of various sizes and situated at + various distances from the sun. Of these eight we know that + one, the earth, is inhabited. The question, then, arises: Are + there any of the others which are inhabited or habitable? Since + it is our intention to discuss the habitability of only one of + the seven to which the question applies, the rest may be + dismissed in a few words. The smallest of them, and the nearest + to the sun, is Mercury, which is regarded as uninhabitable + because it has no perceptible supply of water and air, and + because, owing to the extraordinary eccentricity of its orbit, + it is subjected to excessive and very rapid alterations in the + amount of solar heat and light poured upon its surface, such + alterations being inconsistent with the supposition that it can + support living beings. Even its average temperature is more + than six and a half times that prevailing on the earth! Another + circumstance which militates against its habitability is that, + according to the results of the best telescopic studies, it + always keeps the same face toward the sun, so that one half of + the planet is perpetually exposed to the fierce solar rays, and + the other half faces the unmitigated cold of open space. Venus, + the next in distance from the sun, is almost the exact twin of + the earth in size, and many arguments may be urged in favor of + its habitability, although it is suspected of possessing the + same peculiarity as Mercury, in always keeping the same side + sunward. Unfortunately its atmosphere appears to be so dense + that no permanent markings on its surface are certainly + visible, and the question of its actual condition must, for the + present, be left in abeyance. Mars, the first planet more + distant from the sun than the earth, is the special subject of + this chapter, and will be described and discussed a few lines + further on. Jupiter, Saturn, Uranus, and Neptune, the four + giant planets, all more distant than Mars, and each more + distant than the other in the order named, are all regarded as + uninhabitable because none of them appears to possess any + degree of solidity. They may have solid or liquid nuclei, but + exteriorly they seem to be mere balls of cloud. Of course, one + can imagine what he pleases about the existence of creatures + suited to the physical constitution of such planets as these, + but they must be excluded from the category of habitable worlds + in the ordinary sense of the term. We go back, then, to + Mars.</p> + + <p>It will be best to begin with a description of the planet. + Mars is 4230 miles in diameter; its surface is not much more + than one-quarter as extensive as that of the earth (.285). Its + mean distance from the sun is 141,500,000 miles, 48,500,000 + miles greater than that of the earth. Since radiant energy + varies inversely as the square of distance, Mars receives less + than half as much solar light and heat as the earth gets. Mars' + year (period of revolution round the sun) is 687 days. Its mean + density is 71 per cent of the earth's, and the force of gravity + on its surface is 38 per cent of that on the surface of the + earth; <em>i.e.,</em> a body weighing one hundred pounds on the + earth would, if transported to Mars, weigh but thirty-eight + pounds. The inclination of its equator to the plane of its + orbit differs very little from that of the earth's equator, and + its axial rotation occupies 24 hours 37 minutes. so that the + length of day and night, and the extent of the seasonal changes + on Mars, are almost precisely the same as on the earth. But + owing to the greater length of its year, the seasons of Mars, + while occurring in the same order, are almost twice as long as + ours. The surface of the planet is manifestly solid, like that + of our globe, and the telescope reveals many permanent markings + on it, recalling the appearance of a globe on which + geographical features have been represented in reddish and + dusky tints. Around the poles are plainly to be seen rounded + white areas, which vary in extent with the Martian seasons, + nearly vanishing in summer and extending widely in winter. The + most recent spectroscopic determinations indicate that Mars has + an atmosphere perhaps as dense as that to be found on our + loftiest mountain peaks, and there is a perceptible amount of + watery vapor in this atmosphere. The surface of the planet + appears to be remarkably level, and it has no mountain ranges. + No evidences of volcanic action have been discovered on Mars. + The dusky and reddish areas were regarded by the early + observers as respectively seas and lands, but at present it is + not believed that there are any bodies of water on the planet. + There has never been much doubt expressed that the white areas + about the poles represent snow.</p> + + <p>It will be seen from this brief description that many + remarkable resemblances exist between Mars and the earth, and + there is nothing wonderful in the fact that the question of the + habitability of the former has become one of extreme and + wide-spread interest, giving rise to the most diverse views, to + many extraordinary speculations, and sometimes to regrettably + heated controversy. The first champion of the habitability of + Mars was Sir William Herschel, although even before his time + the idea had been suggested. He was convinced by the + revelations of his telescopes, continually increasing in power, + that Mars was more like the earth than any other planet. He + could not resist the testimony of the polar snows, whose + suggestive conduct was in such striking accord with what occurs + upon the earth. Gradually, as telescopes improved and observers + increased in number, the principal features of the planet were + disclosed and charted, and ``areography,'' as the geography of + Mars was called, took its place among the recognized branches + of astronomical study. But it was not before 1877 that a + fundamentally new discovery in areography gave a truly + sensational turn to speculation about life on ``the red + planet.'' In that year Mars made one of its nearest approaches + to the earth, and was so situated in its orbit that it could be + observed to great advantage from the northern hemisphere of the + earth. The celebrated Italian astronomer, Schiaparelli, took + advantage of this opportunity to make a trigonometrical survey + of the surface of Mars -- as coolly and confidently as if he + were not taking his sights across a thirty-five-million-mile + gulf of empty space -- and in the course of this survey he was + astonished to perceive that the reddish areas, then called + continents, were crossed in many directions by narrow, dusky + lines, to which he gave the suggestive name of ``canals.'' Thus + a kind of firebrand was cast into the field of astronomical + speculation, which has ever since produced disputes that have + sometimes approached the violence of political faction. At + first the accuracy of Schiaparelli's observations was + contested; it required a powerful telescope, and the most + excellent ``seeing,'' to render the enigmatical lines visible + at all, and many searchers were unable to detect them. But + Schiaparelli continued his studies in the serene sky of Italy, + and produced charts of the gridironed face of Mars containing + so much astonishing detail that one had either to reject them + <em>in toto</em> or to confess that Schiaparelli was right. As + subsequent favorable oppositions of Mars occurred, other + observers began to see the ``canals'' and to confirm the + substantial accuracy of the Italian astronomer's work, and + finally few were found who would venture to affirm that the + ``canals'' did not exist, whatever their meaning might be.</p> + + <p>When Schiaparelli began his observations it was generally + believed, as we have said, that the dusky areas on Mars were + seas, and since Schiaparelli thought that the ``canals'' + invariably began and ended at the shores of the ``seas,'' the + appropriateness of the title given to the lines seemed + apparent. Their artificial character was immediately assumed by + many, because they were too straight and too suggestively + geometrical in their arrangement to permit the conclusion that + they were natural watercourses. A most surprising circumstance + noted by Schiaparelli was that the ``canals'' made their + appearance <em>after</em> the melting of the polar snow in the + corresponding hemisphere had begun, and that they grew darker, + longer, and more numerous in proportion as the polar + liquidation proceeded; another very puzzling observation was + that many of them became double as the season advanced; close + beside an already existing ``canal,'' and in perfect + parallelism with it, another would gradually make its + appearance. That these phenomena actually existed and were not + illusions was proved by later observations, and today they are + seen whenever Mars is favorably situated for observation.</p> + + <p>In the closing decade of the nineteenth century, Mr Percival + Lowell took up the work where Schiaparelli had virtually + dropped it, and soon added a great number of ``canals'' to + those previously known, so that in his charts the surface of + the wonderful little planet appears covered as with a spider's + web, the dusky lines criss-crossing in every direction, with + conspicuous knots wherever a number of them come together. Mr + Lowell has demonstrated that the areas originally called seas, + and thus named on the earlier charts, are not bodies of water, + whatever else they may be. He has also found that the + mysterious lines do not, as Schiaparelli supposed, begin and + end at the edges of the dusky regions, but often continue on + across them, reaching in some cases far up into the polar + regions. But Schiaparelli was right in his observation that the + appearance of the ``canals'' is synchronous with the gradual + disappearance of the polar snows, and this fact has become the + basis of the most extraordinary theory that the subject of life + in other worlds has ever given birth to.</p> + + <p>Now, the effect of such discoveries, as we have related, + depends upon the type of mind to whose attention they are + called. Many are content to accept them as strange and + inexplicable at present, and to wait for further light upon + them; others insist upon an immediate inquiry concerning their + probable nature and meaning. Such an inquiry can only be based + upon inference proceeding from analogy. Mars, say Mr Lowell and + those who are of his opinion, is manifestly a solidly incrusted + planet like the earth; it has an atmosphere, though one of + great rarity; it has water vapor, as the snows in themselves + prove; it has the alternation of day and night, and a + succession of seasons closely resembling those of the earth; + its surface is suggestively divided into regions of contrasting + colors and appearance, and upon that surface we see an immense + number of lines geometrically arranged, with a system of + symmetrical intersections where the lines expand into circular + and oval areas -- and all connected with the annual melting of + the polar snows in a way which irresistibly suggests the + interference of intelligence directed to a definite end. Why, + with so many concurrent circumstances to support the + hypothesis, should we not regard Mars as an inhabited + globe?</p> + + <p>But the differences between Mars and the earth are in many + ways as striking as their resemblances. Mars is relatively + small; it gets less than half as much light and heat as we + receive; its atmosphere is so rare that it would be distressing + to us, even if we could survive in it at all; it has no lakes, + rivers, or seas; its surface is an endless prairie. and its + ``canals'' are phenomena utterly unlike anything on the earth. + Yet it is precisely upon these divergences between the earth + and Mars, this repudiation of terrestrial standards, that the + theory of ``life on Mars,'' for which Mr Lowell is mainly + responsible, is based. Because Mars is smaller than the earth, + we are told it must necessarily be more advanced in planetary + evolution, the underlying cause of which is the gradual cooling + and contraction of the planet's mass. Mars has parted with its + internal heat more rapidly than the earth; consequently its + waters and its atmosphere have been mostly withdrawn by + chemical combinations, but enough of both yet remain to render + life still possible on its surface. As the globe of Mars is + evolutionally older than that of the earth, so its forms of + organic life may be proportionally further advanced, and its + inhabitants may have attained a degree of cultivated + intelligence much superior to what at present exists upon the + earth. Understanding the nature and the causes of the + desiccation of their planet, and possessing engineering science + and capabilities far in advance of ours, they may be conceived + to have grappled with the stupendous problem of keeping their + world in a habitable condition as long as possible. Supposing + them to have become accustomed to live in their rarefied + atmosphere (a thing not inconceivable, since men can live for a + time at least in air hardly less rare), the most pressing + problem for them is that of a water-supply, without which plant + life cannot exist, while animal life in turn depends for its + existence upon vegetation. The only direction in which they can + seek water is that of the polar regions, where it is + alternately condensed into snow and released in the liquid form + by the effect of the seasonal changes. It is, then, to the + annual melting of the polar snow-fields that the Martian + engineers are supposed to have recourse in supplying the needs + of their planet, and thus providing the means of prolonging + their own existence. It is imagined that they have for this + purpose constructed a stupendous system of irrigation extending + over the temperate and equatorial regions of the planet. The + ``canals'' represent the lines of irrigation, but the narrow + streaks that we see are not the canals themselves, but the + irrigated bands covered by them. Their dark hue, and their + gradual appearance after the polar melting has begun, are due + to the growth of vegetation stimulated by the water. The + rounded areas visible where several ``canals'' meet and cross + are called by Mr Lowell ``oases.'' These are supposed to be the + principal centers of population and industry. It must be + confessed that some of them, with their complicated systems of + radiating lines, appear to answer very well to such a theory. + No attempt to explain them by analogy with natural phenomena on + the earth has proved successful.</p> + + <p>But a great difficulty yet remains: How to explain the + seemingly miraculous powers of the supposed engineers? Here + recourse is had once more to the relative smallness of the + planet. We have remarked that the force of gravity on Mars is + only thirty-eight per cent of that on the earth. A steam-shovel + driven by a certain horse-power would be nearly three times as + effective there as here. A man of our stature on Mars would + find his effective strength increased in the same proportion. + But just because of the slight force of gravity there, a + Martian might attain to the traditional stature of Goliath + without finding his own weight an encumbrance to his activity, + while at the same time his huge muscles would come into + unimpeded play, enabling him single-handed to perform labors + that would be impossible to a whole gang of terrestrial + workmen. The effective powers of huge machines would be + increased in the same way; and to all this must be added the + fact that the mean density of the materials of which Mars is + composed is much less than that of the constituents of the + earth. Combining all these considerations, it becomes much less + difficult to conceive that public works might be successfully + undertaken on Mars which would be hopelessly beyond the limits + of human accomplishment.</p> + + <p>Certain other difficulties have also to be met; as, for + instance, the relative coldness of the climate of Mars. At its + distance it gets considerably less than half as much light and + heat as we receive. In addition to this, the rarity of its + atmosphere would naturally be expected to decrease the + effective temperature at the planet's surface, since an + atmosphere acts somewhat like the glass cover of a hot-house in + retaining the solar heat which has penetrated it. It has been + calculated that, unless there are mitigating circumstances of + which we know nothing, the average temperature at the surface + of Mars must be far below the freezing-point of water. To this + it is replied that the possible mitigating circumstances spoken + of evidently exist in fact, because we can <em>see</em> that + the watery vapor condenses into snow around the poles in + winter, but melts again when summer comes. The mitigating agent + may be supposed to exist in the atmosphere where the presence + of certain gases would completely alter the temperature + gradients.</p> + + <p>It might also be objected that it is inconceivable that the + Martian engineers, however great may be their physical powers, + and however gigantic the mechanical energies under their + control, could force water in large quantities from the poles + to the equator. This is an achievement that measures up to the + cosmical standard. It is admitted by the champions of the + theory that the difficulty is a formidable one; but they call + attention to the singular fact that on Mars there can be found + no chains of mountains, and it is even doubtful if ranges of + hills exist there. The entire surface of the planet appears to + be almost ``as smooth as a billiard ball,'' and even the broad + regions which were once supposed to be seas apparently lie at + practically the same level as the other parts, since the + ``canals'' in many cases run uninterruptedly across them. + Lowell's idea is that these sombre areas may be expanses of + vegetation covering ground of a more or less marshy character, + for while the largest of them appear to be permanent, there are + some which vary coincidently with the variations of the + canals.</p> + + <p>As to the kind of machinery employed to force the water from + the poles, it has been conjectured that it may have taken the + form of a gigantic system of pumps and conduits; and since the + Martians are assumed to be so far in advance of us in their + mastery of scientific principles, the hypothesis will at least + not be harmed by supposing that they have learned to harness + forces of nature whose very existence in a manageable form is + yet unrecognized on the earth. If we wish to let the + imagination loose, we may conjecture that they have conquered + the secret of those intra-atomic forces whose resistless energy + is beginning to become evident to us, but the possibility of + whose utilization remains a dream, the fulfillment of which + nobody dares to predict.</p> + + <p>Such, in very brief form, is the celebrated theory of Mars + as an inhabited world. It certainly captivates the imagination, + and if we believe it to represent the facts, we cannot but + watch with the deepest sympathy this gallant struggle of an + intellectual race to preserve its planet from the effects of + advancing age and death. We may, indeed, wonder whether our own + humanity, confronted by such a calamity, could be counted on to + meet the emergency with equal stoutness of heart and + inexhaustibleness of resource. Up to the present time we + certainly have shown no capacity to confront Nature toe to toe, + and to seize her by the shoulders and turn her round when she + refuses to go our way. If we could get into wireless telephonic + communication with the Martians we might learn from their own + lips the secret of their more than ``Roman recovery.''</p> + + <p><strong>The Riddle of the Asteroids</strong></p> + + <p>Between the orbits of Mars and Jupiter revolves the most + remarkable system of little bodies with which we are acquainted + -- the Asteroids, or Minor Planets. Some six hundred are now + known, and they may actually number thousands. They form + virtually a ring about the sun. The most striking general fact + about them is that they occupy the place in the sky which + should be occupied, according to Bode's Law, by a single large + planet. This fact, as we shall see, has led to the invention of + one of the most extraordinary theories in astronomy -- + <em>viz.,</em> that of the explosion of a world!</p> + + <p>Bode's Law, so-called, is only an empiric formula, but until + the discovery of Neptune it accorded so well with the distances + of the planets that astronomers were disposed to look upon it + as really representing some underlying principle of planetary + distribution. They were puzzled by the absence of a planet in + the space between Mars and Jupiter, where the ``law'' demanded + that there should be one, and an association of astronomers was + formed to search for it. There was a decided sensation when, in + 1801, Piazzi, of Palermo, announced that he had found a little + planet which apparently occupied the place in the system which + belonged to the missing body. He named it Ceres, and it was the + first of the Asteroids. The next year Olbers, of Bremen, while + looking for Ceres with his telescope, stumbled upon another + small planet which he named Pallas. Immediately he was inspired + with the idea that these two planets were fragments of a larger + one which had formerly occupied the vacant place in the + planetary ranks, and he predicted that others would be found by + searching in the neighborhood of the intersection of the orbits + of the two already discovered. This bold prediction was + brilliantly fulfilled by the finding of two more -- Juno in + 1804, and Vesta in 1807. Olbers would seem to have been led to + the invention of his hypothesis of a planetary explosion by the + faith which astronomers at that time had in Bode's Law. They + appear to have thought that several planets revolving in the + gap where the ``law'' called for but one could only be + accounted for upon the theory that the original <em>one</em> + had been broken up to form the several. Gravitation demanded + that the remnants of a planet blown to pieces, no matter how + their orbits might otherwise differ, should all return at + stated periods to the point where the explosion had occurred; + hence Olbers' prediction that any asteroids that might + subsequently be discovered would be found to have a common + point of orbital intersection. And curiously enough all of the + first asteroids found practically answered to this requirement. + Olbers' theory seemed to be established.</p> + + <p>After the first four, no more asteroids were found until + 1845, when one was discovered; then, in 1847, three more were + added to the list; and after that searchers began to pick them + up with such rapidity that by the close of the century hundreds + were known, and it had become almost impossible to keep track + of them. The first four are by far the largest members of the + group, but their actual sizes remained unknown until less than + twenty years ago. It was long supposed that Vesta was the + largest, because it shines more brightly than any of the + others; but finally, in 1895, Barnard, with the Lick telescope, + definitely measured their diameters, and proved to everybody's + surprise that Ceres is really the chief, and Vesta only the + third in rank. His measures are as follows: Ceres, 477 miles; + Pallas, 304 miles; Vesta, 239 miles; and Juno, 120 miles. They + differ greatly in the reflective power of their surfaces, a + fact of much significance in connection with the question of + their origin. Vesta is, surface for surface, rather more than + three times as brilliant as Ceres, whence the original mistake + about its magnitude.</p> + + <p>Nowadays new asteroids are found frequently by photography, + but physically they are most insignificant bodies, their + average diameter probably not exceeding twenty miles, and some + are believed not to exceed ten. On a planet only ten miles in + diameter, assuming the same mean density as the earth's, which + is undoubtedly too much, the force of gravity would be so + slight that an average man would not weigh more than three + ounces, and could jump off into space whenever he liked.</p> + + <p>Although the asteroids all revolve around the sun in the + same direction as that pursued by the major planets, their + orbits are inclined at a great variety of angles to the general + plane of the planetary system, and some of them are very + eccentric -- almost as much so as the orbits of many of the + periodic comets. It has even been conjectured that the two tiny + moons of Mars and the four smaller satellites of Jupiter may be + asteroids gone astray and captured by those planets. Two of the + asteroids are exceedingly remarkable for the shapes and + positions of their orbits; these are Eros, discovered in 1898, + and T. G., 1906, found eight years later. The latter has a mean + distance from the sun slightly greater than that of Jupiter, + while the mean distance of Eros is less than that of Mars. The + orbit of Eros is so eccentric that at times it approaches + within 15,000,000 miles of the earth, nearer than any other + regular member of the solar system except the moon, thus + affording an unrivaled means of measuring the solar parallax. + But for our present purpose the chief interest of Eros lies in + its extraordinary changes of light.</p> + + <p>These changes, although irregular, have been observed and + photographed many times, and there seems to be no doubt of + their reality. Their significance consists in their possible + connection with the form of the little planet, whose diameter + is generally estimated at not more than twenty miles. Von + Oppolzer found, in 1901, that Eros lost three-fourths of its + brilliancy once in every two hours and thirty-eight minutes. + Other observers have found slightly different periods of + variability, but none as long as three hours. The most + interesting interpretation that has been offered of this + phenomenon is that it is due to a great irregularity of figure, + recalling at once Olbers' hypothesis. According to some, Eros + may be double, the two bodies composing it revolving around + each other at very close quarters; but a more striking, and it + may be said probable, suggestion is that Eros has a form not + unlike that of a dumb-bell, or hour-glass, turning rapidly end + over end so that the area of illuminated surface presented to + our eyes continually changes, reaching at certain times a + minimum when the amount of light that it reflects toward the + earth is reduced to a quarter of its maximum value. Various + other bizarre shapes have been ascribed to Eros, such, for + instance, as that of a flat stone revolving about one of its + longer axes, so that sometimes we see its face and sometimes + its edge.</p> + + <p>All of these explanations proceed upon the assumption that + Eros cannot have a simple globular figure like that of a + typical planet, a figure which is prescribed by the law of + gravitation, but that its shape is what may be called + accidental; in a word, it is a <em>fragment,</em> for it seems + impossible to believe that a body formed in interplanetary + space, either through nebular condensation or through the + aggregation of particles drawn together by their mutual + attractions, should not be practically spherical in shape. Nor + is Eros the only asteroid that gives evidence by variations of + brilliancy that there is something abnormal in its + constitution; several others present the same phenomenon in + varying degrees. Even Vesta was regarded by Olbers as + sufficiently variable in its light to warrant the conclusion + that it was an angular mass instead of a globe. Some of the + smaller ones show very notable variations, and all in short + periods, of three or four hours, suggesting that in turning + about one of their axes they present a surface of variable + extent toward the sun and the earth.</p> + + <p>The theory which some have preferred -- that the variability + of light is due to the differences of reflective power on + different parts of the surface -- would, if accepted, be hardly + less suggestive of the origin of these little bodies by the + breaking up of a larger one, because the most natural + explanation of such differences would seem to be that they + arose from variations in the roughness or smoothness of the + reflecting surface, which would be characteristic of + fragmentary bodies. In the case of a large planet alternating + expanses of land and water, or of vegetation and desert, would + produce a notable variation in the amount of reflection, but on + bodies of the size of the asteroids neither water nor + vegetation could exist, and an atmosphere would be equally + impossible.</p> + + <p>One of the strongest objections to Olbers' hypothesis is + that only a few of the first asteroids discovered travel in + orbits which measurably satisfy the requirement that they + should all intersect at the point where the explosion occurred. + To this it was at first replied that the perturbations of the + asteroidal orbits, by the attractions of the major planets, + would soon displace them in such a manner that they would cease + to intersect. One of the first investigations undertaken by the + late Prof. Simon Newcomb was directed to the solution of this + question, and he arrived at the conclusion that the planetary + perturbations could not explain the actual situation of the + asteroidal orbits. But afterward it was pointed out that the + difficulty could be avoided by supposing that not one but a + series of explosions had produced the asteroids as they now + are. After the primary disruption the fragments themselves, + according to this suggestion, may have exploded, and then the + resulting orbits would be as ``tangled'' as the heart could + wish. This has so far rehabilitated the explosion theory that + it has never been entirely abandoned, and the evidence which we + have just cited of the probably abnormal shapes of Eros and + other asteroids has lately given it renewed life. It is a + subject that needs a thorough rediscussion.</p> + + <p>We must not fail to mention, however, that there is a rival + hypothesis which commends itself to many astronomers -- + <em>viz.,</em> that the asteroids were formed out of a + relatively scant ring of matter, situated between Mars and + Jupiter and resembling in composition the immensely more + massive rings from which, according to Laplace's hypothesis, + the planets were born. It is held by the supporters of this + theory that the attraction of the giant Jupiter was sufficient + to prevent the small, nebulous ring that gave birth to the + asteroids from condensing like the others into a single + planet.</p> + + <p>But if we accept the explosion theory, with its corollary + that minor explosions followed the principal one, we have still + an unanswered question before us: What caused the explosions? + The idea of <em>a world blowing up</em> is too Titanic to be + shocking; it rather amuses the imagination than seriously + impresses it; in a word, it seems essentially chimerical. We + can by no appeal to experience form a mental picture of such an + occurrence. Even the moon did not blow up when it was wrecked + by volcanoes. The explosive nebulæ and new stars are far + away in space, and suggest no connection with such a + catastrophe as the bursting of a planet into hundreds of + pieces. We cannot conceive of a great globe thousands of miles + in diameter resembling a pellet of gunpowder only awaiting the + touch of a match to cause its sudden disruption. Somehow the + thought of human agency obtrudes itself in connection with the + word ``explosion,'' and we smile at the idea that giant powder + or nitro-glycerine could blow up a planet. Yet it would only + need <em>enough</em> of them to do it.</p> + + <p>After all, we may deceive ourselves in thinking, as we are + apt to do, that explosive energies lock themselves up only in + small masses of matter. There are many causes producing + explosions in nature, every volcanic eruption manifests the + activity of some of them. Think of the giant power of confined + steam; if enough steam could be suddenly generated in the + center of the earth by a downpour of all the waters of the + oceans, what might not the consequences be for our globe? In a + smaller globe, and it has never been estimated that the + original asteroid was even as large as the moon, such a + catastrophe would, perhaps, be more easily conceivable; but + since we are compelled in this case to assume that there was a + series of successive explosions, steam would hardly answer the + purpose; it would be more reasonable to suppose that the cause + of the explosion was some kind of chemical reaction, or + something affecting the atoms composing the exploding body. + Here Dr Gustav Le Bon comes to our aid with a most startling + suggestion, based on his theory of the dissipation of + intra-atomic energy. It will be best to quote him at some + length from his book on <em>The Evolution of Forces.</em></p> + + <p>``It does not seem at first sight,'' says Doctor Le Bon,</p> + + <blockquote> + very comprehensible that worlds which appear more and more + stable as they cool could become so unstable as to afterward + dissociate entirely. To explain this phenomenon, we will + inquire whether astronomical observations do not allow us to + witness this dissociation. + + <p>We know that the stability of a body in motion, such as a + top or a bicycle, ceases to be possible when its velocity of + rotation descends below a certain limit. Once this limit is + reached it loses its stability and falls to the ground. Prof. + J. J. Thomson even interprets radio-activity in this manner, + and points out that when the speed of the elements composing + the atoms descends below a certain limit they become unstable + and tend to lose their equilibria. There would result from + this a commencement of dissociation, with diminution of their + potential energy and a corresponding increase of their + kinetic energy sufficient to launch into space the products + of intra-atomic disintegration.</p> + + <p>It must not be forgotten that the atom being an enormous + reservoir of energy is by this very fact comparable with + explosive bodies. These last remain inert so long as their + internal equilibria are undisturbed. So soon as some cause or + other modifies these, they explode and smash everything + around them after being themselves broken to pieces.</p> + + <p>Atoms, therefore, which grow old in consequence of the + diminution of a part of their intra-atomic energy gradually + lose their stability. A moment, then, arrives when this + stability is so weak that the matter disappears by a sort of + explosion more or less rapid. The bodies of the radium group + offer an image of this phenomenon -- a rather faint image, + however, because the atoms of this body have only reached a + period of instability when the dissociation is rather slow. + It probably precedes another and more rapid period of + dissociation capable of producing their final explosion. + Bodies such as radium, thorium, etc., represent, no doubt, a + state of old age at which all bodies must some day arrive, + and which they already begin to manifest in our universe, + since all matter is slightly radio-active. It would suffice + for the dissociation to be fairly general and fairly rapid + for an explosion to occur in a world where it was + manifested.</p> + + <p>These theoretical considerations find a solid support in + the sudden appearances and disappearances of stars. The + explosions of a world which produce them reveal to us, + perhaps, how the universes perish when they become old.</p> + + <p>As astronomical observations show the relative frequency + of these rapid destructions, we may ask ourselves whether the + end of a universe by a sudden explosion after a long period + of old age does not represent its most general ending.</p> + </blockquote> + + <p>Here, perhaps, it will be well to stop, since, entrancing as + the subject may be, we know very little about it, and Doctor Le + Bon's theory affords a limitless field for the reader's + imagination.</p> + <hr> + + <p>A printed version of this book is available from <a href= + "http://csky.sattre-press.com">Sattre Press</a>. It includes + extensive annotations, a new introduction and all the original + photographs and diagrams.</p> + +<pre> +*** END OF THE PROJECT GUTENBERG EBOOK, CURIOSITIES OF THE SKY *** + +This file should be named cusky10h.htm or cusky10h.zip +Corrected EDITIONS of our eBooks get a new NUMBER, cusky11h.htm +VERSIONS based on separate sources get new LETTER, cusky10ah.htm + +Project Gutenberg eBooks are often created from several printed +editions, all of which are confirmed as Public Domain in the US +unless a copyright notice is included. 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