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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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