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diff --git a/old/6630.txt b/old/6630.txt new file mode 100644 index 0000000..5cc0903 --- /dev/null +++ b/old/6630.txt @@ -0,0 +1,5339 @@ +The Project Gutenberg EBook of Curiosities of the Sky, by Garrett Serviss + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Curiosities of the Sky + +Author: Garrett Serviss + +Posting Date: August 25, 2012 [EBook #6630] +Release Date: October, 2004 +First Posted: January 6, 2003 + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY *** + + + + +Produced by William McClain + + + + + + + + + + + Curiosities of the Sky + +by Garrett Serviss + +Curiosities of the Sky was first published in 1909 and the text is in +the public domain. The transcription was done by William McClain +(info@sattre-press.com), 2002. + +A printed version of this book is available from Sattre Press +(http://csky.sattre-press.com). It includes extensive annotations, a +new introduction and all the original photographs and diagrams. +_________________________________________________________________ + +Preface + +What Froude says of history is true also of astronomy: it is the most +impressive where it transcends explanation. It is not the mathematics +of astronomy, but the wonder and the mystery that seize upon the +imagination. The calculation of an eclipse owes all its prestige to +the sublimity of its data; the operation, in itself, requires no more +mental effort than the preparation of a railway time-table. + +The dominion which astronomy has always held over the minds of men is +akin to that of poetry; when the former becomes merely instructive and +the latter purely didactic, both lose their power over the +imagination. Astronomy is known as the oldest of the sciences, and it +will be the longest-lived because it will always have arcana that have +not been penetrated. + +Some of the things described in this book are little known to the +average reader, while others are well known; but all possess the +fascination of whatever is strange, marvelous, obscure, or mysterious +-- magnified, in this case, by the portentous scale of the phenomena. + +The idea of the author is to tell about these things in plain +language, but with as much scientific accuracy as plain language will +permit, showing the wonder that is in them without getting away from +the facts. Most of them have hitherto been discussed only in technical +form, and in treatises that the general public seldom sees and never +reads. + +Among the topics touched upon are: + * The strange unfixedness of the ``fixed stars,'' the vast + migrations of the suns and worlds constituting the universe. + * The slow passing out of existence of those collocations of stars + which for thousands of years have formed famous + ``constellations,'' preserving the memory of mythological heroes + and heroines, and perhaps of otherwise unrecorded history. + * The tendency of stars to assemble in immense clouds, swarms, and + clusters. + * The existence in some of the richest regions of the universe of + absolutely black, starless gaps, deeps, or holes, as if one were + looking out of a window into the murkiest night. + * The marvelous phenomena of new, or temporary, stars, which appear + as suddenly as conflagrations, and often turn into something else + as eccentric as themselves. + * The amazing forms of the ``whirlpool,'' ``spiral,'' ``pinwheel,'' + and ``lace,'' or ``tress,'' nebulae. + * The strange surroundings of the sun, only seen in particular + circumstances, but evidently playing a constant part in the daily + phenomena of the solar system. + * The mystery of the Zodiacal Light and the Gegenschein. + * The extraordinary transformations undergone by comets and their + tails. + * The prodigies of meteorites and masses of stone and metal fallen + from the sky. + * The cataclysms that have wrecked the moon. + * The problem of life and intelligence on the planet Mars. + * The problematical origin and fate of the asteroids. + * The strange phenomena of the auroral lights. + +An attempt has been made to develop these topics in an orderly way, +showing their connection, so that the reader may obtain a broad +general view of the chief mysteries and problems of astronomy, and an +idea of the immense field of discovery which still lies, almost +unexplored, before it. + +The Windows of Absolute Night + +To most minds mystery is more fascinating than science. But when +science itself leads straight up to the borders of mystery and there +comes to a dead stop, saying, ``At present I can no longer see my +way,'' the force of the charm is redoubled. On the other hand, the +illimitable is no less potent in mystery than the invisible, whence +the dramatic effect of Keats' ``stout Cortez'' staring at the +boundless Pacific while all his men look at each other with a wild +surmise, ``silent upon a peak in Darien.'' It is with similar feelings +that the astronomer regards certain places where from the peaks of the +universe his vision seems to range out into endless empty space. He +sees there the shore of his little isthmus, and, beyond, unexplored +immensity. + +The name, ``coal-sacks,'' given to these strange voids is hardly +descriptive. Rather they produce upon the mind the effect of blank +windows in a lonely house on a pitch-dark night, which, when looked at +from the brilliant interior, become appalling in their rayless murk. +Infinity seems to acquire a new meaning in the presence of these black +openings in the sky, for as one continues to gaze it loses its purely +metaphysical quality and becomes a kind of entity, like the ocean. The +observer is conscious that he can actually see the beginning of its +ebon depths, in which the visible universe appears to float like an +enchanted island, resplendent within with lights and life and gorgeous +spectacles, and encircled with screens of crowded stars, but with its +dazzling vistas ending at the fathomless sea of pure darkness which +encloses all. + +The Galaxy, or Milky Way, surrounds the borders of our island in space +like a stellar garland, and when openings appear in it they are, by +contrast, far more impressive than the general darkness of the +interstellar expanse seen in other directions. Yet even that expanse +is not everywhere equally dark, for it contains gloomy deeps +discernable with careful watching. Here, too, contrast plays an +important part, though less striking than within the galactic region. +Some of Sir William Herschel's observations appear to indicate an +association between these tenebrious spots and neighboring star clouds +and nebulae. It is an illuminating bit of astronomical history that +when he was sweeping the then virgin heavens with his great telescopes +he was accustomed to say to his sister who, note-book in hand, waited +at his side to take down his words, fresh with the inspiration of +discovery: ``Prepare to write; the nebulae are coming; here space is +vacant.'' + +The most famous of the ``coal-sacks,'' and the first to be brought to +general attention before astronomers had awakened to the significance +of such things, lies adjacent to the ``Southern Cross,'' and is truly +an amazing phenomenon. It is not alone the conspicuousness of this +celestial vacancy, opening suddenly in the midst of one of the richest +parts of the Galaxy, that has given it its fame, but quite as much the +superstitious awe with which it was regarded by the early explorers of +the South Seas. To them, as well as to those who listened in rapt +wonder to their tales, the ``Coal-sack'' seemed to possess some occult +connection with the mystic ``Cross.'' In the eyes of the sailors it +was not a vacancy so much as a sable reality in the sky, and as, +shuddering, they stared at it, they piously crossed themselves. It was +another of the magical wonders of the unknown South, and as such it +formed the basis of many a ``wild surmise'' and many a sea-dog's yarn. +Scientific investigation has not diminished its prestige, and today no +traveler in the southern hemisphere is indifferent to its fascinating +strangeness, while some find it the most impressive spectacle of the +antarctic heavens. + +All around, up to the very edge of the yawning gap, the sheen of the +Milky Way is surpassingly glorious; but there, as if in obedience to +an almighty edict, everything vanishes. A single faint star is visible +within the opening, producing a curious effect upon the sensitive +spectator, like the sight of a tiny islet in the midst of a black, +motionless, waveless tarn. The dimensions of the lagoon of darkness, +which is oval or pear-shaped, are eight degrees by five, so that it +occupies a space in the sky about one hundred and thirty times greater +than the area of the full moon. It attracts attention as soon as the +eye is directed toward the quarter where it exists, and by virtue of +the rarity of such phenomena it appears a far greater wonder than the +drifts of stars that are heaped around it. Now that observatories are +multiplying in the southern hemisphere, the great austral +``Coal-sack'' will, no doubt, receive attention proportioned to its +importance as one of the most significant features of the sky. Already +at the Sydney Observatory photographs have shown that the southern +portion of this Dead Sea of Space is not quite ``bottomless,'' +although its northern part defies the longest sounding lines of the +astronomer. + +There is a similar, but less perfect, ``coal-sack'' in the northern +hemisphere, in the constellation of ``The Swan,'' which, strange to +say, also contains a well-marked figure of a cross outlined by stars. +This gap lies near the top of the cross-shaped figure. It is best seen +by averted vision, which brings out the contrast with the Milky Way, +which is quite brilliant around it. It does not, however, exercise the +same weird attraction upon the eye as the southern ``Coal-sack,'' for +instead of looking like an absolute void in the sky, it rather appears +as if a canopy of dark gauze had been drawn over the stars. We shall +see the possible significance of this appearance later. + +Just above the southern horizon of our northern middle latitudes, in +summer, where the Milky Way breaks up into vast sheets of nebulous +luminosity, lying over and between the constellations Scorpio and +Sagittarius, there is a remarkable assemblage of ``coal-sacks,'' +though none is of great size. One of them, near a conspicuous +star-cluster in Scorpio, M80, is interesting for having been the first +of these strange objects noted by Herschel. Probably it was its +nearness to M80 which suggested to his mind the apparent connection of +such vacancies with star-clusters which we have already mentioned. + +But the most marvelous of the ``coal-sacks'' are those that have been +found by photography in Sagittarius. One of Barnard's earliest and +most excellent photographs includes two of them, both in the +star-cluster M8. The larger, which is roughly rectangular in outline, +contains one little star, and its smaller neighbor is lune-shaped -- +surely a most singular form for such an object. Both are associated +with curious dark lanes running through the clustered stars like +trails in the woods. Along the borders of these lanes the stars are +ranked in parallel rows, and what may be called the bottoms of the +lanes are not entirely dark, but pebbled with faint stellar points. +One of them which skirts the two dark gaps and traverses the cluster +along its greatest diameter is edged with lines of stars, recalling +the alignment of the trees bordering a French highway. This road of +stars cannot be less than many billions of miles in length! + +All about the cluster the bed of the Galaxy is strangely disturbed, +and in places nearly denuded, as if its contents had been raked away +to form the immense stack and the smaller accumulations of stars +around it. The well-known ``Trifid Nebula'' is also included in the +field of the photograph, which covers a truly marvelous region, so +intricate in its mingling of nebulae, star-clusters, star-swarms, +star-streams, and dark vacancies that no description can do it +justice. Yet, chaotic as it appears, there is an unmistakable +suggestion of unity about it, impressing the beholder with the idea +that all the different parts are in some way connected, and have not +been fortuitously thrown together. Miss Agnes M. Clerke made the +striking remark that the dusky lanes in M8 are exemplified on the +largest scale in the great rift dividing the Milky Way, from Cygnus in +the northern hemisphere all the way to the ``Cross'' in the southern. +Similar lanes are found in many other clusters, and they are generally +associated with flanking rows of stars, resembling in their +arrangement the thick-set houses and villas along the roadways that +traverse the approaches to a great city. + +But to return to the black gaps. Are they really windows in the +star-walls of the universe? Some of them look rather as if they had +been made by a shell fired through a luminous target, allowing the eye +to range through the hole into the void space beyond. If science is +discretely silent about these things, what can the more venturesome +and less responsible imagination suggest? Would a huge ``runaway +sun,'' like Arcturus, for instance, make such an opening if it should +pass like a projectile through the Milky Way? It is at least a +stimulating inquiry. Being probably many thousands of times more +massive than the galactic stars, such a stellar missile would not be +stopped by them, though its direction of flight might be altered. It +would drag the small stars lying close to its course out of their +spheres, but the ultimate tendency of its attraction would be to sweep +them round in its wake, thus producing rather a star-swarm than a +vacancy. Those that were very close to it might be swept away in its +rush and become its satellites, careering away with it in its flight +into outer space; but those that were farther off, and they would, of +course, greatly outnumber the nearer ones, would tend inward from all +sides toward the line of flight, as dust and leaves collect behind a +speeding motor (though the forces operating would be different), and +would fill up the hole, if hole it were. A swarm thus collected should +be rounded in outline and bordered with a relatively barren ring from +which the stars had been ``sucked'' away. In a general sense the M8 +cluster answers to this description, but even if we undertook to +account for its existence by a supposition like the above, the black +gaps would remain unexplained, unless one could make a further draft +on the imagination and suggest that the stars had been thrown into a +vast eddy, or system of eddies, whose vortices appear as dark holes. +Only a maelstrom-like motion could keep such a funnel open, for +without regard to the impulse derived from the projectile, the proper +motions of the stars themselves would tend to fill it. Perhaps some +other cause of the whirling motion may be found. As we shall see when +we come to the spiral nebulae, gyratory movements are exceedingly +prevalent throughout the universe, and the structure of the Milky Way +is everywhere suggestive of them. But this is hazardous sport even for +the imagination -- to play with suns as if they were but thistle-down +in the wind or corks in a mill-race. + +Another question arises: What is the thickness of the hedge of stars +through which the holes penetrate? Is the depth of the openings +proportionate to their width? In other words, is the Milky Way round +in section like a rope, or flat and thin like a ribbon? The answer is +not obvious, for we have little or no information concerning the +relative distances of the faint galactic stars. It would be easier, +certainly, to conceive of openings in a thin belt than in a massive +ring, for in the first case they would resemble mere rifts and breaks, +while in the second they would be like wells or bore-holes. Then, too, +the fact that the Milky Way is not a continuous body but is made up of +stars whose actual distances apart is great, offers another quandary; +persistent and sharply bordered apertures in such an assemblage are a +priori as improbable, if not impossible, as straight, narrow holes +running through a swarm of bees. + +The difficulty of these questions indicates one of the reasons why it +has been suggested that the seeming gaps, or many of them, are not +openings at all, but opaque screens cutting off the light from stars +behind them. That this is quite possible in some cases is shown by +Barnard's later photographs, particularly those of the singular region +around the star Rho Ophiuchi. Here are to be seen somber lanes and +patches, apparently forming a connected system which covers an immense +space, and which their discoverer thinks may constitute a ``dark +nebula.'' This seems at first a startling suggestion; but, after all, +why should their not be dark nebulae as well as visible ones? In truth, +it has troubled some astronomers to explain the luminosity of the +bright nebulae, since it is not to be supposed that matter in so +diffuse a state can be incandescent through heat, and phosphorescent +light is in itself a mystery. The supposition is also in accord with +what we know of the existence of dark solid bodies in space. Many +bright stars are accompanied by obscure companions, sometimes as +massive as themselves; the planets are non-luminous; the same is true +of meteors before they plunge into the atmosphere and become heated by +friction; and many plausible reasons have been found for believing +that space contains as many obscure as shining bodies of great size. +It is not so difficult, after all, then, to believe that there are +immense collections of shadowy gases and meteoric dust whose presence +is only manifested when they intercept the light coming from shining +bodies behind them. + +This would account for the apparent extinguishment of light in open +space, which is indicated by the falling off in relative number of +telescopic stars below the tenth magnitude. Even as things are, the +amount of light coming to us from stars too faint to be seen with the +naked eye is so great that the statement of it generally surprises +persons who are unfamiliar with the inner facts of astronomy. It has +been calculated that on a clear night the total starlight from the +entire celestial sphere amounts to one-sixtieth of the light of the +full moon; but of this less than one-twenty-fifth is due to stars +separately distinguished by the eye. If there were no obscuring medium +in space, it is probable that the amount of starlight would be +noticeably and perhaps enormously increased. + +But while it seems certain that some of the obscure spots in the Milky +Way are due to the presence of ``dark nebulae,'' or concealing veils of +one kind or another, it is equally certain that there are many which +are true apertures, however they may have been formed, and by whatever +forces they may be maintained. These, then, are veritable windows of +the Galaxy, and when looking out of them one is face to face with the +great mystery of infinite space. There the known universe visibly +ends, but manifestly space itself does not end there. It is not within +the power of thought to conceive an end to space, for the instant we +think of a terminal point or line the mind leaps forward to the +beyond. There must be space outside as well as inside. Eternity of +time and infinity of space are ideas that the intellect cannot fully +grasp, but neither can it grasp the idea of a limitation to either +space or time. The metaphysical conceptions of hypergeometry, or +fourth-dimensional space, do not aid us. + +Having, then, discovered that the universe is a thing contained in +something indefinitely greater than itself; having looked out of its +windows and found only the gloom of starless night outside -- what +conclusions are we to draw concerning the beyond? It seems as empty as +a vacuum, but is it really so? If it be, then our universe is a single +atom astray in the infinite; it is the only island in an ocean without +shores; it is the one oasis in an illimitable desert. Then the Milky +Way, with its wide-flung garland of stars, is afloat like a tiny +smoke-wreath amid a horror of immeasurable vacancy, or it is an +evanescent and solitary ring of sparkling froth cast up for a moment +on the viewless billows of immensity. From such conclusions the mind +instinctively shrinks. It prefers to think that there is something +beyond, though we cannot see it. Even the universe could not bear to +be alone -- a Crusoe lost in the Cosmos! As the inhabitants of the +most elegant chateau, with its gardens, parks, and crowds of +attendants, would die of loneliness if they did not know that they +have neighbors, though not seen, and that a living world of indefinite +extent surrounds them, so we, when we perceive that the universe has +limits, wish to feel that it is not solitary; that beyond the hedges +and the hills there are other centers of life and activity. Could +anything be more terrible than the thought of an isolated universe? +The greater the being, the greater the aversion to seclusion. Only the +infinite satisfies; in that alone the mind finds rest. + +We are driven, then, to believe that the universal night which +envelopes us is not tenantless; that as we stare out of the +star-framed windows of the Galaxy and see nothing but uniform +blackness, the fault is with our eyes or is due to an obscuring +medium. Since our universe is limited in extent, there must be other +universes beyond it on all sides. Perhaps if we could carry our +telescopes to the verge of the great ``Coal-sack'' near the ``Cross,'' +being then on the frontier of our starry system, we could discern, +sparkling afar off in the vast night, some of the outer galaxies. They +may be grander than ours, just as many of the suns surrounding us are +immensely greater than ours. If we could take our stand somewhere in +the midst of immensity and, with vision of infinite reach, look about +us, we should perhaps see a countless number of stellar systems, amid +which ours would be unnoticeable, like a single star among the +multitude glittering in the terrestial sky on a clear night. Some +might be in the form of a wreath, like our own; some might be +globular, like the great star-clusters in Hercules and Centaurus; some +might be glittering circles, or disks, or rings within rings. If we +could enter them we should probably find a vast variety of +composition, including elements unknown to terrestrial chemistry; for +while the visible universe appears to contain few if any substances +not existing on the earth or in the sun, we have no warrant to assume +that others may not exist in infinite space. + +And how as to gravitation? We do not know that gravitation acts beyond +the visible universe, but it is reasonable to suppose that it does. At +any rate, if we let go its sustaining hand we are lost, and can only +wander hopelessly in our speculations, like children astray. If the +empire of gravitation is infinite, then the various outer systems must +have some, though measuring by our standards an imperceptible, +attractive influence upon each other, for gravitation never lets go +its hold, however great the space over which it is required to act. +Just as the stars about us are all in motion, so the starry systems +beyond our sight may be in motion, and our system as a whole may be +moving in concert with them. If this be so, then after interminable +ages the aspect of the entire system of systems must change, its +various members assuming new positions with respect to one another. In +the course of time we may even suppose that our universe will approach +relatively close to one of the others; and then, if men are yet living +on the earth, they may glimpse through the openings which reveal +nothing to us now, the lights of another nearing star system, like the +signals of a strange squadron, bringing them the assurance (which can +be but an inference at present) that the ocean of space has other +argosies venturing on its limitless expanse. + +There remains the question of the luminiferous ether by whose agency +the waves of light are borne through space. The ether is as mysterious +as gravitation. With regard to ether we only infer its existence from +the effects which we ascribe to it. Evidently the ether must extend as +far as the most distant visible stars. But does it continue on +indefinitely in outer space? If it does, then the invisibility of the +other systems must be due to their distance diminishing the quantity +of light that comes from them below the limit of perceptibility, or to +the interposition of absorbing media; if it does not, then the reason +why we cannot see them is owing to the absence of a means of +conveyance for the light waves, as the lack of an interplanetary +atmosphere prevents us from hearing the thunder of sun-spots. (It is +interesting to recall that Mr Edison was once credited with the +intention to construct a gigantic microphone which should render the +roar of sun-spots audible by transforming the electric vibrations into +sound-waves). On this supposition each starry system would be +enveloped in its own globule of ether, and no light could cross from +one to another. But the probability is that both the ether and +gravitation are ubiquitous, and that all the stellar systems are +immersed in the former like clouds of phosphorescent organisms in the +sea. + +So astronomy carries the mind from height to greater height. Men were +long in accepting the proofs of the relative insignificance of the +earth; they were more quickly convinced of the comparative littleness +of the solar system; and now the evidence assails their reason that +what they had regarded as the universe is only one mote gleaming in +the sunbeams of Infinity. + +Star-Clouds, Star-Clusters, and Star-Streams + +In the preceding chapter we have seen something of the strangely +complicated structure of the Galaxy, or Milky Way. We now proceed to +study more comprehensively that garlanded ``Pathway of the Gods.'' + +Judged by the eye alone, the Milky Way is one of the most delicately +beautiful phenomena in the entire realm of nature -- a shimmer of +silvery gauze stretched across the sky; but studied in the light of +its revelations, it is the most stupendous object presented to human +ken. Let us consider, first, its appearance to ordinary vision. Its +apparent position in the sky shifts according to the season. On a +serene, cloudless summer evening, in the absence of the moon, whose +light obscures it, one sees the Galaxy spanning the heavens from north +to southeast of the zenith like a phosphorescent arch. In early spring +it forms a similar but, upon the whole, less brilliant arch west of +the zenith. Between spring and summer it lies like a long, faint, +twilight band along the northern horizon. At the beginning of winter +it again forms an arch, this time spanning the sky from east to west, +a little north of the zenith. These are its positions as viewed from +the mean latitude of the United States. Even the beginner in +star-gazing does not have to watch it throughout the year in order to +be convinced that it is, in reality, a great circle, extending +entirely around the celestial sphere. We appear to be situated near +its center, but its periphery is evidently far away in the depths of +space. + +Although to the casual observer it seems but a delicate scarf of +light, brighter in some places than in others, but hazy and indefinite +at the best, such is not its appearance to those who study it with +care. They perceive that it is an organic whole, though marvelously +complex in detail. The telescope shows that it consists of stars too +faint and small through excess of distance to be separately visible. +Of the hundred million suns which some estimates have fixed as the +probable population of the starry universe, the vast majority (at +least thirty to one) are included in this strange belt of misty light. +But they are not uniformly distributed in it; on the contrary, they +are arrayed in clusters, knots, bunches, clouds, and streams. The +appearance is somewhat as if the Galaxy consisted of innumerable +swarms of silver-winged bees, more or less intermixed, some massed +together, some crossing the paths of others, but all governed by a +single purpose which leads them to encircle the region of space in +which we are situated. + +From the beginning of the systematic study of the heavens, the fact +has been recognized that the form of the Milky Way denotes the scheme +of the sidereal system. At first it was thought that the shape of the +system was that of a vast round disk, flat like a cheese, and filled +with stars, our sun and his relatively few neighbors being placed near +the center. According to this view, the galactic belt was an effect of +perspective; for when looking in the direction of the plane of the +disk, the eye ranged through an immense extension of stars which +blended into a glimmering blur, surrounding us like a ring; while when +looking out from the sides of the disk we saw but few stars, and in +those directions the heavens appeared relatively blank. Finally it was +recognized that this theory did not correspond with the observed +appearances, and it became evident that the Milky Way was not a mere +effect of perspective, but an actual band of enormously distant stars, +forming a circle about the sphere, the central opening of the ring +(containing many scattered stars) being many times broader than the +width of the ring itself. Our sun is one of the scattered stars in the +central opening. + +As already remarked, the ring of the Galaxy is very irregular, and in +places it is partly broken. With its sinuous outline, its pendant +sprays, its graceful and accordant curves, its bunching of masses, its +occasional interstices, and the manifest order of a general plan +governing the jumble of its details, it bears a remarkable resemblance +to a garland -- a fact which appears the more wonderful when we recall +its composition. That an elm-tree should trace the lines of beauty +with its leafy and pendulous branches does not surprise us; but we can +only gaze with growing amazement when we behold a hundred million suns +imitating the form of a chaplet! And then we have to remember that +this form furnishes the ground-plan of the universe. + +As an indication of the extraordinary speculations to which the +mystery of the Milky Way has given rise, a theory recently (1909) +proposed by Prof. George C. Comstock may be mentioned. Starting with +the data (first) that the number of stars increases as the Milky Way +is approached, and reaches a maximum in its plane, while on the other +hand the number of nebulae is greatest outside the Milky Way and +increases with distance from it, and (second) that the Milky Way, +although a complete ring, is broad and diffuse on one side through +one-half its course -- that half alone containing nebulae -- and +relatively narrow and well defined on the opposite side, the author of +this singular speculation avers that these facts can best be explained +by supposing that the invisible universe consists of two +interpenetrating parts, one of which is a chaos of indefinite extent, +strewn with stars and nebulous dust, and the other a long, broad but +comparatively thin cluster of stars, including the sun as one of its +central members. This flat star-cluster is conceived to be moving +edgewise through the chaos, and, according to Professor Comstock, it +acts after the manner of a snow-plough sweeping away the cosmic dust +and piling it on either hand above and below the plane of the moving +cluster. It thus forms a transparent rift, through which we see +farther and command a view of more stars than through the intensified +dust-clouds on either hand. This rift is the Milky Way. The dust +thrown aside toward the poles of the Milky Way is the substance of the +nebulae which abound there. Ahead, where the front of the star-plough +is clearing the way, the chaos is nearer at hand, and consequently +there the rift subtends a broader angle, and is filled with primordial +dust, which, having been annexed by the vanguard of the star-swarm, +forms the nebulae seen only in that part of the Milky Way. But behind, +the rift appears narrow because there we look farther away between +dust-clouds produced ages ago by the front of the plough, and no +scattered dust remains in that part of the rift. + +In quoting an outline of this strikingly original theory the present +writer should not be understood as assenting to it. That it appears +bizarre is not, in itself, a reason for rejecting it, when we are +dealing with so problematical and enigmatical a subject as the Milky +Way; but the serious objection is that the theory does not +sufficiently accord with the observed phenomena. There is too much +evidence that the Milky Way is an organic system, however fantastic +its form, to permit the belief that it can only be a rift in chaotic +clouds. As with every organism, we find that its parts are more or +less clearly repeated in its ensemble. Among all the strange things +that the Milky Way contains there is nothing so extraordinary as +itself. Every astronomer must many times have found himself marveling +at it in those comparatively rare nights when it shows all its beauty +and all its strangeness. In its great broken rifts, divisions, and +spirals are found the gigantic prototypes of similar forms in its +star-clouds and clusters. As we have said, it determines the general +shape of the whole sidereal system. Some of the brightest stars in the +sky appear to hang like jewels suspended at the ends of tassels +dropped from the Galaxy. Among these pendants are the Pleiades and the +Hyades. Orion, too, the ``Mighty Hunter,'' is caught in ``a loop of +light'' thrown out from it. The majority of the great first-magnitude +stars seem related to it, as if they formed an inner ring inclined at +an angle of some twenty degrees to its plane. Many of the long curves +that set off from it on both sides are accompanied by corresponding +curves of lucid stars. In a word, it offers every appearance of +structural connection with the entire starry system. That the universe +should have assumed the form of a wreath is certainly a matter for +astonishment; but it would have been still more astonishing if it had +been a cube, a rhomboid, or a dodecahedron, for then we should have +had to suppose that something resembling the forces that shape +crystals had acted upon the stars, and the difficulty of explaining +the universe by the laws of gravitation would have been increased. + +From the Milky Way as a whole we pass to the vast clouds, swarms, and +clusters of stars of which it is made up. It may be, as some +astronomers hold, that most of the galactic stars are much smaller +than the sun, so that their faintness is not due entirely to the +effect of distance. Still, their intrinsic brilliance attests their +solar character, and considering their remoteness, which has been +estimated at not less than ten thousand to twenty thousand light-years +(a light-year is equal to nearly six thousand thousand million miles) +their actual masses cannot be extremely small. The minutest of them +are entitled to be regarded as real suns, and they vary enormously in +magnitude. The effects of their attractions upon one another can only +be inferred from their clustering, because their relative movements +are not apparent on account of the brevity of the observations that we +can make. But imagine a being for whom a million years would be but as +a flitting moment; to him the Milky Way would appear in a state of +ceaseless agitation -- swirling with ``a fury of whirlpool motion.'' + +The cloud-like aspect of large parts of the Galaxy must always have +attracted attention, even from naked-eye observers, but the true +star-clouds were first satisfactorily represented in Barnard's +photographs. The resemblance to actual clouds is often startling. Some +are close-packed and dense, like cumuli; some are wispy or mottled, +like cirri. The rifts and modulations, as well as the general +outlines, are the same as those of clouds of vapor or dust, and one +notices also the characteristic thinning out at the edges. But we must +beware of supposing that the component suns are thickly crowded as the +particles forming an ordinary cloud. They look, indeed, as if they +were matted together, because of the irradiation of light, but in +reality millions and billions of miles separate each star from its +neighbors. Nevertheless they form real assemblages, whose members are +far more closely related to one another than is our sun to the stars +around him, and if we were in the Milky Way the aspect of the +nocturnal sky would be marvelously different from its present +appearance. + +Stellar clouds are characteristic of the Galaxy and are not found +beyond its borders, except in the ``Magellanic Clouds'' of the +southern hemisphere, which resemble detached portions of the Milky +Way. These singular objects form as striking a peculiarity of the +austral heavens as does the great ``Coal-sack'' described in Chapter +1. But it is their isolation that makes them so remarkable, for their +composition is essentially galactic, and if they were included within +its boundaries they would not appear more wonderful than many other +parts of the Milky Way. Placed where they are, they look like masses +fallen from the great stellar arch. They are full of nebulae and +star-clusters, and show striking evidences of spiral movement. + +Star-swarms, which are also characteristic features of the Galaxy, +differ from star-clouds very much in the way that their name would +imply -- i.e., their component stars are so arranged, even when they +are countless in number, that the idea of an exceedingly numerous +assemblage rather than that of a cloud is impressed on the observer's +mind. In a star-swarm the separate members are distinguishable because +they are either larger or nearer than the stars composing a ``cloud.'' +A splendid example of a true star-swarm is furnished by Chi Persei, in +that part of the Milky Way which runs between the constellations +Perseus and Cassiopeia. This swarm is much coarser than many others, +and can be seen by the naked eye. In a small telescope it appears +double, as if the suns composing it had divided into two parties which +keep on their way side by side, with some commingling of their members +where the skirts of the two companies come in contact. + +Smaller than either star-clouds or star-swarms, and differing from +both in their organization, are star-clusters. These, unlike the +others, are found outside as well as inside the Milky Way, although +they are more numerous inside its boundaries than elsewhere. The term +star-cluster is sometimes applied, though improperly, to assemblages +which are rather groups, such, for instance, as the Pleiades. In their +most characteristic aspect star-clusters are of a globular shape -- +globes of suns! A famous example of a globular star-cluster, but one +not included in the Milky Way, is the ``Great Cluster in Hercules.'' +This is barely visible to the naked eye, but a small telescope shows +its character, and in a large one it presents a marvelous spectacle. +Photographs of such clusters are, perhaps, less effective than those +of star-clouds, because the central condensation of stars in them is +so great that their light becomes blended in an indistinguishable +blur. The beautiful effect of the incessant play of infinitesimal rays +over the apparently compact surface of the cluster, as if it were a +globe of the finest frosted silver shining in an electric beam, is +also lost in a photograph. Still, even to the eye looking directly at +the cluster through a powerful telescope, the central part of the +wonderful congregation seems almost a solid mass in which the stars +are packed like the ice crystals in a snowball. + +The same question rises to the lips of every observer: How can they +possibly have been brought into such a situation? The marvel does not +grow less when we know that, instead of being closely compacted, the +stars of the cluster are probably separated by millions of miles; for +we know that their distances apart are slight as compared with their +remoteness from the Earth. Sir William Herschel estimated their number +to be about fourteen thousand, but in fact they are uncountable. If we +could view them from a point just within the edge of the assemblage, +they would offer the appearance of a hollow hemisphere emblazoned with +stars of astonishing brilliancy; the near-by ones unparalleled in +splendor by any celestial object known to us, while the more distant +ones would resemble ordinary stars. An inhabitant of the cluster would +not know, except by a process of ratiocination, that he was dwelling +in a globular assemblage of suns; only from a point far outside would +their spherical arrangement become evident to the eye. Imagine +fourteen-thousand fire-balloons with an approach to regularity in a +spherical space -- say, ten miles in diameter; there would be an +average of less than thirty in every cubic mile, and it would be +necessary to go to a considerable distance in order to see them as a +globular aggregation; yet from a point sufficiently far away they +would blend into a glowing ball. + +Photographs show even better than the best telescopic views that the +great cluster is surrounded with a multitude of dispersed stars, +suggestively arrayed in more or less curving lines, which radiate from +the principle mass, with which their connection is manifest. These +stars, situated outside the central sphere, look somewhat like vagrant +bees buzzing round a dense swarm where the queen bee is sitting. Yet +while there is so much to suggest the operation of central forces, +bringing and keeping the members of the cluster together, the +attentive observer is also impressed with the idea that the whole +wonderful phenomenon may be the result of explosion. As soon as this +thought seizes the mind, confirmation of it seems to be found in the +appearance of the outlying stars, which could be as readily explained +by the supposition that they have been blown apart as that they have +flocked together toward a center. The probable fact that the stars +constituting the cluster are very much smaller than our sun might be +regarded as favoring the hypothesis of an explosion. Of their real +size we know nothing, but, on the basis of an uncertain estimate of +their parallax, it has been calculated that they may average +forty-five thousand miles in diameter -- something more than half the +diameter of the planet Jupiter. Assuming the same mean density, +fourteen thousand such stars might have been formed by the explosion +of a body about twice the size of the sun. This recalls the theory of +Olbers, which has never been altogether abandoned or disproved, that +the Asteroids were formed by the explosion of a planet circulating +between the orbits of Mars and Jupiter. The Asteroids, whatever their +manner of origin, form a ring around the sun; but, of course, the +explosion of a great independent body, not originally revolving about +a superior center of gravitational force, would not result in the +formation of a ring of small bodies, but rather of a dispersed mass of +them. But back of any speculation of this kind lies the problem, at +present insoluble: How could the explosion be produced? (See the +question of explosions in Chapters 6 and 14). + +Then, on the other hand, we have the observation of Herschel, since +abundantly confirmed, that space is unusually vacant in the immediate +neighborhood of condensed star-clusters and nebulae, which, as far as +it goes, might be taken as an indication that the assembled stars had +been drawn together by their mutual attractions, and that the tendency +to aggregation is still bringing new members toward the cluster. But +in that case there must have been an original condensation of stars at +that point in space. This could probably have been produced by the +coagulation of a great nebula into stellar nuclei, a process which +seems now to be taking place in the Orion Nebula. + +A yet more remarkable globular star-cluster exists in the southern +hemisphere, Omega Centauri. In this case the central condensation of +stars presents an almost uniform blaze of light. Like the Hercules +cluster, that in Centaurus is surrounded with stars scattered over a +broad field and showing an appearance of radial arrangement. In fact, +except for its greater richness, Omega Centauri is an exact duplicate +of its northern rival. Each appears to an imaginative spectator as a +veritable ``city of suns.'' Mathematics shrinks from the task of +disentangling the maze of motions in such an assemblage. It would seem +that the chance of collisions is not to be neglected, and this idea +finds a certain degree of confirmation in the appearance of +``temporary stars'' which have more than once blazed out in, or close +by, globular star-clusters. + +This leads up to the notable fact, first established by Professor +Bailey a few years ago, that such clusters are populous with variable +stars. Omega Centauri and the Hercules cluster are especially +remarkable in this respect. The variables found in them are all of +short period and the changes of light show a noteworthy tendency to +uniformity. The first thought is that these phenomena must be due to +collisions among the crowded stars, but, if so, the encounters cannot +be between the stars themselves, but probably between stars and meteor +swarms revolving around them. Such periodic collisions might go on for +ages without the meteors being exhausted by incorporation with the +stars. This explanation appears all the more probable because one +would naturally expect that flocks of meteors would abound in a close +aggregation of stars. It is also consistent with Perrine's discovery +-- that the globular star clusters are powdered with minute stars +strewn thickly among the brighter ones. + +In speaking of Professor Comstock's extraordinary theory of the Milky +Way, the fact was mentioned that, broadly speaking, the nebulae are +less numerous in the galactic belt than in the comparatively open +spaces on either side of it, but that they are, nevertheless, abundant +in the broader half of the Milky Way which he designates as the front +of the gigantic ``plough'' supposed to be forcing its way through the +enveloping chaos. In and around the Sagittarius region the +intermingling of nebulae and galactic star clouds and clusters is +particularly remarkable. That there is a causal connection no +thoughtful person can doubt. We are unable to get away from the +evidence that a nebula is like a seed-ground from which stars spring +forth; or we may say that nebulae resemble clouds in whose bosom +raindrops are forming. The wonderful aspect of the admixtures of +nebulae and star-clusters in Sagittarius has been described in Chapter +1. We now come to a still more extraordinary phenomenon of this kind +-- the Pleiades nebulae. + +The group of the Pleiades, although lying outside the main course of +the Galaxy, is connected with it by a faint loop, and is the scene of +the most remarkable association of stars and nebulous matter known in +the visible universe. The naked eye is unaware of the existence of +nebulae in the Pleiades, or, at the best, merely suspects that there is +something of the kind there; and even the most powerful telescopes are +far from revealing the full wonder of the spectacle; but in +photographs which have been exposed for many hours consecutively, in +order to accumulate the impression of the actinic rays, the revelation +is stunning. The principle stars are seen surrounded by, and, as it +were, drowned in, dense nebulous clouds of an unparalleled kind. The +forms assumed by these clouds seem at first sight inexplicable. They +look like fleeces, or perhaps more like splashes and daubs of luminous +paint dashed carelessly from a brush. But closer inspection shows that +they are, to a large extent, woven out of innumerable threads of filmy +texture, and there are many indications of spiral tendencies. Each of +the bright stars of the group -- Alcyone, Merope, Maia, Electra, +Taygeta, Atlas -- is the focus of a dense fog (totally invisible, +remember, alike to the naked eye and to the telescope), and these +particular stars are veiled from sight behind the strange mists. +Running in all directions across the relatively open spaces are +nebulous wisps and streaks of the most curious forms. On some of the +nebular lines, which are either straight throughout, or if they change +direction do so at an angle, little stars are strung like beads. In +one case seven or eight stars are thus aligned, and, as if to +emphasize their dependence upon the chain which connects them, when it +makes a slight bend the file of stars turns the same way. Many other +star rows in the group suggest by their arrangement that they, too, +were once strung upon similar threads which have now disappeared, +leaving the stars spaced along their ancient tracks. We seem forced to +the conclusion that there was a time when the Pleiades were embedded +in a vast nebula resembling that of Orion, and that the cloud has now +become so rare by gradual condensation into stars that the merest +trace of it remains, and this would probably have escaped detection +but for the remarkable actinic power of the radiant matter of which it +consists. The richness of many of these faint nebulous masses in +ultra-violet radiations, which are those that specifically affect the +photographic plate, is the cause of the marvelous revelatory power of +celestial photography. So the veritable unseen universe, as +distinguished from the ``unseen universe'' of metaphysical +speculation, is shown to us. + +A different kind of association between stars and nebulae is shown in +some surprising photographic objects in the constellation Cygnus, +where long, wispy nebulae, billions of miles in length, some of them +looking like tresses streaming in a breeze, lie amid fields of stars +which seem related to them. But the relation is of a most singular +kind, for notwithstanding the delicate structure of the long nebulae +they appear to act as barriers, causing the stars to heap themselves +on one side. The stars are two, three, or four times as numerous on +one side of the nebulae as on the other. These nebulae, as far as +appearance goes, might be likened to rail fences, or thin hedges, +against which the wind is driving drifts of powdery snow, which, while +scattered plentifully all around, tends to bank itself on the leeward +side of the obstruction. The imagination is at a loss to account for +these extraordinary phenomena; yet there they are, faithfully giving +us their images whenever the photographic plate is exposed to their +radiations. + +Thus the more we see of the universe with improved methods of +observation, and the more we invent aids to human senses, each +enabling us to penetrate a little deeper into the unseen, the greater +becomes the mystery. The telescope carried us far, photography is +carrying us still farther; but what as yet unimagined instrument will +take us to the bottom, the top, and the end? And then, what hitherto +untried power of thought will enable us to comprehend the meaning of +it all? + +Stellar Migrations + +To the untrained eye the stars and the planets are not +distinguishable. It is customary to call them all alike ``stars.'' But +since the planets more or less rapidly change their places in the sky, +in consequence of their revolution about the sun, while the stars +proper seem to remain always in the same relative positions, the +latter are spoken of as ``fixed stars.'' In the beginnings of +astronomy it was not known that the ``fixed stars'' had any motion +independent of their apparent annual revolution with the whole sky +about the earth as a seeming center. Now, however, we know that the +term ``fixed stars'' is paradoxical, for there is not a single really +fixed object in the whole celestial sphere. The apparent fixity in the +positions of the stars is due to their immense distance, combined with +the shortness of the time during which we are able to observe them. It +is like viewing the plume of smoke issuing from a steamer, hull down, +at sea: if one does not continue to watch it for a long time it +appears to be motionless, although in reality it may be traveling at +great speed across the line of sight. Even the planets seem fixed in +position if one watches them for a single night only, and the more +distant ones do not sensibly change their places, except after many +nights of observation. Neptune, for instance, moves but little more +than two degrees in the course of an entire year, and in a month its +change of place is only about one-third of the diameter of the full +moon. + +Yet, fixed as they seem, the stars are actually moving with a speed in +comparison with which, in some cases, the planets might almost be said +to stand fast in their tracks. Jupiter's speed in his orbit is about +eight miles per second, Neptune's is less than three and one-half +miles, and the earth's is about eighteen and one-half miles; while +there are ``fixed stars'' which move two hundred or three hundred +miles per second. They do not all, however, move with so great a +velocity, for some appear to travel no faster than the planets. But in +all cases, notwithstanding their real speed, long-continued and +exceedingly careful observations are required to demonstrate that they +are moving at all. No more overwhelming impression of the frightful +depths of space in which the stars are buried can be obtained than by +reflecting upon the fact that a star whose actual motion across the +line of sight amounts to two hundred miles per second does not change +its apparent place in the sky, in the course of a thousand years, +sufficiently to be noticed by the casual observer of the heavens! + +There is one vast difference between the motions of the stars and +those of the planets to which attention should be at once called: the +planets, being under the control of a central force emanating from +their immediate master, the sun, all move in the same direction and in +orbits concentric about the sun; the stars, on the other hand, move in +every conceivable direction and have no apparent center of motion, for +all efforts to discover such a center have failed. At one time, when +theology had finally to accept the facts of science, a grandiose +conception arose in some pious minds, according to which the Throne of +God was situated at the exact center of His Creation, and, seated +there, He watched the magnificent spectacle of the starry systems +obediently revolving around Him. Astronomical discoveries and +speculations seemed for a time to afford some warrant for this view, +which was, moreover, an acceptable substitute for the abandoned +geocentric theory in minds that could only conceive of God as a +superhuman artificer, constantly admiring his own work. No longer ago +than the middle of the nineteenth century a German astronomer, +Maedler, believed that he had actually found the location of the +center about which the stellar universe revolved. He placed it in the +group of the Pleiades, and upon his authority an extraordinary +imaginative picture was sometimes drawn of the star Alcyone, the +brightest of the Pleiades, as the very seat of the Almighty. This idea +even seemed to gain a kind of traditional support from the mystic +significance, without known historical origin, which has for many +ages, and among widely separated peoples, been attached to the +remarkable group of which Alcyone is the chief. But since Maedler's +time it has been demonstrated that the Pleiades cannot be the center +of revolution of the universe, and, as already remarked, all attempts +to find or fix such a center have proved abortive. Yet so powerful was +the hold that the theory took upon the popular imagination, that even +today astronomers are often asked if Alcyone is not the probable site +of ``Jerusalem the Golden.'' + +If there were a discoverable center of predominant gravitative power, +to which the motions of all the stars could be referred, those motions +would appear less mysterious, and we should then be able to conclude +that the universe was, as a whole, a prototype of the subsidiary +systems of which it is composed. We should look simply to the law of +gravitation for an explanation, and, naturally, the center would be +placed within the opening enclosed by the Milky Way. If it were there +the Milky Way itself should exhibit signs of revolution about it, like +a wheel turning upon its hub. No theory of the star motions as a whole +could stand which failed to take account of the Milky Way as the basis +of all. But the very form of that divided wreath of stars forbids the +assumption of its revolution about a center. Even if it could be +conceived as a wheel having no material center it would not have the +form which it actually presents. As was shown in Chapter 2, there is +abundant evidence of motion in the Milky Way; but it is not motion of +the system as a whole, but motion affecting its separate parts. +Instead of all moving one way, the galactic stars, as far as their +movements can be inferred, are governed by local influences and +conditions. They appear to travel crosswise and in contrary +directions, and perhaps they eddy around foci where great numbers have +assembled; but of a universal revolution involving the entire mass we +have no evidence. + +Most of our knowledge of star motions, called ``proper motions,'' +relates to individual stars and to a few groups which happen to be so +near that the effects of their movements are measurable. In some cases +the motion is so rapid (not in appearance, but in reality) that the +chief difficulty is to imagine how it can have been imparted, and what +will eventually become of the ``runaways.'' Without a collision, or a +series of very close approaches to great gravitational centers, a star +traveling through space at the rate of two hundred or three hundred +miles per second could not be arrested or turned into an orbit which +would keep it forever flying within the limits of the visible +universe. A famous example of these speeding stars is ``1830 +Groombridge,'' a star of only the sixth magnitude, and consequently +just visible to the naked eye, whose motion across the line of sight +is so rapid that it moves upon the face of the sky a distance equal to +the apparent diameter of the moon every 280 years. The distance of +this star is at least 200,000,000,000,000 miles, and may be two or +three times greater, so that its actual speed cannot be less than two +hundred, and may be as much as four hundred, miles per second. It +could be turned into a new course by a close approach to a great sun, +but it could only be stopped by collision, head-on, with a body of +enormous mass. Barring such accidents it must, as far as we can see, +keep on until it has traversed our stellar system, whence in may +escape and pass out into space beyond, to join, perhaps, one of those +other universes of which we have spoken. Arcturus, one of the greatest +suns in the universe, is also a runaway, whose speed of flight has +been estimated all the way from fifty to two hundred miles per second. +Arcturus, we have every reason to believe, possesses hundreds of times +the mass of our sun -- think, then, of the prodigious momentum that +its motion implies! Sirius moves more moderately, its motion across +the line of sight amounting to only ten miles per second, but it is at +the same time approaching the sun at about the same speed, its actual +velocity in space being the resultant of the two displacements. + +What has been said about the motion of Sirius brings us to another +aspect of this subject. The fact is, that in every case of stellar +motion the displacement that we observe represents only a part of the +actual movement of the star concerned. There are stars whose motion +carries them straight toward or straight away from the earth, and such +stars, of course, show no cross motion. But the vast majority are +traveling in paths inclined from a perpendicular to our line of sight. +Taken as a whole, the stars may be said to be flying about like the +molecules in a mass of gas. The discovery of the radial component in +the movements of the stars is due to the spectroscope. If a star is +approaching, its spectral lines are shifted toward the violet end of +the spectrum by an amount depending upon the velocity of approach; if +it is receding, the lines are correspondingly shifted toward the red +end. Spectroscopic observation, then, combined with micrometric +measurements of the cross motion, enables us to detect the real +movement of the star in space. Sometimes it happens that a star's +radial movement is periodically reversed; first it approaches, and +then it recedes. This indicates that it is revolving around a near-by +companion, which is often invisible, and superposed upon this motion +is that of the two stars concerned, which together may be approaching +or receding or traveling across the line of sight. Thus the +complications involved in the stellar motions are often exceedingly +great and puzzling. + +Yet another source of complication exists in the movement of our own +star, the sun. There is no more difficult problem in astronomy than +that of disentangling the effects of the solar motion from those of +the motions of the other stars. But the problem, difficult as it is, +has been solved, and upon its solution depends our knowledge of the +speed and direction of the movement of the solar system through space, +for of course the sun carries its planets with it. One element of the +solution is found in the fact that, as a result of perspective, the +stars toward which we are going appear to move apart toward all points +of the compass, while those behind appear to close up together. Then +the spectroscopic principle already mentioned is invoked for studying +the shift of the lines, which is toward the violet in the stars ahead +of us and toward the red in those that we are leaving behind. Of +course the effects of the independent motions of the stars must be +carefully excluded. The result of the studies devoted to this subject +is to show that we are traveling at a speed of twelve to fifteen miles +per second in a northerly direction, toward the border of the +constellations Hercules and Lyra. A curious fact is that the more +recent estimates show that the direction is not very much out of a +straight line drawn from the sun to the star Vega, one of the most +magnificent suns in the heavens. But it should not be inferred from +this that Vega is drawing us on; it is too distant for its gravitation +to have such an effect. + +Many unaccustomed thoughts are suggested by this mighty voyage of the +solar system. Whence have we come, and whither do we go? Every year of +our lives we advance at least 375,000,000 miles. Since the traditional +time of Adam the sun has led his planets through the wastes of space +no less than 225,000,000,000 miles, or more than 2400 times the +distance that separates him from the earth. Go back in imagination to +the geologic ages, and try to comprehend the distance over which the +earth has flown. Where was our little planet when it emerged out of +the clouds of chaos? Where was the sun when his ``thunder march'' +began? What strange constellations shone down upon our globe when its +masters of life were the monstrous beasts of the ``Age of Reptiles''? +A million years is not much of a span of time in geologic reckoning, +yet a million years ago the earth was farther from its present place +in space than any of the stars with a measurable parallax are now. It +was more than seven times as far as Sirius, nearly fourteen times as +far as Alpha Centauri, three times as far as Vega, and twice as far as +Arcturus. But some geologists demand two hundred, three hundred, even +one thousand million years to enable them to account for the +evolutionary development of the earth and its inhabitants. In a +thousand million years the earth would have traveled farther than from +the remotest conceivable depths of the Milky Way! + +Other curious reflections arise when we think of the form of the +earth's track as it follows the lead of the sun, in a journey which +has neither known beginning nor conceivable end. There are probably +many minds which have found a kind of consolation in the thought that +every year the globe returns to the same place, on the same side of +the sun. This idea may have an occult connection with our traditional +regard for anniversaries. When that period of the year returns at +which any great event in our lives has occurred we have the feeling +that the earth, in its annual round, has, in a manner, brought us back +to the scene of that event. We think of the earth's orbit as a +well-worn path which we traverse many times in the course of a +lifetime. It seems familiar to us, and we grow to have a sort of +attachment to it. The sun we are accustomed to regard as a fixed +center in space, like the mill or pump around which the harnessed +patient mule makes his endless circuits. But the real fact is that the +earth never returns to the place in space where it has once quitted. +In consequence of the motion of the sun carrying the earth and the +other planets along, the track pursued by our globe is a vast spiral +in space continually developing and never returning upon its course. +It is probable that the tracks of the sun and the others stars are +also irregular, and possibly spiral, although, as far as can be at +present determined, they appear to be practically straight. Every +star, wherever it may be situated, is attracted by its fellow-stars +from many sides at once, and although the force is minimized by +distance, yet in the course of many ages its effects must become +manifest. + +Looked at from another side, is there not something immensely +stimulating and pleasing to the imagination in the idea of so +stupendous a journey, which makes all of us the greatest of travelers? +In the course of a long life a man is transported through space thirty +thousand million miles; Halley's Comet does not travel one-quarter as +far in making one of its immense circuits. And there are adventures on +this voyage of which we are just beginning to learn to take account. +Space is full of strange things, and the earth must encounter some of +them as it advances through the unknown. Many singular speculations +have been indulged in by astronomers concerning the possible effects +upon the earth of the varying state of the space that it traverses. +Even the alternation of hot and glacial periods has sometimes been +ascribed to this source. When tropical life flourished around the +poles, as the remains in the rocks assure us, the needed high +temperature may, it has been thought, have been derived from the +presence of the earth in a warm region of space. Then, too, there is a +certain interest for us in the thought of what our familiar planet has +passed through. We cannot but admire it for its long journeying as we +admire the traveler who comes to us from remote and unexplored lands, +or as we gaze with a glow of interest upon the first locomotive that +has crossed a continent, or a ship that has visited the Arctic or +Antarctic regions. If we may trust the indications of the present +course, the earth, piloted by the sun, has come from the Milky Way in +the far south and may eventually rejoin that mighty band of stars in +the far north. + +While the stars in general appear to travel independently of one +another, except when they are combined in binary or trinary systems, +there are notable exceptions to this rule. In some quarters of the sky +we behold veritable migrations of entire groups of stars whose members +are too widely separated to show any indications of revolution about a +common center of gravity. This leads us back again to the wonderful +group of the Pleiades. All of the principle stars composing that group +are traveling in virtually parallel lines. Whatever force set them +going evidently acted upon all alike. This might be explained by the +assumption that when the original projective force acted upon them +they were more closely united than they are at present, and that in +drifting apart they have not lost the impulse of the primal motion. Or +it may be supposed that they are carried along by some current in +space, although it would be exceedingly difficult, in the present +state of our knowledge, to explain the nature of such a current. Yet +the theory of a current has been proposed. As to an attractive center +around which they might revolve, none has been found. Another instance +of similar ``star-drift'' is furnished by five of the seven stars +constituting the figure of the ``Great Dipper.'' In this case the +stars concerned are separated very widely, the two extreme ones by not +less than fifteen degrees, so that the idea of a common motion would +never have been suggested by their aspect in the sky; and the case +becomes the more remarkable from the fact that among and between them +there are other stars, some of the same magnitude, which do not share +their motion, but are traveling in other directions. Still other +examples of the same phenomenon are found in other parts of the sky. +Of course, in the case of compact star-clusters, it is assumed that +all the members share a like motion of translation through space, and +the same is probably true of dense star-swarms and star-clouds. + +The whole question of star-drift has lately assumed a new phase, in +consequence of the investigations of Kapteyn, Dyson, and Eddington on +the ``systematic motions of the stars.'' This research will, it is +hoped, lead to an understanding of the general law governing the +movements of the whole body of stars constituting the visible +universe. Taking about eleven hundred stars whose proper motions have +been ascertained with an approach to certainty, and which are +distributed in all parts of the sky, it has been shown that there +exists an apparent double drift, in two independent streams, moving in +different and nearly opposed directions. The apex of the motion of +what is called ``Stream I'' is situated, according to Professor +Kapteyn, in right ascension 85 deg., declination south 11 deg., which places +it just south of the constellation Orion; while the apex of ``Stream +II'' is in right ascension 260 deg., declination south 48 deg., placing it in +the constellation Ara, south of Scorpio. The two apices differ very +nearly 180 deg. in right ascension and about 120 deg. in declination. The +discovery of these vast star-streams, if they really exist, is one of +the most extraordinary in modern astronomy. It offers the correlation +of stellar movements needed as the basis of a theory of those +movements, but it seems far from revealing a physical cause for them. +As projected against the celestial sphere the stars forming the two +opposite streams appear intermingled, some obeying one tendency and +some the other. As Professor Dyson has said, the hypothesis of this +double movement is of a revolutionary character, and calls for further +investigation. Indeed, it seems at first glance not less surprising +than would be the observation that in a snow-storm the flakes over our +heads were divided into two parties and driving across each other's +course in nearly opposite directions, as if urged by interpenetrating +winds. + +But whatever explanation may eventually be found for the motions of +the stars, the knowledge of the existence of those motions must always +afford a new charm to the contemplative observer of the heavens, for +they impart a sense of life to the starry system that would otherwise +be lacking. A stagnant universe, with every star fixed immovably in +its place, would not content the imagination or satisfy our longing +for ceaseless activity. The majestic grandeur of the evolutions of the +celestial hosts, the inconceivable vastness of the fields of space in +which they are executed, the countless numbers, the immeasurable +distances, the involved convolutions, the flocking and the scattering, +the interpenetrating marches and countermarches, the strange community +of impulsion affecting stars that are wide apart in space and causing +them to traverse the general movement about them like aides and +despatch-bearers on a battle-field -- all these arouse an intensity of +interest which is heightened by the mystery behind them. + +The Passing of the Constellations + +From a historical and picturesque point of view, one of the most +striking results of the motions of the stars described in the last +chapter is their effect upon the forms of the constellations, which +have been watched and admired by mankind from a period so early that +the date of their invention is now unknown. The constellations are +formed by chance combinations of conspicuous stars, like figures in a +kaleidoscope, and if our lives were commensurate with the aeons of +cosmic existence we should perceive that the kaleidoscope of the +heavens was ceaselessly turning and throwing the stars into new +symmetries. Even if the stars stood fast, the motion of the solar +system would gradually alter the configurations, as the elements of a +landscape dissolve and recombine in fresh groupings with the +traveler's progress amid them. But with the stars themselves all in +motion at various speeds and in many directions, the changes occur +more rapidly. Of course, ``rapid'' is here understood in a relative +sense; the wheel of human history to an eye accustomed to the majestic +progression of the universe would appear to revolve with the velocity +of a whirling dynamo. Only the deliberation of geological movements +can be contrasted with the evolution and devolution of the +constellations. + +And yet this secular fluctuation of the constellation figures is not +without keen interest for the meditative observer. It is another +reminder of the swift mutability of terrestial affairs. To the passing +glance, which is all that we can bestow upon these figures, they +appear so immutable that they have been called into service to form +the most lasting records of ancient thought and imagination that we +possess. In the forms of the constellations, the most beautiful, and, +in imaginative quality, the finest, mythology that the world has ever +known has been perpetuated. Yet, in a broad sense, this scroll of +human thought imprinted on the heavens is as evanescent as the summer +clouds. Although more enduring than parchment, tombs, pyramids, and +temples, it is as far as they from truly eternizing the memory of what +man has fancied and done. + +Before studying the effects that the motions of the stars have had and +will have upon the constellations, it is worth while to consider a +little further the importance of the stellar pictures as archives of +history. To emphasize the importance of these effects it is only +necessary to recall that the constellations register the oldest +traditions of our race. In the history of primeval religions they are +the most valuable of documents. Leaving out of account for the moment +the more familiar mythology of the Greeks, based on something older +yet, we may refer for illustration to that of the mysterious Maya race +of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a group +of ruins perched, after the Mexican and Central-American plan, on the +summits of pyramidal mounds which mark the site of an ancient +theogonic center of the Mayas. Here the temples all evidently refer to +a cult based upon the constellations as symbols. The figures and the +names, of course, were not the same as those that we have derived from +our Aryan ancestors, but the star groups were the same or nearly so. +For instance, the loftiest of the temples at Izamal was connected with +the sign of the constellation known to us as Cancer, marking the place +of the sun at the summer solstice, at which period the sun was +supposed to descend at noon like a great bird of fire and consume the +offerings left upon the altar. Our Scorpio was known to the Mayas as a +sign of the ``Death God.'' Our Libra, the ``Balance,'' with which the +idea of a divine weighing out of justice has always been connected, +seems to be identical with the Mayan constellation Teoyaotlatohua, +with which was associated a temple where dwelt the priests whose +special business it was to administer justice and to foretell the +future by means of information obtained from the spirits of the dead. +Orion, the ``Hunter'' of our celestial mythology, was among the Mayas +a ``Warrior,'' while Sagittarius and others of our constellations were +known to them (under different names, of course), and all were endowed +with a religious symbolism. And the same star figures, having the same +significance, were familiar to the Peruvians, as shown by the temples +at Cuzco. Thus the imagination of ancient America sought in the +constellations symbols of the unchanging gods. + +But, in fact, there is no nation and no people that has not recognized +the constellations, and at one period or another in its history +employed them in some symbolic or representative capacity. As handled +by the Greeks from prehistoric times, the constellation myths became +the very soul of poetry. The imagination of that wonderful race +idealized the principal star groups so effectively that the figures +and traditions thus attached to them have, for civilized mankind, +displaced all others, just as Greek art in its highest forms stands +without parallel and eclipses every rival. The Romans translated no +heroes and heroines of the mythical period of their history to the +sky, and the deified Caesars never entered that lofty company, but the +heavens are filled with the early myths of the Greeks. Herakles +nightly resumes his mighty labors in the stars; Zeus, in the form of +the white ``Bull,'' Taurus, bears the fair Europa on his back through +the celestial waves; Andromeda stretches forth her shackled arms in +the star-gemmed ether, beseeching aid; and Perseus, in a blaze of +diamond armor, revives his heroic deeds amid sparkling clouds of +stellar dust. There, too, sits Queen Cassiopeia in her dazzling chair, +while the Great King, Cepheus, towers gigantic over the pole. +Professor Young has significantly remarked that a great number of the +constellations are connected in some way or other with the Argonautic +Expedition -- that strangely fascinating legend of earliest Greek +story which has never lost its charm for mankind. In view of all this, +we may well congratulate ourselves that the constellations will +outlast our time and the time of countless generations to follow us; +and yet they are very far from being eternal. Let us now study some of +the effects of the stellar motions upon them. + +We begin with the familiar figure of the ``Great Dipper.'' He who has +not drunk inspiration from its celestial bowl is not yet admitted to +the circle of Olympus. This figure is made up of seven conspicuous +stars in the constellation Ursa Major, the ``Greater Bear.'' The +handle of the ``Dipper'' corresponds to the tail of the imaginary +``Bear,'' and the bowl lies upon his flank. In fact, the figure of a +dipper is so evident and that of a bear so unevident, that to most +persons the ``Great Dipper'' is the only part of the constellation +that is recognizable. Of the seven stars mentioned, six are of nearly +equal brightness, ranking as of the second magnitude, while the +seventh is of only the third magnitude. The difference is very +striking, since every increase of one magnitude involves an increase +of two-and-a-half times in brightness. There appears to be little +doubt that the faint star, which is situated at the junction of the +bowl and the handle, is a variable of long period, since three hundred +years ago it was as bright as its companions. But however that may be, +its relative faintness at the present time interferes but little with +the perfection of the ``Dipper's'' figure. In order the more readily +to understand the changes which are taking place, it will be well to +mention both the names and the Greek letters which are attached to the +seven stars. Beginning at the star in the upper outer edge of the rim +of the bowl and running in regular order round the bottom and then out +to the end of the handle, the names and letters are as follows: Dubhe +({\alpha}), Merak ({\beta}), Phaed ({\gamma}), Megrez ({\delta}), +Alioth ({\epsilon}), Mizar ({\zeta}), and Benetnasch ({\eta}). Megrez +is the faint star already mentioned at the junction of the bowl and +handle, and Mizar, in the middle of the handle, has a close, naked-eye +companion which is named Alcor. The Arabs called this singular pair of +stars ``The Horse and Rider.'' Merak and Duhbe are called ``The +Pointers,'' because an imaginary line drawn northward through them +indicates the Pole Star. + +Now it has been found that five of these stars -- viz., Merak, Phaed, +Megrez, Alioth, and Mizar (with its comrade) -- are moving with +practically the same speed in an easterly direction, while the other +two, Dubhe and Benetnasch, are simultaneously moving westward, the +motions of Benetnasch being apparently more rapid. The consequence of +these opposed motions is, of course, that the figure of the ``Dipper'' +cannot always have existed and will not continue to exist. In the +accompanying diagrams it has been thought interesting to show the +relative positions of these seven stars, as seen from the point which +the earth now occupies, both in the past and in the future. Arrows +attached to the stars in the figure representing the present +appearance of the ``Dipper'' indicate the directions of the motions +and the distances over which they will carry the stars in a period of +about five hundred centuries. The time, no doubt, seems long, but +remember the vast stretch of ages through which the earth has passed, +and then reflect that no reason is apparent why our globe should not +continue to be a scene of animation for ten thousand centuries yet to +come. The fact that the little star Alcor placed so close to Mizar +should accompany the latter in its flight is not surprising, but that +two of the principal stars of the group should be found moving in a +direction directly opposed to that pursued by the other five is +surprising in the highest degree; and it recalls the strange theory of +a double drift affecting all the stars, to which attention was called +in the preceding chapter. It would appear that Benetnasch and Dubhe +belong to one ``current,'' and Merak, Phaed, Megrez, Alioth, and Mizar +to the other. As far as is known, the motion of the seven stars are +not shared by the smaller stars scattered about them, but on the +theory of currents there should be such a community of motion, and +further investigation may reveal it. + +From the ``Great Dipper'' we turn to a constellation hardly less +conspicuous and situated at an equal distance from the pole on the +other side -- Cassiopeia. This famous star-group commemorating the +romantic Queen of Ethiopia whose vain boasting of her beauty was +punished by the exposure of her daughter Andromeda to the ``Sea +Monster,'' is well-marked by five stars which form an irregular letter +``W'' with its open side toward the pole. Three of these stars are +usually ranked as of the second magnitude, and two of the third; but +to ordinary observation they appear of nearly equal brightness, and +present a very striking picture. They mark out the chair and a part of +the figure of the beautiful queen. Beginning at the right-hand, or +western, end of the ``W,'' their Greek letter designations are: Beta +({\beta}), Alpha ({\alpha}), Gamma ({\gamma}), Delta ({\delta}), and +Epsilon ({\epsilon}). Four of them, Beta, Alpha, Delta, and Epsilon +are traveling eastwardly at various speeds, while the fifth, Gamma, +moves in a westerly direction. The motion of Beta is more rapid than +that of any of the others. It should be said, however, that no little +uncertainty attaches to the estimates of the rate of motion of stars +which are not going very rapidly, and different observers often vary +considerably in their results. + +In the beautiful ``Northern Crown,'' one of the most perfect and +charming of all the figures to be found in the stars, the alternate +combining and scattering effects of the stellar motions are shown by +comparing the appearance which the constellation must have had five +hundred centuries ago with that which it has at present and that which +it will have in the future. The seven principle stars of the asterism, +forming a surprisingly perfect coronet, have movements in three +directions at right angles to one another. That in these circumstances +they should ever have arrived at positions giving them so striking an +appearance of definite association is certainly surprising; from its +aspect one would have expected to find a community of movement +governing the brilliants of the ``Crown,'' but instead of that we find +evidence that they will inevitably drift apart and the beautiful +figure will dissolve. + +A similar fate awaits such asterisms as the ``Northern Cross'' in +Cygnus; the ``Crow'' (Corvus), which stands on the back of the great +``Sea Serpent,'' Hydra, and pecks at his scales; ``Job's Coffin'' +(Delphinus); the ``Great Square of Pegasus''; the ``Twins'' (Gemini); +the beautiful ``Sickle'' in Leo; and the exquisite group of the Hyades +in Taurus. In the case of the Hyades, two controlling movements are +manifest: one, affecting five of the stars which form the well-known +figure of a letter ``V,'' is directed northerly; the other, which +controls the direction of two stars, has an easterly trend. The chief +star of the group, Aldebaran, one of the finest of all stars both for +its brilliance and its color, is the most affected by the easterly +motion. In time it will drift entirely out of connection with its +present neighbors. Although the Hyades do not form so compact a group +as the Pleiades in the same constellation, yet their appearance of +relationship is sufficient to awaken a feeling of surprise over the +fact that, as with the stars of the ``Dipper,'' their association is +only temporary or apparent. + +The great figure of Orion appears to be more lasting, not because its +stars are physically connected, but because of their great distance, +which renders their movements too deliberate to be exactly +ascertained. Two of the greatest of its stars, Betelgeuse and Rigel, +possess, as far as has been ascertained, no perceptible motion across +the line of sight, but there is a little movement perceptible in the +``Belt.'' At the present time this consists of an almost perfect +straight line, a row of second-magnitude stars about equally spaced +and of the most striking beauty. In the course of time, however, the +two right-hand stars, Mintaka and Alnilam (how fine are these Arabic +star names!) will approach each other and form a naked-eye double, but +the third, Alnita, will drift away eastward, so that the ``Belt'' will +no longer exist. + +For one more example, let us go to the southern hemisphere, whose most +celebrated constellation, the ``Southern Cross,'' has found a place in +all modern literatures, although it has no claim to consideration on +account of association with ancient legends. This most attractive +asterism, which has never ceased to fascinate the imagination of +Christendom since it was first devoutly described by the early +explorers of the South, is but a passing collocation of brilliant +stars. Yet even in its transfigurations it has been for hundreds of +centuries, and will continue to be for hundreds of centuries to come, +a most striking object in the sky. Our figures show its appearance in +three successive phases: first, as it was fifty thousand years ago +(viewed from the earth's present location); second, as it is in our +day; and, third, as it will be an equal time in the future. The +nearness of these bright stars to one another -- the length of the +longer beam of the ``Cross'' is only six degrees -- makes this group +very noticeable, whatever the arrangement of its components may be. +The largest star, at the base of the ``Cross,'' is of the first +magnitude, two of the others are of the second magnitude, and the +fourth is of the third. Other stars, not represented in the figures, +increase the effect of a celestial blazonry, although they do not help +the resemblance to a cross. + +But since the motion of the solar system itself will, in the course of +so long a period as fifty thousand years, produce a great change in +the perspective of the heavens as seen from the earth, by carrying us +nearly nineteen trillion miles from our present place, why, it may be +asked, seek to represent future appearances of the constellations +which we could not hope to see, even if we could survive so long? The +answer is: Because these things aid the mind to form a picture of the +effects of the mobility of the starry universe. Only by showing the +changes from some definite point of view can we arrive at a due +comprehension of them. The constellations are more or less familiar to +everybody, so that impending changes of their forms must at once +strike the eye and the imagination, and make clearer the significance +of the movements of the stars. If the future history of mankind is to +resemble its past and if our race is destined to survive yet a million +years, then our remote descendents will see a ``new heavens'' if not a +``new earth,'' and will have to invent novel constellations to +perpetuate their legends and mythologies. + +If our knowledge of the relative distances of the stars were more +complete, it would be an interesting exercise in celestial geometry to +project the constellations probably visible to the inhabitants of +worlds revolving around some of the other suns of space. Our sun is +too insignificant for us to think that he can make a conspicuous +appearance among them, except, perhaps, in a few cases. As seen, for +instance, from the nearest known star, Alpha Centauri, the sun would +appear of the average first magnitude, and consequently from that +standpoint he might be the gem of some little constellation which had +no Sirius, or Arcturus, or Vega to eclipse him with its superior +splendor. But from the distance of the vast majority of the stars the +sun would probably be invisible to the naked eye, and as seen from +nearer systems could only rank as a fifth or sixth magnitude star, +unnoticed and unknown except by the star-charting astronomer. + +Conflagrations in the Heavens + +Suppose it were possible for the world to take fire and burn up -- as +some pessimists think that it will do when the Divine wrath shall have +sufficiently accumulated against it -- nobody out of our own little +corner of space would ever be aware of the catastrophe! With all their +telescopes, the astronomers living in the golden light of Arcturus or +the diamond blaze of Canopus would be unable to detect the least +glimmer of the conflagration that had destroyed the seat of Adam and +his descendents, just as now they are totally ignorant of its +existence. + +But at least fifteen times in the course of recorded history men +looking out from the earth have beheld in the remote depths of space +great outbursts of fiery light, some of them more splendidly luminous +than anything else in the firmament except the sun! If they were +conflagrations, how many million worlds like ours were required to +feed their blaze? + +It is probable that ``temporary'' or ``new'' stars, as these wonderful +apparitions are called, really are conflagrations; not in the sense of +a bonfire or a burning house or city, but in that of a sudden eruption +of inconceivable heat and light, such as would result from the +stripping off the shell of an encrusted sun or the crashing together +of two mighty orbs flying through space with a hundred times the +velocity of the swiftest cannon-shot. + +Temporary stars are the rarest and most erratic of astronomical +phenomena. The earliest records relating to them are not very clear, +and we cannot in every instance be certain that it was one of these +appearances that the ignorant and superstitious old chroniclers are +trying to describe. The first temporary star that we are absolutely +sure of appeared in 1572, and is known as ``Tycho's Star,'' because +the celebrated Danish astronomer (whose remains, with his +gold-and-silver artificial nose -- made necessary by a duel -- still +intact, were disinterred and reburied in 1901) was the first to +perceive it in the sky, and the most assiduous and successful in his +studies of it. As the first fully accredited representative of its +class, this new star made its entry upon the scene with becoming +eclat. It is characteristic of these phenomena that they burst into +view with amazing suddenness, and, of course, entirely unexpectedly. +Tycho's star appeared in the constellation Cassiopeia, near a now +well-known and much-watched little star named Kappa, on the evening of +November 11, 1572. The story has often been repeated, but it never +loses interest, how Tycho, going home that evening, saw people in the +street pointing and staring at the sky directly over their heads, and +following the direction of their hands and eyes he was astonished to +see, near the zenith, an unknown star of surpassing brilliance. It +outshone the planet Jupiter, and was therefore far brighter than the +first magnitude. There was not another star in the heavens that could +be compared with it in splendor. Tycho was not in all respects free +from the superstitions of his time -- and who is? -- but he had the +true scientific instinct, and immediately he began to study the +stranger, and to record with the greatest care every change in its +aspect. First he determined as well as he could with the imperfect +instruments of his day, many of which he himself had invented, the +precise location of the phenomena in the sky. Then he followed the +changes that it underwent. At first it brightened until its light +equaled or exceeded that of the planet Venus at her brightest, a +statement which will be appreciated at its full value by anyone who +has ever watched Venus when she plays her dazzling role of ``Evening +Star,'' flaring like an arc light in the sunset sky. It even became so +brilliant as to be visible in full daylight, since, its position being +circumpolar, it never set in the latitude of Northern Europe. Finally +it began to fade, turning red as it did so, and in March, 1574, it +disappeared from Tycho's searching gaze, and has never been seen again +from that day to this. None of the astronomers of the time could make +anything of it. They had not yet as many bases of speculation as we +possess today. + +Tycho's star has achieved a romantic reputation by being fancifully +identified with the ``Star of Bethlehem,'' said to have led the +wondering Magi from their eastern deserts to the cradle-manger of the +Savior in Palestine. Many attempts have been made to connect this +traditional ``star'' with some known phenomenon of the heavens, and +none seems more idle than this. Yet it persistently survives, and no +astronomer is free from eager questions about it addressed by people +whose imagination has been excited by the legend. It is only necessary +to say that the supposition of a connection between the phenomenon of +the Magi and Tycho's star is without any scientific foundation. It was +originally based on an unwarranted assumption that the star of Tycho +was a variable of long period, appearing once every three hundred and +fifteen years, or thereabout. If that were true there would have been +an apparition somewhere near the traditional date of the birth of +Christ, a date which is itself uncertain. But even the data on which +the assumption was based are inconsistent with the theory. Certain +monkish records speak of something wonderful appearing in the sky in +the years 1264 and 945, and these were taken to have been outbursts of +Tycho's star. Investigation shows that the records more probably refer +to comets, but even if the objects seen were temporary stars, their +dates do not suit the hypothesis; from 945 to 1264 there is a gap of +319 years, and from 1264 to 1572 one of only 308 years; moreover 337 +years have now (1909) elapsed since Tycho saw the last glimmer of his +star. Upon a variability so irregular and uncertain as that, even if +we felt sure that it existed, no conclusion could be found concerning +an apparition occurring 2000 years ago. + +In the year 1600 (the year in which Giordano Bruno was burned at the +stake for teaching that there is more than one physical world), a +temporary star of the third magnitude broke out in the constellation +Cygnus, and curiously enough, considering the rarity of such +phenomena, only four years later another surprisingly brilliant one +appeared in the constellation Ophiuchus. This is often called +``Kepler's star,'' because the great German astronomer devoted to it +the same attention that Tycho had given to the earlier phenomenon. It, +too, like Tycho's, was at first the brightest object in the stellar +heavens, although it seems never to have quite equaled its famous +predecessor in splendor. It disappeared after a year, also turning of +a red color as it became more faint. We shall see the significance of +this as we go on. Some of Kepler's contemporaries suggested that the +outburst of this star was due to a meeting of atoms in space, and idea +bearing a striking resemblance to the modern theory of ``astronomical +collisions.'' + +In 1670, 1848, and 1860 temporary stars made their appearance, but +none of them was of great brilliance. In 1866 one of the second +magnitude broke forth in the ``Northern Crown'' and awoke much +interest, because by that time the spectroscope had begun to be +employed in studying the composition of the stars, and Huggins +demonstrated that the new star consisted largely of incandescent +hydrogen. But this star, apparently unlike the others mentioned, was +not absolutely new. Before its outburst it had shown as a star of the +ninth magnitude (entirely invisible, of course, to the naked eye), and +after about six weeks it faded to its original condition in which it +has ever since remained. In 1876 a temporary star appeared in the +constellation Cygnus, and attained at one time the brightness of the +second magnitude. Its spectrum and its behavior resembled those of its +immediate predecessor. In 1885, astronomers were surprised to see a +sixth-magnitude star glimmering in the midst of the hazy cloud of the +great Andromeda Nebula. It soon absolutely disappeared. Its spectrum +was remarkable for being ``continuous,'' like that of the nebula +itself. A continuous spectrum is supposed to represent a body, or a +mass, which is either solid or liquid, or composed of gas under great +pressure. In January, 1892, a new star was suddenly seen in the +constellation Auriga. It never rose much above the fourth magnitude, +but it showed a peculiar spectrum containing both bright and dark +lines of hydrogen. + +But a bewildering surprise was now in store; the world was to behold +at the opening of the twentieth century such a celestial spectacle as +had not been on view since the times of Tycho and Kepler. Before +daylight on the morning of February 22, 1901, the Rev. Doctor +Anderson, of Edinburgh, an amateur astronomer, who had also been the +first to see the new star in Auriga, beheld a strange object in the +constellation Perseus not far from the celebrated variable star Algol. +He recognized its character at once, and immediately telegraphed the +news, which awoke the startled attention of astronomers all over the +world. When first seen the new star was no brighter than Algol (less +than the second magnitude), but within twenty-four hours it was +ablaze, outshining even the brilliant Capella, and far surpassing the +first magnitude. At the spot in the sky where it appeared nothing +whatever was visible on the night before its coming. This is known +with certainty because a photograph had been made of that very region +on February 21, and this photograph showed everything down to the +twelfth magnitude, but not a trace of the stranger which burst into +view between the 21st and the 22nd like the explosion of a rocket. + +Upon one who knew the stars the apparition of this intruder in a +well-known constellation had the effect of a sudden invasion. The new +star was not far west of the zenith in the early evening, and in that +position showed to the best advantage. To see Capella, the hitherto +unchallenged ruler of that quarter of the sky, abased by comparison +with this stranger of alien aspect, for there was always an unfamiliar +look about the ``nova,'' was decidedly disconcerting. It seemed to +portend the beginning of a revolution in the heavens. One could +understand what the effect of such an apparition must have been in the +superstitious times of Tycho. The star of Tycho had burst forth on the +northern border of the Milky Way; this one was on its southern border, +some forty-five degrees farther east. + +Astronomers were well-prepared this time for the scientific study of +the new star, both astronomical photography and spectroscopy having +been perfected, and the results of their investigations were +calculated to increase the wonder with which the phenomenon was +regarded. The star remained at its brightest only a few days; then, +like a veritable conflagration, it began to languish; and, like the +reflection of a dying fire, as it sank it began to glow with the red +color of embers. But its changes were spasmodic; once about every +three days it flared up only to die away again. During these +fluctuations its light varied alternately in the ratio of one to six. +Finally it took a permanent downward course, and after a few months +the naked eye could no longer perceive it; but it remained visible +with telescopes, gradually fading until it had sunk to the ninth +magnitude. Then another astonishing change happened: in August +photographs taken at the Yerkes Observatory and at Heidelberg showed +that the ``nova'' was surrounded by a spiral nebula! The nebula had +not been there before, and no one could doubt that it represented a +phase of the same catastrophe that had produced the outburst of the +new star. At one time the star seemed virtually to have disappeared, +as if all its substance had been expanded into the nebulous cloud, but +always there remained a stellar nucleus about which the misty spiral +spread wider and ever wider, like a wave expanding around a center of +disturbance. The nebula too showed a variability of brightness, and +four condensations which formed in it seemed to have a motion of +revolution about the star. As time went on the nebula continued to +expand at a rate which was computed to be not less than twenty +thousand miles per second! And now the star itself, showing +indications of having turned into a nebula, behaved in a most erratic +manner, giving rise to the suspicion that it was about to burst out +again. But this did not occur, and at length it sunk into a state of +lethargy from which it has to the present time not recovered. But the +nebulous spiral has disappeared, and the entire phenomena as it now +(1909) exists consists of a faint nebulous star of less than the ninth +magnitude. + +The wonderful transformations just described had been forecast in +advance of the discovery of the nebulous spiral encircling the star by +the spectroscopic study of the latter. At first there was no +suggestion of a nebular constitution, but within a month or two +characteristic nebular lines began to appear, and in less than six +months the whole spectrum had been transformed to the nebular type. In +the mean time the shifting of the spectral lines indicated a +complication of rapid motions in several directions simultaneously. +These motions were estimated to amount to from one hundred to five +hundred miles per second. + +The human mind is so constituted that it feels forced to seek an +explanation of so marvelous a phenomenon as this, even in the absence +of the data needed for a sound conclusion. The most natural +hypothesis, perhaps, is that of a collision. Such a catastrophe could +certainly happen. It has been shown, for instance, that in infinity of +time the earth is sure to be hit by a comet; in the same way it may be +asserted that, if no time limit is fixed, the sun is certain to run +against some obstacle in space, either another star, or a dense meteor +swarm, or one of the dark bodies which there is every reason to +believe abound around us. The consequences of such a collision are +easy to foretell, provided that we know the masses and the velocities +of the colliding bodies. In a preceding chapter we have discussed the +motions of the sun and stars, and have seen that they are so swift +that an encounter between any two of them could not but be disastrous. +But this is not all; for as soon as two stars approached within a few +million miles their speed would be enormously increased by their +reciprocal attractions and, if their motion was directed radially with +respect to their centers, they would come together with a crash that +would reduce them both to nebulous clouds. It is true that the chances +of such a ``head-on'' collision are relatively very small; two stars +approaching each other would most probably fall into closed orbits +around their common center of gravity. If there were a collision it +would most likely be a grazing one instead of a direct front-to-front +encounter. But even a close approach, without any actual collision, +would probably prove disastrous, owing to the tidal influence of each +of the bodies on the other. Suns, in consequence of their enormous +masses and dimensions and the peculiarities of their constitution, are +exceedingly dangerous to one another at close quarters. Propinquity +awakes in them a mutually destructive tendency. Consisting of matter +in the gaseous, or perhaps, in some cases, liquid, state, their tidal +pull upon each other if brought close together might burst them +asunder, and the photospheric envelope being destroyed the internal +incandescent mass would gush out, bringing fiery death to any planets +that were revolving near. Without regard to the resulting disturbance +of the earth's orbit, the close approach of a great star to the sun +would be in the highest degree perilous to us. But this is a danger +which may properly be regarded as indefinitely remote, since, at our +present location in space, we are certainly far from every star except +the sun, and we may feel confident that no great invisible body is +near, for if there were one we should be aware of its presence from +the effects of its attraction. As to dark nebulae which may possibly +lie in the track that the solar system is pursuing at the rate of +375,000,000 miles per year, that is another question -- and they, too, +could be dangerous! + +This brings us directly back to ``Nova Persei,'' for among the many +suggestions offered to explain its outburst, as well as those of other +temporary stars, one of the most fruitful is that of a collision +between a star and a vast invisible nebula. Professor Seeliger, of +Munich, first proposed this theory, but it afterward underwent some +modifications from others. Stated in a general form, the idea is that +a huge dark body, perhaps an extinguished sun, encountered in its +progress through space a widespread flock of small meteors forming a +dark nebula. As it plunged into the swarm the friction of the +innumerable collisions with the meteors heated its surface to +incandescence, and being of vast size it then became visible to us as +a new star. Meanwhile the motion of the body through the nebula, and +its rotation upon itself, set up a gyration in the blazing atmosphere +formed around it by the vaporized meteors; and as this atmosphere +spread wider, under the laws of gyratory motion a rotation in the +opposite direction began in the inflamed meteoric cloud outside the +central part of the vortex. Thus the spectral lines were caused to +show motion in opposite directions, a part of the incandescent mass +approaching the earth simultaneously with the retreat of another part. +So the curious spectroscopic observations before mentioned were +explained. This theory might also account for the appearance of the +nebulous spiral first seen some six months after the original +outburst. The sequent changes in the spectrum of the ``nova'' are +accounted for by this theory on the assumption, reasonable enough in +itself, that at first the invading body would be enveloped in a +vaporized atmosphere of relatively slight depth, producing by its +absorption the fine dark lines first observed; but that as time went +on and the incessant collisions continued, the blazing atmosphere +would become very deep and extensive, whereupon the appearance of the +spectral lines would change, and bright lines due to the light of the +incandescent meteors surrounding the nucleus at a great distance would +take the place of the original dark ones. The vortex of meteors once +formed would protect the flying body within from further immediate +collisions, the latter now occurring mainly among the meteors +themselves, and then the central blaze would die down, and the +original splendor of the phenomenon would fade. + +But the theories about Nova Persei have been almost as numerous as the +astronomers who have speculated about it. One of the most startling of +them assumed that the outburst was caused by the running amuck of a +dark star which had encountered another star surrounded with planets, +the renewed outbreaks of light after the principal one had faded being +due to the successive running down of the unfortunate planets! Yet +another hypothesis is based on what we have already said of the tidal +influence that two close approaching suns would have upon each other. +Supposing two such bodies which had become encrusted, but remained +incandescent and fluid within, to approach within almost striking +distance; they would whirl each other about their common center of +gravity, and at the same time their shells would burst under the tidal +strain, and their glowing nuclei being disclosed would produce a great +outburst of light. Applying this theory to a ``nova,'' like that of +1866 in the ``Northern Crown,'' which had been visible as a small star +before the outbreak, and which afterward resumed its former aspect, we +should have to assume that a yet shining sun had been approached by a +dark body whose attraction temporarily burst open its photosphere. It +might be supposed that in this case the dark body was too far advanced +in cooling to suffer the same fate from the tidal pull of its victim. +But a close approach of that kind would be expected to result in the +formation of a binary system, with orbits of great eccentricity, +perhaps, and after the lapse of a certain time the outburst should be +renewed by another approximation of the two bodies. A temporary star +of that kind would rather be ranked as a variable. + +The celebrated French astronomer, Janssen, had a different theory of +Nova Persei, and of temporary stars in general. According to his idea, +such phenomena might be the result of chemical changes taking place in +a sun without interference by, or collision with, another body. +Janssen was engaged for many years in trying to discover evidence of +the existence of oxygen in the sun, and he constructed his observatory +on the summit of Mount Blanc specially to pursue that research. He +believed that oxygen must surely exist in the sun since we find so +many other familiar elements included in the constitution of the solar +globe, and as he was unable to discover satisfactory evidence of its +presence he assumed that it existed in a form unknown on the earth. If +it were normally in the sun's chromosphere, or coronal atmosphere, he +said, it would combine with the hydrogen which we know is there and +form an obscuring envelope of water vapor. It exists, then, in a +special state, uncombined with hydrogen; but let the temperature of +the sun sink to a critical point and the oxygen will assume its normal +properties and combine with the hydrogen, producing a mighty outburst +of light and heat. This, Janssen thought, might explain the phenomena +of the temporary stars. It would also, he suggested, account for their +brief career, because the combination of the elements would be quickly +accomplished, and then the resulting water vapor would form an +atmosphere cutting off the radiation from the star within. + +This theory may be said to have a livelier human interest than some of +the others, since, according to it, the sun may carry in its very +constitution a menace to mankind; one does not like to think of it +being suddenly transformed into a gigantic laboratory for the +explosive combination of oxygen and hydrogen! But while Janssen's +theory might do for some temporary stars, it is inadequate to explain +all the phenomena of Nova Persei, and particularly the appearance of +the great spiral nebula that seemed to exhale from the heart of the +star. Upon the whole, the theory of an encounter between a star and a +dark nebula seems best to fit the observations. By that hypothesis the +expanding billow of light surrounding the core of the conflagration is +very well accounted for, and the spectroscopic peculiarities are also +explained. + +Dr Gustov Le Bon offers a yet more alarming theory, suggesting that +temporary stars are the result of atomic explosion; but we shall touch +upon this more fully in Chapter 14. + +Twice in the course of this discussion we have called attention to the +change of color invariably undergone by temporary stars in the later +stages of their career. This was conspicuous with Nova Persei which +glowed more and more redly as it faded, until the nebulous light began +to overpower that of the stellar nucleus. Nothing could be more +suggestive of the dying out of a great fire. Moreover, change of color +from white to red is characteristic of all variable stars of long +period, such as ``Mira'' in Cetus. It is also characteristic of stars +believed to be in the later stages of evolution, and consequently +approaching extinction, like Antares and Betelgeuse, and still more +notably certain small stars which ``gleam like rubies in the field of +the telescope.'' These last appear to be suns in the closing period of +existence as self-luminous bodies. Between the white stars, such as +Sirius and Rigel, and the red stars, such as Aldebaran and Alpha +Herculis, there is a progressive series of colors from golden yellow +through orange to deep red. The change is believed to be due to the +increase of absorbing vapors in the stellar atmosphere as the body +cools down. In the case of ordinary stars these changes no doubt +occupy many millions of years, which represent the average duration of +solar life; but the temporary stars run through similar changes in a +few months: they resemble ephemeral insects -- born in the morning and +doomed to perish with the going down of the sun. + +Explosive and Whirling Nebulae + +One of the most surprising triumphs of celestial photography was +Professor Keeler's discovery, in 1899, that the great majority of the +nebulae have a distinctly spiral form. This form, previously known in +Lord Rosse's great ``Whirlpool Nebula,'' had been supposed to be +exceptional; now the photographs, far excelling telescopic views in +the revelation of nebular forms, showed the spiral to be the typical +shape. Indeed, it is a question whether all nebulae are not to some +extent spiral. The extreme importance of this discovery is shown in +the effect that it has had upon hitherto prevailing views of solar and +planetary evolution. For more than three-quarters of a century +Laplace's celebrated hypothesis of the manner of origin of the solar +system from a rotating and contracting nebula surrounding the sun had +guided speculation on that subject, and had been tentatively extended +to cover the evolution of systems in general. The apparent forms of +some of the nebulae which the telescope had revealed were regarded, and +by some are still regarded, as giving visual evidence in favor of this +theory. There is a ``ring nebula'' in Lyra with a central star, and a +``planetary nebula'' in Gemini bearing no little resemblance to the +planet Saturn with its rings, both of which appear to be practical +realizations of Laplace's idea, and the elliptical rings surrounding +the central condensation of the Andromeda Nebula may be cited for the +same kind of proof. + +But since Keeler's discovery there has been a decided turning away of +speculation another way. The form of the spiral nebulae seems to be +entirely inconsistent with the theory of an originally globular or +disk-shaped nebula condensing around a sun and throwing or leaving off +rings, to be subsequently shaped into planets. Some astronomers, +indeed, now reject Laplace's hypothesis in toto, preferring to think +that even our solar system originated from a spiral nebula. Since the +spiral type prevails among the existing nebulae, we must make any +mechanical theory of the development of stars and planetary systems +from them accord with the requirements which that form imposes. A +glance at the extraordinary variations upon the spiral which Professor +Keeler's photographs reveal is sufficient to convince one of the +difficulty of the task of basing a general theory upon them. In truth, +it is much easier to criticize Laplace's hypothesis than to invent a +satisfactory substitute for it. If the spiral nebulae seem to oppose it +there are other nebulae which appear to support it, and it may be that +no one fixed theory can account for all the forms of stellar evolution +in the universe. Our particular planetary system may have originated +very much as the great French mathematician supposed, while others +have undergone, or are now undergoing, a different process of +development. There is always a too strong tendency to regard an +important new discovery and the theories and speculations based upon +it as revolutionizing knowledge, and displacing or overthrowing +everything that went before. Upon the plea that ``Laplace only made a +guess'' more recent guesses have been driven to extremes and treated +by injudicious exponents as ``the solid facts at last.'' + +Before considering more recent theories than Laplace's, let us see +what the nature of the photographic revelations is. The vast celestial +maelstrom discovered by Lord Rosse in the ``Hunting Dogs'' may be +taken as the leading type of the spiral nebulae, although there are +less conspicuous objects of the kind which, perhaps, better illustrate +some of their peculiarities. Lord Rosse's nebula appears far more +wonderful in the photographs than in his drawings made with the aid of +his giant reflecting telescope at Parsonstown, for the photographic +plate records details that no telescope is capable of showing. Suppose +we look at the photograph of this object as any person of common sense +would look at any great and strange natural phenomenon. What is the +first thing that strikes the mind? It is certainly the appearance of +violent whirling motion. One would say that the whole glowing mass had +been spun about with tremendous velocity, or that it had been set +rotating so rapidly that it had become the victim of ``centrifugal +force,'' one huge fragment having broken loose and started to gyrate +off into space. Closer inspection shows that in addition to the +principal focus there are various smaller condensations scattered +through the mass. These are conspicuous in the spirals. Some of them +are stellar points, and but for the significance of their location we +might suppose them to be stars which happen to lie in a line between +us and the nebula. But when we observe how many of them follow most +faithfully the curves of the spirals we cannot but conclude that they +form an essential part of the phenomenon; it is not possible to +believe that their presence in such situations is merely fortuitous. +One of the outer spirals has at least a dozen of these star-like +points strung upon it; some of them sharp, small, and distinct, others +more blurred and nebulous, suggesting different stages of +condensation. Even the part which seems to have been flung loose from +the main mass has, in addition to its central condensation, at least +one stellar point gleaming in the half-vanished spire attached to it. +Some of the more distant stars scattered around the ``whirlpool'' look +as if they too had been shot out of the mighty vortex, afterward +condensing into unmistakable solar bodies. There are at least two +curved rows of minute stars a little beyond the periphery of the +luminous whirl which clearly follow lines concentric with those of the +nebulous spirals. Such facts are simply dumbfounding for anyone who +will bestow sufficient thought upon them, for these are suns, though +they may be small ones; and what a birth is that for a sun! + +Look now again at the glowing spirals. We observe that hardly have +they left the central mass before they begin to coagulate. In some +places they have a ``ropy'' aspect; or they are like peascods filled +with growing seeds, which eventually will become stars. The great +focus itself shows a similar tendency, especially around its +circumference. The sense that it imparts of a tremendous shattering +force at work is overwhelming. There is probably more matter in that +whirling and bursting nebula than would suffice to make a hundred +solar systems! It must be confessed at once that there is no +confirmation of the Laplacean hypothesis here; but what hypothesis +will fit the facts? There is one which it has been claimed does so, +but we shall come to that later. In the meanwhile, as a preparation, +fix in the memory the appearance of that second spiral mass spinning +beside its master which seems to have spurned it away. + +For a second example of the spiral nebulae look at the one in the +constellation Triangulum. God, how hath the imagination of puny man +failed to comprehend Thee! Here is creation through destruction with a +vengeance! The spiral form of the nebula is unmistakable, but it is +half obliterated amid the turmoil of flying masses hurled away on all +sides with tornadic fury. The focus itself is splitting asunder under +the intolerable strain, and in a little while, as time is reckoned in +the Cosmos, it will be gyrating into stars. And then look at the +cyclonic rain of already finished stars whirling round the outskirts +of the storm. Observe how scores of them are yet involved in the +fading streams of the nebulous spirals; see how they have been thrown +into vast loops and curves, of a beauty that half redeems the terror +of the spectacle enclosed within their lines -- like iridescent cirri +hovering about the edges of a hurricane. And so again are suns born! + +Let us turn to the exquisite spiral in Ursa Major; how different its +aspect from that of the other! One would say that if the terrific coil +in Triangulum has all but destroyed itself in its fury, this one on +the contrary has just begun its self-demolition. As one gazes one +seems to see in it the smooth, swift, accelerating motion that +precedes catastrophe. The central part is still intact, dense, and +uniform in texture. How graceful are the spirals that smoothly rise +from its oval rim and, gemmed with little stars, wind off into the +darkness until they have become as delicate as threads of gossamer! +But at bottom the story told here is the same -- creation by gyration! + +Compare with the above the curious mass in Cetus. Here the plane of +the whirling nebula nearly coincides with our line of sight and we see +the object at a low angle. It is far advanced and torn to shreds, and +if we could look at it perpendicularly to its plane it is evident that +it would closely resemble the spectacle in Triangulum. + +Then take the famous Andromeda Nebula (see Frontispiece), which is so +vast that notwithstanding its immense distance even the naked eye +perceives it as an enigmatical wisp in the sky. Its image on the +sensitive plate is the masterpiece of astronomical photography; for +wild, incomprehensible beauty there is nothing that can be compared +with it. Here, if anywhere, we look upon the spectacle of creation in +one of its earliest stages. The Andromeda Nebula is apparently less +advanced toward transformation into stellar bodies than is that in +Triangulum. The immense crowd of stars sprinkled over it and its +neighborhood seem in the main to lie this side of the nebula, and +consequently to have no connection with it. But incipient stars (in +some places clusters of them) are seen in the nebulous rings, while +one or two huge masses seem to give promise of transformation into +stellar bodies of unusual magnitude. I say ``rings'' because although +the loops encompassing the Andromeda Nebula have been called spirals +by those who wish utterly to demolish Laplace's hypothesis, yet they +are not manifestly such, as can be seen on comparing them with the +undoubted spirals of the Lord Rosse Nebula. They look quite as much +like circles or ellipses seen at an angle of, say, fifteen or twenty +degrees to their plane. If they are truly elliptical they accord +fairly well with Laplace's idea, except that the scale of magnitude is +stupendous, and if the Andromeda Nebula is to become a solar system it +will surpass ours in grandeur beyond all possibility of comparison. + +There is one circumstance connected with the spiral nebulae, and +conspicuous in the Andromeda Nebula on account of its brightness, +which makes the question of their origin still more puzzling; they all +show continuous spectra, which, as we have before remarked, indicate +that the mass from which the light comes is either solid or liquid, or +a gas under heavy pressure. Thus nebulae fall into two classes: the +``white'' nebulae, giving a continuous spectrum; and the ``green'' +nebulae whose spectra are distinctly gaseous. The Andromeda Nebula is +the great representative of the former class and the Orion Nebula of +the latter. The spectrum of the Andromeda Nebula has been interpreted +to mean that it consists not of luminous gas, but of a flock of stars +so distant that they are separately indistinguishable even with +powerful telescopes, just as the component stars of the Milky Way are +indistinguishable with the naked eye; and upon this has been based the +suggestion that what we see in Andromeda is an outer universe whose +stars form a series of elliptical garlands surrounding a central mass +of amazing richness. But this idea is unacceptable if for no other +reason than that, as just said, all the spiral nebulae possess the same +kind of spectrum, and probably no one would be disposed to regard them +all as outer universes. As we shall see later, the peculiarity of the +spectra of the spiral nebulae is appealed to in support of a modern +substitute for Laplace's hypothesis. + +Finally, without having by any means exhausted the variety exhibited +by the spiral nebulae, let us turn to the great representative of the +other species, the Orion Nebula. In some ways this is even more +marvelous than the others. The early drawings with the telescope +failed to convey an adequate conception either of its sublimity or of +its complication of structure. It exists in a nebulous region of +space, since photographs show that nearly the whole constellation is +interwoven with faintly luminous coils. To behold the entry of the +great nebula into the field even of a small telescope is a startling +experience which never loses its novelty. As shown by the photographs, +it is an inscrutable chaos of perfectly amazing extent, where spiral +bands, radiating streaks, dense masses, and dark yawning gaps are +strangely intermingled without apparent order. In one place four +conspicuous little stars, better seen in a telescope than in the +photograph on account of the blurring produced by over-exposure, are +suggestively situated in the midst of a dark opening, and no observer +has ever felt any doubt that these stars have been formed from the +substance of the surrounding nebula. There are many other stars +scattered over its expanse which manifestly owe their origin to the +same source. But compare the general appearance of this nebula with +the others that we have studied, and remark the difference. If the +unmistakably spiral nebulae resemble bursting fly-wheels or grindstones +from whose perimeters torrents of sparks are flying, the Orion Nebula +rather recalls the aspect of a cloud of smoke and fragments produced +by the explosion of a shell. This idea is enforced by the look of the +outer portion farthest from the bright half of the nebula, where +sharply edged clouds with dark spaces behind seem to be billowing away +as if driven by a wind blowing from the center. + +Next let us consider what scientific speculation has done in the +effort to explain these mysteries. Laplace's hypothesis can certainly +find no standing ground either in the Orion Nebula or in those of a +spiral configuration, whatever may be its situation with respect to +the grand Nebula of Andromeda, or the ``ring'' and ``planetary'' +nebulae. Some other hypothesis more consonant with the appearances must +be found. Among the many that have been proposed the most elaborate is +the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton. +It is to be remarked that it applies to the spiral nebulae +distinctively, and not to an apparently chaotic mass of gas like the +vast luminous cloud in Orion. The gist of the theory is that these +curious objects are probably the result of close approaches to each +other of two independent suns, reminding us of what was said on this +subject when we were dealing with temporary stars. Of the previous +history of these appulsing suns the theory gives us no account; they +are simply supposed to arrive within what may be called an effective +tide-producing distance, and then the drama begins. Some of the +probable consequences of such an approach have been noticed in Chapter +5; let us now consider them a little more in detail. + +Tides always go in couples; if there is a tide on one side of a globe +there will be a corresponding tide on the other side. The cause is to +be found in the law that the force of gravitation varies inversely as +the square of the distance; the attraction on the nearest surface of +the body exercised by another body is greater than on its center, and +greater yet than on its opposite surface. If two great globes attract +each other, each tends to draw the other out into an ellipsoidal +figure; they must be more rigid than steel to resist this -- and even +then they cannot altogether resist. If they are liquid or gaseous they +will yield readily to the force of distortion, the amount of which +will depend upon their distance apart, for the nearer they are the +greater becomes the tidal strain. If they are encrusted without and +liquid or gaseous in the interior, the internal mass will strive to +assume the figure demanded by the tidal force, and will, if it can, +burst the restraining envelope. Now this is virtually the predicament +of the body we call a sun when in the immediate presence of another +body of similarly great mass. Such a body is presumably gaseous +throughout, the component gases being held in a state of rigidity by +the compression produced by the tremendous gravitational force of +their own aggregate mass. At the surface such a body is enveloped in a +shell of relatively cool matter. Now suppose a great attracting body, +such as another sun, to approach near enough for the difference in its +attraction on the two opposite sides of the body and on its center to +become very great; the consequence will be a tidal deformation of the +whole body, and it will lengthen out along the line of the +gravitational pull and draw in at the sides, and if its shell offers +considerable resistance, but not enough to exercise a complete +restraint, it will be violently burst apart, or blown to atoms, and +the internal mass will leap out on the two opposite sides in great +fiery spouts. In the case of a sun further advanced in cooling than +ours the interior might be composed of molten matter while the +exterior crust had become rigid like the shell of an egg; then the +force of the ``tidal explosion'' produced by the appulse of another +sun would be more violent in consequence of the greater resistance +overcome. Such, then, is the mechanism of the first phase in the +history of a spiral nebula according to the Planetesimal Hypothesis. +Two suns, perhaps extinguished ones, have drawn near together, and an +explosive outburst has occured in one or both. The second phase calls +for a more agile exercise of the imagination. + +To simplify the case, let us suppose that only one of the tugging suns +is seriously affected by the strain. Its vast wings produced by the +outburst are twisted into spirals by their rotation and the contending +attractions exercised upon them, as the two suns, like battleships in +desperate conflict, curve round each other, concentrating their +destructive energies. Then immense quantities of debris are scattered +about in which eddies are created, and finally, as the sun that caused +the damage goes on its way, leaving its victim to repair its injuries +as it may, the dispersed matter cools, condenses, and turns into +streams of solid particles circling in elliptical paths about their +parent sun. These particles, or fragments, are the ``planetesimals'' +of the theory. In consequence of the inevitable intersection of the +orbits of the planetesimals, nodes are formed where the flying +particles meet, and at these nodes large masses are gradually +accumulated. The larger the mass the greater its attraction, and at +last the nodal points become the nuclei of great aggregations from +which planets are shaped. + +This, in very brief form, is the Planetesimal Hypothesis which we are +asked to substitute for that based on Laplace's suggestion as an +explanation of the mode of origin of the solar system; and the +phenomena of the spiral nebulae are appealed to as offering evident +support to the new hypothesis. We are reminded that they are +elliptical in outline, which accords with the hypothesis; that their +spectra are not gaseous, which shows that they may be composed of +solid particles like the planetesimals; and that their central masses +present an oval form, which is what would result from the tidal +effects, as just described. We also remember that some of them, like +the Lord Rosse and the Andromeda nebulae, are visually double, and in +these cases we might suppose that the two masses represent the +tide-burst suns that ventured into too close proximity. It may be +added that the authors of the theory do not insist upon the appulse of +two suns as the only way in which the planetesimals may have +originated, but it is the only supposition that has been worked out. + +But serious questions remain. It needs, for instance, but a glance at +the Triangulum monster to convince the observer that it cannot be a +solar system which is being evolved there, but rather a swarm of +stars. Many of the detached masses are too vast to admit of the +supposition that they are to be transformed into planets, in our sense +of planets, and the distances of the stars which appear to have been +originally ejected from the focal masses are too great to allow us to +liken the assemblage that they form to a solar system. Then, too, no +nodes such as the hypothesis calls for are visible. Moreover, in most +of the spiral nebulae the appearances favor the view that the +supposititious encountering suns have not separated and gone each +rejoicing on its way, after having inflicted the maximum possible +damage on its opponent, but that, on the contrary, they remain in +close association like two wrestlers who cannot escape from each +other's grasp. And this is exactly what the law of gravitation +demands; stars cannot approach one another with impunity, with regard +either to their physical make-up or their future independence of +movement. The theory undertakes to avoid this difficulty by assuming +that in the case of our system the approach of the foreign body to the +sun was not a close one -- just close enough to produce the tidal +extrusion of the relatively insignificant quantity of matter needed to +form the planets. But even then the effect of the appulse would be to +change the direction of flight, both of the sun and of its visitor, +and there is no known star in the sky which can be selected as the +sun's probable partner in their ancient pas deux. That there are +unconquered difficulties in Laplace's hypothesis no one would deny, +but in simplicity of conception it is incomparably more satisfactory, +and with proper modifications could probably be made more consonant +with existing facts in our solar system than that which is offered to +replace it. Even as an explanation of the spiral nebulae, not as solar +systems in process of formation, but as the birthplaces of stellar +clusters, the Planetesimal Hypothesis would be open to many +objections. Granting its assumptions, it has undoubtedly a strong +mathematical framework, but the trouble is not with the mathematics +but with the assumptions. Laplace was one of the ablest mathematicians +that ever lived, but he had never seen a spiral nebula; if he had, he +might have invented a hypothesis to suit its phenomena. His actual +hypothesis was intended only for our solar system, and he left it in +the form of a ``note'' for the consideration of his successors, with +the hope that they might be able to discover the full truth, which he +confessed was hidden from him. It cannot be said that that truth has +yet been found, and when it is found the chances are that intuition +and not logic will have led to it. + +The spiral nebulae, then, remain among the greatest riddles of the +universe, while the gaseous nebulae, like that of Orion, are no less +mysterious, although it seems impossible to doubt that both forms give +birth to stars. It is but natural to look to them for light on the +question of the origin of our planetary system; but we should not +forget that the scale of the phenomena in the two cases is vastly +different, and the forces in operation may be equally different. A +hill may have been built up by a glacier, while a mountain may be the +product of volcanic forces or of the upheaval of the strata of the +planet. + +The Banners of the Sun + +As all the world knows, the sun, a blinding globe pouring forth an +inconceivable quantity of light and heat, whose daily passage through +the sky is caused by the earth's rotation on its axis, constitutes the +most important phenomenon of terrestial existence. Viewed with a dark +glass to take off the glare, or with a telescope, its rim is seen to +be a sharp and smooth circle, and nothing but dark sky is visible +around it. Except for the interference of the moon, we should probably +never have known that there is any more of the sun than our eyes +ordinarily see. + +But when an eclipse of the sun occurs, caused by the interposition of +the opaque globe of the moon, we see its immediate surroundings, which +in some respects are more wonderful than the glowing central orb. +These surroundings, although not in the sense in which we apply the +term to the gaseous envelope of the earth, may be called the sun's +atmosphere. They consist of two very different parts -- first, the red +``prominences,'' which resemble tongues of flame ascending thousands +of miles above the sun's surface; and, second, the ``corona,'' which +extends to distances of millions of miles from the sun, and shines +with a soft, glowing light. The two combined, when well seen, make a +spectacle without parallel among the marvels of the sky. Although many +attempts have been made to render the corona visible when there is no +eclipse, all have failed, and it is to the moon alone that we owe its +revelation. To cover the sun's disk with a circular screen will not +answer the purpose because of the illumination of the air all about +the observer. When the moon hides the sun, on the other hand, the +sunlight is withdrawn from a great cylinder of air extending to the +top of the atmosphere and spreading many miles around the observer. +There is then no glare to interfere with the spectacle, and the corona +appears in all its surprising beauty. The prominences, however, +although they were discovered during an eclipse, can now, with the aid +of the spectroscope, be seen at any time. But the prominences are +rarely large enough to be noticed by the naked eye, while the +streamers of the corona, stretching far away in space, like ghostly +banners blown out from the black circle of the obscuring moon, attract +every eye, and to this weird apparition much of the fear inspired by +eclipses has been due. But if the corona has been a cause of terror in +the past it has become a source of growing knowledge in our time. + +The story of the first scientific observation of the corona and the +prominences is thrillingly interesting, and in fact dramatic. The +observation was made during the eclipse of 1842, which fortunately was +visible all over Central and Southern Europe so that scores of +astronomers saw it. The interest centers in what happened at Pavia in +Northern Italy, where the English astronomer Francis Baily had set up +his telescope. The eclipse had begun and Bailey was busy at his +telescope when, to quote his own words in the account which he wrote +for the Memoirs of the Royal Astronomical Society: + + I was astounded by a tremendous burst of applause from the streets + below, and at the same moment was electrified by the sight of one + of the most brilliant and splendid phenomena that can well be + imagined; for at that instant the dark body of the moon was + suddenly surrounded with a corona, or kind of bright glory, similar + in shape and magnitude to that which painters draw round the heads + of saints... + + Pavia contains many thousand inhabitants, the major part of whom + were at this early hour walking about the streets and squares or + looking out of windows in order to witness this long-talked-of + phenomenon; and when the total obscuration took place, which was + instantaneous, there was a universal shout from every observer + which ``made the welkin ring,'' and for the moment withdrew my + attention from the object with which I was immediately occupied. I + had, indeed, expected the appearance of a luminous circle round the + moon during the time of total obscurity; but I did not expect, from + any of the accounts of preceding eclipses that I had read, to + witness so magnificent an exhibition as that which took place... + + Splendid and astonishing, however, as this remarkable phenomenon + really was, and although it could not fail to call forth the + admiration and applause of every beholder, yet I must confess that + there was at the same time something in its singular and wonderful + appearance that was appalling... + + But the most remarkable circumstance attending the phenomenon was + the appearance of three large protuberances apparently emanating + from the circumference of the moon, but evidently forming a portion + of the corona. They had the appearance of mountains of a prodigious + elevation; their color was red tinged with lilac or purple; perhaps + the color of the peach-blossom would more nearly represent it. They + somewhat resembled the tops of the snowy Alpine mountains when + colored by the rising or the setting sun. They resembled the Alpine + mountains in another respect, inasmuch as their light was perfectly + steady, and had none of that flickering or sparkling motion so + visible in other parts of the corona... + + The whole of these protuberances were visible even to the last + moment of total obscuration, and when the first ray of light was + admitted from the sun they vanished, with the corona, altogether, + and daylight was instantly restored. + +I have quoted nearly all of this remarkable description not alone for +its intrinsic interest, but because it is the best depiction that can +be found of the general phenomena of a total solar eclipse. Still, not +every such eclipse offers an equally magnificent spectacle. The +eclipses of 1900 and 1905, for instance, which were seen by the +writer, the first in South Carolina and the second in Spain, fell far +short of that described by Bailey in splendor and impressiveness. Of +course, something must be allowed for the effect of surprise; Bailey +had not expected to see what was so suddenly disclosed to him. But +both in 1900 and 1905 the amount of scattered light in the sky was +sufficient in itself to make the corona appear faint, and there were +no very conspicuous prominences visible. Yet on both occasions there +was manifest among the spectators that mingling of admiration and awe +of which Bailey speaks. The South Carolinians gave a cheer and the +ladies waved their handkerchiefs when the corona, ineffably delicate +of form and texture, melted into sight and then in two minutes melted +away again. The Spaniards, crowded on the citadel hill of Burgos, with +their king and his royal retinue in their midst, broke out with a +great clapping of hands as the awaited spectacle unfolded itself in +the sky; and on both occasions, before the applause began, after an +awed silence a low murmur ran through the crowds. At Burgos it is said +many made the sign of the cross. + +It was not long before Bailey's idea that the prominences were a part +of the corona was abandoned, and it was perceived that the two +phenomena were to a great extent independent. At the eclipse of 1868, +which the astronomers, aroused by the wonderful scene of 1842, and +eager to test the powers of the newly invented spectroscope, flocked +to India to witness, Janssen conceived the idea of employing the +spectroscope to render the prominences visible when there was no +eclipse. He succeeded the very next day, and these phenomena have been +studied in that way ever since. + +There are recognized two kinds of prominences -- the ``erruptive'' and +the ``quiescent.'' The latter, which are cloud-like in form, may be +seen almost anywhere along the edge of the sun; but the former, which +often shoot up as if hurled from mighty volcanoes, appear to be +associated with sun-spots, and appear only above the zones where spots +abound. Either of them, when seen in projection against the brilliant +solar disk, appears white, not red, as against a background of sky. +The quiescent prominences, whose elevation is often from forty +thousand to sixty thousand miles, consist, as the spectroscope shows, +mainly of hydrogen and helium. The latter, it will be remembered, is +an element which was known to be in the sun many years before the +discovery that it also exists in small quantities on the earth. A fact +which may have a significance which we cannot at present see is that +the emanation from radium gradually and spontaneously changes into +helium, an alchemistical feat of nature that has opened many curious +vistas to speculative thinkers. The eruptive prominences, which do not +spread horizontally like the others, but ascend with marvelous +velocity to elevations of half a million miles or more, are apparently +composed largely of metallic vapors -- i.e. metals which are usually +solid on the earth, but which at solar temperatures are kept in a +volatilized state. The velocity of their ascent occasionally amounts +to three hundred or four hundred miles per second. It is known from +mathematical considerations that the gravitation of the sun would not +be able to bring back any body that started from its surface with a +velocity exceeding three hundred and eighty-three miles per second; so +it is evident that some of the matter hurled forth in eruptive +prominences may escape from solar control and go speeding out into +space, cooling and condensing into solid masses. There seems to be no +reason why some of the projectiles from the sun might not reach the +planets. Here, then, we have on a relatively small scale, explosions +recalling those which it has been imagined may be the originating +cause of some of the sudden phenomena of the stellar heavens. + +Of the sun-spots it is not our intention here specifically to speak, +but they evidently have an intimate connection with eruptive +prominences, as well as some relation, not yet fully understood, with +the corona. Of the real cause of sun-spots we know virtually nothing, +but recent studies by Professor Hale and others have revealed a +strange state of things in the clouds of metallic vapors floating +above them and their surroundings. Evidences of a cyclonic tendency +have been found, and Professor Hale has proved that sun-spots are +strong magnetic fields, and consist of columns of ionized vapors +rotating in opposite directions in the two hemispheres. A fact which +may have the greatest significance is that titanium and vanadium have +been found both in sun-spots and in the remarkable variable Mira Ceti, +a star which every eleven months, or thereabout, flames up with great +brilliancy and then sinks back to invisibility with the naked eye. It +has been suggested that sun-spots are indications of the beginning of +a process in the sun which will be intensified until it falls into the +state of such a star as Mira. Stars very far advanced in evolution, +without showing variability, also exhibit similar spectra; so that +there is much reason for regarding sunspots as emblems of advancing +age. + +The association of the corona with sun-spots is less evident than that +of the eruptive prominences; still such an association exists, for the +form and extent of the corona vary with the sun-spot period of which +we shall presently speak. The constitution of the corona remains to be +discovered. It is evidently in part gaseous, but it also probably +contains matter in the form of dust and small meteors. It includes one +substance altogether mysterious -- ``coronium.'' There are reasons for +thinking that this may be the lightest of all the elements, and +Professor Young, its discoverer, said that it was ``absolutely unique +in nature; utterly distinct from any other known form of matter, +terrestial, solar, or cosmical.'' The enormous extent of the corona is +one of its riddles. Since the development of the curious subject of +the ``pressure of light'' it has been proposed to account for the +sustentation of the corona by supposing that it is borne upon the +billows of light continually poured out from the sun. Experiment has +proved, what mathematical considerations had previously pointed out as +probable, that the waves of light exert a pressure or driving force, +which becomes evident in its effects if the body acted upon is +sufficiently small. In that case the light pressure will prevail over +the attraction of gravitation, and propel the attenuated matter away +from the sun in the teeth of its attraction. The earth itself would be +driven away if, instead of consisting of a solid globe of immense +aggregate mass, it were a cloud of microscopic particles. The reason +is that the pressure varies in proportion to the surface of the body +acted upon, while the gravitational attraction is proportional to the +volume, or the total amount of matter in the body. But the surface of +any body depends upon the square of its diameter, while the volume +depends upon the cube of the diameter. If, for instance, the diameter +is represented by 4, the surface will be proportional to 4 x 4, or 16, +and the volume to 4 x 4 x 4, or 64; but if the diameter is taken as 2, +the surface will be 2 x 2, or 4, and the volume 2 x 2 x 2, or 8. Now, +the ratio of 4 to 8 is twice as great as that of 16 to 64. If the +diameter is still further decreased, the ratio of the surface to the +volume will proportionally grow larger; in other words, the pressure +will gain upon the attraction, and whatever their original ratio may +have been, a time will come, if the diminution of size continues, when +the pressure will become more effective than the attraction, and the +body will be driven away. Supposing the particles of the corona to be +below the critical size for the attraction of a mass like that of the +sun to control them, they would be driven off into the surrounding +space and appear around the sun like the clouds of dust around a mill. +We shall return to this subject in connection with the Zodiacal Light, +the Aurora, and Comets. + +On the other hand, there are parts of the corona which suggest by +their forms the play of electric or magnetic forces. This is +beautifully shown in some of the photographs that have been made of +the corona during recent eclipses. Take, for instance, that of the +eclipse of 1900. The sheaves of light emanating from the poles look +precisely like the ``lines of force'' surrounding the poles of a +magnet. It will be noticed in this photograph that the corona appears +to consist of two portions: one comprising the polar rays just spoken +of, and the other consisting of the broader, longer, and less-defined +masses of light extending out from the equatorial and middle-latitude +zones. Yet even in this more diffuse part of the phenomenon one can +detect the presence of submerged curves bearing more or less +resemblance to those about the poles. Just what part electricity or +electro-magnetism plays in the mechanism of the solar radiation it is +impossible to say, but on the assumption that it is a very important +part is based the hypothesis that there exists a direct solar +influence not only upon the magnetism, but upon the weather of the +earth. This hypothesis has been under discussion for half a century, +and still we do not know just how much truth it represents. It is +certain that the outbreak of great disturbances on the sun, +accompanied by the formation of sun-spots and the upshooting of +eruptive prominences (phenomena which we should naturally expect to be +attended by action), have been instantly followed by corresponding +``magnetic storms'' on the earth and brilliant displays of the auroral +lights. There have been occasions when the influence has manifested +itself in the most startling ways, a great solar outburst being +followed by a mysterious gripping of the cable and telegraph systems +of the world, as if an invisible and irresistible hand had seized +them. Messages are abruptly cut off, sparks leap from the telegraph +instruments, and the entire earth seems to have been thrown into a +magnetic flurry. These occurrences affect the mind with a deep +impression of the dependence of our planet on the sun, such as we do +not derive from the more familiar action of the sunlight on the growth +of plants and other phenomena of life depending on solar influences. + +Perhaps the theory of solar magnetic influence upon the weather is +best known in connection with the ``sun-spot cycle.'' This, at any +rate, is, as already remarked, closely associated with the corona. Its +existence was discovered in 1843 by the German astronomer Schwabe. It +is a period of variable length, averaging about eleven years, during +which the number of spots visible on the sun first increases to a +maximum, then diminishes to a minimum, and finally increases again to +a maximum. For unknown reasons the period is sometimes two or three +years longer than the average and sometimes as much shorter. +Nevertheless, the phenomena always recur in the same order. Starting, +for instance, with a time when the observer can find few or no spots, +they gradually increase in number and size until a maximum, in both +senses, is reached, during which the spots are often of enormous size +and exceedingly active. After two or three years they begin to +diminish in number, magnitude, and activity until they almost or quite +disappear. A strange fact is that when a new period opens, the spots +appear first in high northern and southern latitudes, far from the +solar equator, and as the period advances they not only increase in +number and size, but break out nearer and nearer to the equator, the +last spots of a vanishing period sometimes lingering in the equatorial +region after the advance-guard of its successor has made its +appearance in the high latitudes. Spots are never seen on the equator +nor near the poles. It was not very long after the discovery of the +sun-spot cycle that the curious observation was made that a striking +coincidence existed between the period of the sun-spots and another +period affecting the general magnetic condition of the earth. When a +curved line representing the varying number of sun-spots was compared +with another curve showing the variations in the magnetic state of the +earth the two were seen to be in almost exact accord, a rise in one +curve corresponding to a rise in the other, and a fall to a fall. +Continued observation has proved that this is a real coincidence and +not an accidental one, so that the connection, although as yet +unexplained, is accepted as established. But does the influence extend +further, and directly affect the weather and the seasons as well as +the magnetic elements of the earth? A final answer to this question +cannot yet be given, for the evidence is contradictory, and the +interpretations put upon it depend largely on the predilections of the +judges. + +But, in a broad sense, the sun-spots and the phenomena connected with +them must have a relation to terrestial meteorology, for they prove +the sun to be a variable star. Reference was made, a few lines above, +to the resemblance of the spectra of sun-spots to those of certain +stars which seem to be failing through age. This in itself is +extremely suggestive; but if this resemblance had never been +discovered, we should have been justified in regarding the sun as +variable in its output of energy; and not only variable, but probably +increasingly so. The very inequalities in the sun-spot cycle are +suspicious. When the sun is most spotted its total light may be +reduced by one-thousandth part, although it is by no means certain +that its outgiving of thermal radiations is then reduced. A loss of +one-thousandth of its luminosity would correspond to a decrease of +.0025 of a stellar magnitude, considering the sun as a star viewed +from distant space. So slight a change would not be perceptible; but +it is not alone sun-spots which obscure the solar surface, its entire +globe is enveloped with an obscuring veil. When studied with a +powerful telescope the sun's surface is seen to be thickly mottled +with relatively obscure specks, so numerous that it has been estimated +that they cut off from one-tenth to one-twentieth of the light that we +should receive from it if the whole surface were as brilliant as its +brightest parts. The condition of other stars warrants the conclusion +that this obscuring envelope is the product of a process of +refrigeration which will gradually make the sun more and more variable +until its history ends in extinction. Looking backward, we see a time +when the sun must have been more brilliant than it is now. At that +time it probably shone with the blinding white splendor of such stars +as Sirius, Spica, and Vega; now it resembles the relatively dull +Procyon; in time it will turn ruddy and fall into the closing cycle +represented by Antares. Considering that once it must have been more +radiantly powerful than at present, one is tempted to wonder if that +could have been the time when tropical life flourished within the +earth's polar circles, sustained by a vivific energy in the sun which +it has now lost. + +The corona, as we have said, varies with the sun-spot cycle. When the +spots are abundant and active the corona rises strong above the +spotted zones, forming immense beams or streamers, which on one +occasion, at least, had an observed length of ten million miles. At +the time of a spot minimum the corona is less brilliant and has a +different outline. It is then that the curved polar rays are most +conspicuous. Thus the vast banners of the sun, shaken out in the +eclipse, are signals to tell of its varying state, but it will +probably be long before we can read correctly their messages. + +The Zodiacal Light Mystery + +There is a singular phenomenon in the sky -- one of the most puzzling +of all -- which has long arrested the attention of astronomers, +defying their efforts at explanation, but which probably not one in a +hundred, and possibly not one in a thousand, of the readers of this +book has ever seen. Yet its name is often spoken, and it is a +conspicuous object if one knows when and where to look for it, and +when well seen it exhibits a mystical beauty which at the same time +charms and awes the beholder. It is called ``The Zodiacal Light,'' +because it lies within the broad circle of the Zodiac, marking the +sun's apparent annual path through the stars. What it is nobody has +yet been able to find out with certainty, and books on astronomy +usually speak of it with singular reserve. But it has given rise to +many remarkable theories, and a true explanation of it would probably +throw light on a great many other celestial mysteries. The Milky Way +is a more wonderful object to look upon, but its nature can be +comprehended, while there is a sort of uncanniness about the Zodiacal +Light which immediately impresses one upon seeing it, for its part in +the great scheme of extra-terrestrial affairs is not evident. + +If you are out-of-doors soon after sunset -- say, on an evening late +in the month of February -- you may perceive, just after the angry +flush of the dying winter's day has faded from the sky, a pale ghostly +presence rising above the place where the sun went down. The writer +remembers from boyhood the first time it was pointed out to him and +the unearthly impression that it made, so that he afterward avoided +being out alone at night, fearful of seeing the spectral thing again. +The phenomenon brightens slowly with the fading of the twilight, and +soon distinctly assumes the shape of an elongated pyramid of pearly +light, leaning toward the south if the place of observation is in the +northern hemisphere. It does not impress the observer at all in the +same manner as the Milky Way; that looks far off and is clearly among +the stars, but the Zodiacal Light seems closer at hand, as if it were +something more intimately concerning the earth. To all it immediately +suggests a connection, also, with the sunken sun. If the night is +clear and the moon absent (and if you are in the country, for city +lights ruin the spectacles of the sky), you will be able to watch the +apparition for a long time. You will observe that the light is +brightest near the horizon, gradually fading as the pyramidal beam +mounts higher, but in favorable circumstances it may be traced nearly +to the meridian south of the zenith, where its apex at last vanishes +in the starlight. It continues visible during the evenings of March +and part of April, after which, ordinarily, it is seen no more, or if +seen is relatively faint and unimpressive. But when autumn comes it +appears again, this time not like a wraith hovering above the westward +tomb of the day-god, but rather like a spirit of the morning +announcing his reincarnation in the east. + +The reason why the Zodiacal Light is best seen in our latitudes at the +periods just mentioned is because at those times the Zodiac is more +nearly perpendicular to the horizon, first in the west and then in the +east; and, since the phenomenon is confined within the borders of the +Zodiac, it cannot be favorably placed for observation when the +zodiacal plane is but slightly inclined to the horizon. Its faint +light requires the contrast of a background of dark sky in order to be +readily perceptible. But within the tropics, where the Zodiac is +always at a favorable angle, the mysterious light is more constantly +visible. Nearly all observant travelers in the equatorial regions have +taken particular note of this phenomenon, for being so much more +conspicuous there than in the temperate zones it at once catches the +eye and holds the attention as a novelty. Humboldt mentions it many +times in his works, for his genius was always attracted by things out +of the ordinary and difficult of explanation, and he made many careful +observations on its shape, its brilliancy, and its variations; for +there can be no doubt that it does vary, and sometimes to an +astonishing degree. It is said that it once remained practically +invisible in Europe for several years in succession. During a trip to +South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a +remarkable difference between the appearance of the Zodiacal Light on +his going and coming voyages. In fact, when crossing the equator going +south he did not see it at all; but on returning he had, on March 6th, +when one degree south of the equator, a memorable view of it. + + It was a bright, clear night, and the Zodiacal Light was + extraordinarily brilliant -- brighter than he had ever seen it + before. The Milky Way was not to be compared with it. The brightest + part extended 75 deg. from the sun. There was a faint and much narrower + extension which they could just make out beyond the Pleiades along + the ecliptic, but the greater part of the Zodiacal Light showed as + a broad truncated column, and it did not appear nearly as conical + as he had before seen it. + +When out of the brief twilight of intertropical lands, where the sun +drops vertically to the horizon and night rushes on like a wave of +darkness, the Zodiacal Light shoots to the very zenith, its color is +described as a golden tint, entirely different from the silvery sheen +of the Milky Way. If I may venture again to refer to personal +experiences and impressions, I will recall a view of the Zodiacal +Light from the summit of the cone of Mt Etna in the autumn of the year +1896 (more briefly described in Astronomy with the Naked Eye). There +are few lofty mountains so favorably placed as Etna for observations +of this kind. It was once resorted to by Prof. George E. Hale, in an +attempt to see the solar corona without an eclipse. Rising directly +from sea-level to an elevation of nearly eleven thousand feet, the +observer on its summit at night finds himself, as it were, lost in the +midst of the sky. But for the black flanks of the great cone on which +he stands he might fancy himself to be in a balloon. On the occasion +to which I refer the world beneath was virtually invisible in the +moonless night. The blaze of the constellations overhead was +astonishingly brilliant, yet amid all their magnificence my attention +was immediately drawn to a great tapering light that sprang from the +place on the horizon where the sun would rise later, and that seemed +to be blown out over the stars like a long, luminous veil. It was the +finest view of the Zodiacal light that I had ever enjoyed -- thrilling +in its strangeness -- but I was almost disheartened by the +indifference of my guide, to whom it was only a light and nothing +more. If he had no science, he had less poetry -- rather a remarkable +thing, I thought, for a child of his clime. The Light appeared to me +to be distinctly brighter than the visible part of the Milky Way which +included the brilliant stretches in Auriga and Perseus, and its color, +if one may speak of color in connection with such an object, seemed +richer than that of the galactic band; but I did not think of it as +yellow, although Humboldt has described it as resembling a golden +curtain drawn over the stars, and Du Chaillu in Equatorial Africa +found it of a bright yellow color. It may vary in color as in +conspicuousness. The fascination of that extraordinary sight has never +faded from my memory. I turned to regard it again and again, although +I had never seen the stellar heavens so brilliant, and it was one of +the last things I looked for when the morning glow began softly to +mount in the east, and Sicily and the Mediterranean slowly emerged +from the profound shadow beneath us. + +The Zodiacal Light seems never to have attracted from astronomers in +general the amount of careful attention that it deserves; perhaps +because so little can really be made of it as far as explanation is +concerned. I have referred to the restraint that scientific writers +apparently feel in speaking of it. The grounds for speculation that it +affords may be too scanty to lead to long discussions, yet it piques +curiosity, and as we shall see in a moment has finally led to a most +interesting theory. Once it was the subject of an elaborate series of +studies which carried the observer all round the world. That was in +1845--46, during the United States Exploring Expedition that visited +the then little known Japan. The chaplain of the fleet, the Rev. Mr +Jones, went out prepared to study the mysterious light in all its +phases. He saw it from many latitudes on both sides of the equator, +and the imagination cannot but follow him with keen interest in his +world-circling tour, keeping his eyes every night fixed upon the +phantasm overhead, whose position shifted with that of the hidden sun. +He demonstrated that the flow extends at times completely across the +celestial dome, although it is relatively faint directly behind the +earth. On his return the government published a large volume of his +observations, in which he undertook to show that the phenomenon was +due to the reflection of sunlight from a ring of meteoric bodies +encircling the earth. But, after all, this elaborate investigation +settled nothing. + +Prof. E. E. Barnard has more recently devoted much attention to the +Zodiacal Light, as well as to a strange attendant phenomenon called +the ``Gegenschein,'' or Counterglow, because it always appears at that +point in the sky which is exactly opposite the sun. The Gegenschein is +an extremely elusive phenomenon, suitable only for eyes that have been +specially trained to see it. Professor Newcomb has cautiously remarked +that + + it is said that in that point of the heavens directly opposite the + sun there is an elliptical patch of light... This phenomenon is so + difficult to account for that its existence is sometimes doubted; + yet the testimony in its favor is difficult to set aside. + +It certainly cannot be set aside at all since the observations of +Barnard. I recall an attempt to see it under his guidance during a +visit to Mount Hamilton, when he was occupied there with the Lick +telescope. Of course, both the Gegenschein and the Zodiacal Light are +too diffuse to be studied with telescopes, which, so to speak, magnify +them out of existence. They can only be successfully studied with the +naked eye, since every faintest glimmer that they afford must be +utilized. This is especially true of the Gegenschein. At Mount +Hamilton, Mr Barnard pointed out to me its location with reference to +certain stars, but with all my gazing I could not be sure that I saw +it. To him, on the contrary, it was obvious; he had studied it for +months, and was able to indicate its shape, its boundaries, its +diameter, and the declination of its center with regard to the +ecliptic. There is not, of course, the shadow of a doubt of the +existence of the Gegenschein, and yet I question if one person in a +million has ever seen or ever will see it. The Zodiacal Light, on the +other hand, is plain enough, provided that the time and the +circumstances of the observation are properly chosen. + +In the attempts to explain the Zodiacal Light, the favorite hypothesis +has been that it is an appendage of the sun -- perhaps simply an +extension of the corona in the plane of the ecliptic, which is not +very far from coinciding with that of the sun's equator. This idea is +quite a natural one, because of the evident relation of the light to +the position of the sun. The vast extension of the equatorial wings of +the corona in 1878 gave apparent support to this hypothesis; if the +substance of the corona could extend ten million miles from the sun, +why might it not extend even one hundred million, gradually fading out +beyond the orbit of the earth? A variation of this hypothesis assumes +that the reflection is due to swarms of meteors circling about the +sun, in the plane of its equator, all the way from its immediate +neighborhood to a distance exceeding that of the earth. But in neither +form is the hypothesis satisfactory; there is nothing in the +appearance of the corona to indicate that it extends even as far as +the planet Mercury, while as to meteors, the orbits of the known +swarms do not accord with the hypothesis, and we have no reason to +believe that clouds of others exist traveling in the part of space +where they would have to be in order to answer the requirements of the +theory. The extension of the corona in 1878 did not resemble in its +texture the Zodiacal Light. + +Now, it has so often happened in the history of science that an +important discovery in one branch has thrown unexpected but most +welcome light upon some pending problem in some other branch, that a +strong argument might be based upon that fact alone against the too +exclusive devotion of many investigators to the narrow lines of their +own particular specialty; and the Zodiacal Light affords a case in +point, when it is considered in connection with recent discoveries in +chemistry and physics. From the fact that atoms are compound bodies +made up of corpuscles at least a thousand times smaller than the +smallest known atom -- a fact which astounded most men of science when +it was announced a few years ago -- a new hypothesis has been +developed concerning the nature of the Zodiacal Light (as well as +other astronomical riddles), and this hypothesis comes not from an +astronomer, but from a chemist and physicist, the Swede, Svante +Arrhenius. In considering an outline of this new hypothesis we need +neither accept nor reject it; it is a case rather for suspension of +judgment. + +To begin with, it carries us back to the ``pressure of light'' +mentioned in the preceding chapter. The manner in which this pressure +is believed generally to act was there sufficiently explained, and it +only remains to see how it is theoretically extended to the particles +of matter supposed to constitute the Zodiacal Light. We know that +corpuscles, or ``fragments of atoms'' negatively electrified, are +discharged from hot bodies. Streams of these ``ions'' pour from many +flames and from molten metals; and the impact of the cathode and +ultra-violet rays causes them to gush even from cold bodies. In the +vast laboratory of the sun it is but reasonable to suppose that +similar processes are taking place. ``As a very hot metal emits these +corpuscles,'' says Prof. J. J. Thomson, ``it does not seem an +improbable hypothesis that they are emitted by that very hot body, the +sun.'' Let it be assumed, then, that the sun does emit them; what +happens next? Negatively charged corpuscles, it is known, serve as +nuclei to which particles of matter in the ordinary state are +attracted, and it is probable that those emitted from the sun +immediately pick up loads in this manner and so grow in bulk. If they +grow large enough the gravitation of the sun draws them back, and they +produce a negative charge in the solar atmosphere. But it is probable +that many of the particles do not attain the critical size which, +according to the principles before explained, would enable the +gravitation of the sun to retain them in opposition to the pressure of +the waves of light, and with these particles the light pressure is +dominant. Clouds of them may be supposed to be continually swept away +from the sun into surrounding space, moving mostly in or near the +plane of the solar equator, where the greatest activity, as indicated +by sunspots and related phenomena, is taking place. As they pass +outward into space many of them encounter the earth. If the earth, +like the moon, had no atmosphere the particles would impinge directly +on its surface, giving it a negative electric charge. But the presence +of the atmosphere changes all that, for the first of the flying +particles that encounter it impart to it their negative electricity, +and then, since like electric charges repel like, the storm of +particles following will be sheered off from the earth, and will +stream around it in a maze of hyperbolic paths. Those that continue on +into space beyond the earth may be expected to continue picking up +wandering particles of matter until their bulk has become so great +that the solar attraction prevails again over the light pressure +acting upon them, and they turn again sunward. Passing the earth on +their return they will increase the amount of dust-clouds careering +round it; and these will be further increased by the action of the +ultra-violet rays of the sunlight causing particles to shoot radially +away from the earth when the negative charge of the upper atmosphere +has reached a certain amount, which particles, although starting +sunward, will be swept back to the earth with the oncoming streams. As +the final result of all this accumulation of flying and gyrating +particles in the earth's neighborhood, we are told that the latter +must be transformed into the semblance of a gigantic solid-headed +comet provided with streaming tails, the longest of them stretching +away from the direction of the sun, while another shorter one extends +toward the sun. This shorter tail is due to the particles that we have +just spoken of as being driven sunward from the earth by the action of +ultra-violet light. No doubt this whole subject is too technical for +popular statement; but at any rate the general reader can understand +the picturesque side of the theory, for its advocates assure us that +if we were on the moon we would doubtless be able to see the +comet-like tails of the earth, and then we could appreciate the part +that they play in producing the phenomenon of the Zodiacal Light. + +That the Light as we see it could be produced by the reflection of +sunlight from swarms of particles careering round the earth in the +manner supposed by Arrhenius' hypothesis is evident enough; and it +will be observed that the new theory, after all, is only another +variant of the older one which attributes the Zodiacal Light to an +extension of the solar corona. But it differs from the older theory in +offering an explanation of the manner in which the extension is +effected, and it differentiates between the corona proper and the +streams of negative particles shot away from the sun. In its details +the hypothesis of Arrhenius also affords an explanation of many +peculiarities of the Zodiacal Light, such as that it is confined to +the neighborhood of the ecliptic, and that it is stronger on the side +of the earth which is just turning away from a position under the sun +than on the other side; but it would carry us beyond our limits to go +into these particulars. The Gegenschein, according to this theory, is +a part of the same phenomenon as the Zodiacal Light, for by the laws +of perspective it is evident that the reflection from the streams of +particles situated at a point directly opposite to the sun would be at +a maximum, and this is the place which the Gegenschein occupies. Apart +from its geometrical relations to the position of the sun, the +variability of the Zodiacal Light appears to affirm its solar +dependence, and this too would be accounted for by Arrhenius' +hypothesis better than by the old theory of coronal extension. The +amount of corpuscular discharge from the sun must naturally be +governed by the state of relative activity or inactivity of the +latter, and this could not but be reflected in the varying splendor of +the Zodiacal Light. But much more extended study than has yet been +given to the subject will be required before we can feel that we know +with reasonable certainty what this mysterious phenomenon really is. +By the hypothesis of Arrhenius every planet that has an atmosphere +must have a Zodiacal Light attending it, but the phenomenon is too +faint for us to be able to see it in the case, for instance, of Venus, +whose atmosphere is very abundant. The moon has no corresponding +``comet's tail'' because, as already explained, of the lack of a lunar +atmosphere to repel the streams by becoming itself electrified; but if +there were a lunar Zodiacal Light, no doubt we could see it because of +the relative nearness of our satellite. + +Marvels of the Aurora + +One of the most vivid recollections of my early boyhood is that of +seeing my father return hastily into the house one evening and call +out to the family: ``Come outside and look at the sky!'' Ours was a +country house situated on a commanding site, and as we all emerged +from the doorway we were dumbfounded to see the heavens filled with +pale flames which ran licking and quivering over the stars. Instantly +there sprang into my terrified mind the recollection of an awful +description of ``the Day of Judgment'' (the Dies Irae), which I had +heard with much perturbation of spirit in the Dutch Reformed church +from the lips of a tall, dark-browed, dreadfully-in-earnest preacher +of the old-fashioned type. My heart literally sank at sight of the +spectacle, for it recalled the preacher's very words; it was just as +he had said it would be, and it needed the assured bearing of my +elders finally to convince me that + + That Day of Wrath, O dreadful day, + When Heaven and Earth shall pass away, + As David and the Sibyl say + +had not actually come upon us. And even the older members of the +household were not untouched with misgivings when menacing spots of +crimson appeared, breaking out now here, now there, in the shuddering +sky. Toward the north the spectacle was appalling. A huge arch spanned +an unnaturally dark segment resting on the horizon, and above this +arch sprang up beams and streamers in a state of incessant agitation, +sometimes shooting up to the zenith with a velocity that took one's +breath, and sometimes suddenly falling into long ranks, and marching, +marching, marching, like an endless phalanx of fiery specters, and +moving, as I remember, always from east to west. The absolute silence +with which these mysterious evolutions were performed and the +quavering reflections which were thrown upon the ground increased the +awfulness of the exhibition. Occasionally enormous curtains of lambent +flame rolled and unrolled with a majestic motion, or were shaken to +and fro as if by a mighty, noiseless wind. At times, too, a sudden +billowing rush would be made toward the zenith, and for a minute the +sky overhead would glow so brightly that the stars seemed to have been +consumed. The spectacle continued with varying intensity for hours. + +This exhibition occurred in Central New York, a latitude in which the +Aurora Borealis is seldom seen with so much splendor. I remember +another similar one seen from the city of New York in November, 1882. +On this last occasion some observers saw a great upright beam of light +which majestically moved across the heavens, stalking like an +apparition in the midst of the auroral pageant, of whose general +movements it seemed to be independent, maintaining always its upright +posture, and following a magnetic parallel from east to west. This +mysterious beam was seen by no less than twenty-six observers in +different parts of the country, and a comparison of their observations +led to a curious calculation indicating that the apparition was about +one hundred and thirty-three miles tall and moved at the speed of ten +miles per second! + +But, as everybody knows, it is in the Arctic regions that the Aurora, +or the ``Northern Lights,'' can best be seen. There, in the long polar +night, when for months together the sun does not rise, the strange +coruscations in the sky often afford a kind of spectral daylight in +unison with the weird scenery of the world of ice. The pages in the +narratives of Arctic exploration that are devoted to descriptions of +the wonderful effects of the Northern Lights are second to none that +man has ever penned in their fascination. The lights, as I have +already intimated, display astonishing colors, particularly shades of +red and green, as they flit from place to place in the sky. The +discovery that the magnetic needle is affected by the Aurora, +quivering and darting about in a state of extraordinary excitement +when the lights are playing in the sky, only added to the mystery of +the phenomenon until its electro-magnetic nature had been established. +This became evident as soon as it was known that the focus of the +displays was the magnetic pole; and when the far South was visited the +Aurora Australis was found, having its center at the South Magnetic +Pole. Then, if not before, it was clear that the earth was a great +globular magnet, having its poles of opposite magnetism, and that the +auroral lights, whatever their precise cause might be, were +manifestations of the magnetic activity of our planet. After the +invention of magnetic telegraphy it was found that whenever a great +Aurora occurred the telegraph lines were interrupted in their +operation, and the ocean cables ceased to work. Such a phenomenon is +called a ``magnetic storm.'' + +The interest excited by the Aurora in scientific circles was greatly +stimulated when, in the last half of the nineteenth century, it was +discovered that it is a phenomenon intimately associated with +disturbances on the sun. The ancient ``Zurich Chronicles,'' extending +from the year 1000 to the year 1800, in which both sun-spots visible +to the naked eye and great displays of the auroral lights were +recorded, first set Rudolf Wolf on the track of this discovery. The +first notable proof of the suspected connection was furnished with +dramatic emphasis by an occurrence which happened on September 1, +1859. Near noon on that day two intensely brilliant points suddenly +broke out in a group of sun-spots which were under observation by Mr +R. C. Carrington at his observatory at Redhill, England. The points +remained visible for not more than five minutes, during which interval +they moved thirty-five thousand miles across the solar disk. Mr R. +Hodgson happened to see the same phenomenon at his observatory at +Highgate, and thus all possibility of deception was removed. But +neither of the startled observers could have anticipated what was to +follow, and, indeed, it was an occurrence which has never been +precisely duplicated. I quote the eloquent account given by Miss +Clerke in her History of Astronomy During the Nineteenth Century. + + This unique phenomenon seemed as if specially designed to + accentuate the inference of a sympathetic relation between the + earth and the sun. From August 28 to September 4, 1859, a magnetic + storm of unparalleled intensity, extent, and duration was in + progress over the entire globe. Telegraphic communication was + everywhere interrupted -- except, indeed, that it was in some cases + found practicable to work the lines without batteries by the agency + of the earth-currents alone; sparks issued from the wires; gorgeous + auroras draped the skies in solemn crimson over both hemispheres, + and even in the tropics; the magnetic needle lost all trace of + continuity in its movements and darted to and fro as if stricken + with inexplicable panic. The coincidence was even closer. At the + very instant of the solar outburst witnessed by Carrington and + Hodgson the photographic apparatus at Kew registered a marked + disturbance of all the three magnetic elements; while shortly after + the ensuing midnight the electric agitation culminated, thrilling + the whole earth with subtle vibrations, and lighting up the + atmosphere from pole to pole with coruscating splendors which + perhaps dimly recall the times when our ancient planet itself shone + as a star. + +If this amazing occurrence stood alone, and as I have already said it +has never been exactly duplicated, doubt might be felt concerning some +of the inferences drawn from it; but in varying forms it has been +repeated many times, so that now hardly anyone questions the reality +of the assumed connection between solar outbursts and magnetic storms +accompanied by auroral displays on the earth. It is true that the late +Lord Kelvin raised difficulties in the way of the hypothesis of a +direct magnetic action of the sun upon the earth, because it seemed to +him that an inadmissible quantity of energy was demanded to account +for such action. But no calculation like that which he made is final, +since all calculations depend upon the validity of the data; and no +authority is unshakable in science, because no man can possess +omniscience. It was Lord Kelvin who, but a few years before the thing +was actually accomplished, declared that aerial navigation was an +impracticable dream, and demonstrated its impracticability by +calculation. However the connection may be brought about, it is as +certain as evidence can make it that solar outbursts are coincident +with terrestial magnetic disturbances, and coincident in such a way as +to make the inference of a causal connection irresistible. The sun is +only a little more than a hundred times its own diameter away from the +earth. Why, then, with the subtle connection between them afforded by +the ether which conveys to us the blinding solar light and the +life-sustaining solar heat, should it be so difficult to believe that +the sun's enormous electric energies find a way to us also? No doubt +the impulse coming from the sun acts upon the earth after the manner +of a touch upon a trigger, releasing energies which are already stored +up in our planet. + +But besides the evidence afforded by such occurrences as have been +related of an intimate connection between solar outbreaks and +terrestial magnetic flurries, attended by magnificent auroral +displays, there is another line of proof pointing in the same +direction. Thus, it is known that the sun-spot period, as remarked in +a preceding chapter, coincides in a most remarkable manner with the +periodic fluctuations in the magnetic state of the earth. This +coincidence runs into the most astonishing details. For instance, when +the sun-spot period shortens, the auroral period shortens to precisely +the same extent; as the short sun-spot periods usually bring the most +intense outbreaks of solar activity, so the corresponding short +auroral periods are attended by the most violent magnetic storms; a +secular period of about two hundred and twenty-two years affecting +sun-spots is said to have its auroral duplicate; a shorter period of +fifty-five and a half years, which some observers believe that they +have discovered appears also to be common to the two phenomena; and +yet another ``superposed'' period of about thirty-five years, which +some investigators aver exists, affects sun-spots and aurora alike. In +short, the coincidences are so numerous and significant that one would +have to throw the doctrine of probability to the winds in order to be +able to reject the conclusion to which they so plainly lead. + +But still the question recurs: How is the influence transmitted? Here +Arrhenius comes once more with his hypothesis of negative corpuscles, +or ions, driven away from the sun by light-pressure -- a hypothesis +which seems to explain so many things -- and offers it also as an +explanation of the way in which the sun creates the Aurora. He would +give the Aurora the same lineage with the Zodiacal Light. To +understand the application of this theory we must first recall the +fact that the earth is a great magnet having its two opposite poles of +magnetism, one near the Arctic and the other near the Antarctic +Circle. Like all magnets, the earth is surrounded with ``lines of +force,'' which, after the manner of the curved rays we saw in the +photograph of a solar eclipse, start from a pole, rising at first +nearly vertically, then bend gradually over, passing high above the +equator, and finally descending in converging sheaves to the opposite +pole. Now the axis of the earth is so placed in space that it lies at +nearly a right angle to the direction of the sun, and as the streams +of negatively charged particles come pouring on from the sun (see the +last preceding chapter), they arrive in the greatest numbers over the +earth's equatorial regions. There they encounter the lines of magnetic +force at the place where the latter have their greatest elevation +above the earth, and where their direction is horizontal to the +earth's surface. Obeying a law which has been demonstrated in the +laboratory, the particles then follow the lines of force toward the +poles. While they are above the equatorial regions they do not become +luminescent, because at the great elevation that they there occupy +there is virtually no atmosphere; but as they pass on toward the north +and the south they begin to descend with the lines of force, curving +down to meet at the poles; and, encountering a part of the atmosphere +comparable in density with what remains in an exhausted Crookes tube, +they produce a glow of cathode rays. This glow is conceived to +represent the Aurora, which may consequently be likened to a gigantic +exhibition of vacuum-tube lights. Anybody who recalls his student days +in the college laboratory and who has witnessed a display of Northern +Lights will at once recognize the resemblance between them in colors, +forms, and behavior. This resemblance had often been noted before +Arrhenius elaborated his hypothesis. + +Without intending to treat his interesting theory as more than a +possibly correct explanation of the phenomena of the Aurora, we may +call attention to some apparently confirmatory facts. One of the most +striking of these relates to a seasonal variation in the average +number of aurorae. It has been observed that there are more in March +and September than at any other time of the year, and fewer in June +and December; moreover (and this is a delicate test as applied to the +theory), they are slightly rarer in June than in December. Now all +these facts seem to find a ready explanation in the hypothesis of +Arrhenius, thus: (1) The particles issuing from the sun are supposed +to come principally from the regions whose excitement is indicated by +the presence of sun-spots (which accords with Hale's observation that +sun-spots are columns of ionized vapors), and these regions have a +definite location on either side of the solar equator, seldom +approaching it nearer than within 5 deg. or 10 deg. north or south, and never +extending much beyond 35 deg. toward either pole; (2) The equator of the +sun is inclined about 7 deg. to the plane of the earth's orbit, from which +it results that twice in a year -- viz., in June and December -- the +earth is directly over the solar equator, and twice a year -- viz., in +March and September -- when it is farthest north or south of the solar +equator, it is over the inner edge of the sun-spot belts. Since the +corpuscles must be supposed to be propelled radially from the sun, few +will reach the earth when the latter is over the solar equator in June +and December, but when it is over, or nearly over, the spot belts, in +March and September, it will be in the line of fire of the more active +parts of the solar surface, and relatively rich streams of particles +will reach it. This, as will be seen from what has been said above, is +in strict accord with the observed variations in the frequency of +aurorae. Even the fact that somewhat fewer aurorae are seen in June than +in December also finds its explanation in the known fact that the +earth is about three million miles nearer the sun in the winter than +in the summer, and the number of particles reaching it will vary, like +the intensity of light, inversely as the square of the distance. These +coincidences are certainly very striking, and they have a cumulative +force. If we accept the theory, it would appear that we ought to +congratulate ourselves that the inclination of the sun's equator is so +slight, for as things stand the earth is never directly over the most +active regions of the sun-spots, and consequently never suffers from +the maximum bombardment of charged particles of which the sun is +capable. Incessant auroral displays, with their undulating draperies, +flitting colors, and marching columns might not be objectionable from +the point of view of picturesqueness, but one magnetic storm of +extreme intensity following closely upon the heels of another, for +months on end, crazing the magnetic needle and continually putting the +telegraph and cable lines out of commission, to say nothing of their +effect upon ``wireless telegraphy'', would hardly add to the charms of +terrestrial existence. + +One or two other curious points in connection with Arrhenius' +hypothesis may be mentioned. First, the number of aurorae, according to +his explanation, ought to be greatest in the daytime, when the face of +the earth on the sunward side is directly exposed to the atomic +bombardment. Of course visual observation can give us no information +about this, since the light of the Aurora is never sufficiently +intense to be visible in the presence of daylight, but the records of +the magnetic observatories can be, and have been, appealed to for +information, and they indicate that the facts actually accord with the +theory. Behind the veil of sunlight in the middle of the afternoon, +there is good reason to believe, auroral exhibitions often take place +which would eclipse in magnificence those seen at night if we could +behold them. Observation shows, too, that aurorae are more frequent +before than after midnight, which is just what we should expect if +they originate in the way that Arrhenius supposes. Second, the theory +offers an explanation of the alleged fact that the formation of clouds +in the upper air is more frequent in years when aurorae are most +abundant, because clouds are the result of the condensation of +moisture upon floating particles in the atmosphere (in an absolutely +dustless atmosphere there would be no clouds), and it has been proved +that negative ions like those supposed to come from the sun play a +master part in the phenomena of cloud formation. + +Yet another singular fact, almost mystical in its suggestions, may be +mentioned. It seems that the dance of the auroral lights occurs most +frequently during the absence of the moon from the hemisphere in which +they appear, and that they flee, in greater part, to the opposite +hemisphere when the moon's revolution in an orbit considerably +inclined to the earth's equator brings her into that where they have +been performing. Arrhenius himself discovered this curious relation of +auroral frequency to the position of the moon north or south of the +equator, and he explains it in this way. The moon, like the earth, is +exposed to the influx of the ions from the sun; but having no +atmosphere, or almost none, to interfere with them, they descend +directly upon her surface and charge her with an electric negative +potential to a very high degree. In consequence of this she affects +the electric state of the upper parts of the earth's atmosphere where +they lie most directly beneath her, and thus prevents, to a large +extent, the negative discharges to which the appearance of the Aurora +is due. And so ``the extravagant and erring spirit'' of the Aurora +avoids the moon as Hamlet's ghost fled at the voice of the cock +announcing the awakening of the god of day. + +There are even other apparent confirmations of the hypothesis, but we +need not go into them. We shall, however, find one more application of +it in the next chapter, for it appears to be a kind of cure-all for +astronomical troubles; at any rate it offers a conceivable solution of +the question, How does the sun manage to transmit its electric +influence to the earth? And this solution is so grandiose in +conception, and so novel in the mental pictures that it offers, that +its acceptance would not in the least detract from the impression that +the Aurora makes upon the imagination. + +Strange Adventures of Comets + +The fears and legends of ancient times before Science was born, and +the superstitions of the Dark Ages, sedulously cultivated for +theological purposes by monks and priests, have so colored our ideas +of the influence that comets have had upon the human mind that many +readers may be surprised to learn that it was the apparition of a +wonderful comet, that of 1843, which led to the foundation of our +greatest astronomical institution, the Harvard College Observatory. No +doubt the comet superstition existed half a century ago, as, indeed, +it exists yet today, but in this case the marvelous spectacle in the +sky proved less effective in inspiring terror than in awakening a +desire for knowledge. Even in the sixteenth century the views that +enlightened minds took of comets tended powerfully to inspire popular +confidence in science, and Halley's prediction, after seeing and +studying the motion of the comet which appeared in 1682, that it would +prove to be a regular member of the sun's family and would be seen +returning after a period of about seventy-six years, together with the +fulfillment of that prediction, produced a revulsion from the +superstitious notions which had so long prevailed. + +Then the facts were made plain that comets are subject to the law of +gravitation equally with the planets; that there are many which +regularly return to the neighborhood of the sun (perihelion); and that +these travel in orbits differing from those of the planets only in +their greater eccentricity, although they have the peculiarity that +they do not, like the planets, all go round the sun in the same +direction, and do not keep within the general plane of the planetary +system, but traverse it sometimes from above and sometimes from below. +Other comets, including most of the ``great'' ones, appear to travel +in parabolic or, in a few cases, hyperbolic orbits, which, not being +closed curves, never bring them back again. But it is not certain that +these orbits may not be extremely eccentric ellipses, and that after +the lapse of hundreds, or thousands, of years the comets that follow +them may not reappear. The question is an interesting one, because if +all orbits are really ellipses, then all comets must be permanent +members of the solar system, while in the contrary case many of them +are simply visitors, seen once and never to be seen again. The +hypothesis that comets are originally interlopers might seem to derive +some support from the fact that the certainly periodic ones are +associated, in groups, with the great outer planets, whose attraction +appears to have served as a trap for them by turning them into +elliptical orbits and thus making them prisoners in the solar system. +Jupiter, owing to his great mass and his commanding situation in the +system, is the chief ``comet-catcher;'' but he catches them not for +himself, but for the sun. Yet if comets do come originally from +without the borders of the planetary system, it does not, by any +means, follow that they were wanderers at large in space before they +yielded to the overmastering attraction of the sun. Investigation of +the known cometary orbits, combined with theoretical considerations, +has led some astronomers to the conclusion that as the sun travels +onward through space he ``picks up en route'' cometary masses which, +without belonging strictly to his empire, are borne along in the same +vast ``cosmical current'' that carries the solar system. + +But while no intelligent person any longer thinks that the appearance +of a great comet is a token from the heavenly powers of the +approaching death of a mighty ruler, or the outbreak of a devastating +war, or the infliction of a terrible plague upon wicked mankind, +science itself has discovered mysteries about comets which are not +less fascinating because they are more intellectual than the +irrational fancies that they have displaced. To bring the subject +properly before the mind, let us see what the principal phenomena +connected with a comet are. + +At the present day comets are ordinarily ``picked up'' with the +telescope or the photographic plate before any one except their +discoverer is aware of their existence, and usually they remain so +insignificant in appearance that only astronomers ever see them. Yet +so great is the prestige of the word ``comet'' that the discovery of +one of these inconspicuous wanderers, and its subsequent movements, +become items of the day's news which everybody reads with the feeling, +perhaps, that at least he knows what is going on in the universe even +if he doesn't understand it. But a truly great comet presents quite a +different proposition. It, too, is apt to be detected coming out of +the depths of space before the world at large can get a glimpse of it, +but as it approaches the sun its aspect undergoes a marvelous change. +Agitated apparently by solar influence, it throws out a long streaming +tail of nebulous light, directed away from the sun and looking as if +blown out like a pennon by a powerful wind. Whatever may be the +position of the comet with regard to the sun, as it circles round him +it continually keeps its tail on the off side. This, as we shall soon +see, is a fact of capital importance in relation to the probable +nature of comets' tails. Almost at the same time that the formation of +the tail is observed a remarkable change takes place in the comet's +head, which, by the way, is invariably and not merely occasionally its +most important part. On approaching the sun the head usually +contracts. Coincidently with this contraction a nucleus generally +makes its appearance. This is a bright, star-like point in the head, +and it probably represents the totality of solid matter that the comet +possesses. But it is regarded as extremely unlikely that even the +nucleus consists of a uniformly solid mass. If it were such, comets +would be far more formidable visitors when they pass near the planets +than they have been found to be. The diameter of the nucleus may vary +from a few hundred up to several thousand miles; the heads, on the +average, are from twenty-five thousand to one hundred thousand miles +in diameter, although a few have greatly exceeded these dimensions; +that of the comet of 1811, one of the most stupendous ever seen, was a +million and a quarter miles in diameter! As to the tails, not +withstanding their enormous length -- some have been more than a +hundred million miles long -- there is reason to believe that they are +of extreme tenuity, ``as rare as vacuum.'' The smallest stars have +been seen shining through their most brilliant portions with +undiminished luster. + +After the nucleus has been formed it begins to throw out bright jets +directed toward the sun. A stream, and sometimes several streams, of +light also project sunward from the nucleus, occasionally appearing +like a stunted tail directed oppositely to the real tail. Symmetrical +envelopes which, seen in section, appear as half circles or parabolas, +rise sunward from the nucleus, forming a concentric series. The ends +of these stream backward into the tail, to which they seem to supply +material. Ordinarily the formation of these ejections and envelopes is +attended by intense agitation of the nucleus, which twists and turns, +swinging and gyrating with an appearance of the greatest violence. +Sometimes the nucleus is seen to break up into several parts. The +entire heads of some comets have been split asunder in passing close +around the sun; The comet of 1882 retreated into space after its +perihelion passage with five heads instead of the one that it had +originally, and each of these heads had its own tail! + +The possession of the spectroscope has enabled astronomers during +later years to study the chemical composition of comets by analyzing +their light. At first the only substances thus discovered in them were +hydro-carbon compounds, due evidently to the gaseous envelopes in +which some combination of hydrogen with carbon existed. Behind this +gaseous spectrum was found a faint continuous spectrum ascribed to the +nucleus, which apparently both reflects the sunlight and gives forth +the light of a glowing solid or liquid. Subsequently sodium and iron +lines were found in cometary spectra. The presence of iron would seem +to indicate that some of these bodies may be much more massive than +observations on their attractive effects have indicated. In some +recent comets, such as Morehouse's, in 1908, several lines have been +found, the origin of which is unknown. + +Without going back of the nineteenth century we may find records of +some of the most extraordinary comets that man has ever looked upon. +In 1811, still spoken of as ``the year of the comet,'' because of the +wonderful vintage ascribed to the skyey visitor, a comet shaped like a +gigantic sword amazed the whole world, and, as it remained visible for +seventeen months, was regarded by superstitious persons as a symbol of +the fearful happenings of Napoleon's Russian campaign. This comet, the +extraordinary size of whose head, greatly exceeding that of the sun +itself, has already been mentioned, was also remarkable for exhibiting +so great a brilliancy without approaching even to the earth's distance +from the sun. But there was once a comet (and only once -- in the year +1729) which never got nearer to the sun than four times the distance +of the earth and yet appeared as a formidable object in the sky. As +Professor Young has remarked, ``it must have been an enormous comet to +be visible from such a distance.'' And we are to remember that there +were no great telescopes in the year 1729. That comet affects the +imagination like a phantom of space peering into the solar system, +displaying its enormous train afar off (which, if it had approached as +near as other comets, would probably have become the celestial wonder +of all human memory), and then turning away and vanishing in the +depths of immensity. + +In 1843 a comet appeared which was so brilliant that it could be seen +in broad day close beside the sun! This was the first authenticated +instance of that kind, but the occurrence was to be repeated, as we +shall see in a moment, less than forty years later. + +The splendid comet of 1858, usually called Donati's, is remembered by +many persons yet living. It was, perhaps, both as seen by the naked +eye and with the telescope, the most beautiful comet of which we have +any record. It too marked a rich vintage year, still remembered in the +vineyards of France, where there is a popular belief that a great +comet ripens the grape and imparts to the wine a flavor not attainable +by the mere skill of the cultivator. There are ``comet wines,'' +carefully treasured in certain cellars, and brought forth only when +their owner wishes to treat his guests to a sip from paradise. + +The year 1861 saw another very remarkable comet, of an aspect +strangely vast and diffuse, which is believed to have swept the earth +with its immense tail when it passed between us and the sun on the +night of June 30th, an event which produced no other known effect than +the appearance of an unwonted amount of scattered light in the sky. + +The next very notable comet was the ``Great Southern Comet'' of 1880, +which was not seen from the northern hemisphere. It mimicked the +aspect of the famous comet of 1843, and to the great surprise of +astronomers appeared to be traveling in the same path. This proved to +be the rising of the curtain for an astronomical sensation +unparalleled in its kind; for two years later another brilliant comet +appeared, first in the southern hemisphere, and it too followed the +same track. The startling suggestion was now made that this comet was +identical with those of 1843 and 1880, its return having been hastened +by the resistance experienced in passing twice through the coronal +envelope, and there were some who thought that it would now swing +swiftly round and then plunge straight into the sun, with consequences +that might be disastrous to us on account of the ``flash of heat'' +that would be produced by the impact. Nervous people were frightened, +but observation soon proved that the danger was imaginary, for +although the comet almost grazed the sun, and must have rushed through +two or three million miles of the coronal region, no retardation of +its immense velocity was perceptible, and it finally passed away in a +damaged condition, as before remarked, and has never since appeared. + +Then the probable truth was perceived -- viz., that the three comets +(1843, 1880, and 1882) were not one identical body, but three separate +ones all traveling in the same orbit. It was found, too, that a comet +seen in 1668 bore similar insignia of relationship. The natural +inference was that these four bodies had once formed a single mass +which had been split apart by the disruptive action of the sun. +Strength was lent to this hypothesis by the fact that the comet of +1882 was apparently torn asunder during its perihelion passage, +retreating into space in a dissevered state. But Prof. George Forbes +has a theory that the splitting of the original cometary mass was +effected by an unknown planet, probably greater than Jupiter, situated +at a hundred times the earth's distance from the sun, and revolving in +a period of a thousand years. He supposes that the original comet was +not that of 1668, but one seen in 1556, which has since been +``missing,'' and that its disruption occurred from an encounter with +the supposititious planet about the year 1700. Truly from every point +of view comets are the most extraordinary of adventurers! + +The comet of 1882 was likewise remarkable for being visible, like its +predecessor of 1843, in full daylight in close proximity to the sun. +The story of its detection when almost in contact with the solar disk +is dramatic. It had been discovered in the southern hemisphere only a +couple of weeks before its perihelion, which occurred on September +17th, and on the forenoon of that day it was seen by Doctor Common in +England, and by Doctor Elkin and Mr Finlay at the Cape of Good Hope, +almost touching the sun. It looked like a dazzling white bird with +outspread wings. The southern observers watched it go right into the +sun, when it instantly disappeared. What had happened was that the +comet in passing its perihelion point had swung exactly between the +earth and the sun. On the following morning it was seen from all parts +of the world close by the sun on the opposite side, and it remained +thus visible for three days, gradually receding from the solar disk. +It then became visible for northern observers in the morning sky +before sunrise, brandishing a portentous sword-shaped tail which, if +it had been in the evening sky, would have excited the wonder of +hundreds of millions, but situated where it was, comparatively few +ever saw it. + +The application of photography to the study of comets has revealed +many curious details which might otherwise have escaped detection, or +at best have remained subject to doubt. It has in particular shown not +only the precise form of the tails, but the remarkable vicissitudes +that they undergo. Professor Barnard's photographs of Brooks' comet in +1893 suggested, by the extraordinary changes in the form of the tail +which they revealed, that the comet was encountering a series of +obstructions in space which bent and twisted its tail into fantastic +shapes. The reader will observe the strange form into which the tail +was thrown on the night of October 21st. A cloud of meteors through +which the comet was passing might have produced such deformations of +its tail. In the photograph of Daniels' comet of 1907, a curious +striping of the tail will be noticed. The short bright streaks seen in +the photograph, it may be explained, are the images of stars which are +drawn out into lines in consequence of the fact that the photographic +telescope was adjusted to follow the motion of the comet while the +stars remained at rest. + +But the adventures of comets are not confined to possible encounters +with unknown obstacles. We have referred to the fact that the great +planets, and especially Jupiter, frequently interfere with the motions +of comets. This interference is not limited to the original alteration +of their orbits from possible parabolas to ellipses, but is sometimes +exercised again and again, turning the bewildered comets into +elliptical paths of all degrees of eccentricity. A famous example of +this kind of planetary horse-play is furnished by the story of +Lexell's missing comet. This comet was first seen in 1770. +Investigation showed that it was moving in an orbit which should bring +it back to perihelion every five and a half years; yet it had never +been seen before and, although often searched for, has never been seen +since. Laplace and Leverrier proved mathematically that in 1767 it had +approached so close to Jupiter as to be involved among the orbits of +his satellites. What its track had been before is not known, but on +that occasion the giant planet seized the interloper, threw it into a +short elliptic orbit and sent it, like an arrested vagrant, to receive +sentence at the bar of the sun. On this journey it passed within less +than 1,500,000 miles of the earth. The form of orbit which Jupiter had +impressed required, as we have said, its return in about five and a +half years; but soon after 1770 it had the misfortune a second time to +encounter Jupiter at close range, and he, as if dissatisfied with the +leniency of the sun, or indignant at the stranger's familiarity, +seized the comet and hurled it out of the system, or at any rate so +far away that it has never since been able to rejoin the family circle +that basks in the immediate rays of the solar hearth. Nor is this the +only instance in which Jupiter has dealt summarily with small comets +that have approached him with too little deference. + +The function which Jupiter so conspicuously fulfills as master of the +hounds to the sun is worth considering a little more in detail. To +change the figure, imagine the sun in its voyage through space to be +like a majestic battleship surrounded by its scouts. Small vessels +(the comets, as they are overhauled by the squadron, are taken in +charge by the scouts, with Jupiter for their chief, and are forced to +accompany the fleet, but not all are impressed. If a strange comet +undertakes to run across Jupiter's bows the latter brings it to, and +makes prize of it by throwing it into a relatively small ellipse with +the sun for its focus. Thenceforth, unless, as happened to the unhappy +comet of Lexell, it encounters Jupiter again in such a way as to be +diverted by him into a more distant orbit, it can never get away. +About thirty comets are now known to have thus been captured by the +great planet, and they are called ``Jupiter's Comet Family.'' But, on +the other hand, if a wandering comet crosses the wake of the chief +planetary scout the latter simply drives it away by accelerating its +motion and compels it to steer off into open space. The transformation +of comets into meteors will be considered in the next chapter, but +here, in passing, mention may be made of the strange fate of one +member of Jupiter's family, Biela's comet, which, having become over +bold in its advances to its captor, was, after a few revolutions in is +impressed orbit, torn to pieces and turned into a flock of meteors. + +And now let us return to the mystery of comets' tails. That we are +fully justified in speaking of the tails of comets as mysterious is +proved by the declaration of Sir John Herschel, who averred, in so +many words, that ``there is some profound secret and mystery of nature +concerned in this phenomenon,'' and this profound secret and mystery +has not yet been altogether cleared up. Nevertheless, the +all-explaining hypothesis of Arrhenius offers us once more a certain +amount of aid. Comets' tails, Arrhenius assures us, are but another +result of the pressure of light. The reader will recall the +applications of this theory to the Zodiacal Light and the Aurora. In +the form in which we now have to deal with it, the supposition is made +that as a comet approaches the sun eruptions of vapor, due to the +solar heat, occur in its nucleus. These are naturally most active on +the side which is directly exposed to the sun, whence the appearance +of the immense glowing envelopes that surround the nucleus on the +sunward side. Among the particles of hydro-carbon, and perhaps solid +carbon in the state of fine dust, which are thus set free there will +be many whose size is within the critical limit which enables the +light-waves from the sun to drive them away. Clouds of such particles, +then, will stream off behind the advancing comet, producing the +appearance of a tail. This accounts for the fact that the tails of +comets are always directed away from the sun, and it also explains the +varying forms of the tails and the extraordinary changes that they +undergo. The speed of the particles driven before the light-waves must +depend upon their size and weight, the lightest of a given size +traveling the most swiftly. By accretion certain particles might grow, +thus losing velocity and producing the appearance of bunches in the +tail, such as have been observed. The hypothesis also falls in with +the researches of Bredichin, who has divided the tails of comets into +three principal classes -- viz.: (1) Those which appear as long, +straight rays; (2) Those which have the form of curved plumes or +scimitars; (3) Those which are short, brushy, and curved sharply +backward along the comet's path. In the first type he calculates the +repulsive force at from twelve to fifteen times the force of gravity; +in the second at from two to four times; and in the third at about one +and a half times. The straight tails he ascribes to hydrogen because +the hydrogen atom is the lightest known; the sword-shaped tails to +hydro-carbons; and the stumpy tails to vaporized iron. It will be seen +that, if the force driving off the tails is that which Arrhenius +assumes it to be, the forms of those appendages would accord with +those that Bredichin's theory calls for. At the same time we have an +explanation of the multiple tails with which some comets have adorned +themselves. The comet of 1744, for instance, had at one time no less +than seven tails spread in a wide curved brush behind it. Donati's +comet of 1858 also had at least two tails, the principal one +sword-shaped and the other long, narrow, and as straight as a rule. +According to Bredichin, the straight tail must have been composed of +hydrogen, and the other of some form of hydro-carbon whose atoms are +heavier than those of hydrogen, and, consequently, when swept away by +the storm of light-waves, followed a curvature depending upon the +resultant of the forces operating upon them. The seven tails of the +comet of 1744 presented a kind of diagram graphically exhibiting its +complex composition, and, if we knew a little more about the +constituents of a comet, we might be able to say from the amount of +curvature of the different tails just what were the seven substances +of which that comet consisted. + +If these theories seem to the reader fantastic, at any rate they are +no more fantastic than the phenomena that they seek to explain. + +Meteors, Fire-Balls, and Meteorites + +One of the most terrorizing spectacles with which the heavens have +ever caused the hearts of men to quake occurred on the night of +November 13, 1833. On that night North America, which faced the storm, +was under a continual rain of fire from about ten o'clock in the +evening until daybreak. + +The fragments of a comet had struck the earth. + +But the meaning of what had happened was not discovered until long +afterward. To the astronomers who, with astonishment not less than +that of other people, watched the wonderful scene, it was an +unparalleled ``shower of meteors.'' They did not then suspect that +those meteors had once formed the head of a comet. Light dawned when, +a year later, Prof. Denison Olmsted, of Yale College, demonstrated +that the meteors had all moved in parallel orbits around the sun, and +that these orbits intersected that of the earth at the point where our +planet happened to be on the memorable night of November 13th. +Professor Olmsted even went so far as to suggest that the cloud of +meteors that had encountered the earth might form a diffuse comet; but +full recognition of the fact that they were cometary debris came +later, as the result of further investigation. The key to the secret +was plainly displayed in the spectacle itself, and was noticed without +being understood by thousands of the terror-stricken beholders. It was +an umbrella of fire that had opened overhead and covered the heavens; +in other words, the meteors all radiated from a particular point in +the constellation Leo, and, being countless as the snowflakes in a +winter tempest, they ribbed the sky with fiery streaks. Professor +Olmsted showed that the radiation of the meteors from a fixed point +was an effect of perspective, and in itself a proof that they were +moving in parallel paths when they encountered the earth. The fact was +noted that there had been a similar, but incomparably less brilliant, +display of meteors on the same day of November, 1832, and it was +rightly concluded that these had belonged to the same stream, although +the true relationship of the phenomena was not immediately +apprehended. Olmsted ascribed to the meteors a revolution about the +sun once in every six months, bringing them to the intersection of +their orbit with that of the earth every November 13th; but later +investigators found that the real period was about thirty-three and +one-quarter years, so that the great displays were due three times in +a century, and their return was confidently predicted for the year +1866. The appearance of the meteors in 1832, a year before the great +display, was ascribed to the great length of the stream which they +formed in space -- so great that they required more than two years to +cross the earth's orbit. In 1832 the earth had encountered a +relatively rare part of the stream, but in 1833, on returning to the +crossing-place, it found there the richest part of the stream pouring +across its orbit. This explanation also proved to be correct, and the +predicted return in 1866 was duly witnessed, although the display was +much less brilliant than in 1833. It was followed by another in 1867. + +In the mean time Olmsted's idea of a cometary relationship of the +meteors was demonstrated to be correct by the researches of +Schiaparelli and others, who showed that not only the November +meteors, but those of August, which are seen more or less abundantly +every year, traveled in the tracks of well-known comets, and had +undoubtedly an identical origin with those comets. In other words the +comets and the meteor-swarms were both remnants of original masses +which had probably been split up by the action of the sun, or of some +planet to which they had made close approaches. The annual periodicity +of the August meteors was ascribed to the fact that the separation had +taken place so long ago that the meteors had become distributed all +around the orbit, in consequence of which the earth encountered some +of them every year when it arrived at the crossing-point. Then +Leverrier showed that the original comet associated with the November +meteors was probably brought into the system by the influence of the +planet Uranus in the year 126 of the Christian era. Afterward +Alexander Herschel identified the tracks of no less than seventy-six +meteor-swarms (most of them inconspicuous) with those of comets. The +still more recent researches of Mr W. F. Denning make it probable that +there are no meteors which do not belong to a flock or system probably +formed by the disintegration of a cometary mass; even the apparently +sporadic ones which shoot across the sky, ``lost souls in the night,'' +being members of flocks which have become so widely scattered that the +earth sometimes takes weeks to pass through the region of space where +their paths lie. + +The November meteors should have exhibited another pair of spectacles +in 1899 and 1900, and their failure to do so caused at first much +disappointment, until it was made plain that a good reason existed for +their absence. It was found that after their last appearance, in 1867, +they had been disturbed in their movements by the planets Jupiter and +Saturn, whose attractions had so shifted the position of their orbit +that it no longer intersected that of the earth, as it did before. +Whether another planetary interference will sometime bring the +principal mass of the November meteors back to the former point of +intersection with the earth's orbit is a question for the future to +decide. It would seem that there may be several parallel streams of +the November meteors, and that some of them, like those of August, are +distributed entirely around the orbit, so that every mid-November we +see a few of them. + +We come now to a very remarkable example of the disintegration of a +comet and the formation of a meteor-stream. In 1826 Biela, of +Josephstadt, Austria, discovered a comet to which his name was given. +Calculation showed that it had an orbital period of about six and a +half years, belonging to Jupiter's ``family.'' On one of its returns, +in 1846, it astonished its watchers by suddenly splitting in two. The +two comets thus formed out of one separated to a distance of about one +hundred and sixty thousand miles, and then raced side by side, +sometimes with a curious ligature connecting them, like Siamese twins, +until they disappeared together in interplanetary space. In 1852 they +came back, still nearly side by side, but now the distance between +them had increased to a million and a quarter of miles. After that, at +every recurrence of their period, astronomers looked for them in vain, +until 1872, when an amazing thing happened. On the night of November +28th, when the earth was crossing the plane of the orbit of the +missing comet, a brilliant shower of meteors burst from the northern +sky, traveling nearly in the track which the comet should have +pursued. The astronomers were electrified. Klinkerfues, of Goettingen, +telegraphed to Pogson, of Madras: ``Biela touched earth; search near +Theta Centauri.'' Pogson searched in the place indicated and saw a +cometary mass retreating into the southern heavens, where it was soon +swallowed from sight! + +Since then the Biela meteors have been among the recognized periodic +spectacles of the sky, and few if any doubt that they represent a +portion of the missing comet whose disintegration began with the +separation into two parts in 1846. The comet itself has never since +been seen. The first display of these meteors, sometimes called the +``Andromedes,'' because they radiate from the constellation Andromeda, +was remarkable for the great brilliancy of many of the fire-balls that +shot among the shower of smaller sparks, some of which were described +as equaling the full moon in size. None of them is known to have +reached the earth, but during the display of the same meteors in 1885 +a meteoric mass fell at Mazapil in Northern Mexico (it is now in the +Museum at Vienna), which many have thought may actually be a piece of +the original comet of Biela. This brings us to the second branch of +our subject. + +More rare than meteors or falling stars, and more startling, except +that they never appear in showers, are the huge balls of fire which +occasionally dart through the sky, lighting up the landscapes beneath +with their glare, leaving trains of sparks behind them, often +producing peals of thunder when they explode, and in many cases +falling upon the earth and burying themselves from a few inches to +several feet in the soil, from which, more than once, they have been +picked up while yet hot and fuming. These balls are sometimes called +bolides. They are not really round in shape, although they often look +so while traversing the sky, but their forms are fragmentary, and +occasionally fantastic. It has been supposed that their origin is +different from that of the true meteors; it has even been conjectured +that they may have originated from the giant volcanoes of the moon or +have been shot out from the sun during some of the tremendous +explosions that accompany the formation of eruptive prominences. By +the same reasoning some of them might be supposed to have come from +some distant star. Others have conjectured that they are wanderers in +space, of unknown origin, which the earth encounters as it journeys +on, and Lord Kelvin made a suggestion which has become classic because +of its imaginative reach -- viz., that the first germs of life may +have been brought to the earth by one of these bodies, ``a fragment of +an exploded world.'' + +It is a singular fact that astronomers and scientific men in general +were among the last to admit the possibility of solid masses falling +from the sky. The people had believed in the reality of such phenomena +from the earliest times, but the savants shook their heads and talked +of superstition. This was the less surprising because no +scientifically authenticated instance of such an occurrence was known, +and the stones popularly believed to have fallen from the sky had +become the objects of worship or superstitious reverence, a fact not +calculated to recommend them to scientific credence. The celebrated +``black stone'' suspended in the Kaaba at Mecca is one of these +reputed gifts from heaven; the ``Palladium'' of ancient Troy was +another; and a stone which fell near Ensisheim, in Germany, was placed +in a church as an object to be religiously venerated. Many legends of +falling stones existed in antiquity, some of them curiously +transfigured by the imagination, like the ``Lion of the +Peloponnesus,'' which was said to have sprung down from the sky upon +the Isthmus of Corinth. But near the beginning of the nineteenth +century, in 1803, a veritable shower of falling stones occurred at +L'Aigle, in Northern France, and this time astronomers took note of +the phenomenon and scientifically investigated it. Thousands of the +strange projectiles came from the sky on this occasion, and were +scattered over a wide area of country, and some buildings were hit. +Four years later another shower of stones occurred at Weston, Conn., +numbering thousands of individuals. The local alarm created in both +cases was great, as well it might be, for what could be more +intimidating than to find the blue vault of heaven suddenly hurling +solid missiles at the homes of men? After these occurrences it was +impossible for the most skeptical to doubt any longer, and the regular +study of ``aerolites,'' or ``meteorites,'' began. + +One of the first things recognized was the fact that fire-balls are +solid meteorites in flight, and not gaseous exhalations in the air, as +some had assumed. They burn in the air during their flight, and +sometimes, perhaps, are entirely consumed before reaching the ground. +Their velocity before entering the earth's atmosphere is equal to that +of the planets in their orbits -- viz., from twenty to thirty miles +per second -- a fact which proves that the sun is the seat of the +central force governing them. Their burning in the air is not +difficult to explain; it is the heat of friction which so quickly +brings them to incandescence. Calculation shows that a body moving +through the air at a velocity of about a mile per second will be +brought, superficially, to the temperature of ``red heat'' by friction +with the atmosphere. If its velocity is twenty miles per second the +temperature will become thousands of degrees. This is the state of +affairs with a meteorite rushing into the earth's atmosphere; its +surface is liquefied within a few seconds after the friction begins to +act, and the melted and vaporized portion of its mass is swept +backward, forming the train of sparks that follows every great +fire-ball. However, there is one phenomenon connected with the trains +of meteorites which has never been satisfactorily explained: they +often persist for long periods of time, drifting and turning with the +wind, but not ceasing to glow with a phosphorescent luminosity. The +question is, Whence comes this light? It must be light without heat, +since the fine dust or vapor of which the train can only consist would +not retain sufficient heat to render it luminous for so long a time. +An extremely remarkable incident of this kind occurred on February 22, +1909, when an immense fire-ball that passed over southern England left +a train that remained visible during two hours, assuming many curious +shapes as it was drifted about by currents in the air. + +But notwithstanding the enormous velocity with which meteorites enter +the air they are soon slowed down to comparatively moderate speed, so +that when they disappear they are usually traveling not faster than a +mile a second. The courses of many have been traced by observers +situated along their track at various points, and thus a knowledge has +been obtained of their height above the ground during their flight and +of the length of their visible courses. They generally appear at an +elevation of eighty or a hundred miles, and are seldom visible after +having descended to within five miles of the ground, unless the +observer happens to be near the striking-point, when he may actually +witness the fall. Frequently they burst while high in the air and +their fragments are scattered like shrapnel over the surface of the +ground, sometimes covering an area of several square miles, but of +course not thickly; different fragments of the same meteorite may +reach the ground at points several miles apart. The observed length of +their courses in the atmosphere varies from fifty to five hundred +miles. If they continued a long time in flight after entering the air, +even the largest of them would probably be consumed to the last scrap, +but their fiery career is so short on account of their great speed +that the heat does not have time to penetrate very deeply, and some +that have been picked up immediately after their fall have been found +cold as ice within. Their size after reaching the ground is variable +within wide limits; some are known which weigh several tons, but the +great majority weigh only a few pounds and many only a few ounces. + +Meteorites are of two kinds: stony meteorites and iron meteorites. The +former outnumber the latter twenty to one; but many stone meteorites +contain grains of iron. Nickel is commonly found in iron meteorites, +so that it might be said that that redoubtable alloy nickel-steel is +of cosmical invention. Some twenty-five chemical elements have been +found in meteorites, including carbon and the ``sun-metal,'' helium. +The presence of the latter is certainly highly suggestive in +connection with the question of the origin of meteorites. The iron +meteorites, besides metallic iron and nickel, of which they are almost +entirely composed, contain hydrogen, helium, and carbonic oxide, and +about the only imaginable way in which these gases could have become +absorbed in the iron would be through the immersion of the latter +while in a molten or vaporized state in a hot and dense atmosphere +composed of them, a condition which we know to exist only in the +envelopes of the sun and the stars. + +The existence of carbon in the Canyon Diablo iron meteorites is +attended by a circumstance of the most singular character -- a very +``fairy tale of science.'' In some cases the carbon has become +diamond! These meteoric diamonds are very small; nevertheless, they +are true diamonds, resembling in many ways the little black gems +produced by Moissan's method with the aid of the electric furnace. The +fact that they are found embedded in these iron meteorites is another +argument in favor of the hypothesis of the solar or stellar origin of +the latter. To appreciate this it is necessary to recall the way in +which Moissan made his diamonds. It was by a combination of the +effects of great heat, great pressure, and sudden or rapid superficial +cooling on a mass of iron containing carbon. When he finally broke +open his iron he found it a pudding stuffed with miniature black +diamonds. When a fragment of the Canyon Diablo meteoric iron was +polished in Philadelphia over fifteen years ago it cut the emery-wheel +to pieces, and examination showed that the damage had been effected by +microscopic diamonds peppered through the mass. How were those +diamonds formed? If the sun or Sirius was the laboratory that prepared +them, we can get a glimpse at the process of their formation. There is +plenty of heat, plenty of pressure, and an abundance of vaporized iron +in the sun and the stars. When a great solar eruption takes place, +masses of iron which have absorbed carbon may be shot out with a +velocity which forbids their return. Plunged into the frightful cold +of space, their surfaces are quickly cooled, as Moissan cooled his +prepared iron by throwing it into water, and thus the requisite stress +is set up within, and, as the iron solidifies, the included carbon +crystallizes into diamonds. Whether this explanation has a germ of +truth in it or not, at any rate it is evident that iron meteorites +were not created in the form in which they come to us; they must once +have been parts of immeasurably more massive bodies than themselves. + +The fall of meteorites offers an appreciable, though numerically +insignificant, peril to the inhabitants of the earth. Historical +records show perhaps three or four instances of people being killed by +these bodies. But for the protection afforded by the atmosphere, which +acts as a very effective shield, the danger would doubtless be very +much greater. In the absence of an atmosphere not only would more +meteorites reach the ground, but their striking force would be +incomparably greater, since, as we have seen, the larger part of their +original velocity is destroyed by the resistance of the air. A +meteorite weighing many tons and striking the earth with a velocity of +twenty or thirty miles per second, would probably cause frightful +havoc. + +It is a singular fact that recent investigations seem to have proved +that an event of this kind actually happened in North America -- +perhaps not longer than a thousand or two thousand years ago. The +scene of the supposed catastrophe is in northern central Arizona, at +Coon Butte, where there is a nearly circular crater in the middle of a +circular elevation or small mountain. The crater is somewhat over four +thousand feet in diameter, and the surrounding rim, formed of upturned +strata and ejected rock fragments, rises at its highest point one +hundred and sixty feet above the plain. The crater is about six +hundred feet in depth -- that is, from the rim to the visible floor or +bottom of the crater. There is no evidence that volcanic action has +ever taken place in the immediate neighborhood of Coon Butte. The rock +in which the crater has been made is composed of horizontal sandstone +and limestone strata. Between three hundred and four hundred million +tons of rock fragments have been detached, and a large portion hurled +by some cause out of the crater. These fragments lie concentrically +distributed around the crater, and in large measure form the elevation +known as Coon Butte. The region has been famous for nearly twenty +years on account of the masses of meteoric iron found scattered about +and known as the ``Canyon Diablo'' meteorites. It was one of these +masses, which consist of nickel-iron containing a small quantity of +platinum, and of which in all some ten tons have been recovered for +sale to the various collectors throughout the world, that as before +mentioned destroyed the grinding-tool at Philadelphia through the +cutting power of its embedded diamonds. These meteoric irons are +scattered about the crater-hill, in concentric distribution, to a +maximum distance of about five miles. When the suggestion was first +made in 1896 that a monster meteorite might have created by its fall +this singular lone crater in stratified rocks, it was greeted with +incredulous smiles; but since then the matter has assumed a different +aspect. The Standard Iron Company, formed by Messrs. D. M. Barringer, +B. C. Tilghman, E. J. Bennitt, and S. J. Holsinger, having become, in +1903, the owner of this freak of nature, sunk shafts and bored holes +to a great depth in the interior of the crater, and also trenched the +slopes of the mountain, and the result of their investigations has +proved that the meteoric hypothesis of origin is correct. (See the +papers published in the Proceedings of the Academy of Natural Sciences +of Philadelphia, December, 1905, wherein it is proved that the United +States Geological Survey was wrong in believing this crater to have +been due to a steam explosion. Since that date there has been +discovered a great amount of additional confirmatory proof). Material +of unmistakably meteoric origin was found by means of the drills, +mixed with crushed rock, to a depth of six hundred to seven hundred +feet below the floor of the crater, and a great deal of it has been +found admixed with the ejected rock fragments on the outer slopes of +the mountain, absolutely proving synchronism between the two events, +the formation of this great crater and the falling of the meteoric +iron out of the sky. The drill located in the bottom of the crater was +sent, in a number of cases, much deeper (over one thousand feet) into +unaltered horizontal red sandstone strata, but no meteoric material +was found below this depth (seven hundred feet, or between eleven and +twelve hundred feet below the level of the surrounding plain), which +has been assumed as being about the limit of penetration. It is not +possible to sink a shaft at present, owing to the water which has +drained into the crater, and which forms, with the finely pulverized +sandstone, a very troublesome quicksand encountered at about two +hundred feet below the visible floor of the crater. As soon as this +water is removed by pumping it will be easy to explore the depths of +the crater by means of shafts and drifts. The rock strata (sandstone +and limestone) of which the walls consist present every appearance of +having been violently upturned by a huge body penetrating the earth +like a cannon-ball. The general aspect of the crater strikingly +resembles the impression made by a steel projectile shot into an +armor-plate. Mr Tilghman has estimated that a meteorite about five +hundred feet in diameter and moving with a velocity of about five +miles per second would have made just such a perforation upon striking +rocks of the character of those found at this place. There was some +fusion of the colliding masses, and the heat produced some steam from +the small amount of water in the rocks. As a result there has been +found at depth a considerable amount of fused quartz (original +sandstone), and with it innumerable particles or sparks of fused +nickel-iron (original meteorite). A projectile of that size +penetrating eleven to twelve hundred feet into the rocky shell of the +globe must have produced a shock which was perceptible several hundred +miles away. + +The great velocity ascribed to the supposed meteorite at the moment of +striking could be accounted for by the fact that it probably plunged +nearly vertically downward, for it formed a circular crater in the +rocky crust of the earth. In that case it would have been less +retarded by the resistance of the atmosphere than are meteorites which +enter the air at a lower angle and shoot ahead hundreds of miles until +friction has nearly destroyed their original motion when they drop +upon the earth. Some meteoric masses of great size, such as Peary's +iron meteorite found at Cape York, Greenland, and the almost equally +large mass discovered at Bacubirito, Mexico, appear to have penetrated +but slightly on striking the earth. This may be explained by supposing +that they pursued a long, horizontal course through the air before +falling. The result would be that, their original velocity having been +practically destroyed, they would drop to the ground with a velocity +nearly corresponding to that which gravity would impart within the +perpendicular distance of their final fall. A +six-hundred-and-sixty-pound meteorite, which fell at Knyahinya, +Hungary, striking at an angle of 27 deg. from the vertical, penetrated the +ground to a depth of eleven feet. + +It has been remarked that the Coon Butte meteorite may have fallen not +longer ago than a few thousand years. This is based upon the fact that +the geological indications favor the supposition that the event did +not occur more than five thousand years ago, while on the other hand +the rings of growth in the cedar-trees growing on the slopes of the +crater show that they have existed there about seven hundred years. +Prof. William H. Pickering has recently correlated this with an +ancient chronicle which states that at Cairo, Egypt, in the year 1029, +``many stars passed with a great noise.'' He remarks that Cairo is +about 100 deg., by great circle, from Coon Butte, so that if the meteorite +that made the crater was a member of a flock of similar bodies which +encountered the earth moving in parallel lines, some of them might +have traversed the sky tangent to the earth's surface at Cairo. That +the spectacle spoken of in the chronicle was caused by meteorites he +deems exceedingly probable because of what is said about ``a great +noise;'' meteorites are the only celestial phenomena attended with +perceptible sounds. Professor Pickering conjectures that this supposed +flock of great meteorites may have formed the nucleus of a comet which +struck the earth, and he finds confirmation of the idea in the fact +that out of the ten largest meteorites known, no less than seven were +found within nine hundred miles of Coon Butte. It would be interesting +if we could trace back the history of that comet, and find out what +malicious planet caught it up in its innocent wanderings and hurled it +with so true an aim at the earth! This remarkable crater is one of the +most interesting places in the world, for there is absolutely no +record of such a mass, possibly an iron-headed comet, from outer space +having come into collision with our earth. The results of the future +exploration of the depths of the crater will be awaited with much +interest. + +The Wrecking of the Moon + +There are sympathetic moods under whose influence one gazes with a +certain poignant tenderness at the worn face of the moon; that little +``fossil world'' (the child of our mother earth, too) bears such +terrible scars of its brief convulsive life that a sense of pity is +awakened by the sight. The moon is the wonder-land of the telescope. +Those towering mountains, whose ``proud aspiring peaks'' cast +silhouettes of shadow that seem drawn with india-ink; those vast +plains, enchained with gentle winding hills and bordered with giant +ranges; those oval ``oceans,'' where one looks expectant for the flash +of wind-whipped waves; those enchanting ``bays'' and recesses at the +seaward feet of the Alps; those broad straits passing between guardian +heights incomparably mightier than Gibraltar; those locket-like +valleys as secluded among their mountains as the Vale of Cashmere; +those colossal craters that make us smile at the pretensions of +Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with +the unconcern of Roman roads across mountain, gorge, and valley -- all +these give the beholder an irresistible impression that it is truly a +world into which he is looking, a world akin to ours, and yet no more +like our world than Pompeii is like Naples. Its air, its waters, its +clouds, its life are gone, and only a skeleton remains -- a mute but +eloquent witness to a cosmical tragedy without parallel in the range +of human knowledge. + +One cannot but regret that the moon, if it ever was the seat of +intelligent life, has not remained so until our time. Think what the +consequences would have been if this other world at our very door had +been found to be both habitable and inhabited! We talk rather airily +of communicating with Mars by signals; but Mars never approaches +nearer than 35,000,000 miles, while the moon when nearest is only a +little more than 220,000 miles away. Given an effective magnifying +power of five thousand diameters, which will perhaps be possible at +the mountain observatories as telescopes improve, and we should be +able to bring the moon within an apparent distance of about forty +miles, while the corresponding distance for Mars would be more than +seven thousand miles. But even with existing telescopic powers we can +see details on the moon no larger than some artificial constructions +on the earth. St Peter's at Rome, with the Vatican palace and the +great piazza, if existing on the moon, would unquestionably be +recognizable as something else than a freak of nature. Large cities, +with their radiating lines of communication, would at once betray +their real character. Cultivated tracts, and the changes produced by +the interference of intelligent beings, would be clearly recognizable. +The electric illumination of a large town at night would probably be +markedly visible. Gleams of reflected sunlight would come to us from +the surfaces of the lakes and oceans, and a huge ``liner'' traversing +a lunar sea could probably be followed by its trail of smoke. As to +communications by ``wireless'' signals, which certain enthusiasts have +thought of in connection with Mars, in the case of the moon they +should be a relatively simple matter, and the feat might actually be +accomplished. Think what a literature would grow up about the moon if +it were a living world! Its very differences from the earth would only +accentuate its interest for us. Night and day on the moon are each two +weeks in length; how interesting it would be to watch the manner in +which the lunarians dealt with such a situation as that. Lunar and +terrestrial history would keep step with each other, and we should +record them both. Truly one might well wish to have a neighbor world +to study; one would feel so much the less alone in space. + +It is not impossible that the moon did at one time have inhabitants of +some kind. But, if so, they vanished with the disappearance of its +atmosphere and seas, or with the advent of its cataclysmic age. At the +best, its career as a living world must have been brief. If the water +and air were gradually absorbed, as some have conjectured, by its +cooling interior rocks, its surface might, nevertheless, have retained +them for long ages; but if, as others think, their disappearance was +due to the escape of their gaseous molecules in consequence of the +inability of the relatively small lunar gravitation to retain them, +then the final catastrophe must have been as swift as it was +inevitable. Accepting Darwin's hypothesis, that the moon was separated +from the earth by tidal action while both were yet plastic or +nebulous, we may reasonably conclude that it began its career with a +good supply of both water and air, but did not possess sufficient mass +to hold them permanently. Yet it may have retained them long enough +for life to develop in many forms upon its surface; in fact, there are +so many indications that air and water have not always been lacking to +the lunar world that we are driven to invent theories to explain both +their former presence and their present absence. + +But whatever the former condition of the moon may have been, its +existing appearance gives it a resistless fascination, and it bears so +clearly the story of a vast catastrophe sculptured on its rocky face +that the thoughtful observer cannot look upon it without a feeling of +awe. The gigantic character of the lunar features impresses the +beholder not less than the universality of the play of destructive +forces which they attest. Let us make a few comparisons. Take the +lunar crater called ``Tycho'', which is a typical example of its kind. +In the telescope Tycho appears as a perfect ring surrounding a +circular depression, in the center of which rises a group of +mountains. Its superficial resemblance to some terrestrial volcanic +craters is very striking. Vesuvius, seen from a point vertically +above, would no doubt look something like that (the resemblance would +have been greater when the Monte del Cavallo formed a more complete +circuit about the crater cone). But compare the dimensions. The +remains of the outer crater ring of Vesuvius are perhaps half a mile +in diameter, while the active crater itself is only two or three +hundred feet across at the most; Tycho has a diameter of fifty-four +miles! The group of relatively insignificant peaks in the center of +the crater floor of Tycho is far more massive than the entire mountain +that we call Vesuvius. The largest known volcanic crater on the earth, +Aso San, in Japan, has a diameter of seven miles; it would take sixty +craters like Aso San to equal Tycho in area! And Tycho, though one of +the most perfect, is by no means the largest crater on the moon. +Another, called ``Theophilus,'' has a diameter of sixty-four miles, +and is eighteen thousand feet deep. There are hundreds from ten to +forty miles in diameter, and thousands from one to ten miles. They are +so numerous in many places that they break into one another, like the +cells of a crushed honeycomb. + +The lunar craters differ from those of the earth more fundamentally +than in the matter of mere size; they are not situated on the tops of +mountains. If they were, and if all the proportions were the same, a +crater like Tycho might crown a conical peak fifty or one hundred +miles high! Instead of being cavities in the summits of mountains, the +lunar craters are rather gigantic sink-holes whose bottoms in many +cases lie two or three miles below the general surface of the lunar +world. Around their rims the rocks are piled up to a height of from a +few hundred to two or three thousand feet, with a comparatively gentle +inclination, but on the inner side they fall away in gigantic broken +precipices which make the dizzy cliffs of the Matterhorn seem but +``lover's leaps.'' Down they drop, ridge below ridge, crag under crag, +tottering wall beneath wall, until, in a crater named ``Newton,'' near +the south lunar pole, they attain a depth where the rays of the sun +never reach. Nothing more frightful than the spectacle which many of +these terrible chasms present can be pictured by the imagination. As +the lazy lunar day slowly advances, the sunshine, unmitigated by +clouds or atmospheric veil of any kind, creeps across their rims and +begins to descend the opposite walls. Presently it strikes the ragged +crest of a ridge which had lain hidden in such darkness as we never +know on the earth, and runs along it like a line of kindling fire. +Rocky pinnacles and needles shoot up into the sunlight out of the +black depths. Down sinks the line of light, mile after mile, and +continually new precipices and cliffs are brought into view, until at +last the vast floor is attained and begins to be illuminated. In the +meanwhile the sun's rays, darting across the gulf, have touched the +summits of the central peaks, twenty or thirty miles from the crater's +inmost edge, and they immediately kindle and blaze like huge stars +amid the darkness. So profound are some of these awful craters that +days pass before the sun has risen high enough above them to chase the +last shadows from their depths. + +Although several long ranges of mountains resembling those of the +earth exist on the moon, the great majority of its elevations assume +the crateriform aspect. Sometimes, instead of a crater, we find an +immense mountain ring whose form and aspect hardly suggest volcanic +action. But everywhere the true craters are in evidence, even on the +sea-beds, although they attain their greatest number and size on those +parts of the moon -- covering sixty per cent of its visible surface -- +which are distinctly mountainous in character and which constitute its +most brilliant portions. Broadly speaking, the southwestern half of +the moon is the most mountainous and broken, and the northeastern half +the least so. Right down through the center, from pole to pole, runs a +wonderful line of craters and crateriform valleys of a magnitude +stupendous even for the moon. Another similar line follows the western +edge. Three or four ``seas'' are thrust between these mountainous +belts. By the effects of ``libration'' parts of the opposite +hemisphere of the moon which is turned away from the earth are from +time to time brought into view, and their aspect indicates that that +hemisphere resembles in its surface features the one which faces the +earth. There are many things about the craters which seem to give some +warrant for the hypothesis which has been particularly urged by Mr G. +K. Gilbert, that they were formed by the impact of meteors; but there +are also many things which militate against that idea, and, upon the +whole, the volcanic theory of their origin is to be preferred. + +The enormous size of the lunar volcanoes is not so difficult to +account for when we remember how slight is the force of lunar gravity +as compared with that of the earth. With equal size and density, +bodies on the moon weigh only one-sixth as much as on the earth. +Impelled by the same force, a projectile that would go ten miles on +the earth would go sixty miles on the moon. A lunar giant thirty-five +feet tall would weigh no more than an ordinary son of Adam weighs on +his greater planet. To shoot a body from the earth so that it would +not drop back again, we should have to start it with a velocity of +seven miles per second; a mile and a half per second would serve on +the moon. It is by no means difficult to believe, then, that a lunar +volcano might form a crater ring eight or ten times broader than the +greatest to be found on the earth, especially when we reflect that in +addition to the relatively slight force of gravity, the materials of +the lunar crust are probably lighter than those of our terrestrial +rocks. + +For similar reasons it seems not impossible that the theory mentioned +in a former chapter -- that some of the meteorites that have fallen +upon the earth originated from the lunar volcanoes -- is well founded. +This would apply especially to the stony meteorites, for it is hardly +to be supposed that the moon, at least in its superficial parts, +contains much iron. It is surely a scene most strange that is thus +presented to the mind's eye -- that little attendant of the earth's +(the moon has only one-fiftieth of the volume, and only one-eightieth +of the mass of the earth) firing great stones back at its parent +planet! And what can have been the cause of this furious outbreak of +volcanic forces on the moon? Evidently it was but a passing stage in +its history; it had enjoyed more quiet times before. As it cooled down +from the plastic state in which it parted from the earth, it became +incrusted after the normal manner of a planet, and then oceans were +formed, its atmosphere being sufficiently dense to prevent the water +from evaporating and the would-be oceans from disappearing continually +in mist. This, if any, must have been the period of life in the lunar +world. As we look upon the vestiges of that ancient world buried in +the wreck that now covers so much of its surface, it is difficult to +restrain the imagination from picturing the scenes which were once +presented there; and, in such a case, should the imagination be +fettered? We give it free rein in terrestrial life, and it rewards us +with some of our greatest intellectual pleasures. The wonderful +landscapes of the moon offer it an ideal field with just enough +half-hidden suggestions of facts to stimulate its powers. + +The great plains of the Mare Imbrium and the Mare Serenitatis (the +``Sea of Showers'' and the ``Sea of Serenity''), bordered in part by +lofty mountain ranges precisely like terrestrial mountains, scalloped +along their shores with beautiful bays curving back into the adjoining +highlands, and united by a great strait passing between the nearly +abutting ends of the ``Lunar Apennines'' and the ``Lunar Caucasus,'' +offer the elements of a scene of world beauty such as it would be +difficult to match upon our planet. Look at the finely modulated +bottom of the ancient sea in Mr Ritchey's exquisite photograph of the +western part of the Mare Serenitatis, where one seems to see the play +of the watery currents heaping the ocean sands in waving lines, making +shallows, bars, and deeps for the mariner to avoid or seek, and +affording a playground for the creatures of the main. What geologist +would not wish to try his hammer on those rocks with their stony pages +of fossilized history? There is in us an instinct which forbids us to +think that there was never any life there. If we could visit the moon, +there is not among us a person so prosaic and unimaginative that he +would not, the very first thing, begin to search for traces of its +inhabitants. We would look for them in the deposits on the sea +bottoms; we would examine the shores wherever the configuration seemed +favorable for harbors and the sites of maritime cities -- forgetting +that it may be a little ridiculous to ascribe to the ancient lunarians +the same ideas that have governed the development of our race; we +would search through the valleys and along the seeming courses of +vanished streams; we would explore the mountains, not the terrible +craters, but the pinnacled chains that recall our own Alps and +Rockies; seeking everywhere some vestige of the transforming presence +of intelligent life. Perhaps we should find such traces, and perhaps, +with all our searching, we should find nothing to suggest that life +had ever existed amid that universal ruin. + +Look again at the border of the ``Sea of Serenity'' -- what a name for +such a scene! -- and observe how it has been rent with almost +inconceivable violence, the wall of the colossal crater Posidonius +dropping vertically upon the ancient shore and obliterating it, while +its giant neighbor, Le Monnier, opens a yawning mouth as if to swallow +the sea itself. A scene like this makes one question whether, after +all, those may not be right who have imagined that the so-called sea +bottoms are really vast plains of frozen lava which gushed up in +floods so extensive that even the mighty volcanoes were half drowned +in the fiery sea. This suggestion becomes even stronger when we turn +to another of the photographs of Mr Ritchey's wonderful series, +showing a part of the Mare Tranquilitatis (``Sea of Tranquility''!). +Notice how near the center of the picture the outline of a huge ring +with radiating ridges shows through the sea bottom; a fossil volcano +submerged in a petrified ocean! This is by no means the only instance +in which a buried world shows itself under the great lunar plains. +Yet, as the newer craters in the sea itself prove, the volcanic +activity survived this other catastrophe, or broke out again +subsequently, bringing more ruin to pile upon ruin. + +Yet notwithstanding the evidence which we have just been considering +in support of the hypothesis that the ``seas'' are lava floods, +Messrs. Loewy and Puiseux, the selenographers of the Paris +Observatory, are convinced that these great plains bear characteristic +marks of the former presence of immense bodies of water. In that case +we should be forced to conclude that the later oceans of the moon lay +upon vast sheets of solidified lava; and thus the catastrophe of the +lunar world assumes a double aspect, the earliest oceans being +swallowed up in molten floods issuing from the interior, while the +lands were reduced to chaos by a universal eruption of tremendous +volcanoes; and then a period of comparative quiet followed, during +which new seas were formed, and new life perhaps began to flourish in +the lunar world, only to end in another cataclysm, which finally put a +term to the existence of the moon as a life-supporting world. + +Suppose we examine two more of Mr Ritchey's illuminating photographs, +and, first, the one showing the crater Theophilus and its +surroundings. We have spoken of Theophilus before, citing the facts +that it is sixty-four miles in diameter and eighteen thousand feet +deep. It will be noticed that it has two brother giants -- Cyrillus +the nearer, and Catharina the more distant; but Theophilus is plainly +the youngest of the trio. Centuries, and perhaps thousands of years, +must have elapsed between the periods of their upheaval, for the two +older craters are partly filled with debris, while it is manifest at a +glance that when the south eastern wall of Theophilus was formed, it +broke away and destroyed a part of the more ancient ring of Cyrillus. +There is no more tremendous scene on the moon than this; viewed with a +powerful telescope, it is absolutely appalling. + +The next photograph shows, if possible, a still wilder region. It is +the part of the moon lying between Tycho and the south pole. Tycho is +seen in the lower left-hand part of the picture. To the right, at the +edge of the illuminated portion of the moon, are the crater-rings, +Longomontanus and Wilhelm I, the former being the larger. Between them +are to be seen the ruins of two or three more ancient craters which, +together with portions of the walls of Wilhelm I and Longomontanus, +have been honeycombed with smaller craters. The vast crateriform +depression above the center of the picture is Clavius, an unrivaled +wonder of lunar scenery, a hundred and forty-two miles in its greatest +length, while its whole immense floor has sunk two miles below the +general surface of the moon outside the ring. The monstrous +shadow-filled cavity above Clavius toward the right is Blancanus, +whose aspect here gives a good idea of the appearance of these chasms +when only their rims are in the sunlight. But observe the +indescribable savagery of the entire scene. It looks as though the +spirit of destruction had gone mad in this spot. The mighty craters +have broken forth one after another, each rending its predecessor; and +when their work was finished, a minor but yet tremendous outbreak +occurred, and the face of the moon was gored and punctured with +thousands of smaller craters. These relatively small craters (small, +however, only in a lunar sense, for many of them would appear gigantic +on the earth) recall once more the theory of meteoric impact. It does +not seem impossible that some of them may have been formed by such an +agency. + +One would not wish for our planet such a fate as that which has +overtaken the moon, but we cannot be absolutely sure that something of +the kind may not be in store for it. We really know nothing of the +ultimate causes of volcanic activity, and some have suggested that the +internal energies of the earth may be accumulating instead of dying +out, and may never yet have exhibited their utmost destructive power. +Perhaps the best assurance that we can find that the earth will escape +the catastrophe that has overtaken its satellite is to be found in the +relatively great force of its gravitation. The moon has been the +victim of its weakness; given equal forces, and the earth would be the +better able to withstand them. It is significant, in connection with +these considerations, that the little planet Mercury, which seems also +to have parted with its air and water, shows to the telescope some +indications that it is pitted with craters resembling those that have +torn to pieces the face of the moon. + +Upon the whole, after studying the dreadful lunar landscapes, one +cannot feel a very enthusiastic sympathy with those who are seeking +indications of the continued existence of some kind of life on the +moon; such a world is better without inhabitants. It has met its fate; +let it go! Fortunately, it is not so near that it cannot hide its +scars and appear beautiful -- except when curiosity impels us to look +with the penetrating eyes of the astronomer. + +The Great Mars Problem + +Let any thoughtful person who is acquainted with the general facts of +astronomy look up at the heavens some night when they appear in their +greatest splendor, and ask himself what is the strongest impression +that they make upon his mind. He may not find it easy to frame an +answer, but when he has succeeded it will probably be to the effect +that the stars give him an impression of the universality of +intelligence; they make him feel, as the sun and the moon cannot do, +that his world is not alone; that all this was not made simply to form +a gorgeous canopy over the tents of men. If he is of a devout turn of +mind, he thinks, as he gazes into those fathomless deeps and among +those bewildering hosts, of the infinite multitude of created beings +that the Almighty has taken under his care. The narrow ideas of the +old geocentric theology, which made the earth God's especial +footstool, and man his only rational creature, fall away from him like +a veil that had obscured his vision; they are impossible in the +presence of what he sees above. Thus the natural tendency, in the +light of modern progress, is to regard the universe as everywhere +filled with life. + +But science, which is responsible for this broadening of men's +thoughts concerning the universality of life, itself proceeds to set +limits. Of spiritual existences it pretends to know nothing, but as to +physical beings, it declares that it can only entertain the +supposition of their existence where it finds evidence of an +environment suited to their needs, and such environment may not +everywhere exist. Science, though repelled by the antiquated +theological conception of the supreme isolation of man among created +beings, regards with complacency the probability that there are +regions in the universe where no organic life exists, stars which +shine upon no inhabited worlds, and planets which nourish no animate +creatures. The astronomical view of the universe is that it consists +of matter in every stage of evolution: some nebulous and chaotic; some +just condensing into stars (suns) of every magnitude and order; some +shaped into finished solar bodies surrounded by dependent planets; +some forming stars that perhaps have no planets, and will have none; +some constituting suns that are already aging, and will soon lose +their radiant energy and disappear; and some aggregated into masses +that long ago became inert, cold, and rayless, and that can only be +revivified by means about which we can form conjectures, but of which +we actually know nothing. + +As with the stars, so with the planets, which are the satellites of +stars. All investigations unite to tell us that the planets are not +all in the same state of development. As some are large and some +small, so some are, in an evolutionary sense, young, and some old. As +they depend upon the suns around which they revolve for their light, +heat, and other forms of radiant energy, so their condition varies +with their distance from those suns. Many may never arrive at a state +suitable for the maintenance of life upon their surfaces; some which +are not at present in such a state may attain it later; and the forms +of life themselves may vary with the peculiar environment that +different planets afford. Thus we see that we are not scientifically +justified in affirming that life is ubiquitous, although we are thus +justified in saying that it must be, in a general sense, universal. We +might liken the universe to a garden known to contain every variety of +plant. If on entering it we see no flowers, we examine the species +before us and find that they are not of those which bloom at this +particular season, or perhaps they are such as never bear flowers. Yet +we feel no doubt that we shall find flowers somewhere in the garden, +because there are species which bloom at this season, and the garden +contains all varieties. + +While it is tacitly assumed that there are planets revolving around +other stars than the sun, it would be impossible for us to see them +with any telescope yet invented, and no instrument now in the +possession of astronomers could assure us of their existence; so the +only planetary system of which we have visual knowledge is our own. +Excluding the asteroids, which could not from any point of view be +considered as habitable, we have in the solar system eight planets of +various sizes and situated at various distances from the sun. Of these +eight we know that one, the earth, is inhabited. The question, then, +arises: Are there any of the others which are inhabited or habitable? +Since it is our intention to discuss the habitability of only one of +the seven to which the question applies, the rest may be dismissed in +a few words. The smallest of them, and the nearest to the sun, is +Mercury, which is regarded as uninhabitable because it has no +perceptible supply of water and air, and because, owing to the +extraordinary eccentricity of its orbit, it is subjected to excessive +and very rapid alterations in the amount of solar heat and light +poured upon its surface, such alterations being inconsistent with the +supposition that it can support living beings. Even its average +temperature is more than six and a half times that prevailing on the +earth! Another circumstance which militates against its habitability +is that, according to the results of the best telescopic studies, it +always keeps the same face toward the sun, so that one half of the +planet is perpetually exposed to the fierce solar rays, and the other +half faces the unmitigated cold of open space. Venus, the next in +distance from the sun, is almost the exact twin of the earth in size, +and many arguments may be urged in favor of its habitability, although +it is suspected of possessing the same peculiarity as Mercury, in +always keeping the same side sunward. Unfortunately its atmosphere +appears to be so dense that no permanent markings on its surface are +certainly visible, and the question of its actual condition must, for +the present, be left in abeyance. Mars, the first planet more distant +from the sun than the earth, is the special subject of this chapter, +and will be described and discussed a few lines further on. Jupiter, +Saturn, Uranus, and Neptune, the four giant planets, all more distant +than Mars, and each more distant than the other in the order named, +are all regarded as uninhabitable because none of them appears to +possess any degree of solidity. They may have solid or liquid nuclei, +but exteriorly they seem to be mere balls of cloud. Of course, one can +imagine what he pleases about the existence of creatures suited to the +physical constitution of such planets as these, but they must be +excluded from the category of habitable worlds in the ordinary sense +of the term. We go back, then, to Mars. + +It will be best to begin with a description of the planet. Mars is +4230 miles in diameter; its surface is not much more than one-quarter +as extensive as that of the earth (.285). Its mean distance from the +sun is 141,500,000 miles, 48,500,000 miles greater than that of the +earth. Since radiant energy varies inversely as the square of +distance, Mars receives less than half as much solar light and heat as +the earth gets. Mars' year (period of revolution round the sun) is 687 +days. Its mean density is 71 per cent of the earth's, and the force of +gravity on its surface is 38 per cent of that on the surface of the +earth; i.e., a body weighing one hundred pounds on the earth would, if +transported to Mars, weigh but thirty-eight pounds. The inclination of +its equator to the plane of its orbit differs very little from that of +the earth's equator, and its axial rotation occupies 24 hours 37 +minutes. so that the length of day and night, and the extent of the +seasonal changes on Mars, are almost precisely the same as on the +earth. But owing to the greater length of its year, the seasons of +Mars, while occurring in the same order, are almost twice as long as +ours. The surface of the planet is manifestly solid, like that of our +globe, and the telescope reveals many permanent markings on it, +recalling the appearance of a globe on which geographical features +have been represented in reddish and dusky tints. Around the poles are +plainly to be seen rounded white areas, which vary in extent with the +Martian seasons, nearly vanishing in summer and extending widely in +winter. The most recent spectroscopic determinations indicate that +Mars has an atmosphere perhaps as dense as that to be found on our +loftiest mountain peaks, and there is a perceptible amount of watery +vapor in this atmosphere. The surface of the planet appears to be +remarkably level, and it has no mountain ranges. No evidences of +volcanic action have been discovered on Mars. The dusky and reddish +areas were regarded by the early observers as respectively seas and +lands, but at present it is not believed that there are any bodies of +water on the planet. There has never been much doubt expressed that +the white areas about the poles represent snow. + +It will be seen from this brief description that many remarkable +resemblances exist between Mars and the earth, and there is nothing +wonderful in the fact that the question of the habitability of the +former has become one of extreme and wide-spread interest, giving rise +to the most diverse views, to many extraordinary speculations, and +sometimes to regrettably heated controversy. The first champion of the +habitability of Mars was Sir William Herschel, although even before +his time the idea had been suggested. He was convinced by the +revelations of his telescopes, continually increasing in power, that +Mars was more like the earth than any other planet. He could not +resist the testimony of the polar snows, whose suggestive conduct was +in such striking accord with what occurs upon the earth. Gradually, as +telescopes improved and observers increased in number, the principal +features of the planet were disclosed and charted, and ``areography,'' +as the geography of Mars was called, took its place among the +recognized branches of astronomical study. But it was not before 1877 +that a fundamentally new discovery in areography gave a truly +sensational turn to speculation about life on ``the red planet.'' In +that year Mars made one of its nearest approaches to the earth, and +was so situated in its orbit that it could be observed to great +advantage from the northern hemisphere of the earth. The celebrated +Italian astronomer, Schiaparelli, took advantage of this opportunity +to make a trigonometrical survey of the surface of Mars -- as coolly +and confidently as if he were not taking his sights across a +thirty-five-million-mile gulf of empty space -- and in the course of +this survey he was astonished to perceive that the reddish areas, then +called continents, were crossed in many directions by narrow, dusky +lines, to which he gave the suggestive name of ``canals.'' Thus a kind +of firebrand was cast into the field of astronomical speculation, +which has ever since produced disputes that have sometimes approached +the violence of political faction. At first the accuracy of +Schiaparelli's observations was contested; it required a powerful +telescope, and the most excellent ``seeing,'' to render the +enigmatical lines visible at all, and many searchers were unable to +detect them. But Schiaparelli continued his studies in the serene sky +of Italy, and produced charts of the gridironed face of Mars +containing so much astonishing detail that one had either to reject +them in toto or to confess that Schiaparelli was right. As subsequent +favorable oppositions of Mars occurred, other observers began to see +the ``canals'' and to confirm the substantial accuracy of the Italian +astronomer's work, and finally few were found who would venture to +affirm that the ``canals'' did not exist, whatever their meaning might +be. + +When Schiaparelli began his observations it was generally believed, as +we have said, that the dusky areas on Mars were seas, and since +Schiaparelli thought that the ``canals'' invariably began and ended at +the shores of the ``seas,'' the appropriateness of the title given to +the lines seemed apparent. Their artificial character was immediately +assumed by many, because they were too straight and too suggestively +geometrical in their arrangement to permit the conclusion that they +were natural watercourses. A most surprising circumstance noted by +Schiaparelli was that the ``canals'' made their appearance after the +melting of the polar snow in the corresponding hemisphere had begun, +and that they grew darker, longer, and more numerous in proportion as +the polar liquidation proceeded; another very puzzling observation was +that many of them became double as the season advanced; close beside +an already existing ``canal,'' and in perfect parallelism with it, +another would gradually make its appearance. That these phenomena +actually existed and were not illusions was proved by later +observations, and today they are seen whenever Mars is favorably +situated for observation. + +In the closing decade of the nineteenth century, Mr Percival Lowell +took up the work where Schiaparelli had virtually dropped it, and soon +added a great number of ``canals'' to those previously known, so that +in his charts the surface of the wonderful little planet appears +covered as with a spider's web, the dusky lines criss-crossing in +every direction, with conspicuous knots wherever a number of them come +together. Mr Lowell has demonstrated that the areas originally called +seas, and thus named on the earlier charts, are not bodies of water, +whatever else they may be. He has also found that the mysterious lines +do not, as Schiaparelli supposed, begin and end at the edges of the +dusky regions, but often continue on across them, reaching in some +cases far up into the polar regions. But Schiaparelli was right in his +observation that the appearance of the ``canals'' is synchronous with +the gradual disappearance of the polar snows, and this fact has become +the basis of the most extraordinary theory that the subject of life in +other worlds has ever given birth to. + +Now, the effect of such discoveries, as we have related, depends upon +the type of mind to whose attention they are called. Many are content +to accept them as strange and inexplicable at present, and to wait for +further light upon them; others insist upon an immediate inquiry +concerning their probable nature and meaning. Such an inquiry can only +be based upon inference proceeding from analogy. Mars, say Mr Lowell +and those who are of his opinion, is manifestly a solidly incrusted +planet like the earth; it has an atmosphere, though one of great +rarity; it has water vapor, as the snows in themselves prove; it has +the alternation of day and night, and a succession of seasons closely +resembling those of the earth; its surface is suggestively divided +into regions of contrasting colors and appearance, and upon that +surface we see an immense number of lines geometrically arranged, with +a system of symmetrical intersections where the lines expand into +circular and oval areas -- and all connected with the annual melting +of the polar snows in a way which irresistibly suggests the +interference of intelligence directed to a definite end. Why, with so +many concurrent circumstances to support the hypothesis, should we not +regard Mars as an inhabited globe? + +But the differences between Mars and the earth are in many ways as +striking as their resemblances. Mars is relatively small; it gets less +than half as much light and heat as we receive; its atmosphere is so +rare that it would be distressing to us, even if we could survive in +it at all; it has no lakes, rivers, or seas; its surface is an endless +prairie. and its ``canals'' are phenomena utterly unlike anything on +the earth. Yet it is precisely upon these divergences between the +earth and Mars, this repudiation of terrestrial standards, that the +theory of ``life on Mars,'' for which Mr Lowell is mainly responsible, +is based. Because Mars is smaller than the earth, we are told it must +necessarily be more advanced in planetary evolution, the underlying +cause of which is the gradual cooling and contraction of the planet's +mass. Mars has parted with its internal heat more rapidly than the +earth; consequently its waters and its atmosphere have been mostly +withdrawn by chemical combinations, but enough of both yet remain to +render life still possible on its surface. As the globe of Mars is +evolutionally older than that of the earth, so its forms of organic +life may be proportionally further advanced, and its inhabitants may +have attained a degree of cultivated intelligence much superior to +what at present exists upon the earth. Understanding the nature and +the causes of the desiccation of their planet, and possessing +engineering science and capabilities far in advance of ours, they may +be conceived to have grappled with the stupendous problem of keeping +their world in a habitable condition as long as possible. Supposing +them to have become accustomed to live in their rarefied atmosphere (a +thing not inconceivable, since men can live for a time at least in air +hardly less rare), the most pressing problem for them is that of a +water-supply, without which plant life cannot exist, while animal life +in turn depends for its existence upon vegetation. The only direction +in which they can seek water is that of the polar regions, where it is +alternately condensed into snow and released in the liquid form by the +effect of the seasonal changes. It is, then, to the annual melting of +the polar snow-fields that the Martian engineers are supposed to have +recourse in supplying the needs of their planet, and thus providing +the means of prolonging their own existence. It is imagined that they +have for this purpose constructed a stupendous system of irrigation +extending over the temperate and equatorial regions of the planet. The +``canals'' represent the lines of irrigation, but the narrow streaks +that we see are not the canals themselves, but the irrigated bands +covered by them. Their dark hue, and their gradual appearance after +the polar melting has begun, are due to the growth of vegetation +stimulated by the water. The rounded areas visible where several +``canals'' meet and cross are called by Mr Lowell ``oases.'' These are +supposed to be the principal centers of population and industry. It +must be confessed that some of them, with their complicated systems of +radiating lines, appear to answer very well to such a theory. No +attempt to explain them by analogy with natural phenomena on the earth +has proved successful. + +But a great difficulty yet remains: How to explain the seemingly +miraculous powers of the supposed engineers? Here recourse is had once +more to the relative smallness of the planet. We have remarked that +the force of gravity on Mars is only thirty-eight per cent of that on +the earth. A steam-shovel driven by a certain horse-power would be +nearly three times as effective there as here. A man of our stature on +Mars would find his effective strength increased in the same +proportion. But just because of the slight force of gravity there, a +Martian might attain to the traditional stature of Goliath without +finding his own weight an encumbrance to his activity, while at the +same time his huge muscles would come into unimpeded play, enabling +him single-handed to perform labors that would be impossible to a +whole gang of terrestrial workmen. The effective powers of huge +machines would be increased in the same way; and to all this must be +added the fact that the mean density of the materials of which Mars is +composed is much less than that of the constituents of the earth. +Combining all these considerations, it becomes much less difficult to +conceive that public works might be successfully undertaken on Mars +which would be hopelessly beyond the limits of human accomplishment. + +Certain other difficulties have also to be met; as, for instance, the +relative coldness of the climate of Mars. At its distance it gets +considerably less than half as much light and heat as we receive. In +addition to this, the rarity of its atmosphere would naturally be +expected to decrease the effective temperature at the planet's +surface, since an atmosphere acts somewhat like the glass cover of a +hot-house in retaining the solar heat which has penetrated it. It has +been calculated that, unless there are mitigating circumstances of +which we know nothing, the average temperature at the surface of Mars +must be far below the freezing-point of water. To this it is replied +that the possible mitigating circumstances spoken of evidently exist +in fact, because we can see that the watery vapor condenses into snow +around the poles in winter, but melts again when summer comes. The +mitigating agent may be supposed to exist in the atmosphere where the +presence of certain gases would completely alter the temperature +gradients. + +It might also be objected that it is inconceivable that the Martian +engineers, however great may be their physical powers, and however +gigantic the mechanical energies under their control, could force +water in large quantities from the poles to the equator. This is an +achievement that measures up to the cosmical standard. It is admitted +by the champions of the theory that the difficulty is a formidable +one; but they call attention to the singular fact that on Mars there +can be found no chains of mountains, and it is even doubtful if ranges +of hills exist there. The entire surface of the planet appears to be +almost ``as smooth as a billiard ball,'' and even the broad regions +which were once supposed to be seas apparently lie at practically the +same level as the other parts, since the ``canals'' in many cases run +uninterruptedly across them. Lowell's idea is that these sombre areas +may be expanses of vegetation covering ground of a more or less marshy +character, for while the largest of them appear to be permanent, there +are some which vary coincidently with the variations of the canals. + +As to the kind of machinery employed to force the water from the +poles, it has been conjectured that it may have taken the form of a +gigantic system of pumps and conduits; and since the Martians are +assumed to be so far in advance of us in their mastery of scientific +principles, the hypothesis will at least not be harmed by supposing +that they have learned to harness forces of nature whose very +existence in a manageable form is yet unrecognized on the earth. If we +wish to let the imagination loose, we may conjecture that they have +conquered the secret of those intra-atomic forces whose resistless +energy is beginning to become evident to us, but the possibility of +whose utilization remains a dream, the fulfillment of which nobody +dares to predict. + +Such, in very brief form, is the celebrated theory of Mars as an +inhabited world. It certainly captivates the imagination, and if we +believe it to represent the facts, we cannot but watch with the +deepest sympathy this gallant struggle of an intellectual race to +preserve its planet from the effects of advancing age and death. We +may, indeed, wonder whether our own humanity, confronted by such a +calamity, could be counted on to meet the emergency with equal +stoutness of heart and inexhaustibleness of resource. Up to the +present time we certainly have shown no capacity to confront Nature +toe to toe, and to seize her by the shoulders and turn her round when +she refuses to go our way. If we could get into wireless telephonic +communication with the Martians we might learn from their own lips the +secret of their more than ``Roman recovery.'' + +The Riddle of the Asteroids + +Between the orbits of Mars and Jupiter revolves the most remarkable +system of little bodies with which we are acquainted -- the Asteroids, +or Minor Planets. Some six hundred are now known, and they may +actually number thousands. They form virtually a ring about the sun. +The most striking general fact about them is that they occupy the +place in the sky which should be occupied, according to Bode's Law, by +a single large planet. This fact, as we shall see, has led to the +invention of one of the most extraordinary theories in astronomy -- +viz., that of the explosion of a world! + +Bode's Law, so-called, is only an empiric formula, but until the +discovery of Neptune it accorded so well with the distances of the +planets that astronomers were disposed to look upon it as really +representing some underlying principle of planetary distribution. They +were puzzled by the absence of a planet in the space between Mars and +Jupiter, where the ``law'' demanded that there should be one, and an +association of astronomers was formed to search for it. There was a +decided sensation when, in 1801, Piazzi, of Palermo, announced that he +had found a little planet which apparently occupied the place in the +system which belonged to the missing body. He named it Ceres, and it +was the first of the Asteroids. The next year Olbers, of Bremen, while +looking for Ceres with his telescope, stumbled upon another small +planet which he named Pallas. Immediately he was inspired with the +idea that these two planets were fragments of a larger one which had +formerly occupied the vacant place in the planetary ranks, and he +predicted that others would be found by searching in the neighborhood +of the intersection of the orbits of the two already discovered. This +bold prediction was brilliantly fulfilled by the finding of two more +-- Juno in 1804, and Vesta in 1807. Olbers would seem to have been led +to the invention of his hypothesis of a planetary explosion by the +faith which astronomers at that time had in Bode's Law. They appear to +have thought that several planets revolving in the gap where the +``law'' called for but one could only be accounted for upon the theory +that the original one had been broken up to form the several. +Gravitation demanded that the remnants of a planet blown to pieces, no +matter how their orbits might otherwise differ, should all return at +stated periods to the point where the explosion had occurred; hence +Olbers' prediction that any asteroids that might subsequently be +discovered would be found to have a common point of orbital +intersection. And curiously enough all of the first asteroids found +practically answered to this requirement. Olbers' theory seemed to be +established. + +After the first four, no more asteroids were found until 1845, when +one was discovered; then, in 1847, three more were added to the list; +and after that searchers began to pick them up with such rapidity that +by the close of the century hundreds were known, and it had become +almost impossible to keep track of them. The first four are by far the +largest members of the group, but their actual sizes remained unknown +until less than twenty years ago. It was long supposed that Vesta was +the largest, because it shines more brightly than any of the others; +but finally, in 1895, Barnard, with the Lick telescope, definitely +measured their diameters, and proved to everybody's surprise that +Ceres is really the chief, and Vesta only the third in rank. His +measures are as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, +239 miles; and Juno, 120 miles. They differ greatly in the reflective +power of their surfaces, a fact of much significance in connection +with the question of their origin. Vesta is, surface for surface, +rather more than three times as brilliant as Ceres, whence the +original mistake about its magnitude. + +Nowadays new asteroids are found frequently by photography, but +physically they are most insignificant bodies, their average diameter +probably not exceeding twenty miles, and some are believed not to +exceed ten. On a planet only ten miles in diameter, assuming the same +mean density as the earth's, which is undoubtedly too much, the force +of gravity would be so slight that an average man would not weigh more +than three ounces, and could jump off into space whenever he liked. + +Although the asteroids all revolve around the sun in the same +direction as that pursued by the major planets, their orbits are +inclined at a great variety of angles to the general plane of the +planetary system, and some of them are very eccentric -- almost as +much so as the orbits of many of the periodic comets. It has even been +conjectured that the two tiny moons of Mars and the four smaller +satellites of Jupiter may be asteroids gone astray and captured by +those planets. Two of the asteroids are exceedingly remarkable for the +shapes and positions of their orbits; these are Eros, discovered in +1898, and T. G., 1906, found eight years later. The latter has a mean +distance from the sun slightly greater than that of Jupiter, while the +mean distance of Eros is less than that of Mars. The orbit of Eros is +so eccentric that at times it approaches within 15,000,000 miles of +the earth, nearer than any other regular member of the solar system +except the moon, thus affording an unrivaled means of measuring the +solar parallax. But for our present purpose the chief interest of Eros +lies in its extraordinary changes of light. + +These changes, although irregular, have been observed and photographed +many times, and there seems to be no doubt of their reality. Their +significance consists in their possible connection with the form of +the little planet, whose diameter is generally estimated at not more +than twenty miles. Von Oppolzer found, in 1901, that Eros lost +three-fourths of its brilliancy once in every two hours and +thirty-eight minutes. Other observers have found slightly different +periods of variability, but none as long as three hours. The most +interesting interpretation that has been offered of this phenomenon is +that it is due to a great irregularity of figure, recalling at once +Olbers' hypothesis. According to some, Eros may be double, the two +bodies composing it revolving around each other at very close +quarters; but a more striking, and it may be said probable, suggestion +is that Eros has a form not unlike that of a dumb-bell, or hour-glass, +turning rapidly end over end so that the area of illuminated surface +presented to our eyes continually changes, reaching at certain times a +minimum when the amount of light that it reflects toward the earth is +reduced to a quarter of its maximum value. Various other bizarre +shapes have been ascribed to Eros, such, for instance, as that of a +flat stone revolving about one of its longer axes, so that sometimes +we see its face and sometimes its edge. + +All of these explanations proceed upon the assumption that Eros cannot +have a simple globular figure like that of a typical planet, a figure +which is prescribed by the law of gravitation, but that its shape is +what may be called accidental; in a word, it is a fragment, for it +seems impossible to believe that a body formed in interplanetary +space, either through nebular condensation or through the aggregation +of particles drawn together by their mutual attractions, should not be +practically spherical in shape. Nor is Eros the only asteroid that +gives evidence by variations of brilliancy that there is something +abnormal in its constitution; several others present the same +phenomenon in varying degrees. Even Vesta was regarded by Olbers as +sufficiently variable in its light to warrant the conclusion that it +was an angular mass instead of a globe. Some of the smaller ones show +very notable variations, and all in short periods, of three or four +hours, suggesting that in turning about one of their axes they present +a surface of variable extent toward the sun and the earth. + +The theory which some have preferred -- that the variability of light +is due to the differences of reflective power on different parts of +the surface -- would, if accepted, be hardly less suggestive of the +origin of these little bodies by the breaking up of a larger one, +because the most natural explanation of such differences would seem to +be that they arose from variations in the roughness or smoothness of +the reflecting surface, which would be characteristic of fragmentary +bodies. In the case of a large planet alternating expanses of land and +water, or of vegetation and desert, would produce a notable variation +in the amount of reflection, but on bodies of the size of the +asteroids neither water nor vegetation could exist, and an atmosphere +would be equally impossible. + +One of the strongest objections to Olbers' hypothesis is that only a +few of the first asteroids discovered travel in orbits which +measurably satisfy the requirement that they should all intersect at +the point where the explosion occurred. To this it was at first +replied that the perturbations of the asteroidal orbits, by the +attractions of the major planets, would soon displace them in such a +manner that they would cease to intersect. One of the first +investigations undertaken by the late Prof. Simon Newcomb was directed +to the solution of this question, and he arrived at the conclusion +that the planetary perturbations could not explain the actual +situation of the asteroidal orbits. But afterward it was pointed out +that the difficulty could be avoided by supposing that not one but a +series of explosions had produced the asteroids as they now are. After +the primary disruption the fragments themselves, according to this +suggestion, may have exploded, and then the resulting orbits would be +as ``tangled'' as the heart could wish. This has so far rehabilitated +the explosion theory that it has never been entirely abandoned, and +the evidence which we have just cited of the probably abnormal shapes +of Eros and other asteroids has lately given it renewed life. It is a +subject that needs a thorough rediscussion. + +We must not fail to mention, however, that there is a rival hypothesis +which commends itself to many astronomers -- viz., that the asteroids +were formed out of a relatively scant ring of matter, situated between +Mars and Jupiter and resembling in composition the immensely more +massive rings from which, according to Laplace's hypothesis, the +planets were born. It is held by the supporters of this theory that +the attraction of the giant Jupiter was sufficient to prevent the +small, nebulous ring that gave birth to the asteroids from condensing +like the others into a single planet. + +But if we accept the explosion theory, with its corollary that minor +explosions followed the principal one, we have still an unanswered +question before us: What caused the explosions? The idea of a world +blowing up is too Titanic to be shocking; it rather amuses the +imagination than seriously impresses it; in a word, it seems +essentially chimerical. We can by no appeal to experience form a +mental picture of such an occurrence. Even the moon did not blow up +when it was wrecked by volcanoes. The explosive nebulae and new stars +are far away in space, and suggest no connection with such a +catastrophe as the bursting of a planet into hundreds of pieces. We +cannot conceive of a great globe thousands of miles in diameter +resembling a pellet of gunpowder only awaiting the touch of a match to +cause its sudden disruption. Somehow the thought of human agency +obtrudes itself in connection with the word ``explosion,'' and we +smile at the idea that giant powder or nitro-glycerine could blow up a +planet. Yet it would only need enough of them to do it. + +After all, we may deceive ourselves in thinking, as we are apt to do, +that explosive energies lock themselves up only in small masses of +matter. There are many causes producing explosions in nature, every +volcanic eruption manifests the activity of some of them. Think of the +giant power of confined steam; if enough steam could be suddenly +generated in the center of the earth by a downpour of all the waters +of the oceans, what might not the consequences be for our globe? In a +smaller globe, and it has never been estimated that the original +asteroid was even as large as the moon, such a catastrophe would, +perhaps, be more easily conceivable; but since we are compelled in +this case to assume that there was a series of successive explosions, +steam would hardly answer the purpose; it would be more reasonable to +suppose that the cause of the explosion was some kind of chemical +reaction, or something affecting the atoms composing the exploding +body. Here Dr Gustav Le Bon comes to our aid with a most startling +suggestion, based on his theory of the dissipation of intra-atomic +energy. It will be best to quote him at some length from his book on +The Evolution of Forces. + +``It does not seem at first sight,'' says Doctor Le Bon, + + very comprehensible that worlds which appear more and more stable + as they cool could become so unstable as to afterward dissociate + entirely. To explain this phenomenon, we will inquire whether + astronomical observations do not allow us to witness this + dissociation. + + We know that the stability of a body in motion, such as a top or a + bicycle, ceases to be possible when its velocity of rotation + descends below a certain limit. Once this limit is reached it loses + its stability and falls to the ground. Prof. J. J. Thomson even + interprets radio-activity in this manner, and points out that when + the speed of the elements composing the atoms descends below a + certain limit they become unstable and tend to lose their + equilibria. There would result from this a commencement of + dissociation, with diminution of their potential energy and a + corresponding increase of their kinetic energy sufficient to launch + into space the products of intra-atomic disintegration. + + It must not be forgotten that the atom being an enormous reservoir + of energy is by this very fact comparable with explosive bodies. + These last remain inert so long as their internal equilibria are + undisturbed. So soon as some cause or other modifies these, they + explode and smash everything around them after being themselves + broken to pieces. + + Atoms, therefore, which grow old in consequence of the diminution + of a part of their intra-atomic energy gradually lose their + stability. A moment, then, arrives when this stability is so weak + that the matter disappears by a sort of explosion more or less + rapid. The bodies of the radium group offer an image of this + phenomenon -- a rather faint image, however, because the atoms of + this body have only reached a period of instability when the + dissociation is rather slow. It probably precedes another and more + rapid period of dissociation capable of producing their final + explosion. Bodies such as radium, thorium, etc., represent, no + doubt, a state of old age at which all bodies must some day arrive, + and which they already begin to manifest in our universe, since all + matter is slightly radio-active. It would suffice for the + dissociation to be fairly general and fairly rapid for an explosion + to occur in a world where it was manifested. + + These theoretical considerations find a solid support in the sudden + appearances and disappearances of stars. The explosions of a world + which produce them reveal to us, perhaps, how the universes perish + when they become old. + + As astronomical observations show the relative frequency of these + rapid destructions, we may ask ourselves whether the end of a + universe by a sudden explosion after a long period of old age does + not represent its most general ending. + +Here, perhaps, it will be well to stop, since, entrancing as the +subject may be, we know very little about it, and Doctor Le Bon's +theory affords a limitless field for the reader's imagination. + _________________________________________________________________ + +A printed version of this book is available from Sattre Press +(http://csky.sattre-press.com). 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