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diff --git a/6630-h/6630-h.htm b/6630-h/6630-h.htm new file mode 100644 index 0000000..faade1b --- /dev/null +++ b/6630-h/6630-h.htm @@ -0,0 +1,6009 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" +"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> +<head> +<meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> +<meta http-equiv="Content-Style-Type" content="text/css" /> +<title>The Project Gutenberg eBook of Curiosities of the Sky, by Garrett P. Serviss</title> +<link rel="coverpage" href="images/cover.jpg" /> +<style type="text/css"> + +body { margin-left: 20%; + margin-right: 20%; + text-align: justify; } + +h1, h2, h3, h4, h5 {text-align: center; font-style: normal; font-weight: +normal; line-height: 1.5; margin-top: .5em; margin-bottom: .5em;} + +h1 {font-size: 300%; + margin-top: 0.6em; + margin-bottom: 0.6em; + letter-spacing: 0.12em; + word-spacing: 0.2em; + text-indent: 0em;} +h2 {font-size: 150%; margin-top: 2em; margin-bottom: 1em;} +h3 {font-size: 130%; margin-top: 1em;} +h4 {font-size: 120%;} +h5 {font-size: 110%;} + +.no-break {page-break-before: avoid;} /* for epubs */ + +div.chapter {page-break-before: always; margin-top: 4em;} + +hr {width: 80%; margin-top: 2em; margin-bottom: 2em;} + +p {text-indent: 1em; + margin-top: 0.25em; + margin-bottom: 0.25em; } + +p.poem {text-indent: 0%; + margin-left: 10%; + font-size: 90%; + margin-top: 1em; + margin-bottom: 1em; } + +p.letter {text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +.letter {text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em;} + +p.noindent {text-indent: 0% } + +div.fig { display:block; + margin:0 auto; + text-align:center; + margin-top: 1em; + margin-bottom: 1em;} + +p.caption {font-weight: bold; + text-align: center; } + +a:link {color:blue; text-decoration:none} +a:visited {color:blue; text-decoration:none} +a:hover {color:red} + +</style> + +</head> + +<body> + +<div style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of Curiosities of the Sky, by Garrett P. Serviss</div> +<div style='display:block; margin:1em 0'> +This eBook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online +at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you +are not located in the United States, you will have to check the laws of the +country where you are located before using this eBook. +</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: Curiosities of the Sky</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Garrett P. Serviss</div> +<div style='display:block; margin:1em 0'>Release Date: January 6, 2003 [eBook #6630]<br /> +[Most recently updated: December 28, 2022]</div> +<div style='display:block; margin:1em 0'>Language: English</div> +<div style='display:block; margin:1em 0'>Character set encoding: UTF-8</div> +<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: William McClain</div> +<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY ***</div> + +<div class="fig" style="width:55%;"> +<img src="images/cover.jpg" style="width:100%;" alt="[Illustration]" /> +</div> + +<h1>Curiosities of the Sky</h1> + +<h2 class="no-break">by Garrett P. Serviss</h2> + +<hr /> + +<div class="chapter"> + +<h2>Contents</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#pref01">PREFACE</a></td> +</tr> + +<tr> +<td> <a href="#chap00">CURIOSITIES OF THE SKY</a></td> +</tr> + +<tr> +<td> <a href="#chap01">I. The Windows of Absolute Night</a></td> +</tr> + +<tr> +<td> <a href="#chap02">II. Star-Clouds, Star-Clusters, and Star-Streams</a></td> +</tr> + +<tr> +<td> <a href="#chap03">III. Stellar Migrations</a></td> +</tr> + +<tr> +<td> <a href="#chap04">IV. The Passing of the Constellations</a></td> +</tr> + +<tr> +<td> <a href="#chap05">V. Conflagrations in the Heavens</a></td> +</tr> + +<tr> +<td> <a href="#chap06">VI. Explosive and Whirling Nebulæ</a></td> +</tr> + +<tr> +<td> <a href="#chap07">VII. The Banners of the Sun</a></td> +</tr> + +<tr> +<td> <a href="#chap08">VIII. The Zodiacal Light Mystery</a></td> +</tr> + +<tr> +<td> <a href="#chap09">IX. Marvels of the Aurora</a></td> +</tr> + +<tr> +<td> <a href="#chap10">X. Strange Adventures of Comets</a></td> +</tr> + +<tr> +<td> <a href="#chap11">XI. Meteors, Fire-Balls, and Meteorites</a></td> +</tr> + +<tr> +<td> <a href="#chap12">XII. The Wrecking of the Moon</a></td> +</tr> + +<tr> +<td> <a href="#chap13">XIII. The Great Mars Problem</a></td> +</tr> + +<tr> +<td> <a href="#chap14">XIV. The Riddle of the Asteroids</a></td> +</tr> + +</table> + +<div class="fig" style="width:100%;"> +<a name="illus01"></a> +<a href="images/image01.jpg"> +<img src="images/image01.jpg" width="376" height="600" alt="Illustration:" /></a> +<p class="caption">Great spiral nebula in Andromeda</p> +</div> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2>Illustrations</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#illus01">Great spiral nebula in Andromeda</a></td> +</tr> + +<tr> +<td> <a href="#illus02">The Milky Way</a></td> +</tr> + +<tr> +<td> <a href="#illus03">Star-cluster in Hercules</a></td> +</tr> + +<tr> +<td> <a href="#illus04">Great southern star-cluster, Omega Centauri</a></td> +</tr> + +<tr> +<td> <a href="#illus05">The Pleiades</a></td> +</tr> + +<tr> +<td> <a href="#illus06">The “Great Dipper”</a></td> +</tr> + +<tr> +<td> <a href="#illus07">Cassiopeia</a></td> +</tr> + +<tr> +<td> <a href="#illus08">The “Northern Crown”</a></td> +</tr> + +<tr> +<td> <a href="#illus09">The “Southern Cross”</a></td> +</tr> + +<tr> +<td> <a href="#illus10">Chart showing location of Tycho’s star, 1572, and Nova Persei of 1901</a></td> +</tr> + +<tr> +<td> <a href="#illus11">Nova Persei, with its nebular rings</a></td> +</tr> + +<tr> +<td> <a href="#illus12">Lord Rosse’s nebula</a></td> +</tr> + +<tr> +<td> <a href="#illus13">Wonderful spiral in triangulum</a></td> +</tr> + +<tr> +<td> <a href="#illus14">Spiral in Ursa Major</a></td> +</tr> + +<tr> +<td> <a href="#illus15">Nebula in Cetus</a></td> +</tr> + +<tr> +<td> <a href="#illus16">The Orion nebula</a></td> +</tr> + +<tr> +<td> <a href="#illus17">The Corona</a></td> +</tr> + +<tr> +<td> <a href="#illus18">A solar “prominence”</a></td> +</tr> + +<tr> +<td> <a href="#illus19">Auroral beams seen in England</a></td> +</tr> + +<tr> +<td> <a href="#illus20">Auroral arches of an eliptic form seen in the Arctic regions</a></td> +</tr> + +<tr> +<td> <a href="#illus21">Auroral curtain seen in Scandinavia</a></td> +</tr> + +<tr> +<td> <a href="#illus22">Auroral arches seen in Scandinavia</a></td> +</tr> + +<tr> +<td> <a href="#illus23">Swift’s comet</a></td> +</tr> + +<tr> +<td> <a href="#illus24">Daniels’ comet</a></td> +</tr> + +<tr> +<td> <a href="#illus25">Brooks’ comet</a></td> +</tr> + +<tr> +<td> <a href="#illus26">Curious forms of meteorite trains</a></td> +</tr> + +<tr> +<td> <a href="#illus27">Section of the atmosphere up to 100 kilometers</a></td> +</tr> + +<tr> +<td> <a href="#illus28">A meteor photographed in flight</a></td> +</tr> + +<tr> +<td> <a href="#illus29">Looking across Coon Butte crater from northern rim</a></td> +</tr> + +<tr> +<td> <a href="#illus30">Trail on south side, Coon Butte crater</a></td> +</tr> + +<tr> +<td> <a href="#illus31">The craters Clavius, Longomontanus, Tycho, etc.</a></td> +</tr> + +<tr> +<td> <a href="#illus32">Western part of the Mare Serenitatis</a></td> +</tr> + +<tr> +<td> <a href="#illus33">Mare Tranquilitatis and surroundings</a></td> +</tr> + +<tr> +<td> <a href="#illus34">Lunar craters Theophilus and surrounding region</a></td> +</tr> + +<tr> +<td> <a href="#illus35">Mare Crisium</a></td> +</tr> + +<tr> +<td> <a href="#illus36">Schiaparelli’s chart of Mars, showing the so-called system of canals</a></td> +</tr> + +</table> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="pref01"></a>PREFACE</h2> + +<p> +What Froude says of history is true also of astronomy: it is the most +impressive where it transcends explanation. It is not the mathematics of +astronomy, but the wonder and the mystery that seize upon the imagination. The +calculation of an eclipse owes all its prestige to the sublimity of its data; +the operation, in itself, requires no more mental effort than the preparation +of a railway time-table. +</p> + +<p> +The dominion which astronomy has always held over the minds of men is akin to +that of poetry; when the former becomes merely instructive and the latter +purely didactic, both lose their power over the imagination. Astronomy is known +as the oldest of the sciences, and it will be the longest-lived because it will +always have arcana that have not been penetrated. +</p> + +<p> +Some of the things described in this book are little known to the average +reader, while others are well known; but all possess the fascination of +whatever is strange, marvelous, obscure, or mysterious—magnified, in this case, +by the portentous scale of the phenomena. +</p> + +<p> +The idea of the author is to tell about these things in plain language, but +with as much scientific accuracy as plain language will permit, showing the +wonder that is in them without getting away from the facts. Most of them have +hitherto been discussed only in technical form, and in treatises that the +general public seldom sees and never reads. +</p> + +<p> +Among the topics touched upon are: +</p> + +<ul> <li>The strange unfixedness of the “fixed stars,” the vast migrations of +the suns and worlds constituting the universe.</li> + +<li>The slow passing out of existence of those collocations of stars which for +thousands of years have formed famous “constellations,” preserving the memory +of mythological heroes and heroines, and perhaps of otherwise unrecorded +history.</li> + +<li>The tendency of stars to assemble in immense clouds, swarms, and +clusters.</li> + +<li>The existence in some of the richest regions of the universe of absolutely +black, starless gaps, deeps, or holes, as if one were looking out of a window +into the murkiest night.</li> + +<li>The marvelous phenomena of new, or temporary, stars, which appear as +suddenly as conflagrations, and often turn into something else as eccentric as +themselves.</li> + +<li>The amazing forms of the “whirlpool,” “spiral,” “pinwheel,” and “lace,” or +“tress,” nebulæ.</li> + +<li>The strange surroundings of the sun, only seen in particular circumstances, +but evidently playing a constant part in the daily phenomena of the solar +system.</li> + +<li>The mystery of the Zodiacal Light and the Gegenschein.</li> + +<li>The extraordinary transformations undergone by comets and their tails.</li> + +<li>The prodigies of meteorites and masses of stone and metal fallen from the +sky.</li> + +<li>The cataclysms that have wrecked the moon.</li> + +<li>The problem of life and intelligence on the planet Mars.</li> + +<li>The problematical origin and fate of the asteroids.</li> + +<li>The strange phenomena of the auroral lights.</li> </ul> + +<p> +An attempt has been made to develop these topics in an orderly way, showing +their connection, so that the reader may obtain a broad general view of the +chief mysteries and problems of astronomy, and an idea of the immense field of +discovery which still lies, almost unexplored, before it. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap00"></a>CURIOSITIES OF THE SKY</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap01"></a>I<br/> +The Windows of Absolute Night</h2> + +<p> +To most minds mystery is more fascinating than science. But when science itself +leads straight up to the borders of mystery and there comes to a dead stop, +saying, “At present I can no longer see my way,” the force of the charm is +redoubled. On the other hand, the illimitable is no less potent in mystery than +the invisible, whence the dramatic effect of Keats’ “stout Cortez” staring at +the boundless Pacific while all his men look at each other with a wild surmise, +“silent upon a peak in Darien.” It is with similar feelings that the astronomer +regards certain places where from the peaks of the universe his vision seems to +range out into endless empty space. He sees there the shore of his little +isthmus, and, beyond, unexplored immensity. +</p> + +<p> +The name, “coal-sacks,” given to these strange voids is hardly descriptive. +Rather they produce upon the mind the effect of blank windows in a lonely house +on a pitch-dark night, which, when looked at from the brilliant interior, +become appalling in their rayless murk. Infinity seems to acquire a new meaning +in the presence of these black openings in the sky, for as one continues to +gaze it loses its purely metaphysical quality and becomes a kind of entity, +like the ocean. The observer is conscious that he can actually <i>see</i> the +beginning of its ebon depths, in which the visible universe appears to float +like an enchanted island, resplendent within with lights and life and gorgeous +spectacles, and encircled with screens of crowded stars, but with its dazzling +vistas ending at the fathomless sea of pure darkness which encloses all. +</p> + +<p> +The Galaxy, or Milky Way, surrounds the borders of our island in space like a +stellar garland, and when openings appear in it they are, by contrast, far more +impressive than the general darkness of the interstellar expanse seen in other +directions. Yet even that expanse is not everywhere equally dark, for it +contains gloomy deeps discernable with careful watching. Here, too, contrast +plays an important part, though less striking than within the galactic region. +Some of Sir William Herschel’s observations appear to indicate an association +between these tenebrious spots and neighboring star clouds and nebulæ. It is an +illuminating bit of astronomical history that when he was sweeping the then +virgin heavens with his great telescopes he was accustomed to say to his sister +who, note-book in hand, waited at his side to take down his words, fresh with +the inspiration of discovery: “Prepare to write; the nebulæ are coming; here +space is vacant.” +</p> + +<p> +The most famous of the “coal-sacks,” and the first to be brought to general +attention before astronomers had awakened to the significance of such things, +lies adjacent to the “Southern Cross,” and is truly an amazing phenomenon. It +is not alone the conspicuousness of this celestial vacancy, opening suddenly in +the midst of one of the richest parts of the Galaxy, that has given it its +fame, but quite as much the superstitious awe with which it was regarded by the +early explorers of the South Seas. To them, as well as to those who listened in +rapt wonder to their tales, the “Coal-sack” seemed to possess some occult +connection with the mystic “Cross.” In the eyes of the sailors it was not a +vacancy so much as a sable reality in the sky, and as, shuddering, they stared +at it, they piously crossed themselves. It was another of the magical wonders +of the unknown South, and as such it formed the basis of many a “wild surmise” +and many a sea-dog’s yarn. Scientific investigation has not diminished its +prestige, and today no traveler in the southern hemisphere is indifferent to +its fascinating strangeness, while some find it the most impressive spectacle +of the antarctic heavens. +</p> + +<p> +All around, up to the very edge of the yawning gap, the sheen of the Milky Way +is surpassingly glorious; but there, as if in obedience to an almighty edict, +everything vanishes. A single faint star is visible within the opening, +producing a curious effect upon the sensitive spectator, like the sight of a +tiny islet in the midst of a black, motionless, waveless tarn. The dimensions +of the lagoon of darkness, which is oval or pear-shaped, are eight degrees by +five, so that it occupies a space in the sky about one hundred and thirty times +greater than the area of the full moon. It attracts attention as soon as the +eye is directed toward the quarter where it exists, and by virtue of the rarity +of such phenomena it appears a far greater wonder than the drifts of stars that +are heaped around it. Now that observatories are multiplying in the southern +hemisphere, the great austral “Coal-sack” will, no doubt, receive attention +proportioned to its importance as one of the most significant features of the +sky. Already at the Sydney Observatory photographs have shown that the southern +portion of this Dead Sea of Space is not quite “bottomless,” although its +northern part defies the longest sounding lines of the astronomer. +</p> + +<p> +There is a similar, but less perfect, “coal-sack” in the northern hemisphere, +in the constellation of “The Swan,” which, strange to say, also contains a +well-marked figure of a cross outlined by stars. This gap lies near the top of +the cross-shaped figure. It is best seen by averted vision, which brings out +the contrast with the Milky Way, which is quite brilliant around it. It does +not, however, exercise the same weird attraction upon the eye as the southern +“Coal-sack,” for instead of looking like an absolute void in the sky, it rather +appears as if a canopy of dark gauze had been drawn over the stars. We shall +see the possible significance of this appearance later. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus02"></a> +<a href="images/image02.jpg"> +<img src="images/image02.jpg" width="481" height="600" alt="Illustration:" /></a> +<p class="caption">The Milky Way. Region near M.S. Photographed by Professor +Barnard</p> +</div> + +<p> +Just above the southern horizon of our northern middle latitudes, in summer, +where the Milky Way breaks up into vast sheets of nebulous luminosity, lying +over and between the constellations Scorpio and Sagittarius, there is a +remarkable assemblage of “coal-sacks,” though none is of great size. One of +them, near a conspicuous star-cluster in Scorpio, M80, is interesting for +having been the first of these strange objects noted by Herschel. Probably it +was its nearness to M80 which suggested to his mind the apparent connection of +such vacancies with star-clusters which we have already mentioned. +</p> + +<p> +But the most marvelous of the “coal-sacks” are those that have been found by +photography in Sagittarius. One of Barnard’s earliest and most excellent +photographs includes two of them, both in the star-cluster M8. The larger, +which is roughly rectangular in outline, contains one little star, and its +smaller neighbor is lune-shaped—surely a most singular form for such an object. +Both are associated with curious dark lanes running through the clustered stars +like trails in the woods. Along the borders of these lanes the stars are ranked +in parallel rows, and what may be called the bottoms of the lanes are not +entirely dark, but pebbled with faint stellar points. One of them which skirts +the two dark gaps and traverses the cluster along its greatest diameter is +edged with lines of stars, recalling the alignment of the trees bordering a +French highway. This <i>road of stars</i> cannot be less than many billions of +miles in length! +</p> + +<p> +All about the cluster the bed of the Galaxy is strangely disturbed, and in +places nearly denuded, as if its contents had been raked away to form the +immense stack and the smaller accumulations of stars around it. The well-known +“Trifid Nebula” is also included in the field of the photograph, which covers a +truly marvelous region, so intricate in its mingling of nebulæ, star-clusters, +star-swarms, star-streams, and dark vacancies that no description can do it +justice. Yet, chaotic as it appears, there is an unmistakable suggestion of +unity about it, impressing the beholder with the idea that all the different +parts are in some way connected, and have not been fortuitously thrown +together. Miss Agnes M. Clerke made the striking remark that the dusky lanes in +M8 are exemplified on the largest scale in the great rift dividing the Milky +Way, from Cygnus in the northern hemisphere all the way to the “Cross” in the +southern. Similar lanes are found in many other clusters, and they are +generally associated with flanking rows of stars, resembling in their +arrangement the thick-set houses and villas along the roadways that traverse +the approaches to a great city. +</p> + +<p> +But to return to the black gaps. Are they really windows in the star-walls of +the universe? Some of them look rather as if they had been made by a shell +fired through a luminous target, allowing the eye to range through the hole +into the void space beyond. If science is discretely silent about these things, +what can the more venturesome and less responsible imagination suggest? Would a +huge “runaway sun,” like Arcturus, for instance, make such an opening if it +should pass like a projectile through the Milky Way? It is at least a +stimulating inquiry. Being probably many thousands of times more massive than +the galactic stars, such a stellar missile would not be stopped by them, though +its direction of flight might be altered. It would drag the small stars lying +close to its course out of their spheres, but the ultimate tendency of its +attraction would be to sweep them round in its wake, thus producing rather a +star-swarm than a vacancy. Those that were very close to it might be swept away +in its rush and become its satellites, careering away with it in its flight +into outer space; but those that were farther off, and they would, of course, +greatly outnumber the nearer ones, would tend inward from all sides toward the +line of flight, as dust and leaves collect behind a speeding motor (though the +forces operating would be different), and would fill up the hole, if hole it +were. A swarm thus collected should be rounded in outline and bordered with a +relatively barren ring from which the stars had been “sucked” away. In a +general sense the M8 cluster answers to this description, but even if we +undertook to account for its existence by a supposition like the above, the +black gaps would remain unexplained, unless one could make a further draft on +the imagination and suggest that the stars had been thrown into a vast eddy, or +system of eddies, whose vortices appear as dark holes. Only a maelstrom-like +motion could keep such a funnel open, for without regard to the impulse derived +from the projectile, the proper motions of the stars themselves would tend to +fill it. Perhaps some other cause of the whirling motion may be found. As we +shall see when we come to the spiral nebulæ, gyratory movements are exceedingly +prevalent throughout the universe, and the structure of the Milky Way is +everywhere suggestive of them. But this is hazardous sport even for the +imagination—to play with <i>suns</i> as if they were but thistle-down in the +wind or corks in a mill-race. +</p> + +<p> +Another question arises: What is the thickness of the hedge of stars through +which the holes penetrate? Is the depth of the openings proportionate to their +width? In other words, is the Milky Way round in section like a rope, or flat +and thin like a ribbon? The answer is not obvious, for we have little or no +information concerning the relative distances of the faint galactic stars. It +would be easier, certainly, to conceive of openings in a thin belt than in a +massive ring, for in the first case they would resemble mere rifts and breaks, +while in the second they would be like wells or bore-holes. Then, too, the fact +that the Milky Way is not a <i>continuous</i> body but is made up of stars +whose actual distances apart is great, offers another quandary; persistent and +sharply bordered apertures in such an assemblage are <i>a priori</i> as +improbable, if not impossible, as straight, narrow holes running through a +swarm of bees. +</p> + +<p> +The difficulty of these questions indicates one of the reasons why it has been +suggested that the seeming gaps, or many of them, are not openings at all, but +opaque screens cutting off the light from stars behind them. That this is quite +possible in some cases is shown by Barnard’s later photographs, particularly +those of the singular region around the star Rho Ophiuchi. Here are to be seen +somber lanes and patches, apparently forming a connected system which covers an +immense space, and which their discoverer thinks may constitute a “dark +nebula.” This seems at first a startling suggestion; but, after all, why should +their not be dark nebulæ as well as visible ones? In truth, it has troubled +some astronomers to explain the luminosity of the bright nebulæ, since it is +not to be supposed that matter in so diffuse a state can be incandescent +through heat, and phosphorescent light is in itself a mystery. The supposition +is also in accord with what we know of the existence of dark solid bodies in +space. Many bright stars are accompanied by obscure companions, sometimes as +massive as themselves; the planets are non-luminous; the same is true of +meteors before they plunge into the atmosphere and become heated by friction; +and many plausible reasons have been found for believing that space contains as +many obscure as shining bodies of great size. It is not so difficult, after +all, then, to believe that there are immense collections of shadowy gases and +meteoric dust whose presence is only manifested when they intercept the light +coming from shining bodies behind them. +</p> + +<p> +This would account for the apparent extinguishment of light in open space, +which is indicated by the falling off in relative number of telescopic stars +below the tenth magnitude. Even as things are, the amount of light coming to us +from stars too faint to be seen with the naked eye is so great that the +statement of it generally surprises persons who are unfamiliar with the inner +facts of astronomy. It has been calculated that on a clear night the total +starlight from the entire celestial sphere amounts to one-sixtieth of the light +of the full moon; but of this less than one-twenty-fifth is due to stars +separately distinguished by the eye. If there were no obscuring medium in +space, it is probable that the amount of starlight would be noticeably and +perhaps enormously increased. +</p> + +<p> +But while it seems certain that some of the obscure spots in the Milky Way are +due to the presence of “dark nebulæ,” or concealing veils of one kind or +another, it is equally certain that there are many which are true apertures, +however they may have been formed, and by whatever forces they may be +maintained. These, then, are veritable windows of the Galaxy, and when looking +out of them one is face to face with the great mystery of infinite space. +<i>There</i> the known universe visibly ends, but manifestly space itself does +not end there. It is not within the power of thought to conceive an end to +space, for the instant we think of a terminal point or line the mind leaps +forward to the <i>beyond.</i> There must be space outside as well as inside. +Eternity of time and infinity of space are ideas that the intellect cannot +fully grasp, but neither can it grasp the idea of a limitation to either space +or time. The metaphysical conceptions of hypergeometry, or fourth-dimensional +space, do not aid us. +</p> + +<p> +Having, then, discovered that the universe is a thing <i>contained</i> in +something indefinitely greater than itself; having looked out of its windows +and found only the gloom of starless night outside—what conclusions are we to +draw concerning the beyond? It <i>seems</i> as empty as a vacuum, but is it +really so? If it be, then our universe is a single atom astray in the infinite; +it is the only island in an ocean without shores; it is the one oasis in an +illimitable desert. Then the Milky Way, with its wide-flung garland of stars, +is afloat like a tiny smoke-wreath amid a horror of immeasurable vacancy, or it +is an evanescent and solitary ring of sparkling froth cast up for a moment on +the viewless billows of immensity. From such conclusions the mind instinctively +shrinks. It prefers to think that there is <i>something</i> beyond, though we +cannot see it. Even the universe could not bear to be alone—a Crusoe lost in +the Cosmos! As the inhabitants of the most elegant château, with its gardens, +parks, and crowds of attendants, would die of loneliness if they did not know +that they have neighbors, though not seen, and that a living world of +indefinite extent surrounds them, so we, when we perceive that the universe has +limits, wish to feel that it is not solitary; that beyond the hedges and the +hills there are other centers of life and activity. Could anything be more +terrible than the thought of an <i>isolated universe?</i> The greater the +being, the greater the aversion to seclusion. Only the infinite satisfies; in +that alone the mind finds rest. +</p> + +<p> +We are driven, then, to believe that the universal night which envelopes us is +not tenantless; that as we stare out of the star-framed windows of the Galaxy +and see nothing but uniform blackness, the fault is with our eyes or is due to +an obscuring medium. Since <i>our</i> universe is limited in extent, there must +be <i>other</i> universes beyond it on all sides. Perhaps if we could carry our +telescopes to the verge of the great “Coal-sack” near the “Cross,” being then +on the frontier of our starry system, we could discern, sparkling afar off in +the vast night, some of the outer galaxies. They may be grander than ours, just +as many of the suns surrounding us are immensely greater than ours. If we could +take our stand somewhere in the midst of immensity and, with vision of infinite +reach, look about us, we should perhaps see a countless number of stellar +systems, amid which ours would be unnoticeable, like a single star among the +multitude glittering in the terrestial sky on a clear night. Some might be in +the form of a wreath, like our own; some might be globular, like the great +star-clusters in Hercules and Centaurus; some might be glittering circles, or +disks, or rings within rings. If we could enter them we should probably find a +vast variety of composition, including elements unknown to terrestrial +chemistry; for while the <i>visible</i> universe appears to contain few if any +substances not existing on the earth or in the sun, we have no warrant to +assume that others may not exist in infinite space. +</p> + +<p> +And how as to gravitation? We do not <i>know</i> that gravitation acts beyond +the visible universe, but it is reasonable to suppose that it does. At any +rate, if we let go <i>its</i> sustaining hand we are lost, and can only wander +hopelessly in our speculations, like children astray. If the empire of +gravitation is infinite, then the various outer systems must have <i>some,</i> +though measuring by our standards an imperceptible, attractive influence upon +each other, for gravitation never lets go its hold, however great the space +over which it is required to act. Just as the stars about us are all in motion, +so the starry systems beyond our sight may be in motion, and our system as a +whole may be moving in concert with them. If this be so, then after +interminable ages the aspect of the entire system of systems must change, its +various members assuming new positions with respect to one another. In the +course of time we may even suppose that our universe will approach relatively +close to one of the others; and then, if men are yet living on the earth, they +may glimpse through the openings which reveal nothing to us now, the lights of +another nearing star system, like the signals of a strange squadron, bringing +them the assurance (which can be but an inference at present) that the ocean of +space has other argosies venturing on its limitless expanse. +</p> + +<p> +There remains the question of the luminiferous ether by whose agency the waves +of light are borne through space. The ether is as mysterious as gravitation. +With regard to ether we only infer its existence from the effects which we +ascribe to it. Evidently the ether must extend as far as the most distant +visible stars. But does it continue on indefinitely in outer space? If it does, +then the invisibility of the other systems must be due to their distance +diminishing the quantity of light that comes from them below the limit of +perceptibility, or to the interposition of absorbing media; if it does not, +then the reason why we cannot see them is owing to the absence of a means of +conveyance for the light waves, as the lack of an interplanetary atmosphere +prevents us from hearing the thunder of sun-spots. (It is interesting to recall +that Mr Edison was once credited with the intention to construct a gigantic +microphone which should render the roar of sun-spots audible by transforming +the electric vibrations into sound-waves). On this supposition each starry +system would be enveloped in its own globule of ether, and no light could cross +from one to another. But the probability is that both the ether and gravitation +are ubiquitous, and that all the stellar systems are immersed in the former +like clouds of phosphorescent organisms in the sea. +</p> + +<p> +So astronomy carries the mind from height to greater height. Men were long in +accepting the proofs of the relative insignificance of the earth; they were +more quickly convinced of the comparative littleness of the solar system; and +now the evidence assails their reason that what they had regarded as <i>the</i> +universe is only one mote gleaming in the sunbeams of Infinity. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap02"></a>II<br/> +Star-Clouds, Star-Clusters, and Star-Streams</h2> + +<p> +In the preceding chapter we have seen something of the strangely complicated +structure of the Galaxy, or Milky Way. We now proceed to study more +comprehensively that garlanded “Pathway of the Gods.” +</p> + +<p> +Judged by the eye alone, the Milky Way is one of the most delicately beautiful +phenomena in the entire realm of nature—a shimmer of silvery gauze stretched +across the sky; but studied in the light of its revelations, it is the most +stupendous object presented to human ken. Let us consider, first, its +appearance to ordinary vision. Its apparent position in the sky shifts +according to the season. On a serene, cloudless summer evening, in the absence +of the moon, whose light obscures it, one sees the Galaxy spanning the heavens +from north to southeast of the zenith like a phosphorescent arch. In early +spring it forms a similar but, upon the whole, less brilliant arch west of the +zenith. Between spring and summer it lies like a long, faint, twilight band +along the northern horizon. At the beginning of winter it again forms an arch, +this time spanning the sky from east to west, a little north of the zenith. +These are its positions as viewed from the mean latitude of the United States. +Even the beginner in star-gazing does not have to watch it throughout the year +in order to be convinced that it is, in reality, a great circle, extending +entirely around the celestial sphere. We appear to be situated near its center, +but its periphery is evidently far away in the depths of space. +</p> + +<p> +Although to the casual observer it seems but a delicate scarf of light, +brighter in some places than in others, but hazy and indefinite at the best, +such is not its appearance to those who study it with care. They perceive that +it is an organic whole, though marvelously complex in detail. The telescope +shows that it consists of stars too faint and small through excess of distance +to be separately visible. Of the hundred million suns which some estimates have +fixed as the probable population of the starry universe, the vast majority (at +least thirty to one) are included in this strange belt of misty light. But they +are not uniformly distributed in it; on the contrary, they are arrayed in +clusters, knots, bunches, clouds, and streams. The appearance is somewhat as if +the Galaxy consisted of innumerable swarms of silver-winged bees, more or less +intermixed, some massed together, some crossing the paths of others, but all +governed by a single purpose which leads them to encircle the region of space +in which we are situated. +</p> + +<p> +From the beginning of the systematic study of the heavens, the fact has been +recognized that the form of the Milky Way denotes the scheme of the sidereal +system. At first it was thought that the shape of the system was that of a vast +round disk, flat like a cheese, and filled with stars, our sun and his +relatively few neighbors being placed near the center. According to this view, +the galactic belt was an effect of perspective; for when looking in the +direction of the plane of the disk, the eye ranged through an immense extension +of stars which blended into a glimmering blur, surrounding us like a ring; +while when looking out from the sides of the disk we saw but few stars, and in +those directions the heavens appeared relatively blank. Finally it was +recognized that this theory did not correspond with the observed appearances, +and it became evident that the Milky Way was not a mere effect of perspective, +but an actual band of enormously distant stars, forming a circle about the +sphere, the central opening of the ring (containing many scattered stars) being +many times broader than the width of the ring itself. Our sun is one of the +scattered stars in the central opening. +</p> + +<p> +As already remarked, the ring of the Galaxy is very irregular, and in places it +is partly broken. With its sinuous outline, its pendant sprays, its graceful +and accordant curves, its bunching of masses, its occasional interstices, and +the manifest order of a general plan governing the jumble of its details, it +bears a remarkable resemblance to a garland—a fact which appears the more +wonderful when we recall its composition. That an elm-tree should trace the +lines of beauty with its leafy and pendulous branches does not surprise us; but +we can only gaze with growing amazement when we behold <i>a hundred million +suns imitating the form of a chaplet!</i> And then we have to remember that +this form furnishes the ground-plan of the universe. +</p> + +<p> +As an indication of the extraordinary speculations to which the mystery of the +Milky Way has given rise, a theory recently (1909) proposed by Prof. George C. +Comstock may be mentioned. Starting with the data (first) that the number of +stars increases as the Milky Way is approached, and reaches a maximum in its +plane, while on the other hand the number of nebulæ is greatest outside the +Milky Way and increases with distance from it, and (second) that the Milky Way, +although a complete ring, is broad and diffuse on one side through one-half its +course—that half alone containing nebulæ—and relatively narrow and well defined +on the opposite side, the author of this singular speculation avers that these +facts can best be explained by supposing that the invisible universe consists +of two interpenetrating parts, one of which is a chaos of indefinite extent, +strewn with stars and nebulous dust, and the other a long, broad but +comparatively thin cluster of stars, including the sun as one of its central +members. This flat star-cluster is conceived to be moving edgewise through the +chaos, and, according to Professor Comstock, it acts after the manner of a +snow-plough sweeping away the cosmic dust and piling it on either hand above +and below the plane of the moving cluster. It thus forms a transparent rift, +through which we see farther and command a view of more stars than through the +intensified dust-clouds on either hand. This rift is the Milky Way. The dust +thrown aside toward the poles of the Milky Way is the substance of the nebulæ +which abound there. Ahead, where the front of the star-plough is clearing the +way, the chaos is nearer at hand, and consequently there the rift subtends a +broader angle, and is filled with primordial dust, which, having been annexed +by the vanguard of the star-swarm, forms the nebulæ seen only in that part of +the Milky Way. But behind, the rift appears narrow because there we look +farther away between dust-clouds produced ages ago by the front of the plough, +and no scattered dust remains in that part of the rift. +</p> + +<p> +In quoting an outline of this strikingly original theory the present writer +should not be understood as assenting to it. That it appears bizarre is not, in +itself, a reason for rejecting it, when we are dealing with so problematical +and enigmatical a subject as the Milky Way; but the serious objection is that +the theory does not sufficiently accord with the observed phenomena. There is +too much evidence that the Milky Way is an organic system, however fantastic +its form, to permit the belief that it can only be a rift in chaotic clouds. As +with every organism, we find that its parts are more or less clearly repeated +in its ensemble. Among all the strange things that the Milky Way contains there +is nothing so extraordinary as itself. Every astronomer must many times have +found himself marveling at it in those comparatively rare nights when it shows +all its beauty and all its strangeness. In its great broken rifts, divisions, +and spirals are found the gigantic prototypes of similar forms in its +star-clouds and clusters. As we have said, it determines the general shape of +the whole sidereal system. Some of the brightest stars in the sky appear to +hang like jewels suspended at the ends of tassels dropped from the Galaxy. +Among these pendants are the Pleiades and the Hyades. Orion, too, the “Mighty +Hunter,” is caught in “a loop of light” thrown out from it. The majority of the +great first-magnitude stars seem related to it, as if they formed an inner ring +inclined at an angle of some twenty degrees to its plane. Many of the long +curves that set off from it on both sides are accompanied by corresponding +curves of lucid stars. In a word, it offers every appearance of structural +connection with the entire starry system. That the universe should have assumed +the form of a wreath is certainly a matter for astonishment; but it would have +been still more astonishing if it had been a cube, a rhomboid, or a +dodecahedron, for then we should have had to suppose that something resembling +the forces that shape crystals had acted upon the stars, and the difficulty of +explaining the universe by the laws of gravitation would have been increased. +</p> + +<p> +From the Milky Way as a whole we pass to the vast clouds, swarms, and clusters +of stars of which it is made up. It may be, as some astronomers hold, that most +of the galactic stars are much smaller than the sun, so that their faintness is +not due entirely to the effect of distance. Still, their intrinsic brilliance +attests their solar character, and considering their remoteness, which has been +estimated at not less than ten thousand to twenty thousand light-years (a +light-year is equal to nearly six thousand thousand million miles) their actual +masses cannot be extremely small. The minutest of them are entitled to be +regarded as real suns, and they vary enormously in magnitude. The effects of +their attractions upon one another can only be inferred from their clustering, +because their relative movements are not apparent on account of the brevity of +the observations that we can make. But imagine a being for whom a million years +would be but as a flitting moment; to him the Milky Way would appear in a state +of ceaseless agitation—swirling with “a fury of whirlpool motion.” +</p> + +<p> +The cloud-like aspect of large parts of the Galaxy must always have attracted +attention, even from naked-eye observers, but the true star-clouds were first +satisfactorily represented in Barnard’s photographs. The resemblance to actual +clouds is often startling. Some are close-packed and dense, like cumuli; some +are wispy or mottled, like cirri. The rifts and modulations, as well as the +general outlines, are the same as those of clouds of vapor or dust, and one +notices also the characteristic thinning out at the edges. But we must beware +of supposing that the component suns are thickly crowded as the particles +forming an ordinary cloud. They <i>look,</i> indeed, as if they were matted +together, because of the irradiation of light, but in reality millions and +billions of miles separate each star from its neighbors. Nevertheless they form +real assemblages, whose members are far more closely related to one another +than is our sun to the stars around him, and if we were in the Milky Way the +aspect of the nocturnal sky would be marvelously different from its present +appearance. +</p> + +<p> +Stellar clouds are characteristic of the Galaxy and are not found beyond its +borders, except in the “Magellanic Clouds” of the southern hemisphere, which +resemble detached portions of the Milky Way. These singular objects form as +striking a peculiarity of the austral heavens as does the great “Coal-sack” +described in Chapter 1. But it is their isolation that makes them so +remarkable, for their composition is essentially galactic, and if they were +included within its boundaries they would not appear more wonderful than many +other parts of the Milky Way. Placed where they are, they look like masses +fallen from the great stellar arch. They are full of nebulæ and star-clusters, +and show striking evidences of spiral movement. +</p> + +<p> +Star-swarms, which are also characteristic features of the Galaxy, differ from +star-clouds very much in the way that their name would imply—<i>i.e.,</i> their +component stars are so arranged, even when they are countless in number, that +the idea of an exceedingly numerous assemblage rather than that of a cloud is +impressed on the observer’s mind. In a star-swarm the separate members are +distinguishable because they are either larger or nearer than the stars +composing a “cloud.” A splendid example of a true star-swarm is furnished by +Chi Persei, in that part of the Milky Way which runs between the constellations +Perseus and Cassiopeia. This swarm is much coarser than many others, and can be +seen by the naked eye. In a small telescope it appears double, as if the suns +composing it had divided into two parties which keep on their way side by side, +with some commingling of their members where the skirts of the two companies +come in contact. +</p> + +<p> +Smaller than either star-clouds or star-swarms, and differing from both in +their organization, are star-clusters. These, unlike the others, are found +outside as well as inside the Milky Way, although they are more numerous inside +its boundaries than elsewhere. The term star-cluster is sometimes applied, +though improperly, to assemblages which are rather groups, such, for instance, +as the Pleiades. In their most characteristic aspect star-clusters are of a +globular shape—globes of suns! A famous example of a globular star-cluster, but +one not included in the Milky Way, is the “Great Cluster in Hercules.” This is +barely visible to the naked eye, but a small telescope shows its character, and +in a large one it presents a marvelous spectacle. Photographs of such clusters +are, perhaps, less effective than those of star-clouds, because the central +condensation of stars in them is so great that their light becomes blended in +an indistinguishable blur. The beautiful effect of the incessant play of +infinitesimal rays over the apparently compact surface of the cluster, as if it +were a globe of the finest frosted silver shining in an electric beam, is also +lost in a photograph. Still, even to the eye looking directly at the cluster +through a powerful telescope, the central part of the wonderful congregation +seems almost a solid mass in which the stars are packed like the ice crystals +in a snowball. +</p> + +<p> +The same question rises to the lips of every observer: How can they possibly +have been brought into such a situation? The marvel does not grow less when we +know that, instead of being closely compacted, the stars of the cluster are +probably separated by millions of miles; for we know that their distances apart +are slight as compared with their remoteness from the Earth. Sir William +Herschel estimated their number to be about fourteen thousand, but in fact they +are uncountable. If we could view them from a point just within the edge of the +assemblage, they would offer the appearance of a hollow hemisphere emblazoned +with stars of astonishing brilliancy; the near-by ones unparalleled in splendor +by any celestial object known to us, while the more distant ones would resemble +ordinary stars. An inhabitant of the cluster would not know, except by a +process of ratiocination, that he was dwelling in a globular assemblage of +suns; only from a point far outside would their spherical arrangement become +evident to the eye. Imagine fourteen-thousand fire-balloons with an approach to +regularity in a spherical space—say, ten miles in diameter; there would be an +average of less than thirty in every cubic mile, and it would be necessary to +go to a considerable distance in order to see them as a globular aggregation; +yet from a point sufficiently far away they would blend into a glowing ball. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus03"></a> +<a href="images/image03.jpg"> +<img src="images/image03.jpg" width="409" height="600" alt="Illustration:" /></a> +<p class="caption">Star-cluster in Hercules<br/> +(Photographed with a two-foot reflector)</p> +</div> + +<p> +Photographs show even better than the best telescopic views that the great +cluster is surrounded with a multitude of dispersed stars, suggestively arrayed +in more or less curving lines, which radiate from the principle mass, with +which their connection is manifest. These stars, situated outside the central +sphere, look somewhat like vagrant bees buzzing round a dense swarm where the +queen bee is sitting. Yet while there is so much to suggest the operation of +central forces, bringing and keeping the members of the cluster together, the +attentive observer is also impressed with the idea that the whole wonderful +phenomenon may be <i>the result of explosion.</i> As soon as this thought +seizes the mind, confirmation of it seems to be found in the appearance of the +outlying stars, which could be as readily explained by the supposition that +they have been blown apart as that they have flocked together toward a center. +The probable fact that the stars constituting the cluster are very much smaller +than our sun might be regarded as favoring the hypothesis of an explosion. Of +their real size we know nothing, but, on the basis of an uncertain estimate of +their parallax, it has been calculated that they may average forty-five +thousand miles in diameter—something more than half the diameter of the planet +Jupiter. Assuming the same mean density, fourteen thousand such stars might +have been formed by the explosion of a body about twice the size of the sun. +This recalls the theory of Olbers, which has never been altogether abandoned or +disproved, that the Asteroids were formed by the explosion of a planet +circulating between the orbits of Mars and Jupiter. The Asteroids, whatever +their manner of origin, form a ring around the sun; but, of course, the +explosion of a great independent body, not originally revolving about a +superior center of gravitational force, would not result in the formation of a +ring of small bodies, but rather of a dispersed mass of them. But back of any +speculation of this kind lies the problem, at present insoluble: How could the +explosion be produced? (See the question of explosions in Chapters 6 and 14). +</p> + +<p> +Then, on the other hand, we have the observation of Herschel, since abundantly +confirmed, that space is unusually vacant in the immediate neighborhood of +condensed star-clusters and nebulæ, which, as far as it goes, might be taken as +an indication that the assembled stars had been drawn together by their mutual +attractions, and that the tendency to aggregation is still bringing new members +toward the cluster. But in that case there must have been an original +condensation of stars at that point in space. This could probably have been +produced by the coagulation of a great nebula into stellar nuclei, a process +which seems now to be taking place in the Orion Nebula. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus04"></a> +<a href="images/image04.jpg"> +<img src="images/image04.jpg" width="417" height="600" alt="Illustration:" /></a> +<p class="caption">Great southern star-cluster, Omega Centauri</p> +</div> + +<p> +A yet more remarkable globular star-cluster exists in the southern hemisphere, +Omega Centauri. In this case the central condensation of stars presents an +almost uniform blaze of light. Like the Hercules cluster, that in Centaurus is +surrounded with stars scattered over a broad field and showing an appearance of +radial arrangement. In fact, except for its greater richness, Omega Centauri is +an exact duplicate of its northern rival. Each appears to an imaginative +spectator as a veritable “city of suns.” Mathematics shrinks from the task of +disentangling the maze of motions in such an assemblage. It would seem that the +chance of collisions is not to be neglected, and this idea finds a certain +degree of confirmation in the appearance of “temporary stars” which have more +than once blazed out in, or close by, globular star-clusters. +</p> + +<p> +This leads up to the notable fact, first established by Professor Bailey a few +years ago, that such clusters are populous with variable stars. Omega Centauri +and the Hercules cluster are especially remarkable in this respect. The +variables found in them are all of short period and the changes of light show a +noteworthy tendency to uniformity. The first thought is that these phenomena +must be due to collisions among the crowded stars, but, if so, the encounters +cannot be between the stars themselves, but probably between stars and meteor +swarms revolving around them. Such periodic collisions might go on for ages +without the meteors being exhausted by incorporation with the stars. This +explanation appears all the more probable because one would naturally expect +that flocks of meteors would abound in a close aggregation of stars. It is also +consistent with Perrine’s discovery—that the globular star clusters are +powdered with minute stars strewn thickly among the brighter ones. +</p> + +<p> +In speaking of Professor Comstock’s extraordinary theory of the Milky Way, the +fact was mentioned that, broadly speaking, the nebulæ are less numerous in the +galactic belt than in the comparatively open spaces on either side of it, but +that they are, nevertheless, abundant in the broader half of the Milky Way +which he designates as the front of the gigantic “plough” supposed to be +forcing its way through the enveloping chaos. In and around the Sagittarius +region the intermingling of nebulæ and galactic star clouds and clusters is +particularly remarkable. That there is a causal connection no thoughtful person +can doubt. We are unable to get away from the evidence that a nebula is like a +seed-ground from which stars spring forth; or we may say that nebulæ resemble +clouds in whose bosom raindrops are forming. The wonderful aspect of the +admixtures of nebulæ and star-clusters in Sagittarius has been described in +Chapter 1. We now come to a still more extraordinary phenomenon of this +kind—the Pleiades nebulæ. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus05"></a> +<a href="images/image05.jpg"> +<img src="images/image05.jpg" width="600" height="436" alt="Illustration:" /></a> +<p class="caption">The Pleiades</p> +</div> + +<p> +The group of the Pleiades, although lying outside the main course of the +Galaxy, is connected with it by a faint loop, and is the scene of the most +remarkable association of stars and nebulous matter known in the visible +universe. The naked eye is unaware of the existence of nebulæ in the Pleiades, +or, at the best, merely suspects that there is something of the kind there; and +even the most powerful telescopes are far from revealing the full wonder of the +spectacle; but in photographs which have been exposed for many hours +consecutively, in order to accumulate the impression of the actinic rays, the +revelation is stunning. The principle stars are seen surrounded by, and, as it +were, <i>drowned in,</i> dense nebulous clouds of an unparalleled kind. The +forms assumed by these clouds seem at first sight inexplicable. They look like +fleeces, or perhaps more like splashes and daubs of luminous paint dashed +carelessly from a brush. But closer inspection shows that they are, to a large +extent, <i>woven</i> out of innumerable threads of filmy texture, and there are +many indications of spiral tendencies. Each of the bright stars of the +group—Alcyone, Merope, Maia, Electra, Taygeta, Atlas—is the focus of a dense +fog (totally invisible, remember, alike to the naked eye and to the telescope), +and these particular stars are veiled from sight behind the strange mists. +Running in all directions across the relatively open spaces are nebulous wisps +and streaks of the most curious forms. On some of the nebular lines, which are +either straight throughout, or if they change direction do so at an angle, +little stars are strung like beads. In one case seven or eight stars are thus +aligned, and, as if to emphasize their dependence upon the chain which connects +them, when it makes a slight bend the file of stars turns the same way. Many +other star rows in the group suggest by their arrangement that they, too, were +once strung upon similar threads which have now disappeared, leaving the stars +spaced along their ancient tracks. We seem forced to the conclusion that there +was a time when the Pleiades were embedded in a vast nebula resembling that of +Orion, and that the cloud has now become so rare by gradual condensation into +stars that the merest trace of it remains, and this would probably have escaped +detection but for the remarkable actinic power of the radiant matter of which +it consists. The richness of many of these faint nebulous masses in +ultra-violet radiations, which are those that specifically affect the +photographic plate, is the cause of the marvelous revelatory power of celestial +photography. So the veritable unseen universe, as distinguished from the +“unseen universe” of metaphysical speculation, is shown to us. +</p> + +<p> +A different kind of association between stars and nebulæ is shown in some +surprising photographic objects in the constellation Cygnus, where long, wispy +nebulæ, billions of miles in length, some of them looking like tresses +streaming in a breeze, lie amid fields of stars which seem related to them. But +the relation is of a most singular kind, for notwithstanding the delicate +structure of the long nebulæ they appear to act as barriers, causing the stars +to heap themselves on one side. The stars are two, three, or four times as +numerous on one side of the nebulæ as on the other. These nebulæ, as far as +appearance goes, might be likened to rail fences, or thin hedges, against which +the wind is driving drifts of powdery snow, which, while scattered plentifully +all around, tends to bank itself on the leeward side of the obstruction. The +imagination is at a loss to account for these extraordinary phenomena; yet +there they are, faithfully giving us their images whenever the photographic +plate is exposed to their radiations. +</p> + +<p> +Thus the more we see of the universe with improved methods of observation, and +the more we invent aids to human senses, each enabling us to penetrate a little +deeper into the unseen, the greater becomes the mystery. The telescope carried +us far, photography is carrying us still farther; but what as yet unimagined +instrument will take us to the bottom, the top, and the end? And then, what +hitherto untried power of thought will enable us to comprehend the meaning of +it all? +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap03"></a>III<br/> +Stellar Migrations</h2> + +<p> +To the untrained eye the stars and the planets are not distinguishable. It is +customary to call them all alike “stars.” But since the planets more or less +rapidly change their places in the sky, in consequence of their revolution +about the sun, while the stars proper seem to remain always in the same +relative positions, the latter are spoken of as “fixed stars.” In the +beginnings of astronomy it was not known that the “fixed stars” had any motion +independent of their apparent annual revolution with the whole sky about the +earth as a seeming center. Now, however, we know that the term “fixed stars” is +paradoxical, for there is not a single really fixed object in the whole +celestial sphere. The apparent fixity in the positions of the stars is due to +their immense distance, combined with the shortness of the time during which we +are able to observe them. It is like viewing the plume of smoke issuing from a +steamer, hull down, at sea: if one does not continue to watch it for a long +time it appears to be motionless, although in reality it may be traveling at +great speed across the line of sight. Even the planets seem fixed in position +if one watches them for a single night only, and the more distant ones do not +sensibly change their places, except after many nights of observation. Neptune, +for instance, moves but little more than two degrees in the course of an entire +year, and in a month its change of place is only about one-third of the +diameter of the full moon. +</p> + +<p> +Yet, fixed as they seem, the stars are actually moving with a speed in +comparison with which, in some cases, the planets might almost be said to stand +fast in their tracks. Jupiter’s speed in his orbit is about eight miles per +second, Neptune’s is less than three and one-half miles, and the earth’s is +about eighteen and one-half miles; while there are “fixed stars” which move two +hundred or three hundred miles per second. They do not all, however, move with +so great a velocity, for some appear to travel no faster than the planets. But +in all cases, notwithstanding their real speed, long-continued and exceedingly +careful observations are required to demonstrate that they are moving at all. +No more overwhelming impression of the frightful depths of space in which the +stars are buried can be obtained than by reflecting upon the fact that a star +whose actual motion across the line of sight amounts to two hundred miles per +second does not change its apparent place in the sky, in the course of a +thousand years, sufficiently to be noticed by the casual observer of the +heavens! +</p> + +<p> +There is one vast difference between the motions of the stars and those of the +planets to which attention should be at once called: the planets, being under +the control of a central force emanating from their immediate master, the sun, +all move in the same direction and in orbits concentric about the sun; the +stars, on the other hand, move in every conceivable direction and have no +apparent center of motion, for all efforts to discover such a center have +failed. At one time, when theology had finally to accept the facts of science, +a grandiose conception arose in some pious minds, according to which the Throne +of God was situated at the exact center of His Creation, and, seated there, He +watched the magnificent spectacle of the starry systems obediently revolving +around Him. Astronomical discoveries and speculations seemed for a time to +afford some warrant for this view, which was, moreover, an acceptable +substitute for the abandoned geocentric theory in minds that could only +conceive of God as a superhuman artificer, constantly admiring his own work. No +longer ago than the middle of the nineteenth century a German astronomer, +Maedler, believed that he had actually found the location of the center about +which the stellar universe revolved. He placed it in the group of the Pleiades, +and upon his authority an extraordinary imaginative picture was sometimes drawn +of the star Alcyone, the brightest of the Pleiades, as the very seat of the +Almighty. This idea even seemed to gain a kind of traditional support from the +mystic significance, without known historical origin, which has for many ages, +and among widely separated peoples, been attached to the remarkable group of +which Alcyone is the chief. But since Maedler’s time it has been demonstrated +that the Pleiades cannot be the center of revolution of the universe, and, as +already remarked, all attempts to find or fix such a center have proved +abortive. Yet so powerful was the hold that the theory took upon the popular +imagination, that even today astronomers are often asked if Alcyone is not the +probable site of “Jerusalem the Golden.” +</p> + +<p> +If there were a discoverable center of predominant gravitative power, to which +the motions of all the stars could be referred, those motions would appear less +mysterious, and we should then be able to conclude that the universe was, as a +whole, a prototype of the subsidiary systems of which it is composed. We should +look simply to the law of gravitation for an explanation, and, naturally, the +center would be placed within the opening enclosed by the Milky Way. If it were +there the Milky Way itself should exhibit signs of revolution about it, like a +wheel turning upon its hub. No theory of the star motions as a whole could +stand which failed to take account of the Milky Way as the basis of all. But +the very form of that divided wreath of stars forbids the assumption of its +revolution about a center. Even if it could be conceived as a wheel having no +material center it would not have the form which it actually presents. As was +shown in Chapter 2, there is abundant evidence of motion in the Milky Way; but +it is not motion of the system as a whole, but motion affecting its separate +parts. Instead of all moving one way, the galactic stars, as far as their +movements can be inferred, are governed by local influences and conditions. +They appear to travel crosswise and in contrary directions, and perhaps they +eddy around foci where great numbers have assembled; but of a universal +revolution involving the entire mass we have no evidence. +</p> + +<p> +Most of our knowledge of star motions, called “proper motions,” relates to +individual stars and to a few groups which happen to be so near that the +effects of their movements are measurable. In some cases the motion is so rapid +(not in appearance, but in reality) that the chief difficulty is to imagine how +it can have been imparted, and what will eventually become of the “runaways.” +Without a collision, or a series of very close approaches to great +gravitational centers, a star traveling through space at the rate of two +hundred or three hundred miles per second could not be arrested or turned into +an orbit which would keep it forever flying within the limits of the visible +universe. A famous example of these speeding stars is “1830 Groombridge,” a +star of only the sixth magnitude, and consequently just visible to the naked +eye, whose motion across the line of sight is so rapid that it moves upon the +face of the sky a distance equal to the apparent diameter of the moon every 280 +years. The distance of this star is at least 200,000,000,000,000 miles, and may +be two or three times greater, so that its actual speed cannot be less than two +hundred, and may be as much as four hundred, miles per second. It could be +turned into a new course by a close approach to a great sun, but it could only +be stopped by collision, head-on, with a body of enormous mass. Barring such +accidents it must, as far as we can see, keep on until it has traversed our +stellar system, whence in may escape and pass out into space beyond, to join, +perhaps, one of those other universes of which we have spoken. Arcturus, one of +the greatest suns in the universe, is also a runaway, whose speed of flight has +been estimated all the way from fifty to two hundred miles per second. +Arcturus, we have every reason to believe, possesses hundreds of times the mass +of our sun—think, then, of the prodigious momentum that its motion implies! +Sirius moves more moderately, its motion across the line of sight amounting to +only ten miles per second, but it is at the same time approaching the sun at +about the same speed, its actual velocity in space being the resultant of the +two displacements. +</p> + +<p> +What has been said about the motion of Sirius brings us to another aspect of +this subject. The fact is, that in every case of stellar motion the +displacement that we observe represents only a part of the actual movement of +the star concerned. There are stars whose motion carries them straight toward +or straight away from the earth, and such stars, of course, show no cross +motion. But the vast majority are traveling in paths inclined from a +perpendicular to our line of sight. Taken as a whole, the stars may be said to +be flying about like the molecules in a mass of gas. The discovery of the +radial component in the movements of the stars is due to the spectroscope. If a +star is approaching, its spectral lines are shifted toward the violet end of +the spectrum by an amount depending upon the velocity of approach; if it is +receding, the lines are correspondingly shifted toward the red end. +Spectroscopic observation, then, combined with micrometric measurements of the +cross motion, enables us to detect the real movement of the star in space. +Sometimes it happens that a star’s radial movement is periodically reversed; +first it approaches, and then it recedes. This indicates that it is revolving +around a near-by companion, which is often invisible, and superposed upon this +motion is that of the two stars concerned, which together may be approaching or +receding or traveling across the line of sight. Thus the complications involved +in the stellar motions are often exceedingly great and puzzling. +</p> + +<p> +Yet another source of complication exists in the movement of our own star, the +sun. There is no more difficult problem in astronomy than that of disentangling +the effects of the solar motion from those of the motions of the other stars. +But the problem, difficult as it is, has been solved, and upon its solution +depends our knowledge of the speed and direction of the movement of the solar +system through space, for of course the sun carries its planets with it. One +element of the solution is found in the fact that, as a result of perspective, +the stars toward which we are going appear to move apart toward all points of +the compass, while those behind appear to close up together. Then the +spectroscopic principle already mentioned is invoked for studying the shift of +the lines, which is toward the violet in the stars ahead of us and toward the +red in those that we are leaving behind. Of course the effects of the +independent motions of the stars must be carefully excluded. The result of the +studies devoted to this subject is to show that we are traveling at a speed of +twelve to fifteen miles per second in a northerly direction, toward the border +of the constellations Hercules and Lyra. A curious fact is that the more recent +estimates show that the direction is not very much out of a straight line drawn +from the sun to the star Vega, one of the most magnificent suns in the heavens. +But it should not be inferred from this that Vega is drawing us on; it is too +distant for its gravitation to have such an effect. +</p> + +<p> +Many unaccustomed thoughts are suggested by this mighty voyage of the solar +system. Whence have we come, and whither do we go? Every year of our lives we +advance at least 375,000,000 miles. Since the traditional time of Adam the sun +has led his planets through the wastes of space no less than 225,000,000,000 +miles, or more than 2400 times the distance that separates him from the earth. +Go back in imagination to the geologic ages, and try to comprehend the distance +over which the earth has flown. Where was our little planet when it emerged out +of the clouds of chaos? Where was the sun when his “thunder march” began? What +strange constellations shone down upon our globe when its masters of life were +the monstrous beasts of the “Age of Reptiles”? A million years is not much of a +span of time in geologic reckoning, yet a million years ago the earth was +farther from its present place in space than any of the stars with a measurable +parallax are now. It was more than seven times as far as Sirius, nearly +fourteen times as far as Alpha Centauri, three times as far as Vega, and twice +as far as Arcturus. But some geologists demand two hundred, three hundred, even +one thousand million years to enable them to account for the evolutionary +development of the earth and its inhabitants. In a thousand million years the +earth would have traveled farther than from the remotest conceivable depths of +the Milky Way! +</p> + +<p> +Other curious reflections arise when we think of the form of the earth’s track +as it follows the lead of the sun, in a journey which has neither known +beginning nor conceivable end. There are probably many minds which have found a +kind of consolation in the thought that every year the globe returns to the +same place, on the same side of the sun. This idea may have an occult +connection with our traditional regard for anniversaries. When that period of +the year returns at which any great event in our lives has occurred we have the +feeling that the earth, in its annual round, has, in a manner, brought us back +to the scene of that event. We think of the earth’s orbit as a well-worn path +which we traverse many times in the course of a lifetime. It seems familiar to +us, and we grow to have a sort of attachment to it. The sun we are accustomed +to regard as a fixed center in space, like the mill or pump around which the +harnessed patient mule makes his endless circuits. But the real fact is that +the earth never returns to the place in space where it has once quitted. In +consequence of the motion of the sun carrying the earth and the other planets +along, the track pursued by our globe is a vast spiral in space continually +developing and never returning upon its course. It is probable that the tracks +of the sun and the others stars are also irregular, and possibly spiral, +although, as far as can be at present determined, they appear to be practically +straight. Every star, wherever it may be situated, is attracted by its +fellow-stars from many sides at once, and although the force is minimized by +distance, yet in the course of many ages its effects must become manifest. +</p> + +<p> +Looked at from another side, is there not something immensely stimulating and +pleasing to the imagination in the idea of so stupendous a journey, which makes +all of us the greatest of travelers? In the course of a long life a man is +transported through space thirty thousand million miles; Halley’s Comet does +not travel one-quarter as far in making one of its immense circuits. And there +are adventures on this voyage of which we are just beginning to learn to take +account. Space is full of strange things, and the earth must encounter some of +them as it advances through the unknown. Many singular speculations have been +indulged in by astronomers concerning the possible effects upon the earth of +the varying state of the space that it traverses. Even the alternation of hot +and glacial periods has sometimes been ascribed to this source. When tropical +life flourished around the poles, as the remains in the rocks assure us, the +needed high temperature may, it has been thought, have been derived from the +presence of the earth in a warm region of space. Then, too, there is a certain +interest for us in the thought of what our familiar planet has passed through. +We cannot but admire it for its long journeying as we admire the traveler who +comes to us from remote and unexplored lands, or as we gaze with a glow of +interest upon the first locomotive that has crossed a continent, or a ship that +has visited the Arctic or Antarctic regions. If we may trust the indications of +the present course, the earth, piloted by the sun, has come from the Milky Way +in the far south and may eventually rejoin that mighty band of stars in the far +north. +</p> + +<p> +While the stars in general appear to travel independently of one another, +except when they are combined in binary or trinary systems, there are notable +exceptions to this rule. In some quarters of the sky we behold veritable +migrations of entire groups of stars whose members are too widely separated to +show any indications of revolution about a common center of gravity. This leads +us back again to the wonderful group of the Pleiades. All of the principle +stars composing that group are traveling in virtually parallel lines. Whatever +force set them going evidently acted upon all alike. This might be explained by +the assumption that when the original projective force acted upon them they +were more closely united than they are at present, and that in drifting apart +they have not lost the impulse of the primal motion. Or it may be supposed that +they are carried along by some current in space, although it would be +exceedingly difficult, in the present state of our knowledge, to explain the +nature of such a current. Yet the theory of a current has been proposed. As to +an attractive center around which they might revolve, none has been found. +Another instance of similar “star-drift” is furnished by five of the seven +stars constituting the figure of the “Great Dipper.” In this case the stars +concerned are separated very widely, the two extreme ones by not less than +fifteen degrees, so that the idea of a common motion would never have been +suggested by their aspect in the sky; and the case becomes the more remarkable +from the fact that among and between them there are other stars, some of the +same magnitude, which do not share their motion, but are traveling in other +directions. Still other examples of the same phenomenon are found in other +parts of the sky. Of course, in the case of compact star-clusters, it is +assumed that all the members share a like motion of translation through space, +and the same is probably true of dense star-swarms and star-clouds. +</p> + +<p> +The whole question of star-drift has lately assumed a new phase, in consequence +of the investigations of Kapteyn, Dyson, and Eddington on the “systematic +motions of the stars.” This research will, it is hoped, lead to an +understanding of the general law governing the movements of the whole body of +stars constituting the visible universe. Taking about eleven hundred stars +whose proper motions have been ascertained with an approach to certainty, and +which are distributed in all parts of the sky, it has been shown that there +exists an apparent double drift, in two independent streams, moving in +different and nearly opposed directions. The apex of the motion of what is +called “Stream I” is situated, according to Professor Kapteyn, in right +ascension 85°, declination south 11°, which places it just south of the +constellation Orion; while the apex of “Stream II” is in right ascension +260°, declination south 48°, placing it in the constellation Ara, south +of Scorpio. The two apices differ very nearly 180° in right ascension and +about 120° in declination. The discovery of these vast star-streams, if +they really exist, is one of the most extraordinary in modern astronomy. It +offers the correlation of stellar movements needed as the basis of a theory of +those movements, but it seems far from revealing a physical cause for them. As +projected against the celestial sphere the stars forming the two opposite +streams appear intermingled, some obeying one tendency and some the other. As +Professor Dyson has said, the hypothesis of this double movement is of a +revolutionary character, and calls for further investigation. Indeed, it seems +at first glance not less surprising than would be the observation that in a +snow-storm the flakes over our heads were divided into two parties and driving +across each other’s course in nearly opposite directions, as if urged by +interpenetrating winds. +</p> + +<p> +But whatever explanation may eventually be found for the motions of the stars, +the knowledge of the existence of those motions must always afford a new charm +to the contemplative observer of the heavens, for they impart a sense of life +to the starry system that would otherwise be lacking. A stagnant universe, with +every star fixed immovably in its place, would not content the imagination or +satisfy our longing for ceaseless activity. The majestic grandeur of the +evolutions of the celestial hosts, the inconceivable vastness of the fields of +space in which they are executed, the countless numbers, the immeasurable +distances, the involved convolutions, the flocking and the scattering, the +interpenetrating marches and countermarches, the strange community of impulsion +affecting stars that are wide apart in space and causing them to traverse the +general movement about them like aides and despatch-bearers on a +battle-field—all these arouse an intensity of interest which is heightened by +the mystery behind them. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap04"></a>IV<br/> +The Passing of the Constellations</h2> + +<p> +From a historical and picturesque point of view, one of the most striking +results of the motions of the stars described in the last chapter is their +effect upon the forms of the constellations, which have been watched and +admired by mankind from a period so early that the date of their invention is +now unknown. The constellations are formed by chance combinations of +conspicuous stars, like figures in a kaleidoscope, and if our lives were +commensurate with the æons of cosmic existence we should perceive that the +kaleidoscope of the heavens was ceaselessly turning and throwing the stars into +new symmetries. Even if the stars stood fast, the motion of the solar system +would gradually alter the configurations, as the elements of a landscape +dissolve and recombine in fresh groupings with the traveler’s progress amid +them. But with the stars themselves all in motion at various speeds and in many +directions, the changes occur more rapidly. Of course, “rapid” is here +understood in a relative sense; the wheel of human history to an eye accustomed +to the majestic progression of the universe would appear to revolve with the +velocity of a whirling dynamo. Only the deliberation of geological movements +can be contrasted with the evolution and devolution of the constellations. +</p> + +<p> +And yet this secular fluctuation of the constellation figures is not without +keen interest for the meditative observer. It is another reminder of the swift +mutability of terrestial affairs. To the passing glance, which is all that we +can bestow upon these figures, they appear so immutable that they have been +called into service to form the most lasting records of ancient thought and +imagination that we possess. In the forms of the constellations, the most +beautiful, and, in imaginative quality, the finest, mythology that the world +has ever known has been perpetuated. Yet, in a broad sense, this scroll of +human thought imprinted on the heavens is as evanescent as the summer clouds. +Although more enduring than parchment, tombs, pyramids, and temples, it is as +far as they from truly eternizing the memory of what man has fancied and done. +</p> + +<p> +Before studying the effects that the motions of the stars have had and will +have upon the constellations, it is worth while to consider a little further +the importance of the stellar pictures as archives of history. To emphasize the +importance of these effects it is only necessary to recall that the +constellations register the oldest traditions of our race. In the history of +primeval religions they are the most valuable of documents. Leaving out of +account for the moment the more familiar mythology of the Greeks, based on +something older yet, we may refer for illustration to that of the mysterious +Maya race of America. At Izamal, in Yucatan, says Mr Stansbury Hagar, is a +group of ruins perched, after the Mexican and Central-American plan, on the +summits of pyramidal mounds which mark the site of an ancient theogonic center +of the Mayas. Here the temples all evidently refer to a cult based upon the +constellations as symbols. The figures and the names, of course, were not the +same as those that we have derived from our Aryan ancestors, but the star +groups were the same or nearly so. For instance, the loftiest of the temples at +Izamal was connected with the sign of the constellation known to us as Cancer, +marking the place of the sun at the summer solstice, at which period the sun +was supposed to descend at noon like a great bird of fire and consume the +offerings left upon the altar. Our Scorpio was known to the Mayas as a sign of +the “Death God.” Our Libra, the “Balance,” with which the idea of a divine +weighing out of justice has always been connected, seems to be identical with +the Mayan constellation Teoyaotlatohua, with which was associated a temple +where dwelt the priests whose special business it was to administer justice and +to foretell the future by means of information obtained from the spirits of the +dead. Orion, the “Hunter” of our celestial mythology, was among the Mayas a +“Warrior,” while Sagittarius and others of our constellations were known to +them (under different names, of course), and all were endowed with a religious +symbolism. And the same star figures, having the same significance, were +familiar to the Peruvians, as shown by the temples at Cuzco. Thus the +imagination of ancient America sought in the constellations symbols of the +unchanging gods. +</p> + +<p> +But, in fact, there is no nation and no people that has not recognized the +constellations, and at one period or another in its history employed them in +some symbolic or representative capacity. As handled by the Greeks from +prehistoric times, the constellation myths became the very soul of poetry. The +imagination of that wonderful race idealized the principal star groups so +effectively that the figures and traditions thus attached to them have, for +civilized mankind, displaced all others, just as Greek art in its highest forms +stands without parallel and eclipses every rival. The Romans translated no +heroes and heroines of the mythical period of their history to the sky, and the +deified Cæsars never entered that lofty company, but the heavens are filled +with the early myths of the Greeks. Herakles nightly resumes his mighty labors +in the stars; Zeus, in the form of the white “Bull,” Taurus, bears the fair +Europa on his back through the celestial waves; Andromeda stretches forth her +shackled arms in the star-gemmed ether, beseeching aid; and Perseus, in a blaze +of diamond armor, revives his heroic deeds amid sparkling clouds of stellar +dust. There, too, sits Queen Cassiopeia in her dazzling chair, while the Great +King, Cepheus, towers gigantic over the pole. Professor Young has significantly +remarked that a great number of the constellations are connected in some way or +other with the Argonautic Expedition—that strangely fascinating legend of +earliest Greek story which has never lost its charm for mankind. In view of all +this, we may well congratulate ourselves that the constellations will outlast +our time and the time of countless generations to follow us; and yet they are +very far from being eternal. Let us now study some of the effects of the +stellar motions upon them. +</p> + +<p> +We begin with the familiar figure of the “Great Dipper.” He who has not drunk +inspiration from its celestial bowl is not yet admitted to the circle of +Olympus. This figure is made up of seven conspicuous stars in the constellation +Ursa Major, the “Greater Bear.” The handle of the “Dipper” corresponds to the +tail of the imaginary “Bear,” and the bowl lies upon his flank. In fact, the +figure of a dipper is so evident and that of a bear so unevident, that to most +persons the “Great Dipper” is the only part of the constellation that is +recognizable. Of the seven stars mentioned, six are of nearly equal brightness, +ranking as of the second magnitude, while the seventh is of only the third +magnitude. The difference is very striking, since every increase of one +magnitude involves an increase of two-and-a-half times in brightness. There +appears to be little doubt that the faint star, which is situated at the +junction of the bowl and the handle, is a variable of long period, since three +hundred years ago it was as bright as its companions. But however that may be, +its relative faintness at the present time interferes but little with the +perfection of the “Dipper’s” figure. In order the more readily to understand +the changes which are taking place, it will be well to mention both the names +and the Greek letters which are attached to the seven stars. Beginning at the +star in the upper outer edge of the rim of the bowl and running in regular +order round the bottom and then out to the end of the handle, the names and +letters are as follows: Dubhe (α), Merak (β), Phaed (γ), Megrez +(δ), Alioth (ε), Mizar (ζ), and Benetnasch (η). Megrez is +the faint star already mentioned at the junction of the bowl and handle, and +Mizar, in the middle of the handle, has a close, naked-eye companion which is +named Alcor. The Arabs called this singular pair of stars “The Horse and +Rider.” Merak and Duhbe are called “The Pointers,” because an imaginary line +drawn northward through them indicates the Pole Star. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus06"></a> +<a href="images/image06.jpg"> +<img src="images/image06.jpg" width="515" height="600" alt="Illustration:" /></a> +<p class="caption">The “Great Dipper”</p> +</div> + +<p> +Now it has been found that five of these stars—<i>viz.,</i> Merak, Phaed, +Megrez, Alioth, and Mizar (with its comrade)—are moving with practically the +same speed in an easterly direction, while the other two, Dubhe and Benetnasch, +are simultaneously moving westward, the motions of Benetnasch being apparently +more rapid. The consequence of these opposed motions is, of course, that the +figure of the “Dipper” cannot always have existed and will not continue to +exist. In the accompanying diagrams it has been thought interesting to show the +relative positions of these seven stars, as seen from the point which the earth +now occupies, both in the past and in the future. Arrows attached to the stars +in the figure representing the present appearance of the “Dipper” indicate the +directions of the motions and the distances over which they will carry the +stars in a period of about five hundred centuries. The time, no doubt, seems +long, but remember the vast stretch of ages through which the earth has passed, +and then reflect that no reason is apparent why our globe should not continue +to be a scene of animation for ten thousand centuries yet to come. The fact +that the little star Alcor placed so close to Mizar should accompany the latter +in its flight is not surprising, but that two of the principal stars of the +group should be found moving in a direction directly opposed to that pursued by +the other five is surprising in the highest degree; and it recalls the strange +theory of a double drift affecting all the stars, to which attention was called +in the preceding chapter. It would appear that Benetnasch and Dubhe belong to +one “current,” and Merak, Phaed, Megrez, Alioth, and Mizar to the other. As far +as is known, the motion of the seven stars are not shared by the smaller stars +scattered about them, but on the theory of currents there should be such a +community of motion, and further investigation may reveal it. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus07"></a> +<a href="images/image07.jpg"> +<img src="images/image07.jpg" width="519" height="600" alt="Illustration:" /></a> +<p class="caption">Cassiopeia</p> +</div> + +<p> +From the “Great Dipper” we turn to a constellation hardly less conspicuous and +situated at an equal distance from the pole on the other side—Cassiopeia. This +famous star-group commemorating the romantic Queen of Ethiopia whose vain +boasting of her beauty was punished by the exposure of her daughter Andromeda +to the “Sea Monster,” is well-marked by five stars which form an irregular +letter “W” with its open side toward the pole. Three of these stars are usually +ranked as of the second magnitude, and two of the third; but to ordinary +observation they appear of nearly equal brightness, and present a very striking +picture. They mark out the chair and a part of the figure of the beautiful +queen. Beginning at the right-hand, or western, end of the “W,” their Greek +letter designations are: Beta (β), Alpha (α), Gamma (γ), Delta +(δ), and Epsilon (ε). Four of them, Beta, Alpha, Delta, and Epsilon +are traveling eastwardly at various speeds, while the fifth, Gamma, moves in a +westerly direction. The motion of Beta is more rapid than that of any of the +others. It should be said, however, that no little uncertainty attaches to the +estimates of the rate of motion of stars which are not going very rapidly, and +different observers often vary considerably in their results. +</p> + +<p> +In the beautiful “Northern Crown,” one of the most perfect and charming of all +the figures to be found in the stars, the alternate combining and scattering +effects of the stellar motions are shown by comparing the appearance which the +constellation must have had five hundred centuries ago with that which it has +at present and that which it will have in the future. The seven principle stars +of the asterism, forming a surprisingly perfect coronet, have movements in +three directions at right angles to one another. That in these circumstances +they should ever have arrived at positions giving them so striking an +appearance of definite association is certainly surprising; from its aspect one +would have expected to find a community of movement governing the brilliants of +the “Crown,” but instead of that we find evidence that they will inevitably +drift apart and the beautiful figure will dissolve. +</p> + +<p> +A similar fate awaits such asterisms as the “Northern Cross” in Cygnus; the +“Crow” (Corvus), which stands on the back of the great “Sea Serpent,” Hydra, +and pecks at his scales; “Job’s Coffin” (Delphinus); the “Great Square of +Pegasus”; the “Twins” (Gemini); the beautiful “Sickle” in Leo; and the +exquisite group of the Hyades in Taurus. In the case of the Hyades, two +controlling movements are manifest: one, affecting five of the stars which form +the well-known figure of a letter “V,” is directed northerly; the other, which +controls the direction of two stars, has an easterly trend. The chief star of +the group, Aldebaran, one of the finest of all stars both for its brilliance +and its color, is the most affected by the easterly motion. In time it will +drift entirely out of connection with its present neighbors. Although the +Hyades do not form so compact a group as the Pleiades in the same +constellation, yet their appearance of relationship is sufficient to awaken a +feeling of surprise over the fact that, as with the stars of the “Dipper,” +their association is only temporary or apparent. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus08"></a> +<a href="images/image08.jpg"> +<img src="images/image08.jpg" width="560" height="600" alt="Illustration:" /></a> +<p class="caption">The “Northern Crown”</p> +</div> + +<p> +The great figure of Orion appears to be more lasting, not because its stars are +physically connected, but because of their great distance, which renders their +movements too deliberate to be exactly ascertained. Two of the greatest of its +stars, Betelgeuse and Rigel, possess, as far as has been ascertained, no +perceptible motion across the line of sight, but there is a little movement +perceptible in the “Belt.” At the present time this consists of an almost +perfect straight line, a row of second-magnitude stars about equally spaced and +of the most striking beauty. In the course of time, however, the two right-hand +stars, Mintaka and Alnilam (how fine are these Arabic star names!) will +approach each other and form a naked-eye double, but the third, Alnita, will +drift away eastward, so that the “Belt” will no longer exist. +</p> + +<p> +For one more example, let us go to the southern hemisphere, whose most +celebrated constellation, the “Southern Cross,” has found a place in all modern +literatures, although it has no claim to consideration on account of +association with ancient legends. This most attractive asterism, which has +never ceased to fascinate the imagination of Christendom since it was first +devoutly described by the early explorers of the South, is but a passing +collocation of brilliant stars. Yet even in its transfigurations it has been +for hundreds of centuries, and will continue to be for hundreds of centuries to +come, a most striking object in the sky. Our figures show its appearance in +three successive phases: first, as it was fifty thousand years ago (viewed from +the earth’s present location); second, as it is in our day; and, third, as it +will be an equal time in the future. The nearness of these bright stars to one +another—the length of the longer beam of the “Cross” is only sixdegrees—makes +this group very noticeable, whatever the arrangement of its components may be. +The largest star, at the base of the “Cross,” is of the first magnitude, two of +the others are of the second magnitude, and the fourth is of the third. Other +stars, not represented in the figures, increase the effect of a celestial +blazonry, although they do not help the resemblance to a cross. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus09"></a> +<a href="images/image09.jpg"> +<img src="images/image09.jpg" width="500" height="444" alt="Illustration:" /></a> +<p class="caption">The “Southern Cross”</p> +</div> + +<p> +But since the motion of the solar system itself will, in the course of so long +a period as fifty thousand years, produce a great change in the perspective of +the heavens as seen from the earth, by carrying us nearly nineteen trillion +miles from our present place, why, it may be asked, seek to represent future +appearances of the constellations which we could not hope to see, even if we +could survive so long? The answer is: Because these things aid the mind to form +a picture of the effects of the mobility of the starry universe. Only by +showing the changes from some definite point of view can we arrive at a due +comprehension of them. The constellations are more or less familiar to +everybody, so that impending changes of their forms must at once strike the eye +and the imagination, and make clearer the significance of the movements of the +stars. If the future history of mankind is to resemble its past and if our race +is destined to survive yet a million years, then our remote descendents will +see a “new heavens” if not a “new earth,” and will have to invent novel +constellations to perpetuate their legends and mythologies. +</p> + +<p> +If our knowledge of the relative distances of the stars were more complete, it +would be an interesting exercise in celestial geometry to project the +constellations probably visible to the inhabitants of worlds revolving around +some of the other suns of space. Our sun is too insignificant for us to think +that he can make a conspicuous appearance among them, except, perhaps, in a few +cases. As seen, for instance, from the nearest known star, Alpha Centauri, the +sun would appear of the average first magnitude, and consequently from that +standpoint he might be the gem of some little constellation which had no +Sirius, or Arcturus, or Vega to eclipse him with its superior splendor. But +from the distance of the vast majority of the stars the sun would probably be +invisible to the naked eye, and as seen from nearer systems could only rank as +a fifth or sixth magnitude star, unnoticed and unknown except by the +star-charting astronomer. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap05"></a>V<br/> +Conflagrations in the Heavens</h2> + +<p> +Suppose it were possible for the world to take fire and burn up—as some +pessimists think that it will do when the Divine wrath shall have sufficiently +accumulated against it—nobody out of our own little corner of space would ever +be aware of the catastrophe! With all their telescopes, the astronomers living +in the golden light of Arcturus or the diamond blaze of Canopus would be unable +to detect the least glimmer of the conflagration that had destroyed the seat of +Adam and his descendents, just as now they are totally ignorant of its +existence. +</p> + +<p> +But at least fifteen times in the course of recorded history men looking out +from the earth have beheld in the remote depths of space great outbursts of +fiery light, some of them more splendidly luminous than anything else in the +firmament except the sun! If <i>they</i> were conflagrations, how many million +worlds like ours were required to feed their blaze? +</p> + +<p> +It is probable that “temporary” or “new” stars, as these wonderful apparitions +are called, really are conflagrations; not in the sense of a bonfire or a +burning house or city, but in that of a sudden eruption of inconceivable heat +and light, such as would result from the stripping off the shell of an +encrusted sun or the crashing together of two mighty orbs flying through space +with a hundred times the velocity of the swiftest cannon-shot. +</p> + +<p> +Temporary stars are the rarest and most erratic of astronomical phenomena. The +earliest records relating to them are not very clear, and we cannot in every +instance be certain that it was one of these appearances that the ignorant and +superstitious old chroniclers are trying to describe. The first temporary star +that we are absolutely sure of appeared in 1572, and is known as “Tycho’s +Star,” because the celebrated Danish astronomer (whose remains, with his +gold-and-silver artificial nose—made necessary by a duel—still intact, were +disinterred and reburied in 1901) was the first to perceive it in the sky, and +the most assiduous and successful in his studies of it. As the first fully +accredited representative of its class, this new star made its entry upon the +scene with becoming <i>éclat.</i> It is characteristic of these phenomena that +they burst into view with amazing suddenness, and, of course, entirely +unexpectedly. Tycho’s star appeared in the constellation Cassiopeia, near a now +well-known and much-watched little star named Kappa, on the evening of November +11, 1572. The story has often been repeated, but it never loses interest, how +Tycho, going home that evening, saw people in the street pointing and staring +at the sky directly over their heads, and following the direction of their +hands and eyes he was astonished to see, near the zenith, an unknown star of +surpassing brilliance. It outshone the planet Jupiter, and was therefore far +brighter than the first magnitude. There was not another star in the heavens +that could be compared with it in splendor. Tycho was not in all respects free +from the superstitions of his time—and who is?—but he had the true scientific +instinct, and immediately he began to study the stranger, and to record with +the greatest care every change in its aspect. First he determined as well as he +could with the imperfect instruments of his day, many of which he himself had +invented, the precise location of the phenomena in the sky. Then he followed +the changes that it underwent. At first it brightened until its light equaled +or exceeded that of the planet Venus at her brightest, a statement which will +be appreciated at its full value by anyone who has ever watched Venus when she +plays her dazzling rôle of “Evening Star,” flaring like an arc light in the +sunset sky. It even became so brilliant as to be visible in full daylight, +since, its position being circumpolar, it never set in the latitude of Northern +Europe. Finally it began to fade, turning red as it did so, and in March, 1574, +it disappeared from Tycho’s searching gaze, and has never been seen again from +that day to this. None of the astronomers of the time could make anything of +it. They had not yet as many bases of speculation as we possess today. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus10"></a> +<a href="images/image10.jpg"> +<img src="images/image10.jpg" width="500" height="408" alt="Illustration:" /></a> +<p class="caption">Chart showing location of Tycho’s star, 1572, and Nova +Persei of 1901</p> +</div> + +<p> +Tycho’s star has achieved a romantic reputation by being fancifully identified +with the “Star of Bethlehem,” said to have led the wondering Magi from their +eastern deserts to the cradle-manger of the Savior in Palestine. Many attempts +have been made to connect this traditional “star” with some known phenomenon of +the heavens, and none seems more idle than this. Yet it persistently survives, +and no astronomer is free from eager questions about it addressed by people +whose imagination has been excited by the legend. It is only necessary to say +that the supposition of a connection between the phenomenon of the Magi and +Tycho’s star is without any scientific foundation. It was originally based on +an unwarranted assumption that the star of Tycho was a variable of long period, +appearing once every three hundred and fifteen years, or thereabout. If that +were true there would have been an apparition somewhere near the traditional +date of the birth of Christ, a date which is itself uncertain. But even the +data on which the assumption was based are inconsistent with the theory. +Certain monkish records speak of something wonderful appearing in the sky in +the years 1264 and 945, and these were taken to have been outbursts of Tycho’s +star. Investigation shows that the records more probably refer to comets, but +even if the objects seen were temporary stars, their dates do not suit the +hypothesis; from 945 to 1264 there is a gap of 319 years, and from 1264 to 1572 +one of only 308 years; moreover 337 years have now (1909) elapsed since Tycho +saw the last glimmer of his star. Upon a variability so irregular and uncertain +as that, even if we felt sure that it existed, no conclusion could be found +concerning an apparition occurring 2000 years ago. +</p> + +<p> +In the year 1600 (the year in which Giordano Bruno was burned at the stake for +teaching that there is more than one physical world), a temporary star of the +third magnitude broke out in the constellation Cygnus, and curiously enough, +considering the rarity of such phenomena, only four years later another +surprisingly brilliant one appeared in the constellation Ophiuchus. This is +often called “Kepler’s star,” because the great German astronomer devoted to it +the same attention that Tycho had given to the earlier phenomenon. It, too, +like Tycho’s, was at first the brightest object in the stellar heavens, +although it seems never to have quite equaled its famous predecessor in +splendor. It disappeared after a year, also turning of a red color as it became +more faint. We shall see the significance of this as we go on. Some of Kepler’s +contemporaries suggested that the outburst of this star was due to a meeting of +atoms in space, and idea bearing a striking resemblance to the modern theory of +“astronomical collisions.” +</p> + +<p> +In 1670, 1848, and 1860 temporary stars made their appearance, but none of them +was of great brilliance. In 1866 one of the second magnitude broke forth in the +“Northern Crown” and awoke much interest, because by that time the spectroscope +had begun to be employed in studying the composition of the stars, and Huggins +demonstrated that the new star consisted largely of incandescent hydrogen. But +this star, apparently unlike the others mentioned, was not absolutely new. +Before its outburst it had shown as a star of the ninth magnitude (entirely +invisible, of course, to the naked eye), and after about six weeks it faded to +its original condition in which it has ever since remained. In 1876 a temporary +star appeared in the constellation Cygnus, and attained at one time the +brightness of the second magnitude. Its spectrum and its behavior resembled +those of its immediate predecessor. In 1885, astronomers were surprised to see +a sixth-magnitude star glimmering in the midst of the hazy cloud of the great +Andromeda Nebula. It soon absolutely disappeared. Its spectrum was remarkable +for being “continuous,” like that of the nebula itself. A continuous spectrum +is supposed to represent a body, or a mass, which is either solid or liquid, or +composed of gas under great pressure. In January, 1892, a new star was suddenly +seen in the constellation Auriga. It never rose much above the fourth +magnitude, but it showed a peculiar spectrum containing both bright and dark +lines of hydrogen. +</p> + +<p> +But a bewildering surprise was now in store; the world was to behold at the +opening of the twentieth century such a celestial spectacle as had not been on +view since the times of Tycho and Kepler. Before daylight on the morning of +February 22, 1901, the Rev. Doctor Anderson, of Edinburgh, an amateur +astronomer, who had also been the first to see the new star in Auriga, beheld a +strange object in the constellation Perseus not far from the celebrated +variable star Algol. He recognized its character at once, and immediately +telegraphed the news, which awoke the startled attention of astronomers all +over the world. When first seen the new star was no brighter than Algol (less +than the second magnitude), but within twenty-four hours it was ablaze, +outshining even the brilliant Capella, and far surpassing the first magnitude. +At the spot in the sky where it appeared nothing whatever was visible on the +night before its coming. This is known with certainty because a photograph had +been made of that very region on February 21, and this photograph showed +everything down to the twelfth magnitude, but not a trace of the stranger which +burst into view between the 21st and the 22nd like the explosion of a rocket. +</p> + +<p> +Upon one who knew the stars the apparition of this intruder in a well-known +constellation had the effect of a sudden invasion. The new star was not far +west of the zenith in the early evening, and in that position showed to the +best advantage. To see Capella, the hitherto unchallenged ruler of that quarter +of the sky, abased by comparison with this stranger of alien aspect, for there +was always an unfamiliar look about the “nova,” was decidedly disconcerting. It +seemed to portend the beginning of a revolution in the heavens. One could +understand what the effect of such an apparition must have been in the +superstitious times of Tycho. The star of Tycho had burst forth on the northern +border of the Milky Way; this one was on its southern border, some forty-five +degrees farther east. +</p> + +<p> +Astronomers were well-prepared this time for the scientific study of the new +star, both astronomical photography and spectroscopy having been perfected, and +the results of their investigations were calculated to increase the wonder with +which the phenomenon was regarded. The star remained at its brightest only a +few days; then, like a veritable conflagration, it began to languish; and, like +the reflection of a dying fire, as it sank it began to glow with the red color +of embers. But its changes were spasmodic; once about every three days it +flared up only to die away again. During these fluctuations its light varied +alternately in the ratio of one to six. Finally it took a permanent downward +course, and after a few months the naked eye could no longer perceive it; but +it remained visible with telescopes, gradually fading until it had sunk to the +ninth magnitude. Then another astonishing change happened: in August +photographs taken at the Yerkes Observatory and at Heidelberg showed that the +“nova” was <i>surrounded by a spiral nebula!</i> The nebula had not been there +before, and no one could doubt that it represented a phase of the same +catastrophe that had produced the outburst of the new star. At one time the +star seemed virtually to have disappeared, as if all its substance had been +expanded into the nebulous cloud, but always there remained a stellar nucleus +about which the misty spiral spread wider and ever wider, like a wave expanding +around a center of disturbance. The nebula too showed a variability of +brightness, and four condensations which formed in it seemed to have a motion +of revolution about the star. As time went on the nebula continued to expand at +a rate which was computed to be not less than twenty thousand miles per second! +And now the star itself, showing indications of having turned into a nebula, +behaved in a most erratic manner, giving rise to the suspicion that it was +about to burst out again. But this did not occur, and at length it sunk into a +state of lethargy from which it has to the present time not recovered. But the +nebulous spiral has disappeared, and the entire phenomena as it now (1909) +exists consists of a faint nebulous star of less than the ninth magnitude. +</p> + +<p> +The wonderful transformations just described had been forecast in advance of +the discovery of the nebulous spiral encircling the star by the spectroscopic +study of the latter. At first there was no suggestion of a nebular +constitution, but within a month or two characteristic nebular lines began to +appear, and in less than six months the whole spectrum had been transformed to +the nebular type. In the mean time the shifting of the spectral lines indicated +a complication of rapid motions in several directions simultaneously. These +motions were estimated to amount to from one hundred to five hundred miles per +second. +</p> + +<p> +The human mind is so constituted that it feels forced to seek an explanation of +so marvelous a phenomenon as this, even in the absence of the data needed for a +sound conclusion. The most natural hypothesis, perhaps, is that of a collision. +Such a catastrophe could certainly happen. It has been shown, for instance, +that in infinity of time the earth is sure to be hit by a comet; in the same +way it may be asserted that, if no time limit is fixed, the sun is certain to +run against some obstacle in space, either another star, or a dense meteor +swarm, or one of the dark bodies which there is every reason to believe abound +around us. The consequences of such a collision are easy to foretell, provided +that we know the masses and the velocities of the colliding bodies. In a +preceding chapter we have discussed the motions of the sun and stars, and have +seen that they are so swift that an encounter between any two of them could not +but be disastrous. But this is not all; for as soon as two stars approached +within a few million miles their speed would be enormously increased by their +reciprocal attractions and, if their motion was directed radially with respect +to their centers, they would come together with a crash that would reduce them +both to nebulous clouds. It is true that the chances of such a “head-on” +collision are relatively very small; two stars approaching each other would +most probably fall into closed orbits around their common center of gravity. If +there were a collision it would most likely be a grazing one instead of a +direct front-to-front encounter. But even a close approach, without any actual +collision, would probably prove disastrous, owing to the tidal influence of +each of the bodies on the other. Suns, in consequence of their enormous masses +and dimensions and the peculiarities of their constitution, are exceedingly +dangerous to one another at close quarters. Propinquity awakes in them a +mutually destructive tendency. Consisting of matter in the gaseous, or perhaps, +in some cases, liquid, state, their tidal pull upon each other if brought close +together might burst them asunder, and the photospheric envelope being +destroyed the internal incandescent mass would gush out, bringing fiery death +to any planets that were revolving near. Without regard to the resulting +disturbance of the earth’s orbit, the close approach of a great star to the sun +would be in the highest degree perilous to us. But this is a danger which may +properly be regarded as indefinitely remote, since, at our present location in +space, we are certainly far from every star except the sun, and we may feel +confident that no great invisible body is near, for if there were one we should +be aware of its presence from the effects of its attraction. As to dark nebulæ +which may possibly lie in the track that the solar system is pursuing at the +rate of 375,000,000 miles per year, that is another question—and they, too, +could be dangerous! +</p> + +<div class="fig" style="width:100%;"> +<a name="illus11"></a> +<a href="images/image11.jpg"> +<img src="images/image11.jpg" width="600" height="563" alt="Illustration:" /></a> +<p class="caption">Nova Persei, with its nebular rings</p> +</div> + +<p> +This brings us directly back to “Nova Persei,” for among the many suggestions +offered to explain its outburst, as well as those of other temporary stars, one +of the most fruitful is that of a collision between a star and a vast invisible +nebula. Professor Seeliger, of Munich, first proposed this theory, but it +afterward underwent some modifications from others. Stated in a general form, +the idea is that a huge dark body, perhaps an extinguished sun, encountered in +its progress through space a widespread flock of small meteors forming a dark +nebula. As it plunged into the swarm the friction of the innumerable collisions +with the meteors heated its surface to incandescence, and being of vast size it +then became visible to us as a new star. Meanwhile the motion of the body +through the nebula, and its rotation upon itself, set up a gyration in the +blazing atmosphere formed around it by the vaporized meteors; and as this +atmosphere spread wider, under the laws of gyratory motion a rotation in the +opposite direction began in the inflamed meteoric cloud outside the central +part of the vortex. Thus the spectral lines were caused to show motion in +opposite directions, a part of the incandescent mass approaching the earth +simultaneously with the retreat of another part. So the curious spectroscopic +observations before mentioned were explained. This theory might also account +for the appearance of the nebulous spiral first seen some six months after the +original outburst. The sequent changes in the spectrum of the “nova” are +accounted for by this theory on the assumption, reasonable enough in itself, +that at first the invading body would be enveloped in a vaporized atmosphere of +relatively slight depth, producing by its absorption the fine dark lines first +observed; but that as time went on and the incessant collisions continued, the +blazing atmosphere would become very deep and extensive, whereupon the +appearance of the spectral lines would change, and bright lines due to the +light of the incandescent meteors surrounding the nucleus at a great distance +would take the place of the original dark ones. The vortex of meteors once +formed would protect the flying body within from further immediate collisions, +the latter now occurring mainly among the meteors themselves, and then the +central blaze would die down, and the original splendor of the phenomenon would +fade. +</p> + +<p> +But the theories about Nova Persei have been almost as numerous as the +astronomers who have speculated about it. One of the most startling of them +assumed that the outburst was caused by the running amuck of a dark star which +had encountered another star surrounded with planets, the renewed outbreaks of +light after the principal one had faded being due to the successive running +down of the unfortunate planets! Yet another hypothesis is based on what we +have already said of the tidal influence that two close approaching suns would +have upon each other. Supposing two such bodies which had become encrusted, but +remained incandescent and fluid within, to approach within almost striking +distance; they would whirl each other about their common center of gravity, and +at the same time their shells would burst under the tidal strain, and their +glowing nuclei being disclosed would produce a great outburst of light. +Applying this theory to a “nova,” like that of 1866 in the “Northern Crown,” +which had been visible as a small star before the outbreak, and which afterward +resumed its former aspect, we should have to assume that a yet shining sun had +been approached by a dark body whose attraction temporarily burst open its +photosphere. It might be supposed that in this case the dark body was too far +advanced in cooling to suffer the same fate from the tidal pull of its victim. +But a close approach of that kind would be expected to result in the formation +of a binary system, with orbits of great eccentricity, perhaps, and after the +lapse of a certain time the outburst should be renewed by another approximation +of the two bodies. A temporary star of that kind would rather be ranked as a +variable. +</p> + +<p> +The celebrated French astronomer, Janssen, had a different theory of Nova +Persei, and of temporary stars in general. According to his idea, such +phenomena might be the result of chemical changes taking place in a sun without +interference by, or collision with, another body. Janssen was engaged for many +years in trying to discover evidence of the existence of oxygen in the sun, and +he constructed his observatory on the summit of Mount Blanc specially to pursue +that research. He believed that oxygen must surely exist in the sun since we +find so many other familiar elements included in the constitution of the solar +globe, and as he was unable to discover satisfactory evidence of its presence +he assumed that it existed in a form unknown on the earth. If it were normally +in the sun’s chromosphere, or coronal atmosphere, he said, it would combine +with the hydrogen which we know is there and form an obscuring envelope of +water vapor. It exists, then, in a special state, uncombined with hydrogen; but +let the temperature of the sun sink to a critical point and the oxygen will +assume its normal properties and combine with the hydrogen, producing a mighty +outburst of light and heat. This, Janssen thought, might explain the phenomena +of the temporary stars. It would also, he suggested, account for their brief +career, because the combination of the elements would be quickly accomplished, +and then the resulting water vapor would form an atmosphere cutting off the +radiation from the star within. +</p> + +<p> +This theory may be said to have a livelier human interest than some of the +others, since, according to it, the sun may carry in its very constitution a +menace to mankind; one does not like to think of it being suddenly transformed +into a gigantic laboratory for the explosive combination of oxygen and +hydrogen! But while Janssen’s theory might do for some temporary stars, it is +inadequate to explain all the phenomena of Nova Persei, and particularly the +appearance of the great spiral nebula that seemed to exhale from the heart of +the star. Upon the whole, the theory of an encounter between a star and a dark +nebula seems best to fit the observations. By that hypothesis the expanding +billow of light surrounding the core of the conflagration is very well +accounted for, and the spectroscopic peculiarities are also explained. +</p> + +<p> +Dr Gustov Le Bon offers a yet more alarming theory, suggesting that temporary +stars are the result of <i>atomic explosion;</i> but we shall touch upon this +more fully in Chapter 14. +</p> + +<p> +Twice in the course of this discussion we have called attention to the change +of color invariably undergone by temporary stars in the later stages of their +career. This was conspicuous with Nova Persei which glowed more and more redly +as it faded, until the nebulous light began to overpower that of the stellar +nucleus. Nothing could be more suggestive of the dying out of a great fire. +Moreover, change of color from white to red is characteristic of all variable +stars of long period, such as “Mira” in Cetus. It is also characteristic of +stars believed to be in the later stages of evolution, and consequently +approaching extinction, like Antares and Betelgeuse, and still more notably +certain small stars which “gleam like rubies in the field of the telescope.” +These last appear to be suns in the closing period of existence as +self-luminous bodies. Between the white stars, such as Sirius and Rigel, and +the red stars, such as Aldebaran and Alpha Herculis, there is a progressive +series of colors from golden yellow through orange to deep red. The change is +believed to be due to the increase of absorbing vapors in the stellar +atmosphere as the body cools down. In the case of ordinary stars these changes +no doubt occupy many millions of years, which represent the average duration of +solar life; but the temporary stars run through similar changes in a few +months: they resemble ephemeral insects—born in the morning and doomed to +perish with the going down of the sun. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap06"></a>VI<br/> +Explosive and Whirling Nebulæ</h2> + +<p> +One of the most surprising triumphs of celestial photography was Professor +Keeler’s discovery, in 1899, that the great majority of the nebulæ have a +distinctly spiral form. This form, previously known in Lord Rosse’s great +“Whirlpool Nebula,” had been supposed to be exceptional; now the photographs, +far excelling telescopic views in the revelation of nebular forms, showed the +spiral to be the typical shape. Indeed, it is a question whether all nebulæ are +not to some extent spiral. The extreme importance of this discovery is shown in +the effect that it has had upon hitherto prevailing views of solar and +planetary evolution. For more than three-quarters of a century Laplace’s +celebrated hypothesis of the manner of origin of the solar system from a +rotating and contracting nebula surrounding the sun had guided speculation on +that subject, and had been tentatively extended to cover the evolution of +systems in general. The apparent forms of some of the nebulæ which the +telescope had revealed were regarded, and by some are still regarded, as giving +visual evidence in favor of this theory. There is a “ring nebula” in Lyra with +a central star, and a “planetary nebula” in Gemini bearing no little +resemblance to the planet Saturn with its rings, both of which appear to be +practical realizations of Laplace’s idea, and the elliptical rings surrounding +the central condensation of the Andromeda Nebula may be cited for the same kind +of proof. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus12"></a> +<a href="images/image12.jpg"> +<img src="images/image12.jpg" width="425" height="600" alt="Illustration:" /></a> +<p class="caption">Lord Rosse’s nebula</p> +</div> + +<p> +But since Keeler’s discovery there has been a decided turning away of +speculation another way. The form of the spiral nebulæ seems to be entirely +inconsistent with the theory of an originally globular or disk-shaped nebula +condensing around a sun and throwing or leaving off rings, to be subsequently +shaped into planets. Some astronomers, indeed, now reject Laplace’s hypothesis +<i>in toto,</i> preferring to think that even our solar system originated from +a spiral nebula. Since the spiral type prevails among the existing nebulæ, we +must make any mechanical theory of the development of stars and planetary +systems from them accord with the requirements which that form imposes. A +glance at the extraordinary variations upon the spiral which Professor Keeler’s +photographs reveal is sufficient to convince one of the difficulty of the task +of basing a general theory upon them. In truth, it is much easier to criticize +Laplace’s hypothesis than to invent a satisfactory substitute for it. If the +spiral nebulæ seem to oppose it there are other nebulæ which appear to support +it, and it may be that no one fixed theory can account for all the forms of +stellar evolution in the universe. Our particular planetary system may have +originated very much as the great French mathematician supposed, while others +have undergone, or are now undergoing, a different process of development. +There is always a too strong tendency to regard an important new discovery and +the theories and speculations based upon it as revolutionizing knowledge, and +displacing or overthrowing everything that went before. Upon the plea that +“Laplace only made a guess” more recent guesses have been driven to extremes +and treated by injudicious exponents as “the solid facts at last.” +</p> + +<div class="fig" style="width:100%;"> +<a name="illus13"></a> +<a href="images/image13.jpg"> +<img src="images/image13.jpg" width="482" height="600" alt="Illustration:" /></a> +<p class="caption">Wonderful spiral in triangulum</p> +</div> + +<p> +Before considering more recent theories than Laplace’s, let us see what the +nature of the photographic revelations is. The vast celestial maelstrom +discovered by Lord Rosse in the “Hunting Dogs” may be taken as the leading type +of the spiral nebulæ, although there are less conspicuous objects of the kind +which, perhaps, better illustrate some of their peculiarities. Lord Rosse’s +nebula appears far more wonderful in the photographs than in his drawings made +with the aid of his giant reflecting telescope at Parsonstown, for the +photographic plate records details that no telescope is capable of showing. +Suppose we look at the photograph of this object as any person of common sense +would look at any great and strange natural phenomenon. What is the first thing +that strikes the mind? It is certainly the appearance of violent whirling +motion. One would say that the whole glowing mass had been spun about with +tremendous velocity, or that it had been set rotating so rapidly that it had +become the victim of “centrifugal force,” one huge fragment having broken loose +and started to gyrate off into space. Closer inspection shows that in addition +to the principal focus there are various smaller condensations scattered +through the mass. These are conspicuous in the spirals. Some of them are +stellar points, and but for the significance of their location we might suppose +them to be stars which happen to lie in a line between us and the nebula. But +when we observe how many of them follow most faithfully the curves of the +spirals we cannot but conclude that they form an essential part of the +phenomenon; it is not possible to believe that their presence in such +situations is merely fortuitous. One of the outer spirals has at least a dozen +of these star-like points strung upon it; some of them sharp, small, and +distinct, others more blurred and nebulous, suggesting different stages of +condensation. Even the part which seems to have been flung loose from the main +mass has, in addition to its central condensation, at least one stellar point +gleaming in the half-vanished spire attached to it. Some of the more distant +stars scattered around the “whirlpool” look as if they too had been shot out of +the mighty vortex, afterward condensing into unmistakable solar bodies. There +are at least two curved rows of minute stars a little beyond the periphery of +the luminous whirl which clearly follow lines concentric with those of the +nebulous spirals. Such facts are simply dumbfounding for anyone who will bestow +sufficient thought upon them, for these are <i>suns,</i> though they may be +small ones; and what a birth is that for a sun! +</p> + +<p> +Look now again at the glowing spirals. We observe that hardly have they left +the central mass before they begin to coagulate. In some places they have a +“ropy” aspect; or they are like peascods filled with growing seeds, which +eventually will become stars. The great focus itself shows a similar tendency, +especially around its circumference. The sense that it imparts of a tremendous +shattering force at work is overwhelming. There is probably more matter in that +whirling and bursting nebula than would suffice to make a hundred solar +systems! It must be confessed at once that there is no confirmation of the +Laplacean hypothesis here; but what hypothesis will fit the facts? There is one +which it has been claimed does so, but we shall come to that later. In the +meanwhile, as a preparation, fix in the memory the appearance of that second +spiral mass spinning beside its master which seems to have spurned it away. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus14"></a> +<a href="images/image14.jpg"> +<img src="images/image14.jpg" width="407" height="600" alt="Illustration:" /></a> +<p class="caption">Spiral in Ursa Major</p> +</div> + +<p> +For a second example of the spiral nebulæ look at the one in the constellation +Triangulum. <i>God, how hath the imagination of puny man failed to comprehend +Thee!</i> Here is creation through destruction with a vengeance! The spiral +form of the nebula is unmistakable, but it is half obliterated amid the turmoil +of flying masses hurled away on all sides with tornadic fury. The focus itself +is splitting asunder under the intolerable strain, and in a little while, as +time is reckoned in the Cosmos, it will be gyrating into stars. And then look +at the cyclonic rain of already finished stars whirling round the outskirts of +the storm. Observe how scores of them are yet involved in the fading streams of +the nebulous spirals; see how they have been thrown into vast loops and curves, +of a beauty that half redeems the terror of the spectacle enclosed within their +lines—like iridescent cirri hovering about the edges of a hurricane. And so +again are suns born! +</p> + +<p> +Let us turn to the exquisite spiral in Ursa Major; how different its aspect +from that of the other! One would say that if the terrific coil in Triangulum +has all but destroyed itself in its fury, this one on the contrary has just +begun its self-demolition. As one gazes one seems to see in it the smooth, +swift, accelerating motion that precedes catastrophe. The central part is still +intact, dense, and uniform in texture. How graceful are the spirals that +smoothly rise from its oval rim and, gemmed with little stars, wind off into +the darkness until they have become as delicate as threads of gossamer! But at +bottom the story told here is the same—creation by gyration! +</p> + +<p> +Compare with the above the curious mass in Cetus. Here the plane of the +whirling nebula nearly coincides with our line of sight and we see the object +at a low angle. It is far advanced and torn to shreds, and if we could look at +it perpendicularly to its plane it is evident that it would closely resemble +the spectacle in Triangulum. +</p> + +<p> +Then take the famous Andromeda Nebula (see Frontispiece), which is so vast that +notwithstanding its immense distance even the naked eye perceives it as an +enigmatical wisp in the sky. Its image on the sensitive plate is the +masterpiece of astronomical photography; for wild, incomprehensible beauty +there is nothing that can be compared with it. Here, if anywhere, we look upon +the spectacle of creation in one of its earliest stages. The Andromeda Nebula +is apparently less advanced toward transformation into stellar bodies than is +that in Triangulum. The immense crowd of stars sprinkled over it and its +neighborhood seem in the main to lie this side of the nebula, and consequently +to have no connection with it. But incipient stars (in some places clusters of +them) are seen in the nebulous rings, while one or two huge masses seem to give +promise of transformation into stellar bodies of unusual magnitude. I say +“rings” because although the loops encompassing the Andromeda Nebula have been +called spirals by those who wish utterly to demolish Laplace’s hypothesis, yet +they are not manifestly such, as can be seen on comparing them with the +undoubted spirals of the Lord Rosse Nebula. They look quite as much like +circles or ellipses seen at an angle of, say, fifteen or twenty degrees to +their plane. If they are truly elliptical they accord fairly well with +Laplace’s idea, except that the scale of magnitude is stupendous, and if the +Andromeda Nebula is to become a solar system it will surpass ours in grandeur +beyond all possibility of comparison. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus15"></a> +<a href="images/image15.jpg"> +<img src="images/image15.jpg" width="457" height="600" alt="Illustration:" /></a> +<p class="caption">Nebula in Cetus</p> +</div> + +<p> +There is one circumstance connected with the spiral nebulæ, and conspicuous in +the Andromeda Nebula on account of its brightness, which makes the question of +their origin still more puzzling; they all show continuous spectra, which, as +we have before remarked, indicate that the mass from which the light comes is +either solid or liquid, or a gas under heavy pressure. Thus nebulæ fall into +two classes: the “white” nebulæ, giving a continuous spectrum; and the “green” +nebulæ whose spectra are distinctly gaseous. The Andromeda Nebula is the great +representative of the former class and the Orion Nebula of the latter. The +spectrum of the Andromeda Nebula has been interpreted to mean that it consists +not of luminous gas, but of a flock of stars so distant that they are +separately indistinguishable even with powerful telescopes, just as the +component stars of the Milky Way are indistinguishable with the naked eye; and +upon this has been based the suggestion that what we see in Andromeda is an +outer universe whose stars form a series of elliptical garlands surrounding a +central mass of amazing richness. But this idea is unacceptable if for no other +reason than that, as just said, all the spiral nebulæ possess the same kind of +spectrum, and probably no one would be disposed to regard them all as outer +universes. As we shall see later, the peculiarity of the spectra of the spiral +nebulæ is appealed to in support of a modern substitute for Laplace’s +hypothesis. +</p> + +<p> +Finally, without having by any means exhausted the variety exhibited by the +spiral nebulæ, let us turn to the great representative of the other species, +the Orion Nebula. In some ways this is even more marvelous than the others. The +early drawings with the telescope failed to convey an adequate conception +either of its sublimity or of its complication of structure. It exists in a +nebulous region of space, since photographs show that nearly the whole +constellation is interwoven with faintly luminous coils. To behold the entry of +the great nebula into the field even of a small telescope is a startling +experience which never loses its novelty. As shown by the photographs, it is an +inscrutable chaos of perfectly amazing extent, where spiral bands, radiating +streaks, dense masses, and dark yawning gaps are strangely intermingled without +apparent order. In one place four conspicuous little stars, better seen in a +telescope than in the photograph on account of the blurring produced by +over-exposure, are suggestively situated in the midst of a dark opening, and no +observer has ever felt any doubt that these stars have been formed from the +substance of the surrounding nebula. There are many other stars scattered over +its expanse which manifestly owe their origin to the same source. But compare +the general appearance of this nebula with the others that we have studied, and +remark the difference. If the unmistakably spiral nebulæ resemble bursting +fly-wheels or grindstones from whose perimeters torrents of sparks are flying, +the Orion Nebula rather recalls the aspect of a cloud of smoke and fragments +produced by the explosion of a shell. This idea is enforced by the look of the +outer portion farthest from the bright half of the nebula, where sharply edged +clouds with dark spaces behind seem to be billowing away as if driven by a wind +blowing from the center. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus16"></a> +<a href="images/image16.jpg"> +<img src="images/image16.jpg" width="600" height="595" alt="Illustration:" /></a> +<p class="caption">The Orion nebula</p> +</div> + +<p> +Next let us consider what scientific speculation has done in the effort to +explain these mysteries. Laplace’s hypothesis can certainly find no standing +ground either in the Orion Nebula or in those of a spiral configuration, +whatever may be its situation with respect to the grand Nebula of Andromeda, or +the “ring” and “planetary” nebulæ. Some other hypothesis more consonant with +the appearances must be found. Among the many that have been proposed the most +elaborate is the “Planetesimal Hypothesis” of Professors Chamberlin and +Moulton. It is to be remarked that it applies to the spiral nebulæ +distinctively, and not to an apparently chaotic mass of gas like the vast +luminous cloud in Orion. The gist of the theory is that these curious objects +are probably the result of close approaches to each other of two independent +suns, reminding us of what was said on this subject when we were dealing with +temporary stars. Of the previous history of these appulsing suns the theory +gives us no account; they are simply supposed to arrive within what may be +called an effective tide-producing distance, and then the drama begins. Some of +the probable consequences of such an approach have been noticed in Chapter 5; +let us now consider them a little more in detail. +</p> + +<p> +Tides always go in couples; if there is a tide on one side of a globe there +will be a corresponding tide on the other side. The cause is to be found in the +law that the force of gravitation varies inversely as the square of the +distance; the attraction on the nearest surface of the body exercised by +another body is greater than on its center, and greater yet than on its +opposite surface. If two great globes attract each other, each tends to draw +the other out into an ellipsoidal figure; they must be more rigid than steel to +resist this—and even then they cannot altogether resist. If they are liquid or +gaseous they will yield readily to the force of distortion, the amount of which +will depend upon their distance apart, for the nearer they are the greater +becomes the tidal strain. If they are encrusted without and liquid or gaseous +in the interior, the internal mass will strive to assume the figure demanded by +the tidal force, and will, if it can, burst the restraining envelope. Now this +is virtually the predicament of the body we call a sun when in the immediate +presence of another body of similarly great mass. Such a body is presumably +gaseous throughout, the component gases being held in a state of rigidity by +the compression produced by the tremendous gravitational force of their own +aggregate mass. At the surface such a body is enveloped in a shell of +relatively cool matter. Now suppose a great attracting body, such as another +sun, to approach near enough for the difference in its attraction on the two +opposite sides of the body and on its center to become very great; the +consequence will be a tidal deformation of the whole body, and it will lengthen +out along the line of the gravitational pull and draw in at the sides, and if +its shell offers considerable resistance, but not enough to exercise a complete +restraint, it will be violently burst apart, or blown to atoms, and the +internal mass will leap out on the two opposite sides in great fiery spouts. In +the case of a sun further advanced in cooling than ours the interior might be +composed of molten matter while the exterior crust had become rigid like the +shell of an egg; then the force of the “tidal explosion” produced by the +appulse of another sun would be more violent in consequence of the greater +resistance overcome. Such, then, is the mechanism of the first phase in the +history of a spiral nebula according to the Planetesimal Hypothesis. Two suns, +perhaps extinguished ones, have drawn near together, and an explosive outburst +has occured in one or both. The second phase calls for a more agile exercise of +the imagination. +</p> + +<p> +To simplify the case, let us suppose that only one of the tugging suns is +seriously affected by the strain. Its vast wings produced by the outburst are +twisted into spirals by their rotation and the contending attractions exercised +upon them, as the two suns, like battleships in desperate conflict, curve round +each other, concentrating their destructive energies. Then immense quantities +of débris are scattered about in which eddies are created, and finally, as the +sun that caused the damage goes on its way, leaving its victim to repair its +injuries as it may, the dispersed matter cools, condenses, and turns into +streams of solid particles circling in elliptical paths about their parent sun. +These particles, or fragments, are the “planetesimals” of the theory. In +consequence of the inevitable intersection of the orbits of the planetesimals, +nodes are formed where the flying particles meet, and at these nodes large +masses are gradually accumulated. The larger the mass the greater its +attraction, and at last the nodal points become the nuclei of great +aggregations from which planets are shaped. +</p> + +<p> +This, in very brief form, is the Planetesimal Hypothesis which we are asked to +substitute for that based on Laplace’s suggestion as an explanation of the mode +of origin of the solar system; and the phenomena of the spiral nebulæ are +appealed to as offering evident support to the new hypothesis. We are reminded +that they are elliptical in outline, which accords with the hypothesis; that +their spectra are not gaseous, which shows that they may be composed of solid +particles like the planetesimals; and that their central masses present an oval +form, which is what would result from the tidal effects, as just described. We +also remember that some of them, like the Lord Rosse and the Andromeda nebulæ, +are visually double, and in these cases we might suppose that the two masses +represent the tide-burst suns that ventured into too close proximity. It may be +added that the authors of the theory do not insist upon the appulse of two suns +as the <i>only</i> way in which the planetesimals may have originated, but it +is the only supposition that has been worked out. +</p> + +<p> +But serious questions remain. It needs, for instance, but a glance at the +Triangulum monster to convince the observer that it cannot be a solar system +which is being evolved there, but rather a swarm of stars. Many of the detached +masses are too vast to admit of the supposition that they are to be transformed +into planets, in our sense of planets, and the distances of the stars which +appear to have been originally ejected from the focal masses are too great to +allow us to liken the assemblage that they form to a solar system. Then, too, +no nodes such as the hypothesis calls for are visible. Moreover, in most of the +spiral nebulæ the appearances favor the view that the supposititious +encountering suns have not separated and gone each rejoicing on its way, after +having inflicted the maximum possible damage on its opponent, but that, on the +contrary, they remain in close association like two wrestlers who cannot escape +from each other’s grasp. And this is exactly what the law of gravitation +demands; stars cannot approach one another with impunity, with regard either to +their physical make-up or their future independence of movement. The theory +undertakes to avoid this difficulty by assuming that in the case of our system +the approach of the foreign body to the sun was not a close one—just close +enough to produce the tidal extrusion of the relatively insignificant quantity +of matter needed to form the planets. But even then the effect of the appulse +would be to change the direction of flight, both of the sun and of its visitor, +and there is no known star in the sky which can be selected as the sun’s +probable partner in their ancient <i>pas deux.</i> That there are unconquered +difficulties in Laplace’s hypothesis no one would deny, but in simplicity of +conception it is incomparably more satisfactory, and with proper modifications +could probably be made more consonant with existing facts in our solar system +than that which is offered to replace it. Even as an explanation of the spiral +nebulæ, not as solar systems in process of formation, but as the birthplaces of +stellar clusters, the Planetesimal Hypothesis would be open to many objections. +Granting its assumptions, it has undoubtedly a strong mathematical framework, +but the trouble is not with the mathematics but with the assumptions. Laplace +was one of the ablest mathematicians that ever lived, but he had never seen a +spiral nebula; if he had, he might have invented a hypothesis to suit its +phenomena. His actual hypothesis was intended only for our solar system, and he +left it in the form of a “note” for the consideration of his successors, with +the hope that they might be able to discover the full truth, which he confessed +was hidden from him. It cannot be said that that truth has yet been found, and +when it is found the chances are that intuition and not logic will have led to +it. +</p> + +<p> +The spiral nebulæ, then, remain among the greatest riddles of the universe, +while the gaseous nebulæ, like that of Orion, are no less mysterious, although +it seems impossible to doubt that both forms give birth to stars. It is but +natural to look to them for light on the question of the origin of our +planetary system; but we should not forget that the scale of the phenomena in +the two cases is vastly different, and the forces in operation may be equally +different. A hill may have been built up by a glacier, while a mountain may be +the product of volcanic forces or of the upheaval of the strata of the planet. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap07"></a>VII<br/> +The Banners of the Sun</h2> + +<p> +As all the world knows, the sun, a blinding globe pouring forth an +inconceivable quantity of light and heat, whose daily passage through the sky +is caused by the earth’s rotation on its axis, constitutes the most important +phenomenon of terrestial existence. Viewed with a dark glass to take off the +glare, or with a telescope, its rim is seen to be a sharp and smooth circle, +and nothing but dark sky is visible around it. Except for the interference of +the moon, we should probably never have known that there is any more of the sun +than our eyes ordinarily see. +</p> + +<p> +But when an eclipse of the sun occurs, caused by the interposition of the +opaque globe of the moon, we see its immediate surroundings, which in some +respects are more wonderful than the glowing central orb. These surroundings, +although not in the sense in which we apply the term to the gaseous envelope of +the earth, may be called the sun’s atmosphere. They consist of two very +different parts—first, the red “prominences,” which resemble tongues of flame +ascending thousands of miles above the sun’s surface; and, second, the +“corona,” which extends to distances of millions of miles from the sun, and +shines with a soft, glowing light. The two combined, when well seen, make a +spectacle without parallel among the marvels of the sky. Although many attempts +have been made to render the corona visible when there is no eclipse, all have +failed, and it is to the moon alone that we owe its revelation. To cover the +sun’s disk with a circular screen will not answer the purpose because of the +illumination of the air all about the observer. When the moon hides the sun, on +the other hand, the sunlight is withdrawn from a great cylinder of air +extending to the top of the atmosphere and spreading many miles around the +observer. There is then no glare to interfere with the spectacle, and the +corona appears in all its surprising beauty. The prominences, however, although +they were discovered during an eclipse, can now, with the aid of the +spectroscope, be seen at any time. But the prominences are rarely large enough +to be noticed by the naked eye, while the streamers of the corona, stretching +far away in space, like ghostly banners blown out from the black circle of the +obscuring moon, attract every eye, and to this weird apparition much of the +fear inspired by eclipses has been due. But if the corona has been a cause of +terror in the past it has become a source of growing knowledge in our time. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus17"></a> +<a href="images/image17.jpg"> +<img src="images/image17.jpg" width="600" height="454" alt="Illustration:" /></a> +<p class="caption">The Corona</p> +</div> + +<p> +The story of the first scientific observation of the corona and the prominences +is thrillingly interesting, and in fact dramatic. The observation was made +during the eclipse of 1842, which fortunately was visible all over Central and +Southern Europe so that scores of astronomers saw it. The interest centers in +what happened at Pavia in Northern Italy, where the English astronomer Francis +Baily had set up his telescope. The eclipse had begun and Bailey was busy at +his telescope when, to quote his own words in the account which he wrote for +the <i>Memoirs of the Royal Astronomical Society:</i> +</p> + +<div class="letter"> +<p> +I was astounded by a tremendous burst of applause from the streets below, and +at the same moment was electrified by the sight of one of the most brilliant +and splendid phenomena that can well be imagined; for at that instant the dark +body of the moon was suddenly surrounded with a corona, or kind of bright +glory, similar in shape and magnitude to that which painters draw round the +heads of saints... +</p> + +<p> +Pavia contains many thousand inhabitants, the major part of whom were at this +early hour walking about the streets and squares or looking out of windows in +order to witness this long-talked-of phenomenon; and when the total obscuration +took place, which was <i>instantaneous,</i> there was a universal shout from +every observer which “made the welkin ring,” and for the moment withdrew my +attention from the object with which I was immediately occupied. I had, indeed, +expected the appearance of a luminous circle round the moon during the time of +total obscurity; but I did not expect, from any of the accounts of preceding +eclipses that I had read, to witness so magnificent an exhibition as that which +took place... +</p> + +<p> +Splendid and astonishing, however, as this remarkable phenomenon really was, +and although it could not fail to call forth the admiration and applause of +every beholder, yet I must confess that there was at the same time something in +its singular and wonderful appearance that was appalling... +</p> + +<p> +But the most remarkable circumstance attending the phenomenon was the +appearance of <i>three large protuberances</i> apparently emanating from the +circumference of the moon, but evidently forming a portion of the corona. They +had the appearance of mountains of a prodigious elevation; their color was red +tinged with lilac or purple; perhaps the color of the peach-blossom would more +nearly represent it. They somewhat resembled the tops of the snowy Alpine +mountains when colored by the rising or the setting sun. They resembled the +Alpine mountains in another respect, inasmuch as their light was perfectly +steady, and had none of that flickering or sparkling motion so visible in other +parts of the corona... +</p> + +<p> +The whole of these protuberances were visible even to the last moment of total +obscuration, and when the first ray of light was admitted from the sun they +vanished, with the corona, altogether, and daylight was instantly restored. +</p> +</div> + +<p> +I have quoted nearly all of this remarkable description not alone for its +intrinsic interest, but because it is the best depiction that can be found of +the general phenomena of a total solar eclipse. Still, not every such eclipse +offers an equally magnificent spectacle. The eclipses of 1900 and 1905, for +instance, which were seen by the writer, the first in South Carolina and the +second in Spain, fell far short of that described by Bailey in splendor and +impressiveness. Of course, something must be allowed for the effect of +surprise; Bailey had not expected to see what was so suddenly disclosed to him. +But both in 1900 and 1905 the amount of scattered light in the sky was +sufficient in itself to make the corona appear faint, and there were no very +conspicuous prominences visible. Yet on both occasions there was manifest among +the spectators that mingling of admiration and awe of which Bailey speaks. The +South Carolinians gave a cheer and the ladies waved their handkerchiefs when +the corona, ineffably delicate of form and texture, <i>melted</i> into sight +and then in two minutes melted away again. The Spaniards, crowded on the +citadel hill of Burgos, with their king and his royal retinue in their midst, +broke out with a great clapping of hands as the awaited spectacle unfolded +itself in the sky; and on both occasions, before the applause began, after an +awed silence a low murmur ran through the crowds. At Burgos it is said many +made the sign of the cross. +</p> + +<p> +It was not long before Bailey’s idea that the prominences were a part of the +corona was abandoned, and it was perceived that the two phenomena were to a +great extent independent. At the eclipse of 1868, which the astronomers, +aroused by the wonderful scene of 1842, and eager to test the powers of the +newly invented spectroscope, flocked to India to witness, Janssen conceived the +idea of employing the spectroscope to render the prominences visible when there +was no eclipse. He succeeded the very next day, and these phenomena have been +studied in that way ever since. +</p> + +<p> +There are recognized two kinds of prominences—the “erruptive” and the +“quiescent.” The latter, which are cloud-like in form, may be seen almost +anywhere along the edge of the sun; but the former, which often shoot up as if +hurled from mighty volcanoes, appear to be associated with sun-spots, and +appear only above the zones where spots abound. Either of them, when seen in +projection against the brilliant solar disk, appears white, not red, as against +a background of sky. The quiescent prominences, whose elevation is often from +forty thousand to sixty thousand miles, consist, as the spectroscope shows, +mainly of hydrogen and helium. The latter, it will be remembered, is an element +which was known to be in the sun many years before the discovery that it also +exists in small quantities on the earth. A fact which may have a significance +which we cannot at present see is that the emanation from radium gradually and +spontaneously changes into helium, an alchemistical feat of nature that has +opened many curious vistas to speculative thinkers. The eruptive prominences, +which do not spread horizontally like the others, but ascend with marvelous +velocity to elevations of half a million miles or more, are apparently composed +largely of metallic vapors—<i>i.e.</i> metals which are usually solid on the +earth, but which at solar temperatures are kept in a volatilized state. The +velocity of their ascent occasionally amounts to three hundred or four hundred +miles per second. It is known from mathematical considerations that the +gravitation of the sun would not be able to bring back any body that started +from its surface with a velocity exceeding three hundred and eighty-three miles +per second; so it is evident that some of the matter hurled forth in eruptive +prominences may escape from solar control and go speeding out into space, +cooling and condensing into solid masses. There seems to be no reason why some +of the projectiles from the sun might not reach the planets. Here, then, we +have on a relatively small scale, <i>explosions</i> recalling those which it +has been imagined may be the originating cause of some of the sudden phenomena +of the stellar heavens. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus18"></a> +<a href="images/image18.jpg"> +<img src="images/image18.jpg" width="478" height="600" alt="Illustration:" /></a> +<p class="caption">A solar “prominence.” Photographed May 21, 1907</p> +</div> + +<p> +Of the sun-spots it is not our intention here specifically to speak, but they +evidently have an intimate connection with eruptive prominences, as well as +some relation, not yet fully understood, with the corona. Of the real cause of +sun-spots we know virtually nothing, but recent studies by Professor Hale and +others have revealed a strange state of things in the clouds of metallic vapors +floating above them and their surroundings. Evidences of a cyclonic tendency +have been found, and Professor Hale has proved that sun-spots are strong +magnetic fields, and consist of columns of ionized vapors rotating in opposite +directions in the two hemispheres. A fact which may have the greatest +significance is that titanium and vanadium have been found both in sun-spots +and in the remarkable variable Mira Ceti, a star which every eleven months, or +thereabout, flames up with great brilliancy and then sinks back to invisibility +with the naked eye. It has been suggested that sun-spots are indications of the +beginning of a process in the sun which will be intensified until it falls into +the state of such a star as Mira. Stars very far advanced in evolution, without +showing variability, also exhibit similar spectra; so that there is much reason +for regarding sunspots as emblems of advancing age. +</p> + +<p> +The association of the corona with sun-spots is less evident than that of the +eruptive prominences; still such an association exists, for the form and extent +of the corona vary with the sun-spot period of which we shall presently speak. +The constitution of the corona remains to be discovered. It is evidently in +part gaseous, but it also probably contains matter in the form of dust and +small meteors. It includes one substance altogether mysterious—“coronium.” +There are reasons for thinking that this may be the lightest of all the +elements, and Professor Young, its discoverer, said that it was “absolutely +unique in nature; utterly distinct from any other known form of matter, +terrestial, solar, or cosmical.” The enormous extent of the corona is one of +its riddles. Since the development of the curious subject of the “pressure of +light” it has been proposed to account for the sustentation of the corona by +supposing that it is borne upon the billows of light continually poured out +from the sun. Experiment has proved, what mathematical considerations had +previously pointed out as probable, that the waves of light exert a pressure or +driving force, which becomes evident in its effects if the body acted upon is +sufficiently small. In that case the light pressure will prevail over the +attraction of gravitation, and propel the attenuated matter away from the sun +in the teeth of its attraction. The earth itself would be driven away if, +instead of consisting of a solid globe of immense aggregate mass, it were a +cloud of microscopic particles. The reason is that the pressure varies in +proportion to the <i>surface</i> of the body acted upon, while the +gravitational attraction is proportional to the <i>volume,</i> or the total +amount of matter in the body. But the surface of any body depends upon the +<i>square</i> of its diameter, while the volume depends upon the <i>cube</i> of +the diameter. If, for instance, the diameter is represented by 4, the surface +will be proportional to 4 × 4, or 16, and the volume to 4 × 4 +× 4, or 64; but if the diameter is taken as 2, the surface will be 2 +× 2, or 4, and the volume 2 × 2 × 2, or 8. Now, the ratio of +4 to 8 is twice as great as that of 16 to 64. If the diameter is still further +decreased, the ratio of the surface to the volume will proportionally grow +larger; in other words, the pressure will gain upon the attraction, and +whatever their original ratio may have been, a time will come, if the +diminution of size continues, when the pressure will become more effective than +the attraction, and the body will be driven away. Supposing the particles of +the corona to be below the critical size for the attraction of a mass like that +of the sun to control them, they would be driven off into the surrounding space +and appear around the sun like the clouds of dust around a mill. We shall +return to this subject in connection with the Zodiacal Light, the Aurora, and +Comets. +</p> + +<p> +On the other hand, there are parts of the corona which suggest by their forms +the play of electric or magnetic forces. This is beautifully shown in some of +the photographs that have been made of the corona during recent eclipses. Take, +for instance, that of the eclipse of 1900. The sheaves of light emanating from +the poles look precisely like the “lines of force” surrounding the poles of a +magnet. It will be noticed in this photograph that the corona appears to +consist of two portions: one comprising the polar rays just spoken of, and the +other consisting of the broader, longer, and less-defined masses of light +extending out from the equatorial and middle-latitude zones. Yet even in this +more diffuse part of the phenomenon one can detect the presence of submerged +curves bearing more or less resemblance to those about the poles. Just what +part electricity or electro-magnetism plays in the mechanism of the solar +radiation it is impossible to say, but on the assumption that it is a very +important part is based the hypothesis that there exists a direct solar +influence not only upon the magnetism, but upon the weather of the earth. This +hypothesis has been under discussion for half a century, and still we do not +know just how much truth it represents. It is certain that the outbreak of +great disturbances on the sun, accompanied by the formation of sun-spots and +the upshooting of eruptive prominences (phenomena which we should naturally +expect to be attended by action), have been instantly followed by corresponding +“magnetic storms” on the earth and brilliant displays of the auroral lights. +There have been occasions when the influence has manifested itself in the most +startling ways, a great solar outburst being followed by a mysterious gripping +of the cable and telegraph systems of the world, as if an invisible and +irresistible hand had seized them. Messages are abruptly cut off, sparks leap +from the telegraph instruments, and the entire earth seems to have been thrown +into a magnetic flurry. These occurrences affect the mind with a deep +impression of the dependence of our planet on the sun, such as we do not derive +from the more familiar action of the sunlight on the growth of plants and other +phenomena of life depending on solar influences. +</p> + +<p> +Perhaps the theory of solar magnetic influence upon the weather is best known +in connection with the “sun-spot cycle.” This, at any rate, is, as already +remarked, closely associated with the corona. Its existence was discovered in +1843 by the German astronomer Schwabe. It is a period of variable length, +averaging about eleven years, during which the number of spots visible on the +sun first increases to a maximum, then diminishes to a minimum, and finally +increases again to a maximum. For unknown reasons the period is sometimes two +or three years longer than the average and sometimes as much shorter. +Nevertheless, the phenomena always recur in the same order. Starting, for +instance, with a time when the observer can find few or no spots, they +gradually increase in number and size until a maximum, in both senses, is +reached, during which the spots are often of enormous size and exceedingly +active. After two or three years they begin to diminish in number, magnitude, +and activity until they almost or quite disappear. A strange fact is that when +a new period opens, the spots appear first in high northern and southern +latitudes, far from the solar equator, and as the period advances they not only +increase in number and size, but break out nearer and nearer to the equator, +the last spots of a vanishing period sometimes lingering in the equatorial +region after the advance-guard of its successor has made its appearance in the +high latitudes. Spots are never seen on the equator nor near the poles. It was +not very long after the discovery of the sun-spot cycle that the curious +observation was made that a striking coincidence existed between the period of +the sun-spots and another period affecting the general magnetic condition of +the earth. When a curved line representing the varying number of sun-spots was +compared with another curve showing the variations in the magnetic state of the +earth the two were seen to be in almost exact accord, a rise in one curve +corresponding to a rise in the other, and a fall to a fall. Continued +observation has proved that this is a real coincidence and not an accidental +one, so that the connection, although as yet unexplained, is accepted as +established. But does the influence extend further, and directly affect the +weather and the seasons as well as the magnetic elements of the earth? A final +answer to this question cannot yet be given, for the evidence is contradictory, +and the interpretations put upon it depend largely on the predilections of the +judges. +</p> + +<p> +But, in a broad sense, the sun-spots and the phenomena connected with them +<i>must</i> have a relation to terrestial meteorology, for they prove the sun +to be a variable star. Reference was made, a few lines above, to the +resemblance of the spectra of sun-spots to those of certain stars which seem to +be failing through age. This in itself is extremely suggestive; but if this +resemblance had never been discovered, we should have been justified in +regarding the sun as variable in its output of energy; and not only variable, +but probably increasingly so. The very inequalities in the sun-spot cycle are +suspicious. When the sun is most spotted its total light may be reduced by +one-thousandth part, although it is by no means certain that its outgiving of +thermal radiations is then reduced. A loss of one-thousandth of its luminosity +would correspond to a decrease of .0025 of a stellar magnitude, considering the +sun as a star viewed from distant space. So slight a change would not be +perceptible; but it is not alone sun-spots which obscure the solar surface, its +entire globe is enveloped with an obscuring veil. When studied with a powerful +telescope the sun’s surface is seen to be thickly mottled with relatively +obscure specks, so numerous that it has been estimated that they cut off from +one-tenth to one-twentieth of the light that we should receive from it if the +whole surface were as brilliant as its brightest parts. The condition of other +stars warrants the conclusion that this obscuring envelope is the product of a +process of refrigeration which will gradually make the sun more and more +variable until its history ends in extinction. Looking backward, we see a time +when the sun must have been more brilliant than it is now. At that time it +probably shone with the blinding white splendor of such stars as Sirius, Spica, +and Vega; now it resembles the relatively dull Procyon; in time it will turn +ruddy and fall into the closing cycle represented by Antares. Considering that +once it must have been more radiantly powerful than at present, one is tempted +to wonder if that could have been the time when tropical life flourished within +the earth’s polar circles, sustained by a vivific energy in the sun which it +has now lost. +</p> + +<p> +The corona, as we have said, varies with the sun-spot cycle. When the spots are +abundant and active the corona rises strong above the spotted zones, forming +immense beams or streamers, which on one occasion, at least, had an observed +length of <i>ten million miles.</i> At the time of a spot minimum the corona is +less brilliant and has a different outline. It is then that the curved polar +rays are most conspicuous. Thus the vast banners of the sun, shaken out in the +eclipse, are signals to tell of its varying state, but it will probably be long +before we can read correctly their messages. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap08"></a>VIII<br/> +The Zodiacal Light Mystery</h2> + +<p> +There is a singular phenomenon in the sky—one of the most puzzling of all—which +has long arrested the attention of astronomers, defying their efforts at +explanation, but which probably not one in a hundred, and possibly not one in a +thousand, of the readers of this book has ever seen. Yet its name is often +spoken, and it is a conspicuous object if one knows when and where to look for +it, and when well seen it exhibits a mystical beauty which at the same time +charms and awes the beholder. It is called “The Zodiacal Light,” because it +lies within the broad circle of the Zodiac, marking the sun’s apparent annual +path through the stars. What it is nobody has yet been able to find out with +certainty, and books on astronomy usually speak of it with singular reserve. +But it has given rise to many remarkable theories, and a true explanation of it +would probably throw light on a great many other celestial mysteries. The Milky +Way is a more wonderful object to look upon, but its nature can be +comprehended, while there is a sort of uncanniness about the Zodiacal Light +which immediately impresses one upon seeing it, for its part in the great +scheme of extra-terrestrial affairs is not evident. +</p> + +<p> +If you are out-of-doors soon after sunset—say, on an evening late in the month +of February—you may perceive, just after the angry flush of the dying winter’s +day has faded from the sky, a pale ghostly presence rising above the place +where the sun went down. The writer remembers from boyhood the first time it +was pointed out to him and the unearthly impression that it made, so that he +afterward avoided being out alone at night, fearful of seeing the spectral +thing again. The phenomenon brightens slowly with the fading of the twilight, +and soon distinctly assumes the shape of an elongated pyramid of pearly light, +leaning toward the south if the place of observation is in the northern +hemisphere. It does not impress the observer at all in the same manner as the +Milky Way; that looks far off and is clearly among the stars, but the Zodiacal +Light seems closer at hand, as if it were something more intimately concerning +the earth. To all it immediately suggests a connection, also, with the sunken +sun. If the night is clear and the moon absent (and if you are in the country, +for city lights ruin the spectacles of the sky), you will be able to watch the +apparition for a long time. You will observe that the light is brightest near +the horizon, gradually fading as the pyramidal beam mounts higher, but in +favorable circumstances it may be traced nearly to the meridian south of the +zenith, where its apex at last vanishes in the starlight. It continues visible +during the evenings of March and part of April, after which, ordinarily, it is +seen no more, or if seen is relatively faint and unimpressive. But when autumn +comes it appears again, this time not like a wraith hovering above the westward +tomb of the day-god, but rather like a spirit of the morning announcing his +reincarnation in the east. +</p> + +<p> +The reason why the Zodiacal Light is best seen in our latitudes at the periods +just mentioned is because at those times the Zodiac is more nearly +perpendicular to the horizon, first in the west and then in the east; and, +since the phenomenon is confined within the borders of the Zodiac, it cannot be +favorably placed for observation when the zodiacal plane is but slightly +inclined to the horizon. Its faint light requires the contrast of a background +of dark sky in order to be readily perceptible. But within the tropics, where +the Zodiac is always at a favorable angle, the mysterious light is more +constantly visible. Nearly all observant travelers in the equatorial regions +have taken particular note of this phenomenon, for being so much more +conspicuous there than in the temperate zones it at once catches the eye and +holds the attention as a novelty. Humboldt mentions it many times in his works, +for his genius was always attracted by things out of the ordinary and difficult +of explanation, and he made many careful observations on its shape, its +brilliancy, and its variations; for there can be no doubt that it does vary, +and sometimes to an astonishing degree. It is said that it once remained +practically invisible in Europe for several years in succession. During a trip +to South Africa in 1909 an English astronomer, Mr E. W. Maunder, found a +remarkable difference between the appearance of the Zodiacal Light on his going +and coming voyages. In fact, when crossing the equator going south he did not +see it at all; but on returning he had, on March 6th, when one degree south of +the equator, a memorable view of it. +</p> + +<p class="letter"> +It was a bright, clear night, and the Zodiacal Light was extraordinarily +brilliant—brighter than he had ever seen it before. The Milky Way was not to be +compared with it. The brightest part extended 75° from the sun. There was a +faint and much narrower extension which they could just make out beyond the +Pleiades along the ecliptic, but the greater part of the Zodiacal Light showed +as a broad truncated column, and it did not appear nearly as conical as he had +before seen it. +</p> + +<p> +When out of the brief twilight of intertropical lands, where the sun drops +vertically to the horizon and night rushes on like a wave of darkness, the +Zodiacal Light shoots to the very zenith, its color is described as a golden +tint, entirely different from the silvery sheen of the Milky Way. If I may +venture again to refer to personal experiences and impressions, I will recall a +view of the Zodiacal Light from the summit of the cone of Mt Etna in the autumn +of the year 1896 (more briefly described in <i>Astronomy with the Naked +Eye</i>). There are few lofty mountains so favorably placed as Etna for +observations of this kind. It was once resorted to by Prof. George E. Hale, in +an attempt to see the solar corona without an eclipse. Rising directly from +sea-level to an elevation of nearly eleven thousand feet, the observer on its +summit at night finds himself, as it were, lost in the midst of the sky. But +for the black flanks of the great cone on which he stands he might fancy +himself to be in a balloon. On the occasion to which I refer the world beneath +was virtually invisible in the moonless night. The blaze of the constellations +overhead was astonishingly brilliant, yet amid all their magnificence my +attention was immediately drawn to a great tapering light that sprang from the +place on the horizon where the sun would rise later, and that seemed to be +blown out over the stars like a long, luminous veil. It was the finest view of +the Zodiacal light that I had ever enjoyed—thrilling in its strangeness—but I +was almost disheartened by the indifference of my guide, to whom it was only a +light and nothing more. If he had no science, he had less poetry—rather a +remarkable thing, I thought, for a child of his clime. The Light appeared to me +to be distinctly brighter than the visible part of the Milky Way which included +the brilliant stretches in Auriga and Perseus, and its color, if one may speak +of color in connection with such an object, seemed richer than that of the +galactic band; but I did not think of it as yellow, although Humboldt has +described it as resembling a golden curtain drawn over the stars, and Du +Chaillu in Equatorial Africa found it of a bright yellow color. It may vary in +color as in conspicuousness. The fascination of that extraordinary sight has +never faded from my memory. I turned to regard it again and again, although I +had never seen the stellar heavens so brilliant, and it was one of the last +things I looked for when the morning glow began softly to mount in the east, +and Sicily and the Mediterranean slowly emerged from the profound shadow +beneath us. +</p> + +<p> +The Zodiacal Light seems never to have attracted from astronomers in general +the amount of careful attention that it deserves; perhaps because so little can +really be made of it as far as explanation is concerned. I have referred to the +restraint that scientific writers apparently feel in speaking of it. The +grounds for speculation that it affords may be too scanty to lead to long +discussions, yet it piques curiosity, and as we shall see in a moment has +finally led to a most interesting theory. Once it was the subject of an +elaborate series of studies which carried the observer all round the world. +That was in 1845—46, during the United States Exploring Expedition that visited +the then little known Japan. The chaplain of the fleet, the Rev. Mr Jones, went +out prepared to study the mysterious light in all its phases. He saw it from +many latitudes on both sides of the equator, and the imagination cannot but +follow him with keen interest in his world-circling tour, keeping his eyes +every night fixed upon the phantasm overhead, whose position shifted with that +of the hidden sun. He demonstrated that the flow extends at times completely +across the celestial dome, although it is relatively faint directly behind the +earth. On his return the government published a large volume of his +observations, in which he undertook to show that the phenomenon was due to the +reflection of sunlight from a ring of meteoric bodies encircling the earth. +But, after all, this elaborate investigation settled nothing. +</p> + +<p> +Prof. E. E. Barnard has more recently devoted much attention to the Zodiacal +Light, as well as to a strange attendant phenomenon called the “Gegenschein,” +or Counterglow, because it always appears at that point in the sky which is +exactly opposite the sun. The Gegenschein is an extremely elusive phenomenon, +suitable only for eyes that have been specially trained to see it. Professor +Newcomb has cautiously remarked that +</p> + +<p class="letter"> +it is said that in that point of the heavens directly opposite the sun there is +an elliptical patch of light... This phenomenon is so difficult to account for +that its existence is sometimes doubted; yet the testimony in its favor is +difficult to set aside. +</p> + +<p> +It certainly cannot be set aside at all since the observations of Barnard. I +recall an attempt to see it under his guidance during a visit to Mount +Hamilton, when he was occupied there with the Lick telescope. Of course, both +the Gegenschein and the Zodiacal Light are too diffuse to be studied with +telescopes, which, so to speak, magnify them out of existence. They can only be +successfully studied with the naked eye, since every faintest glimmer that they +afford must be utilized. This is especially true of the Gegenschein. At Mount +Hamilton, Mr Barnard pointed out to me its location with reference to certain +stars, but with all my gazing I could not be sure that I saw it. To him, on the +contrary, it was obvious; he had studied it for months, and was able to +indicate its shape, its boundaries, its diameter, and the declination of its +center with regard to the ecliptic. There is not, of course, the shadow of a +doubt of the existence of the Gegenschein, and yet I question if one person in +a million has ever seen or ever will see it. The Zodiacal Light, on the other +hand, is plain enough, provided that the time and the circumstances of the +observation are properly chosen. +</p> + +<p> +In the attempts to explain the Zodiacal Light, the favorite hypothesis has been +that it is an appendage of the sun—perhaps simply an extension of the corona in +the plane of the ecliptic, which is not very far from coinciding with that of +the sun’s equator. This idea is quite a natural one, because of the evident +relation of the light to the position of the sun. The vast extension of the +equatorial wings of the corona in 1878 gave apparent support to this +hypothesis; if the substance of the corona could extend ten million miles from +the sun, why might it not extend even one hundred million, gradually fading out +beyond the orbit of the earth? A variation of this hypothesis assumes that the +reflection is due to swarms of meteors circling about the sun, in the plane of +its equator, all the way from its immediate neighborhood to a distance +exceeding that of the earth. But in neither form is the hypothesis +satisfactory; there is nothing in the appearance of the corona to indicate that +it extends even as far as the planet Mercury, while as to meteors, the orbits +of the known swarms do not accord with the hypothesis, and we have no reason to +believe that clouds of others exist traveling in the part of space where they +would have to be in order to answer the requirements of the theory. The +extension of the corona in 1878 did not resemble in its texture the Zodiacal +Light. +</p> + +<p> +Now, it has so often happened in the history of science that an important +discovery in one branch has thrown unexpected but most welcome light upon some +pending problem in some other branch, that a strong argument might be based +upon that fact alone against the too exclusive devotion of many investigators +to the narrow lines of their own particular specialty; and the Zodiacal Light +affords a case in point, when it is considered in connection with recent +discoveries in chemistry and physics. From the fact that atoms are compound +bodies made up of corpuscles at least a thousand times smaller than the +smallest known atom—a fact which astounded most men of science when it was +announced a few years ago—a new hypothesis has been developed concerning the +nature of the Zodiacal Light (as well as other astronomical riddles), and this +hypothesis comes not from an astronomer, but from a chemist and physicist, the +Swede, Svante Arrhenius. In considering an outline of this new hypothesis we +need neither accept nor reject it; it is a case rather for suspension of +judgment. +</p> + +<p> +To begin with, it carries us back to the “pressure of light” mentioned in the +preceding chapter. The manner in which this pressure is believed generally to +act was there sufficiently explained, and it only remains to see how it is +theoretically extended to the particles of matter supposed to constitute the +Zodiacal Light. We know that corpuscles, or “fragments of atoms” negatively +electrified, are discharged from hot bodies. Streams of these “ions” pour from +many flames and from molten metals; and the impact of the cathode and +ultra-violet rays causes them to gush even from cold bodies. In the vast +laboratory of the sun it is but reasonable to suppose that similar processes +are taking place. “As a very hot metal emits these corpuscles,” says Prof. J. +J. Thomson, “it does not seem an improbable hypothesis that they are emitted by +that very hot body, the sun.” Let it be assumed, then, that the sun does emit +them; what happens next? Negatively charged corpuscles, it is known, serve as +nuclei to which particles of matter in the ordinary state are attracted, and it +is probable that those emitted from the sun immediately pick up loads in this +manner and so grow in bulk. If they grow large enough the gravitation of the +sun draws them back, and they produce a negative charge in the solar +atmosphere. But it is probable that many of the particles do not attain the +critical size which, according to the principles before explained, would enable +the gravitation of the sun to retain them in opposition to the pressure of the +waves of light, and with these particles the light pressure is dominant. Clouds +of them may be supposed to be continually swept away from the sun into +surrounding space, moving mostly in or near the plane of the solar equator, +where the greatest activity, as indicated by sunspots and related phenomena, is +taking place. As they pass outward into space many of them encounter the earth. +If the earth, like the moon, had no atmosphere the particles would impinge +directly on its surface, giving it a negative electric charge. But the presence +of the atmosphere changes all that, for the first of the flying particles that +encounter it impart to it their negative electricity, and then, since like +electric charges repel like, the storm of particles following will be sheered +off from the earth, and will stream around it in a maze of hyperbolic paths. +Those that continue on into space beyond the earth may be expected to continue +picking up wandering particles of matter until their bulk has become so great +that the solar attraction prevails again over the light pressure acting upon +them, and they turn again sunward. Passing the earth on their return they will +increase the amount of dust-clouds careering round it; and these will be +further increased by the action of the ultra-violet rays of the sunlight +causing particles to shoot radially away from the earth when the negative +charge of the upper atmosphere has reached a certain amount, which particles, +although starting sunward, will be swept back to the earth with the oncoming +streams. As the final result of all this accumulation of flying and gyrating +particles in the earth’s neighborhood, we are told that the latter must be +transformed into the semblance of a gigantic solid-headed comet provided with +streaming tails, the longest of them stretching away from the direction of the +sun, while another shorter one extends toward the sun. This shorter tail is due +to the particles that we have just spoken of as being driven sunward from the +earth by the action of ultra-violet light. No doubt this whole subject is too +technical for popular statement; but at any rate the general reader can +understand the picturesque side of the theory, for its advocates assure us that +if we were on the moon we would doubtless be able to see the comet-like tails +of the earth, and then we could appreciate the part that they play in producing +the phenomenon of the Zodiacal Light. +</p> + +<p> +That the Light as we see it could be produced by the reflection of sunlight +from swarms of particles careering round the earth in the manner supposed by +Arrhenius’ hypothesis is evident enough; and it will be observed that the new +theory, after all, is only another variant of the older one which attributes +the Zodiacal Light to an extension of the solar corona. But it differs from the +older theory in offering an explanation of the manner in which the extension is +effected, and it differentiates between the corona proper and the streams of +negative particles shot away from the sun. In its details the hypothesis of +Arrhenius also affords an explanation of many peculiarities of the Zodiacal +Light, such as that it is confined to the neighborhood of the ecliptic, and +that it is stronger on the side of the earth which is just turning away from a +position under the sun than on the other side; but it would carry us beyond our +limits to go into these particulars. The Gegenschein, according to this theory, +is a part of the same phenomenon as the Zodiacal Light, for by the laws of +perspective it is evident that the reflection from the streams of particles +situated at a point directly opposite to the sun would be at a maximum, and +this is the place which the Gegenschein occupies. Apart from its geometrical +relations to the position of the sun, the variability of the Zodiacal Light +appears to affirm its solar dependence, and this too would be accounted for by +Arrhenius’ hypothesis better than by the old theory of coronal extension. The +amount of corpuscular discharge from the sun must naturally be governed by the +state of relative activity or inactivity of the latter, and this could not but +be reflected in the varying splendor of the Zodiacal Light. But much more +extended study than has yet been given to the subject will be required before +we can feel that we know with reasonable certainty what this mysterious +phenomenon really is. By the hypothesis of Arrhenius every planet that has an +atmosphere must have a Zodiacal Light attending it, but the phenomenon is too +faint for us to be able to see it in the case, for instance, of Venus, whose +atmosphere is very abundant. The moon has no corresponding “comet’s tail” +because, as already explained, of the lack of a lunar atmosphere to repel the +streams by becoming itself electrified; but if there were a lunar Zodiacal +Light, no doubt we could see it because of the relative nearness of our +satellite. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap09"></a>IX<br/> +Marvels of the Aurora</h2> + +<p> +One of the most vivid recollections of my early boyhood is that of seeing my +father return hastily into the house one evening and call out to the family: +“Come outside and look at the sky!” Ours was a country house situated on a +commanding site, and as we all emerged from the doorway we were dumbfounded to +see the heavens filled with pale flames which ran licking and quivering over +the stars. Instantly there sprang into my terrified mind the recollection of an +awful description of “the Day of Judgment” (the <i>Dies Iræ</i>), which I had +heard with much perturbation of spirit in the Dutch Reformed church from the +lips of a tall, dark-browed, dreadfully-in-earnest preacher of the +old-fashioned type. My heart literally sank at sight of the spectacle, for it +recalled the preacher’s very words; it was just as he had said it would be, and +it needed the assured bearing of my elders finally to convince me that +</p> + +<p class="poem"> +That Day of Wrath, O dreadful day,<br/> +When Heaven and Earth shall pass away,<br/> +As David and the Sibyl say +</p> + +<p class="noindent"> +had not actually come upon us. And even the older members of the household were +not untouched with misgivings when menacing spots of crimson appeared, breaking +out now here, now there, in the shuddering sky. Toward the north the spectacle +was appalling. A huge arch spanned an unnaturally dark segment resting on the +horizon, and above this arch sprang up beams and streamers in a state of +incessant agitation, sometimes shooting up to the zenith with a velocity that +took one’s breath, and sometimes suddenly falling into long ranks, and +<i>marching, marching, marching,</i> like an endless phalanx of fiery specters, +and moving, as I remember, always from east to west. The absolute silence with +which these mysterious evolutions were performed and the quavering reflections +which were thrown upon the ground increased the awfulness of the exhibition. +Occasionally enormous curtains of lambent flame rolled and unrolled with a +majestic motion, or were shaken to and fro as if by a mighty, noiseless wind. +At times, too, a sudden billowing rush would be made toward the zenith, and for +a minute the sky overhead would glow so brightly that the stars seemed to have +been consumed. The spectacle continued with varying intensity for hours. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus19"></a> +<a href="images/image19.jpg"> +<img src="images/image19.jpg" width="600" height="459" alt="Illustration:" /></a> +<p class="caption">Auroral beams seen in England</p> +</div> + +<p> +This exhibition occurred in Central New York, a latitude in which the Aurora +Borealis is seldom seen with so much splendor. I remember another similar one +seen from the city of New York in November, 1882. On this last occasion some +observers saw a great upright beam of light which majestically moved across the +heavens, stalking like an apparition in the midst of the auroral pageant, of +whose general movements it seemed to be independent, maintaining always its +upright posture, and following a magnetic parallel from east to west. This +mysterious beam was seen by no less than twenty-six observers in different +parts of the country, and a comparison of their observations led to a curious +calculation indicating that the apparition was about <i>one hundred and +thirty-three miles tall</i> and moved at the speed of ten miles per second! +</p> + +<div class="fig" style="width:100%;"> +<a name="illus20"></a> +<a href="images/image20.jpg"> +<img src="images/image20.jpg" width="600" height="403" alt="Illustration:" /></a> +<p class="caption">Auroral arches of an eliptic form seen in the Arctic regions</p> +</div> + +<p> +But, as everybody knows, it is in the Arctic regions that the Aurora, or the +“Northern Lights,” can best be seen. There, in the long polar night, when for +months together the sun does not rise, the strange coruscations in the sky +often afford a kind of spectral daylight in unison with the weird scenery of +the world of ice. The pages in the narratives of Arctic exploration that are +devoted to descriptions of the wonderful effects of the Northern Lights are +second to none that man has ever penned in their fascination. The lights, as I +have already intimated, display astonishing colors, particularly shades of red +and green, as they flit from place to place in the sky. The discovery that the +magnetic needle is affected by the Aurora, quivering and darting about in a +state of extraordinary excitement when the lights are playing in the sky, only +added to the mystery of the phenomenon until its electro-magnetic nature had +been established. This became evident as soon as it was known that the focus of +the displays was the magnetic pole; and when the far South was visited the +Aurora Australis was found, having its center at the South Magnetic Pole. Then, +if not before, it was clear that the earth was a great globular magnet, having +its poles of opposite magnetism, and that the auroral lights, whatever their +precise cause might be, were manifestations of the magnetic activity of our +planet. After the invention of magnetic telegraphy it was found that whenever a +great Aurora occurred the telegraph lines were interrupted in their operation, +and the ocean cables ceased to work. Such a phenomenon is called a “magnetic +storm.” +</p> + +<p> +The interest excited by the Aurora in scientific circles was greatly stimulated +when, in the last half of the nineteenth century, it was discovered that it is +a phenomenon intimately associated with disturbances on the sun. The ancient +“Zurich Chronicles,” extending from the year 1000 to the year 1800, in which +both sun-spots visible to the naked eye and great displays of the auroral +lights were recorded, first set Rudolf Wolf on the track of this discovery. The +first notable proof of the suspected connection was furnished with dramatic +emphasis by an occurrence which happened on September 1, 1859. Near noon on +that day two intensely brilliant points suddenly broke out in a group of +sun-spots which were under observation by Mr R. C. Carrington at his +observatory at Redhill, England. The points remained visible for not more than +five minutes, during which interval they moved <i>thirty-five thousand +miles</i> across the solar disk. Mr R. Hodgson happened to see the same +phenomenon at his observatory at Highgate, and thus all possibility of +deception was removed. But neither of the startled observers could have +anticipated what was to follow, and, indeed, it was an occurrence which has +never been precisely duplicated. I quote the eloquent account given by Miss +Clerke in her <i>History of Astronomy During the Nineteenth Century.</i> +</p> + +<p class="letter"> +This unique phenomenon seemed as if specially designed to accentuate the +inference of a sympathetic relation between the earth and the sun. From August +28 to September 4, 1859, a magnetic storm of unparalleled intensity, extent, +and duration was in progress over the entire globe. Telegraphic communication +was everywhere interrupted—except, indeed, that it was in some cases found +practicable to work the lines <i>without batteries</i> by the agency of the +earth-currents alone; sparks issued from the wires; gorgeous auroras draped the +skies in solemn crimson over both hemispheres, and even in the tropics; the +magnetic needle lost all trace of continuity in its movements and darted to and +fro as if stricken with inexplicable panic. The coincidence was even closer. +<i>At the very instant</i> of the solar outburst witnessed by Carrington and +Hodgson the photographic apparatus at Kew registered a marked disturbance of +all the three magnetic elements; while shortly after the ensuing midnight the +electric agitation culminated, thrilling the whole earth with subtle +vibrations, and lighting up the atmosphere from pole to pole with coruscating +splendors which perhaps dimly recall the times when our ancient planet itself +shone as a star. +</p> + +<p> +If this amazing occurrence stood alone, and as I have already said it has never +been exactly duplicated, doubt might be felt concerning some of the inferences +drawn from it; but in varying forms it has been repeated many times, so that +now hardly anyone questions the reality of the assumed connection between solar +outbursts and magnetic storms accompanied by auroral displays on the earth. It +is true that the late Lord Kelvin raised difficulties in the way of the +hypothesis of a direct magnetic action of the sun upon the earth, because it +seemed to him that an inadmissible quantity of energy was demanded to account +for such action. But no calculation like that which he made is final, since all +calculations depend upon the validity of the data; and no authority is +unshakable in science, because no man can possess omniscience. It was Lord +Kelvin who, but a few years before the thing was actually accomplished, +declared that aerial navigation was an impracticable dream, and demonstrated +its impracticability by calculation. However the connection may be brought +about, it is as certain as evidence can make it that solar outbursts are +coincident with terrestial magnetic disturbances, and coincident in such a way +as to make the inference of a causal connection irresistible. The sun is only a +little more than a hundred times its own diameter away from the earth. Why, +then, with the subtle connection between them afforded by the ether which +conveys to us the blinding solar light and the life-sustaining solar heat, +should it be so difficult to believe that the sun’s enormous electric energies +find a way to us also? No doubt the impulse coming from the sun acts upon the +earth after the manner of a touch upon a trigger, releasing energies which are +already stored up in our planet. +</p> + +<p> +But besides the evidence afforded by such occurrences as have been related of +an intimate connection between solar outbreaks and terrestial magnetic +flurries, attended by magnificent auroral displays, there is another line of +proof pointing in the same direction. Thus, it is known that the sun-spot +period, as remarked in a preceding chapter, coincides in a most remarkable +manner with the periodic fluctuations in the magnetic state of the earth. This +coincidence runs into the most astonishing details. For instance, when the +sun-spot period shortens, the auroral period shortens to precisely the same +extent; as the short sun-spot periods usually bring the most intense outbreaks +of solar activity, so the corresponding short auroral periods are attended by +the most violent magnetic storms; a secular period of about two hundred and +twenty-two years affecting sun-spots is said to have its auroral duplicate; a +shorter period of fifty-five and a half years, which some observers believe +that they have discovered appears also to be common to the two phenomena; and +yet another “superposed” period of about thirty-five years, which some +investigators aver exists, affects sun-spots and aurora alike. In short, the +coincidences are so numerous and significant that one would have to throw the +doctrine of probability to the winds in order to be able to reject the +conclusion to which they so plainly lead. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus21"></a> +<a href="images/image21.jpg"> +<img src="images/image21.jpg" width="600" height="395" alt="Illustration:" /></a> +<p class="caption">Auroral curtain seen in Scandinavia</p> +</div> + +<p> +But still the question recurs: How is the influence transmitted? Here Arrhenius +comes once more with his hypothesis of negative corpuscles, or ions, driven +away from the sun by light-pressure—a hypothesis which seems to explain so many +things—and offers it also as an explanation of the way in which the sun creates +the Aurora. He would give the Aurora the same lineage with the Zodiacal Light. +To understand the application of this theory we must first recall the fact that +the earth is a great magnet having its two opposite poles of magnetism, one +near the Arctic and the other near the Antarctic Circle. Like all magnets, the +earth is surrounded with “lines of force,” which, after the manner of the +curved rays we saw in the photograph of a solar eclipse, start from a pole, +rising at first nearly vertically, then bend gradually over, passing high above +the equator, and finally descending in converging sheaves to the opposite pole. +Now the axis of the earth is so placed in space that it lies at nearly a right +angle to the direction of the sun, and as the streams of negatively charged +particles come pouring on from the sun (see the last preceding chapter), they +arrive in the greatest numbers over the earth’s equatorial regions. There they +encounter the lines of magnetic force at the place where the latter have their +greatest elevation above the earth, and where their direction is horizontal to +the earth’s surface. Obeying a law which has been demonstrated in the +laboratory, the particles then follow the lines of force toward the poles. +While they are above the equatorial regions they do not become luminescent, +because at the great elevation that they there occupy there is virtually no +atmosphere; but as they pass on toward the north and the south they begin to +descend with the lines of force, curving down to meet at the poles; and, +encountering a part of the atmosphere comparable in density with what remains +in an exhausted Crookes tube, they produce a glow of cathode rays. This glow is +conceived to represent the Aurora, which may consequently be likened to a +gigantic exhibition of vacuum-tube lights. Anybody who recalls his student days +in the college laboratory and who has witnessed a display of Northern Lights +will at once recognize the resemblance between them in colors, forms, and +behavior. This resemblance had often been noted before Arrhenius elaborated his +hypothesis. +</p> + +<p> +Without intending to treat his interesting theory as more than a possibly +correct explanation of the phenomena of the Aurora, we may call attention to +some apparently confirmatory facts. One of the most striking of these relates +to a seasonal variation in the average number of auroræ. It has been observed +that there are more in March and September than at any other time of the year, +and fewer in June and December; moreover (and this is a delicate test as +applied to the theory), they are slightly rarer in June than in December. Now +all these facts seem to find a ready explanation in the hypothesis of +Arrhenius, thus: (1) The particles issuing from the sun are supposed to come +principally from the regions whose excitement is indicated by the presence of +sun-spots (which accords with Hale’s observation that sun-spots are columns of +ionized vapors), and these regions have a definite location on either side of +the solar equator, seldom approaching it nearer than within 5° or 10° +north or south, and never extending much beyond 35° toward either pole; (2) +The equator of the sun is inclined about 7° to the plane of the earth’s +orbit, from which it results that twice in a year—<i>viz.,</i> in June and +December—the earth is directly over the solar equator, and twice a +year—<i>viz.,</i> in March and September—when it is farthest north or south of +the solar equator, it is over the inner edge of the sun-spot belts. Since the +corpuscles must be supposed to be propelled radially from the sun, few will +reach the earth when the latter is over the solar equator in June and December, +but when it is over, or nearly over, the spot belts, in March and September, it +will be in the line of fire of the more active parts of the solar surface, and +relatively rich streams of particles will reach it. This, as will be seen from +what has been said above, is in strict accord with the observed variations in +the frequency of auroræ. Even the fact that somewhat fewer auroræ are seen in +June than in December also finds its explanation in the known fact that the +earth is about three million miles nearer the sun in the winter than in the +summer, and the number of particles reaching it will vary, like the intensity +of light, inversely as the square of the distance. These coincidences are +certainly very striking, and they have a cumulative force. If we accept the +theory, it would appear that we ought to congratulate ourselves that the +inclination of the sun’s equator is so slight, for as things stand the earth is +never directly over the most active regions of the sun-spots, and consequently +never suffers from the maximum bombardment of charged particles of which the +sun is capable. Incessant auroral displays, with their undulating draperies, +flitting colors, and marching columns might not be objectionable from the point +of view of picturesqueness, but one magnetic storm of extreme intensity +following closely upon the heels of another, for months on end, crazing the +magnetic needle and continually putting the telegraph and cable lines out of +commission, to say nothing of their effect upon “wireless telegraphy”, would +hardly add to the charms of terrestrial existence. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus22"></a> +<a href="images/image22.jpg"> +<img src="images/image22.jpg" width="600" height="462" alt="Illustration:" /></a> +<p class="caption">Auroral arches seen in Scandinavia</p> +</div> + +<p> +One or two other curious points in connection with Arrhenius’ hypothesis may be +mentioned. First, the number of auroræ, according to his explanation, ought to +be greatest in the daytime, when the face of the earth on the sunward side is +directly exposed to the atomic bombardment. Of course visual observation can +give us no information about this, since the light of the Aurora is never +sufficiently intense to be visible in the presence of daylight, but the records +of the magnetic observatories can be, and have been, appealed to for +information, and they indicate that the facts actually accord with the theory. +Behind the veil of sunlight in the middle of the afternoon, there is good +reason to believe, auroral exhibitions often take place which would eclipse in +magnificence those seen at night if we could behold them. Observation shows, +too, that auroræ are more frequent before than after midnight, which is just +what we should expect if they originate in the way that Arrhenius supposes. +Second, the theory offers an explanation of the alleged fact that the formation +of clouds in the upper air is more frequent in years when auroræ are most +abundant, because clouds are the result of the condensation of moisture upon +floating particles in the atmosphere (in an absolutely dustless atmosphere +there would be no clouds), and it has been proved that negative ions like those +supposed to come from the sun play a master part in the phenomena of cloud +formation. +</p> + +<p> +Yet another singular fact, almost mystical in its suggestions, may be +mentioned. It seems that the dance of the auroral lights occurs most frequently +during the absence of the moon from the hemisphere in which they appear, and +that they flee, in greater part, to the opposite hemisphere when the moon’s +revolution in an orbit considerably inclined to the earth’s equator brings her +into that where they have been performing. Arrhenius himself discovered this +curious relation of auroral frequency to the position of the moon north or +south of the equator, and he explains it in this way. The moon, like the earth, +is exposed to the influx of the ions from the sun; but having no atmosphere, or +almost none, to interfere with them, they descend directly upon her surface and +charge her with an electric negative potential to a very high degree. In +consequence of this she affects the electric state of the upper parts of the +earth’s atmosphere where they lie most directly beneath her, and thus prevents, +to a large extent, the negative discharges to which the appearance of the +Aurora is due. And so “the extravagant and erring spirit” of the Aurora avoids +the moon as Hamlet’s ghost fled at the voice of the cock announcing the +awakening of the god of day. +</p> + +<p> +There are even other apparent confirmations of the hypothesis, but we need not +go into them. We shall, however, find one more application of it in the next +chapter, for it appears to be a kind of cure-all for astronomical troubles; at +any rate it offers a conceivable solution of the question, How does the sun +manage to transmit its electric influence to the earth? And this solution is so +grandiose in conception, and so novel in the mental pictures that it offers, +that its acceptance would not in the least detract from the impression that the +Aurora makes upon the imagination. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap10"></a>X<br/> +Strange Adventures of Comets</h2> + +<p> +The fears and legends of ancient times before Science was born, and the +superstitions of the Dark Ages, sedulously cultivated for theological purposes +by monks and priests, have so colored our ideas of the influence that comets +have had upon the human mind that many readers may be surprised to learn that +it was the apparition of a wonderful comet, that of 1843, which led to the +foundation of our greatest astronomical institution, the Harvard College +Observatory. No doubt the comet superstition existed half a century ago, as, +indeed, it exists yet today, but in this case the marvelous spectacle in the +sky proved less effective in inspiring terror than in awakening a desire for +knowledge. Even in the sixteenth century the views that enlightened minds took +of comets tended powerfully to inspire popular confidence in science, and +Halley’s prediction, after seeing and studying the motion of the comet which +appeared in 1682, that it would prove to be a regular member of the sun’s +family and would be seen returning after a period of about seventy-six years, +together with the fulfillment of that prediction, produced a revulsion from the +superstitious notions which had so long prevailed. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus23"></a> +<a href="images/image23.jpg"> +<img src="images/image23.jpg" width="508" height="600" alt="Illustration:" /></a> +<p class="caption">Swift’s comet. Taken at Arequipa, March 30 1892</p> +</div> + +<p> +Then the facts were made plain that comets are subject to the law of +gravitation equally with the planets; that there are many which regularly +return to the neighborhood of the sun (perihelion); and that these travel in +orbits differing from those of the planets only in their greater eccentricity, +although they have the peculiarity that they do not, like the planets, all go +round the sun in the same direction, and do not keep within the general plane +of the planetary system, but traverse it sometimes from above and sometimes +from below. Other comets, including most of the “great” ones, appear to travel +in parabolic or, in a few cases, hyperbolic orbits, which, not being closed +curves, never bring them back again. But it is not certain that these orbits +may not be extremely eccentric ellipses, and that after the lapse of hundreds, +or thousands, of years the comets that follow them may not reappear. The +question is an interesting one, because if all orbits are really ellipses, then +all comets must be permanent members of the solar system, while in the contrary +case many of them are simply visitors, seen once and never to be seen again. +The hypothesis that comets are originally interlopers might seem to derive some +support from the fact that the certainly periodic ones are associated, in +groups, with the great outer planets, whose attraction appears to have served +as a trap for them by turning them into elliptical orbits and thus making them +prisoners in the solar system. Jupiter, owing to his great mass and his +commanding situation in the system, is the chief “comet-catcher;” but he +catches them not for himself, but for the sun. Yet if comets do come originally +from without the borders of the planetary system, it does not, by any means, +follow that they were wanderers at large in space before they yielded to the +overmastering attraction of the sun. Investigation of the known cometary +orbits, combined with theoretical considerations, has led some astronomers to +the conclusion that as the sun travels onward through space he “picks up <i>en +route</i>” cometary masses which, without belonging strictly to his empire, are +borne along in the same vast “cosmical current” that carries the solar system. +</p> + +<p> +But while no intelligent person any longer thinks that the appearance of a +great comet is a token from the heavenly powers of the approaching death of a +mighty ruler, or the outbreak of a devastating war, or the infliction of a +terrible plague upon wicked mankind, science itself has discovered mysteries +about comets which are not less fascinating because they are more intellectual +than the irrational fancies that they have displaced. To bring the subject +properly before the mind, let us see what the principal phenomena connected +with a comet are. +</p> + +<p> +At the present day comets are ordinarily “picked up” with the telescope or the +photographic plate before any one except their discoverer is aware of their +existence, and usually they remain so insignificant in appearance that only +astronomers ever see them. Yet so great is the prestige of the word “comet” +that the discovery of one of these inconspicuous wanderers, and its subsequent +movements, become items of the day’s news which everybody reads with the +feeling, perhaps, that at least he knows what is going on in the universe even +if he doesn’t understand it. But a truly great comet presents quite a different +proposition. It, too, is apt to be detected coming out of the depths of space +before the world at large can get a glimpse of it, but as it approaches the sun +its aspect undergoes a marvelous change. Agitated apparently by solar +influence, it throws out a long streaming tail of nebulous light, directed away +from the sun and looking as if blown out like a pennon by a powerful wind. +Whatever may be the position of the comet with regard to the sun, as it circles +round him it continually keeps its tail on the off side. This, as we shall soon +see, is a fact of capital importance in relation to the probable nature of +comets’ tails. Almost at the same time that the formation of the tail is +observed a remarkable change takes place in the comet’s head, which, by the +way, is invariably and not merely occasionally its most important part. On +approaching the sun the head usually contracts. Coincidently with this +contraction a nucleus generally makes its appearance. This is a bright, +star-like point in the head, and it probably represents the totality of solid +matter that the comet possesses. But it is regarded as extremely unlikely that +even the nucleus consists of a uniformly solid mass. If it were such, comets +would be far more formidable visitors when they pass near the planets than they +have been found to be. The diameter of the nucleus may vary from a few hundred +up to several thousand miles; the heads, on the average, are from twenty-five +thousand to one hundred thousand miles in diameter, although a few have greatly +exceeded these dimensions; that of the comet of 1811, one of the most +stupendous ever seen, was a million and a quarter miles in diameter! As to the +tails, not withstanding their enormous length—some have been more than a +hundred million miles long—there is reason to believe that they are of extreme +tenuity, “as rare as vacuum.” The smallest stars have been seen shining through +their most brilliant portions with undiminished luster. +</p> + +<p> +After the nucleus has been formed it begins to throw out bright jets directed +toward the sun. A stream, and sometimes several streams, of light also project +sunward from the nucleus, occasionally appearing like a stunted tail directed +oppositely to the real tail. Symmetrical envelopes which, seen in section, +appear as half circles or parabolas, rise sunward from the nucleus, forming a +concentric series. The ends of these stream backward into the tail, to which +they seem to supply material. Ordinarily the formation of these ejections and +envelopes is attended by intense agitation of the nucleus, which twists and +turns, swinging and gyrating with an appearance of the greatest violence. +Sometimes the nucleus is seen to break up into several parts. The entire heads +of some comets have been split asunder in passing close around the sun; The +comet of 1882 retreated into space after its perihelion passage with <i>five +heads</i> instead of the one that it had originally, and each of these heads +had its own tail! +</p> + +<p> +The possession of the spectroscope has enabled astronomers during later years +to study the chemical composition of comets by analyzing their light. At first +the only substances thus discovered in them were hydro-carbon compounds, due +evidently to the gaseous envelopes in which some combination of hydrogen with +carbon existed. Behind this gaseous spectrum was found a faint continuous +spectrum ascribed to the nucleus, which apparently both reflects the sunlight +and gives forth the light of a glowing solid or liquid. Subsequently sodium and +iron lines were found in cometary spectra. The presence of iron would seem to +indicate that some of these bodies may be much more massive than observations +on their attractive effects have indicated. In some recent comets, such as +Morehouse’s, in 1908, several lines have been found, the origin of which is +unknown. +</p> + +<p> +Without going back of the nineteenth century we may find records of some of the +most extraordinary comets that man has ever looked upon. In 1811, still spoken +of as “the year of the comet,” because of the wonderful vintage ascribed to the +skyey visitor, a comet shaped like a gigantic sword amazed the whole world, +and, as it remained visible for seventeen months, was regarded by superstitious +persons as a symbol of the fearful happenings of Napoleon’s Russian campaign. +This comet, the extraordinary size of whose head, greatly exceeding that of the +sun itself, has already been mentioned, was also remarkable for exhibiting so +great a brilliancy without approaching even to the earth’s distance from the +sun. But there was once a comet (and only once—in the year 1729) which never +got nearer to the sun than four times the distance of the earth and yet +appeared as a formidable object in the sky. As Professor Young has remarked, +“it must have been an enormous comet to be visible from such a distance.” And +we are to remember that there were no great telescopes in the year 1729. That +comet affects the imagination like a phantom of space peering into the solar +system, displaying its enormous train afar off (which, if it had approached as +near as other comets, would probably have become <i>the</i> celestial wonder of +all human memory), and then turning away and vanishing in the depths of +immensity. +</p> + +<p> +In 1843 a comet appeared which was so brilliant that it could be seen in broad +day close beside the sun! This was the first authenticated instance of that +kind, but the occurrence was to be repeated, as we shall see in a moment, less +than forty years later. +</p> + +<p> +The splendid comet of 1858, usually called Donati’s, is remembered by many +persons yet living. It was, perhaps, both as seen by the naked eye and with the +telescope, the most beautiful comet of which we have any record. It too marked +a rich vintage year, still remembered in the vineyards of France, where there +is a popular belief that a great comet ripens the grape and imparts to the wine +a flavor not attainable by the mere skill of the cultivator. There are “comet +wines,” carefully treasured in certain cellars, and brought forth only when +their owner wishes to treat his guests to a sip from paradise. +</p> + +<p> +The year 1861 saw another very remarkable comet, of an aspect strangely vast +and diffuse, which is believed to have swept the earth with its immense tail +when it passed between us and the sun on the night of June 30th, an event which +produced no other known effect than the appearance of an unwonted amount of +scattered light in the sky. +</p> + +<p> +The next very notable comet was the “Great Southern Comet” of 1880, which was +not seen from the northern hemisphere. It mimicked the aspect of the famous +comet of 1843, and to the great surprise of astronomers appeared to be +traveling in the same path. This proved to be the rising of the curtain for an +astronomical sensation unparalleled in its kind; for two years later another +brilliant comet appeared, first in the southern hemisphere, <i>and it too +followed the same track.</i> The startling suggestion was now made that this +comet was identical with those of 1843 and 1880, its return having been +hastened by the resistance experienced in passing twice through the coronal +envelope, and there were some who thought that it would now swing swiftly round +and then plunge straight into the sun, with consequences that might be +disastrous to us on account of the “flash of heat” that would be produced by +the impact. Nervous people were frightened, but observation soon proved that +the danger was imaginary, for although the comet almost grazed the sun, and +must have rushed through two or three million miles of the coronal region, no +retardation of its immense velocity was perceptible, and it finally passed away +in a damaged condition, as before remarked, and has never since appeared. +</p> + +<p> +Then the probable truth was perceived—<i>viz.,</i> that the three comets (1843, +1880, and 1882) were not one identical body, but three separate ones all +traveling in the same orbit. It was found, too, that a comet seen in 1668 bore +similar insignia of relationship. The natural inference was that these four +bodies had once formed a single mass which had been split apart by the +disruptive action of the sun. Strength was lent to this hypothesis by the fact +that the comet of 1882 was apparently torn asunder during its perihelion +passage, retreating into space in a dissevered state. But Prof. George Forbes +has a theory that the splitting of the original cometary mass was effected by +an unknown planet, probably greater than Jupiter, situated at a hundred times +the earth’s distance from the sun, and revolving in a period of a thousand +years. He supposes that the original comet was not that of 1668, but one seen +in 1556, which has since been “missing,” and that its disruption occurred from +an encounter with the supposititious planet about the year 1700. Truly from +every point of view comets are the most extraordinary of adventurers! +</p> + +<p> +The comet of 1882 was likewise remarkable for being visible, like its +predecessor of 1843, in full daylight in close proximity to the sun. The story +of its detection when almost in contact with the solar disk is dramatic. It had +been discovered in the southern hemisphere only a couple of weeks before its +perihelion, which occurred on September 17th, and on the forenoon of that day +it was seen by Doctor Common in England, and by Doctor Elkin and Mr Finlay at +the Cape of Good Hope, almost touching the sun. It looked like a dazzling white +bird with outspread wings. The southern observers watched it go <i>right into +the sun,</i> when it instantly disappeared. What had happened was that the +comet in passing its perihelion point had swung exactly between the earth and +the sun. On the following morning it was seen from all parts of the world close +by the sun on the opposite side, and it remained thus visible for three days, +gradually receding from the solar disk. It then became visible for northern +observers in the morning sky before sunrise, brandishing a portentous +sword-shaped tail which, if it had been in the evening sky, would have excited +the wonder of hundreds of millions, but situated where it was, comparatively +few ever saw it. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus24"></a> +<a href="images/image24.jpg"> +<img src="images/image24.jpg" width="285" height="650" alt="Illustration:" /></a> +<p class="caption">Daniels’ comet. August 11, 1907</p> +</div> + +<p> +The application of photography to the study of comets has revealed many curious +details which might otherwise have escaped detection, or at best have remained +subject to doubt. It has in particular shown not only the precise form of the +tails, but the remarkable vicissitudes that they undergo. Professor Barnard’s +photographs of Brooks’ comet in 1893 suggested, by the extraordinary changes in +the form of the tail which they revealed, that the comet was encountering a +series of obstructions in space which bent and twisted its tail into fantastic +shapes. The reader will observe the strange form into which the tail was thrown +on the night of October 21st. A cloud of meteors through which the comet was +passing might have produced such deformations of its tail. In the photograph of +Daniels’ comet of 1907, a curious striping of the tail will be noticed. The +short bright streaks seen in the photograph, it may be explained, are the +images of stars which are drawn out into lines in consequence of the fact that +the photographic telescope was adjusted to follow the motion of the comet while +the stars remained at rest. +</p> + +<p> +But the adventures of comets are not confined to possible encounters with +unknown obstacles. We have referred to the fact that the great planets, and +especially Jupiter, frequently interfere with the motions of comets. This +interference is not limited to the original alteration of their orbits from +possible parabolas to ellipses, but is sometimes exercised again and again, +turning the bewildered comets into elliptical paths of all degrees of +eccentricity. A famous example of this kind of planetary horse-play is +furnished by the story of Lexell’s missing comet. This comet was first seen in +1770. Investigation showed that it was moving in an orbit which should bring it +back to perihelion every five and a half years; yet it had never been seen +before and, although often searched for, has never been seen since. Laplace and +Leverrier proved mathematically that in 1767 it had approached so close to +Jupiter as to be involved among the orbits of his satellites. What its track +had been before is not known, but on that occasion the giant planet seized the +interloper, threw it into a short elliptic orbit and sent it, like an arrested +vagrant, to receive sentence at the bar of the sun. On this journey it passed +within less than 1,500,000 miles of the earth. The form of orbit which Jupiter +had impressed required, as we have said, its return in about five and a half +years; but soon after 1770 it had the misfortune a second time to encounter +Jupiter at close range, and he, as if dissatisfied with the leniency of the +sun, or indignant at the stranger’s familiarity, seized the comet and hurled it +out of the system, or at any rate so far away that it has never since been able +to rejoin the family circle that basks in the immediate rays of the solar +hearth. Nor is this the only instance in which Jupiter has dealt summarily with +small comets that have approached him with too little deference. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus25"></a> +<a href="images/image25.jpg"> +<img src="images/image25.jpg" width="600" height="463" alt="Illustration:" /></a> +<p class="caption">Brooks’ comet. Photographed by Barnard, October 21, 1893</p> +</div> + +<p> +The function which Jupiter so conspicuously fulfills as master of the hounds to +the sun is worth considering a little more in detail. To change the figure, +imagine the sun in its voyage through space to be like a majestic battleship +surrounded by its scouts. Small vessels (the comets, as they are overhauled by +the squadron, are taken in charge by the scouts, with Jupiter for their chief, +and are forced to accompany the fleet, but not all are impressed. If a strange +comet undertakes to run across Jupiter’s bows the latter brings it to, and +makes prize of it by throwing it into a relatively small ellipse with the sun +for its focus. Thenceforth, unless, as happened to the unhappy comet of Lexell, +it encounters Jupiter again in such a way as to be diverted by him into a more +distant orbit, it can never get away. About thirty comets are now known to have +thus been captured by the great planet, and they are called “Jupiter’s Comet +Family.” But, on the other hand, if a wandering comet crosses the wake of the +chief planetary scout the latter simply drives it away by accelerating its +motion and compels it to steer off into open space. The transformation of +comets into meteors will be considered in the next chapter, but here, in +passing, mention may be made of the strange fate of one member of Jupiter’s +family, Biela’s comet, which, having become over bold in its advances to its +captor, was, after a few revolutions in is impressed orbit, torn to pieces and +turned into a flock of meteors. +</p> + +<p> +And now let us return to the mystery of comets’ tails. That we are fully +justified in speaking of the tails of comets as mysterious is proved by the +declaration of Sir John Herschel, who averred, in so many words, that “there is +some profound secret and mystery of nature concerned in this phenomenon,” and +this profound secret and mystery has not yet been altogether cleared up. +Nevertheless, the all-explaining hypothesis of Arrhenius offers us once more a +certain amount of aid. Comets’ tails, Arrhenius assures us, are but another +result of the pressure of light. The reader will recall the applications of +this theory to the Zodiacal Light and the Aurora. In the form in which we now +have to deal with it, the supposition is made that as a comet approaches the +sun eruptions of vapor, due to the solar heat, occur in its nucleus. These are +naturally most active on the side which is directly exposed to the sun, whence +the appearance of the immense glowing envelopes that surround the nucleus on +the sunward side. Among the particles of hydro-carbon, and perhaps solid carbon +in the state of fine dust, which are thus set free there will be many whose +size is within the critical limit which enables the light-waves from the sun to +drive them away. Clouds of such particles, then, will stream off behind the +advancing comet, producing the appearance of a tail. This accounts for the fact +that the tails of comets are always directed away from the sun, and it also +explains the varying forms of the tails and the extraordinary changes that they +undergo. The speed of the particles driven before the light-waves must depend +upon their size and weight, the lightest of a given size traveling the most +swiftly. By accretion certain particles might grow, thus losing velocity and +producing the appearance of bunches in the tail, such as have been observed. +The hypothesis also falls in with the researches of Bredichin, who has divided +the tails of comets into three principal classes—<i>viz.:</i> (1) Those which +appear as long, straight rays; (2) Those which have the form of curved plumes +or scimitars; (3) Those which are short, brushy, and curved sharply backward +along the comet’s path. In the first type he calculates the repulsive force at +from twelve to fifteen times the force of gravity; in the second at from two to +four times; and in the third at about one and a half times. The straight tails +he ascribes to hydrogen because the hydrogen atom is the lightest known; the +sword-shaped tails to hydro-carbons; and the stumpy tails to vaporized iron. It +will be seen that, if the force driving off the tails is that which Arrhenius +assumes it to be, the forms of those appendages would accord with those that +Bredichin’s theory calls for. At the same time we have an explanation of the +multiple tails with which some comets have adorned themselves. The comet of +1744, for instance, had at one time no less than seven tails spread in a wide +curved brush behind it. Donati’s comet of 1858 also had at least two tails, the +principal one sword-shaped and the other long, narrow, and as straight as a +rule. According to Bredichin, the straight tail must have been composed of +hydrogen, and the other of some form of hydro-carbon whose atoms are heavier +than those of hydrogen, and, consequently, when swept away by the storm of +light-waves, followed a curvature depending upon the resultant of the forces +operating upon them. The seven tails of the comet of 1744 presented a kind of +diagram graphically exhibiting its complex composition, and, if we knew a +little more about the constituents of a comet, we might be able to say from the +amount of curvature of the different tails just what were the seven substances +of which that comet consisted. +</p> + +<p> +If these theories seem to the reader fantastic, at any rate they are no more +fantastic than the phenomena that they seek to explain. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap11"></a>XI<br/> +Meteors, Fire-Balls, and Meteorites</h2> + +<p> +One of the most terrorizing spectacles with which the heavens have ever caused +the hearts of men to quake occurred on the night of November 13, 1833. On that +night North America, which faced the storm, was under a continual rain of fire +from about ten o’clock in the evening until daybreak. +</p> + +<p> +<i>The fragments of a comet had struck the earth.</i> +</p> + +<p> +But the meaning of what had happened was not discovered until long afterward. +To the astronomers who, with astonishment not less than that of other people, +watched the wonderful scene, it was an unparalleled “shower of meteors.” They +did not then suspect that those meteors had once formed the head of a comet. +Light dawned when, a year later, Prof. Denison Olmsted, of Yale College, +demonstrated that the meteors had all moved in parallel orbits around the sun, +and that these orbits intersected that of the earth at the point where our +planet happened to be on the memorable night of November 13th. Professor +Olmsted even went so far as to suggest that the cloud of meteors that had +encountered the earth might form a diffuse comet; but full recognition of the +fact that they were cometary débris came later, as the result of further +investigation. The key to the secret was plainly displayed in the spectacle +itself, and was noticed without being understood by thousands of the +terror-stricken beholders. It was <i>an umbrella of fire</i> that had opened +overhead and covered the heavens; in other words, the meteors all radiated from +a particular point in the constellation Leo, and, being countless as the +snowflakes in a winter tempest, they ribbed the sky with fiery streaks. +Professor Olmsted showed that the radiation of the meteors from a fixed point +was an effect of perspective, and in itself a proof that they were moving in +parallel paths when they encountered the earth. The fact was noted that there +had been a similar, but incomparably less brilliant, display of meteors on the +same day of November, 1832, and it was rightly concluded that these had +belonged to the same stream, although the true relationship of the phenomena +was not immediately apprehended. Olmsted ascribed to the meteors a revolution +about the sun once in every six months, bringing them to the intersection of +their orbit with that of the earth every November 13th; but later investigators +found that the real period was about thirty-three and one-quarter years, so +that the great displays were due three times in a century, and their return was +confidently predicted for the year 1866. The appearance of the meteors in 1832, +a year before the great display, was ascribed to the great length of the stream +which they formed in space—so great that they required more than two years to +cross the earth’s orbit. In 1832 the earth had encountered a relatively rare +part of the stream, but in 1833, on returning to the crossing-place, it found +there the richest part of the stream pouring across its orbit. This explanation +also proved to be correct, and the predicted return in 1866 was duly witnessed, +although the display was much less brilliant than in 1833. It was followed by +another in 1867. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus26"></a> +<a href="images/image26.jpg"> +<img src="images/image26.jpg" width="600" height="459" alt="Illustration:" /></a> +<p class="caption">Curious forms of meteorite trains<br/> +Nos. 1 to 6 show the changes undergone by a train left by a meteorite which passed near the “Great Dipper”; +7 shows the changes and drift of a train seen in the constellation Virgo; 8 is the singular train +of the meteorite of February 22, 1909, near the Pole Star. (From <i>La Nature</i>.)</p> +</div> + +<p> +In the mean time Olmsted’s idea of a cometary relationship of the meteors was +demonstrated to be correct by the researches of Schiaparelli and others, who +showed that not only the November meteors, but those of August, which are seen +more or less abundantly every year, traveled in the tracks of well-known +comets, and had undoubtedly an identical origin with those comets. In other +words the comets and the meteor-swarms were both remnants of original masses +which had probably been split up by the action of the sun, or of some planet to +which they had made close approaches. The annual periodicity of the August +meteors was ascribed to the fact that the separation had taken place so long +ago that the meteors had become distributed all around the orbit, in +consequence of which the earth encountered some of them every year when it +arrived at the crossing-point. Then Leverrier showed that the original comet +associated with the November meteors was probably brought into the system by +the influence of the planet Uranus in the year 126 of the Christian era. +Afterward Alexander Herschel identified the tracks of no less than seventy-six +meteor-swarms (most of them inconspicuous) with those of comets. The still more +recent researches of Mr W. F. Denning make it probable that there are no +meteors which do not belong to a flock or system probably formed by the +disintegration of a cometary mass; even the apparently sporadic ones which +shoot across the sky, “lost souls in the night,” being members of flocks which +have become so widely scattered that the earth sometimes takes weeks to pass +through the region of space where their paths lie. +</p> + +<p> +The November meteors should have exhibited another pair of spectacles in 1899 +and 1900, and their failure to do so caused at first much disappointment, until +it was made plain that a good reason existed for their absence. It was found +that after their last appearance, in 1867, they had been disturbed in their +movements by the planets Jupiter and Saturn, whose attractions had so shifted +the position of their orbit that it no longer intersected that of the earth, as +it did before. Whether another planetary interference will sometime bring the +principal mass of the November meteors back to the former point of intersection +with the earth’s orbit is a question for the future to decide. It would seem +that there may be several parallel streams of the November meteors, and that +some of them, like those of August, are distributed entirely around the orbit, +so that every mid-November we see a few of them. +</p> + +<p> +We come now to a very remarkable example of the disintegration of a comet and +the formation of a meteor-stream. In 1826 Biela, of Josephstadt, Austria, +discovered a comet to which his name was given. Calculation showed that it had +an orbital period of about six and a half years, belonging to Jupiter’s +“family.” On one of its returns, in 1846, it astonished its watchers by +suddenly splitting in two. The two comets thus formed out of one separated to a +distance of about one hundred and sixty thousand miles, and then raced side by +side, sometimes with a curious ligature connecting them, like Siamese twins, +until they disappeared together in interplanetary space. In 1852 they came +back, still nearly side by side, but now the distance between them had +increased to a million and a quarter of miles. After that, at every recurrence +of their period, astronomers looked for them in vain, until 1872, when an +amazing thing happened. On the night of November 28th, when the earth was +crossing the plane of the orbit of the missing comet, a brilliant shower of +meteors burst from the northern sky, traveling nearly in the track which the +comet should have pursued. The astronomers were electrified. Klinkerfues, of +Göttingen, telegraphed to Pogson, of Madras: <i>“Biela touched earth; search +near Theta Centauri.”</i> Pogson searched in the place indicated and saw a +cometary mass retreating into the southern heavens, where it was soon swallowed +from sight! +</p> + +<div class="fig" style="width:100%;"> +<a name="illus27"></a> +<a href="images/image27.jpg"> +<img src="images/image27.jpg" width="151" height="700" alt="Illustration:" /></a> +<p class="caption">Section of the atmosphere up to 100 kilometers.<br/> +Showing the mean elevation at which meteorites and meteors make their +appearance. Below are shown the elevation of Mount Everest, the highest manned +balloon ascent by M. Berson; the height of cirrus clouds; the highest free +balloon ascent; and the elevation attained by the clouds of fire-dust ejected +by the Krakatoa eruption in 1883. (From <i>La Nature</i>) +</p> +</div> + +<p> +Since then the Biela meteors have been among the recognized periodic spectacles +of the sky, and few if any doubt that they represent a portion of the missing +comet whose disintegration began with the separation into two parts in 1846. +The comet itself has never since been seen. The first display of these meteors, +sometimes called the “Andromedes,” because they radiate from the constellation +Andromeda, was remarkable for the great brilliancy of many of the fire-balls +that shot among the shower of smaller sparks, some of which were described as +equaling the full moon in size. None of them is known to have reached the +earth, but during the display of the same meteors in 1885 a meteoric mass fell +at Mazapil in Northern Mexico (it is now in the Museum at Vienna), which many +have thought may actually be a piece of the original comet of Biela. This +brings us to the second branch of our subject. +</p> + +<p> +More rare than meteors or falling stars, and more startling, except that they +never appear in showers, are the huge balls of fire which occasionally dart +through the sky, lighting up the landscapes beneath with their glare, leaving +trains of sparks behind them, often producing peals of thunder when they +explode, and in many cases falling upon the earth and burying themselves from a +few inches to several feet in the soil, from which, more than once, they have +been picked up while yet hot and fuming. These balls are sometimes called +bolides. They are not really round in shape, although they often look so while +traversing the sky, but their forms are fragmentary, and occasionally +fantastic. It has been supposed that their origin is different from that of the +true meteors; it has even been conjectured that they may have originated from +the giant volcanoes of the moon or have been shot out from the sun during some +of the tremendous explosions that accompany the formation of eruptive +prominences. By the same reasoning some of them might be supposed to have come +from some distant star. Others have conjectured that they are wanderers in +space, of unknown origin, which the earth encounters as it journeys on, and +Lord Kelvin made a suggestion which has become classic because of its +imaginative reach—<i>viz.,</i> that the first germs of life may have been +brought to the earth by one of these bodies, “a fragment of an exploded world.” +</p> + +<p> +It is a singular fact that astronomers and scientific men in general were among +the last to admit the possibility of solid masses falling from the sky. The +people had believed in the reality of such phenomena from the earliest times, +but the savants shook their heads and talked of superstition. This was the less +surprising because no scientifically authenticated instance of such an +occurrence was known, and the stones popularly believed to have fallen from the +sky had become the objects of worship or superstitious reverence, a fact not +calculated to recommend them to scientific credence. The celebrated “black +stone” suspended in the Kaaba at Mecca is one of these reputed gifts from +heaven; the “Palladium” of ancient Troy was another; and a stone which fell +near Ensisheim, in Germany, was placed in a church as an object to be +religiously venerated. Many legends of falling stones existed in antiquity, +some of them curiously transfigured by the imagination, like the “Lion of the +Peloponnesus,” which was said to have sprung down from the sky upon the Isthmus +of Corinth. But near the beginning of the nineteenth century, in 1803, a +veritable shower of falling stones occurred at L’Aigle, in Northern France, and +this time astronomers took note of the phenomenon and scientifically +investigated it. Thousands of the strange projectiles came from the sky on this +occasion, and were scattered over a wide area of country, and some buildings +were hit. Four years later another shower of stones occurred at Weston, Conn., +numbering thousands of individuals. The local alarm created in both cases was +great, as well it might be, for what could be more intimidating than to find +the blue vault of heaven suddenly hurling solid missiles at the homes of men? +After these occurrences it was impossible for the most skeptical to doubt any +longer, and the regular study of “aerolites,” or “meteorites,” began. +</p> + +<p> +One of the first things recognized was the fact that fire-balls are solid +meteorites in flight, and not gaseous exhalations in the air, as some had +assumed. They burn in the air during their flight, and sometimes, perhaps, are +entirely consumed before reaching the ground. Their velocity before entering +the earth’s atmosphere is equal to that of the planets in their +orbits—<i>viz.,</i> from twenty to thirty miles per second—a fact which proves +that the sun is the seat of the central force governing them. Their burning in +the air is not difficult to explain; it is the heat of friction which so +quickly brings them to incandescence. Calculation shows that a body moving +through the air at a velocity of about a mile per second will be brought, +superficially, to the temperature of “red heat” by friction with the +atmosphere. If its velocity is twenty miles per second the temperature will +become thousands of degrees. This is the state of affairs with a meteorite +rushing into the earth’s atmosphere; its surface is liquefied within a few +seconds after the friction begins to act, and the melted and vaporized portion +of its mass is swept backward, forming the train of sparks that follows every +great fire-ball. However, there is one phenomenon connected with the trains of +meteorites which has never been satisfactorily explained: they often persist +for long periods of time, drifting and turning with the wind, but not ceasing +to glow with a phosphorescent luminosity. The question is, Whence comes this +light? It must be light without heat, since the fine dust or vapor of which the +train can only consist would not retain sufficient heat to render it luminous +for so long a time. An extremely remarkable incident of this kind occurred on +February 22, 1909, when an immense fire-ball that passed over southern England +left a train that remained visible during two hours, assuming many curious +shapes as it was drifted about by currents in the air. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus28"></a> +<a href="images/image28.jpg"> +<img src="images/image28.jpg" width="474" height="600" alt="Illustration:" /></a> +<p class="caption">A meteor photographed in flight</p> +</div> + +<p> +But notwithstanding the enormous velocity with which meteorites enter the air +they are soon slowed down to comparatively moderate speed, so that when they +disappear they are usually traveling not faster than a mile a second. The +courses of many have been traced by observers situated along their track at +various points, and thus a knowledge has been obtained of their height above +the ground during their flight and of the length of their visible courses. They +generally appear at an elevation of eighty or a hundred miles, and are seldom +visible after having descended to within five miles of the ground, unless the +observer happens to be near the striking-point, when he may actually witness +the fall. Frequently they burst while high in the air and their fragments are +scattered like shrapnel over the surface of the ground, sometimes covering an +area of several square miles, but of course not thickly; different fragments of +the same meteorite may reach the ground at points several miles apart. The +observed length of their courses in the atmosphere varies from fifty to five +hundred miles. If they continued a long time in flight after entering the air, +even the largest of them would probably be consumed to the last scrap, but +their fiery career is so short on account of their great speed that the heat +does not have time to penetrate very deeply, and some that have been picked up +immediately after their fall have been found cold as ice within. Their size +after reaching the ground is variable within wide limits; some are known which +weigh several tons, but the great majority weigh only a few pounds and many +only a few ounces. +</p> + +<p> +Meteorites are of two kinds: <i>stony</i> meteorites and <i>iron</i> +meteorites. The former outnumber the latter twenty to one; but many stone +meteorites contain grains of iron. Nickel is commonly found in iron meteorites, +so that it might be said that that redoubtable alloy nickel-steel is of +cosmical invention. Some twenty-five chemical elements have been found in +meteorites, including carbon and the “sun-metal,” helium. The presence of the +latter is certainly highly suggestive in connection with the question of the +origin of meteorites. The iron meteorites, besides metallic iron and nickel, of +which they are almost entirely composed, contain hydrogen, helium, and carbonic +oxide, and about the only imaginable way in which these gases could have become +absorbed in the iron would be through the immersion of the latter while in a +molten or vaporized state in a hot and dense atmosphere composed of them, a +condition which we know to exist only in the envelopes of the sun and the +stars. +</p> + +<p> +The existence of carbon in the Canyon Diablo iron meteorites is attended by a +circumstance of the most singular character—a very “fairy tale of science.” In +some cases <i>the carbon has become diamond!</i> These meteoric diamonds are +very small; nevertheless, they are true diamonds, resembling in many ways the +little black gems produced by Moissan’s method with the aid of the electric +furnace. The fact that they are found embedded in these iron meteorites is +another argument in favor of the hypothesis of the solar or stellar origin of +the latter. To appreciate this it is necessary to recall the way in which +Moissan made his diamonds. It was by a combination of the effects of great +heat, great pressure, and sudden or rapid superficial cooling on a mass of iron +containing carbon. When he finally broke open his iron he found it a pudding +stuffed with miniature black diamonds. When a fragment of the Canyon Diablo +meteoric iron was polished in Philadelphia over fifteen years ago it cut the +emery-wheel to pieces, and examination showed that the damage had been effected +by microscopic diamonds peppered through the mass. How were those diamonds +formed? If the sun or Sirius was the laboratory that prepared them, we can get +a glimpse at the process of their formation. There is plenty of heat, plenty of +pressure, and an abundance of vaporized iron in the sun and the stars. When a +great solar eruption takes place, masses of iron which have absorbed carbon may +be shot out with a velocity which forbids their return. Plunged into the +frightful cold of space, their surfaces are quickly cooled, as Moissan cooled +his prepared iron by throwing it into water, and thus the requisite stress is +set up within, and, as the iron solidifies, the included carbon crystallizes +into diamonds. Whether this explanation has a germ of truth in it or not, at +any rate it is evident that iron meteorites were not created in the form in +which they come to us; they must once have been parts of immeasurably more +massive bodies than themselves. +</p> + +<p> +The fall of meteorites offers an appreciable, though numerically insignificant, +peril to the inhabitants of the earth. Historical records show perhaps three or +four instances of people being killed by these bodies. But for the protection +afforded by the atmosphere, which acts as a very effective shield, the danger +would doubtless be very much greater. In the absence of an atmosphere not only +would more meteorites reach the ground, but their striking force would be +incomparably greater, since, as we have seen, the larger part of their original +velocity is destroyed by the resistance of the air. A meteorite weighing many +tons and striking the earth with a velocity of twenty or thirty miles per +second, would probably cause frightful havoc. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus29"></a> +<a href="images/image29.jpg"> +<img src="images/image29.jpg" width="600" height="406" alt="Illustration:" /></a> +<p class="caption">Looking across Coon Butte crater from northern rim</p> +</div> + +<p> +It is a singular fact that recent investigations seem to have proved that an +event of this kind actually happened in North America—perhaps not longer than a +thousand or two thousand years ago. The scene of the supposed catastrophe is in +northern central Arizona, at Coon Butte, where there is a nearly circular +crater in the middle of a circular elevation or small mountain. The crater is +somewhat over four thousand feet in diameter, and the surrounding rim, formed +of upturned strata and ejected rock fragments, rises at its highest point one +hundred and sixty feet above the plain. The crater is about six hundred feet in +depth—that is, from the rim to the visible floor or bottom of the crater. There +is no evidence that volcanic action has ever taken place in the immediate +neighborhood of Coon Butte. The rock in which the crater has been made is +composed of horizontal sandstone and limestone strata. Between three hundred +and four hundred million tons of rock fragments have been detached, and a large +portion hurled by some cause out of the crater. These fragments lie +concentrically distributed around the crater, and in large measure form the +elevation known as Coon Butte. The region has been famous for nearly twenty +years on account of the masses of meteoric iron found scattered about and known +as the “Canyon Diablo” meteorites. It was one of these masses, which consist of +nickel-iron containing a small quantity of platinum, and of which in all some +ten tons have been recovered for sale to the various collectors throughout the +world, that as before mentioned destroyed the grinding-tool at Philadelphia +through the cutting power of its embedded diamonds. These meteoric irons are +scattered about the crater-hill, in concentric distribution, to a maximum +distance of about five miles. When the suggestion was first made in 1896 that a +monster meteorite might have created by its fall this singular lone crater +<i>in stratified rocks,</i> it was greeted with incredulous smiles; but since +then the matter has assumed a different aspect. The Standard Iron Company, +formed by Messrs. D. M. Barringer, B. C. Tilghman, E. J. Bennitt, and S. J. +Holsinger, having become, in 1903, the owner of this freak of nature, sunk +shafts and bored holes to a great depth in the interior of the crater, and also +trenched the slopes of the mountain, and the result of their investigations has +proved that the meteoric hypothesis of origin is correct. (See the papers +published in the <i>Proceedings of the Academy of Natural Sciences of +Philadelphia,</i> December, 1905, wherein it is proved that the United States +Geological Survey was wrong in believing this crater to have been due to a +steam explosion. Since that date there has been discovered a great amount of +additional confirmatory proof). Material of unmistakably meteoric origin was +found by means of the drills, mixed with crushed rock, to a depth of six +hundred to seven hundred feet below the floor of the crater, and a great deal +of it has been found admixed with the ejected rock fragments on the outer +slopes of the mountain, absolutely proving synchronism between the two events, +the formation of this great crater and the falling of the meteoric iron out of +the sky. The drill located in the bottom of the crater was sent, in a number of +cases, much deeper (over one thousand feet) into unaltered horizontal red +sandstone strata, but no meteoric material was found below this depth (seven +hundred feet, or between eleven and twelve hundred feet below the level of the +surrounding plain), which has been assumed as being about the limit of +penetration. It is not possible to sink a shaft at present, owing to the water +which has drained into the crater, and which forms, with the finely pulverized +sandstone, a very troublesome quicksand encountered at about two hundred feet +below the visible floor of the crater. As soon as this water is removed by +pumping it will be easy to explore the depths of the crater by means of shafts +and drifts. The rock strata (sandstone and limestone) of which the walls +consist present every appearance of having been violently upturned by a huge +body penetrating the earth like a cannon-ball. The general aspect of the crater +strikingly resembles the impression made by a steel projectile shot into an +armor-plate. Mr Tilghman has estimated that a meteorite about five hundred feet +in diameter and moving with a velocity of about five miles per second would +have made just such a perforation upon striking rocks of the character of those +found at this place. There was some fusion of the colliding masses, and the +heat produced some steam from the small amount of water in the rocks. As a +result there has been found at depth a considerable amount of fused quartz +(original sandstone), and with it innumerable particles or sparks of fused +nickel-iron (original meteorite). A projectile of that size penetrating eleven +to twelve hundred feet into the rocky shell of the globe must have produced a +shock which was perceptible several hundred miles away. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus30"></a> +<a href="images/image30.jpg"> +<img src="images/image30.jpg" width="600" height="405" alt="Illustration:" /></a> +<p class="caption">Trail on south side, Coon Butte crater</p> +</div> + +<p> +The great velocity ascribed to the supposed meteorite at the moment of striking +could be accounted for by the fact that it probably plunged nearly vertically +downward, for it formed a circular crater in the rocky crust of the earth. In +that case it would have been less retarded by the resistance of the atmosphere +than are meteorites which enter the air at a lower angle and shoot ahead +hundreds of miles until friction has nearly destroyed their original motion +when they drop upon the earth. Some meteoric masses of great size, such as +Peary’s iron meteorite found at Cape York, Greenland, and the almost equally +large mass discovered at Bacubirito, Mexico, appear to have penetrated but +slightly on striking the earth. This may be explained by supposing that they +pursued a long, horizontal course through the air before falling. The result +would be that, their original velocity having been practically destroyed, they +would drop to the ground with a velocity nearly corresponding to that which +gravity would impart within the perpendicular distance of their final fall. A +six-hundred-and-sixty-pound meteorite, which fell at Knyahinya, Hungary, +striking at an angle of 27° from the vertical, penetrated the ground to a +depth of eleven feet. +</p> + +<p> +It has been remarked that the Coon Butte meteorite may have fallen not longer +ago than a few thousand years. This is based upon the fact that the geological +indications favor the supposition that the event did not occur more than five +thousand years ago, while on the other hand the rings of growth in the +cedar-trees growing on the slopes of the crater show that they have existed +there about seven hundred years. Prof. William H. Pickering has recently +correlated this with an ancient chronicle which states that at Cairo, Egypt, in +the year 1029, “many stars passed with a great noise.” He remarks that Cairo is +about 100°, by great circle, from Coon Butte, so that if the meteorite that +made the crater was a member of a flock of similar bodies which encountered the +earth moving in parallel lines, some of them might have traversed the sky +tangent to the earth’s surface at Cairo. That the spectacle spoken of in the +chronicle was caused by meteorites he deems exceedingly probable because of +what is said about “a great noise;” meteorites are the only celestial phenomena +attended with perceptible sounds. Professor Pickering conjectures that this +supposed flock of great meteorites may have formed the nucleus of a comet which +struck the earth, and he finds confirmation of the idea in the fact that out of +the ten largest meteorites known, no less than seven were found within nine +hundred miles of Coon Butte. It would be interesting if we could trace back the +history of that comet, and find out what malicious planet caught it up in its +innocent wanderings and hurled it with so true an aim at the earth! This +remarkable crater is one of the most interesting places in the world, for there +is absolutely no record of such a mass, possibly an iron-headed comet, from +outer space having come into collision with our earth. The results of the +future exploration of the depths of the crater will be awaited with much +interest. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap12"></a>XII<br/> +The Wrecking of the Moon</h2> + +<p> +There are sympathetic moods under whose influence one gazes with a certain +poignant tenderness at the worn face of the moon; that little “fossil world” +(the child of our mother earth, too) bears such terrible scars of its brief +convulsive life that a sense of pity is awakened by the sight. The moon is the +wonder-land of the telescope. Those towering mountains, whose “proud aspiring +peaks” cast silhouettes of shadow that seem drawn with india-ink; those vast +plains, enchained with gentle winding hills and bordered with giant ranges; +those oval “oceans,” where one looks expectant for the flash of wind-whipped +waves; those enchanting “bays” and recesses at the seaward feet of the Alps; +those broad straits passing between guardian heights incomparably mightier than +Gibraltar; those locket-like valleys as secluded among their mountains as the +Vale of Cashmere; those colossal craters that make us smile at the pretensions +of Vesuvius, Etna, and Cotopaxi; those strange white ways which pass with the +unconcern of Roman roads across mountain, gorge, and valley—all these give the +beholder an irresistible impression that it is truly a world into which he is +looking, a world akin to ours, and yet no more like our world than Pompeii is +like Naples. Its air, its waters, its clouds, its life are gone, and only a +skeleton remains—a mute but eloquent witness to a cosmical tragedy without +parallel in the range of human knowledge. +</p> + +<p> +One cannot but regret that the moon, if it ever was the seat of intelligent +life, has not remained so until our time. Think what the consequences would +have been if this other world at our very door had been found to be both +habitable and inhabited! We talk rather airily of communicating with Mars by +signals; but Mars never approaches nearer than 35,000,000 miles, while the moon +when nearest is only a little more than 220,000 miles away. Given an effective +magnifying power of five thousand diameters, which will perhaps be possible at +the mountain observatories as telescopes improve, and we should be able to +bring the moon within an apparent distance of about forty miles, while the +corresponding distance for Mars would be more than seven thousand miles. But +even with existing telescopic powers we can see details on the moon no larger +than some artificial constructions on the earth. St Peter’s at Rome, with the +Vatican palace and the great piazza, if existing on the moon, would +unquestionably be recognizable as something else than a freak of nature. Large +cities, with their radiating lines of communication, would at once betray their +real character. Cultivated tracts, and the changes produced by the interference +of intelligent beings, would be clearly recognizable. The electric illumination +of a large town at night would probably be markedly visible. Gleams of +reflected sunlight would come to us from the surfaces of the lakes and oceans, +and a huge “liner” traversing a lunar sea could probably be followed by its +trail of smoke. As to communications by “wireless” signals, which certain +enthusiasts have thought of in connection with Mars, in the case of the moon +they should be a relatively simple matter, and the feat might actually be +accomplished. Think what a literature would grow up about the moon if it were a +living world! Its very differences from the earth would only accentuate its +interest for us. Night and day on the moon are each two weeks in length; how +interesting it would be to watch the manner in which the lunarians dealt with +such a situation as that. Lunar and terrestrial history would keep step with +each other, and we should record them both. Truly one might well wish to have a +neighbor world to study; one would feel so much the less alone in space. +</p> + +<p> +It is not impossible that the moon did at one time have inhabitants of some +kind. But, if so, they vanished with the disappearance of its atmosphere and +seas, or with the advent of its cataclysmic age. At the best, its career as a +living world must have been brief. If the water and air were gradually +absorbed, as some have conjectured, by its cooling interior rocks, its surface +might, nevertheless, have retained them for long ages; but if, as others think, +their disappearance was due to the escape of their gaseous molecules in +consequence of the inability of the relatively small lunar gravitation to +retain them, then the final catastrophe must have been as swift as it was +inevitable. Accepting Darwin’s hypothesis, that the moon was separated from the +earth by tidal action while both were yet plastic or nebulous, we may +reasonably conclude that it began its career with a good supply of both water +and air, but did not possess sufficient mass to hold them permanently. Yet it +may have retained them long enough for life to develop in many forms upon its +surface; in fact, there are so many indications that air and water have not +always been lacking to the lunar world that we are driven to invent theories to +explain both their former presence and their present absence. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus31"></a> +<a href="images/image31.jpg"> +<img src="images/image31.jpg" width="405" height="600" alt="Illustration:" /></a> +<p class="caption">The craters Clavius, Longomontanus, Tycho, etc.</p> +</div> + +<p> +But whatever the former condition of the moon may have been, its existing +appearance gives it a resistless fascination, and it bears so clearly the story +of a vast catastrophe sculptured on its rocky face that the thoughtful observer +cannot look upon it without a feeling of awe. The gigantic character of the +lunar features impresses the beholder not less than the universality of the +play of destructive forces which they attest. Let us make a few comparisons. +Take the lunar crater called “Tycho”, which is a typical example of its kind. +In the telescope Tycho appears as a perfect ring surrounding a circular +depression, in the center of which rises a group of mountains. Its superficial +resemblance to some terrestrial volcanic craters is very striking. Vesuvius, +seen from a point vertically above, would no doubt look something like that +(the resemblance would have been greater when the Monte del Cavallo formed a +more complete circuit about the crater cone). But compare the dimensions. The +remains of the outer crater ring of Vesuvius are perhaps half a mile in +diameter, while the active crater itself is only two or three hundred feet +across at the most; Tycho has a diameter of fifty-four miles! The group of +relatively insignificant peaks in the center of the crater floor of Tycho is +far more massive than the entire mountain that we call Vesuvius. The largest +known volcanic crater on the earth, Aso San, in Japan, has a diameter of seven +miles; it would take <i>sixty</i> craters like Aso San to equal Tycho in area! +And Tycho, though one of the most perfect, is by no means the largest crater on +the moon. Another, called “Theophilus,” has a diameter of sixty-four miles, and +is eighteen thousand feet deep. There are hundreds from ten to forty miles in +diameter, and thousands from one to ten miles. They are so numerous in many +places that they break into one another, like the cells of a crushed honeycomb. +</p> + +<p> +The lunar craters differ from those of the earth more fundamentally than in the +matter of mere size; <i>they are not situated on the tops of mountains.</i> If +they were, and if all the proportions were the same, a crater like Tycho might +crown a conical peak fifty or one hundred miles high! Instead of being cavities +in the summits of mountains, the lunar craters are rather gigantic sink-holes +whose bottoms in many cases lie two or three miles below the general surface of +the lunar world. Around their rims the rocks are piled up to a height of from a +few hundred to two or three thousand feet, with a comparatively gentle +inclination, but on the inner side they fall away in gigantic broken precipices +which make the dizzy cliffs of the Matterhorn seem but “lover’s leaps.” Down +they drop, ridge below ridge, crag under crag, tottering wall beneath wall, +until, in a crater named “Newton,” near the south lunar pole, they attain a +depth where the rays of the sun never reach. Nothing more frightful than the +spectacle which many of these terrible chasms present can be pictured by the +imagination. As the lazy lunar day slowly advances, the sunshine, unmitigated +by clouds or atmospheric veil of any kind, creeps across their rims and begins +to descend the opposite walls. Presently it strikes the ragged crest of a ridge +which had lain hidden in such darkness as we never know on the earth, and runs +along it like a line of kindling fire. Rocky pinnacles and needles shoot up +into the sunlight out of the black depths. Down sinks the line of light, mile +after mile, and continually new precipices and cliffs are brought into view, +until at last the vast floor is attained and begins to be illuminated. In the +meanwhile the sun’s rays, darting across the gulf, have touched the summits of +the central peaks, twenty or thirty miles from the crater’s inmost edge, and +they immediately kindle and blaze like huge stars amid the darkness. So +profound are some of these awful craters that days pass before the sun has +risen high enough above them to chase the last shadows from their depths. +</p> + +<p> +Although several long ranges of mountains resembling those of the earth exist +on the moon, the great majority of its elevations assume the crateriform +aspect. Sometimes, instead of a crater, we find an immense mountain ring whose +form and aspect hardly suggest volcanic action. But everywhere the true craters +are in evidence, even on the sea-beds, although they attain their greatest +number and size on those parts of the moon—covering sixty per cent of its +visible surface—which are distinctly mountainous in character and which +constitute its most brilliant portions. Broadly speaking, the southwestern half +of the moon is the most mountainous and broken, and the northeastern half the +least so. Right down through the center, from pole to pole, runs a wonderful +line of craters and crateriform valleys of a magnitude stupendous even for the +moon. Another similar line follows the western edge. Three or four “seas” are +thrust between these mountainous belts. By the effects of “libration” parts of +the opposite hemisphere of the moon which is turned away from the earth are +from time to time brought into view, and their aspect indicates that that +hemisphere resembles in its surface features the one which faces the earth. +There are many things about the craters which seem to give some warrant for the +hypothesis which has been particularly urged by Mr G. K. Gilbert, that they +were formed by the impact of meteors; but there are also many things which +militate against that idea, and, upon the whole, the volcanic theory of their +origin is to be preferred. +</p> + +<p> +The enormous size of the lunar volcanoes is not so difficult to account for +when we remember how slight is the force of lunar gravity as compared with that +of the earth. With equal size and density, bodies on the moon weigh only +one-sixth as much as on the earth. Impelled by the same force, a projectile +that would go ten miles on the earth would go sixty miles on the moon. A lunar +giant thirty-five feet tall would weigh no more than an ordinary son of Adam +weighs on his greater planet. To shoot a body from the earth so that it would +not drop back again, we should have to start it with a velocity of seven miles +per second; a mile and a half per second would serve on the moon. It is by no +means difficult to believe, then, that a lunar volcano might form a crater ring +eight or ten times broader than the greatest to be found on the earth, +especially when we reflect that in addition to the relatively slight force of +gravity, the materials of the lunar crust are probably lighter than those of +our terrestrial rocks. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus32"></a> +<a href="images/image32.jpg"> +<img src="images/image32.jpg" width="409" height="600" alt="Illustration:" /></a> +<p class="caption">Western part of the Mare Serenitatis</p> +</div> + +<p> +For similar reasons it seems not impossible that the theory mentioned in a +former chapter—that some of the meteorites that have fallen upon the earth +originated from the lunar volcanoes—is well founded. This would apply +especially to the stony meteorites, for it is hardly to be supposed that the +moon, at least in its superficial parts, contains much iron. It is surely a +scene most strange that is thus presented to the mind’s eye—that little +attendant of the earth’s (the moon has only one-fiftieth of the volume, and +only one-eightieth of the mass of the earth) firing great stones back at its +parent planet! And what can have been the cause of this furious outbreak of +volcanic forces on the moon? Evidently it was but a passing stage in its +history; it had enjoyed more quiet times before. As it cooled down from the +plastic state in which it parted from the earth, it became incrusted after the +normal manner of a planet, and then oceans were formed, its atmosphere being +sufficiently dense to prevent the water from evaporating and the would-be +oceans from disappearing continually in mist. This, if any, must have been the +period of life in the lunar world. As we look upon the vestiges of that ancient +world buried in the wreck that now covers so much of its surface, it is +difficult to restrain the imagination from picturing the scenes which were once +presented there; and, in such a case, should the imagination be fettered? We +give it free rein in terrestrial life, and it rewards us with some of our +greatest intellectual pleasures. The wonderful landscapes of the moon offer it +an ideal field with just enough half-hidden suggestions of facts to stimulate +its powers. +</p> + +<p> +The great plains of the <i>Mare Imbrium</i> and the <i>Mare Serenitatis</i> +(the “Sea of Showers” and the “Sea of Serenity”), bordered in part by lofty +mountain ranges precisely like terrestrial mountains, scalloped along their +shores with beautiful bays curving back into the adjoining highlands, and +united by a great strait passing between the nearly abutting ends of the “Lunar +Apennines” and the “Lunar Caucasus,” offer the elements of a scene of world +beauty such as it would be difficult to match upon our planet. Look at the +finely modulated bottom of the ancient sea in Mr Ritchey’s exquisite photograph +of the western part of the <i>Mare Serenitatis,</i> where one seems to see the +play of the watery currents heaping the ocean sands in waving lines, making +shallows, bars, and deeps for the mariner to avoid or seek, and affording a +playground for the creatures of the main. What geologist would not wish to try +his hammer on those rocks with their stony pages of fossilized history? There +is in us an instinct which forbids us to think that there was never any life +there. If we could visit the moon, there is not among us a person so prosaic +and unimaginative that he would not, the very first thing, begin to search for +traces of its inhabitants. We would look for them in the deposits on the sea +bottoms; we would examine the shores wherever the configuration seemed +favorable for harbors and the sites of maritime cities—forgetting that it may +be a little ridiculous to ascribe to the ancient lunarians the same ideas that +have governed the development of our race; we would search through the valleys +and along the seeming courses of vanished streams; we would explore the +mountains, not the terrible craters, but the pinnacled chains that recall our +own Alps and Rockies; seeking everywhere some vestige of the transforming +presence of intelligent life. Perhaps we should find such traces, and perhaps, +with all our searching, we should find nothing to suggest that life had ever +existed amid that universal ruin. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus33"></a> +<a href="images/image33.jpg"> +<img src="images/image33.jpg" width="421" height="600" alt="Illustration:" /></a> +<p class="caption">Mare Tranquilitatis and surroundings</p> +</div> + +<p> +Look again at the border of the “Sea of Serenity”—what a name for such a +scene!—and observe how it has been rent with almost inconceivable violence, the +wall of the colossal crater Posidonius dropping vertically upon the ancient +shore and obliterating it, while its giant neighbor, Le Monnier, opens a +yawning mouth as if to swallow the sea itself. A scene like this makes one +question whether, after all, those may not be right who have imagined that the +so-called sea bottoms are really vast plains of frozen lava which gushed up in +floods so extensive that even the mighty volcanoes were half drowned in the +fiery sea. This suggestion becomes even stronger when we turn to another of the +photographs of Mr Ritchey’s wonderful series, showing a part of the <i>Mare +Tranquilitatis</i> (“Sea of Tranquility”!). Notice how near the center of the +picture the outline of a huge ring with radiating ridges shows through the sea +bottom; a fossil volcano submerged in a petrified ocean! This is by no means +the only instance in which a buried world shows itself under the great lunar +plains. Yet, as the newer craters in the sea itself prove, the volcanic +activity survived this other catastrophe, or broke out again subsequently, +bringing more ruin to pile upon ruin. +</p> + +<p> +Yet notwithstanding the evidence which we have just been considering in support +of the hypothesis that the “seas” are lava floods, Messrs. Loewy and Puiseux, +the selenographers of the Paris Observatory, are convinced that these great +plains bear characteristic marks of the former presence of immense bodies of +water. In that case we should be forced to conclude that the later oceans of +the moon lay upon vast sheets of solidified lava; and thus the catastrophe of +the lunar world assumes a double aspect, the earliest oceans being swallowed up +in molten floods issuing from the interior, while the lands were reduced to +chaos by a universal eruption of tremendous volcanoes; and then a period of +comparative quiet followed, during which new seas were formed, and new life +perhaps began to flourish in the lunar world, only to end in another cataclysm, +which finally put a term to the existence of the moon as a life-supporting +world. +</p> + +<p> +Suppose we examine two more of Mr Ritchey’s illuminating photographs, and, +first, the one showing the crater Theophilus and its surroundings. We have +spoken of Theophilus before, citing the facts that it is sixty-four miles in +diameter and eighteen thousand feet deep. It will be noticed that it has two +brother giants—Cyrillus the nearer, and Catharina the more distant; but +Theophilus is plainly the youngest of the trio. Centuries, and perhaps +thousands of years, must have elapsed between the periods of their upheaval, +for the two older craters are partly filled with débris, while it is manifest +at a glance that when the south eastern wall of Theophilus was formed, it broke +away and destroyed a part of the more ancient ring of Cyrillus. There is no +more tremendous scene on the moon than this; viewed with a powerful telescope, +it is absolutely appalling. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus34"></a> +<a href="images/image34.jpg"> +<img src="images/image34.jpg" width="425" height="600" alt="Illustration:" /></a> +<p class="caption">Lunar craters Theophilus and surrounding region</p> +</div> + +<p> +The next photograph shows, if possible, a still wilder region. It is the part +of the moon lying between Tycho and the south pole. Tycho is seen in the lower +left-hand part of the picture. To the right, at the edge of the illuminated +portion of the moon, are the crater-rings, Longomontanus and Wilhelm I, the +former being the larger. Between them are to be seen the ruins of two or three +more ancient craters which, together with portions of the walls of Wilhelm I +and Longomontanus, have been honeycombed with smaller craters. The vast +crateriform depression above the center of the picture is Clavius, an unrivaled +wonder of lunar scenery, a hundred and forty-two miles in its greatest length, +while its whole immense floor has sunk two miles below the general surface of +the moon outside the ring. The monstrous shadow-filled cavity above Clavius +toward the right is Blancanus, whose aspect here gives a good idea of the +appearance of these chasms when only their rims are in the sunlight. But +observe the indescribable savagery of the entire scene. It looks as though the +spirit of destruction had gone mad in this spot. The mighty craters have broken +forth one after another, each rending its predecessor; and when their work was +finished, a minor but yet tremendous outbreak occurred, and the face of the +moon was gored and punctured with thousands of smaller craters. These +relatively small craters (small, however, only in a lunar sense, for many of +them would appear gigantic on the earth) recall once more the theory of +meteoric impact. It does not seem impossible that some of them may have been +formed by such an agency. +</p> + +<p> +One would not wish for our planet such a fate as that which has overtaken the +moon, but we cannot be absolutely sure that something of the kind may not be in +store for it. We really know nothing of the ultimate causes of volcanic +activity, and some have suggested that the internal energies of the earth may +be accumulating instead of dying out, and may never yet have exhibited their +utmost destructive power. Perhaps the best assurance that we can find that the +earth will escape the catastrophe that has overtaken its satellite is to be +found in the relatively great force of its gravitation. The moon has been the +victim of its weakness; given equal forces, and the earth would be the better +able to withstand them. It is significant, in connection with these +considerations, that the little planet Mercury, which seems also to have parted +with its air and water, shows to the telescope some indications that it is +pitted with craters resembling those that have torn to pieces the face of the +moon. +</p> + +<p> +Upon the whole, after studying the dreadful lunar landscapes, one cannot feel a +very enthusiastic sympathy with those who are seeking indications of the +continued existence of some kind of life on the moon; such a world is better +without inhabitants. It has met its fate; let it go! Fortunately, it is not so +near that it cannot hide its scars and appear beautiful—except when curiosity +impels us to look with the penetrating eyes of the astronomer. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus35"></a> +<a href="images/image35.jpg"> +<img src="images/image35.jpg" width="484" height="600" alt="Illustration:" /></a> +<p class="caption">Mare Crisium</p> +</div> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap13"></a>XIII<br/> +The Great Mars Problem</h2> + +<p> +Let any thoughtful person who is acquainted with the general facts of astronomy +look up at the heavens some night when they appear in their greatest splendor, +and ask himself what is the strongest impression that they make upon his mind. +He may not find it easy to frame an answer, but when he has succeeded it will +probably be to the effect that the stars give him an impression of the +universality of intelligence; they make him feel, as the sun and the moon +cannot do, that his world is not alone; that all this was not made simply to +form a gorgeous canopy over the tents of men. If he is of a devout turn of +mind, he thinks, as he gazes into those fathomless deeps and among those +bewildering hosts, of the infinite multitude of created beings that the +Almighty has taken under his care. The narrow ideas of the old geocentric +theology, which made the earth God’s especial footstool, and man his only +rational creature, fall away from him like a veil that had obscured his vision; +they are impossible in the presence of what he sees above. Thus the natural +tendency, in the light of modern progress, is to regard the universe as +everywhere filled with life. +</p> + +<p> +But science, which is responsible for this broadening of men’s thoughts +concerning the universality of life, itself proceeds to set limits. Of +spiritual existences it pretends to know nothing, but as to physical beings, it +declares that it can only entertain the supposition of their existence where it +finds evidence of an environment suited to their needs, and such environment +may not everywhere exist. Science, though repelled by the antiquated +theological conception of the supreme isolation of man among created beings, +regards with complacency the probability that there are regions in the universe +where no organic life exists, stars which shine upon no inhabited worlds, and +planets which nourish no animate creatures. The astronomical view of the +universe is that it consists of matter in every stage of evolution: some +nebulous and chaotic; some just condensing into stars (suns) of every magnitude +and order; some shaped into finished solar bodies surrounded by dependent +planets; some forming stars that perhaps have no planets, and will have none; +some constituting suns that are already aging, and will soon lose their radiant +energy and disappear; and some aggregated into masses that long ago became +inert, cold, and rayless, and that can only be revivified by means about which +we can form conjectures, but of which we actually know nothing. +</p> + +<p> +As with the stars, so with the planets, which are the satellites of stars. All +investigations unite to tell us that the planets are not all in the same state +of development. As some are large and some small, so some are, in an +evolutionary sense, young, and some old. As they depend upon the suns around +which they revolve for their light, heat, and other forms of radiant energy, so +their condition varies with their distance from those suns. Many may never +arrive at a state suitable for the maintenance of life upon their surfaces; +some which are not at present in such a state may attain it later; and the +forms of life themselves may vary with the peculiar environment that different +planets afford. Thus we see that we are not scientifically justified in +affirming that life is ubiquitous, although we are thus justified in saying +that it must be, in a general sense, universal. We might liken the universe to +a garden known to contain every variety of plant. If on entering it we see no +flowers, we examine the species before us and find that they are not of those +which bloom at this particular season, or perhaps they are such as never bear +flowers. Yet we feel no doubt that we shall find flowers somewhere in the +garden, because there <i>are</i> species which bloom at this season, and the +garden contains <i>all</i> varieties. +</p> + +<p> +While it is tacitly assumed that there are planets revolving around other stars +than the sun, it would be impossible for us to see them with any telescope yet +invented, and no instrument now in the possession of astronomers could assure +us of their existence; so the only planetary system of which we have visual +knowledge is our own. Excluding the asteroids, which could not from any point +of view be considered as habitable, we have in the solar system eight planets +of various sizes and situated at various distances from the sun. Of these eight +we know that one, the earth, is inhabited. The question, then, arises: Are +there any of the others which are inhabited or habitable? Since it is our +intention to discuss the habitability of only one of the seven to which the +question applies, the rest may be dismissed in a few words. The smallest of +them, and the nearest to the sun, is Mercury, which is regarded as +uninhabitable because it has no perceptible supply of water and air, and +because, owing to the extraordinary eccentricity of its orbit, it is subjected +to excessive and very rapid alterations in the amount of solar heat and light +poured upon its surface, such alterations being inconsistent with the +supposition that it can support living beings. Even its average temperature is +more than six and a half times that prevailing on the earth! Another +circumstance which militates against its habitability is that, according to the +results of the best telescopic studies, it always keeps the same face toward +the sun, so that one half of the planet is perpetually exposed to the fierce +solar rays, and the other half faces the unmitigated cold of open space. Venus, +the next in distance from the sun, is almost the exact twin of the earth in +size, and many arguments may be urged in favor of its habitability, although it +is suspected of possessing the same peculiarity as Mercury, in always keeping +the same side sunward. Unfortunately its atmosphere appears to be so dense that +no permanent markings on its surface are certainly visible, and the question of +its actual condition must, for the present, be left in abeyance. Mars, the +first planet more distant from the sun than the earth, is the special subject +of this chapter, and will be described and discussed a few lines further on. +Jupiter, Saturn, Uranus, and Neptune, the four giant planets, all more distant +than Mars, and each more distant than the other in the order named, are all +regarded as uninhabitable because none of them appears to possess any degree of +solidity. They may have solid or liquid nuclei, but exteriorly they seem to be +mere balls of cloud. Of course, one can imagine what he pleases about the +existence of creatures suited to the physical constitution of such planets as +these, but they must be excluded from the category of habitable worlds in the +ordinary sense of the term. We go back, then, to Mars. +</p> + +<p> +It will be best to begin with a description of the planet. Mars is 4230 miles +in diameter; its surface is not much more than one-quarter as extensive as that +of the earth (.285). Its mean distance from the sun is 141,500,000 miles, +48,500,000 miles greater than that of the earth. Since radiant energy varies +inversely as the square of distance, Mars receives less than half as much solar +light and heat as the earth gets. Mars’ year (period of revolution round the +sun) is 687 days. Its mean density is 71 per cent of the earth’s, and the force +of gravity on its surface is 38 per cent of that on the surface of the earth; +<i>i.e.,</i> a body weighing one hundred pounds on the earth would, if +transported to Mars, weigh but thirty-eight pounds. The inclination of its +equator to the plane of its orbit differs very little from that of the earth’s +equator, and its axial rotation occupies 24 hours 37 minutes. so that the +length of day and night, and the extent of the seasonal changes on Mars, are +almost precisely the same as on the earth. But owing to the greater length of +its year, the seasons of Mars, while occurring in the same order, are almost +twice as long as ours. The surface of the planet is manifestly solid, like that +of our globe, and the telescope reveals many permanent markings on it, +recalling the appearance of a globe on which geographical features have been +represented in reddish and dusky tints. Around the poles are plainly to be seen +rounded white areas, which vary in extent with the Martian seasons, nearly +vanishing in summer and extending widely in winter. The most recent +spectroscopic determinations indicate that Mars has an atmosphere perhaps as +dense as that to be found on our loftiest mountain peaks, and there is a +perceptible amount of watery vapor in this atmosphere. The surface of the +planet appears to be remarkably level, and it has no mountain ranges. No +evidences of volcanic action have been discovered on Mars. The dusky and +reddish areas were regarded by the early observers as respectively seas and +lands, but at present it is not believed that there are any bodies of water on +the planet. There has never been much doubt expressed that the white areas +about the poles represent snow. +</p> + +<p> +It will be seen from this brief description that many remarkable resemblances +exist between Mars and the earth, and there is nothing wonderful in the fact +that the question of the habitability of the former has become one of extreme +and wide-spread interest, giving rise to the most diverse views, to many +extraordinary speculations, and sometimes to regrettably heated controversy. +The first champion of the habitability of Mars was Sir William Herschel, +although even before his time the idea had been suggested. He was convinced by +the revelations of his telescopes, continually increasing in power, that Mars +was more like the earth than any other planet. He could not resist the +testimony of the polar snows, whose suggestive conduct was in such striking +accord with what occurs upon the earth. Gradually, as telescopes improved and +observers increased in number, the principal features of the planet were +disclosed and charted, and “areography,” as the geography of Mars was called, +took its place among the recognized branches of astronomical study. But it was +not before 1877 that a fundamentally new discovery in areography gave a truly +sensational turn to speculation about life on “the red planet.” In that year +Mars made one of its nearest approaches to the earth, and was so situated in +its orbit that it could be observed to great advantage from the northern +hemisphere of the earth. The celebrated Italian astronomer, Schiaparelli, took +advantage of this opportunity to make a trigonometrical survey of the surface +of Mars—as coolly and confidently as if he were not taking his sights across a +thirty-five-million-mile gulf of empty space—and in the course of this survey +he was astonished to perceive that the reddish areas, then called continents, +were crossed in many directions by narrow, dusky lines, to which he gave the +suggestive name of “canals.” Thus a kind of firebrand was cast into the field +of astronomical speculation, which has ever since produced disputes that have +sometimes approached the violence of political faction. At first the accuracy +of Schiaparelli’s observations was contested; it required a powerful telescope, +and the most excellent “seeing,” to render the enigmatical lines visible at +all, and many searchers were unable to detect them. But Schiaparelli continued +his studies in the serene sky of Italy, and produced charts of the gridironed +face of Mars containing so much astonishing detail that one had either to +reject them <i>in toto</i> or to confess that Schiaparelli was right. As +subsequent favorable oppositions of Mars occurred, other observers began to see +the “canals” and to confirm the substantial accuracy of the Italian +astronomer’s work, and finally few were found who would venture to affirm that +the “canals” did not exist, whatever their meaning might be. +</p> + +<div class="fig" style="width:100%;"> +<a name="illus36"></a> +<a href="images/image36.jpg"> +<img src="images/image36.jpg" width="750" height="419" alt="Illustration:" /></a> +<p class="caption">Schiaparelli’s chart of Mars, showing the so-called system +of canals</p> +</div> + +<p> +When Schiaparelli began his observations it was generally believed, as we have +said, that the dusky areas on Mars were seas, and since Schiaparelli thought +that the “canals” invariably began and ended at the shores of the “seas,” the +appropriateness of the title given to the lines seemed apparent. Their +artificial character was immediately assumed by many, because they were too +straight and too suggestively geometrical in their arrangement to permit the +conclusion that they were natural watercourses. A most surprising circumstance +noted by Schiaparelli was that the “canals” made their appearance <i>after</i> +the melting of the polar snow in the corresponding hemisphere had begun, and +that they grew darker, longer, and more numerous in proportion as the polar +liquidation proceeded; another very puzzling observation was that many of them +became double as the season advanced; close beside an already existing “canal,” +and in perfect parallelism with it, another would gradually make its +appearance. That these phenomena actually existed and were not illusions was +proved by later observations, and today they are seen whenever Mars is +favorably situated for observation. +</p> + +<p> +In the closing decade of the nineteenth century, Mr Percival Lowell took up the +work where Schiaparelli had virtually dropped it, and soon added a great number +of “canals” to those previously known, so that in his charts the surface of the +wonderful little planet appears covered as with a spider’s web, the dusky lines +criss-crossing in every direction, with conspicuous knots wherever a number of +them come together. Mr Lowell has demonstrated that the areas originally called +seas, and thus named on the earlier charts, are not bodies of water, whatever +else they may be. He has also found that the mysterious lines do not, as +Schiaparelli supposed, begin and end at the edges of the dusky regions, but +often continue on across them, reaching in some cases far up into the polar +regions. But Schiaparelli was right in his observation that the appearance of +the “canals” is synchronous with the gradual disappearance of the polar snows, +and this fact has become the basis of the most extraordinary theory that the +subject of life in other worlds has ever given birth to. +</p> + +<p> +Now, the effect of such discoveries, as we have related, depends upon the type +of mind to whose attention they are called. Many are content to accept them as +strange and inexplicable at present, and to wait for further light upon them; +others insist upon an immediate inquiry concerning their probable nature and +meaning. Such an inquiry can only be based upon inference proceeding from +analogy. Mars, say Mr Lowell and those who are of his opinion, is manifestly a +solidly incrusted planet like the earth; it has an atmosphere, though one of +great rarity; it has water vapor, as the snows in themselves prove; it has the +alternation of day and night, and a succession of seasons closely resembling +those of the earth; its surface is suggestively divided into regions of +contrasting colors and appearance, and upon that surface we see an immense +number of lines geometrically arranged, with a system of symmetrical +intersections where the lines expand into circular and oval areas—and all +connected with the annual melting of the polar snows in a way which +irresistibly suggests the interference of intelligence directed to a definite +end. Why, with so many concurrent circumstances to support the hypothesis, +should we not regard Mars as an inhabited globe? +</p> + +<p> +But the differences between Mars and the earth are in many ways as striking as +their resemblances. Mars is relatively small; it gets less than half as much +light and heat as we receive; its atmosphere is so rare that it would be +distressing to us, even if we could survive in it at all; it has no lakes, +rivers, or seas; its surface is an endless prairie. and its “canals” are +phenomena utterly unlike anything on the earth. Yet it is precisely upon these +divergences between the earth and Mars, this repudiation of terrestrial +standards, that the theory of “life on Mars,” for which Mr Lowell is mainly +responsible, is based. Because Mars is smaller than the earth, we are told it +must necessarily be more advanced in planetary evolution, the underlying cause +of which is the gradual cooling and contraction of the planet’s mass. Mars has +parted with its internal heat more rapidly than the earth; consequently its +waters and its atmosphere have been mostly withdrawn by chemical combinations, +but enough of both yet remain to render life still possible on its surface. As +the globe of Mars is evolutionally older than that of the earth, so its forms +of organic life may be proportionally further advanced, and its inhabitants may +have attained a degree of cultivated intelligence much superior to what at +present exists upon the earth. Understanding the nature and the causes of the +desiccation of their planet, and possessing engineering science and +capabilities far in advance of ours, they may be conceived to have grappled +with the stupendous problem of keeping their world in a habitable condition as +long as possible. Supposing them to have become accustomed to live in their +rarefied atmosphere (a thing not inconceivable, since men can live for a time +at least in air hardly less rare), the most pressing problem for them is that +of a water-supply, without which plant life cannot exist, while animal life in +turn depends for its existence upon vegetation. The only direction in which +they can seek water is that of the polar regions, where it is alternately +condensed into snow and released in the liquid form by the effect of the +seasonal changes. It is, then, to the annual melting of the polar snow-fields +that the Martian engineers are supposed to have recourse in supplying the needs +of their planet, and thus providing the means of prolonging their own +existence. It is imagined that they have for this purpose constructed a +stupendous system of irrigation extending over the temperate and equatorial +regions of the planet. The “canals” represent the lines of irrigation, but the +narrow streaks that we see are not the canals themselves, but the irrigated +bands covered by them. Their dark hue, and their gradual appearance after the +polar melting has begun, are due to the growth of vegetation stimulated by the +water. The rounded areas visible where several “canals” meet and cross are +called by Mr Lowell “oases.” These are supposed to be the principal centers of +population and industry. It must be confessed that some of them, with their +complicated systems of radiating lines, appear to answer very well to such a +theory. No attempt to explain them by analogy with natural phenomena on the +earth has proved successful. +</p> + +<p> +But a great difficulty yet remains: How to explain the seemingly miraculous +powers of the supposed engineers? Here recourse is had once more to the +relative smallness of the planet. We have remarked that the force of gravity on +Mars is only thirty-eight per cent of that on the earth. A steam-shovel driven +by a certain horse-power would be nearly three times as effective there as +here. A man of our stature on Mars would find his effective strength increased +in the same proportion. But just because of the slight force of gravity there, +a Martian might attain to the traditional stature of Goliath without finding +his own weight an encumbrance to his activity, while at the same time his huge +muscles would come into unimpeded play, enabling him single-handed to perform +labors that would be impossible to a whole gang of terrestrial workmen. The +effective powers of huge machines would be increased in the same way; and to +all this must be added the fact that the mean density of the materials of which +Mars is composed is much less than that of the constituents of the earth. +Combining all these considerations, it becomes much less difficult to conceive +that public works might be successfully undertaken on Mars which would be +hopelessly beyond the limits of human accomplishment. +</p> + +<p> +Certain other difficulties have also to be met; as, for instance, the relative +coldness of the climate of Mars. At its distance it gets considerably less than +half as much light and heat as we receive. In addition to this, the rarity of +its atmosphere would naturally be expected to decrease the effective +temperature at the planet’s surface, since an atmosphere acts somewhat like the +glass cover of a hot-house in retaining the solar heat which has penetrated it. +It has been calculated that, unless there are mitigating circumstances of which +we know nothing, the average temperature at the surface of Mars must be far +below the freezing-point of water. To this it is replied that the possible +mitigating circumstances spoken of evidently exist in fact, because we can +<i>see</i> that the watery vapor condenses into snow around the poles in +winter, but melts again when summer comes. The mitigating agent may be supposed +to exist in the atmosphere where the presence of certain gases would completely +alter the temperature gradients. +</p> + +<p> +It might also be objected that it is inconceivable that the Martian engineers, +however great may be their physical powers, and however gigantic the mechanical +energies under their control, could force water in large quantities from the +poles to the equator. This is an achievement that measures up to the cosmical +standard. It is admitted by the champions of the theory that the difficulty is +a formidable one; but they call attention to the singular fact that on Mars +there can be found no chains of mountains, and it is even doubtful if ranges of +hills exist there. The entire surface of the planet appears to be almost “as +smooth as a billiard ball,” and even the broad regions which were once supposed +to be seas apparently lie at practically the same level as the other parts, +since the “canals” in many cases run uninterruptedly across them. Lowell’s idea +is that these sombre areas may be expanses of vegetation covering ground of a +more or less marshy character, for while the largest of them appear to be +permanent, there are some which vary coincidently with the variations of the +canals. +</p> + +<p> +As to the kind of machinery employed to force the water from the poles, it has +been conjectured that it may have taken the form of a gigantic system of pumps +and conduits; and since the Martians are assumed to be so far in advance of us +in their mastery of scientific principles, the hypothesis will at least not be +harmed by supposing that they have learned to harness forces of nature whose +very existence in a manageable form is yet unrecognized on the earth. If we +wish to let the imagination loose, we may conjecture that they have conquered +the secret of those intra-atomic forces whose resistless energy is beginning to +become evident to us, but the possibility of whose utilization remains a dream, +the fulfillment of which nobody dares to predict. +</p> + +<p> +Such, in very brief form, is the celebrated theory of Mars as an inhabited +world. It certainly captivates the imagination, and if we believe it to +represent the facts, we cannot but watch with the deepest sympathy this gallant +struggle of an intellectual race to preserve its planet from the effects of +advancing age and death. We may, indeed, wonder whether our own humanity, +confronted by such a calamity, could be counted on to meet the emergency with +equal stoutness of heart and inexhaustibleness of resource. Up to the present +time we certainly have shown no capacity to confront Nature toe to toe, and to +seize her by the shoulders and turn her round when she refuses to go our way. +If we could get into wireless telephonic communication with the Martians we +might learn from their own lips the secret of their more than “Roman recovery.” +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap14"></a>XIV<br/> +The Riddle of the Asteroids</h2> + +<p> +Between the orbits of Mars and Jupiter revolves the most remarkable system of +little bodies with which we are acquainted—the Asteroids, or Minor Planets. +Some six hundred are now known, and they may actually number thousands. They +form virtually a ring about the sun. The most striking general fact about them +is that they occupy the place in the sky which should be occupied, according to +Bode’s Law, by a single large planet. This fact, as we shall see, has led to +the invention of one of the most extraordinary theories in +astronomy—<i>viz.,</i> that of the explosion of a world! +</p> + +<p> +Bode’s Law, so-called, is only an empiric formula, but until the discovery of +Neptune it accorded so well with the distances of the planets that astronomers +were disposed to look upon it as really representing some underlying principle +of planetary distribution. They were puzzled by the absence of a planet in the +space between Mars and Jupiter, where the “law” demanded that there should be +one, and an association of astronomers was formed to search for it. There was a +decided sensation when, in 1801, Piazzi, of Palermo, announced that he had +found a little planet which apparently occupied the place in the system which +belonged to the missing body. He named it Ceres, and it was the first of the +Asteroids. The next year Olbers, of Bremen, while looking for Ceres with his +telescope, stumbled upon another small planet which he named Pallas. +Immediately he was inspired with the idea that these two planets were fragments +of a larger one which had formerly occupied the vacant place in the planetary +ranks, and he predicted that others would be found by searching in the +neighborhood of the intersection of the orbits of the two already discovered. +This bold prediction was brilliantly fulfilled by the finding of two more—Juno +in 1804, and Vesta in 1807. Olbers would seem to have been led to the invention +of his hypothesis of a planetary explosion by the faith which astronomers at +that time had in Bode’s Law. They appear to have thought that several planets +revolving in the gap where the “law” called for but one could only be accounted +for upon the theory that the original <i>one</i> had been broken up to form the +several. Gravitation demanded that the remnants of a planet blown to pieces, no +matter how their orbits might otherwise differ, should all return at stated +periods to the point where the explosion had occurred; hence Olbers’ prediction +that any asteroids that might subsequently be discovered would be found to have +a common point of orbital intersection. And curiously enough all of the first +asteroids found practically answered to this requirement. Olbers’ theory seemed +to be established. +</p> + +<p> +After the first four, no more asteroids were found until 1845, when one was +discovered; then, in 1847, three more were added to the list; and after that +searchers began to pick them up with such rapidity that by the close of the +century hundreds were known, and it had become almost impossible to keep track +of them. The first four are by far the largest members of the group, but their +actual sizes remained unknown until less than twenty years ago. It was long +supposed that Vesta was the largest, because it shines more brightly than any +of the others; but finally, in 1895, Barnard, with the Lick telescope, +definitely measured their diameters, and proved to everybody’s surprise that +Ceres is really the chief, and Vesta only the third in rank. His measures are +as follows: Ceres, 477 miles; Pallas, 304 miles; Vesta, 239 miles; and Juno, +120 miles. They differ greatly in the reflective power of their surfaces, a +fact of much significance in connection with the question of their origin. +Vesta is, surface for surface, rather more than three times as brilliant as +Ceres, whence the original mistake about its magnitude. +</p> + +<p> +Nowadays new asteroids are found frequently by photography, but physically they +are most insignificant bodies, their average diameter probably not exceeding +twenty miles, and some are believed not to exceed ten. On a planet only ten +miles in diameter, assuming the same mean density as the earth’s, which is +undoubtedly too much, the force of gravity would be so slight that an average +man would not weigh more than three ounces, and could jump off into space +whenever he liked. +</p> + +<p> +Although the asteroids all revolve around the sun in the same direction as that +pursued by the major planets, their orbits are inclined at a great variety of +angles to the general plane of the planetary system, and some of them are very +eccentric—almost as much so as the orbits of many of the periodic comets. It +has even been conjectured that the two tiny moons of Mars and the four smaller +satellites of Jupiter may be asteroids gone astray and captured by those +planets. Two of the asteroids are exceedingly remarkable for the shapes and +positions of their orbits; these are Eros, discovered in 1898, and T. G., 1906, +found eight years later. The latter has a mean distance from the sun slightly +greater than that of Jupiter, while the mean distance of Eros is less than that +of Mars. The orbit of Eros is so eccentric that at times it approaches within +15,000,000 miles of the earth, nearer than any other regular member of the +solar system except the moon, thus affording an unrivaled means of measuring +the solar parallax. But for our present purpose the chief interest of Eros lies +in its extraordinary changes of light. +</p> + +<p> +These changes, although irregular, have been observed and photographed many +times, and there seems to be no doubt of their reality. Their significance +consists in their possible connection with the form of the little planet, whose +diameter is generally estimated at not more than twenty miles. Von Oppolzer +found, in 1901, that Eros lost three-fourths of its brilliancy once in every +two hours and thirty-eight minutes. Other observers have found slightly +different periods of variability, but none as long as three hours. The most +interesting interpretation that has been offered of this phenomenon is that it +is due to a great irregularity of figure, recalling at once Olbers’ hypothesis. +According to some, Eros may be double, the two bodies composing it revolving +around each other at very close quarters; but a more striking, and it may be +said probable, suggestion is that Eros has a form not unlike that of a +dumb-bell, or hour-glass, turning rapidly end over end so that the area of +illuminated surface presented to our eyes continually changes, reaching at +certain times a minimum when the amount of light that it reflects toward the +earth is reduced to a quarter of its maximum value. Various other bizarre +shapes have been ascribed to Eros, such, for instance, as that of a flat stone +revolving about one of its longer axes, so that sometimes we see its face and +sometimes its edge. +</p> + +<p> +All of these explanations proceed upon the assumption that Eros cannot have a +simple globular figure like that of a typical planet, a figure which is +prescribed by the law of gravitation, but that its shape is what may be called +accidental; in a word, it is a <i>fragment,</i> for it seems impossible to +believe that a body formed in interplanetary space, either through nebular +condensation or through the aggregation of particles drawn together by their +mutual attractions, should not be practically spherical in shape. Nor is Eros +the only asteroid that gives evidence by variations of brilliancy that there is +something abnormal in its constitution; several others present the same +phenomenon in varying degrees. Even Vesta was regarded by Olbers as +sufficiently variable in its light to warrant the conclusion that it was an +angular mass instead of a globe. Some of the smaller ones show very notable +variations, and all in short periods, of three or four hours, suggesting that +in turning about one of their axes they present a surface of variable extent +toward the sun and the earth. +</p> + +<p> +The theory which some have preferred—that the variability of light is due to +the differences of reflective power on different parts of the surface—would, if +accepted, be hardly less suggestive of the origin of these little bodies by the +breaking up of a larger one, because the most natural explanation of such +differences would seem to be that they arose from variations in the roughness +or smoothness of the reflecting surface, which would be characteristic of +fragmentary bodies. In the case of a large planet alternating expanses of land +and water, or of vegetation and desert, would produce a notable variation in +the amount of reflection, but on bodies of the size of the asteroids neither +water nor vegetation could exist, and an atmosphere would be equally +impossible. +</p> + +<p> +One of the strongest objections to Olbers’ hypothesis is that only a few of the +first asteroids discovered travel in orbits which measurably satisfy the +requirement that they should all intersect at the point where the explosion +occurred. To this it was at first replied that the perturbations of the +asteroidal orbits, by the attractions of the major planets, would soon displace +them in such a manner that they would cease to intersect. One of the first +investigations undertaken by the late Prof. Simon Newcomb was directed to the +solution of this question, and he arrived at the conclusion that the planetary +perturbations could not explain the actual situation of the asteroidal orbits. +But afterward it was pointed out that the difficulty could be avoided by +supposing that not one but a series of explosions had produced the asteroids as +they now are. After the primary disruption the fragments themselves, according +to this suggestion, may have exploded, and then the resulting orbits would be +as “tangled” as the heart could wish. This has so far rehabilitated the +explosion theory that it has never been entirely abandoned, and the evidence +which we have just cited of the probably abnormal shapes of Eros and other +asteroids has lately given it renewed life. It is a subject that needs a +thorough rediscussion. +</p> + +<p> +We must not fail to mention, however, that there is a rival hypothesis which +commends itself to many astronomers—<i>viz.,</i> that the asteroids were formed +out of a relatively scant ring of matter, situated between Mars and Jupiter and +resembling in composition the immensely more massive rings from which, +according to Laplace’s hypothesis, the planets were born. It is held by the +supporters of this theory that the attraction of the giant Jupiter was +sufficient to prevent the small, nebulous ring that gave birth to the asteroids +from condensing like the others into a single planet. +</p> + +<p> +But if we accept the explosion theory, with its corollary that minor explosions +followed the principal one, we have still an unanswered question before us: +What caused the explosions? The idea of <i>a world blowing up</i> is too +Titanic to be shocking; it rather amuses the imagination than seriously +impresses it; in a word, it seems essentially chimerical. We can by no appeal +to experience form a mental picture of such an occurrence. Even the moon did +not blow up when it was wrecked by volcanoes. The explosive nebulæ and new +stars are far away in space, and suggest no connection with such a catastrophe +as the bursting of a planet into hundreds of pieces. We cannot conceive of a +great globe thousands of miles in diameter resembling a pellet of gunpowder +only awaiting the touch of a match to cause its sudden disruption. Somehow the +thought of human agency obtrudes itself in connection with the word +“explosion,” and we smile at the idea that giant powder or nitro-glycerine +could blow up a planet. Yet it would only need <i>enough</i> of them to do it. +</p> + +<p> +After all, we may deceive ourselves in thinking, as we are apt to do, that +explosive energies lock themselves up only in small masses of matter. There are +many causes producing explosions in nature, every volcanic eruption manifests +the activity of some of them. Think of the giant power of confined steam; if +enough steam could be suddenly generated in the center of the earth by a +downpour of all the waters of the oceans, what might not the consequences be +for our globe? In a smaller globe, and it has never been estimated that the +original asteroid was even as large as the moon, such a catastrophe would, +perhaps, be more easily conceivable; but since we are compelled in this case to +assume that there was a series of successive explosions, steam would hardly +answer the purpose; it would be more reasonable to suppose that the cause of +the explosion was some kind of chemical reaction, or something affecting the +atoms composing the exploding body. Here Dr Gustav Le Bon comes to our aid with +a most startling suggestion, based on his theory of the dissipation of +intra-atomic energy. It will be best to quote him at some length from his book +on <i>The Evolution of Forces.</i> +</p> + +<p> +“It does not seem at first sight,” says Doctor Le Bon, +</p> + +<div class="letter"> +<p> +very comprehensible that worlds which appear more and more stable as they cool +could become so unstable as to afterward dissociate entirely. To explain this +phenomenon, we will inquire whether astronomical observations do not allow us +to witness this dissociation. +</p> + +<p> +We know that the stability of a body in motion, such as a top or a bicycle, +ceases to be possible when its velocity of rotation descends below a certain +limit. Once this limit is reached it loses its stability and falls to the +ground. Prof. J. J. Thomson even interprets radio-activity in this manner, and +points out that when the speed of the elements composing the atoms descends +below a certain limit they become unstable and tend to lose their equilibria. +There would result from this a commencement of dissociation, with diminution of +their potential energy and a corresponding increase of their kinetic energy +sufficient to launch into space the products of intra-atomic disintegration. +</p> + +<p> +It must not be forgotten that the atom being an enormous reservoir of energy is +by this very fact comparable with explosive bodies. These last remain inert so +long as their internal equilibria are undisturbed. So soon as some cause or +other modifies these, they explode and smash everything around them after being +themselves broken to pieces. +</p> + +<p> +Atoms, therefore, which grow old in consequence of the diminution of a part of +their intra-atomic energy gradually lose their stability. A moment, then, +arrives when this stability is so weak that the matter disappears by a sort of +explosion more or less rapid. The bodies of the radium group offer an image of +this phenomenon—a rather faint image, however, because the atoms of this body +have only reached a period of instability when the dissociation is rather slow. +It probably precedes another and more rapid period of dissociation capable of +producing their final explosion. Bodies such as radium, thorium, etc., +represent, no doubt, a state of old age at which all bodies must some day +arrive, and which they already begin to manifest in our universe, since all +matter is slightly radio-active. It would suffice for the dissociation to be +fairly general and fairly rapid for an explosion to occur in a world where it +was manifested. +</p> + +<p> +These theoretical considerations find a solid support in the sudden appearances +and disappearances of stars. The explosions of a world which produce them +reveal to us, perhaps, how the universes perish when they become old. +</p> + +<p> +As astronomical observations show the relative frequency of these rapid +destructions, we may ask ourselves whether the end of a universe by a sudden +explosion after a long period of old age does not represent its most general +ending. +</p> +</div> + +<p> +Here, perhaps, it will be well to stop, since, entrancing as the subject may +be, we know very little about it, and Doctor Le Bon’s theory affords a +limitless field for the reader’s imagination. +</p> + +</div><!--end chapter--> + +<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK CURIOSITIES OF THE SKY ***</div> +<div style='text-align:left'> + +<div style='display:block; margin:1em 0'> +Updated editions will replace the previous one—the old editions will +be renamed. +</div> + +<div style='display:block; margin:1em 0'> +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the United +States without permission and without paying copyright +royalties. 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