The Project Gutenberg eBook, Other Worlds, by Garrett P. Serviss This eBook is for the use of anyone anywhere at no cost and with almost no restrictions whatsoever. You may copy it, give it away or re-use it under the terms of the Project Gutenberg License included with this eBook or online at www.gutenberg.org Title: Other Worlds Their Nature, Possibilities and Habitability in the Light of the Latest Discoveries Author: Garrett P. Serviss Release Date: May 22, 2006 [eBook #18431] Language: English Character set encoding: ISO-8859-1 ***START OF THE PROJECT GUTENBERG EBOOK OTHER WORLDS*** E-text prepared by Suzanne Lybarger, Brian Janes, and the Project Gutenberg Online Distributed Proofreading Team (http://www.pgdp.net/) OTHER WORLDS BY GARRETT P. SERVISS. OTHER WORLDS. Their Nature and Possibilities in the Light of the Latest Discoveries. Illustrated. 12mo. Cloth, $1.20 net; postage additional. No science has ever equaled astronomy in its appeal to the imagination, and recently popular interest in the wonders of the starry heavens has been stimulated by surprising discoveries and imaginary discoveries, as well as by a marked tendency of writers of fiction to include other worlds and their possible inhabitants within the field of romance. Mr. Serviss's new book on "Other Worlds, their Nature and Possibilities in the Light of the Latest Discoveries," summarizes what is known. With helpful illustrations, the most interesting facts about the planets Venus, Mars, Jupiter, Saturn, etc., as well as about the nearest of all other worlds, the moon, are presented in a popular manner, and always from the point of view of human interest—a point that is too seldom taken by writers on science. ASTRONOMY WITH AN OPERA-GLASS. A Popular Introduction to the Study of the Starry Heavens with the simplest of Optical Instruments. Illustrated. 8vo. Cloth, $1.50. "By its aid thousands of people who have resigned themselves to the ignorance in which they were left at school, by our wretched system of teaching by the book only, will thank Mr. Serviss for the suggestions he has so well carried out."—New York Times. PLEASURES OF THE TELESCOPE. A Descriptive Guide to Amateur Astronomers and All Lovers of the Stars. Illustrated. 8vo. Cloth, $1.50. "The volume will be found interesting by those for whom it is written, and will inspire many with a love for the study of astronomy, one of the most far-reaching of the sciences."—Milwaukee Journal. D. APPLETON AND COMPANY, NEW YORK. CHART OF MARS. After Schiaparelli. Other Worlds Their Nature, Possibilities and Habitability in the light of the latest discoveries. By GARRETT P. SERVISS Author of "Astronomy with an Opera-glass" and "Pleasures of the Telescope" With Charts and Illustrations "Shall we measure the councils of heaven by the narrow impotence of human faculties, or conceive that silence and solitude reign throughout the mighty empire of nature?" —Dr. Thomas Chalmers. New York D. APPLETON AND COMPANY 1901 COPYRIGHT, 1901, BY D. APPLETON AND COMPANY. TO The Memory OF WILLIAM JAY YOUMANS. PREFACE The point of view of this book is human interest in the other worlds around us. It presents the latest discoveries among the planets of the solar system, and shows their bearing upon the question of life in those planets. It points out the resemblances and the differences between the earth and the other worlds that share with it in the light of the sun. It shows what we should see and experience if we could visit those worlds. While basing itself upon facts, it does not exclude the discussion of interesting probabilities and theories that have commanded wide popular attention. It points out, for instance, what is to be thought of the idea of interplanetary communication. It indicates what must be the outlook of the possible inhabitants of some of the other planets toward the earth. As far as may be, it traces the origin and development of the other worlds of our system, and presents a graphic picture of their present condition as individuals, and of their wonderful contrasts as members of a common family. In short, the aim of the author has been to show how wide, and how rich, is the field of interest opened to the human mind by man's discoveries concerning worlds, which, though inaccessible to him in a physical sense, offer intellectual conquests of the noblest description. And, finally, in order to assist those who may wish to recognize for themselves these other worlds in the sky, this book presents a special series of charts to illustrate a method of finding the planets which requires no observatory and no instruments, and only such knowledge of the starry heavens as anybody can easily acquire. G.P.S. Borough of Brooklyn, New York City, September, 1901. CONTENTS CHAPTER I 1 INTRODUCTORY Remarkable popular interest in questions concerning other worlds and their inhabitants—Theories of interplanetary communication— The plurality of worlds in literature—Romances of foreign planets—Scientific interest in the subject—Opposing views based on telescopic and spectroscopic revelations—Changes of opinion—Desirability of a popular presentation of the latest facts—The natural tendency to regard other planets as habitable—Some of the conditions and limitations of the problem—The solar system viewed from outer space—The resemblances and contrasts of its various planets—Three planetary groups recognized—The family character of the solar system CHAPTER II 18 Grotesqueness of Mercury considered as a world—Its dimensions, mass, and movements—The question of an atmosphere— Mercury's visibility from the earth—Its eccentric orbit, and rapid changes of distance from the sun—Momentous consequences of these peculiarities—A virtual fall of fourteen million miles toward the sun in six weeks—The tremendous heat poured upon Mercury and its great variations—The little planet's singular manner of rotation on its axis—Schiaparelli's astonishing discovery—A day side and a night side—Interesting effects of libration—The heavens as viewed from Mercury—Can it support life? CHAPTER III 46 A planet that matches ours in size—Its beauty in the sky—Remarkable circularity of its orbit—Probable absence of seasons and stable conditions of temperature and weather on Venus—Its dense and abundant atmosphere—Seeing the atmosphere of Venus from the earth—Is the real face of the planet hidden under an atmospheric veil?—Conditions of habitability—All planetary life need not be of the terrestrial type—The limit fixed by destructive temperature—Importance of air and water in the problem—Reasons why Venus may be a more agreeable abode than the earth—Splendor of our globe as seen from Venus—What astronomers on Venus might learn about the earth—A serious question raised—Does Venus, like Mercury, rotate but once in the course of a revolution about the sun?—Reasons for and against that view CHAPTER IV 85 Resemblances between Mars and the earth—Its seasons and its white polar caps—Peculiar surface markings—Schiaparelli's discovery of the canals—His description of their appearance and of their duplication—Influence of the seasons on the aspect of the canals—What are the canals?—Mr. Lowell's observations—The theory of irrigation—How the inhabitants of Mars are supposed to have taken advantage of the annual accession of water supplied by the melting of the polar caps—Wonderful details shown in charts of Mars—Curious effects that may follow from the small force of gravity on Mars—Imaginary giants—Reasons for thinking that Mars may be, in an evolutionary sense, older than the earth—Speculations about interplanetary signals from Mars, and their origin—Mars's atmosphere—The question of water—The problem of temperature—Eccentricities of Mars's moons CHAPTER V 129 Only four asteroids large enough to be measured—Remarkable differences in their brightness irrespective of size—Their widely scattered and intermixed orbits—Eccentric orbit of Eros—the nearest celestial body to the earth except the moon—Its existence recorded by photography before it was discovered—Its great and rapid fluctuations in light, and the curious hypotheses based upon them—Is it a fragment of an exploded planet?—The startling theory of Olbers as to the origin of the asteroids revived—Curious results of the slight force of gravity on an asteroid—An imaginary visit to a world only twelve miles in diameter CHAPTER VI 160 Jupiter compared with our globe—His swift rotation on his axis—Remarkable lack of density—The force of gravity on Jupiter— Wonderful clouds—Strange phenomena of the great belts—Brilliant display of colors—The great red spot and the many theories it has given rise to—Curious facts about the varying rates of rotation of the huge planet's surface—The theory of a hidden world in Jupiter—When Jupiter was a companion star to the sun—The miracle of world-making before our eyes—Are Jupiter's satellites habitable?—Magnificent spectacles in the Jovian system CHAPTER VII 185 The wonder of the great rings—Saturn's great distance and long year—The least dense of all the planets—It would float in water— What kind of a world is it?—Sir Humphry Davy's imaginary inhabitants of Saturn—Facts about the rings, which are a phenomenon unparalleled in the visible universe—The surprising nature of the rings, as revealed by mathematics and the spectroscope—The question of their origin and ultimate fate—Dr. Dick's idea of their habitability—Swedenborg's curious description of the appearance of the rings from Saturn—Is Saturn a globe of vapor, or of dust?—The nine satellites and "Roche's limit"—The play of spectacular shadows in the Saturnian system—Uranus and Neptune—Is there a yet undiscovered planet greater than Jupiter? CHAPTER VIII 212 The moon a favorite subject for intellectual speculation—Its nearness to the earth graphically illustrated—Ideas of the ancients— Galileo's discoveries—What first raised a serious question as to its habitability—Singularity of the moon's motions—Appearance of its surface to the naked eye and with the telescope—The "seas" and the wonderful mountains and craters—A terrible abyss described— Tycho's mysterious rays—Difference between lunar and terrestrial volcanoes—Mountain-ringed valleys—Gigantic cracks in the lunar globe—Slight force of gravity of the moon and some interesting deductions—The moon a world of giantism—What kind of atmospheric gases can the moon contain—The question of water and of former oceans—The great volcanic cataclysm in the moon's history—Evidence of volcanic and other changes now occurring—Is there vegetation on the moon?—Lunar day and night—The earth as seen from the moon—Discoveries yet to be made MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS VENUS, THE TWIN OF THE EARTH MARS, A WORLD MORE ADVANCED THAN OURS THE ASTEROIDS, A FAMILY OF DWARF WORLDS JUPITER, THE GREATEST OF KNOWN WORLDS SATURN, A PRODIGY AMONG PLANETS THE MOON, CHILD OF THE EARTH AND THE SUN CHAPTER IX 256 It is easy to make acquaintance with the planets and to follow them among the stars—The first step a knowledge of the constellations —How this is to be acquired—How to use the Nautical Almanac in connection with the charts in this book—The visibility of Mercury and Venus—The oppositions of Mars, Jupiter, and Saturn 277 LIST OF ILLUSTRATIONS PAGE Chart of Mars Frontispiece Diagram showing causes of day and night on portions of Mercury 35 Regions of day and night on Mercury 38 Venus's atmosphere seen as a ring of light 56 View of Jupiter facing 168 Three views of Saturn facing 186 Diagram showing the moon's path through space 217 The lunar Alps, Apennines, and Caucasus facing 222 The moon at first and last quarter facing 226 Phases and rotation of the moon 250 Charts showing the zodiacal constellations: 1. From right ascension 0 hours to 4 hours 259 2. " " 4 " " 8 " 261 3. " " 8 " " 12 " 263 4. " " 12 " " 16 " 265 5. " " 16 " " 20 " 267 6. " " 20 " " 24 " 269 OTHER WORLDS CHAPTER I INTRODUCTORY Other worlds and their inhabitants are remarkably popular subjects of speculation at the present time. Every day we hear people asking one another if it is true that we shall soon be able to communicate with some of the far-off globes, such as Mars, that circle in company with our earth about the sun. One of the masters of practical electrical science in our time has suggested that the principle of wireless telegraphy may be extended to the transmission of messages across space from planet to planet. The existence of intelligent inhabitants in some of the other planets has become, with many, a matter of conviction, and for everybody it presents a question of fascinating interest, which has deeply stirred the popular imagination. The importance of this subject as an intellectual phenomenon of the opening century is clearly indicated by the extent to which it has entered into recent literature. Poets feel its inspiration, and novelists and romancers freely select other planets as the scenes of their stories. One tells us of a visit paid by men to the moon, and of the wonderful things seen, and adventures had, there. Lucian, it is true, did the same thing eighteen hundred years ago, but he had not the aid of hints from modern science to guide his speculations and lend verisimilitude to his narrative. Another startles us from our sense of planetary security with a realistic account of the invasion of the earth by the terrible sons of warlike Mars, seeking to extend their empire by the conquest of foreign globes. Sometimes it is a trip from world to world, a kind of celestial pleasure yachting, with depictions of creatures more wonderful than— "The anthropophagi and men whose heads Do grow beneath their shoulders"— that is presented to our imagination; and sometimes we are informed of the visions beheld by the temporarily disembodied spirits of trance mediums, or other modern thaumaturgists, flitting about among the planets. Then, to vary the theme, we find charming inhabitants of other worlds represented as coming down to the earth and sojourning for a time on our dull planet, to the delight of susceptible successors of father Adam, who become, henceforth, ready to follow their captivating visitors to the ends of the universe. In short, writers of fiction have already established interplanetary communication to their entire satisfaction, thus vastly and indefinitely enlarging the bounds of romance, and making us so familiar with the peculiarities of our remarkable brothers and sisters of Mars, Venus, and the moon, that we can not help feeling, notwithstanding the many divergences in the descriptions, that we should certainly recognize them on sight wherever we might meet them. But the subject is by no means abandoned to the tellers of tales and the dreamers of dreams. Men of science, also, eagerly enter into the discussion HOW TO FIND THE PLANETS INDEX [Pg 1] [Pg 2] [Pg 3] [Pg 4] of the possibilities of other worlds, and become warm over it. Around Mars, in particular, a lively war of opinions rages. Not all astronomers have joined in the dispute—some have not imagination enough, and some are waiting for more light before choosing sides—but those who have entered the arena are divided between two opposed camps. One side holds that Mars is not only a world capable of having inhabitants, but that it actually has them, and that they have given visual proof of their existence and their intelligence through the changes they have produced upon its surface. The other side maintains that Mars is neither inhabited nor habitable, and that what are taken for vast public works and engineering marvels wrought by its industrious inhabitants, are nothing but illusions of the telescope, or delusions of the observer's mind. Both adduce numerous observations, telescopic and spectroscopic, and many arguments, scientific and theoretic, to support their respective contentions, but neither side has yet been able to convince or silence the other, although both have made themselves and their views intensely interesting to the world at large, which would very much like to know what the truth really is. And not only Mars, but Venus—the beauteous twin sister of the earth, who, when she glows in the evening sky, makes everybody a lover of the stars—and even Mercury, the Moor among the planets, wearing "the shadowed livery of the burnished sun," to whom he is "a neighbor and near bred," and Jupiter, Saturn, and the moon itself—all these have their advocates, who refuse to believe that they are lifeless globes, mere reflectors of useless sunshine. The case of the moon is, in this respect, especially interesting, on account of the change that has occurred in the opinions held concerning its physical condition. For a very long time our satellite was confidently, and almost universally, regarded as an airless, waterless, lifeless desert, a completely "dead world," a bare, desiccated skull of rock, circling about the living earth. But within a few years there has been a reaction from this extreme view of the lifelessness of the moon. Observers tell us of clouds suddenly appearing and then melting to invisibility over volcanic craters; of evidences of an atmosphere, rare as compared with ours, yet manifest in its effects; of variations of color witnessed in certain places as the sunlight drifts over them at changing angles of incidence; of what seem to be immense fields of vegetation covering level ground, and of appearances indicating the existence of clouds of ice crystals and deposits of snow among the mountainous lunar landscapes. Thus, in a manner, the moon is rehabilitated, and we are invited to regard its silvery beams not as the reflections of the surface of a desert, but as sent back to our eyes from the face of a world that yet has some slight remnants of life to brighten it. The suggestion that there is an atmosphere lying close upon the shell of the lunar globe, filling the deep cavities that pit its face and penetrating to an unknown depth in its interior, recalls a speculation of the ingenious and entertaining Fontenelle, in the seventeenth century—recently revived and enlarged upon by the author of one of our modern romances of adventure in the moon—to the effect that the lunar inhabitants dwell beneath the surface of their globe instead of on the top of it. Now, because of this widespread and continually increasing interest in the subject of other worlds, and on account of the many curious revelations that we owe to modern telescopes and other improved means of investigation, it is certainly to be desired that the most important and interesting discoveries that have lately been made concerning the various globes which together with the earth constitute the sun's family, should be assembled in a convenient and popular form—and that is the object of this book. Fact is admittedly often stranger and more wonderful than fiction, and there are no facts that appeal more powerfully to the imagination than do those of astronomy. Technical books on astronomy usually either ignore the subject of the habitability of the planets, or dismiss it with scarcely any recognition of the overpowering human interest that it possesses. Hence, a book written specially from the point of view of that subject would appear calculated to meet a popular want; and this the more, because, since Mr. Proctor wrote his Other Worlds than Ours and M. Flammarion his Pluralité des Mondes Habités, many most important and significant discoveries have been made that, in several notable instances, have completely altered the aspect in which the planets present themselves for our judgment as to their conditions of habitability. No doubt the natural tendency of the mind is to regard all the planets as habitable worlds, for there seems to be deeply implanted in human nature a consciousness of the universality of life, giving rise to a conviction that one world, even in the material sense, is not enough for it, but that every planet must belong to its kingdom. We are apt to say to ourselves: "The earth is one of a number of planets, all similarly circumstanced; the earth is inhabited, why should not the others also be inhabited?" What has been learned of the unity in chemical constitution and mechanical operation prevailing throughout the solar system, together with the continually accumulating evidence of the common origin of its various members, and the identity of the evolutionary processes that have brought them into being, all tends to strengthen the a priori hypothesis that life is a phenomenon general to the entire system, and only absent where its essential and fundamental conditions, for special and local, and perhaps temporary, reasons, do not exist. If we look for life in the sun, for instance, while accepting the prevalent conception of the sun as a center of intense thermal action, we must abandon all our ideas of the physical organization of life formed upon what we know of it from experimental evidence. We can not imagine any form of life that has ever been presented to our senses as existing in the sun. But this is not generally true of the planets. Life, in our sense of it, is a planetary, not a solar, phenomenon, and while we may find reasons for believing that on some of the planets the conditions are such that creatures organized like ourselves could not survive, yet we can not positively say that every form of living organism must necessarily be excluded from a world whose environment would be unsuited for us and our contemporaries in terrestrial life. Although our sole knowledge of animated nature is confined to what we learn by experience on the earth, yet it is a most entertaining, and by no means unedifying, occupation, to seek to apply to the exceedingly diversified conditions prevailing in the other planets, as astronomical observations reveal them to us, the principles, types, and limitations that govern the living creatures of our world, and to judge, as best we can, how far those types and limits may be modified or extended so that those other planets may reasonably be included among the probable abodes of life. In order to form such judgments each planet must be examined by itself, but first it is desirable to glance at the planetary system as a whole. To do this we may throw off, in imagination, the dominance of the sun, and suppose ourselves to be in the midst of open space, far removed both from the sun and the other stars. In this situation it is only by chance, or through foreknowledge, that we can distinguish our sun at all, for it is lost among the stars; and when we discover it we find that it is only one of the smaller and less conspicuous members of the sparkling host. We rapidly approach, and when we have arrived within a distance comparable with that of its planets, we see that the sun has increased in apparent magnitude, until now it enormously outshines all the other stars, and its rays begin to produce the effect of daylight upon the orbs that they reach. But we are in no danger of mistaking its apparent superiority to its fellow stars for a real one, because we clearly perceive that our nearness alone makes it seem so great and overpowering. And now we observe that this star that we have drawn near to has attending it a number of minute satellites, faintly shining specks, that circle about it as if charmed, like night-wandering insects, by its splendor. It is manifest to us at the first glance that without the sun these obedient little planets would not exist; it is his attraction that binds them together in a system, and his rays that make them visible to one another in the abyss of space. Although they vary in relative size, yet we observe a striking similarity among them. They are all globular bodies, they all turn upon their axes, they all travel about the sun in the same direction, and their paths all lie very nearly in one plane. Some of them have one or more moons, or satellites, circling about them in imitation of their own revolution about the sun. Their family relationship to one another and to the sun is so evident that it colors our judgment about them as individuals; and when we happen to find, upon closer approach, that one of them, the earth, is covered with vegetation and water and filled with thousands of species of animated creatures, we are disposed to believe, without further examination, that they are all alike in this respect, just as they are all alike in receiving light and heat from the sun. [Pg 5] [Pg 6] [Pg 7] [Pg 8] [Pg 9] [Pg 10] [Pg 11] [Pg 12] [Pg 13] This preliminary judgment, arising from the evident unity of the planetary system, can only be varied by an examination of its members in detail. One striking fact that commands our attention as soon as we have entered the narrow precincts of the solar system is the isolation of the sun and its attendants in space. The solar system occupies a disk-shaped, or flat circular, expanse, about 5,580,000,000 miles across and relatively very thin, the sun being in the center. From the sun to the nearest star, or other sun, the distance is approximately five thousand times the entire diameter of the solar system. But the vast majority of the stars are probably a hundred times yet more remote. In other words, if the Solar system be represented by a circular flower-bed ten feet across, the nearest star must be placed at a distance of nine and a half miles, and the great multitude of the stars at a distance of nine hundred miles! Or, to put it in another way, let us suppose the sun and his planets to be represented by a fleet of ships at sea, all included within a space about half a mile across; then, in order that there might be no shore relatively nearer than the nearest fixed star is to the sun, we should have to place our fleet in the middle of the Pacific Ocean, while the distance of the main shore of the starry universe would be so immense that the whole surface of the earth would be far too small to hold the expanse of ocean needed to represent it! From these general considerations we next proceed to recall some of the details of the system of worlds amid which we dwell. Besides the earth, the sun has seven other principal planets in attendance. These eight planets fall into two classes—the terrestrial planets and the major, or jovian, planets. The former class comprises Mercury, Venus, the earth, and Mars, and the latter Jupiter, Saturn, Uranus, and Neptune. I have named them all in the order of their distance from the sun, beginning with the nearest. The terrestrial planets, taking their class name from terra, the earth, are relatively close to the sun and comparatively small. The major planets—or the jovian planets, if we give them a common title based upon the name of their chief, Jupiter or Jove—are relatively distant from the sun and are characterized both by great comparative size and slight mean density. The terrestrial planets are all included within a circle, having the sun for a center, about 140,000,000 miles in radius. The space, or gap, between the outermost of them, Mars, and the innermost of the jovian planets, Jupiter, is nearly two and a half times as broad as the entire radius of the circle within which they are included. And not only is the jovian group of planets widely separated from the terrestrial group, but the distances between the orbits of its four members are likewise very great and progressively increasing. Between Jupiter and Saturn is a gap 400,000,000 miles across, and this becomes 900,000,000 miles between Saturn and Uranus, and more than 1,000,000,000 miles between Uranus and Neptune. All of these distances are given in round numbers. Finally, we come to some very extraordinary worlds—if we can call them worlds at all—the asteroids. They form a third group, characterized by the extreme smallness of its individual members, their astonishing number, and the unusual eccentricities and inclinations of their orbits. They are situated in the gap between the terrestrial and the jovian planets, and about 500 of them have been discovered, while there is reason to think that their real number may be many thousands. The largest of them is less than 500 miles in diameter, and many of those recently discovered may be not more than ten or twenty miles in diameter. What marvelous places of abode such little planets would be if it were possible to believe them inhabited, we shall see more clearly when we come to consider them in their turn. But without regard to the question of habitability, the asteroids will be found extremely interesting. In the next chapter we proceed to take up the planets for study as individuals, beginning with Mercury, the one nearest the sun. CHAPTER II MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS Mercury, the first of the other worlds that we are going to consider, fascinates by its grotesqueness, like a piece of Chinese ivory carving, so small is it for its kind and so finished in its eccentric details. In a little while we shall see how singular Mercury is in many of the particulars of planetary existence, but first of all let us endeavor to obtain a clear idea of the actual size and mass of this strange little planet. Compared with the earth it is so diminutive that it looks as if it had been cut out on the pattern of a satellite rather than that of an independent planet. Its diameter, 3,000 miles, only exceeds the moon's by less than one half, while both Jupiter and Saturn, among their remarkable collections of moons, have each at least one that is considerably larger than the planet Mercury. But, insignificant though it be in size, it holds the place of honor, nearest to the sun. It was formerly thought that Mercury possessed a mass greatly in excess of that which its size would seem to imply, and some estimates, based upon the apparent effect of its attraction on comets, made it equal in mean density to lead, or even to the metal mercury. This led to curious speculations concerning its probable metallic composition, and the possible existence of vast quantities of such heavy elements as gold in the frame of the planet. But more recent, and probably more correct, computations place Mercury third in the order of density among the members of the solar system, the earth ranking as first and Venus as second. Mercury's density is now believed to be less than the earth's in the ratio of 85 to 100. Accepting this estimate, we find that the force of gravity upon the surface of Mercury is only one third as great as upon the surface of the earth—i.e., a body weighing 300 pounds on the earth would weigh only 100 pounds on Mercury. This is an important matter, because not only the weight of bodies, but the density of the atmosphere and even the nature of its gaseous constituents, are affected by the force of gravity, and if we could journey from world to world, in our bodily form, it would make a great difference to us to find gravity considerably greater or less upon other planets than it is upon our own. This alone might suffice to render some of the planets impossible places of abode for us, unless a decided change were effected in our present physical organization. One of the first questions that we should ask about a foreign world to which we proposed to pay a visit, would relate to its atmosphere. We should wish to know in advance if it had air and water, and in what proportions and quantities. However its own peculiar inhabitants might be supposed able to dispense with these things, to us their presence would be essential, and if we did not find them, even a planet that blazed with gold and diamonds only waiting to be seized would remain perfectly safe from our invasion. Now, in the case of Mercury, some doubt on this point exists. Messrs. Huggins, Vogel, and others have believed that they found spectroscopic proof of the existence of both air and the vapor of water on Mercury. But the necessary observations are of a very delicate nature, and difficult to make, and some astronomers doubt whether we possess sufficient proof that Mercury has an atmosphere. At any rate, its atmosphere is very rare as compared with the earth's, but we need not, on that account, conclude that Mercury is lifeless. Possibly, in view of certain other peculiarities soon to be explained, a rare atmosphere would be decidedly advantageous. Being much nearer the sun than the earth is, Mercury can be seen by us only in the same quarter of the sky where the sun itself appears. As it revolves in its orbit about the sun it is visible, alternately, in the evening for a short time after sunset and in the morning for a short time before sunrise, but it can never be seen, as the outer planets are seen, in the mid-heaven or late at night. When seen low in the twilight, at evening or morning, it glows with the brilliance of a bright first-magnitude star, and is a beautiful object, though few casual watchers of the stars ever catch sight of it. When it is nearest the earth and is about to pass between the earth and the sun, it temporarily disappears in the glare of the sunlight; and likewise, when it it is farthest from the earth and passing around in its orbit on the opposite side of the sun, it is concealed by the blinding solar rays. Consequently, except with the instruments of an observatory, which are able to show it in broad day, Mercury is never visible save during the comparatively brief periods of time when it is near its greatest apparent distance east or west from the sun. The nearer a planet is to the sun the more rapidly it is compelled to move in its orbit, and Mercury, being the nearest to the sun of all the planets, is [Pg 14] [Pg 15] [Pg 16] [Pg 17] [Pg 18] [Pg 19] [Pg 20] [Pg 21] [Pg 22] [Pg 23] by far the swiftest footed among them. But its velocity is subject to remarkable variation, owing to the peculiar form of the orbit in which the planet travels. This is more eccentric than the orbit of any other planet, except some of the asteroids. The sun being situated in one focus of the elliptical orbit, when Mercury is at perihelion, or nearest to the sun, its distance from that body is 28,500,000 miles, but when it is at aphelion, or farthest from the sun, its distance is 43,500,000 miles. The difference is no less than 14,000,000 miles! When nearest the sun Mercury darts forward in its orbit at the rate of twenty-nine miles in a second, while when farthest from the sun the speed is reduced to twenty-three miles. Now, let us return for a moment to the consideration of the wonderful variations in Mercury's distance from the sun, for we shall find that their effects are absolutely startling, and that they alone suffice to mark a wide difference between Mercury and the earth, considered as the abodes of sentient creatures. The total change of distance amounts, as already remarked, to 14,000,000 miles, which is almost half the entire distance separating the planet from the sun at perihelion. This immense variation of distance is emphasized by the rapidity with which it takes place. Mercury's periodic time, i.e., the period required for it to make a single revolution about the sun—or, in other words, the length of its year—is eighty-eight of our days. In just one half of that time, or in about six weeks, it passes from aphelion to perihelion; that is to say, in six weeks the whole change in its distance from the sun takes place. In six weeks Mercury falls 14,000,000 miles—for it is a fall, though in a curve instead of a straight line—falls 14,000,000 miles toward the sun! And, as it falls, like any other falling body it gains in speed, until, having reached the perihelion point, its terrific velocity counteracts its approach and it begins to recede. At the end of the next six weeks it once more attains its greatest distance, and turns again to plunge sunward. Of course it may be said of every planet having an elliptical orbit that between aphelion and perihelion it is falling toward the sun, but no other planet than Mercury travels in an orbit sufficiently eccentric, and approaches sufficiently near to the sun, to give to the mind so vivid an impression of an actual, stupendous fall! Next let us consider the effects of this rapid fall, or approach, toward the sun, which is so foreign to our terrestrial experience, and so appalling to the imagination. First, we must remember that the nearer a planet is to the sun the greater is the amount of heat and light that it receives, the variation being proportional to the inverse square of the distance. The earth's distance from the sun being 93,000,000 miles, while Mercury's is only 36,000,000, it follows, to begin with, that Mercury gets, on the average, more than six and a half times as much heat from the sun as the earth does. That alone is enough to make it seem impossible that Mercury can be the home of living forms resembling those of the earth, for imagine the heat of the sun in the middle of a summer's day increased six or seven fold! If there were no mitigating influences, the face of the earth would shrivel as in the blast of a furnace, the very stones would become incandescent, and the oceans would turn into steam. Still, notwithstanding the tremendous heat poured upon Mercury as compared with that which our planet receives, we can possibly, and for the sake of a clearer understanding of the effects of the varying distance, which is the object of our present inquiry, find a loophole to admit the chance that yet there may be living beings there. We might, for instance, suppose that, owing to the rarity of its atmosphere, the excessive heat was quickly radiated away, or that there was something in the constitution of the atmosphere that greatly modified the effective temperature of the sun's rays. But, having satisfied our imagination on this point, and placed our supposititious inhabitants in the hot world of Mercury, how are we going to meet the conditions imposed by the rapid changes of distance—the swift fall of the planet toward the sun, followed by the equally swift rush away from it? For change of distance implies change of heat and temperature. It is true that we have a slight effect of this kind on the earth. Between midsummer (of the northern hemisphere) and midwinter our planet draws 3,000,000 miles nearer the sun, but the change occupies six months, and, at the earth's great average distance, the effect of this change is too slight to be ordinarily observable, and only the astronomer is aware of the consequent increase in the apparent size of the sun. It is not to this variation of the sun's distance, but rather to the changes of the seasons, depending on the inclination of the earth's axis, that we owe the differences of temperature that we experience. In other words, the total supply of heat from the sun is not far from uniform at all times of the year, and the variations of temperature depend upon the distribution of that supply between the northern and southern hemispheres, which are alternately inclined sunward. But on Mercury the supply of solar heat is itself variable to an enormous extent. In six weeks, as we have seen, Mercury diminishes its distance from the sun about one third, which is proportionally ten times as great a change of distance as the earth experiences in six months. The inhabitants of Mercury in those six pregnant weeks see the sun expand in the sky to more than two and a half times its former magnitude, while the solar heat poured upon them swiftly augments from something more than four and a half times to above eleven times the amount received upon the earth! Then, immediately, the retreat of the planet begins, the sun visibly shrinks, as a receding balloon becomes smaller in the eyes of its watchers, the heat falls off as rapidly as it had previously increased, until, the aphelion point being reached, the process is again reversed. And thus it goes on unceasingly, the sun growing and diminishing in the sky, and the heat increasing and decreasing by enormous amounts with astonishing rapidity. It is difficult to imagine any way in which atmospheric influences could equalize the effects of such violent changes, or any adjustments in the physical organization of living beings that could make such changes endurable. But we have only just begun the story of Mercury's peculiarities. We come next to an even more remarkable contrast between that planet and our own. During the Paris Exposition of 1889 a little company of astronomers was assembled at the Juvisy observatory of M. Flammarion, near the French capital, listening to one of the most surprising disclosures of a secret of nature that any savant ever confided to a few trustworthy friends while awaiting a suitable time to make it public. It was a secret as full of significance as that which Galileo concealed for a time in his celebrated anagram, which, when at length he furnished the key, still remained a riddle, for then it read: "The Mother of the Loves imitates the Shapes of Cynthia," meaning that the planet Venus, when viewed with a telescope, shows phases like those of the moon. The secret imparted in confidence to the knot of astronomers at Juvisy came from a countryman of Galileo's, Signor G. V. Schiaparelli, the Director of the Observatory of Milan, and its purport was that the planet Mercury always keeps the same face directed toward the sun. Schiaparelli had satisfied himself, by a careful series of observations, of the truth of his strange announcement, but before giving it to the world he determined to make doubly sure. Early in 1890 he withdrew the pledge of secrecy from his friends and published his discovery. No one can wonder that the statement was generally received with incredulity, for it was in direct contradiction to the conclusions of other astronomers, who had long believed that Mercury rotated on its axis in a period closely corresponding with that of the earth's rotation—that is to say, once every twenty-four hours. Schiaparelli's discovery, if it were received as correct, would put Mercury, as a planet, in a class by itself, and would distinguish it by a peculiarity which had always been recognized as a special feature of the moon, viz., that of rotating on its axis in the same period of time required to perform a revolution in its orbit, and, while this seemed natural enough for a satellite, almost nobody was prepared for the ascription of such eccentric conduct to a planet. The Italian astronomer based his discovery upon the observation that certain markings visible on the disk of Mercury remained in such a position with reference to the direction of the sun as to prove that the planet's rotation was extremely slow, and he finally satisfied himself that there was but one rotation in the course of a revolution about the sun. That, of course, means that one side of Mercury always faces toward the sun while the opposite side always faces away from it, and neither side experiences the alternation of day and night, one having perpetual day and the other perpetual night. The older observations, from which had been deduced the long accepted opinion that Mercury rotated, like the earth, once in about twenty-four hours, had also been made upon the markings on the planet's disk, but these are not easily seen, and their appearances had evidently been misinterpreted. The very fact of the difficulty of seeing any details on Mercury tended to prevent or delay corroboration of Schiaparelli's discovery. But there were two circumstances that contributed to the final acceptance of his results. One of these was his well-known experience as an observer and the high reputation that he enjoyed among astronomers, and the other was the development by Prof. George Darwin of the theory of tidal friction in its [Pg 23] [Pg 24] [Pg 25] [Pg 26] [Pg 27] [Pg 28] [Pg 29] [Pg 30] [Pg 31] [Pg 32] application to planetary evolution, for this furnished a satisfactory explanation of the manner in which a body, situated as near the sun as Mercury is, could have its axial rotation gradually reduced by the tidal attraction of the sun until it coincided in period with its orbital revolution. Accepting the accuracy of Schiaparelli's discovery, which was corroborated in every particular in 1896 by Percival Lowell in a special series of observations on Mercury made with his 24-inch telescope at Flagstaff, Arizona, and which has also been corroborated by others, we see at once how important is its bearing on the habitability of the planet. It adds another difficulty to that offered by the remarkable changes of distance from the sun, and consequent variations of heat, which we have already discussed. In order to bring the situation home to our experience, let us, for a moment, imagine the earth fallen into Mercury's dilemma. There would then be no succession of day and night, such as we at present enjoy, and upon which not alone our comfort but perhaps our very existence depends, but, instead, one side of our globe—it might be the Asiatic or the American half—would be continually in the sunlight, and the other side would lie buried in endless night. And this condition, so suggestive of the play of pure imagination, this plight of being a two-faced world, like the god Janus, one face light and the other face dark, must be the actual state of things on Mercury. There is one interesting qualification. In the case just imagined for the earth, supposing it to retain the present inclination of its axis while parting with its differential rotation, there would be an interchange of day and night once a year in the polar regions. On Mercury, whose axis appears to be perpendicular, a similar phenomenon, affecting not the polar regions but the eastern and western sides of the planet, is produced by the extraordinary eccentricity of its orbit. As the planet alternately approaches and recedes from the sun its orbital velocity, as we have already remarked, varies between the limits of twenty-three and thirty-five miles per second, being most rapid at the point nearest the sun. But this variation in the speed of its revolution about the sun does not, in any manner, affect the rate of rotation on its axis. The latter is perfectly uniform and just fast enough to complete one axial turn in the course of a single revolution about the sun. The accompanying figure may assist the explanation. Diagram showing that, owing to the Eccentricity of its Orbit, and its Varying Velocity, Mercury, although making but One Turn on its Axis in the Course of a Revolution about the Sun, nevertheless experiences on Parts of its Surface the Alternation of Day and Night. Let us start with Mercury in perihelion at the point A. The little cross on the planet stands exactly under the sun and in the center of the illuminated hemisphere. The large arrows show the direction in which the planet travels in its revolution about the sun, and the small curved arrows the direction in which it rotates on its axis. Now, in moving along its orbit from A to B the planet, partly because of its swifter motion when near the sun, and partly because of the elliptical nature of the orbit, traverses a greater angular interval with reference to the sun than the cross, moving with the uniform rotation of the planet on its axis, is able to traverse in the same time. As drawn in the diagram, the cross has moved through exactly ninety degrees, or one right angle, while the planet in its orbit has moved through considerably more than a right angle. In consequence of this gain of the angle of revolution upon the angle of rotation, the cross at B is no longer exactly under the sun, nor in the center of the illuminated hemisphere. It appears to have shifted its position toward the west, while the hemispherical cap of sunshine has slipped eastward over the globe of the planet. In the next following section of the orbit the planet rotates through another right angle, but, owing to increased distance from the sun, the motion in the orbit now becomes slower until, when the planet arrives at aphelion, C, the angular difference disappears and the cross is once more just under the sun. On returning from aphelion to perihelion the same phenomena recur in reverse order and the line between day and night on the planet first shifts westward, attaining its limit in that respect at D, and then, at perihelion, returns to its original position. Now, if we could stand on the sunward hemisphere of Mercury what, to our eyes, would be the effect of this shifting of the sun's position with regard to a fixed point on the planet's surface? Manifestly it would cause the sun to describe a great arc in the sky, swinging to and fro, in an east and west line, like a pendulum bob, the angular extent of the swing being a little more than forty-seven degrees, and the time required for the sun to pass from its extreme eastern to its extreme western position and back again being eighty-eight days. But, owing to the eccentricity of the orbit, the sun swings much faster toward the east than toward the west, the eastward motion occupying about thirty-seven days and the westward motion about fifty-one days. The Regions of Perpetual Day, Perpetual Night, and Alternate Day and Night on Mercury. In the Left-Hand View the Observer looks at the Planet in the Plane of its Equator; in the Right-Hand View he looks down on its North Pole.. Another effect of the libratory motion of the sun as seen from Mercury is represented in the next figure, where we have a view of the planet showing both the day and the night hemisphere, and where we see that between the two there is a region upon which the sun rises and sets once every eighty- eight days. There are, in reality, two of these lune-shaped regio...