All this variety of tone and color makes of a telescopic view of Jupiter a picture that will not quickly fade from the memory; while if an instrument of considerable power is used, so that the wonderful details of the belts, with their scalloped edges, their diagonal filaments, their many divisions, and their curious light and dark spots, are made plain, the observer is deeply impressed with the strangeness of the spectacle, and the more so as he reflects upon the enormous real magnitude of that which is spread before his eye. The whole earth flattened out would be but a small blotch on that gigantic disk!
Then, the visible rotation of the great Jovian globe, whose effects become evident to a practised eye after but a few minutes' watching, heightens the impression. And the presence of the four satellites, whose motions in their orbits are also evident, through the change in their positions, during the course of a single not prolonged observation, adds its influence to the effectiveness of the scene. Indeed, color and motion are so conspicuous in the immense spectacle presented by Jupiter that they impart to it a powerful suggestion of life, which the mind does not readily divest itself of when compelled to face the evidence that Jupiter is as widely different from the earth, and as diametrically opposed to lifelike conditions, as we comprehend them, as a planet possibly could be.
The great belts lie in latitudes about corresponding to those in which the trade-winds blow upon the earth, and it has often been suggested that their existence indicates a similarity between the atmospheric circulation of Jupiter and that of the world in which we live. No doubt there are times when the earth, seen with a telescope from a distant planet, would present a belted appearance somewhat resembling that of Jupiter, but there would almost certainly be no similar display of colors in the clouds, and the latter would exhibit no such persistence in general form and position as characterizes those of Jupiter. Our clouds are formed by the action of the sun, producing evaporation of water; on Jupiter, whose mean distance from the sun is more than five times as great as ours, the intensity of the solar rays is reduced to less than one twenty-fifth part of their intensity on the earth, so that the evaporation can not be equally active there, and the tendency to form aerial currents and great systems of winds must be proportionally slight. In brief, the clouds of Jupiter are probably of an entirely different origin from that of terrestrial clouds, and rather resemble the chaotic masses of vapor that enveloped the earth when it was still in a seminebulous condition, and before its crust had formed.
Although the strongest features of the disk of Jupiter are the great cloud belts, and the white or colored spots in the equatorial zone, yet the telescope shows many markings north and south of the belts, including a number of narrower and fainter belts, and small light or dark spots. None of them is absolutely fixed in position with reference to others. In other words, all of the spots, belts, and markings shift their places to a perceptible extent, the changes being generally very slow and regular, but occasionally quite rapid. The main belts never entirely disappear, and never depart very far from their mean positions with respect to the equator, but the smaller belts toward the north and south are more or less evanescent. Round or oblong spots, as distinguished from belts, are still more variable and transient. The main belts themselves show great internal commotion, frequently splitting up, through a considerable part of their length, and sometimes apparently throwing out projections into the lighter equatorial zone, which occasionally resemble bridges, diagonally spanning the broad space between the belts.
Perhaps the most puzzling phenomenon that has ever made its appearance on Jupiter is the celebrated "great red spot," which was first noticed in 1878, although it has since been shown to be probably identical with a similar spot seen in 1869, and possibly with one noticed in 1857. This spot, soon after its discovery in 1878, became a clearly defined red oval, lying near the southern edge of the south belt in latitude about 30 deg. Its length was nearly one third of the diameter of the disk and its width almost one quarter as great as its length. Translated into terrestrial measure, it was about 30,000 miles long and 7,000 miles broad.
In 1879 it seemed to deepen in color until it became a truly wonderful object, its redness of hue irresistibly suggesting the idea that it was something hot and glowing. During the following years it underwent various changes of appearance, now fading almost to invisibility and now brightening again, but without ever completely vanishing, and it is still (1901) faintly visible.
Nobody has yet suggested an altogether probable and acceptable theory as to its nature. Some have said that it might be a part of the red-hot crust of the planet elevated above the level of the clouds; others that its appearance might be due to the clearing off of the clouds above a heated region of the globe beneath, rendering the latter visible through the opening; others that it was perhaps a mass of smoke and vapor ejected from a gigantic volcano, or from the vents covering a broad area of volcanic action; others that it might be a vast incandescent slag floating upon the molten globe of the planet and visible through, or above, the enveloping clouds; and others have thought that it could be nothing but a cloud among clouds, differing, for unknown reasons, in composition and cohesion from its surroundings. All of these hypotheses except the last imply the existence, just beneath the visible cloud shell, of a more or less stable and continuous surface, either solid or liquid.
When the red spot began to lose distinctness a kind of veil seemed to be drawn over it, as if light clouds, floating at a superior elevation, had drifted across it. At times it has been reduced in this manner to a faint oval ring, the rim remaining visible after the central part has faded from sight.
One of the most remarkable phenomena connected with the mysterious spot is a great bend, or scallop, in the southern edge of the south belt adjacent to the spot. This looks as if it were produced by the spot, or by the same cause to which the spot owes its existence. If the spot were an immense mountainous elevation, and the belt a current of liquid, or of clouds, flowing past its base, one would expect to see some such bend in the stream. The visual evidence that the belt is driven, or forced, away from the neighborhood of the spot seems complete. The appearance of repulsion between them is very striking, and even when the spot fades nearly to invisibility the curve remains equally distinct, so that in using a telescope too small to reveal the spot itself one may discover its location by observing the bow in the south belt. The suggestion of a resemblance to the flowing of a stream past the foot of an elevated promontory, or mountain, is strengthened by the fact, which was observed early in the history of the spot, that markings involved in the south belt have a quicker rate of rotation about the planet's axis than that of the red spot, so that such markings, first seen in the rear of the red spot, gradually overtake and pass it, and eventually leave it behind, as boats in a river drift past a rock lying in the midst of the current.
This leads us to another significant fact concerning the peculiar condition of Jupiter's surface. Not only does the south belt move perceptibly faster than the red spot, but, generally speaking, the various markings on the surface of the planet move at different rates according as they are nearer to or farther from the equator. Between the equator and latitude 30 deg. or 40 deg. there is a difference of six minutes in the rotation period—i.e., the equatorial parts turn round the axis so much faster than the parts north and south of them, that in one rotation they gain six minutes of time. In other words, the clouds over Jupiter's equator flow past those in the middle latitudes with a relative velocity of 270 miles per hour. But there are no sharp lines of separation between the different velocities; on the contrary, the swiftness of rotation gradually diminishes from the equator toward the poles, as it manifestly could not do if the visible surface of Jupiter were solid.
In this respect Jupiter resembles the sun, whose surface also has different rates of rotation diminishing from the equator. Measured by the motion of spots on or near the equator, Jupiter's rotation period is about nine hours fifty minutes; measured by the motion of spots in the middle latitudes, it is about nine hours fifty-six minutes. The red spot completes a rotation in a little less than nine hours and fifty-six minutes, but its period can not be positively given for the singular reason that it is variable. The variation amounts to only a few seconds in the course of several years, but it is nevertheless certain. The phenomenon of variable motion is not, however, peculiar to the red spot. Mr. W.F. Denning, who has studied Jupiter for a quarter of a century, says:
"It is well known that in different latitudes of Jupiter there are currents, forming the belts and zones, moving at various rates of speed. In many instances the velocity changes from year to year. And it is a singular circumstance that in the same current a uniform motion is not maintained in all parts of the circumference. Certain spots move faster than others, so that if we would obtain a fair value for the rotation period of any current it is not sufficient to derive it from one marking alone; we must follow a number of objects distributed in different longitudes along the current and deduce a mean from the whole."
[Footnote 10: The Observatory, No. 286, December, 1899.]
Nor is this all. Observation indicates that if we could look at a vertical section of Jupiter's atmosphere we should behold an equally remarkable contrast and conflict of motions. There is evidence that some of the visible spots, or clouds, lie at a greater elevation than others, and it has been observed that the deeper ones move more rapidly. This fact has led some observers to conclude that the deep-lying spots may be a part of the actual surface of the planet. But if we could think that there is any solid nucleus, or core, in the body of Jupiter, it would seem, on account of the slight mean density of the planet, that it can not lie so near the visible surface, but must be at a depth of thousands, perhaps tens of thousands, of miles. Since the telescope is unable to penetrate the cloudy envelope we can only guess at the actual constitution of the interior of Jupiter's globe. In a spirit of mere speculative curiosity it has been suggested that deep under the clouds of the great planet there may be a comparatively small solid globe, even a habitable world, closed round by a firmament all its own, whose vault, raised 30,000 or 40,000 miles above the surface of the imprisoned planet, appears only an unbroken dome, too distant to reveal its real nature to watchers below, except, perhaps, under telescopic scrutiny; enclosing, as in a shell, a transparent atmosphere, and deriving its illumination partly from the sunlight that may filter through, but mainly from some luminous source within.
But is not Jupiter almost equally fascinating to the imagination, if we dismiss all attempts to picture a humanly impossible world shut up within it, and turn rather to consider what its future may be, guided by the not unreasonable hypothesis that, because of its immense size and mass, it is still in a chaotic condition? Mention has been made of the resemblance of Jupiter to the sun by virtue of their similar manner of rotation. This is not the only reason for looking upon Jupiter as being, in some respects, almost as much a solar as a planetary body. Its exceptional brightness rather favors the view that a small part of the light by which it shines comes from its own incandescence. In size and mass it is half-way between the earth and the sun. Jupiter is eleven times greater than the earth in diameter and thirteen hundred times greater in volume; the sun is ten times greater than Jupiter in diameter and a thousand times greater in volume. The mean density of Jupiter, as we have seen, is almost exactly the same as the sun's.
Now, the history of the solar system, according to the nebular hypothesis, is a history of cooling and condensation. The sun, a thousand times larger than Jupiter, has not yet sufficiently cooled and contracted to become incrusted, except with a shell of incandescent metallic clouds; Jupiter, a thousand times smaller than the sun, has cooled and contracted until it is but slightly, if at all, incandescent at its surface, while its thickening shell, although still composed of vapor and smoke, and still probably hot, has grown so dense that it entirely cuts off the luminous radiation from within; the earth, to carry the comparison one step further, being more than a thousand times smaller than Jupiter, has progressed so far in the process of cooling that its original shell of vapor has given place to one of solid rock.
A sudden outburst of light from Jupiter, such as occurs occasionally in a star that is losing its radiance through the condensation of absorbing vapors around it, would furnish strong corroboration of the theory that Jupiter is really an extinguished sun which is now on the way to become a planet in the terrestrial sense.
Not very long ago, as time is reckoned in astronomy, our sun, viewed from the distance of the nearer fixed stars, may have appeared as a binary star, the brighter component of the pair being the sun itself and the fainter one the body now called the planet Jupiter. Supposing the latter to have had the same intrinsic brilliance, surface for surface, as the sun, it would have radiated one hundred times less light than the sun. A difference of one hundredfold between the light of two stars means that they are six magnitudes apart; or, in other words, from a point in space where the sun appeared as bright as what we call a first-magnitude star, its companion, Jupiter, would have shone as a sixth-magnitude star. Many stars have companions proportionally much fainter than that. The companion of Sirius, for instance, is at least ten thousand times less bright than its great comrade.
Looking at Jupiter in this way, it interests us not as the probable abode of intelligent life, but as a world in the making, a world, moreover, which, when it is completed—if it ever shall be after the terrestrial pattern—will dwarf our globe into insignificance. That stupendous miracle of world-making which is dimly painted in the grand figures employed by the writers of Genesis, and the composers of other cosmogonic legends, is here actually going on before our eyes. The telescope shows us in the cloudy face of Jupiter the moving of the spirit upon the face of the great deep. What the final result will be we can not tell, but clearly the end of the grand processes there in operation has not yet been reached.
The interesting suggestion was made and urged by Mr. Proctor that if Jupiter itself is in no condition at present to bear life, its satellites may be, in that respect, more happily circumstanced. It can not be said that very much has been learned about the satellites of Jupiter since Proctor's day, and his suggestion is no less and no more probable now than it was when first offered.
There has been cumulative evidence that Jupiter's satellites obey the same law that governs the rotation of our moon, viz., that which compels them always to keep the same face turned toward their primary, and this would clearly affect, although it might not preclude, their habitability. With the exception of the minute fifth satellite discovered by Barnard in 1892, they are all of sufficient size to retain at least some traces of an atmosphere. In fact, one of them is larger than the planet Mars, and another is of nearly the same size as that planet, while the smallest of the four principal ones is about equal to our moon. Under the powerful attraction of Jupiter they travel rapidly, and viewed from the surface of that planet they would offer a wonderful spectacle.
They are continually causing solar eclipses and themselves undergoing eclipse in Jupiter's shadow, and their swiftly changing aspects and groupings would be watched by an astronomer on Jupiter with undying interest.
But far more wonderful would be the spectacle presented by Jupiter to inhabitants dwelling on his moons. From the nearer moon, in particular, which is situated less than 220,000 miles from Jupiter's surface, the great planet would be an overwhelming phenomenon in the sky.
Its immense disk, hanging overhead, would cover a circle of the firmament twenty degrees in diameter, or, in round numbers, forty times the diameter of the full moon as seen from the earth! It would shed a great amount of light and heat, and thus would more or less effectively supply the deficit of solar radiation, for we must remember that Jupiter and his satellites receive from the sun less than one twenty-fifth as much light and heat as the earth receives.
The maze of contending motions, the rapid flow and eddying of cloud belts, the outburst of strange fiery spots, the display of rich, varied, and constantly changing colors, which astonish and delight the telescopic observer on the earth, would be exhibited to the naked eye of an inhabitant of Jupiter's nearest moon far more clearly than the greatest telescope is able to reveal them to us.
Here, again, the mind is carried back to long past ages in the history of the planet on which we dwell. It is believed by some that our moon may have contained inhabitants when the earth was still hot and glowing, as Jupiter appears to be now, and that, as the earth cooled and became habitable, the moon gradually parted with its atmosphere and water so that its living races perished almost coincidently with the beginning of life on the earth. If we accept this view and apply it to the case of Jupiter we may conclude that when that enormous globe has cooled and settled down to a possibly habitable condition, its four attendant moons will suffer the fate that overtook the earth's satellite, and in their turn become barren and death-stricken, while the great orb that once nurtured them with its light and heat receives the Promethean fire and begins to bloom with life.
SATURN, A PRODIGY AMONG PLANETS
One of the first things that persons unaccustomed to astronomical observations ask to see when they have an opportunity to look through a telescope is the planet Saturn. Many telescopic views in the heavens disappoint the beginner, but that of Saturn does not. Even though the planet may not look as large as he expects to see it from what he has been told of the magnifying power employed, the untrained observer is sure to be greatly impressed by the wonderful rings, suspended around it as if by a miracle. No previous inspection of pictures of these rings can rob them of their effect upon the eye and the mind. They are overwhelming in their inimitable singularity, and they leave every spectator truly amazed. Sir John Herschel has remarked that they have the appearance of an "elaborately artificial mechanism." They have even been regarded as habitable bodies! What we are to think of that proposition we shall see when we come to consider their composition and probable origin. In the meantime let us recall the main facts of Saturn's dimensions and situation in the solar system.
Saturn is the second of the major, or Jovian, group of planets, and is situated at a mean distance from the sun of 886,000,000 miles. We need not consider the eccentricity of its orbit, which, although relatively not very great, produces a variation of 50,000,000 miles in its distance from the sun, because, at its immense mean distance, this change would not be of much importance with regard to the planet's habitability or non-habitability. Under the most favorable conditions Saturn can never be nearer than 744,000,000 miles to the earth, or eight times the sun's distance from us. It receives from the sun about one ninetieth of the light and heat that we get.
Saturn takes twenty-nine and a half years to complete a journey about the sun. Like Jupiter, it rotates very rapidly on its axis, the period being ten hours and fourteen minutes. Its axis of rotation is inclined not far from the same angle as that of the earth's axis (26 deg. 49 min.), so that its seasons should resemble ours, although their alternations are extremely slow in consequence of the enormous length of Saturn's year.
Not including the rings in the calculation, Saturn exceeds the earth in size 760 times. The addition of the rings would not, however, greatly alter the result of the comparison, because, although the total surface of the rings, counting both faces, exceeds the earth's surface about 160 times, their volume, owing to their surprising thinness, is only about six times the volume of the earth, and their mass, in consequence of their slight density, is very much less than the earth's, perhaps, indeed, inappreciable in comparison.
Saturn's mean diameter is 73,000 miles, and its polar compression is even greater than that of Jupiter, a difference of 7,000 miles—almost comparable with the entire diameter of the earth—existing between its equatorial and its polar diameter, the former being 75,000 and the latter 68,000 miles.
We found the density of Jupiter astonishingly slight, but that of Saturn is slighter still. Jupiter would sink if thrown into water, but Saturn would actually float, if not "like a cork," yet quite as buoyantly as many kinds of wood, for its mean density is only three quarters that of water, or one eighth of the earth's. In fact, there is no known planet whose density is so slight as Saturn's. Thus it happens that, notwithstanding its vast size and mass, the force of gravity upon Saturn is nearly the same as upon our globe. Upon visiting Venus we should find ourselves weighing a little less than at home, and upon visiting Saturn a little more, but in neither case would the difference be very important. If the relative weight of bodies on the surfaces of planets formed the sole test of their habitability, Venus and Saturn would both rank with the earth as suitable abodes for men.
But the exceedingly slight density of Saturn seems to be most reasonably accounted for on the supposition that, like Jupiter, it is in a vaporous condition, still very hot within—although but slightly, if at all, incandescent at the surface—and, therefore, unsuited to contain life. It is hardly worth while to speculate about any solid nucleus within, because, even if such a thing were possible, or probable, it must lie forever hidden from our eyes. But if we accept the theory that Saturn is in an early formative stage, and that, millions of years hence, it may become an incrusted and habitable globe, we shall, at least, follow the analogy of what we believe to have been the history of the earth, except that Saturn's immense distance from the sun will always prevent it from receiving an amount of solar radiation consistent with our ideas of what is required by a living world. Of course, since one can imagine what he chooses, it is possible to suppose inhabitants suited to existence in a world composed only of whirling clouds, and a poet with the imagination of a Milton might give us very imposing and stirring images of such creatures and their chaotic surroundings, but fancies like these can have no basis in human experience, and consequently can make no claim upon scientific recognition.
Or, as an alternative, it might be assumed that Saturn is composed of lighter elements and materials than those which constitute the earth and the other solid planets in the more immediate neighborhood of the sun. But such an assumption would put us entirely at sea as regards the forms of organic life that could exist upon a planet of that description, and, like Sir Humphry Davy in the Vision, that occupies the first chapter of his quaintly charming Consolations in Travel, or, the Last Days of a Philosopher, we should be thrown entirely upon the resources of the imagination in representing to ourselves the nature and appearance of its inhabitants. Yet minds of unquestioned power and sincerity have in all ages found pleasure and even profit in such exercises, and with every fresh discovery arises a new flight of fancies like butterflies from a roadside pool. As affording a glimpse into the mind of a remarkable man, as well as a proof of the fascination of such subjects, it will be interesting to quote from the book just mentioned Davy's description of his imaginary inhabitants of Saturn:
"I saw below me a surface infinitely diversified, something like that of an immense glacier covered with large columnar masses, which appeared as if formed of glass, and from which were suspended rounded forms of various sizes which, if they had not been transparent, I might have supposed to be fruit. From what appeared to me to be analogous to bright-blue ice, streams of the richest tint of rose color or purple burst forth and flowed into basins, forming lakes or seas of the same color. Looking through the atmosphere toward the heavens, I saw brilliant opaque clouds, of an azure color, that reflected the light of the sun, which had to my eyes an entirely new aspect and appeared smaller, as if seen through a dense blue mist.
"I saw moving on the surface below me immense masses, the forms of which I find it impossible to describe. They had systems for locomotion similar to those of the morse, or sea-horse, but I saw, with great surprise, that they moved from place to place by six extremely thin membranes, which they used as wings. Their colors were varied and beautiful, but principally azure and rose color. I saw numerous convolutions of tubes, more analogous to the trunk of the elephant than to anything else I can imagine, occupying what I supposed to be the upper parts of the body. It was with a species of terror that I saw one of them mounting upward, apparently flying toward those opaque clouds which I have before mentioned.
"'I know what your feelings are,' said the Genius; 'you want analogies, and all the elements of knowledge to comprehend the scene before you. You are in the same state in which a fly would be whose microscopic eye was changed for one similar to that of man, and you are wholly unable to associate what you now see with your former knowledge. But those beings who are before you, and who appear to you almost as imperfect in their functions as the zoophytes of the polar sea, to which they are not unlike in their apparent organization to your eyes, have a sphere of sensibility and intellectual enjoyment far superior to that of the inhabitants of your earth. Each of those tubes, which appears like the trunk of an elephant, is an organ of peculiar motion or sensation. They have many modes of perception of which you are wholly ignorant, at the same time that their sphere of vision is infinitely more extended than yours, and their organs of touch far more perfect and exquisite.'"
After descanting upon the advantages of Saturn's position for surveying some of the phenomena of the solar system and of outer space, and the consequent immense advances that the Saturnians have made in astronomical knowledge, the Genius continues:
"'If I were to show you the different parts of the surface of this planet you would see the marvelous results of the powers possessed by these highly intellectual beings, and of the wonderful manner in which they have applied and modified matter. Those columnar masses, which seem to you as if rising out of a mass of ice below, are results of art, and processes are going on within them connected with the formation and perfection of their food. The brilliant-colored fluids are the results of such operations as on the earth would be performed in your laboratories, or more properly in your refined culinary apparatus, for they are connected with their system of nourishment. Those opaque azure clouds, to which you saw a few minutes ago one of those beings directing his course, are works of art, and places in which they move through different regions of their atmosphere, and command the temperature and the quantity of light most fitted for their philosophical researches, or most convenient for the purposes of life.'"
[Footnote 11: Davy, of course, was aware that, owing to increase of distance, the sun would appear to an inhabitant of Saturn with a disk only one ninetieth as great in area as that which it presents to our eyes.]
But, while Saturn does not appear, with our present knowledge, to hold out any encouragement to those who would regard it as the abode of living creatures capable of being described in any terms except those of pure imagination, yet it is so unique a curiosity among the heavenly bodies that one returns again and again to the contemplation of its strange details. Saturn has nine moons, but some of them are relatively small bodies—the ninth, discovered photographically by Professor Pickering in 1899, being especially minute—and others are situated at great distances from the planet, and for these reasons, together with the fact that the sunlight is so feeble upon them that, surface for surface, they have only one ninetieth as much illumination as our moon receives, they can not make a very brilliant display in the Saturnian sky. To astronomers on Saturn they would, of course, be intensely interesting because of their perturbations and particularly the effect of their attraction on the rings.
This brings us again to the consideration of those marvelous appendages, and to the statement of facts about them which we have not yet recalled.
If the reader will take a ball three inches in diameter to represent the globe of Saturn, and, out of the center of a circular piece of writing-paper seven inches in diameter, will cut a round hole three and three quarter inches across, and will then place the ball in the middle of the hole in the paper, he will have a very fair representation of the relative proportions of Saturn and its rings. To represent the main gap or division in the rings he might draw, a little more than three eighths of an inch from the outer edge of the paper disk, a pencil line about a sixteenth of an inch broad.
Perhaps the most striking fact that becomes conspicuous in making such a model of the Saturnian system is the exceeding thinness of the rings as compared with their enormous extent. They are about 170,000 miles across from outer edge to outer edge, and about 38,000 miles broad from outer edge to inner edge—including the gauze ring presently to be mentioned—yet their thickness probably does not surpass one hundred miles! In fact, the sheet of paper in our imaginary model is several times too thick to represent the true relative thickness of Saturn's rings.
Several narrow gaps in the rings have been detected from time to time, but there is only one such gap that is always clearly to be seen, the one already mentioned, situated about 10,000 miles from the outer edge and about 1,600 miles in width. Inside of this gap the broadest and brightest ring appears, having a width of about 16,500 miles. For some reason this great ring is most brilliant near the gap, and its brightness gradually falls off toward its inner side. At a distance of something less than 20,000 miles from the planet—or perhaps it would be more correct to say above the planet, for the rings hang directly over Saturn's equator—the broad, bright ring merges into a mysterious gauzelike object, also in the form of a ring, which extends to within 9,000 or 10,000 miles of the planet's surface, and therefore itself has a width of say 10,000 miles.
In consequence of the thinness of the rings they completely disappear from the range of vision of small telescopes when, as occurs once in every fifteen years, they are seen exactly edgewise from the earth. In a telescope powerful enough to reveal them when in that situation they resemble a thin, glowing needle run through the ball of the planet. The rings will be in this position in 1907, and again in 1922.
The opacity of the rings is proved by the shadow which they cast upon the ball of the planet. This is particularly manifest at the time when they are edgewise to the earth, for the sun being situated slightly above or below the plane of the rings then throws their shadow across Saturn close to its equator. When they are canted at a considerable angle to our line of sight their shadow is seen on the planet, bordering their outer edge where they cross the ball.
The gauze ring, the detection of which as a faintly luminous phenomenon requires a powerful telescope, can be seen with slighter telescopic power in the form of a light shade projected against the planet at the inner edge of the broad bright ring. The explanation of the existence of this peculiar object depends upon the nature of the entire system, which, instead of being, as the earliest observers thought it, a solid ring or series of concentric rings, is composed of innumerable small bodies, like meteorites, perhaps, in size, circulating independently but in comparatively close juxtaposition to one another about Saturn, and presenting to our eyes, because of their great number and of our enormous distance, the appearance of solid, uniform rings. So a flock of ducks may look from afar like a continuous black line or band, although if we were near them we should perceive that a considerable space separates each individual from his neighbors.
The fact that this is the constitution of Saturn's rings can be confidently stated because it has been mathematically proved that they could not exist if they were either solid or liquid bodies in a continuous form, and because the late Prof. James E. Keeler demonstrated with the spectroscope, by means of the Doppler principle, already explained in the chapter on Venus, that the rings circulate about the planet with varying velocities according to their distance from Saturn's center, exactly as independent satellites would do.
It might be said, then, that Saturn, instead of having nine satellites only, has untold millions of them, traveling in orbits so closely contiguous that they form the appearance of a vast ring.
As to their origin, it may be supposed that they are a relic of a ring of matter left in suspension during the contraction of the globe of Saturn from a nebulous mass, just as the rings from which the various planets are supposed to have been formed were left off during the contraction of the main body of the original solar nebula. Other similar rings originally surrounding Saturn may have become satellites, but the matter composing the existing rings is so close to the planet that it falls within the critical distance known as "Roche's limit," within which, owing to the tidal effect of the planet's attraction, no body so large as a true satellite could exist, and accordingly in the process of formation of the Saturnian system this matter, instead of being aggregated into a single satellite, has remained spread out in the form of a ring, although its substance long ago passed from the vaporous and liquid to the solid form. We have spoken of the rings as being composed of meteorites, but perhaps their component particles may be so small as to answer more closely to the definition of dust. In these rings of dust, or meteorites, disturbances are produced by the attraction of the planet and that of the outer satellites, and it is yet a question whether they are a stable and permanent feature of Saturn, or will, in the course of time, be destroyed.
[Footnote 12: For further details about Saturn's rings, see The Tides, by G.H. Darwin, chap. xx.]
It has been thought that the gauze ring is variable in brightness. This would tend to show that it is composed of bodies which have been drawn in toward the planet from the principal mass of the rings, and these bodies may end their career by falling upon the planet. This process, indefinitely continued, would result in the total disappearance of the rings—Saturn would finally swallow them, as the old god from whom the planet gets its name is fabled to have swallowed his children.
Near the beginning of this chapter reference was made to the fact that Saturn's rings have been regarded as habitable bodies. That, of course, was before the discovery that they were not solid. Knowing what we now know about them, even Dr. Thomas Dick, the great Scotch popularizer of astronomy in the first half of the nineteenth century, would have been compelled to abandon his theory that Saturn's rings were crowded with inhabitants. At the rate of 280 to the square mile he reckoned that they could easily contain 8,078,102,266,080 people.
He even seems to have regarded their edges—in his time their actual thinness was already well known—as useful ground for the support of living creatures, for he carefully calculated the aggregate area of these edges and found that it considerably exceeded the area of the entire surface of the earth. Indeed, Dr. Dick found room for more inhabitants on Saturn's rings than on Saturn itself, for, excluding the gauze ring, undiscovered in his day, the two surfaces of the rings are greater in area than the surface of the globe of the planet. He did not attack the problem of the weight of bodies on worlds in the form of broad, flat, thin, surfaces like Saturn's rings, or indulge in any reflections on the interrelations of the inhabitants of the opposite sides, although he described the wonderful appearance of Saturn and other celestial objects as viewed from the rings.
But all these speculations fall to the ground in face of the simple fact that if we could reach Saturn's rings we should find nothing to stand upon, except a cloud of swiftly flying dust or a swarm of meteors, swayed by contending attractions. And, indeed, it is likely that upon arriving in the immediate neighborhood of the rings they would virtually disappear! Seen close at hand their component particles might be so widely separated that all appearance of connection between them would vanish, and it has been estimated that from Saturn's surface the rings, instead of presenting a gorgeous arch spanning the heavens, may be visible only as a faintly gleaming band, like the Milky Way or the zodiacal light. In this respect the mystic Swedenborg appears to have had a clearer conception of the true nature of Saturn's rings than did Dr. Dick, for in his book on The Earths in the Universe he says—using the word "belt" to describe the phenomenon of the rings:
"Being questioned concerning that great belt which appears from our earth to rise above the horizon of that planet, and to vary its situations, they [the inhabitants of Saturn] said that it does not appear to them as a belt, but only as somewhat whitish, like snow in the heaven, in various directions."
In view of such observations as that of Prof. E.E. Barnard, in 1892, showing that a satellite passing through the shadow of Saturn's rings does not entirely disappear—a fact which proves that the rings are partially transparent to the sunlight—one might be tempted to ask whether Saturn itself, considering its astonishing lack of density, is not composed, at least in its outer parts, of separate particles of matter revolving independently about their center of attraction, and presenting the appearance of a smooth, uniform shell reflecting the light of the sun. In other words, may not Saturn be, exteriorly, a globe of dust instead of a globe of vapor? Certainly the rings, incoherent and translucent though they be, reflect the sunlight to our eyes, at least from the brighter part of their surface, with a brilliance comparable with that of the globe of the planet itself.
As bearing on the question of the interior condition of Saturn and Jupiter, it should, perhaps, be said that mathematical considerations, based on the figures of equilibrium of rotating liquid masses, lead to the conclusion that those planets are comparatively very dense within. Professor Darwin puts the statement very strongly, as follows: "In this way it is known with certainty that the central portions of the planets Jupiter and Saturn are much denser, compared to their superficial portions, than is the case with the earth."
[Footnote 13: The Tides, by G.H. Darwin, p. 333.]
The globe and rings of Saturn witness an imposing spectacle of gigantic moving shadows. The great ball stretches its vast shade across the full width of the rings at times, and the rings, as we have seen, throw their shadow in a belt, whose position slowly changes, across the ball, sweeping from the equator, now toward one pole and now toward the other. The sun shines alternately on each side of the rings for a space of nearly fifteen years—a day fifteen years long! And then, when that face of the ring is turned away from the sun, there ensues a night of fifteen years' duration also.
Whatever appearance the rings may present from the equator and the middle latitudes on Saturn, from the polar regions they would be totally invisible. As one passed toward the north, or the south, pole he would see the upper part of the arch of the rings gradually sink toward the horizon until at length, somewhere in the neighborhood of the polar circle, it would finally disappear, hidden by the round shoulder of the great globe.
URANUS, NEPTUNE, AND THE SUSPECTED ULTRANEPTUNIAN PLANET
What has been said of Jupiter and Saturn applies also to the remaining members of the Jovian group of planets, Uranus and Neptune, viz., that their density is so small that it seems probable that they can not, at the present time, be in a habitable planetary condition. All four of these outer, larger planets have, in comparatively recent times, been solar orbs, small companions of the sun. The density of Uranus is about one fifth greater than that of water, and slightly greater than that of Neptune. Uranus is 32,000 miles in diameter, and Neptune 35,000 miles. Curiously enough, the force of gravity upon each of these two large planets is a little less than upon the earth. This arises from the fact that in reckoning gravity on the surface of a planet not only the mass of the planet, but its diameter or radius, must be considered. Gravity varies directly as the mass, but inversely as the square of the radius, and for this reason a large planet of small density may exercise a less force of gravity at its surface than does a small planet of great density.
The mean distance of Uranus from the sun is about 1,780,000,000 miles, and its period of revolution is eighty-four years; Neptune's mean distance is about 2,800,000,000 miles, and its period of revolution is about 164 years.
Uranus has four satellites, and Neptune one. The remarkable thing about these satellites is that they revolve backward, or contrary to the direction in which all the other satellites belonging to the solar system revolve, and in which all the other planets rotate on their axis. In the case of Uranus, the plane in which the satellites revolve is not far from a position at right angles to the plane of the ecliptic; but in the case of Neptune, the plane of revolution of the satellites is tipped much farther backward. Since in every other case the satellites of a planet are situated nearly in the plane of the planet's equator, it may be assumed that the same rule holds with Uranus and Neptune; and, that being so, we must conclude that those planets rotate backward on their axes. This has an important bearing on the nebular hypothesis of the origin of the solar system, and at one time was thought to furnish a convincing argument against that hypothesis; but it has been shown that by a modification of Laplace's theory the peculiar behavior of Uranus and Neptune can be reconciled with it.
Very little is known of the surfaces of Uranus and Neptune. Indications of the existence of belts resembling those of Jupiter have been found in the case of both planets. There are similar belts on Saturn, and as they seem to be characteristic of large, rapidly rotating bodies of small density, it was to be expected that they would be found on Uranus and Neptune.
The very interesting opinion is entertained by some astronomers that there is at least one other great planet beyond Neptune. The orbits of certain comets are relied upon as furnishing evidence of the existence of such a body. Prof. George Forbes has estimated that this, as yet undiscovered, planet may be even greater than Jupiter in mass, and may be situated at a distance from the sun one hundred times as great as the earth's, where it revolves in an orbit a single circuit of which requires a thousand years.
Whether this planet, with a year a thousand of our years in length, will ever be seen with a telescope, or whether its existence will ever, in some other manner, be fully demonstrated, can not yet be told. It will be remembered that Neptune was discovered by means of computations based upon its disturbing attraction on Uranus before it had ever been recognized with the telescope. But when the astronomers in the observatories were told by their mathematical brethren where to look they found the planet within half an hour after the search began. So it is possible the suspected great planet beyond Neptune may be within the range of telescopic vision, but may not be detected until elaborate calculations have deduced its place in the heavens. As a populous city is said to furnish the best hiding-place for a man who would escape the attention of his fellow beings, so the star-sprinkled sky is able to conceal among its multitudes worlds both great and small until the most painstaking detective methods bring them to recognition.
THE MOON, CHILD OF THE EARTH AND THE SUN
Very naturally the moon has always been a great favorite with those who, either in a scientific or in a literary spirit, have speculated about the plurality of inhabited worlds. The reasons for the preference accorded to the moon in this regard are evident. Unless a comet should brush us—as a comet is suspected of having done already—no celestial body, of any pretensions to size, can ever approach as near to the earth as the moon is, at least while the solar system continues to obey the organic laws that now control it. It is only a step from the earth to the moon. What are 240,000 miles in comparison with the distances of the stars, or even with the distances of the planets? Jupiter, driving between the earth and the moon, would occupy more than one third of the intervening space with the chariot of his mighty globe; Saturn, with broad wings outspread, would span more than two thirds of the distance; and the sun, so far from being able to get through at all, would overlap the way more than 300,000 miles on each side.
In consequence, of course, of its nearness, the moon is the only member of the planetary system whose principal features are visible to the naked eye. In truth, the naked eye perceives the larger configurations of the lunar surface more clearly than the most powerful telescope shows the details on the disk of Mars. Long before the time of Galileo and the invention of the telescope, men had noticed that the face of the moon bears a resemblance to the appearance that the earth would present if viewed from afar off. In remote antiquity there were philosophers who thought that the moon was an inhabited world, and very early the romancers took up the theme. Lucian, the Voltaire of the second century of our era, mercilessly scourged the pretenders of the earth from an imaginary point of vantage on the moon, which enabled him to peer down into their secrets. Lucian's description of the appearance of the earth from the moon shows how clearly defined in his day had become the conception of our globe as only an atom in space.
"Especially did it occur to me to laugh at the men who were quarreling about the boundaries of their land, and at those who were proud because they cultivated the Sikyonian plain, or owned that part of Marathon around Oenoe, or held possession of a thousand acres at Acharnae. Of the whole of Greece, as it then appeared to me from above, being about the size of four fingers, I think Attica was in proportion a mere speck. So that I wondered on what condition it was left to these rich men to be proud."
[Footnote 14: Ikaromenippus; or, Above the Clouds. Prof. D.C. Brown's translation.]
Such scenes as Lucian beheld, in imagination, upon the earth while looking from the moon, many would fain behold, with telescopic aid, upon the moon while looking from the earth. Galileo believed that the details of the lunar surface revealed by his telescope closely resembled in their nature the features of the earth's surface, and for a long time, as the telescope continued to be improved, observers were impressed with the belief that the moon possessed not only mountains and plains, but seas and oceans also.
It was the discovery that the moon has no perceptible atmosphere that first seriously undermined the theory of its habitability. Yet, as was remarked in the introductory chapter, there has of late been some change of view concerning a lunar atmosphere; but the change has been not so much in the ascertained facts as in the way of looking at those facts.
But before we discuss this matter, it will be well to state what is known beyond peradventure about the moon.
Its mean distance from the earth is usually called, for the sake of a round number, 240,000 miles, but more accurately stated it is 238,840 miles. This is variable to the extent of more than 31,000 miles, on account of the eccentricity of its orbit, and the eccentricity itself is variable, in consequence of the perturbing attractions of the earth and the sun, so that the distance of the moon from the earth is continually changing. It may be as far away as 253,000 miles and as near as 221,600 miles.
Although the orbit of the moon is generally represented, for convenience, as an ellipse about the earth, it is, in reality, a varying curve, having the sun for its real focus, and always concave toward the latter. This is a fact that can be more readily explained with the aid of a diagram.
In the accompanying cut, when the earth is at A the moon is between it and the sun, in the phase called new moon. At this point the earth's orbit about the sun is more curved than the moon's, and the earth is moving relatively faster than the moon, so that when it arrives at B it is ahead of the moon, and we see the latter to the right of the earth, in the phase called first quarter. The earth being at this time ahead of the moon, the effect of its attraction, combined with that of the sun, tends to hasten the moon onward in its orbit about the sun, and the moon begins to travel more swiftly, until it overtakes the earth at C, and appears on the side opposite the sun, in the phase called full moon. At this point the moon's orbit about the sun has a shorter radius of curvature than the earth's. In traveling from C to D the moon still moves more rapidly than the earth, and, having passed it, appears at D to the left of the earth, in the phase called third quarter. Now, the earth being behind the moon, the effect of its attraction combined with the sun's tends to retard the moon in its orbit about the sun, with the result that the moon moves again less rapidly than the earth, and the latter overtakes it, so that, upon reaching E, the two are once more in the same relative positions that they occupied at A, and it is again new moon. Thus it will be seen that, although the real orbit of the moon has the sun for its center of revolution, nevertheless, in consequence of the attraction of the earth, combined in varying directions with that of the sun, the moon, once every month, makes a complete circuit of our globe.
The above explanation should not be taken for a mathematical demonstration of the moon's motion, but simply for a graphical illustration of how the moon appears to revolve about the earth while really obeying the sun's attraction as completely as the earth does.
There is no other planet that has a moon relatively as large as ours. The moon's diameter is 2,163 miles. Its volume, compared with the earth's, is in the ratio of 1 to 49, and its density is about six tenths of the earth's. This makes its mass to that of our globe about as 1 to 81. In other words, it would take eighty-one moons to counterbalance the earth. Before speaking of the force of gravity on the moon we will examine the character of the lunar surface.
To the naked eye the moon's face appears variegated with dusky patches, while a few points of superior brilliance shine amid the brighter portions, especially in the southern and eastern quarters, where immense craters like Tycho and Copernicus are visible to a keen eye, gleaming like polished buttons. With a telescope, even of moderate power, the surface of the moon presents a scene of astonishing complexity, in which strangeness, beauty, and grandeur are all combined. The half of the moon turned earthward contains an area of 7,300,000 square miles, a little greater than the area of South America and a little less than that of North America. Of these 7,300,000 square miles, about 2,900,000 square miles are occupied by the gray, or dusky, expanses, called in lunar geography, or selenography, maria—i.e., "seas." Whatever they may once have been, they are not now seas, but dry plains, bordered in many places by precipitous cliffs and mountains, varied in level by low ridges and regions of depression, intersected occasionally by immense cracks, having the width and depth of our mightiest river canons, and sprinkled with bright points and crater pits. The remaining 4,400,000 square miles are mainly occupied by mountains of the most extraordinary character. Owing partly to roughness of the surface and partly to more brilliant reflective power, the mountainous regions of the moon appear bright in comparison with the dull-colored plains.
Some of the lunar mountains lie in long, massive chains, with towering peaks, profound gorges, narrow valleys, vast amphitheaters, and beetling precipices. Looking at them with a powerful telescope, the observer might well fancy himself to be gazing down from an immense height into the heart of the untraveled Himalayas. But these, imposing though they are, do not constitute the most wonderful feature of the mountain scenery of the moon.
Appearing sometimes on the shores of the "seas," sometimes in the midst of broad plains, sometimes along the course of mountain chains, and sometimes in magnificent rows, following for hundreds of miles the meridians of the lunar globe, are tremendous, mountain-walled, circular chasms, called craters. Frequently they have in the middle of their depressed interior floors a peak, or a cluster of peaks. Their inner and outer walls are seamed with ridges, and what look like gigantic streams of frozen lava surround them. The resemblance that they bear to the craters of volcanoes is, at first sight, so striking that probably nobody would ever have thought of questioning the truth of the statement that they are such craters but for their incredible magnitude. Many of them exceed fifty miles in diameter, and some of them sink two, three, four, and more miles below the loftiest points upon their walls! There is a chasm, 140 miles long and 70 broad, named Newton, situated about 200 miles from the south pole of the moon, whose floor lies 24,000 feet below the summit of a peak that towers just above it on the east! This abyss is so profound that the shadows of its enclosing precipices never entirely quit it, and the larger part of its bottom is buried in endless night.
One can not but shudder at the thought of standing on the broken walls of Newton, and gazing down into a cavity of such stupendous depth that if Chimborazo were thrown into it, the head of the mighty Andean peak would be thousands of feet beneath the observer.
A different example of the crater mountains of the moon is the celebrated Tycho, situated in latitude about 43 deg. south, corresponding with the latitude of southern New Zealand on the earth. Tycho is nearly circular and a little more than 54 miles across. The highest point on its wall is about 17,000 feet above the interior. In the middle of its floor is a mountain 5,000 or 6,000 feet high. Tycho is especially remarkable for the vast system of whitish streaks, or rays, which starting from its outer walls, spread in all directions over the face of the moon, many of them, running, without deviation, hundreds of miles across mountains, craters, and plains. These rays are among the greatest of lunar mysteries, and we shall have more to say of them.
Copernicus, a crater mountain situated about 10 deg. north of the equator, in the eastern hemisphere of the moon, is another wonderful object, 56 miles in diameter, a polygon appearing, when not intently studied, as a circle, 11,000 or 12,000 feet deep, and having a group of relatively low peaks in the center of its floor. Around Copernicus an extensive area of the moon's surface is whitened with something resembling the rays of Tycho, but more irregular in appearance. Copernicus lies within the edge of the great plain named the Oceanus Procellarum, or "Ocean of Storms," and farther east, in the midst of the "ocean," is a smaller crater mountain, named Kepler, which is also enveloped by a whitish area, covering the lunar surface as if it were the result of extensive outflows of light-colored lava.
In one important particular the crater mountains of the moon differ from terrestrial volcanoes. This difference is clearly described by Nasmyth and Carpenter in their book on The Moon:
"While the terrestrial crater is generally a hollow on a mountain top, with its flat bottom high above the level of the surrounding country, those upon the moon have their lowest points depressed more or less deeply below the general surface of the moon, the external height being frequently only a half or one third of the internal depth."
It has been suggested that these gigantic rings are only "basal wrecks" of volcanic mountains, whose conical summits have been blown away, leaving vast crateriform hollows where the mighty peaks once stood; but the better opinion seems to be that which assumes that the rings were formed by volcanic action very much as we now see them. If such a crater as Copernicus or the still larger one named Theophilus, which is situated in the western hemisphere of the moon, on the shore of the "Sea of Nectar," ever had a conical mountain rising from its rim, the height attained by the peak, if the average slope were about 30 deg., would have been truly stupendous—fifteen or eighteen miles!
There is a kind of ring mountains, found in many places on the moon, whose forms and surroundings do not, as the craters heretofore described do, suggest at first sight a volcanic origin. These are rather level plains of an oval or circular outline, enclosed by a wall of mountains. The finest example is, perhaps, the dark-gray Plato, situated in 50 deg. of north latitude, near an immense mountain uplift named the Lunar Alps, and on the northern shore of the Mare Imbrium, or "Sea of Showers." Plato appears as an oval plain, very smooth and level, about 60 miles in length, and completely surrounded by mountains, quite precipitous on the inner side, and rising in their highest peaks to an elevation of 6,000 to 7,000 feet. Enclosed plains, bearing more or less resemblance to Plato—sometimes smooth within, and sometimes broken with small peaks and craters or hilly ridges—are to be found scattered over almost all parts of the moon. If our satellite was ever an inhabited world like the earth, while its surface was in its present condition, these valleys must have presented an extraordinary spectacle. It has been thought that they may once have been filled with water, forming lakes that recall the curious Crater Lake of Oregon.
It is not my intention to give a complete description of the various lunar features, and I mention but one other—the "clefts" or "rills," which are to be seen running across the surface like cracks. One of the most remarkable of these is found in the Oceanus Procellarum, near the crater-mountain Aristarchus, which is famed for the intense brilliance of its central peak, whose reflective power is so great that it was once supposed to be aflame with volcanic fire. The cleft, or crack, in question is very erratic in its course, and many miles in length, and it terminates in a ringed plain named Herodotus not far east of Aristarchus, breaking through the wall of the plain and entering the interior. Many other similar chasms or canons exist on the moon, some crossing plains, some cleaving mountain walls, and some forming a network of intersecting clefts. Mr. Thomas Gwyn Elger has this to say on the subject of the lunar clefts:
"If, as seems most probable, these gigantic cracks are due to contractions of the moon's surface, it is not impossible, in spite of the assertions of the text-books to the effect that our satellite is now a 'changeless world,' that emanations may proceed from these fissures, even if, under the monthly alternations of extreme temperatures, surface changes do not now occasionally take place from this cause also. Should this be so, the appearance of new rills and the extension and modification of those already existing may reasonably be looked for."
Mr. Elger then proceeds to describe his discovery in 1883, in the ring-plain Mersenius, of a cleft never noticed before, and which seems to have been of recent formation.
[Footnote 15: The Moon, a Full Description and Map of its Principal Features, by Thomas Gwyn Elger, 1895.
Those who desire to read detailed descriptions of lunar scenery may consult, in addition to Mr. Elger's book, the following: The Moon, considered as a Planet, a World, and a Satellite, by James Nasmyth and James Carpenter, 1874; The Moon, and the Condition and Configurations of its Surface, by Edmund Neison, 1876. See also Annals of Harvard College Observatory, vol. xxxii, part ii, 1900, for observations made by Prof. William H. Pickering at the Arequipa Observatory.]
We now return to the question of the force of lunar gravity. This we find to be only one sixth as great as gravity on the surface of the earth. It is by far the smallest force of gravity that we have found anywhere except on the asteroids. Employing the same method of comparison that was made in the case of Mars, we compute that a man on the moon could attain a height of thirty-six feet without being relatively more unwieldy than a six-foot descendant of Adam is on the earth.
Whether this furnishes a sound reason for assuming that the lunar inhabitants, if any exist or have ever existed, should be preposterous giants is questionable; yet such an assumption receives a certain degree of support from the observed fact that the natural features of the moon are framed on an exaggerated scale as compared with the earth's. We have just observed that the moon is characterized by vast mountain rings, attaining in many cases a diameter exceeding fifty miles. If these are volcanic craters, it is evident, at a glance, that the mightiest volcanoes of the earth fall into insignificance beside them. Now, the slight force of gravity on the moon has been appealed to as a reason why volcanic explosions on the lunar globe should produce incomparably greater effects than upon the earth, where the ejected materials are so much heavier. The same force that would throw a volcanic bomb a mile high on the earth could throw it six miles high on the moon. The giant cannon that we have placed in one of our coast forts, which is said to be able to hurl a projectile to a distance of fifteen miles, could send the same projectile ninety miles on the moon. An athlete who can clear a horizontal bar at a height of six feet on the earth could clear the same bar at a height of thirty-six feet on the moon. In other words, he could jump over a house, unless, indeed, the lunarians really are giants, and live in houses proportioned to their own dimensions and to the size of their mountains. In that case, our athlete would have to content himself with jumping over a lunarian, whose head he could just clear—with the hat off.
These things are not only amusing, but important. There can be no question that the force of gravity on the moon actually is as slight as it has just been described. So, even without calling in imaginary inhabitants to lend it interest, the comparative inability of the moon to arrest bodies in motion becomes a fact of much significance. It has led to the theory that meteorites may have originally been shot out of the moon's great volcanoes, when those volcanoes were active, and may have circulated about the sun until various perturbations have brought them down upon the earth. A body shot radially from the surface of the moon would need to have a velocity of only about a mile and a half in a second in order to escape from the moon's control, and we can believe that a lunar volcano when in action could have imparted such a velocity, all the more readily because with modern gunpowders we have been able to give to projectiles a speed one half as great as that needed for liberation from lunar gravity.
Another consequence of the small gravitative power of the moon bears upon the all-important question of atmosphere. According to the theory of Dr. Johnstone Stoney, heretofore referred to, oxygen, nitrogen, and water vapor would all gradually escape from the moon, if originally placed upon it, because, by the kinetic theory, the maximum velocities of their molecules are greater than a mile and a half per second. The escape would not occur instantly, nor all at once, for it would be only the molecules at the upper surface of the atmosphere which were moving with their greatest velocity, and in a direction radial to the center of the moon, that would get away; but in the course of time this gradual leakage would result in the escape of all of those gases.
[Footnote 16: The discovery of free hydrogen in the earth's atmosphere, by Professor Dewar, 1901, bears upon the theory of the escape of gases from a planet, and may modify the view above expressed. Since hydrogen is theoretically incapable of being permanently retained in the free state by the earth, its presence in the atmosphere indicates either that there is an influx from space or that it emanates from the earth's crust. In a similar way it may be assumed that atmospheric gases can be given off from the crust of the moon, thus, to a greater or less extent, supplying the place of the molecules that escape.]
After it had been found that, to ordinary tests, the moon offered no evidence of the possession of an atmosphere, and before Dr. Stoney's theory was broached, it was supposed by many that the moon had lost its original supply of air by absorption into its interior. The oxygen was supposed to have entered into combination with the cooling rocks and minerals, thus being withdrawn from the atmosphere, and the nitrogen was imagined to have disappeared also within the lunar crust. For it seems to have always been tacitly assumed that the phenomenon to be accounted for was not so much the absence of a lunar atmosphere as its disappearance. But disappearance, of course, implies previous existence. In like manner it has always been a commonly accepted view that the moon probably once had enough water to form lakes and seas.
These, it has been calculated, could have been absorbed into the lunar globe as it cooled off. But Johnstone Stoney's theory offers another method by which they could have escaped, through evaporation and the gradual flight of the molecules into open space. Possibly both methods have been in operation, a portion of the constituents of the former atmosphere and oceans having entered into chemical combinations in the lunar crust, and the remainder having vanished in consequence of the lack of sufficient gravitative force to retain them.
But why, it may be asked, should it be assumed that the moon ever had things which it does not now possess? Perhaps no entirely satisfactory reply can be made. Some observers have believed that they detected unmistakable indications of alluvial deposits on lunar plains, and of the existence of beaches on the shores of the "seas." Messrs. Loewy and Puiseux, of the Paris Observatory, whose photographs of the moon are perhaps the finest yet made, say on this subject:
"There exists, from the point of view of relief, a general similarity between the 'seas' of the moon and the plateaux which are covered to-day by terrestrial oceans. In these convex surfaces are more frequent than concave basins, thrown back usually toward the verge of the depressed space. In the same way the 'seas' of the moon present, generally at the edges, rather pronounced depressions. In one case, as in the other, we observe normal deformations of a shrinking globe shielded from the erosive action of rain, which tends, on the contrary, in all the abundantly watered parts of the earth to make the concave surfaces predominate. The explanation of this structure, such as is admitted at present by geologists, seems to us equally valid for the moon."
[Footnote 17: Comptes Rendus, June 26, July 3, 1899.]
It might be urged that there is evidence of former volcanic activity on the moon of such a nature that explosions of steam must have played a part in the phenomena, and if there was steam, of course there was water.
But perhaps the most convincing argument tending to show that the moon once had a supply of water, of which some remnant may yet remain below the surface of the lunar globe, is based upon the probable similarity in composition of the earth and the moon. This similarity results almost equally whether we regard the moon as having originated in a ring of matter left off from the contracting mass that became the earth, or whether we accept the suggestion of Prof. G.H. Darwin, that the moon is the veritable offspring of the earth, brought into being by the assistance of the tidal influence of the sun. The latter hypothesis is the more picturesque of the two, and, at present, is probably the more generally favored. It depends upon the theory of tidal friction, which was referred to in Chapter III, as offering an explanation of the manner in which the rotation of the planet Mercury has been slowed down until its rotary period coincides with that of its revolution.
The gist of the hypothesis in question is that at a very early period in its history, when the earth was probably yet in a fluid condition, it rotated with extreme rapidity on its axis, and was, at the same time, greatly agitated by the tidal attraction of the sun, and finally huge masses were detached from the earth which, ultimately uniting, became the moon.
[Footnote 18: The Tides, by G.H. Darwin, chapter xvi.]
Born in this manner from the very substance of the earth, the moon would necessarily be composed, in the main, of the same elements as the globe on which we dwell, and is it conceivable that it should not have carried with it both air and water, or the gases from which they were to be formed? If the moon ever had enough of these prime requisites to enable it to support forms of life comparable with those of the earth, the disappearance of that life must have been a direct consequence of the gradual vanishing of the lunar air and water. The secular drying up of the oceans and wasting away of the atmosphere on our little neighbor world involved a vast, all-embracing tragedy, some of the earlier scenes of which, if theories be correct, are now reenacted on the half-desiccated planet Mars—a planet, by the way, which in size, mass, and ability to retain vital gases stands about half-way between the earth and the moon.
One of the most interesting facts about the moon is that its surface affords evidence of a cataclysm which has wiped out many, and perhaps nearly all, of the records of its earlier history, that were once written upon its face. Even on the earth there have been geological catastrophes destroying or burying the accumulated results of ages of undisturbed progress, but on the moon these effects have been transcendent. The story of the tremendous disaster that overtook the moon is partly written in its giant volcanoes. Although it may be true, as some maintain, that there is yet volcanic action going on upon the lunar surface, it is evident that such action must be insignificant in comparison with that which took place ages ago.
There is a spot in the western hemisphere of the moon, on the border of a placid bay or "sea," that I can never look at without a feeling of awe and almost of shrinking. There, within a space about 250 miles in length by 100 in width, is an exhibition of the most terrifying effects of volcanic energy that the eye of man can anywhere behold. Three immense craters—Theophilus, 64 miles across and 3-1/2 miles deep; Cyrillus, 60 miles across and 15,000 feet deep; and Catharina, 70 miles across and from 8,000 to 16,000 feet deep—form an interlinked chain of mountain rings, ridges, precipices, chasms, and bottomless pits that take away one's breath.
But when the first impression of astonishment and dismay produced by this overwhelming spectacle has somewhat abated, the thoughtful observer will note that here the moon is telling him a part of her wonderful story, depicted in characters so plain that he needs no instruction in order to decipher their meaning. He will observe that this ruin was not all wrought at once or simultaneously. Theophilus, the crater-mountain at the northwestern end of the chain, whose bottom lies deepest of all, is the youngest of these giants, though the most imposing. For a distance of forty miles the lofty wall of Theophilus has piled itself upon the ruins of the wall of Cyrillus, and the circumference of the circle of its tremendous crater has been forcibly thrust within the original rim of the more ancient crater, which was thus rudely compelled to make room for its more vigorous rival and successor.
The observer will also notice that Catharina, the huge pit at the southeastern end of the chain, bears evidence of yet greater age. Its original walls, fragments of which still stand in broken grandeur, towering to a height of 16,000 feet, have, throughout the greater part of their circuit, been riddled by the outbreak of smaller craters, and torn asunder and thrown down on all sides.
In the vast enclosure that was originally the floor of the crater-mountain Catharina, several crater rings, only a third, a quarter, or a fifth as great in diameter, have broken forth, and these in turn have been partially destroyed, while in the interior of the oldest of them yet smaller craters, a nest of them, mere Etnas, Cotopaxis, and Kilaueas in magnitude, simple pinheads on the moon, have opened their tiny jaws in weak and ineffective expression of the waning energies of a still later epoch, which followed the truly heroic age of lunar vulcanicity.
This is only one example among hundreds, scattered all over the moon, which show how the surface of our satellite has suffered upheaval after upheaval. It is possible that some of the small craters, not included within the walls of the greater ones, may represent an early stage in the era of volcanic activity that wrecked the moon, but where larger and smaller are grouped together a certain progression can be seen, tending finally to extinction. The internal energies reached a maximum and then fell off in strength until they died out completely.
It can hardly be supposed that the life-bearing phase of lunar history—if there ever was one—could survive the outbreak of the volcanic cataclysm. North America, or Europe, if subjected to such an experience as the continental areas of the moon have passed through, would be, in proportion, worse wrecked than the most fearfully battered steel victim of a modern sea fight, and one can readily understand that, in such circumstances, those now beautiful and populous continents would exhibit, from a distance, scarcely any token of their present topographical features, to say nothing of any relics of their occupation by living creatures.
There are other interesting glimpses to be had of an older world in the moon than that whose scarred face is now beautified for us by distance. Not far from Theophilus and the other great crater-mountains just described, at the upper, or southern, end of the level expanse called the "Sea of Nectar," is a broad, semicircular bay whose shores are formed by the walls of a partially destroyed crater named Fracastorius. It is evident that this bay, and the larger part of the "Sea of Nectar," have been created by an outwelling of liquid lavas, which formed a smooth floor over a portion of the pre-existing surface of the moon, and broke down and submerged a large part of the mountain ring of Fracastorius, leaving the more ancient walls standing at the southern end, while, outlined by depressions and corrugations in the rocky blanket, are certain half-defined forms belonging to the buried world beneath.
Near Copernicus, some years ago, as Dr. Edward S. Holden pointed out, photographs made with the great Lick telescope, then under his direction, showed, in skeleton outline, a huge ring buried beneath some vast outflow of molten matter and undiscerned by telescopic observers. And Mr. Elger, who was a most industrious observer and careful interpreter of lunar scenery, speaks of "the undoubted existence of the relics of an earlier lunar world beneath the smooth superficies of the maria."
Although, as already remarked, it seems necessary to assume that any life existing in the moon prior to its great volcanic outburst must have ceased at that time, yet the possibility may be admitted that life could reappear upon the moon after its surface had again become quiet and comparatively undisturbed. Germs of the earlier life might have survived, despite the terrible nature of the catastrophe. But the conditions on the moon at present are such that even the most confident advocates of the view that the lunar world is not entirely dead do not venture to assume that anything beyond the lowest and simplest organic forms—mainly, if not wholly, in the shape of vegetation—can exist there. The impression that even such life is possible rests upon the accumulating evidence of the existence of a lunar atmosphere, and of visible changes, some apparently of a volcanic character and some not, on the moon's surface.
Prof. William H. Pickering, who is, perhaps, more familiar with the telescopic and photographic aspects of the moon than any other American astronomer, has recorded numberless instances of change in minute details of the lunar landscapes. He regards some of his observations made at Arequipa as "pointing very strongly to the existence of vegetation upon the surface of the moon in large quantities at the present time." The mountain-ringed valley of Plato is one of the places in the lunar world where the visible changes have been most frequently observed, and more than one student of the moon has reached the conclusion that something very like the appearances that vegetation would produce is to be seen in that valley.
Professor Pickering has thoroughly discussed the observations relating to a celebrated crater named Linne in the Mare Serenitatis, and after reading his description of its changes of appearance one can hardly reject his conclusion that Linne is an active volcanic vent, but variable in its manifestations. This is only one of a number of similar instances among the smaller craters of the moon. The giant ones are evidently entirely extinct, but some of the minor vents give occasional signs of activity. Nor should it be assumed that these relatively slight manifestations of volcanic action are really insignificant. As Professor Pickering shows, they may be regarded as comparable with the greatest volcanic phenomena now witnessed on the earth, and, speaking again of Plato, he says of its evidences of volcanic action:
"It is, I believe, more active than any area of similar size upon the earth. There seems to be no evidences of lava, but the white streaks indicate apparently something analogous to snow or clouds. There must be a certain escape of gases, presumably steam and carbonic acid, the former of which, probably, aids in the production of the white markings."
[Footnote 19: Annals of Harvard College Observatory, vol. xxxii, part ii, 1900.]
To Professor Pickering we owe the suggestion that the wonderful rays emanating from Tycho consist of some whitish substance blown by the wind, not from Tycho itself, but from lines of little volcanic vents or craters lying along the course of the rays. This substance may be volcanic powder or snow, in the form of minute ice crystals. Mr. Elger remarks of this theory that the "confused network of streaks" around Copernicus seems to respond to it more happily than the rays of Tycho do, because of the lack of definiteness of direction so manifest in the case of the rays.
As an encouragement to amateur observers who may be disposed to find out for themselves whether or not changes now take place in the moon, the following sentence from the introduction to Professor Pickering's chapter on Plato in the Harvard Observatory Annals, volume xxxii, will prove useful and interesting:
"In reviewing the history of selenography, one must be impressed by the singular fact that, while most of the astronomers who have made a special study of the moon, such as Schroeter, Maedler, Schmidt, Webb, Neison, and Elger, have all believed that its surface was still subject to changes readily visible from the earth, the great majority of astronomers who have paid little attention to the subject have quite as strenuously denied the existence of such changes."
In regard to the lunar atmosphere, it may be said, in a word, that even those who advocate the existence of vegetation and of clouds of dust or ice crystals on the moon do not predicate any greater amount, or greater density, of atmosphere than do those who consider the moon to be wholly dead and inert. Professor Pickering himself showed, from his observations, that the horizontal refraction of the lunar atmosphere, instead of being less than 2 sec., as formerly stated, was less than 0.4 sec. Yet he found visual evidence that on the sunlit side of the moon this rare atmosphere was filled to a height of four miles with some absorbing medium which was absent on the dark side, and which was apparently an emanation from the lunar crust, occurring after sunrise. And Messrs. Loewy and Puiseux, of the Paris Observatory, say, after showing reasons for thinking that the great volcanic eruptions belong to a recent period in the history of the moon, that "the diffusion of cinders to great distances infers a gaseous envelope of a certain density.... The resistance of the atmosphere must have been sufficient to retard the fall of this dust [the reference is to the white trails, like those from Tycho], during its transport over a distance of more than 1,000 kilometers [620 miles]."
[Footnote 20: Comptes Rendus, June 23, July 3, 1899.]
We come now to a brief consideration of certain peculiarities in the motions of the moon, and in the phenomena of day and night on its surface. The moon keeps the same side forever turned toward the earth, behaving, in this respect, as Mercury does with regard to the sun. The consequence is that the lunar globe makes but one rotation on its axis in the course of a month, or in the course of one revolution about the earth. Some of the results of this practical identity of the periods of rotation and revolution are illustrated in the diagram on page 250. The moon really undergoes considerable libration, recalling the libration of Mercury, which was explained in the chapter on that planet, and in consequence we are able to see a little way round into the opposite lunar hemisphere, now on this side and now on the other, but in the diagram this libration has been neglected. If it had been represented we should have found that, instead of only one half, about three fifths of the total superficies of the moon are visible from the earth at one time or another.
Perhaps it should be remarked that in drawing the moon's orbit about the earth as a center we offer no contradiction to what was shown earlier in this chapter. The moon does travel around the earth, and its orbit about our globe may, for our present purpose, be treated independently of its motion about the sun. Let the central globe, then, represent the earth, and let the sun be supposed to shine from the left-hand side of the diagram. A little cross is erected at a fixed spot on the globe of the moon.
At A the moon is between the earth and the sun, or in the phase of new moon. The lunar hemisphere facing the earth is now buried in night, except so far as the light reflected from the earth illuminates it, and this illumination, it is interesting to remember, is about fourteen times as great—reckoned by the relative areas of the reflecting surfaces—as that which the full moon sends to the earth. An inhabitant of the moon, standing beside the cross, sees the earth in the form of a huge full moon directly above his head, but, as far as the sun is concerned, it is midnight for him.
In the course of about seven days the moon travels to B. In the meantime it has turned one quarter of the way around its axis, and the spot marked by the cross is still directly under the earth. For the lunar inhabitant standing on that spot the sun is now on the point of rising, and he sees the earth no longer in the shape of a full moon, but in that of a half-moon. The lunar globe itself appears, at the same time from the earth, as a half-moon, being in the position or phase that we call first quarter.
Seven more days elapse, and the moon arrives at C, opposite to the position of the sun, and with the earth between it and the solar orb. It is now high noon for our lunarian standing beside the cross, while the earth over his head appears, if he sees it at all, only as a black disk close to the sun, or—as would sometimes be the case—covering the sun, and encircled with a beautiful ring of light produced by the refraction of its atmosphere. (Recall the similar phenomenon in the case of Venus.) The moon seen from the earth is now in the phase called full moon.