Since the _ma.s.s_ of the moon is only about one-eightieth that of the earth, it will be understood that the force of gravity which she exercises is much less. It is calculated that, at her surface, this is only about one-sixth of what we experience. A man transported to the moon would thus be able to jump _six times as high_ as he can here. A building could therefore be six times as tall as upon our earth, without causing any more strain upon its foundations. It should not, then, be any subject for wonder, that the highest peaks in the Lunar Apennines attain to such heights as 22,000 feet. Such a height, upon a comparatively small body like the moon, for her _volume_ is only one-fiftieth that of the earth, is relatively very much in excess of the 29,000 feet of Himalayan structure, Mount Everest, the boast of our planet, 8000 miles across!

High as are the Lunar Apennines, the highest peaks on the moon are yet not found among them. There is, for instance, on the extreme southern edge of the lunar disc, a range known as the Leibnitz Mountains; several peaks of which rise to a height of nearly 30,000 feet, one peak in particular being said to attain to 36,000 feet (see Plate IX., p. 198).

[Ill.u.s.tration: PLATE X. ONE OF THE MOST INTERESTING REGIONS ON THE MOON

We have here (see "Map," Plate IX., p. 198) the mountain ranges of the Apennines, the Caucasus and the Alps; also the craters Plato, Aristotle, Eudoxus, Ca.s.sini, Aristillus, Autolycus, Archimedes and Linne. The crater Linne is the very bright spot in the dark area at the upper left hand side of the picture. From a photograph taken at the Paris Observatory by M.M. Loewy and Puiseux.

(Page 200)]

But the reader will surely ask the question: "How is it possible to determine the actual height of a lunar mountain, if one cannot go upon the moon to measure it?" The answer is, that we can calculate its height from noting the length of the shadow which it casts. Any one will allow that the length of a shadow cast by the sun depends upon two things: firstly, upon the height of the object which causes the shadow, and secondly, upon the elevation of the sun at the moment in the sky. The most casual observer of nature upon our earth can scarcely have failed to notice that shadows are shortest at noonday, when the sun is at its highest in the sky; and that they lengthen out as the sun declines towards its setting. Here, then, we have the clue. To ascertain, therefore, the height of a lunar mountain, we have first to consider at what elevation the sun is at that moment above the horizon of the place where the mountain in question is situated. Then, having measured the actual length in miles of the shadow extended before us, all that is left is to ask ourselves the question: "What height must an object be whose shadow cast by the sun, when at that elevation in the sky, will extend to this length?"

There is no trace whatever of water upon the moon. The opinion, indeed, which seems generally held, is that water has never existed upon its surface. Erosions, sedimentary deposits, and all those marks which point to a former occupation by water are notably absent.

Similarly there appears to be no atmosphere on the moon; or, at any rate, such an excessively rare one, as to be quite inappreciable. Of this there are several proofs. For instance, in a solar eclipse the moon's disc always stands out quite clear-cut against that of the sun.

Again during occultations, stars disappear behind the moon with a suddenness, which could not be the case were there any appreciable atmosphere. Lastly, we see no traces of twilight upon the lunar surface, nor any softening at the edges of shadows; both which effects would be apparent if there were an atmosphere.

The moon's surface is rough and rocky, and displays no marks of the "weathering" that would necessarily follow, had it possessed anything of an atmosphere in the past. This makes us rather inclined to doubt that it ever had one at all. Supposing, however, that it did possess an atmosphere in the past, it is interesting to inquire what may have become of it. In the first place it might have gradually disappeared, in consequence of the gases which composed it uniting chemically with the materials of which the lunar body is constructed; or, again, its const.i.tuent gases may have escaped into s.p.a.ce, in accordance with the principles of that kinetic theory of which we have already spoken. The latter solution seems, indeed, the most reasonable of the two, for the force of gravity at the lunar surface appears too weak to hold down any known gases. This argument seems also to dispose of the question of absence of water; for Dr. George Johnstone Stoney, in a careful investigation of the subject, has shown that the liquid in question, when in the form of vapour, will escape from a planet if its ma.s.s is less than _one-fourth_ that of our earth. And the ma.s.s of the moon is very much less than this; indeed only the _one-eightieth_, as we have already stated.

In consequence of this lack of atmosphere, the condition of things upon the moon will be in marked contrast to what we experience upon the earth. The atmosphere here performs a double service in shielding us from the direct rays of the sun, and in bottling the heat as a gla.s.s-house does. On the moon, however, the sun beats down in the day-time with a merciless force; but its rays are reflected away from the surface as quickly as they are received, and so the cold of the lunar night is excessive. It has been calculated that the day temperature on the moon may, indeed, be as high as our boiling-point, while the night temperature may be more than twice as low as the greatest cold known in our arctic regions.

That a certain amount of solar heat is reflected to us from the moon is shown by the sharp drop in temperature which certain heat-measuring instruments record when the moon becomes obscured in a lunar eclipse.

The solar heat which is thus reflected to us by the moon is, however, on the whole extremely small; more light and heat, indeed, reach us _direct_ from the sun in half a minute than we get by _reflection_ from the moon during the entire course of the year.

With regard to the origin of the lunar craters there has been much discussion. Some have considered them to be evidence of violent volcanic action in the dim past; others, again, as the result of the impact of meteorites upon the lunar surface, when the moon was still in a plastic condition; while a third theory holds that they were formed by the bursting of huge bubbles during the escape into s.p.a.ce of gases from the interior. The question is, indeed, a very difficult one. Though volcanic action, such as would result in craters of the size of Ptolemaeus, is hard for us to picture, and though the lone peaks which adorn the centres of many craters have nothing reminiscent of them in our terrestrial volcanoes, nevertheless the volcanic theory seems to receive more favour than the others.

In addition to the craters there are two more features which demand notice, namely, what are known as _rays_ and _rills_. The rays are long, light-coloured streaks which radiate from several of the large craters, and extend to a distance of some hundreds of miles. That they are mere markings on the surface is proved by the fact that they cast no shadows of any kind. One theory is, that they were originally great cracks which have been filled with lighter coloured material, welling up from beneath. The rills, on the other hand, are actually fissures, about a mile or so in width and about a quarter of a mile in depth.

The rays are seen to the best advantage in connection with the craters Tycho and Copernicus (see Plate XI., p. 204). In consequence of its fairly forward position on the lunar disc, and of the remarkable system of rays which issue from it like spokes from the axle of a wheel, Tycho commands especial attention. The late Rev. T.W. Webb, a famous observer, christened it, very happily, the "metropolitan crater of the moon."

[Ill.u.s.tration: PLATE XI. THE MOON

The systems of rays from the craters Tycho, Copernicus and Kepler are well shown here. From a photograph taken at the Paris Observatory by M.P. Puiseux.

(Page 204)]

A great deal of attention is, and has been, paid by certain astronomers to the moon, in the hope of finding out if any changes are actually in progress at present upon her surface. Sir William Herschel, indeed, once thought that he saw a lunar volcano in eruption, but this proved to be merely the effect of the sunlight striking the top of the crater Aristarchus, while the region around it was still in shadow--sunrise upon Aristarchus, in fact! No change of any real importance has, however, been noted, although it is suspected that some minor alterations have from time to time taken place. For instance, slight variations of tint have been noticed in certain areas of the lunar surface. Professor W.H. Pickering puts forward the conjecture that these may be caused by the growth and decay of some low form of vegetation, brought into existence by vapours of water, or carbonic acid gas, making their way out from the interior through cracks near at hand.

Again, during the last hundred years one small crater known as Linne (Linnaeus), situated in the Mare Serenitatis (Sea of Serenity), has appeared to undergo slight changes, and is even said to have been invisible for a while (see Plate X., p. 200). It is, however, believed that the changes in question may be due to the varying angles at which the sunlight falls upon the crater; for it is an understood fact that the irregularities of the moon's motion give us views of her surface which always differ slightly.

The suggestion has more than once been put forward that the surface of the moon is covered with a thick layer of ice. This is generally considered improbable, and consequently the idea has received very little support. It first originated with the late Mr. S.E. Peal, an English observer of the moon, and has recently been resuscitated by the German observer, Herr Fauth.

The most unfavourable time for telescopic study of the moon is when she is full. The sunlight is then falling directly upon her visible hemisphere, and so the mountains cast no shadows. We thus do not get that impression of hill and hollow which is so very noticeable in the other phases.

The first map of the moon was constructed by Galileo. Tobias Mayer published another in 1775; while during the nineteenth century greatly improved ones were made by Beer and Madler, Schmidt, Neison and others.

In 1903, Professor W.H. Pickering brought out a complete photographic lunar atlas; and a similar publication has recently appeared, the work of MM. Loewy and Puiseux of the Observatory of Paris.

The so-called "seas" of the moon are, as we have seen, merely dark areas, and there appears to be no proof that they were ever occupied by any liquid. They are for the most part found in the _northern_ portion of the moon; a striking contrast to our seas and oceans, which take up so much of the _southern_ hemisphere of the earth.

There are many erroneous ideas popularly held with regard to certain influences which the moon is supposed to exercise upon the earth. For instance, a change in the weather is widely believed to depend upon a change in the moon. But the word "change" as here used is meaningless, for the moon is continually changing her phase during the whole of her monthly round. Besides, the moon is visible over a great portion of the earth _at the same moment_, and certainly all the places from which it can then be seen do not get the same weather! Further, careful observations, and records extending over the past one hundred years and more, fail to show any reliable connection between the phases of the moon and the condition of the weather.

It has been stated, on very good authority, that no telescope ever shows the surface of the moon as clearly as we could see it with the naked eye were it only 240 miles distant from us.

Supposing, then, that we were able to approach our satellite, and view it without optical aid at such comparatively close quarters, it is interesting to consider what would be the smallest detail which our eye could take in. The question of the limit of what can be appreciated with the naked eye is somewhat uncertain, but it appears safe to say that at a distance of 240 miles the _minutest speck_ visible would have to be _at least_ some 60 yards across.

Atmosphere and liquid both wanting, the lunar surface must be the seat of an eternal calm; where no sound breaks the stillness and where change, as we know it, does not exist. The sun beats down upon the arid rocks, and inky shadows lie athwart the valleys. There is no mellowing of the harsh contrasts.

We cannot indeed absolutely affirm that Life has no place at all upon this airless and waterless globe, since we know not under what strange conditions it may manifest its presence; and our most powerful telescopes, besides, do not bring the lunar surface sufficiently near to us to disprove the existence there of even such large creatures as disport themselves upon our planet. Still, we find it hard to rid ourselves of the feeling that we are in the presence of a dead world. On she swings around the earth month after month, with one face ever turned towards us, leaving a certain mystery to hang around that hidden side, the greater part of which men can never hope to see. The rotation of the moon upon her axis--the lunar day--has become, as we have seen, equal to her revolution around the earth. An epoch may likewise eventually be reached in the history of our own planet, when the length of the terrestrial day has been so slowed down by tidal friction that it will be equal to the year. Then will the earth revolve around the central orb, with one side plunged in eternal night and the other in eternal sunshine. But such a vista need not immediately distress us. It is millions of years forward in time.

[14] _Journal of the British Astronomical a.s.sociation_, vol. x.

(1899-1900), Nos. 1 and 3.

[15] Certain of the ancient Greeks thought the markings on the moon to be merely the reflection of the seas and lands of our earth, as in a badly polished mirror.

[16] Mare Imbrium, Sinus Iridum, Lacus Somniorum.

[17] The lunar craters have, as a rule, received their names from celebrated persons, usually men of science. This system of nomenclature was originated by Riccioli, in 1651.

CHAPTER XVII

THE SUPERIOR PLANETS

Having, in a previous chapter, noted the various aspects which an inferior planet presents to our view, in consequence of its...o...b..t being nearer to the sun than the orbit of the earth, it will be well here to consider in the same way the case of a superior planet, and to mark carefully the difference.

To begin with, it should be quite evident that we cannot ever have a transit of a superior planet. The orbit of such a body being entirely _outside_ that of the earth, the body itself can, of course, never pa.s.s between us and the sun.

A superior planet will be at its greatest distance from us when on the far side of the sun. It is said then to be in _conjunction_. As it comes round in its...o...b..t it eventually pa.s.ses, so to speak, at the _back_ of us. It is then at its nearest, or in _opposition_, as this is technically termed, and therefore in the most favourable position for telescopic observation of its surface. Being, besides, seen by us at that time in the direction of the heavens exactly opposite to where the sun is, it will thus at midnight be high up in the south side of the sky, a further advantage to the observer.

Last of all, a superior planet cannot show crescent shapes like an interior; for whether it be on the far side of the sun, or behind us, or again to our right or left, the sunlight must needs appear to fall more or less full upon its face.

THE PLANETOID EROS

The nearest to us of the superior planets is the tiny body, Eros, which, as has been already stated, was discovered so late as the year 1898. In point of view, however, of its small size, it can hardly be considered as a true planet, and the name "planetoid" seems much more appropriate to it.

Eros was not discovered, like Ura.n.u.s, in the course of telescopic examination of the heavens, nor yet, like Neptune, as the direct result of difficult calculations, but was revealed by the impress of its light upon a photographic plate, which had been exposed for some length of time to the starry sky. Since many of the more recent additions to the asteroids have been discovered in the same manner, we shall have somewhat more to say about this special employment of photography when we come to deal with those bodies later on.

The path of Eros around the sun is so very elliptical, or, to use the exact technical term, so very "eccentric," that the planetoid does not keep all the time entirely in the s.p.a.ce between our orbit and that of Mars, which latter happens to be the next body in the order of planetary succession outwards. In portions of its journey Eros, indeed, actually goes outside the Martian orbit. The paths of the planetoid and of Mars are, however, _not upon the same plane_, so the bodies always pa.s.s clear of each other, and there is thus as little chance of their dashing together as there would be of trains which run across a bridge at an upper level, colliding with those which pa.s.s beneath it at a lower level.

When Eros is in opposition, it comes within about 13-1/2 million miles of our earth, and, after the moon, is therefore by a long way our nearest neighbour in s.p.a.ce. It is, however, extremely small, not more, perhaps, than twenty miles in diameter, and is subject to marked variations in brightness, which do not appear up to the present to meet with a satisfactory explanation. But, insignificant as is this little body, it is of great importance to astronomy; for it happens to furnish the best method known of calculating the sun's distance from our earth--a method which Galle, in 1872, and Sir David Gill, in 1877, suggested that asteroids might be employed for, and which has in consequence supplanted the old one founded upon transits of Venus. The sun's distance is now an ascertained fact to within 100,000 miles, or less than half the distance of the moon.

THE PLANET MARS

We next come to the planet Mars. This body rotates in a period of slightly more than twenty-four hours. The inclination, or slant, of its axis is about the same as that of the earth, so that, putting aside its greater distance from the sun, the variations of season which it experiences ought to be very much like ours.