CHAPTER XVI

[13]Every one knows the simple experiment in which a coin lying at the bottom of an empty basin, and hidden from the eye by its side, becomes visible when a certain quantity of water has been poured in. This is an example of refraction. The rays of light coming from the coin oughtnotto reach the eye, on account of the basin's side being in the way; yet by the action of the water they arerefracted, or bent over its edge, in such a manner that they do.

Whatwe call the moon's "phases" are merely the various ways in which we see the sun shining upon her surface during the course of her monthly revolutions around the earth (see Fig. 14, p. 184). When she passes in the neighbourhood of the sun all his light falls upon that side which is turned away from us, and so the side which is turned towards us is unillumined, and therefore invisible. When in this position the moon is spoken of asnew.

As she continues her motion around the earth, she draws gradually to the east of the sun's place in the sky. The sunlight then comes somewhat from the side; and so we see a small portion of the right side of the lunar disc illuminated. This is the phase known as thecrescentmoon.

As she moves on in her orbit more and more of her illuminated surface is brought into view; and so the crescent of light becomes broader and broader, until we get what is called half-moon, orfirst quarter, when we see exactly one-half of her surface lit up by the sun's rays. As she draws still further round yet more of her illuminated surface is brought into view, until three-quarters of the disc appear lighted up. She is then said to begibbous.

Eventually she moves round so that she faces thesun completely, and the whole of her disc appears illuminated. She is then spoken of asfull. When in this position it is clear that she is on the contrary side of the earth to the sun, and therefore rises about the same time that he is setting. She is now, in fact, at her furthest from the sun.

Direction from which the sun's rays are coming.Fig. 14aVarious positions and illumination of the mooon by the sun during her revolution around the earth.

Fig. 14bThe corresponding positions as viewed from the earth, showing the consequent phases.Fig. 14.—Orbit and Phases of the Moon.

After this, the motion of the moon in her orbit carries her on back again in the direction of the sun.She thus goes through her phases as before, only these of course arein the reverse order. The full phase is seen to give place to the gibbous, and this in turn to the half-moon and to the crescent; after which her motion carries her into the neighbourhood of the sun, and she is once more new, and lost to our sight in the solar glare. Following this she draws away to the east of the sun again, and the old order of phases repeat themselves as before.

The early Babylonians imagined that the moon had a bright and a dark side, and that her phases were caused by the bright side coming more and more into view during her movement around the sky. The Greeks, notably Aristotle, set to work to examine the question from a mathematical standpoint, and came to the conclusion that the crescent and other appearances were such as would necessarily result if the moon were a dark body of spherical shape illumined merely by the light of the sun.

Although the true explanation of the moon's phases has thus been known for centuries, it is unfortunately not unusual to see pictures—advertisement posters, for instance—in which stars appearwithinthe horns of a crescent moon! Can it be that there are to-day educated persons who believe that the moon is a thing whichgrowsto a certain size and then wastes away again; who, in fact, do not know that the entire body of the moon is there all the while?

When the moon shows a very thin crescent, we are able dimly to see her still dark portion standing out against the sky. This appearance is popularly known as the "old moon in the new moon's arms." The dark part of her surface must, indeed, be to somedegree illumined, or we should not be able to see it at all. Whence then comes the light which illumines it, since it clearly cannot come from the sun? The riddle is easily solved, if we consider what kind of view of our earth an observer situated on this darkened part of the moon would at that moment get. He would, as a matter of fact, just then see nearly the whole disc of the earth brightly lit up by sunlight. The lunar landscape all around would, therefore, be bathed in what tohimwould be "earthlight," which of course takes the place there of whatwecall moonlight. If, then, we recollect how much greater in size the earth is than the moon, it should not surprise us that this earthlight will be many times brighter than moonlight. It is considered, indeed, to be some twenty times brighter. It is thus not at all astonishing that we can see the dark portion of the moon illumined merely by sunlight reflected upon it from our earth.

The ancients were greatly exercised in their minds to account for this "earthlight," or "earthshine," as it is also called. Posidonius (135–51B.C.) tried to explain it by supposing that the moon was partially transparent, and that some sunlight consequently filtered through from the other side. It was not, however, until the fifteenth century that the correct solution was arrived at.

Fig. 15aOne side of the moon only is ever presented to the earth. This side is here indicated by the letters S.F.E. (side facing earth).

One side of the moon only is ever presented to the earth. This side is here indicated by the letters S.F.E. (side facing earth).

Fig. 15By placing the above positions in a row, we can see at once that the moon makes one complete rotation on her axis in exactly the same time as she revolves around the earth.Fig. 15.—The Rotation of the Moon on her Axis.

By placing the above positions in a row, we can see at once that the moon makes one complete rotation on her axis in exactly the same time as she revolves around the earth.

Perhaps the most remarkable thing which one notices about the moon is that she always turns the same side towards us, and so we never see her other side. One might be led from this to jump to the conclusion that she does not rotate upon an axis, as do the other bodies which we see; but, paradoxical as it may appear, the fact that she turns one face always towards the earth, is actually a proof that shedoesrotate upon an axis. The rotation, however, takes place with such slowness, that she turns round but once during the time in which she revolves around the earth (see Fig. 15). In order to understand the matter clearly, let the reader place an object in the centre of a room and walk around it once,keeping his face turned towards it the whole time, While he is doing this, it is evident that he will face every one of the four walls of the room in succession.Now in order to face each of the four walls of a room in succession one would be obligedto turn oneself entirely round. Therefore, during the act of walking round an object with his face turned directly towards it, a person at the same time turns his body once entirely round.

In the long, long past the moon must have turned round much faster than this. Her rate of rotation has no doubt been slowed down by the action of some force. It will be recollected how, in the course of the previous chapter, we found that the tides were tending, though exceedingly gradually, to slow down the rotation of the earth upon its axis. But, on account of the earth's much greater mass, the force of gravitation exercised by it upon the surface of the moon is, of course, much more powerful than that which the moon exercises upon the surface of the earth. The tendency to tidal action on the moon itself must, therefore, be much in excess of anything which we here experience. It is, in consequence, probable that such a tidal drag, extending over a very long period of time, has resulted in slowing down the moon's rotation to its present rate.

The fact that we never see but one side of the moon has given rise from time to time to fantastic speculations with regard to the other side. Some, indeed, have wished to imagine that our satellite is shaped like an egg, the more pointed end being directed away from us. We are here, of course, faced with a riddle, which is all the more tantalising from its appearing for ever insoluble to men, chained as they are to the earth. However, it seems going too far to suppose that any abnormal conditions necessarilyexist at the other side of the moon. As a matter of fact, indeed, small portions of that side are brought into our view from time to time in consequence of slight irregularities in the moon's movement; and these portions differ in no way from those which we ordinarily see. On the whole, we obtain a view of about 60 per cent. of the entire lunar surface; that is to say, a good deal more than one-half.

The actual diameter of the moon is about 2163 miles, which is somewhat more than one-quarter the diameter of the earth. For a satellite, therefore, she seems very large compared with her primary, the earth; when we consider that Jupiter's greatest satellite, although nearly twice as broad as our moon, has a diameter only one twenty-fifth that of Jupiter. Furthermore, the moon moves around the earth comparatively slowly, making only about thirteen revolutions during the entire year. Seen from space, therefore, she would not give the impression of a circling body, as other satellites do. Her revolutions are, indeed, relatively so very slow that she would appear rather like a smaller planet accompanying the earth in its orbit. In view of all this, some astronomers are inclined to regard the earth and moon rather as a "double planet" than as a system of planet and satellite.

When the moon is full she attracts more attention perhaps than in any of her other phases. The moon, in order to be full, must needs be in that region of the heavens exactly opposite to the sun. The sunappearsto go once entirely round the sky in the course of a year, and the moon performs the same journey in the space of about a month. The moon,when full, having got half-way round this journey, occupies, therefore, that region of the sky which the sun itself will occupy half a year later. Thus in winter the full moon will be found roughly to occupy the sun's summer position in the sky, and in summer the sun's winter position. It therefore follows that the full moon in winter time is high up in the heavens, while in summer time it is low down. We thus get the greatest amount of full moonlight when it is the most needed.

The great French astronomer, Laplace, being struck by the fact that the "lesser light" did not rule the night to anything like the same extent that the "greater light" ruled the day, set to work to examine the conditions under which it might have been made to do so. The result of his speculations showed that if the moon were removed to such a distance that she took a year instead of a month to revolve around the earth; and if she were started off in her orbit at full moon, she would always continue to remain full—a great advantage for us. Whewell, however, pointed out that in order to get the moon to move with the requisite degree of slowness, she would have to revolve so far from the earth that she would only look one-sixteenth as large as she does at present, which rather militates against the advantage Laplace had in mind! Finally, however, it was shown by M. Liouville, in 1845, that the position of aperennial full moon, such as Laplace dreamed of, would be unstable—that is to say, the body in question could not for long remain undisturbed in the situation suggested (see Fig. 16, p. 191).

Fig. 16aVarious positions of Laplace's "Moon" with regard to the earth and sun during the course of a year.

Fig. 16.The same positions of Laplace's "Moon," arranged around the earth, show that it would make only one revolution in a year.Fig. 16.—Laplace's "Perennial Full Moon."

There is a well-known phenomenon calledharvest moon, concerning the nature of which there seems to be much popular confusion. An idea in fact appears to prevail among a good many people that the moon is a harvest moon when, at rising, it looks bigger and redder than usual. Such an appearance has, however, nothing at all to say to the matter; for themoon alwayslookslarger when low down in the sky, and, furthermore, it usually looks red in the later months of the year, when there is more mist and fog about than there is in summer. What astronomers actually term the harvest moon is, indeed, something entirely different from this. About the month of September the slant at which the full moon comes up from below the horizon happens to be such that, during several evenings together, sherises almost at the same hour, instead of some fifty minutes later, as is usually the case. As the harvest is being gathered in about that time, it has come to be popularly considered that this is a provision of nature, according to which the sunlight is, during several evenings, replaced without delay by more or less full-moonlight, in order that harvesters may continue their work straight on into the night, and not be obliged to break off after sunset to wait until the moon rises. The same phenomenon is almost exactly repeated a month later, but by reason of the pursuits then carried on it is known as the "hunter's moon."

In this connection should be mentioned that curious phenomenon above alluded to, and which seems to attract universal notice, namely, that the moonlooks much larger when near the horizon—at its rising, for instance, than when higher up in the sky. This seeming enlargement is, however, by no means confined to the moon. That the sun also looks much larger when low down in the sky than when high up, seems to strike even the most casual watcher of a sunset. The same kind of effect will, indeed, be noted if close attention be paid to the stars themselves. A constellation, for instance, appears morespread out when low down in the sky than when high up. This enlargement of celestial objects when in the neighbourhood of the horizon is, however, onlyapparentand not real. It must be entirely anillusion; for the most careful measurements of the discs of the sun and of the moon fail to show that the bodies are any larger when near the horizon than when high up in the sky. In fact, if there be any difference in measurements with regard to the moon, it will be found to be the other way round; for her disc, when carefully measured, is actually the slightest degreegreaterwhenhighin the sky, than when low down. The reason for this is that, on account of the rotundity of the earth's surface, she is a trifle nearer the observer when overhead of him.

This apparent enlargement of celestial objects, when low down in the sky, is granted on all sides to be an illusion; but although the question has been discussed with animation time out of mind, none of the explanations proposed can be said to have received unreserved acceptance. The one which usually figures in text-books is that we unconsciously compare the sun and moon, when low down in the sky, with the terrestrial objects in the same field of view, and are therefore inclined to exaggerate the size of these orbs. Some persons, on the other hand, imagine the illusion to have its source in the structure of the human eye; while others, again, put it down to the atmosphere, maintaining that the celestial objects in questionloomlarge in the thickened air near the horizon, in the same way that they do when viewed through fog or mist.

The writer[14]ventures, however, to think that the illusion has its origin in our notion of the shape of the celestial vault. One would be inclined, indeed, to suppose that this vault ought to appear to us as the half of a hollow sphere; but he maintains that it does not so appear, as a consequence of the manner in which the eyes of men are set quite close together in their heads. If one looks, for instance, high up in the sky, the horizon cannot come within the field of view, and so there is nothing to make one think that the expanse then gazed upon is other than quiteflat—let us say like the ceiling of a room. But, as the eyes are lowered, a portion of theencirclinghorizon comes gradually into the field of view, and the region of the sky then gazed upon tends in consequence to assume ahollowed-outform. From this it would seem that our idea of the shape of the celestial vault is, that it isflattened down over our heads and hollowed out all around in the neighbourhood of the horizon(see Fig. 17, p. 195). Now, as a consequence of their very great distance, all the objects in the heavens necessarily appear to us to move as if they were placed on the background of the vault; the result being that the mind is obliged to conceive them as expanded or contracted, in its unconscious attempts to make them always fill their due proportion of space in the various parts of this abnormally shaped sky.

From such considerations the writer concludes that the apparent enlargement in question is merely the natural consequence of the idea we have of the shape of the celestial vault—an idea gradually built up inchildhood, to become later on what is called "second nature." And in support of this contention, he would point to the fact that the enlargement is not by any means confined to the sun and moon, but is every whit as marked in the case of the constellations. To one who has not noticed this before, it is really quite a revelation to compare the appearance of one of the large constellations (Orion, for instance) when high up in the sky and when low down. The widening apart of the various stars composing the group, when in the latter position, is very noticeable indeed.

Fig. 17.Fig. 17.—Illustrating the author's explanation of the apparent enlargement of celestial objects.

Further, if a person were to stand in the centre of a large dome, he would be exactly situated as if he were beneath the vaulted heaven, and one would consequently expect him to suffer the same illusion as to the shape of the dome. Objects fixed upon its background would therefore appear to him under thesame conditions as objects in the sky, and the illusions as to their apparent enlargement should hold good here also.

Some years ago a Belgian astronomer, M. Stroobant, in an investigation of the matter at issue, chanced to make a series of experiments under the very conditions just detailed. To various portions of the inner surface of a large dome he attached pairs of electric lights; and on placing himself at the centre of the building, he noticed that, in every case, those pairs which were high up appeared closer together than those which were low down! He does not, however, seem to have sought for the cause in the vaulted expanse. On the contrary, he attributed the effect to something connected with our upright stature, to some physiological reason which regularly makes us estimate objects as larger when in front than when overhead.

In connection with this matter, it may be noted that it always appears extremely difficult to estimate with the eye the exact height above the horizon at which any object (say a star) happens to be. Even skilled observers find themselves in error in attempting to do so. This seems to bear out the writer's contention that the form under which the celestial vault really appears to us is a peculiar one, and tends to give rise to false judgments.

Before leaving this question, it should also be mentioned that nothing perhaps is more deceptive than the size which objects in the sky appear to present. The full moon looks so like a huge plate, that it astonishes one to find that a threepenny bit held at arm's length will a long way more than cover its disc.

Plate VIII.Plate VIII. The MoonFrom a photograph taken at the Paris Observatory by M.P. Puiseux.(Page 197)

The moon is just too far off to allow us to see the actual detail on her surface with the naked eye. When thus viewed she merely displays a patchy appearance,[15]and the imaginary forms which her darker markings suggest to the fancy are popularly expressed by the term "Man in the Moon." An examination of her surface with very moderate optical aid is, however, quite a revelation, and the view we then get is not easily comparable to what we see with the unaided eye.

Even with an ordinary opera-glass, an observer will be able to note a good deal of detail upon the lunar disc. If it be his first observation of the kind, he cannot fail to be struck by the fact to which we have just made allusion, namely, the great change which the moon appears to undergo when viewed with magnifying power. "Cain and his Dog," the "Man in the Moon gathering sticks," or whatever indeed his fancy was wont to conjure up from the lights and shades upon the shining surface, have now completely disappeared; and he sees instead a silvery globe marked here and there with extensive dark areas, and pitted all over with crater-like formations (see Plate VIII., p. 196). The dark areas retain even to the present day their ancient name of "seas," for Galileo and the early telescopic observers believed them to be such, and they are still catalogued under the mystic appellations given to them in the long ago; as, for instance, "Sea of Showers," "Bay of Rainbows," "Lake of Dreams."[16]The improved telescopes of latertimes showed, however, that they were not really seas (there is no water on the moon), but merely areas of darker material.

The crater-like formations above alluded to are the "lunar mountains." A person examining the moon for the first time with telescopic aid, will perhaps not at once grasp the fact that his view of lunar mountains must needs be what is called a "bird's-eye" one, namely, a view from above, like that from a balloon and that he cannot, of course, expect to see them from the side, as he does the mountains upon the earth. But once he has realised this novel point of view, he will no doubt marvel at the formations which lie scattered as it were at his feet. The type of lunar mountain is indeed in striking contrast to the terrestrial type. On our earth the range-formation is supreme; on the moon the crater-formation is the rule, and is so-called from analogy to our volcanoes. A typical lunar crater may be described as a circular wall, enclosing a central plain, or "floor," which is often much depressed below the level of the surface outside. These so-called "craters," or "ring-mountains," as they are also termed, are often of gigantic proportions. For instance, the central plain of one of them, known as Ptolemæus,[17]is about 115 miles across, while that of Plato is about 60. The walls of craters often rise to great heights; which, in proportion to the small size of the moon, are very much in excess of our highest terrestrial elevations. Nevertheless, a person posted at the centre of one of the larger craters might be surprised to find that he could not see the encompassing crater-walls, which would in every direction be below his horizon. This would arise not alone from the great breadth of the crater itself, but also from the fact that the curving of the moon's surface is very sharp compared with that of our earth.

Plate IX.Plate IX. Map of the Moon, showing the principal "Craters," Mountain Ranges, and "Seas"In this, as in the other plates of the Moon, theSouthwill be found at the top of the picture; such being the view given by the ordinary astronomical telescope, in which all objects are seeninverted.(Page 199)

In this, as in the other plates of the Moon, theSouthwill be found at the top of the picture; such being the view given by the ordinary astronomical telescope, in which all objects are seeninverted.(Page 199)

We have mentioned Ptolemæus as among the very large craters, or ring-mountains, on the moon. Its encompassing walls rise to nearly 13,000 feet, and it has the further distinction of being almost in the centre of the lunar disc. There are, however, several others much wider, but they are by no means in such a conspicuous position. For instance, Schickard, close to the south-eastern border, is nearly 130 miles in diameter, and its wall rises in one point to over 10,000 feet. Grimaldi, almost exactly at the east point, is nearly as large as Schickard. Another crater, Clavius, situated near the south point, is about 140 miles across; while its neighbour Bailly—named after a famous French astronomer of the eighteenth century—is 180, and the largest of those which we can see (see Plate IX., p. 198).

Many of the lunar craters encroach upon one another; in fact there is not really room for them all upon the visible hemisphere of the moon. About 30,000 have been mapped; but this is only a small portion, for according to the American astronomer, Professor W.H. Pickering, there are more than 200,000 in all.

Notwithstanding the fact that the crater is the type of mountain associated in the mind with the moon, it must not be imagined that upon our satellite thereare no mountains at all of the terrestrial type. There are indeed many isolated peaks, but strangely enough they are nearly always to be found in the centres of craters. Some of these peaks are of great altitude, that in the centre of the crater Copernicus being over 11,000 feet high. A few mountain ranges also exist; the best known of which are styled, the Lunar Alps and Lunar Apennines (see Plate X., p. 200).

Since themassof 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 jumpsix times as highas 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 hervolumeis 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).

Plate X.Plate X. One of the most interesting regions on the MoonWe 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, Cassini, Aristillus, Autolycus, Archimedes and Linné. The crater Linné 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)

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, Cassini, Aristillus, Autolycus, Archimedes and Linné. The crater Linné 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 themoon'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 constituent gases may have escaped into space, 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 mass is less thanone-fourththat of our earth. And the mass of the moon is very much less than this; indeed only theone-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 glass-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 usdirectfrom the sun in half a minute than we get byreflectionfrom 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 space of gases from the interior. The question is, indeed, a very difficultone. Though volcanic action, such as would result in craters of the size of Ptolemæus, 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 asraysandrills. 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."

Plate XI.Plate XI. The MoonThe 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 Linné (Linnæus), 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 Mädler, 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 thenorthernportion of the moon; a striking contrast to our seas and oceans, which take up so much of thesouthernhemisphere 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 earthat 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 theminutest speckvisible would have to beat leastsome 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 monthafter 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 Association, 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.

[14]Journal of the British Astronomical Association, 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.

Having, in a previous chapter, noted the various aspects which an inferior planet presents to our view, in consequence of its orbit 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 entirelyoutsidethat of the earth, the body itself can, of course, never pass 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 inconjunction. As it comes round in its orbit it eventually passes, so to speak, at thebackof us. It is then at its nearest, or inopposition, 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 farside 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 Uranus, 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 space 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 pass 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 pass beneath it at a lower level.

When Eros is in opposition, it comes within about 13½ million miles of our earth, and, after the moon, is therefore by a long way our nearest neighbour in space. 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.

The first marking detected upon Mars was the notable one called the Syrtis Major, also known, onaccount of its shape, as the Hour-Glass Sea. This observation was made by the famous Huyghens in 1659; and, from the movement of the marking in question across the disc, he inferred that the planet rotated on its axis in a period of about twenty-four hours.

There appears to be very little atmosphere upon Mars, the result being that we almost always obtain a clear view of the detail on its surface. Indeed, it is only to be expected from the kinetic theory that Mars could not retain much of an atmosphere, as the force of gravity at its surface is less than one-half of what we experience upon the earth. It should here be mentioned that recent researches with the spectroscope seem to show that, whatever atmosphere there may be upon Mars, its density at the surface of the planet cannot be more than the one-fourth part of the density of the air at the surface of the earth. Professor Lowell, indeed, thinks it may be more rarefied than that upon our highest mountain-tops.

Seen with the naked eye, Mars appears of a red colour. Viewed in the telescope, its surface is found to be in general of a ruddy hue, varied here and there with darker patches of a bluish-green colour. These markings are permanent, and were supposed by the early telescopic observers to imply a distribution of the planet's surface into land and water, the ruddy portions being considered as continental areas (perhaps sandy deserts), and the bluish-green as seas. The similarity to our earth thus suggested was further heightened by the fact that broad white caps, situated at the poles, were seen to vary with the planet's seasons, diminishing greatly in extent during theMartian summer (the southern cap in 1894 even disappearing altogether), and developing again in the Martian winter.[18]Readers of Oliver Wendell Holmes will no doubt recollect that poet's striking lines:—

"The snows that glittered on the disc of MarsHave melted, and the planet's fiery orbRolls in the crimson summer of its year."

A state of things so strongly analogous to what we experience here, naturally fired the imaginations of men, and caused them to look on Mars as a world like ours, only upon a much smaller scale. Being smaller, it was concluded to have cooled quicker, and to be now long past its prime; and its "inhabitants" were, therefore, pictured as at a later stage of development than the inhabitants of our earth.

Notwithstanding the strong temptation to assume that the whiteness of the Martian polar caps is due to fallen snow, such a solution is, however, by no means so simple as it looks. The deposition of water in the form of snow, or even of hoar frost, would at least imply that the atmosphere of Mars should now and then display traces of aqueous vapour, which it does not appear to do.[19]It has, indeed, been suggested that the whiteness may not after all be due to this cause, but to carbonic acid gas (carbon dioxide), which is known to freeze at avery lowtemperature. The suggestion is plainlybased upon the assumption that, as Mars is so much further from the sun than we are, it would receive much less heat, and that the little thus received would be quickly radiated away into space through lack of atmosphere to bottle it in.

We now come to those well-known markings, popularly known as the "canals" of Mars, which have been the subject of so much discussion since their discovery thirty years ago.

It was, in fact, in the year 1877, when Mars was in opposition, and thus at its nearest to us, that the famous Italian astronomer, Schiaparelli, announced to the world that he had found that the ruddy areas, thought to be continents, were intersected by a network of straight dark lines. These lines, he reported, appeared in many cases to be of great length, so long, indeed, as several thousands of miles, and from about twenty to sixty miles in width. He christened the lineschannels, the Italian word for which, "canali," was unfortunately translated into English as "canals." The analogy, thus accidentally suggested, gave rise to the idea that they might be actual waterways.[20]

In the winter of 1881–1882, when Mars was again in opposition, Schiaparelli further announced that he had found some of these lines doubled; that is to say, certain of them were accompanied by similar lines running exactly parallel at no great distance away. There was at first a good deal of scepticism on the subject of Schiaparelli's discoveries, but gradually other observers found themselves seeing both thelines and their doublings. We have in this a good example of a curious circumstance in astronomical observation, namely, the fact that when fine detail has once been noted by a competent observer, it is not long before other observers see the same detail with ease.

An immense amount of close attention has been paid to the planet Mars during recent years by the American observer, Professor Percival Lowell, at his famous observatory, 7300 feet above the sea, near the town of Flagstaff, Arizona, U.S.A. His observations have not, like those of most astronomers, been confined merely to "oppositions," but he has systematically kept the planet in view, so far as possible, since the year 1894.

The instrumental equipment of his observatory is of the very best, and the "seeing" at Flagstaff is described as excellent. In support of the latter statement, Mr. Lampland, of the Lowell Observatory, maintains that the faintest stars shown on charts made at the Lick Observatory with the 36–inch telescope there, areperfectly visiblewith the 24–inch telescope at Flagstaff.

Professor Lowell is, indeed, generally at issue with the other observers of Mars. He finds the canals extremely narrow and sharply defined, and he attributes the blurred and hazy appearance, which they have presented to other astronomers, to the unsteady and imperfect atmospheric conditions in which their observations have been made. He assigns to the thinnest a width of two or three miles, and from fifteen to twenty to the larger. Relatively to their width, however, he finds their length enormous.Many of them are 2000 miles long, while one is even as much as 3540. Such lengths as these are very great in comparison with the smallness of the planet. He considers that the canals stand in some peculiar relation to the polar cap, for they crowd together in its neighbourhood. In place, too, of ill-defined condensations, he sees sharp black spots where the canals meet and intersect, and to these he gives the name of "Oases." He further lays particular stress upon a dark band of a blue tint, which is always seen closely to surround the edges of the polar caps all the time that they are disappearing; and this he takes to be a proof that the white material is something which actuallymelts. Of all substances which we know, water alone, he affirms, would act in such a manner.

The question of melting at all may seem strange in a planet which is situated so far from the sun, and possesses such a rarefied atmosphere. But Professor Lowell considers that this very thinness of the atmosphere allows the direct solar rays to fall with great intensity upon the planet's surface, and that this heating effect is accentuated by the great length of the Martian summer. In consequence he concludes that, although the general climate of Mars is decidedly cold, it is above the freezing point of water.

The observations at Flagstaff appear to do away with the old idea that the darkish areas are seas, for numerous lines belonging to the so-called "canal system" are seen to traverse them. Again, there is no star-like image of the sun reflected from them, as there would be, of course, from the surface of a great sheet of water. Lastly, they are observed to vary in tone and colour with the changing Martian seasons, the blue-green changing into ochre, and later on back again into blue-green. Professor Lowell regards these areas as great tracts of vegetation, which are brought into activity as the liquid reaches them from the melting snows.

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