Before we leave our satellite, something must be said as to the conditions prevailing on her surface. The early astronomers who devoted attention to lunar study were drawn on in their labours largely by the hope of detecting resemblances to our own earth, or even traces of human habitation. Schröter and Gruithuisen imagined that they had discovered not only indications of a lunar atmosphere, but also evidence of change upon the surface, and traces of the handiwork of lunarian inhabitants. Gruithuisen, in particular, was confident that in due time it would become possible to trace the cities and the works of the Lunarians. Gradually these hopes have receded into the distance. The existence of a lunar atmosphere is, indeed, no longer positively denied now, as it was a few years ago; but it is certain that such atmosphere as may exist is of extreme rarity, quite inadequate to support animal life as we understand such a thing. Certain delicate changes of colour which take place within some of the craters—Plato for instance—have been referred to vegetation; and Professor Pickering has intimated his observation of something which he considers to be the forming and melting of hoar-frost within certain areas, Messier and a small crater near Herodotus among others. But the observations at best are very delicate and the inferences uncertain. It cannot be denied that the moon may have an atmosphere; but positive traces of its existence are so faint that, even if their reality be admitted, very little can be built upon them.
At the same time when the affirmation is made that the moon is 'a world where there is no weather, and where nothing ever happens,' the most careful modern students of lunar matters would be the first to question such a statement. Even supposing it to be true that no concrete evidence of change upon the lunar surface can be had, this would not necessarily mean that no change takes place. The moon has certainly never been studied to advantage with any power exceeding 1,000, and the average powers employed have been much less. Nasmyth puts 300 as about the profitable limit, and 500 would be almost an outside estimate for anything like regular work. But even assuming the use of a power of 1,000, that means that the moon is seen as large as though she were only 240 miles distant from us. The reader can judge how entirely all but the very largest features of our world would be lost to sight at such a distance, and how changes involving the destruction of large areas might take place and the observer be none the wiser. When it is remembered that even at this long range we are viewing our object through a sea of troubled air of which every tremor is magnified in proportion to the telescopic power employed, until the finer details are necessarily blurred and indistinct, it will be seen that the case has been understated. Indeed it may be questioned if the moon has ever been as well seen as though it had been situated at a distance of 500 miles from the earth. At such a distance nothing short of the vastest cataclysms would be visible; and it is therefore going quite beyond the mark to assume that nothing ever happens on the moon simply because we do not see it happening. Moreover, the balance of evidence does appear to be inclining, slightly perhaps, but still almost unquestionably, towards the view that change does occur upon the moon. Some of the observations which seem to imply change may be explained on other grounds; but there is a certain residuum which appears to defy explanation, and it is very noteworthy that while those who at once dismiss the idea of lunar change are, generally speaking, those who have made no special study of the moon's surface, the contrary opinion is most strongly maintained by eminent observers who have devoted much time to our satellite with the best modern instruments to aid them in their work.
The admission of the possibility of change does not, however, imply anything like fitness for human habitation. The moon, to use Beer and Mädler's oft-quoted phrase, is 'no copy of the earth'; and the conditions of her surface differ widely from anything that we are acquainted with. The extreme rarity of her atmosphere must render her, were other conditions equally favourable, an ideal situation for an observatory. From her surface the stars, which are hidden from us in the daytime by the diffused light in our air, would be visible at broad noonday; while multitudes of the smaller magnitudes which here require telescopic power would there be plain to the unaided eye. The lunar night would be lit by our own earth, a gigantic moon, presenting a surface more than thirteen times as large as that which the full moon offers us, and hanging almost stationary in the heavens, while exhibiting all the effects of rapid rotation upon its own axis. Those appendages of the sun, which only the spectroscope or the fleeting total eclipse can reveal to us, the corona, the chromosphere, and the prominences, would there be constantly visible.
Our astronomers who are painfully wrestling with atmospheric disturbance, and are gradually being driven from the plains to the summits of higher and higher hills in search of suitable sites for the giant telescopes of to-day, may well long for a world where atmospheric disturbance must be unknown, or at least a negligible quantity.
[*] See drawings by Colonel Markwick with 2¾-inch refractor, of the eclipse of August 30, 1905, 'The Total Solar Eclipse, 1905,' British Astronomical Association, pp. 59, 60.
CHAPTER VIII
MARS
The Red Planet is our nearest neighbour on the further, as Venus is on the hither side. He is also in some ways the planet best situated for our observation; for while the greatest apparent diameter of his disc is considerably less than that of Venus, he does not hide close to the sun's rays like the inferior planets, but may be seen all night when in opposition.[*] Not all oppositions, however, are equally favourable. Under the best circumstances he may come as near to us as 35,000,000 miles; when less favourably situated, he may come no nearer than 61,000,000. This very considerable variation in his distance arises from the eccentricity of the planet's orbit, which amounts to nearly one-tenth, and, so far as we are concerned, it means that his disc is three times larger when he comes to opposition at his least distance from the sun than it is when the conditions are reversed. Under the most favourable circumstances—i.e., when opposition and perihelion[†] occur together, he presents, it has been calculated, a disc of the same diameter as a half sovereign held up 2,000 yards from the spectator. Periods of opposition recur at intervals of about 780 days, and at the more favourable ones the planet's brilliancy is very striking. The 1877 opposition was very notable in this respect, and in others connected with the study of Mars, and that which preceded the Crimean War was also marked by great brilliancy. Readers of Tennyson will remember how Maud
'Seem'd to divide in a dream from a band of the blest,