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 Linné in the Mare Serenitatis, and after reading his description of its changes of appearance one can hardly reject his conclusion that Linné 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."[19]
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″, as formerly stated, was less than 0.4″. 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]."[20]
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.