Fig. 134.
B is the centre of the moon, M the top of the mountain, and SAM a ray of sunlight which just grazes the terminator at A, and then strikes the top of the mountain at M. The triangle BAM is right-angled at A. BA is the radius of the moon, and AM is known by measurement; BM, the hypothenuse, may then be found by computation. BM is evidently equal to the radius of the moon plus the height of the mountain.
By one or the other of these methods, the heights of the lunar mountains have been found with a great degree of accuracy. It is claimed that the heights of the lunar mountains are more accurately known than those of the mountains on the earth. Compared with the size of the moon, lunar mountains attain a greater height than those on the earth.
115. General Aspect of the Lunar Surface.—A cursory examination of the moon with a low power is sufficient to show the prevalence of crater-like inequalities and the general tendency to circular shape which is apparent in nearly all the surface markings; for even the large "seas" and the smaller patches of the same character repeat in their outlines the round form of the craters. It is along the terminator that we see these crater-like spots to the best advantage; as it is there that the rising or setting sun casts long shadows over the lunar landscape, and brings elevations into bold relief. They vary greatly in size; some being so large as to bear a sensible proportion to the moon's diameter, while the smallest are so minute as to need the most powerful telescopes and the finest conditions of atmosphere to perceive them.
Fig. 135.
The prevalence of ring-shaped mountains and plains will be evident from Fig. 135, which is from a photograph of a model of the moon constructed by Nasmyth.
This same feature is nearly as marked in Figs. 131 and 132, which are copies of Rutherfurd's photographs of the moon.
116. Lunar Craters.—The smaller saucer-shaped formations on the surface of the moon are called craters. They are of all sizes, from a mile to a hundred and fifty miles in diameter; and they are supposed to be of volcanic origin. A high telescopic power shows that these craters vary remarkably, not only in size, but also in structure and arrangement. Some are considerably elevated above the surrounding surface, others are basins hollowed out of that surface, and with low surrounding ramparts; some are like walled plains, while the majority have their lowest depression considerably below the surrounding surface; some are isolated upon the plains, others are thickly crowded together, overlapping and intruding upon each other; some have elevated peaks or cones in their centres, and some are without these central cones, while others, again, contain several minute craters instead; some have their ramparts whole and perfect, others have them broken or deformed, and many have them divided into terraces, especially on their inner sides.