Page:The Moon (Pickering).djvu/70

46 degree through melting, has sunk into the hollows and crevices of the surface, leaving the projecting irregularities exposed. On account of the rarity of the atmosphere the lunar sky is absolutely black, and no substance, however white, will be visible until it is illuminated by the direct rays of the Sun. This explains why the partly bright regions are invisible at sunrise, since the white material which causes them can become visible only when the sun has risen high enough to shine into the crevices and cavities in which it lies.

The central range of mountains in Plate 9E is known as the Apennines. The photograph shows them at the close of the lunar day. It will be noticed that many of the peaks, although within a few hours of sunset, are still very brilliant, while other lower surfaces, equally inclined to the Sun's rays, are much darker. The brilliancy of the polar regions at the bottom of the picture is also quite noticeable, and is worthy of further attention. Thus, if we examine the photograph of the full moon (Plate C, Fig. 1), taken at a time when the two poles were about equally illuminated by the Sun, we shall find that it contains three large bright regions and numerous comparatively small ones. The latter are always associated with mountain peaks or craters. Of the three large areas, by far the most conspicuous surrounds the great crater Tycho, and extends northwestward from it as far as the Moon's equator. This whole region is elevated and mountainous. The two other large areas surround the two poles of the Moon; that at the south pole is virtually continuous with the region surrounding Tycho, although the space between them is slightly less luminous than either of these regions. The north polar region is quite isolated from the others. The entire limb or edge of the Moon is dark except for these two bright polar spots.

The position of the Moon's poles is shown by the two white lines. Plate C, Figure 2, is from the same original negative as Figure 1, but is so printed as to show the Moon as it appears to the eye in a small telescope. In comparison with Figure 1, it will be noted that the polar regions are but little brighter than the left-hand edge.

A comparison of Plates 6A and 6C shows a portion of the Moon taken at the time of lunar sunrise, and the same region five and a half days later in the lunation when the surface has become partially obscured by the appearance of the white material. The latter picture was taken two days before full moon, when nearly all the shadows had disappeared. A few small regions can be identified which are particularly bright in both pictures, and are permanently bright under all illuminations, but most of the