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74 confuse the bright interior with its white outer walls. This confusion of the white frost with the bright interior of Messier was well shown in a sketch made July 1, 1901, which looked very much like the photograph, Figure 7. Indeed, the two were made at exactly the same number of days after sunrise. Another sketch was made the day following, when the hoarfrost had still further evaporated, and one could then distinctly trace the rounded brilliant crater outline, with the fainter hazy blur extending to the north and south of it. A close inspection of Figure 7 enables one, though with some difficulty, to make this distinction between the crater outline and the hazy blur to the south of it. In Figure 8 the true outlines of Messier and of A, l, m and n combined in one are again shown. In a drawing made September 1, 1898, 12.5 days after sunrise, the line separating A from the three smaller divisions is clearly shown.

The indistinctness of the western walls of the two craters upon the fifth day after sunrise, and shown partly for Messier in Figure 5, is due, not as some have contended, to fog or mist rising from the craters, but simply because the regions inside and outside of them at these places are of equal brightness and the boundary line between them cannot therefore be distinguished. The varying irregular shapes of A and of the small patch of vegetation in its bottom, upon the fifth and sixth days, are due to the irregular deposition of hoarfrost on the crater walls at the time that the shadow of the crater upon its inner western wall disappears. The fact that neither Messier nor the vegetation in its bottom participates to the same extent in these irregular changes at this time is of interest, and may be due to more uniform aqueous conditions and to a different shape of the crater floor, which in this case is long and narrow.

The interior brightness of these craters presents rather unusual phenomena. The interiors of most small snow-craters are equally bright when the Sun shines on them from sunrise to sunset. At noon the bright areas are somewhat enlarged by extending over the crater walls. In Messier and A, however, the interior eastern walls are dark for the first two days after sunrise, as already noted. They brighten and then darken again later, and disappear, as shown in Figure 7, only reappearing when their shadows make them visible. They seem, in fact, to be brilliant only for a few days during the early middle part of the lunation. The interior western walls, on the other hand, at least of Messier, are brilliant as soon as the Sun strikes them, and remain so till sunset This difference in the appearance of the eastern and western walls is perhaps due to the difference in their slope, the western walls being nearly vertical while the eastern ones are very