Page:The Moon (Pickering).djvu/85

Rh it the floor is bright. It soon darkens uniformly, but on the third day the extreme northeastern portion begins to fade and by the sixth day the faded region covers one-quarter of the floor. On the fourth day a slight darkening is noticed to the south of the central peak. This rapidly develops, and the next day the region is seen to contain two very dark spots, each located in the vicinity of an elongated crevice which may be observed earlier in the lunation. These spots remain virtually unchanged until the twelfth day. They then suddenly fade and by the next day have completely vanished, leaving only the gray tint in the southwestern half of the floor, which disappears at sunset.

Other craters, such as Atlas, 5A [1.0, 2.9], Alphonsus, 8A [3.3, 3.1], and Riccioli, 15 A [2.7, 7.6], present similar phenomena. In the last, which is near the equator, the changes are exceptionally rapid, and occur immediately after sunrise. Thus while in the case of Atlas, in latitude +46°, the changes require six days for their completion, in Riccioli they are completed in about twenty-four hours, and a noticeable change takes place in less than five hours. Since it is located near the eastern edge of the disk, sunrise occurs a day or two before full moon, and the complete series of changes can therefore be watched in the course of a single night. These changes are very striking, but can be seen to advantage on only two or three nights in the course of a year when the libration is favourable—that is to say, when that edge of the moon is turned slightly toward the Earth so as to bring the crater well into view.

The three drawings here reproduced are upon a scale of $1⁄2,000,000$ The first drawing was made between 11$h$ 10$m$ and 11$h$ 20$m$ Greenwich Mean Time on the night of