Page:The Moon (Pickering).djvu/103

Rh mean time at which the negative was taken. The fifth column gives the colongitude of the sunrise terminator, the sixth the number of terrestrial days that had elapsed since the Sun rose on the craters, and the seventh the east and west libration of the Moon at the time. Since the craters are near the equator, the north and south libration has but little influence on their appearance. When the libration is positive the effect is to bring them toward the centre of the Moon's disk, and therefore lengthen them in an east-andwest direction. The longitude of the craters is +47° and their latitude is -2°. Messier is the western or left-hand crater, and is situated nearest to the limb.

If now we examine the photographs closely we shall find that in Figures 3, 4 and 8 A appears to be the larger of the two craters. On the other hand, in Figures 5, 6 and 7 Messier appears the larger. The craters are lengthened sometimes in one direction and sometimes in another. Thus in Figures 5 and 8 the lengthening is in an east-and-west direction, while in Figure 7 the direction is north and south. That this difference is not due to libration, as might at first be supposed, is shown, since the libration at the time the last-mentioned plate was taken lies between that of the two others. We may express this fact another way by saying that the libration tends to expand the craters in an east-and-west direction in Figure 7 and to contract them in Figure 8. Since the effect actually observed is the exact reverse of this, it is obvious that it cannot be due to libration.

If we measure the distance from the little craterlet north of Messier to that south of A on these two figures we shall find that they are on very nearly the same scale. Nevertheless, the increased size of A in Figure 8 is very noticeable. An examination of the original negatives confirms this observation.

With regard to their shapes, Messier is elliptical in Figures 4 and 7, pear-shaped