Page:The Moon (Pickering).djvu/104

72 in 3 and 8, and triangular in 5 and 6. A is elliptical in Figure 7, round in 8, irregular in 4 and 6, triangular in 3 and 5. In Figures 5 and 7 the two craters are similar in shape, but in 3, 4, 6 and 8 they are quite different. In Figure 4 the major axes are clearly inclined to one another; in 7 they are parallel.

Many changes not visible in these photographs are described in Volume XXXII. of the "Harvard Annals," and are shown in the accompanying drawings. In many cases the photographs did not happen to be taken at the proper time to show the changes to the best advantage. Thus we have no photograph showing the craters exactly alike. This resemblance, first noted by Beer and Madler, occurs only at 5.0 days and between 8.5 and 9.5 days after lunar sunrise. Again, although Figure 5 shows Messier as larger than A, yet the time when the difference is most marked, and is even more conspicuous, occurs a day later in the lunation. We have no negative taken at that time. No photographs are capable of showing the changes in the interior spots to advantage.

Nearly 100 drawings of these craters have been made at various times during the past ten years, besides numerous studies, when descriptions, but no drawings, were secured. As a result of the study of this material, and of the various photographs of the craters, I would suggest the following explanation of the various phenomena and changes observed.

Figure 6 is a copy of a drawing made at Arequipa with the thirteen-inch telescope and a power of 795 diameters, on May 4, 1892, at 12$h$ 50$m$, G. M. T. Time elapsed since sunrise, 4.5 days. Figure 7 is a diagram upon the scale of $1⁄500,000$, or eight miles to the inch, measured on the Moon's axis. The full lines represent the outlines of these craters, their sizes and shapes, as I believe they should be drawn, and are based upon a study of the photographs. Owing to the inferior seeing, the separation of the crescent at the right into three divisions, l, m and n, has never been observed at Cambridge, although sometimes the central division and sometimes the three divisions, taken as a whole, can be seen. The broken and dotted lines represent