Page:The Moon (Pickering).djvu/86

58 October 9, 1897. Colongitude of the sunrise terminator, 77°. The eastern wall of Riccioli was just visible. Most of the interior of the crater was darker than anything in its immediate vicinity excepting the shadows, but it was of a nearly uniform tint, and no trace whatever could be fotmd of the eastern boundary of the dark spot, which in a few hours was to become very conspicuous. The northeastern boundary, later equally conspicuous, was described as a delicate shaded band, barely visible. The next observation was made two and a half hours later. The northeastern boundary of the spot was now clearly marked and the contrast throughout had begun to increase.

The second drawing was made four and a half hours after the first one. The outline of the dark spot was now much more nearly complete, and the range of hills just east of Riccioli was on the terminator. Two hours later the spot was drawn, and described as very distinct and strongly marked throughout its entire extent, although not as much so as it was later in the limation. It resembled in general the third drawing.

The third drawing was made upon the next day, October 10th, between 14$h$ o$m$ and 14$h$ 15$m$. The spot was then fully developed, and no further change, either in shape or density, took place in it from that time on until just before sunset, at which time it rapidly faded out. A well-defined dark spot southeast of the larger one is shown in the last drawing which is not fotmd in either of the others. At what time it developed is not certain, nor whether it actually coincides in position with the bright spot shown in the first drawing.

In the course of a few days the entire crater of Riccioli became practically invisible with the exception of the black spot, 15C [2.8, 7.5], by which it may always under these circumstances be found. The same is the case with Alphonsus, near full moon, whose location at that time can best be determined by means of the three conspicuous variable spots which it contains. Since the latitude of Alphonsus is -13°, we should expect these spots also to develop very rapidly, and such in fact is found to be the case, although no visual observations have as yet been secured which are as complete as those relating to Riccioli. We may study the variable spots in Atlas and also in Hercules, 5 A [1.4, 2.9], as they increase in intensity on Plate 5C and as they disappear on Plate 3E. The formation and disappearance of the spots in Alphonsus is shown on Plates 8A, 10C [1.5, 2.1] and 10E [1.4, 3.4].

The word "canal" as used in astronomy is applied to a dark, narrow, straight or smoothly curved surface marking. The term does not necessarily imply the presence