Crater-Hopping: Observing the Moon on Day 4

Abstract

About 70 additional named features generally become exposed to our view on the fourth day of a lunation. The selenographic colongitude on the fourth day ranges from about 314° to 334°. The terminator continues as a steep concave curve that reveals the spherical nature of the Moon, as shown in Fig. 9.1. On days 4, 5, and 6 of a lunation, a problem can arise in trying to list the features visible on the particular day, when the sunrise terminator moves west of a feature thus, making it initially available for observation. The sunrise terminator appears to move rapidly across the equatorial areas, while nearly standing still at the cusps. Therefore, some features that I list for day 4 will not be visible on day 4, but will sometimes be viewable on day 5; likewise, there will be times when a feature listed for day 5 will actually become observable a day earlier or a day later. In addition to the varying speed of the terminator, libration plays a major role in the location of the colongitude of the terminator on these days. We will start this crater hop near the terminus of the southern cusp at the crater Hagecius.