twenty-one day moon

About ¹/₃ of the way down the terminator, the Mare Serenitatis is half lost to darkness. On its north-west edge the Caucasus Mountains run roughly north-south, marking the boundary with the Mare Imbrium. Toward their southern end they gradually submerge beneath the smooth mare.
After a short interval their line is continued, curving to the south-west, by the Apennines, which define the south-eastern rim of Imbrium and are even more rugged and spectacular. These two ranges are tonight lit to fabulous effect by the setting Sun, which casts them into sharp relief.

On the southern edge of Serenitatis, near the terminator, is Menelaus, and to its west-south-west at roughly ¹/₃ of the length of the Apennines is its near-namesake Manilius. Both of these class 1 craters remain conspicuous, although their floors have become dark as they have lost their full moon brilliance.

Due south of Menelaus by the length of Serenitatis lies larger class 2 Delambre, a light-rimmed but dark-floored circle. To the south-west at the same distance again is the large class 5 ring mountain Albategnius. It shows well in this light, with its large central peak, as does the smaller and relatively more recent (but still very old) Klein, which interrupts its west wall. To the south by about two Humorum lengths, class 2 Maurolycus is slightly smaller than Albategnius, and briefly shines even brighter than Tycho, two Humorum widths to its west.