twenty day moon

Around the centre of the terminator, the Mare Tranquillitatis is now half in darkness. To the north of it, the rough oval of the Mare Serenitatis is still complete. At their junction, class 1 Plinius (Pliny) stands out well against the smooth darkness around it. At the north-east edge of Serenitatis, the larger class 5 Posidonius shows as a bright ring.

To the south of Posidonius, and north-east of Plinius, the Sun will tonight set on the landing site of Apollo 17, where in 1972 Eugene Cernan and Harrison Schmitt became the last men to set foot on the Moon... so far.

North-west of Posidonius and about one Posidonius length beyond the northern edge of Serenitatis is Eudoxus and, at twice the distance from the Mare, its northern neighbour, ring mountain Aristoteles (Aristotle), both class 1 and conspicuous in the low-angle light. The latter, larger crater is especially clear against the smooth backdrop of the Mare Frigoris, which has itself begun to fall behind the terminator.

On the western edge of Tranquillitatis, the tiny Dionysius still shines out brightly. At the end of its large southern bay is the most northerly of a distinctive series of ring mountains: the large class 1 Theophilus, which shows in strong relief. On its south-western side it slightly overlaps the older, similarly sized class 3 Cyrillus, slightly to the south of which lies the still older and again similarly sized class 4 Catharina.

To the west of Catharina, at a separation roughly equal to its radius, an irregular dark line begins. It curves south-east towards the terminator. This is the Altai Scarp, a 300 mile long, mile high cliff. Its darkness shows that it faces east - compare its lighting with that of the nearby craters. Two weeks from now it will be bright as the rising Sun shines on its face.