Twenty Day Old Moon‡

Tonight's Gibbous Moon will be visible during the late evening and for much of morning. It transits during the last hours of darkness.


Key Features to Observe Tonight

Tonight Mare Tranquillitatis is split with about half in darkness. North of it, Mare Serenitatis is still completed illuminated. At their junction, Plinius (Pliny) stands out well against the smooth maria surrounding it. At the northeast edge of Mare Serenitatis, the larger Posidonius shows as a bright ring.

Northwest of Posidonius and about its diameter beyond the northern edge of Mare Serenitatis is the crater Eudoxus. North of it is the large ring mountain crater Aristotle. Both of these are eye-catching in the low-angle light. Aristotle is especially distinct against the velvety surface of Mare Frigoris, which is beginning to disappear into the night behind the terminator.

South of Posidonius, and northeast of Plinius, where the Sun will soon set, is the landing site of Apollo 17, where Eugene Cernan and Harrison Schmitt became the two last men to set foot on the Moon (no one has been back since their trip in 1972).

On the western edge of Mare Tranquillitatis, the small crater Dionysius still appears quite brightly and makes a nice landmark. Further south is the beginnings of a distinctive series of ring mountain craters. The large Theophilus, is the northernmost, which shows strong relief tonight. It slightly overlaps the older, similarly sized Cyrillus, to its southwest. Further south yet lies the still older and similarly sized crater Catharina.

West of Catharina, half its diameter away lies the Altai Scarp, which tonight appears as an irregular dark line which curves southeast towards the terminator. This is actually a cliff, nearly 2 km high and over 500 km long (1 mile high by 300 miles long.) It appears dark at this time as the cliff wall faces east; you can check this by comparing its shadow with that of nearby craters. The lighting will be opposite in two weeks, when it appears as a bright line with the rising Sun shining on the cliff face.


‡with permission from Lunar Discoverer User's Manual by Robert Duvall, 2013