Twenty-One Day Old Moon (Third Quarter)‡

Tonight's Moon is very near Last Quarter. It rises around midnight and remains in the sky until nearly noon. It will transit around sunrise, so the best time to observe will be during the last hours of darkness. It is be bright enough to be prominent in daylight, but contrast is markedly reduced on features, due to the scattered light from our atmosphere.


Key Features to Observe Tonight

From the north, about half way to the equator, Mare Serenitatis is about cut in half by the terminator, with the east covered in darkness. On its northwest edge the Caucasus Range runs roughly north to south, marking the boundary with Mare Imbrium. It appears to be the crater rim of the Imbrium impact, forming an arc. They gradually disappear beneath the smooth mare at their southern end. Just to their west lies the Cassini crater, easily identified by the two craters within its floor.

After a short break the crater rim continues, curving to the southwest, now named as the Apennines. This range defines the southeastern rim of the Imbrium impact. They are even more rugged and spectacular than the Caucasus Mountains. Together, these two ranges make a marvelous sight under the low setting Sun, which throws long shadows from the towering peaks, creating sharp relief.

Back on the southern edge of Serenitatis, not far from the terminator, is the crater Menelaus, and to its west-southwest at roughly half of the diameter of Serenitatis is Manilius. Both of these craters remain conspicuous, although their floors have become darker away from their Full Moon brilliance as the Sun and the terminator has moved progressively west.

Due south of Menelaus, close to the terminator, just below the equator, lies larger Delambre, a light-rimmed but dark-floored circle. To the southwest at the same distance again is the large ring mountain crater Albategnius. It appears nicely tonight, with its large central peak, as does the smaller and more recent crater Klein, which impacted its west rim.

To the south, close to the terminator, lies the crater Maurolycus, slightly smaller than Albategnius, and which briefly outshines even Tycho, the easily identified bright crater to its west.


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