Twenty-Three Day Old Moon‡

The broad crescent Moon does not rise until after midnight, but will be visible for most of morning, through noon. It will transit around sunrise, so the best time to observe will be during the last hours of darkness.

 

Key Features to Observe Tonight

Beginning in the north, in the lunar arctic circle, about a third of the way to the equator, is Mare Frigoris. The dark floored band of Mare Frigoris lies three-fourths in darkness tonight, engulfed by the western advancing terminator. Past its south shore, the crisp oval of Plato remains distinctively dark-floored on the north edge of Mare Imbrium.

Distinctively seen tonight to the south of Plato are the isolated mountains on the northern Imbrium plain. Mons Pico, a peak of massive size, juts out from below the lava plain. It is far more conspicuous than the nearby Teneriffe Mountains to its northwest. Further south, the isolated smallish but bright-walled Timocharis shows well tonight against Mare Imbrium, although it appears less dark than it was earlier in the lunar cycle.

When Galileo first looked at the Moon through a telescope four centuries ago, it is quite possible that he saw it as it appears tonight. Certainly, this is how he drew it in three of the five illustrations in his Siderius Nuncius (Celestial Messenger) of 1610. He clearly drew the bright arcs of the Alps and the Apennines extending into the dark side from the northern and southern edges of Mare Imbrium, as they generally appear on this night.

Southwest of the Apennines, where they end is Eratosthenes, a crater which is particularly impressive. It lies just outside the rim wall of the Imbrium impact basin.

Tonight, southwest of Eratosthenes, the magnificent ring mountain crater Copernicus can be seen for the last time this lunation, showing strong contrast in the low light angle near the terminator. Traces of its ray system can be still be found on Mare Insularum and the Oceanus Procellarum, even under this low illumination.

Further south, near the terminator and about a third of the way north from the southern cusp is the irregular shaped Mare Nubium. In its southeast quarter a light hairline may be seen running northeast to southwest. This is Rupes Recta (the Straight Wall), a 75 mile long fault, facing towards the west. Despite its sharp appearance, it is not a precipitous cliff but a moderately steep slope. It is at its brightest in this light, just before local sunset.

About half a Mare Nubium diameter further south lies ring mountain Tycho. With a deep interior it displays strong contrast, but the extensive ray system which made it the focal point at Full Moon now appears very muted.

To the southeast of Tycho, the large walled plain Maginus looks very nice tonight. To the southwest at a similar distance again lies the walled plain Clavius, the second-largest crater on the visible side of the Moon. Within it, a series of six sequentially shrinking craters runs in an attractive arc from the south to the west wall.

________________________________________

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