Fifteen Day Old Moon‡

The Waning Gibbous Moon will be visible during the late afternoon, setting after dawn. It transits after midnight, which favors observing tonight, as one views through the least atmosphere in dark skies.

The Moon is now just past full. The terminator does not just disappear from the western limb and reappear on the eastern one, but appears "roll" or "rock" from west to east by pivoting about the north or south pole. This rolling effect may be noticeable tonight and, as with libration effects, will affect the apparent position and visibility of features near the limb.


Key Features to Observe Tonight

With favorable libration, south of the equator on the western limb, irregularities may be seen in the otherwise smooth edge of the lunar disc. These are the Cordillera Mountains, with four mile high peaks. They are part of the rugged outer ring surrounding the Mare Orientale (the Eastern Sea), an unusually named feature, since it lies on the western limb. The mare is enormous but lies primarily on the far-side, seen only under extreme libration.

We begin to shift our attention from the western limb to the eastern limb tonight, following the terminator where shadows are most intense. Appearing in a darker sky, many of the features seen shortly after the New Moon are again under low incidence lighting, but rather than westward falling shadows, they are opposite, falling east ( at sunset now, versus sunrise). For many of them, this is the best time to view them in detail.

North from the prominent Mare Crisium in the northern hemisphere, about twice its length way is the distinctive dark crater Endymion, elliptical and similar in appearance to Grimaldi opposite it on the western limb. Near Endymion lies another large, dark, flat region, the Mare Humboltianum (the Humboldt Sea).

Very near the eastern limb and the terminator is the large walled plain Gauss, northeast of the Mare Crisium. It appears well in the long shadows of the lunar sunset.

In the far south, past Mare Fecunditatis, is the large crater Humboldt (not to be confused with the mare by similar name in the far north, classified as a walled plain). Tonight will be the last chance to see it.

Midway to the south pole further south is the small, dark crater Oken. It is the northernmost and of a cluster of lava flooded dark patches, collectively named the Mare Australe. Oken is arguably the best defined of them.


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