Tuesday, September 10, 2024

Ptolemaeus, Alphonsus and Arzachel (evening)

On Tuesday night, September 10, the terminator boundary on the waxing gibbous moon will fall just to the west of a trio of large craters located a short distance south of the moon's center. Ptolemaeus, Alphonsus, and Arzachel are all large enough to see with binoculars and any size of telescope. The northernmost crater Ptolemaeus (96 miles or 154 km wide) has been battered by later impacts that confirm its older age. The flat, almost featureless floor has been filled by lava flows, submerging its central peak and elevating its floor. Alphonsus (74 miles or 119 km wide) is older yet, and only partially filled, allowing its central peak to remain visible. Alphonsus contains a triangle of dark spots that are most prominent when the moon is full - ash deposits from long-ago volcanic venting. Relatively young Arzachel (60 miles
or 96 km wide) has an unaltered floor and a terraced rim. Numerous
north-south lineations carved by ejecta blasted out during the powerful
Imbrium Basin impact event surround those craters.

(Data Courtesy of Starry Night)
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