Tuesday, April 16, 2024

Ptolemaeus, Alphonsus and Arzachel (all night)

On Tuesday night, April 16, the moon’s waxing gibbous phase will favor a look at 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. A large number of north-south lineations surround the craters - carved by ejecta blasted out during the powerful Imbrium Basin impact event.

(Data courtesy Starry Night)
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