With nearby (and smaller) Alphonsus and Arzachel, this is huge "circular formation" is one of the most interesting and impressive vistas on the Moon. Ptolemaeus is 93-miles in diameter, and features a flat floor that includes ghost craters and many small craterlets. The largest of these floor craters is Ammonius, which is a young-looking 5-mile-across feature. Beside Ammonius is the barely perceptible Ptolemaeus B, which was drowned in the lava flow that created Ptolemaeus' smooth floor. The north wall of Ptolemaeus is graced by the complex and well-defined Herschel (named after Sir William Herschel, natch).
Image Data: Celestron NS11 GPS, Celestron Neximage webcam. Processed with Registax 4 and Paintshop Pro.
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