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A large and prominent ringed plain near the centre of the visible disc of the Moon, the northernmost of a chain of signifcant features running down the eastern side of the Mare Nubium. Ptolemaeus is a circular feature a little over 150 km from edge to edge. Its southern wall has been obliterated by a later event, which formed the crater Alphonsus, which (with a diameter of 118 km) is only a little smaller than Ptolemaeus itself.
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