Ptolemaeus is an ancient lunar impact crater close to the center of the near side, named for Claudius Ptolemy, the Greco-Roman writer, mathematician, astronomer, geographer and astrologer.

[1] To the south-southwest, Ptolemaeus is joined to the rim of the crater Alphonsus by a section of rugged, irregular terrain, and these form a prominent chain with Arzachel to the south.

These leave only a slight rise where the rim existed, and are difficult to detect except at low angles of sunlight.

On either side of this crater are linear, irregular gashes in the lunar surface, forming valley-like features.

By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Ptolemaeus.