Ptolemaeus (lunar crater)

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For the Ptolemaeus crater on Mars, see Ptolemaeus (crater on Mars).

Crater characteristics
Ptolemaeus from Apollo 16. NASA photo.
Coordinates	9.2° S, 1.8° W
Diameter	153 km
Depth	2.4 km
Colongitude	3° at sunrise
Eponym	Claudius Ptolemaeus

Ptolemaeus is an ancient lunar impact crater close to the center of the near side. To the south-southeast Ptolemaeus is joined to the rim of Alphonsus crater by a section of rugged, irregular terrain, and these form a prominent chain with Arzachel to the south. To the southeast is Albategnius crater and to the north is the smaller but well-defined Herschel crater.

The features of Ptolemaeus are highlighted when the sun is at low angles during the first and last quarter. During the full moon the sun is directly overhead and the crater contours become more difficult to discern.

The crater has a low, irregular outer rim that is heavily worn and impacted with multiple smaller craters. The rim has a discernible polygonal shape, although overall it remains circular. The largest of the peaks along the rim, designated Ptolemaeus Gamma (γ), has an altitude of 2.9 km and is located along the northwest rim. The crater has no central peak, a lava-flooded floor, and lacks a ray system. Impact sites of this form are often classified as a "walled-plain", due to their resemblance to the maria.

The somewhat dark-hued floor of Ptolemaeus is notable for several "ghost" craters, formed when lava flow covers a pre-existing crater. These leave only a slight rise where the rim existed, and are difficult to detect except at low angles of sunlight. There are also multiple smaller craters across the floor surface, most notably Ammonius crater in the northeastern quadrant.

On either of this crater are linear, irregular gashes in the lunar surface, forming valley-like features. These features are approximately parallel to each other and radiate from the direction of Mare Imbrium to the north-northwest.

[edit] Satellite craters

Ammonius crater on the floor of Ptolemaeus. Herschel crater lies to the right. The Apollo 12 lunar module is visible near top of image. NASA photo.

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

The following crater has been renamed by the IAU.

• Ptolemaeus A — See Ammonius crater.

[edit] References

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• Wlasuk, Peter T. (2000). Observing the Moon. Springer. ISBN 1852331933.