is an ancient lunar impact crater
located in the southern highlands region to the southeast of Mare Nubium
. It is joined at the chaotic northern rim by the crater Purbach
, and to the south-southeast is Walther
The crater is heavily worn from impact erosion, and the rim is irregular, mountainous and distinctly oval in outline. The walls on the southern and western edges have been almost completely degraded. A prominent ridge lies off-center to the northwest, with the small crater Regiomontanus A at its summit. The crater floor is lava
-flooded, and has a number of small crater impacts.
The summit crater Regiomontanus A was once considered proof of volcanic
activity on the Moon. However this feature is actually an impact crater, and is offset slightly from the summit of the central peak.
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 Regiomontanus.