Geological Chronicle
Regiomontanus is an ancient lunar impact crater located in the southern highlands to the southeast of Mare Nubium. Measuring 126.64 kilometers in diameter, the crater is joined at its chaotic northern rim by Purbach, with Walther situated to the south-southeast. Impact erosion has heavily worn the formation, leaving an irregular, mountainous rim with a distinctly oval outline whose southern and western edges are almost completely degraded.
The crater floor is lava-flooded and marked by several small impact craters. A prominent ridge lies off-center to the northwest, capped at its summit by the small crater designated Regiomontanus A. Although once considered proof of volcanic activity on the Moon, this summit feature is an impact crater offset slightly from the peak's true summit.
The crater is named for the 15th-century German astronomer and mathematician Johannes Müller von Königsberg, known as Regiomontanus. The feature received its name from Giovanni Riccioli in 1651, whose nomenclature system became standardized across the Moon's near side. Earlier cartographers had assigned different names: Michael van Langren's 1645 map labeled it "Christinae Reg. Suec." after Queen Christina of Sweden, while Johannes Hevelius grouped it with Purbach and Walther as "Mons Libanus" after Mount Lebanon.