Geological Chronicle
Purbach is a large lunar impact crater situated in the rugged southern highlands of the Moon, bordered by Regiomontanus to the south and Thebit to the northwest. Measuring 114.97 kilometers in diameter, the formation dates to the Pre-Nectarian epoch on the lunar geologic timescale.
The outer wall of Purbach is heavily worn, with the most intact section surviving along the east and northeast sides. Its northern wall is almost completely destroyed, and the irregular crater Purbach G lies across the northwest rim. Along the western edge, distortion creates the appearance of a double rim distended towards the west. The crater floor is relatively smooth across its eastern half, while low ridges and a partial ghost crater outline occupy the area just west of the midpoint.
For a few hours before the first quarter, the crater's rim contributes to the Lunar X visual phenomenon. Sunlight glancing on the tops of shadowed hills creates an illuminated letter shape along the lunar terminator.
The crater is named after the 15th-century Austrian astronomer Georg von Peuerbach. Giovanni Riccioli introduced the Latin name Purbachius in his 1651 nomenclature system, which later became standardized. Earlier cartographers assigned different names: Michael van Langren's 1645 map designated it Christierni IV Reg. Daniae, while Johannes Hevelius grouped it with Walther and Regiomontanus under the name Mons Libanus.