Geological Chronicle
Located in the rugged southern highlands to the southwest of the bright ray crater Tycho, Longomontanus is an ancient circular depression classified as a walled plain. The structure measures 145.5 kilometers in diameter. Because of its location, Longomontanus appears distinctly oval in shape due to foreshortening.
On the lunar geologic timescale, Longomontanus is one of the largest craters of Nectarian age. To the southeast lies the even larger formation Clavius, Maginus is located to the east, and north of the rim is the irregular crater Montanari, which is joined at its northern rim by Wilhelm.
The wall of Longomontanus is heavily worn and incised by past impacts, leaving the rim essentially level with the surrounding terrain. The northern rim especially is impacted with multiple overlapping craterlets, while a semi-circular ridge to the east has the appearance of an overlapped crater rim. The crater floor is relatively flat, with a low cluster of central peaks somewhat offset to the west.