Geological Chronicle
Langrenus lies on the eastern shore of Mare Fecunditatis, near the Moon’s eastern limb. Its circular outline appears oblong through foreshortening. The inner wall is wide, irregularly terraced, and averages about 20 kilometers in width. British astronomer T. W. Webb called it “a superb object under oblique sunshine, with its terraced ring.”
The outer ramparts are irregular and hilly, with a bright, fragmented ray system spread westward across the mare. The interior has a higher albedo than its surroundings, so Langrenus appears bright when the Sun is overhead. Boulders cover the floor, which is slightly irregular in the northwest half. The central peaks rise about a kilometer above the floor, and an eastern-rim peak reaches 3 km. The infrared spectrum of pure crystalline plagioclase has been identified on the central peak.
On December 30, 1992, Audouin Dollfus of the Observatoire de Paris observed a series of glows on the floor with a one-meter telescope. The glows changed form with time, and Dollfus believed they were likely a gaseous emission. The crater’s cracked floor may have been the source of the gas. In 1645, Flemish astronomer Michael van Langren drew a lunar map and named many features. Langrenus is the only one of his named features to retain his original designation.