Billy is a lunar impact crater that is located at the southern fringes of the Oceanus Procellarum, in the western hemisphere of the Moon. It lies to the southeast of the similar-sized crater Hansteen, and west-southwest of the similar-sized flooded Letronne.
The interior floor of Billy has been flooded by basaltic lava, leaving a dark surface due to the low albedo. This feature makes it easier to identify under high illumination. The portion of the rim remaining above the surface is narrow and low, with a thin inner wall that is somewhat polygonal in outline. Only a few tiny craterlets mark the interior, with a pair in the southern half showing gray ejecta collars.
This formation dates to 3.88 ga, placing it in the Nectarian or Early Imbrian epoch of the lunar geologic timescale, with the resurfacing event occurring at 3.86 ga. The ejecta blanket surrounding Billy is relatively rich in FeO and TiO2. To the north of the crater is a triangular mountainous formation named Mons Hansteen, after the nearby crater. Southeast of Billy is a rille, designated Rima Billy, that runs 70 kilometers to the south.
This crater was named after French mathematician Jacques de Billy (1602–1679). The name was incorporated into lunar nomenclature by Italian astronomer Giovanni Riccioli in 1651. Its designation was formally adopted by the International Astronomical Union in 1935.
Satellite craters
By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Billy.