Geological Chronicle
Posidonius is a lunar impact crater of Upper (Late) Imbrian age located on the northeastern edge of Mare Serenitatis, south of Lacus Somniorum, spanning 95.06 kilometers in diameter. T. W. Webb described it as a "walled plain" that is "a good object about 6d after" a new Moon. Its rim is shallow and obscured, especially on the western edge, with crater ramparts still observable to the south and east and to a lesser degree to the north. Crater Chacornac is attached to the southeast rim, and Daniell lies to the north.
The interior of Posidonius has been overlain by a lava flow in the past, leaving the floor slightly bulged and laced with a rille system named Rimae Posidonius. The past lava uplift also likely produced the complex of rilles. There is no central peak, but the floor is hilly and contains a smaller, semi-circular rim of a concentric, flooded crater offset towards the eastern edge. In addition, the infrared spectrum of pure crystalline plagioclase has been identified on the central hills and on the southeast and southwest rims.
Smaller impact craters interrupt the formation and its surroundings. Northeast along the rim lies Posidonius B, while another smaller crater, Posidonius A, sits offset just west of center within the main rim. Nearby on the surface of Mare Serenitatis, wrinkle ridges designated Dorsa Smirnov parallel the shore. At the peak of these ridges sits Posidonius Y, a two-kilometer crater surrounded by a ray system and high-albedo patch that Julius Schmidt observed in 1867.