Posidonius (Moon)

Named after Of Apamea Posidonius - Greek geographer (135(?)-51(?) B.C.).

Posidonius is a lunar impact crater on the north-eastern edge of Mare Serenitatis, to the south of Lacus Somniorum. The crater Chacornac is attached to the southeast rim, and to the north is Daniell. The rim is shallow and obscured, especially on the western edge, and the interior has been overlain by a lava flow in the past. The crater ramparts can still be observed to the south and east of the crater rim, and to a lesser degree to the north. (wikipedia entry)

A crater is a circular depression likely created by an impact event. On the Moon they are named after deceased scientists, polar explorers, astronauts or cosmonauts.