Dionysius is a lunar impact crater that lies on the western edge of the Mare Tranquillitatis. To the southeast is the crater pair of Ritter and Sabine. Just to the northwest is the system of rilles designated Rimae Ritter. These clefts follow a generally northwest direction. (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.
