Named after Pedro Nuñez Salaciense Nonius - Portuguese mathematician (1502-1578).
Nonius is a lunar impact crater. It is located in the rugged southern highlands 's near side. The northwestern side and about half the interior floor are overlain by the rim and outer rampart of the walled plain Walther. About half its diameter to the southeast is the smaller crater Kaiser, and slightly farther to the south lies Fernelius. (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.
