Geological Chronicle
Positioned on the southwestern edge of Mare Nubium, Campanus spans 46.41 kilometers in diameter and forms a crater pair with Mercator just to the southeast. Along its southern rampart lies the small lunar mare Palus Epidemiarum, while the rille systems Rimae Hippalus and Rimae Ramsden lie to the west and south. Dating to the Lower (Early) Imbrian age on the lunar geologic timescale, the crater has a roughly circular rim that has not been significantly eroded, showing an outward bulge along the western rim and an inward bulge to the north-northwest.
Unlike Mercator, Campanus contains a system of terraces along its inner wall. Basaltic lava resurfaced the interior floor to match the low albedo of the nearby mare, producing a dark center noted by observer T. W. Webb. Only a small central peak projects above the surface, flanked to the east by a slender rille crossing the floor from north to south between the tiny interior craterlets Campanus X and Y.
The crater is named after Italian astronomer Campanus of Novara. Italian astronomer Giovanni Riccioli included the name in lunar nomenclature in 1651, and the International Astronomical Union formally adopted the designation in 1935.