Geological Chronicle
Kepler is a 31-kilometer impact crater situated between Oceanus Procellarum to the west and Mare Insularum to the east. Formed during the Copernican period, the crater is geologically young. Its excavated rock has experienced minimal space weathering from the solar wind, preserving high-albedo ejecta across the surrounding dark volcanic maria.
The crater rim exhibits a slightly polygonal outline bordered by an outer ejecta rampart. Terraced inner walls descend 2.6 kilometers to an uneven floor that features a minor central rise. Kepler's most prominent attribute is an expansive ray system extending over 300 kilometers across the lunar plains, overlapping ejecta rays from Copernicus to the east.
In 1651, Italian astronomer Giovanni Riccioli named the crater after Johannes Kepler, the German astronomer who developed the laws of planetary motion. Riccioli deliberately positioned the name where a faint ray from Tycho intersects the mare, connecting Kepler to Tycho Brahe, whose planetary observations Kepler analyzed. On Riccioli's lunar map, the bright terrain surrounding Kepler was designated Insulara Ventorum, or Islands of the Winds.
Spacecraft have mapped Kepler extensively from orbit. Lunar Orbiter 3 and Lunar Orbiter 4 photographed its terraced walls in 1967, followed by orbital observations from Apollo 12 in November 1969. High-resolution data from NASA's Lunar Reconnaissance Orbiter (LRO) later documented ponds of frozen impact melt and boulder fields distributed across the crater floor.