Kepler is most notable for the prominent ray system that covers the surrounding mare. The rays extend for well over 300 kilometers, overlapping the rays from other craters. Kepler has a small rampart of ejecta surrounding the exterior of its high rim. The outer wall is not quite circular, and possesses a slightly polygonal form. The interior walls of Kepler are slumped and slightly terraced, descending to an uneven floor and a minor central rise.
One of the rays from Tycho, when extended across the Oceanus Procellarum, intersects this crater. This was a factor in the choice of the crater's name when Giovanni Riccioli was creating his system of lunar nomenclature, as Kepler used the observations of Tycho Brahe while devising his three laws of planetary motion. On Riccioli's maps, this crater was named Keplerus, and the surrounding skirt of higher albedo terrain was named Insulara Ventorum.
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