A large and prominent crater on the Moon, connecting to the walled plain of Ptolemaeus directly to the north (whose southern wall Alphonsus in fact partially obscures). These structures, together with Arzachel to the south, are three of the most distinct features of the highlands east of Mare Nubium, the Sea of Clouds.
The crater Alphonsus has a diameter of 118km, with a clear central peak. The crater floor is pitted by much smaller craters formed by lesser impacts after the Alphonsus' creation, as well as several clefts, with the most prominent running north to south. It was in the northern part of Alphonsus that the lunar probe Ranger 9 was deliberately impacted onto the Moon's surface in 1965, transmitting high quality images of the areas around and within Alphonsus as it descended.