Named after Alfonso X (El Sabio) Alphonsus - Spanish astronomer (1221-1284).
Alphonsus is an ancient impact crater that dates from the pre-Nectarian era. It is located on the lunar highlands on the eastern end of Mare Nubium, west of the Imbrian Highlands, and slightly overlaps the crater Ptolemaeus to the north. To the northwest is the smaller Alpetragius. (wikipedia entry)
