Ptolemaeus and Alphonsus craters. Artwork of the Ptolemaeus crater (upper right) and the Alphonsus crater (lower centre) on the Moon. The Ptolemaeus crater is 153 kilometres in diameter and is named for the 2nd century Roman-Greek astronomer Claudius Ptolemaeus (more commonly known as Ptolemy). The Alphonsus crater is 119 kilometres in diameter and is named for Alfonso X of Castile, a 13th century Spanish ruler who encouraged work being done on astrology and astronomy. These craters are located on the visible side of the Moon, near its equator.