Geological Chronicle
Theophilus lies between Sinus Asperitatis in the north and Mare Nectaris to the southeast, measuring 98.59 kilometres across. It partially intrudes into the comparably sized crater Cyrillus to the southwest, while Mädler lies to the east and Beaumont is further to the south-southeast. Together with Cyrillus and Catharina, it forms a group of large craters visible on the terminator 5 days after the new moon. Named after the 4th-century Coptic Pope Theophilus I of Alexandria, British astronomer T. W. Webb called this "the deepest of all visible craters".
Created during the Eratosthenian period, from 3.2 to 1.1 billion years ago, Theophilus has massive walls and has broken into a second formation, Cyrillus. The rim of Theophilus has a wide, terraced inner surface that shows indications of landslips. The crater is 4,200 metres deep, with an imposing central mountain 1,400 metres high with four summits. Its floor is relatively flat, and the triple-peaked central mountain climbs to a height of about 2 kilometres above the floor.
The western slopes of this ridge are wider and more irregular, whereas the peaks descend more sharply to the floor on the northern and western faces. The western peak is designated Psi (ψ), the eastern Phi (φ), and the northern peak is Alpha (α) Theophilus. In these peaks, detections include anorthosite with a very low mafic abundance and the infrared spectrum of pure crystalline plagioclase. During lunar exploration, the Apollo 16 mission collected several pieces of basalt that are believed to be ejecta from the formation of Theophilus.