Geological Chronicle
Cyrillus lies on the northwest edge of Mare Nectaris, with Catharina to the south. The slightly larger and younger Theophilus intrudes into its northeastern rim, where Cyrillus's broken walls remain intact. Theophilus is slightly larger and younger.
The floor contains a reduced central hill and the considerable crater Cyrillus A. Its central peak and ejecta to the southwest are olivine-rich. Northeast of the center, three rounded mountains, Cyrillus Alpha, Delta and Eta, rise 1,000 metres above the floor.
The formation dates to the Nectarian epoch. Giovanni B. Riccioli included “S. Cyrillus Alex.” in lunar nomenclature in 1651, and the International Astronomical Union formally adopted the modern designation in 1935. The modern designation was adopted in 1935. East of Shioli, a tiny crater with bright rays, a point was proposed as the landing site for Japan's SLIM lander. SLIM landed on January 19, 2024, within 100 metres of the targeted landing zone.