Geological Chronicle
Measuring 230.8 kilometers across and 4.8 kilometers deep, Clavius is the second-largest impact crater on the visible lunar near side, behind only the Bailly basin near the southwest limb. Located in the heavily cratered southern highlands south of the prominent ray crater Tycho, Clavius appears as an elongated ellipse when viewed from Earth because of its high southern latitude. Under low-angle solar illumination, the formation is large enough to be spotted with the unaided eye as a distinct notch on the terminator one to two days after first quarter.
The crater formed during the Nectarian period roughly 3.9 billion years ago. The original collision punched through the upper lunar crust to a depth of 20 to 25 kilometers, excavating approximately 68,000 cubic kilometers of material. Clavius has a low, worn outer rim broken on the northeast by the younger crater Porter. Its floor retains a modest central mountain complex between satellite craters Clavius C and N. Spectroscopic measurements indicate that this central uplift consists of anorthositic gabbro originating from depths between 24.5 and 42.4 kilometers.
Across the convex interior floor lies one of the most recognized crater chains on the Moon: a counterclockwise arc of descending craterlets. The sequence begins on the south-southwest rim with Rutherfurd (54 km), then curves inward across the floor through Clavius D (28 km), Clavius C (21 km), Clavius N (13 km), Clavius J (12 km), and Clavius JA (5 km). Because these craters decrease in size in an orderly progression, observational astronomers often use them as resolution targets to evaluate optical telescope optics.
In October 2020, NASA reported that the airborne Stratospheric Observatory for Infrared Astronomy (SOFIA) detected the 6-micrometer spectroscopic signature of molecular water (H2O) on the sunlit floor of Clavius. The observations showed water concentrations of 100 to 412 parts per million, sequestered within impact melt glasses or lodged between mineral grains. The crater carries the name of Christopher Clavius (Christoph Klau), a 16th-century German mathematician and Jesuit astronomer who was the principal architect of the Gregorian calendar.