Eratosthenes and the Lunar Timescale

The northeast rim of Eratosthenes crater, displaying a change in reflectance due to differing slopes. LROC NAC image M117562615L, image width is 600 m [NASA/GSFC/Arizona State University].

Drew Enns
LROC News System

Eratosthenes is located on the Near side of the Moon, northeast of Copernicus, at 14.5°N, 348.7°E. Eratosthenes is named for the ancient Greek who measured the Earth's circumference around 240 BC. In the 1960's, Eratosthenes was used by Gene Shoemaker and Robert Hackman as the benchmark and namesake for the Eratosthenian period of the lunar timescale.

This period represents "middle-aged" craters on the Moon. Even though they are "middle-aged," these craters are inferred to be anywhere from 3.2 to 1.1 billion years old!

LROC Wide Angle Camera mosaic showing Eratosthenes crater (E), northeast of Copernicus (C). Ejecta rays from Copernicus can be seen on Eratosthenes, showing Eratosthenes as the older of the two craters [NASA/GFSC/Arizona State University].

Like Copernicus, Eratosthenes has a well defined rim, walls, and a central peak. However, it lacks rays, and in fact is intersected by rays from Copernicus. Shoemaker & Hackman invoked the geologic law of superposition, which says that younger layers lie on top of older layers, and established Eratosthenes to be the older of the two craters, and thus the Eratosthenian period is the second youngest geologic age on the Moon.

Take a look at the features in Eratosthenes in the Narrow Angle Camera (NAC) image and the Wide Angle Camera (WAC) mosaic!