NASA - National Aeronautics and Space Administration

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The northeast rim of Eratosthenes crater is easily seen as the change in reflectance due to differing slopes - higher reflectance terrain in the lower left is the steeply sloping interior. LROC NAC image M117562615L, image width is 600 m. Credit: NASA/GSFC/Arizona State University

Eratosthenes crater is found in the central nearside, east-northeast of Copernicus crater (latitude 14.5°, longitude 348.7°). Eratosthenes is named for the ancient Greek who first estimated the Earth's circumference around 240 BC. In the 1960's, Eratosthenes crater was identified by Gene Shoemaker and Robert Hackman as a key stratigraphic marker delimiting the longest period (Eratosthenian) of the lunar timescale, thought to range from about 3.2 to 1.1 billion years old! However we have very few absolute age dates (from samples) for this period in the lunar time scale, thus its actual age is not well constrained. Sampling key Eratosthenian units (craters, mare, domes, etc) is a prime goal of future lunar exploration.

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Boulders resting on the top of a wrinkle ridge in the middle of Sinus Iridum. Where did they come from? LROC NAC M124749832R. Credit: NASA/GSFC/Arizona State University

Like Copernicus, Eratosthenes has a well defined rim, walls, and a central peak. However, it lacks rays, and in fact is crossed by rays from Copernicus crater. Shoemaker and 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.

Explore all of LRO's incredible images at the LRO site at Arizona State University.