Iapetus

noun

Iap·​e·​tus ī-ˈa-pə-təs 

astronomy

: the third largest satellite of Saturn with a diameter of approximately 890 miles (1440 km)

Ever since J.D. Cassini discovered the object in 1671, astronomers have puzzled over why Iapetus' leading hemisphere (facing forward along its orbit) is dark and its trailing one bright.

—J. Kelly Beatty, Sky & Telescope, November 1983

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