Definition of Iapetus
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
