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Iapetus[Iap´itus] Pronunciation Key, in astronomy, one of the 18 named moons, or natural satellites, of Saturn. Also known as Saturn VIII (or S8), Iapetus is 907 mi (1460 km) in diameter, orbits Saturn at a mean distance of 2,212,885 mi (3,561,300 km), and has equal orbital and rotational periods of 79.33 earth days. Saturn's third largest moon, it was discovered in 1671 by the Italian-French astronomer Gian Domenico Cassini. With a density of only 1.1, Iapetus is believed to consist almost completely of water ice. The reflectivity and surface features of its leading and trailing hemispheres are noticeably different. The leading hemisphere is remarkably dark, while the trailing hemisphere is light : the asymmetry is so marked that Cassini wrote that he could see Iapetus on one side of Saturn but not on the other. The trailing hemisphere is also heavily cratered, while the leading hemisphere is not. Unlike all the other moons but Phoebe, Iapetus's orbit is inclined to the plane of Saturn's equator.
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