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Phoebe[fE´bE] Pronunciation Key, in astronomy, one of the 18 named moons, or natural satellites, of Saturn. Also known as Saturn IX (or S9), Phoebe is 137 mi (220 km) in diameter, orbits Saturn at a mean distance of 8,047,985 mi (12,952,000 km), has an orbital period of 550.5 earth days, and rotates on its axis in about nine hours. The outermost of the known satellites, Phoebe was discovered by American astronomer William H. Pickering in 1898. Phoebe's reflectivity is very low. Its orbit is inclined 175° to Saturn; that is, its north pole is almost aligned with the planet's south pole. Phoebe is the only one of the moons that orbits with retrograde motion, i.e., opposite to that of the planet's rotation. The sharp inclination of its orbit and its retrograde motion encourage the belief that Phoebe is a captured object similar to an asteroid rather than a native satellite.
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