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Neptune, in astronomy, 8th planet from the sun at a mean distance of about 2.8 billion mi (4.5 billion km) with an orbit lying between those of Uranus and Pluto; its period of revolution is about 165 years. (Pluto, the 9th planet, has such a highly elliptical orbit that from 1979 to 1999 it was closer to the sun than Neptune; it will remain farther from the sun for 220 years, when it will again pass inside Neptune's orbit.) Neptune was discovered as the result of observed irregularities in the motion of Uranus and was the first planet to be discovered on the basis of theoretical calculations. J. C. Adams of Britain and U. J. Leverrier of France independently predicted the position of Neptune, and it was discovered by J. C. Galle in 1846, the day after he received Leverrier's prediction. Neptune has an equatorial diameter of about 30,700 mi (49,400 km), nearly four times that of the earth, and a mass about 17 times the earth's mass. It is much like Uranus and the other giant planets, with a thick atmosphere of hydrogen, helium, methane, and ammonia, a relatively low density, and a rapid period of rotation. On Aug. 24–25, 1989, the U.S. spacecraft Voyager 2 observed Neptune and its moons. It discovered that Neptune's atmosphere has zones like Jupiter's as well as giant storm systems as dark spots on its surface. Although Neptune receives a much smaller fraction of the sun's radiation than does Uranus, its surface temperature is similar: -350°F (-212°C). This may indicate a possible internal heat source. Voyager also confirmed the existence of eight moons and a ring system with three bands. Neptune's largest moon, Triton, was discovered in 1846, a month after the discovery of the planet itself. Triton has a diameter of about 1,700 mi (2,700 km), and its motion is retrograde (see retrograde motion), i.e., opposite to that of the planet's rotation. Its surface temperature is -400°F (-240°C), making it one of the coldest objects in the solar system. Nereid, discovered in 1949, has a diameter of about 210 mi (338 km), is very faint, and has a highly elliptic orbit; astronomers consider that it may be of asteroid origin. Voyager discovered six smaller dark moons orbiting between the plant and Triton: Naiad, Thalassa, Despina, Galatea, Larissa, and Proteus : all irregularly shaped, ranging from 35–260 mi (58–418 km) in diameter.
See P. Moore, Planet Neptune (1989); E. Burgess, Far Encounter: The Neptune System (1992); G. E. Hunt et al., Atlas of Neptune (1994).
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