satellite, natural

satellite, natural,

celestial body orbiting a planet, dwarf planet, asteroid, or star of a larger size. The most familiar natural satellite is the earth's moonmoon,
natural satellite of a planet (see satellite, natural) or dwarf planet, in particular, the single natural satellite of the earth. The Earth-Moon System

The moon is the earth's nearest neighbor in space.
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; thus, satellites of other planets are often referred to as moons. Within the solar system the earth's moon is the largest satellite in relation to its planet and Charon is even larger relative to the dwarf planet Pluto, although neither is the largest in actual size. The largest natural satellite, Jupiter's Ganymede, is 3,268 mi (5,262 km) in diameter, and it and Saturn's Titan are both larger than the planet Mercury. In comparison, some satellites are quite small, e.g., Deimos, the outer satellite of Mars, is c.4 mi (6 km) in diameter. Neither of the inferior planets, Mercury or Venus, has a known satellite; all of the superior planets (those whose orbits lie beyond the orbit of the earth) have at least two known satellites (MarsMars,
in astronomy, 4th planet from the sun, with an orbit next in order beyond that of the earth. Physical Characteristics

Mars has a striking red appearance, and in its most favorable position for viewing, when it is opposite the sun, it is twice as bright as
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, 2; JupiterJupiter
, in astronomy, 5th planet from the sun and largest planet of the solar system. Astronomical and Physical Characteristics

Jupiter's orbit lies beyond the asteroid belt at a mean distance of 483.6 million mi (778.
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, 79; SaturnSaturn,
in astronomy, 6th planet from the sun. Astronomical and Physical Characteristics of Saturn

Saturn's orbit lies between those of Jupiter and Uranus; its mean distance from the sun is c.886 million mi (1.
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, 82; UranusUranus
, in astronomy, 7th planet from the sun, at a mean distance of 1.78 billion mi (2.87 billion km), with an orbit lying between those of Saturn and Neptune; its period of revolution is slightly more than 84 years.
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, 27; NeptuneNeptune,
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 the dwarf planet Pluto; its period of revolution is about 165 years.
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, 13). A number of satellites, e.g., Phoebe of Saturn, Triton of Neptune, and most of the small outer moons of Jupiter and Uranus, have retrograde motionretrograde motion,
in astronomy, real or apparent movement of a planet, dwarf planet, moon, asteroid, or comet from east to west relative to the fixed stars. The most common direction of motion in the solar system, both for orbital revolution and axial rotation, is from west to
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 and may be asteroids that were captured by the planet's gravitation. The asteroid Ida has a tiny moon, Dactyl, that is about a mile (1.6 km) in diameter and orbits about 60 mi (97 km) above Ida's surface.
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satellite, natural

(moon) A natural body that orbits a planet. The nine planets of the Solar System have a total of more than 150 known satellites between them. In addition, the numerous small bodies that comprise the planetary rings of Saturn, Jupiter, Uranus, and Neptune may be regarded as natural satellites. A satellite may arise from one of two mechanisms, both of which probably applied in the Solar System: a body may grow by accretion from material (planetesimals) gravitationally attracted toward, and entering orbit around, a protoplanet; secondly, bodies, such as asteroids, may be captured gravitationally by a planet. Another mechanism, involving formation from the debris of a major impact on the primary planet, may explain the Moon's origin. Most of the known satellites occur in the outer Solar System. Because of the low ambient temperature both now and during formation, many of these satellites are composed largely of water ice or of roughly equal mixtures of water ice and rock; these are the icy satellites. Three satellites – Titan, Triton, and Io – are now known to have atmospheres. For satellite properties, see Table 2, backmatter.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
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