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moon : a natural satellite of a planet — Webster
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Natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets. Of the inner planets, Mercury and Venus have no moons; Earth has one large moon, known as the Moon; and Mars has two tiny moons, Phobos and Deimos. The large gas giants have extensive systems of moons, including half a dozen comparable in size to Earth’s moon: the four Galilean moons, Saturn’s Titan, and Neptune’s Triton. Saturn has an additional six mid-sized moons massive enough to have achieved hydrostatic equilibrium, and Uranus has five. The Earth-Moon system is unique in that the ratio of the mass of the Moon to the Earth is much greater than any other planet-moon ratio in the Solar System, and the Moon’s orbit with respect to the Sun is always concave. — Wikipedia
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