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The average centre-to-centre distance from the Earth to the Moon is 384,403 kilometres (238,857 mi), about thirty times the diameter of the Earth. The common centre of mass of the system (the barycentre) is located at about 1,700 kilometres (1,100 mi)—a quarter the Earth's radius—beneath the surface of the Earth. The Moon makes a complete orbit around the Earth every 27.3 days[nb 3] (the orbital period), and the periodic variations in the geometry of the Earth–Moon–Sun system are responsible for the phases of the Moon, which repeat every 29.5 days[nb 4] (the synodic period). or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify non-artificial satellites of planets, dwarf planets, and minor planets. As of July 2009, 336 bodies are formally classified as moons. They include 168 orbiting six of the eight planets, 6 orbiting three of the five dwarf planets, 104 asteroid moons, and 58 satellites of Trans-Neptunian objects, some of which will likely turn out to be dwarf planets. Some 150 additional small bodies were observed within Saturn's ring system, but they were not tracked long enough to establish orbits. Planets around other stars are likely to have natural satellites as well, although none have been observed. 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. Of the inner planets, Mercury and Venus have no moons at all; Earth has one large moon, known as the Moon; and Mars has two tiny moons, Phobos and Deimos. It has been suggested that a few moons, notably Europa, one of Jupiter's Galilean moons, may harbour life, though there is currently no direct evidence to support this claim. Among the dwarf planets, Ceres has no moons (though many objects in the asteroid belt do). Pluto has three known satellites, the rather large Charon and the smaller Nix and Hydra. Haumea has two moons, and Eris has one. The Pluto-Charon system is unusual in that the centre of mass lies in open space between the two, a characteristic of a double planet system. The Man in the Moon is a shape resembling a human face or figure perceived in the full Moon.