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The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. There are, at least, 140 moons within Earth's solar system, and presumably many others orbiting the planets of other stars. Typically the larger gas giants have extensive systems of moons. Mercury and Venus have no moons at all, Earth has one large moon, Mars has two tiny moons, and Pluto a large companion called Charon (sometimes considered to be a double planet).
Most moons are assumed to have been formed out of the same collapsing region of protoplanetary disk that gave rise to its primary. However, there are many exceptions and variations to this standard model of moon formation that are known or theorized. Several moons are thought to be captured foreign objects, fragments of larger moons shattered by large impacts, or (in the case of Earth's Moon) a portion of the planet itself blasted into orbit by a large impact. As most moons are known only through a few distant observations through probes or telescopes, most theories about them are still uncertain.
Most moons in the solar system are tidally lockedA separate article treats the phenomenon of tidal resonance in oceanography. See the article tidal acceleration for a more quantitative description of the Earth-Moon system. Tidal locking makes one side of an astronomical body always face another, like th to their primaries; an exception is SaturnSaturn is the sixth planet from the Sun. It is a gas giant, the second-largest planet in the solar system after Jupiter. It was named after the Roman god Saturn. Its symbol is a stylized representation of the god's sickle ( Unicode: ♄). Physical ch's moon HyperionHyperion ("hy PEER ee un") is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is named after Hyperion, a Titan in Greek mythology. It is also designated Saturn VII. Hyperion's discovery came not too, which rotates chaotically due to a variety of external influences. No moons have moons of their own; the tidal effects of their primaries make orbits around them unstable. However, several moons have companions in their Lagrangian pointIn celestial mechanics, the Lagrangian point s, (also Lagrange point L-point or libration point are the five stationary solutions of the circular restricted three-body problem. given two massive bodies in circular orbits around their common center of masss (eg, Saturn's moons TethysTethys ("TETH iss" or "TEE thiss") is a moon of Saturn that was discovered by Giovanni Domenico Cassini in 1684. Name Tethys is named after the titan Tethys of Greek mythology. It is also designated Saturn III. It should not be confused with the asteroid and DioneDione ("dye OH nee") is a moon of Saturn discovered by Giovanni Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV. There is also an asteroid called 106 Dione. Cassini named the four moons he discovered).
The recent discovery of Ida243 Ida is an asteroid that was imaged by the Galileo probe on August 28, 1993. The probe discovered that Ida had a small moon, an asteroid dubbed Dactyl, only 1. 4 km in diameter; this was the first asteroid moon to be discovered. Some researchers believ's moon Dactyl243 Ida (left) and Dacytl (right), as photographed by Galileo. Dactyl discovered in 1993, is a tiny asteroid (diameter 1. 4 km) that orbits asteroid 243 Ida with a period of 1. 54 days at an average distance of 108 km. The origins of Dactyl are unclear. confirms that some asteroids also have moons. Some, like 90 Antiope, are double asteroids with two equal-sized components.