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Natural Satellites or moons orbit planets.
Natural satellites are objects orbiting planets or other bodies larger than itself and which is not man-made. Natural satellites are often called moons. Technically, the term could also refer to a planet orbiting a star, or even to a star orbiting a Galactic center, but these uses are rare. Instead, the term is normally used to identify non-artificial satellites of planets, dwarf planets, or minor planets.
There are 240 known moons within the Solar System, including 162 orbiting the planets, 4 orbiting dwarf planets, and dozens more orbiting small solar system bodies. Other stars and their planets also have natural satellites.
The large gas giants have extensive systems of moons, including half a dozen comparable in size to Earth's Moon. Of the inner planets, Mercury and Venus have no moons at all; Earth has one large moon ("the Moon" or "Luna"); and Mars has two tiny moons, Phobos and Deimos. Among the dwarf planets, Ceres has no moons (though many objects in the asteroid belt do), Eris has one, Dysnomia, and Pluto has three known satellites, Nix, Hydra, and a large companion called Charon. Charon is also unusual in that it does not orbit Pluto, but rather both bodies orbit a barycenter in empty space, making Pluto-Charon a binary system.
Origin of natural satellites.
Moons orbiting relatively close to the planet on prograde orbits (regular satellites) are believed to have been formed out of the same collapsing region of protoplanetary disk that gave rise to its primary. Most Irregular satellites (orbiting on distant, inclined, eccentric often retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. Many special scenarios exist, including origin in a break-up of the planet. For example, the Earth's Moon is theorized to have formed from material blasted into orbit by a large impact. In contrast, the large Neptunian satellite Triton is known to be a captured Trans-Neptunian object because of its retrograde orbit, even though it orbits quite close to Neptune.
Orbital characteristics of natural satellites.
Most regular moons in the solar system are tidally locked to their primaries, meaning that one side of the moon is always turned toward the planet. Exceptions include Saturn's moon Hyperion, which rotates chaotically because of a variety of external influences.
In contrast, the outer moons of the gas giants (irregular satellites) are too far away to become 'locked'. For example, Jupiter's moon Himalia, Saturn's moon Phoebe and Neptune's Nereid have rotation period in the range of 10 hours compared with their orbital periods of hundreds of days.
Satellites of satellites.
No "moons of moons" (natural satellites that orbit the natural satelite of another body) are known. It is uncertain whether such objects can be stable in the long term. In most cases, the tidal effects of their primaries make such a system unstable; the gravity from other nearby objects (most notably the primary) would perturb the orbit of the moon's moon until it broke away or impacted its primary. In theory, a secondary satellite could exist in a primary satellite's Hill sphere, outside of which it would be lost because of the greater gravitational pull of the planet (or other object) that the primary satellite orbits. For example, the Moon orbits the Earth because the Moon is 370,000 km from Earth, well within Earth's Hill sphere, which has a radius of 1.5 million km (0.01 AU or 235 Earth radii). If a Moon-sized object were to orbit the Earth outside its Hill sphere, it would soon be captured by the Sun and become a Dwarf planet in a near-Earth orbit.
However, two moons are known have small companions in their L4 and L5 Lagrangian points of their orbits, about sixty degrees ahead of and behind their positions. These companions are called Trojan moons, because their positions are comparable to the positions of the Trojan asteroids relative to Jupiter. They are Telesto and Calypso, the leading and following companions respectively of Tethys; and Helene and Polydeuces, the leading and following companions of Dione.
The recent discovery of 243 Ida's moon Dactyl confirms that some asteroids also have moons. Some, like 90 Antiope, are double asteroids with two equal-sized components. The asteroid 87 Sylvia has two moons. See Asteroid moon for further information.
Moons of the Solar system.
The largest moons in the solar system (those bigger than about 3000 km across) are Earth's Moon, Jupiter's Galilean moons (Io, Europa, Ganymede, and Callisto), Saturn's moon Titan, and Neptune's captured moon Triton. For smaller moons see the articles on the appropriate planet. In addition to the moons of the various planets there are also over 80 known moons of the dwarf planets, asteroids and other small solar system bodies. Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites.
The following is a comparative table classifying the moons of the solar system by diameter. The column on the right includes some notable planets, dwarf planets, asteroids, and Trans-Neptunian Objects for comparison.
Terminology of natural satellites.
The first known natural satellite was the Moon (luna in Latin). Until the discovery of the Galilean moons in 1610, however, there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetę ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited.
Christiaan Huygens, the discoverer of Titan, was the first to use the term moon for such objects, calling Titan Luna Saturni or Luna Saturnia - "Saturn's Moon" or "The Saturnian Moon", because it stood in the same relation to Saturn as the Moon did to the Earth.
As additional moons of Saturn were discovered, however, this term was abandoned. Giovanni Domenico Cassini sometimes referred to his discoveries as plančtes in French, but more often as satellites, using a term derived from the Latin satelles, meaning "guard", "attendant", or "companion", because the satellites accompanied their primary planet in their journey through the heavens.
The term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of "moon". In 1957, however, the launching of the artificial object Sputnik created a need for new terminology. The terms man-made satellite or artificial moon were very quickly abandoned in favor of the simpler satellite, and as a consequence, the term has come to be linked primarily with artificial objects flown in space - including, sometimes, even those which are not in orbit around a planet.
As a consequence of this shift in meaning, the term moon, which had continued to be used in a generic sense in works of popular science and in fiction, has regained respectability and is now used interchangeably with satellite, even in scientific articles. When it is necessary to avoid both the ambiguity of confusion with the Earth's Moon on the one hand, and artificial satellites on the other, the term natural satellite (using "natural" in a sense opposed to "artificial") is used.
The definition of a moon.
There has been some debate about the precise definition of a moon. This debate has been caused by the presence of orbital systems where the difference in mass between the larger body and its satellite, are not as pronounced as in normal systems (a commonly accepted defining point is where the satellite is larger than 1/40th the mass of the larger body) . Two examples include the Pluto-Charon system, and the Earth-Luna System. The presence of these systems has caused a debate about where to precisely draw the line between a double body system, and a main body-satellite system, the most commonly agreed upon definition is whether the barycentre is below the surface of the larger body, though this is not official. Another definition is whether the Sun or the larger body in the system has the most gravitational influence over the satellite in question. At the other end of the spectrum there are many ice/rock clumps that form the ring systems on the Solar System's gas giants and there is no set point to define when one of these clumps is large enough to be classified as a moon, though the term "moonlet" may be sometimes used to refer to extremely small objects in orbit around a larger body, there is again no official definition.
Natural satellites of planets and dwarf planets.
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