Posted - 2008.02.18 04:20:00 - [3]

 

A natural satellite or moon is a celestial body that orbits a planet 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. (There are no known natural satellites of moons.)

240 bodies, all in the Solar System, are classified as moons. They include 166 orbiting the eight planets,[1] 4 orbiting dwarf planets, and dozens more orbiting small solar system bodies. Other stars and their planets are very likely to have natural satellites, although none have yet 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 (the Moon); 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. The Pluto-Charon system is unusual in that the center of mass lies in open space between the two, a characteristic of a double planet system.

Limits on the size of a moon

There is no established lower limit on what should be considered a moon: Every satellite with an identified orbit, some as small as a kilometer across, has been identified as a moon, though clumps a tenth that size within Saturn's rings (which may not be solid bodies) have been called moonlets. (Small asteroid moons, such as Dactyl, some up to tens of kilometers across, have also been called moonlets.) The upper limit is also vague: When the masses of two orbiting bodies are similar enough that one cannot be said to orbit the other, they are described as a dual body rather than primary and satellite. However, with the exception of Pluto-Charon among some authors, only asteroids are such as 90 Antiope are considered dual bodies, and they have not yet forced a clear definition as to what constitutes a moon.

Origin

The natural satellites orbiting relatively close to the planet on prograde orbits (regular satellites) are generally believed to have been formed out of the same collapsing region of the protoplanetary disk that gave rise to its primary. In contrast, irregular satellites (generally orbiting on distant, inclined, eccentric and/or retrograde orbits) are thought to be captured asteroids possibly further fragmented by collisions. The Earth-Moon[2] and possibly Pluto-Charon systems[3] are exceptions among large bodies in that they are believed to have originated by the collision of two large proto-planetary objects (see the giant impact hypothesis). The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting moons. As opposed to planetary-sized bodies, asteroid moons are thought to commonly form by this process.

Orbital characteristics

Tidal locking

Most regular natural satellites 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 moon Nereid have rotation period in the range of 10 hours compared with their orbital periods of hundreds of days.

Posted - 2008.02.18 04:55:00 - [5]

 

Edited by: Ikasu on 18/02/2008 04:55:11

Originally by: An Anarchyyt

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A natural satellite or moon is a celestial body that orbits a planet 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. (There are no known natural satellites of moons.)

240 bodies, all in the Solar System, are classified as moons. They include 166 orbiting the eight planets,[1] 4 orbiting dwarf planets, and dozens more orbiting small solar system bodies. Other stars and their planets are very likely to have natural satellites, although none have yet 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 (the Moon); 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. The Pluto-Charon system is unusual in that the center of mass lies in open space between the two, a characteristic of a double planet system.

Limits on the size of a moon

There is no established lower limit on what should be considered a moon: Every satellite with an identified orbit, some as small as a kilometer across, has been identified as a moon, though clumps a tenth that size within Saturn's rings (which may not be solid bodies) have been called moonlets. (Small asteroid moons, such as Dactyl, some up to tens of kilometers across, have also been called moonlets.) The upper limit is also vague: When the masses of two orbiting bodies are similar enough that one cannot be said to orbit the other, they are described as a dual body rather than primary and satellite. However, with the exception of Pluto-Charon among some authors, only asteroids are such as 90 Antiope are considered dual bodies, and they have not yet forced a clear definition as to what constitutes a moon.

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This

Posted - 2008.02.18 07:24:00 - [7]

 

for some dumb reason they are what you take to own space..

not planets...

because planets wouldn't make any sense...