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A natural satellite is an object that orbits a planet or other body larger than itself and which is not man-made. Such objects are often called moons. 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 163 orbiting the planets, four orbiting dwarf planets, and dozens more orbiting small solar system bodies.
The large gas giants have extensive systems of natural satellites, including half a dozen comparable in size to the Earth's moon. Of the inner planets, Mercury and Venus have no moon 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.
The orbital properties and compositions of natural satellites provides us with important information on the origin and evolution of the satellite system. Especially a system of natural satellites orbiting around a gas giant can be regarded as a miniature solar system that contains precious clues for studying the formation of solar systems.
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 and possibly Pluto-Charon systems 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.
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 Nereid have rotation period in the range of ten hours compared with their orbital periods of hundreds of days.
Satellites of satellites
No "moons of moons" (natural satellites that orbit the natural satellite 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.
Two moons are known to have small companions at their L4 and L5 Lagrangian points, which are about sixty degrees ahead of and behind the body in its orbit. These companions are called Trojan moons, because their positions are comparable to the positions of the Trojan asteroids relative to Jupiter. Such objects are Telesto and Calypso, which are the leading and following companions respectively of Tethys; and Helene and Polydeuces, which are the leading and following companions of Dione.
The discovery of 243 Ida's moon Dactyl in the early 1990s 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.
Natural satellites of the solar system
The largest natural satellites in the solar system (those bigger than about 3,000 kilometers 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 percent 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.
|Satellites of planets||Dwarf planet satellites||Satellites of|
|(136472) 2005 FY9|
|Charon||(136108) 2003 EL61|
2 Pallas, 4 Vesta
many more TNOs
|Dysnomia||S/2005 (2003 EL61) 1|
S/2005 (79360) 1
and many others
|S/2005 (2003 EL61) 2|
many more TNOs
S/2000 (90) 1
many more TNOs
S/2000 (762) 1
S/2002 (121) 1
S/2003 (283) 1
S/2004 (1313) 1
and many TNOs
|less than 10||at least 47||at least 21||many||many|
The first known natural satellite was the Moon (Luna in Latin). Until the discovery of the Galilean satellites in 1610, however, there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ ("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 planètes 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 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 more typical systems. Two examples are the Pluto-Charon system and the Earth-Moon 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 common definition rests upon whether the barycentre is below the surface of the larger body, though this is unofficial and somewhat arbitrary. At the other end of the spectrum there are many ice/rock clumps that form ring systems around 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. The term "moonlet" is sometimes used to refer to extremely small objects in orbit around a larger body, but again there is no official definition.
- ↑Canup, R. and E. Asphaug (2001). Origin of the Moon in a giant impact near the end of the Earth's formation. Nature 412: 708-712.
- ↑Stern, S., H. Weaver, A. Steffl, M. Mutchler, W. Merline, M. Buie, E. Young, L. Young, and J. Spencer (2006). A giant impact origin for Pluto’s small moons and satellite multiplicity in the Kuiper belt. Nature 439: 946-949.
- ↑Marchis, F., P. Descamps, D. Hestroffer and J. Berthier (2005). Discovery of the triple asteroidal system 87 Sylvia. Nature 436: 822-824. Retrieved July 2, 2007.
- ↑ This column lists objects that are moons of small solar system bodies, not small solar system bodies themselves.
- ↑ Sometimes referred to as "Luna".
- ↑ 6.06.1 Diameters of the new Plutonian satellites are still very poorly known, but they are estimated to lie between 44 and 130 km.
- ↑ (617) Patroclus I Menoetius
- ↑ (22) Kalliope I Linus
- ↑ (87) Sylvia I Romulus
- ↑ (45) Eugenia I Petit-Prince
- Karttunen, H., et al. (eds.). 2003. Fundamental Astronomy, 4th ed. Helsinki: Springer-Verlag. ISBN 3540001794
- Bakich, Michael E. 2000. The Cambridge Planetary Handbook. New York: Cambridge University Press. ISBN 0521632803
- Beatty, J. Kelly, et al. (eds.). 1999. The New Solar System, 4th ed. New York: Cambridge University Press. ISBN 0521645875
All links retrieved December 23, 2014.
|Natural satellites of the Solar System|
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A natural satellite in astronomy is a smaller body which moves around a larger body. The smaller body is held in orbit by gravitation. The term is used for moons which go around planets, and it is also used for small galaxies which orbit larger galaxies.SMA,KYS
Bodies which orbit planets are called moons. They vary in size, and are ball-shaped. The Earth has only one moon. Some other planets have many moons, and some have none. When people write just "the moon", they are usually talking about the moon of the Earth. Earth's moon is written with a capital letter, Moon. The Latin word for the moon is luna, which is why the adjective used to talk about the moon is "lunar". For example, lunar eclipse.
Anything that goes around a planet is called a satellite. Moons are natural satellites. People also use rockets to send machines into orbit around the Earth. These machines are called artificial (man-made) satellites.
Earth's moon[change | change source]
Main article: The Moon
Moons do not make their own light. We can see the Earth's moon because it acts like a mirror, and reflects the light of the Sun. The same half of the moon faces toward Earth at all times, no matter where it moves. But different parts of the moon are lit up by the Sun, so it looks different at different times of the month. This change as seen from Earth is called the phases of the moon, or lunar phases.
A moon's cycle is the time the moon takes to change from looking very bright and round to looking very small and thin, and then back to bright and round again. In the case of the Earth's moon, this is about four weeks. It does this about 13 times in one year. The moon's cycle is about 28 days, a bit shorter than a calendar month.
The Apollo 11 mission helped Neil Armstrong and Buzz Aldrin become the first people to walk on the Moon. They did this on July 20, 1969.
Orbits[change | change source]
The orbit of a moon or other satellite is affected by two forces: gravity, and the centripetal force. For example, the Earth's moon is kept in orbit by the gravitational pull from the Earth. This is also the way the Earth is attracted to the Sun, and stays in its orbit. The orbit of the Earth's moon actually causes the tides and waves on Earth.
Moons of moons[change | change source]
No moons that belong to moons have been found. In most cases, the tidal effects of the main body would make such as unstable.so there are no moons of moons
However, math completed after the recent finding  of a possible ring system around Saturn's moon Rhea show that Rhean orbits would be stable. Also, the rings are thought to be narrow, something that is known with shepherd moons.
Asteroid moons[change | change source]
The finding of 243 Ida's moon Dactyl in the early 1990s was the proof that some asteroids have moons; indeed, 87 Sylvia has two. Some, such as 90 Antiope, are double asteroids with two same-sized parts.
Moons of the Solar System[change | change source]
The biggest 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.
The following is a table grouping the moons of the solar system by diameter. The column on the right has some notable planets, dwarf planets, asteroids, and Trans-Neptunian Objects for comparing. It is normal for moons to be named after people from mythology.
Planets that have moons[change | change source]
Planets in our Solar System that have moon(s):
Dwarf planets that have moons[change | change source]
Planets that do not have moons[change | change source]
Planets in our Solar System that do not have moons:
Galaxies[change | change source]
Galaxies are found in groups called galaxy clusters which are also held together by gravitation. Our own Milky Way is the second largest galaxy in our Local Group (the largest is Andromeda). Many smaller galaxies and star clusters are also held in the Local Group, outside the two main galaxies. They are all in orbits round one of the centres of gravity. That means most of them move round either Andromeda or the Milky Way. so it seems natural for astronomers to use the term 'satellite' for these as well.
Our Local Group is itself part of an even larger group, the Virgo Supercluster. There are other, even larger, groups of galaxies: see the Great Wall for an example.