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A moon is a big fat purple monkey.

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+A moon is a big fat purple monkey.
-{{redirect|Moons}}
-[[Image:Moons of solar system v7.jpg|thumb|400px|Selected moons, with the Earth to scale. Nineteen moons are large enough to be round, and one, Titan, has a substantial atmosphere.]]
-A '''natural satellite''' or '''moon''' is a [[celestial body]] that [[orbit]]s 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 [[satellite]]s of planets, [[dwarf planet]]s, and [[minor planet]]s.
-{{As of|2008|alt=As of September 2008}}, 327 bodies are formally classified as moons. They include 165 orbiting six of the eight planets,<ref>[http://www.dtm.ciw.edu/sheppard/satellites/ List of natural satellites orbiting the planets.]</ref> 6 orbiting three of the five dwarf planets, 104 [[asteroid moon]]s, and 58 satellites of [[Trans-Neptunian object]]s, 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. Other stars and their planets are likely to have natural satellites as well, although none have been observed.
-The large [[gas giant]]s have extensive systems of moons, including half a dozen comparable in size to [[Earth]]'s moon: the four [[Galilean moon]]s, [[Saturn]]'s [[Titan (moon)|Titan]], and [[Neptune]]'s [[Triton (moon)|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 (planet)|Mercury]] and [[Venus]] have no moons at all; Earth has one large moon (the [[Moon]]); and [[Mars]] has two tiny moons, [[Phobos (moon)|Phobos]] and [[Deimos (moon)|Deimos]].
-Among the dwarf planets, [[Ceres (dwarf planet)|Ceres]] has no moons (though many objects in the asteroid belt do). [[Pluto]] has three known satellites, the rather large [[Charon (moon)|Charon]] and the smaller [[Nix (moon)|Nix]] and [[Hydra (moon)|Hydra]]. [[Haumea (dwarf planet)|Haumea]] has [[Moons of Haumea|two moons]], and [[Eris (dwarf planet)|Eris]] has [[Dysnomia (moon)|one]]. The Pluto-Charon system is unusual in that the [[barycenter|center of mass]] lies in open space between the two, a characteristic of a [[double planet]] system.
-[[Image:Occulting Enceladus PIA10500.jpg|thumb|300px|right|Two moons: Saturn's moon [[Dione (moon)|Dione]] occults [[Enceladus (moon)|Enceladus]]]]
-==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 satellite]]s (generally orbiting on distant, [[orbital inclination|inclined]], [[orbital eccentricity|eccentric]] and/or [[retrograde motion|retrograde]] orbits) are thought to be captured [[asteroid]]s possibly further fragmented by collisions. The Earth's Moon<ref>{{cite journal | last = R. Canup and E. Asphaug | title = Origin of the Moon in a giant impact near the end of the Earth's formation  | journal = Nature | volume = 412 | pages = 708–712 | date = 2001 | doi = 10.1038/35089010 }}</ref> and possibly Charon<ref>{{cite journal | title = A giant impact origin for Pluto's small moons and satellite multiplicity in the Kuiper belt |last = S. Stern, H. Weaver, A. Steffl, M. Mutchler, W. Merline, M. Buie, E. Young,
-L. Young, and J. Spencer | journal = Nature| volume = 439 | date = 2006 | pages = 946–949 | doi = 10.1038/nature04548}}</ref> 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 moon]]s are thought to commonly form by this process. Triton is another exception, which although large and in a close, circular orbit, is thought to be a captured [[dwarf planet]].
-==Geological activity==
-Of the nineteen known moons massive enough to have lapsed into hydrostatic equilibrium, several remain geologically active today. [[io (moon)|Io]] is the most volcanically active body in the Solar System, while [[triton (moon)|Triton]] and [[enceladus (moon)|Enceledus]] have geysers. [[titan (moon)|Titan]] and Triton have significant atmospheres; Titan also has [[lakes of Titan|methane lakes]], and presumably rain. Four of the largest moons, [[europa (moon)|Europa]], [[ganymede (moon)|Ganymede]], [[callisto (moon)|Callisto]], and Titan, are thought to have subsurface oceans of liquid water, while smaller Enceladus may have localized subsurface water. Many other moons, such as [[Tethys (moon)|Tethys]], show evidence of past geological activity. In these cases, the geological activity comes mainly from the [[Tidal_heating#Tidal_heating|tidal heating]] from orbiting their gas giant primaries.
-==Orbital characteristics==
-===Tidal locking===
-The regular natural satellites in the solar system are [[tidal locking|tidally locked]] to their primaries, meaning that the same side of the moon always faces the planet. The only known exception is [[Saturn]]'s moon [[Hyperion (moon)|Hyperion]], which rotates chaotically because of the gravitational influence of Titan.
-In contrast, the outer moons of the gas giants (irregular satellites) are too far away to have become locked. For example, Jupiter's moon [[Himalia (moon)|Himalia]], Saturn's moon [[Phoebe (moon)|Phoebe]], and Neptune's moon [[Nereid (moon)|Nereid]] have rotation period in the range of ten hours, while their orbital periods are hundreds of days.
-===Satellites of satellites===
-[[Image:Rhean rings PIA10246 Full res.jpg|thumb|Artist impression of Rhea's rings]]
-No moons of moons (natural satellites that orbit the natural satellite of another body) are known. In most cases, the tidal effects of the primary would make such a system unstable.
-However, calculations performed after the recent detection <ref>{{cite web
-|url=http://www.sciencemag.org/cgi/content/abstract/319/5868/1380?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=rhea&searchid=1&FIRSTINDEX=0&resourcetype=HWCIT
-|title=The Dust Halo of Saturn's Largest Icy Moon, Rhea -- Jones et al. 319 (5868): 1380 -- Science
-|accessdate=2008-03-06
-|format=
-|work=
-}}</ref> of a possible [[Rings_of_Rhea|ring system]] around Saturn's moon [[Rhea (moon)|Rhea]] indicate that Rhean orbits would be stable. Furthermore, the suspected rings are thought to be narrow,<ref>{{cite web
-|url=http://www.newscientist.com/article/dn13421-saturn-satellite-reveals-first-moon-rings.html
-|title=Saturn satellite reveals first moon rings - 06 March 2008 - New Scientist
-|accessdate=2008-03-06
-|format=
-|work=
-}}</ref> a phenomenon normally associated with [[shepherd moon]]s.
-===Trojan satellites===
-Two moons are known to have small companions at their L<sub>4</sub> and L<sub>5</sub> [[Lagrangian point]]s, sixty degrees ahead and behind the body in its orbit.  These companions are called [[Trojan moon]]s, as their orbits are analogous to the [[Trojan asteroid]]s of [[Jupiter]]. The Trojan moons are [[Telesto (moon)|Telesto]] and [[Calypso (moon)|Calypso]], which are the leading and following companions respectively of [[Tethys (moon)|Tethys]]; and [[Helene (moon)|Helene]] and [[Polydeuces (moon)|Polydeuces]], the leading and following companions of [[Dione (moon)|Dione]].
-===Asteroid satellites===
-{{main|asteroid moon}}
-The discovery of [[243 Ida]]'s moon [[Dactyl (moon)|Dactyl]] in the early 1990s confirmed that some [[asteroid]]s have moons; indeed, [[87 Sylvia]] has two. Some, such as [[90 Antiope]], are double asteroids with two comparably sized components.
-==Natural satellites of the Solar System==
-[[Image:Masses of all moons.png|thumb|300px|The relative masses of the moons of the Solar system. Mimas, Enceladus, and Miranda are too small to be visible at this scale. All the irregularly shaped moons, even added together, would also be too small to be visible.]]
-The largest natural satellites in the Solar System (those bigger than about 3000&nbsp;km across) are Earth's [[moon]], [[Jupiter]]'s [[Galilean moon]]s ([[Io (moon)|Io]], [[Europa (moon)|Europa]], [[Ganymede (moon)|Ganymede]], and [[Callisto (moon)|Callisto]]), [[Saturn]]'s moon [[Titan (moon)|Titan]], and [[Neptune]]'s captured moon [[Triton (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 planet]]s, [[asteroid]]s and other [[small solar system bodies]]. Some studies estimate that up to 15% of all [[trans-Neptunian object]]s 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.
-The moons of the planets are named after mythological figures. These are predominately Greek, except for the [[moons of Uranus|Uranian moons]], which are named after Shakespearean characters. The nineteen bodies massive enough to have achieved [[hydrostatic equilibrium]] are in bold in the chart below and labeled on the chart at right, though a few of the smaller ones are not visible at the scale of the chart. Minor planets suspected but not proven to have achieved a hydrostatic equilibrium are italicized in the table below.
-{{clear}}
-{{Natural satellites}}
-==Terminology==
-The first known natural satellite was the [[Moon]] (''luna'' in [[Latin]]). Until the discovery of the [[Galilean moons|Galilean satellites]] in 1610, however, there was no opportunity for referring to such objects as a class. [[Galileo Galilei|Galileo]] chose to refer to his discoveries as ''Planetæ'' ("[[planet]]s"), but later discoverers chose other terms to distinguish them from the objects they orbited.
-[[Christiaan Huygens]], the discoverer of [[Titan (moon)|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 language|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===
-[[Image:Moon Earth Comparison.png|thumb|150px|right|Comparison of [[Earth]] and the [[Moon]]]]
-[[Image:Plutoncharon1.jpg|thumb|150px|right|Comparison of [[Pluto]] and [[Charon (moon)|Charon]]]]
-[[Image:Jupiter.moons2.jpg|thumb|150px|right|Comparison of [[Jupiter]]'s [[Great Red Spot]] and [[Galilean moons|Jupiter's four largest moons]]. Compared to Earth/Luna and Pluto/Charon, there is a much greater difference in [[mass]].]]
-There is no established lower limit on what should be considered a moon. Every body with an identified orbit, some as small as a kilometer across, has been identified as a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called ''[[moonlet]]s.'' Small asteroid moons, such as [[Dactyl (moon)|Dactyl]], 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 [[double planet|double body]] rather than primary and satellite. [[Asteroid]]s such as [[90 Antiope]] are considered double asteroids, but they have not forced a clear definition as to what constitutes a moon. Some authors consider the Pluto-Charon system to be a double (dwarf) planet. The most common dividing line on what is considered a moon rests upon whether the [[barycentre]] is below the surface of the larger body, though this is somewhat arbitrary, as it relies on distance as well as relative mass.
-==See also==
-* [[Co-orbital moon]]
-* [[Extrasolar moon]]
-* [[Inner moon]]
-* [[Irregular moon]]
-* [[List of moons]]
-* [[List of moons by diameter]]
-* [[Naming of moons]]
-* [[Quasi-satellite]]
-* [[Timeline of discovery of Solar System planets and their moons]]
-* [[Trojan moon]]
-=====Moons of planets =====
-* [[Moon|Earth's moon]]
-* [[Moons of Mars]]
-* [[Moons of Jupiter]]
-* [[Moons of Saturn]]
-* [[Moons of Uranus]]
-* [[Moons of Neptune]]
-=====Moons of dwarf planets and small Solar System bodies=====
-* [[Asteroid moon]]
-* [[Moons of Pluto]]
-* [[Dysnomia (moon)|Eris' moon]]
-* [[Moons of Haumea]]
-==Notes and references==
-<!-- ----------------------------------------------------------
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-<div class="references-small">
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-==External links==
-===Jupiter's moons===
-*[http://www.dtm.ciw.edu/sheppard/satellites/jupsatdata.html Scott Sheppard on Jupiter's satellites]
-*[http://www.ifa.hawaii.edu/faculty/jewitt/jmoons/jmoons.html Jupiter's new moons (discovered in 2000)]
-*[http://www.ifa.hawaii.edu/~sheppard/satellites/jup.html Jupiter's new moons (discovered in 2002)]
-*[http://www.ifa.hawaii.edu/~sheppard/satellites/jup2003.html Jupiter's new moons (discovered in 2003)]
-===Saturn's moons===
-*[http://www.news.cornell.edu/releases/Oct00/Saturn.moons.deb.html Saturn's new moons (discovered in 2000)]
-*[http://www.ifa.hawaii.edu/~sheppard/satellites/sat2003.html Saturn's new moon (discovered in 2003)]
-===All moons===
-*[http://ssd.jpl.nasa.gov/?sat_phys_par Natural Satellite Physical Parameters] (JPL-NASA, with refs—last updated July 2006)
-*[http://www.planetary.org/explore/topics/compare_the_planets/moon_numbers.html Moons of the Solar System] (The Planetary Society, as of July 2006)
-*[http://www.dtm.ciw.edu/sheppard/satellites/ Scott Sheppard's page]
-*[http://www.hudsonfla.com/spaceplanets.htm Major moons in order from the Sun]
-*[http://ssd.jpl.nasa.gov JPL's Solar System Dynamics page]
-*[http://www.space.com/scienceastronomy/planet_photo_040910.html Moon of an Object? First Photo of Satellite Beyond the Solar System]
-*[http://planetarynames.wr.usgs.gov/append7.html USGS list of named moons]
-*[http://www.newscientistspace.com/article.ns?id=dn9336&feedId=online-news_rss20 Upper size limit for moons explained]
-*[http://www.johnstonsarchive.net/astro/asteroidmoons.html Asteroids with Satellites]
-{{Solar System moons}}
-{{Solar System}}
-[[Category:Moons| ]]
-[[af:Natuurlike satelliet]]
-[[als:Satellit (Astronomie)]]
-[[ar:قمر طبيعي]]
-[[bs:Prirodni satelit]]
-[[br:Adplanedenn]]
-[[bg:Естествен спътник]]
-[[ca:Satèl·lit natural]]
-[[cs:Měsíc (satelit)]]
-[[da:Måne]]
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-[[et:Planeedi kaaslane]]
-[[es:Satélite natural]]
-[[eo:Natura satelito]]
-[[eu:Satelite]]
-[[fr:Satellite naturel]]
-[[gl:Satélite natural]]
-[[zh-classical:衛星]]
-[[ko:자연위성]]
-[[hr:Prirodni satelit]]
-[[id:Satelit alami]]
-[[is:Fylgihnöttur]]
-[[it:Satellite naturale]]
-[[he:ירח]]
-[[ht:Satelit]]
-[[la:Satelles]]
-[[lv:Dabiskais pavadonis]]
-[[lb:Mound]]
-[[lt:Palydovas]]
-[[li:Maon (satelliet)]]
-[[hu:Hold (égitesttípus)]]
-[[mk:Сателит]]
-[[mr:नैसर्गिक उपग्रह]]
-[[ms:Satelit semula jadi]]
-[[nl:Natuurlijke maan]]
-[[ja:衛星]]
-[[no:Naturlig satellitt]]
-[[nn:Naturleg satellitt]]
-[[nds:Maand (Astronomie)]]
-[[pl:Naturalny satelita]]
-[[pt:Satélite natural]]
-[[ksh:Moond (Astronomie)]]
-[[qu:Killa Satiliti]]
-[[ru:Спутники планет]]
-[[simple:Natural satellite]]
-[[sk:Mesiac (družica)]]
-[[sl:Naravni satelit]]
-[[fi:Kuu (yleisnimi)]]
-[[sr:Природни сателит]]
-[[sv:Måne]]
-[[tl:Likas na satelayt]]
-[[ta:துணைக்கோள்]]
-[[th:ดวงจันทร์บริวาร]]
-[[vi:Vệ tinh tự nhiên]]
-[[tr:Uydu]]
-[[uk:Супутник]]
-[[ur:قدرتی سیارچہ]]
-[[zh-yue:衞星]]
-[[zh:衛星 (天體)]]