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A characteristic satellite or moon is, inside the most average utilization, A galactic body that circles a planet or planetoid.
In the system there square measure six planetary satellite systems containing one hundred seventy five well-known natural satellites. Four IAU-listed dwarf planets also are well-known to possess natural satellites: Pluto, Haumea, Makemake, and Eris. As of Oct 2016, there square measure over three hundred minor planets well-known to possess moons.
The Earth–Moon system is exclusive in this the quantitative relation of the mass of the Moon to the mass of Earth is far bigger than that of the other natural-satellite–planet quantitative relation within the system. At 3,474 kilometre across, Earth's Moon is zero.27 times the diameter of Earth.
Terminology
The first well-known natural satellite was the Moon, however it absolutely was thought-about a planet till Copernicus' introduction of heliocentrism in 1543. till the invention of the Galilean satellites in 1610, however, there was no chance for pertaining to such objects as a category. astronomer selected to confer with his discoveries as Planetæ, however later discoverers selected alternative terms to differentiate them from the objects they orbited.
The first to use of the term satellite Delawarescribe|to explain} orbiting bodies was the German uranologist stargazer in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus in 1610. He derived the term from the Latin word satelles, that means guard, attendant, or companion, as a result of the satellites accompanied their primary planet in their journey through the heavens.
The term satellite so became the traditional one for pertaining to AN object orbiting a planet, because it avoided the anomaly of moon. In 1957, however, the launching of the substitute object orbiter created a desire for brand spanking new nomenclature. The terms synthetic satellite and artificial moon were terribly quickly abandoned in favor of the easier satellite, and as a consequence, the term has become connected primarily with artificial objects flown in house – together with, sometimes, even those not in orbit around a planet.
Because of this shift in that means, the term moon, that had continued to be employed in a generic sense in works of widespread science and in fiction, has regained honourableness and is currently used interchangeably with natural satellite, even in scientific articles. once it's necessary to avoid each the anomaly of confusion with Earth's natural satellite the Moon and also the natural satellites of the opposite planets on the one hand, and artificial satellites on the opposite, the term natural satellite is employed. To more avoid ambiguity, the convention is to capitalize the word Moon once pertaining to Earth's natural satellite, however not once pertaining to alternative natural satellites.
Many creators diagram satellite or normal satellite as circling some planet or planetoid, substitutable with moon by such a definition every single regular satellite square measure moons, however Earth and option planets aren't satellites. Many recent authors outline moon as a satellite of a planet or planetoid, and planet as a satellite of a star such authors think about Earth as a natural satellite of the sun.
Definition of a moon
There is no established lower limit on what's thought-about a moon. each natural heavenly body with AN known orbit around a planet of the system, some as tiny as a kilometre across, has been thought-about a moon, although objects a tenth that size at intervals Saturn's rings, that haven't been directly determined, are known as moonlets. tiny asteroid moons, like Dactyl, have conjointly been known as moonlets.
The higher limit is additionally imprecise. 2 orbiting bodies square measure generally represented as a double body instead of primary and satellite. Asteroids like ninety Antiope square measure thought-about double asteroids, however they need not forced a transparent definition of what constitutes a moon. Some authors think about the Pluto–Charon system to be a double planet. the foremost common demarcation on what's thought-about a moon rests upon whether or not the barycentre is below the surface of the larger body, although this is often somewhat capricious, as a result of it depends on distance additionally as relative mass.
Origin and orbital characteristics
The natural satellites orbiting comparatively near the world on prograde, uninclined circular orbits square measure typically thought to possess been shaped out of identical collapsing region of the protoplanetary disk that created its primary. In distinction, irregular satellites square measure thought to be captured asteroids probably more fragmented by collisions. Most of the foremost natural satellites of the system have regular orbits, whereas most of the tiny natural satellites have irregular orbits. The Moon and probably Charon square measure exceptions among giant bodies in this they're thought to possess originated by the collision of 2 giant proto-planetary objects. the fabric that will are placed in orbit round the centrosome is foretold to possess reaccreted to make one or additional orbiting natural satellites. As against planetary-sized bodies, asteroid moons square measure thought to normally kind by this method. Triton is another exception; though giant and in an exceedingly shut, circular orbit, its motion is retrograde and it's thought to be a captured dwarf planet.
Tidal protection
Most regular moons within the system square measure tidally barred to their several primaries, that means that identical aspect of the natural satellite invariably faces its planet. the sole well-known exception is Saturn's natural satellite Hyperion, that rotates chaotically attributable to the gravitative influence of Titan.
In distinction, the outer natural satellites of the enormous planets square measure too remote to possess become barred. for instance, Jupiter's Himalia, Saturn's Phoebe, and Neptune's sea nymph have rotation periods within the vary of 10 hours, whereas their orbital periods square measure many days.
Satellites of satellites
No moons of moons square measure presently referred to as of 2017. In most cases, the periodic event effects of the world would build such a system unstable.
However, calculations performed when the recent detection of a doable ring system around Saturn's moon Rhea indicate that satellites orbiting Rhea might have stable orbits. what is more, the suspected rings square measure thought to be slender, a development commonly related to shepherd moons. However, targeted pictures taken by the Cassini orbiter did not discover rings around Rhea.
It has conjointly been planned that Saturn's moon Iapetus had a satellite within the past; this is often one among many hypotheses that are suggests to account for its equatorial ridge.
Trojan satellites
Two natural satellites square measure well-known to possess tiny companions at each their L4 and L5 Lagrangian points, sixty degrees ahead and behind the body in its orbit. These companions square measure known as trojan moons, as their orbits square measure analogous to the trojan asteroids of Jupiter. The trojan moons square measure Telesto and Calypso, that square measure the leading and following companions, severally, of the Saturnian moon Tethys; and Helene and Polydeuces, the leading and following companions of the Saturnian moon Dione.
Asteroid satellites
The discovery of 243 Ida's natural satellite Dactyl within the early Nineteen Nineties confirmed that some asteroids have natural satellites; so, eighty seven Sylvia has 2. Some, like ninety Antiope, square measure double asteroids with 2 comparably sized parts.
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