Mass
Mass is both a property of a physical body and a measure of its resistance to acceleration when a net force is applied. The object's mass determines the strength of its gravitational attraction to other bodies; the basic SI unit of mass is the kilogram. In physics, mass is not the same as weight though mass is determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass; this is because weight is a force, while mass is the property that determines the strength of this force. There are several distinct phenomena. Although some theorists have speculated that some of these phenomena could be independent of each other, current experiments have found no difference in results regardless of how it is measured: Inertial mass measures an object's resistance to being accelerated by a force. Active gravitational mass measures the gravitational force exerted by an object.
Passive gravitational mass measures the gravitational force exerted on an object in a known gravitational field. The mass of an object determines its acceleration in the presence of an applied force; the inertia and the inertial mass describe the same properties of physical bodies at the qualitative and quantitative level by other words, the mass quantitatively describes the inertia. According to Newton's second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A body's mass determines the degree to which it generates or is affected by a gravitational field. If a first body of mass mA is placed at a distance r from a second body of mass mB, each body is subject to an attractive force Fg = GmAmB/r2, where G = 6.67×10−11 N kg−2 m2 is the "universal gravitational constant". This is sometimes referred to as gravitational mass. Repeated experiments since the 17th century have demonstrated that inertial and gravitational mass are identical.
The standard International System of Units unit of mass is the kilogram. The kilogram is 1000 grams, first defined in 1795 as one cubic decimeter of water at the melting point of ice. However, because precise measurement of a decimeter of water at the proper temperature and pressure was difficult, in 1889 the kilogram was redefined as the mass of the international prototype kilogram of cast iron, thus became independent of the meter and the properties of water. However, the mass of the international prototype and its identical national copies have been found to be drifting over time, it is expected that the re-definition of the kilogram and several other units will occur on May 20, 2019, following a final vote by the CGPM in November 2018. The new definition will use only invariant quantities of nature: the speed of light, the caesium hyperfine frequency, the Planck constant. Other units are accepted for use in SI: the tonne is equal to 1000 kg. the electronvolt is a unit of energy, but because of the mass–energy equivalence it can be converted to a unit of mass, is used like one.
In this context, the mass has units of eV/c2. The electronvolt and its multiples, such as the MeV, are used in particle physics; the atomic mass unit is 1/12 of the mass of a carbon-12 atom 1.66×10−27 kg. The atomic mass unit is convenient for expressing the masses of molecules. Outside the SI system, other units of mass include: the slug is an Imperial unit of mass; the pound is a unit of both mass and force, used in the United States. In scientific contexts where pound and pound need to be distinguished, SI units are used instead; the Planck mass is the maximum mass of point particles. It is used in particle physics; the solar mass is defined as the mass of the Sun. It is used in astronomy to compare large masses such as stars or galaxies; the mass of a small particle may be identified by its inverse Compton wavelength. The mass of a large star or black hole may be identified with its Schwarzschild radius. In physical science, one may distinguish conceptually between at least seven different aspects of mass, or seven physical notions that involve the concept of mass.
Every experiment to date has shown these seven values to be proportional, in some cases equal, this proportionality gives rise to the abstract concept of mass. There are a number of ways mass can be measured or operationally defined: Inertial mass is a measure of an object's resistance to acceleration when a force is applied, it is determined by applying a force to an object and measuring the acceleration that results from that force. An object with small inertial mass will accelerate more than an object with large inertial mass when acted upon by the same force. One says. Active gravitational mass is a measure of the strength of an object's gravitational flux. Gravitational field can be measured by allowing a small "test object" to fall and measuring its free-fall acceleration. For example, an object in free fall near the Moon is subject to a smaller gravitational field, hence
Rings of Jupiter
The planet Jupiter has a system of rings known as the rings of Jupiter or the Jovian ring system. It was the third ring system to be discovered after those of Saturn and Uranus, it was first observed in 1979 by the Voyager 1 space probe and investigated in the 1990s by the Galileo orbiter. It has been observed by the Hubble Space Telescope and from Earth for several years. Ground-based observation of the rings requires the largest available telescopes; the Jovian ring system is faint and consists of dust. It has four main components: a thick inner torus of particles known as the "halo ring"; the main and halo rings consist of dust ejected from the moons Metis and other unobserved parent bodies as the result of high-velocity impacts. High-resolution images obtained in February and March 2007 by the New Horizons spacecraft revealed a rich fine structure in the main ring. In visible and near-infrared light, the rings have a reddish color, except the halo ring, neutral or blue in color; the size of the dust in the rings varies, but the cross-sectional area is greatest for nonspherical particles of radius about 15 μm in all rings except the halo.
The halo ring is dominated by submicrometre dust. The total mass of the ring system is poorly known, but is in the range of 1011 to 1016 kg; the age of the ring system is not known. A ring could exist in Himalia's orbit. One possible explanation is that a small moon had crashed into Himalia and the force of the impact caused material to blast off Himalia. Jupiter's ring system was the third to be discovered in the Solar System, after those of Saturn and Uranus, it was first observed in 1979 by the Voyager 1 space probe. It is composed of four main components: a thick inner torus of particles known as the "halo ring"; the principal attributes of the known Jovian Rings are listed in the table. The narrow and thin main ring is the brightest part of Jupiter's ring system, its outer edge is located at a radius of about 129000 km and coincides with the orbit of Jupiter's smallest inner satellite, Adrastea. Its inner edge is located at about 122500 km, thus the width of the main ring is around 6500 km. The appearance of the main ring depends on the viewing geometry.
In forward-scattered light the brightness of the main ring begins to decrease steeply at 128600 km and reaches the background level at 129300 km—just outward of the Adrastean orbit. Therefore, Adrastea at 129000 km shepherds the ring; the brightness continues to increase in the direction of Jupiter and has a maximum near the ring's center at 126000 km, although there is a pronounced gap near the Metidian orbit at 128000 km. The inner boundary of the main ring, in contrast, appears to fade off from 124000 to 120000 km, merging into the halo ring. In forward-scattered light all Jovian rings are bright. In back-scattered light the situation is different; the outer boundary of the main ring, located at 129100 km, or beyond the orbit of Adrastea, is steep. The orbit of the moon is marked by a gap in the ring so there is a thin ringlet just outside its orbit. There is another ringlet just inside Adrastean orbit followed by a gap of unknown origin located at about 128500 km; the third ringlet is found inward outside the orbit of Metis.
The ring's brightness drops just outward of the Metidian orbit, forming the Metis notch. Inward of the orbit of Metis, the brightness of the ring rises much less than in forward-scattered light. So in the back-scattered geometry the main ring appears to consist of two different parts: a narrow outer part extending from 128000 to 129000 km, which itself includes three narrow ringlets separated by notches, a fainter inner part from 122500 to 128000 km, which lacks any visible structure like in the forward-scattering geometry; the Metis notch serves as their boundary. The fine structure of the main ring was discovered in data from the Galileo orbiter and is visible in back-scattered images obtained from New Horizons in February–March 2007; the early observations by Hubble Space Telescope and the Cassini spacecraft failed to detect it due to insufficient spatial resolution. However the fine structure was observed by the Keck telescope using adaptive optics in 2002–2003. Observed in back-scattered light the main ring appears to be razor thin, extending in the vertical direction no more than 30 km.
In the side scatter geometry the ring thickness is 80–160 km, increasing somewhat in the direction of Jupiter. The ring appears to be much thicker in the forward-scattered light—about 300 km. One of the discoveries of the Galileo orbiter was the bloom of the main ring—a faint thick cloud of material which surrounds its inner part; the bloom grows in thickness towards the inner boundary of the main ring, where it transitions into the halo. Detailed analysis of the Galileo images revealed longitudinal variations of the main ring's brightness unconnected with the viewing geometry; the Galileo images showed some patchiness in the ring on the scales 500–1000 km. In February–March 2007 New Horizons spacecraft conducted a deep search for new small moons inside the mai
James Lick telescope
The James Lick Telescope is a refracting telescope built in 1888. It has a lens 36 inches in diameter- a major achievement in its day; the instrument remains in operation and public viewing is allowed on a limited basis. Called the "Great Lick Refractor" or "Lick Refractor", it was the largest refracting telescope in the world until 1897 and now ranks third, after the 40-inch unit at the Yerkes Observatory and the Swedish 1-m Solar Telescope; the telescope is located at the University of California's Lick Observatory atop Mount Hamilton at an elevation of 4,209 feet above sea level. The instrument is housed inside a dome, powered by hydraulic systems that raise and lower the floor, rotate the dome and drive the clock mechanism to track the Earth's rotation; the original hydraulic arrangement still operates today, with the exception that the original wind-powered pumps that once filled the reservoirs have been replaced with electric pumps. James Lick is entombed below the floor of the observing room of the telescope.
Here are some excerpts from an 1894 book describing the telescope: The height of the marble floor of the main building above mean sea level is 4209 feet. On a connected peak half a mile to the east of the Observatory, 50 feet higher, are the reservoirs from which water for household and photographic purposes is distributed. A spring about 350 feet below and one mile to the northeast of the Observatory supplies excellent water. Another peak seven-eighths of a mile to the east is the summit of Mount Hamilton; this system receives its supply from the winter rains falling on the roofs. The movable floor in the dome is the first of the kind to be constructed, it is 60 feet in diameter, can be raised or lowered through a distance of 16 1⁄2 feet, its purpose being to bring the observer within convenient reach of the eye end of the telescope. The fabrication of the two-element achromatic objective lens, the largest lens made at the time, caused years of delay; the famous large telescope maker Alvan Clark was in charge of the optical design.
He gave the contract for casting the high quality optical glass blanks, of a size never before attempted, to the firm of Charles Feil in Paris. One of the huge glass disks broke during shipping, making a replacement was delayed. After 18 failed attempts, the lens was finished, transported safely across country, on December 31, 1887, was installed in the telescope tube; the builders had to wait for three days for a break in the clouds to test it. On the evening of January 3 the telescope saw first light, users found that the instrument couldn't be focused. An error in the estimation of the lens' focal length had caused the tube to be built too long. A hacksaw was procured, the great tube was unceremoniously cut back to the proper length and the star Aldebaran came into focus; these are some of the discoveries made with the Lick telescope, as described in the same 1894 book: Amalthea, the fifth satellite of Jupiter was discovered in September, 1892. The speed of the planetary nebulae in their motions through space is of the same order of magnitude as the speed of the stars.
Twenty-five comets—17 unexpected and 8 periodic—have been discovered. The unequaled Lick series of comet photographs has taught us more as to the structure and dissolution of comets' tails than had been learned in all previous time. About 1300 new double stars have been discovered; the period of revolution of the double star delta Equulei has been shown to be 53⁄4 years, the shortest period known for any double star being 11.4 years. It is therefore in many ways the most interesting double star under observation. Spectroscopic observations have shown that the atmosphere of Mars is of low density—probably much less dense at the surface of Mars than the Earth's atmosphere at the summit of the highest peak in the Himalayas; the average speed of the brighter stars is 21 miles per second. The North Polar Star was found to be a triple star, in 1899, by means of spectroscopic observations. Two of its members are invisible in our largest telescopes; the bright star and one dark companion revolve around each other in four days.
Capella was discovered, in 1899, to be a spectroscopic binary star, period 104 days, the two nearly equal components being inseparable in our largest telescopes. About 40 spectroscopic binaries—that is, stars seen single in ordinary telescopes, but proven to be double by means of the spectroscope—were discovered in 1898–1902. At least one star in seven has an invisible component, observable thus far only by spectroscopic means. About 10,000 nebulae have been discovered in the past at the various observatories; these photographs led to the unexpected discovery that the majority of the nebulae have a spiral form—undoubted evidence of their rotation. The light of the inner portion of the solar corona is inherent, whereas the light of the outer portion is reflected sunlight, as proven at the Sumatra eclipse by means of spectroscopic and polariscopic observations, it has been shown. The extraordinary motion in the nebula surrounding Nova Persei was discovered from the photograph of November 7–8, 1901.
Many thousands of accurate positions of stars have been secured with the meridian circle. Extensive and accurate observations of double st
Moons of Jupiter
There are 79 known moons of Jupiter. This gives Jupiter the largest number of known moons with reasonably stable orbits of any planet in the Solar System, if one doesn't count the moonlets within Saturn's rings; the most massive of the moons are the four Galilean moons, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body, neither Earth nor the Sun. From the end of the 19th century, dozens of much smaller Jovian moons have been discovered and have received the names of lovers or daughters of the Roman god Jupiter or his Greek equivalent Zeus; the Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 75 known moons and the rings together comprising just 0.003% of the total orbiting mass. Of Jupiter's moons, eight are regular satellites with prograde and nearly circular orbits that are not inclined with respect to Jupiter's equatorial plane; the Galilean satellites are nearly spherical in shape due to their planetary mass, so would be considered at least dwarf planets if they were in direct orbit around the Sun.
The other four regular satellites are closer to Jupiter. The remainder of Jupiter's moons are irregular satellites whose prograde and retrograde orbits are much farther from Jupiter and have high inclinations and eccentricities; these moons were captured by Jupiter from solar orbits. Twenty-seven of the irregular satellites have not yet been named; the physical and orbital characteristics of the moons vary widely. The four Galileans are all over 3,100 kilometres in diameter. All other Jovian moons are less than 250 kilometres in diameter, with most exceeding 5 kilometres, their orbital shapes range from nearly circular to eccentric and inclined, many revolve in the direction opposite to Jupiter's spin. Orbital periods range to some three thousand times more. Jupiter's regular satellites are believed to have formed from a circumplanetary disk, a ring of accreting gas and solid debris analogous to a protoplanetary disk, they may be the remnants of a score of Galilean-mass satellites that formed early in Jupiter's history.
Simulations suggest that, while the disk had a high mass at any given moment, over time a substantial fraction of the mass of Jupiter captured from the solar nebula was passed through it. However, only 2% of the proto-disk mass of Jupiter is required to explain the existing satellites, thus there may have been several generations of Galilean-mass satellites in Jupiter's early history. Each generation of moons might have spiraled into Jupiter, because of drag from the disk, with new moons forming from the new debris captured from the solar nebula. By the time the present generation formed, the disk had thinned so that it no longer interfered with the moons' orbits; the current Galilean moons were still affected, falling into and being protected by an orbital resonance with each other, which still exists for Io, Ganymede. Ganymede's larger mass means that it would have migrated inward at a faster rate than Io; the outer, irregular moons are thought to have originated from captured asteroids, whereas the protolunar disk was still massive enough to absorb much of their momentum and thus capture them into orbit.
Many are believed to have broken up by mechanical stresses during capture, or afterward by collisions with other small bodies, producing the moons we see today. Some scholars propose that the earliest record of a Jovian moon is a note by Chinese astronomer Gan De of an observation around 364 BC. However, the first certain observations of Jupiter's satellites were those of Galileo Galilei in 1609. By January 1610, he had sighted the four massive Galilean moons with his 30× magnification telescope, he published his results in March 1610. Simon Marius had independently discovered the moons one day after Galileo, although he did not publish his book on the subject until 1614. So, the names Marius assigned are used today: Ganymede. No additional satellites were discovered until E. E. Barnard observed Amalthea in 1892. With the aid of telescopic photography, further discoveries followed over the course of the 20th century. Himalia was discovered in 1904, Elara in 1905, Pasiphae in 1908, Sinope in 1914, Lysithea and Carme in 1938, Ananke in 1951, Leda in 1974.
By the time that the Voyager space probes reached Jupiter, around 1979, 13 moons had been discovered, not including Themisto, observed in 1975, but was lost until 2000 due to insufficient initial observation data. The Voyager spacecraft discovered an additional three inner moons in 1979: Metis. No additional moons were discovered for two decades during the 1980s and 1990s, but between October 1999 and February 2003, researchers found another 34 moons using sensitive ground-based detectors; these are tiny moons, in long, eccentric retrograde orbits, averaging 3 km in diameter, with the largest being just 9 km across. All of these moons are thought to have been captured asteroidal or comet bodies fragmented into several pieces. By 2015, a total of 15 additional moons were discovered. Two more were discovered in 2016 by the team led by Scott S. Sheppard at the Carnegie Institution for
International Astronomical Union
The International Astronomical Union is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the internationally recognized authority for assigning designations and names to celestial bodies and any surface features on them; the IAU is a member of the International Council for Science. Its main objective is to promote and safeguard the science of astronomy in all its aspects through international cooperation; the IAU maintains friendly relations with organizations that include amateur astronomers in their membership. The IAU has its head office on the second floor of the Institut d'Astrophysique de Paris in the 14th arrondissement of Paris. Working groups include the Working Group for Planetary System Nomenclature, which maintains the astronomical naming conventions and planetary nomenclature for planetary bodies, the Working Group on Star Names, which catalogs and standardizes proper names for stars.
The IAU is responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams. The Minor Planet Center operates under the IAU, is a "clearinghouse" for all non-planetary or non-moon bodies in the Solar System; the Working Group for Meteor Shower Nomenclature and the Meteor Data Center coordinate the nomenclature of meteor showers. The IAU was founded on 28 July 1919, at the Constitutive Assembly of the International Research Council held in Brussels, Belgium. Two subsidiaries of the IAU were created at this assembly: the International Time Commission seated at the International Time Bureau in Paris and the International Central Bureau of Astronomical Telegrams seated in Copenhagen, Denmark; the 7 initial member states were Belgium, France, Great Britain, Greece and the United States, soon to be followed by Italy and Mexico. The first executive committee consisted of Benjamin Baillaud, Alfred Fowler, four vice presidents: William Campbell, Frank Dyson, Georges Lecointe, Annibale Riccò.
Thirty-two Commissions were appointed at the Brussels meeting and focused on topics ranging from relativity to minor planets. The reports of these 32 Commissions formed the main substance of the first General Assembly, which took place in Rome, Italy, 2–10 May 1922. By the end of the first General Assembly, ten additional nations had joined the Union, bringing the total membership to 19 countries. Although the Union was formed eight months after the end of World War I, international collaboration in astronomy had been strong in the pre-war era; the first 50 years of the Union's history are well documented. Subsequent history is recorded in the form of reminiscences of past IAU Presidents and General Secretaries. Twelve of the fourteen past General Secretaries in the period 1964-2006 contributed their recollections of the Union's history in IAU Information Bulletin No. 100. Six past IAU Presidents in the period 1976–2003 contributed their recollections in IAU Information Bulletin No. 104. The IAU includes a total of 12,664 individual members who are professional astronomers from 96 countries worldwide.
83% of all individual members are male, while 17% are female, among them the union's former president, Mexican astronomer Silvia Torres-Peimbert. Membership includes 79 national members, professional astronomical communities representing their country's affiliation with the IAU. National members include the Australian Academy of Science, the Chinese Astronomical Society, the French Academy of Sciences, the Indian National Science Academy, the National Academies, the National Research Foundation of South Africa, the National Scientific and Technical Research Council, KACST, the Council of German Observatories, the Royal Astronomical Society, the Royal Astronomical Society of New Zealand, the Royal Swedish Academy of Sciences, the Russian Academy of Sciences, the Science Council of Japan, among many others; the sovereign body of the IAU is its General Assembly. The Assembly determines IAU policy, approves the Statutes and By-Laws of the Union and elects various committees; the right to vote on matters brought before the Assembly varies according to the type of business under discussion.
The Statutes consider such business to be divided into two categories: issues of a "primarily scientific nature", upon which voting is restricted to individual members, all other matters, upon which voting is restricted to the representatives of national members. On budget matters, votes are weighted according to the relative subscription levels of the national members. A second category vote requires a turnout of at least two-thirds of national members in order to be valid. An absolute majority is sufficient for approval in any vote, except for Statute revision which requires a two-thirds majority. An equality of votes is resolved by the vote of the President of the Union. Since 1922, the IAU General Assembly meets every three years, with the ex
Orbital eccentricity
The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values between 0 and 1 form an elliptic orbit, 1 is a parabolic escape orbit, greater than 1 is a hyperbola; the term derives its name from the parameters of conic sections, as every Kepler orbit is a conic section. It is used for the isolated two-body problem, but extensions exist for objects following a Klemperer rosette orbit through the galaxy. In a two-body problem with inverse-square-law force, every orbit is a Kepler orbit; the eccentricity of this Kepler orbit is a non-negative number. The eccentricity may take the following values: circular orbit: e = 0 elliptic orbit: 0 < e < 1 parabolic trajectory: e = 1 hyperbolic trajectory: e > 1 The eccentricity e is given by e = 1 + 2 E L 2 m red α 2 where E is the total orbital energy, L is the angular momentum, mred is the reduced mass, α the coefficient of the inverse-square law central force such as gravity or electrostatics in classical physics: F = α r 2 or in the case of a gravitational force: e = 1 + 2 ε h 2 μ 2 where ε is the specific orbital energy, μ the standard gravitational parameter based on the total mass, h the specific relative angular momentum.
For values of e from 0 to 1 the orbit's shape is an elongated ellipse. The limit case between an ellipse and a hyperbola, when e equals 1, is parabola. Radial trajectories are classified as elliptic, parabolic, or hyperbolic based on the energy of the orbit, not the eccentricity. Radial orbits hence eccentricity equal to one. Keeping the energy constant and reducing the angular momentum, elliptic and hyperbolic orbits each tend to the corresponding type of radial trajectory while e tends to 1. For a repulsive force only the hyperbolic trajectory, including the radial version, is applicable. For elliptical orbits, a simple proof shows that arcsin yields the projection angle of a perfect circle to an ellipse of eccentricity e. For example, to view the eccentricity of the planet Mercury, one must calculate the inverse sine to find the projection angle of 11.86 degrees. Next, tilt any circular object by that angle and the apparent ellipse projected to your eye will be of that same eccentricity; the word "eccentricity" comes from Medieval Latin eccentricus, derived from Greek ἔκκεντρος ekkentros "out of the center", from ἐκ- ek-, "out of" + κέντρον kentron "center".
"Eccentric" first appeared in English in 1551, with the definition "a circle in which the earth, sun. Etc. deviates from its center". By five years in 1556, an adjectival form of the word had developed; the eccentricity of an orbit can be calculated from the orbital state vectors as the magnitude of the eccentricity vector: e = | e | where: e is the eccentricity vector. For elliptical orbits it can be calculated from the periapsis and apoapsis since rp = a and ra = a, where a is the semimajor axis. E = r a − r p r a + r p = 1 − 2 r a r p + 1 where: ra is the radius at apoapsis. Rp is the radius at periapsis; the eccentricity of an elliptical orbit can be used to obtain the ratio of the periapsis to the apoapsis: r p r a = 1 − e 1 + e For Earth, orbital eccentricity ≈ 0.0167, apoapsis= aphelion and periapsis= perihelion relative to sun. For Earth's annual orbit path, ra/rp ratio = longest_radius / shortest_radius ≈ 1.034 relative to center point of path. The eccentricity of the Earth's orbit is about 0.0167.
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Natural satellite
A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet. In the Solar System there are six planetary satellite systems containing 185 known natural satellites. Four IAU-listed dwarf planets are known to have natural satellites: Pluto, Haumea and Eris; as of September 2018, there are 334 other minor planets known to have moons. The Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km across, the Moon is 0.27 times the diameter of Earth. The first known natural satellite was the Moon, but it was considered a "planet" until Copernicus' introduction of De revolutionibus orbium coelestium in 1543; until the discovery of the Galilean satellites in 1610, there was no opportunity for referring to such objects as a class. Galileo chose to refer to his discoveries as Planetæ, but discoverers chose other terms to distinguish them from the objects they orbited.
The first to use of the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus in 1610. He derived the term from the Latin word 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, the launching of the artificial object Sputnik created a need for new terminology. Sputnik was created by Soviet Union, it was the first satellite ever; the terms man-made satellite and artificial moon were quickly abandoned in favor of the simpler satellite, as a consequence, the term has become linked with artificial objects flown in space – including, sometimes those not in orbit around a planet. Because 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 natural satellite in scientific articles.
When it is necessary to avoid both the ambiguity of confusion with Earth's natural satellite the Moon and the natural satellites of the other planets on the one hand, artificial satellites on the other, the term natural satellite is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite, but not when referring to other natural satellites. Many authors define "satellite" or "natural satellite" as orbiting some planet or minor planet, synonymous with "moon" – by such a definition all natural satellites are moons, but Earth and other planets are not satellites. A few recent authors define "moon" as "a satellite of a planet or minor planet", "planet" as "a satellite of a star" – such authors consider Earth as a "natural satellite of the sun". There is no established lower limit on what is considered a "moon"; every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn's rings, which have not been directly observed, have been called moonlets.
Small asteroid moons, such as Dactyl, have been called moonlets. The upper limit is vague. Two orbiting bodies are sometimes described as a double planet rather than satellite. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon; some authors consider the Pluto–Charon system to be a double 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, because it depends on distance as well as relative mass; the natural satellites orbiting close to the planet on prograde, uninclined circular orbits are thought to have been formed out of the same collapsing region of the protoplanetary disk that created its primary. In contrast, irregular satellites are thought to be captured asteroids further fragmented by collisions. Most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits.
The Moon and Charon are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects. The material that would have been placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites; as opposed to planetary-sized bodies, asteroid moons are thought to form by this process. Triton is another exception; the capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, temporary satellites should be a common phenomenon; the only observed example is 2006 RH120, a temporary satellite of Earth for nine months in 2006 and 2007. Most regular moons (natural satellites following close and prograde orbits with small orb
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