50-451: Heinrich Wilhelm Matthias Olbers ( / ˈ ɔː l b ər z / ; German: [ˈɔlbɐs] ; 11 October 1758 – 2 March 1840) was a German astronomer . He found a convenient method of calculating the orbit of comets, and in 1802 and 1807, discovered the second and the fourth asteroids Pallas and Vesta . Olbers was born in Arbergen , Germany, today part of Bremen , and studied to be
100-543: A physician at Göttingen (1777–80). While he was at Göttingen, he studied mathematics with Abraham Gotthelf Kästner . In 1779, while attending to a sick fellow student, he devised a method of calculating cometary orbits which made an epoch in the treatment of the subject, as it was the first satisfactory method of calculating cometary orbits. After his graduation in 1780, he began practicing medicine in Bremen. At night he dedicated his time to astronomical observation, making
150-463: 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 natural satellites massive enough to have achieved hydrostatic equilibrium , and Uranus has five. It has been suggested that some satellites may potentially harbour life . Among the objects generally agreed by astronomers to be dwarf planets, Ceres and Sedna have no known natural satellites. Pluto has
200-427: A few hours a month to stargazing and reading the latest developments in research. However, amateurs span the range from so-called "armchair astronomers" to the highly ambitious people who own science-grade telescopes and instruments with which they are able to make their own discoveries, create astrophotographs , and assist professional astronomers in research. Natural satellite A natural satellite is, in
250-438: A few weeks per year. Analysis of observed phenomena, along with making predictions as to the causes of what they observe, takes the majority of observational astronomers' time. Astronomers who serve as faculty spend much of their time teaching undergraduate and graduate classes. Most universities also have outreach programs, including public telescope time and sometimes planetariums , as a public service to encourage interest in
300-478: A generic sense in works of popular science and fiction, has regained respectability and is now used interchangeably with natural satellite , even 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, and artificial satellites on the other, the term natural satellite (using "natural" in
350-433: A global subsurface ocean of liquid water. Besides planets and dwarf planets objects within our Solar System known to have natural satellites are 76 in the asteroid belt (five with two each), four Jupiter trojans , 39 near-Earth objects (two with two satellites each), and 14 Mars-crossers . There are also 84 known natural satellites of trans-Neptunian objects . Some 150 additional small bodies have been observed within
400-466: A natural satellite of a planet) are currently known. In most cases, the tidal effects of the planet would make such a system unstable. However, calculations performed after the 2008 detection of a possible ring system around Saturn's moon Rhea indicate that satellites orbiting Rhea could have stable orbits. Furthermore, the suspected rings are thought to be narrow, a phenomenon normally associated with shepherd moons . However, targeted images taken by
450-438: A periodic comet , now named after him (formally designated 13P/Olbers ). Olbers' paradox , described by him in 1823 (and then reformulated in 1826), states that the darkness of the night sky conflicts with the supposition of an infinite and eternal static universe . In July 1804, the young Friedrich Wilhelm Bessel contacted Olbers to get his opinion of Bessel's treatise on orbit calculation of Halley's Comet . Olbers noticed
500-784: A planet or minor planet", and "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 (natural satellites of asteroids), such as Dactyl , have also been called moonlets. The upper limit
550-465: A se quatuor Iouis satellitibus erronibus ("Narration About Four Satellites of Jupiter Observed") 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
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#1732772236833600-535: A sense opposed to "artificial") is used. To further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earth's natural satellite (a proper noun ), but not when referring to other natural satellites ( common nouns ). 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
650-738: A system of stars or a galaxy to complete a life cycle, astronomers must observe snapshots of different systems at unique points in their evolution to determine how they form, evolve, and die. They use this data to create models or simulations to theorize how different celestial objects work. Further subcategories under these two main branches of astronomy include planetary astronomy , astrobiology , stellar astronomy , astrometry , galactic astronomy , extragalactic astronomy , or physical cosmology . Astronomers can also specialize in certain specialties of observational astronomy, such as infrared astronomy , neutrino astronomy , x-ray astronomy , and gravitational-wave astronomy . Historically , astronomy
700-505: Is a statue of Olbers of 1850 in the former rampart area in Bremen. Astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth . Astronomers observe astronomical objects , such as stars , planets , moons , comets and galaxies – in either observational (by analyzing the data) or theoretical astronomy . Examples of topics or fields astronomers study include planetary science , solar astronomy ,
750-463: Is also vague. Two orbiting bodies are sometimes described as a double planet rather than primary and 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 (dwarf) planet. The most common dividing line on what is considered a moon rests upon whether
800-479: Is known to be high enough that it is probably a solid ellipsoid as well.) The larger natural satellites, being tidally locked, tend toward ovoid (egg-like) shapes: squat at their poles and with longer equatorial axes in the direction of their primaries (their planets) than in the direction of their motion. Saturn's moon Mimas , for example, has a major axis 9% greater than its polar axis and 5% greater than its other equatorial axis. Methone , another of Saturn's moons,
850-419: Is not always permanent. According to simulations, temporary satellites should be a common phenomenon. The only observed examples are 1991 VG , 2006 RH 120 , 2020 CD 3 . 2006 RH 120 was a temporary satellite of Earth for nine months in 2006 and 2007. Most regular moons (natural satellites following relatively close and prograde orbits with small orbital inclination and eccentricity) in
900-519: Is only around 3 km in diameter and visibly egg-shaped . The effect is smaller on the largest natural satellites, where their gravity is greater relative to the effects of tidal distortion, especially those that orbit less massive planets or, as in the case of the Moon, at greater distances. Of the twenty known natural satellites in the Solar System that are large enough to be gravitationally rounded, several remain geologically active today. Io
950-420: Is the most volcanically active body in the Solar System, while Europa , Enceladus , Titan and Triton display evidence of ongoing tectonic activity and cryovolcanism . In the first three cases, the geological activity is powered by the tidal heating resulting from having eccentric orbits close to their giant-planet primaries. (This mechanism would have also operated on Triton in the past before its orbit
1000-605: The Cassini spacecraft failed to detect rings around Rhea. It has also been proposed that Saturn's moon Iapetus had a satellite in the past; this is one of several hypotheses that have been put forward to account for its equatorial ridge . Light-curve analysis suggests that Saturn's irregular satellite Kiviuq is extremely prolate, and is likely a contact binary or even a binary moon. Two natural satellites are known to have small companions at both their L 4 and L 5 Lagrangian points , sixty degrees ahead and behind
1050-754: The Saturn – Titan system at 0.044 (with the second mass ratio next to the Earth–Moon system, 1 to 4220), the Jupiter – Ganymede system at 0.038, and the Uranus – Titania system at 0.031. For the category of dwarf planets , Charon has the largest ratio, being 0.52 the diameter and 12.2% the mass of Pluto . 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
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#17327722368331100-492: The baptism of Napoleon II of France on 9 June 1811. He was a member of the corps legislatif in Paris 1812–13. He died in Bremen aged 81. He was twice married, and one son survived him. Olbers' paradox , the argument that the dark sky at night shows that stars cannot be evenly distributed through infinite space, is named for him, though others had also advanced it. The following celestial features are named for Olbers: There
1150-625: 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 relatively close to the planet on prograde , uninclined circular orbits ( regular satellites ) are generally thought to have been formed out of the same collapsing region of the protoplanetary disk that created 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. Most of
1200-568: The origin or evolution of stars , or the formation of galaxies . A related but distinct subject is physical cosmology , which studies the Universe as a whole. Astronomers usually fall under either of two main types: observational and theoretical . Observational astronomers make direct observations of celestial objects and analyze the data. In contrast, theoretical astronomers create and investigate models of things that cannot be observed. Because it takes millions to billions of years for
1250-520: The rings of Saturn , but only a few were tracked long enough to establish orbits. Planets around other stars are likely to have satellites as well, and although numerous candidates have been detected to date, none have yet been confirmed. Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos . The giant planets have extensive systems of natural satellites, including half
1300-545: The PhD level and beyond (as of 2024). Contrary to the classical image of an old astronomer peering through a telescope through the dark hours of the night, it is far more common to use a charge-coupled device (CCD) camera to record a long, deep exposure, allowing a more sensitive image to be created because the light is added over time. Before CCDs, photographic plates were a common method of observation. Modern astronomers spend relatively little time at telescopes, usually just
1350-513: The Solar System (those bigger than 2,500 km across) are Jupiter's Galilean moons (Ganymede, Callisto , Io, and Europa ), Saturn's moon Titan, Earth's moon, and Neptune's captured natural satellite Triton. Triton, the smallest of these, has more mass than all smaller natural satellites together. Similarly in the next size group of nine mid-sized natural satellites, between 1,000 km and 1,600 km across, Titania , Oberon , Rhea , Iapetus , Charon, Ariel , Umbriel , Dione , and Tethys,
1400-423: The Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. This phenomenon comes about through a loss of energy due to tidal forces raised by the planet, slowing the rotation of the satellite until it is negligible. Exceptions are known; one such exception is Saturn 's natural satellite Hyperion , which rotates chaotically because of
1450-433: 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 and artificial moon were very quickly abandoned in favor of the simpler satellite , and as a consequence, the term has become linked primarily with artificial objects flown in space. Because of this shift in meaning, the term moon , which had continued to be used in
1500-493: The body in its orbit. These companions are called trojan moons , as their orbits are analogous to the trojan asteroids of Jupiter . The trojan moons are Telesto and Calypso , which are the leading and following companions, respectively, of the Saturnian moon Tethys ; and Helene and Polydeuces , the leading and following companions of the Saturnian moon Dione . The discovery of 243 Ida 's natural satellite Dactyl in
1550-460: The case of Triton solar heating appears to provide the energy. Titan and Triton have significant atmospheres; Titan also has hydrocarbon lakes . All four of the Galilean moons have atmospheres, though they are extremely thin. Four of the largest natural satellites, Europa, Ganymede, Callisto , and Titan, are thought to have subsurface oceans of liquid water, while smaller Enceladus also supports
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1600-407: 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 commonly form by this process. Triton is another exception; although large and in a close, circular orbit, its motion is retrograde and it is thought to be a captured dwarf planet . The capture of an asteroid from a heliocentric orbit
1650-421: The core sciences, a competency examination, experience with teaching undergraduates and participating in outreach programs, work on research projects under the student's supervising professor, completion of a PhD thesis , and passing a final oral exam . Throughout the PhD training, a successful student is financially supported with a stipend . While there is a relatively low number of professional astronomers,
1700-539: 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 ("planets"), but later discoverers chose other terms to distinguish them from the objects they orbited. The first to use the term satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis
1750-611: The early 1990s confirmed that some asteroids have natural satellites; indeed, 87 Sylvia has two. Some, such as 90 Antiope , are double asteroids with two comparably sized components. Neptune's moon Proteus is the largest irregularly shaped natural satellite; the shapes of Eris' moon Dysnomia and Orcus ' moon Vanth are unknown. All other known natural satellites that are at least the size of Uranus's Miranda have lapsed into rounded ellipsoids under hydrostatic equilibrium , i.e. are "round/rounded satellites" and are sometimes categorized as planetary-mass moons . (Dysnomia's density
1800-474: The field is popular among amateurs . Most cities have amateur astronomy clubs that meet on a regular basis and often host star parties . The Astronomical Society of the Pacific is the largest general astronomical society in the world, comprising both professional and amateur astronomers as well as educators from 70 different nations. As with any hobby , most people who practice amateur astronomy may devote
1850-489: The field. Those who become astronomers usually have a broad background in physics, mathematics , sciences, and computing in high school. Taking courses that teach how to research, write, and present papers are part of the higher education of an astronomer, while most astronomers attain both a Master's degree and eventually a PhD degree in astronomy, physics or astrophysics . PhD training typically involves 5-6 years of study, including completion of upper-level courses in
1900-505: The gravitational influence of Titan . Pluto's four, circumbinary small moons also rotate chaotically due to Charon's influence. In contrast, the outer natural satellites of the giant planets (irregular satellites) are too far away to have become locked. For example, Jupiter's Himalia , Saturn's Phoebe , and Neptune's Nereid have rotation periods in the range of ten hours, whereas their orbital periods are hundreds of days. No "moons of moons" or subsatellites (natural satellites that orbit
1950-488: The major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon and the Moons of Pluto are exceptions among large bodies in that they are thought to have originated from the collision of two large protoplanetary objects early in the Solar System's history (see the giant impact hypothesis ). The material that would have been placed in orbit around
2000-694: The majority of their time working on research, although they quite often have other duties such as teaching, building instruments, or aiding in the operation of an observatory. The American Astronomical Society , which is the major organization of professional astronomers in North America , has approximately 8,200 members (as of 2024). This number includes scientists from other fields such as physics, geology , and engineering , whose research interests are closely related to astronomy. The International Astronomical Union comprises about 12,700 members from 92 countries who are involved in astronomical research at
2050-459: The mass of any natural satellites that orbit it, with a correspondingly much larger diameter. The Earth–Moon system is a unique exception in the Solar System; at 3,474 kilometres (2,158 miles) across, the Moon is 0.273 times the diameter of Earth and about 1 ⁄ 80 of its mass. The next largest ratios are the Neptune – Triton system at 0.055 (with a mass ratio of about 1 to 4790),
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2100-706: The most common usage, an astronomical body that orbits a planet , dwarf planet , or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons , a derivation from the Moon of Earth . In the Solar System , there are six planetary satellite systems containing 288 known natural satellites altogether. Seven objects commonly considered dwarf planets by astronomers are also known to have natural satellites: Orcus , Pluto , Haumea , Quaoar , Makemake , Gonggong , and Eris . As of January 2022, there are 447 other minor planets known to have natural satellites . A planet usually has at least around 10,000 times
2150-498: The natural satellites in the Solar System by diameter. The column on the right includes some notable planets, dwarf planets, asteroids, and trans-Neptunian objects for comparison. The natural satellites of the planets are named after mythological figures. These are predominantly Greek, except for the Uranian natural satellites , which are named after Shakespearean characters. The twenty satellites massive enough to be round are in bold in
2200-770: The outstanding quality of this work and arranged for its publication. In 1804, Olbers was elected a Fellow of the Royal Society of London, in 1809, corresponding member living abroad of the Royal Institute of the Netherlands in 1822, a Foreign Honorary Member of the American Academy of Arts and Sciences , and in 1827, a foreign member of the Royal Swedish Academy of Sciences . Olbers was deputed by his fellow citizens to assist at
2250-442: The relatively large natural satellite Charon and four smaller natural satellites; Styx , Nix , Kerberos , and Hydra . Haumea has two natural satellites; Orcus , Quaoar , Makemake , Gonggong , and Eris have one each. The Pluto–Charon system is unusual in that the center of mass lies in open space between the two, a characteristic sometimes associated with a double-planet system. The seven largest natural satellites in
2300-408: The smallest, Tethys, has more mass than all smaller natural satellites together. As well as the natural satellites of the various planets, there are also over 80 known natural satellites of the dwarf planets , minor planets and other small Solar System bodies . Some studies estimate that up to 15% of all trans-Neptunian objects could have satellites. The following is a comparative table classifying
2350-416: The upper story of his home into an observatory . In 1800, Olbers was one of 24 astronomers invited to participate in the group known as the " celestial police ", dedicated to finding new planets in the solar system. On 28 March 1802, Olbers discovered and named the asteroid Pallas . Five years later, on 29 March 1807, he discovered the asteroid Vesta , which he allowed Carl Friedrich Gauss to name. As
2400-430: The word "asteroid" was not yet coined, the literature of the time referred to these minor planets as planets in their own right. He proposed that the asteroid belt , where these objects lay, was the remnants of a planet that had been destroyed . The current view of most scientists is that tidal effects from the planet Jupiter disrupted the planet-formation process in the asteroid belt. On 6 March 1815, Olbers discovered
2450-462: Was circularized .) Many other natural satellites, such as Earth's Moon, Ganymede , Tethys, and Miranda, show evidence of past geological activity, resulting from energy sources such as the decay of their primordial radioisotopes , greater past orbital eccentricities (due in some cases to past orbital resonances ), or the differentiation or freezing of their interiors. Enceladus and Triton both have active features resembling geysers , although in
2500-498: Was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using physical laws . Today, that distinction has mostly disappeared and the terms "astronomer" and "astrophysicist" are interchangeable. Professional astronomers are highly educated individuals who typically have a PhD in physics or astronomy and are employed by research institutions or universities. They spend
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