Glossary of astronomy#Common proper motion

This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.

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A

{{defn|In the {{gli|Harvard class|Harvard spectral classification}} system, a class of {{gli|main-sequence}} star having spectra dominated by Balmer absorption lines of hydrogen. Stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 times {{gli|solar mass|the mass of the Sun}}, and have surface temperatures of 7,600–10,000 kelvin.}}

{{defn|A measure of a {{gli|star}}'s absolute brightness. It is defined as the {{gli|apparent magnitude}} the star would show if it were located at a distance of 10 {{gli|parsec|parsecs}}, or 32.6 {{gli|light-year|light-years}}.}}

{{defn|A roughly circular mass of diffuse material in {{gli|orbit}} around a central object, such as a {{gli|star}} or {{gli|black hole}}. The material is acquired from a source external to the central object, and friction causes it to spiral inward towards the object.}}

{{defn|A compact region in the center of a {{gli|galaxy}} displaying a much higher than normal {{gli|luminosity}} over some part of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by {{gli|star|stars}}. A galaxy hosting an AGN is called an active galaxy.}}

{{defn|A measure of the proportion of the total solar radiation received by an {{gli|astronomical body}}, such as a {{gli|planet}}, that is diffusely reflected away from the body. It is a dimensionless quantity typically measured on a scale from 0 (indicating total absorption of all incident radiation, as by a {{gli|black body}}) to 1 (indicating total reflection). The albedo reported for an astronomical body may vary widely by the spectral and angular distribution of the incident radiation, by the "layer" of the body being measured (e.g. upper atmosphere versus surface), and by local variation within these layers (e.g. cloud cover and geological or environmental surface features).}}

File:Mars and Syrtis Major - GPN-2000-000923.jpg (center) is a prominent dark {{gli|albedo feature}} on Mars.]]

{{defn|A large area on the surface of a reflecting object that shows a significant contrast in brightness or darkness ({{gli|albedo}}) compared to adjacent areas.}}

{{defn|The boundary separating a star's corona from the {{gli|stellar wind}} defined as where the coronal plasma's Alfvén speed and the large-scale stellar wind speed are equal.}}

{{defn|A chemically peculiar star belonging to the more general class of {{gli|A-type star|A-type stars}}. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals. See {{gli|metallicity}}.}}

{{defn|The point at which a body orbiting the Earth's {{gli|Sun}} is furthest from the Sun. Contrast {{gli|perihelion}}.}}

{{defn|The point at which an orbiting body is furthest from its {{gli|primary}}. Contrast {{gli|periapsis}}.}}

{{defn|The point at which a body orbiting the Earth (such as the {{gli|Moon}} or an {{gli|artificial satellite}}) is furthest from the Earth. Contrast {{gli|perigee}}.}}

{{defn|A measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere. The brighter the object appears, the lower its magnitude.}}

{{defn|The closest approach of one celestial object to another, as viewed from a third body.}}

{{defn|In the {{gli|orbit}} of a {{gli|planetary body}}, one of the two extreme points of distance between the body and its {{gli|primary}} – either the point of minimal distance, called the {{gli|periapsis}}, or the point of maximal distance, called the {{gli|apoapsis}}. The term may also be used to refer to the value of the distance rather than the point itself. All {{gli|elliptical orbit|elliptical orbits}} have exactly two apsides.}}

{{ghat|Also argument of perifocus or argument of pericenter.}}

{{defn|The angle from an {{gli|orbit|orbiting}} body's {{gli|ascending node}} to its {{gli|periapsis}}, measured in the direction of motion. It is one of six canonical {{gli|orbital elements}} used to characterize an orbit.}}

{{defn|An object that has been intentionally placed into {{gli|orbit}} by humans, often around the Earth but also around other bodies within the {{gli|Solar System}}. Contrast {{gli|natural satellite}}.}}

{{defn|The {{gli|orbital node}} at which an orbiting object moves north through the {{gli|plane of reference}} (in geocentric and heliocentric orbits) or at which the orbiting object moves away from the observer (in orbits outside of the {{gli|Solar System}}). The position of the ascending node with respect to a reference direction, called the {{gli|longitude of the ascending node}}, is used along with other {{gli|orbital elements|parameters}} to describe an orbit. Contrast {{gli|descending node}}.}}

{{defn|The position of a {{gli|planet}} or Earth's {{gli|Moon}} with respect to the {{gli|Sun}}, as viewed from Earth.}}

{{defn|Any pattern of {{gli|star|stars}} recognizable in Earth's {{gli|night sky}}. An asterism may form part of an official {{gli|constellation}} or it may be composed of stars from more than one constellation.}}

{{defn|A {{gli|minor planet}} of the inner {{gli|Solar System}}, i.e. one that orbits the {{gli|Sun}} at a distance no greater than the orbit of Jupiter. Asteroids are somewhat arbitrarily distinguished from many different types of similar objects: {{gli|small Solar System body|small Solar System bodies}} primarily composed of dust and ice instead of mineral and rock are known as {{gli|comet|comets}}; bodies less than one meter in diameter are known as {{gli|meteoroid|meteoroids}}; very large asteroids are sometimes called {{gli|planetoid|planetoids}} or {{gli|planetesimal|planetesimals}}; and bodies similar to asteroids in size and composition but which lie beyond Jupiter are known as {{gli|distant minor planet|distant minor planets}}.}}

{{defn|The {{gli|circumstellar disc}} in the {{gli|Solar System}} located roughly between the orbits of Mars and Jupiter that is occupied by numerous irregularly shaped {{gli|small Solar System body|small Solar System bodies}} ranging in size from dust particles to {{gli|asteroid|asteroids}} and {{gli|minor planet|minor planets}}. The asteroid belt is often called the main asteroid belt or main belt to distinguish it from other asteroid populations in other parts of the Solar System.}}

{{defn|An interdisciplinary field that studies the origins, evolution, distribution, and future of living systems in the universe, encompassing research on organic compounds in {{gli|outer space|space}}, abiogenesis and extreme-environment adaptation on Earth, the habitability of {{gli|extrasolar planet|extrasolar planets}}, the possible existence of extraterrestrial life, and how humans might be able to detect extraterrestrial biosignatures, among other topics.}}

{{defn|See {{gli|orbital mechanics}}.}}

{{defn|A field that studies the geology of solidified bodies such as the planets and their moons, asteroids, comets, and meteorites. Investigations are centered around the composition, structure, processes, and history of these objects.}}

{{defn|A type of {{gli|binary system}} where evidence for an unseen orbiting companion is revealed by its periodic {{gli|perturbation|gravitational perturbation}} of the visible component. See also {{gli|spectroscopic binary}}.}}

{{defn|The branch of astronomy that involves precise measurements of the positions and movements of {{gli|star|stars}} and other {{gli|astronomical body|celestial bodies}}.}}

{{defn|A type of naturally occurring physical entity, association, or structure within the {{gli|observable universe}} that is a single, tightly bound, contiguous structure, such as a {{gli|star}}, {{gli|planet}}, {{gli|moon}}, or {{gli|asteroid}}. Though the terms astronomical "body" and {{gli|astronomical object|astronomical "object"}} are often used interchangeably, there are technical distinctions.}}

{{defn|A list of astronomical objects, typically grouped together because they share a common type, morphology, origin, means of detection, or method of discovery.}}

{{anchor|astronomical objects|celestial object|celestial objects}}

{{defn|A type of naturally occurring physical entity, association, or structure that exists within the {{gli|observable universe}} but is a more complex, less cohesively bound structure than an {{gli|astronomical body}}, consisting perhaps of multiple bodies or even other objects with substructures, such as a {{gli|planetary system}}, {{gli|star cluster}}, {{gli|nebula}}, or {{gli|galaxy}}. Though the terms astronomical "object" and astronomical "body" are often used interchangeably, there are technical distinctions.}}

{{defn|Any abstract pictorial symbol used to represent one or more {{gli|astronomical objects}}, events, or theoretical constructs, e.g. those of the {{gli|planets}} of the {{gli|Solar System}}, the {{gli|phases of the Moon}}, the {{gli|zodiacal}} {{gli|constellations}}, and the {{gli|solstices}} and {{gli|equinoxes}}. Many of these symbols were commonly used historically, though in the modern era they are usually limited to almanacs and astrology, and their appearance in scientific literature has become increasingly infrequent. Exceptions include the symbols for the Sun (☉), the Earth (🜨), and the Moon (☾), which are sometimes used for {{gli|astronomical constants}} and in other forms of shorthand.}}

{{defn|A unit of length used primarily for measuring distances within the {{gli|Solar System}} or secondarily between the Earth and distant stars. Originally conceived as the semimajor axis of the Earth's orbit around the Sun, the astronomical unit is now more rigidly defined as exactly {{convert|149597870.7|km|mi pc ly|sigfig=5|abbr=off}}.}}

{{defn|The scientific study of {{gli|astronomical object|celestial objects}} and {{gli|celestial event|phenomena}}, the origins of those objects and phenomena, and their evolution.}}

{{defn|The photography of astronomical objects, celestial events, or areas of the night sky.}}

{{defn|The branch of astronomy that employs principles of physics and chemistry to determine the nature of {{gli|astronomical object|astronomical objects}} and phenomena, examining properties such as luminosity, density, temperature, and chemical composition (rather than the positions or motions of objects in space, which is more specifically the emphasis of {{gli|celestial mechanics}}).}}

{{defn|A gaseous envelope held in place by the gravity of a planet. This shell of gas has no clearly defined exterior boundary, but instead grows increasingly tenuous with altitude. The term can also be applied to a stellar atmosphere, referring to the visible outer layers of a star.}}

{{defn|A slow, continuous, gravity-induced change (a {{gli|precession}}) in the orientation of an {{gli|astronomical body}}'s {{gli|axis of rotation}}. The term often refers in particular to the gradual shift in the orientation of Earth's rotational axis with respect to {{gli|ecliptic|its orbital plane}} over a cycle of approximately 25,772 years, which is caused predominantly by the gravitational influence of the {{gli|Moon}} and the {{gli|Sun}} on the Earth's equatorial bulge. The phenomenon is similar to but much larger in magnitude than other changes in the alignment of Earth's axis such as {{gli|nutation}} and polar motion, and is the cause of the apparent {{gli|precession of the equinoxes}} in the {{gli|night sky}}.}}

{{defn|The angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its {{gli|equator|equatorial}} plane and {{gli|orbital plane}}. Axial tilt usually does not change considerably during a single {{gli|orbital period}}; Earth's axial tilt is the cause of the seasons. Axial tilt is distinct from {{gli|orbital inclination}}.}}

{{defn|An angular measurement of an object's orientation along the horizon of the observer, relative to the direction of true north. When combined with the altitude above the horizon, it defines an object's current position in the spherical coordinate system.}}

B

{{defn|A model that attempts to explain magnetic and sunspot patterns observed on the Sun.}}

File:Orbit5.gif

{{defn|The common center of mass about which any two or more bodies of a gravitationally bound system {{gli|orbit}}. The barycenter is one of the foci of the {{gli|elliptical orbit}} of each body participating in the system; its location is strongly influenced by the mass of each body and the distances between them. For example, in a {{gli|planetary system}} where the mass of the central {{gli|star}} is significantly larger than the mass of an orbiting {{gli|planet}}, the barycenter may actually be located within the radius of the star, such that the planet appears to orbit the star itself, though both bodies actually orbit the shared barycenter.}}

{{defn|The process by which the class of subatomic particles known as baryons were generated in the early Universe, including the means by which baryons outnumber antibaryons.}}

{{defn|The prevailing {{gli|cosmogony|cosmological model}} for the origin of the {{gli|observable universe}}. It depicts a starting condition of extremely high density and temperature, followed by an ongoing expansion that led to the current conditions.}}

{{defn|A {{gli|star system}} consisting of exactly two {{gli|star|stars}} orbiting around their common {{gli|barycenter}}. The term is often used interchangeably with {{gli|double star}}, though the latter can also refer to an optical double star, a type of optical illusion which is entirely distinct from true binary star systems.}}

{{defn|A concentration of mass so compact that it creates a region of space from which not even light can escape. The outer boundary of this region is called the {{gli|event horizon}}.}}

{{ghat|Also {{gli|critical velocity}} or critical rotation.}}

{{defn|The surface velocity at which the centrifugal force generated by a rapidly spinning star matches the force of Newtonian gravity. At rotational velocities beyond this point, the star begins to eject matter from its surface.}}

{{defn|A {{gli|substellar object}} that is too low in mass to sustain the nuclear fusion of hydrogen-1 in its core, with the latter being a characteristic of stars on the {{gli|main sequence}}. Brown dwarfs can still generate energy from {{gli|Kelvin–Helmholtz mechanism|gravitational contraction}} and by the fusion of deuterium.}}

C

{{defn|This is a spectrograph placed at the Coudé focus of a reflecting telescope. The focus remains stationary as the telescope is re-oriented, which is advantageous for the stable mounting of heavy spectroscopic instruments.}}

{{defn|The imaginary great circle of a body's {{gli|celestial sphere}} that is coplanar with the body's terrestrial {{gli|equator}}. On Earth, the plane of the celestial equator is the basis of the {{gli|equatorial coordinate system}}. Due to Earth's {{gli|axial tilt}}, this plane is currently inclined at an angle of 23.44 degrees with respect to the {{gli|ecliptic}}.}}

File:AxialTiltObliquity.png

{{defn|The branch of {{gli|astronomy}} that studies the motions of all types of {{gli|astronomical object|astronomical objects}}, including {{gli|star|stars}}, {{gli|planet|planets}}, and {{gli|natural satellite|natural}} and {{gli|artificial satellite|artificial satellites}}, among others.}}

{{defn|See {{gli|meridian}}.}}

{{defn|One of two coordinates in the Earth's {{gli|sky}} at which a hypothetical indefinite extension of the Earth's {{gli|axis of rotation}} "intersects" the {{gli|celestial sphere}}, i.e. the two points in the sky that are directly overhead the terrestrial North and South Poles, around which all {{gli|fixed stars}} appear to revolve during the course of a day. The celestial poles form the north and south poles of the {{gli|equatorial coordinate system}}.}}

{{defn|An imaginary sphere that encompasses the Earth's entire {{gli|sky}} and is stationary with respect to the {{gli|fixed stars|background stars}}. It is the basis for {{gli|spherical astronomy}}.}}

{{defn|A {{gli|small Solar System body}} with either a {{gli|perihelion}} or a {{gli|semi-major axis}} between those of the {{gli|outer planets}}, i.e. generally inward of the {{gli|Kuiper belt}} but beyond the {{gli|Jupiter trojan|Jupiter trojans}}. Centaurs are {{gli|cis-Neptunian object|cis-Neptunian objects}} that typically exhibit characteristics of both {{gli|asteroid|asteroids}} and {{gli|comet|comets}}, and generally also have unstable orbits because they cross the orbits of one or more of the {{gli|giant planet|giant planets}}.}}

{{defn|Any very large concentration of mass at the center of a {{gli|galaxy}}, typically either a {{gli|supermassive black hole}} or a {{gli|nuclear star cluster|compact stellar nucleus}}, but sometimes both.}}

{{defn|A parameter indicating the magnetic activity in a star's {{gli|chromosphere}}. One measure of this activity is {{math|log R′_HK}}, where {{math|R′_HK}} is the ratio of the equivalent width of a star's singly ionized calcium H and K lines, after correction for photospheric light, to the bolometric flux. Schröder et al. (2009) divide solar-type stars into four groups depending on their activity index: very active ({{math|log R′_HK}} above −4.2), active (−4.2 to −4.75), inactive (−4.75 to −5.1), and very inactive (below −5.1).}}

{{defn|A numeric value that is used to compare the brightness of a star measured from different frequency bands of the electromagnetic spectrum. Because the energy output of a star varies by frequency as a function of temperature, the color index can be used to indicate the star's temperature.}}

{{defn|A relatively small, icy body that displays extended features when it approaches the {{gli|Sun}}. The energy from the Sun vaporizes volatiles on a comet's surface, producing a visible coma around the cometary body. Sometimes a comet can produce a long tail radiating away from the Sun.}}

{{defn|A property of two objects orbiting the same body whose {{gli|orbital period|orbital periods}} are in a rational proportion. For example, the orbital period of Saturn around the Sun is very nearly 5/2 the orbital period of Jupiter.}}

{{defn|A term used to indicate that two or more stars share the same motion through space, within the margin of observational error. That is, either they have nearly the same {{gli|proper motion}} and {{gli|radial velocity}} parameters, which may suggest that they are gravitationally bound or share a common origin, or they are known to be gravitationally bound (in which case their proper motions may be rather different but average to be the same over time).}}

{{defn|Any {{gli|astronomical body}} with a very high mass relative to its radius, compared to most ordinary atomic matter. The term typically refers to very high-density objects such as {{gli|white dwarf|white dwarfs}}, {{gli|neutron star|neutron stars}}, and {{gli|black hole|black holes}}, or to {{gli|stellar remnant|stellar remnants}} with very small radii.}}

{{defn|See {{gli|nuclear star cluster}}.}}

{{defn|A phenomenon during which two {{gli|astronomical object|astronomical objects}} or spacecraft have either the same {{gli|right ascension}} or the same {{gli|ecliptic coordinate system|ecliptic longitude}} as observed from a third body (usually the Earth), such that, from the observer's perspective, the objects appear to closely approach each other in the sky.}}

{{defn|A region on the {{gli|celestial sphere}} surrounding a specific and identifiable grouping of stars. The names of constellations are assigned by tradition and often have an associated folklore based in mythology, while the modern demarcation of their borders was established by the {{gli|International Astronomical Union}} in 1930. Compare {{gli|asterism}}.}}

{{defn|An aura of plasma that surrounds cooler stars such as the {{gli|Sun}}. It can be observed during a {{gli|solar eclipse}} as a bright glow surrounding the lunar disk. The temperature of the corona is much higher than that of the stellar surface, and the mechanism that creates this heat remains subject to debate among astronomers.}}

{{defn|A significant release of plasma and the accompanying magnetic field from the {{gli|Sun}}'s {{gli|corona}}, often following a {{gli|solar flare}} or present during a {{gli|solar prominence}} eruption.}}

{{defn|Dust which exists in {{gli|outer space}} or has fallen on Earth, generally composed of fine particles of solid matter far smaller than those found in terrestrial dust.}}

{{defn|A type of radiation consisting of high-energy protons and atomic nuclei which move through space at nearly the speed of light, and which may originate from the {{gli|Sun}} or from outside the {{gli|Solar System}}. Collisions of cosmic rays with the Earth's atmosphere can produce dramatic effects both in the air and on the surface.}}

{{defn|Any model concerning the origin of either the {{gli|universe}} or the cosmos.}}

{{defn|The scientific study of the origin, evolution, and eventual fate of the {{gli|Universe}}.}}

{{defn|The surface velocity at the equator of a rotating body where the centrifugal force balances the Newtonian gravity. At this rotation rate, mass can be readily lost from the equator, forming a {{gli|circumstellar disc}}. See also {{gli|break-up velocity}}.}}

{{defn|The apparent movement of an {{gli|astronomical object}} (e.g. the {{gli|Sun}}, the {{gli|Moon}}, a {{gli|planet}}, a {{gli|star}}, a {{gli|constellation}}, etc.) across the observer's local {{gli|meridian}}. During each day, the Earth's rotation causes every astronomical object to {{gli|appear to move}} along a circular path on the {{gli|celestial sphere}}, creating two points at which it crosses the meridian: an upper culmination, at which the object reaches its highest point above the {{gli|horizon}}, and a lower culmination, at which it reaches its lowest point, nearly 12 hours later. When not otherwise qualified, the time of culmination typically refers to the time at which the upper culmination occurs.{{cite book|title=The Facts on File Dictionary of Astronomy |first1=John |last1=Daintith |first2=William |last2=Gould |publisher=Infobase Publishing |year=2009 |isbn=978-1438109329}}}}

D

{{defn|A ring-shaped {{gli|circumstellar disc}} of dust and debris orbiting its host star. It is created by collisions between {{gli|planetesimal|planetesimals}}. A debris disk can be discerned from an infrared excess being emitted from the star system, as the orbiting debris re-radiates the star's energy into space as heat.}}

File:NASA-14114-HubbleSpaceTelescope-DebrisDisks-20140424.jpg, and artist's impressions of the disks' orientations around their host stars]]

{{defn|In the {{gli|equatorial coordinate system}}, the celestial equivalent of terrestrial latitude. Coordinates north of the {{gli|celestial equator}} are measured in positive degrees from 0° to 90°, while coordinates to the south are measured in negative degrees. See also {{gli|right ascension}}.}}

{{defn|A {{gli|circumstellar disc}} formed from gas ejected from a central star that now follows a nearly {{gli|Kepler orbit|Keplerian orbit}} around it. This type of disk can be found around many Be stars.}}

{{defn|Any {{gli|astronomical object}} that is not an individual {{gli|star}} or an object within the Earth's {{gli|Solar System}}. The classification is used mostly in amateur {{gli|observational astronomy}} to distinguish faint objects in the {{gli|night sky}} such as {{gli|star cluster|star clusters}}, {{gli|nebula|nebulae}}, and {{gli|galaxy|galaxies}}.}}

{{defn|A star composed of degenerate matter, e.g. a {{gli|white dwarf}} or a {{gli|neutron star}}. These stars are in an advanced state of {{gli|stellar evolution|evolution}} and have suffered extreme {{gli|gravitational collapse}}, such that normal atoms cannot exist in them.}}

{{defn|The {{gli|orbital node}} at which an orbiting object moves south through the {{gli|plane of reference}} (in geocentric and heliocentric orbits) or at which the orbiting object moves toward the observer (in orbits outside of the {{gli|Solar System}}). Contrast {{gli|ascending node}}.}}

{{ghat|Also distant detached object and extended scattered disc object.}}

{{defn|A {{gli|astrodynamics|dynamical}} class of {{gli|minor planet}} in the outer reaches of the {{gli|Solar System}} whose {{gli|perihelion|point of closest approach to the Sun}} is so distant that the object is only moderately or weakly affected by the gravitational influence of Neptune and the other known planets, such that it appears to be "detached" from the rest of the Solar System. Detached objects are thus distinct from other populations of {{gli|trans-Neptunian object|trans-Neptunian objects}}, such as {{gli|classical Kuiper belt object|cubewanos}} and {{gli|scattered disc}} objects.}}

{{defn|See {{gli|prograde motion}}.}}

{{defn|The apparent motion of an {{gli|astronomical object}} (e.g. the {{gli|Sun}}, a {{gli|planet}}, or a distant {{gli|star}}) around the two {{gli|celestial pole|celestial poles}} in the Earth's {{gli|night sky}} over the course of one day. Diurnal motion is caused by Earth's rotation about its own axis, such that every object appears to follow a circular path called the diurnal circle.}}

{{defn|Any pair of {{gli|star|stars}} which appear near each other on the celestial sphere, either because the two stars coincidentally lie along nearly the same line of sight from the Earth, though they are in fact physically distant from each other, or because the two stars are actually located in physical proximity to each other, by which they may form a co-moving pair or a {{gli|binary star}} system.}}

{{defn|Any {{gli|star}} belonging to a category of ordinary {{gli|main-sequence}} stars like the {{gli|Sun}}, in contrast to {{gli|stellar evolution|evolved}} giant stars like Betelgeuse and Antares. Confusingly, the term has also come to include {{gli|stellar remnant|stellar remnants}} known as {{gli|white dwarf|white dwarfs}} as well as low-mass {{gli|substellar object|substellar objects}} known as {{gli|brown dwarf|brown dwarfs}}.}}

E

{{defn|A hotter and more massive star, in contrast to {{gli|late-type star|late-type stars}} that are cooler and less massive. The term originated from historical stellar models that assumed stars began their early life at a high temperature then gradually cooled off as they aged. It may be used to refer to the higher-temperature members of any particular population or category of stars, rather than of all stars in general.}}

{{defn|See {{gli|orbital eccentricity}}.}}

{{defn|The plane defined by the Earth's orbit around the {{gli|Sun}}. Hence, the position of the Sun as viewed from the Earth defines the intersection of this plane with the {{gli|celestial sphere}}. The ecliptic is widely used as a {{gli|plane of reference|reference plane}} for describing the position of other {{gli|Solar System}} bodies within various {{gli|celestial coordinate system|celestial coordinate systems}}. It differs from the {{gli|celestial equator}} because of the {{gli|axial tilt}} of the Earth.}}

File:Earths orbit and ecliptic.svg

{{defn|An {{gli|astronomical coordinate system}} commonly used to specify the apparent positions, orbits, and axial orientations of objects within the {{gli|Solar System}}, with an origin at the geometric center of either the {{gli|Sun}} or the Earth, a fundamental plane defined by the plane of Earth's orbit around the Sun (i.e. the {{gli|ecliptic|plane of the ecliptic}}), a {{gli|primary direction}} towards the {{gli|vernal equinox}}, and a right-handed convention. This system is convenient because most of the {{gli|planet|planets}} and many {{gli|small Solar System body|small Solar System bodies}} orbit the Sun with only slight {{gli|orbital inclination|inclinations}} to the ecliptic. It may be implemented in either spherical or rectangular coordinates.}}

{{defn|(of a star or planet) The temperature of an ideal black body that would emit the same total amount of electromagnetic radiation.}}

{{defn|A type of {{gli|galaxy}} with an approximately ellipsoidal shape and a smooth, nearly featureless appearance. They are one of three main morphological classes of galaxy, along with {{gli|spiral galaxy|spiral}} and {{gli|lenticular galaxy|lenticular galaxies}}.}}

{{defn|A type of {{gli|Kepler orbit}} with an {{gli|orbital eccentricity}} of less than 1 (often inclusive of {{gli|circular orbit|circular orbits}}, which have eccentricity equal to 0), or one with negative {{gli|specific orbital energy|energy}}. Elliptical orbits take the shape of an ellipse, and are very common in two-body astronomical systems.}}

File:Elliptic_orbit.gif

{{defn|The angular separation between the {{gli|Sun}} and an orbiting body, such as a {{gli|planet}}, as it appears from Earth.}}

{{defn|A list or table of the expected positions of {{gli|astronomical object|astronomical objects}} or {{gli|artificial satellite|artificial satellites}} in the {{gli|sky}} at various dates and times. Modern ephemerides are often provided by computer software.}}

{{defn|A moment in time used as a reference point for some time-varying astronomical quantity, such as the {{gli|celestial coordinate system|celestial coordinates}} or {{gli|orbital elements}} of an {{gli|astronomical object}}, because such quantities are subject to {{gli|perturbation|perturbations}} and change over time. The primary use of astronomical quantities specified by epochs is to calculate other relevant parameters of motion in order to predict future positions and velocities. In modern usage, astronomical quantities are often specified as a polynomial function of a particular time interval, with a given epoch as the temporal point of origin.}}

{{defn|The imaginary line on a gravitationally rounded spheroid such as a {{gli|planet}} that represents the intersection of the spheroid's surface with a plane perpendicular to its {{gli|axis of rotation}} and equidistant from its geographical poles. The plane of the Earth's terrestrial equator is the basis for the {{gli|celestial equator}}.}}

{{defn|An {{gli|astronomical coordinate system}} defined by an origin at the geometric center of the Earth, a fundamental plane created by projecting the Earth's terrestrial {{gli|equator}} onto the {{gli|celestial sphere}} (forming the {{gli|celestial equator}}), a {{gli|primary direction}} towards the {{gli|vernal equinox}}, and a right-handed convention. This system is widely used to specify the positions of celestial objects as viewed from Earth. It may be implemented in either spherical or rectangular coordinates.}}

{{defn|Of, relating to, or occurring at an {{gli|equinox}}.}}

{{defn|Either of the two precise times of year when the imaginary plane of the Earth's {{gli|equator}}, {{gli|celestial equator|extended indefinitely in all directions}}, passes through the center of the {{gli|Sun}} (i.e. the two points at which this plane intersects the {{gli|plane of the ecliptic}}); or, equivalently, when the Sun's apparent geocentric {{gli|ecliptic coordinate system|longitude}} is either 0 degrees or 180 degrees.{{cite book |title=Astronomical Almanac |at=Glossary |publisher=United States Naval Observatory |year=2008}} The two equinoxes, known as the {{gli|vernal equinox}} and the {{gli|autumnal equinox}}, occur on or near {{gli|March equinox|March 20}} and {{gli|September equinox|September 22}} each year. On the day of an equinox, the center of the visible Sun appears to be {{gli|zenith|directly above}} the equator, and the durations of day and night are approximately equal all over the planet. Compare {{gli|solstice}}.}}

{{defn|The minimum speed that must be achieved for a free, non-propelled object to escape from the gravitational influence of a massive body, i.e. to achieve an infinite distance from it; more generally, escape velocity is the speed at which the sum of an object's kinetic energy and gravitational potential energy is equal to zero. It is a function of the mass of the body and of the distance between the object and the body's center of mass. An object which has achieved escape velocity is neither on the surface nor in a closed {{gli|orbit}} of any radius.}}

{{defn|A curve on the {{gli|Hertzsprung–Russell diagram}} that a solitary {{gli|star}} of a particular mass and composition is expected to follow during the course of its {{gli|stellar evolution|evolution}}. This curve predicts the combination of temperature and {{gli|luminosity}} that a star will have during part or all of its lifetime.}}

File:Stellar evolutionary tracks-en.svg

{{defn|The absorption and scattering of electromagnetic radiation by matter (dust and gas) between an emitting {{gli|astronomical object}} and the observer. Atmospheric extinction varies by the wavelength of the radiation, with the attenuation being greater for blue light than for red.}}

{{defn|The branch of astronomy that studies objects and phenomena outside of the {{gli|Milky Way}} {{gli|galaxy}}, i.e. all objects not covered by {{gli|galactic astronomy}}.}}

{{defn|Any {{gli|astronomical object}} that exists outside the {{gli|Solar System}}. The term is generally not applied to {{gli|star|stars}} or any objects larger than a star or the Solar System itself, such as {{gli|galaxy|galaxies}}.}}

{{defn|Any {{gli|planet}} outside the Earth's {{gli|Solar System}}.}}

{{defn|See {{gli|astrobiology}}.}}

F

{{term|facula (disambiguation){{!}}facula}}

{{defn|A bright spot on a {{gli|star}}'s {{gli|photosphere}} formed by concentrations of magnetic field lines. For the {{gli|Sun}} in particular, faculae (see {{gli|solar facula}}) are most readily observed near the {{gli|limb darkening|solar limb}}. An increase in faculae as a result of a stellar cycle increases the star's total irradiance.}}

{{defn|Any {{gli|galaxy}} that does not belong to a larger cluster of galaxies and is gravitationally isolated.}}

{{defn|A randomly situated {{gli|star}} that lies along the line of sight to a group of physically associated stars under study, such as a {{gli|star cluster}}. These field stars are important to identify in order to prevent them from contaminating the results of a study.}}

{{defn|defn=The angular extent of the observable world that is seen at any given moment. In astronomy, the field of view is usually expressed as an angular area viewed by the instrument, in square degrees, or for higher magnification instruments, in square arc-minutes.}}

{{defn|The first use of a newly constructed {{gli|telescope}} or other instrument to take an {{gli|astrophotography|astronomical image}}.}}

{{defn|A term used to classify the brightest stars in the night sky, with apparent magnitudes lower (i.e. brighter) than 1.50. There are 22 stars that are classified as first magnitude stars.}}

{{defn|The location of the {{gli|March equinox}} upon the {{gli|celestial sphere}}, used as a reference point in {{gli|celestial coordinate system|celestial coordinate systems}}. Located in the {{gli|constellation}} Pisces, the First Point of Aries defines the {{gli|ecliptic coordinate system|ecliptic coordinate}} of (0°, 0°) and represents the point at which the {{gli|Sun}} meets the {{gli|celestial equator}} while traveling from south to north each year. It is directly opposite the {{gli|First Point of Libra}}.}}

{{defn|The location of the {{gli|September equinox}} upon the {{gli|celestial sphere}}, used as a reference point in {{gli|celestial coordinate system|celestial coordinate systems}}. Located in the {{gli|constellation}} Virgo, the First Point of Libra represents the point at which the {{gli|Sun}} meets the {{gli|celestial equator}} while traveling from north to south each year. It is directly opposite the {{gli|First Point of Aries}}.}}

{{defn|The "background" of {{gli|astronomical object|astronomical objects}} in the {{gli|night sky}} which are so distant from observers on Earth that they do not appear to move relative to each other, as opposed to the "foreground" of objects within the {{gli|Solar System}} which do. The fixed stars are typically taken to include all {{gli|star|stars}} other than the {{gli|Sun}}, as well as all other {{gli|extrasolar object|extrasolar}} and {{gli|deep-sky object|deep-sky objects}}.}}

{{defn|A class of {{gli|variable star}} that undergoes sudden, dramatic increases in brightness due to magnetic activity on its surface. This change in brightness occurs across the electromagnetic spectrum from radio waves to X-rays. Most flare stars are faint {{gli|red dwarf|red dwarfs}}.}}

{{defn|The apparent uncommonness of {{gli|planet|planets}} having a size between 1.5 and 2 times that of the Earth.}}

G

{{defn|The branch of astronomy that studies objects and phenomena within the {{gli|Milky Way}} {{gli|galaxy}}, as opposed to everything outside of the Milky Way, which is the domain of {{gli|extragalactic astronomy}}.}}

{{defn|The direction in space that is directly opposite the {{gli|Galactic Center|center}} of the {{gli|Milky Way|Milky Way Galaxy}}, as viewed from Earth; considered as a point on the {{gli|celestial sphere}}, the Milky Way's anticenter is in the {{gli|constellation}} Auriga.}}

{{defn|The rotational {{gli|central massive object|center}} of the {{gli|Milky Way}} {{gli|galaxy}}, consisting of a {{gli|supermassive black hole}} of 4.100 ± 0.034 million {{gli|solar mass|solar masses}}. It is approximately {{convert|8,200|pc|ly}} away from Earth in the direction of the {{gli|constellation|constellations}} Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest.}}

{{defn|The region at the center of a {{gli|galaxy}}, usually home to a very dense concentration of stars and gas. It almost always includes a {{gli|supermassive black hole}} which, when active, can generate a much higher {{gli|luminosity}} in a compact region than its surroundings. This excess luminosity is known as an {{gli|active galactic nucleus}}, and the brightest such active galaxies are known as {{gli|quasars}}.}}

{{defn|The time a given {{gli|astronomical object}} within a {{gli|galaxy}} takes to complete one {{gli|orbit}} around the {{gli|galactic center}}. Estimates of the duration of one revolution of the {{gli|Solar System}} about the center of the {{gli|Milky Way}} range from 225 to 250 million terrestrial years.}}

{{defn|The {{gli|tidal force}} experienced by objects subject to the gravitational field of a {{gli|galaxy}} such as the {{gli|Milky Way}}.}}

{{defn|A star or cluster's distance from the {{gli|central massive object|gravitational center}} of a particular {{gli|galaxy}}. For example, the {{gli|Sun}} is about 27,000 {{gli|light-years}} (approximately 8 kiloparsecs) away from the {{gli|Galactic Center}} of the {{gli|Milky Way}}. Galactocentric distance may also refer to a galaxy's distance from another galaxy.}}

{{defn|A large, gravitationally bound system of {{gli|star|stars}}, {{gli|stellar remnant|stellar remnants}}, {{gli|interstellar medium|interstellar gas}}, {{gli|cosmic dust|dust}}, and {{gli|dark matter}}, each of which orbits a {{gli|galactic nucleus|center of mass}}. Galaxies may contain hundreds of billions of stars and are categorized according to their visual morphology as {{gli|elliptical galaxy|elliptical}}, {{gli|spiral galaxy|spiral}}, or {{gli|irregular galaxy|irregular}}. Most of the galaxies in the {{gli|observable universe}} are between {{convert|1000|and|3000|pc|ly}} in diameter though some, including the {{gli|Milky Way}}, are much larger.}}

{{defn|A large-scale structure consisting of hundreds or thousands of {{gli|galaxy|galaxies}} bound together by gravity. Galaxy clusters are distinct from similarly named {{gli|open cluster|galactic clusters}} and other types of {{gli|star clusters}} and from smaller aggregates of galaxies known as {{gli|galaxy groups}}. Galaxy groups and galaxy clusters can themselves cluster together to form {{gli|superclusters}}.}}

{{defn|A gravitationally bound aggregation of up to 50 {{gli|galaxies}}, each at least as luminous as the {{gli|Milky Way Galaxy}}. Larger aggregations may be called {{gli|galaxy clusters}}, and galaxy groups and clusters can themselves cluster together to form {{gli|superclusters}}.}}

{{defn|A collective name for the four {{gli|natural satellite|moons}} of Jupiter discovered by Galileo Galilei in 1610: Io, Europa, Ganymede, and Callisto.}}

{{defn|The subfield of {{gli|astronomy}} that studies {{gli|astronomical objects}} detectable at gamma-ray wavelengths.}}

{{defn|A cataclysmic event that generates a brief but intense outburst of gamma ray radiation which can be detected from billions of {{gli|light-years}} away. The source of most GRBs is theorized to be {{gli|supernova}} or {{gli|hypernova}} explosions of high-mass stars. Short GRBs may also result from the collision of {{gli|neutron stars}}.}}

{{defn|A {{gli|giant planet}} composed mainly of hydrogen and helium gases rather than heavier elements, e.g. Jupiter and Saturn in the {{gli|Solar System}}.}}

{{defn|The geometric center of the Earth, i.e. the arithmetic mean position of all points within the oblate spheroid that is the precise shape of the Earth.}}

{{defn|With reference to, or pertaining to, the {{gli|geocenter|geometric center}} of the Earth; centered upon the Earth, e.g. a geocentric orbit.}}

{{defn|The point projected upon the {{gli|celestial sphere}} by a straight line that passes through the {{gli|geocenter}} and an observer; i.e. the observer's {{gli|zenith}} as defined with respect to the center of the Earth.}}

{{defn|The ratio of the brightness of an astronomical body at a {{gli|phase angle}} of zero to an idealized flat, fully reflecting, diffusively scattering (Lambertian) disk with the same cross-section. It is a measure of how much of the incoming illumination is being scattered back toward an observer and has a value between zero and one.}}

{{defn|The position of an object ({{gli|celestial object|celestial}} or otherwise) with respect to the {{gli|geocenter|center of the Earth}} or to the position of an observer, i.e. as defined by a straight line between the center of the Earth (or the observer) and the object at a given time, without any corrections for {{gli|light-time}}, {{gli|aberration}}, etc.}}

{{defn|A {{gli|circular orbit|circular}} {{gli|geosynchronous orbit}}, which maintains a constant altitude of {{convert|35786|km|mi}} directly above Earth's {{gli|equator}} in the {{gli|prograde motion|same direction}} as Earth's rotation such that, to an observer on Earth's surface, the orbiting object appears motionless, in a fixed position in the sky. {{gli|artificial satellite|Artificial satellites}} are often placed in geostationary orbit so that antennas on Earth do not have to rotate to track them.}}

{{defn|A {{gli|synchronous orbit}} about the Earth, i.e. with an {{gli|orbital period}} equal to Earth's {{gli|rotation period|rotational period}}, such that the orbiting object appears to return to exactly the same position in the sky after a period of one {{gli|sidereal day}}. All geosynchronous orbits have a {{gli|semi-major axis}} equal to {{convert|35786|km|mi}}; {{gli|geostationary orbit|geostationary orbits}} are a special case of geosynchronous orbits.}}

{{defn|Any very large or massive {{gli|planet}}, including {{gli|gas giants}} and {{gli|ice giants}}.}}

{{defn|A tight, spherical conglomeration of many thousands of {{gli|stars}} which are gravitationally bound to each other and which {{gli|orbit}} a {{gli|galactic nucleus|galactic core}} as a {{gli|satellite}}. They differ from {{gli|open clusters}} in having a much higher combined mass, with a typical lifespan extending for billions of years.}}

{{defn|Any very large distribution of mass, such as a {{gli|galactic cluster}}, which can bend passing light from a distant source by a noticeable degree. The effect, known as gravitational lensing, can make background objects appear to an observer to take on a ring or arc shape.}}

File:A_Horseshoe_Einstein_Ring_from_Hubble.JPG]]

{{defn|A branch of {{gli|observational astronomy}} which analyzes minute distortions in the curvature of spacetime known as gravitational waves to collect observational data about astronomical objects and events such as {{gli|neutron stars}}, {{gli|black holes}}, {{gli|supernovae}}, and the {{gli|Big Bang}}.}}

H

{{defn|An ionized {{gli|nebula}} powered by young, massive O-type stars. Ultraviolet photons from these hot stars ionize gas in the surrounding environment, and the nebular gas shines brightly in spectral lines of hydrogen and other elements. Because O-type stars have relatively short lifetimes (typically a few million years), the presence of an H II region indicates that massive star formation has taken place recently at that location. H II regions are often found in the arms of {{gli|spiral galaxy|spiral galaxies}} and in star-forming {{gli|irregular galaxy|irregular galaxies}}.}}

{{defn|The precise geometric center of the Earth's {{gli|Sun}}, i.e. the arithmetic mean position of all points within the approximate spheroid that is the shape of the Sun.}}

{{defn|With reference to, or pertaining to, the {{gli|heliocenter|geometric center}} of the Earth's {{gli|Sun}}; centered upon the Sun, e.g. a heliocentric orbit.}}

{{defn|The vast, bubble-like cavity in the {{gli|interstellar medium}} which surrounds and is created by the {{gli|solar wind|plasma}} emanating from the Earth's {{gli|Sun}}. The heliosphere encompasses the entirety of the {{gli|Solar System}} and a vast region of space beyond it. Its outer limit is often considered the boundary between matter originating from the Sun and matter originating from the rest of the galaxy.}}

{{defn|A plot of {{gli|luminosity}} versus {{gli|effective temperature}} for a population of {{gli|star|stars}}; depending on the usage, the star's {{gli|absolute magnitude}} may be substituted for luminosity, and its {{gli|color index}} or {{gli|spectral type}} for temperature. Single stars of known mass and composition follow predictable {{gli|evolutionary track|tracks}} across this chart over the course of their {{gli|stellar evolution|evolution}}. Hence, knowing a star's mass and {{gli|metallicity}} allows its age to be estimated. Stars of similar types are also found grouped together in specific regions of the chart, including {{gli|main-sequence}}, {{gli|red giant}}, and {{gli|white dwarf}} stars.}}

{{defn|The approximate region around an {{gli|astronomical object}} within which its gravitational attraction dominates the motions of {{gli|satellites}}. It is computed with respect to the next most gravitationally attractive object, such as the nearest star or the {{gli|galactic nucleus|galactic core}}. Satellites moving outside this radius tend to be perturbed away from the main body.}}

{{defn|The apparent boundary between the surface of a celestial body and its sky when viewed from the perspective of an observer on or near that body's surface; more specifically, the plane perpendicular to a line from an observer to the {{gli|zenith}} that passes through the point of observation.}}

{{defn|For a given {{gli|celestial object}}, the angular distance on the {{gli|celestial sphere}} measured westward along the {{gli|celestial equator}} from the observer's local {{gli|meridian}} to the {{gli|hour circle}} that passes through the celestial object; or, equivalently, the angle between the plane containing Earth's {{gli|axis of rotation|rotational axis}} and the {{gli|zenith}}, and the plane containing Earth's rotational axis and the object of interest. Analogous to {{gli|right ascension}}, the hour angle is one of many ways commonly used to specify the longitudinal position of an object upon the celestial sphere.}}

{{defn|Any imaginary great circle drawn upon the celestial sphere that passes through both of the {{gli|celestial pole|celestial poles}} and is therefore perpendicular to the {{gli|celestial equator}}. Similar to a {{gli|meridian}} but additionally taking into account the terrain and the depth to the {{gli|geocenter}} at a ground observer's particular location, the concept of the hour circle is employed to describe the longitudinal position of a celestial object relative to the observer's local meridian.}}

{{defn|These hybrid stars are G and K giant and supergiant stars that display the spectra of a hot coronae found in more massive giants and the cool stellar winds of M-type giants. They can be a source of X-ray emission.}}

{{defn|This is a hybrid class of pulsating stars that display pulsation frequencies of two different classes of variables. An example are variables displaying characteristic frequencies of both Delta Scuti and Gamma Doradus variables. On the {{gli|Hertzsprung–Russell diagram}}, these stars are positioned where the instability strips of both variable classes overlap.}}

{{defn|A critical mass below which an astronomical object cannot sustain its surface {{gli|luminosity}} through nuclear fusion. This mass limit, equal to about 7% of the {{gli|solar mass|mass of the Sun}}, forms the dividing line between {{gli|brown dwarf|brown dwarfs}} and hydrogen-fusing stars.}}

{{defn|A system consisting of a large {{gli|galaxy}} accompanied by multiple smaller {{gli|satellite galaxy|satellite galaxies}} (often elliptical) as well as its {{gli|galactic corona}}. The {{gli|Milky Way}} and Andromeda systems are examples of hypergalaxies.}}

I

{{defn|A {{gli|giant planet}} composed mainly of elements heavier than hydrogen or helium (such as oxygen, carbon, nitrogen, and sulfur), especially chemical {{gli|volatile|volatiles}} with freezing points above {{convert|100|K|C}}, e.g. Uranus and Neptune in the {{gli|Solar System}}.}}

{{defn|See {{gli|orbital inclination}}.}}

{{defn|An archaic term that is sometimes used to refer to the planets Mercury and Venus. The name originated from the fact that these planets orbit closer to the {{gli|Sun}} than the Earth and hence, in the geocentric {{gli|cosmology}} of Ptolemy, both appear to travel with the Sun across the sky. This is in contrast to the so-called {{gli|superior planet|superior planets}}, such as Mars, which appear to move independently of the Sun.}}

{{defn|The subfield of {{gli|astronomy}} that studies {{gli|astronomical object|astronomical objects}} detectable at infrared wavelengths.}}

{{defn|The matter that exists in the space between the {{gli|star|stars}} in a {{gli|galaxy}}. This medium mainly consists of hydrogen and helium, but is enhanced by traces of other elements contributed by matter expelled from stars.}}

{{defn|An effect produced by the incremental absorption and scattering of electromagnetic energy from interstellar matter, known as {{gli|extinction}}. This effect causes more distant objects such as stars to appear redder and dimmer than expected. It is not to be confused with the separate phenomenon of {{gli|redshift}}.}}

{{ghat|Also Laplace's invariable plane or the Laplace plane.}}

{{defn|The imaginary plane passing through the {{gli|barycenter}} of a {{gli|planetary system}} and perpendicular to its angular momentum vector, and which may be regarded as the weighted average of all planetary {{gli|orbital plane|orbital}} and rotational planes comprising the system.}}

{{defn|A {{gli|natural satellite}} following a distant, {{gli|orbital inclination|inclined}}, and often {{gli|orbital eccentricity|eccentric}} and {{gli|retrograde motion|retrograde}} orbit about its {{gli|primary}}. Irregular moons are thought to be captured from other orbits, as opposed to {{gli|regular moon|regular moons}}, which are thought to form in situ.}}

{{defn|A curve on the {{gli|Hertzsprung–Russell diagram}} that represents the {{gli|stellar evolution|evolutionary}} positions of stars having the same age but differing masses. This is in contrast to an {{gli|evolutionary track}}, which is a plot of stars having the same mass but differing ages. In fact, multiple evolutionary tracks can be used to build isochrones by putting curves through equal-age points along the tracks. When the mass of a star can be determined, an isochrone can be used to estimate the star's age.}}

J

{{defn|A physical state in which an {{gli|interstellar cloud}} of gas will begin to undergo collapse and form stars. A cloud can become unstable against collapse when it cools sufficiently or has perturbations of density, allowing gravity to overcome the gas pressure.}}

{{defn|A unit of time defined as exactly 365.25 days of 86,400 SI seconds each. Because these are units of constant duration, the Julian year is also constant and does not vary with a specific calendar or with any of the other means of determining the length of a year, such as the {{gli|tropical year}}. It is therefore widely used as the basis for defining the standard astronomical {{gli|epoch}} and the {{gli|light-year}}.}}

K

{{defn|The motion of one {{gli|orbit|orbiting}} body relative to another, as an ellipse, parabola, or hyperbola, which forms a two-dimensional {{gli|orbital plane}} (or sometimes a straight line) in three-dimensional space. Kepler orbits are idealized mathematical constructions which consider only the point-like gravitational attraction of two bodies, neglecting more complex orbital {{gli|perturbation|perturbations}} that may exist in reality.}}

{{defn|A {{gli|circumstellar disc}} of {{gli|small Solar System body|small Solar System bodies}} such as {{gli|asteroid|asteroids}}, {{gli|trojan|trojans}}, and {{gli|centaur|centaurs}} in the outer {{gli|Solar System}}, extending between 30 and 50 {{gli|astronomical unit|AU}} from the {{gli|Sun}}. It is similar to the {{gli|asteroid belt}} but far larger, and is home to several {{gli|dwarf planet|dwarf planets}}, including Pluto.}}

L

{{ghat|Also Lagrange point, libration point, or L-point.}}

{{defn|Any of a set of points near two large {{gli|astronomical body|bodies}} in {{gli|orbit}} at which a smaller object will maintain a constant position relative to the larger bodies. At other locations, a small object would eventually be pulled into its own orbit around one of the large bodies, but at the Lagrangian points the gravitational forces of the large bodies, the centripetal force of orbital motion, and (in certain scenarios) the Coriolis acceleration all align in a way that causes the small object to become "locked" in a stable or nearly stable relative position. For each combination of two orbital bodies, there are five such Lagrangian points, typically identified with the labels L₁ to L₅. The phenomenon is the basis for the stable orbits of {{gli|trojan satellite|trojan satellites}} and is commonly exploited by {{gli|artificial satellite|man-made satellites}}.}}

{{ghat|Also the Lenakaeia Supercluster, Local Supercluster, or Local SCI.}}

{{defn|A slight oscillating motion of the {{gli|Moon}} as seen from the Earth, a result of the Moon's elliptical orbit. It can allow normally hidden parts of the Moon's far side to be visible along the limbs of the lunar disk.}}

{{defn|A unit of length used to express astronomical distances that is equivalent to the distance that an object moving at the {{gli|speed of light}} in vacuum would travel in one {{gli|Julian year}}: approximately 9.46 trillion kilometres ({{val|9.46|e=12|u=km}}) or 5.88 trillion miles ({{val|5.88|e=12|u=mi}}). Though the light-year is often used to measure {{gli|galaxy|galactic}}-scale distances in non-specialist publications, the unit of length most commonly used in professional {{gli|astrometry}} is the {{gli|parsec}}.}}

{{defn|An optical effect seen in stars (including the {{gli|Sun}}), where the center part of the disk appears brighter than the edge or limb of the image.}}

{{defn|The imaginary line connecting the two {{gli|apsis|apsides}} (the {{gli|periapsis}} and the {{gli|apoapsis}}) of an {{gli|elliptical orbit}}, and which therefore represents the distance of the orbit's longest axis.}}

{{defn|defn=An X-ray optics design with an ultra wide field of view, based on the structure of the eyes of a lobster. It allows X-ray light to enter from multiple angles, capturing more X-rays from a larger area than other X-ray telescopes.}}

{{defn|The angle between a specified reference direction, called the {{gli|origin of longitude}}, and the direction of an {{gli|orbit}}'s {{gli|ascending node}}, as measured on a specified {{gli|plane of reference}}. The angle is typically measured eastwards from the reference direction to the ascending node (i.e. counterclockwise as seen from the north). It is one of six canonical {{gli|orbital elements}} used to characterize an orbit.}}

{{defn|The total amount of energy emitted per unit time by a {{gli|star}}, {{gli|galaxy}}, or other {{gli|astronomical object}}. In SI units, luminosity is measured in joules per second or watts, and is often given in terms of {{gli|magnitude|astronomical magnitude}}. Luminosity is related to but distinct from {{gli|apparent magnitude|visual brightness}}.}}

{{defn|Of or relating to the Earth's {{gli|Moon}}.}}

{{defn|The shape of the portion of the {{gli|Moon}} that is illuminated by direct sunlight as viewed from Earth. This shape is referred to as a phase because it gradually changes in a regular cycle over the course of a {{gli|synodic time|synodic month}}: as the orbital positions of the Moon around Earth and Earth around the {{gli|Sun}} change, the visibility of the side of the Moon that {{gli|tidal locking|constantly faces}} Earth alternates between completely illuminated (known as a full moon) and completely darkened by the Moon's own shadow (known as a new moon). There are also intermediate phases, during which the visible side may be only partially sunlit, e.g. when the Moon appears as a crescent. During the part of the lunar cycle in which the illuminated portion is growing larger, the Moon is said to be {{gli|waxing}}; when the illuminated portion is becoming smaller, it is said to be {{gli|waning}}. The phase of the Moon at any particular time appears the same from every point on Earth.}}

File:Moon Phase Diagram for Simple English Wikipedia.GIF

M

{{defn|A kind of {{gli|astronomical body}} that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body that emits little or no radiation and drifts through interstellar space unassociated with any {{gli|planetary system}}. Examples of MACHOs include {{gli|black hole|black holes}} or {{gli|neutron star|neutron stars}} as well as {{gli|brown dwarf|brown dwarfs}} and {{gli|rogue planet|rogue planets}}.}}

{{defn|A sudden reversals in the magnetic field of the {{gli|solar wind}}.}}

{{defn|A mostly convex region formed when a plasma, such as the {{gli|solar wind}}, interacts with the magnetic field of a body, such as a {{gli|planet}} or {{gli|star}}.}}

{{defn|A numerical logarithmic scale indicating the brightness of an {{gli|astronomical object}}, where the lower the value, the brighter the object. By convention, a first magnitude star is 100 times as bright as a sixth magnitude star. Magnitude 6 is considered the lower limit of objects that can be seen with the naked eye, although this can vary depending on sky conditions and eyesight.}}

{{defn|A category of {{gli|stars}} which form a continuous and distinctive band on plots of stellar temperature versus {{gli|luminosity}}, in particular the {{gli|Hertzsprung–Russell diagram}}. These stars are characterized by being in hydrostatic equilibrium and undergoing nuclear fusion of hydrogen-1 in their core region. The {{gli|Sun}} is a main-sequence star.}}

{{defn|See {{gli|semi-major axis}}.}}

{{defn|The precise time of year on Earth when the Sun appears to cross the {{gli|celestial equator}}, while generally trending northward at each {{gli|zenith}} passage. It represents the moment at which the North Pole of the Earth begins to tilt toward the Sun, and typically occurs on or near March 20 each year. It is the vernal equinox in the Northern Hemisphere and the autumnal equinox in the Southern Hemisphere. Contrast {{gli|September equinox}}.}}

{{anchor|mean anomaly}}{{term|term=mean anomaly|content=mean anomaly ({{math|M}})}}

{{defn|The fraction of an {{gli|elliptical orbit}}'s {{gli|orbital period|period}} that has elapsed since the orbiting body passed {{gli|periapsis}}, expressed as the angular distance from the {{gli|pericenter}} which a fictitious body would have if it moved in a perfectly {{gli|circular orbit}} in the same {{gli|orbital period}} as the actual body in its elliptical orbit. Unlike the {{gli|true anomaly}}, the mean anomaly does not correspond to a real geometric angle but is instead a contrived parameter used to make calculating the position of the orbiting body in the {{gli|two-body problem}} mathematically convenient.}}

{{defn|See {{gli|orbital resonance}}.}}

{{defn|A line running north–south across the {{gli|sky}} and passing through the point directly overhead known as the {{gli|zenith}}.}}

{{defn|The measurement of positions of celestial objects based on observation of the times of their {{gli|transit}} across the {{gli|meridian}} and of their {{gli|zenith}} distance at those times, with the intention of obtaining accurate {{gli|star}} positions which are self-consistent over large areas of sky.{{cite book |last1=Ridpath |first1=Ian |title=Oxford Dictionary of Astronomy |date=2007 |publisher=Oxford University Press |location=New York |isbn=9780199214938 |ol=18293428M |edition=2nd |url=https://openlibrary.org/books/OL18293428M/Oxford_dictionary_of_astronomy}}}}

{{defn|One of a set of 110 "nebulous" {{gli|astronomical object|astronomical objects}}, 103 of which were catalogued as non-{{gli|comet|comets}} by French comet hunter Charles Messier between 1771 and 1781. The Messier catalogue includes most of the {{gli|deep-sky object|deep-sky objects}} easily visible from the Northern Hemisphere.}}

{{defn|The visible passage of a glowing {{gli|meteoroid}}, {{gli|micrometeoroid}}, {{gli|comet}}, or {{gli|asteroid}} through the Earth's {{gli|atmosphere}}, usually as a long streak of light produced when such an object is heated to incandescence by collisions with air molecules in the upper atmosphere, leaving an ionization trail as a result of its rapid motion and sometimes also shedding material in its wake.}}

{{defn|A solid piece of debris from a {{gli|meteor}} that originated in outer space and survived its passage through the atmosphere to reach the surface of a planet or moon.}}

{{defn|A small rock or boulder that has entered a planetary {{gli|atmosphere}}. If it survives to reach the surface, it is then termed a {{gli|meteorite}}.}}

{{defn|A series of {{gli|meteor|meteors}} that seemingly radiate from a single area in the {{gli|night sky}}. These are produced by debris left over from a larger body, such as a {{gli|comet}}, and hence they follow roughly the same {{gli|orbit}}. This makes many meteor showers predictable events, as they recur every year.}}

{{defn|A measure of the abundance of elements other than hydrogen and helium within an astronomical object. Note that this definition includes elements that are not traditionally considered metallic by chemical convention.}}

{{defn|A very small {{gli|meteorite}} that has survived its passage through the atmosphere to reach the surface of a planet or moon, usually ranging in size from 50 μm to 2 mm. Micrometeorites are a major component of {{gli|cosmic dust}}.}}

{{defn|A very small {{gli|meteoroid}}, usually weighing less than one gram. If it survives to reach a planetary surface, it is then termed a {{gli|micrometeorite}}.}}

{{defn|A stellar object such as a {{gli|variable star}} that undergoes very small variations in {{gli|luminosity}}, in which the amplitude of the fluctuations amounts to just a few thousandths of a {{gli|magnitude}}. Detecting microvariability typically requires a sufficient number of observations to rule out random error as a source.}}

{{defn|no=1|The {{gli|spiral galaxy|barred spiral}} {{gli|galaxy}} that includes the Earth's {{gli|Solar System}}. The name describes the galaxy's appearance from the Earth: a hazy band of light visible in the {{gli|night sky}}, formed from billions of {{gli|star|stars}} that cannot be individually distinguished by the naked eye. The Milky Way Galaxy has a diameter of 100,000–200,000 {{gli|light-year|light-years}} and is estimated to contain 100–400 billion stars and at least that number of planets. The Solar System is located on the inner edge of one of the Milky Way's spiral arms, about 27,000 light-years from the {{gli|Galactic Center}}, which the {{gli|Sun}} orbits with a period of 240 million years.}}

{{defn|no=2|The hazy band of light itself, which from Earth appears as a band because the galaxy's disk-shaped structure is viewed side-on from within.}}

File:ALMA and the centre of the Milky Way.jpg.]]

{{defn|See {{gli|semi-minor axis}}.}}

{{defn|An object in direct {{gli|orbit}} around the {{gli|Sun}} that is neither a dominant {{gli|planet}} nor originally classified as a {{gli|comet}}. A {{gli|natural satellite|moon}} is not a minor planet because it orbits another body instead of the Sun.}}

{{defn|A {{gli|natural satellite}} that orbits a {{gli|minor planet}}. See also {{gli|moonlet}} and {{gli|subsatellite}}.}}

{{defn|An {{gli|interstellar cloud}} in which the prevailing physical conditions allow molecules to form, including molecular hydrogen.}}

{{defn|A dimensionless quantity that characterizes the radial distribution of mass inside a planet or moon.}}

{{defn|See {{gli|natural satellite}}.}}

{{defn|The solid, rocky body that orbits the Earth as its only {{gli|natural satellite}}, completing a full orbit every 27.3 days. The Moon's gravitational influence is responsible for {{gli|tide|tides}} on Earth; because of {{gli|tidal locking}}, only one side of the Moon is ever visible from the Earth. Sunlight reflected from its surface makes the Moon appear very bright in the {{gli|night sky}}, though its orbital position with respect to the Earth and the {{gli|Sun}} causes its visibility to change in a regular cycle of {{gli|lunar phases|phases}} when viewed from the Earth. The adjectival lunar is often used specifically to describe the orbit, gravity, and other properties of the Earth's Moon.}}

{{defn|An especially small {{gli|natural satellite}} orbiting a {{gli|planet}}, {{gli|dwarf planet}}, or other {{gli|minor planet}}. See also {{gli|minor-planet moon}} and {{gli|subsatellite}}.}}

{{defn|See {{gli|subsatellite}}.}}

{{defn|The horizontal angular distance between the rise {{gli|azimuth}} of a {{gli|celestial body}} and the east direction.{{cite book |title=Astronomy for beginners : all you need to know to get started in astronomy |date=2015 |isbn=978-1785461477 |page=151 |edition=Third |url=https://archive.org/details/AstronomyForBeginnersThirdEdition |language=en|last1=Westlake |first1=Hannah |publisher=Imagine Publishing Limited }}{{cite web |last1=Esken |first1=Thomas |title=Gcal 4.1: Appendix N Glossary |url=https://www.gnu.org/software/gcal/manual/html_node/Glossary.html |website=www.gnu.org |publisher=GNU |access-date=19 January 2020}}{{cite web |last1=Umar |first1=Khalid Bin |title=Al Khwarizmi: A Medieval Polymath |url=http://www.tawarikhkhwani.com/al-khwarizmi-a-medieval-polymath/ |website=TAWARIKHKHWANI |publisher=www.tawarikhkhwani.com |access-date=19 January 2020 |date=18 August 2019}}}}

{{defn|A loose grouping of {{gli|star|stars}} which travel together through space. Although the members were formed together in the same {{gli|molecular cloud}}, they have since moved too far apart to be gravitationally bound as a {{gli|star cluster|cluster}}.}}

{{defn|A type of {{gli|astronomy}} based on the acquisition of information about {{gli|astronomical object|astronomical objects}} through the coordinated observation and interpretation of four disparate classes of "messenger" signals with {{gli|extrasolar}} origins: electromagnetic radiation, gravitational waves, {{gli|neutrino|neutrinos}}, and {{gli|cosmic ray|cosmic rays}}. Because these four extrasolar messengers are created by different astrophysical processes, their presence or absence during a celestial event can reveal useful information about their sources.}}

N

{{defn|An early classification for active galaxies that had the visual appearance of a galaxy with a particularly bright, star-like nucleus. As a group, they are intermediate between Seyfert galaxies and Quasar. Most are giant ellipticals that are radio sources and display prominent emission lines.}}

{{defn|The point on the {{gli|celestial sphere}} exactly opposite from the {{gli|zenith}}. Thus, where the zenith is directly above an observer, the nadir is underfoot. The zenith and nadir form the two poles of the {{gli|horizon}} line.}}

{{defn|The human eye as used without any magnifying or light-collecting optical aid, such as a telescope, nor any eye protection. Many astronomical objects emit or reflect visible light that is sufficiently bright to fall within the limits of normal human visual perception, allowing observers to see them from the Earth's surface without any special equipment. Vision corrected to normal acuity using eyeglasses or contact lenses is still considered unaided.}}

{{defn|Any {{gli|astronomical body}} that {{gli|orbit|orbits}} a {{gli|planet}}, {{gli|minor planet}}, or sometimes another {{gli|small Solar System body}}.}}

{{defn|Any {{gli|small Solar System body}}, such as an {{gli|asteroid}} or {{gli|comet}}, whose orbit brings it into proximity with Earth, generally by being less than 1.3 {{gli|astronomical unit|AU}} from the {{gli|Sun}} at its {{gli|perihelion|closest approach}}.}}

{{defn|Any {{gli|astronomical object}} of indistinct nebulosity. In modern usage, the term typically refers to an {{gli|interstellar cloud}} of {{gli|cosmic dust|dust}}, hydrogen, helium, and other ionized gases. Historically, it was also used to refer to extended sources of {{gli|luminosity}} that could not be resolved into their individual components, such as {{gli|star cluster|star clusters}} and {{gli|galaxy|galaxies}}.}}

{{defn|A type of elementary particle, electrically neutral and with an extremely small rest mass, that interacts with other particles only via the weak interaction and the gravitational interaction. Neutrinos therefore typically pass through normal matter unimpeded and undetected.}}

{{defn|A type of {{gli|compact star}} that is composed almost entirely of neutrons, which are a type of subatomic particle with no electrical charge. Typically, neutron stars have a mass between about 1.35 and 2.0 times the mass of the {{gli|Sun}}, but with a radius of only {{Convert|12|km|mi|abbr=on|lk=off}}, making them among the densest known objects in the universe.}}

{{defn|The appearance of the Earth's {{gli|sky}} at nighttime, when the {{gli|Sun}} is below the horizon, and more specifically when clear weather and low levels of ambient light permit visibility of {{gli|astronomical object|celestial objects}} such as {{gli|star|stars}}, {{gli|planet|planets}}, and the {{gli|Moon}}. The night sky remains a fundamental setting for both amateur and professional {{gli|observational astronomy}}.}}

{{defn|Any {{gli|orbit}} that is coplanar with a specified {{gli|plane of reference}}, such that the {{gli|orbital inclination}} is 0 degrees for {{gli|prograde motion|prograde}} orbits and 180 degrees for {{gli|retrograde motion|retrograde}} ones.}}

{{defn|A compact and dense concentration of stars located at the center of a galaxy.}}

{{defn|The quantity of some specified particle or object class per unit volume. For atoms, molecules, or subatomic particles, the volume is typically in cm³ or m³. With stars, cubic parsecs (pc³) are often used.}}

{{defn|A continuous, gravity-induced change in the orientation of an {{gli|astronomical body}}'s {{gli|axis of rotation}} which results from the combined effects of small, short-term variations. Nutation is distinguished from {{gli|precession}}, which is a similar but longer-term change in axial orientation.}}

O

{{defn|A diagram of observed minus calculated values over time, showing how observed data differ from theoretical values which have been calculated according to a particular scientific model. It is often used as a diagnostic tool to determine the accuracy of the model. With a variable star, it is typically used to compare phase differences over time.}}

{{defn|A group of massive {{gli|star|stars}} which are not gravitationally bound to each other, but move together through space in a loose association. The OB in the name is a reference to stars of {{gli|stellar classification|stellar classifications}} O and B.}}

{{defn|See {{gli|axial tilt}}.}}

{{defn|The duration of time between the earliest and latest {{gli|observational astronomy|observations}} made by astronomers of an object within the {{gli|Solar System}}, which defines the length of the path traced by the object between these same observations. The term is primarily used in the discovery and tracking of {{gli|asteroid|asteroids}} and {{gli|comet|comets}}, which can be difficult to continuously track because of their size and great distance from Earth. Very short observation arcs, e.g. where the time between the initial observation and the most recent observation is less than 30 days, are of limited descriptive power because they represent only a very small fraction of the total path traced by the object in its {{gli|orbit}} around the Sun (or other {{gli|primary}}), and therefore result in a high degree of {{gli|uncertainty parameter|uncertainty}} when estimating the shape and characteristics of the object's orbit.}}

{{defn|The practice and study of directly observing {{gli|astronomical object|astronomical objects}} with the use of {{gli|telescope|telescopes}} and other astronomical instruments. It is concerned with recording data about the {{gli|observable universe}}, as opposed to {{gli|theoretical astronomy}}, which is concerned with calculating the measurable implications of astronomical models.}}

{{defn|A celestial event that occurs when a distant {{gli|astronomical body}} or {{gli|astronomical object|object}} is hidden by another, nearer body or object that passes between it and the observer, thereby blocking the first object from view. {{gli|solar eclipse|Solar}} and {{gli|lunar eclipse|lunar}} eclipses are specific types of occultations.}}

{{defn|A vast theoretical cloud of predominantly icy {{gli|planetesimal|planetesimals}} hypothesized to surround the {{gli|Sun}} at distances ranging from 2,000 to 200,000 {{gli|astronomical unit|AU}}. It is thought to be divided into two regions: a disc-shaped inner Oort cloud and a spherical outer Oort cloud. The outer limit of the Oort cloud is often considered the cosmographical boundary of the {{gli|Solar System}}.}}

{{defn|A measure of the resistance of a medium to the radiative transmission of energy. Within a star, it is an important factor in determining whether convection occurs.}}

{{defn|A gravitationally bound group of up to one thousand stars that formed together in the same {{gli|molecular cloud}}.}}

{{defn|The positioning of two celestial objects on opposite sides of the sky, from the perspective of an observer. This occurs, for example, when a planet makes its closest approach to the Earth, placing it in opposition to the Sun.}}

{{defn|The gravitationally curved trajectory of an {{gli|astronomical object|object}}, such as the trajectory of a {{gli|planet}} around a {{gli|star}} or a {{gli|natural satellite}} around a planet. Though the smaller body is often said to orbit the larger body itself, both bodies actually follow approximately {{gli|elliptical orbit|elliptical orbits}} around a common {{gli|barycenter|center of mass}} positioned at a focal point of each ellipse. The word "orbit" can variously refer to the elliptical trajectory itself or the act of following this trajectory, and can refer to a stable, regularly repeating trajectory as well as a non-repeating trajectory.}}

{{defn|A schematic diagram of a complete {{gli|orbit}}. For a binary system, it is typically presented from the {{gli|primary}}'s frame of reference.}}

{{defn|A parameter that determines how much an {{gli|orbit}} deviates from a perfect circle. For an {{gli|elliptical orbit}}, the eccentricity ranges from greater than zero to less than one.}}

{{defn|The set of parameters that uniquely define an {{gli|orbit}}.}}

File:Orbit1.svg

{{defn|The tilt of an object's {{gli|orbit}} around an astronomical body, expressed as the angle between the {{gli|orbital plane}} or axis of direction of the orbiting object and a {{gli|plane of reference}}.}}

{{defn|One of two points at which the {{gli|orbital plane|plane}} of an {{gli|orbit}} intersects a specified {{gli|plane of reference}} to which it is {{gli|orbital inclination|inclined}}; in some contexts, the two nodes may be distinguished as the {{gli|ascending node}} and the {{gli|descending node}}. A {{gli|non-inclined orbit}}, which is coplanar with the reference plane, has no nodes.}}

{{defn|The time a given {{gli|astronomical object}} takes to complete one {{gli|orbit}} around another object. For objects in the {{gli|Solar System}}, the orbital period is often referred to as the {{gli|sidereal period}}.}}

{{defn|The imaginary geometric plane defined by the {{gli|orbit}} of an {{gli|astronomical body}} around its {{gli|primary}}. The Earth's orbital plane, which defines the {{gli|ecliptic}}, is commonly used as a {{gli|plane of reference}} for the orbits of other objects in the {{gli|Solar System}}.}}

{{defn|The situation that occurs when two or more {{gli|orbit|orbiting}} bodies exert regular, periodic gravitational influences on each other such that one or more of their {{gli|orbital elements|orbital parameters}} (e.g. {{gli|orbital eccentricity|eccentricity}}, {{gli|semi-major axis}}, {{gli|inclination}}, etc., or any combination thereof) exist in some definite mathematical relationship with each other. Most commonly, the term refers to mean-motion orbital resonance, in which the bodies' {{gli|orbital period|orbital periods}} are related by a ratio of small integers. For example, the {{gli|dwarf planet}} Pluto exists in a stable 2:3 resonance with Neptune, such that Pluto completes two orbits around the {{gli|Sun}} in the same time it takes Neptune to complete three. Resonance may act on any time scale, from short-term to {{gli|secular}}, and often leads to either long-term stabilization of the orbits or their eventual destabilization.}}

File:Resonant planetary system.gif

{{defn|The speed at which an astronomical body or object {{gli|orbit|orbits}} around a {{gli|barycenter}}, or its speed relative to the center of mass of the most massive body in the system. The term may be used to refer to either the mean orbital speed, i.e. the average speed over the entire {{gli|orbital period}}, or the instantaneous speed at a particular point in the orbit. Maximum instantaneous orbital speed typically occurs at {{gli|periapsis}}.}}

{{defn|The hypothetical, idealized {{gli|Kepler orbit}} that an orbiting object would follow around its {{gli|primary}} if all {{gli|perturbation|perturbations}} were absent, i.e. the orbit that coincides with the instantaneous orbital state vectors at a given moment in time.{{cite book |first=Forest R. |last=Moulton |title=Introduction to Celestial Mechanics |orig-year=1902 |publisher=Dover |year=1970 |pages=322–23 |isbn=0486646874 |edition=2nd revised |location=Mineola, New York}}}}

{{defn|The vast, nearly empty expanse that exists beyond the Earth and between all {{gli|astronomical object|celestial bodies}}, characterized generally by extremely low densities of particles, extremely low temperatures, and minimal gravity. Most of the volume of the {{gli|Universe}} is intergalactic space, and even galaxies and star systems consist almost entirely of empty space.}}

P

File:Stellarparallax parsec1.svg

{{defn|A unit of length defined as the distance at which a star would show a {{gli|stellar parallax|parallax}} shift of exactly one arcsecond as observed from Earth's orbit. It is equal to 3.2616 {{gli|light-year|light-years}} or 206,265 {{gli|astronomical unit|astronomical units}}. The word "parsec" is a portmanteau of the words parallax and second.}}

{{defn|For a variable star, this is the highest amplitude achieved during a rise in luminosity, followed by a decline. This data point can provide useful distance information for a cataclysmic variable. It can be determined from a light curve of the stellar variability.}}

{{defn|The point at which an orbiting body is closest to its {{gli|primary}}. Contrast {{gli|apoapsis}}.}}

{{defn|The point at which a body orbiting the Earth (such as the {{gli|Moon}} or an {{gli|artificial satellite}}) is closest to the Earth. Contrast {{gli|apogee}}.}}

{{defn|The point at which a body orbiting the Earth's {{gli|Sun}} is closest to the Sun. Contrast {{gli|aphelion}}.}}

{{defn|The complex motion of an astronomical body that is subject to forces other than the gravitational attraction of its {{gli|primary}} alone, or any force which complicates the {{gli|orbit|orbital}} characteristics of the body such that the idealized {{gli|Kepler orbit}} of the {{gli|two-body problem}} is not an accurate representation of the body's actual orbit. Perturbing forces may include the gravitational forces exerted by any number of additional bodies, the off-center gravitational forces which are consequences of bodies not being perfectly spherical, and/or atmospheric resistance.}}

{{defn|The {{gli|elongation}} or angle between an orbiting body and the Sun as viewed from a particular perspective, such as the Earth. It determines the amount of a planet or moon's visible surface that lies in shadow. {{gli|inferior planet|Inferior planets}} such as Venus generally have low phase angles as seen from Earth, so they often appear as a slim crescent; {{gli|superior planet|superior planets}} such as Mars and Jupiter usually have high phase angles, so that little of the shadowed side is visible.}}

{{defn|An arbitrarily chosen, imaginary plane from which to measure and define {{gli|orbital elements}} such as {{gli|inclination}} and {{gli|longitude of the ascending node}}. The {{gli|ecliptic plane}}, {{gli|invariable plane}}, and {{gli|equatorial plane}} are all commonly used as reference planes in various contexts.}}

{{defn|An imaginary plane that is perpendicular to the line of sight. Typically this is used as a reference plane for the {{gli|inclination}} of an orbital plane of a distant star system.}}

{{defn|A type of {{gli|astronomical body}} {{gli|orbit|orbiting}} the Sun, which is massive enough to be rounded by its own gravity (but not massive enough to achieve thermonuclear fusion) and has {{gli|clearing the neighbourhood|cleared its neighbouring region}} of all {{gli|planetesimal|planetesimals}}. The term exoplanet is used in reference to a planet-like object that is not orbiting the Sun.}}

{{defn|Of or relating to a {{gli|planet}} or planets.}}

{{defn|Any {{gli|secondary body}} that is geologically differentiated or in hydrostatic equilibrium and therefore has a {{gli|planet}}-like geology, such as a {{gli|planet}}, {{gli|dwarf planet}}, or other {{gli|planetary-mass object}}, but excluding smaller objects such as {{gli|planetesimal|planetesimals}}.}}

{{defn|The process of separating out different constituents of a planetary body, causing it to develop compositionally distinct layers (such as a metallic core).}}

{{defn|A type of emission nebula formed from a glowing shell of expanding plasma that has been ejected from a {{gli|red giant}} star late in its life. The name derives from their resemblance to a {{gli|planet}}. An example is the Ring Nebula.}}

{{defn|The scientific study of {{gli|planet|planets}}, {{gli|natural satellite|moons}}, and {{gli|planetary system|planetary systems}}, with the aim of understanding their formation, composition, topography, dynamics, and interactions with other bodies.}}

{{defn|Any set of gravitationally bound non-{{gli|stellar}} objects in or out of {{gli|orbit}} around a {{gli|star}} or {{gli|star system}}. In general, planetary systems include one or more {{gli|planet|planets}}, though such systems may also consist of {{gli|dwarf planet|dwarf planets}}, {{gli|natural satellite|moons}}, {{gli|asteroid|asteroids}}, {{gli|meteoroid|meteoroids}}, {{gli|planetesimal|planetesimals}}, and {{gli|circumstellar disc|debris discs}}, among other objects.}}

{{defn|Any solid object (generally larger than {{convert|1|km}} in diameter) that arises during the formation of a {{gli|planet}} whose internal strength is dominated by self-gravity and whose orbital dynamics are not significantly affected by gas drag. The term is most commonly applied to small bodies thought to exist in {{gli|protoplanetary disk|protoplanetary disks}} and {{gli|debris disk|debris disks}} during the process of planet formation, but is also sometimes used to refer to various types of {{gli|small Solar System body|small Solar System bodies}} which are left over from the formation process. There is no precise distinction between a planetesimal and a {{gli|protoplanet}}.}}

{{defn|Another name for a {{gli|minor planet}} or {{gli|dwarf planet}}.}}

{{defn|An {{gli|orbit}} in which the orbiting object passes directly over or nearly over both poles of the body being orbited during each revolution. It therefore has an {{gli|orbital inclination|inclination}} equal or nearly equal to 90 degrees to the body's {{gli|equator}}.}}

{{defn|Any slow change in the orientation of an object's {{gli|axis of rotation}}. For the Earth in particular, this phenomenon is referred to as the {{gli|precession of the equinoxes}}. Apsidal precession refers to a steady change in the orientation of an orbit, such as the precession in the orbit of Mercury that was explained by the theory of general relativity.}}

{{ghat|Also gravitational primary, primary body, or central body.}}

{{defn|The main physical body of a gravitationally bound, multi-object system. The primary constitutes most of the system's mass and is generally located near the system's {{gli|barycenter}}.}}

{{defn|Orbital or rotational motion of an object in the same direction as the rotation of the object's {{gli|primary}}. The direction of rotation is determined by an inertial frame of reference such as the {{gli|fixed stars}}. Contrast {{gli|retrograde motion}}.}}

{{defn|The observed physical separation between two astronomical objects, as determined from their angular separation and estimated distance. For planets and {{gli|double star|double stars}}, this distance is usually given in {{gli|astronomical unit|astronomical units}}. The actual separation of the two objects depends on the angle of the line between the two objects to the line-of-sight of the observer.}}

{{defn|The rate of angular motion of an object over an interval of time, usually years. For stars, this is typically given in milliarcseconds per year.}}

{{defn|A large {{gli|planetary}} embryo that originated within a {{gli|protoplanetary disk}} and has since undergone internal melting to produce an interior of non-uniform composition. Protoplanets represent an intermediate step in the formation of a full-sized {{gli|planet}}; they are thought to form out of smaller {{gli|planetesimal|planetesimals}} as they collide with each other and gradually coalesce into larger bodies.}}

{{defn|A concentration of mass formed out of the contraction of a collapsing interstellar cloud. Once sufficient mass has fallen onto this central core, it becomes a pre-main-sequence star.}}

{{defn|For an eccentric orbit, this is a near synchronization of revolution and rotation at periastron.{{citation | postscript=.

| title=Tidal evolution in close binary systems.

| last=Hut | first=P.

| journal=Astronomy and Astrophysics

| volume=99 | pages=126–140 | date=June 1981

| bibcode=1981A&A....99..126H }}}}

{{defn|A highly magnetized rotating {{gli|neutron star}} or {{gli|white dwarf}} that emits a beam of electromagnetic radiation. This beam is observed only when it is pointing toward Earth, making the object appear to pulse.}}

Q

{{defn|A method of computing the mean strength of a varying stellar magnetic field. It is determined by calculating the root mean square of a series of longitudinal magnetic field strength measurements taken at different times.}}

{{defn|A configuration in which two celestial bodies have apparent {{gli|ecliptic coordinate system|ecliptic longitudes}} that differ by 90 degrees as viewed from a third body, e.g. when a planet's {{gli|elongation}} is perpendicular to the direction of the {{gli|Sun}} as viewed from the Earth. The term is used especially to describe the position of a {{gli|superior planet}} or the {{gli|Moon}} at its first and last quarter phases.}}

File:Quadrature (astronomy).svg

{{defn|A distant, point-like energy source originating from a powerful {{gli|active galactic nucleus}}. Its {{gli|luminosity}} is generated by the {{gli|accretion disk|accretion}} of gas onto a {{gli|supermassive black hole}}. Quasars emit radiation across the electromagnetic spectrum from radio waves to X-rays, and their ultraviolet and optical spectra are characterized by strong, broad emission lines.}}

R

{{defn|The velocity of an object along the line of sight to the observer, which in astronomy is usually determined via Doppler spectroscopy. Positive values are used to indicate a receding object. An object such as a star can undergo changes in its radial velocity because of the {{gli|perturbation|gravitational perturbation}} of another body, or because of radial pulsations of its surface. The latter, for example, occurs with a Beta Cephei variable star.}}

{{defn|The subfield of {{gli|astronomy}} that studies {{gli|astronomical object|astronomical objects}} at radio frequencies, using large radio antennas known as radio telescopes.}}

{{defn|Any {{gli|astronomical object}} that emits strong radio waves into space. These objects are the basis for {{gli|radio astronomy}}.}}

{{defn|A conspicuous trail of enlarged red stars found on the {{gli|Hertzsprung–Russell diagram}} for a typical {{gli|globular cluster}}. It begins at the main-sequence turnoff point and extends toward the higher luminosity and lower temperature range until reaching the red-giant tip. This branch consists of older stars that have evolved away from the main sequence but have not yet initiated helium fusion in their core region.}}

{{defn|An increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation.}}

{{defn|A {{gli|natural satellite}} following a relatively close and {{gli|prograde motion|prograde}} orbit with little or no {{gli|orbital inclination}} or {{gli|orbital eccentricity}}. Regular moons are thought to form in situ about their {{gli|primary}}, as opposed to {{gli|irregular moon|irregular moons}}, which are thought to be captured.}}

{{defn|A beam of ionised matter accelerated close to the speed of light. Most have been observationally associated with central black holes of some active galaxies, radio galaxies or quasars.}}

{{defn|A grid of fine lines or crosshatches engraved upon a transparent glass plate, which when placed in front of film during a photographic exposure produces a corresponding grid in the resulting photograph by creating permanent shadows on the film negative. These grids are used in some photographic telescopes to produce reference markers in photographs of distant stars, allowing precise and convenient measurement of astrometric positions.}}

{{defn|Orbital or rotational motion of an object in the direction opposite the rotation of the object's {{gli|primary}}. The direction of rotation is determined by an inertial frame of reference such as the {{gli|fixed stars}}. Contrast {{gli|prograde motion}}.}}

File:Retrogradeorbit.gif

{{defn|In the {{gli|equatorial coordinate system}}, the celestial equivalent of terrestrial longitude. It divides the {{gli|celestial equator}} into 24 hours, each of 60 minutes.}}

{{defn|A disk- or ring-shaped accumulation of various solid material such as {{gli|cosmic dust|dust}} and {{gli|moonlet|moonlets}} that orbits an {{gli|astronomical body}} such as a {{gli|planet}}. Ring systems are common components of {{gli|satellite system|satellite systems}} around {{gli|giant planet|giant planets}}, as with the Rings of Saturn. See also {{gli|circumplanetary disk}}.}}

{{defn|The distance from an astronomical object at which the {{gli|tidal force}} matches an orbiting body's gravitational self-attraction. Inside this limit, the tidal forces will cause the orbiting body to disintegrate, usually to disperse and form a {{gli|ring system|ring}}. Outside this limit, loose material will tend to coalesce.}}

{{ghat|Also interstellar planet, nomad planet, orphan planet, and starless planet.}}

{{defn|Any {{gli|planetary-mass object}} that orbits a {{gli|galactic nucleus|galactic center}} directly rather than a {{gli|star}} or {{gli|substellar object}}. Such objects have often been ejected from the {{gli|planetary system}} in which they formed, or otherwise have never been gravitationally bound to any star system.}}

{{defn|{{anchor|Rosseland optical depth}}An extinction coefficient of an {{gli|atmosphere}}, which describes the net {{gli|opacity}} to radiation at a given depth. See {{gli|optical depth}}.}}

{{defn|The time that an object takes to complete a single revolution about its own {{gli|axis of rotation}} relative to the {{gli|background stars}}. It is not necessarily the same as the object's {{gli|synodic day}} or {{gli|sidereal day}}.}}

{{defn|A phenomenon which causes the {{gli|luminosity}} of a star to vary as rotation carries {{gli|star spot|star spots}} or other localized activity across the line of sight. Examples include RS CVn and BY Dra variables.}}

S

{{defn|A smaller companion {{gli|galaxy}} that orbits within the gravitational potential of a more massive and luminous host galaxy; e.g. the Large Magellanic Cloud is a satellite galaxy of the {{gli|Milky Way}}.}}

{{defn|A distant circumstellar disc in the {{gli|Solar System}} that is sparsely populated by icy {{gli|small Solar System bodies}}, which are a subset of the broader family of {{gli|trans-Neptunian object}}s.}}

{{defn|Rapid variations in the apparent {{gli|apparent magnitude|brightness}}, color, or position of a {{gli|star}} (or any other distant luminous object) as viewed through a medium, such as the Earth's atmosphere, caused by the passing of light through layers of turbulence in the medium. Most terrestrial scintillation effects are the result of atmospheric refraction caused by small-scale fluctuations in air density, and are much more pronounced near the {{gli|horizon}}, since light rays near the horizon must travel longer paths through the atmosphere before reaching the observer.}}

{{defn|Continuing, or changing in a non-periodic way, over a long period of time.{{cite book |last1=Mitton |first1=Jacqueline |title=Cambridge Illustrated Dictionary of Astronomy |date=2007 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=978-0-521-82364-7 |url=https://openlibrary.org/works/OL1826779W/Cambridge_Illustrated_Dictionary_of_Astronomy?edition=ia%3Aunset0000unse_f5i7}}}}

{{defn|Any change in movement that happens over a very long time period. Examples include the perihelion precession of Mercury, the tidal acceleration of the Earth–Moon system, and {{gli|axial precession|precession of the Earth's axis}}.}}

{{defn|With reference to, or pertaining to, the geometric center of the Earth's {{gli|Moon}}.}}

{{defn|The angle at the position of an observer subtended by the equatorial radius of the Sun, the Moon, or a planet.}}

{{defn|One half of the longest diameter (the major axis) of an ellipse. It is expressed in units of length and often used to give a physical dimension to a two-body {{gli|elliptical orbit|elliptical}} {{gli|Kepler orbit}}, such as for a {{gli|binary star}} system or star–planet system. When the distance between the orbiting bodies is unknown, the semi-major axis may be given as an angle.}}

{{defn|The precise time of year on Earth when the Sun appears to cross the {{gli|celestial equator}}, while generally trending southward at each {{gli|zenith}} passage. It represents the moment at which the North Pole of the Earth begins to tilt away from the Sun, and typically occurs on or near September 22 each year. It is the autumnal equinox in the Northern Hemisphere and the vernal equinox in the Southern Hemisphere. Contrast {{gli|March equinox}}.}}

{{defn|The {{gli|rotation period}} of an object (e.g. the Earth) with respect to the distant {{gli|fixed stars}} of its own {{gli|celestial sphere}} (rather than to its primary star, e.g. the {{gli|Sun}}), measured as the time it takes for the fixed stars, as viewed from a particular point on the object's surface, to return to the same position in the {{gli|sky}} on consecutive nights. The Earth's sidereal day is equal to approximately 86,164.09 seconds (23 hours, 56 minutes, 4.09 seconds), about four minutes shorter than the {{gli|solar day}}, which instead reckons time based on the Sun's position in the sky.}}

{{defn|The {{gli|orbital period}} of an object within the {{gli|Solar System}}, e.g. the Earth's orbital period around the Sun. The name "sidereal" implies that the object returns to the same position relative to the {{gli|fixed stars}} of the {{gli|celestial sphere}} as observed from the Earth.}}

{{defn|The calculation of the passage of time based on the {{gli|diurnal motion}} of the {{gli|fixed stars}} in the Earth's {{gli|sky}}.{{cite web |title=Section M: Glossary |url=https://asa.hmnao.com/SecM/Glossary.html |website=The Astronomical Almanac Online |publisher=U.S. Naval Observatory and H.M. Nautical Almanac Office |access-date=2021-11-22 |archive-date=2022-02-23 |archive-url=https://web.archive.org/web/20220223151156/http://asa.hmnao.com/SecM/Glossary.html |url-status=dead }} The fundamental unit of sidereal time is the {{gli|sidereal day}}, i.e. the time interval between two successive returns of the fixed stars to the local {{gli|meridian}}, as viewed from a given location on the Earth's surface.}}

{{defn|The time that Earth or another planetary body takes to orbit the Sun once with respect to the fixed stars.}}

{{defn|Everything that lies above the surface of the Earth, including the atmosphere and {{gli|outer space}}. In the context of {{gli|astronomy}}, the term "sky" is also used as another name for the {{gli|celestial sphere}}. See also {{gli|night sky}}.}}

{{defn|An object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The SSSBs are: the comets; the classical asteroids, with the exception of the dwarf planet Ceres; the trojans; and the centaurs and trans-Neptunian objects, with the exception of the dwarf planets.}}

{{defn|A {{gli|synodic day}} on Earth, i.e. the {{gli|rotation}} of the Earth with respect to the {{gli|Sun}}, measured as the time it takes for the Sun, as viewed from a particular point on the Earth's surface, to return to the same position in the {{gli|sky}} (e.g. to cross the same {{gli|meridian}}) on consecutive days. Because the Earth's orbit around the Sun affects the angle at which the Sun is seen from the Earth, the Sun appears to take slightly longer to return to the same position than do the {{gli|fixed stars}}, which results in the solar day being on average about four minutes longer than the {{gli|sidereal day}}. The length of the solar day is also not constant, but rather changes over the course of the year because the Earth's orbit is {{gli|eccentricity|not perfectly circular}} and because its rotational axis is {{gli|axial tilt|not perpendicular}} to its orbital plane. One {{gli|mean solar time|mean solar day}} (averaged over the Earth's orbital period) is currently equal to 86,400 seconds, or exactly 24 hours.}}

{{defn|An {{gli|occultation}} of the {{gli|Sun}} by the Earth's {{gli|Moon}}, in which a portion of the Earth passes through the shadow cast by the Moon, temporarily blocking sunlight, fully or partially, from reaching that portion of the Earth's surface. A solar eclipse occurs when the Moon is {{gli|syzygy|precisely aligned}} between the Sun and the Earth. Because all three bodies are continuously moving, the shadow of the Moon traces out a narrow path across the Earth's surface, and from any given location within or very close to this path, the eclipse is visible only for a short duration. Depending on the observer's location and on the apparent sizes of the solar and lunar disks in the sky, an eclipse may appear to be {{gli|total solar eclipse|total}}, {{gli|partial solar eclipse|partial}}, or {{gli|annular eclipse|annular}}.}}

{{defn|Solar faculae are bright spots in the {{gli|photosphere}} of the Sun that form in the canyons between solar granules. They are produced by concentrations of magnetic field lines. The Sun's faculae are most readily observed near the {{gli|limb darkening|solar limb}}. Faculae occur on other stars (see facula).}}

{{defn|An intense, localized emission of electromagnetic radiation in the Sun's atmosphere.}}

{{defn|Transient, collimated flows of plasma in the Sun's atmosphere.}}

{{anchor|solar mass}}{{term|term=solar mass|content=solar mass ({{solar mass}})}}

{{defn|A standard unit of mass equal to the mass of the Earth's {{gli|Sun}}, or approximately {{val|1.98847|e=30|ul=kg}}. It is commonly used to express the masses of other {{gli|star|stars}} and astronomical objects relative to the Sun.}}

{{defn|A feature in the Sun's atmosphere that appears as bright, "sponge-like" patches in extreme ultraviolet light, occurring above the Sun's visible surface at the base of hot coronal loops in active regions.}}

{{defn|A large, bright, transient feature, often in the shape of a {{gli|coronal loop|loop}}, consisting of plasma extending outward from the {{gli|Sun}}'s {{gli|visible surface|photosphere}} into the {{gli|corona}}. Prominences may be hundreds of thousands of kilometers long.}}

{{anchor|solar radius}}{{term|term=solar radius|content=solar radius ({{solar radius}})}}

{{defn|A standard unit of distance equal to the radius of the Earth's {{gli|Sun}} (typically measured from the Sun's center to the layer in the {{gli|photosphere}} at which the optical depth equals 2/3), or approximately {{convert|695,700|km|mi}}. It is commonly used to express the radii of other {{gli|star|stars}} and astronomical objects relative to the Sun.}}

{{defn|See {{gli|geomagnetic storm}}.}}

{{defn|A dynamic jet of plasma in the Sun's chromosphere about 300 km in diameter.}}

{{defn|The gravitationally bound {{gli|planetary system}} of the Earth's {{gli|Sun}} and all of the objects that orbit it, either directly or indirectly, including the eight true {{gli|planet|planets}}, five {{gli|dwarf planet|dwarf planets}}, and numerous {{gli|small Solar System body|small Solar System bodies}} such as {{gli|asteroid|asteroids}}, {{gli|comet|comets}}, and {{gli|natural satellite|natural satellites}}.}}

{{defn|The calculation of the passage of time based on the {{gli|diurnal motion}} of the {{gli|Sun}} in the Earth's {{gli|sky}}.{{cite web |title=Section M: Glossary |url=https://asa.hmnao.com/SecM/Glossary.html |website=The Astronomical Almanac Online |publisher=U.S. Naval Observatory and H.M. Nautical Almanac Office |access-date=2021-11-22 |archive-date=2022-02-23 |archive-url=https://web.archive.org/web/20220223151156/http://asa.hmnao.com/SecM/Glossary.html |url-status=dead }} The fundamental unit of solar time is the {{gli|synodic day|solar day}}, i.e. the time interval between two successive returns of the Sun to the local {{gli|meridian}}, as viewed from a given location on the Earth's surface. Because the duration of this interval changes during the Earth's orbit around the Sun, {{gli|apparent solar time}} is distinguished from {{gli|mean solar time}}. Solar time and {{gli|sidereal time}} were employed by astronomers as time reckoning systems before the introduction of {{gli|ephemeris time}}.}}

{{defn|A stream of charged particles, primarily protons, electrons, and alpha particles, released from the {{gli|Sun}}'s {{gli|corona}} and flowing outwards at up to {{convert|900|km/s|mph}} into interplanetary space. Phenomena influenced by the solar wind include {{gli|aurora}}, {{gli|geomagnetic storm|geomagnetic storms}}, and the plasma tails of {{gli|comet|comets}}.}}

{{defn|Either of the two precise times of year when the {{gli|Sun}} reaches its most northerly or most southerly point in the {{gli|sky}} as seen from Earth; or, equivalently, when the Sun's apparent geocentric {{gli|ecliptic coordinate system|longitude}} is either 90 degrees or 270 degrees. The solstices occur on or near June 20 and December 21 each year. The {{gli|June solstice}}, called the {{gli|summer solstice}} in the Northern Hemisphere, is the annual date featuring the longest duration of daylight and the shortest duration of nighttime for any given point in the Northern Hemisphere; the reverse is true in the Southern Hemisphere, where the June date is the {{gli|winter solstice}}.}}

{{defn|See {{gli|stellar classification}}.}}

{{defn|A type of {{gli|binary star}} system where the individual components have not been resolved with a {{gli|telescope}}. Instead, the evidence for the binarity comes from shifts observed in the spectrum. This is caused by the Doppler effect as the {{gli|radial velocity}} of the components change over the course of each orbit.}}

File:Atmospheric electromagnetic opacity.svg

{{defn|A branch of {{gli|observational astronomy}} which is used to locate the positions of astronomical objects on the {{gli|celestial sphere}} as they would appear from a particular date, time, and location on Earth. It relies on the mathematical methods of spherical geometry and the measurements of {{gli|astrometry}}.}}

{{anchor|standard gravity}}{{term|term=standard gravity|content=standard gravity ({{math|ɡ₀}} or {{math|ɡ_n}})}}

{{defn|The nominal gravitational acceleration of an object in a vacuum near the surface of the Earth, as a result of Earth's gravity and, less importantly, the centrifugal force generated by its rotation. It is by definition equal to {{val|9.80665|u=m/s2}} (approximately {{val|32.17405|u=ft/s2}}).}}

{{defn|A massive, luminous spheroid of plasma held together by its own gravity which, for at least a portion of its life, radiates energy into {{gli|outer space}} due to the thermonuclear fusion of hydrogen into helium within its core. Astronomers can determine the mass, age, platemperature, chemical composition, and many other properties of a star by observing its motion through space, its {{gli|luminosity}}, and its {{gli|spectroscopy|emission spectrum}}.}}

{{defn|Any small number of {{gli|star|stars}} that orbit each other, bound by gravitational attraction, such as a {{gli|binary star}} system. In the broadest sense, very large groups of stars bound by gravitation such as {{gli|star cluster|star clusters}} and {{gli|galaxy|galaxies}} are also star systems. Star systems are distinct from {{gli|planetary system|planetary systems}}, which include {{gli|planet|planets}} and other bodies such as {{gli|comet|comets}}.}}

{{defn|Any {{gli|galaxy}} that has an anomalously high rate of star formation. The criteria for a starburst is a star formation rate that would normally consume the galaxy's available supply of unbound gas within a time period shorter than the age of the galaxy. Most starbursts occur as a result of galactic interactions, such as a merger.}}

{{defn|Any set of {{gli|star|stars}} visible in an arbitrarily sized field of view of a {{gli|telescope}}, usually in the context of some region of interest within the {{gli|celestial sphere}}. For example, the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.}}

{{defn|Of or relating to a {{gli|star}} or {{gli|star system}}.}}

{{defn|The outermost region of a {{gli|star}}, located above the stellar core, radiation zone, and convection zone. Although it constitutes only a small portion of the star's mass, for some evolved stars the stellar envelope can encompass a significant fraction of the radius.}}

{{defn|The categorization of {{gli|star|stars}} based upon their {{gli|spectrum|spectra}}. The modern {{gli|Morgan–Keenan spectral classification}} scheme is a two-dimensional classification based on temperature and {{gli|luminosity}}.}}

{{defn|no=1|The region within the volume of a star that transports energy from the stellar core to the {{gli|stellar atmosphere}}; or another name for the stellar atmosphere itself.}}

{{defn|no=2|The {{gli|common envelope}} of gases encompassing a {{gli|binary star}} system.}}

{{defn|An {{gli|astrophysics|astrophysical}} model of a star's {{gli|stellar evolution}} over time based upon its mass and chemical composition.}}

{{defn|A magnetic field generated by the convective motion of plasma inside a {{gli|star}}, responsible for phenomena such as {{gli|starspot|starspots}} and {{gli|coronal loop|coronal loops}}.}}

{{defn|The subfield of {{gli|astronomy}} that studies {{gli|astronomical object|astronomical objects}} detectable at submillimetre wavelengths (i.e. terahertz radiation).}}

{{defn|Any {{gli|natural satellite|natural}} or {{gli|artificial satellite}} that orbits another natural satellite, i.e. "a moon of a moon".}}

{{defn|An {{gli|astronomical object}} whose mass is smaller than the smallest mass at which the fusion of hydrogen nuclei can be sustained (equivalent to approximately 0.08 {{gli|solar mass|solar masses}}), including {{gli|brown dwarf|brown dwarfs}} and some {{gli|stellar remnant|stellar remnants}}, as well as certain {{gli|planetary-mass object|planetary-mass objects}}.}}

{{defn|An archaic term that is sometimes used to refer to {{gli|planet|planets}} that orbit further from the Sun than the Earth, such as Saturn. The name originated from the geocentric {{gli|cosmology}} of Ptolemy. Contrast {{gli|inferior planet}}.}}

{{defn|One of a class of very large {{gli|black hole|black holes}} which possess masses ranging from hundreds of thousands to many billions of times the {{gli|solar mass|mass of the Sun}}. These are typically found at a {{gli|galactic nucleus|galactic core}}, where they can have a profound effect upon the evolution of the surrounding {{gli|galaxy}}.}}

{{defn|An extremely {{gli|luminosity|luminous}}, {{gli|transient}} stellar explosion occurring during a massive {{gli|star}}'s final {{gli|stellar evolution|evolutionary}} stages or when a {{gli|white dwarf}} is triggered into runaway nuclear fusion.}}

{{defn|The gravitational acceleration experienced at the equatorial surface of an {{gli|astronomical body}} or other {{gli|astronomical object|object}}, including that produced by the effects of rotation. It is typically expressed in units of acceleration such as meters per second squared (m/s²) or as a multiple of the Earth's {{gli|standard gravity}}, which is equal to {{val|9.80665|u=m/s2}}.}}

{{defn|Any {{gli|orbit}} in which an object orbits its {{gli|primary}} with an {{gli|orbital period}} equal to the average {{gli|rotation period|rotational period}} of the primary and in the {{gli|prograde motion|same direction}} as the primary's rotation.}}

{{defn|The time it takes for an object to rotate once about its own {{gli|axis of rotation|axis}} (i.e. its {{gli|rotation period}}) relative to the {{gli|primary}} it is orbiting (rather than to the much more distant {{gli|fixed stars}}). The synodic day may be described as the time between two consecutive sunrises (in the case where the primary is a {{gli|star}}), which is not necessarily the same as the {{gli|sidereal day}}. As it does on Earth, an object's synodic day may change slightly in duration over the course of the {{gli|orbital period}} due to {{gli|orbital eccentricity|eccentricity}} and {{gli|axial tilt}}; Earth's synodic day is often called a {{gli|solar day}}.}}

{{defn|The time it takes for a body visible from another body (often the Earth) to complete a cycle with respect to the {{gli|fixed stars|background stars}} visible in the second body's {{gli|celestial sphere}}. Synodic period is most commonly used to indicate the elapsed time between a given body's consecutive appearances in the {{gli|aspect|same location}} in the {{gli|night sky}} as observed from Earth, but can in principle be calculated with respect to the sky as observed from any body. It is related to but distinct from the {{gli|orbital period}}, a result of the fact that both the body being studied (e.g. Jupiter) and the body from which it is being observed (e.g. Earth) are independently orbiting a third body (the {{gli|Sun}}).}}

{{defn|The calculation of the passage of time based on successive {{gli|conjunction|conjunctions}} of an astronomical object, such as a {{gli|planet}} (i.e. successive returns of the object to the same {{gli|aspect}} in the Earth's {{gli|sky}}).}}

{{defn|The straight-line configuration of three celestial bodies in a gravitational system.}}

T

{{defn|The component of the velocity of a star or other {{gli|astronomical body}} that is perpendicular to the line of sight of the {{gli|observer}} (i.e. in the tangent plane). This component can be computed from the body's observed {{gli|proper motion}} and its measured distance from the observer.}}

{{defn|A device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation.}}

{{defn|A {{gli|star}} with nearly featureless continuum spectra that can be used to correct for the effect of telluric contamination of the Earth's atmosphere on the spectra of other stars. For example, water vapor in the atmosphere creates significant telluric absorption bands at wavelengths above 6800 Å. These features need to be corrected for in order to more accurately measure the spectrum.}}

{{defn|The boundary within the {{gli|heliosphere}}, approximately 75 to 90 {{gli|astronomical unit|AU}} from the {{gli|Sun}}, beyond which the {{gli|solar wind}} slows to subsonic speeds (relative to the Sun) as a result of interactions with the local {{gli|interstellar medium}}.}}

{{defn|The line that divides the illuminated side of a {{gli|moon}} or {{gli|planet}} from its dark side. The line moves as the object rotates with respect to its parent {{gli|star}}.}}

{{defn|A branch of astronomy that uses analytical and computational models based on principles from physics and chemistry to describe, explain, and model the properties of astronomical objects and phenomena, with the ultimate goal of accurately predicting the observable or testable consequences of those models.}}

{{defn|One of the structural components of about 2/3 of all disk galaxies, including the Milky Way.}}

{{defn|The layer of the {{gli|Milky Way}} galaxy where the spiral arms are found and where most of the star formation takes place. It is about {{Convert|300|–|400|pc|ly|abbr=off|lk=off}} deep and centered on the {{gli|galactic plane}}. Stars belonging to this population generally follow orbits that lie close to this plane. This is in contrast to members of the {{gli|thick disk population}} and {{gli|halo star|halo stars}}.}}

{{defn|The transfer of momentum between an astronomical body and an orbiting satellite as the result of {{gli|tidal force|tidal forces}}. This can cause changes in the {{gli|rotation period|rotation periods}} for both bodies as well as modification of their mutual orbit. A satellite in a {{gli|prograde motion|prograde}} orbit will gradually recede from its primary while slowing the rotation rate of both bodies.}}

{{defn|A gravitational effect that stretches a body along the line towards and away from the center of mass of another body due to spatial variations in strength in gravitational field from the other body.}}

{{defn|The net result of continued {{gli|tidal braking}} such that, over the course of an orbit, there is no net transfer of angular momentum between an astronomical body and its gravitational partner. When the {{gli|eccentricity|orbital eccentricity}} is low, the result is that the {{gli|natural satellite|satellite}} orbits with the same face always pointed toward its {{gli|primary}}. An example is the {{gli|Moon}}, which is tidally locked with the Earth.}}

{{defn|A stream of {{gli|star|stars}} and gases which are stripped from gas clouds and star clusters because of interaction with the gravitational field of a {{gli|galaxy}} such as the {{gli|Milky Way}}.}}

{{defn|The gradual reduction of the {{gli|axial tilt|obliquity}} of an orbiting {{gli|natural satellite|satellite}} due to tidal interactions.}}

{{anchor|Tisserand's parameter}}{{term|term=Tisserand's parameter|content=Tisserand's parameter ({{mvar|T}})}}

{{defn|A measure of the orbital motion of a relatively small body (e.g. an {{gli|asteroid}} or {{gli|comet}}) with respect to a larger, perturbing body (e.g. a {{gli|planet}}), used for {{gli|restricted three-body problem|restricted three-body problems}} in which the three bodies all differ greatly in mass. The parameter is calculated from the {{gli|orbital elements}} of each body, including the small body's {{gli|semimajor axis}}, {{gli|eccentricity}}, and {{gli|orbital inclination|inclination}}, and is useful in specifically identifying small bodies observed before and after planetary encounters, as its numerical value remains largely constant throughout the body's lifetime. It is also used to distinguish between different kinds of orbits which are characteristic of different classes of bodies.}}

{{defn|With reference to, or pertaining to, a point on the surface of the Earth.}}

File:Solar eclipse 1999 4.jpg

{{defn|Any minor planet in the {{gli|Solar System}} that orbits the Sun at a greater average distance than Neptune.}}

{{defn|no=1|The passage of a particular {{gli|celestial object}} across a particular {{gli|meridian}}.}}

{{defn|no=2|An astronomical event during which a celestial body or object passes visibly across the face of a much larger body. An example is the transit of Venus across the face of the {{gli|Sun}}, which was visible from Earth in 2004 and 2012. Because a transit results in a decrease in the net {{gli|luminosity}} from the two objects, the transit method can be used to detect {{gli|extrasolar planet|extrasolar planets}} as they pass in front of their host stars. A transit by an object that appears roughly the same size or larger than the body it is transiting is called an {{gli|occultation}} or {{gli|eclipse}}.}}

{{defn|A small celestial body (mostly asteroids) that shares the orbit of a larger body, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of its {{gli|Lagrangian point}}s.}}

{{defn|The time that the Sun takes to return to the same position in the sky.}}

{{anchor|true anomaly}}{{term|term=true anomaly|content=true anomaly ({{mvar|ν}}, {{mvar|θ}}, or {{mvar|f}})}}

{{defn|The angle between the direction of {{gli|periapsis}} and the current position of an orbiting body as it moves along an {{gli|elliptical orbit}}, as measured from the nearest focus of the ellipse. The true anomaly is one of three angular parameters that define a position along an orbital path, the other two being the {{gli|eccentric anomaly}} and the {{gli|mean anomaly}}, and also one of six canonical {{gli|orbital elements}} used to characterize an orbit.}}

{{defn|An empirical relationship between the mass or intrinsic {{gli|luminosity}} of a {{gli|spiral galaxy}} and its angular velocity or emission line width. It can be used to estimate the distance of the galaxy, and hence forms a rung on the {{gli|cosmic distance ladder}}.}}

{{defn|The time period immediately before sunrise and after sunset during which, despite the Sun being completely below the horizon, the scattering of sunlight by the Earth's atmosphere supplies significant illumination to the ambient environment. Several definitions of twilight are commonly distinguished, including {{gli|astronomical twilight|astronomical}}, {{gli|civil twilight|civil}}, and {{gli|nautical twilight}}.}}

{{defn|A problem of classical mechanics to calculate and predict the motion of two massive bodies that are orbiting each other in space.}}

U

{{ghat|Also the Johnson system or Johnson–Morgan system.}}

{{defn|defn=A photometric system usually employed for classifying stars according to their colors.}}

{{defn|no=1|The entirety of space and time and their contents, including {{gli|galaxy|galaxies}}, {{gli|star|stars}}, {{gli|planet|planets}}, all other forms of matter and energy, and the physical laws and constants that describe them. When not otherwise qualified, "the Universe" usually refers to the entire Universe, whose spatial extent is unknown because it is not directly measurable; this is distinguished from the {{gli|observable universe}}, whose size it is possible to measure.}}

{{defn|no=2|One of many hypothetical parallel universes which exist as causally disconnected constituent parts of a larger {{gli|multiverse}}, which itself comprises all of space and time and their contents.}}

V

{{defn|Any {{gli|star}} that is observed to vary in brightness. This variation may be periodic, with one or more cycles that last hours, days, months, or even years. Some stars vary in an irregular manner, while others undergo cataclysmic changes in brightness. Other forms of variability are intrinsic changes to the star's {{gli|radial velocity}} or its profile of spectral lines.}}

{{defn|The statistical dispersion of velocities about the mean velocity for a group of objects, such as stars in a {{gli|globular cluster}} or galaxies in a {{gli|open cluster|galactic cluster}}. This value can be used to derive the combined mass of the group by using the virial theorem.}}

W

{{defn|A reference to the faintness of the spectral lines for a star compared to standard stars with the same {{gli|stellar classification}}. Since most absorption lines are caused by elements other than hydrogen and helium—what astronomers refer to as "metals"—these are sometimes called metal weak stars.}}

{{defn|A type of {{gli|stellar remnant}} composed mostly of electron-degenerate matter. A white dwarf lacks the mass needed to continue the nuclear fusion process with its constituent atoms, so the object's energy output normally comes from radiative cooling. See {{gli|nova}} and Type Ia supernova.}}

{{defn|A correlation between the width of the singly ionized calcium K-line (Ca II K) at 3933 Å and the {{gli|absolute magnitude|absolute visual magnitude}} of the emitting {{gli|late-type star|late-type stars}}. This linear relation makes it useful for determining the distances of G, K, and M-type stars.}}

X

{{defn|A seemingly normal {{gli|galaxy}} that does not appear to have an {{gli|active galactic nucleus}}, yet displays an anomalous level of excess X-ray emission.}}

{{defn|A source of X-rays. They are usually produced when a high-mass object, usually a neutron star or black hole and a companion star are in a binary system.}}

Z

{{defn|The point in the {{gli|sky}} that is directly overhead from the perspective of a particular location on the Earth.}}

{{defn|The sequence of positions along the {{gli|Hertzsprung–Russell diagram}} achieved by newly formed, chemically homogeneous {{gli|star|stars}} which have finished contracting and have reached hydrostatic equilibrium, with energy being derived solely from nuclear fusion.}}

{{defn|The area of the {{gli|sky}} that extends approximately 8 degrees north or south (in {{gli|celestial sphere|celestial latitude}}) of the {{gli|ecliptic plane|ecliptic}}, the apparent path of the {{gli|Sun}} across the {{gli|celestial sphere}} over the course of the year as observed from Earth. The Sun, {{gli|Moon}}, and visible planets appear to travel across a band of twelve Zodiac constellations within this belt as the Earth orbits the Sun.}}

{{defn|A band of light in the night sky, thought to be sunlight reflected from cometary dust concentrated in the plane of the zodiac, or ecliptic. }}

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Astronomy

Category:Wikipedia glossaries using description lists