An Etymological Dictionary of Astronomy and Astrophysics

1. Physics: That part of the boundless four dimensional continuum in which matter can be physically extended.
   
2. Astro.: That part of the Universe which lies beyond the Earth’s atmosphere and in which the density of matter is low. Also known as outer space.

Etymology (EN): M.E., from O.Fr. espace, from L. spatium “room, area, distance, stretch of time,” of unknown origin.

Etymology (PE): Fazâ, loan from Ar.

1. Physics: That part of the boundless four dimensional continuum in which matter can be physically extended.
   
2. Astro.: That part of the Universe which lies beyond the Earth’s atmosphere and in which the density of matter is low. Also known as outer space.

Etymology (EN): M.E., from O.Fr. espace, from L. spatium “room, area, distance, stretch of time,” of unknown origin.

Etymology (PE): Fazâ, loan from Ar.

Electricity: An electric charge belonging to a cloud of electrons lying between a cathode and plate within an electric tube.
Geophysics: An excess of either negatively or positively charged ions in a layer of the atmosphere, giving that layer either a negative or positive charge.

See also: → space; → charge.

Electricity: An electric charge belonging to a cloud of electrons lying between a cathode and plate within an electric tube.
Geophysics: An excess of either negatively or positively charged ions in a layer of the atmosphere, giving that layer either a negative or positive charge.

See also: → space; → charge.

Man-made objects in orbit around the Earth that no longer serve any useful purpose. The estimated number of debris include about 22,000 tractable objects larger than 10 cm in all orbits, of which 2,200 are dead satellites and last stages of the rocket that put them in orbit. There are also left-overs from spacecraft and mission operations, such as bolts, lens caps, clamp bands, auxiliary motors, etc. The debris presents a threat because of their high speeds, which ranges between 15 and 20 km/sec. Also called space junk, space waste, orbital debris.

See also: → space; → debris.

Man-made objects in orbit around the Earth that no longer serve any useful purpose. The estimated number of debris include about 22,000 tractable objects larger than 10 cm in all orbits, of which 2,200 are dead satellites and last stages of the rocket that put them in orbit. There are also left-overs from spacecraft and mission operations, such as bolts, lens caps, clamp bands, auxiliary motors, etc. The debris presents a threat because of their high speeds, which ranges between 15 and 20 km/sec. Also called space junk, space waste, orbital debris.

See also: → space; → debris.

A travel outside the Earth by manned or unmanned vehicle requiring space technology.

See also: → space; → flight.

A travel outside the Earth by manned or unmanned vehicle requiring space technology.

See also: → space; → flight.

Set of operations (rotation about an axis, reflection across a plane, translation, or combination of these) which when carried out on a periodic arrangement of points in space brings the system of points to self-coincidence.

Etymology (EN): The word group comes from the mathematical notion of a group.

Set of operations (rotation about an axis, reflection across a plane, translation, or combination of these) which when carried out on a periodic arrangement of points in space brings the system of points to self-coincidence.

Etymology (EN): The word group comes from the mathematical notion of a group.

A manned or unmanned space flight outside the Earth’s atmosphere.

Etymology (EN): → space; mission, from L. missionem (nominative missio) “act of sending,” from mittere “to send,” of unknown origin.

Etymology (PE): Gosilân, from gosil, variant gosi “sending away, dismission;” Mid.Pers. wisé “to despatch” (Parthian Mid.Pers. wsys- “to despatch;” Buddhist Mid.Pers. wsydy “to despatch;” Sogdian ‘ns’yd- “to exhort”), from Proto-Iranian *vi-sid- “to despatch, send off,” from prefix vi- “apart, away, out,” + *sid- “to call” + -ân nuance suffix; fazâyi adj. of fazâ, → space.

A manned or unmanned space flight outside the Earth’s atmosphere.

Etymology (EN): → space; mission, from L. missionem (nominative missio) “act of sending,” from mittere “to send,” of unknown origin.

Etymology (PE): Gosilân, from gosil, variant gosi “sending away, dismission;” Mid.Pers. wisé “to despatch” (Parthian Mid.Pers. wsys- “to despatch;” Buddhist Mid.Pers. wsydy “to despatch;” Sogdian ‘ns’yd- “to exhort”), from Proto-Iranian *vi-sid- “to despatch, send off,” from prefix vi- “apart, away, out,” + *sid- “to call” + -ân nuance suffix; fazâyi adj. of fazâ, → space.

The velocity and direction of motion of a star or celestial object with respect to the Local Standard of Rest. Same as → peculiar velocity.

See also: → space; → motion.

The velocity and direction of motion of a star or celestial object with respect to the Local Standard of Rest. Same as → peculiar velocity.

See also: → space; → motion.

A spacecraft carrying instruments intended for use in exploration of the physical properties of outer space or celestial bodies other than Earth.

See also: → space; → probe.

A spacecraft carrying instruments intended for use in exploration of the physical properties of outer space or celestial bodies other than Earth.

See also: → space; → probe.

A reusable space vehicle designed to travel between the Earth and an orbiting space station for specific missions (carrying a crew and a cargo deploying and retrieving satellites) and then to return.

Etymology (EN): → space; M.E. shotil (n.); O.E. scytel “a dart, arrow;” cf. O.N. skutill “harpoon;” akin to shut, shoot.

Etymology (PE): Nâvak “small ship; ship like,” from nâv “ship” (O.Pers./Av. *nāv-, O.Pers. nāviyā- “fleet;” cf. Skt. nau-, nava- “ship, boat;” Gk. naus “ship;” L. nauticus “pertaining to ships or sailors”) + -ak diminutive/similarity suffix. Nâvak also means “a small arrow, an arrow flying swift,” which may have a different origin.

A reusable space vehicle designed to travel between the Earth and an orbiting space station for specific missions (carrying a crew and a cargo deploying and retrieving satellites) and then to return.

Etymology (EN): → space; M.E. shotil (n.); O.E. scytel “a dart, arrow;” cf. O.N. skutill “harpoon;” akin to shut, shoot.

Etymology (PE): Nâvak “small ship; ship like,” from nâv “ship” (O.Pers./Av. *nāv-, O.Pers. nāviyā- “fleet;” cf. Skt. nau-, nava- “ship, boat;” Gk. naus “ship;” L. nauticus “pertaining to ships or sailors”) + -ak diminutive/similarity suffix. Nâvak also means “a small arrow, an arrow flying swift,” which may have a different origin.

A program aimed at monitoring the near-Earth environment for recognizing and
preventing space hazards by means of radar and optical observations from either space or the ground. The objective of the → European Space Agency initiative is to support the European independent utilization of, and access to, space for research or services, through the provision of timely and quality data, information, services and knowledge regarding the space environment, the threats and the sustainable exploitation of the outer space surrounding our planet Earth.

The SSA Program was authorized at the November 2008 Ministerial Council and formally launched on 1 January 2009. The mandate was extended at the 2012 and 2016 Ministerial Councils, and the program is funded through to 2020. The program comprises three segments:

1. Space Surveillance and Tracking (SST), which is
   the monitoring and tracking of every object orbiting the Earth, such as satellites, space stations and debris. The objective is the prediction and warning of collisions and re-entry events.

2. → Space Weather (SWE), which aims at detection and forecasting of space weather and its effects through
   monitoring of the Sun, solar wind, magnetosphere, radiation belts, ionosphere and disturbances in the geomagnetic field.

3. → Near-Earth Objects (NEOs), which provides warning services against potential asteroid impact risks, including discovery, identification, orbit prediction and civil alert capabilities.

See also: → space; → situation; → -al; → awareness.

A program aimed at monitoring the near-Earth environment for recognizing and
preventing space hazards by means of radar and optical observations from either space or the ground. The objective of the → European Space Agency initiative is to support the European independent utilization of, and access to, space for research or services, through the provision of timely and quality data, information, services and knowledge regarding the space environment, the threats and the sustainable exploitation of the outer space surrounding our planet Earth.

The SSA Program was authorized at the November 2008 Ministerial Council and formally launched on 1 January 2009. The mandate was extended at the 2012 and 2016 Ministerial Councils, and the program is funded through to 2020. The program comprises three segments:

1. Space Surveillance and Tracking (SST), which is
   the monitoring and tracking of every object orbiting the Earth, such as satellites, space stations and debris. The objective is the prediction and warning of collisions and re-entry events.

2. → Space Weather (SWE), which aims at detection and forecasting of space weather and its effects through
   monitoring of the Sun, solar wind, magnetosphere, radiation belts, ionosphere and disturbances in the geomagnetic field.

3. → Near-Earth Objects (NEOs), which provides warning services against potential asteroid impact risks, including discovery, identification, orbit prediction and civil alert capabilities.

See also: → space; → situation; → -al; → awareness.

A large satellite equipped to support a human crew and designed to remain in orbit around Earth for an extended period and be used for a variety of purposes (conducting research, repairing satellites, performing other space-related activities).

See also: → space; → station.

A large satellite equipped to support a human crew and designed to remain in orbit around Earth for an extended period and be used for a variety of purposes (conducting research, repairing satellites, performing other space-related activities).

See also: → space; → station.

The systematic application of science, technology, and engineering to the exploration and utilization of outer space.

See also: → space; → technology.

The systematic application of science, technology, and engineering to the exploration and utilization of outer space.

See also: → space; → technology.

A telescope which is placed in an orbit around the → Earth and operates through commands from sent from the control center on Earth, such as → Hubble space telescope, → Herschel satellite, → Infrared Astronomical Satellite (IRAS), → Infrared Space Observatory (ISO), → International Ultraviolet Explorer (IUE), → Planck Satellaite , → Spitzer Space Telescope.

See also: → space; → telescope.

A telescope which is placed in an orbit around the → Earth and operates through commands from sent from the control center on Earth, such as → Hubble space telescope, → Herschel satellite, → Infrared Astronomical Satellite (IRAS), → Infrared Space Observatory (ISO), → International Ultraviolet Explorer (IUE), → Planck Satellaite , → Spitzer Space Telescope.

See also: → space; → telescope.

The velocity of a star relative to the Sun.

See also: → space; → velocity.

The velocity of a star relative to the Sun.

See also: → space; → velocity.

The varying conditions in space and specifically in the near-Earth environment. Space weather is chiefly solar driven, resulting from solar activities such as → solar flares, → solar wind, and → coronal mass ejections
that affect → magnetosphere, → ionosphere, and → thermosphere. Non-solar sources such as Galactic → cosmic rays, → meteoroids, and → space debris can all be considered as altering space weather conditions at the Earth. Space weather may affect the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health.
The research in this field aims at monitoring and diagnosis of space weather conditions and constructing reliable numerical prediction models. See also → Space Situational Awareness.

See also: → space; → weather; → meteorology.

The varying conditions in space and specifically in the near-Earth environment. Space weather is chiefly solar driven, resulting from solar activities such as → solar flares, → solar wind, and → coronal mass ejections
that affect → magnetosphere, → ionosphere, and → thermosphere. Non-solar sources such as Galactic → cosmic rays, → meteoroids, and → space debris can all be considered as altering space weather conditions at the Earth. Space weather may affect the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health.
The research in this field aims at monitoring and diagnosis of space weather conditions and constructing reliable numerical prediction models. See also → Space Situational Awareness.

See also: → space; → weather; → meteorology.

The slight erosion of Solar System bodies (planets, moons, asteroids) caused by the → solar wind, → cosmic rays, and → micrometeorite bombardments. Space weathering affects the physical and optical properties of the surfaces of these bodies. Understanding this process is therefore important for the interpretation of remotely obtained spectral data, such as space probe photographs of outer Solar System moons.

See also: → space; → weathering.

The slight erosion of Solar System bodies (planets, moons, asteroids) caused by the → solar wind, → cosmic rays, and → micrometeorite bombardments. Space weathering affects the physical and optical properties of the surfaces of these bodies. Understanding this process is therefore important for the interpretation of remotely obtained spectral data, such as space probe photographs of outer Solar System moons.

See also: → space; → weathering.

A physical entity resulting from the union of space and time concepts. In its most simple version space-time is the four-dimensional continuum, having three spatial coordinates and one temporal coordinate, in which any → event or physical object is located. In → special relativity it is Minkowski’s flat space-time. In → general relativity, it is described by a curved entity characterized by a → metric. Free-fall motion describes the → geodesic of this curved space-time. It may have additional dimensions in the context of speculative theories, such as → string theory.

See also: → space; → time.

A physical entity resulting from the union of space and time concepts. In its most simple version space-time is the four-dimensional continuum, having three spatial coordinates and one temporal coordinate, in which any → event or physical object is located. In → special relativity it is Minkowski’s flat space-time. In → general relativity, it is described by a curved entity characterized by a → metric. Free-fall motion describes the → geodesic of this curved space-time. It may have additional dimensions in the context of speculative theories, such as → string theory.

See also: → space; → time.

→ curvature of space-time.

See also: → space; → time; → curvature.

→ curvature of space-time.

See also: → space; → time; → curvature.

A simple way of representing the → space-time continuum, usually including time and only one spatial dimension. The curve of a particle’s equation of motion on a space-time diagram is called a → world line. Same as → Minkowski diagram.

See also: → space; → time; → diagram.

A simple way of representing the → space-time continuum, usually including time and only one spatial dimension. The curve of a particle’s equation of motion on a space-time diagram is called a → world line. Same as → Minkowski diagram.

See also: → space; → time; → diagram.

1. In Einstein’s → theory of relativity, ds² = c²dt² - dx² - dy² - dz².
   

2. In → Newtonian mechanics, time interval: dt; space interval at fixed time: dl² = dx² + dy² + dz².

See also: → space-time; → interval.

1. In Einstein’s → theory of relativity, ds² = c²dt² - dx² - dy² - dz².
   

2. In → Newtonian mechanics, time interval: dt; space interval at fixed time: dl² = dx² + dy² + dz².

See also: → space-time; → interval.

A vehicle designed to travel or operate, with or without a → crew, in a controlled → flight pattern in space beyond the Earth’s atmosphere or in → orbit around the Earth. Also called spaceship.

See also: → space; → craft.

A vehicle designed to travel or operate, with or without a → crew, in a controlled → flight pattern in space beyond the Earth’s atmosphere or in → orbit around the Earth. Also called spaceship.

See also: → space; → craft.

Of, pertaining to, or describing an → event being outside the → light cone.

See also: → space; → like.

Of, pertaining to, or describing an → event being outside the → light cone.

See also: → space; → like.

The → space-time interval between two → events if it is imaginary, i.e.
ds²< 0.

See also: → spacelike; → interval.

The → space-time interval between two → events if it is imaginary, i.e.
ds²< 0.

See also: → spacelike; → interval.

→ space-time.

See also: → space; → time.

→ space-time.

See also: → space; → time.

A digging tool with a flat blade attached to a shaft so that
it can be pushed into the ground with the foot.

Etymology (EN): M.E., from O.E. spadu; cognate with Gk. spathe “blade of a sword or oar.”

Etymology (PE): Bil “spade,” variants Kurd. bêr, Baluci bard, Gabri bard(a); Mid.Pers. bêl, bêr; Proto-Iranian *barda- metathesis of *badar-; cf. Av. vadar- “weapon” (Gershevitch 1962).

A digging tool with a flat blade attached to a shaft so that
it can be pushed into the ground with the foot.

Etymology (EN): M.E., from O.E. spadu; cognate with Gk. spathe “blade of a sword or oar.”

Etymology (PE): Bil “spade,” variants Kurd. bêr, Baluci bard, Gabri bard(a); Mid.Pers. bêl, bêr; Proto-Iranian *barda- metathesis of *badar-; cf. Av. vadar- “weapon” (Gershevitch 1962).

1. A chip or splinter.
   
2. To break up into small chips, flakes, or splinters, or to cause to break off in flakes.

Etymology (EN): M.E. spalle “a chip,” verb spald “to split,” from M.L.G. spalden, cognate with O.H.G. spaltan “to split.”

Etymology (PE): Terišé “a chip,” from tarâšidan “to cut, hew; scape; shave;” Mid.Pers. tâšitan “to cut, cleave; create by putting together different elements;” Av. taš- “to cut off, fashion, shape, create,” taša- “axe” (Mod.Pers. taš, tišé “axe”),
tašan- “creator;” cf. Skt. taks- “to form by cutting, tool, hammer, form,” taksan- “wood-cutter, carpenter;” Gk. tekton “carpenter,”
tekhne “art, skill, craft, method, system;” L. textere “to weave;” PIE *teks- “to fashion.”

1. A chip or splinter.
   
2. To break up into small chips, flakes, or splinters, or to cause to break off in flakes.

Etymology (EN): M.E. spalle “a chip,” verb spald “to split,” from M.L.G. spalden, cognate with O.H.G. spaltan “to split.”

Etymology (PE): Terišé “a chip,” from tarâšidan “to cut, hew; scape; shave;” Mid.Pers. tâšitan “to cut, cleave; create by putting together different elements;” Av. taš- “to cut off, fashion, shape, create,” taša- “axe” (Mod.Pers. taš, tišé “axe”),
tašan- “creator;” cf. Skt. taks- “to form by cutting, tool, hammer, form,” taksan- “wood-cutter, carpenter;” Gk. tekton “carpenter,”
tekhne “art, skill, craft, method, system;” L. textere “to weave;” PIE *teks- “to fashion.”

A nuclear reaction in which a high energy particle that collides with a nucleus causes the target to eject several particles, thus changing both its mass number and its atomic number.

Etymology (EN): From → spall + -ation.

Etymology (PE): Verbal noun from terišidan, → spall.

A nuclear reaction in which a high energy particle that collides with a nucleus causes the target to eject several particles, thus changing both its mass number and its atomic number.

Etymology (EN): From → spall + -ation.

Etymology (PE): Verbal noun from terišidan, → spall.

1. Aeronautics: The distance between the wing tips of an airplane.
   

2. Math.: The smallest subspace of a → vector space that contains a given element or set of elements.

Etymology (EN): M.E. spanne, sponne, spayn; O.E. span(n), spon(n) “distance between the thumb and little finger of an extended hand;” cf. Ger. Spanne, Du. span.

Etymology (PE): Bâzé “extension of both arms when streched out,” related to bâzu “arm” (Mid.Pers. bâzûk “arm;” Av. bāzu- “arm;” cf. Skt. bāhu- “arm, forearm;” Gk. pechys “forearm, arm, ell;”
O.H.G. buog “shoulder;” Ger. Bug “shoulder;” Du. boeg; O.E. bôg, bôh “shoulder, bough;” E. bough " a branch of a tree;" PIE *bhaghu- “arm”); from Av. vībāzu- “fathom, measure of the outstretched arms.”

1. Aeronautics: The distance between the wing tips of an airplane.
   

2. Math.: The smallest subspace of a → vector space that contains a given element or set of elements.

Etymology (EN): M.E. spanne, sponne, spayn; O.E. span(n), spon(n) “distance between the thumb and little finger of an extended hand;” cf. Ger. Spanne, Du. span.

Etymology (PE): Bâzé “extension of both arms when streched out,” related to bâzu “arm” (Mid.Pers. bâzûk “arm;” Av. bāzu- “arm;” cf. Skt. bāhu- “arm, forearm;” Gk. pechys “forearm, arm, ell;”
O.H.G. buog “shoulder;” Ger. Bug “shoulder;” Du. boeg; O.E. bôg, bôh “shoulder, bough;” E. bough " a branch of a tree;" PIE *bhaghu- “arm”); from Av. vībāzu- “fathom, measure of the outstretched arms.”

Visible disruptive discharge of electricity between two places at opposite high potential. It is preceded by ionization of the path.

Etymology (EN): M.E., from O.E. spearca; cf. M.L.G. sparke, M.Du. spranke.

Etymology (PE): Laki âger “fire accompanied by flame,” Lori azgel daaneh-ye aatash-e sorx shodeh va godaaxteh Kurd. agir “fire” Gilaki val “prominence, flame” Tâleši kel “blazing flame” standard Pers. gorr Laki gorron “flame;”
jaraqé, probably word made by sound imitation.

Visible disruptive discharge of electricity between two places at opposite high potential. It is preceded by ionization of the path.

Etymology (EN): M.E., from O.E. spearca; cf. M.L.G. sparke, M.Du. spranke.

Etymology (PE): Laki âger “fire accompanied by flame,” Lori azgel daaneh-ye aatash-e sorx shodeh va godaaxteh Kurd. agir “fire” Gilaki val “prominence, flame” Tâleši kel “blazing flame” standard Pers. gorr Laki gorron “flame;”
jaraqé, probably word made by sound imitation.

A device consisting of two electrodes separated by a small gap that is filled by a gas, usually air. A high → potential difference applied to the electrodes ionizes the gas and current flows across it for a brief time causing a spark across the gap. Spark gaps have a wide application. As spark plugs, they are used to ignite a mixture of fuel and air in the piston cylinders of an internal combustion engine. The electricity is provided by the battery and ignition coil, and the spark timing is controlled by the distributor. Spark gaps are also used as safety devices on equipment to prevent damage from voltage surges.

See also: → spark; → gap.

A device consisting of two electrodes separated by a small gap that is filled by a gas, usually air. A high → potential difference applied to the electrodes ionizes the gas and current flows across it for a brief time causing a spark across the gap. Spark gaps have a wide application. As spark plugs, they are used to ignite a mixture of fuel and air in the piston cylinders of an internal combustion engine. The electricity is provided by the battery and ignition coil, and the spark timing is controlled by the distributor. Spark gaps are also used as safety devices on equipment to prevent damage from voltage surges.

See also: → spark; → gap.

The emission spectrum produced through a gas or vapor as a result of a high-voltage discharge between metallic electrodes.

See also: → spark; → spectrum.

The emission spectrum produced through a gas or vapor as a result of a high-voltage discharge between metallic electrodes.

See also: → spark; → spectrum.

In a wave train, a correlation between the phases of waves at points separated in space at a given time.

See also: → spatial; → coherence.

In a wave train, a correlation between the phases of waves at points separated in space at a given time.

See also: → spatial; → coherence.

The smallest detail that can be seen in an image. Same as → angular resolution.

See also: → spatial; → resolution.

The smallest detail that can be seen in an image. Same as → angular resolution.

See also: → spatial; → resolution.

To utter words with the ordinary voice (not singing) to communicate; to talk.

Etymology (EN): From M.E. speken “to speak,” from O.E. specan, alteration of earlier sprecan “to speak;” cf. Low Germ. spreken “to speak,” Du. spreken, Ger. sprechen “to speak;” ultimately from PIE *spreg- “to make a sound, utter, speak.”

Etymology (PE): From M.P. saxwanitan “to speak, to talk,” → speech.

To utter words with the ordinary voice (not singing) to communicate; to talk.

Etymology (EN): From M.E. speken “to speak,” from O.E. specan, alteration of earlier sprecan “to speak;” cf. Low Germ. spreken “to speak,” Du. spreken, Ger. sprechen “to speak;” ultimately from PIE *spreg- “to make a sound, utter, speak.”

Etymology (PE): From M.P. saxwanitan “to speak, to talk,” → speech.

Of a distinct or particular kind or character; having a particular function
or purpose; not common, usual, or general.

Etymology (EN): M.E., from O.Fr. especial, from L. specialis “individual, particular,” from → species “appearance, kind, sort.”

Etymology (PE): Vižé, from Mid.Pers. apēcak “pure, sacred,” from *apa-vēcak “set apart,” from prefix apa- + vēcak, from vēxtan (Mod.Pers. bixtan) “to detach, separate, sift, remove,” Av. vaēk- “to select, sort out, sift,” pr. vaēca-, Skt. vic-, vinakti “to sift, winnow, separate; to inquire.”

Of a distinct or particular kind or character; having a particular function
or purpose; not common, usual, or general.

Etymology (EN): M.E., from O.Fr. especial, from L. specialis “individual, particular,” from → species “appearance, kind, sort.”

Etymology (PE): Vižé, from Mid.Pers. apēcak “pure, sacred,” from *apa-vēcak “set apart,” from prefix apa- + vēcak, from vēxtan (Mod.Pers. bixtan) “to detach, separate, sift, remove,” Av. vaēk- “to select, sort out, sift,” pr. vaēca-, Skt. vic-, vinakti “to sift, winnow, separate; to inquire.”

Of, relating to, or subject to the theory of → special relativity.

See also: → special; → relativistic.

Of, relating to, or subject to the theory of → special relativity.

See also: → special; → relativistic.

The theory formulated by A. Einstein in 1905, which is based on the following two → postulates:

1. → Principle of relativity:
   The laws of physical phenomena are the same when studied in terms of two reference systems moving at a constant velocity relative to each other.
   

2. → Principle of constancy: The → velocity of light in free space is the same for all observers and is independent of the relative velocity of the source of light and the observer.
   

The term “special theory of relativity” refers to the restriction in the first postulate to reference systems moving at a constant velocity relative to each other (→ inertial reference frame). See also → general relativity.

See also: → special; → relativity.

The theory formulated by A. Einstein in 1905, which is based on the following two → postulates:

1. → Principle of relativity:
   The laws of physical phenomena are the same when studied in terms of two reference systems moving at a constant velocity relative to each other.
   

2. → Principle of constancy: The → velocity of light in free space is the same for all observers and is independent of the relative velocity of the source of light and the observer.
   

The term “special theory of relativity” refers to the restriction in the first postulate to reference systems moving at a constant velocity relative to each other (→ inertial reference frame). See also → general relativity.

See also: → special; → relativity.

1. A class of individuals having some common characteristics or qualities; distinct sort or kind.
   

2. Biology: The major subdivision of a genus or subgenus, regarded as the basic category of biological classification, composed of related individuals that resemble one another, are able to breed among themselves, but are not able to breed with members of another species.
   

3. Logic: One of the classes of things included with other classes in a genus. The set of things within one of these classes (Dictionary.com).

Etymology (EN): From L. species “a particular sort, kind, or type,” originally “a sight, look, view, appearance,” from specere “to look at, to see, behold;” PIE root spek- “to look around,” → scope.

Etymology (PE): Âraz, from intensive/nuance â- + raz-, from Av. razan “order, → rule,” from r&#257z- “to put in line, direct set,” cf. Mod.Pers. raj “line, row,” variants raž, rak, râk, rezg (Lori), radé, râdé “line, rule, row,”
rasté, râsté “row, a market with regular ranges of shops;” ris, risé “straight;” → right.

1. A class of individuals having some common characteristics or qualities; distinct sort or kind.
   

2. Biology: The major subdivision of a genus or subgenus, regarded as the basic category of biological classification, composed of related individuals that resemble one another, are able to breed among themselves, but are not able to breed with members of another species.
   

3. Logic: One of the classes of things included with other classes in a genus. The set of things within one of these classes (Dictionary.com).

Etymology (EN): From L. species “a particular sort, kind, or type,” originally “a sight, look, view, appearance,” from specere “to look at, to see, behold;” PIE root spek- “to look around,” → scope.

Etymology (PE): Âraz, from intensive/nuance â- + raz-, from Av. razan “order, → rule,” from r&#257z- “to put in line, direct set,” cf. Mod.Pers. raj “line, row,” variants raž, rak, râk, rezg (Lori), radé, râdé “line, rule, row,”
rasté, râsté “row, a market with regular ranges of shops;” ris, risé “straight;” → right.

1. Clearly defined or identified; precise; particular.
   

2. Belonging or relating uniquely to a particular subject; not general.
   

3. Biology: Relating to species or a species.
   

4. Physics: Of or denoting a physical quantity expressed in terms of a unit mass, volume, or other measure, in order to give a value independent of the properties or scale of the particular system studied. → specific angular momentum; → specific charge; → specific density; → specific gravity; → specific heat; → specific humidity; → specific intensity; → specific volume.

Etymology (EN): From Fr. spécifique and directly from L.L. specificus “constituting a kind or sort,” from L. species “kind, sort,” → species.

Etymology (PE): Âbizé, from Mid.Pers. apēcak “pure, sacred” (older form of vižé, → special), from *apa-vēcak “set apart,” from prefix apa- + vēcak, from vēxtan (Mod.Pers. bixtan) “to detach, separate, sift, remove,” Av. vaēk- “to select, sort out, sift,” pr. vaēca-, Skt. vic-, vinakti “to sift, winnow, separate; to inquire.”

1. Clearly defined or identified; precise; particular.
   

2. Belonging or relating uniquely to a particular subject; not general.
   

3. Biology: Relating to species or a species.
   

4. Physics: Of or denoting a physical quantity expressed in terms of a unit mass, volume, or other measure, in order to give a value independent of the properties or scale of the particular system studied. → specific angular momentum; → specific charge; → specific density; → specific gravity; → specific heat; → specific humidity; → specific intensity; → specific volume.

Etymology (EN): From Fr. spécifique and directly from L.L. specificus “constituting a kind or sort,” from L. species “kind, sort,” → species.

Etymology (PE): Âbizé, from Mid.Pers. apēcak “pure, sacred” (older form of vižé, → special), from *apa-vēcak “set apart,” from prefix apa- + vēcak, from vēxtan (Mod.Pers. bixtan) “to detach, separate, sift, remove,” Av. vaēk- “to select, sort out, sift,” pr. vaēca-, Skt. vic-, vinakti “to sift, winnow, separate; to inquire.”

→ Angular momentum per unit mass.

See also: → specific; → angular; → momentum.

→ Angular momentum per unit mass.

See also: → specific; → angular; → momentum.

The electric charge to mass ratio of an elementary particle.

See also: → specific; → charge.

The electric charge to mass ratio of an elementary particle.

See also: → specific; → charge.

Same as → relative density.

See also: → specific; → density.

Same as → relative density.

See also: → specific; → density.

The ratio of the density of a substance at the temperature under consideration to the density of water at the temperature of its maximum density (4 °C).

See also: → specific; → gravity.

The ratio of the density of a substance at the temperature under consideration to the density of water at the temperature of its maximum density (4 °C).

See also: → specific; → gravity.

1. The quantity of heat required to raise the temperature of 1 gm of a substance through 1 °C. More generally, the → heat capacity of a unit mass of a substance. For a homogeneous body it is expressed as: C = dQ/M dT, where dQ is the quantity of heat transferred to a mass of M to raise the temperature by dT. It is often convenient to use the gram-mole as a unit of mass, → molar heat capacity.
   

2. For a gas there are two principal specific heats depending on the way in which the temperature is increased: i) that measured at constant pressure, C_P, and ii) that measured at constant volume, C_V. The specific heat C_P is greater than C_V, because a gas heated at constant pressure expands, and heat energy must be supplied equivalent to the work done in the expansion. The ratio γ = C_P/C_V is called the → adiabatic index. It varies from 1.66 for mono-atomic gases to a little over 1 for gases with complex molecules.

See also: → specific; → heat.

1. The quantity of heat required to raise the temperature of 1 gm of a substance through 1 °C. More generally, the → heat capacity of a unit mass of a substance. For a homogeneous body it is expressed as: C = dQ/M dT, where dQ is the quantity of heat transferred to a mass of M to raise the temperature by dT. It is often convenient to use the gram-mole as a unit of mass, → molar heat capacity.
   

2. For a gas there are two principal specific heats depending on the way in which the temperature is increased: i) that measured at constant pressure, C_P, and ii) that measured at constant volume, C_V. The specific heat C_P is greater than C_V, because a gas heated at constant pressure expands, and heat energy must be supplied equivalent to the work done in the expansion. The ratio γ = C_P/C_V is called the → adiabatic index. It varies from 1.66 for mono-atomic gases to a little over 1 for gases with complex molecules.

See also: → specific; → heat.

The dimensionless ratio of the mass of water vapor to the total mass in a particular volume. → humidity

See also: → specific; → humidity.

The dimensionless ratio of the mass of water vapor to the total mass in a particular volume. → humidity

See also: → specific; → humidity.

A measure of the amount of radiation received per unit solid angle per unit time per unit area normally from an element of surface.

See also: → specific; → intensity.

A measure of the amount of radiation received per unit solid angle per unit time per unit area normally from an element of surface.

See also: → specific; → intensity.

Star formation rate per unit → mass. More specifically, the → star formation rate in a galaxy divided by the → stellar mass of the galaxy. Observations of galaxies over a wide range of → redshifts suggest that the slope of the SFR-M_* relation is about unity, which implies that their sSFR does not depend strongly on stellar mass. Specific star formation rates increase out to z ~ 2 and are constant, or perhaps slowly increasing, from z = 2 out to z = 6, though with a large scatter, sSFR ~ 2-10 Gyr^-1 (Lehnert et al., 2015, A&A 577, A112, and references therein).

See also: → specific; → star; → formation; → rate.

Star formation rate per unit → mass. More specifically, the → star formation rate in a galaxy divided by the → stellar mass of the galaxy. Observations of galaxies over a wide range of → redshifts suggest that the slope of the SFR-M_* relation is about unity, which implies that their sSFR does not depend strongly on stellar mass. Specific star formation rates increase out to z ~ 2 and are constant, or perhaps slowly increasing, from z = 2 out to z = 6, though with a large scatter, sSFR ~ 2-10 Gyr^-1 (Lehnert et al., 2015, A&A 577, A112, and references therein).

See also: → specific; → star; → formation; → rate.

The volume occupied by unit mass of a substance. Specific volume is the reciprocal of density.

See also: → specific; → volume.

The volume occupied by unit mass of a substance. Specific volume is the reciprocal of density.

See also: → specific; → volume.

1. The act of specifying.
   

2. A particular item, aspect, calculation, etc., in such a description.
   

3. Something specified, as in a bill of particulars; a specified particular, item, or article (Dictionary.com).

See also: Verbal noun of → specify.

1. The act of specifying.
   

2. A particular item, aspect, calculation, etc., in such a description.
   

3. Something specified, as in a bill of particulars; a specified particular, item, or article (Dictionary.com).

See also: Verbal noun of → specify.

The state or character of being → specific.

See also: → specific; → -ity.

The state or character of being → specific.

See also: → specific; → -ity.

1. To mention or name specifically or definitely; state in detail.
   

2. To give a specific character to; to set forth as a specification (Dictionary.com).

See also: → specific; → -fy.

1. To mention or name specifically or definitely; state in detail.
   

2. To give a specific character to; to set forth as a specification (Dictionary.com).

See also: → specific; → -fy.

A part or an individual taken as exemplifying a whole mass or number; a typical animal, plant, mineral, part, etc. → sample.

Etymology (EN): From L. specimen “mark, example, indication, sign, evidence,” from speci- stem of specere “to look at,” → -scope,

• -men noun suffix denoting result or means.

Etymology (PE): Nemuné, from nemudan “to show;” Mid.Pers. nimūdan, nimây- “to show,” from O.Pers./Av. ni- “down; in, into,” → ni- (PIE), + māy- “to measure;” cf. Skt. mati “measures,” matra- “measure;”
Gk. metron “measure;” L. metrum; PIE base *me- “to measure.”

A part or an individual taken as exemplifying a whole mass or number; a typical animal, plant, mineral, part, etc. → sample.

Etymology (EN): From L. specimen “mark, example, indication, sign, evidence,” from speci- stem of specere “to look at,” → -scope,

• -men noun suffix denoting result or means.

Etymology (PE): Nemuné, from nemudan “to show;” Mid.Pers. nimūdan, nimây- “to show,” from O.Pers./Av. ni- “down; in, into,” → ni- (PIE), + māy- “to measure;” cf. Skt. mati “measures,” matra- “measure;”
Gk. metron “measure;” L. metrum; PIE base *me- “to measure.”

1. Optics: An image defect, one of a large number of bright and dark spots, that appears when an object is illuminated by monochromatic, highly → coherent light. This phenomenon results from the → interference of a number of randomly phased complex contributions of electromagnetic → wavefronts scattered from an object with
   rough structure, such as a piece of paper, a display screen, or a metallic surface. In particular, whenever the object is rough on the scale of an optical wavelength, the image has a grainy appearance. Also called speckle noise.
   

2. Astro.: The pattern produced by a short-exposure image of a → point source, such as a star, when the → wavefront is torn apart under the effect of the → atmospheric turbulence. Speckles change very rapidly with time as a function of the atmospheric turbulence. → speckle lifetime.
   Long exposure images of these changing speckle patterns result in a blurred image of the star, called a → seeing disk.
   → Fried parameter.

Etymology (EN): Speckle “a speck or small spot, as a natural dot of color on skin, plumage, or foliage,”
from M.E.speck (from O.E. specca “small spot, stain,” of unknown origin; probably related to Du. speckel “speck, speckle”) + -le a noun suffix having originally a diminutive meaning.

Etymology (PE): Pakâl, from pak “spot” (Lâri, Gerâši), pašy “mingled, confused” (Tâleši), probably related to pisé “dappled, variegated,” pis, pisi “leprosy,” neveštan “to write,” pišé “profession,” → professional astronomer; Mid.Pers. parš “speckled, spotted,” pēsīdan “to color, adorn,” pēsit “adorned;”
O.Pers. pais- “to adorn, cut, engrave;”
Av. paēs- “to paint, adorn,” paēsa- “adornment;” cf. Skt. peś- “to adorn, hew out, decorate,” piśáti “adorns; cuts;” Gk. poikilos “multicolored;” L. pingit “embroiders, paints;” O.C.S. pisati “to write;” O.H.G. fēh “multicolored;” Lith. piēšti “to draw, adorn;” PIE base *peik- “colored, speckled.”

1. Optics: An image defect, one of a large number of bright and dark spots, that appears when an object is illuminated by monochromatic, highly → coherent light. This phenomenon results from the → interference of a number of randomly phased complex contributions of electromagnetic → wavefronts scattered from an object with
   rough structure, such as a piece of paper, a display screen, or a metallic surface. In particular, whenever the object is rough on the scale of an optical wavelength, the image has a grainy appearance. Also called speckle noise.
   

2. Astro.: The pattern produced by a short-exposure image of a → point source, such as a star, when the → wavefront is torn apart under the effect of the → atmospheric turbulence. Speckles change very rapidly with time as a function of the atmospheric turbulence. → speckle lifetime.
   Long exposure images of these changing speckle patterns result in a blurred image of the star, called a → seeing disk.
   → Fried parameter.

Etymology (EN): Speckle “a speck or small spot, as a natural dot of color on skin, plumage, or foliage,”
from M.E.speck (from O.E. specca “small spot, stain,” of unknown origin; probably related to Du. speckel “speck, speckle”) + -le a noun suffix having originally a diminutive meaning.

Etymology (PE): Pakâl, from pak “spot” (Lâri, Gerâši), pašy “mingled, confused” (Tâleši), probably related to pisé “dappled, variegated,” pis, pisi “leprosy,” neveštan “to write,” pišé “profession,” → professional astronomer; Mid.Pers. parš “speckled, spotted,” pēsīdan “to color, adorn,” pēsit “adorned;”
O.Pers. pais- “to adorn, cut, engrave;”
Av. paēs- “to paint, adorn,” paēsa- “adornment;” cf. Skt. peś- “to adorn, hew out, decorate,” piśáti “adorns; cuts;” Gk. poikilos “multicolored;” L. pingit “embroiders, paints;” O.C.S. pisati “to write;” O.H.G. fēh “multicolored;” Lith. piēšti “to draw, adorn;” PIE base *peik- “colored, speckled.”

A technique for generating a clear composite image of a celestial object blurred by
→ atmospheric turbulence in which a large number of short-exposure photographs are mathematically correlated by a computer. By comparing the behavior of the → speckles in a series of images it is possible to approach the theoretical resolution of the telescope.

See also: → speckle; → interferometry.

A technique for generating a clear composite image of a celestial object blurred by
→ atmospheric turbulence in which a large number of short-exposure photographs are mathematically correlated by a computer. By comparing the behavior of the → speckles in a series of images it is possible to approach the theoretical resolution of the telescope.

See also: → speckle; → interferometry.

The time scale on which a stellar image changes significantly due to → atmospheric turbulence. It is proportional to the ratio r₀/Δv, where r₀ is the → Fried parameter and Δv the standard deviation of the distribution of wind velocities weighted by the turbulence structure coefficient. Typical lifetimes in the visible range from about 3 to 30 milliseconds.

Etymology (EN): → speckle; → life; → time.

Etymology (PE): Omr “life-time;” from Ar. ‘umr; pakâl, → speckle.

The time scale on which a stellar image changes significantly due to → atmospheric turbulence. It is proportional to the ratio r₀/Δv, where r₀ is the → Fried parameter and Δv the standard deviation of the distribution of wind velocities weighted by the turbulence structure coefficient. Typical lifetimes in the visible range from about 3 to 30 milliseconds.

Etymology (EN): → speckle; → life; → time.

Etymology (PE): Omr “life-time;” from Ar. ‘umr; pakâl, → speckle.

An image defect associated with the → speckle phenomenon.

See also: → speckle; → noise.

An image defect associated with the → speckle phenomenon.

See also: → speckle; → noise.

Of or pertaining to a → spectrum.

See also: → spectrum; → -al.

Of or pertaining to a → spectrum.

See also: → spectrum; → -al.

A system that assigns a → spectral type to a star according to characteristics of its spectrum. The earliest attempt to divide stars on the basis of their spectra was the → Secchi classification in the 1860s. This scheme paved the way for the → Harvard classification that led to the current → Morgan-Keenan classification of spectral types. In the Harvard system stars were originally thought to follow an evolutionary sequence from the “early” O and B types to the “late” K and M types. Although this is now known to be wrong, the terms
→ early-type star and → late-type star are still in use. In the Morgan-Keenan system stars are classified as type O, B, A, F, G, K, or M in order of decreasing → effective temperature, and each type further subdivided into subclasses from 0 (hottest, except for → O-type stars) to 9 (coolest).
They are also accompanied by a → luminosity class.
In the late 1990s, spectral types L and T were added to the sequence to accommodate the coolest stars and → brown dwarfs (with class Y reserved for the coolest brown dwarfs of all, as yet unobserved).

See also: → spectral; → classification.

A system that assigns a → spectral type to a star according to characteristics of its spectrum. The earliest attempt to divide stars on the basis of their spectra was the → Secchi classification in the 1860s. This scheme paved the way for the → Harvard classification that led to the current → Morgan-Keenan classification of spectral types. In the Harvard system stars were originally thought to follow an evolutionary sequence from the “early” O and B types to the “late” K and M types. Although this is now known to be wrong, the terms
→ early-type star and → late-type star are still in use. In the Morgan-Keenan system stars are classified as type O, B, A, F, G, K, or M in order of decreasing → effective temperature, and each type further subdivided into subclasses from 0 (hottest, except for → O-type stars) to 9 (coolest).
They are also accompanied by a → luminosity class.
In the late 1990s, spectral types L and T were added to the sequence to accommodate the coolest stars and → brown dwarfs (with class Y reserved for the coolest brown dwarfs of all, as yet unobserved).

See also: → spectral; → classification.

The → range of → wavelengths or frequencies (→ frequency) at which a → detector is sensitive. Same as → bandwidth.

See also: → spectral; → coverage.

The → range of → wavelengths or frequencies (→ frequency) at which a → detector is sensitive. Same as → bandwidth.

See also: → spectral; → coverage.

For a specified → bandwidth of radiation consisting of a continuous → frequency spectrum, the total → power in the bandwidth divided by the bandwidth. Spectral density describes how the power (or variance) of a time series is distributed with frequency. Also called power spectral density.

See also: → spectral; → density.

For a specified → bandwidth of radiation consisting of a continuous → frequency spectrum, the total → power in the bandwidth divided by the bandwidth. Spectral density describes how the power (or variance) of a time series is distributed with frequency. Also called power spectral density.

See also: → spectral; → density.

→ dispersion.

See also: → spectral; → dispersion.

→ dispersion.

See also: → spectral; → dispersion.

A plot showing the energy emitted by a source as a function of the radiation wavelength or frequency. It is used in many branches of astronomy to characterize astronomical sources, in particular mainly in → near infrared and → middle infrared to study → protostars or → young stellar objects. The SED of these objects is divided in four classes.

Class 0 in which the SED represents a very embedded protostar, where the mass of the central core is small in comparison to the mass of the → accreting envelope. The SED is characterized by the → blackbody radiation of the envelope and peaks at → submillimeter wavelengths.

Class I objects possess a SED that peaks in the → far infrared and is characterized by a weak contribution of the blackbody of the central protostar (detected in near infrared) and the emission of a thick disk and dense envelope. These objects have less mass in the envelope and more massive central cores with respect to Class 0.

Class II objects are the → classical T Tauri stars
with a SED due to the emission of a thin disk and the central star.
They have accumulated most of their final mass and have dispersed almost completely their circumstellar envelope.

Finally, Class III objects have pure photospheric spectra. Their SED is peaked in the optical and is well approximated by a blackbody emission with a faint → infrared excess due to the presence of a residual optically thin disk that may be the origin of → planetesimals.
This classification scheme can be made more quantitative by defining a → spectral index.

See also: → spectral; → energy; → distribution.

A plot showing the energy emitted by a source as a function of the radiation wavelength or frequency. It is used in many branches of astronomy to characterize astronomical sources, in particular mainly in → near infrared and → middle infrared to study → protostars or → young stellar objects. The SED of these objects is divided in four classes.

Class 0 in which the SED represents a very embedded protostar, where the mass of the central core is small in comparison to the mass of the → accreting envelope. The SED is characterized by the → blackbody radiation of the envelope and peaks at → submillimeter wavelengths.

Class I objects possess a SED that peaks in the → far infrared and is characterized by a weak contribution of the blackbody of the central protostar (detected in near infrared) and the emission of a thick disk and dense envelope. These objects have less mass in the envelope and more massive central cores with respect to Class 0.

Class II objects are the → classical T Tauri stars
with a SED due to the emission of a thin disk and the central star.
They have accumulated most of their final mass and have dispersed almost completely their circumstellar envelope.

Finally, Class III objects have pure photospheric spectra. Their SED is peaked in the optical and is well approximated by a blackbody emission with a faint → infrared excess due to the presence of a residual optically thin disk that may be the origin of → planetesimals.
This classification scheme can be made more quantitative by defining a → spectral index.

See also: → spectral; → energy; → distribution.

An emission or absorption mark in the spectrum of an astronomical object, of known or unknown origin, usually with complex structure.

See also: → spectral; → feature.

An emission or absorption mark in the spectrum of an astronomical object, of known or unknown origin, usually with complex structure.

See also: → spectral; → feature.

1. The → exponent of the → frequency on which depends
   the intensity of the → continuum emission, that is: F_ν∝ ν^α. The exponent (α) typically takes positive values from 0 to 2 for → thermal emission,
   while → non-thermal emission, such as → synchrotron radiation, leads to negative values of the spectral index ranging from about -0.5 to -1.5.
   

2. The ratio α_IR = dlog(λF_λ)/dlogλ, where F represents the flux and λ the wavelength, in the range 2.2 μm ≤ λ ≤ 25 μm, particularly used in the classification of → protostars (→ Class I, → Class II, and → Class III).

See also: → spectral; → index.

1. The → exponent of the → frequency on which depends
   the intensity of the → continuum emission, that is: F_ν∝ ν^α. The exponent (α) typically takes positive values from 0 to 2 for → thermal emission,
   while → non-thermal emission, such as → synchrotron radiation, leads to negative values of the spectral index ranging from about -0.5 to -1.5.
   

2. The ratio α_IR = dlog(λF_λ)/dlogλ, where F represents the flux and λ the wavelength, in the range 2.2 μm ≤ λ ≤ 25 μm, particularly used in the classification of → protostars (→ Class I, → Class II, and → Class III).

See also: → spectral; → index.

A dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow wavelength range, compared with the nearby wavelengths.

See also: → spectral; → line.

A dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow wavelength range, compared with the nearby wavelengths.

See also: → spectral; → line.

The observable spectral range provided by a spectroscope, as determined by the grating dispersion, camera focal length, and detector size.

See also: → spectral; → range.

The observable spectral range provided by a spectroscope, as determined by the grating dispersion, camera focal length, and detector size.

See also: → spectral; → range.

An extent of wavelengths into which the electromagnetic spectrum is divided; e.g. infrared or ultraviolet region.

See also: → spectral; → region.

An extent of wavelengths into which the electromagnetic spectrum is divided; e.g. infrared or ultraviolet region.

See also: → spectral; → region.

The capacity of a spectrograph to separate two adjacent spectral lines. The theoretical spectral resolution depends on the grating dispersion, grating position, pixel size, collimator and camera focal length, and the entrance slit-width.

See also: → spectral; → resolution.

The capacity of a spectrograph to separate two adjacent spectral lines. The theoretical spectral resolution depends on the grating dispersion, grating position, pixel size, collimator and camera focal length, and the entrance slit-width.

See also: → spectral; → resolution.

Domain of the electromagnetic spectrum over which a detector is sensitive. Same as spectral sensitivity.

See also: → spectral;
→ response.

Domain of the electromagnetic spectrum over which a detector is sensitive. Same as spectral sensitivity.

See also: → spectral;
→ response.

Spectral lines or group of lines occurring in sequence.

See also: → spectral; → series.

Spectral lines or group of lines occurring in sequence.

See also: → spectral; → series.

The process of computing line strengths in stellar spectra using an appropriate stellar atmosphere model, atomic and molecular data, and the numerical solution of the → radiative transfer equation at each point in the spectrum.

See also: → spectral; → synthesis.

The process of computing line strengths in stellar spectra using an appropriate stellar atmosphere model, atomic and molecular data, and the numerical solution of the → radiative transfer equation at each point in the spectrum.

See also: → spectral; → synthesis.

A group into which stars may be classified according to the characteristics of their spectra. Spectral type correlates with the star’s
→ effective temperature and → color.
There are seven main spectral types. From hot
and blue to cool and red, they are O, B, A, F, G, K, and M. Each spectral type is divided into several subtypes. For example, from warmest to coolest, spectral type G is G0, G1, G2, G3, and so on to G9. A precise → spectral classification requires determining the → luminosity class. The Sun is spectral type G2 V.

See also: → spectral; → type.

A group into which stars may be classified according to the characteristics of their spectra. Spectral type correlates with the star’s
→ effective temperature and → color.
There are seven main spectral types. From hot
and blue to cool and red, they are O, B, A, F, G, K, and M. Each spectral type is divided into several subtypes. For example, from warmest to coolest, spectral type G is G0, G1, G2, G3, and so on to G9. A precise → spectral classification requires determining the → luminosity class. The Sun is spectral type G2 V.

See also: → spectral; → type.

The state of a spectrum from an astronomical object in which the lines change with time as far as their intensity, profile, and wavelength are concerned.

See also: → spectral; → variability.

The state of a spectrum from an astronomical object in which the lines change with time as far as their intensity, profile, and wavelength are concerned.

See also: → spectral; → variability.

A combining form representing → spectrum in compound words.
→ spectrogram, → spectrograph, → spectroheliogram, → spectroheliograph, → spectrometer, → spectrophotometer, → spectropolarimeric, → spectropolarimetry, → spectroscope, → spectroscopy, → spectroscopic.

See also: → spectrum

A combining form representing → spectrum in compound words.
→ spectrogram, → spectrograph, → spectroheliogram, → spectroheliograph, → spectrometer, → spectrophotometer, → spectropolarimeric, → spectropolarimetry, → spectroscope, → spectroscopy, → spectroscopic.

See also: → spectrum

The → extreme adaptive optics system and → coronagraphic facility at the → European Southern Observatory (ESO) → Very Large Telescope (VLT) (UT3) available from May 2014. Its primary science goal is imaging, low-resolution spectroscopic, and polarimetric characterization of → exoplanetary system at → visible and → near-infrared wavelengths (0.5-2.32 μm).

SPHERE is capable of obtaining → diffraction-limited images at 0’’.02 to 0’’.08 resolution depending on the wavelength. Its → spectral resolution is 30 to 350, depending on the mode.

See also: → spectro-; → polarimetric; → high; → contrast; → exoplanet.

The → extreme adaptive optics system and → coronagraphic facility at the → European Southern Observatory (ESO) → Very Large Telescope (VLT) (UT3) available from May 2014. Its primary science goal is imaging, low-resolution spectroscopic, and polarimetric characterization of → exoplanetary system at → visible and → near-infrared wavelengths (0.5-2.32 μm).

SPHERE is capable of obtaining → diffraction-limited images at 0’’.02 to 0’’.08 resolution depending on the wavelength. Its → spectral resolution is 30 to 350, depending on the mode.

See also: → spectro-; → polarimetric; → high; → contrast; → exoplanet.

A plot of the intensity of light at different wavelengths obtained using a spectrograph.

See also: → spectro-; → -gram.

A plot of the intensity of light at different wavelengths obtained using a spectrograph.

See also: → spectro-; → -gram.

An instrument that disperses the light into spectral lines and
records them.

See also: → spectro-; → -graph.

An instrument that disperses the light into spectral lines and
records them.

See also: → spectro-; → -graph.

An image of the Sun taken in the light of one particular wavelength.

See also: → spectro-; → heliogram.

An image of the Sun taken in the light of one particular wavelength.

See also: → spectro-; → heliogram.

An instrument for recording monochromatic images of the Sun.

See also: → spectro-; → heliograph.

An instrument for recording monochromatic images of the Sun.

See also: → spectro-; → heliograph.

1. A spectrograph in which the spectrum is recorded by electronic means so that wavelength, intensity, etc. can be measured.
   
2. An instrument for determining the distribution of energies in a beam of particles.

See also: → spectro-; → -meter.

1. A spectrograph in which the spectrum is recorded by electronic means so that wavelength, intensity, etc. can be measured.
   
2. An instrument for determining the distribution of energies in a beam of particles.

See also: → spectro-; → -meter.

An instrument designed to measure the intensity of a particular spectral line or a series of spectral lines.

See also: → spectro-; → photometer.

An instrument designed to measure the intensity of a particular spectral line or a series of spectral lines.

See also: → spectro-; → photometer.

Of or relating to → spectrophotometry.

See also: → spectrum; → photometry.

Of or relating to → spectrophotometry.

See also: → spectrum; → photometry.

In astronomy, measurement of the absolute fluxes of the components of different frequencies in the spectrum of a light source.

See also: → spectrum; → photometry.

In astronomy, measurement of the absolute fluxes of the components of different frequencies in the spectrum of a light source.

See also: → spectrum; → photometry.

Of or relating to → spectropolarimetry.

See also: → spectropolarimetry; → -ic.

Of or relating to → spectropolarimetry.

See also: → spectropolarimetry; → -ic.

A technique of observation in → astrophysics which combines → spectroscopy and → polarization measurements. Spectropolarimetry has a wide range of applications in astrophysics, including → stellar magnetic field studies. → ESPaDOnS, → HARPSpol.

See also: → spectro-; → polarimetry.

A technique of observation in → astrophysics which combines → spectroscopy and → polarization measurements. Spectropolarimetry has a wide range of applications in astrophysics, including → stellar magnetic field studies. → ESPaDOnS, → HARPSpol.

See also: → spectro-; → polarimetry.

An optical instrument for forming and examining the spectrum of a light source. The instrument contains a narrow slit through which the light enters. The slit is placed at the focus of a positive lens called the collimator lens to form a beam of parallel rays. The beam of light falls on a dispersing element (prism, grating, or grism) which separates the light into its colors. This spectrum can be observed with an ocular (in the spectroscope) or recorded on a detector (in the spectrograph).

See also: → spectro-; → -scope.

An optical instrument for forming and examining the spectrum of a light source. The instrument contains a narrow slit through which the light enters. The slit is placed at the focus of a positive lens called the collimator lens to form a beam of parallel rays. The beam of light falls on a dispersing element (prism, grating, or grism) which separates the light into its colors. This spectrum can be observed with an ocular (in the spectroscope) or recorded on a detector (in the spectrograph).

See also: → spectro-; → -scope.

Of or relating to → spectroscopy.

See also: → spectro-; → -scopy; → -ic.

Of or relating to → spectroscopy.

See also: → spectro-; → -scopy; → -ic.

A binary system that cannot be resolved by a telescope, but can be identified by means of the Doppler shift of the spectral lines. As stars revolve, they alternately approach and recede in the line of sight. This motion is shown up in their spectra as a periodic oscillation or doubling of spectral lines.

See also: → spectroscopic; → binary.

A binary system that cannot be resolved by a telescope, but can be identified by means of the Doppler shift of the spectral lines. As stars revolve, they alternately approach and recede in the line of sight. This motion is shown up in their spectra as a periodic oscillation or doubling of spectral lines.

See also: → spectroscopic; → binary.

The situation in which spectroscopic features in a certain optical region are not sensitive enough to distinguish adjacent → luminosity classes, for instance → dwarf stars from → giant stars.

See also: → spectroscopic; → degeneracy.

The situation in which spectroscopic features in a certain optical region are not sensitive enough to distinguish adjacent → luminosity classes, for instance → dwarf stars from → giant stars.

See also: → spectroscopic; → degeneracy.

A spacial → Hertzsprung-Russell diagram (HRD) which is independent of distance and extinction measurements. The sHRD is derived from the classical HRD by replacing the luminosity (L) to the quantity ℒ = T ⁴_eff/g which is the inverse of the flux-weighted gravity introduced by Kudritzki et al. (2003). The value of ℒ can be calculated from stellar atmosphere analyses without prior knowledge of the distance or the extinction. In contrast to the classical T_eff-log g diagram (→ Kiel diagram), the sHRD sorts stars according to their proximity to the → Eddington limit, because ℒ is proportional to the Eddington factor Γ = L/L_Edd according to the relation

ℒ = (1/4πσG)(L/M) = (c/(σκ)Γ,

where σ is the → Stefan-Boltzmann constant, κ is the electron → scattering  → opacity in the stellar envelope, and the other symbols have their usual meanings

(Langer, N., Kudritzki, R. P., 2014, A&A 564, A52, arXive:1403.2212,

Castro et al., 2014, A&A 570, L13.

See also: → spectroscopic; → H-R diagram.

A spacial → Hertzsprung-Russell diagram (HRD) which is independent of distance and extinction measurements. The sHRD is derived from the classical HRD by replacing the luminosity (L) to the quantity ℒ = T ⁴_eff/g which is the inverse of the flux-weighted gravity introduced by Kudritzki et al. (2003). The value of ℒ can be calculated from stellar atmosphere analyses without prior knowledge of the distance or the extinction. In contrast to the classical T_eff-log g diagram (→ Kiel diagram), the sHRD sorts stars according to their proximity to the → Eddington limit, because ℒ is proportional to the Eddington factor Γ = L/L_Edd according to the relation

ℒ = (1/4πσG)(L/M) = (c/(σκ)Γ,

where σ is the → Stefan-Boltzmann constant, κ is the electron → scattering  → opacity in the stellar envelope, and the other symbols have their usual meanings

(Langer, N., Kudritzki, R. P., 2014, A&A 564, A52, arXive:1403.2212,

Castro et al., 2014, A&A 570, L13.

See also: → spectroscopic; → H-R diagram.

The stellar mass derived from → gravity (g) and radius (R), expressed by M = gR²/G, where G is the → gravitational constant. Spectroscopic mass conveys the actual mass of the star, in contrast with its → initial mass.

See also: → spectroscopic; → mass.

The stellar mass derived from → gravity (g) and radius (R), expressed by M = gR²/G, where G is the → gravitational constant. Spectroscopic mass conveys the actual mass of the star, in contrast with its → initial mass.

See also: → spectroscopic; → mass.

The measurement of a stellar distance by the absolute magnitude derived from the luminosity criteria of the spectrum and the apparent magnitude of the star.

See also: → spectroscopic; → parallax.

The measurement of a stellar distance by the absolute magnitude derived from the luminosity criteria of the spectrum and the apparent magnitude of the star.

See also: → spectroscopic; → parallax.

A → variable star that displays changes in its → spectrum. In such stars line intensities may vary and new lines may appear. Examples include → AG Carinae, HD 108, HD 191612, and HD 148937.

See also: → spectroscopic; → variable.

A → variable star that displays changes in its → spectrum. In such stars line intensities may vary and new lines may appear. Examples include → AG Carinae, HD 108, HD 191612, and HD 148937.

See also: → spectroscopic; → variable.

The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the physical and chemical properties of astronomical objects.

See also: Spectro- combining form of → spectrum + → -scopy.

The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the physical and chemical properties of astronomical objects.

See also: Spectro- combining form of → spectrum + → -scopy.

The → electromagnetic radiation divided into its constituting wavelengths or frequencies.

Etymology (EN): From L. spectrum “appearance, image, apparition,” from specere “to look at, view;” Gk. skopein “to behold, look, consider,”
skeptesthai “to look at;” PIE base *spek- “to see;” cf. Av. spas-, spaš- “to attend to; to serve; spy,” spasiieiti “looks at, perceives” (Mod.Pers. sepâs “kindness, favor, thanksgiving,” sepâsidan “to praise for benefits received);” Skt. paś- “to see, watch,” spasati “sees;” L. specere “to look at;” O.H.G. spehônn “to spy,” Ger. spähen “to spy.”

Etymology (PE): Binâb “a vision;” Mid.Pers. wênâb “vision,” from wên-, present stem of didan “to see;”
O.Pers. vain- “to see;” Av. vaēn- “to see;”
cf. Skt. veda “I know;” Gk. oida “I know,” idein “to see;” L. videre “to see;” PIE base *weid- “to know, to see.”

The → electromagnetic radiation divided into its constituting wavelengths or frequencies.

Etymology (EN): From L. spectrum “appearance, image, apparition,” from specere “to look at, view;” Gk. skopein “to behold, look, consider,”
skeptesthai “to look at;” PIE base *spek- “to see;” cf. Av. spas-, spaš- “to attend to; to serve; spy,” spasiieiti “looks at, perceives” (Mod.Pers. sepâs “kindness, favor, thanksgiving,” sepâsidan “to praise for benefits received);” Skt. paś- “to see, watch,” spasati “sees;” L. specere “to look at;” O.H.G. spehônn “to spy,” Ger. spähen “to spy.”

Etymology (PE): Binâb “a vision;” Mid.Pers. wênâb “vision,” from wên-, present stem of didan “to see;”
O.Pers. vain- “to see;” Av. vaēn- “to see;”
cf. Skt. veda “I know;” Gk. oida “I know,” idein “to see;” L. videre “to see;” PIE base *weid- “to know, to see.”

The reflection of light waves in which the reflected waves travel in a definite direction, and the directions of the incident and reflected waves make equal angles with a line perpendicular to the reflecting surface. Same as → regular reflection; opposite of → diffuse reflection.

Etymology (EN): From L. specularis, from speculum “mirror;”
→ reflection.

Etymology (PE): Bâztâb, → reflection; âyenevâr “mirror-like,” from âyené, → mirror + -vâr similarity suffix.

The reflection of light waves in which the reflected waves travel in a definite direction, and the directions of the incident and reflected waves make equal angles with a line perpendicular to the reflecting surface. Same as → regular reflection; opposite of → diffuse reflection.

Etymology (EN): From L. specularis, from speculum “mirror;”
→ reflection.

Etymology (PE): Bâztâb, → reflection; âyenevâr “mirror-like,” from âyené, → mirror + -vâr similarity suffix.

To guess possible answers to a question when there are not enough information to be certain.

Etymology (EN): Back formation from O.Fr. speculation, from L. speculatus, p.p. of speculari “to watch over, observe,” from specula “watch tower,” from specere “to look at, regard,” cognate with Av. spas- “to attend; to serve,” spasiieiti “looks at, perceives;” Pers. sepâs “kindness, thanksgiving;” Skt. spasati “sees;” Gk. skopein “to behold, look, consider,” skeptesthai “to look at;” O.H.G. spehhon “to spy;” Ger. spähen “to spy;” PIE *spek- “to look around, observe.”

Etymology (PE): Gâsidan infinitive from gâs, from Av. kas- “to look at, see,” with extension of the vowel and change of the last phoneme from k to g, as in and cognate with negâh (Mid.Pers. nikâh), → look, âgâh (Mid.Pers. âkâh) “aware” (→ Space Situational Awareness),
pargast “God forbid!,” and maybe (Lori, Laki, Torbat-Heydarie-yi) gâs “perhaps,” (Shirâzi, Fasâyi) gâsam “maybe;” cf. Skt. kāś- “to become visible, appear;” Gk. tekmor, tekmar “sign, mark;” PIE base *k^wek- “to appear, show.”

To guess possible answers to a question when there are not enough information to be certain.

Etymology (EN): Back formation from O.Fr. speculation, from L. speculatus, p.p. of speculari “to watch over, observe,” from specula “watch tower,” from specere “to look at, regard,” cognate with Av. spas- “to attend; to serve,” spasiieiti “looks at, perceives;” Pers. sepâs “kindness, thanksgiving;” Skt. spasati “sees;” Gk. skopein “to behold, look, consider,” skeptesthai “to look at;” O.H.G. spehhon “to spy;” Ger. spähen “to spy;” PIE *spek- “to look around, observe.”

Etymology (PE): Gâsidan infinitive from gâs, from Av. kas- “to look at, see,” with extension of the vowel and change of the last phoneme from k to g, as in and cognate with negâh (Mid.Pers. nikâh), → look, âgâh (Mid.Pers. âkâh) “aware” (→ Space Situational Awareness),
pargast “God forbid!,” and maybe (Lori, Laki, Torbat-Heydarie-yi) gâs “perhaps,” (Shirâzi, Fasâyi) gâsam “maybe;” cf. Skt. kāś- “to become visible, appear;” Gk. tekmor, tekmar “sign, mark;” PIE base *k^wek- “to appear, show.”

The act or an instance of speculating.

See also: Verbal noun of → speculate.

The act or an instance of speculating.

See also: Verbal noun of → speculate.

1. The faculty or power of speaking; oral communication; ability to express one’s thoughts and emotions by speech sounds and gesture (Dictionary.com).
   

2. A form of communication in spoken language, made by a speaker before an audience for a given purpose (Dictionary.com).

Etymology (EN): M.E. speche; O.E. spæc; cf. Dan. sprog, O.S. spraca, O.Fris. spreke, Du. spraak, O.H.G. sprahha, Ger. Sprache “speech.”

Etymology (PE): Soxan “speech, utterance, word;” Mid.Pers. saxwan “word, speech;” O.Pers. θaⁿh- “to declare, say,” aθaham “I said;”
Av. səngh- “to declare,” sənghāmi “I say;” cf. Skt. śams- “to praise, declare;” L. censere “to estimate, think; decide.”

1. The faculty or power of speaking; oral communication; ability to express one’s thoughts and emotions by speech sounds and gesture (Dictionary.com).
   

2. A form of communication in spoken language, made by a speaker before an audience for a given purpose (Dictionary.com).

Etymology (EN): M.E. speche; O.E. spæc; cf. Dan. sprog, O.S. spraca, O.Fris. spreke, Du. spraak, O.H.G. sprahha, Ger. Sprache “speech.”

Etymology (PE): Soxan “speech, utterance, word;” Mid.Pers. saxwan “word, speech;” O.Pers. θaⁿh- “to declare, say,” aθaham “I said;”
Av. səngh- “to declare,” sənghāmi “I say;” cf. Skt. śams- “to praise, declare;” L. censere “to estimate, think; decide.”

The ratio of the distance covered to the time taken by a moving body. Speed in a specified direction is → velocity.

Etymology (EN): M.E. spede “good luck, prosperity, rapidity;”
O.E. sped “success, prosperity, advancement;”
cf. O.S. spod “success,” Du. spoed “haste, speed,” O.H.G. spuot “success,” O.H.G. spuoten “to haste;” from PIE base *spe- “to thrive, prosper” (cf. Skt. sphā- “to increase, become fat;” L. sperare “to hope;” O.C.S. spechu “endeavor;” Lith. speju “to have leisure”).

Etymology (PE): Tondi “speed,” from tond “swift, rapid, brisk; fierce, severe” (Mid.Pers. tund “sharp, violent;” Sogdian tund “violent;” cf. Skt. tod- “to thrust, give a push,” tudáti “he thrusts;” L. tundere “to thrust, to hit” (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere “to thrust or bore through;”
PIE base *(s)teud- “to thrust, to beat”) + noun suffix -i.

The ratio of the distance covered to the time taken by a moving body. Speed in a specified direction is → velocity.

Etymology (EN): M.E. spede “good luck, prosperity, rapidity;”
O.E. sped “success, prosperity, advancement;”
cf. O.S. spod “success,” Du. spoed “haste, speed,” O.H.G. spuot “success,” O.H.G. spuoten “to haste;” from PIE base *spe- “to thrive, prosper” (cf. Skt. sphā- “to increase, become fat;” L. sperare “to hope;” O.C.S. spechu “endeavor;” Lith. speju “to have leisure”).

Etymology (PE): Tondi “speed,” from tond “swift, rapid, brisk; fierce, severe” (Mid.Pers. tund “sharp, violent;” Sogdian tund “violent;” cf. Skt. tod- “to thrust, give a push,” tudáti “he thrusts;” L. tundere “to thrust, to hit” (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere “to thrust or bore through;”
PIE base *(s)teud- “to thrust, to beat”) + noun suffix -i.

Same as → velocity of light.

See also: → speed; → light.

Same as → velocity of light.

See also: → speed; → light.

To name or write in order the letters constituting a word

Etymology (EN): M.E. spellen, from O.Fr. espeller, from Proto-Germanic spellan “to tell,” which also gave rise to the O.E. spellian; ultimately from PIE *spel- “to say aloud, recite.”

Etymology (PE): Vâbidan, from vâb, from vab- ultimately from Proto-Ir. *uab/f- “to call,” which has given rise to several words in Iranian languages, mainly Pers. gap “word, talk,” gapidan “to talk,” buf “owl,” zand-vâf “nightingale,” literally “song teller, ode singer;” Baluchi gwâp-/gwâpt “to summon, call together;” Sogd. waβ-/wab- “to speak, to talk;” Pash. wây-/wayəl “to speak;” Yaghnobi wov-/wovta “to speak, call.”

To name or write in order the letters constituting a word

Etymology (EN): M.E. spellen, from O.Fr. espeller, from Proto-Germanic spellan “to tell,” which also gave rise to the O.E. spellian; ultimately from PIE *spel- “to say aloud, recite.”

Etymology (PE): Vâbidan, from vâb, from vab- ultimately from Proto-Ir. *uab/f- “to call,” which has given rise to several words in Iranian languages, mainly Pers. gap “word, talk,” gapidan “to talk,” buf “owl,” zand-vâf “nightingale,” literally “song teller, ode singer;” Baluchi gwâp-/gwâpt “to summon, call together;” Sogd. waβ-/wab- “to speak, to talk;” Pash. wây-/wayəl “to speak;” Yaghnobi wov-/wovta “to speak, call.”

A person who spells words.

See also: → spell; → -er.

A person who spells words.

See also: → spell; → -er.

1. The manner in which words are spelled; orthography; a group of letters representing a word.
   

   2. The act of a speller.

See also: → spell; → -ing.

1. The manner in which words are spelled; orthography; a group of letters representing a word.
   

   2. The act of a speller.

See also: → spell; → -ing.

1. To pay out, disburse, or expend; dispose of (money, wealth, resources, etc.).
   

2. To employ (labor, thought, words, time, etc.), as on some object or in some proceeding (Dictionary.com).

Etymology (EN): M.E. spenden, from O.En. -spendan (in forspendan “use up”), from M.L. spendere, from expendere “to pay out, weigh out money,” from → ex- “out” + pendere “to pay, weigh.”

Etymology (PE): Ziyâmidan, from Sogd. zyâm “to consume, spend,” ultimately from Proto-Ir. *uz-iam-, from *uz- “out, away,” → ex-,

• *iam- “to hold, take; stretch, reach out;” cf. Av. yam- “to hold, keep,” (+ *apa-) “to take away;” Skt. yam- “to hold, restrain.”

1. To pay out, disburse, or expend; dispose of (money, wealth, resources, etc.).
   

2. To employ (labor, thought, words, time, etc.), as on some object or in some proceeding (Dictionary.com).

Etymology (EN): M.E. spenden, from O.En. -spendan (in forspendan “use up”), from M.L. spendere, from expendere “to pay out, weigh out money,” from → ex- “out” + pendere “to pay, weigh.”

Etymology (PE): Ziyâmidan, from Sogd. zyâm “to consume, spend,” ultimately from Proto-Ir. *uz-iam-, from *uz- “out, away,” → ex-,

• *iam- “to hold, take; stretch, reach out;” cf. Av. yam- “to hold, keep,” (+ *apa-) “to take away;” Skt. yam- “to hold, restrain.”

A solid geometric figure generated by the revolution of a semicircle about its diameter; equation: x² + y² + z² = r².

Etymology (EN): M.E. spere, from O.Fr. espere, from L. sphæra “globe, ball, celestial sphere,” from Gk. sphaira “globe, ball,” of unknown origin.

Etymology (PE): Koré, loan from Ar. kurat.
Sepehr “sphere, celestial globe, heavens, sky;” Mid.Pers. spihr “sphere, sky, firmament, fate;” Av. spiti- in compounds: spiti-dôiθra- “with clear eyes;” Proto-Iranian *spiθra- (in proper name); cf. Skt. śvitrá- “white, whitish.”

A solid geometric figure generated by the revolution of a semicircle about its diameter; equation: x² + y² + z² = r².

Etymology (EN): M.E. spere, from O.Fr. espere, from L. sphæra “globe, ball, celestial sphere,” from Gk. sphaira “globe, ball,” of unknown origin.

Etymology (PE): Koré, loan from Ar. kurat.
Sepehr “sphere, celestial globe, heavens, sky;” Mid.Pers. spihr “sphere, sky, firmament, fate;” Av. spiti- in compounds: spiti-dôiθra- “with clear eyes;” Proto-Iranian *spiθra- (in proper name); cf. Skt. śvitrá- “white, whitish.”

The region of space around one of the bodies in a system of two celestial bodies where a third body of much smaller mass is influenced by the gravitational field of that body. The sphere of influence of a planet with respect to the Sun has a radius given by: R = R_P(M_P/M_S)^2/3, where R_P is the radius of the planet’s orbit around the Sun, M_P is the mass of the planet, and M_S is the solar mass. The sphere of influence of the Earth has a radius of about 927,000 km or slightly under 150 Earth radii. Beyond this limit, a space probe will come under the influence of the Sun.

See also: → sphere; → influence.

The region of space around one of the bodies in a system of two celestial bodies where a third body of much smaller mass is influenced by the gravitational field of that body. The sphere of influence of a planet with respect to the Sun has a radius given by: R = R_P(M_P/M_S)^2/3, where R_P is the radius of the planet’s orbit around the Sun, M_P is the mass of the planet, and M_S is the solar mass. The sphere of influence of the Earth has a radius of about 927,000 km or slightly under 150 Earth radii. Beyond this limit, a space probe will come under the influence of the Sun.

See also: → sphere; → influence.

A series of spheres with varying radii centred on the Earth, each rotating uniformly about an axis fixed with respect to the surface of the next larger sphere, all comprising a model in Greek astronomy to describe the motions of the heavenly bodies. The spheres turned with different speeds about axes with different orientations.
The fixed stars revolved around the Earth by the motion of the most distant sphere to which the stars were thought to be attached. Each of the five planets’ (Mercury, Venus, Mars, Jupiter, and Saturn) motion could be described using four spheres. The Sun and the Moon required three spheres each to explain their motions. Therefore, a total of 27 spheres described the behavior of the heavenly bodies in terms of circular motion.
Eudoxus was the first person to devise a model that could explain the → retrograde motion of the planets in the sky along a looped curve known as the → hippopede.

See also: → sphere; Eudoxus (Ευδοξοσ) of Cnidus (c 408 BC - c 355 BC), Greek astronomer and mathematician.

A series of spheres with varying radii centred on the Earth, each rotating uniformly about an axis fixed with respect to the surface of the next larger sphere, all comprising a model in Greek astronomy to describe the motions of the heavenly bodies. The spheres turned with different speeds about axes with different orientations.
The fixed stars revolved around the Earth by the motion of the most distant sphere to which the stars were thought to be attached. Each of the five planets’ (Mercury, Venus, Mars, Jupiter, and Saturn) motion could be described using four spheres. The Sun and the Moon required three spheres each to explain their motions. Therefore, a total of 27 spheres described the behavior of the heavenly bodies in terms of circular motion.
Eudoxus was the first person to devise a model that could explain the → retrograde motion of the planets in the sky along a looped curve known as the → hippopede.

See also: → sphere; Eudoxus (Ευδοξοσ) of Cnidus (c 408 BC - c 355 BC), Greek astronomer and mathematician.

Having the form of a sphere; of or pertaining to a sphere or spheres.

See also: From → sphere + → -ic + → -al.

Having the form of a sphere; of or pertaining to a sphere or spheres.

See also: From → sphere + → -ic + → -al.

An aberration of a spherical lens or spherical mirror in which light rays converge not to a single point but to a series of points with different distances from the lens or mirror. Spherical aberration is corrected by using parabolic reflecting and refracting surface.

See also: → spherical; → aberration.

An aberration of a spherical lens or spherical mirror in which light rays converge not to a single point but to a series of points with different distances from the lens or mirror. Spherical aberration is corrected by using parabolic reflecting and refracting surface.

See also: → spherical; → aberration.

An angle formed on the surface of a sphere by the intersection of two great circles of the sphere.

See also: → spherical; → angle.

An angle formed on the surface of a sphere by the intersection of two great circles of the sphere.

See also: → spherical; → angle.

A type of → astrolabe in which the observer’s horizon is drawn on the surface of a globe, mounted with a freely rotating spherical lattice work or ‘spider’ representing the celestial sphere. The earliest description of the spherical astrolabe dates back to  the Iranian astronomer Nayrizi (865-922).

See also: → spherical; → astrolabe.

A type of → astrolabe in which the observer’s horizon is drawn on the surface of a globe, mounted with a freely rotating spherical lattice work or ‘spider’ representing the celestial sphere. The earliest description of the spherical astrolabe dates back to  the Iranian astronomer Nayrizi (865-922).

See also: → spherical; → astrolabe.

The branch of astronomy that is concerned with determining the apparent positions and motions of celestial bodies on the celestial sphere. Same as → positional astronomy.

See also: → spherical; → astronomy.

The branch of astronomy that is concerned with determining the apparent positions and motions of celestial bodies on the celestial sphere. Same as → positional astronomy.

See also: → spherical; → astronomy.

A coordinate system using an origin (O) and three perpendicular axes (Ox, Oy, Oz), in which the position of a point (P) is given by three numbers (r, θ, φ). The coordinate r is the distance from the origin, θ the angle between the z-axis and the r direction, and φ the angle between the projection of r on the xy-plane and the Ox-axis. The coordinate φ is also called the → azimuthal angle.

See also: → spherical; → coordinate.

A coordinate system using an origin (O) and three perpendicular axes (Ox, Oy, Oz), in which the position of a point (P) is given by three numbers (r, θ, φ). The coordinate r is the distance from the origin, θ the angle between the z-axis and the r direction, and φ the angle between the projection of r on the xy-plane and the Ox-axis. The coordinate φ is also called the → azimuthal angle.

See also: → spherical; → coordinate.

The difference between the sum of the three angles of a → spherical triangle and 180° (π radians).

See also: → spherical; → excess.

The difference between the sum of the three angles of a → spherical triangle and 180° (π radians).

See also: → spherical; → excess.

The branch of geometry that deals with figures on the surface of a sphere (such as the spherical triangle and spherical polygon). It is an example of a non-Euclidean geometry.

See also: → spherical; → geometry.

The branch of geometry that deals with figures on the surface of a sphere (such as the spherical triangle and spherical polygon). It is an example of a non-Euclidean geometry.

See also: → spherical; → geometry.

A solution of some mathematical equations when → spherical polar coordinates are used in investigating physical problems in three dimensions. For example, solutions of → Laplace’s equation treated in spherical polar coordinates. Spherical harmonics are ubiquitous in atomic and molecular physics and appear in quantum mechanics as → eigenfunctions of → orbital angular momentum. They are also important in the representation of the gravitational and magnetic fields of planetary bodies, the characterization of the → cosmic microwave background anisotropy, the description of electrical potentials due to charge distributions, and in certain types of fluid motion.

See also: The term spherical harmonics was first used by William Thomson (Lord Kelvin) and Peter Guthrie Tait in their 1867 Treatise on Natural Philosophy; → spherical; → harmonic.

A solution of some mathematical equations when → spherical polar coordinates are used in investigating physical problems in three dimensions. For example, solutions of → Laplace’s equation treated in spherical polar coordinates. Spherical harmonics are ubiquitous in atomic and molecular physics and appear in quantum mechanics as → eigenfunctions of → orbital angular momentum. They are also important in the representation of the gravitational and magnetic fields of planetary bodies, the characterization of the → cosmic microwave background anisotropy, the description of electrical potentials due to charge distributions, and in certain types of fluid motion.

See also: The term spherical harmonics was first used by William Thomson (Lord Kelvin) and Peter Guthrie Tait in their 1867 Treatise on Natural Philosophy; → spherical; → harmonic.

The angle between the → normal to a spherical reference surface and the → equatorial plane.

See also: → spherical; → latitude.

The angle between the → normal to a spherical reference surface and the → equatorial plane.

See also: → spherical; → latitude.

A lens with a refractng surface which is a portion of a sphere. Spherical lenses can be of various types: → biconvex, → biconcave, → plano-convex, → plano-concave, → concavo-convex, and → convexo-concave.

See also: → spherical; → lens.

A lens with a refractng surface which is a portion of a sphere. Spherical lenses can be of various types: → biconvex, → biconcave, → plano-convex, → plano-concave, → concavo-convex, and → convexo-concave.

See also: → spherical; → lens.

A mirror whose reflecting surface is a portion of a sphere. Spherical mirrors can be of two types: → concave mirror and → convex mirror.

See also: → spherical; → mirror.

A mirror whose reflecting surface is a portion of a sphere. Spherical mirrors can be of two types: → concave mirror and → convex mirror.

See also: → spherical; → mirror.

Same as → spherical coordinates.

See also: → spherical; → polar; → coordinate

Same as → spherical coordinates.

See also: → spherical; → polar; → coordinate

A configuration in which the constituting parts are arranged concentrically around the center of a sphere.

See also: → spherical; → symmetry.

A configuration in which the constituting parts are arranged concentrically around the center of a sphere.

See also: → spherical; → symmetry.

A triangle drawn on the → surface of a → sphere. A spherical triangle, like a plane triangle, may be right, obtuse, acute, equilateral, isosceles, or scalene. The sum of the angles of a spherical triangle is greater than 180° (π) and less than 540° (3π). See also → spherical excess.

See also: → spherical; → triangle.

A triangle drawn on the → surface of a → sphere. A spherical triangle, like a plane triangle, may be right, obtuse, acute, equilateral, isosceles, or scalene. The sum of the angles of a spherical triangle is greater than 180° (π) and less than 540° (3π). See also → spherical excess.

See also: → spherical; → triangle.

A body that is shaped like a sphere but is not perfectly round, especially an ellipsoid that is generated by revolving an ellipse around one of its axes.

See also: → sphere; → -oid.

A body that is shaped like a sphere but is not perfectly round, especially an ellipsoid that is generated by revolving an ellipse around one of its axes.

See also: → sphere; → -oid.

Shaped like a → spheroid.

See also: → spheroid; → -al.

Shaped like a → spheroid.

See also: → spheroid; → -al.

Any of many vitrified droplets of rock formed by the solidification of molten meteoritic material that flows off a meteorite during its passage through the Earth’s atmosphere. Sizes range typically from 10 to 200 microns.

Etymology (EN): “Small sphere,” from → sphere + diminutive suffix → -ule.

Etymology (PE): Guyel “small globe,” from guy “ball, sphere” (variants golulé, gullé, goruk, gulu, gudé; cf. Skt. guda- “ball, mouthful, lump, tumour,” Pali gula- “ball,”
Gk. gloutos “rump,” L. glomus “ball,” globus “globe,” Ger. Kugel, E. clot; PIE *gel- “to make into a ball”) + -el diminutive suffix, → -ule.

Any of many vitrified droplets of rock formed by the solidification of molten meteoritic material that flows off a meteorite during its passage through the Earth’s atmosphere. Sizes range typically from 10 to 200 microns.

Etymology (EN): “Small sphere,” from → sphere + diminutive suffix → -ule.

Etymology (PE): Guyel “small globe,” from guy “ball, sphere” (variants golulé, gullé, goruk, gulu, gudé; cf. Skt. guda- “ball, mouthful, lump, tumour,” Pali gula- “ball,”
Gk. gloutos “rump,” L. glomus “ball,” globus “globe,” Ger. Kugel, E. clot; PIE *gel- “to make into a ball”) + -el diminutive suffix, → -ule.

The brightest star in the constellation → Virgo, and the 15th brightest star in the night sky. Also known as HD 116658. It is 260 → light-years distant from Earth. A → blue giant, it is a variable
→ eclipsing binary, with a period of 4.014 days. Both components are → B-type stars, the → primary being a → Beta Cephei variable near to core hydrogen exhaustion
(→ spectral type B1 III-IV) and the → secondary a → main sequence star (B2 V). See, e.g., R.S. Schnerr et al., 2010, arXiv:1008.4260.

Etymology (EN): From L. spica “ear of grain,” related to spina “thorn,” corresponding to Gk. stakhys “grapes.”

Etymology (PE): Sonbolé, from sonbol “an ear of corn; a hyacinth,” from Ar. sumbul.

The brightest star in the constellation → Virgo, and the 15th brightest star in the night sky. Also known as HD 116658. It is 260 → light-years distant from Earth. A → blue giant, it is a variable
→ eclipsing binary, with a period of 4.014 days. Both components are → B-type stars, the → primary being a → Beta Cephei variable near to core hydrogen exhaustion
(→ spectral type B1 III-IV) and the → secondary a → main sequence star (B2 V). See, e.g., R.S. Schnerr et al., 2010, arXiv:1008.4260.

Etymology (EN): From L. spica “ear of grain,” related to spina “thorn,” corresponding to Gk. stakhys “grapes.”

Etymology (PE): Sonbolé, from sonbol “an ear of corn; a hyacinth,” from Ar. sumbul.

Any of numerous vertical → spikes of → gas visible in the → monochromatic light of certain strong → spectral lines beyond the → Sun’s limb. Spicules are short-lived phenomena, corresponding to rising → jets of gas that move upward at about 30km/sec up to 10,000 km and last only about 10 minutes.

Etymology (EN): From L. spiculum “spearhead, arrowhead, bee stinger,” from spica “ear of grain” + -ulum, → -ule.

Etymology (PE): Sixak, from six “spur, spit; thorn; any pointed thing.”

Any of numerous vertical → spikes of → gas visible in the → monochromatic light of certain strong → spectral lines beyond the → Sun’s limb. Spicules are short-lived phenomena, corresponding to rising → jets of gas that move upward at about 30km/sec up to 10,000 km and last only about 10 minutes.

Etymology (EN): From L. spiculum “spearhead, arrowhead, bee stinger,” from spica “ear of grain” + -ulum, → -ule.

Etymology (PE): Sixak, from six “spur, spit; thorn; any pointed thing.”

One of, usually three or four, diagonal supports that hold the → secondary mirror in a → reflecting telescope. Also called support vane.

Etymology (EN): M.E. spithre, O.E. M.E. spithra, akin to spinnan “to spin;”
cf. M.L.G., M.Du., M.H.G., Ger. spinne, Du. spin “spider;” → vane.

Etymology (PE): Parré, → vane; târtan “spider,” literally “weaver,” composite word of with two cognate elements, the first one târ
“thread, warp, string,” related to tur “net, fishing net, snare,”
tâl “thread” (Borujerdi dialect), tân “thread, warp of a web,” from the second element tan-, tanidan
“to spin, twist, weave;” Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to stretch, extend;" tanoti “stretches,” tántra- “warp; essence, main point;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
Lith. tiñklas “net, fishing net, snare,” Latv. tikls “net;” PIE base *ten- “to stretch.”

One of, usually three or four, diagonal supports that hold the → secondary mirror in a → reflecting telescope. Also called support vane.

Etymology (EN): M.E. spithre, O.E. M.E. spithra, akin to spinnan “to spin;”
cf. M.L.G., M.Du., M.H.G., Ger. spinne, Du. spin “spider;” → vane.

Etymology (PE): Parré, → vane; târtan “spider,” literally “weaver,” composite word of with two cognate elements, the first one târ
“thread, warp, string,” related to tur “net, fishing net, snare,”
tâl “thread” (Borujerdi dialect), tân “thread, warp of a web,” from the second element tan-, tanidan
“to spin, twist, weave;” Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to stretch, extend;" tanoti “stretches,” tántra- “warp; essence, main point;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
Lith. tiñklas “net, fishing net, snare,” Latv. tikls “net;” PIE base *ten- “to stretch.”

1. A long, pointed → metal  → bar.
   

2. Physics: A → transient variation in → voltage or → current in an → electric circuit.

Etymology (EN): M.E. spik(e) from O.N. spikr “nail;” akin to M.L.G. spiker “nail.”

Etymology (PE): Sixak, from six “spur, spit; thorn; any pointed thing,” + -ak a suffix of similarity and nuance.

1. A long, pointed → metal  → bar.
   

2. Physics: A → transient variation in → voltage or → current in an → electric circuit.

Etymology (EN): M.E. spik(e) from O.N. spikr “nail;” akin to M.L.G. spiker “nail.”

Etymology (PE): Sixak, from six “spur, spit; thorn; any pointed thing,” + -ak a suffix of similarity and nuance.

1. Mechanics: The rotation of a body about an axis through the body.
   To cause to turn around rapidly, as on an axis. To revolve or rotate rapidly,
   
2. Quantum mechanics: See → spin quantum number; → spin angular momentum.

Etymology (EN): M.E. spinnen; O.E. spinnan “to draw out and twist fibers into thread” (cf. O.N., O.Fris. spinna, Dan. spinde, Du. spinnen, O.H.G. spinnan, Ger. spinnen); cognate with Pers. tan-, tanidan
“to spin, twist, weave” (Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to stretch, extend;" tanoti “stretches,” tántra- “warp; essence, main point;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
Lith. tiñklas “net, fishing net, snare,” Latv. tikls “net;” PIE base *ten- “to stretch”).

Etymology (PE): Espin, loan from E., as above.

1. Mechanics: The rotation of a body about an axis through the body.
   To cause to turn around rapidly, as on an axis. To revolve or rotate rapidly,
   
2. Quantum mechanics: See → spin quantum number; → spin angular momentum.

Etymology (EN): M.E. spinnen; O.E. spinnan “to draw out and twist fibers into thread” (cf. O.N., O.Fris. spinna, Dan. spinde, Du. spinnen, O.H.G. spinnan, Ger. spinnen); cognate with Pers. tan-, tanidan
“to spin, twist, weave” (Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to stretch, extend;" tanoti “stretches,” tántra- “warp; essence, main point;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
Lith. tiñklas “net, fishing net, snare,” Latv. tikls “net;” PIE base *ten- “to stretch”).

Etymology (PE): Espin, loan from E., as above.

An intrinsic quantum mechanical characteristic of a particle that has no classical counterpart but may loosely be likened to the classical → angular momentum of a particle arising from rotation about its own axis. The magnitude of spin angular momentum is given by the expression S = ħ √ s(s + 1), where s is the → spin quantum number. As an example, the spin of an electron is s = 1/2; this means that its spin angular momentum is (ħ /2) √ 3 or 0.91 x 10^-34 J.s. In addition, the projection of an angular momentum onto some defined axis is also quantized, with a z-component S_z = m_sħ. The only values of m_s (magnetic quantum number) are ± 1/2. See also → Stern-Gerlach experiment.

See also: → spin; → angular; → momentum.

An intrinsic quantum mechanical characteristic of a particle that has no classical counterpart but may loosely be likened to the classical → angular momentum of a particle arising from rotation about its own axis. The magnitude of spin angular momentum is given by the expression S = ħ √ s(s + 1), where s is the → spin quantum number. As an example, the spin of an electron is s = 1/2; this means that its spin angular momentum is (ħ /2) √ 3 or 0.91 x 10^-34 J.s. In addition, the projection of an angular momentum onto some defined axis is also quantized, with a z-component S_z = m_sħ. The only values of m_s (magnetic quantum number) are ± 1/2. See also → Stern-Gerlach experiment.

See also: → spin; → angular; → momentum.

The magnetic moment associated with the → spin angular momentum of a charged particle. The direction of the magnetic moment is opposite to the direction of the angular momentum. The magnitude of the magnetic moment is given by: μ = -g(q / 2m)J, where q is the charge, m is the mass, and J the angular momentum. The parameter g is a characteristic of the state of the atom. It would be 1 for a pure orbital moment, or 2 for a spin moment, or some other number in between for a complicated system like an atom. The quantity in the parenthesis for the electron is the → Bohr magneton. The electron spin magnetic moment is important in the → spin-orbit interaction which splits atomic energy levels and gives rise to → fine structure in the spectra of atoms. It is also a factor in the interaction of atom with external fields, → Zeeman effect.

See also: → spin; → magnetic moment.

The magnetic moment associated with the → spin angular momentum of a charged particle. The direction of the magnetic moment is opposite to the direction of the angular momentum. The magnitude of the magnetic moment is given by: μ = -g(q / 2m)J, where q is the charge, m is the mass, and J the angular momentum. The parameter g is a characteristic of the state of the atom. It would be 1 for a pure orbital moment, or 2 for a spin moment, or some other number in between for a complicated system like an atom. The quantity in the parenthesis for the electron is the → Bohr magneton. The electron spin magnetic moment is important in the → spin-orbit interaction which splits atomic energy levels and gives rise to → fine structure in the spectra of atoms. It is also a factor in the interaction of atom with external fields, → Zeeman effect.

See also: → spin; → magnetic moment.

An integer or half-integer on which the magnitude of a particle’s → spin angular momentum depends. It is expressed in units of → Planck’s constant divided by 2π. Called also spin, denoted s. The spin of a particle can only have a value that is zero or a multiple of 1/2. Particles with half-integer spins, 1/2, 3/2, 5/2, …, are → fermions. Particles with integer spin (0, 1, 2, …) are called → bosons.

See also: → spin; → quantum; → number.

An integer or half-integer on which the magnitude of a particle’s → spin angular momentum depends. It is expressed in units of → Planck’s constant divided by 2π. Called also spin, denoted s. The spin of a particle can only have a value that is zero or a multiple of 1/2. Particles with half-integer spins, 1/2, 3/2, 5/2, …, are → fermions. Particles with integer spin (0, 1, 2, …) are called → bosons.

See also: → spin; → quantum; → number.

The → excitation temperature of the → hyperfine structure levels of the → neutral hydrogen→ 21-centimeter line.

See also: → spin; → temperature.

The → excitation temperature of the → hyperfine structure levels of the → neutral hydrogen→ 21-centimeter line.

See also: → spin; → temperature.

A phenomenon in which the rotation period of a pulsar steadily decreases with the pulsar age. The cause of the spin-down is magnetic torque due to the strong fields threading out from the pulsar. The magnetic energy is being converted to high-energy particles and radiation from the nebula. Observed spin-down rates range from about 10^-5 seconds/year for the youngest pulsars to about 10^-12 seconds/year for recycled pulsars. The Crab pulsar is slowing down at a rate of about 10^-5 seconds/year. Knowing the rotation period and the lengthening rate of a pulsar leads to its age.

Etymology (EN): → spin; down, M.E.; O.E. ofdune “downward,” from dune “from the hill.”

Etymology (PE): Kond-carxi, from kond “slow; dull” + carx→ rotate + -i noun suffix.

A phenomenon in which the rotation period of a pulsar steadily decreases with the pulsar age. The cause of the spin-down is magnetic torque due to the strong fields threading out from the pulsar. The magnetic energy is being converted to high-energy particles and radiation from the nebula. Observed spin-down rates range from about 10^-5 seconds/year for the youngest pulsars to about 10^-12 seconds/year for recycled pulsars. The Crab pulsar is slowing down at a rate of about 10^-5 seconds/year. Knowing the rotation period and the lengthening rate of a pulsar leads to its age.

Etymology (EN): → spin; down, M.E.; O.E. ofdune “downward,” from dune “from the hill.”

Etymology (PE): Kond-carxi, from kond “slow; dull” + carx→ rotate + -i noun suffix.

Quantum mechanics: The scattering of a particle that reverses the spin direction.

Etymology (EN): → spin; flip, from flip-flap;
→ scattering.

Etymology (PE): Parâkaneš, → scattering; bâ “with;” vâruni, noun from vârun, → inverse; espin, → spin.

Quantum mechanics: The scattering of a particle that reverses the spin direction.

Etymology (EN): → spin; flip, from flip-flap;
→ scattering.

Etymology (PE): Parâkaneš, → scattering; bâ “with;” vâruni, noun from vârun, → inverse; espin, → spin.

1. Astro.: A relationship between the orbital period of one body around another and its rotational period on its axis. The relationship results from tidal forces between the two bodies. For example, the rotation period of the Moon equals its revolution period around the Earth.
   
2. Quantum mechanics: The interaction between a particle’s → spin angular momentum and its → orbital angular momentum.

See also: → spin; → orbit; → coupling.

1. Astro.: A relationship between the orbital period of one body around another and its rotational period on its axis. The relationship results from tidal forces between the two bodies. For example, the rotation period of the Moon equals its revolution period around the Earth.
   
2. Quantum mechanics: The interaction between a particle’s → spin angular momentum and its → orbital angular momentum.

See also: → spin; → orbit; → coupling.

1. A rounded rod, usually of wood, tapering toward each end, used in hand-spinning to twist into thread the fibers drawn from the mass on the distaff, and on which the thread is wound as it is spun (Dictionary.com).
   

2. → Spindle Galaxy.

Etymology (EN): M.E. spindel, O.E. spin(e)l, from spinnan, → spin.

Etymology (PE): Duk “spindle,” variants dêk, dik, ultimately from Proto-Ir. *dau- “to run;” cf. Pers. dow-, davidan “to run” (Cheung 2007).

1. A rounded rod, usually of wood, tapering toward each end, used in hand-spinning to twist into thread the fibers drawn from the mass on the distaff, and on which the thread is wound as it is spun (Dictionary.com).
   

2. → Spindle Galaxy.

Etymology (EN): M.E. spindel, O.E. spin(e)l, from spinnan, → spin.

Etymology (PE): Duk “spindle,” variants dêk, dik, ultimately from Proto-Ir. *dau- “to run;” cf. Pers. dow-, davidan “to run” (Cheung 2007).

Same as → NGC 5866.

See also: → spindle; → galaxy.

Same as → NGC 5866.

See also: → spindle; → galaxy.

1. In 3D → magnetic reconnection models of solar plasma, a field line crossing the → fan at the
   → magnetic null point. See also → fan (Lau & Finn. 1990, ApJ 350, 672; Parnell et al. 1996, Physics of Plasmas 3, 759).
   

2. A very narrow line of light extending back from the coma into the tail of some → comets.

Etymology (EN): M.E., from O.Fr. espine, from L. spina “backbone,” originally “thorn, prickle,” cf. L. spica “ear of corn,” O.N. spikr “nail;” from PIE *spei- “sharp point.”

Etymology (PE): Xâr “spine, thorn,” related to xal-, xalidan “to prick, to pierce,” xâridan “to scratch, itch;” Av. x^vara- “wound, sore.”

1. In 3D → magnetic reconnection models of solar plasma, a field line crossing the → fan at the
   → magnetic null point. See also → fan (Lau & Finn. 1990, ApJ 350, 672; Parnell et al. 1996, Physics of Plasmas 3, 759).
   

2. A very narrow line of light extending back from the coma into the tail of some → comets.

Etymology (EN): M.E., from O.Fr. espine, from L. spina “backbone,” originally “thorn, prickle,” cf. L. spica “ear of corn,” O.N. spikr “nail;” from PIE *spei- “sharp point.”

Etymology (PE): Xâr “spine, thorn,” related to xal-, xalidan “to prick, to pierce,” xâridan “to scratch, itch;” Av. x^vara- “wound, sore.”

A mineral, MgAl₂O₄, occurring in various colors, used as a gem, the most valuable being red. The famous “Black Prince’s Ruby” which forms part of the Crown Jewels of England, is, in fact, a red spinel. Spinel has often been confounded with → ruby. The most famous source of spinel is the historic region of Badakhshan (today northeastern Afghanistan and southeastern Tajikistan). The Badakhshan mines were mentioned by Persian writers as early as the 10th century. According to a Persian tradition, these mines were first disclosed when the mountain was broken open by an earthquake.

Etymology (EN): From Fr. spinelle, from It. spinella of unknown origin.  

Etymology (PE): Lâl, la’l “spinel; red,” originally “red” (cf. Tabari âl “red”); cf. Av. raoidita- “red, reddish;” Skt. rudhirá- “red, bloody;” L. ruber “red;” Gk. erythros “red;” akin to E. → red.

A mineral, MgAl₂O₄, occurring in various colors, used as a gem, the most valuable being red. The famous “Black Prince’s Ruby” which forms part of the Crown Jewels of England, is, in fact, a red spinel. Spinel has often been confounded with → ruby. The most famous source of spinel is the historic region of Badakhshan (today northeastern Afghanistan and southeastern Tajikistan). The Badakhshan mines were mentioned by Persian writers as early as the 10th century. According to a Persian tradition, these mines were first disclosed when the mountain was broken open by an earthquake.

Etymology (EN): From Fr. spinelle, from It. spinella of unknown origin.  

Etymology (PE): Lâl, la’l “spinel; red,” originally “red” (cf. Tabari âl “red”); cf. Av. raoidita- “red, reddish;” Skt. rudhirá- “red, bloody;” L. ruber “red;” Gk. erythros “red;” akin to E. → red.

A toy that with a quick or vigorous twist spins around its symmetry axis and balances on a point. Suppose a top is perfectly fashioned so that its → rotation axis passes through its
→ center of mass. If it is spun carefully such that it remains perfectly upright while rotating, it will spin at a steady → angular velocity almost indefinitely in the absence of → friction. Rotation creates an → angular momentum which is directed upward along the rotation axis, opposite to the → gravity vector. However, a slight mismatch between the rotation axis and the center of mass causes gravity to exert a → torque on the top due to its weight, acting through the center of mass. The torque gives rise to a time rate of change of angular momentum, so the top experiences → precession about its point of contact. The tip of the angular momentum vector can be perceived as precessing about the → vertical, thus
describing the → precessional circle. The top’s precession period is given by:

T_p = (4π²I)/(mgrT_s),

where I is the → moment of inertia, m the mass of the top, g gravity, r the distance between the center of mass and the contact point, and T_s is the spinning period of the top. Precession is accompanied by another oscillatory phenomenon, called → nutation. Nutation is less influenced by the gravity torque and is determined by the inertia forces
acting on the spinning body.

Etymology (EN): → spin; → -ing; top M.E., from O.E. top, maybe related to Fr. toupie.

Etymology (PE): Farmuk, ferferé “spinning top” (Dehxodâ), two words of unknown etymology.

A toy that with a quick or vigorous twist spins around its symmetry axis and balances on a point. Suppose a top is perfectly fashioned so that its → rotation axis passes through its
→ center of mass. If it is spun carefully such that it remains perfectly upright while rotating, it will spin at a steady → angular velocity almost indefinitely in the absence of → friction. Rotation creates an → angular momentum which is directed upward along the rotation axis, opposite to the → gravity vector. However, a slight mismatch between the rotation axis and the center of mass causes gravity to exert a → torque on the top due to its weight, acting through the center of mass. The torque gives rise to a time rate of change of angular momentum, so the top experiences → precession about its point of contact. The tip of the angular momentum vector can be perceived as precessing about the → vertical, thus
describing the → precessional circle. The top’s precession period is given by: