An Etymological Dictionary of Astronomy and Astrophysics

A block of the Earth’s crust, bounded by two normal faults, that has dropped downward in relation to adjacent portions.

Etymology (EN): Graben, from Ger. Graben “ditch, trench;” O.H.G. graban “ditch,” grab “grave, tomb;” Goth. graban “ditch;” P.Gmc. *graban; cf. O.E. græf “grave, ditch;” E. a grave; PIE base *ghrebh-/*ghrobh- “to dig, to scratch, to scrape.”

Etymology (PE): Foruzamin, from foru- + zamin. The first component foru- “down, downward; below; beneath;” Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” cf. Skt. pravát- “a sloping path, the slope of a mountain.” The second component zamin, variant
zami “earth, ground,” from Mid.Pers. zamig “earth;”
Av. zam- “the earth;” cf. Skt. ksam; Gk. khthôn, khamai “on the ground;” L. homo “earthly being” and humus “the earth” (as in homo sapiens or homicide, humble, humus, exhume);
PIE root *dh(e)ghom “earth.”

A block of the Earth’s crust, bounded by two normal faults, that has dropped downward in relation to adjacent portions.

Etymology (EN): Graben, from Ger. Graben “ditch, trench;” O.H.G. graban “ditch,” grab “grave, tomb;” Goth. graban “ditch;” P.Gmc. *graban; cf. O.E. græf “grave, ditch;” E. a grave; PIE base *ghrebh-/*ghrobh- “to dig, to scratch, to scrape.”

Etymology (PE): Foruzamin, from foru- + zamin. The first component foru- “down, downward; below; beneath;” Mid.Pers. frôt “down, downward;” O.Pers. fravata “forward, downward;” cf. Skt. pravát- “a sloping path, the slope of a mountain.” The second component zamin, variant
zami “earth, ground,” from Mid.Pers. zamig “earth;”
Av. zam- “the earth;” cf. Skt. ksam; Gk. khthôn, khamai “on the ground;” L. homo “earthly being” and humus “the earth” (as in homo sapiens or homicide, humble, humus, exhume);
PIE root *dh(e)ghom “earth.”

1. A degree or step in a scale, as of rank, advancement, quality, value, or intensity.
   

2. To arrange in a series of grades; class; sort (Dictionary.com).

Etymology (EN): From Fr. grade “grade, degree,” from L. gradus “step, pace, gait, walk;” figuratively “a step, stage, degree,” related to gradi “to walk, step, go,” and second element in congress, progress, etc.;
from PIE *ghredh-; cf. Lith. gridiju “to go, wander,” O.C.S. gredo “to come,” O.Ir. in-greinn “he pursues.”

Etymology (PE): Padâk, from Baluci padâk “step, stair, ladder” (ultimately from Proto-Ir. *padaka-), older form of Pers. pâyé “step, base,” from Mid.Pers. pâd, pây;
Av. pad-, cf. Skt. pat: Gk. pos, genitive podos; L. pes; PIE *pod-/*ped-.

1. A degree or step in a scale, as of rank, advancement, quality, value, or intensity.
   

2. To arrange in a series of grades; class; sort (Dictionary.com).

Etymology (EN): From Fr. grade “grade, degree,” from L. gradus “step, pace, gait, walk;” figuratively “a step, stage, degree,” related to gradi “to walk, step, go,” and second element in congress, progress, etc.;
from PIE *ghredh-; cf. Lith. gridiju “to go, wander,” O.C.S. gredo “to come,” O.Ir. in-greinn “he pursues.”

Etymology (PE): Padâk, from Baluci padâk “step, stair, ladder” (ultimately from Proto-Ir. *padaka-), older form of Pers. pâyé “step, base,” from Mid.Pers. pâd, pây;
Av. pad-, cf. Skt. pat: Gk. pos, genitive podos; L. pes; PIE *pod-/*ped-.

1. General: Degree of slope.
   

2. Physics: Change in the value of a quantity (as temperature, pressure) with change in a given variable.
   

3. Math.: A differential → operator (symbol → nabla, ∇) that, operating upon a function (f) of several variables, creates a → vector whose coordinates are the → partial derivatives of the function: ∇f = (∂f/∂x)i

• (∂f/∂y)j + (∂f/∂z)k. The gradient of a → scalar function is a vector function.

Etymology (EN): From L. gradient-, gradiens, pr.p. of gradi “to walk, go,” from grad- “walk” + -i- thematic vowel + -ent suffix of conjugation.

Etymology (PE): Ziné “ladder, steps, stair,” may be related to ciné, from
cidan “to place (something) above/upon (another similar thing);” cf. Lori râ-zina, Yazdi râ-cina “stairs,” Nâyini orcen “stairs, ladder;” the phoneme change -c- into -z-, as in gozidan, gozin-/cidan, cin- both deriving from Proto-Ir. *cai- “to heap up, gather, collect.”

1. General: Degree of slope.
   

2. Physics: Change in the value of a quantity (as temperature, pressure) with change in a given variable.
   

3. Math.: A differential → operator (symbol → nabla, ∇) that, operating upon a function (f) of several variables, creates a → vector whose coordinates are the → partial derivatives of the function: ∇f = (∂f/∂x)i

• (∂f/∂y)j + (∂f/∂z)k. The gradient of a → scalar function is a vector function.

Etymology (EN): From L. gradient-, gradiens, pr.p. of gradi “to walk, go,” from grad- “walk” + -i- thematic vowel + -ent suffix of conjugation.

Etymology (PE): Ziné “ladder, steps, stair,” may be related to ciné, from
cidan “to place (something) above/upon (another similar thing);” cf. Lori râ-zina, Yazdi râ-cina “stairs,” Nâyini orcen “stairs, ladder;” the phoneme change -c- into -z-, as in gozidan, gozin-/cidan, cin- both deriving from Proto-Ir. *cai- “to heap up, gather, collect.”

Proceeding, taking place, changing by small degrees.

Etymology (EN): From M.L. gradualis, from L. gradus “step.”

Etymology (PE): Padâkvâr, from padâk “grade,” + -vâr a suffix which denotes “suiting, befitting, resembling, in the manner of, possession.”
Pâypâyé “step by step,” from pây, pâ “foot, step,” → foot.

Proceeding, taking place, changing by small degrees.

Etymology (EN): From M.L. gradualis, from L. gradus “step.”

Etymology (PE): Padâkvâr, from padâk “grade,” + -vâr a suffix which denotes “suiting, befitting, resembling, in the manner of, possession.”
Pâypâyé “step by step,” from pây, pâ “foot, step,” → foot.

A burst that happens gradually, in contrast to a sudden burst.

See also: → gradual; → burst.

A burst that happens gradually, in contrast to a sudden burst.

See also: → gradual; → burst.

1. To divide into or mark with degrees or other divisions, as the scale of a thermometer.
   

2. To receive a degree or diploma on completing a course of study (often followed by from).
   

3. To confer a degree upon, or to grant a diploma to, at the close of a course of study, as in a university, college, or school.
   

4. A person who has received a degree or diploma on completing a course of study, as in a university, college, or school (Dictionary.com).

Etymology (EN): M.E., from M.L. graduatus, p.pa. of graduari “to take a degree,” from L. gradus “step, → grade.”

Etymology (PE): 1, 3) Padâk dâdan, compound infinitive, padâkidan simple infinitive, both from padâk, → grade, + dâdan “to give, grant,” → datum, and -idan, → -fy.

2. Padâk gereftan, from padâk + gereftan
   “→ take, hold.”
   

3. Padâkmand, from padâk + -mand suffix of possession and ability, → -al; padâkidé, p.p. of padâkidan, as above.

1. To divide into or mark with degrees or other divisions, as the scale of a thermometer.
   

2. To receive a degree or diploma on completing a course of study (often followed by from).
   

3. To confer a degree upon, or to grant a diploma to, at the close of a course of study, as in a university, college, or school.
   

4. A person who has received a degree or diploma on completing a course of study, as in a university, college, or school (Dictionary.com).

Etymology (EN): M.E., from M.L. graduatus, p.pa. of graduari “to take a degree,” from L. gradus “step, → grade.”

Etymology (PE): 1, 3) Padâk dâdan, compound infinitive, padâkidan simple infinitive, both from padâk, → grade, + dâdan “to give, grant,” → datum, and -idan, → -fy.

2. Padâk gereftan, from padâk + gereftan
   “→ take, hold.”
   

3. Padâkmand, from padâk + -mand suffix of possession and ability, → -al; padâkidé, p.p. of padâkidan, as above.

1. Marking the scale of an instrument, e.g. the stem of a thermometer is graduated in degrees.
   

2. An act of graduating; the state of being graduated.

See also: Verbal noun of → graduate.

1. Marking the scale of an instrument, e.g. the stem of a thermometer is graduated in degrees.
   

2. An act of graduating; the state of being graduated.

See also: Verbal noun of → graduate.

1. A small, hard seed of plants, especially the seed of cereals.
   

2. A tiny portion or particle of something such as sand or salt; → dust grain.

Etymology (EN): M.E. grain, grein, from O.Fr. grein, from L. granum “seed;” akin to corn.

Etymology (PE): Dâné “grain, seed;” Mid.Pers. dân, dânag “seed, corn;” Av. dānô- in dānô.karš- “carrying grains; an ant;” cf.
Skt. dhânâ- “corn, grain;” Tokharian B tāno “grain;” Lith. duona “corn, bread.”

1. A small, hard seed of plants, especially the seed of cereals.
   

2. A tiny portion or particle of something such as sand or salt; → dust grain.

Etymology (EN): M.E. grain, grein, from O.Fr. grein, from L. granum “seed;” akin to corn.

Etymology (PE): Dâné “grain, seed;” Mid.Pers. dân, dânag “seed, corn;” Av. dānô- in dānô.karš- “carrying grains; an ant;” cf.
Skt. dhânâ- “corn, grain;” Tokharian B tāno “grain;” Lith. duona “corn, bread.”

Sticking together of micron- to centimetre-sized grains occurring in the interstellar and protoplanetary environments to form larger grain agglomerates.

See also: → grain; → coagulation.

Sticking together of micron- to centimetre-sized grains occurring in the interstellar and protoplanetary environments to form larger grain agglomerates.

See also: → grain; → coagulation.

Conversion of dust grains into smaller grains due to high environmental temperatures.

See also: → grain; → evaporation.

Conversion of dust grains into smaller grains due to high environmental temperatures.

See also: → grain; → evaporation.

The process by which dust grains are assembled or produced.

See also: → grain; → formation.

The process by which dust grains are assembled or produced.

See also: → grain; → formation.

The increase of dust grains to micron sizes in the interstellar environments due to various physical processes, for example mutual collisions and accumulation of ice mantles.

See also: → grain; → growth.

The increase of dust grains to micron sizes in the interstellar environments due to various physical processes, for example mutual collisions and accumulation of ice mantles.

See also: → grain; → growth.

A layer of icy molecules covering interstellar dust grains.

Etymology (EN): → grain; mantle, from O.E. mentel “loose, sleeveless cloak,” from L. mantellum “cloak,” perhaps from a Celtic source.

Etymology (PE): Rupuš “overgarment, cloak,” from ru “surface, face; aspect; appearance” (Mid.Pers. rôy, rôdh “face;” Av. raoδa- “growth,” in plural form “appearance,” from raod- “to grow, sprout, shoot;” cf. Skt. róha- “rising, height”) + puš “covering, mantle,” from pušidan “to cover; to put on” (Mid.Pers. pôšidan, pôš- “to cover; to wear;” cf. Mid.Pers. pôst; Mod.Pers. pust “skin, hide;” O.Pers. pavastā- “thin clay envelope used to protect unbaked clay tablets;” Skt. pavásta- “cover,” Proto-Indo-Iranian *pauastā- “cloth”).

A layer of icy molecules covering interstellar dust grains.

Etymology (EN): → grain; mantle, from O.E. mentel “loose, sleeveless cloak,” from L. mantellum “cloak,” perhaps from a Celtic source.

Etymology (PE): Rupuš “overgarment, cloak,” from ru “surface, face; aspect; appearance” (Mid.Pers. rôy, rôdh “face;” Av. raoδa- “growth,” in plural form “appearance,” from raod- “to grow, sprout, shoot;” cf. Skt. róha- “rising, height”) + puš “covering, mantle,” from pušidan “to cover; to put on” (Mid.Pers. pôšidan, pôš- “to cover; to wear;” cf. Mid.Pers. pôst; Mod.Pers. pust “skin, hide;” O.Pers. pavastā- “thin clay envelope used to protect unbaked clay tablets;” Skt. pavásta- “cover,” Proto-Indo-Iranian *pauastā- “cloth”).

The ejection of atoms from interstellar dust grains due to impact by
gas ions, which leads to grain destruction.

Etymology (EN): → grain; sputtering, from sputter “to spit with explosive sounds,” cognate with Du. sputteren.

Etymology (PE): Osparâni, verbal noun of osparândan, from os- “out of, outside,” → ex- + parândan “to eject,” transitive of
paridan “to fly” (from Mid./Mod.Pers. par(r) “feather, wing,” Av. parəna- “feather, wing;” cp. Skt. parna “feather,” E. fern; PIE *porno- “feather”).

The ejection of atoms from interstellar dust grains due to impact by
gas ions, which leads to grain destruction.

Etymology (EN): → grain; sputtering, from sputter “to spit with explosive sounds,” cognate with Du. sputteren.

Etymology (PE): Osparâni, verbal noun of osparândan, from os- “out of, outside,” → ex- + parândan “to eject,” transitive of
paridan “to fly” (from Mid./Mod.Pers. par(r) “feather, wing,” Av. parəna- “feather, wing;” cp. Skt. parna “feather,” E. fern; PIE *porno- “feather”).

A unit of mass equal to one thousandth of a kilogram.

Etymology (EN): From Fr. gramme, from L.L. gramma “small weight,” from Gk. gramma “small weight,” originally “letter of the alphabet,” from stem of graphein “to draw, write.”

Etymology (PE): Geram, loanword from Fr. gramme, as above.

A unit of mass equal to one thousandth of a kilogram.

Etymology (EN): From Fr. gramme, from L.L. gramma “small weight,” from Gk. gramma “small weight,” originally “letter of the alphabet,” from stem of graphein “to draw, write.”

Etymology (PE): Geram, loanword from Fr. gramme, as above.

1. The study of the way the sentences of a language are constructed; → morphology and → syntax.
   

   2. These features or constructions themselves (Dictionary.com).

Etymology (EN): M.E. gramarye, from O.Fr. gramaire “grammar; learning,” especially Latin and philology, an “irregular semi-popular adoption” of L. grammatica, from Gk. grammatike (tekhne) “(art) of letters” with a sense of both philology and literature, from grammatikos “pertaining to or versed in letters or learning,” from gramma “letter,” → -gram.

Etymology (PE): Dastur-e zabân, literally “language rule,” from dastur “rule; mandate, command; religious authority (of the Zoroastrians);” Mid.Pers. dast “able, capable;” Av. danh- “to teach, instruct;” cf. Skt. dams- “to show or teach wonderful skills, perform wise;” Gk. didasko “I learn;” PIE *dens- “to become skilfull; to teach, instruct” (Cheung 2007); + zabân, → language.

1. The study of the way the sentences of a language are constructed; → morphology and → syntax.
   

   2. These features or constructions themselves (Dictionary.com).

Etymology (EN): M.E. gramarye, from O.Fr. gramaire “grammar; learning,” especially Latin and philology, an “irregular semi-popular adoption” of L. grammatica, from Gk. grammatike (tekhne) “(art) of letters” with a sense of both philology and literature, from grammatikos “pertaining to or versed in letters or learning,” from gramma “letter,” → -gram.

Etymology (PE): Dastur-e zabân, literally “language rule,” from dastur “rule; mandate, command; religious authority (of the Zoroastrians);” Mid.Pers. dast “able, capable;” Av. danh- “to teach, instruct;” cf. Skt. dams- “to show or teach wonderful skills, perform wise;” Gk. didasko “I learn;” PIE *dens- “to become skilfull; to teach, instruct” (Cheung 2007); + zabân, → language.

A specialist or expert in grammar.

Etymology (EN): From O.Fr. gramairien “learned man, person who knows Latin,” agent noun from grammaire, → grammar.

Etymology (PE): Zabân-dasturdân, literally “knower of grammar,” from zabân-dastur→ grammar + dân agent noun and present stem of dânestan, → know.

A specialist or expert in grammar.

Etymology (EN): From O.Fr. gramairien “learned man, person who knows Latin,” agent noun from grammaire, → grammar.

Etymology (PE): Zabân-dasturdân, literally “knower of grammar,” from zabân-dastur→ grammar + dân agent noun and present stem of dânestan, → know.

Of or relating to → grammar; conforming to standard usage.

Etymology (EN): From M.Fr. grammatical and directly from L. grammaticalis “of a scholar,” from grammaticus “pertaining to → grammar.”

Etymology (PE): Dastur-e zabâni, zabân-dasturi</i<, adj. from dastur-e zabân, zabân-dastur, → grammar.

Of or relating to → grammar; conforming to standard usage.

Etymology (EN): From M.Fr. grammatical and directly from L. grammaticalis “of a scholar,” from grammaticus “pertaining to → grammar.”

Etymology (PE): Dastur-e zabâni, zabân-dasturi</i<, adj. from dastur-e zabân, zabân-dastur, → grammar.

An inflectional category, basically pertaing to nouns and pronoun, which marks their relationship with other parts of the sentence.
sentence.
→ accusative case, → nominative case, → genitive case, → dative case, → ablative case, → vocative case, → imperative case.

See also: → grammatical; → case.

An inflectional category, basically pertaing to nouns and pronoun, which marks their relationship with other parts of the sentence.
sentence.
→ accusative case, → nominative case, → genitive case, → dative case, → ablative case, → vocative case, → imperative case.

See also: → grammatical; → case.

A galaxy with prominent → arms that are clearly attached to the central → bulge or → bar spiraling continuously outward until they reach the edge of the visible disk. Some examples are: → Whirlpool galaxy (M51), M74 (NGC 628), and NGC 2997.

Etymology (EN): M.E. graunt, from O.Fr. grant, grand, from L. grandis “big, great,” also “full-grown;” design, from M.E. designen, from L. designare “mark out, designate, appoint,” from → de- “out” + signare “to mark,” from signum→ sign; → spiral; → galaxy.

Etymology (PE): Kahkešân, → galaxy; mârpic→ spiral; farsâz, → perfect.

A galaxy with prominent → arms that are clearly attached to the central → bulge or → bar spiraling continuously outward until they reach the edge of the visible disk. Some examples are: → Whirlpool galaxy (M51), M74 (NGC 628), and NGC 2997.

Etymology (EN): M.E. graunt, from O.Fr. grant, grand, from L. grandis “big, great,” also “full-grown;” design, from M.E. designen, from L. designare “mark out, designate, appoint,” from → de- “out” + signare “to mark,” from signum→ sign; → spiral; → galaxy.

Etymology (PE): Kahkešân, → galaxy; mârpic→ spiral; farsâz, → perfect.

Any physical theory that unites the strong, electromagnetic, and weak interactions at high energy. It is hoped that GUTs can ultimately be extended to incorporate gravity. → theory of everything.

Etymology (EN): M.E. graunt, from O.Fr. grant, grand, from L. grandis “big, great,” also “full-grown;” unified, p.p. of → unify; → theory.

Etymology (PE): Negâré, → theory; yegâneš, verbal noun of yegânestan, → unify; bozorg→ great.

Any physical theory that unites the strong, electromagnetic, and weak interactions at high energy. It is hoped that GUTs can ultimately be extended to incorporate gravity. → theory of everything.

Etymology (EN): M.E. graunt, from O.Fr. grant, grand, from L. grandis “big, great,” also “full-grown;” unified, p.p. of → unify; → theory.

Etymology (PE): Negâré, → theory; yegâneš, verbal noun of yegânestan, → unify; bozorg→ great.

A very hard, granular, → igneous rock of visibly crystalline texture consisting mainly of → quartz, → mica, and → feldspar that constitutes the bulk of the → continental crust.

See also: From Fr. granit(e) or directly from It. granito “granite,” originally “grained,” p.p. adj. from granire “granulate, make grainy,” from grano “grain,” from L. granum, → grain.

A very hard, granular, → igneous rock of visibly crystalline texture consisting mainly of → quartz, → mica, and → feldspar that constitutes the bulk of the → continental crust.

See also: From Fr. granit(e) or directly from It. granito “granite,” originally “grained,” p.p. adj. from granire “granulate, make grainy,” from grano “grain,” from L. granum, → grain.

The mottled appearance of the solar → photosphere, caused by → convective cells, resembling → granules, which rises from the interior of the Sun. Each granule has a mean size of about 1,000 km and an upward velocity of about 0.5 km/sec. Granules are separated by intergranular walls about 400 K colder. They emerge from the fragments of the preceding granules and their lifetimes are about 20 minutes.

Etymology (EN): From → granule + -ation a combination of -ate and -ion, used to form nouns from stems in -ate.

Etymology (PE): Dâne-bandi, from dâné, → grain, + bandi verbal noun of bastan, vastan “to bind, shut;” O.Pers./Av. band- “to bind, fetter,” banda- “band, tie” (cf.
Skt. bandh- “to bind, tie, fasten;” PIE *bhendh- “to bind;” Ger. binden; E. bind).

The mottled appearance of the solar → photosphere, caused by → convective cells, resembling → granules, which rises from the interior of the Sun. Each granule has a mean size of about 1,000 km and an upward velocity of about 0.5 km/sec. Granules are separated by intergranular walls about 400 K colder. They emerge from the fragments of the preceding granules and their lifetimes are about 20 minutes.

Etymology (EN): From → granule + -ation a combination of -ate and -ion, used to form nouns from stems in -ate.

Etymology (PE): Dâne-bandi, from dâné, → grain, + bandi verbal noun of bastan, vastan “to bind, shut;” O.Pers./Av. band- “to bind, fetter,” banda- “band, tie” (cf.
Skt. bandh- “to bind, tie, fasten;” PIE *bhendh- “to bind;” Ger. binden; E. bind).

1. Geology: A term used for a sedimentary particle that is between 2 and 4 millimeters in size. Granules are larger than → sand but smaller than → pebbles. Granules have typically been rounded by abrasion during sedimentary transport (geology.com/dictionary).
   

   2. One of the → convective cells constituting the solar → granulation.

See also: → grain + → -ule.

1. Geology: A term used for a sedimentary particle that is between 2 and 4 millimeters in size. Granules are larger than → sand but smaller than → pebbles. Granules have typically been rounded by abrasion during sedimentary transport (geology.com/dictionary).
   

   2. One of the → convective cells constituting the solar → granulation.

See also: → grain + → -ule.

The edible, pulpy, smooth-skinned berry or fruit that grows in clusters on vines of the genus Vitis, and from which wine is made (Dictionary.com).

Etymology (EN): M.E., from O.Fr. grape “bunch of grapes, grape.”

Etymology (PE): Angur “grape,” from Mid.Pers. angur “grape;” cf. Khwarazmi ‘nkyδ, Yidgha agidro, Munji aglero, Shughni angûrδ, related to quré “unripe grape.”

The edible, pulpy, smooth-skinned berry or fruit that grows in clusters on vines of the genus Vitis, and from which wine is made (Dictionary.com).

Etymology (EN): M.E., from O.Fr. grape “bunch of grapes, grape.”

Etymology (PE): Angur “grape,” from Mid.Pers. angur “grape;” cf. Khwarazmi ‘nkyδ, Yidgha agidro, Munji aglero, Shughni angûrδ, related to quré “unripe grape.”

1. A visual representation of data that displays the relationship among variables, usually cast along X and Y axes.
   

2. In → graph theory, a graph G = (V, E) consists of a set of objects V called vertices and a set E which contains unordered pairs of distinct elements of V called edges.

Etymology (EN): Short for graphic (formula), from L. graphicus “of painting or drawing,” from Gk. graphikos “able to draw or paint,” from graph(ein) “to draw, write” + -ikos, → ic.

Etymology (PE): Negâré, from negâr “picture, figure” (verb negârdan, negâštan “to paint”), from prefix ne-, O.Pers./Av. ni- “down; into,” → ni-, + gâr, from kar-, kardan “to do, to make” (Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base *k^wer- “to do, to make”).

1. A visual representation of data that displays the relationship among variables, usually cast along X and Y axes.
   

2. In → graph theory, a graph G = (V, E) consists of a set of objects V called vertices and a set E which contains unordered pairs of distinct elements of V called edges.

Etymology (EN): Short for graphic (formula), from L. graphicus “of painting or drawing,” from Gk. graphikos “able to draw or paint,” from graph(ein) “to draw, write” + -ikos, → ic.

Etymology (PE): Negâré, from negâr “picture, figure” (verb negârdan, negâštan “to paint”), from prefix ne-, O.Pers./Av. ni- “down; into,” → ni-, + gâr, from kar-, kardan “to do, to make” (Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base *k^wer- “to do, to make”).

The branch of → mathematics dealing with → graphs. In particular, it involves the ways in which sets of points (→ vertex) can be connected by lines or arcs (→ edge).

See also: → graph; → theory.

The branch of → mathematics dealing with → graphs. In particular, it involves the ways in which sets of points (→ vertex) can be connected by lines or arcs (→ edge).

See also: → graph; → theory.

(Adj.) Pertaining to the use of diagrams, graphs, mathematical curves, or the like.
Math.: Pertaining to the determination of values, solution of problems, etc., by direct measurement on diagrams instead of by ordinary calculations.
(n.) A product of the graphic arts, as a drawing or print. A computer-generated image.

See also: → graph + → -ic.

(Adj.) Pertaining to the use of diagrams, graphs, mathematical curves, or the like.
Math.: Pertaining to the determination of values, solution of problems, etc., by direct measurement on diagrams instead of by ordinary calculations.
(n.) A product of the graphic arts, as a drawing or print. A computer-generated image.

See also: → graph + → -ic.

A particular crystalline form of → carbon occurring as a soft, black, lustrous mineral. The carbon atoms in graphite are strongly bonded together in sheets. Because the bonds between the sheets are weak, other atoms can easily fit between them, causing graphite to be soft and slippery to the touch. Graphite conducts electricity and is used in lead pencils and electrolytic anodes, as a lubricant, and as a moderator in nuclear reactors. If graphite is subjected to high pressure, it will be transformed into → diamond.

Graphite is present in the → interstellar medium; it forms in circumstellar shells and supernova ejecta. In particular, the 2175 Å interstellar extinction feature is accounted for by small graphite grains.

See also: From Ger. Graphit, from Gk. graph(ein) “to write, draw,” so called because it was used for pencils, → graph + -it a suffix of chemical compounds, equivalent to E. -ite.

A particular crystalline form of → carbon occurring as a soft, black, lustrous mineral. The carbon atoms in graphite are strongly bonded together in sheets. Because the bonds between the sheets are weak, other atoms can easily fit between them, causing graphite to be soft and slippery to the touch. Graphite conducts electricity and is used in lead pencils and electrolytic anodes, as a lubricant, and as a moderator in nuclear reactors. If graphite is subjected to high pressure, it will be transformed into → diamond.

Graphite is present in the → interstellar medium; it forms in circumstellar shells and supernova ejecta. In particular, the 2175 Å interstellar extinction feature is accounted for by small graphite grains.

See also: From Ger. Graphit, from Gk. graph(ein) “to write, draw,” so called because it was used for pencils, → graph + -it a suffix of chemical compounds, equivalent to E. -ite.

Same as → diffraction grating.

Etymology (EN): M.E. grating, M.L. grata “a grating,” variant of crata, from crat-, stem of cratis “wickerwork.”

Etymology (PE): Turi, from tur “fishing net, net, snare,” variants târ “thread, warp, string,” tâl “thread” (Borujerdi dialect), cognate with tanidan, tan- “to spin, twist, weave” (Mid.Pers. tanitan; Av. tan- to stretch, extend;" Skt. tan- to stretch, extend;" tanoti “stretches,” tantram “loom;” 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”).

Same as → diffraction grating.

Etymology (EN): M.E. grating, M.L. grata “a grating,” variant of crata, from crat-, stem of cratis “wickerwork.”

Etymology (PE): Turi, from tur “fishing net, net, snare,” variants târ “thread, warp, string,” tâl “thread” (Borujerdi dialect), cognate with tanidan, tan- “to spin, twist, weave” (Mid.Pers. tanitan; Av. tan- to stretch, extend;" Skt. tan- to stretch, extend;" tanoti “stretches,” tantram “loom;” 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”).

The angle between the incident optical beam and the normal to the grating. It is the angle to which the grating must be set to place the desired wavelength at the center of the detector.

See also: → grating; → angle.

The angle between the incident optical beam and the normal to the grating. It is the angle to which the grating must be set to place the desired wavelength at the center of the detector.

See also: → grating; → angle.

The measure of the light intensity diffracted from a grating.

See also: → grating; → efficiency.

The measure of the light intensity diffracted from a grating.

See also: → grating; → efficiency.

One of thousands of long, narrow indentations in the surface of a → diffraction grating.

See also: → grating; → groove.

One of thousands of long, narrow indentations in the surface of a → diffraction grating.

See also: → grating; → groove.

To move or tend to move under the influence of gravitational force.

Etymology (EN): From L. gravitatus, p.p. of gravitâre, from gravis “heavy,” → gravity.

Etymology (PE): Gerânidan, infinitive of gerân, → gravity.

To move or tend to move under the influence of gravitational force.

Etymology (EN): From L. gravitatus, p.p. of gravitâre, from gravis “heavy,” → gravity.

Etymology (PE): Gerânidan, infinitive of gerân, → gravity.

1. The universal phenomenon of attraction between material bodies.
   → Newton’s law of gravitation.
   
2. The act or process of moving under the influence of this attraction.

See also: Verbal noun of → gravitate.

1. The universal phenomenon of attraction between material bodies.
   → Newton’s law of gravitation.
   
2. The act or process of moving under the influence of this attraction.

See also: Verbal noun of → gravitate.

Of or relating to or caused by → gravitation.

See also: Adj. of → gravitation.

Of or relating to or caused by → gravitation.

See also: Adj. of → gravitation.

The acceleration caused by the force of gravity. At the Earth’s surface it is determined by the distance of the object form the center of the Earth: g = GM/R², where G is the → gravitational constant, and M and R are the Earth’s mass and radius respectively. It is approximately equal to 9.8 m s^-2. The value varies slightly with latitude and elevation. Also known as the → acceleration of gravity.

See also: → gravitational; → acceleration.

The acceleration caused by the force of gravity. At the Earth’s surface it is determined by the distance of the object form the center of the Earth: g = GM/R², where G is the → gravitational constant, and M and R are the Earth’s mass and radius respectively. It is approximately equal to 9.8 m s^-2. The value varies slightly with latitude and elevation. Also known as the → acceleration of gravity.

See also: → gravitational; → acceleration.

The force that pulls material bodies toward one another because of → gravitation.

See also: → gravitational; → attraction.

The force that pulls material bodies toward one another because of → gravitation.

See also: → gravitational; → attraction.

Collapse of a mass of material as a result of the mutual → gravitational attraction of all its constituents.

See also: → gravitational; → collapse.

Collapse of a mass of material as a result of the mutual → gravitational attraction of all its constituents.

See also: → gravitational; → collapse.

A fundamental constant that appears in → Newton’s law of gravitation. It is the force of attraction between two bodies of unit mass separated by unit distance:
G = 6.673 x 10^-8 dyn cm² g^-2 or 6.673 x 10^-8 cm³s^-2g^-1, or 6.673 x 10^-11 N m² kg^-2 or
6.673 x 10^-11 m³s^-2kg^-1. It was first measured in 1798 by Henry Cavendish (1731-1810), 71 years after Newton’s death. Same as the → Newtonian constant of gravitation.

See also: → gravitational; → constant.

A fundamental constant that appears in → Newton’s law of gravitation. It is the force of attraction between two bodies of unit mass separated by unit distance:
G = 6.673 x 10^-8 dyn cm² g^-2 or 6.673 x 10^-8 cm³s^-2g^-1, or 6.673 x 10^-11 N m² kg^-2 or
6.673 x 10^-11 m³s^-2kg^-1. It was first measured in 1798 by Henry Cavendish (1731-1810), 71 years after Newton’s death. Same as the → Newtonian constant of gravitation.

See also: → gravitational; → constant.

Decrease in the volume of an astronomical object under the action of a dominant, central gravitational force.

See also: → gravitational; → contraction.

Decrease in the volume of an astronomical object under the action of a dominant, central gravitational force.

See also: → gravitational; → contraction.

The dimensionless gravitational constant defined as the gravitational attraction between pair of electrons and normally given by: α_G = (Gm_e²) / (ħc) = (m_e / m_P)² ~ 1.7518 × 10^-45, where ħ is → Planck’s reduced constant, c the → speed of light, m_e is the → electron mass, and m_P is the → Planck mass.

See also: → gravitational; → coupling; → constant.

The dimensionless gravitational constant defined as the gravitational attraction between pair of electrons and normally given by: α_G = (Gm_e²) / (ħc) = (m_e / m_P)² ~ 1.7518 × 10^-45, where ħ is → Planck’s reduced constant, c the → speed of light, m_e is the → electron mass, and m_P is the → Planck mass.

See also: → gravitational; → coupling; → constant.

An encounter in which two moving bodies alter each other’s direction and velocity by mutual → gravitational attraction.

See also: → gravitational; → encounter.

An encounter in which two moving bodies alter each other’s direction and velocity by mutual → gravitational attraction.

See also: → gravitational; → encounter.

Same as → gravitational potential energy.

See also: → gravitational; → energy.

Same as → gravitational potential energy.

See also: → gravitational; → energy.

The condition in a celestial body when gravitational forces acting on each point are balanced by some outward pressure, such as radiation pressure or electron degeneracy pressure, so that no vertical motion results.

See also: → gravitational; → equilibrium.

The condition in a celestial body when gravitational forces acting on each point are balanced by some outward pressure, such as radiation pressure or electron degeneracy pressure, so that no vertical motion results.

See also: → gravitational; → equilibrium.

The region of space in which → gravitational attraction exists.

See also: → gravitational; → field.

The region of space in which → gravitational attraction exists.

See also: → gravitational; → field.

The weakest of the four fundamental forces of nature. Described by → Newton’s law of gravitation and subsequently by Einstein’s → general relativity.

See also: → gravitational; → force.

The weakest of the four fundamental forces of nature. Described by → Newton’s law of gravitation and subsequently by Einstein’s → general relativity.

See also: → gravitational; → force.

The process by which fluctuations in an infinite medium of size greater than a certain length scale (the Jeans length) grow by self-gravitation.

See also: → gravitational; → instability.

The process by which fluctuations in an infinite medium of size greater than a certain length scale (the Jeans length) grow by self-gravitation.

See also: → gravitational; → instability.

Mutual attraction between any two bodies that have mass.

See also: → gravitational; → interaction.

Mutual attraction between any two bodies that have mass.

See also: → gravitational; → interaction.

A concentration of matter, such as a galaxy or a cluster of galaxies, that bends light rays from a background object, resulting in production of multiple images. If the two objects and the Earth are perfectly aligned, the light from the distant object appears as a ring from Earth. This is called an Einstein Ring, since its existence was predicted by Einstein in his theory of general relativity.

See also: → gravitational; → lens.

A concentration of matter, such as a galaxy or a cluster of galaxies, that bends light rays from a background object, resulting in production of multiple images. If the two objects and the Earth are perfectly aligned, the light from the distant object appears as a ring from Earth. This is called an Einstein Ring, since its existence was predicted by Einstein in his theory of general relativity.

See also: → gravitational; → lens.

The main equation of gravitational lens theory that sets a relation between the angular position of the point source and the observable position of its image.

See also: → gravitational; → lens; → equation.

The main equation of gravitational lens theory that sets a relation between the angular position of the point source and the observable position of its image.

See also: → gravitational; → lens; → equation.

The act of producing or the state of a → gravitational lens.

See also: → gravitational; → lensing.

The act of producing or the state of a → gravitational lens.

See also: → gravitational; → lensing.

The difference in light travel times along the various light paths from the source to the observer when the source image is divided into several images because of → gravitational lensing.

According to the theory of → general relativity, light rays are deflected in the vicinity of massive objects. If the light source and the deflector are sufficiently well aligned with the observer, and obey some conditions on their distances (→ Einstein radius), we can observe several (generally distorted and magnified) images of the source.

A property of → strong lensing is that the light travel time from the source to the observer is generally not identical for the different images. In other words, we not only see several images of one same object, but we also see this object, in each image, at different times. This means, in one image the lensed object will be observed before the other image.

Given a physical model of the gravitational lens, the light travel time for each image can be computed.

The expression giving the time delay has two components: a term is called → geometric delay, and the second term, known as the → Shapiro time delay. The latter is due to time dilation by the gravitational field of the lens, a direct consequence of general relativity. See also → time delay distance.

See also: → gravitational; → lensing; → time; → delay.

The difference in light travel times along the various light paths from the source to the observer when the source image is divided into several images because of → gravitational lensing.

According to the theory of → general relativity, light rays are deflected in the vicinity of massive objects. If the light source and the deflector are sufficiently well aligned with the observer, and obey some conditions on their distances (→ Einstein radius), we can observe several (generally distorted and magnified) images of the source.

A property of → strong lensing is that the light travel time from the source to the observer is generally not identical for the different images. In other words, we not only see several images of one same object, but we also see this object, in each image, at different times. This means, in one image the lensed object will be observed before the other image.

Given a physical model of the gravitational lens, the light travel time for each image can be computed.

The expression giving the time delay has two components: a term is called → geometric delay, and the second term, known as the → Shapiro time delay. The latter is due to time dilation by the gravitational field of the lens, a direct consequence of general relativity. See also → time delay distance.

See also: → gravitational; → lensing; → time; → delay.

The mass of an object measured using the effect of a gravitational field on the object.

See also: → gravitational; → mass.

The mass of an object measured using the effect of a gravitational field on the object.

See also: → gravitational; → mass.

1. The energy that an object possesses because of its position in a → gravitational field, especially an object near the surface of the Earth where the → gravitational acceleration can be assumed to be constant, at about 9.8 m s^-2.
   

2. In a two body system. It is the amount of work done in bringing the mass
   m to the distance R from M:
   E_P = -GMm/R, where G is the → gravitational constant.
   

3. For a uniform sphere. It is E_P = -(3/5)GM²/R, where G is the gravitational constant and M is the mass contained in the sphere of radius R.

See also: → gravitational; → potential; → energy.

1. The energy that an object possesses because of its position in a → gravitational field, especially an object near the surface of the Earth where the → gravitational acceleration can be assumed to be constant, at about 9.8 m s^-2.
   

2. In a two body system. It is the amount of work done in bringing the mass
   m to the distance R from M:
   E_P = -GMm/R, where G is the → gravitational constant.
   

3. For a uniform sphere. It is E_P = -(3/5)GM²/R, where G is the gravitational constant and M is the mass contained in the sphere of radius R.

See also: → gravitational; → potential; → energy.

The → energy transported by → gravitational waves. Gravitational radiation is to → gravity what light is to → electromagnetism.

See also: → gravitational; → radiation.

The → energy transported by → gravitational waves. Gravitational radiation is to → gravity what light is to → electromagnetism.

See also: → gravitational; → radiation.

The change in the wavelength or frequency of electromagnetic radiation in a gravitational field predicted by general relativity.

See also: → gravitational; → redshift.

The change in the wavelength or frequency of electromagnetic radiation in a gravitational field predicted by general relativity.

See also: → gravitational; → redshift.

A physical process occurring in → stellar atmospheres whereby in a very stable atmosphere → heavy elements are gravitationally pulled down preferentially. If such an atmosphere is stable for long periods of time, the → absorption lines of heavy elements may therefore become very weak. Observationally, the star seems to contain only → hydrogen and → helium. Gravitational settling takes place in the Sun at the bottom of the outer → convective zone where helium is dragged down, leading to a surface He abundant smaller than the cosmic value. It occurs also in the atmospheres of → brown dwarfs and → planets. See also → radiative levitation, → element diffusion, → thermal diffusion.

See also: → gravitational; → settling.

A physical process occurring in → stellar atmospheres whereby in a very stable atmosphere → heavy elements are gravitationally pulled down preferentially. If such an atmosphere is stable for long periods of time, the → absorption lines of heavy elements may therefore become very weak. Observationally, the star seems to contain only → hydrogen and → helium. Gravitational settling takes place in the Sun at the bottom of the outer → convective zone where helium is dragged down, leading to a surface He abundant smaller than the cosmic value. It occurs also in the atmospheres of → brown dwarfs and → planets. See also → radiative levitation, → element diffusion, → thermal diffusion.

See also: → gravitational; → settling.

Same as → gravity assist.

Etymology (EN): → gravitational; slingshot, from sling, from M.E. slyngen, from O.N. slyngva “to sling, fling” + shot, from M.E., from O.E. sc(e)ot, (ge)sceot; cf. Ger. Schoss, Geschoss.

Etymology (PE): Falâxan “sling;” from Av. fradaxšana- “sling,” fradaxšanya- “sling, sling-stone;”
→ gravitational.

Same as → gravity assist.

Etymology (EN): → gravitational; slingshot, from sling, from M.E. slyngen, from O.N. slyngva “to sling, fling” + shot, from M.E., from O.E. sc(e)ot, (ge)sceot; cf. Ger. Schoss, Geschoss.

Etymology (PE): Falâxan “sling;” from Av. fradaxšana- “sling,” fradaxšanya- “sling, sling-stone;”
→ gravitational.

A → space-time oscillation created by the motion of matter, as predicted by Einstein’s → general relativity. When an object accelerates, it creates ripples in space-time, just like a boat causes ripples in a lake. Gravitational waves are extremely weak even for the most massive objects like → supermassive black holes. They had been inferred from observing a → binary pulsar in which the components slow down, due to losing energy from emitting gravitational waves. Gravitational waves were directly detected for the first time on September 14, 2015 by the → Laser Interferometer Gravitational-Wave Observatory (LIGO) (Abbott et al., 2016, Phys. Rev. Lett. 116, 061102).

Since then several other events have been detected by LIGO and → Laser Interferometer Space Antenna (LISA).

The Nobel Prize in physics 2017 was awarded to three physicists who had leading roles in the first detection of gravitational waves using LIGO. They were Rainer Weiss (MIT), Barry C. Barish, and Kip S. Thorne (both Caltech).

2. Not to be confounded with → gravity wave.

See also: → gravitational; → wave.

A → space-time oscillation created by the motion of matter, as predicted by Einstein’s → general relativity. When an object accelerates, it creates ripples in space-time, just like a boat causes ripples in a lake. Gravitational waves are extremely weak even for the most massive objects like → supermassive black holes. They had been inferred from observing a → binary pulsar in which the components slow down, due to losing energy from emitting gravitational waves. Gravitational waves were directly detected for the first time on September 14, 2015 by the → Laser Interferometer Gravitational-Wave Observatory (LIGO) (Abbott et al., 2016, Phys. Rev. Lett. 116, 061102).

Since then several other events have been detected by LIGO and → Laser Interferometer Space Antenna (LISA).

The Nobel Prize in physics 2017 was awarded to three physicists who had leading roles in the first detection of gravitational waves using LIGO. They were Rainer Weiss (MIT), Barry C. Barish, and Kip S. Thorne (both Caltech).

2. Not to be confounded with → gravity wave.

See also: → gravitational; → wave.

A theory that treats gravity as a field rather than a force acting at a distance.

See also: → gravitational; → field.

A theory that treats gravity as a field rather than a force acting at a distance.

See also: → gravitational; → field.

Objects held in orbit about each other by their → gravitational attraction. Such objects are part of a → bound system.

See also: → gravitational; → bound.

Objects held in orbit about each other by their → gravitational attraction. Such objects are part of a → bound system.

See also: → gravitational; → bound.

A hypothetical force-carrying particle predicted by supersymmetry theories. The gravitino’s spin would be 1/2; its mass is unknown.

See also: From gravit(on) + (neutr)ino.

A hypothetical force-carrying particle predicted by supersymmetry theories. The gravitino’s spin would be 1/2; its mass is unknown.

See also: From gravit(on) + (neutr)ino.

A hypothetical elementary particle associated with the gravitational interactions. This quantum of gravitational radiation is a stable particle, which
travels with the speed of light, and has zero rest mass, zero charge, and a spin of ± 2.

See also: From gravit(y), → gravity

• → -on a suffix used in the names of subatomic particles.

A hypothetical elementary particle associated with the gravitational interactions. This quantum of gravitational radiation is a stable particle, which
travels with the speed of light, and has zero rest mass, zero charge, and a spin of ± 2.

See also: From gravit(y), → gravity

• → -on a suffix used in the names of subatomic particles.

1. The apparent force of → gravitation on an object at or near the surface of a star, planet, satellite, etc.
   

2. Same as → gravitation and → gravitational interaction.

Etymology (EN): From L. gravitatem (nom. gravitas) “weight, heaviness,” from gravis “heavy,” from PIE base *g^wrə- “heavy” (cf. Mod.Pers. gerân “heavy;” Av. gouru- “heavy;” Skt. guru- “heavy, weighty, venerable;” Gk. baros “weight,” barys “heavy;” Goth. kaurus “heavy”).

Etymology (PE): Gerâni, noun of gerân “heavy, ponderous, valuable,” from Mid.Pers. garân “heavy, hard, difficult;” Av. gouru- “heavy” (in compounds), from Proto-Iranian *garu-; cognate with gravity, as above.

1. The apparent force of → gravitation on an object at or near the surface of a star, planet, satellite, etc.
   

2. Same as → gravitation and → gravitational interaction.

Etymology (EN): From L. gravitatem (nom. gravitas) “weight, heaviness,” from gravis “heavy,” from PIE base *g^wrə- “heavy” (cf. Mod.Pers. gerân “heavy;” Av. gouru- “heavy;” Skt. guru- “heavy, weighty, venerable;” Gk. baros “weight,” barys “heavy;” Goth. kaurus “heavy”).

Etymology (PE): Gerâni, noun of gerân “heavy, ponderous, valuable,” from Mid.Pers. garân “heavy, hard, difficult;” Av. gouru- “heavy” (in compounds), from Proto-Iranian *garu-; cognate with gravity, as above.

An important astronautical technique whereby a → spacecraft takes up a tiny fraction of the → orbital energy of a planet it is flying by, allowing it to change → trajectory and → speed. Since the planet is not at rest but gravitating around the Sun, the spacecraft uses both the orbital energy and the gravitational pull of the planet. Also known as the slingshot effect or → gravitational slingshot. More specifically, as the spacecraft approaches the planet, it is accelerated by the planet’s gravity. If the spacecraft’s velocity is too low, or if it is heading too close to the planet, then the planet’s → gravitational force will pull it down to the planet. But if its speed is large enough, and its orbit does not bring it too close to the planet, then the gravitational attraction will just bend the spacecraft’s trajectory around, and the accelerated spacecraft will pass rapidly by the planet and start to move away. In the absence of other gravitational forces, the planet’s gravity would start to slow down the spacecraft as it moves away. If the planet were stationary, the slow-down effect would be equal to the initial acceleration, so there would be no net gain in speed. But the planets are themselves moving through space at high speeds, and this is what gives the “slingshot” effect. Provided the spacecraft is traveling through space in the same direction as the planet, the spacecraft will emerge from the gravity assist maneuver moving faster than before.

Etymology (EN): → gravity; assist, from M.Fr. assister “to stand by, help, assist,” from L. assistere “assist, stand by,” from → ad- “to” + sistere “to cause to stand,” from PIE *siste-, from *sta- “to stand” (cognate with Pers. istâdan “to stand”).

Etymology (PE): Yâri “assistance, help; friendship,” from yâr “assistant, helper, friend,” from Mid.Pers. hayyâr “helper,” hayyârêh “help, aid, assistance,” Proto-Iranian *adyāva-bara-, cf. Av. aidū- “helpful, useful.”

An important astronautical technique whereby a → spacecraft takes up a tiny fraction of the → orbital energy of a planet it is flying by, allowing it to change → trajectory and → speed. Since the planet is not at rest but gravitating around the Sun, the spacecraft uses both the orbital energy and the gravitational pull of the planet. Also known as the slingshot effect or → gravitational slingshot. More specifically, as the spacecraft approaches the planet, it is accelerated by the planet’s gravity. If the spacecraft’s velocity is too low, or if it is heading too close to the planet, then the planet’s → gravitational force will pull it down to the planet. But if its speed is large enough, and its orbit does not bring it too close to the planet, then the gravitational attraction will just bend the spacecraft’s trajectory around, and the accelerated spacecraft will pass rapidly by the planet and start to move away. In the absence of other gravitational forces, the planet’s gravity would start to slow down the spacecraft as it moves away. If the planet were stationary, the slow-down effect would be equal to the initial acceleration, so there would be no net gain in speed. But the planets are themselves moving through space at high speeds, and this is what gives the “slingshot” effect. Provided the spacecraft is traveling through space in the same direction as the planet, the spacecraft will emerge from the gravity assist maneuver moving faster than before.

Etymology (EN): → gravity; assist, from M.Fr. assister “to stand by, help, assist,” from L. assistere “assist, stand by,” from → ad- “to” + sistere “to cause to stand,” from PIE *siste-, from *sta- “to stand” (cognate with Pers. istâdan “to stand”).

Etymology (PE): Yâri “assistance, help; friendship,” from yâr “assistant, helper, friend,” from Mid.Pers. hayyâr “helper,” hayyârêh “help, aid, assistance,” Proto-Iranian *adyāva-bara-, cf. Av. aidū- “helpful, useful.”

→ gravity darkening.

See also: → gravity; → brightening.

→ gravity darkening.

See also: → gravity; → brightening.

The darkening, or brightening, of a region on a star due to localized decrease, or increase, in the → effective gravity. Gravity darkening is explained by the → von Zeipel theorem, whereby on stellar surface the → radiative flux is proportional to the effective gravity. This means that in → rotating stars regions close to the pole are brighter (and have higher temperature) than regions close to the equator. Gravity darkening occurs also in corotating → binary systems, where the → tidal force leads to both gravity darkening and gravity brightening.
The effects are often seen in binary star → light curves. See also → gravity darkening exponent. Recent theoretical work (Espinosa Lara & Rieutord, 2011, A&A 533, A43) has shown that gravity darkening is not well represented by the von Zeipel theorem. This is supported by new interferometric observations of some rapidly rotating stars indicating that the von Zeipel theorem seems to overestimate the temperature difference between the poles and equator.

See also: → gravity; → darkening

The darkening, or brightening, of a region on a star due to localized decrease, or increase, in the → effective gravity. Gravity darkening is explained by the → von Zeipel theorem, whereby on stellar surface the → radiative flux is proportional to the effective gravity. This means that in → rotating stars regions close to the pole are brighter (and have higher temperature) than regions close to the equator. Gravity darkening occurs also in corotating → binary systems, where the → tidal force leads to both gravity darkening and gravity brightening.
The effects are often seen in binary star → light curves. See also → gravity darkening exponent. Recent theoretical work (Espinosa Lara & Rieutord, 2011, A&A 533, A43) has shown that gravity darkening is not well represented by the von Zeipel theorem. This is supported by new interferometric observations of some rapidly rotating stars indicating that the von Zeipel theorem seems to overestimate the temperature difference between the poles and equator.

See also: → gravity; → darkening

According to the → von Zeipel theorem, the emergent flux, F, of total radiation at any point over the surface of a rotationally or tidally distorted star in → hydrostatic equilibrium
varies proportionally to the local gravity acceleration:
F ∝ g_eff^α, where g_eff is the → effective gravity and α is the gravity darkening coefficient. See also the → gravity darkening exponent.

See also: → gravity; → darkening; → coefficient.

According to the → von Zeipel theorem, the emergent flux, F, of total radiation at any point over the surface of a rotationally or tidally distorted star in → hydrostatic equilibrium
varies proportionally to the local gravity acceleration:
F ∝ g_eff^α, where g_eff is the → effective gravity and α is the gravity darkening coefficient. See also the → gravity darkening exponent.

See also: → gravity; → darkening; → coefficient.

The exponent appearing in the power law that describes the → effective temperature of a → rotating star as a function of the → effective gravity, as deduced from the → von Zeipel theorem or law. Generalizing this law, the effective temperature is usually expressed as T_eff∝ g_eff^β, where β is the gravity darkening exponent with a value of 0.25. It has, however, been shown that the relation between the effective temperature and gravity is not exactly a power law. Moreover, the value of β = 0.25 is appropriate only in the limit of slow rotators and is
smaller for fast rotating stars (Espinosa Lara & Rieutord, 2011, A&A 533, A43).

See also: → gravity; → darkening; → exponent.

The exponent appearing in the power law that describes the → effective temperature of a → rotating star as a function of the → effective gravity, as deduced from the → von Zeipel theorem or law. Generalizing this law, the effective temperature is usually expressed as T_eff∝ g_eff^β, where β is the gravity darkening exponent with a value of 0.25. It has, however, been shown that the relation between the effective temperature and gravity is not exactly a power law. Moreover, the value of β = 0.25 is appropriate only in the limit of slow rotators and is
smaller for fast rotating stars (Espinosa Lara & Rieutord, 2011, A&A 533, A43).

See also: → gravity; → darkening; → exponent.

Same as → g mode

See also: → gravity; → mode.

Same as → g mode

See also: → gravity; → mode.

Transient → streamers which form when → clumps of particles begin to collapse under their own → self-gravity but are sheared out by → differential rotation. This phenomenon is believed to be the source of → azimuthal asymmetry in → Saturn’s → A ring (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → gravity; → wake.

Transient → streamers which form when → clumps of particles begin to collapse under their own → self-gravity but are sheared out by → differential rotation. This phenomenon is believed to be the source of → azimuthal asymmetry in → Saturn’s → A ring (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → gravity; → wake.

1. A wave that forms and propagates at the free → surface of a body of → fluid after that surface has been disturbed and the fluid particles have been displaced from their original positions. The motion of such waves is controlled by the restoring force of gravity rather than by the surface tension of the fluid.
   

2. Not to be confounded with → gravitational wave.

See also: → gravity; → wave.

1. A wave that forms and propagates at the free → surface of a body of → fluid after that surface has been disturbed and the fluid particles have been displaced from their original positions. The motion of such waves is controlled by the restoring force of gravity rather than by the surface tension of the fluid.
   

2. Not to be confounded with → gravitational wave.

See also: → gravity; → wave.

The interplay between supersonic turbulence and self-gravity in star forming gas.

See also: Gravo-, from grav-, from → gravity + epenthetic vowel -o- + → turbulence.

The interplay between supersonic turbulence and self-gravity in star forming gas.

See also: Gravo-, from grav-, from → gravity + epenthetic vowel -o- + → turbulence.

(n.) A color between white and black. (adj.) Having a neutral hue.

Etymology (EN): M.E., O.E. græg, from P.Gmc. *græwyaz; cf. O.N. grar, O.Fris. gre, Du. graw, Ger. grau; Frank. *gris, Fr. gris.

Etymology (PE): Xâkestari, “ash-colored,” from xâkestar “ashes,” from Mid.Pers. *xâkâtur, from xâk “earth, dust” + âtur “fire,” varaint âtaxš (Mod.Pers. âtaš, âzar, taš), from Av. ātar-, āθr- “fire,” singular nominative ātarš-; O.Pers. ātar- “fire;” Av. āθaurvan- “fire priest;” Skt. átharvan- “fire priest;” cf. L. ater “black” (“blackened by fire”); Arm. airem “burns;” Serb. vatra “fire;” PIE base *āter- “fire.”

(n.) A color between white and black. (adj.) Having a neutral hue.

Etymology (EN): M.E., O.E. græg, from P.Gmc. *græwyaz; cf. O.N. grar, O.Fris. gre, Du. graw, Ger. grau; Frank. *gris, Fr. gris.

Etymology (PE): Xâkestari, “ash-colored,” from xâkestar “ashes,” from Mid.Pers. *xâkâtur, from xâk “earth, dust” + âtur “fire,” varaint âtaxš (Mod.Pers. âtaš, âzar, taš), from Av. ātar-, āθr- “fire,” singular nominative ātarš-; O.Pers. ātar- “fire;” Av. āθaurvan- “fire priest;” Skt. átharvan- “fire priest;” cf. L. ater “black” (“blackened by fire”); Arm. airem “burns;” Serb. vatra “fire;” PIE base *āter- “fire.”

An SI unit of absorbed radiation dose. One gray is equivalent to an energy absorption of 1 → joule/kg. It has replaced the → rad (rd), an older standard. One gray is equivalent to 100 rad. See also → sievert (Sv).

See also: Named for Louis Harold Gray (1905-1965), British radiologist and the pioneer of use of radiation in cancer treatment.

An SI unit of absorbed radiation dose. One gray is equivalent to an energy absorption of 1 → joule/kg. It has replaced the → rad (rd), an older standard. One gray is equivalent to 100 rad. See also → sievert (Sv).

See also: Named for Louis Harold Gray (1905-1965), British radiologist and the pioneer of use of radiation in cancer treatment.

A simplifying assumption in the models of stellar atmosphere, according to which
the absorption coefficient has the same value at all wavelengths.

See also: → gray; → atmosphere.

A simplifying assumption in the models of stellar atmosphere, according to which
the absorption coefficient has the same value at all wavelengths.

See also: → gray; → atmosphere.

A hypothetical body which emits radiation at each wavelength in a constant ratio, less than unity, to that emitted by a black body at the same temperature.

See also: → gray; → body.

A hypothetical body which emits radiation at each wavelength in a constant ratio, less than unity, to that emitted by a black body at the same temperature.

See also: → gray; → body.

To touch or rub lightly in passing.

Etymology (EN): Perhaps special use of graze “to feed on grass,” from M.E. grasen, O.E. grasian.

Etymology (PE): Barmažidan, from Choresmian parmž “to touch, to rub,” variants barmajidan, majidan, parmâsidan, Mid.Pers. pahrmâh- “to touch, to feel;” ultimately from Proto-Ir. *pari-mars-,
from *Hmars-, *Hmarz- “to touch, rub, wipe;” probably related to marz “border, frontier,” mâlidan “to rub, polish.”

To touch or rub lightly in passing.

Etymology (EN): Perhaps special use of graze “to feed on grass,” from M.E. grasen, O.E. grasian.

Etymology (PE): Barmažidan, from Choresmian parmž “to touch, to rub,” variants barmajidan, majidan, parmâsidan, Mid.Pers. pahrmâh- “to touch, to feel;” ultimately from Proto-Ir. *pari-mars-,
from *Hmars-, *Hmarz- “to touch, rub, wipe;” probably related to marz “border, frontier,” mâlidan “to rub, polish.”

A thing that grazes.

See also: Agent noun of → graze.

A thing that grazes.

See also: Agent noun of → graze.

1. Describing something that grazes. → grazing angle, → grazing occultation.
   

2. The act of touching or rubbing lightly in passing.

See also: → graze; → -ing.

1. Describing something that grazes. → grazing angle, → grazing occultation.
   

2. The act of touching or rubbing lightly in passing.

See also: → graze; → -ing.

Light striking a surface at an angle almost perpendicular to the normal. → grazing-incidence telescope.

See also: → grazing; → incidence.

Light striking a surface at an angle almost perpendicular to the normal. → grazing-incidence telescope.

See also: → grazing; → incidence.

A special type of occultation that occurs when the star appears to pass tangentially on the → edge of the → Moon.

See also: → grazing; → occultation.

A special type of occultation that occurs when the star appears to pass tangentially on the → edge of the → Moon.

See also: → grazing; → occultation.

A telescope design used for focusing → extreme ultraviolet, → X-rays, and → gamma rays by means of → grazing incidence. Such short wavelengths do not reflect in the same manner as at the large incidence angles employed in optical and radio telescopes. Instead, they are mostly absorbed. To bring X-rays to a → focus, one has to use a different approach from → Cassegrain or other typical → reflecting telescopes. In a grazing-incidence telescope, incoming light is almost → parallel to the → mirror surface and strikes the mirror → surface at a very → shallow angle. Much like skipping a stone on the water by throwing it at a low angle to the surface, X-rays may be → deflected by mirrors arranged at low incidence angles to the incoming energy. Several designs of grazing-incidence mirrors have been used in various → X-ray telescopes, including → plane mirrors or combinations of → parabolic and → hyperbolic surfaces. To increase the collecting area a number of mirror elements are often nested inside one another. For example, the → Chandra X-ray Observatory uses two sets of four nested grazing-incidence mirrors to bring X-ray photons to focus onto two → detector instruments. → Bragg’s law; → X-ray astronomy.

See also: → grazing incidence; → telescope.

A telescope design used for focusing → extreme ultraviolet, → X-rays, and → gamma rays by means of → grazing incidence. Such short wavelengths do not reflect in the same manner as at the large incidence angles employed in optical and radio telescopes. Instead, they are mostly absorbed. To bring X-rays to a → focus, one has to use a different approach from → Cassegrain or other typical → reflecting telescopes. In a grazing-incidence telescope, incoming light is almost → parallel to the → mirror surface and strikes the mirror → surface at a very → shallow angle. Much like skipping a stone on the water by throwing it at a low angle to the surface, X-rays may be → deflected by mirrors arranged at low incidence angles to the incoming energy. Several designs of grazing-incidence mirrors have been used in various → X-ray telescopes, including → plane mirrors or combinations of → parabolic and → hyperbolic surfaces. To increase the collecting area a number of mirror elements are often nested inside one another. For example, the → Chandra X-ray Observatory uses two sets of four nested grazing-incidence mirrors to bring X-ray photons to focus onto two → detector instruments. → Bragg’s law; → X-ray astronomy.

See also: → grazing incidence; → telescope.

Unusual or considerable in degree, power, intensity, number, etc.

Etymology (EN): O.E. great “big, coarse, stout,” from W.Gmc. *grautaz (cf. Du. groot, Ger. groß “great”).

Etymology (PE): Bozorg “great, large, immense, grand, magnificient;” Mid.Pers. vazurg “great, big, high, lofty;” O.Pers. vazarka- “great;” Av. vazra- “club, mace” (Mod.Pers. gorz “mace”); cf. Skt. vájra- “(Indra’s) thunderbolt,” vaja- “strength, speed;” L. vigere “be lively, thrive,” velox “fast, lively,” vegere “to enliven,” vigil “watchful, awake;”
P.Gmc. *waken (Du. waken; O.H.G. wahhen; Ger. wachen “to be awake;” E. wake); PIE base *weg- “to be strong, be lively.”

Unusual or considerable in degree, power, intensity, number, etc.

Etymology (EN): O.E. great “big, coarse, stout,” from W.Gmc. *grautaz (cf. Du. groot, Ger. groß “great”).

Etymology (PE): Bozorg “great, large, immense, grand, magnificient;” Mid.Pers. vazurg “great, big, high, lofty;” O.Pers. vazarka- “great;” Av. vazra- “club, mace” (Mod.Pers. gorz “mace”); cf. Skt. vájra- “(Indra’s) thunderbolt,” vaja- “strength, speed;” L. vigere “be lively, thrive,” velox “fast, lively,” vegere “to enliven,” vigil “watchful, awake;”
P.Gmc. *waken (Du. waken; O.H.G. wahhen; Ger. wachen “to be awake;” E. wake); PIE base *weg- “to be strong, be lively.”

A hypothesized large concentration of mass (about 10¹⁶  → solar masses), some hundred million → light-years from Earth, in the direction of the → Centaurus → supercluster, that seems to be affecting the motions of many nearby galaxies by virtue of its gravity.

See also: → great; → attractor.

A hypothesized large concentration of mass (about 10¹⁶  → solar masses), some hundred million → light-years from Earth, in the direction of the → Centaurus → supercluster, that seems to be affecting the motions of many nearby galaxies by virtue of its gravity.

See also: → great; → attractor.

A circle on a sphere whose plane passes through the center of the sphere.

See also: → great; → circle.

A circle on a sphere whose plane passes through the center of the sphere.

See also: → great; → circle.

One of a series of dark spots on → Neptune similar in appearance to Jupiter’s → Great Red Spot. It was discovered in 1989 by NASA’s Voyager 2 space probe. Also known as GDS-89. The dark, oval spot had initial dimensions of 13,000 × 6,600 km, about the same size as Earth. Although it appears similar to Jupiter’s spot, which is an → anticyclonic storm, it is believed that the Great Dark Spot is an atmospheric hole similar to the hole in Earth’s → ozone layer ozone layer. Moreover, unlike Jupiter’s spot, which has lasted for hundreds of years, the lifetimes of Great Dark Spots appear to be much shorter, forming and disappearing once every few years or so. Based on pictures taken by Voyager and since then with the → Hubble Space Telescope, Neptune appears to spend somewhat more than half its time with a Great Dark Spot.
Around the Great Dark Spot, winds were measured blowing up to 2,400 km an hour, the fastest in the solar system.

See also: → great; → dark; → spot.

One of a series of dark spots on → Neptune similar in appearance to Jupiter’s → Great Red Spot. It was discovered in 1989 by NASA’s Voyager 2 space probe. Also known as GDS-89. The dark, oval spot had initial dimensions of 13,000 × 6,600 km, about the same size as Earth. Although it appears similar to Jupiter’s spot, which is an → anticyclonic storm, it is believed that the Great Dark Spot is an atmospheric hole similar to the hole in Earth’s → ozone layer ozone layer. Moreover, unlike Jupiter’s spot, which has lasted for hundreds of years, the lifetimes of Great Dark Spots appear to be much shorter, forming and disappearing once every few years or so. Based on pictures taken by Voyager and since then with the → Hubble Space Telescope, Neptune appears to spend somewhat more than half its time with a Great Dark Spot.
Around the Great Dark Spot, winds were measured blowing up to 2,400 km an hour, the fastest in the solar system.

See also: → great; → dark; → spot.

An anticyclonic storm on the planet Jupiter
akin to a hurricane on Earth, but it is enormous (three Earths would fit within its boundaries) and it has persisted for at least the 400 years that humans have observed it through telescopes.

See also: → great; → red; → spot.

An anticyclonic storm on the planet Jupiter
akin to a hurricane on Earth, but it is enormous (three Earths would fit within its boundaries) and it has persisted for at least the 400 years that humans have observed it through telescopes.

See also: → great; → red; → spot.

An apparent fissure in the bright clouds of the Milky Way between → Cygnus and → Sagittarius caused by a series of large, dark, overlapping clouds.

See also: → great; → rift.

An apparent fissure in the bright clouds of the Milky Way between → Cygnus and → Sagittarius caused by a series of large, dark, overlapping clouds.

See also: → great; → rift.

The Greatest → elongation of an inferior planet occurring after sunset.

See also: Superlative of → great; → eastern; → elongation.

The Greatest → elongation of an inferior planet occurring after sunset.

See also: Superlative of → great; → eastern; → elongation.

The instant when the axis of the Moon’s → shadow cone passes closest to Earth’s center. For → total eclipses, the instant of greatest eclipse is virtually identical to the instants of greatest magnitude and greatest
duration. However, for → annular eclipses, the instant of greatest duration may occur at either the time of greatest eclipse or near the sunrise and sunset points of the eclipse path (F. Espenak, NASA).

See also: Superlative of → great; → eclipse.

The instant when the axis of the Moon’s → shadow cone passes closest to Earth’s center. For → total eclipses, the instant of greatest eclipse is virtually identical to the instants of greatest magnitude and greatest
duration. However, for → annular eclipses, the instant of greatest duration may occur at either the time of greatest eclipse or near the sunrise and sunset points of the eclipse path (F. Espenak, NASA).

See also: Superlative of → great; → eclipse.

The largest → elongation of an inferior planet from the Sun. It may be → greatest eastern elongation or → greatest western elongation. The greatest elongation of Mercury is about 28°, and thus Mercury can only be observed 112 minutes after sunset or before sunrise. For Venus, it is about 47°, making it visible at most about 3 hours after sunset or before sunrise.

See also: Superlative of → great; → eastern; → elongation.

The largest → elongation of an inferior planet from the Sun. It may be → greatest eastern elongation or → greatest western elongation. The greatest elongation of Mercury is about 28°, and thus Mercury can only be observed 112 minutes after sunset or before sunrise. For Venus, it is about 47°, making it visible at most about 3 hours after sunset or before sunrise.

See also: Superlative of → great; → eastern; → elongation.

The Greatest → elongation of an inferior planet occurring before sunrise.

See also: Superlative of → great; → western; → elongation.

The Greatest → elongation of an inferior planet occurring before sunrise.

See also: Superlative of → great; → western; → elongation.

A → numeral system in which letters represent numbers. In an earlier system, called acrophonic, the symbols for numerals came from the first letter of the number name. Subsequently, the numerals were based on giving values to the letters of alphabet. For example α, β, γ, and δ represented 1, 2, 3, and 4; while ι, κ, λ, and μ stood for 10, 20, 30, and 40, and ρ, σ, τ, and υ for 100, 200, 300, and 400. The Greek also used the additive principle. For example 11, 12, 13, 14, and 374 were written ια, ιβ, ιγ, ιδ, and τοδ. The numbers between 1000 and 9000 were expressed by adding a subscript or superscript ι (iota) to the symbols for 1 to 9. For example ιA and ιΘ for 1000 and 9000. Numbers greater than 9999 were expressed using M, which was the myriad, 10,000. Therefore, since 123 was represented by ρκγ, 123,000 was written as M^ρκγ.

See also: → numeral; → system.

A → numeral system in which letters represent numbers. In an earlier system, called acrophonic, the symbols for numerals came from the first letter of the number name. Subsequently, the numerals were based on giving values to the letters of alphabet. For example α, β, γ, and δ represented 1, 2, 3, and 4; while ι, κ, λ, and μ stood for 10, 20, 30, and 40, and ρ, σ, τ, and υ for 100, 200, 300, and 400. The Greek also used the additive principle. For example 11, 12, 13, 14, and 374 were written ια, ιβ, ιγ, ιδ, and τοδ. The numbers between 1000 and 9000 were expressed by adding a subscript or superscript ι (iota) to the symbols for 1 to 9. For example ιA and ιΘ for 1000 and 9000. Numbers greater than 9999 were expressed using M, which was the myriad, 10,000. Therefore, since 123 was represented by ρκγ, 123,000 was written as M^ρκγ.

See also: → numeral; → system.

A color intermediate in the spectrum between yellow and blue (wavelength between 5000 and 5700 Å). The color of most grasses and leaves while growing.

Etymology (EN): Green, from O.E. grene, related to growan “to grow,” from W.Gmc. *gronja- (cf. Dan. grøn, Du. groen, Ger. grün), from PIE base *gro- “to grow.”

Etymology (PE): Sabz “green,” from Mid.Pers. sabz “green, fresh,” related to sabzi “grass.”

A color intermediate in the spectrum between yellow and blue (wavelength between 5000 and 5700 Å). The color of most grasses and leaves while growing.

Etymology (EN): Green, from O.E. grene, related to growan “to grow,” from W.Gmc. *gronja- (cf. Dan. grøn, Du. groen, Ger. grün), from PIE base *gro- “to grow.”

Etymology (PE): Sabz “green,” from Mid.Pers. sabz “green, fresh,” related to sabzi “grass.”

A brilliant green color that occasionally appears on the upper limb of the Sun as it rises or sets.

See also: → green; → flash.

A brilliant green color that occasionally appears on the upper limb of the Sun as it rises or sets.

See also: → green; → flash.

A pea harvested and eaten while still green, soft, and unripe; a garden pea; usually in plural.

See also: → green; → pea.

A pea harvested and eaten while still green, soft, and unripe; a garden pea; usually in plural.

See also: → green; → pea.

A member of a class of galaxies of relatively small size (→ compact galaxy) having very strong → emission lines especially the → [O III] doublet and an unusually large → equivalent width of up to 1000 Å. They were first noted because of their peculiar bright green color and small size, unresolved in → Sloan Digital Sky Survey imaging.

Green Peas are similar to high-→ redshift  → Lyman alpha emitting galaxies (LAEs) in many respects (small sizes, low → stellar masses, 10^8-10 → solar masses (Msun), low metallicities for their stellar masses, high → specific star formation rates (sSFR), and large [O III] λ5007/[O II]λ3727 ratios. Green Peas are relatively luminous and massive galaxies compared to the faint-end → dwarf starburst galaxies and LAEs (See Yang et al, 2017, arxiv/1706.02819, and references therein).

See also: Such called because of their appearance and green color (mainly due to very strong optical emission line [O III] 5007 Å) in composite images;
→ green; → pea;
→ galaxy.

A member of a class of galaxies of relatively small size (→ compact galaxy) having very strong → emission lines especially the → [O III] doublet and an unusually large → equivalent width of up to 1000 Å. They were first noted because of their peculiar bright green color and small size, unresolved in → Sloan Digital Sky Survey imaging.

Green Peas are similar to high-→ redshift  → Lyman alpha emitting galaxies (LAEs) in many respects (small sizes, low → stellar masses, 10^8-10 → solar masses (Msun), low metallicities for their stellar masses, high → specific star formation rates (sSFR), and large [O III] λ5007/[O II]λ3727 ratios. Green Peas are relatively luminous and massive galaxies compared to the faint-end → dwarf starburst galaxies and LAEs (See Yang et al, 2017, arxiv/1706.02819, and references therein).

See also: Such called because of their appearance and green color (mainly due to very strong optical emission line [O III] 5007 Å) in composite images;
→ green; → pea;
→ galaxy.

A building with transparent walls and roof, usually of glass, for the cultivation and exhibition of plants under controlled conditions (Dictionary.com).

Etymology (EN): → green; → house.

Etymology (PE): Garmxâné, literally “warm house,” from garm, → warm,

• xâné, → house.

A building with transparent walls and roof, usually of glass, for the cultivation and exhibition of plants under controlled conditions (Dictionary.com).

Etymology (EN): → green; → house.

Etymology (PE): Garmxâné, literally “warm house,” from garm, → warm,

• xâné, → house.

An increase in → temperature caused when incoming → solar radiation is passed but outgoing → thermal radiation is trapped by the → atmosphere. The major factors for this effect are → carbon dioxide and → water vapor. The greenhouse effect is very important on Venus and Earth but very weak on Mars. On average, about one third of the solar radiation that hits the Earth is reflected back to space. The Earth’s surface becomes warm and emits → infrared radiation.
The → greenhouse gases trap the infrared radiation, thus warming the atmosphere. Without the greenhouse effect the Earth’s average global temperature would be -18° Celsius, rather than the present 15° Celsius. However, human activities are causing greenhouse gas levels in the atmosphere to increase.

See also: → greenhouse; → effect.

An increase in → temperature caused when incoming → solar radiation is passed but outgoing → thermal radiation is trapped by the → atmosphere. The major factors for this effect are → carbon dioxide and → water vapor. The greenhouse effect is very important on Venus and Earth but very weak on Mars. On average, about one third of the solar radiation that hits the Earth is reflected back to space. The Earth’s surface becomes warm and emits → infrared radiation.
The → greenhouse gases trap the infrared radiation, thus warming the atmosphere. Without the greenhouse effect the Earth’s average global temperature would be -18° Celsius, rather than the present 15° Celsius. However, human activities are causing greenhouse gas levels in the atmosphere to increase.

See also: → greenhouse; → effect.

Gases responsible for the greenhouse effect. These gases include: water vapor (H₂O), carbon dioxide (CO₂); methane (CH₄); nitrous oxide (N₂O); chlorofluorocarbons (CF_xCl_x); and tropospheric ozone (O₃).

See also: → greenhouse; → gas.

Gases responsible for the greenhouse effect. These gases include: water vapor (H₂O), carbon dioxide (CO₂); methane (CH₄); nitrous oxide (N₂O); chlorofluorocarbons (CF_xCl_x); and tropospheric ozone (O₃).

See also: → greenhouse; → gas.

The → Greenwich Mean Sidereal Time corrected for → nutation. Therefore, it is measured with respect to the → true vernal equinox. GAST and GMST differ by the → equation of the equinoxes.

See also: → Greenwich Meridian; → apparent; → sidereal; → time.

The → Greenwich Mean Sidereal Time corrected for → nutation. Therefore, it is measured with respect to the → true vernal equinox. GAST and GMST differ by the → equation of the equinoxes.

See also: → Greenwich Meridian; → apparent; → sidereal; → time.

The → sidereal time related to the angle between the → prime meridian and the → mean vernal equinox, measured in the plane of the equator.

See also: → mean; → Greenwich Meridian; → sidereal; → time.

The → sidereal time related to the angle between the → prime meridian and the → mean vernal equinox, measured in the plane of the equator.

See also: → mean; → Greenwich Meridian; → sidereal; → time.

The → prime meridian that separates east from west in the same way that the Equator separates north from south. It is defined by the position of the → Airy transit circle.

See also: A borough in southeast London, England, on the Thames River. It is the site of the original Royal Observatory, through which passes the prime meridian, or longitude 0°; → meridian.

The → prime meridian that separates east from west in the same way that the Equator separates north from south. It is defined by the position of the → Airy transit circle.

See also: A borough in southeast London, England, on the Thames River. It is the site of the original Royal Observatory, through which passes the prime meridian, or longitude 0°; → meridian.

The number and fraction of → mean sidereal days elapsed on the → Greenwich meridian since 12h January 1, 4773 BC (mean sidereal).

See also: → Greenwich meridian; → sidereal; → date.

The number and fraction of → mean sidereal days elapsed on the → Greenwich meridian since 12h January 1, 4773 BC (mean sidereal).

See also: → Greenwich meridian; → sidereal; → date.

The integral part of the → Greenwich sidereal date.

See also: → Greenwich; → sidereal; → day; → number.

The integral part of the → Greenwich sidereal date.

See also: → Greenwich; → sidereal; → day; → number.

A → solar calendar in which the year length is assumed to be 365.2425 solar days. It is now used as the civil calendar in most countries. The Gregorian calendar is a revision of the → Julian calendar instituted in a papal bull by Pope Gregory XIII in 1582. The reason for the calendar change was to correct for drift in the dates of significant religious observations (primarily Easter) and to prevent further drift in the dates.

See also: Named after Pope Gregory XIII (1502-1585), an Italian, born Ugo Boncompagni, Pope from 1572 to 1585, who ordered the reform of the Julian calendar; → calendar.

A → solar calendar in which the year length is assumed to be 365.2425 solar days. It is now used as the civil calendar in most countries. The Gregorian calendar is a revision of the → Julian calendar instituted in a papal bull by Pope Gregory XIII in 1582. The reason for the calendar change was to correct for drift in the dates of significant religious observations (primarily Easter) and to prevent further drift in the dates.

See also: Named after Pope Gregory XIII (1502-1585), an Italian, born Ugo Boncompagni, Pope from 1572 to 1585, who ordered the reform of the Julian calendar; → calendar.

A reflecting telescope in which the light rays are reflected from the primary mirror to a concave secondary mirror, from which the light is reflected back to the primary mirror and through the central hole behind the primary mirror. Compare with the → Cassegrain telescope, in which the secondary mirror is convex.

See also: Named after the Scottish mathematician and astronomer James Gregory (1638-1675), who devised the telescope, but did not succeed in constructing it;
→ telescope.

A reflecting telescope in which the light rays are reflected from the primary mirror to a concave secondary mirror, from which the light is reflected back to the primary mirror and through the central hole behind the primary mirror. Compare with the → Cassegrain telescope, in which the secondary mirror is convex.

See also: Named after the Scottish mathematician and astronomer James Gregory (1638-1675), who devised the telescope, but did not succeed in constructing it;
→ telescope.

A theoretical limit of approximately 6 × 10¹⁹  → electron-volts for the energy of → cosmic rays above which they would lose energy in their interaction with the → cosmic microwave radiation background photons. Cosmic ray protons with these energies produce → pions on blackbody photons via the Δ resonance according to: γ_CMB + p → p + π⁰, or γ_CMB +
p → n + π⁺, thereby losing a large fraction of their energy. These interactions would reduce the energy of the cosmic rays to below the GZK limit. Due to this phenomenon, → Ultra-high-energy cosmic rays are absorbed within about 50 Mpc.

See also: Named after Kenneth Greisen (1966), Physical Review Letters 16, 748 and Georgiy Zatsepin & Vadim Kuzmin (1966),
Journal of Experimental and Theoretical Physics Letters 4, 78; → limit.

A theoretical limit of approximately 6 × 10¹⁹  → electron-volts for the energy of → cosmic rays above which they would lose energy in their interaction with the → cosmic microwave radiation background photons. Cosmic ray protons with these energies produce → pions on blackbody photons via the Δ resonance according to: γ_CMB + p → p + π⁰, or γ_CMB +
p → n + π⁺, thereby losing a large fraction of their energy. These interactions would reduce the energy of the cosmic rays to below the GZK limit. Due to this phenomenon, → Ultra-high-energy cosmic rays are absorbed within about 50 Mpc.

See also: Named after Kenneth Greisen (1966), Physical Review Letters 16, 748 and Georgiy Zatsepin & Vadim Kuzmin (1966),
Journal of Experimental and Theoretical Physics Letters 4, 78; → limit.

1. A → grating of crossed bars; gridiron.
   

2. A → network of → horizontal and → perpendicular lines, uniformly spaced, for locating points on a map, chart, or aerial photograph by means of a system of coordinates.
   

3. Electricity: i) A metallic framework employed in a storage cell or battery for conducting the electric current and supporting the active material.
   ii) A system of electrical distribution serving a large area, especially by means of high-tension lines.

Etymology (EN): Shortening of gridiron “a utensil consisting of parallel metal bars on which to broil meat or other food,” from M.E. griderne, from gridel, from O.Fr. gredil, gridil, from L. craticula “gridiron, small griddle,” diminutive of cratis “wickerwork.”

Etymology (PE): Šabâk, from Laki šowâk “a net woven from goat fleece used for carrying chaff or fruits like melon,” variants šâvâk (Lori), šavak (Nahâvand).

1. A → grating of crossed bars; gridiron.
   

2. A → network of → horizontal and → perpendicular lines, uniformly spaced, for locating points on a map, chart, or aerial photograph by means of a system of coordinates.
   

3. Electricity: i) A metallic framework employed in a storage cell or battery for conducting the electric current and supporting the active material.
   ii) A system of electrical distribution serving a large area, especially by means of high-tension lines.

Etymology (EN): Shortening of gridiron “a utensil consisting of parallel metal bars on which to broil meat or other food,” from M.E. griderne, from gridel, from O.Fr. gredil, gridil, from L. craticula “gridiron, small griddle,” diminutive of cratis “wickerwork.”

Etymology (PE): Šabâk, from Laki šowâk “a net woven from goat fleece used for carrying chaff or fruits like melon,” variants šâvâk (Lori), šavak (Nahâvand).

A first step in making a telescope mirror, which consists of rubbing the glass blank with hard tools (glass, tile, or metal) and abrasive grit to produce a concave form. → figuring; → polishing.

Etymology (EN): Grinding, verbal noun of grind, from O.E. grindan, forgrindan “destroy by crushing,” from P.Gmc. *grindanan (cf. Du. grenden), from PIE *ghrendh- “crushing” (cf. L. frendere “to crush, grind;” Gk. khondros “granule, groats”).

Etymology (PE): Sâbeš, verbal noun of sâbidan, variants sâyidan, pasâvidan “to touch” (Khotanese sauy- “to rub;” Sogdian ps’w- “to touch;” Proto-Iranian *sau- “to rub”).

A first step in making a telescope mirror, which consists of rubbing the glass blank with hard tools (glass, tile, or metal) and abrasive grit to produce a concave form. → figuring; → polishing.

Etymology (EN): Grinding, verbal noun of grind, from O.E. grindan, forgrindan “destroy by crushing,” from P.Gmc. *grindanan (cf. Du. grenden), from PIE *ghrendh- “crushing” (cf. L. frendere “to crush, grind;” Gk. khondros “granule, groats”).

Etymology (PE): Sâbeš, verbal noun of sâbidan, variants sâyidan, pasâvidan “to touch” (Khotanese sauy- “to rub;” Sogdian ps’w- “to touch;” Proto-Iranian *sau- “to rub”).

A minor → complaint.

Etymology (EN): M.E. gripen, from O.E. gripan; cognate with Du. grijpen, Ger. griefen.

Etymology (PE): Gelé, → complain.

A minor → complaint.

Etymology (EN): M.E. gripen, from O.E. gripan; cognate with Du. grijpen, Ger. griefen.

Etymology (PE): Gelé, → complain.

An optical dispersing device used in a spectrograph. It is a combination of a prism and a grating, in the sense that the grating is placed side by side to one surface of a small-angle prism.

See also: Grism, from gr(ating) + (pr)ism.

An optical dispersing device used in a spectrograph. It is a combination of a prism and a grating, in the sense that the grating is placed side by side to one surface of a small-angle prism.

See also: Grism, from gr(ating) + (pr)ism.

Abrasive particles or granules, classified into predetermined sizes, typically of Silicon Carbide or Aluminum Oxide, used between the mirror and tile tool to grind the glass.

Etymology (EN): Grit, from O.E. greot “sand, dust, earth, gravel,” from P.Gmc. *greutan “tiny particles of crushed rock” (cf. O.S. griot; O.N. grjot “rock, stone;” Ger. Grieß “grit, sand”); PIE base *ghreu- “to rub, pound, crush.”

Etymology (PE): Šen “sand, grit.”

Abrasive particles or granules, classified into predetermined sizes, typically of Silicon Carbide or Aluminum Oxide, used between the mirror and tile tool to grind the glass.

Etymology (EN): Grit, from O.E. greot “sand, dust, earth, gravel,” from P.Gmc. *greutan “tiny particles of crushed rock” (cf. O.S. griot; O.N. grjot “rock, stone;” Ger. Grieß “grit, sand”); PIE base *ghreu- “to rub, pound, crush.”

Etymology (PE): Šen “sand, grit.”

Anatomy: The depression on either side of the front of the body between the thigh and the abdomen.

Etymology (EN): M.E. grynde “groin,” originally “depression in the ground,” from O.E. grynde “abyss,” perhaps also “depression, hollow,” related to → ground.

Etymology (PE): Kašâl, kašâlé, literally “side, edge, margin,” cf. Dari Kermâni kašâr, Kermâni kešâl “side, edge,” from kašidan “to draw, pull, trace, trail,” → galaxy.

Anatomy: The depression on either side of the front of the body between the thigh and the abdomen.

Etymology (EN): M.E. grynde “groin,” originally “depression in the ground,” from O.E. grynde “abyss,” perhaps also “depression, hollow,” related to → ground.

Etymology (PE): Kašâl, kašâlé, literally “side, edge, margin,” cf. Dari Kermâni kašâr, Kermâni kešâl “side, edge,” from kašidan “to draw, pull, trace, trail,” → galaxy.

An instrument composed of a vertical staff and a horizontal cross with a plumb line at the end of each arm. It was used in ancient Roman empire to survey straight lines, squares, and rectangles.

See also: From L. groma, gruma, from Gk. → gnomon, possibly through Etruscan.

An instrument composed of a vertical staff and a horizontal cross with a plumb line at the end of each arm. It was used in ancient Roman empire to survey straight lines, squares, and rectangles.

See also: From L. groma, gruma, from Gk. → gnomon, possibly through Etruscan.

→ grating groove.

Etymology (EN): Groove, from O.N. grod “pit,” or M.Du. groeve “furrow, ditch,” from P.Gmc. *grobo (cf. O.H.G. gruoba “ditch,” Goth. groba “pit, cave,” O.E. græf “ditch”), related to grave (n.).

Etymology (PE): Šiyâr “furrow, ploughed ground,” from Av. karši-, karša- “furrow,” karšuiiā “plowed (land),” related to Mod.Pers. kašidan/kešidan “to carry, draw, protract,
trail, drag;” Mid.Pers. kešidan “to draw, pull;” from Av. karš- “to draw; to plow;” cf. Skt. kars-, kársati “to pull, drag, plough,” karṣū- “furrow, trench;” Gk. pelo, pelomai “to be in motion, to bustle;” PIE base k^wels- “to plow.”

→ grating groove.

Etymology (EN): Groove, from O.N. grod “pit,” or M.Du. groeve “furrow, ditch,” from P.Gmc. *grobo (cf. O.H.G. gruoba “ditch,” Goth. groba “pit, cave,” O.E. græf “ditch”), related to grave (n.).

Etymology (PE): Šiyâr “furrow, ploughed ground,” from Av. karši-, karša- “furrow,” karšuiiā “plowed (land),” related to Mod.Pers. kašidan/kešidan “to carry, draw, protract,
trail, drag;” Mid.Pers. kešidan “to draw, pull;” from Av. karš- “to draw; to plow;” cf. Skt. kars-, kársati “to pull, drag, plough,” karṣū- “furrow, trench;” Gk. pelo, pelomai “to be in motion, to bustle;” PIE base k^wels- “to plow.”

1. The surface of the Earth; soil.
   
2. The foundation or basis on which a belief or action rests.

Etymology (EN): From O.E. grund “foundation, ground, surface of the earth,” from P.Gmc. *grundus (cf. Du. grond, Ger. Grund “ground, soil, bottom”).

Etymology (PE): 1) Zamin, variant
zami “earth, ground,” from Mid.Pers. zamig “earth;”
Av. zam- “the earth;” cf. Skt. ksam; Gk. khthôn, khamai “on the ground;” L. homo “earthly being” and humus “the earth” (as in homo sapiens or homicide, humble, humus, exhume);
PIE root *dh(e)ghom “earth.”
2) Zaminé, from zamin + nuance suffix -é.

1. The surface of the Earth; soil.
   
2. The foundation or basis on which a belief or action rests.

Etymology (EN): From O.E. grund “foundation, ground, surface of the earth,” from P.Gmc. *grundus (cf. Du. grond, Ger. Grund “ground, soil, bottom”).

Etymology (PE): 1) Zamin, variant
zami “earth, ground,” from Mid.Pers. zamig “earth;”
Av. zam- “the earth;” cf. Skt. ksam; Gk. khthôn, khamai “on the ground;” L. homo “earthly being” and humus “the earth” (as in homo sapiens or homicide, humble, humus, exhume);
PIE root *dh(e)ghom “earth.”
2) Zaminé, from zamin + nuance suffix -é.

The lowest energy state of an atom, molecule, or ion, when all electrons are in their lowest possible energy levels, i.e. not excited.

See also: → ground; → state.

The lowest energy state of an atom, molecule, or ion, when all electrons are in their lowest possible energy levels, i.e. not excited.

See also: → ground; → state.

An astronomical observation carried out using a telescope on Earth, as opposed to that from an orbiting satellite.

Etymology (EN): → ground; based, adj. of base, from
O.Fr. bas, from L. basis “foundation,” from Gk. basis “step, pedestal,” from bainein “to step;” → observation.

Etymology (PE): Nepâheš, → observation; az “from,” → ex-; zamin, → ground.

An astronomical observation carried out using a telescope on Earth, as opposed to that from an orbiting satellite.

Etymology (EN): → ground; based, adj. of base, from
O.Fr. bas, from L. basis “foundation,” from Gk. basis “step, pedestal,” from bainein “to step;” → observation.

Etymology (PE): Nepâheš, → observation; az “from,” → ex-; zamin, → ground.

1a) Any collection or assemblage of persons or things considered together or regarded as belonging together; e.g. → Local Group of galaxies.

1b) Math.: A set of elements a, b, c, …, finite or infinite in number, with a rule for combining any two of them to form a “product,” subject to the following four axioms: → closure axiom, → associative axiom, → identity axiom, and → inverse axiom.

2a) (v.tr.) To place or associate together in a group.
2b) (v.intr.) To be part of a group.

Etymology (EN): From Fr. groupe “cluster, group,” from It. gruppo “cluster, packet, knot,” likely from P.Gmc. *kruppa “round mass, lump.”

Etymology (PE): Goruh “group,” from Mid.Pers. grôh “group, crowd.”

1a) Any collection or assemblage of persons or things considered together or regarded as belonging together; e.g. → Local Group of galaxies.

1b) Math.: A set of elements a, b, c, …, finite or infinite in number, with a rule for combining any two of them to form a “product,” subject to the following four axioms: → closure axiom, → associative axiom, → identity axiom, and → inverse axiom.

2a) (v.tr.) To place or associate together in a group.
2b) (v.intr.) To be part of a group.

Etymology (EN): From Fr. groupe “cluster, group,” from It. gruppo “cluster, packet, knot,” likely from P.Gmc. *kruppa “round mass, lump.”

Etymology (PE): Goruh “group,” from Mid.Pers. grôh “group, crowd.”

A branch of mathematics concerned with structures called → groups and the description of their properties. Group theory provides a powerful formal method of analyzing abstract and physical systems in which → symmetry is present. It has a very considerable use in physics, especially → quantum mechanics, notably in analyzing the → eigenstates of energy of a physical system.

See also: → group; → theory.

A branch of mathematics concerned with structures called → groups and the description of their properties. Group theory provides a powerful formal method of analyzing abstract and physical systems in which → symmetry is present. It has a very considerable use in physics, especially → quantum mechanics, notably in analyzing the → eigenstates of energy of a physical system.

See also: → group; → theory.

The velocity at which the envelope of a → wave packet propagates, v_gr = dω/dk, at k₀ (the central value of k). The group velocity can be equal to, larger, or smaller than the → phase velocity.

See also: → group; → velocity.

The velocity at which the envelope of a → wave packet propagates, v_gr = dω/dk, at k₀ (the central value of k). The group velocity can be equal to, larger, or smaller than the → phase velocity.

See also: → group; → velocity.

The act or process of uniting into groups.
A collection of things assembled into a group.
The occurence of several astronomical objects, usually of the same category, in a region of the sky.

See also: Verbal noun of → group.

The act or process of uniting into groups.
A collection of things assembled into a group.
The occurence of several astronomical objects, usually of the same category, in a region of the sky.

See also: Verbal noun of → group.

To increase by natural development, as any living organism or part by assimilation of nutriment; increase in size or substance (Dictionary.com).

Etymology (EN): From M.E. growen, O.E. growan; cf. Du. groeien, O.H.G. grouwan;
PIE base *ghre- “to grow, become green,” from which is also derived grass.

Etymology (PE): Ruyidan, rostan “to grow,” from Mid.Pers. rôditan, rustan “to grow;” Av. raod- “to grow, sprout, shoot,” with fra- “to grow up, shoot forth;” cf. Skt. ruh- “to grow, develop, ascend, climb,” rohati “grows,” rudh- “to grow, sprout, shoot,” rodhati “grows.”

To increase by natural development, as any living organism or part by assimilation of nutriment; increase in size or substance (Dictionary.com).

Etymology (EN): From M.E. growen, O.E. growan; cf. Du. groeien, O.H.G. grouwan;
PIE base *ghre- “to grow, become green,” from which is also derived grass.

Etymology (PE): Ruyidan, rostan “to grow,” from Mid.Pers. rôditan, rustan “to grow;” Av. raod- “to grow, sprout, shoot,” with fra- “to grow up, shoot forth;” cf. Skt. ruh- “to grow, develop, ascend, climb,” rohati “grows,” rudh- “to grow, sprout, shoot,” rodhati “grows.”

The act or process, or a manner of growing; development; gradual increase. → curve of growth; → grain growth.

See also: Ruyeš, verbal noun of → grow; rost, past stem of ruyidan, → grow, used as verbal noun.

The act or process, or a manner of growing; development; gradual increase. → curve of growth; → grain growth.

See also: Ruyeš, verbal noun of → grow; rost, past stem of ruyidan, → grow, used as verbal noun.

The Crane. A constellation in the Southern Hemisphere , located at 22h 30m right ascension, -45° declination. Its brightest star, of magnitude 1.7 and spectral type B7. Abbreviation: Gru; genitive: Gruis

Etymology (EN): From L. grus “crane;” akin to Gk. geranos “crane;” Welsh garan; Lith. garnys “heron, stork;” O.E. cran; E. crane. Named by Johann Bayer in 1603.

Etymology (PE): Dornâ “crane,” from Turkish, a bird of the family Gruidae.

The Crane. A constellation in the Southern Hemisphere , located at 22h 30m right ascension, -45° declination. Its brightest star, of magnitude 1.7 and spectral type B7. Abbreviation: Gru; genitive: Gruis

Etymology (EN): From L. grus “crane;” akin to Gk. geranos “crane;” Welsh garan; Lith. garnys “heron, stork;” O.E. cran; E. crane. Named by Johann Bayer in 1603.

Etymology (PE): Dornâ “crane,” from Turkish, a bird of the family Gruidae.