An Etymological Dictionary of Astronomy and Astrophysics

→ optical fiber.

Etymology (EN): From Fr. fibre, from O.Fr. fibre, from L. fibra “a fiber, filament,” of uncertain origin, perhaps related to L. filum “thread.”

Etymology (PE): Fibr, loan from Fr., as above.

→ optical fiber.

Etymology (EN): From Fr. fibre, from O.Fr. fibre, from L. fibra “a fiber, filament,” of uncertain origin, perhaps related to L. filum “thread.”

Etymology (PE): Fibr, loan from Fr., as above.

One of the numbers in the → Fibonacci sequence.

See also: → Fibonacci sequence; → number.

One of the numbers in the → Fibonacci sequence.

See also: → Fibonacci sequence; → number.

An infinite sequence of integers, starting with 0 and 1, where each element is the sum of the two previous numbers. For example: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, … As the sequence develops, the ratio of the consecutive terms converges to the → golden ratio, about 1.618.

See also: Leonardo Pisano Fibonacci (1170-1250), medieval Italian mathematician who wrote Liber abaci (1202; Book of the Abacus), the first European work on Indian and Arabian mathematics, which introduced “Arabic” numerals in Europe; → sequence.

An infinite sequence of integers, starting with 0 and 1, where each element is the sum of the two previous numbers. For example: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, … As the sequence develops, the ratio of the consecutive terms converges to the → golden ratio, about 1.618.

See also: Leonardo Pisano Fibonacci (1170-1250), medieval Italian mathematician who wrote Liber abaci (1202; Book of the Abacus), the first European work on Indian and Arabian mathematics, which introduced “Arabic” numerals in Europe; → sequence.

A linear pattern in the → chromosphere of the → Sun, as seen through an → H-alpha filter, occurring near strong → sunspots and → plages or in → filament channels. They are magnetically confined tubes of hot → plasma. Individually, they are about 10,000 km long and last for 10 to 20 minutes.

Etymology (EN): From Mod.L. fibrilla, from fibr(a) “fiber” + -illa diminutive suffix.

Etymology (PE): Târcé, from târ “thread, warp, → string”

• diminutive suffix -cé, from Mid.Pers. -cak, variants -êžak (as in kanicak “little girl,” sangcak “small stone,” xôkcak “small pig”), also Mod.Pers. -ak.

A linear pattern in the → chromosphere of the → Sun, as seen through an → H-alpha filter, occurring near strong → sunspots and → plages or in → filament channels. They are magnetically confined tubes of hot → plasma. Individually, they are about 10,000 km long and last for 10 to 20 minutes.

Etymology (EN): From Mod.L. fibrilla, from fibr(a) “fiber” + -illa diminutive suffix.

Etymology (PE): Târcé, from târ “thread, warp, → string”

• diminutive suffix -cé, from Mid.Pers. -cak, variants -êžak (as in kanicak “little girl,” sangcak “small stone,” xôkcak “small pig”), also Mod.Pers. -ak.

Anatomy: The outer and thinner of the two bones of the human leg, extending from the knee to the ankle.

Etymology (EN): Fom L. fibula “clasp, brooch; bolt, peg, pin,” related to figere “to drive in, insert, fasten,” → fix.

Etymology (PE): Nâzok-ney, literally “fine reed,” from nâzok “→ fine” + ney “reed, pipe, flute.”

Anatomy: The outer and thinner of the two bones of the human leg, extending from the knee to the ankle.

Etymology (EN): Fom L. fibula “clasp, brooch; bolt, peg, pin,” related to figere “to drive in, insert, fasten,” → fix.

Etymology (PE): Nâzok-ney, literally “fine reed,” from nâzok “→ fine” + ney “reed, pipe, flute.”

1. Literary works invented by the imagination, such as novels or short stories.
   

2. An invented story or explanation; lie.
   

3. The act of inventing a story or explanation.
   

4. Law: Something assumed to be true for the sake of convenience, though probably false (Dictionary.com).

Etymology (EN): M.E., from O.Fr. ficcion “dissimulation, ruse; invention, fabrication” and directly from L. fictionem “a fashioning or feigning,” noun of action from p.p. stem of fingere “to shape, form, devise, feign,” originally “to knead, form out of clay,” from PIE *dheigh- “to build, form, knead;” akin to Skt. dehah “body,” literally “that which is formed,” dih- “to besmear;” Gk. teikhos “wall;” L. fingere “to form, fashion,” Gothic deigan “to smear;” O.Irish digen “firm, solid.”

Etymology (PE): Formed on the model of fiction, as above, from diz- “to build, to form;” (related to Pers. dež, dez “fortress”);
cf. Mid.Pers. dys-/dēs- “to build;” Sogd. dys “to build;” Av. (+ *pari-) daēz- “to build (around);” Proto-Ir. *daiz- “to build, form;” from PIE *dheigh- “to build, form,” as above, + suffix -an.

1. Literary works invented by the imagination, such as novels or short stories.
   

2. An invented story or explanation; lie.
   

3. The act of inventing a story or explanation.
   

4. Law: Something assumed to be true for the sake of convenience, though probably false (Dictionary.com).

Etymology (EN): M.E., from O.Fr. ficcion “dissimulation, ruse; invention, fabrication” and directly from L. fictionem “a fashioning or feigning,” noun of action from p.p. stem of fingere “to shape, form, devise, feign,” originally “to knead, form out of clay,” from PIE *dheigh- “to build, form, knead;” akin to Skt. dehah “body,” literally “that which is formed,” dih- “to besmear;” Gk. teikhos “wall;” L. fingere “to form, fashion,” Gothic deigan “to smear;” O.Irish digen “firm, solid.”

Etymology (PE): Formed on the model of fiction, as above, from diz- “to build, to form;” (related to Pers. dež, dez “fortress”);
cf. Mid.Pers. dys-/dēs- “to build;” Sogd. dys “to build;” Av. (+ *pari-) daēz- “to build (around);” Proto-Ir. *daiz- “to build, form;” from PIE *dheigh- “to build, form,” as above, + suffix -an.

1. General: An expanse of anything.
   

2. Physics: A region or space defined by the presence of a physical force, such as electric, magnetic, or gravitational.
   

3. Math.: A mathematical entity for which addition, subtraction, multiplication, and division are well-defined.

Etymology (EN): M.E., from O.E. feld “plain, open land,” probably related to O.E. folde “earth, land,” from P.Gmc. *felthuz “flat land” (cf. Ger. Feld), from PIE *pel(e)-tu-,from base *pele- “flat, to sprea;” cf. L. planus “flat, level,” → plane.

Etymology (PE): Meydân “field, arena, extensive plain; town square; gathering place” from Mid.Pers. mêdân “arena, field.” Meydân has been borrowed into various languages: Ar. maydân, Turkish meydan, Crimean Tatar, Ukrainian maidan, Polish maidan, also in Pakistan, India, Indonesia, Nepal.

1. General: An expanse of anything.
   

2. Physics: A region or space defined by the presence of a physical force, such as electric, magnetic, or gravitational.
   

3. Math.: A mathematical entity for which addition, subtraction, multiplication, and division are well-defined.

Etymology (EN): M.E., from O.E. feld “plain, open land,” probably related to O.E. folde “earth, land,” from P.Gmc. *felthuz “flat land” (cf. Ger. Feld), from PIE *pel(e)-tu-,from base *pele- “flat, to sprea;” cf. L. planus “flat, level,” → plane.

Etymology (PE): Meydân “field, arena, extensive plain; town square; gathering place” from Mid.Pers. mêdân “arena, field.” Meydân has been borrowed into various languages: Ar. maydân, Turkish meydan, Crimean Tatar, Ukrainian maidan, Polish maidan, also in Pakistan, India, Indonesia, Nepal.

An aberration in an optical instrument, common in Schmidt telescopes, in which the focus changes from the center to the edge of the field of view. Owing to this aberration, a straight object looks curved in the image.

See also: → field; → curvature.

An aberration in an optical instrument, common in Schmidt telescopes, in which the focus changes from the center to the edge of the field of view. Owing to this aberration, a straight object looks curved in the image.

See also: → field; → curvature.

In a physical theory, an equation that describe how a fundamental force interacts with matter. Einstein’s equations of → general relativity are called field equations since they describe the → gravitational field. Similarly, → Maxwell’s equations describe the electromagnetic field.

See also: → field; → equation.

In a physical theory, an equation that describe how a fundamental force interacts with matter. Einstein’s equations of → general relativity are called field equations since they describe the → gravitational field. Similarly, → Maxwell’s equations describe the electromagnetic field.

See also: → field; → equation.

A galaxy that lies in the direction of a → cluster of galaxies, but is not a member of the cluster. Field galaxies are rare, less than about 5% of all galaxies.

See also: → field; → galaxy.

A galaxy that lies in the direction of a → cluster of galaxies, but is not a member of the cluster. Field galaxies are rare, less than about 5% of all galaxies.

See also: → field; → galaxy.

A → horizontal branch star with high velocity.

See also: → field; → horizontal; → branch; → star.

A → horizontal branch star with high velocity.

See also: → field; → horizontal; → branch; → star.

A lens placed at or near the focal plane of a telescope to create an image of the primary mirror inside the instrument.

See also: → field; → lens.

A lens placed at or near the focal plane of a telescope to create an image of the primary mirror inside the instrument.

See also: → field; → lens.

An → O-type star which is apparently not associated with a → star cluster. A significant fraction of → massive stars in the → Milky Way and other galaxies are located far from star clusters and → star-forming regions. It is known that some of these stars are → runaways, i.e. possess high → space velocities (determined through the → proper motion and/or → radial velocity measurements), and therefore most likely were formed in embedded clusters and then ejected into the field because of dynamical interactions or → binary-→ supernova explosions. However, there exists a group of field O stars whose runaway status is difficult to prove via direct proper motion measurements or whose low space velocities and/or young ages appear to be incompatible with their large separation from known star clusters. The existence of this group led some authors to believe that these stars can form → in situ. The question of whether or not O stars can form in isolation (→ isolated massive star formation) is of crucial importance for → star formation models (Gvaramadze et al., 2012, MNRAS, 424, 3037).

See also: → field; → O star.

An → O-type star which is apparently not associated with a → star cluster. A significant fraction of → massive stars in the → Milky Way and other galaxies are located far from star clusters and → star-forming regions. It is known that some of these stars are → runaways, i.e. possess high → space velocities (determined through the → proper motion and/or → radial velocity measurements), and therefore most likely were formed in embedded clusters and then ejected into the field because of dynamical interactions or → binary-→ supernova explosions. However, there exists a group of field O stars whose runaway status is difficult to prove via direct proper motion measurements or whose low space velocities and/or young ages appear to be incompatible with their large separation from known star clusters. The existence of this group led some authors to believe that these stars can form → in situ. The question of whether or not O stars can form in isolation (→ isolated massive star formation) is of crucial importance for → star formation models (Gvaramadze et al., 2012, MNRAS, 424, 3037).

See also: → field; → O star.

An astronomical object that is seen in the direction of a group but not physically belonging to the group. → field galaxy.

See also: → field; → object.

An astronomical object that is seen in the direction of a group but not physically belonging to the group. → field galaxy.

See also: → field; → object.

The region of → space surrounding a body, such as a mass of → matter, a → charged particle, or a → magnet, within which it can exert a → force on another similar body not in contact with it.

See also: → field; → force.

The region of → space surrounding a body, such as a mass of → matter, a → charged particle, or a → magnet, within which it can exert a → force on another similar body not in contact with it.

See also: → field; → force.

The entire angular expanse of the sky viewed by an optical instrument.

See also: → field; → view.

The entire angular expanse of the sky viewed by an optical instrument.

See also: → field; → view.

The effect of the Earth’s rotation on the position of the image formed on the → focal plane of a telescope during long exposures. In the case of → equatorial mounting, the image remains fixed, whereas it turns continuously with an → altazimuth mounting. In the latter case the image motion must be compensated by an appropriate mechanism, → field rotator.

See also: → field; → rotation.

The effect of the Earth’s rotation on the position of the image formed on the → focal plane of a telescope during long exposures. In the case of → equatorial mounting, the image remains fixed, whereas it turns continuously with an → altazimuth mounting. In the latter case the image motion must be compensated by an appropriate mechanism, → field rotator.

See also: → field; → rotation.

A device used on a telescope to correct for the → field rotation while tracking an object.

See also: → field; → rotator.

A device used on a telescope to correct for the → field rotation while tracking an object.

See also: → field; → rotator.

A star that does not belong to a stellar cluster, but happens to be adjacent to it.

See also: → field; → star.

A star that does not belong to a stellar cluster, but happens to be adjacent to it.

See also: → field; → star.

A diaphragm located at an image plane of an optical system that determines the size and shape of the image. → aperture stop.

See also: → field; → stop.

A diaphragm located at an image plane of an optical system that determines the size and shape of the image. → aperture stop.

See also: → field; → stop.

1. A theory which uses the concept of → field to describe physical phenomena. It consists of two types:
   → classical field theory and → quantum field theory.
   

2. Math.: A branch of mathematics which studies the → fields.

See also: → field; → theory.

1. A theory which uses the concept of → field to describe physical phenomena. It consists of two types:
   → classical field theory and → quantum field theory.
   

2. Math.: A branch of mathematics which studies the → fields.

See also: → field; → theory.

The precise curve required on the surface of an optical element, especially the mirror of a reflecting telescope.

Etymology (EN): From O.Fr. figure, from L. figura “a shape, form.”

Etymology (PE): Šekl from Ar. šakl “figure.”

The precise curve required on the surface of an optical element, especially the mirror of a reflecting telescope.

Etymology (EN): From O.Fr. figure, from L. figura “a shape, form.”

Etymology (PE): Šekl from Ar. šakl “figure.”

The final stage of mirror making, a process of altering the mirror’s shape into the one required for proper function. → grinding;
→ polishing.

Etymology (EN): Figuring, noun of → figure.

Etymology (PE): Šekldahi, “giving form,” from šekl, → figure + dahi, a verbal noun of dâdan “to give,” from
Mid.Pers. dâdan “to give,” O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives;” cf. Skt. dadáti “he gives,” Gk. didomi “I give,” tithenai “to put, set, place;” L. dare “to give, offer;” facere “to do, to make;” Rus. delat “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do.”
PIE base *dhe- “to put, to do.”

The final stage of mirror making, a process of altering the mirror’s shape into the one required for proper function. → grinding;
→ polishing.

Etymology (EN): Figuring, noun of → figure.

Etymology (PE): Šekldahi, “giving form,” from šekl, → figure + dahi, a verbal noun of dâdan “to give,” from
Mid.Pers. dâdan “to give,” O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives;” cf. Skt. dadáti “he gives,” Gk. didomi “I give,” tithenai “to put, set, place;” L. dare “to give, offer;” facere “to do, to make;” Rus. delat “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do.”
PIE base *dhe- “to put, to do.”

1. A long tongue of a relatively cold matter (10,000 K), suspended in the → solar corona (2 million K). Filaments seem dark in the Hα light when they are seen projected on the solar disk; at the limb they look as what they actually are: → prominences.
   

2. Electricity, Electronics: In electric light bulbs and thermionic valves, a wire of metal of high melting point, which is heated by the passage of an electric current.
   

3. Cosmology: → cosmic filament.

Etymology (EN): From Mod.L. filamentum, from L.L. filare “to spin, draw out in a long line,” from L. filum “thread,” from PIE base *g^whi- “thread, tendon;” cf. Gk. bios “bow;” Skt. jiyā- “bow-string;” Av. jiiā- “bow-string;” Mod.Pers. zeh “string, bow-string” (dialectal Qomi zij “mason’s cord”);
Arm. jil “string, line;” Lith. gijà “thread;” Russ. žica “thread.”

Etymology (PE): Rešté “thread, line, file,” from reštan, ristan “to spin;” Mid.Pers. ‘rws- “to turn to,” abrešom “silk;” Sogdian rwyšt “spun;” Av. uruuaēs- “to twist, turn;” Proto-Iranian *uris- “to turn, spin.”

1. A long tongue of a relatively cold matter (10,000 K), suspended in the → solar corona (2 million K). Filaments seem dark in the Hα light when they are seen projected on the solar disk; at the limb they look as what they actually are: → prominences.
   

2. Electricity, Electronics: In electric light bulbs and thermionic valves, a wire of metal of high melting point, which is heated by the passage of an electric current.
   

3. Cosmology: → cosmic filament.

Etymology (EN): From Mod.L. filamentum, from L.L. filare “to spin, draw out in a long line,” from L. filum “thread,” from PIE base *g^whi- “thread, tendon;” cf. Gk. bios “bow;” Skt. jiyā- “bow-string;” Av. jiiā- “bow-string;” Mod.Pers. zeh “string, bow-string” (dialectal Qomi zij “mason’s cord”);
Arm. jil “string, line;” Lith. gijà “thread;” Russ. žica “thread.”

Etymology (PE): Rešté “thread, line, file,” from reštan, ristan “to spin;” Mid.Pers. ‘rws- “to turn to,” abrešom “silk;” Sogdian rwyšt “spun;” Av. uruuaēs- “to twist, turn;” Proto-Iranian *uris- “to turn, spin.”

A nebula, generally ionized, consisting of filament-like structures of gas, such as the
Veil Nebula (NGC 6960) or the supernova remnant IC 443.

See also: Filamentary adj. of → filament; → nebula.

A nebula, generally ionized, consisting of filament-like structures of gas, such as the
Veil Nebula (NGC 6960) or the supernova remnant IC 443.

See also: Filamentary adj. of → filament; → nebula.

An instrument used with a telescope for accurately measuring small angular separations between two celestial bodies (as between binary
stars). The instrument consists of two parallel fine wires with one wire being fixed and the other movable by means of a finely threaded screw.

Etymology (EN): Filar, from L. fil(um) “a thread” (see the paragraph below)

• -ar variant of the adjective-forming suffix → -al;
  → micrometer.

Etymology (PE): Kehsanj, → micrometer; zehi adj. of zeh “string, bow-string;” Mid.Pers. zih “bow-string,” zig “string; astronomical table” (loaned into Ar. as zij);
Av. jiiā- “bow-string;” cf. Skt. jiyā- “bow-string;” Gk. bios “bow;” L. filum “thread;” Arm. jil “string, line;” Lith. gijà “thread;” Russ. žica “thread;” PIE base *g^whi- “thread, tendon.”

An instrument used with a telescope for accurately measuring small angular separations between two celestial bodies (as between binary
stars). The instrument consists of two parallel fine wires with one wire being fixed and the other movable by means of a finely threaded screw.

Etymology (EN): Filar, from L. fil(um) “a thread” (see the paragraph below)

• -ar variant of the adjective-forming suffix → -al;
  → micrometer.

Etymology (PE): Kehsanj, → micrometer; zehi adj. of zeh “string, bow-string;” Mid.Pers. zih “bow-string,” zig “string; astronomical table” (loaned into Ar. as zij);
Av. jiiā- “bow-string;” cf. Skt. jiyā- “bow-string;” Gk. bios “bow;” L. filum “thread;” Arm. jil “string, line;” Lith. gijà “thread;” Russ. žica “thread;” PIE base *g^whi- “thread, tendon.”

In computer science, a collection of related data or program records stored on a support.

Etymology (EN): From M.E. filen, from M.F. filer “to string documents on a thread,” O.F. filer “to wind or spin thread,” from L.L. filare “to spin, draw out in a long line,” from L. filum “thread,” from PIE base *g^whi- “thread, tendon;” cf. Gk. bios “bow;” Skt. jiyā- “bow-string;” Av. jiiā- “bow-string;” Mod.Pers. zeh “bow-string;” Arm. jil “string, line,” Lith. gijà “thread;” Russ. žica “thread.”

Etymology (PE): Parvandé “file,” initially “a bundle, roll, truss (of clothes),” from Mid.Pers. parwastan, parwand- “to surround, enclose, contain, comprise,” from par-, variants far-, par-, pirâ- “around, about” (Mid.Pers. pêrâ; O.Pers. pariy “around, about,” Av. pairi “around, over;” Skt. pari; Indo-Iranian *pari- “around;” PIE base *per- “through, across, beyond;” cf. Gk. peri “around, about, beyond;” L. per “through”) + vand-/band- stem of vastan/bastan “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).

In computer science, a collection of related data or program records stored on a support.

Etymology (EN): From M.E. filen, from M.F. filer “to string documents on a thread,” O.F. filer “to wind or spin thread,” from L.L. filare “to spin, draw out in a long line,” from L. filum “thread,” from PIE base *g^whi- “thread, tendon;” cf. Gk. bios “bow;” Skt. jiyā- “bow-string;” Av. jiiā- “bow-string;” Mod.Pers. zeh “bow-string;” Arm. jil “string, line,” Lith. gijà “thread;” Russ. žica “thread.”

Etymology (PE): Parvandé “file,” initially “a bundle, roll, truss (of clothes),” from Mid.Pers. parwastan, parwand- “to surround, enclose, contain, comprise,” from par-, variants far-, par-, pirâ- “around, about” (Mid.Pers. pêrâ; O.Pers. pariy “around, about,” Av. pairi “around, over;” Skt. pari; Indo-Iranian *pari- “around;” PIE base *per- “through, across, beyond;” cf. Gk. peri “around, about, beyond;” L. per “through”) + vand-/band- stem of vastan/bastan “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).

Of a molecular cloud or a nebula, the ratio of the volumes filled with matter to the total volume of the cloud.

Etymology (EN): Filling, from fill, from O.E. fyllan, from P.Gmc. *fullijan (cf. Du. vullen, Ger. füllen “to fill”), a derivative of adj. *fullaz→ full; → factor.

Etymology (PE): Karvand, → factor; pori, from por,
→ full.

Of a molecular cloud or a nebula, the ratio of the volumes filled with matter to the total volume of the cloud.

Etymology (EN): Filling, from fill, from O.E. fyllan, from P.Gmc. *fullijan (cf. Du. vullen, Ger. füllen “to fill”), a derivative of adj. *fullaz→ full; → factor.

Etymology (PE): Karvand, → factor; pori, from por,
→ full.

1. General: A device for separating the constituents of a system according to a physical property. In particular, a device for removing solid matter suspended in a liquid by forcing the suspension through a material which retains the solid matter while allowing the liquid to pass.
   

2. Optics: A device for transmitting electromagnetic radiation with restricted ranges of wavelength. Two types are generally used, → glass filters and → interference filters. See also → neutral density filter.
   

3. Electronics: A circuit or equipment allowing the transmission of desired frequencies and the elimination of others.

Etymology (EN): From M.L. filtrum “felt,” which was used to strain impurities from liquid, from W.Gmc. *filtiz.

Etymology (PE): Pâlâyé, from pâlây present stem of pâludan “to filter, purify;” Mid.Pers. pâludan, pâlây- “to strain, filter, purify,” from Proto-Iranian *pari-harz-, from prefix *pari- “around,” → peri-,

• harz- “to let go, release;” cf. Av. pairi.harəz- “to filter,” from pairi- “around, over” (cf. Skt. pari; Gk. peri “around, about, beyond;” L. per “through;” PIE base *per- “through, across, beyond”)
• harəz- “to release” (especially water, liquid; cf. Chorasmian pžy- “to purify, filter;” Arm. (loanword) parzel “to filter, purify;”

1. General: A device for separating the constituents of a system according to a physical property. In particular, a device for removing solid matter suspended in a liquid by forcing the suspension through a material which retains the solid matter while allowing the liquid to pass.
   

2. Optics: A device for transmitting electromagnetic radiation with restricted ranges of wavelength. Two types are generally used, → glass filters and → interference filters. See also → neutral density filter.
   

3. Electronics: A circuit or equipment allowing the transmission of desired frequencies and the elimination of others.

Etymology (EN): From M.L. filtrum “felt,” which was used to strain impurities from liquid, from W.Gmc. *filtiz.

Etymology (PE): Pâlâyé, from pâlây present stem of pâludan “to filter, purify;” Mid.Pers. pâludan, pâlây- “to strain, filter, purify,” from Proto-Iranian *pari-harz-, from prefix *pari- “around,” → peri-,

• harz- “to let go, release;” cf. Av. pairi.harəz- “to filter,” from pairi- “around, over” (cf. Skt. pari; Gk. peri “around, about, beyond;” L. per “through;” PIE base *per- “through, across, beyond”)
• harəz- “to release” (especially water, liquid; cf. Chorasmian pžy- “to purify, filter;” Arm. (loanword) parzel “to filter, purify;”

Pertaining to or coming at the end; last in place, order, or time.

Etymology (EN): M.E., from O.Fr. final from L. finalis “of or pertaining to an end, concluding,” from finis “end.”

Etymology (PE): Pâyâni from pâyân “end, extremity; limit, boundary,” from pâ(y) “foot; step; track,” → foot.

Pertaining to or coming at the end; last in place, order, or time.

Etymology (EN): M.E., from O.Fr. final from L. finalis “of or pertaining to an end, concluding,” from finis “end.”

Etymology (PE): Pâyâni from pâyân “end, extremity; limit, boundary,” from pâ(y) “foot; step; track,” → foot.

A meteorite that was not seen to fall, but was found at some later date, as opposed to a → fall.

Etymology (EN): O.E. findan “to come upon,” from P.Gmc. *finthanan (cf. M.Du. vinden, Ger. finden), from PIE *pent- “to go, pass, path, bridge;” cf. Av. paθ-, variants paθi-, paθā-, pantay-; Mid/Mod.Pers. pand “path, advice, councel;” Khotanese pande “road, path;” Ossetic fœndœg “path, road;” cf. Skt. pánthā- “road, path, course;” Gk. patos “path, way,” pontos “sea;” L. pons “bridge, path.”

Etymology (PE): Yâft, past stem of yâftan, yâbidan “to find, discover; to obtain, acquire;” Mid.Pers. ayâftan, ayâpênitan “to reach, attain;” Manichean Mid.Pers. ‘y’b “to attain;” Parthian, Sogdian (+ *pati-) pty’b “to reach, obtain;” Av. ap- “to reach, overtake,” apayeiti “achieved, reached;” Skt. âp- “to reach, gain,”
âpnoti “reaches, gains;” Gk. hapto, haptomai “to touch, cling to, adhere to;” L. apiscor “touch, reach;” PIE base *ap- “to take, reach.”

A meteorite that was not seen to fall, but was found at some later date, as opposed to a → fall.

Etymology (EN): O.E. findan “to come upon,” from P.Gmc. *finthanan (cf. M.Du. vinden, Ger. finden), from PIE *pent- “to go, pass, path, bridge;” cf. Av. paθ-, variants paθi-, paθā-, pantay-; Mid/Mod.Pers. pand “path, advice, councel;” Khotanese pande “road, path;” Ossetic fœndœg “path, road;” cf. Skt. pánthā- “road, path, course;” Gk. patos “path, way,” pontos “sea;” L. pons “bridge, path.”

Etymology (PE): Yâft, past stem of yâftan, yâbidan “to find, discover; to obtain, acquire;” Mid.Pers. ayâftan, ayâpênitan “to reach, attain;” Manichean Mid.Pers. ‘y’b “to attain;” Parthian, Sogdian (+ *pati-) pty’b “to reach, obtain;” Av. ap- “to reach, overtake,” apayeiti “achieved, reached;” Skt. âp- “to reach, gain,”
âpnoti “reaches, gains;” Gk. hapto, haptomai “to touch, cling to, adhere to;” L. apiscor “touch, reach;” PIE base *ap- “to take, reach.”

A low-power telescope with a wide field of view attached to a larger telescope with the optical axes of both telescopes parallel. The finder is used to help point the larger telescope to the desired viewing location.

See also: Agent noun of → find.

A low-power telescope with a wide field of view attached to a larger telescope with the optical axes of both telescopes parallel. The finder is used to help point the larger telescope to the desired viewing location.

See also: Agent noun of → find.

A sketch or image used to recognize objects in the field of view of a telescope.

Etymology (EN): Finding, noun of → find; chart,
from M.Fr. charte “card, map,” from L. charta
“leaf of paper, tablet,” from Gk. khartes “layer of papyrus.”

Etymology (PE): Negâre-ye yâbeš, from negâré, from negâr “picture, figure” (verb negârdan, negâštan “to paint”), from prefix ne- “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”); yâbeš, verbal noun of yâftan, → find.

A sketch or image used to recognize objects in the field of view of a telescope.

Etymology (EN): Finding, noun of → find; chart,
from M.Fr. charte “card, map,” from L. charta
“leaf of paper, tablet,” from Gk. khartes “layer of papyrus.”

Etymology (PE): Negâre-ye yâbeš, from negâré, from negâr “picture, figure” (verb negârdan, negâštan “to paint”), from prefix ne- “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”); yâbeš, verbal noun of yâftan, → find.

Very thin or slender. → fine structure, → fine-structure constant.

Etymology (EN): M.E. fin, from O.Fr. fin “perfected, of highest quality,” from L. finis “end, limit.”

Etymology (PE): Nâzok “thin, slender, subtle,” from Mid.Pers. nâzuk “tender, gentle,” variant nâzik, from nâz “joy, pride, glory” + → -ik.

Very thin or slender. → fine structure, → fine-structure constant.

Etymology (EN): M.E. fin, from O.Fr. fin “perfected, of highest quality,” from L. finis “end, limit.”

Etymology (PE): Nâzok “thin, slender, subtle,” from Mid.Pers. nâzuk “tender, gentle,” variant nâzik, from nâz “joy, pride, glory” + → -ik.

Meteorology: An → inhomogeneous  → mixture of tiny, part → solid, part → liquid or → gaseous  → particles that are, in average, smaller than ten → microns. The constituents are soot, heavy metals, organic substances, and dioxins. The smaller these dust particles, the deeper they penetrate into the lung. Larger particles are intercepted by mucous membrane in nose, mouth, and throat but smaller particles can penetrate the smallest lung bronchioles and may cause severe damage (various respiratory disorders, lung cancer) → particulate matter.

See also: → fine; → dust.

Meteorology: An → inhomogeneous  → mixture of tiny, part → solid, part → liquid or → gaseous  → particles that are, in average, smaller than ten → microns. The constituents are soot, heavy metals, organic substances, and dioxins. The smaller these dust particles, the deeper they penetrate into the lung. Larger particles are intercepted by mucous membrane in nose, mouth, and throat but smaller particles can penetrate the smallest lung bronchioles and may cause severe damage (various respiratory disorders, lung cancer) → particulate matter.

See also: → fine; → dust.

Closely spaced components seen at high resolution in a → spectral line. The phenomenon is explained by the fact that instead of a single → energy level corresponding to a given value of the → quantum number  n, there are actually a number of energy levels lying close to one another. → fine-structure constant, → fine-structure line.

See also: → fine; → structure.

Closely spaced components seen at high resolution in a → spectral line. The phenomenon is explained by the fact that instead of a single → energy level corresponding to a given value of the → quantum number  n, there are actually a number of energy levels lying close to one another. → fine-structure constant, → fine-structure line.

See also: → fine; → structure.

A measure of the strength of → interaction between a → charged particle and the → electromagnetic field. It is a → dimensionless number expressed (in → cgs units) by α = e²/ħc, where e is the → electron charge, ħ is the → reduced Planck’s constant, and c is the → speed of light. It is approximately equal to 1/137 or 7.3 × 10^-3.
The smallness of this number is of great importance since it determines the size of → atoms and the → stability of → matter. Same as → electromagnetic coupling constant.

See also: → fine structure; → constant.

A measure of the strength of → interaction between a → charged particle and the → electromagnetic field. It is a → dimensionless number expressed (in → cgs units) by α = e²/ħc, where e is the → electron charge, ħ is the → reduced Planck’s constant, and c is the → speed of light. It is approximately equal to 1/137 or 7.3 × 10^-3.
The smallness of this number is of great importance since it determines the size of → atoms and the → stability of → matter. Same as → electromagnetic coupling constant.

See also: → fine structure; → constant.

A → spectral line whose → energy levels have a → fine structure. Examples are [C II] 157.7 μm (→ singly ionized carbon), [O III] 88 μm, and [Ne II] 12.8 μm.

See also: → fine; → structure; → line.

A → spectral line whose → energy levels have a → fine structure. Examples are [C II] 157.7 μm (→ singly ionized carbon), [O III] 88 μm, and [Ne II] 12.8 μm.

See also: → fine; → structure; → line.

Any of the terminal members of the hand, especially one other than the thumb (Dictionary.com).

Etymology (EN): M.E., from O.E. fingor, cognate with Ger. Finger, Du. vinger,
O.N. fingr, Goth. figgrs.

Etymology (PE): Angošt, variants angol, angul (also angal “loop”);
Mid.Pers. angust; Av. angušta- “toe,”
ank- “curved, crooked;” cf. Skt. angustha- “thumb,” angula- “finger,” ankah “hook, bent;” Gk. angkon “elbow,” angkura “anchor;” L. angulum “corner;” Lith. anka “loop;” O.E. ancleo “ankle;” O.H.G. ango “hook;” PIE *ang-/*ank- “to bend”.

Any of the terminal members of the hand, especially one other than the thumb (Dictionary.com).

Etymology (EN): M.E., from O.E. fingor, cognate with Ger. Finger, Du. vinger,
O.N. fingr, Goth. figgrs.

Etymology (PE): Angošt, variants angol, angul (also angal “loop”);
Mid.Pers. angust; Av. angušta- “toe,”
ank- “curved, crooked;” cf. Skt. angustha- “thumb,” angula- “finger,” ankah “hook, bent;” Gk. angkon “elbow,” angkura “anchor;” L. angulum “corner;” Lith. anka “loop;” O.E. ancleo “ankle;” O.H.G. ango “hook;” PIE *ang-/*ank- “to bend”.

A weak yet important kind of mixing that results from → fingering instability in stars within → radiative zones that have an unstable mean → molecular weight  → gradient. Also called
→ thermohaline convection.

See also: → finger; → -ing; → convection.

A weak yet important kind of mixing that results from → fingering instability in stars within → radiative zones that have an unstable mean → molecular weight  → gradient. Also called
→ thermohaline convection.

See also: → finger; → -ing; → convection.

A type of instability that often occurs in fluids which are thermally stably stratified, but have an inhomogeneous composition. A well-known example, found in upper layers of the Earth’s oceans, is
→ salt fingers. Similar fingering instabilities can occur in any other thermally stably stratified solution, provided the concentration of the slower-diffusing solute increases with height. The saturated state of this instability, → fingering convection, takes the form of tightly-packed, vertically-elongated plumes of sinking dense fluid and rising light fluid, and significantly enhances the vertical transport of both heat and chemical composition. The fingering instability occurs in stars within radiation zones that have an unstable mean → molecular weight  → gradient (μ gradient). This situation is often found as a result of material accretion onto a star by anything from a single or multiple planets, to material from a dust-enriched or debris accretion disk, or material from a more evolved companion. It also naturally arises in the vicinity of the → hydrogen shell burning in → red giant branch (RGB) stars, and in thin element-rich layers near the surface of intermediate-mass stars. The fingering instability initially takes the form of thin tubes, hence the name “finger,” within which the fluid moves vertically. The tubes rapidly break down, however, as a result of parasitic shear instabilities that develop inbetween them, and the fingering instability eventually saturates into a state of homogeneous fingering convection where the typical aspect ratio of the eddies is closer to one (P. Garaud et al., 2015, arXiv:1505.07759).

See also: → finger; → -ing; → instability.

A type of instability that often occurs in fluids which are thermally stably stratified, but have an inhomogeneous composition. A well-known example, found in upper layers of the Earth’s oceans, is
→ salt fingers. Similar fingering instabilities can occur in any other thermally stably stratified solution, provided the concentration of the slower-diffusing solute increases with height. The saturated state of this instability, → fingering convection, takes the form of tightly-packed, vertically-elongated plumes of sinking dense fluid and rising light fluid, and significantly enhances the vertical transport of both heat and chemical composition. The fingering instability occurs in stars within radiation zones that have an unstable mean → molecular weight  → gradient (μ gradient). This situation is often found as a result of material accretion onto a star by anything from a single or multiple planets, to material from a dust-enriched or debris accretion disk, or material from a more evolved companion. It also naturally arises in the vicinity of the → hydrogen shell burning in → red giant branch (RGB) stars, and in thin element-rich layers near the surface of intermediate-mass stars. The fingering instability initially takes the form of thin tubes, hence the name “finger,” within which the fluid moves vertically. The tubes rapidly break down, however, as a result of parasitic shear instabilities that develop inbetween them, and the fingering instability eventually saturates into a state of homogeneous fingering convection where the typical aspect ratio of the eddies is closer to one (P. Garaud et al., 2015, arXiv:1505.07759).

See also: → finger; → -ing; → instability.

A → redshift space distortion which causes the dense central regions of → galaxy clusters to appear elongated along the → line of sight. This effect is attributed to random velocities in clusters of galaxies deviating from pure → Hubble flow. For an observer galaxies with peculiar velocity perpendicular to the line of sight do not change the redshift, which is given just by the normal Hubble expansion. On the contrary, galaxies with peculiar velocity along the line of sight appear with a different redshift, resulting from the Hubble expansion velocity plus the peculiar velocity. Since this affects only redshift and not position on the sky, the stretching occurs only radially, toward the observer. See also → Kaiser effect, → peculiar velocity.

See also: → finger; → God.

A → redshift space distortion which causes the dense central regions of → galaxy clusters to appear elongated along the → line of sight. This effect is attributed to random velocities in clusters of galaxies deviating from pure → Hubble flow. For an observer galaxies with peculiar velocity perpendicular to the line of sight do not change the redshift, which is given just by the normal Hubble expansion. On the contrary, galaxies with peculiar velocity along the line of sight appear with a different redshift, resulting from the Hubble expansion velocity plus the peculiar velocity. Since this affects only redshift and not position on the sky, the stretching occurs only radially, toward the observer. See also → Kaiser effect, → peculiar velocity.

See also: → finger; → God.

1. Math: The opposite of → infinite.
   

2. Physics: Either non-infinite or non-zero.

Etymology (EN): From L. finitus, p.p. of finire “to limit, set bounds, end.”

Etymology (PE): Karânmand, from karân “boundary, side, end, coast” + -mand adjective suffix. Karân, variants karâné, kenâr, from Mid.Pers. karân, karânak, kenâr “edge, limit, boundary,” Av. karana- “side, boundary, end.”

1. Math: The opposite of → infinite.
   

2. Physics: Either non-infinite or non-zero.

Etymology (EN): From L. finitus, p.p. of finire “to limit, set bounds, end.”

Etymology (PE): Karânmand, from karân “boundary, side, end, coast” + -mand adjective suffix. Karân, variants karâné, kenâr, from Mid.Pers. karân, karânak, kenâr “edge, limit, boundary,” Av. karana- “side, boundary, end.”

A → statistical population consisting of individuals or items which are finite in number.

See also: → finite; → population.

A → statistical population consisting of individuals or items which are finite in number.

See also: → finite; → population.

A sum a₁ + a₂ + a₃ + · · · + a_N, where the a_i’s are real numbers. In terms of Σ-notation, it is written as

a₁ + a₂ + a₃ + · · · + a_N = Σ (n = 1 to N).  See also → infinite series.

See also: → finite; → series.

A sum a₁ + a₂ + a₃ + · · · + a_N, where the a_i’s are real numbers. In terms of Σ-notation, it is written as

a₁ + a₂ + a₃ + · · · + a_N = Σ (n = 1 to N).  See also → infinite series.

See also: → finite; → series.

A → set whose elements can be numbered from 1 to n, for some positive integer n.

See also: → finite; → set.

A → set whose elements can be numbered from 1 to n, for some positive integer n.

See also: → finite; → set.

A state, process, or instance of combustion in which a substance combines with oxygen producing heat, light, and flame.

Etymology (EN): O..E. fyr, from P.Gmc. *fuir (cf. O.N. fürr, M.Du. vuur, Ger. Feuer), from PIE *paewr-; cf. Mod.Pers. Lori porpor “blazing charcoal,” Gilaki bur, biur “smokeless red fire” (Lori perisk, periska “spark,” Kurd. biriske “spark,” Lârestâni pelita “spark”); Tokharian por, puwar “fire;” Gk. pyr “fire;” Hitt. pahhur “fire;” Skt. pū- “to cleanse.”

Etymology (PE): Âtaš, variants âzar, taš, from Mid.Pers. âtaxš, âtur “fire;” 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.”

A state, process, or instance of combustion in which a substance combines with oxygen producing heat, light, and flame.

Etymology (EN): O..E. fyr, from P.Gmc. *fuir (cf. O.N. fürr, M.Du. vuur, Ger. Feuer), from PIE *paewr-; cf. Mod.Pers. Lori porpor “blazing charcoal,” Gilaki bur, biur “smokeless red fire” (Lori perisk, periska “spark,” Kurd. biriske “spark,” Lârestâni pelita “spark”); Tokharian por, puwar “fire;” Gk. pyr “fire;” Hitt. pahhur “fire;” Skt. pū- “to cleanse.”

Etymology (PE): Âtaš, variants âzar, taš, from Mid.Pers. âtaxš, âtur “fire;” 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.”

A → meteor that is brighter than the brightest planets, i.e. with an apparent magnitude of -5 or greater. Fireballs are often followed by → meteorite falls. Also called → bolide.

Etymology (EN): From → fire + ball, from O.E., from O.N. bollr “ball,” from P.Gmc. *balluz (cf. O.H.G. ballo, Ger. Ball), from PIE base *bhel- “to swell.”

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

A → meteor that is brighter than the brightest planets, i.e. with an apparent magnitude of -5 or greater. Fireballs are often followed by → meteorite falls. Also called → bolide.

Etymology (EN): From → fire + ball, from O.E., from O.N. bollr “ball,” from P.Gmc. *balluz (cf. O.H.G. ballo, Ger. Ball), from PIE base *bhel- “to swell.”

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

1. Having a solid structure that resists pressure.
   

   2. Solidly or securely fixed in place.

Etymology (EN): M.E. ferm, from O.Fr. ferm “strong, vigorous; healthy, sound; steadfast,” from L. firmus “strong, steadfast, stable,” from PIE root *dher- “to hold firmly, support.”

Etymology (PE): Deš (Hamadâni) “firm, steady,” (Ilâmi) dež “intact, entire,” (Hamadâni) daj “intact, full,” (Šuštari) dec “full,” may be related to Proto-Ir. *dar- “to hold, keep;” Av. dār- “to hold, keep;” Pers./ Mid.Pers. dâštan/dâr- “to have, hold, keep;” cf. Skt. dhar- “to hold, keep, preserve;” L. firmus “firm, steady, as above.”

1. Having a solid structure that resists pressure.
   

   2. Solidly or securely fixed in place.

Etymology (EN): M.E. ferm, from O.Fr. ferm “strong, vigorous; healthy, sound; steadfast,” from L. firmus “strong, steadfast, stable,” from PIE root *dher- “to hold firmly, support.”

Etymology (PE): Deš (Hamadâni) “firm, steady,” (Ilâmi) dež “intact, entire,” (Hamadâni) daj “intact, full,” (Šuštari) dec “full,” may be related to Proto-Ir. *dar- “to hold, keep;” Av. dār- “to hold, keep;” Pers./ Mid.Pers. dâštan/dâr- “to have, hold, keep;” cf. Skt. dhar- “to hold, keep, preserve;” L. firmus “firm, steady, as above.”

Being before all others with respect to time, order, importance, etc., used as the ordinal number of one.

Etymology (EN): O.E. fyrst “foremost,” superlative of fore, from P.Gmc. *furisto (cf. O.H.G. furist, O.N. fyrstr, Dan. første, M.Du. vorste “first,” Ger. Fürst “prince”), superlative of *fur-/*for-, from PIE *pro- (cf. Av. pouruua- “first,” fra- “forward, forth;” Skt. pūrva- “first,” pra- “before, formerly,” Gk. pro; L. pro; E. fore).

Etymology (PE): Naxost, from Mid.Pers. naxust “the first,” Parthian Mid.Pers. nxwšt, from naxu, Manichean Parthian nwx “beginning” + -ist superlative suffix, Av. -išta-, cf. Skt. -istha-, Gk. -istos, O.H.G. -isto, -osto, O.E. -st, -est, -ost; naxostin, from naxost + suffix -in.
Yekom, from yek, → one, + -om suffix of ordinal numbers.
Âqâz “beginning,” from Proto-Iranian *āgāza-, from prefix ā-

• *gāz- “to take, receive,” cf. Sogdian āγāz “beginning, start,” pcγz “reception, taking.”

Being before all others with respect to time, order, importance, etc., used as the ordinal number of one.

Etymology (EN): O.E. fyrst “foremost,” superlative of fore, from P.Gmc. *furisto (cf. O.H.G. furist, O.N. fyrstr, Dan. første, M.Du. vorste “first,” Ger. Fürst “prince”), superlative of *fur-/*for-, from PIE *pro- (cf. Av. pouruua- “first,” fra- “forward, forth;” Skt. pūrva- “first,” pra- “before, formerly,” Gk. pro; L. pro; E. fore).

Etymology (PE): Naxost, from Mid.Pers. naxust “the first,” Parthian Mid.Pers. nxwšt, from naxu, Manichean Parthian nwx “beginning” + -ist superlative suffix, Av. -išta-, cf. Skt. -istha-, Gk. -istos, O.H.G. -isto, -osto, O.E. -st, -est, -ost; naxostin, from naxost + suffix -in.
Yekom, from yek, → one, + -om suffix of ordinal numbers.
Âqâz “beginning,” from Proto-Iranian *āgāza-, from prefix ā-

• *gāz- “to take, receive,” cf. Sogdian āγāz “beginning, start,” pcγz “reception, taking.”

1. Generally, an expression to indicate that a comment or result is only approximate.
   

2. Math.: In calculus, limiting a differential equation to its → first derivative, for example: e^x≅ 1 + x. Also called → linear approximation.

See also: → first; → approximation.

1. Generally, an expression to indicate that a comment or result is only approximate.
   

2. Math.: In calculus, limiting a differential equation to its → first derivative, for example: e^x≅ 1 + x. Also called → linear approximation.

See also: → first; → approximation.

An early phase in the process of star formation which begins when the mass of a → molecular cloud → clump exceeds the → Jeans mass.
The collapse is initially → optically thin to the thermal emission from → dust grains, and the compressional heating rate is much smaller than the cooling rate by the → thermal radiation. The collapse proceeds
→ isothermally. The isothermal condition is broken when the central density reaches about 10^-13 g cm^-3 and a small region at the center of the cloud starts to become → opaque. The heat generated by the collapse in this region is no longer freely radiated away, and the compression becomes approximately → adiabatic. The central temperature and pressure then begin to rise rapidly, soon becoming sufficient to decelerate and stop the collapse at the center.
There then arises a small central core, called the → first core, in which the material has stopped collapsing and is approaching → hydrostatic equilibrium.
Outside this core, the material is still nearly isothermal and continues to fall inward almost in → free fall.
Consequently a shock front arises at the boundary of the core, where the infalling material is suddenly stopped. The initial mass and radius of the core are about 10³¹ g and 6 x 10¹³ cm, respectively, and the central density and temperature are about 2 x 10^-10 g cm^-3 and 170 K, respectively. As the collapse proceeds, the core grows in mass due to the infall of the surrounding material; at the same time, however, the core radius decreases because of radiative energy losses from the outer layers of the core. The process leads to the → second collapse (R. B. Larson, 1969, MNRAS 145, 271).

See also: → first; → collapse.

An early phase in the process of star formation which begins when the mass of a → molecular cloud → clump exceeds the → Jeans mass.
The collapse is initially → optically thin to the thermal emission from → dust grains, and the compressional heating rate is much smaller than the cooling rate by the → thermal radiation. The collapse proceeds
→ isothermally. The isothermal condition is broken when the central density reaches about 10^-13 g cm^-3 and a small region at the center of the cloud starts to become → opaque. The heat generated by the collapse in this region is no longer freely radiated away, and the compression becomes approximately → adiabatic. The central temperature and pressure then begin to rise rapidly, soon becoming sufficient to decelerate and stop the collapse at the center.
There then arises a small central core, called the → first core, in which the material has stopped collapsing and is approaching → hydrostatic equilibrium.
Outside this core, the material is still nearly isothermal and continues to fall inward almost in → free fall.
Consequently a shock front arises at the boundary of the core, where the infalling material is suddenly stopped. The initial mass and radius of the core are about 10³¹ g and 6 x 10¹³ cm, respectively, and the central density and temperature are about 2 x 10^-10 g cm^-3 and 170 K, respectively. As the collapse proceeds, the core grows in mass due to the infall of the surrounding material; at the same time, however, the core radius decreases because of radiative energy losses from the outer layers of the core. The process leads to the → second collapse (R. B. Larson, 1969, MNRAS 145, 271).

See also: → first; → collapse.

1. The beginning of a → solar eclipse when the eastern part of the lunar limb touches the western limb of the Sun, marking the beginning of an eclipse.
   

2. For a → lunar eclipse, the moment when the eastern limb of the Moon is the first to enter the Earth’s shadow.

See also: → first; → contact.

1. The beginning of a → solar eclipse when the eastern part of the lunar limb touches the western limb of the Sun, marking the beginning of an eclipse.
   

2. For a → lunar eclipse, the moment when the eastern limb of the Moon is the first to enter the Earth’s shadow.

See also: → first; → contact.

A first object in → hydrostatic equilibrium predicted to form during early dynamical contraction of a → molecular cloud → clump in the course of the → first collapse.

See also: → first; → core.

A first object in → hydrostatic equilibrium predicted to form during early dynamical contraction of a → molecular cloud → clump in the course of the → first collapse.

See also: → first; → core.

A equation in which the highest → exponent of the → variable is 1. Same as
→ linear equation.

See also: → first; → degree; → equation.

A equation in which the highest → exponent of the → variable is 1. Same as
→ linear equation.

See also: → first; → degree; → equation.

The simplest mathematical → differentiation of one quantity relative to another; df(x)/dx. It is the same as → derivative, contrasted with the → second derivative or the nth derivative.

See also: → first; → derivative.

The simplest mathematical → differentiation of one quantity relative to another; df(x)/dx. It is the same as → derivative, contrasted with the → second derivative or the nth derivative.

See also: → first; → derivative.

In → calculus, a method for determining whether an → inflection point is a → local minimum, → local maximum, or neither.

See also: → first; → derivative; → test.

In → calculus, a method for determining whether an → inflection point is a → local minimum, → local maximum, or neither.

See also: → first; → derivative; → test.

The → dredge-up occurring after core hydrogen burning as the core contracts before helium burning ignites (on the ascending giant branch). The hydrogen envelope becomes convective and this convective zone penetrates deep into the core dredging up material that has been processed by the central nuclear reactions. As a result the abundances of helium and nitrogen are boosted.

See also: → first; → dredge-up.

The → dredge-up occurring after core hydrogen burning as the core contracts before helium burning ignites (on the ascending giant branch). The hydrogen envelope becomes convective and this convective zone penetrates deep into the core dredging up material that has been processed by the central nuclear reactions. As a result the abundances of helium and nitrogen are boosted.

See also: → first; → dredge-up.

The total energy of a → closed system is constant. This means that energy can be changed from one form to another, or transferred from one system to another, but it cannot be created or destroyed. A mathematical formulation of the first law is: δQ = δU + δW, where δQ is the heat transferred to the system, δU the change in internal energy (resulting in a rise or fall of temperature), and δW is the work done by the system.

See also: → first; → law; → thermodynamics.

The total energy of a → closed system is constant. This means that energy can be changed from one form to another, or transferred from one system to another, but it cannot be created or destroyed. A mathematical formulation of the first law is: δQ = δU + δW, where δQ is the heat transferred to the system, δU the change in internal energy (resulting in a rise or fall of temperature), and δW is the work done by the system.

See also: → first; → law; → thermodynamics.

The first astronomical observation done with a major newly built telescope.

See also: → first; → light.

The first astronomical observation done with a major newly built telescope.

See also: → first; → light.

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → vernal equinox several thousands years ago.

See also: → first; → point; → Aries.

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → vernal equinox several thousands years ago.

See also: → first; → point; → Aries.

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → autumnal equinox several thousands years ago. Because of → precession, this equinoctial point no longer lies in Libra but in neighboring Virgo.

One of the two points on the celestial sphere where the ecliptic and the celestial equator crossed one another at → autumnal equinox several thousands years ago. Because of → precession, this equinoctial point no longer lies in Libra but in neighboring Virgo.

A → lunar phase that occurs in the middle of the interval between the → new Moon and the → full Moon, when half of the Moon’s disk is illuminated. At first quarter, the Moon is situated at 90° east of the Sun.

See also: → first; → quarter.

A → lunar phase that occurs in the middle of the interval between the → new Moon and the → full Moon, when half of the Moon’s disk is illuminated. At first quarter, the Moon is situated at 90° east of the Sun.

See also: → first; → quarter.

A member of the → Population III → massive stars that formed some 500 Myr after the → Big Bang. First stars had a simple chemical composition consisting only of H, He, and traces of ⁷Li and were very short-lived. They are the most likely sources of → reionization of the Universe,
which put an end to the → Cosmic Dark Age.

See also: → first; → star.

A member of the → Population III → massive stars that formed some 500 Myr after the → Big Bang. First stars had a simple chemical composition consisting only of H, He, and traces of ⁷Li and were very short-lived. They are the most likely sources of → reionization of the Universe,
which put an end to the → Cosmic Dark Age.

See also: → first; → star.

A → differential equation containing only the first → derivative. For example, dy/dx = 3x and 2y(dy/dx) + 3x = 5.

See also: → first; → order; → differential; → equation.

A → differential equation containing only the first → derivative. For example, dy/dx = 3x and 2y(dy/dx) + 3x = 5.

See also: → first; → order; → differential; → equation.

A system of → formal logic that is an extension of → propositional logic. It is also known as → first-order predicate calculus and → predicate logic. FOL eliminates deficiencies of propositional logic by: representing → objects (their properties, relations and statements about them), introducing → variables, and introducing → quantifiers.

See also: → first; → order; → predicate; → logic.

A system of → formal logic that is an extension of → propositional logic. It is also known as → first-order predicate calculus and → predicate logic. FOL eliminates deficiencies of propositional logic by: representing → objects (their properties, relations and statements about them), introducing → variables, and introducing → quantifiers.

See also: → first; → order; → predicate; → logic.

For a → function with several → variables, the derivative, done only once, with respect to one of the variables. For example ∂u/∂x for u = u(x,y).

See also: → first; → order; → partial; → derivative.

For a → function with several → variables, the derivative, done only once, with respect to one of the variables. For example ∂u/∂x for u = u(x,y).

See also: → first; → order; → partial; → derivative.

Same as → first-order logic.

See also: → first; → order; → predicate; → calculus.

Same as → first-order logic.

See also: → first; → order; → predicate; → calculus.

An optical spectrum, produced by a diffraction grating, in which the difference in path length of light from adjacent slits is one wavelength.

See also: → first; → order;
→ spectrum.

An optical spectrum, produced by a diffraction grating, in which the difference in path length of light from adjacent slits is one wavelength.

See also: → first; → order;
→ spectrum.

A limbless cold-blooded vertebrate animal with gills and fins living wholly in water (OxfordDictionaries.com).

Etymology (EN): M.E. fis(c)h, fyssh, O.E. fisc; cognate with Du. vis, Ger. Fisch, O.Norse fiskr, Goth. fisks; akin to L. piscis; PIE root *pisk- “a fish.”

Etymology (PE): Mâhi “fish,” from Mid.Pers. mâhik; Av. masya-; cf. Skt. matsya-, Pali maccha-.

A limbless cold-blooded vertebrate animal with gills and fins living wholly in water (OxfordDictionaries.com).

Etymology (EN): M.E. fis(c)h, fyssh, O.E. fisc; cognate with Du. vis, Ger. Fisch, O.Norse fiskr, Goth. fisks; akin to L. piscis; PIE root *pisk- “a fish.”

Etymology (PE): Mâhi “fish,” from Mid.Pers. mâhik; Av. masya-; cf. Skt. matsya-, Pali maccha-.

Any material that is capable of undergoing → nuclear fission by → thermal neutrons. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239. Although sometimes used as a synonym for → fissionable material, this term has acquired a more restricted meaning.

Etymology (EN): From L. fissilis, from fiss(us), + -illis a suffix of adjectives expressing capability, susceptibility, liability, aptitude, etc.

Etymology (PE): Šekâftani, from šekâft, → fission + -i a suffix expressing capability, aptitude, etc.

Any material that is capable of undergoing → nuclear fission by → thermal neutrons. The three primary fissile materials are uranium-233, uranium-235, and plutonium-239. Although sometimes used as a synonym for → fissionable material, this term has acquired a more restricted meaning.

Etymology (EN): From L. fissilis, from fiss(us), + -illis a suffix of adjectives expressing capability, susceptibility, liability, aptitude, etc.

Etymology (PE): Šekâftani, from šekâft, → fission + -i a suffix expressing capability, aptitude, etc.

An isotope that is capable of undergoing nuclear fission after capturing either fast neutron or thermal neutron. Typical fissionable isotopes: ²³⁸U, ²⁴⁰Pu, but also ²³⁵U, ²³³U, ²³⁹Pu, ²⁴¹Pu

See also: → fissile; → isotope .

An isotope that is capable of undergoing nuclear fission after capturing either fast neutron or thermal neutron. Typical fissionable isotopes: ²³⁸U, ²⁴⁰Pu, but also ²³⁵U, ²³³U, ²³⁹Pu, ²⁴¹Pu

See also: → fissile; → isotope .

1. The act or process of splitting or breaking into parts.
   

2. Splitting of the nucleus of an atom into two or more fragments of comparable size, usually as the result of the impact of a neutron on the nucleus. Same as → nuclear fission.

Etymology (EN): Fission, from L. fissionem “a breaking up, cleaving,” from root of findere “to split.”

Etymology (PE): Šekâft, stem of šekâftan “to split, break, tear,” akin to kaftan, kâftan “to split; to dig,” Parthian Mid.Pers. q’f- “to split;” Sogdian kβ “to split;” Chorasmian kf- “to split, be split;” Proto-Iranian *kap-, *kaf- “to split.”

1. The act or process of splitting or breaking into parts.
   

2. Splitting of the nucleus of an atom into two or more fragments of comparable size, usually as the result of the impact of a neutron on the nucleus. Same as → nuclear fission.

Etymology (EN): Fission, from L. fissionem “a breaking up, cleaving,” from root of findere “to split.”

Etymology (PE): Šekâft, stem of šekâftan “to split, break, tear,” akin to kaftan, kâftan “to split; to dig,” Parthian Mid.Pers. q’f- “to split;” Sogdian kβ “to split;” Chorasmian kf- “to split, be split;” Proto-Iranian *kap-, *kaf- “to split.”

Nuclides generated by the fission of higher mass elements or by subsequent radioactive decay of nuclides directly generated by fission.

See also: → fission; → product.

Nuclides generated by the fission of higher mass elements or by subsequent radioactive decay of nuclides directly generated by fission.

See also: → fission; → product.

A theory that suggests the Moon was formed at the same time as Earth. A spinning Earth ejected a large piece of its material into space which then developed into the shape and orbit of the Moon. This event was also thought to be at the origin of the Pacific Ocean.

This first modern idea about the formation of the Moon is due to George Darwin, the son of the great naturalist Charles Darwin.

The fission theory explained the lack of volatile substances on the Earth. The volatile materials on the Earth would have been thrown out into space

The fission theory is almost completely abandoned today. The analysis of lunar rocks brought to Earth by NASA astronauts showed that the Moon rocks are older than the rocks at the bottom of the Pacific Ocean. Moreover, modern → plate tectonics gives a better explanation of the origin of the Pacific Ocean. See also → giant impact hypothesis, → capture theory, → co-formation theory.

See also: → fission; → theory.

A theory that suggests the Moon was formed at the same time as Earth. A spinning Earth ejected a large piece of its material into space which then developed into the shape and orbit of the Moon. This event was also thought to be at the origin of the Pacific Ocean.

This first modern idea about the formation of the Moon is due to George Darwin, the son of the great naturalist Charles Darwin.

The fission theory explained the lack of volatile substances on the Earth. The volatile materials on the Earth would have been thrown out into space

The fission theory is almost completely abandoned today. The analysis of lunar rocks brought to Earth by NASA astronauts showed that the Moon rocks are older than the rocks at the bottom of the Pacific Ocean. Moreover, modern → plate tectonics gives a better explanation of the origin of the Pacific Ocean. See also → giant impact hypothesis, → capture theory, → co-formation theory.

See also: → fission; → theory.

The material that can be fissioned by fast neutrons, such as uranium-238. Commonly used as a synonym for → fissile material.

See also: From → fission + → -able.

The material that can be fissioned by fast neutrons, such as uranium-238. Commonly used as a synonym for → fissile material.

See also: From → fission + → -able.

1. (v.) To pass a mathematical line or curve through data points.
   
2. (n.) The act of fitting.

Etymology (EN): From M.E. fitten; akin to M.Du. vitten “to befit.”

Etymology (PE): Saz, from sazidan “to suit, fit, be worthy,” sazâ “suitable, agreeing with, congruous, deserving of,” Mid.Pers. sacitan/sazidan “to fit,” sazešn “fitness,” sazâg “fitting, worth;” Av. ¹sak- “to understand or know a thing; to mark;” cf. Skt. śak- “to be able, powerful” śakta- “able, competent,” śakti- “ability, power;” alternatively from Av. ²sak- “to go by, pass, pass away; to be up or over (of time).”

1. (v.) To pass a mathematical line or curve through data points.
   
2. (n.) The act of fitting.

Etymology (EN): From M.E. fitten; akin to M.Du. vitten “to befit.”

Etymology (PE): Saz, from sazidan “to suit, fit, be worthy,” sazâ “suitable, agreeing with, congruous, deserving of,” Mid.Pers. sacitan/sazidan “to fit,” sazešn “fitness,” sazâg “fitting, worth;” Av. ¹sak- “to understand or know a thing; to mark;” cf. Skt. śak- “to be able, powerful” śakta- “able, competent,” śakti- “ability, power;” alternatively from Av. ²sak- “to go by, pass, pass away; to be up or over (of time).”

The process or instance of adapting a mathematical curve to data points.

See also: Verbal form of → fit.

The process or instance of adapting a mathematical curve to data points.

See also: Verbal form of → fit.

The discrepancy between the mathematical curve and data points. → fit.

See also: → fitting; → error.

The discrepancy between the mathematical curve and data points. → fit.

See also: → fitting; → error.

A cardinal number whose symbol is 5, V, or ۵.

Etymology (EN): Five, from
O.E. fif, from P.Gmc. *fimfe (cf. O.S. fif, O.H.G. funf), from PIE *penk^we “five;” cognate with Pers. panj, as below.

Etymology (PE): Panj, from Mid.Pers. panj; Av. panca; cf.
Skt. páñca; Gk. pente; L. quinque; E. five, as above.

A cardinal number whose symbol is 5, V, or ۵.

Etymology (EN): Five, from
O.E. fif, from P.Gmc. *fimfe (cf. O.S. fif, O.H.G. funf), from PIE *penk^we “five;” cognate with Pers. panj, as below.

Etymology (PE): Panj, from Mid.Pers. panj; Av. panca; cf.
Skt. páñca; Gk. pente; L. quinque; E. five, as above.

A photometric system which uses five filters, from ultraviolet to the red part of the visual spectrum: U, B, V, R and I.

See also: → five; → color; → system.

A photometric system which uses five filters, from ultraviolet to the red part of the visual spectrum: U, B, V, R and I.

See also: → five; → color; → system.

The 500 m diameter → radio telescope which is the largest → single-dish antenna in the world. It is an Arecibo type telescope nestled within a natural basin in China’s remote and mountainous Dawodang, Kedu Town, in southeastern China’s Guizhou Province. The → reflector consists of 4,450 triangular panels, each with a side length of 11 m. More than 2,000 → actuators are used, according to the feedback from the measuring system, to deform the whole reflector surface and directly correct for → spherical aberration. Several detectors are used to cover a frequency range of 70 MHz to 3 GHz.

See also: → five; → hundred; → meter; → aperture; → spherical; → radio; → telescope.

The 500 m diameter → radio telescope which is the largest → single-dish antenna in the world. It is an Arecibo type telescope nestled within a natural basin in China’s remote and mountainous Dawodang, Kedu Town, in southeastern China’s Guizhou Province. The → reflector consists of 4,450 triangular panels, each with a side length of 11 m. More than 2,000 → actuators are used, according to the feedback from the measuring system, to deform the whole reflector surface and directly correct for → spherical aberration. Several detectors are used to cover a frequency range of 70 MHz to 3 GHz.

See also: → five; → hundred; → meter; → aperture; → spherical; → radio; → telescope.

1. To make fast, firm, or stable.
   

2. To place definitely and more or less permanently (Dictionary.com).

Etymology (EN): M.E. fixen, probably from O.Fr. fixe “fixed,” from L. fixus “fixed, fast, established, settled,” p.p. of figere “to fix, fasten.”

Etymology (PE): Infinitive, from barjâ, → fixed.

1. To make fast, firm, or stable.
   

2. To place definitely and more or less permanently (Dictionary.com).

Etymology (EN): M.E. fixen, probably from O.Fr. fixe “fixed,” from L. fixus “fixed, fast, established, settled,” p.p. of figere “to fix, fasten.”

Etymology (PE): Infinitive, from barjâ, → fixed.

The act of fixing or the state of being fixed.

See also: Verbal noun of → fix; → -tion.

The act of fixing or the state of being fixed.

See also: Verbal noun of → fix; → -tion.

1. Fastened, attached, or placed so as to be firm and not readily movable; firmly implanted; stationary; rigid.
   

2. Rendered stable or permanent, as color.
   

3. Set or intent upon something; steadily directed (Dictionary.com).

Etymology (EN): Past participle from → fix.

Etymology (PE): Barjâ “fixed; in place; properly placed,” from bar “on, upon, up,” → object,

• jâ, → place,

1. Fastened, attached, or placed so as to be firm and not readily movable; firmly implanted; stationary; rigid.
   

2. Rendered stable or permanent, as color.
   

3. Set or intent upon something; steadily directed (Dictionary.com).

Etymology (EN): Past participle from → fix.

Etymology (PE): Barjâ “fixed; in place; properly placed,” from bar “on, upon, up,” → object,

• jâ, → place,

A → heavenly body that, in → contrast to the → planets, does not appear to change its
relative → position on the → celestial sphere.

Etymology (EN): Fixed, p.p. of → fix; → star.

Etymology (PE): Setâré, → star; barjâ, → fixed.
Istâdé (Biruni in at-Tafhim) “standing,” from istâdan “to stand;” O.Pers./Av. sta- “to stand, stand still; set” (cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh;” stasis “a standing still;” Lith. statau “place;” Goth. standan; L. stare “to stand;” PIE base *sta- “to stand”);
Biyâbâni (Biruni in at-Tafhim), from Mid.Pers. awiyâbânig “fixed,” from negation prefix → a- + wiyâbânig “wandering,” from wiyâbân “deluded, seduced,” wiyâbânênidan “to lead astray, deceive,” from Av. vi- “apart, away from, out” (O.Pers. viy- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”)

• dab- “to deceive;” cf. Skt. vimugdha- “confused, bewildered,” vimohita- “confused, infatuated.” Note that biyâbân “desert” is from an other origin.

A → heavenly body that, in → contrast to the → planets, does not appear to change its
relative → position on the → celestial sphere.

Etymology (EN): Fixed, p.p. of → fix; → star.

Etymology (PE): Setâré, → star; barjâ, → fixed.
Istâdé (Biruni in at-Tafhim) “standing,” from istâdan “to stand;” O.Pers./Av. sta- “to stand, stand still; set” (cf. Skt. sthâ- “to stand;” Gk. histemi “put, place, weigh;” stasis “a standing still;” Lith. statau “place;” Goth. standan; L. stare “to stand;” PIE base *sta- “to stand”);
Biyâbâni (Biruni in at-Tafhim), from Mid.Pers. awiyâbânig “fixed,” from negation prefix → a- + wiyâbânig “wandering,” from wiyâbân “deluded, seduced,” wiyâbânênidan “to lead astray, deceive,” from Av. vi- “apart, away from, out” (O.Pers. viy- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”)

• dab- “to deceive;” cf. Skt. vimugdha- “confused, bewildered,” vimohita- “confused, infatuated.” Note that biyâbân “desert” is from an other origin.

The determination of the shape of an optical surface by means of → interferometry using a → fringe pattern formed with respect to a reference surface.

See also: Named after the French physicist Armand Hippolyte Louis Fizeau (1819-1896), see also → toothed-wheel experiment; → test.

The determination of the shape of an optical surface by means of → interferometry using a → fringe pattern formed with respect to a reference surface.

See also: Named after the French physicist Armand Hippolyte Louis Fizeau (1819-1896), see also → toothed-wheel experiment; → test.