An Etymological Dictionary of Astronomy and Astrophysics

A geometrical or physical structure that repeats itself or nearly repeats itself on many different scales of magnification.

Etymology (EN): From Fr. fractale, term coined by Benoit Mandelbrot (1975), from frac(tus) “broken, uneven”, → fraction, + -ale→ -al.

Etymology (PE): Barxâl, from barx, → fraction, + -âl→ -al.

A geometrical or physical structure that repeats itself or nearly repeats itself on many different scales of magnification.

Etymology (EN): From Fr. fractale, term coined by Benoit Mandelbrot (1975), from frac(tus) “broken, uneven”, → fraction, + -ale→ -al.

Etymology (PE): Barxâl, from barx, → fraction, + -âl→ -al.

The postulate that the concentrations of matter in the Universe follow a → fractal structure over a wide range of scales.

See also: → fractal; → cosmology.

The postulate that the concentrations of matter in the Universe follow a → fractal structure over a wide range of scales.

See also: → fractal; → cosmology.

A → hierarchial structure that can be likened to fractals.

See also: → fractal; → structure

A → hierarchial structure that can be likened to fractals.

See also: → fractal; → structure

A rational number of the form a/b where a is called the numerator and b is called the denominator.

Etymology (EN): From L.L. fractionem (nom. fractio) “a breaking in pieces,” from frangere “to break,” from PIE base *bhreg- “to break” (cf. Goth. brikan, O.E. brecan “to break;” Lith. brasketi
“crash, crack”).

Etymology (PE): Barxé, from barx “lot, portion,” variant bahr, from Mid.Pers. bahr “lot, share, portion,” Av. baxəδra- “portion.”

A rational number of the form a/b where a is called the numerator and b is called the denominator.

Etymology (EN): From L.L. fractionem (nom. fractio) “a breaking in pieces,” from frangere “to break,” from PIE base *bhreg- “to break” (cf. Goth. brikan, O.E. brecan “to break;” Lith. brasketi
“crash, crack”).

Etymology (PE): Barxé, from barx “lot, portion,” variant bahr, from Mid.Pers. bahr “lot, share, portion,” Av. baxəδra- “portion.”

1. Math.: Pertaining to fractions; constituting a fraction.
   

2. Chemistry: Of or relating to any process by which parts of a mixture are separated by exploiting differences in their physical properties, such as their boiling points, solubility, or other
   characteristics.

See also: → fraction; → -al.

1. Math.: Pertaining to fractions; constituting a fraction.
   

2. Chemistry: Of or relating to any process by which parts of a mixture are separated by exploiting differences in their physical properties, such as their boiling points, solubility, or other
   characteristics.

See also: → fraction; → -al.

The portion of the 4π → steradians of the sky that a radiotelescope can observe from a given location on Earth over a 24-hour time interval.

See also: → fractional; → sky; → coverage.

The portion of the 4π → steradians of the sky that a radiotelescope can observe from a given location on Earth over a 24-hour time interval.

See also: → fractional; → sky; → coverage.

1. To break something up into smaller parts.
   

2. To separate a mixture into ingredients or portions having different properties, as by distillation or otherwise.

Etymology (EN): From → fraction + -ate a suffix forming verbs or nouns, from L. -atus, -ata, -atum.

Etymology (PE): Barxândan, from barx, barxé, → fraction,

• -ândan suffix of transitive verbs.

1. To break something up into smaller parts.
   

2. To separate a mixture into ingredients or portions having different properties, as by distillation or otherwise.

Etymology (EN): From → fraction + -ate a suffix forming verbs or nouns, from L. -atus, -ata, -atum.

Etymology (PE): Barxândan, from barx, barxé, → fraction,

• -ândan suffix of transitive verbs.

1. Any of various methods of separating the components of a mixture into fractions of different properties.
   

2. → isotope fractionation

See also: Verbal noun from → fractionate.

1. Any of various methods of separating the components of a mixture into fractions of different properties.
   

2. → isotope fractionation

See also: Verbal noun from → fractionate.

1. (n.) A part broken off or detached.
   
2. (v.intr.) To collapse or break into pieces.
   
3. (v.tr.) To break something into pieces.

Etymology (EN): From L. fragmentum, from frangere “to break.”

Etymology (PE): 1) Latté, lat, variant laxt, laxté “piece, part;” pâré “piece, part, portion, fragment;” Mid.Pers. pârag “piece, part, portion; gift, offering, bribe;” Av. pāra- “debt,” from par- “to remunerate, equalize; to condemn;” PIE *per- “to sell, hand over, distribute; to assigne;” cf. L. pars “part, piece, side, share,” portio “share, portion;” Gk. peprotai “it has been granted;” Skt. purti- “reward;” Hitt. pars-, parsiya- “to break, crumble.”
2) and 3) Verbal forms.

1. (n.) A part broken off or detached.
   
2. (v.intr.) To collapse or break into pieces.
   
3. (v.tr.) To break something into pieces.

Etymology (EN): From L. fragmentum, from frangere “to break.”

Etymology (PE): 1) Latté, lat, variant laxt, laxté “piece, part;” pâré “piece, part, portion, fragment;” Mid.Pers. pârag “piece, part, portion; gift, offering, bribe;” Av. pāra- “debt,” from par- “to remunerate, equalize; to condemn;” PIE *per- “to sell, hand over, distribute; to assigne;” cf. L. pars “part, piece, side, share,” portio “share, portion;” Gk. peprotai “it has been granted;” Skt. purti- “reward;” Hitt. pars-, parsiya- “to break, crumble.”
2) and 3) Verbal forms.

Generally, the process of breaking up into smaller parts. In particular, the splitting
of a large molecular cloud into smaller, denser clumps. → cloud fragmentation.

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

Etymology (PE): Latpâreš, verbal noun from latpâridan, → fragment.

Generally, the process of breaking up into smaller parts. In particular, the splitting
of a large molecular cloud into smaller, denser clumps. → cloud fragmentation.

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

Etymology (PE): Latpâreš, verbal noun from latpâridan, → fragment.

The succession of physical events that results in the breaking of a → molecular cloud into several → fragments.

See also: → fragmentation; → process.

The succession of physical events that results in the breaking of a → molecular cloud into several → fragments.

See also: → fragmentation; → process.

1. A border or case for enclosing a picture, mirror, etc.; a structure for admitting or enclosing something.
   
2. One of the successive pictures on a roll of movie film or videotape.
   
3. In computers, the information or image on a screen or monitor at any one time.

Etymology (EN): Frame, from M.E. verb framen “to prepare (timber),” from O.E. framian “to avail, profit.”; cf. O.H.G. (gi)framon “to do.”

Etymology (PE): 1) Cârcub “frame,” from câr, contraction of cahâr “four” (→ four) + cub “stick, satff, beam,” Mid.Pers. côp “wood, stick.” 2) Tasvirak from Ar. tasvir “image” + -ak
suffix of relation and similarity (as in poštak, dastak, nâxonak), → fibril.

1. A border or case for enclosing a picture, mirror, etc.; a structure for admitting or enclosing something.
   
2. One of the successive pictures on a roll of movie film or videotape.
   
3. In computers, the information or image on a screen or monitor at any one time.

Etymology (EN): Frame, from M.E. verb framen “to prepare (timber),” from O.E. framian “to avail, profit.”; cf. O.H.G. (gi)framon “to do.”

Etymology (PE): 1) Cârcub “frame,” from câr, contraction of cahâr “four” (→ four) + cub “stick, satff, beam,” Mid.Pers. côp “wood, stick.” 2) Tasvirak from Ar. tasvir “image” + -ak
suffix of relation and similarity (as in poštak, dastak, nâxonak), → fibril.

The alteration in the → free fall motion of a test → mass in the presence of a massive → rotating object, as compared to the identical case of a non-rotating object. This dragging of → inertial frames is predicted by → general relativity. Also called → Lense-Thirring effect.

See also: → frame; → drag.

The alteration in the → free fall motion of a test → mass in the presence of a massive → rotating object, as compared to the identical case of a non-rotating object. This dragging of → inertial frames is predicted by → general relativity. Also called → Lense-Thirring effect.

See also: → frame; → drag.

The number of times per second that the frame is scanned in television. Also known as picture frequency.

See also: → frame; → frequency.

The number of times per second that the frame is scanned in television. Also known as picture frequency.

See also: → frame; → frequency.

A set of axes to which positions and motions in a system can be referred.

See also: → frame; → reference.

A set of axes to which positions and motions in a system can be referred.

See also: → frame; → reference.

The process of adjusting a television picture to a desired position in the direction of progression.

Etymology (EN): From → frame + → -ing.

Etymology (PE): Cârcubeš, verbal noun of cârcubidan, from cârcub→ frame.

The process of adjusting a television picture to a desired position in the direction of progression.

Etymology (EN): From → frame + → -ing.

Etymology (PE): Cârcubeš, verbal noun of cârcubidan, from cârcub→ frame.

An extremely rare radioactive chemical element; symbol Fr. Atomic number 87; atomic weight of most stable isotope 223; melting point about 27°C; boiling point about 677°C. Its most stable isotope (half-life about 22 minutes) occurs naturally, to a very limited extent, in uranium minerals. More than 30 other isotopes of francium are known; some are prepared by bombarding thorium with protons, deuterons, or alpha particles.

See also: From France, where the French physicist Marguerite Perey (1919-1975) discovered it in 1939 in the alpha particle decay of actinium.

An extremely rare radioactive chemical element; symbol Fr. Atomic number 87; atomic weight of most stable isotope 223; melting point about 27°C; boiling point about 677°C. Its most stable isotope (half-life about 22 minutes) occurs naturally, to a very limited extent, in uranium minerals. More than 30 other isotopes of francium are known; some are prepared by bombarding thorium with protons, deuterons, or alpha particles.

See also: From France, where the French physicist Marguerite Perey (1919-1975) discovered it in 1939 in the alpha particle decay of actinium.

One of many absorption lines and bands in the spectrum of the Sun. The most prominent features are labeled with capital letters A to K, starting at the red end. The A and B bands are now known to be caused by absorption in Earth’s atmosphere, while the rest are due to absorption in the Sun’s → photosphere. C and F are now better known as H-alpha and H-beta (→ Balmer series); the → D lines are of sodium, the → H and K lines of calcium, and the G band of neutral iron and the interstellar → CH (methylidine) molecule.

See also: Named after Joseph von Fraunhofer (1787-1826), German optician and physicist, who discovered these lines in 1814; → line.

One of many absorption lines and bands in the spectrum of the Sun. The most prominent features are labeled with capital letters A to K, starting at the red end. The A and B bands are now known to be caused by absorption in Earth’s atmosphere, while the rest are due to absorption in the Sun’s → photosphere. C and F are now better known as H-alpha and H-beta (→ Balmer series); the → D lines are of sodium, the → H and K lines of calcium, and the G band of neutral iron and the interstellar → CH (methylidine) molecule.

See also: Named after Joseph von Fraunhofer (1787-1826), German optician and physicist, who discovered these lines in 1814; → line.

The first modern refracting telescope which had an outstanding quality. It was built in 1824 by Fraunhofer for the Russian Imperial Observatory in Dorpat, now Tartu in Estonia. It had a 23-cm → achromatic lens and a German-type → equatorial mounting driven by a clockwork. Wilhelm Struve (1793-1864) used the refractor to observe many → visual binaries, and attempted to measure the distances of stars through their visual → parallaxes. He also obtaibned accurate values for the diameters of the → Galilean satellites of → Jupiter.

See also: Named after Joseph von Fraunhofer (1787-1826), German optician and physicist;
→ refractor.

The first modern refracting telescope which had an outstanding quality. It was built in 1824 by Fraunhofer for the Russian Imperial Observatory in Dorpat, now Tartu in Estonia. It had a 23-cm → achromatic lens and a German-type → equatorial mounting driven by a clockwork. Wilhelm Struve (1793-1864) used the refractor to observe many → visual binaries, and attempted to measure the distances of stars through their visual → parallaxes. He also obtaibned accurate values for the diameters of the → Galilean satellites of → Jupiter.

See also: Named after Joseph von Fraunhofer (1787-1826), German optician and physicist;
→ refractor.

Not physically bound by something.
→ free atmosphere, → free body, → free charge, → free electron, → free expansion phase, → free fall, → free flow, → free occurrence, → free oscillation, → free radical, → free system, → free-bound emission, → free-fall time, → free-floating object, → free-free emission, → freedom, → bound-free transition, → force-free magnetic field, → Gibbs free energy, → Helmholtz free energy, → mean free path, → universality of free fall.

Etymology (EN): From M.E. fre, O.E. freo “free, exempt from,” also “noble, joyful;” cf. Ger. frei, Du. vrij; ultimately from PIE *prijos “dear, beloved;” cf. Av. frāy- “to satisfy, propitiate,” friθa- “beloved; dear,” friθô.tara- “more beloved,” Mod.Pers. fari “happy, fortunate, blessed; pleasing, good,” Skt. priyá- “beloved, wished for;” Gk. praos “mild, gentle.”

Etymology (PE): Âzâd “free,” from Mid.Pers. âzât “free, noble;” Av. āzāta- “high-born, noble,” from zan- “to bear, give birth to a child, be born,” infinitive zazāite, zāta- “born,” āsna- “innate, natural;” cf. Skt. janati “begets, bears;” Gk. gignesthai “to become, happen,” genes “born;” L. gignere “to beget;” PIE base *gen- “to give birth, beget.”

Not physically bound by something.
→ free atmosphere, → free body, → free charge, → free electron, → free expansion phase, → free fall, → free flow, → free occurrence, → free oscillation, → free radical, → free system, → free-bound emission, → free-fall time, → free-floating object, → free-free emission, → freedom, → bound-free transition, → force-free magnetic field, → Gibbs free energy, → Helmholtz free energy, → mean free path, → universality of free fall.

Etymology (EN): From M.E. fre, O.E. freo “free, exempt from,” also “noble, joyful;” cf. Ger. frei, Du. vrij; ultimately from PIE *prijos “dear, beloved;” cf. Av. frāy- “to satisfy, propitiate,” friθa- “beloved; dear,” friθô.tara- “more beloved,” Mod.Pers. fari “happy, fortunate, blessed; pleasing, good,” Skt. priyá- “beloved, wished for;” Gk. praos “mild, gentle.”

Etymology (PE): Âzâd “free,” from Mid.Pers. âzât “free, noble;” Av. āzāta- “high-born, noble,” from zan- “to bear, give birth to a child, be born,” infinitive zazāite, zāta- “born,” āsna- “innate, natural;” cf. Skt. janati “begets, bears;” Gk. gignesthai “to become, happen,” genes “born;” L. gignere “to beget;” PIE base *gen- “to give birth, beget.”

That part of the atmosphere where the effects of the ground on the → turbulence conditions are negligible.

See also: → free, → atmosphere.

That part of the atmosphere where the effects of the ground on the → turbulence conditions are negligible.

See also: → free, → atmosphere.

A → rigid body not constrained with other bodies and which from any given position can be displaced in any direction in space. Opposite of → constrained body.

See also: → free; → body.

A → rigid body not constrained with other bodies and which from any given position can be displaced in any direction in space. Opposite of → constrained body.

See also: → free; → body.

An electric charge which is not held by another charge, in contrast to a → bound charge.

See also: → free; → charge.

An electric charge which is not held by another charge, in contrast to a → bound charge.

See also: → free; → charge.

An electron that is not attached to an → atom, → molecule, or → ion and is free to move under the influence of a present electric or magnetic field.

See also: → free; → electron.

An electron that is not attached to an → atom, → molecule, or → ion and is free to move under the influence of a present electric or magnetic field.

See also: → free; → electron.

The first phase of → supernova remnant (SNR) evolution in which the surrounding → interstellar medium (ISM) has no influence on the expansion of the → shock wave, and the pressure of the interstellar gas is negligible. The shock wave created by the → supernova explosion moves outward into the
interstellar gas at highly → supersonic speed. Assuming that most of the → supernova energy  E_SN is transformed into → kinetic energy of the ejected gas, the ejection velocity
v_e can be estimated from E_SN by using

E_SN = (1/2) M_ev_e², which leads to
v_e = (2E_SN / M_e)^(1/2),
where M_e is the ejected mass.

The schematic structure of the SNR at this phase can be described as follows: behind the strong → shock front which moves outward into the ISM, compressed interstellar gas accumulates forming a → shell of interstellar gas.
This shell of swept-up material in front of shock does not represent a significant increase in the mass of the system. After some time the accumulated mass equals the ejected mass of stellar material, and it will start to affect the expansion of the SNR. By definition, this is the end of the free expansion phase, and the corresponding radius of the SNR, called → sweep-up radius,

R_SW, is defined by

M_e = (4π/3) R_SW³ρ₀, that is R_SW = (3M_e / 4πρ₀)^(1/3),

where ρ₀ is the initial density of the ISM.
This radius is reached at the sweep-up time t_SW = R_SW/v_e.
The free expansion phase lasts some 100-200 years until the mass of the material swept up by the shock wave exceeds the mass of the ejected material. Then the following → snowplow phase starts.

See also: → free; → expansion; → phase.

The first phase of → supernova remnant (SNR) evolution in which the surrounding → interstellar medium (ISM) has no influence on the expansion of the → shock wave, and the pressure of the interstellar gas is negligible. The shock wave created by the → supernova explosion moves outward into the
interstellar gas at highly → supersonic speed. Assuming that most of the → supernova energy  E_SN is transformed into → kinetic energy of the ejected gas, the ejection velocity
v_e can be estimated from E_SN by using

E_SN = (1/2) M_ev_e², which leads to
v_e = (2E_SN / M_e)^(1/2),
where M_e is the ejected mass.

The schematic structure of the SNR at this phase can be described as follows: behind the strong → shock front which moves outward into the ISM, compressed interstellar gas accumulates forming a → shell of interstellar gas.
This shell of swept-up material in front of shock does not represent a significant increase in the mass of the system. After some time the accumulated mass equals the ejected mass of stellar material, and it will start to affect the expansion of the SNR. By definition, this is the end of the free expansion phase, and the corresponding radius of the SNR, called → sweep-up radius,

R_SW, is defined by

M_e = (4π/3) R_SW³ρ₀, that is R_SW = (3M_e / 4πρ₀)^(1/3),

where ρ₀ is the initial density of the ISM.
This radius is reached at the sweep-up time t_SW = R_SW/v_e.
The free expansion phase lasts some 100-200 years until the mass of the material swept up by the shock wave exceeds the mass of the ejected material. Then the following → snowplow phase starts.

See also: → free; → expansion; → phase.

The motion of a body under the influence of → gravity alone. See also → free-fall time.

See also: → free; → fall.

The motion of a body under the influence of → gravity alone. See also → free-fall time.

See also: → free; → fall.

A fluid flow which develops when density differences within the fluid are the only driving forces. See also → forced flow.

See also: → free; → flow.

A fluid flow which develops when density differences within the fluid are the only driving forces. See also → forced flow.

See also: → free; → flow.

An → occurrence of a → variable in a → wff, → iff it is not a → bound occurrence.

See also: → bound; → occurrence.

An → occurrence of a → variable in a → wff, → iff it is not a → bound occurrence.

See also: → bound; → occurrence.

Oscillation of any system in stable equilibrium under the influence of internal forces only, or of a constant force originating outside the system, or of both.

See also: → free; → oscillation.

Oscillation of any system in stable equilibrium under the influence of internal forces only, or of a constant force originating outside the system, or of both.

See also: → free; → oscillation.

A chemical radical that can exist independently from atoms or group of atoms.

See also: → free; → radical.

A chemical radical that can exist independently from atoms or group of atoms.

See also: → free; → radical.

A → mechanical system if all of its constituent particles or bodies can occupy arbitrary points in space or have arbitrary velocities. Otherwise, it is called a → constrained system.

See also: → free; → system.

A → mechanical system if all of its constituent particles or bodies can occupy arbitrary points in space or have arbitrary velocities. Otherwise, it is called a → constrained system.

See also: → free; → system.

The radiation emitted when a → free electron is captured by an → ion. See also:
→ free-free emission; → bound-free transition.

See also: → free; → bound; → emission.

The radiation emitted when a → free electron is captured by an → ion. See also:
→ free-free emission; → bound-free transition.

See also: → free; → bound; → emission.

The characteristic time it would take a body to collapse under its own → gravitational attraction, if no other forces existed to oppose the collapse. It is given by: t_ff = (3π/32 ρ₀ G)^1/2,
where ρ₀ denotes the initial density and G the → gravitational constant. Free-fall time is independent of the starting radius. Also known as → dynamical time scale.

See also: → free fall; → time.

The characteristic time it would take a body to collapse under its own → gravitational attraction, if no other forces existed to oppose the collapse. It is given by: t_ff = (3π/32 ρ₀ G)^1/2,
where ρ₀ denotes the initial density and G the → gravitational constant. Free-fall time is independent of the starting radius. Also known as → dynamical time scale.

See also: → free fall; → time.

A population of → substellar objects which are not bound to stars; they are detected in young star clusters. Their masses, estimated from their fluxes, is several Jupiter masses, lower than those of → brown dwarfs. Their formation is not yet explained. Among the envisaged possibilities: 1) These objects form like stars, from protostellar core collapse and subsequent accretion; 2) they form as low-mass members of small groups, and are ejected from the group; 3) they form like planets within circumstellar disks of higher-mass objects, but are ejected either due to internal dynamics or external interactions.

Etymology (EN): → free; floating, from M.E. float, from O.E. flotian “to float” (cf. O.N. flota, M.Du. vloten); → object.

Etymology (PE): Barâxt, → object; šenâvar “that swims, floats,” from šenâ “swimming;” Mid.Pers. šnâz “swim,” šnâzidan “to swim;” Av. snā- “to wash, swim;” cf. Skt. snā- “to bathe, to wash;” L. nare, natare “to swim” (Fr. nage, nager, natation; Sp.nadar, natacion).

A population of → substellar objects which are not bound to stars; they are detected in young star clusters. Their masses, estimated from their fluxes, is several Jupiter masses, lower than those of → brown dwarfs. Their formation is not yet explained. Among the envisaged possibilities: 1) These objects form like stars, from protostellar core collapse and subsequent accretion; 2) they form as low-mass members of small groups, and are ejected from the group; 3) they form like planets within circumstellar disks of higher-mass objects, but are ejected either due to internal dynamics or external interactions.

Etymology (EN): → free; floating, from M.E. float, from O.E. flotian “to float” (cf. O.N. flota, M.Du. vloten); → object.

Etymology (PE): Barâxt, → object; šenâvar “that swims, floats,” from šenâ “swimming;” Mid.Pers. šnâz “swim,” šnâzidan “to swim;” Av. snā- “to wash, swim;” cf. Skt. snā- “to bathe, to wash;” L. nare, natare “to swim” (Fr. nage, nager, natation; Sp.nadar, natacion).

→ Electromagnetic radiation produced in a → plasma by → free electrons scattering off → ions
without being captured. The electrons are free before the interaction and remain free afterward.

See also: → free; → emission.

→ Electromagnetic radiation produced in a → plasma by → free electrons scattering off → ions
without being captured. The electrons are free before the interaction and remain free afterward.

See also: → free; → emission.

The state of being free or at liberty.

Etymology (EN): From M.E. fredom, from O.E. freodom, from
freo “free; noble, joyful,” → free.

Etymology (PE): Âzâdi “freedom,” noun from âzâd, → free.

The state of being free or at liberty.

Etymology (EN): From M.E. fredom, from O.E. freodom, from
freo “free; noble, joyful,” → free.

Etymology (PE): Âzâdi “freedom,” noun from âzâd, → free.

A statistical finding about “normal” → spiral galaxies,
whereby there is an upper limit on the mean central → surface brightness of disks. This value is constant for different spiral types, amounting to
21.65 ± 0.30 mag arcsec² in the B band.

See also: Named after K. C. Freeman (1970, Ap.J. 160, 811); → law.

A statistical finding about “normal” → spiral galaxies,
whereby there is an upper limit on the mean central → surface brightness of disks. This value is constant for different spiral types, amounting to
21.65 ± 0.30 mag arcsec² in the B band.

See also: Named after K. C. Freeman (1970, Ap.J. 160, 811); → law.

1. To change from the liquid to the solid state by loss of heat; become hardened into ice.
   
2. Become hardened into a solid body. → coagulate.

Etymology (EN): Freeze, from O.E. freosan “turn to ice,” from P.Gmc. *freusanan (cf. O.H.G. friosan, Ger. frieren “to freeze”), from *freus-, from PIE base *preus- “to freeze” also “to burn” (cf. Skt. pruśva- “hoar-frost, ice;” L. pruina “hoar-frost,” Skt. pruśta- “burnt;” Albanian prus “burning coals;” L. pruna “a live coal”).

Etymology (PE): 1) Yax bastan, from yax “ice” + bastan “to bind, shut; to congeal, coagulate.” The first component yax, from Av. aexa- “ice, frost,” isav-, isu- “icy, chilly,” cf. Sarikoli (Pamir dialect) īš “cold,”
P.Gmc. *isa- (O.N. iss, O.Fris. is, Du. ijs, Ger. Eis). The second component bastan, from Mid.Pers. bastan/vastan “to bind, shut;” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie” (cf.
Skt. bandh- “to bind, tie, fasten;” PIE *bhendh- “to bind;” Ger. binden; E. bind). 2) Rocidan, → coagulate.

1. To change from the liquid to the solid state by loss of heat; become hardened into ice.
   
2. Become hardened into a solid body. → coagulate.

Etymology (EN): Freeze, from O.E. freosan “turn to ice,” from P.Gmc. *freusanan (cf. O.H.G. friosan, Ger. frieren “to freeze”), from *freus-, from PIE base *preus- “to freeze” also “to burn” (cf. Skt. pruśva- “hoar-frost, ice;” L. pruina “hoar-frost,” Skt. pruśta- “burnt;” Albanian prus “burning coals;” L. pruna “a live coal”).

Etymology (PE): 1) Yax bastan, from yax “ice” + bastan “to bind, shut; to congeal, coagulate.” The first component yax, from Av. aexa- “ice, frost,” isav-, isu- “icy, chilly,” cf. Sarikoli (Pamir dialect) īš “cold,”
P.Gmc. *isa- (O.N. iss, O.Fris. is, Du. ijs, Ger. Eis). The second component bastan, from Mid.Pers. bastan/vastan “to bind, shut;” Av./O.Pers. band- “to bind, fetter,” banda- “band, tie” (cf.
Skt. bandh- “to bind, tie, fasten;” PIE *bhendh- “to bind;” Ger. binden; E. bind). 2) Rocidan, → coagulate.

1. The phase transition of a substance passing from the liquid to the solid state; the opposite of → fusion. In meteorology, the freezing of water.
   
2. Solidification.

See also: Verbal noun from → freeze.

1. The phase transition of a substance passing from the liquid to the solid state; the opposite of → fusion. In meteorology, the freezing of water.
   
2. Solidification.

See also: Verbal noun from → freeze.

1. The temperature at which a liquid of specified composition changes into a solid under a specified pressure.
   
2. The temperature at which the liquid and solid phases of a substance of specified composition coexist in equilibrium.

See also: → freezing; → point.

1. The temperature at which a liquid of specified composition changes into a solid under a specified pressure.
   
2. The temperature at which the liquid and solid phases of a substance of specified composition coexist in equilibrium.

See also: → freezing; → point.

A calendar composed by Fabre d’Eglantine and others during the French Revolution which divided the year into 12 months of 30 days each, with five odd days called → Sansculottides. The year started at → autumnal equinox and the months were: Vendémiaire (Vintage), Brumaire (Fog), Frimaire (Frost), Nivôse (Snow), Pluviôse (Rain), Ventôse (Wind), Germinal (Buds), Floréal (Flowers), Prairial (Meadows), Messidor (Harvest), Termidor (Heat), Fructidor (Fruits). The week consisted of 10 days, and was called a Décade; each 10th day of Décade (called Décadi) was a day of rest. The calendar was used by the French government for about 12 years, from late 1793 to 1805, when it was suppressed by Napoleon.

See also: M.E. Frensh, French, O.E. Frencisc “of the Franks,” from Frank; republican, from republic, from Fr. république, from L. respublica, from res publica “public interest, the state,” from res “affair, matter, thing” + publica, feminine of publicus “public;” → calendar.

A calendar composed by Fabre d’Eglantine and others during the French Revolution which divided the year into 12 months of 30 days each, with five odd days called → Sansculottides. The year started at → autumnal equinox and the months were: Vendémiaire (Vintage), Brumaire (Fog), Frimaire (Frost), Nivôse (Snow), Pluviôse (Rain), Ventôse (Wind), Germinal (Buds), Floréal (Flowers), Prairial (Meadows), Messidor (Harvest), Termidor (Heat), Fructidor (Fruits). The week consisted of 10 days, and was called a Décade; each 10th day of Décade (called Décadi) was a day of rest. The calendar was used by the French government for about 12 years, from late 1793 to 1805, when it was suppressed by Napoleon.

See also: M.E. Frensh, French, O.E. Frencisc “of the Franks,” from Frank; republican, from republic, from Fr. république, from L. respublica, from res publica “public interest, the state,” from res “affair, matter, thing” + publica, feminine of publicus “public;” → calendar.

The number of complete oscillations per unit time of a vibrating system. The reciprocal of the → period, T.

Etymology (EN): From L. frequentia “assembly, multitude, crowd.”

Etymology (PE): Basâmad, from bas “many, much” (Mid.Pers. vas “many, much;” O.Pers. vasiy “at will, greatly, utterly;” Av. varəmi “I wish,” vasô, vasə “at one’s pleasure or will,” from vas- “to will, desire, wish”)

• âmad past stem of âmadan “to occur, to come, to become” (Mid.Pers. âmatan;
  O.Pers. gam- “to come; to go,” Av. gam- “to come; to go,” jamaiti “goes;” Proto-Iranian *āgmatani; Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE root *gwem- “to go, come”).
  Feregi, from feré “much, more; increase; priority,” farâ(vân) “abundant,” Mid.Pers. frêh “more, much;” O.Pers. fra- “before, forth;”
  Av. frā, fərrā “forth, forward;” PIE base *pro-; cf. L. pro “on behalf of, in place of, before, for;” Gk. pro- “before, in front of.”

The number of complete oscillations per unit time of a vibrating system. The reciprocal of the → period, T.

Etymology (EN): From L. frequentia “assembly, multitude, crowd.”

Etymology (PE): Basâmad, from bas “many, much” (Mid.Pers. vas “many, much;” O.Pers. vasiy “at will, greatly, utterly;” Av. varəmi “I wish,” vasô, vasə “at one’s pleasure or will,” from vas- “to will, desire, wish”)

• âmad past stem of âmadan “to occur, to come, to become” (Mid.Pers. âmatan;
  O.Pers. gam- “to come; to go,” Av. gam- “to come; to go,” jamaiti “goes;” Proto-Iranian *āgmatani; Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE root *gwem- “to go, come”).
  Feregi, from feré “much, more; increase; priority,” farâ(vân) “abundant,” Mid.Pers. frêh “more, much;” O.Pers. fra- “before, forth;”
  Av. frā, fərrā “forth, forward;” PIE base *pro-; cf. L. pro “on behalf of, in place of, before, for;” Gk. pro- “before, in front of.”

A range of frequencies that is continuous between two specified limits, selected from a more extended range of frequencies.

See also: → frequency; → band.

A range of frequencies that is continuous between two specified limits, selected from a more extended range of frequencies.

See also: → frequency; → band.

An undesired progressive change in an oscillator’s frequency with time.

See also: → frequency; → drift.

An undesired progressive change in an oscillator’s frequency with time.

See also: → frequency; → drift.

The change in the frequency of a wave motion due to the → Doppler effect.

See also: → frequency; → shift.

The change in the frequency of a wave motion due to the → Doppler effect.

See also: → frequency; → shift.

A graphical display of the intensity of radiation or energy versus frequency.

See also: → frequency; → spectrum.

A graphical display of the intensity of radiation or energy versus frequency.

See also: → frequency; → spectrum.

A mode of observation in radio astronomy in which the telescope remains at a fixed position and data is accumulated while the center of the receiver passband is switched between signal and offset frequencies. Data accumulated at the offset frequency is subtracted from the signal frequency data before storage as a frequency switched scan. → beam switching.

See also: → frequency; → switching.

A mode of observation in radio astronomy in which the telescope remains at a fixed position and data is accumulated while the center of the receiver passband is switched between signal and offset frequencies. Data accumulated at the offset frequency is subtracted from the signal frequency data before storage as a frequency switched scan. → beam switching.

See also: → frequency; → switching.

Deriving the → wavelength of an undulatory phenomenon from its → frequency, and vice versa.

1. For → electromagnetic waves: λ = c / f, where λ is the wavelength, c is the → speed of light in → meters per second
   and f the frequency in → hertz. It can be written as: λ (m) = 2.998 × 10⁸ / f (Hz).
   

2. For → sound waves: λ = C / f, where C is the → sound speed. For air at temperature 0°C, λ (m) = 332 / f (Hz).

See also: → frequency; → wavelength; → conversion.

Deriving the → wavelength of an undulatory phenomenon from its → frequency, and vice versa.

1. For → electromagnetic waves: λ = c / f, where λ is the wavelength, c is the → speed of light in → meters per second
   and f the frequency in → hertz. It can be written as: λ (m) = 2.998 × 10⁸ / f (Hz).
   

2. For → sound waves: λ = C / f, where C is the → sound speed. For air at temperature 0°C, λ (m) = 332 / f (Hz).

See also: → frequency; → wavelength; → conversion.

The diffraction effects obtained when either the source of light or observing screen, or both, are at a finite distance from diffracting aperture or obstacle. → Fraunhofer diffraction.

See also: Named after Jean Augustin Fresnel (1788-1827), French physicist, a key figure in establishing the wave theory of light. His earlier work on interference was carried out in ignorance of that of Thomas Young (1773-1829), English physician and physicist, but later they corresponded and were allies; → diffraction.

The diffraction effects obtained when either the source of light or observing screen, or both, are at a finite distance from diffracting aperture or obstacle. → Fraunhofer diffraction.

See also: Named after Jean Augustin Fresnel (1788-1827), French physicist, a key figure in establishing the wave theory of light. His earlier work on interference was carried out in ignorance of that of Thomas Young (1773-1829), English physician and physicist, but later they corresponded and were allies; → diffraction.

For an electromagnetic wave incident upon the interface between two media with different indices of refraction, one of a set of equations that give the → reflection coefficient and → transmission coefficient at the optical interface.
These coefficients depend on the polarization degree of the incident wave.

See also: → Fresnel diffraction; → equation.

For an electromagnetic wave incident upon the interface between two media with different indices of refraction, one of a set of equations that give the → reflection coefficient and → transmission coefficient at the optical interface.
These coefficients depend on the polarization degree of the incident wave.

See also: → Fresnel diffraction; → equation.

Two integrals that involve quadratic equations in the sine and cosine functions and are defined as:

C(x) = ∫ cos (πt²/2) dt and

C(y) = ∫ sin (πt²/2) dt, integrated from 0 to x.

They are quite frequently used in optics studying → Fresnel diffraction and similar topics. The Fresnel integrals are also used in railway and freeway constructions.
These integrals may be evaluated to arbitrary precision using → power series. Alternatively the amplitudes may be found graphically by use of → Cornu’s spiral.

See also: → Fresnel diffraction; → integral.

Two integrals that involve quadratic equations in the sine and cosine functions and are defined as:

C(x) = ∫ cos (πt²/2) dt and

C(y) = ∫ sin (πt²/2) dt, integrated from 0 to x.

They are quite frequently used in optics studying → Fresnel diffraction and similar topics. The Fresnel integrals are also used in railway and freeway constructions.
These integrals may be evaluated to arbitrary precision using → power series. Alternatively the amplitudes may be found graphically by use of → Cornu’s spiral.

See also: → Fresnel diffraction; → integral.

An optical lens composed of a series of rings of glass so curved that they all have the same focus. It is flat on one side and ridged on the other making it possible that nearly every ray of light from the source be re-directed out on a horizontal path.
This design enables the construction of lenses of large aperture and short focal length avoiding thus the large weights and volumes of material which would be required in conventional lenses. The first Fresnel lens, designed for use in a lighthouse on the river Gironde,
was installed in France in 1823, and by the 1850s many examples were in use everywhere. Fresnel lenses are most often used in light gathering applications, such as condenser systems or emitter/detector setups. They can also be used as magnifiers and projection lenses.
Nowadays, Fresnel lenses made of optical plastics are widely used for various applications.

See also: → Fresnel diffraction; → lens.

An optical lens composed of a series of rings of glass so curved that they all have the same focus. It is flat on one side and ridged on the other making it possible that nearly every ray of light from the source be re-directed out on a horizontal path.
This design enables the construction of lenses of large aperture and short focal length avoiding thus the large weights and volumes of material which would be required in conventional lenses. The first Fresnel lens, designed for use in a lighthouse on the river Gironde,
was installed in France in 1823, and by the 1850s many examples were in use everywhere. Fresnel lenses are most often used in light gathering applications, such as condenser systems or emitter/detector setups. They can also be used as magnifiers and projection lenses.
Nowadays, Fresnel lenses made of optical plastics are widely used for various applications.

See also: → Fresnel diffraction; → lens.

A pair of plane mirrors which are slightly inclined to one another. It is used for producing two coherent images in interference experiments.

See also: → Fresnel diffraction; → mirror.

A pair of plane mirrors which are slightly inclined to one another. It is used for producing two coherent images in interference experiments.

See also: → Fresnel diffraction; → mirror.

A piece of special glass in the form of an oblique → parallelepiped so cut that a ray of light entering one of its faces at right angles shall emerge at right angles at the opposite face, after undergoing two internal reflections. It is a type of → quarter-wave retarder used to produce a → circularly polarized light from a → plane polarized light, or the reverse.

See also: → Fresnel diffraction; → rhombus.

A piece of special glass in the form of an oblique → parallelepiped so cut that a ray of light entering one of its faces at right angles shall emerge at right angles at the opposite face, after undergoing two internal reflections. It is a type of → quarter-wave retarder used to produce a → circularly polarized light from a → plane polarized light, or the reverse.

See also: → Fresnel diffraction; → rhombus.

An optical element consisting of two small angle → prisms,
joined together at their bases, used to produce two
→ coherent sources. The thin double prism refracts the light from a source into two overlapping beams, which produce
→ interference fringes. With this experiment Fresnel was able to produce interference without relying upon → diffraction to bring the interfering beams together.

See also: → Fresnel diffraction; → bi-; → prism.

An optical element consisting of two small angle → prisms,
joined together at their bases, used to produce two
→ coherent sources. The thin double prism refracts the light from a source into two overlapping beams, which produce
→ interference fringes. With this experiment Fresnel was able to produce interference without relying upon → diffraction to bring the interfering beams together.

See also: → Fresnel diffraction; → bi-; → prism.

Two plane mirrors, fitted side by side at a small angle, used to create two mutually → coherent sources in a famous → interference experiment first suggested by A. Fresnel. A point source reflected at the mirrors appears as a pair of → virtual light sources, positioned close together, which interfere with each other due to their → coherence. This arrangement removes the problem that two separate light sources do not produce observable interference on account of their incoherence. Same as Fresnel’s double mirror. See also → Fresnel’s biprism, → Lloyd’s mirror.

See also: → Fresnel diffraction; → mirror.

Two plane mirrors, fitted side by side at a small angle, used to create two mutually → coherent sources in a famous → interference experiment first suggested by A. Fresnel. A point source reflected at the mirrors appears as a pair of → virtual light sources, positioned close together, which interfere with each other due to their → coherence. This arrangement removes the problem that two separate light sources do not produce observable interference on account of their incoherence. Same as Fresnel’s double mirror. See also → Fresnel’s biprism, → Lloyd’s mirror.

See also: → Fresnel diffraction; → mirror.

The resisting force offered by one body to the relative motion of another body in contact with the first.

Etymology (EN): From L. frictionem “a rubbing, rubbing down,” from fricare “to rub.”

Etymology (PE): Mâleš, verbal noun of mâlidan “to rub,” from, variants
parmâs “contact, touching,” marz “frontier, border, boundary,” Mid.Pers. mâlitan, muštan “to rub, sweep;” Av. marəz- “to rub, wipe,” marəza- “border, district;” PIE base *merg- “boundary, border;” cf. L. margo “edge” (Fr. marge “margin”); P.Gmc. *marko;
Ger. Mark; E. mark, margin.

The resisting force offered by one body to the relative motion of another body in contact with the first.

Etymology (EN): From L. frictionem “a rubbing, rubbing down,” from fricare “to rub.”

Etymology (PE): Mâleš, verbal noun of mâlidan “to rub,” from, variants
parmâs “contact, touching,” marz “frontier, border, boundary,” Mid.Pers. mâlitan, muštan “to rub, sweep;” Av. marəz- “to rub, wipe,” marəza- “border, district;” PIE base *merg- “boundary, border;” cf. L. margo “edge” (Fr. marge “margin”); P.Gmc. *marko;
Ger. Mark; E. mark, margin.

One of the parameters that characterize atmospheric → seeing. It is the diameter of the largest aperture that can be used before → turbulence starts to degrade the image quality. As the turbulence gets stronger, the Fried parameter, denoted r₀, becomes smaller. The Fried parameter is wavelength dependent:
r₀ ∝ λ^6/5.
On best astronomical mountain tops it ranges between 20 and 30 cm for λ = 5000 A.

See also: Named after David L. Fried, who defined the parameter 10 1966; → parameter.

One of the parameters that characterize atmospheric → seeing. It is the diameter of the largest aperture that can be used before → turbulence starts to degrade the image quality. As the turbulence gets stronger, the Fried parameter, denoted r₀, becomes smaller. The Fried parameter is wavelength dependent:
r₀ ∝ λ^6/5.
On best astronomical mountain tops it ranges between 20 and 30 cm for λ = 5000 A.

See also: Named after David L. Fried, who defined the parameter 10 1966; → parameter.

An equation that expresses energy conservation in an → expanding Universe. It is formally derived from → Einstein’s field equations of → general relativity by requiring the Universe to be everywhere → homogeneous and → isotropic. It is expressed by

H²(t) = (8πG)/(3c²)ε(t) - (kc²)/R²(t), where H(t) is the → Hubble parameter, G is the → gravitational constant, c is the → speed of light, ε(t) is the → energy density, k is the → curvature of space-time, and R(t) is the → cosmic scale factor. See also → Big Bang, → accelerating Universe. See also → Friedmann-Lemaitre Universe.

See also: Named after the Russian mathematician and physical scientist Aleksandr Aleksandrovich Friedmann (1888-1925), who was the first to formulate an → expanding Universe based on Einstein’s theory of → general relativity ; → equation.

An equation that expresses energy conservation in an → expanding Universe. It is formally derived from → Einstein’s field equations of → general relativity by requiring the Universe to be everywhere → homogeneous and → isotropic. It is expressed by

H²(t) = (8πG)/(3c²)ε(t) - (kc²)/R²(t), where H(t) is the → Hubble parameter, G is the → gravitational constant, c is the → speed of light, ε(t) is the → energy density, k is the → curvature of space-time, and R(t) is the → cosmic scale factor. See also → Big Bang, → accelerating Universe. See also → Friedmann-Lemaitre Universe.

See also: Named after the Russian mathematician and physical scientist Aleksandr Aleksandrovich Friedmann (1888-1925), who was the first to formulate an → expanding Universe based on Einstein’s theory of → general relativity ; → equation.

One of the first → cosmological models to incorporate Einstein’s → general relativity, predicting that → galaxies should be → receding from each other due to → cosmic expansion.

See also: → Friedmann equation; Georges Edouard Lemaître (1894-1966), a
Belgian cosmologist and priest who proposed a first sketch of the → Big Bang theory; → universe.

One of the first → cosmological models to incorporate Einstein’s → general relativity, predicting that → galaxies should be → receding from each other due to → cosmic expansion.

See also: → Friedmann equation; Georges Edouard Lemaître (1894-1966), a
Belgian cosmologist and priest who proposed a first sketch of the → Big Bang theory; → universe.

Very cold in temperature.

Etymology (EN): From Latin frigidus “cold, chill, cool,” from stem of frigere “be cold;” related to noun frigus “cold, coldness, frost,” from PIE root *srig- “cold;” cf. Gk. rhigos “cold, frost.”

Etymology (PE): Sajan “very cold,” variants šaja,, sajâm, šajad,
Oss. I. syjyn/syd, D. sujun/sud “to freeze,” Yaghnobi ši-, Yazghulami šed/šiy- “to freeze,” Shughni
šitô , Sariqoli š(i)tu “cold, ice.” Proto-Ir. *saiH-/siH- “to freeze.” Skt. syā- “to freeze, coagulate, become rigid.”

Very cold in temperature.

Etymology (EN): From Latin frigidus “cold, chill, cool,” from stem of frigere “be cold;” related to noun frigus “cold, coldness, frost,” from PIE root *srig- “cold;” cf. Gk. rhigos “cold, frost.”

Etymology (PE): Sajan “very cold,” variants šaja,, sajâm, šajad,
Oss. I. syjyn/syd, D. sujun/sud “to freeze,” Yaghnobi ši-, Yazghulami šed/šiy- “to freeze,” Shughni
šitô , Sariqoli š(i)tu “cold, ice.” Proto-Ir. *saiH-/siH- “to freeze.” Skt. syā- “to freeze, coagulate, become rigid.”

1. One of the alternating bright or dark bands produced by → interference or → diffraction.
   

2. Wavy patterns due to the layered structure of → CCDs. These interference effects are prominent when emission lines such as the night-sky emissions are present.

Etymology (EN): From M.E. frenge, from O.Fr. frange, from V.L. *frimbia, metathesis of L. fimbriæ “fibers, threads, fringe,” of uncertain origin.

Etymology (PE): Fariz, contraction of farâviz “fringe, lace, edging,” from far-, par-, variant 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”)

• âviz “anything suspended; a place where things are hung up; a border, margin,” from âvixtan, âvizidan “to hang, suspend;” Mid.Pers. âwextan “to hang;” Av. vij- “to shake, swing,” frauuaēγa- “swinging forward;” cf. Skt. vej- “to dart up or back, move up;” Proto-Iranian *uij- “to shake, swing.”

1. One of the alternating bright or dark bands produced by → interference or → diffraction.
   

2. Wavy patterns due to the layered structure of → CCDs. These interference effects are prominent when emission lines such as the night-sky emissions are present.

Etymology (EN): From M.E. frenge, from O.Fr. frange, from V.L. *frimbia, metathesis of L. fimbriæ “fibers, threads, fringe,” of uncertain origin.

Etymology (PE): Fariz, contraction of farâviz “fringe, lace, edging,” from far-, par-, variant 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”)

• âviz “anything suspended; a place where things are hung up; a border, margin,” from âvixtan, âvizidan “to hang, suspend;” Mid.Pers. âwextan “to hang;” Av. vij- “to shake, swing,” frauuaēγa- “swinging forward;” cf. Skt. vej- “to dart up or back, move up;” Proto-Iranian *uij- “to shake, swing.”

Optics: If the intensity in an interference fringe pattern has the maximum and minimum values I_max and I_min, the visibility is defined by the relation ν = (I_max - I_min) / (I_max

• I_min), where 0 ≤ ν ≤ 1. In terms of the intensities of the two interfering waves: ν = 2(I₁ . I₂)^1/2 / (I₁ + I₂).

See also: → fringe; → visibility

Optics: If the intensity in an interference fringe pattern has the maximum and minimum values I_max and I_min, the visibility is defined by the relation ν = (I_max - I_min) / (I_max

• I_min), where 0 ≤ ν ≤ 1. In terms of the intensities of the two interfering waves: ν = 2(I₁ . I₂)^1/2 / (I₁ + I₂).

See also: → fringe; → visibility

A tailless amphibian with a short squat body, moist smooth skin, a large head, and very long hind legs for leaping.

Etymology (PE): Qurbâqe “frog,” prefixed bâq, variants Tabari, Aftari vak, Tabari vag, Lori, Laki qorvâ, korvâx, Kurd. baq, Zâzâ baqa; Mid.Pers. vazak, vak; Av. vazaγa- “frog.”

A tailless amphibian with a short squat body, moist smooth skin, a large head, and very long hind legs for leaping.

Etymology (PE): Qurbâqe “frog,” prefixed bâq, variants Tabari, Aftari vak, Tabari vag, Lori, Laki qorvâ, korvâx, Kurd. baq, Zâzâ baqa; Mid.Pers. vazak, vak; Av. vazaγa- “frog.”

1. The part or side of anything that faces forward. → ionization front.
   

2. Meteo.: A narrow zone of transition between air masses of contrasting density, that is, air masses of different temperature or different water vapor concentration or both.
   

3. The side of the → planispheric astrolabe that displays the → limb of the → mater, the → tympanum, the → rete, and, in some models, the → rule. By setting the front, i.e., by rotating the rete around the mater, one can depict the appearance of the heavens as determined by observation in order to obtain a time value from the instrument. Alternatively, by configuring the rete for a given day, one can perform several astronomical computations such as the rising, culmination, and setting of the Sun, Moon, planets, and stars (online museo galileo, VirtualMuseum).

Etymology (EN): From O.Fr. front “forehead, brow,” from L. frontem “forehead,” perhaps lit. “that which projects,” from PIE *bhront-, from base *bhren- “to project, stand out.”

Etymology (PE): Pišân, from pišâni “front, forehead,” from piš “before; in front,” from Mid.Pers. pêš “before, earlier;” O.Pers. paišiya “before; in the presence of” + -ân suffix of place and time.
Ru “face,” → surface.

1. The part or side of anything that faces forward. → ionization front.
   

2. Meteo.: A narrow zone of transition between air masses of contrasting density, that is, air masses of different temperature or different water vapor concentration or both.
   

3. The side of the → planispheric astrolabe that displays the → limb of the → mater, the → tympanum, the → rete, and, in some models, the → rule. By setting the front, i.e., by rotating the rete around the mater, one can depict the appearance of the heavens as determined by observation in order to obtain a time value from the instrument. Alternatively, by configuring the rete for a given day, one can perform several astronomical computations such as the rising, culmination, and setting of the Sun, Moon, planets, and stars (online museo galileo, VirtualMuseum).

Etymology (EN): From O.Fr. front “forehead, brow,” from L. frontem “forehead,” perhaps lit. “that which projects,” from PIE *bhront-, from base *bhren- “to project, stand out.”

Etymology (PE): Pišân, from pišâni “front, forehead,” from piš “before; in front,” from Mid.Pers. pêš “before, earlier;” O.Pers. paišiya “before; in the presence of” + -ân suffix of place and time.
Ru “face,” → surface.

A device containing a radio-frequency amplifier and associated cryogenic systems, routers, and converters (mixers), whose input is the voltage from a receptor and whose output is an intermediate-frequency signal. → back-end.

Etymology (EN): → front + end, from
O.E. ende, from P.Gmc. *andja, originally “the opposite side,” from PIE *antjo “end, boundary,” from base *anta-/*anti- “opposite, in front of, before.”

Etymology (PE): Piš-tah, from piš, → front, + tah “end;” Mid.Pers. tah “bottom.” The origin of this term is not clear. It may be related to Gk. tenagos “bottom, swamp,” Latvian tigas “depth;” PIE *tenegos “water bottom.”

A device containing a radio-frequency amplifier and associated cryogenic systems, routers, and converters (mixers), whose input is the voltage from a receptor and whose output is an intermediate-frequency signal. → back-end.

Etymology (EN): → front + end, from
O.E. ende, from P.Gmc. *andja, originally “the opposite side,” from PIE *antjo “end, boundary,” from base *anta-/*anti- “opposite, in front of, before.”

Etymology (PE): Piš-tah, from piš, → front, + tah “end;” Mid.Pers. tah “bottom.” The origin of this term is not clear. It may be related to Gk. tenagos “bottom, swamp,” Latvian tigas “depth;” PIE *tenegos “water bottom.”

A border between two countries.
A line of division between different or opposed things.
The farthermost limits of knowledge or achievement in a particular subject.

Etymology (EN): From O.Fr. fronter, from front “forehead, brow,” → front.

Etymology (PE): Marz, from Mid.Pers. marz “boundary;” Av. marəza- “border, district,” marəz- “to rub, wipe;” Mod.Pers. parmâs “contact, touching” (→ contact), mâl-, mâlidan “to rub;” PIE base *merg- “boundary, border;” cf. L. margo “edge” (Fr. marge “margin”); P.Gmc. *marko;
Ger. Mark; E. mark, margin.

A border between two countries.
A line of division between different or opposed things.
The farthermost limits of knowledge or achievement in a particular subject.

Etymology (EN): From O.Fr. fronter, from front “forehead, brow,” → front.

Etymology (PE): Marz, from Mid.Pers. marz “boundary;” Av. marəza- “border, district,” marəz- “to rub, wipe;” Mod.Pers. parmâs “contact, touching” (→ contact), mâl-, mâlidan “to rub;” PIE base *merg- “boundary, border;” cf. L. margo “edge” (Fr. marge “margin”); P.Gmc. *marko;
Ger. Mark; E. mark, margin.

An observing project using the → Hubble Space Telescope (HST) and the → Spitzer Space Telescope to obtain deep images for cosmological studies. The Frontier Fields combines the power of HST and Spitzer with the natural gravitational telescopes of massive high-magnification clusters of galaxies to produce the deepest observations of clusters and their lensed galaxies ever obtained. Six clusters (Abell 2744, MACSJ0416.1-2403, MACSJ0717.5+3745, MACSJ1149.5+2223, Abell S1063, and Abell 370) were selected based on their lensing strength, sky darkness, Galactic extinction, parallel field suitability, accessibility to ground-based facilities, HST, Spitzer and JWST observability, and preexisting ancillary data. (Lotz et al., 2016, arxiv/1605.06567 and references therein).

See also: → frontier; → field.

An observing project using the → Hubble Space Telescope (HST) and the → Spitzer Space Telescope to obtain deep images for cosmological studies. The Frontier Fields combines the power of HST and Spitzer with the natural gravitational telescopes of massive high-magnification clusters of galaxies to produce the deepest observations of clusters and their lensed galaxies ever obtained. Six clusters (Abell 2744, MACSJ0416.1-2403, MACSJ0717.5+3745, MACSJ1149.5+2223, Abell S1063, and Abell 370) were selected based on their lensing strength, sky darkness, Galactic extinction, parallel field suitability, accessibility to ground-based facilities, HST, Spitzer and JWST observability, and preexisting ancillary data. (Lotz et al., 2016, arxiv/1605.06567 and references therein).

See also: → frontier; → field.

Ice crystals that are formed by deposition of water vapor on a relatively cold surface.
The condition that exists when the temperature of the earth’s surface and earthbound objects fall below freezing.

Etymology (EN): O.E. forst, frost “a freezing, becoming frozen, extreme cold,” from P.Gmc. *frusta- (cf. O.H.G. frost, Du. vorst), related to freosan “to freeze.”

Etymology (PE): Bašmé, from bašm “hoar-frost; dew,” variants bažm, bašk, pašak “frost; dew,” may be related to (štiyâni, Qomi dialects) bašand, vašand, vašan “rain” (vašan-sâl “rainy year”), (Lori, Laki) vašt “rain shower,” (Gurâni) wašt, wišani “rain” (Tâti Karingâni) vurasten “to rain;” Av. -varšta- “rain,” aiwi-varšta- “rained upon;” Skt. vars- “to rain,” varsá- “rain;” M.Irish frass “rain shower, torrent;” Gk. eérse “dew,” oureo “to urinate.”

Ice crystals that are formed by deposition of water vapor on a relatively cold surface.
The condition that exists when the temperature of the earth’s surface and earthbound objects fall below freezing.

Etymology (EN): O.E. forst, frost “a freezing, becoming frozen, extreme cold,” from P.Gmc. *frusta- (cf. O.H.G. frost, Du. vorst), related to freosan “to freeze.”

Etymology (PE): Bašmé, from bašm “hoar-frost; dew,” variants bažm, bašk, pašak “frost; dew,” may be related to (štiyâni, Qomi dialects) bašand, vašand, vašan “rain” (vašan-sâl “rainy year”), (Lori, Laki) vašt “rain shower,” (Gurâni) wašt, wišani “rain” (Tâti Karingâni) vurasten “to rain;” Av. -varšta- “rain,” aiwi-varšta- “rained upon;” Skt. vars- “to rain,” varsá- “rain;” M.Irish frass “rain shower, torrent;” Gk. eérse “dew,” oureo “to urinate.”

A → dimensionless number that gives the ratio of local acceleration to gravitational acceleration in the vertical.

See also: Named after William Froude (1810-1879), English engineer.

A → dimensionless number that gives the ratio of local acceleration to gravitational acceleration in the vertical.

See also: Named after William Froude (1810-1879), English engineer.

1. Turned into or covered with ice. → frozen water.
   

   2. Attached or → fixed so as to be immovable.
      → frozen magnetic field line.

See also: Past participle of → freeze.

1. Turned into or covered with ice. → frozen water.
   

   2. Attached or → fixed so as to be immovable.
      → frozen magnetic field line.

See also: Past participle of → freeze.

A → magnetic field line in a → fluid
when the motion of the fluid carries the magnetic field along with it.

See also: Frozen, p.p. of → freeze; → magnetic; → line.

A → magnetic field line in a → fluid
when the motion of the fluid carries the magnetic field along with it.

See also: Frozen, p.p. of → freeze; → magnetic; → line.

Turned into or covered with → ice. See also: → water ice.

See also: Past participle of → freeze.

Turned into or covered with → ice. See also: → water ice.

See also: Past participle of → freeze.