An Etymological Dictionary of Astronomy and Astrophysics

Of or relating to a focus; placed at or measured from a focus.

See also: → focus + → -al.

same as → focal length.

See also: → focal; → distance.

The distance between the optical center of a lens, or the surface of a mirror, and its focus.

See also: → focal; → distance.

A plane at right angles to the principal axis of a lens or mirror on which the best image is formed.

See also: → focal; → plane.

Same as → focus.

See also: → focal; → point.

The ratio of the → focal length of a reflecting surface or lens to its effective diameter, i.e. to its → aperture. The smaller the focal ratio, the smaller the image scale and the more luminous the image for a given aperture.

See also: → focal; → ratio.

An optical component or system for changing the image scale of a telescope to achieve a better match between the → seeing disk and the → pixel size.

See also: → focal; → reducer.

1. (n.) A point where parallel light rays from an object are gathered together by a lens or a concave mirror. It is the place where the clearest image of a distant object forms. Also called focal point. See also → focal distance.
   

2. (v.) To adjust the eyepiece or objective of a telescope so that the image is clearly seen by the observer.

Etymology (EN): From L. focus “hearth, fireplace,” of unknown origin,

Etymology (PE): Kânun “hearth, fireplace.”

Of an optical system, being in focus or brought into focus; adjusted to produce a clear image.

See also: Past participle of → focus.

The act of bringing into focus.

See also: Noun of → focus.

A mass or layer of suspended water droplets or ice crystals near the surface of the earth, reducing visibility.

Etymology (EN): From Dan. fog “spray, shower, snowdrift,” related to O.N. fok “snow flurry.”

Etymology (PE): Meh “fog” (variants miq, mož, Tabari miyâ, Lori/Laki (kara) mozy, Ossetic mig/megæ), from Mid.Pers. mēq “cloud, mist,” Av. mēγa- “cloud;” cf. Skt. meghá- “cloud, overcast weather;” Gk. omikhle “mist;” Lith. miglà “mist, haze;” PIE base *mighlā- “cloud.”

A large, faintly colored, circular arc formed by light (usually sunlight) falling on cloud or fog. Also called → cloudbow.

See also: → fog; → bow.

A modified form of → Boltzmann’s equation allowing for collision terms in an approximate way. It describes the rate of change of a particle’s velocity as a result of small-angle collisional deflections.

See also: After Dutch physicist Adriaan Fokker (1887-1972) and the German physicist Max Planck (1858-1947); → equation.

1. Usually, folks. (used with a plural verb) people in general.
   

2. (used with a plural verb) people as the carriers of culture, especially as representing the composite of social mores, customs, forms of behavior, etc., in a society (Dictionary.com).

Etymology (EN): M.E.; O.E. folc; cognate with O.Sax., O.Norse folk, O.H.G. folk (Ger. Volk).

Etymology (PE): Palg, from (Pashto) parrk “group of people,” with variants:
(Dehxodâ) parré “group of people; a circular disposition of troops for hunting or other purposes; a rank or file of soldiers;” (Lori, Torbart-Heydariye-yi, Qomi) borr “group of people, crowd;” (Qomi) borré; (Laki) berr “group of people;” (Qâyeni) bor “group, flock, herd;” (Kurd. Kurmanji) âpora “crowd;” transformation of -r- into -l- (as por = bol, → poly-)
in Tabari bəlik, əllik “herd, flock;” ultimately from Proto-Ir. *paraka-, from *par- “to fill;” cf. Av. pər- “to fill, stuff with,” pouru- “full, much, many;” O.Pers. paru- “much, many;” Pers. anbâr “ricks, storehouse,” por, bol “full, much, many;” PIE *pel- “to fill;” → population.

The traditional beliefs, legends, customs, etc., of a people; lore of a people (Dictionary.com).

Etymology (EN): Coined by English scholar and antiquary William John Thoms (1803-1885), from → folk, + lore “traditional knowledge or belief,”
from M.E., O.E. lar cognate with Du. leer, Ger. Lehre “teaching,” E. learn.

Etymology (PE): Palgvâr, literally “customs of people,” from palg, → folk, + vâr “custom, rule, law” (Dehxodâ).

An observation which expands previous observations and aims at obtaining complementary data in particular with other telescopes/instruments.

Etymology (EN): Follow-up, from follow, from O.E. folgian, fylgan “to follow, pursue,” from W.Gmc. *fulg- (cf. O.Fris. folgia, M.Du. volghen, Ger. folgen “to follow”) + up,
O.E. up, uppe
(cf. Du. op, Ger. auf “up, upward”), from PIE base *upo “up from below;” cf. O.Pers./Av. upā;
Skt. úpa; Gk. hypo; L. sub, → hypo-; → observation.

Etymology (PE): Nepâheš, → observation; peygir, from pey “after; step,” related to pâ “foot, step, track,” → foot, + gir present stem of gereftan “to take, seize” (Mid.Pers. griftan, Av./O.Pers. grab- “to take, seize,” cf. Skt. grah-, grabh- “to seize, take,” graha “seizing, holding, perceiving,” M.L.G. grabben “to grab,” from P.Gmc. *grab, E. grab “to take or grasp suddenly;” PIE base *ghrebh- “to seize”).

A → sunspot that follows the → leader spot. Sunspots tend to appear in groups consisting of one leader and several follower spots. The leader and the follower spots having opposite polarities.

Etymology (EN): Agent noun of follow, from O.E. folgian, fylgan “to follow, pursue,” from W.Gmc. *fulg- (cf. O.Fris. folgia, M.Du. volghen, Ger. folgen “to follow”).

Etymology (PE): Laké, → spot; peyrow “follower,” from pey “step; after,” related to pâ “foot, step, track,” → foot, + row “going; which goes,” present stem of raftan “to go, walk, proceed,” Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- “to go; to attack.”

The brightest star (V = 1.17) in the constellation → Piscis Austrinus. Fomalhaut is a white A3 main-sequence star about 25 light-years away. It is encircled by a dust belt (→ debris disk) of about 25 A.U. wide at a radial distance of about 140 A.U.

Etymology (EN): From Ar. Fam al-Hut (فم‌الحوت) “the fish’s mouth,” from fam “mouth” + hut “fish.”

Etymology (PE): Famelhut, from Ar. as above; Mâhidahân, from mâhi “fish,” from Mid.Pers. mâhik; Av. masya-; cf. Skt. matsya-, Pali maccha-

• dahân “mouth;” Mid.Pers. dahân, from Av. zafan-, zafar- “mouth (of Ahrimanic beings).”

An assortment or set of type or characters all of one style and sometimes one size (Merriam-Webster.com).

Etymology (EN): From M.Fr. fonte “act of founding, casting,” from fondre “to melt,” so called because all the letters in a given set were cast at the same time (etymonline.com).

Etymology (PE): Rixtâr, from rixtan “to cast, to pour,” → morphology.

1. The terminal part of the vertebrate leg upon which an individual stands.
   

2. Something resembling a foot in position or use.

Etymology (EN): M.E., from O.E. fot; cf. O.S. fot, O.N. fotr, Du. voet, O.H.G. fuoz, Ger. Fuß, Goth. fotus “foot;” cognate with Pers. pâ, pây, as below.

Etymology (PE): Pâ “foot;” Mid.Pers. pâd, pây; Khotanese fad; Av. pad- “foot;” cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; E. foot, as above;
PIE *pod-/*ped-.

A preposition used to indicate the object or purpose of an action.

Etymology (EN): M.E., from O.E. for “before, in the sight of, in the presence of” (source also of Old Saxon furi “before,” Old Frisian for, Middle Dutch vore, Dutch voor “for, before;” German für “for;” Danish for “for,” før “before;” Gothic faur “for,” faura “before”), from PIE root *per- “forward,” hence “in front of, before,” etc.

Etymology (PE): Barâye “because of,” related to Pers. cerâ “for what reason, why?,” irâ “for this reason, therefore,” zirâ “because, on account of;” also rây “opinion, consult,” Mid.Pers. râd, rây “because of, for the sake of, on behalf of;” O.Pers. rā “reason, cause,” in rādiy “for this reason,” → reason.

Increased reddening and monochromaticity of light as the path length in the air increases.

See also: After the Scottish physicist James David Forbes (1809-1868); → effect.

1. General: Not allowed; prohibited.
   

2. Physics: Involving a change in → quantum numbers that is not permitted by the → selection rules, → forbidden transition.

Etymology (EN): Past participle of forbid, from O.E. forbeodan, from for- “against” + beodan “to command” (O.E. biddan“to beg, ask, demand;” cf. Ger. bitten “to ask”); cf. Du. verbieden, O.H.G. farbiotan, Ger. verbieten.

Etymology (PE): Bažkam in classical Persian dictionaries “a keeping back, a preventing,” from Av. paitigam- “to prevent,” from paiti- “opposite, against, back, in return” (O.Pers. paity “agaist, back, opposite to, toward, face to face, in front of;” Mod.Pers. variants pâd- (→ anti-); cf. Skt. práti “opposite, against, again, back, in return, toward;” Gk. proti, pros “face to face with, toward, in addition to, near;” PIE *proti)

• gam- “to come; to go,” jamaiti “goes;” O.Pers. gam- “to come; to go;” Mod./Mid.Pers. gâm
  “step, pace,” âmadan “to come” (cf. 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”).

In → solid state physics, a range of → energy levels which is not attained by any electrons in a → crystal. In the energy level diagram forbidden bands appear as gaps between → allowed bands.

See also: → forbidden; → band .

A → forbidden line in → emission.

See also: → forbidden; → emission; → line.

A spectral line emitted by atoms undergoing energy transitions not normally allowed by the → selection rules of → quantum mechanics. Forbidden emission lines form in low-density interstellar gas and are collisionally excited. They are designated by enclosing in brackets, e.g. [O III] and [N II], O and N representing oxygen and nitrogen atoms respectively.

See also: → forbidden; → line.

A transition between two quantum mechanical → states
that → violates the quantum mechanical → selection rules.

See also: → forbidden; → transition.

A decrease in the Galactic cosmic flux around sunspot maximum as a consequence of Solar flare activity. When sunspots explode, they often hurl massive clouds of hot gas away from the Sun, a phenomenon called → coronal mass ejections (CME). The CMEs contain not only gas but also magnetic fields. So when a CME sweeps past Earth, it also sweeps away many of the electrically-charged cosmic rays.

Etymology (EN): After Scott E. Forbush, American physicist who studied cosmic rays in the 1930s and 40s; → effect; decrease, from O.Fr. stem of descreistre, from L. decrescere, from → de- “away from”

• crescere “to grow.”

Etymology (PE): Kâheš, verbal noun of kâstan, kâhidan “to decrease,” from Mid.Pers. kâhitan, kâstan, kâhênitan “to decrease, diminish, lessen;” Av. kasu- “small, little” (Mod.Pers. keh), Proto-Iranian *kas- “to be small, diminish, lessen.”

1. The cause of a change in motion or shape of a body.
   

2. A → vector quantity defined by → Newton’s second law as the rate of change of → momentum or, equivalently, as the product of → mass and → acceleration. A force is completely specified by giving its magnitude, direction, and point of application. In mechanics, force represents the quantitative measure of the mechanical interaction of material bodies.

Etymology (EN): From O.Fr. force, from L.L. fortia, from neut. pl. of L. fortis “strong,” from forctus, from PIE base *bhergh- “high” (cf. Av. barəz- “high, mount,” barezan- “height;”
O.Pers. baršan- “height;” Mod.Pers. borz in (the mountain chain) Alborz, and borz “height, magnitude,” bâlâ “up, above, high, elevated, height,” Lori dialect berg “hill, mountain;” Skt. bhrant- “high;” O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg).

Etymology (PE): Niru, from Mid.Pers. nêrok, Av. nairya- “manly, male” (cf. Skt. nárya-), from nar- “man, male” (Mid./Mod.Pers. nar- “male,” Skt. nár- “male”).

Same as → field of force.

See also: → force; → field.

One of the three quantities (k, α, and δ) which are used in the → radiation-driven wind theory to express the radiation pressure due to spectral lines. These coefficients parametrize the radiation acceleration as: g_rad≅ k(dv/dr)^αg_e,

where g_e = σ_eL/4πcR_*² is the radiative acceleration by electron scattering. The parameter k is dependent on the number of lines that produce the radiation pressure. The parameter α depends on the optical depth of the driving lines and varies between 0 (optically thin) and 1 (optically thick). The parameter δ describes the dependence of k on the density with k ≅ ρ^δ. The velocity law of radiation-driven winds depends on α and δ, but not on k. The → mass loss rate depends on k, α, and δ (Castor et al. 1975, ApJ 195, 157; Lamers et al., 1995, ApJ 455, 269 and references therein).

See also: → force; → multiplier; → parameter.

Any set of forces acting on a → rigid body.

See also: → force; → system.

The condition in a plasma when the → Lorentz force is zero, that is when the electric current flows along the magnetic field. Force-free magnetic fields are encountered in astrophysical plasmas with negligible gas pressure. The solar corona is the best available example of such fields in action in a plasma.

See also: → force; → free; → magnetic; → field.

1. Physics: Caused by an external force. → forced oscillation.
   

2. General: Subjected to force, compulsory.

Etymology (EN): Adjective from → force.

Etymology (PE): Adjective from zur, → strength.

A fluid flow generated when external forces cause the fluid to flow, for example when a flow is caused by a pump. It contrasts with → free flow.

See also: → forced; → flow.

The oscillation of a system or object induced by an external periodic force. See also → free oscillation.

See also: → forced; → oscillation.

The part of the arm between the → elbow and the → wrist.

Etymology (EN): From fore- a prefix meaning “before; front; superior,” related to → per-; → arm.

Etymology (PE): Araš, variant ârenj “elbow;” Mid.Pers. âranj; O.Pers. arašan- “cubit;” Av. arəθnâ- “elbow;” Skt. aratni- “elbow,” Iranian stem aratan-, araθn-, borrowed from Iranian into General Slavic as aršin “ell.”

The ground or parts situated, or represented as situated, in the front; the portion of a scene or stellar field nearest to the viewer. → background.

Etymology (EN): From fore, from O.E. fore (prep.) “before, in front of;” (adv.) “before, previously” (cf. O.H.G. fora, Ger. vor, Goth. faiura); from PIE *per-/*pr- (cf. Skt. pura “before, formerly;” Av. paro “before;” Hittite para- “on, forth;” Gk. paros “before,” para “from beside, beyond,” peri “around, about, toward,” pro “before;” L. pro “before, for, instead of,” prae “before,” per “through, for”) + ground,
from M.E., from O.E. grund; akin to O.H.G. grunt “ground.”

Etymology (PE): Pišzaminé, from piš- “before; in front,” from Mid.Pers. pêš “before, earlier;” O.Pers. paišiya “before; in the presence of”

• ziminé “ground,” from zamin “earth, ground” → earth.

Loss of radiant energy received from an astronomical object due to the presence of absorbing matter situated between the object and observer.

See also: → foreground; → absorption.

1. A large tract of land covered with trees and underbrush; woodland (Dictionary.com).
   

   2. In → graph theory, a → graph which contains no → cycles. The connected components of a forest are → trees.

Etymology (EN): M.E., from O.Fr. forest, probably from L.L. forestis (silva) “the outside woods,” from L. foris “outside.”

Etymology (PE): Jangal “a wood, forest, a vast land with plenty of trees;” cf. Skt. jangala- “arid , sparingly grown with trees and plants.”

1. An instrument having two or more prongs or tines, for holding, lifting, etc.,
   as an implement for handling food or any of various agricultural tools (dictionary.com).
   

2. Something resembling or suggesting this in form. → fork mounting.

Etymology (EN): Fork, from O.E. forca, from L. furca “pitchfork,” of uncertain origin; → mounting.

Etymology (PE): Došâx, došâxé “two-pronged; fork,” from do “two” (Mid.Pers. do, Av. dva-,
Skt. dvi-, Gk. duo, L. duo, E. two, Ger. zwei, Fr. deux) + šâx “branch; horn,” from Mid.Pers. šâk “branch;” cf. Lith. šaka “branch;” O.S. soxa “plough;” Gothic hoha “plough.”
Cangâl “claws, fingers,” from cang “claws, fingers,”

• -al, → -al.

A type of → equatorial mounting consisting of a two-pronged fork, which forms the → polar axis, with the → telescope pivoted between the prongs of the fork on a → declination axis.

See also: → fork; → mounting.

1. (n.) General: The shape and structure of something as distinguished from its material.
   Philosophy: The structure, pattern, organization, or essential nature of anything. Structure or pattern as distinguished from matter.
   Logic: The abstract relations of terms in a proposition, and of propositions to one another.
   
2. (v.intr.) of form.
   
3. (v.tr.) of form.

Etymology (EN): From O.Fr. forme, from L. forma “form, mold, shape, case,” origin unknown.

Etymology (PE): 1) Dis, disé “form, appearance,” variants -diz, -diš (tandis “body form, like a body; effigy;” mâhdis “moon-like;” šabdiz “night color; a horse of dark rusty color;” andiš- “to think, contemplate”), from Mid.Pers. dêsag “form, appearance,” dêsidan “to form, build;” Av. daēs- “to show,” daēsa- “sign, omen;” cf. Skt. deś- “to show, point out;” PIE *deik- “to show” (cf. Gk. deiknumi “to show,” dike “manner, custom;” L. dicere “to utter, say;” O.H.G. zeigon, Ger. zeigen “to show;” O.E. teon “to accuse,” tæcan “to teach”).
2) and 3) corresponding infintives of dis, as above.

1. According to, or following established or prescribed forms, conventions, etc.
   

2. Math., logic: Of a proof, in strict logical form with a justification for every step.
   

3. Math., logic: Of a calculation, correct in form; made with strict justification for every step; of or pertaining to manipulation of symbols without regard to their meaning.
   

4. → formal language.
   

5. → formal logic.

Etymology (EN): M.E. formal, formel, from L. formalis, from → form

• → -al.

Etymology (PE): Diseyi, desevar, from dis, → form, + adj. suffixes -i and -var.

A language designed for use in situations in which natural language is unsuitable, as for example in → mathematics, → logic, or → computer  → programming. The symbols and formulas of such languages stand in precisely specified syntactic and semantic relations to one another (Dictionary.com).

See also: → formal; → language.

The traditional or → classical logic in which the → validity or → invalidity of a conclusion is deduced from two or more statements (→ premises). Based on Aristotle’s (384-322 BC) theory of → syllogism, systematized in his book “Organon,” its focus is not on what is stated (the content) but on the structure (form) of the → argument and the validity of the inference drawn from the premises of the argument; if the premises are true then the logical consequence must also be true. Formal logic is → bivalent, that is it recognizes only two → truth values: → true and → false. The basic principles of formal logic are: 1) → principle of identity, 2) → principle of excluded middle, and 3) → principle of non-contradiction. See also → symbolic logic, → fuzzy logic.

See also: → formal; → logic.

In logic and mathematics, a system in which statements can be constructed and manipulated with logical rules.

See also: → formal; → system.

A colorless gas with a pungent, suffocating odor used as an adhering component of glues in many wood products. Formaldehyde (H₂CO) is obtained most commonly by the oxidation of methanol or petroleum gases such as methane, ethane, etc.

In the → interstellar medium, formaldehyde is formed the surface of → dust grains. It is
then converted into gas phase by → photodesorption. H₂CO was the first → organic molecule discovered in the ISM (Lewis E. Snyder et al. 1969, Phys. Rev. Lett., 22, 679).

See also: From form(ic) acid, from Fr. formique, + → aldehyde.

1. Excessive adherence to prescribed forms.
   

2. Math., Logic: → Formalized expression of a set of elements; the nature of such expression.
   

3. A philosophical view of mathematics in which mathematics, including the logic used in proofs, can be based on the formal manipulation of symbols irrespective of their meaning.

See also: From → formal + → -ism.

1. Condition or quality of being formal; accordance with required or traditional rules, procedures, etc.
   

   2. Strict adherence to established rules and procedures (Dictionary.com).

See also: → formal; → -ity.

1. The act of giving something a form or structure by introducing rules and procedures.
   

   2. The act of formalizing.

See also: → formalize; → -tion.

1. To state in symbolic form; to give a definite structure to.
   

2. To give something legal or formal status.

Etymology (EN): From → formal + → -ize.

Etymology (PE): Compound verb, from disevar, → formal,

• kardan “to do, to make;” Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make.”

1. General: The way in which something is presented, organized, or arranged.
   Publishing: The shape and size of a book, magazine, or newspaper, including the typeface, binding, quality of paper, margins, etc.
   Computers: The structure or arrangement of digital data in a specific type of file. A computer program accepts data as input in a certain format, processes it, and provides it as output in the same or another format.
   

2. Computers: To prepare a storage medium, usually a disk, for reading and writing.

Etymology (EN): From Fr. format, from Mod.L. liber formatus “a book formed” (in such and such a way), referring to shape, size; from formatus p.p. of formare “to form,” → form.

Etymology (PE): 1) Disâr, from dis, → form + -âr contraction of âvar agent noun of âvardan “to bring; to cause, produce” (Mid.Pers. âwurtan, âvaritan; Av. ābar- “to bring; to possess,” from prefix ā- + Av./O.Pers. bar- “to bear, carry,” bareθre “to bear (infinitive),” bareθri “a female that bears (children), a mother;” Mod.Pers. bordan “to carry;” Skt. bharati “he carries;” Gk. pherein; L. fero “to carry”).
2) Infinitive from dis, → form.

1. The act or process of forming or the state of being formed, such as → star formation.
   
2. Geology: A laterally continuous rock unit with a distinctive set of characteristics that make it possible to recognize and map from one → outcrop or well to another. The basic rock unit of stratigraphy.

Etymology (EN): Verbal noun of → form, + -ate + -ion.

Etymology (PE): Diseš, verbal noun of disidan, → from dis-, → form, + -eš.

The process of preparing a hard disk or other storage medium for use by an operating system. Before a hard disk can be used, it needs to be formatted so that it will be able to store files and applications.

See also: → format; → -ing.

A colorless, corrosive fuming liquid with pungent odor. It occurs in various plants and in the venom of many ant species. Used in dyeing, tanning, and electroplating. Also called methanoic acid. HCOOH is the simplest organic acid and the first identified in the interstellar medium (Zuckerman et al. 1971, ApJ, 163, L41). It has been observed principally in star-forming regions such as Orion KL, Sgr B2, Sgr A, and W51 and is associated with → hot molecular cores and → massive star formation. Recently, it has also been shown to be present in some → hot corinos associated with formation of stars similar to our Sun. Due to the presence of carboxyl radical (COOH), it plays an important role in the pathway formation of → prebiotic molecules like amino acids, in the interstellar clouds and comets (see, e.g., Lattanzi et al. 2008, ApJS 176, 536).

See also: From L. formica “ant,” ultimately from from PIE *morwi-, *wormiko- “ant;” cf. Av. maoiri-; Mid.Per. môr; Pers. mur, murcé “ant;” Skt. vamra- “ant;” Gk. murmeks, wormikas; O.C.S. mraviji; O.Ir. moirb; O.N. maurr.

1. Physics, Math.: A statement of facts in a symbolical or general form, by substitution in which a result applicable to particular data may be obtained.
   
2. Chemistry: An expression of the constituents of a compound by symbols and figures.

Etymology (EN): From L. formula “form, rule, method, formula,” literally “small form,” from forma, → form,

• → -ule diminutive suffix.

Etymology (PE): Disul, from dis, → form, + -ul,
→ -ule.

To express in precise → form; state definitely or systematically. To reduce to or express in a → formula.

See also: Verbal form of → form.

1. The act or process of formulating.
   

2. A particular expression of an idea, thought, or theory.
   

3. Something prepared according to a → formula.

See also: → formulate; → -tion.

The Chemical Furnace. A faint → constellation in the southern sky, representing a chemist’s furnace. Its brightest star, Alpha Fornacis, is a double of magnitudes 4.0 and 6.5. Abbreviation: For; genitive: Fornacis.

Etymology (EN): L. fornax “oven, kiln,” related to fornus, furnus “oven,” and to formus “warm,” from PIE base *ghworm-/*ghwerm- “warm” (cf. Mod./Mid.Pers. garm “warm;” O.Pers./Av. garəma- “hot, warm;” Skt. gharmah “heat;” Gk. thermos “warm;” cf. O.E. wearm; O.H.G., Ger. warm).

Fornax was created and named by Abbé Nicolas Louis de Lacaille (1713-1762) in honor of Antoine Lavoisier, the French chemist, who was guillotined in the French Revolution in 1794. He created it out of several faint stars in the constellation → Eridanus.

Etymology (PE): Kuré “furnace;” Tabari kalə “furnace,” kəlen “ash;” Laki koira; Kurd. kulan, kulandan “to cook;”
Laki koira; Kurd. kulan, kulandan “to cook;” related to garm “warm;” cf. Skt. ghar- “to burn;” E. kiln “furnace, oven,” from L. culina “cooking stove, kitchen;” PIE root *gwher- “to warm, be warm,” → warm.

The second richest → cluster of galaxies within 100 million light-years, although it is much smaller than the → Virgo cluster.

See also: → Fornax; → cluster.

A cardinal number, ten times four.

Etymology (EN): M.E. fourti, O.E. feowertig, from feower, → four,

• tig “group of ten” (cf. O.S. fiwartig, Du. veertig, O.H.G. fiorzug, Ger. vierzig, Goth. fidwor tigjus.

Etymology (PE): Cehel (short form cel); Mid.Pers. cehel “forty;” Av. caθwarəsa(n)t- “forty,” from caθwar-, → four, + sant-, sat “ten;” cf. Skt. catvārimśát- “forty.”

1a) To or toward what is ahead or in front.

1b) Directed toward a point in advance.

2. To send forward; transmit, especially (a letter or email) to a different address.

Etymology (EN): From fore “before, in front of,” cognate with Pers. farâ, → pro-,+ → -ward.

Etymology (PE): Piš-su, from piš “forward; in front of; before;” Mid.Pers. pêš, + su, → direction.

Scattering in which photons emerge from the → scattering medium travelling predominantly in the same direction as they entered. The → halos around the Sun and Moon in wet weather are caused by forward scattering by water droplets in the Earth’s atmosphere.
→ backscattering.

See also: → forward; → scattering.

1. Geology: The process whereby a geologic section (subsurface model of one-, two-, or three dimensions) is transformed into a synthetic seismogram (synthetic seismic record).
   

   2. In → asteroseismology, a model that takes the physical properties of a star as input parameters and predicts the star’s oscillations. Then, by finding parameters that yield oscillation frequencies (ω_nlm) close to those observed, one can infer the properties of the observed star. The quantum numbers n, l, and m must be identified before any meaningful comparison between seismic data and model predictions can be made. That mode identification requires a physical interpretation of the measured frequencies.

See also: → forward; → seismic; → modeling.

A highly → supersonic → shock wave created in a → supernova remnant as the expanding stellar ejecta runs into the → interstellar medium (ISM).

This forward shock wave produces sudden, large changes in pressure and temperature behind the shock wave.

The forward shock wave also accelerates electrons and other charged particles to extremely high energies.

The forward shock front has a velocity of 10⁴ km s^-1 and can heat the shocked gas to temperatures ~ 10⁹ K.

While the forward shock continues to expand into the ISM, it creates a → reverse shock that travels back into the freely expanding → supernova ejecta.

See also: → forward; → shock.

A relic, remnant, or representation of an organism that existed in a past geological age.

Etymology (EN): From Fr. fossile, from L. fossilis “dug up,” from fossus, p.p. of fodere “to dig.”

Etymology (PE): Sangâré literally “resembling stone,” from sang, → stone, + -vâré, from -vâr, → -oid.

In a physical system, the → magnetic field belonging to an earlier magnetic process or event. A fossil magnetic field may be a vanished one or exist in relic forms. As an example, the solar magnetic field, which was present during the formation of the Sun, has disappeared over the last 4.6 billions years.

See also: → fossil;→ magnetic; → field;

Same as → eddy current.

See also: → Foucault; → current.

A method used to test the → image quality of → mirrors and → lenses. The test is performed by moving a knife edge laterally into the → image of a small → point source. The → eye, or a → camera, is placed
immediately behind the knife edge, and the → exit pupil of the system is observed.

See also: Named after the French physicist Léon Foucault (1819-1868),
who invented the method; → knife; → edge; → test.

A → pendulum consisting of a heavy weight on a very long wire attached to a support, that shows the rotation of Earth. The support must be nearly frictionless
in order that the pendulum can continue to swing freely for long periods of time. The pendulum will swing in the same plane as it started. The → Earth’s rotation is reflected in the slow turning of the plane of the pendulum’s motion,
which appears to rotate through 360° in T hours. The rotation time is
given by the expression: T = T₀/sin φ, where T₀ = 23.9344 hours is the → sidereal day and φ the → latitude of the place. At the poles the rotation period is 23h 56m 04s, and at the equator is ∞, i.e. the swing plane does not move. For regions near the equator it is very long; for example at Quito, the capital city of Ecuador, with φ = 00°15’S, it is 5485 days or more than 15 years! This phenomenon shows that the Earth is a → non-inertial frame.

See also: The experiment was performed for the first time by the French physicist Léon Foucault (1819-1868) in 1851, who set up, in the Pantheon in Paris, a simple pendulum consisting of a lead ball weighing 28 kg, suspended by a fine steel wire 67m long. At the latitude of Paris, the pendulum takes 31h 47m 38s to complete a precession cycle;
→ pendulum.

The first functioning → reflecting telescope with a silvered glass mirror. It was built by Léon Foucault in 1826 for the Marseille Observatory. The mirror of 80-cm in diameter (f/d = 5) had an excellent quality. The telescope was used for a century as a visual instrument. Edouard Stéphan (1837-1923) used it from 1871 to 1884 to find 800 high-brightness galaxies, among which the → Stephan’s Quintet. From 1906 to 1962 the telescope was used by Robert Jonckheere (1888-1927) to discover 3,350 new binary stars. In 1873, following an idea of Hippolyte Fizeau (1819-1896), Stéphan attempted to use it as an → interferometer to measure the diameter of a number of stars. In 1914 Charles Fabry (1867-1945) and Henri Buisson (1873-1944) used the telescope to obtain the first astronomical Fabry-Pérot interferogram, on the → Orion Nebula.

See also: After the French physicist and optician Léon Foucault (1819-1868); Marseille (Observatory), the second largest city of France, located on the south east coast of the Mediterranean Sea, from L. Massalia, from Gk. Massalia; → reflector.

Etymology (EN): O.E. feower, from P.Gmc. *petwor- (cf. O.S. fiwar, Du. and Ger. vier, O.N. fjorir, Dan. fire, Sw. fyra), cognate with Pers. cahâr, as below, from PIE *qwetwor.

Etymology (PE): Cahâr, variant câr, from Mid.Pers. cahâr; Av. caθwarô, catur-; cf. Skt. catvārah;
Gk. tessares; cognate with L. quattuor; E. four, as above.

An operator defined as: ▫ = (∂/∂x, ∂/∂y, ∂/∂z, 1/(jc∂/∂t).

See also: → four; → dimensional; → operator.

The process of decomposing any function of time or space into a sum of sinusoidal
functions using the → Fourier series and → Fourier transforms. In other words, any data analysis procedure that describes or measures the fluctuations in a time series by comparing them with sinusoids.
Fourier analysis is an essential component of much of modern applied and pure mathematics. It forms an exceptionally powerful analytical tool for solving various problems in many areas of mathematics, physics, engineering, biology, finance, etc. and has opened up new realms of knowledge.

See also: After the French mathematician Baron Jean Baptiste Joseph Fourier (1768-1830), whose work had a tremendous impact on the physical applications of mathematics; → analysis.

One of the coefficients a_n or b_n of cos (nx) and sin (nx) respectively in the → Fourier series representation of a function. They are expressed by:
a_n = (1/π) ∫f(x) cos nx dx, for n≥ 0, summed over 0 to 2π
b_n = (1/π) ∫f(x) sin nx dx, for n≥ 1, summed over 0 to 2π.

See also: → Fourier analysis; → series.

An integral used in the → Fourier transform.

See also: → Fourier analysis; → integral.

A mathematical tool used for decomposing a → periodic function into an infinite sum of sine and cosine functions. The general form of the Fourier series for a function f(x) with period 2π is:

(1/2) a₀ + Σ (a_n cos (nx) + b_n sin (nx), summed from n = 1 to ∞,

where a_n and b_n are the → Fourier coefficients, measuring
the strength of contribution from each harmonic. The functions cos (nx) and sin (nx) can be used in this way because they satisfy the → orthogonality conditions. For the problem of convergence of the Fourier series see → Dirichlet conditions. The Fourier series play a very important role in the study of periodic phenomena, because they allow one to decompose a large number of complex problems into simpler ones. The generalization of this method, called the → Fourier transform, makes it possible to also decompose non-periodic functions into harmonic components. See also → complex Fourier series, → Parseval’s theorem.

See also: → Fourier analysis; → series.

Any finite periodic motion may be analyzed into components, each of which is a simple harmonic motion of definite and determinable amplitudes and phase.

See also: → Fourier analysis; → theorem.

A powerful mathematical tool which is the generalization of the → Fourier series for the analysis of non-periodic functions. The Fourier transform transforms a function defined on physical space into a function defined on the space of frequencies, whose values quantify the “amount” of each periodic frequency contained in the original function. The inverse Fourier transform then reconstructs the original function from its transformed frequency components. The integral F(α) = ∫ f(u)e^-iαudu
is called the Fourier transform of
F(x) = (1/2π)∫ f(α)e^iαxdx,
both integrals from -∞ to + ∞.

See also: → Fourier analysis; → transform.

The end of a solar eclipse marked by the disk of the Moon completely passing away from the disk of the Sun.

Etymology (EN): From M.E. fourthe, O.E. féowertha, from four, from
O.E. feower, from P.Gmc. *petwor- (cf. Du. and Ger. vier, O.N. fjorir, Dan. fire, Sw. fyra), from PIE *qwetwor (cf. Mod.Pers. cahâr, Av. caθwar-, catur-, Skt. catvarah, Gk. tessares, L. quattuor)

• -th a suffix used in the formation of ordinal numbers, from
  M.E. -the, -te, O.E. -tha, -the; cf. O.N. -thi, -di; L. -tus; Gk -tos; → contact.

Etymology (PE): Parmâs, → contact; cahârom cardinal form from cahâr “four,” cognate with E. four, as above.

A small depression, approximately 1.5 millimeters in diameter, at the back of the → retina. It forms the area of the most acute vision.

Etymology (EN): Contraction of fovea centralis, from L. fovea “small pit,” of unknown origin.

Etymology (PE): Lake-ye zard, literally “yellow spot,” from laké, → spot, zard, → yellow.

Any of several carnivores of the dog family, especially those of the genus Vulpes, smaller than wolves, having a pointed, slightly upturned muzzle, erect ears, and a long, bushy tail (Dictionary.com).

Etymology (EN): M.E., from O.E. fox “a fox;” cognate with O.Saxon vohs, M.Du. vos, O.H.G. fuhs, Ger. Fuchs, from Proto-Germanic *fuh-, from PIE *puk- “tail;” cf. Skt. puccha- “tail,” Toch. B päkā- “tail.”

Etymology (PE): Rubâh, from Mid.Pers. rôbâh (Sogd. ropas), Av. raopi- “fox.” This Iranian word was borrowed into Nordic languages as Old Norse rew, Icelandic refur, Faroese revur, Swedish räv, Danish ræv, Finnish repo. The transmission would have taken place through Sarmatian or Alanic intermediaries (cf. Ossetic ruvas / robas “fox”).