An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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.

→ 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.

→ 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.

From L. focus "hearth, fireplace," of unknown origin,

Kânun "hearth, fireplace."

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

Past participle of → focus.

The act of bringing into focus.

Noun of → focus.

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

From Dan. fog "spray, shower, snowdrift," related to O.N. fok "snow flurry."

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.

→ 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.

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).

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

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).

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.

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.

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.

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.

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").

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.

From Ar. Fam al-Hut (فم‌الحوت) "the fish's mouth," from fam "mouth" + hut "fish."

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).

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).

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.

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.

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.

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.

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.

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.

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.

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.

→ forbidden; → band .

A → forbidden line in → emission.

→ forbidden; → emission; → line.