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

An imaginary line directed along the path that the light follows. In other words, light pictured simply in terms of straight lines.

→ light; → ray.

A type of → light pollution which is light falling where it is not wanted or needed. Light trespass occurs when poorly shielded or poorly aimed fixtures cast light into unwanted areas, such as buildings, neighboring property, and homes. This light is a main contributor to → skyglow which interferes with astronomical instruments.

→ light; → trespass.

→ light-year

→ light; → year.

The most important function of an astronomical telescope, which is directly related to the area (or to the square of the diameter) of the main mirror or lens.

→ light; gathering, from O.E. gadrian, gædrian "to gather, collect;" → power.

Tavân, → power; gerdâvari, verbal noun of gerd âvardan, from gerd "round; around" (Mid.Pers. girt "round, all around," O.Iranian *gart- "to twist, to wreathe," cf. Skt. krt "to twist threads, spin, to wind, to surround," kata- "a twist of straw;" Pali kata- "ring, bracelet;" Gk. kartalos "a kind of basket," kyrtos "curved") + âvardan "to bring," 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;" nur, → light.

The distance travelled by light in free space in one second. It is equivalent to 2.997924580 × 10⁸ m or 2.998 × 10⁵ km. This unit of length is mainly used in astronomy, telecommunications, and relativistic physics. Some quantities expressed in this unit are as follows. The mean diameter of the Earth: about 0.0425 light-seconds. The average distance from the Earth to the Moon: about 1.282 light-seconds. The diameter of the Sun: about 4.643 light-seconds. The average distance from the Earth to the Sun: 499.0 light-seconds.

→ light; → second.

The time it takes for light, travelling at about 300 000 km per second, to travel a certain distance.

→ light; → time.

The distance traversed by a photon between the time it is emitted and the time it reaches the observer. It is also referred to as the → look-back time.

→ light; → travel; → distance.

The distance that light travels in one year at about 300,000 km per second, i.e. 9.5 x 10¹² km. It is equal to about 63,000 → astronomical units. See also → parsec.

→ light; → year.

A bundle of optical fibers arranged randomly for the purpose of transmitting energy, not an image.

→ light; → guide.

Of, pertaining to, or describing an → event on the → light cone.

→ light; → -like.

The space-time interval between two events if it is zero, i.e. ds² = 0.

→ lightlike; → interval.

A → flash of light produced by an → electric discharge in response to the buildup of an → electric potential between → cloud and → Earth's surface, or between different portions of the same cloud.

Lightning, pr.p. of lightnen "make bright," extended form of O.E. lihting, from leht, → light.

Âzaraxš, from âzar "fire," variants âtaš, taš (Mid.Pers. âtaxš, âtur "fire;" Av. ātar-, āθr- "fire," singular nominative ātarš-; O.Pers. ātar- "fire;" Av. āθaurvan- "fire priest;" Skt. átharvan- "fire priest;" cf. L. ater "black" ("blackened by fire"); Arm. airem "burns;" Serb. vatra "fire;" PIE base *āter- "fire") + raxš "lightning, reflection of light," raxšidan "to shine, flash," variant deraxš, deraxšidan "to shine, radiate" (O.Pers. raucah-, Av. raocah- "light" (cf. Skt. roka- "brightness, light," Gk. leukos "white, clear," L. lux "light" (also lumen, luna), E. light, Ger. Licht, Fr. lumière; PIE base *leuk- "light, brightness"); cognate with Mod.Pers. words ruz "day," rowšan "bright, clear," foruq "light," and afruxtan "to light, kindle").

1) The state of being likely or probable; a probability or chance of something.
2) In technical language likelihood is not synonymous for probability. Same as → likelihood function.

From → likely + -hood a suffix denoting state, condition, character, nature, etc., from M.E. -hode, -hod, O.E. -hād (cf. Ger. -heit).

Šodvâri, noun of šodvâr, → likely.

A function that allows one to estimate unknown parameters based on known outcomes. Opposed to → probability, which allows one to predict unknown outcomes based on known parameters. More specifically, a probability refers to the occurrence of future events, while a likelihood refers to past events with known outcomes.

→ likelihood; → function.

Probably or apparently destined; having a high probability of occurring or being true.

Perhaps from O.N. likligr "likely," from likr "like" (adj.).

Šodvâr, from šod past stem of šodan "to become, to be, to be doing, to go, to pass" + -vâr a suffix with several meanings "resembling, like, in the manner of; having, endowed with." The first element from Mid.Pers. šudan, šaw- "to go;" Av. šiyav-, š(ii)auu- "to move, go," šiyavati "goes," šyaoθna- "activity; action; doing, working;" O.Pers. šiyav- "to go forth, set," ašiyavam "I set forth;" cf. Skt. cyu- "to move to and fro, shake about; to stir," cyávate "stirs himself, goes;" Gk. kinein "to move;" Goth. haitan "call, be called;" O.E. hatan "command, call;" PIE base *kei- "to move to and fro."

1) The outer edge or border of the apparent disk of a celestial body. → limb brightening, → limb darkening.
2) The raised edge of the mater of a → planispheric astrolabe, bearing a scale divided into 360°. The limb is the reference against which the rete is rotated in the computation process so that the planispheric astrolabe will simulate the sky's appearance on a given day at a specific hour. Vice versa, the limb can be configured from the observation of the altitude of an celestial body on the horizon. In that case, the limb will indicate an angle that, thanks to the hour scale on the back, can be converted into the hour of day or night (online museo galileo, VirtualMuseum).

From L. limbus "border, hem, fringe, edge," cognate with Skt. lambate "hangs down."

Labé "limb, edge," from lab "lip;" Mid.Pers. lap; cognate with L. labium, E. lip; Ger. Lefze.

An observed increase in the intensity of radio, extreme ultraviolet, or X-radiation from the Sun from its center to its limb.

→ limb; → brightening.

An apparent decrease in brightness of the Sun near its edge as compared to its brightness toward the center. Limb darkening is readily apparent in photographs of the Sun. The reason is that when we look toward the disk's center we look into deeper and hence hotter layers along the line of sight. Toward the limb, we get radiation from higher and hence cooler and less bright layers of the → photosphere. Limb darkening has been detected in the case of several other stars. A similar phenomenon occurs in → eclipsing binaries where the effect of limb darkening on one or both components manifests itself in the shape of the system's → light curve.

→ limb; → darkening.

A white or grayish-white substance obtained by burning → limestone, used in mortars, plasters, cements, and in the manufacture of steel, paper, glass, and various chemicals of calcium.

M.E., O.E. lim; akin to Du. lijm, Ger. Leim, O.N. lim "glue;" L. limus "slime."

Âhak, probably a variant of xâk, → soil.

A → sedimentary rock composed principally of calcium carbonate. Limestone is usually formed from shells of once-living organisms or other organic processes, but may also form by inorganic precipitation.

→ lime; → stone.