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

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").

Light exhibiting → linear polarization.

→ linearly; → polarized; → light.

Of a → supernova, → peak luminosity.

→ maximum; → light.

The light of the Moon.

From → moon + → light.

Mahtâb (Gilaki mângtâb) from mah, mâh (mâng), → moon, + tâb "light," from tâbidan, tâftan "to shine," tafsidan "to become hot" (Av. tāp-, taf- "to warm up, heat," tafsat "became hot," tāpaiieiti "to create warmth;" cf. Skt. tap- "to spoil, injure, damage; to suffer; to heat, be/become hot," tapati "burns;" L. tepere "to be warm," tepidus "warm;" PIE base *tep- "warm").

One of the three twilight phases which is the period before sunrise and after sunset when the center of the Sun's disk is between 6° and 12° below the horizon. This twilight phase is followed or preceded by → civil twilight. See also → astronomical twilight. In clear weather conditions, the horizon is faintly visible during this phase. Many of the brighter stars can also be seen, making it possible to use the position of the stars in relation to the horizon to navigate at sea. This is why it is called nautical twilight.

→ nautical astronomy; → twilight.

A small bundle of → rays of light. See also → beam of light.

M.E. pencel, from M.Fr. pincel, from L. penicillus "painter's brush or pencil," diminutive of peniculus "little tail," diminutive of penis "tail;" → light.

Bâriké, from bârik, → narrow, + nuance suffix -é; nur, → light.

Light exhibiting → plane polarization. Same as → linearly polarized light.

→ plane; → polarized; → light.

→ Electromagnetic radiation in the optical region which has undergone → polarization.

Past participle of → polarize; → light.

The motion of light in the first approximation, as evidenced from the formation of shadows and other every day experience. However, → diffraction

→ rectilinear; → propagation; → light.

Solar radiation which reaches the observer from the general sky. It is sunlight which has undergone multiple scattering events with the molecules of the Earth's atmosphere (Rayleigh scattering) or with clouds or other aerosols in the atmosphere. High levels of skylight reduce the contrast of a shadow. Also known as diffuse skylight, diffuse sky radiation.

→ sky; → light.