aberration of light
Fr.: aberration de la lumière
→ aberration; → light.
aberration of starlight
birâheš-e nur-e setâré
Fr.: aberration de la lumière d'étoile
An apparent displacement in the observed position of a star. It is a result of the finite speed of light combined with the relative motion of the Earth through space. Suppose that you walk through a vertically falling rain with an umbrella over your head. The faster you walk, the further you must lower the umbrella in front of yourself to prevent the rain from striking your face. For starlight to enter a telescope, a similar phenomenon must occur, because the Earth is in motion. The telescope must be tilted in the direction of motion by an angle: tan θ =(v/c), where v the Earth velocity and c the speed of light. The aberration of starlight was discovered by the English astronomer James Bradley (1693-1762) in 1729 by observing → Gamma Draconis. The tilt angle is θ = 20''.50, from which the Earth's orbital speed, 29.80 km s-1, can be deduced, using the above equation. See also → annual aberration; → diurnal aberration; → secular aberration. → Special relativity modifies the classical formula for aberration, predicting results which differ substantially from those of classical physics for objects moving at a substantial fraction of the speed of light; → relativistic aberration.
→ aberration; → star; → light.
arc of light
Fr.: arc de lumière
The apparent angular separation (→ elongation) between the → centers of the → Sun and the → Moon.
Fr.: lumière artificielle
Any light other than that which proceeds from the heavenly bodies.
→ artificial; → light.
nur-e xâkestari (#)
Fr.: lumière cendrée
The faint glow occasionally observed on the unlit area of Venus in its crescent phase. Its cause is not known with certainty, but it might result from bombardment of atmospheric atoms and molecules by energetic particles and radiation, as with terrestrial airglow.
nimtâb-e axtaršenâsik, ~ axtarsnâxti
Fr.: crépuscule astronomique
One of the twilight phases when the Sun's center lies between 12 and 18 degrees below the horizon. Astronomical twilight is followed or preceded by → nautical twilight. Most stars and other celestial objects can be seen during this phase. However, some of the fainter stars and galaxies may not be observable as long as the Sun is less than 18 degrees below the horizon. See also → civil twilight.
→ astronomical; → twilight.
Fr.: lumière rétrodiffusée
The light that has undergone → backscattering.
→ backscatter; → backscattering.
gu-ye âzaraxš (#)
Fr.: foudre en bulle
A rare form of lightning occurring as a bright red globe observed floating or moving through the atmosphere close to the ground. It usually is seen shortly before or after, or during, a → thunderstorm. Its duration varies from a few seconds to a few minutes. See also → Saint Elmo's fire.
beam of light
tâbe-ye nur (#)
Fr.: faisceau lumineux
A relatively large bundle of → rays of light. See also → pencil of light.
Fr.: lumière catoptrique
Light that is reflected from a curved surface mirror.
→ catoprtics; → light.
circularly polarized light
nur-e qotbide-ye dâyere-yi
Fr.: lumière polarisée circulairement
Light exhibiting → circular polarization.
Fr.: crépuscule civil
The time between sunset or sunrise and the moment when the Sun's center lies 6° below the horizon. It is followed or preceded by → nautical twilight. See also → astronomical twilight. In the morning, this twilight phase ends at sunrise. In the evening it begins at sunset. Civil twilight is the brightest of the three twilight phases. As the Earth's atmosphere scatters and reflects much of the Sun's rays, artificial lighting is generally not required in clear weather conditions to carry out most outdoor activities. Only the brightest stars and planets, like Venus and Jupiter, can be seen with the naked eye.
Fr.: vol d'accostage
The unpowered flight of a spacecraft or missile after propulsion cutoff or between the burnout of one stage and the ignition of the next.
nur-e hamdus (#)
Fr.: lumière cohérente
Light waves that have the same wavelength and possess a fixed phase relationship, as in a laser.
corpuscular theory of light
negare-ye karpuli-ye nur
Fr.: théorie corpusculaire de la lumière
Newton's theory according to which light is made up of point-like particles without any mass. It failed to explains several phenomena: simultaneous reflection and refraction at a semi-transparent boundary, interference, diffraction and polarization. Moreover, it requested that the speed of light be greater in a denser medium than in a rarer medium; this prediction is contrary to experimental results. In 1924 Louis de Broglie postulated that matter has not only a corpuscular nature but also a wave nature, and subsequent experiments confirmed de Broglie's model.
Fr.: météore de jour
A → meteor detected using → radar techniques during daylight or when skies are cloudy.
daylight saving time
vaxt-e nur anduzi, vaqt-e ~
Fr.: heure d'été
A system of adjusting the official local time in some countries in order to provide a better match between the hours of daylight and the active hours of work and school. The "saved" daylight is spent on evening activities which get more daylight, rather than being "wasted" while people sleep past dawn. Although known also as summer time, it includes the spring season and nearly half of autumn.
→ day; → light; saving, from save, from O.Fr. sauver, from L.L. salvare "to secure," from L. salvus "safe," PIE *solwos, from base *sol- "whole" (cf. O.Pers. haruva-, Av. haurva- "whole, intact," Mod.Pers. har "every, all; any," Skt. sarva- "whole, entire," Gk. holos "whole"); → time.
Vaxt, written vaqt
deflection of light
Fr.: déflexion de la lumière
The bending of a light ray under the gravitational effect of a massive body. → deflection angle.
→ deflection; → light.
diffuse galactic light
nur-e kahkašâni-ye paxšidé
Fr.: lumière galactique diffuse
A minor component of galactic light resulting from the diffusion of starlight by → interstellar dust near the → galactic plane.
Fr.: lumière de Drummond
A very brilliant white light which is the ignited flame of a mixture of oxygen and hydrogen projected against a block of calcium oxide (lime). Also called limelight. First working version produced by Lieutenant of the Royal Engineers, upon the Ordnance Trigonometrical Survey of Ireland (1826). It was used at night as a substitute for solar light. It was first employed in a theater in 1837 and was in wide use by the 1860s, among which in photography.
Named after Scottish engineer Thomas Drummond (1797-1840); → light.