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aberration birâheš Fr.: aberration 1) An imperfection in the imaging properties of a → lens
or → mirror.
The main aberrations are → chromatic aberration,
→ spherical aberration, → coma,
→ astigmatism, → distortion,
and → field curvature. Aberration, from L. aberrationem, from aberrare "go astray," → aberrate. Birâheš, from birâidan, → aberrate. |
aberration angle zâviye-ye birâheš Fr.: angle d'aberration The angle tilt required by the → stellar aberration phenomenon in order that a moving telescope points directly to a star. → aberration; → angle. |
aberration constant pâyâ-ye birâheš Fr.: constante d'aberration Same as → constant of aberration. → aberration; → constant. |
aberration of light birâheš-e nur 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. |
aberration orbit madâr-e birâheš Fr.: orbite d'aberration The apparent path described by a star on the → celestial sphere due → annual aberration. A star at the → ecliptic pole is seen to move around a circle of angular radius about 20".50, once a year. A star on the → ecliptic oscillates to and fro along a line of angular half-length 20".50. At an intermediate → celestial latitude, β, the aberration orbit is an ellipse, with semi-major axis 20".50 and semi-minor axis (20".50) sin β. → aberration; → orbit. |
aberrational birâheši Fr.: aberrationnel Of or pertaining to → aberration. → aberration; → -al. |
aberrational day number šomâre-ye ruz-e birâheši Fr.: nombre de jours d'aberration A → Besselian day number denoted by C or D. → aberration; → -al; → day; → number. |
aberrational ellipse beyzi-ye birâheši Fr.: ellipse d'aberration The → locus of points on the → celestial sphere occupied by a star during the annual → revolution of the → Earth about the → Sun due to → annual aberration. annual aberration. → aberrational; → ellipse. |
absolute acceleration šetâb-e avast Fr.: accélération absolue For a body that moves with respect to a rotating → reference frame, the vector sum of the observed acceleration, the → Coriolis acceleration, and the → centrifugal acceleration. See also the → Coriolis theorem. → absolute; → acceleration. |
absorptivity-emissivity ratio vâbar-e daršamâyi/gosilâyi Fr.: rapport absorptivité-émissivité For a spatial object, the ratio between its absorption of solar radiation and its infrared emission. → absorptivity; → emissivity; → ratio. |
acceleration šetâb (#) Fr.: accélération The rate at which the velocity of an object changes with time. Acceleration, from accelerate, from L. accelerare "quicken," from → ad- "to" + celerare "hasten," from celer "swift" (cf. Skt. car, carati "to move, go, drive," Gk. keles "fast horse, horse race," Av. kar- "to walk, move, go around," Mod.Pers. cal, calidan "to move, to go, to walk" (jald? "quick, active, brisk"), Gilaki/Hamadâni jal "quick, fast,"Lori žil "motion, impulse"); PIE *kel- "to drive, set in swift motion." Šetâb "quickness, haste, speed," Mid.Pers. ôštâp "hurry, haste," ôštâftan "to hurry, hasten," from *abi.stap-, from abi- " to; in addition to; against" + *stap- "to oppress," Arm. (loanword) štap "haste, trouble." |
acceleration of gravity šetâb-e gerâni Fr.: accélération de la gravité The acceleration that an object experiences because of gravity when it falls freely close to the surface of a massive body, such as a planet. Same as → gravitational acceleration. → acceleration; → gravity. |
acceleration parameter pârâmun-e šetâb Fr.: paramètre d'accéleration A measure of the departure from a constant rate of the acceleration of the Universe, expressed by: q(t) = R(t)R^{ ..}(t)/R^{ .}^{2}(t), where R(t) represents the size of the Universe at time t. Traditionally, a negative sign is inserted in the above equation for the → deceleration parameter. → acceleration; → parameter. |
agglomeration bargolemeš Fr.: agglomération 1) A jumbled cluster or mass of varied parts. Verbal noun of → agglomerate. |
alteration degarguni, degarguneš Fr.: altération 1) The act or process of altering; the state of being altered. Verbal noun of → alter. |
angular acceleration šetâb-e zâviye-yi Fr.: accélération angulaire The rate of change of → angular velocity. It is equal to the → first derivative of the → angular velocity: α = dω/dt =d^{2}θ/dt^{2} = a_{t}/r, where θ is the angle rotated, a_{t} is the linear tangential acceleration, and r is the radius of circular path. → angular; → acceleration. |
angular separation jodâyiye zâviye-yi Fr.: séparation angulaire Same as → angular distance. → angular; → separation. |
annual aberration birâheš-e sâlâné Fr.: aberration annuelle The apparent, small displacement in position of a star during the year due to the → aberration of starlight. It depends on the → celestial latitude, and its maximum value is about 20''.50. See also → constant of aberration; → aberration orbit. → annual; → aberration. |
aperture ratio vâbar-e dahâné Fr.: rapport d'ouverture The ratio of the effective diameter of a lens or mirror to its focal length. |
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