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. |
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. |
chromatic aberration birâheš-e rangi Fr.: aberration chromatique A defect in a lens that causes it to concentrate the various colors in a beam of light at various point, thus producing color fringes. → chromatic; → aberration. |
constant of aberration pâyâ-ye birâheš Fr.: constante d'aberration The maximum amount of the apparent yearly displacement of a star, resulting from the → aberration of starlight. The value of the constant of aberration, κ, at J2000.0 is 20".49552. κ = (v/c) csc 1", where v is the average speed of the Earth about the Sun and c is the → speed of light in vacuum. The Earth's speed is given by: v = 2πa / [P(1 - e^{2})^{1/2}], where a is the → semi-major axis of the Earth's orbit, e is the → eccentricity of the Earth's orbit, and P is the → sidereal period of the Earth. Same as → constant of annual aberration. See also → constant of diurnal aberration. → constant; → aberration. |
constant of annual aberration pâyâ-ye birâheš sâlâné Fr.: constante d'aberration annuelle Same as → constant of aberration. → constant; → annual; → aberration. |
constant of diurnal aberration pâyâ-ye birâheš ruzâné Fr.: constante d'aberration diurne The quantity 0''.3200 ρ cos φ', where ρ is the geocentric distance of the observer measured in units of → equatorial radius the Earth and φ' is the observer's → geocentric latitude. The numerical part is equal to 2πa csc1'' / (cP), where a is the equatorial radius of the Earth, P is its → sidereal period of rotation, and c is the → speed of light in vacuum. → constant; → diurnal; → aberration. |
diurnal aberration birâheš-e ruzâné Fr.: aberration diurne The aberration of a star's position due to the rotation of the Earth. Its value depends on the latitude of the observer, and is only 0''.32 in the case of an observer at the equator, where the rotational velocity is greatest. → diurnal; → aberration. |
e-term of aberration birâheš-e tarm-e e Fr.: aberration elliptique The same as → elliptic aberration. e, → elliptic; → term; → aberration. |
elliptic aberration birâheš-e biyzigun Fr.: aberration elliptique That part of → annual aberration proportional to the → eccentricity of the Earth's orbit. → elliptic; → aberration. |
optical aberration birâheš-e nuri Fr.: aberration optique An imperfection in the imaging properties of a lens or mirror. The main aberrations are → chromatic aberration, → spherical aberration, → coma, → astigmatism, → field curvature, → distortion. → optical; → aberration. |
planetary aberration birâheš-e sayyâreyi Fr.: aberration planétaire The difference between the true position of a planet and its apparent position, due to the time required for light to travel the distance from the planet to Earth. Correction for planetary aberration is necessary in determining orbits. → planetary; → aberration. |
relativistic aberration birâheš-e bâzânigimand Fr.: aberration relativiste The aberration of light for an object moving with
→ relativistic speed. In contrast to the classical case,
the → Lorentz transformation between the
→ rest frame of the observer and that of the object
must be used. Relativistic aberration is expressed by the equation:
cos φ' = (cos φ - → relativistic; → aberration. |