annual parallax didgašt-e sâlâné Fr.: parallaxe annuelle The difference in position of a star as seen from the → Earth and → Sun, i.e. the angle subtended at a star by the mean → radius of the Earth's orbit around the Sun. Same as → heliocentric parallax. Because the Earth revolves around the Sun, we observe the sky from a constantly moving position in space. Therefore, we should expect to see an annual effect, in which the positions of nearby objects appear to oscillate back and forth in response to our motion around the Sun. This does in fact happen, but the distances to even the nearest stars are so great that we need to make careful observations with a telescope to detect it. The annual parallax of the nearest star, → Proxima Centauri, is 0.762 arcsec, which is too small for our → acuity of vision. |
diurnal parallax didgašt-e ruzâné Fr.: parallaxe diurne The apparent difference between the position of a celestial object measured from the Earth's surface and the position that would be recorded by a hypothetical observer at the center of the Earth. Same as → geocentric parallax. |
dynamical parallax didgašt-e tavânik Fr.: parallaxe dynamique A method for deriving the distance to a binary star. The angular diameter of the orbit of the stars around each other and their apparent brightness are observed. By applying Kepler's laws and the mass-luminosity relation, the distance of the binary star can be calculated. |
geocentric parallax didgašt-e zamin-markazi Fr.: parallaxe géocentrique The difference between the direction of an object as seen from a point on the surface of the Earth and the direction in which it would be seen from the Earth's center. Also known as → diurnal parallax. → geocentric; → parallax. |
heliocentric parallax didgašt-e hurmarkazi Fr.: parallaxe héliocentrique The parallax of a celestial body when viewed from two points in the Earth's orbit around the Sun. More specifically, the angular difference in a celestial object's position as seen from the center of the Sun and the center of the Earth. Also called → annual parallax. → heliocentric; → parallax. |
horizontal parallax didgašt-e ofoqi Fr.: parallaxe horizontale The angle under which the radius of the Earth at the place of observation would be seen from a celestial body when it is in the horizon (at the instant of rising or setting). The amount varies with the latitude since the Earth is not exactly spherical, and is greatest at equator. → horizontal; → parallax. |
lunar parallax didgašt-e mângi Fr.: parallaxe lunaire The apparent shift in the → Moon's position relative to the background stars when observed from different places on Earth. The first parallax determination was for the Moon, by Hipparchus (150 B.C.). He determined that one-fifth of the Sun's angular diameter corresponded to the lunar parallax between Hellespont and Alexandria. |
mean parallax didgašt-e miyângin Fr.: parallaxe moyenne The parallax, derived by means of statistical studies of brightness and motions, for a large group of stars whose individual parallaxes cannot be measured. |
parallax didgašt (#) Fr.: parallaxe The apparent → shift of a nearby object's → position in relation to more distant ones when the nearby object is observed from different → viewing angles. See also → stellar parallax. From M.Fr. parallaxe, from Gk. parallaxis "change, alteration," from parallassein "to alter, make things alternate," from → para- "beside" + allassein "to change," from allos "other;" → alias. Didgašt, literally "view change," from did "sight, view; eye," from didan "to see" (Mid.Pers. ditan "to see, regard, catch sight of, contemplate, experience;" O.Pers. dī- "to see;" Av. dā(y)- "to see," didāti "sees;" cf. Skt. dhī- "to perceive, think, ponder; thought, reflection, meditation," dādhye; Gk. dedorka "have seen") + gašt "change, alteration," past stem of gaštan, gardidan "to turn, to change" (Mid.Pers. vartitan; Av. varət- "to turn, revolve;" Skt. vrt- "to turn, roll," vartate "it turns round, rolls;" L. vertere "to turn;" O.H.G. werden "to become;" PIE base *wer- "to turn, bend"). |
parallax angle zâviye-ye didgašt Fr.: angle de parallaxe The angular displacement associated with → parallax. |
photometric parallax didgašt-e šidsanjik Fr.: parallaxe photométrique A method of deriving the distance of a star using its → apparent magnitude and the → absolute magnitude inferred from its → spectral type. This is a misnomer, because the method has nothing to do with parallax; → photometric; → parallax. |
secular parallax didgašt-e diryâz Fr.: parallaxe séculaire The angle subtended at a star by a baseline that is the distance the Sun moves in a given interval of time with respect to the local standard of rest (4.09 AU per year). |
solar parallax didgašt-e xoršidi Fr.: parallaxe solaire The angle subtended (8''.79) by the → equatorial radius of the Earth at a distance of 1 → astronomical unit. |
spectroscopic parallax didgašt-e binâbnamâyi Fr.: parallaxe spectroscopique The measurement of a stellar distance by the absolute magnitude derived from the luminosity criteria of the spectrum and the apparent magnitude of the star. → spectroscopic; → parallax. |
statistical parallax didgašt-e âmâri Fr.: parallaxe statistique The mean parallax of a group of stars that are all at approximately the same distance, as determined from their radial velocities and proper motions. → statistical; → parallax. |
stellar parallax didgašt-e setâré Fr.: parallaxe stellaire The apparent → difference in the → position
of a → celestial object as seen by an → observer
from two widely separated → locations.
The parallax of an object can be used to derive its → distance.
The relationship between the → parallax angle
p (measured in seconds of arc) and the distance d (measured in
→ astronomical units)
is given by d = 206,264 / p.
For a parallax angle p = 1'', the distance to the
star would correspond to 206,264 AU. By convention, the distance unit
→ parsec is defined to be equivalent to 206,264
AU. Therefore, the parallax relation takes the much
simpler form: d (in pc) = 1/p (in seconds of arc).
The first star whose parallax was measured was → 61 Cygni
(Bessel, 1838). |
trigonometric parallax didgašt-e sebarsanji, ~ sebarsanjik Fr.: parallaxe trigonométrique The → parallax of a nearby star (less than 300 → light-years) against the background of more distant stars resulting from the motion of the Earth in its orbit around the Sun. → trigonometric; → parallax. |