An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



Number of Results: 17 Search : parallax
annual parallax
  دیدگشت ِ سالانه   
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.

annual; → parallax.

diurnal parallax
  دیدگشت ِ روزانه   
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.

diurnal; → parallax.

dynamical parallax
  دیدگشت ِ توانیک   
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.

dynamical; → parallax.

geocentric parallax
  دیدگشت ِ زمین‌مرکزی   
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
  دیدگشت ِ هورمرکزی   
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
  دیدگشت ِ افقی   
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
  دیدگشت ِ مانگی   
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.

lunar; → parallax.

mean parallax
  دیدگشت ِ میانگین   
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.

mean; → 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
  زاویه‌ی ِ دیدگشت   
zâviye-ye didgašt

Fr.: angle de parallaxe   

The angular displacement associated with → parallax.

parallax; → angle.

photometric parallax
  دیدگشت ِ شیدسنجیک   
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
  دیدگشت ِ دیریاز   
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).

secular; → parallax.

solar parallax
  دیدگشت ِ خورشیدی   
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.

solar; → parallax.

spectroscopic parallax
  دیدگشت ِ بیناب‌نمایی   
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
  دیدگشت ِ آماری   
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
  دیدگشت ِ ستاره   
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).
See also:
annual parallax, → diurnal parallax, → dynamical parallax, → geocentric parallax, → heliocentric parallax, → horizontal parallax, → lunar parallax, → mean parallax, → parallactic ellipse, → parsec, → photometric parallax, → secular parallax, → solar parallax, → spectroscopic parallax, → statistical parallax, → trigonometric parallax.

stellar; → parallax.

trigonometric parallax
  دیدگشت ِ سه‌برسنجی، ~ سه‌برسنجیک   
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.