An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 26 Search : distance
action at a distance
  ژیرش از دور   
žireš az dur

Fr.: action à distance   

The instantaneous action of a body on another body independently of the distance separating them. The description of → gravity by → Newton's law and → electrostatics by → Coulomb's law are examples of action at a distance. According to Newton, → gravitation acts directly and instantaneously between two objects. For example, if the Sun should suddenly break apart, the Earth's orbit would be affected instantaneously. However, action at a distance violates the → principle of relativistic causality. According to → general relativity, gravitational effects travel at the → speed of light. For modern physics there is no instantaneous action at a distance.

action; → distance.

angular diameter distance
  اپست ِ زاویه‌ای   
apest-e zâviye-yi

Fr.: distance angulaire   

1) The ratio of an object's → linear size to its → angular size (in → radians). It is used to convert observed angular separations into proper separations at the source.
2) In a → homogeneous Universe the angular diameter distance is solely a function of → redshift. In inhomogeneous cosmologies, → gravitational lensing by intervening metric fluctuations can cause magnification of the angular size - with associated change of flux density, since surface brightness is unaffected by lensing. Thus the apparent distance to objects at a given z becomes in effect a randomly fluctuating quantity.

angular; → diameter; → distance.

angular distance
  اپست ِ زاویه‌ای   
apest-e zâviye-yi

Fr.: distance angulaire   

Between two points A and B, the angle → subtended by lines drawn from an observing point O to A and B. Same as → angular separation.

angular; → distance.

angular size distance
  اپست ِ زاویه‌ای   
apest-e zâviye-yi

Fr.: distance angulaire   

Same as → angular diameter distance.

angular; → size; → distance.

aphelion distance
  اپست ِ اپاهوری   
apest-e apâhuri

Fr.: distance à l'aphélie   

The distance between the → Sun and an → object in orbit around it when they are at their farthest approach.

perihelion; → distance.

apparent distance
  دورا‌ی ِ پدیدار   
durâ-ye padidâr

Fr.: distance apparente   

The angular distance between two celestial bodies (e.g. the components of a binary star system), expressed in degrees, minutes and seconds of arc.

apparent; → distance.

comoving distance
  دورای ِ هم‌جنب   
durâ-ye hamjonb

Fr.: distance comobile   

1) A distance in → comoving coordinates between two points in space at a given cosmological time. In other words, the distance between two nearby objects in the Universe which remains constant with epoch if the two objects are moving with the → Hubble flow. Comoving time is deduced from the → Robertson Walker metric. The comoving distance corresponds to the current size of the → cosmic horizon, i.e. about 48 × 109light-years. See also → look-back time.
2) Line-of-sight comoving distance: The total line-of-sight comoving distance from us to a distant object is computed by integrating the infinitesimal comoving distance contributions between nearby events along the radial ray from redshift 0 to the object.
3) Transverse comoving distance: The comoving distance between two events at the same → redshift but separated on the sky by some angle.

comoving; → distance.

cosmic distance scale
  مرپل ِ دورای ِ کیهانی   
marpel-e durâ-ye keyhâni

Fr.: échelle des distances cosmiques   

Measurement of the distances to the farthest objects in the Universe based on a bootstrapping series of methods, each applicable to more distant objects, and each dependent on the previous methods.

cosmic; → distance; → scale.

cosmological distance
  دورای ِ کیهان‌شناسیک، ~ کیهان‌شناختی   
durâ-ye keyhânšenâsik, ~ keyhânšenâxti

Fr.: distance cosmologique   

The distance to a remote galaxy based on its redshift assuming that the redshift is caused by the → Doppler effect and reflects the general expansion of the Universe.

cosmological; → distance .

distance
  اپست، دورا، دوری   
apest, durâ (#), duri (#)

Fr.: distance   

1) The → separation/→ length in → space/→ time between two → things/→ events.
2) The state of being apart in space or time.
3) In cosmology three different distance definitions are used: → angular diameter distance, → proper motion distance, and → luminosity distance. In a → flat Universe these three approaches give the same result. However, in the → Robertson-Walker metric they give three different but related values. In particular, angular diameter distance has a maximum value; therefore an object that lies further away than a closer object can appear larger.
See also:
angular size distance, → apparent distance, → comoving distance, → cosmic distance scale, → cosmological distance, → distance function, → distance modulus, → distance to the horizon, → focal distance, → Hubble distance, → perihelion distance, → polar distance, → redshift-distance relation, → velocity-distance relation, → zenith distance.

M.E., from O.Fr. destance, from L. distantia "a standing apart," from distantem (nominative distans) "standing apart, separate, distant," pr.p. of distare "to stand apart," from → dis- "apart, off" + stare "to stand," (cf. Mod.Pers. istâdan "to stand," O.Pers./Av. sta- "to stand, stand still; set," Skt. sthâ- "to stand," Gk. histemi "put, place, weigh," stasis "a standing still").

Apest, literally "standing apart," from apa- prefix denoting "separation, away, off," → dis-, + est variant of ist, present stem of istâdan, to stand," as above;" cf. Choresmian bst "to stand apart," from *apa- + st- "to stand," → stand.
Durâ, duri, noun from dur "far, distant, remote," Mid.Pers. dur, O.Pers. dūra- "far (in time or space)," Av. dūra-, from dav- "to move away," cf. Skt. dūrá- "distant, far."

distance function
  کریای ِ اپست   
karyâ-ye apest

Fr.: fonction de distance   

Same as → metric.

distance + → function.

distance modulus
  پیمون ِ اپست   
peymun-e apest

Fr.: module de distance   

The difference between the → apparent magnitude (m) of a star or galaxy and its → absolute magnitude (M). It is given by m - M = 5 log d - 5, where d is the distance in → parsecs. For an object that is 10 pc away, the distance modulus is zero.

distance; → modulus.

distance to the horizon
  اپست ِ افق   
apest-e ofoq

Fr.: distance à l'horizon   

The distance separating an observer and the → apparent horizon of the place. Neglecting the → atmospheric refraction, it is given by: d = (2Rh)1/2, where R is the radius of the Earth and h is the observer's height. This can be approximated to: d (km) = 3.57(h)1/2 for a typical value of R = 6378 km. The atmospheric refraction, however, makes the thing more complex, depending on the temperature and density variations along the line of sight. Generally, refraction pushes the apparent horizon about 10% farther.

distance; → horizon.

focal distance
  دورای ِ کانونی   
durâ-ye kânuni

Fr.: distance focale   

same as → focal length.

focal; → distance.

galactocentric distance
  دورای ِ کهکشان-مرکزی   
durâ-ye kahkešân-markazi

Fr.: distance galactocentrique   

The distance from the center of a galaxy.

galactocentric; → distance.

Hubble distance
  دورای ِ هابل   
durâ-ye Hubble

Fr.: distance de Hubble   

The distance from the Earth to the → cosmic horizon which marks the edge of the → observable Universe. Same as → Hubble radius, → Hubble length, and → cosmic horizon.

Hubble; → distance.

luminosity distance
  دورای ِ تابندگی   
durâ-ye tâbandegi

Fr.: distance de luminosité   

Distance derived by comparison of → observed and → intrinsic luminosities. if an object has a known luminosity L, and the observed flux is S, the luminosity distance is defined by dL = (L/4πS)1/2. For very distant galaxies, spatial curvature is important and the luminosity distance differs from other measures of distance.

luminosity; → distance.

minimum orbit intersection distance (MOID)
  دورای کمینه‌ی ِ اندرسکنج ِ مدار   
durâ-ye kamine-ye andarsekanj-e madâr

Fr.: distance minimale d'intersection d'une orbite   

The minimum distance between the paths of two orbiting objects around a → primary. Such distance between an object and Earth is called Earth MOID.

minimum; → orbit; → intersection; → distance.

perihelion distance
  اپست ِ پیراهوری   
apest-e pirâhuri

Fr.: distance au périhélie   

The distance between the → Sun and an → object in orbit around it when they are at their closest approach.

perihelion; → distance.

polar distance
  دورای ِ قطبی   
durâ-ye qotbi

Fr.: distance polaire   

The angular distance of an object from a celestial pole. It is equal to 90° minus the object's declination.

polar; → distance.

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