action at a distance 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. |
angular diameter distance apest-e zâviye-yi Fr.: distance angulaire 1) The ratio of an object's → linear size (l)
to its → angular size (δθ, in
→ radians), that is
D_{A} = l/δθ.
It is used to convert observed angular separations into proper separations
at the source. |
angular distance 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 size distance apest-e zâviye-yi Fr.: distance angulaire Same as → angular diameter distance. |
aphelion distance 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 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. |
comoving distance apest-e ham-miyâv 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.
More specifically, it is the → proper distance
divided by the ratio of the → scale factor
of the Universe between then, a(t)_{em},
and now, a(t)_{obs}:
D_{C} = D_{proper} .
[a(t)_{obs}/a(t)_{em}].
In terms of → redshift (z),
it is the proper distance multiplied by (1 + z).
At the present epoch, i.e. a = a(t_{obs}) = 1,
D_{C} = D_{proper}.
If the objects have no peculiar velocity their comoving distance at
any time is the same as their distance today. |
cosmic distance scale 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. |
cosmological distance 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 apest, durâ (#), duri (#) Fr.: distance 1) The separation/length in space/time between two things/events. M.E., from O.Fr., 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. |
distance function karyâ-ye apest Fr.: fonction de distance Same as → metric. |
distance modulus 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 to the horizon 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. |
focal distance durâ-ye kânuni Fr.: distance focale same as → focal length. |
galactocentric distance durâ-ye kahkešân-markazi Fr.: distance galactocentrique The distance from the center of a galaxy. → galactocentric; → distance. |
Hubble distance 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. |
light-travel distance apest-e safar-e nur Fr.: distance du voyage de la lumière The distance traversed by a photon between the time it is emitted and the time it reaches the observer. It is also referred to as the → look-back time. |
luminosity distance apast-e tâbandegi Fr.: distance de luminosité 1) 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
D_{L} = (L/4πS)^{1/2}. → luminosity; → distance. |
minimum orbit intersection distance (MOID) 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 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. |