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
Fr.: distance angulaire
1) The ratio of an object's → linear size (l)
to its → angular size (δθ, in
→ radians), that is
DA = l/δθ.
It is used to convert observed angular separations into proper separations
at the source.
Fr.: distance angulaire
angular size distance
Fr.: distance angulaire
Same as → angular diameter distance.
Fr.: distance à l'aphélie
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.
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:
DC = Dproper .
In terms of → redshift (z),
it is the proper distance multiplied by (1 + z).
At the present epoch, i.e. a = a(tobs) = 1,
DC = Dproper.
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.
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.
apest, durâ (#), duri (#)
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.
Fr.: fonction de distance
Same as → metric.
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
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.
Fr.: distance focale
same as → focal length.
Fr.: distance galactocentrique
The distance from the center of a galaxy.
Fr.: distance de Hubble
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.
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
DL = (L/4πS)1/2.
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.
Fr.: distance au périhélie