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geologic time zamân-e zaminšenâxti (#) Fr.: temps géologique The long span of time from the end of the formation of Earth during which our planet underwent its major transformations. |
gravitational lensing time delay derang-e zâyide-ye lenzeš-e gerâneši Fr.: retard dû à l'effet de lentille gravitationnelle The difference in light travel times along the various light paths from the source to the observer when the source image is divided into several images because of → gravitational lensing. According to the theory of → general relativity, light rays are deflected in the vicinity of massive objects. If the light source and the deflector are sufficiently well aligned with the observer, and obey some conditions on their distances (→ Einstein radius), we can observe several (generally distorted and magnified) images of the source. A property of → strong lensing is that the light travel time from the source to the observer is generally not identical for the different images. In other words, we not only see several images of one same object, but we also see this object, in each image, at different times. This means, in one image the lensed object will be observed before the other image. Given a physical model of the gravitational lens, the light travel time for each image can be computed. The expression giving the time delay has two components: a term is called → geometric delay, and the second term, known as the → Shapiro time delay. The latter is due to time dilation by the gravitational field of the lens, a direct consequence of general relativity. See also → time delay distance. → gravitational; → lensing; → time; → delay. |
Greenwich Apparent Sidereal Time (GAST) zamân-e axtari-ye padidâr-e Greenwich Fr.: temps sidéral apparent de Greenwich The → Greenwich Mean Sidereal Time corrected for → nutation. Therefore, it is measured with respect to the → true vernal equinox. GAST and GMST differ by the → equation of the equinoxes. → Greenwich Meridian; → apparent; → sidereal; → time. |
Greenwich Mean Sidereal Time (GMST) zamân-e axtari-ye miyângin-e Greenwich Fr.: temps sidéral moyen de Greenwich The → sidereal time related to the angle between the → prime meridian and the → mean vernal equinox, measured in the plane of the equator. → mean; → Greenwich Meridian; → sidereal; → time. |
Hubble time zamân-e Hubble (#) Fr.: temps de Hubble An estimate for the age of the Universe by presuming that the Universe has always expanded at the same rate as it is expanding today. It is the inverse of the → Hubble-Lemaitre constant: t_{H} = 1/H_{0}. Also called the Hubble age or the Hubble period. |
integration time zamân-e dorostâleš Fr.: temps d'intégration The time during which a detector integrates the incoming photons. → integration; → time. |
International Atomic Time (TAI) zamân-e atomi-ye jahâni (#) Fr.: Temps Atomique International (TAI) A weighted average of the time kept by about 200 caesium atomic clocks in over 50 national laboratories worldwide. It has been available since 1955, and became the international standard on which UTC is based on January 1972. → international; → atomic; → time. |
Kelvin-Helmholtz time zamân-e Kelvin-Helmholtz Fr.: échelle de temps de Kelvin-Helmholtz The characteristic time that would be required for a contracting spherical cloud of gas to transform all its → gravitational energy into → thermal energy. It is given by the relation: t_{KH} ≅ GM^{2}/RL, where G is the → gravitational constant, M is the mass of the cloud, R the initial radius, and L the → luminosity. The Kelvin-Helmholtz time scale determines how quickly a pre-main sequence star contracts before → nuclear fusion starts. For the Sun it is 3 x 10^{7} years, which also represents the time scale on which the Sun would contract if its nuclear source were turned off. Moreover, this time scale indicates that the gravitational energy cannot account for the solar luminosity. For a → massive star of M = 30 Msun, the Kelvin-Helmholtz time is only about 3 x 10^{4} years. |
light-time nur-zamân, zamân-e nuri (#) Fr.: temps-lumière The time it takes for light, travelling at about 300 000 km per second, to travel a certain distance. |
local sidereal time zamân-e axtari-ye mahali Fr.: temps sidéral local Local time measured by the apparent motion of the stars. It is the most useful form of sidereal time since it gives the right ascension of a transiting celestial object at a given location. |
local time zamân-e mahali (#) Fr.: temps local Time based upon the local meridian as reference, in contrast to that of the time zone within which the place is located, or the Greenwich time. |
look-back time zamân-e negâh bé gozašté Fr.: temps de regard en arrière The time that has elapsed since the light was emitted from a distant object (of → redshift z). Because → light moves at a → constant → speed, it takes a finite time to travel from distant objects. Hence, we "see" distant objects at a point in time in their past. In other words, look-back time is the difference between the age of the Universe now and the age of the Universe at the time the photons were emitted from the object. See also → comoving distance. Zamân, → time; negâh, → look; gozašté "past, passed" (from gozaštan "to pass, proceed, go on," variant gozâštan "to put, to place, let, allow;" Mid.Pers. widardan, widâštan "to pass, to let pass (by);" O.Pers. vitar- "to pass across," viyatarayam "I put across;" Av. vi-tar- "to pass across," from vi- "apart, away from" (O.Pers. viy- "apart, away;" Av. vi- "apart, away;" cf. Skt. vi- "apart, asunder, away, out;" L. vitare "to avoid, turn aside") + O.Pers./Av. tar- "to cross over"). |
mean sidereal time zamân-e axtari-ye miyângin (#) Fr.: temps sidéral moyen The hour angle of the mean equinox for a given observer. |
mean solar time zamân-e xoršidi-ye miyângin (#) Fr.: temps solaire moyen The time since the mean Sun crossed the meridian with 12 hours added to make the day begin at midnight. |
Minkowski space-time fazâ-zamân-e Minkowski (#) Fr.: espace-temps de Minkowski A completely flat four-dimensional space, which contains no gravitating matter, used in the theory of special relativity. → Minkowski metric; → space-time. |
multivariate time series seri-ye zamâni-ye basvartâ Fr.: série temporelle multivariée A → time series consisting of two or more → univariate time series which share the same time period. As an example, if we record wind velocity and wind direction at the same instant of time, we have a multi-variate time series, specifically a bivariate one. → multivariate; → time; → series. |
nuclear time scale marpel-e zamâni-ye haste-yi Fr.: échelle de temps nucléaire The time required for a star to exhaust its hydrogen (H) supply in → nuclear fusion. The nuclear time scale is given by the relation t = E/L, where E is the total nuclear energy that can be generated by a star and L is the stellar → luminosity. Assuming that the end point of fusion is → iron (Fe), the → atomic mass difference between H and Fe is Δm = 0.008 m_{H}. Therefore, the maximum amount of energy a star with a hydrogen mass M can release is Δ M = 0.008 Mc^{2}. The nuclear time scale is then: t = 0.008 c^{2}M/L. However, stars use up only a fraction of their hydrogen supply, because only the inner part of the star is hot enough for fusion. For example, the Sun will spend only about 10% of its hydrogen supply before evolving into a → red giant. In other words, the solar life time on the → main sequence is about 10^{10} years. |
Ohmic decay time zamân-e tabâhi-ye Ohmi Fr.: temps de dissipation ohmique An upper bound on the time scale on which the magnetic field of a system would decay in the absence of any other agent. It is expressed as: τ_{μ} = R^{2} / μ, where R is the scale size of the system, η the magnetic diffusivity (η = 1 / μσ, where μ is the magnetic permeability and σ the electrical conductivity). For a star like the Sun, τ_{μ} ≅ 10^{10} years, so a fossil magnetic field could survive for the star's lifetime on the main sequence. For the Earth, τ_{μ} ≅ 10^{4} years, so a → dynamo is required to explain the persistence of the geomagnetic field. |
overhead time zamân-e bâlâsar Fr.: The part of observing time at a telescope which is not directly used for science, such as the time spent for detector read-out, changing instruments, focusing, etc. Bâlâ "up, above, high, elevated, height" (variants boland "high, tall, elevated, sublime," borz "height, magnitude" (it occurs also in the name of the mountain chain Alborz), Laki dialect berg "hill, mountain;" Mid.Pers. buland "high;" O.Pers. baršan- "height;" Av. barəz- "high, mount," barezan- "height;" cf. Skt. bhrant- "high;" L. fortis "strong" (Fr. and E. force); O.E. burg, burh "castle, fortified place," from P.Gmc. *burgs "fortress;" Ger. Burg "castle," Goth. baurgs "city," E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg; PIE base *bhergh- "high") + sar, → head. |
photon escape time zamân-e goriz-e foton Fr.: temps d'échappement des photons The time required for a photon created in the Sun's core to attain the → photosphere and leave the Sun. If the photons were free to escape, they would take a time of only R/c (a couple of seconds) to reach the surface, where R is the Solar radius and c the speed of light. The solar material is, however, very opaque, so that photons travel only a short distance before interacting with other particles. Therefore, photons undergo a very large number of → random walks before arriving at the surface by chance. The typical time is approximately 5 x 10^{4} years for a constant density Sun. |
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