An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 104 Search : time
geologic time
  زمان ِ زمین‌شناختی   
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.

geologic; → time.

gravitational lensing time delay
  درنگ ِ زاییده‌ی ِ لنزش ِ گرانشی   
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)
  زمان ِ اختری ِ پدیدار ِ گرینویچ   
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)
  زمان ِ اختری ِ میانگین ِ گرینویچ   
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
  زمان ِ هابل   
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: tH = 1/H0. Also called the Hubble age or the Hubble period.

Hubble; → time.

integration time
  زمان ِ دُرُستالش   
zamân-e dorostâleš

Fr.: temps d'intégration   

The time during which a detector integrates the incoming photons.

integration; → time.

International Atomic Time (TAI)
  زمان ِ اتمی ِ جهانی   
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
  زمان ِ کلوین-هلمهولتس   
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: tKH ≅ GM2/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 107 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 104 years.

Kelvin-Helmholtz contraction; → time.

  نور-زمان، زمان ِ نوری   
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.

light; → time.

local sidereal time
  زمان ِ اختری ِ محلی   
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; → sidereal; → time.

local time
  زمان ِ محلی   
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.

local; → time.

look-back time
  زمان ِ نگاه به گذشته   
zamân-e negâh bé gozašté

Fr.: temps de retour 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.

look; → back; → time.

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
  زمان ِ اختری ِ میانگین   
zamân-e axtari-ye miyângin (#)

Fr.: temps sidéral moyen   

The hour angle of the mean equinox for a given observer.

mean; → sidereal; → time.

mean solar time
  زمان ِ خورشیدی ِ میانگین   
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.

mean; → solar; → time.

Minkowski space-time
  فضا-زمان ِ مینکوفسکی   
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
  سری ِ زمانی ِ بسورتا   
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
  مرپل ِ زمانی ِ هسته‌ای   
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 mH. Therefore, the maximum amount of energy a star with a hydrogen mass M can release is Δ M = 0.008 Mc2. The nuclear time scale is then: t = 0.008 c2M/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 1010 years.

nuclear; → time; → scale.

Ohmic decay time
  زمان ِ تباهی ِ اُهمی   
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: τμ = R2 / 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, τμ  ≅ 1010 years, so a fossil magnetic field could survive for the star's lifetime on the main sequence. For the Earth, τμ  ≅ 104 years, so a → dynamo is required to explain the persistence of the geomagnetic field.

Ohmic; → decay; → time.

overhead time
  زمانِ بالاسر   
zamân-e bâlâsar


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.

over-; → head; → time.

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
  زمان ِ گریز ِ فوتون   
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 104 years for a constant density Sun.

photon; → escape; → time.

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