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duration pâyeš Fr.: durée Continuance in time; a period of existence or persistence; length of time during which anything continues. Noun of action from L. durare "to harden," → during. Pâyeš, noun of action from pâyidan, → last (v.). |
dust coagulation mâseš-e qobâr, roceš-e ~ Fr.: coagulation de la poussière A process of formation of → dust grains in → interstellar medium and → protoplanetary disks, in which randomly colliding aggregates may stick together. → dust; → coagulation. |
dust emission gosil-e qobâr Fr.: émission des poussières Thermal emission in infrared from interstellar → dust grains receiving photons. Dust grains absorb ultraviolet and visible light emitted by nearby stars and re-radiate in the infrared wavelengths. Since the infrared light is of lower energy than the ultraviolet/visible light, the difference goes into heating the dust grain. Typical temperatures for interstellar grains are tens of degrees Kelvin. |
dust obscuration tirešod pat qobâr Fr.: obscurcissement par la poussière The → absorption of → electromagnetic radiation from an astrophysical object by → dust grains associated with that object. → dust; → obscuration. Tiregi, → obscuration, pat, → by; qobâr, → dust. |
dynamical disruption gosixt-e tavânik Fr.: rupture dynamique The process whereby a → bound system, such as a → binary system or a → globular cluster, is broken apart. → dynamical; → disruption. |
dynamical friction mâleš-e tavânik Fr.: frottement dynamique The gravitational interaction between a relatively massive body and a field of much less massive bodies through which the massive body travels. As a result, the moving body loses → momentum and → kinetic energy. An example of dynamical friction is the sinking of massive stars to the center of a → star cluster, a process called → mass segregation. Dynamical friction plays an important role in → stellar dynamics. It was first quantified by Chandrasekhar (1943). |
dynamical relaxation vâheleš-e tavânik Fr.: relaxation dynamique The evolution over time of a gravitationally → bound system consisting of N components because of encounters between the components, as studied in → stellar dynamics. Due to this process, in a → star cluster, → low-mass stars may acquire larger random velocities, and consequently occupy a larger volume than → high-mass stars. As a result, massive stars sink to the cluster centre on a time-scale that is inversely proportional to their mass. See also → mass segregation. → dynamical; → relaxation. |
E-mode polarization qotbeš-e tarz-e E Fr.: polarisation en mode E A → polarization component in the → cosmic microwave background radiation that depends only on → gradient, is independent of → curl and does not have → handedness. In contrast to the → B-mode, the E-mode may be due to both the → scalar perturbations and → tensor perturbations. E, indicating electric-field like; → mode; → polarization. |
e-term of aberration birâheš-e tarm-e e Fr.: aberration elliptique The same as → elliptic aberration. e, → elliptic; → term; → aberration. |
Earth's rotation carxeš-e zamin (#) Fr.: rotation de la Terre The natural motion of the Earth around its own axis, which takes place once in a → sidereal day. The Earth rotates toward the → east, in the same direction as it revolves around the Sun. If viewed from the north celestial pole, the Earth turns → counterclockwise. The opposite is true when the Earth is viewed from the south celestial pole. The Earth's rotation is responsible for the diurnal cycles of day and night, and also causes the apparent movement of the Sun across the sky. The Earth's rotation velocity at the → equator is 1,673 km h^{-1} or about 465 m s^{-1}. More generally, at the → latitude φ it is given by: v_{φ} = v_{eq} cos φ, where v_{eq} is the rotation velocity at the equator. The Earth's rotation is gradually slowing down under the action of the → tides, which are generated by the → gravitational attraction of the → Moon. As the result of this → tidal friction, the day is becoming longer at a rate of about 2 milliseconds, or 0.002 seconds, per century (or one second every 50,000 years). Moreover, the loss of the Earth's → rotational angular momentum increases the Moon's → orbital angular momentum, because the angular momentum of the → Earth-Moon system is conserved. In consequence, the Moon slowly recedes from the Earth by about 4 cm per year, which leads to increasing its orbital period and the length of a month as well. |
eastern elongation derâzeš-e xâvari Fr.: élongation est The position of a planet when it can be seen in the western sky just after sunset. → eastern; → elongation. |
eclipse obscuration tirešod-e xorgereft Fr.: obscuration de l'éclipse The fraction of the Sun's area occulted by the Moon. It should not be confused with → eclipse magnitude, which is the fraction of the Sun's diameter occulted by the Moon. Eclipse obscuration may be expressed as either a percentage or a decimal fraction (e.g., 50% or 0.50) (F. Espenak, NASA). |
eddy diffusion paxš-e gižâvi Fr.: diffusion turbulente A macroscopic process that occurs in a → fluid because of the relative motions induced by the non-uniform → turbulent motions of the fluid. Also known as turbulent → diffusion. Eddy diffusion may occur in an atmosphere if it is unstable against turbulence. It dominates the atmosphere below the homopause. See also → molecular diffusion. |
edition 1) virâyeš (#); 2) virâst (#) Fr.: édition 1) The act or process of editing. Verbal noun of → edit. |
education farhizeš (#) Fr.: éducation The act or process of educating. Verbal noun of → educate. |
eigenfunction viž-karyâ Fr.: fonction propre 1) Math.: An → eigenvector for a linear
→ operator on a → vector space
whose vectors are → functions. Also known as
proper function. From Ger. Eigenfunktion, from eigen- "characteristic, particular, own" (from P.Gmc. *aigana- "possessed, owned," Du. eigen, O.E. agen "one's own") + → function. Viž-karyâ, from viž, contraction of vižé "particular, charcteristic" + karyâ, → function. Vižé, from Mid.Pers. apēcak "pure, sacred," from *apa-vēcak "set apart," from prefix apa- + vēcak, from vēxtan (Mod.Pers. bixtan) "to detach, separate, sift, remove," Av. vaēk- "to select, sort out, sift," pr. vaēca-, Skt. vic-, vinakti "to sift, winnow, separate; to inquire." |
Einstein notation namâdgân-e Einstein Fr.: convention Einstein A notation convention in → tensor analysis whereby whenever there is an expression with a repeated → index, the summation is done over that index from 1 to 3 (or from 1 to n, where n is the space dimension). For example, the dot product of vectors a and b is usually written as: a.b = Σ (i = 1 to 3) a_{i}.b_{i}. In the Einstein notation this is simply written as a.b = a_{i}.b_{i}. This notation makes operations much easier. Same as Einstein summation convention. |
Einstein's field equations hamugešhâ-ye meydân-e Einstein Fr.: équations de champ d'Einstein A system of ten non-linear → partial differential equations in the theory of → general relativity which relate the curvature of → space-time with the distribution of matter-energy. They have the form: G_{μν} = -κ T_{μν}, where G_{μν} is the → Einstein tensor (a function of the → metric tensor), κ is a coupling constant called the → Einstein gravitational constant, and T_{μν} is the → energy-momentum tensor. The field equations mean that the curvature of space-time is due to the distribution of mass-energy in space. A more general form of the field equations proposed by Einstein is: G_{μν} + Λg_{μν} = - κT_{μν}, where Λ is the → cosmological constant. Named after Albert Einstein (1879-1955); → field; → equation. |
Einstein's gravitational constant pâyâ-ye gerâneši-ye Einstein (#) Fr.: constante gravitationnelle d'Einstein The coupling constant appearing in → Einstein's field equations, expressed by: κ = 8πG/c^{4}, where G is the Newtonian → gravitational constant and c the → speed of light. → einstein; → gravitational; → constant. |
Einstein-Hilbert action žireš-e Einstein-Hilbert Fr.: action de Einstein-Hilbert In → general relativity, the → action
that yields → Einstein's field equations.
It is expressed by: → Einstein; → Hilbert space; → action. |
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