# An Etymological Dictionary of Astronomy and AstrophysicsEnglish-French-Persian

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

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 radial velocity method   روش ِ تندای ِ شعاعی   raveš-e tondâ-ye šo'â'iFr.: méthode de vitesses radiales   The technique based on the analysis of the → radial velocity curve, used to detect the presence of an invisible secondary around a host star. This method holds the majority of exoplanet discoveries. recession velocity   تندای ِ دورشدن   tondâ-ye dur šodanFr.: vitesse d'éloignement   The velocity with which an object moves away from another object or a reference point.→ recession; → velocity. reciprocity theorem   فربین ِ دوسویگی   farbin-e dosuyegiFr.: théorème de réciprocité   1) General: Any theorem that expresses various reciprocal relations for the behavior of some physical systems, in which input and output can be interchanged without altering the response of the system to a given excitation. 2) In classical electromagnetism, the theorem stating that the current in a detector divided by the voltage at the source remains constant when source and detector are interchanged, as long as the frequency and all the impedances are left unchanged.Reciprocity, from L. reciproc(us) "returning the same way, alternating" + → -ity; → theorem.Farbin, → theorem; dosuyegi, quality noun of dosuyé nuanced term of dosu "two-sided," from do, → two, + su "direction, side," from Mid.Pers. sôk "direction, side." relative velocity   تندای ِ بازانی   tondâ-ye bâzâniFr.: vitesse relative   For two objects A and B, the velocity which B, supposing itself at rest, assigns to A.→ relative; → velocity. relativistic velocity   تندای ِ بازانیگی‌مند   tondâ-ye bâzânigimandFr.: vitesse relativiste   The velocity of a body when it is a significant fraction of the → speed of light.→ relativistic; → velocity. Rosseland mean opacity   کدری ِ میانگین ِ روسلاند   kederi-ye miyângin-e RosselandFr.: opacité moyenne de Rosseland   The → opacity of a gas of given composition, temperature, and density averaged over the various wavelengths of the radiation being absorbed and scattered. The radiation is assumed to be in → thermal equilibrium with the gas, and hence have a → blackbody spectrum. Since → monochromatic opacity in stellar plasma has a complex frequency dependence, the Rosseland mean opacity facilitates the analysis. Denoted κR, it is defined by: 1/κR = (π/4σT3) ∫(1/kν) (∂B/∂T)νdν, summed from 0 to ∞, where σ is the → Stefan-Boltzmann constant, T temperature, B(T,ν) the → Planck function, and kν monochromatic opacity (See Rogers, F.J., Iglesias, C. A. Radiative atomic Rosseland mean opacity tables, 1992, ApJS 79, 507).Named after Svein Rosseland (1894-1985), a Norwegian astrophysicist, who obtained the expression in 1924; → mean; → opacity. rotational velocity   تندای ِ چرخشی   tondâ-ye catxešiFr.: vitesse de rotation   The velocity of a → rotational motion; same as → angular velocity.→ rotational; → velocity. scientificity   دانشیگی   dânešigiFr.: scientificité   The quality of the practices and theories that aim at establishing reproducible regularities in phenomena by using experimental method and providing a clearly formulated description.→ scientific + → -ity. solar metallicity   فلزیگی ِ خورشیدی   felezigi-ye xoršidiFr.: métallicité solaire   The proportion of the solar matter made up of → chemical elements heavier than → helium. It is denoted by Z, which represents the sum of all elements heavier than → helium, in mass fraction. The most recent determination of the solar Z gives a value of 0.0134 (Asplund et al. 2009, ARAA 47, 481), corresponding to the present-day photospheric composition.→ solar; → metallicity. solar velocity   تندای ِ خورشید، ~ خورشیدی   tondâ-ye xoršid, ~ xoršidiFr.: vitesse solaire   The rate of change of the Sun's position with respect to the local standard of rest toward the → solar apex.→ solar; → velocity. space velocity   تندای ِ فضایی   tondâ-ye fazâyFr.: vitesse spatiale   The velocity of a star relative to the Sun.→ space; → velocity. stellar metallicity   فلزیگی ِ ستاره‌ای   felezigi-ye setâre-yiFr.: métallicité stellaire   The metallicity derived from observations of stars in galaxies. It is mainly based on spectral → absorption lines in → ultraviolet (UV) and optical ranges. Stellar metallicity is a direct measure of the amount of metals in a galaxy, since large part of heavy elements lies in its stars.→ stellar; → metallicity. tangential velocity   تندای ِ سایانی   tondâ-ye sâyâniFr.: vitesse tangentielle   1) The instantaneous linear velocity of a body moving in a circular path. It is equal to the → angular velocity multiplied by the radius: vt = ωr. 3) The component of the velocity of an object, such as a star, that is at right-angles to the observer's → line of sight; also known as transverse velocity. See also → radial velocity.→ tangential; → velocity. terminal velocity   تندای ِ پایانی   tondâ-ye pâyâniFr.: vitesse terminale   1) The constant maximum velocity reached by a body falling under gravity through a liquid or gas, especially the atmosphere. The body ceases to accelerate downward because the force of gravity is equal to the opposing force of resistance by the medium. 2) The velocity acquired at the end of a body's motion. 3) The velocity attained by → stellar wind at very large distance from the star. The material that escapes from the outer layers of the stars is accelerated outward from a small radial velocity at the → photosphere of the star, to some high velocity at large distance from the star. The theory of → radiation-driven winds predicts that the terminal velocity scales with the → escape velocity as: v∞  ≅ 2-3 vesc. The winds of hot → O stars experience fast acceleration and reach 80% of their terminal velocity near the star (some 3 R*). The terminal velocity ranges from about 10 km s-1 for a cool → supergiant star to 3000 km s-1 for a luminous hot star. Terminal velocities are quite accurately measured from the violet trough of saturated → P Cygni line profiles in the ultraviolet (mainly N V λ1239, C IV λ1548, Si IV λ1394).→ terminal; → velocity. thermoelectricity   دمابرق   damâbarq (#)Fr.: thermo-éléctricité   The electricity produced by heat or temperature difference in a conductor.→ thermo- + → electricity. transverse velocity   تندای ِ تراگذر   tondâ-ye tarâgozarFr.: vitesse transverse   Same as → tangential velocity.→ transverse; → velocity. univocity   یکوازی   yekvâziFr.: monosémie   The character of being → univocal. Same as → monosemy.→ univocal; → ity. vector angular velocity   بردار ِ تندای ِ زاویه‌ای   bordâr-e tondâ-ye zâviye-yiFr.: vecteur de vitesse angulaire   Of a rotating body, a vector of magnitude ω (→ angular velocity) pointing in the direction of advance of a right-hand screw which is turned in the direction of rotation.→ vector; → angular; → velocity. velocity   تندا   tondâFr.: vitesse   The time rate of change of position in a given direction, measured as length per unit time. → speed.L. velocitatem (nominative velocitas) "swiftness, speed," from velox (genitive velocis) "swift."Tondâ, from tond "swift, rapid, brisk; fierce, severe" (Mid.Pers. tund "sharp, violent;" Sogdian tund "violent;" cf. Skt. tod- "to thrust, give a push," tudáti "he thrusts;" L. tundere "to thrust, to hit" (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere "to thrust or bore through;" PIE base *(s)teud- "to thrust, to beat") + noun suffix -â. velocity curve   خم ِ تندا   xam-e tondâFr.: courbe de vitesse   A plot of the radial velocity of an object against time, derived from the Doppler shift of spectral lines.→ velocity; → curve.