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

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

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 velocity dispersion   پاشش ِ تندا   pâšeš-e tondâFr.: dispersion de vitesses   The → standard deviation of a velocity → distribution. It indicates how objects of the sample move relative to one another. Objects with similar velocities have a small velocity dispersion, whereas objects with very different velocities have a large velocity dispersion.→ velocity; → dispersion. velocity gradient   زینه‌ی ِ تندا   zine-ye tondâFr.: gradient de vitesse   Fluid Mechanics: The rate at which the velocity changes with the distance across the flow. When a fluid flows past a stationary wall, the fluid right close to the wall does not move. However, away from the wall the flow speed is not zero. Therefore a velocity gradient exists, which is due to adhesive, cohesive, and frictional forces. The amount of the velocity gradient is characteristic of the fluid.→ velocity; → gradient. velocity law   قانون ِ تندا   qânun-e tondâFr.: loi de vitesse   In the theory of → radiation-driven winds, an equation that describes the behavior of the → wind velocity of → hot stars as a function of distance from the star. This velocity β-law is given by the expression: v(r) = v∞(1 - R*/r)β, where v∞ is the → terminal velocity, R* is the stellar radius, and r the distance from the center. For → O-type stars, the exponent is estimated to be β = 0.8. → velocity; → law. velocity of light   تندی ِ نور، تندای ِ ~   tondi-ye nur, tondâ-ye ~Fr.: vitesse de la lumière   A → physical constant which represents the ultimate speed limit for anything moving through space, according to the theory of → special relativity. It is the speed of propagation of → electromagnetic waves in a vacuum, equal to 299,792.458 km/s (nearly 3 x 108 m/s). The velocity of light appears as the connecting link between mass and energy in the → mass-energy relation. Usually denoted by c, from L. celeritas "swiftness," from celer "swift," → acceleration.→ velocity; → speed; → light. velocity pressure   فشار ِ توانیک   fešâr-e tavânikFr.: pression dynamique   → velocity; → pressure. velocity profile   فراپال ِ تندا   farâpâl-e tondâFr.: profil de vitesse   A plot of the fluid velocity as a function of position.→ velocity; → profile. velocity space   فضای ِ تندا، ~ تنداها   fazâ-ye tondâyi, ~ tondâhâFr.: espace de vitesses   Of a dynamical system, a three-dimensional space which consists of the set of values that the velocity can take (vx, vy, vz). → phase space.→ velocity; → space. velocity-distance relation   باز‌آنش ِ تندا-دورا   bâzâneš-e tondâ-durâFr.: relation vitesse-distance   The linear relation wherein all galaxies are moving away from one another, with velocities that are greater with increasing distance of the galaxy. Same as → Hubble's law.→ velocity; → distance; → relation. vorticity   گردشاریگی   gerdšârigiFr.: vorticité   In fluid mechanics, a measure of the rate of rotational spin in a fluid. Mathematically, vorticity is a vector field defined as the curl of the velocity field: ω = ∇ x v. Meteo.: The rotation of air around a vertical axis.From L. vortic-, from → vortex + → -ity.Gerdšârigi, from gerdšâr→ vortex + -igi, → -ity. wind velocity   تندای ِ باد   tondâ-ye bâdFr.: vitesse de vent   The speed at which the → stellar wind is forced away from the star. Wind velocities of → hot stars are directly measured from → P Cygni profiles, which indicate velocities from several hundred to several thousand km s-1. See also → escape velocity, → terminal velocity, → velocity law, → radiation-driven wind, → CAK model.→ wind; → velocity. zero-velocity surface   رویه‌ی ِ تندای ِ صفر   ruye-ye tondâ-ye sefrFr.: surface de vitesse nulle   In the → restricted three-body problem, a surface which limits the region of space in which a small body can move. In the expression for the → Jacobi integral, the left side value is always positive or nul; hence the particle motion is confined to the region where U ≤ CJ. The surface that limits this region, defined by U = CJ, is called the zero-velocity surface.→ zero; → velocity; → surface.

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