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

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

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 dimensionless quantity   چندای ِ بی‌وامون   candâ-ye bivâmunFr.: quantité sans dimension   A quantity without an associated → physical dimension. Dimensionless quantities are defined as the ratio of two quantities with the same dimension. The magnitude of such quantities is independent of the system of units used. A dimensionless quantity is not always a ratio; for instance, the number of people in a room is a dimensionless quantity. Examples include the → Alfven Mach number, → Ekman number, → Froude number, → Mach number, → Prandtl number, → Rayleigh number, → Reynolds number, → Richardson number, → Rossby number, → Toomre parameter. See also → large number.→ dimension; → quantity. Dione (Saturn IV)   دیونه   DionéFr.: Dioné   The fourth largest moon of Saturn and the second densest after Titan. Its diameter is 1,120 km and its orbit 377,400 km from Saturn. It is composed primarily of water ice but must have a considerable fraction of denser material like silicate rock.Discovered in 1684 by Jean-Dominique Cassini, Italian born French astronomer (1625-1712). In Gk. mythology Dione was the mother of Aphrodite (Venus) by Zeus (Jupiter). dip of the horizon   نشیب ِ افق   našib-e ofoqFr.: inclinaison de l'horizon   The angle created by the observer's line of sight to the → apparent horizon and t he → true horizon. Neglecting the → atmospheric refraction, dip of the horizon can be expressed by θ (radians) = (2h/R)1/2, where h is the observer's height and R the Earth's radius. An an example, for a height of 1.5m above the sea, and R = 6.4 x 106 m, the dip angle is about 0.00068 radians, or 0.039 degrees, about 2.3 minutes of arc, quite appreciable by the eye. See also → distance to the horizon. Same as → dip angle.→ dip; → horizon. diphthong   دوواکه   dovâkéFr.: diphthongue   Phonetics: A → vowel sound produced by a blended sequence of two separate vowels in a single syllable, where the sound begins as one vowel and moves toward another (as in loud, light, and lair).From M.Fr. diphthongue, from L.L. diphthongus, from Gk. diphthongos "having two sounds," from → di- "double" + phthongos "sound, voice."Dovâké, from do "two, → di-" + vâké, vâk, → voice. dipole radiation   تابش ِ دوقطبه   tâbeš-e doqotbéFr.: rayonnement dipolaire   The electromagnetic radiation produced by an oscillating → electric dipole or → magnetic dipole.→ dipole; → radiation. diproton   دیپروتون   diprotonFr.: diproton   An → isotope of → helium that consists of two → protons, without any → neutrons. It is extremely → unstable.→ di-; → proton. Dirac annihilation   نابودی ِ دیراک   nâbudi-ye DiracFr.: annihilation de Dirac   Same as → pair annihilation.→ Dirac; → annihilation. Dirac constant   پایای ِ دیراک   pâyâ-ye DiracFr.: constante de Dirac   The → Planck constant divided by 2π and denoted ħ, pronounced h-bar. Also called → reduced Planck constant.→ Dirac; → constant. Dirac equation   هموگش ِ دیراک   hamugeš-e DirâkFr.: équation de Dirac   The equation that describes the behavior of an → electron in a way that combines the requirements of → quantum mechanics with the requirements of → special relativity. The Dirac equation predicted the existence of antimatter→ Dirac; → equation. Dirac function   کریای ِ دیراک   karyâ-ye DirâkFr.: fonction de Dirac   A function of x defined as being zero for all values of x other than x = x0 and having the definite integral from x = -∞ to x = +∞ equal to unity.→ Dirac; → function Dirac's constant   پایای ِ دیراک   pâyâ-ye DiracFr.: constante de Dirac   → Dirac; → constant. direct correlation   هم‌باز‌آنش ِ سرراست   hambâzâneš-e sarrâstFr.: corrélation directe   A correlation between two variables such that as one variable becomes large, the other also becomes large, and vice versa. The correlation coefficient is between 0 and +1. Also called positive correlation.→ direct; → correlation. direct motion   جنبش ِ فرارو، ~ سرراست   jonb eš-e farârow, ~ sarrâstFr.: mouvement direct   The motion of a solar system body from West to East across the sky against the background stars. It is the "normal" direction of motion within the solar system. For rotating or orbiting solar system objects it is anti-clockwise as seen from above the solar system in the direction of the North Pole. The same as → prograde motion. See also → retrograde motion.→ direct; → motion. direction   راستا، سو؛ ۲) راشتاری   1) râstâ, su (#); 2) râštâriFr.: direction   1) A position to which motion or another position is referred. 2) The act or an instance of directing.M.E. direccioun, from M.Fr., from L. direction-, stem of directio "arranging in line, straightening," → direct.1) Râstâ, from direct→ direct + -â dimension suffix; su, from Mid.Pers. sôk "side." 2) Râštâri, noun from râštâr→ director + suffix -i. direction angle   زاویه‌ی ِ راستا   zâviye-ye râstâFr.: angle de direction   An angle made by a given vector and a coordinate axis. The angle between the North and a chosen direction.→ direct; → angle. Dirichlet condition   بوتار ِ دیریکله   butâr-e DirichletFr.: condition de Dirichlet   One of the following conditions for a → Fourier series to converge: 1) The function f(x) is defined and single valued, except possibly at a finite number of points in the interval -π, +π. 2) f(x) has a period of 2π. 3) f(x) and f'(x) are → piecewise continuous functions on -π, +π. Then, the Fourier series converges to: (a) f(x) if x is a point of continuity. (b) (f(x + 0) + f(x - 0))/2, if x is a point of discontinuity.Named after Peter Gustav Lejeune Dirichlet (1805-1859), German mathematician who made valuable contributions to → number theory, → analysis, and → mechanics; → condition. disallocation   واتسک   vâteskFr.: desallouation   The act of disallocating or the state of being disallocated.→ dis- + → allocate. discontinuity   ناپیوستگی   nâpeyvastegi (#)Fr.: discontinuité   A break in sequence or continuity of anything. → Balmer discontinuityMath.: A point at which a function is not continuous. Geology: A surface separating rock layers of differing properties or compositions.M.L. discontinuitas, from discontinuus, from → dis- + continuus, → continuous.Nâpeyvastegi, noun from nâpeyvasté "discontinuous," from nâ- "non, un-," → a-, + peyvasté, → continuous. discrete absorption component (DAC)   همنه‌ی ِ درشمی ِ گسسته   hamne-ye daršami-ye gosastéFr.: composante d'absorption discrète   The rapid, systematic changes in the absorption parts of the → P Cygni profiles of the ultraviolet → resonance lines (Si IV, C IV, and N V) observed in a majority of massive → hot stars. DACs are typically seen to accelerate to the → blue wing of the profile over a few days, becoming narrower as they approach the → terminal velocity.→ discrete; → absorption; → component. discrete transition   گذرش ِ گسسته   gozareš-e gosastéFr.: transition discrète   A transition between two quantum-mechanical energy levels. See also → discrete spectrum.→ discrete; → transition.