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dimensionless quantity candâ-ye bivâmun Fr.: 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 |
Dione (Saturn IV) Dioné Fr.: 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 našib-e ofoq Fr.: 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 10^{6} 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. |
diphthong dovâké Fr.: 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." |
dipole radiation tâbeš-e doqotbé Fr.: rayonnement dipolaire The electromagnetic radiation produced by an oscillating → electric dipole or → magnetic dipole. |
diproton diproton Fr.: diproton An → isotope of → helium that consists of two → protons, without any → neutrons. It is extremely → unstable. |
Dirac annihilation nâbudi-ye Dirac Fr.: annihilation de Dirac Same as → pair annihilation. → Dirac; → annihilation. |
Dirac constant pâyâ-ye Dirac Fr.: constante de Dirac The → Planck constant divided by 2π and denoted ħ, pronounced h-bar. Also called → reduced Planck constant. |
Dirac equation hamugeš-e Dirâk Fr.: 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 function karyâ-ye Dirâk Fr.: fonction de Dirac A function of x defined as being zero for all values of x other than x = x_{0} and having the definite integral from x = -∞ to x = +∞ equal to unity. |
Dirac's constant pâyâ-ye Dirac Fr.: constante de Dirac |
direct correlation hambâzâneš-e sarrâst Fr.: 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 jonb eš-e farârow, ~ sarrâst Fr.: 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. |
direction 1) râstâ, su (#); 2) râštâri Fr.: direction 1) A position to which motion or another position is referred. 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." |
direction angle zâviye-ye râstâ Fr.: angle de direction An angle made by a given vector and a coordinate axis. |
Dirichlet condition butâr-e Dirichlet Fr.: condition de Dirichlet One of the following conditions for a → Fourier series
to converge: Named after Peter Gustav Lejeune Dirichlet (1805-1859), German mathematician who made valuable contributions to → number theory, → analysis, and → mechanics; → condition. |
disallocation vâtesk Fr.: desallouation The act of disallocating or the state of being disallocated. |
discontinuity nâpeyvastegi (#) Fr.: discontinuité A break in sequence or continuity of anything.
→ Balmer discontinuity 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) hamne-ye daršami-ye gosasté Fr.: 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 gozareš-e gosasté Fr.: transition discrète A transition between two quantum-mechanical energy levels. See also → discrete spectrum. → discrete; → transition. |
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