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

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

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 Debye length   درازای ِ دبی   derâzâ-ye Debye (#)Fr.: longueur de Debye   A characteristic length scale in a → plasma, determined by the temperature and number density of the charged particles. The Debye length (in cm) is given by the expression: λD = 743(Te/ne)1/2, where Te is the electron temperature in → electron-volts and ne is the electron density in cm-3. Particles which pass each other at distances smaller than the Debye length interact directly. Outside the Debye length particle interactions are dominated by collective effects. In typical laboratory plasmas the Debye length is small compared with the plasma dimension, so that collective effects are important.→ debye (D); → length. Debye model   مدل ِ دبی   model-e Debye (#)Fr.: modèle de Debye   An extension of the → Einstein model accounting for → specific heats, based on the concept of → elastic waves in → crystals. In this model specific heat is given by: CV = 9R[(4/x2)∫ y2/(ey - 1)dy - x/(ex - 1)], integrating from 0 to x, where R is the → gas constant, k is → Boltzmann's constant, x = hνmax/k, and y = hν/k. The parameter TD = hνmax/k is the characteristic → Debye temperature of the crystal. At low temperatures the specific heat prediction by this model is in good agreement with observations (→ Debye law), in contrast to Einstein's model.→ debye; → temperature. Debye temperature   دمای ِ دبی   damâ-ye Debye (#)Fr.: température de Debye   The characteristic → temperature of the → crystal as given by the → Debye model of → specific heats.→ debye; → temperature. deca-   دکا-   dekâ- (#)Fr.: déca-   Prefix meaning "ten" and "ten times" used in terms belonging to the metric system.From Fr. déca-, from L. deca-, dec-, from Gk. deka "ten;" cf. Av. dasa "ten," Mod.Pers. dah "ten," Skt. dasa, PIE *dekm (Dan. ti, Du. tien, Ger. zehn, E. ten, Fr. dix).Dekâ-, from Fr. as above. decay   ۱) تباهیدن؛ ۲) تباهی، فروپاشی   1) tabâhidan (#); 2) tabâhi (#), forupâši (#)Fr.: 1) se désintegrer, se désexciter; 2) désintegration, désexcitation   1a) To become decomposed. 1b) Of a radioactive nucleus, to disintegrate spontaneously into one or more different nuclei, accompanied by the emission of → alpha particles, → beta particles, → positrons, and/or → gamma rays. 2a) Decomposition. 2b) The → transition of a system from an → excited state to a less excited one. Radiative decay refers to the process when the energy difference between the states is taken away by radiation. Likewise, collisional decay is when the energy difference is removed by a target during a collision. 2c) Of a spacecraft, a gradual decrease in the radius of its orbit over time, caused by aerodynamic drag of the atmosphere and other forces. The rate of orbit decay rises as the spacecraft falls and encounters increasing atmospheric density, eventually resulting in reentry. → alpha decay; → beta decay; → dark matter decay; → decay chain; → decay constant; → decay mode; → decay product; → decay time; → inverse beta decay; → Ohmic decay time; → orbit decay; → radioactive decay; → turbulence decay.From O.Fr. decair, from V.L. *decadere "to fall off," from L. cadere "to fall," PIE base *kad- "to fall" (cf. Pers. Gilaki katan "to fall," ba.ka.tam "I fell," dakatan "to fall (in a marsh, in a pit)," vakatan "to fall from tiredness, be exhausted," fakatan "to fall from (lose) reputation," Pers. Laki: katen "to fall," kat "he fell," beko "fall!," Pers. Tabari: dakətə "fallen," dakətən "to crash down," dakət.gu "stray cow," Arm. chacnum "to fall").Tabâhidan, verbal form of tabâhi, noun form of tabâh "spoiled, ruined, destroyed," Mid.Pers. tapâh "spoiled, destroyed." Maybe related to Mod.Pers. tâb "affliction, pain, torment; heat, burning," tab "fever," tâbidan, tâftan "to shine," tafsidan "to become hot," Av. tāp-, taf- "to warm up, heat," tafsat "became hot," tāpaiieiti "to create warmth," cf. Skt. tap- "to spoil, injure, damage; to suffer; to heat, be/become hot," tapati "burns," L. tepere "to be warm," tepidus "warm," PIE base *tep- "warm." decay chain   زنجیره‌ی ِ تباهی   zanjire-ye tabâhiFr.: chaîne de désintégration   A series of nuclear decays produced by successive → daughter products, when the daughters are themselves → radioactive. For example, the decay chain N1→ N2→ N3→ ... in which the parent nuclide N1 decays to the daughter N2, which in turn decays to N3. Each → radionuclide in the decay chain can → branch to more than one daughter.→ decay; → chain. decay constant   پایای ِ تباهی   pâyâ-ye tabâhiFr.: constante de désintégration   A constant of proportionality occurring in the formula expressing spontaneous → decay of → radionuclides. The number of atoms decaying is given by N = N0e-kt, where N0 is the number of nuclei in the given volume of the substance at instant t = 0, N is the number of nuclei at t, and k is decay constant. Decay constant is related to → half-life by τ = ln2/k, roughly 0.693/k.→ decay; → constant. decay mode   ترز ِ تباهی، مد ِ ~   tarz-e tabâhi, mod-e ~Fr.: mode de désintégration   A possible type of decay of a → radionuclide or → elementary particle. In general, a given particle may decay by more than one decay mode. Usually the number of decay modes is one or two. There are, however, → nuclides which have many decay modes; for example the nuclide 11Li has seven decay modes.→ decay; → mode. decay product   فر‌آورده‌ی ِ تباهی   farâvarde-ye tabâhiFr.: produit de désintégration   A → stable nuclide or → radioactive nuclide formed by the → disintegration of a → radioactive isotope, either directly or as a result of a → decay chain. Also called → daughter product. For example, the decay product of 238U is 206Pb, after passing through the following chain: 238U → 234Th (4.5 billion yr) → 234Pa (24 days) → 234U (1 min) → 230Th (245,000 yr) → 226Ra (76,000 yr) → 222Rn (1,600 yr) → 218Po (3.8 d) → 214Pb (3 m) → 214Bi (27 m) → 214Po (160 microseconds) → 210Pb (22 yr) → 210Bi (5 d) → 210Po (138 d) → 206Pb.→ decay; → product. decay time   زمان ِ تباهی   zamân-e tabâhi (#)Fr.: temps d'amortissement   The time required for the amplitude of a vibrating system to decrease to 1/e of its initial value. Of an orbiting object, its lifetime in a non stable orbit.→ decay; → time. decelerate   واشتابیدن؛ واشتاباندن   vâšetâbidan (#); vâšetâbândan (#)Fr.: décélérer   (v.intr.) To slow down. (v.tr.) To decrease the velocity of.From → de- + (ac)celerate, from → accelerate.Vâšetâbidan, from vâ-→ de- + šetâbidan, → accelerate. deceleration   واشتاب   vâšetâb (#)Fr.: décéleration   The act or process of moving, or of causing to move, with decreasing speed. Sometimes called negative acceleration.Verbal noun of decelerate. deceleration parameter   پارامون ِ واشتاب   pârâmun-e vâšetâbFr.: paramètre de décéleration   A parameter designating the rate at which the expansion of the Universe would slow down owing to the braking gravitational effect of the matter content of the Universe. It is expressed by: q(t) = -R(t)R ..(t)/R .2(t), where R(t) represents the size of the Universe at time t. See also → expansion parameter; compare with → acceleration parameter.→ deceleration; → parameter. deci-   دسی-   desi- (#)Fr.: déci-   Prefix used in the metric system to mean on-tenth.From Fr. déci-, from L. decimus "tenth," from decem "ten," cf. Av. dasa "ten," Mod.Pers. dah "ten," Gk. deka, Skt. dasa, PIE *dekm (Dan. ti, Du. tien, Ger. zehn, E. ten, Fr. dix).Desi-, from Fr. déci-, as above. decibel (dB)   دسی‌بل   desibel (#)Fr.: décibel   A dimensionless unit used to express relative difference in power or intensity, usually between two acoustic or electric signals. It is defined as n [dB] = 10 log (P1/P0), with → natural logarithm. One decibel equals one-tenth of a → bel (B).→ deci-; → bel (B). decide   واسونیدن   vâsunidanFr.: décider   To make a choice or come to a conclusion about something. → decision, → rule of decision.M.E. deciden, from O.Fr. decider, from L. decidere "to decide, determine," literally "to cut off," from → de-"off" + caedere "to cut, chop, beat, hew."Vâsunidan, from vâ- "off, away," → de-, + sun "to cut;" cf. Kurd. Soriani su, sun "to sharpen, whet;" Kurd. Kurmanji (prefixed ha-, contraction of *ham-) hasun "to sharpen, whet;" Mod.Pers. sân "whetstone," variants fasân, afsân, awsân; Av. si-, sā- "to cut;" cf. Skt. śā- "to sharpen, whet;" + -idan infinitive suffix. decider   واسوننده   vâsunandéFr.: décideur   Someone who decides, or something that allows to determine.→ decide; → -er. decimal   دهدهی   dahdahi (#)Fr.: décimal   Referring to or based in the number 10; relating to tens or tenths.M.L. decimalis "of tithes or tenths," from L. decimus "tenth," from decem, → ten.Dahi , of or relating to dah, → ten; dahdahi literally "ten by ten." decimal fraction   برخه‌ی ِ دهدهی   barxe-ye dahdahiFr.: fraction décimale   A fraction expressed by using → decimal representation, as opposed to a vulgar fraction. For example, 2/5 is a vulgar fraction; 0.40 is a decimal fraction.→ decimal; → fraction. decimal logarithm   لگاریتم ِ دهدهی   logâritm-e dahdahiFr.: logarithme décimal   → decimal; → logarithm.