An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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doubly convex
  دو-کوژ   
do-kuž

Fr.: double-convexe   

Describing a → lens which is → convex on both sides.

doubly; → convex.

doubly ionized
  دوبار یونیده   
dobâr yonidé (#)

Fr.: deux fois ionisé   

An atom that has lost two of its external electrons, for example O++ ([O III]).

doubly; → ionized.

draconic month
  ماه ِ گوزهری، ~ ِ گرهی   
mâh-e gowzahri (#), ~ gerehi (#)

Fr.: mois draconitique   

The time interval between two successive passages of the Moon through its → ascending node, 27.212 220 days (27d 5h 5 m 35.8s). Draconic month is important for predicting → eclipses. Also called draconitic month, nodical month.

Draconic, adj. of dragon, → Draco, referring to a mythological dragon for the following reason. Since an eclipse occurs when the Earth, the Sun, and a node are aligned and moreover the Moon is situated near the node, it was believed that a dragon that resided in the node swallowed the Sun or the Moon. → month.

Mâh, → month.
Gowzahri, related to gowzahr, from Mid.Pers. gowzihr "a node of the lunar orbit" [gowzihr sar ("head") = ascending node, gowzihr dumb ("tail") = descending node], also the astrological dragon, from Av. gao-ciθra- "keeping in it the seed of the ox," epithet of the Moon, since according to Iranian mythology the Moon keeps the seed/sperm of bovine animals; from gao- "cow, ox, bull" (Mod.Pers. gâv, Skt. gaus-, Gk. bous "ox," Arm. kov, O.E. cu, PIE *gwou-) + ciθra- "origin, seed, lineage" (Mod.Pers. cehr "origin"). Gowzahr was loaned into Arabic astronomical texts as jawzahr.
Gerehi, adj. of gerehnode.

Draconids
  اژدهاییان   
eždahâiyân

Fr.: Draconides   

Two meteor showers with radiants in the constellation → Draco. One appears early in October and the other late in June.

Draconids, from → Draco constellation + → -ids suffix denoting "descendant of, belonging to the family of."

Eždahâyiyân, from eždahâ, → Draco, + -iyân-ids.

Drake equation
  هموگش ِ دریک   
hamugeš-e Drake

Fr.: équation de Drake   

A probabilistic argument used to estimate the number of → intelligent, communicating → extraterrestrial civilizations in the → Milky Way galaxy. The Drake equation is:
N = R* . fp . ne . fl . fi . fc . L, where:
N = the number of → civilizations in our Galaxy with which → communication might be possible,
R* = the average rate of → star formation in our Galaxy,
fp = the fraction of those stars that have → planets,
ne = the average number of planets that can potentially support → life per star that has planets,
fl = the fraction of planets that could develop life at some point,
fi = the fraction of planets with life that actually go on to develop intelligent life,
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space,
L = the length of time for which such civilizations release detectable signals into space.
The first three terms of the equation have been successfully investigated by astronomers and are to some extent known. In contrast, values for the last four are very speculative. Drake himself estimates that N might be as high as 10,000. Carl Sagan was more optimistic, and came up with the value of a million or more for N. These estimates may be too optimistic. A pessimistic choice of parameters leads to N smaller than 1, which means that we might be the only technically sophisticated civilization in the Galaxy.

Frank Donald Drake (1930-); → equation.

Drummond light
  نور ِ درامن   
nur-e Drummond

Fr.: lumière de Drummond   

A very brilliant white light which is the ignited flame of a mixture of oxygen and hydrogen projected against a block of calcium oxide (lime). Also called limelight. First working version produced by Lieutenant of the Royal Engineers, upon the Ordnance Trigonometrical Survey of Ireland (1826). It was used at night as a substitute for solar light. It was first employed in a theater in 1837 and was in wide use by the 1860s, among which in photography.

Named after Scottish engineer Thomas Drummond (1797-1840); → light.

Dulong-Petit law
  قانون ِ دولون-پتی   
qânun-e Dulong-Petit

Fr.: loi de Dulong et Petit   

The product of the → specific heat and → atomic weight of most solid elements at room → temperature is nearly the same. In other words, specific heat is constant for a solid and independent of temperature. Experiment shows that at moderate temperatures this law is satisfied for → crystals with rather simple structure. However, the law fails for crystals with more complex structures. More specifically the law cannot explain the variation of specific heat with temperature. The specific heat drops to zero as the temperature approaches 0 K. This behavior is explained only with the quantum theory. → Debye model.

Named after Pierre L. Dulong (1785-1838) and Alexis T. Petit (1797-1820), French chemists, who proposed the law in 1819. They collaborated in several important investigations, including studies of thermal expansion of gases and of liquids and the specific heats of substances; → law.

duplication
  دوتایش، دوتاکرد   
dotâyeš, dotâkard

Fr.: duplication   

An act or instance of duplicating; the state of being duplicated.

Verbal noun of → duplicate.

duration
  پایش   
pâyeš

Fr.: durée   

Continuance in time; a period of existence or persistence; length of time during which anything continues.

Noun of action from L. durare "to harden," → during.

Pâyeš, noun of action from pâyidan, → last (v.).

dust coagulation
  ماسش ِ غبار، رچش ِ ~   
mâseš-e qobâr, roceš-e ~

Fr.: coagulation de la poussière   

A process of formation of → dust grains in → interstellar medium and → protoplanetary disks, in which randomly colliding aggregates may stick together.

dust; → coagulation.

dust emission
  گسیل ِ غبار   
gosil-e qobâr

Fr.: émission des poussières   

Thermal emission in infrared from interstellar → dust grains receiving photons. Dust grains absorb ultraviolet and visible light emitted by nearby stars and re-radiate in the infrared wavelengths. Since the infrared light is of lower energy than the ultraviolet/visible light, the difference goes into heating the dust grain. Typical temperatures for interstellar grains are tens of degrees Kelvin.

dust; → emission.

dust obscuration
  تیره‌شد پت غبار   
tirešod pat qobâr

Fr.: obscurcissement par la poussière   

The → absorption of → electromagnetic radiation from an astrophysical object by → dust grains associated with that object.

dust; → obscuration.

Tiregi, → obscuration, pat, → by; qobâr, → dust.

dynamical disruption
  گسیخت ِ توانیک   
gosixt-e tavânik

Fr.: rupture dynamique   

The process whereby a → bound system, such as a → binary system or a → globular cluster, is broken apart.

dynamical; → disruption.

dynamical friction
  مالش ِ توانیک   
mâleš-e tavânik

Fr.: frottement dynamique   

The gravitational interaction between a relatively massive body and a field of much less massive bodies through which the massive body travels. As a result, the moving body loses → momentum and → kinetic energy. An example of dynamical friction is the sinking of massive stars to the center of a → star cluster, a process called → mass segregation. Dynamical friction plays an important role in → stellar dynamics. It was first quantified by Chandrasekhar (1943).

dynamical; → friction.

dynamical relaxation
  واهلش ِ توانیک   
vâheleš-e tavânik

Fr.: relaxation dynamique   

The evolution over time of a gravitationally → bound system consisting of N components because of encounters between the components, as studied in → stellar dynamics. Due to this process, in a → star cluster, → low-mass stars may acquire larger random velocities, and consequently occupy a larger volume than → high-mass stars. As a result, massive stars sink to the cluster centre on a time-scale that is inversely proportional to their mass. See also → mass segregation.

dynamical; → relaxation.

Dyson shell
  پوسته‌ی ِ دایسون   
puste-ye Dyson

Fr.: couche de Dyson   

Dyson sphere.

Dyson; → shell.

Dyson sphere
  سپهر ِ دایسون   
sepehr-e Dyson

Fr.: sphère de Dyson   

A hypothetical structure built around a → star by an advanced → civilization to utilize most or all of the → energy radiated by their star. The idea of such a sphere was first formalized and popularized by theoretical physicist Freeman Dyson in 1960, though it was originally put forward by a 1945 science fiction novel. Dyson assumed that the power needs of → intelligent civilizations never stops increasing. He also proposed that searching for the existence of such structures might lead to the discovery of advanced civilizations elsewhere in the Galaxy. Sometimes referred to as a → Dyson shell or → megastructure.

Freeman John Dyson (1923-). His article, entitled "Search for Artificial Stellar Sources of Infrared Radiation," appeared in the 1960 issue of Science, 131 (3414), 1667-1668; → sphere.

E corona
  تاج ِ E   
tâj-e E

Fr.: couronne E   

A part of the → solar corona defined by the → emission lines of hot gases. These emission lines include the so-called → forbidden lines of the strongly ionized atoms of iron, calcium, and other elements. The E corona is thinner than the → K corona and the → F corona (M.S.: SDE).

E indicating → emission; → corona.

E-mode polarization
  قطبش ِ ترز ِ E   
qotbeš-e tarz-e E

Fr.: polarisation en mode E   

A → polarization component in the → cosmic microwave background radiation that depends only on → gradient, is independent of → curl and does not have → handedness. In contrast to the → B-mode, the E-mode may be due to both the → scalar perturbations and → tensor perturbations.

E, indicating electric-field like; → mode; → polarization.

e-term of aberration
  بیراهش ِ ترم ِ e   
birâheš-e tarm-e e

Fr.: aberration elliptique   

The same as → elliptic aberration.

e, → elliptic; → term; → aberration.

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