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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Number of Results: 3079 Search : on
curvature constant
  پارامون ِ خمیدگی   
pârâmun-e xamidegi

Fr.: paramètre de courbure   

A parameter occurring in the → Friedmann equations of → general relativity describing the geometry of → space-time. A spatially → open Universe is defined by k = -1, a → closed Universe by k = + 1 and a → flat Universe by k = 0. See also the → Robertson-Walker metric. See also → curvature of space-time.

curvature; → parameter.

cyclone
  چرخند   
carxand (#)

Fr.: cyclone   

Any circulatory wind system in the atmosphere in which the motion is anticlockwise in the northern hemisphere (that is in the same sense as that of Earth) and clockwise in the southern hemisphere, around a center of low pressure.

From Gk. kyklon "moving in a circle, whirling around," pr.p. of kykloun "move in a circle, whirl," from kyklos "circle," cognate with Pers. carxcycle.

Carxand "moving in a circle," from carxidanrotate, from carx, → cycle.

cyclonic
  چرخندی   
carxandi

Fr.: cyclonique   

Having a sense of rotation about the local vertical the same as that of the Earth's rotation. More specifically, as viewed from above, → counterclockwise in the Northern Hemisphere, → clockwise in the Southern Hemisphere, and undefined at the equator. The opposite of → anticyclonic (Meteorology Glossary, American Meteorological Society).

cyclone; → -ic.

cyclotron
  سیکلوترون   
siklotron (#)

Fr.: cyclotron   

An → accelerator in which charged subatomic particles generated at a central source are accelerated to acquire energies up to several tens of millions of → electron-volts. The cyclotron consists of two flat, semicircular metal boxes or electrodes, called dees or D's because of their shape. An alternating electric field between the dees continuously accelerates the particles from one dee to the other, while the magnetic field bends their direction guiding them in a circular path. As the speed of the particles increases, so does the radius of their path, and the particles spiral outward. See also → cyclotron frequency, → synchrotron.

From cyclo- a combining form meaning → cycle + → -tron.

cyclotron frequency
  بسامد ِ سیکلوترون   
basâmad-e siklotron

Fr.: fréquence cyclotron   

The frequency with which a → non-relativistic particle of charge q and mass m turns in a cyclotron with → uniform magnetic field B. The equality of the → centripetal force to the → Lorentz force leads to the expression: by: fcycl = qB/2πm. For → relativistic case, the frequency is smaller and is called → synchrotron frequency.

cyclotron; → frequency.

cyclotron radius
  شعاع ِ سیکلوترون   
šo'â'-e siklotron

Fr.: rayon de cyclotron   

Same as → Larmor radius.

cyclotron; → radius.

Cygnus OB association
  آهزش ِ OB ماکیان   
âhazeš-e OB Mâkiyân

Fr.: association OB Cygne   

One of nine → OB associations located in the Cygnus → constellation. The central association, → Cygnus OB2, is the most famous and the youngest of the Cygnus region. (L. Mahy et al. 2013, astro-ph/1301.0500 and references therein).

Cygnus; → OB star; → association.

D-type ionization front
  پیشان ِ یونش ِ گونه‌ی ِ D   
pišân-e yoneš-e gune-ye D

Fr.: front d'ionisation de type D   

An → ionization front of → H II regions whose expansion speed is comparable to the → sound speed in the gas (~ 10 km/sec for hydrogen at 104 K). A D-type ionization front results from → R-type ionization front when its propagation speed decreases as the volume of gas ahead of the ionization front grows. If front velocity is equal to a lower limit (C12 / 2C2, where C1 and C2 are the sound speed ahead and behind the front respectively), the front is called D critical.

D referring to a dense gas; → type; → ionization; → front.

Dalton minimum
  کمینه‌ی ِ دالتون   
kamine-ye Dalton

Fr.: minimum de Dalton   

A 40-year period of unusually low → solar activity, from about 1790 to 1830. See also the → Maunder minimum.

Named after John Dalton (1766-1844), British meteorologist; → minimum.

Danjon astrolabe
  اسطرلاب ِ دانژون   
ostorlab-e Danjon

Fr.: astrolabe de Danjon   

A modern unportable astrolabe which is used for high precision measuring of stellar and geographical coordinates. The instrument uses the simultaneous observations of two images of the same star, one of the images formed directly by the lower face of a prism and the other by the light rays reflected first from a mercury bath and then by the upper face of the prism. The images coincide when the zenithal distance of the star attains a prefixed value (Gauss method of equal altitudes, → almucantar). Apart from astrometry, the Danjon astrolabe was used for studying the Earth's rotation and is currently used for solar radius measurements.

After André Danjon (1890-1967), French astronomer, who developed the instrument at the Strasbourg Observatory before the Second World War and at the Paris Observatory in 1948. The concept of prism astrolabe was initially invented by the French Auguste Claude (1858-1938) around 1900 and was later modified in collaboration with Ludovic Driencourt (1861-1940); → astrolabe.

Danjon scale
  مرپل ِ دانژون   
marpel-e danjon

Fr.: échelle de Danjon   

A scale to evaluate as exactly as possible the darkening degree of a total → lunar eclipse. The five steps of the scale run from 0 (extremely dark, invisible Moon) to 4 (extremely bright, the eclipse having a very weak effect on the Moon's visibility). The darkening at a lunar eclipse is determined to a great extent by the transparency of the terrestrial atmosphere, which is affected by clouds and the dust from the volcanic eruptions (M.S.: SDE).

Named after André Danjon, who set up the scale, → Danjon astrolabe; → scale.

dark adaptation
  نیاوش به تاریکی   
niyâveš bé târiki

Fr.: adaptation à l'obscurité   

The automatic adjustment of the iris and retina of the eye to allow maximum vision in the dark, following exposure of the eye to a relatively brighter illumination.

dark; → adaptation.

dark matter annihilation
  نابودی ِ ماده‌ی ِ تاریک   
nâbudi-ye mâde-ye târik

Fr.: annihilation de la matière noire   

A hypothetical process whereby hypothetical → non-baryonic dark matter particles undergo → annihilation interactions with themselves. The process results in observable by-products such as high-energy photons, neutrinos, and other detectable particles. See also → dark matter decay.

dark; → matter; → annihilation.

data acquisition
  الفنجش ِ داده‌ها   
alfanješ-e dâdehâ

Fr.: acquisition de données   

The process of controlling telescope operations during observation and obtaining data.

data; acquisition, from L. acquisitionem, from acquirere "get in addition," from → ad- "extra" + quærere "to search for, obtain."

Alfanješ, verbal noun of alfanjidan (variant alfaqdan) "to acquire, get," Bactrian αλφανζ "to acquire," Sogdian δβ'yz "to acquire, gain, get" (Cheung 2007); → data.

data reduction
  بازهازش ِ داده‌ها   
bâzhâzeš-e dâdehâ

Fr.: réduction de données   

The process of converting crude observational data into usable information for scientific interpretation, by correcting, rearranging, ordering, and simplifying.

data; → reduction.

Davisson-Germer experiment
  آزمایش ِ دیویسون-جرمر   
âzmâyeš-e Davisson-Germer

Fr.: expérience de Davisson-Germer   

The experiment carried out in 1927 that confirmed the → de Broglie hypothesis as to the → wave nature of the → electron. It showed that electrons scattering off crystals form a → diffraction pattern. The experimental setup consisted of a → nickle chloride → crystal as → target, an electron gun, and a → detector placed on a graduated circular scale. The intensity of the reflected electrons was measured as a function of angle and electron energy. The observations showed a strong intensity peak at a certain angle. The nickel crystal acted as a → diffraction grating. → Constructive interference occurred at a particular angle, where the peak intensity was observed in accord with → Bragg's law. Interestingly, the intent of the initial experiment was was not to confirm the de Broglie hypothesis. In fact, the discovery was made by accident.

Carried out by American physicists Clinton Davisson (1881-1958) and Lester Germer (1896-1971); → experiment.

de Broglie equation
  هموگش ِ دوبروی   
hamugeš-e de Broglie

Fr.: équation de de Broglie   

According to the → de Broglie hypothesis, which has been verified by experiments, every → particle of matter, whatever its nature, has a characteristic → wavelength associated with its wavelike quantum aspect. The de Broglie equation gives the equivalent wavelength of a moving particle: λ = h/mv, where h is → Planck's constant, m the mass of the particle, and v its velocity.
See also: → de Broglie wavelength, → Davisson-Germer experiment.

Named after Louis Victor de Broglie (1892-1987), French physicist, creator of a new field in physics, wave mechanics, who won the Nobel prize in physics in 1929. → equation

de-excitation
  فرو انگیزش   
foru-angizeš

Fr.: désexcitation   

Transition from an excited energy state to a lower energy level, as in spectral line formation or particle emission from an atomic nucleus.

From → de- + → excitation.

de-projection
  وا-فراشانش   
vâ-farâšâneš

Fr.: déprojection   

A method for estimating the real orientation of a field and/or related velocities/separations from two-dimensional images.

de-; → projection.

decay constant
  پایای ِ تباهی   
pâyâ-ye tabâhi

Fr.: 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.

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