An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 699
ray of class 4
  پرتو ِ رده‌ی ِ ۳   
partw-e rade-ye 4

Fr.: rayon de classe 4   

In → rainbows, a sun ray that emerges from a water → droplet after two internal reflections. Rays of class 4 give rise to the → secondary rainbow.

ray; → class.

rayleigh (#)

Fr.: rayleigh   

A c.g.s. unit of light intensity used in astronomy and physics to measure the brightness of the night sky, auroras, etc. One rayleigh (R) represents the light intensity of one million photons of light emitted in all directions per square centimeter of receiver per second; or, in SI units, 795.775 x 106 photons per square meter per steradian (m-2·sr-1). A dark night sky has a light intensity of roughly 250 R.

In honor of the English mathematician and physicist Lord Rayleigh (1842-1919), surname of John William Strutt, Third Baron Rayleigh, whose research ranged over several fields of physics.

Rayleigh line
  خط ریلی   
xatt-e Rayleigh

Fr.: 1) droite de Rayleigh; 2) raie de Rayleigh   

1) A straight line that connects the points corresponding to the initial and final states on a graph of pressure versus specific volume for a substance subjected to a → shock wave. The slope of the Rayleigh line is proportional to the square of shock speed. Steeper Rayleigh lines correspond to higher shock speeds. See also → Hugoniot curve.
2) In → Raman scattering, the spectral line in scattered radiation which has the same frequency as the corresponding incident monochromatic radiation.

rayleigh; → line.

Rayleigh number (Ra)
  عدد ِ ریلی   
adad-e Rayleigh

Fr.: nombre de Rayleigh   

The ratio of the buoyancy force to the viscous force in a medium. This dimensionless number is used to estimate when convection commences in a fluid. It depends on the density and depth of the fluid, the coefficient of thermal expansion, the gravitational field, the temperature gradient, the thermal diffusivity, and the kinematic viscosity. Convection usually starts when Ra is 1000 or more, while heat transfer is entirely by conduction when Ra is less than 10.

rayleigh; → number.

Rayleigh scattering
  پراکنش ِ ریلی   
parâkaneš-e Rayleigh

Fr.: diffusion Rayleigh   

The scattering of light by → particles of size small compared with the → wavelength of light. The intensity of the light scattered by unit volume of the medium at an angle θ to the direction of propagation of the incident light is: Iθ = 8 π4α2 N I0 (1 + cos2θ)/(R2λ4), where α is the → molecular polarizability, N is the number of scattering molecules, I0 is intensity of the incident light, λ is the wavelength, and R is the distance from the scatterer. The fourth power dependence on wavelength means that blue light is much more strongly scattered than red light from a medium containing very fine particles. The air molecules, mostly → nitrogen (78%) and → oxygen (21%) are some 1,000 times larger than → visible light wavelengths. This accounts for the bluish appearance of smoke and of clear sky when the observation is not along the direction of illumination. The setting Sun, seen through a considerable thickness of atmosphere appears reddish because long wave radiation predominates in the transmitted light.
Historically, John Tyndall first discovered this phenomenon in 1859 (→ Tyndall effect), but Lord Rayleigh studied it in more detail in 1871.

rayleigh; → scattering.

Rayleigh-Jeans law
  قانون ِ ریلی-جینز   
qânun-e Rayleigh-Jeans(#)

Fr.: loi de Rayleigh-Jeans   

A classical law approximately describing the intensity of radiation emitted by a → blackbody. It states that this intensity is proportional to the temperature divided by the fourth power of the wavelength (8πkT4). The Rayleigh-Jeans law is a good approximation to the experimentally verified Planck radiation formula only at long wavelengths. At short wavelengths it runs into a paradox named the → ultraviolet catastrophe.

Rayleigh; → Jeans; → law.

Rayleigh-Jeans spectrum
  بیناب ِ ریلی-جینز   
binâb-e Rayleigh-Jeans

Fr.: spectre Rayleigh-Jeans   

The part of → electromagnetic spectrum approximated by the → Rayleigh-Jeans law.

Rayleigh; → Jeans; → spectrum.

Rayleigh-Taylor instability
  ناپایداری ِ ریلی-تیلر   
nâpâydâri-ye Rayleigh-Taylor

Fr.: instabilité Rayleigh-Taylor   

A type of hydrodynamical instability between two fluids of different densities, which occurs when the heavy fluid lies above the lighter fluid in a gravitational field. More generally a material interface is said to be Rayleigh-Taylor unstable whenever the fluid acceleration has an opposite direction to the density gradient.

rayleigh; → Taylor number; → instability.

Réaumur scale
  مرپل ِ ریءومور   
marpel-e Réaumur

Fr.: échelle Réaumur   

A temperature scale in which the → freezing point and the → boiling point of → water are set to 0 and 80 degrees respectively. See also → Celsius scale, → Fahrenheit scale, → Kelvin scale, → Rankine scale.

Named after René Antoine Ferchault de Réaumur (1683-1757), who first proposed it in 1730; → scale.

RCW catalogue
  کاتالوگ ِ WCR   
kâtâlog-e RCW

Fr.: catalogue RCW   

A catalog of → H II regions in the → southern  → Milky Way based on observations obtained at Mount Stromlo Observatory (Australia). It contains 181 → H-alpha emission objects characterized by their positions, dimensions, and estimated brightness.

Rodgers,A.W., Campbell, C.T., Whiteoak, J.B., 1960, MNRAS, 121, 103; → catalog.

  وا-، باز-   
vâ- (#), bâz- (#)

Fr.: re-   

A prefix meaning "again, anew" to indicate repetition (as rebuild, retell), "back, backward" to indicate "withdrawal" or "backward motion" (react, recall), or sometimes merely intensive (refine, resolve).

M.E., from O.Fr., from and L. re- "again, back."

Vâ-, variant bâz-, prefix denoting "reversal, opposition; separation; repetition; open; off; away," from Mid.Pers. abâz-, apâc-; O.Pers. apa- [pref.] "away, from;" Av. apa- [pref.] "away, from," apaš [adv.] "toward the back;" cf. Skt. ápāñc "situated behind."


Fr.: réagir   

1) To act in response to an agent, influence, or stimulus.
2) To act in a reverse direction or manner; to act in opposition, as against some force.
3) Chem.: To undergo a → chemical reaction.

re-; → act.


Fr.: réactance   

The opposition to the flow of alternating current caused by the inductance and capacitance in a circuit rather than by resistance.

From → react + -ance a suffix of nouns.

Vâžirâyi state noun of vâžirâ agent noun of vâžiridanreact.



1) General: A person or thing that reacts.
2) A substance on the left side of a → chemical equation.

From → react + -ant.

  واژیرش، واکنش   
vâžireš, vâkoneš

Fr.: réaction   

1) General: Action in response to some influence, event, stimulus.
2) An equal and opposite force exerted by a body against a force acting upon it. → principle of action and reaction.
3) A → nuclear reaction; → chain reaction.
4) → chemical reaction.

Verbal noun of → react.

reaction rate
  نرخ ِ واژیرش   
nerx-e vâžireš

Fr.: taux de réaction   

The speed with which a → chemical reaction takes place. In other words, the change in → concentration of a → substance divided by the → time interval during which the change is observed.

reaction; → rate.

  واژیرنده؛ واژیرشی   
vâžirandé; vâžireši

Fr.: réactif   

Tending to react. Pertaining to or characterized by reaction.

From → react + -ive a suffix of adjectives expressing tendency, disposition, function, connection, etc.

Vâžirandé verbal adj. of vâžiridanreact; vâžireši, adj. of vâžirešreaction.


Fr.: réactivité   

1) General: The quality or condition of being reactive.
2) Physics: A measure of the deviation from the condition at which a → nuclear reactor is critical.
3) Chem.: The relative → capacity of an → atom, → molecule, or → redical to undergo a → chemical reaction with another atom, molecule, or → compound.

From → reactive + → -ity.


Fr.: réacteur   

1) A device for introducing reactance into an electrical circuit (e.g. a capacitor).
2) Same as → nuclear reactor.

From → react + → -or.

reactor core
  مغزه‌ی ِ واژیرگر   
maqze-ye vâžirgar

Fr.: cœur de réacteur   

The part of a → nuclear reactor in which → nuclear fission takes place and huge quantities of heat energy are generated.

reactor; → core.

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