An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



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Number of Results: 397
ohm (#)

Fr.: ohm   

A unit of electrical resistance equal to that of a conductor in which a current of one ampere is produced by a potential of one volt across its terminals.

Named after Georg Simon Ohm (1789-1854), the German physicist who discovered the law which bears his name.

Ohm's law
  قانونِ اُهم   
qânun-e Ohm (#)

Fr.: loi d'Ohm   

1) For a → conductor at rest, the → voltage across the conductor is equal to the product of the current flowing through it and its → resistance. In other words, when such a conductor is subjected to an electric field E, the resulting → current density, J, is proportional to the electric field E: J = σE, where σ is the → conductivity, i.e. the reciprocal of → resistivity, ρ = 1/σ.
2) Ohm's law for a moving conductor is expressed by: J = σ(E + v x B), where v is the velocity and B the → magnetic induction.

ohm; → law.

Ohmi (#)

Fr.: ohmique   

Of or relating to a system which obeys Ohm's law.

ohm + → -ic.

Ohmic decay time
  زمان ِ تباهی ِ اُهمی   
zamân-e tabâhi-ye Ohmi

Fr.: temps de dissipation ohmique   

An upper bound on the time scale on which the magnetic field of a system would decay in the absence of any other agent. It is expressed as: τμ = R2 / μ, where R is the scale size of the system, η the magnetic diffusivity (η = 1 / μσ, where μ is the magnetic permeability and σ the electrical conductivity). For a star like the Sun, τμ  ≅ 1010 years, so a fossil magnetic field could survive for the star's lifetime on the main sequence. For the Earth, τμ  ≅ 104 years, so a → dynamo is required to explain the persistence of the geomagnetic field.

Ohmic; → decay; → time.

Ohmic dissipation
  افتالِ اُهمی   
eftâl-e ohmi

Fr.: dissipation ohmique   

1) A loss of electric energy due to conversion into heat when a current flows through a resistance. Same as Ohmic loss.
2) In plasma physics, the energy released by charged particles as they make collisions with other particles.

Ohmic; → dissipation.

Ohmic loss
  دسترفت ِ اهمی   
dastraft-e Ohmi

Fr.: perte ohmique   

Same as → Ohmic dissipation.

Ohmic; → loss.

Olbers' paradox
  پارادخشِ اُلبرس   
pârâdaxš-e Olbers (#)

Fr.: paradoxe d'Olbers   

The puzzle of why the night sky is not as uniformly bright as the surface of the Sun if, as used to be assumed, the Universe is infinitely large and filled uniformly with stars. It can be traced as far back as Johannes Kepler (1571-1630), was discussed by Edmond Halley (1656-1742) and Philippe Loys de Chéseaux (1718-1751), but was not popularized as a paradox until Heinrich Olbers took up the issue in the nineteenth century. This paradox has been resolved by the → Big Bang theory. In a Universe with a beginning, we can receive light only from that part of the Universe close enough so that light has had time to travel from there to here since the Big Bang. The night sky is dark because the galaxies are only about ten billion years old and have emitted only a limited amount of light, not because that light has been weakened by the expansion of the Universe (P. S. Wesson et al., 1987, ApJ 317, 601).

Formulated in 1826 by Heinrich Wilhelm Olbers (1758-1840), German physician and amateur astronomer, who discovered the asteroids Pallas and Vesta as well as five comets; → paradox.

  کهن، پیر   
kohan (#), pir (#)

Fr.: vieux   

Of an astronomical object, having existed as specified with relation to younger or newer objects of the same category; e.g. → old star.

From M.E., from O.E. eald, ald; cf. Du. old, Ger. alt, Goth. altheis; akin to O.N. ala "to nourish."

Kohan "old, ancient," kohné "worn;" Mid.Pers. kahwan "old, aged, worn."
Pir, from Mid.Pers. pir "old, aged, ancient;" Av. parô (adv.) "before, before (of time), in front (of space);" cf. Skt. puáh, combining form of puras "before (of time and place), in front, in advance."

old star
  ستاره‌یِ کهن، کهن‌ستاره، ستاره‌ی پیر   
setâre-ye kohan (#), kohan-setâré (#), setâre-ye pir (#)

Fr.: vielle étoile   

A member of a population of stars that, according to stellar evolution theories, are almost as aged as the galaxy in which it resides.

old; → star.

old stellar population
  پرینش ِ ستاره‌ای ِ کهن   
porineš-e setâre-yi-ye kohan

Fr.: population stellaire vielle   

A population of stars in a stellar system that have definitely left the → main sequence.

old; → stellar; → population.

olivin (#)

Fr.: olivine   

A silicate mineral of magnesium (Mg2SiO4) and iron (Fe2SiO4,) found commonly in basalt and in carbonaceous chondrites.

From Ger. Olivin, from olive, because of its olive-green to gray-green color, + -in equivalent to -ine a noun suffix used in chemical and mineralogical nomenclature.

Olympus Mons
  کوهِ اُلومپوس   
kuh-e Olumpos

Fr.: Olympus Mons   

The highest peak on Mars, and the largest volcano in the solar system. It rises to a height of 27 kilometres above the datum level selected on the basis of atmospheric pressure.This gigantic shield volcano, 700 kilometres across, is similar in nature to volcanoes on Earth but its volume is at least fifty times greater than its nearest terrestrial equivalent.

From L. Mons, → mountain, + Olympus, from Gk. Olympos a mountain (2966 m) in north-east Greece, on the boundary between Thessaly and Macedonia, mythical abode of the greater Grecian gods.

Omega Centauri (ω Cen)
  ا ُمگا کنتاؤروس   
Omegâ Kentawros

Fr.: Omega centauri   

The largest and most luminous → globular cluster associated with the Milky Way Galaxy. Omega Centauri is located about 18,300 → light-years away and contains several million old stars. The stars in its center are so crowded that they are believed to be only 0.1 light-year away from each other. It is about 12 billion years old. Omega Centauri was first listed in Ptolemy's catalog nearly two thousand years ago. In 1677 Edmond Halley reported it as a nebula, and in the 1830s John Herschel was the first to correctly identify it as a globular cluster. Also called NGC 5139.

Omega, Gk. alphabet letter; Centauri, → Centaurus.

omega effect
  اسکر ِ امگا   
oskar-e omega

Fr.: effet ω   

In the → solar dynamo model, the process whereby the → meridional magnetic field is stretched into an → azimuthal magnetic field by → differential rotation. See also → alpha effect.

Omega (ω), Gk. letter of alphabet; → effect.

Omega Nebula
  میغِ اُمگا   
miq-e omegâ

Fr.: nébuleuse Omega   

An → H II region located in the rich star fields of the → Sagittarius area of the Milky Way. Its distance from the Earth is between 5,000 and 6,000 → light-years and spans some 15 light-years in diameter. Its other designations are: Swan Nebula, Messier 17, and NGC 6618.

Omega, Gk. alphabet letter; → nebula.

Omicron Ceti
  ا ُمیکرون کتوس   
Omikron Ketus

Fr.: Omicron Ceti   

Another name for → Mira.

Omicron, Gk. alphabet letter; Ceti, → Cetus.

Omicron Lupi

Fr.: Omicron (ο) Lupi   

A bright star of → apparent visual magnitude  V = 4.3 lying in the constellation → Lupus. Among its other designations: HD 130807, HR 5528, HIP 72683. It is an → early-type star of → spectral type B5 IV and a member of the → Scorpius-Centaurus association. Ο Lupi is in fact a → binary system whose components have an angular separation of 0.043 arcsec, corresponding to a physical separation of 5.3 → astronomical units, with a mass ratio of 0.91.

Bayer designation; Lupi, genitive of → Lupus.

visp- (#)

Fr.: omni-   

All, universally.

L. omni-, combining form of omnis "all, every," of unknown origin.

Visp-, from Mid.Pers. visp- "all;" O.Pers. visa-, vispa- "all;" Av. vīspa- "all, every, entire, universal" (vīspô.ayāra- "lasting all the days," vīspô.vīδvah- "knowing everything, omniscient"); cf. Skt. vīśva- "all, every; whole, universal."

visp-tavân (#)

Fr.: omnipotent   

Having very great or unlimited power or authority.

omni-; → power.

  ویسپ‌باشنده، ویسپ‌باش، ویسپ‌باشا   
visp-bâšandé, visp-bâš, visp-bâšâ

Fr.: omniprésent   

Same as → ubiquitous.

omni-; → present.

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