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

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 27 Search : parameter
 Rossby parameter   پارامون ِ روسبی   pârâmun-e RossbyFr.: paramètre de Rossby   The northward variation of the Coriolis parameter, arising from the sphericity of the Earth. size parameter   پارامون ِ اندازه   pârâmun-e andâzéFr.: paramètre de taille   A quantity that defines the type of → scattering.→ size; → parameter. slope parameter   پارامون ِ شیب   pârâmun-e šibFr.: paramètre de pente   In a → power-law distribution or → regression, the → exponent that represents the effect of the → independent variable, x, on the → dependent variable, y. X has no association with y if the slope parameter = 0 and x has strong association with y if the slope parameter is large.→ slope; → parameter. Stokes parameters   پارامون‌های ِ استوکس   pârâmunhâ-ye StokesFr.: paramètres de Stokes   Four parameters which are needed to fully describe the → polarization state of → electromagnetic radiation. They involve the maximum and minimum intensity, the ellipticity, and the direction of polarization. The four Stokes parameters are traditionally defined as follows: I ≡ total intensity. Q ≡ I0 - I90 = difference in intensities between → horizontal and → vertical  → linearly polarized components. U ≡ I+45 - I-45 = difference in intensities between linearly polarized components oriented at +45° and -45° (or 135°). V ≡ Ircp - Ilcp = difference in intensities between right and left → circularly polarized polarized components.→ Stokes; → parameter. Tisserand's parameter   پارامون ِ تیسران   pârâmun-e TisserandFr.: paramètre de Tisserand   In celestial mechanics, a combination of orbital elements commonly used to distinguish between comets and asteroids. Objects whose Tisserand's parameter value is smaller than 3 are considered to be dynamically cometary, and those with a value larger than 3 asteroidal. Also called Tisserand's invariant.Named after François Félix Tisserand (1845-1896), French astronomer, Director of the Paris Observatory (1892). Toomre parameter   پارامون ِ تومره   pârâmun-e ToomreFr.: paramètre de Toomre   A quantity that measures the stability of a differentially rotating disk of matter against → gravitational collapse. It is expressed by the relation: Q = csκ / πGΣ, where cs is the → sound speed, κ the → epicyclic frequency, G the → gravitational constant, and Σ the → surface density. The disk is linearly stable for Q > 1 and linearly unstable for Q < 1.After Alar Toomre (1936-), an American astrophysicist of Estonian origin, professor of mathematics at the Massachusetts Institute of Technology; → parameter. virial parameter   پارامون ِ ویریال   pârâmun-e viriyâlFr.: paramètre du viriel   A dimensionless parameter that measures the ratio of thermal plus kinetic energies to gravitational energy of a physical system, such as a molecular cloud. The virial parameter is expressed as: αvir = 5σ2R / GM, where R and M are the radius and mass of the cloud respectively, σ is the one-dimensional → velocity dispersion inside the cloud, and G the → gravitational constant. It indicates whether a cloud could be bound or not. For molecular clouds that are confined by their surface pressure and for which self-gravity is unimportant, αvir is much larger than unity, whereas αvir is ~ 1 when the gravitational energy of a clump becomes comparable to its kinetic energy. See, e.g., Bertoldi & McKee, 1992 (ApJ 395, 140). See also → virial theorem.→ virial; → parameter.

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