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: 44 Search : rotation
axis of rotation
  آسه‌ی ِ چرخش   
âse-ye carxeš

Fr.: axe de rotation   

Same as → rotation axis and → rotational axis.

axis; → rotation.

Carrington rotation
  چرخش ِ کرینگتون   
carxeš-e Carrington

Fr.: rotation de Carrington   

A system for counting rotations of the Sun based on the mean → synodic rotation period of the Sun. Initially, Lord Carrington determined the solar rotation rate by watching low-latitude → sunspots. He defined a fixed solar coordinate system that rotates in a sidereal frame exactly once every 25.38 days. This means that the solar rotation period, as viewed from the Earth, is assumed to be constant. However, the synodic rotation rate varies during the year because of the changing speed of the Earth in its orbit and the mean synodic period is about 27.2753 days. Carrington rotation number 1 began on November 9, 1853.

Named for Richard C. Harrington (1826-1875), British astronomer, who initiated the system; → rotation.

corotation
  هم‌چرخش   
hamcarxeš

Fr.: corotation   

The act of corotating.

Verbal noun of → corotate.

corotation radius
  شعاع ِ هم‌چرخش   
šoâ'-e ham-carxeš

Fr.: rayon de corotation   

1) In the → X-wind model of → accretion, the distance from the star where the → centrifugal force on a particle corotating with the star balances the → gravitational attraction; in other words, where the → accretion disk rotates at the same → angular velocity as the star.
2) In a → spiral galaxy, the place where the spiral → pattern speed has the same velocity as the → rotation curve of the → galactic disk. In the frame rotating with the wave, particles inside this radius will appear to revolve in the direction of the frame rotation (prograde) while outside this corotation radius, they will be retrograde.

corotation; → radius.

corotation resonance
  باز‌آوایی ِ هم‌چرخش   
bâzâvâyi-ye ham-carxeš

Fr.: résonance de corotation   

That condition of a → galactic disk at an orbital radius in which the → angular velocity of the disk equals the → pattern speed. It is significant that the spiral wave pattern rotates as a rigid body (ΩP = const), whereas the galactic disk rotates differentially (Ω is a function of galactocentric distance r). The distance rC at which the two angular velocities coincide (Ω(rC) = ΩP) is referred to as the → corotation radius. The corotation resonance and its position within the galaxy is one of the fundamental properties of a spiral galaxy.

corotation; → resonance.

dextrorotation
  راست‌چرخش   
râstcarxeš

Fr.: dextrorotation   

The clockwise rotation of the → plane of polarization of light (as viewed by an observer looking straight in the incoming light) by certain substances. See also → levorotation.

dextro-; → rotation.

differential rotation
  چرخش ِ دگرسانه‌ای   
carxeš-e degarsâneyi

Fr.: rotation différentielle   

1) Of a single body (such as a star or a gaseous planet), the axial rotation of equatorial latitudes faster than polar latitudes.
2) Of a galaxy, the orbiting of stars nearer the center faster than those at the edge.

differential; → rotation.

Earth's rotation
  چرخش ِ زمین   
carxeš-e zamin (#)

Fr.: rotation de la Terre   

The natural motion of the Earth around its own axis, which takes place once in a → sidereal day. The Earth rotates toward the → east, in the same direction as it revolves around the Sun. If viewed from the north celestial pole, the Earth turns → counterclockwise. The opposite is true when the Earth is viewed from the south celestial pole. The Earth's rotation is responsible for the diurnal cycles of day and night, and also causes the apparent movement of the Sun across the sky. The Earth's rotation velocity at the → equator is 1,673 km h-1 or about 465 m s-1. More generally, at the → latitude  φ it is given by: vφ = veq cos φ, where veq is the rotation velocity at the equator. The Earth's rotation is gradually slowing down under the action of the → tides, which are generated by the → gravitational attraction of the → Moon. As the result of this → tidal friction, the day is becoming longer at a rate of about 2 milliseconds, or 0.002 seconds, per century (or one second every 50,000 years). Moreover, the loss of the Earth's → rotational angular momentum increases the Moon's → orbital angular momentum, because the angular momentum of the → Earth-Moon system is conserved. In consequence, the Moon slowly recedes from the Earth by about 4 cm per year, which leads to increasing its orbital period and the length of a month as well.

Earth; → rotation.

Faraday rotation
  چرخش ِ فارادی   
carxeš-e Faraday (#)

Fr.: rotation Faraday   

The rotation of the plane of → polarization experienced by a beam of → linearly polarized radiation when the radiation passes through a material containing a magnetic field with a component in the direction of propagation. This effect occurs in → H II regions in which a magnetic field causes a change in the polarized waves passing through. Same as → Faraday effect.

farad; → rotation.

field rotation
  چرخش ِ میدان   
carxeš-e meydân

Fr.: rotation de champ   

The effect of the Earth's rotation on the position of the image formed on the → focal plane of a telescope during long exposures. In the case of → equatorial mounting, the image remains fixed, whereas it turns continuously with an → altazimuth mounting. In the latter case the image motion must be compensated by an appropriate mechanism, → field rotator.

field; → rotation.

flat rotation curve
  خم ِ چرخش ِ تخت   
xam-e carxeš-e taxt

Fr.: courbe de rotation plate   

A galactic → rotation curve in which the → rotation velocity is constant in the outer parts. The flat component is preceded by a rising curve that shows solid body rotation in the very center of the → galaxy. A flat rotation curve implies that the mass is still increasing linearly with radius. See also → dark matter.

flat; → rotation; → curve.

galactic rotation
  چرخش ِ کهکشان   
carxeš-e kahkešân

Fr.: rotation galactique   

The revolving of the gaseous and stellar content of a galaxy around its central nucleus. The rotation is not uniform, but differential. One revolution of the Sun within our own Galaxy takes about 220 million years, or one cosmic year.

galactic; → rotation.

Keplerian rotation curve
  خم ِ چرخش ِ کپلری   
xam-e carxeš-e Kepleri (#)

Fr.: courbe de rotation keplérienne   

A → rotation curve in which the speed of the orbiting body is inversely proportional to the → square root of its distance from the mass concentrated at the center of the system.

Keplerian; → rotation; → curve.

levorotation
  چپ‌چرخش   
capcarxeš

Fr.: lévorotation   

The counterclockwise rotation of the → plane of polarization of light (as observed when looking straight through the incoming light) by certain substances.

Adj. related to → levorotation.

magnetorotational instability (MRI)
  ناپایداری ِ مغنات-و-چرخشی   
nâpâydâri-ye meqnâtocarxeši

Fr.: instabilité magnétorotationnelle   

An instability that arises from the action of a weak → poloidal magnetic field in a → differentially rotating system, such as a → Keplerian disk. The MRI provides a mechanism to account for the additional outward → angular momentum transport. To put it simply, the → frozen magnetic field line acts as a spring connecting two radially neighboring fluid parcels. In a Keplerian disk the inner fluid parcel orbits more rapidly than the outer, causing the spring to stretch. The magnetic tension forces the inner parcel to slow down reducing its angular momentum by moving it to a lower orbit. The outer fluid parcel is forced by the spring to speed up, increase its angular momentum, and therefore move to a higher orbit. The spring tension increases as the two fluid parcels grow further apart, and eventually the process runs away. The MRI was first noted in a non-astrophysical context by E. Velikhov in 1959 when considering the stability of → Couette flow of an ideal hydromagnetic fluid. His result was later generalized by S. Chandrasekhar in 1960. The MRI was rediscovered by Balbus and Hawley 1991 (ApJ 376, 214) who demonstrated that this instability does indeed manifest itself in → accretion disks, and could account for the turbulent mixing needed to explain the observed mass → accretion rates.

magneto-; → rotational; → instability.

plane of rotation
  هامن ِ چرخش   
hâmon-e carxeš

Fr.: plan de rotation   

For a rotating object, the plane → perpendicular to the → rotation axis.

plane; → rotation.

projected rotational velocity
  تندای چرخشی ِ فراشانده   
tondâ-ye carxeši-ye farâšândé

Fr.: vitesse rotationnelle projetée   

The → angular velocity of a star deduced from the → rotational broadening of its → spectral lines. It is expressed as v sini, where i is the → inclination of the rotational axis with respect to the normal to the → plane of the sky. The real equatorial rotational velocity can be determined only if the inclination of the rotational axis is known.

Projected, p.p. of → project; → rotational; → velocity.

rotation
  چرخش   
carxeš (#)

Fr.: rotation   

The motion of a body about its axis.

Verbal noun of → rotate.

rotation axis
  آسه‌ی ِ چرخش   
âse-ye carxeš

Fr.: axe de rotation   

The imaginary line around which an object rotates. Same as → rotational axis and → axis of rotation.

rotation; → axis.

rotation curve
  خم ِ چرخش   
xam-e carxeš

Fr.: courbe de rotation   

A plot of the variation in → orbital velocity of stars and → interstellar matter with distance from the center of a → galaxy. A "flat" rotation curve indicates that the mass of the galaxy increases linearly with distance from its center. See also: farsi→ Keplerian rotation curve

Rotation; → curve.

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