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
lepton number
  عدد ِ لپتونی   
adad-e leptoni (#)

Fr.: nombre leptonique   

In particle physics, a quantum number attributed to elementary particles which is conserved in nuclear reactions. It is +1 for a lepton, -1 for an antilepton and 0 for other particles.

lepton; → number.

Lesath (Upsilon Scorpii)
  لسعه   
Las'é

Fr.: Lesath   

A bright blue star of → apparent visual magnitudeV = 2.70, that with → Shaula (Lambda Scorpii) makes up the Scorpion's stinger. Among its other designations: HR 6508 and HIP 85696. Lesath is 580 → light-years away. Lesath and Shaula appear very close on the sky (less than a degree apart), but they are not physically related. Lesath is a → subgiant of → spectral type B2 IV with a → luminosity of about 7,380 Msun. It has a radius of about 6 Rsun, and a → surface temperature of about 22,000 K.

Lesath, from Ar. al-Las'ah (اللسعه) "the sting."

levitation
  ۱) بالا‌کرد؛ ۲) بالا‌شد   
1) bâlâ-kard; 2) bâlâ-šod

Fr.: lévitation   

1) The action of rising or causing something to rise and float in the air, typically by means of supposed magical powers.
2) → radiative levitation.

levitate; → -tion.

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.

libration
  هلازان، رخگرد   
halâzân, roxgard (#)

Fr.: libration   

Small oscillations of a → celestial body about its mean position. The term is used mainly to mean the Moon's libration caused by the apparent wobble of the Moon as it orbits the Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of the Moon's surface can be seen over a period of time. This results from three kinds of libration working in combination: → libration in longitude, → libration in latitude, and → diurnal libration. See also: → geometrical libration, → physical libration.

L. libration- "a balancing."

Halâzân "to and fro motion, oscillation," literally "a swing: a seat suspended by ropes on which a person may sit for swinging," from Gilaki halâcin "a swing," Ilâmi harazân "a swing," variants (Dehxodâ) holucin, holu "a swing," probably from Proto-Ir. *harz- "to send, to set."
Roxgard, literally "turning the face," from rox, variant ru(y) "face, surface; aspect; appearance" (Mid.Pers. rôy, rôdh "face;" Av. raoδa- "growth," in plural form "appearance," from raod- "to grow, sprout, shoot;" cf. Skt. róha- "rising, height") + gard "turning, changing," from gardidan "to turn, to change" (Mid.Pers. vartitan; Av. varət- "to turn, revolve;" Skt. vrt- "to turn, roll," vartate "it turns round, rolls;" L. vertere "to turn;" O.H.G. werden "to become;" PIE base *wer- "to turn, bend").

libration in latitude
  هلازان ِ ورونایی   
halâzân-e varunâ-yi

Fr.: libration en latitude   

A tiny oscillating motion of the Moon arising from the fact that the Moon's axis is slightly inclined relative to the Earth's. More specifically, the Moon's polar axis is tilted nearly 7° with respect to the plane of its orbit around Earth. Hence for half of each orbit we see slightly more of the north pole when its tipped toward us, and for the other half we see slightly more of its south pole. Libration in latitude displaces the mean center of the Moon north-south by between 6°.5 and 6°.9.

libration; → latitude.

libration in longitude
  هلازان ِ درژنایی   
halâzân-e derežnâyi

Fr.: libration en longitude   

A tiny oscillating motion of the → Moon arising from the fact that the Moon's orbit is not a precise circle but rather an → ellipse. Therefore, Moon is sometimes a little closer to the Earth than at other times, and as a result its → orbital velocity varies a bit. Since the Moon's rotation on its own axis is more regular, the difference appears as a slight east-west oscillation. Libration in longitude is the most significant kind of libration. It varies between about 4°.5 and 8°.1 because of gravitational perturbations in the Moon's orbit caused by the Sun.

libration; → longitude.

light adaptation
  نیاوش به روشنایی   
niyâveš bé rowšanâyi

Fr.: adaptation à la lumière   

The reflex adaptation of the eye to bright light, consisting of an increase in the number of functioning cones, accompanied by a decrease in the number of functioning rods; opposed to dark adaptation.

light; → adaptation.

light cone
  مخروط ِ نور   
maxrut-e nur (#)

Fr.: cône de lumière   

The set of all directions in which a light signal can travel toward an event (past light cone) or from an event (future light cone).

light; → cone.

light deflection
  واچفت ِ نور   
vâcaft-e nur

Fr.: déflexion de la lumière   

The deviation of a light ray by the gravitational field of a massive body. For example, stellar light passing near the Sun will be deviated by 1''.75 at the Sun's limb.

light; → deflection.

light pollution
  آلودگی ِ نوری   
âludegi-ye nuri (#)

Fr.: pollution lumineuse   

The inappropriate or excessive use of artificial light. It brightens the sky and has a particularly damaging effect on astronomical observations. More generally, light pollution can have serious environmental consequences for humans, wildlife, and our climate. Types of light pollution include: → glare, → skyglow, → light trespass, and → light clutter.

light; → pollution.

light-second
  نور-ثانیه، ثانیه‌ی ِ نوری   
nur-sâniyé, sâniye-ye nuri

Fr.: second-lumière   

The distance travelled by light in free space in one second. It is equivalent to 2.997924580 × 108 m or 2.998 × 105 km. This unit of length is mainly used in astronomy, telecommunications, and relativistic physics. Some quantities expressed in this unit are as follows. The mean diameter of the Earth: about 0.0425 light-seconds. The average distance from the Earth to the Moon: about 1.282 light-seconds. The diameter of the Sun: about 4.643 light-seconds. The average distance from the Earth to the Sun: 499.0 light-seconds.

light; → second.

likelihood function
  کریای ِ شدواری   
karyâ-ye šodvâri

Fr.: fonction de vraisemblance   

A function that allows one to estimate unknown parameters based on known outcomes. Opposed to → probability, which allows one to predict unknown outcomes based on known parameters. More specifically, a probability refers to the occurrence of future events, while a likelihood refers to past events with known outcomes.

likelihood; → function.

limestone
  سنگ ِ آهک   
sang-e âhak (#)

Fr.: castine, calcaire    

A → sedimentary rock composed principally of calcium carbonate. Limestone is usually formed from shells of once-living organisms or other organic processes, but may also form by inorganic precipitation.

lime; → stone.

limonite
  لیمونیت   
limonit (#)

Fr.: limonite   

A widely occurring ore of yellowish-brown to black color that consists of amorphous oxides of iron.

Gk. leimon "meadow" in reference to its occurrence as "bog ore" in meadows and marshes + → -ite.

Lindblad resonance
  باز‌آوایی ِ لیندبلاد   
bâzâvâyi-ye Lindblad

Fr.: résonance de Lindblad   

A kinematic resonance hypothesized to explain the existence of galactic → spiral arms. It occurs when the frequency at which a star encounters the spiral → density wave is a multiple of its → epicyclic frequency. Orbital resonances occur at the location in the disk where Ωp = Ω ± κ/m, where Ωp is → pattern speed, κ → epicyclic frequency, and m an integer representing the number of spiral arms. The minus sign corresponds to the inner Lindblad resonance (ILR) and the plus sign to the outer Lindblad resonance (OLR). The corotation resonance corresponds to Ωp = Ω. In general, the Lindblad resonances are defined for two spiral arms (m = 2), and low order. There are other less important resonances corresponding to higher m values. These resonances tend to increase the object's orbital eccentricity and to cause its longitude of periapse to line up in phase with the perturbing force. Lindblad resonances drive spiral density waves both in galaxies (where stars are subject to forcing by the spiral arms themselves) and in Saturn's rings (where ring particles are subject to forcing by Saturn's moons).

After the originator of the model, Bertil Lindblad (1895-1965), a Swedish astronomer, who made important contributions to the study of the rotation of the Galaxy; → resonance.

line identification
  ایدانش ِ خط   
idâneš-e xatt

Fr.: identification de raies   

The process of recognizing the spectral lines in a spectrum.

line; → identification.

line of action
  خط ِ ژیرش، ~ کنش   
xatt-e žireš, ~ koneš

Fr.: ligne d'action   

Of a force, the straight line along which the force → vector is directed. The action of a force on a → rigid body does not change when its point of application is displaced along the line of action. Hence, forces applied to a rigid body can be regarded as non-localized, or sliding, vectors.

line; → action.

line of induction
  خط ِ درهازش   
xatt-e darhâzeš

Fr.: ligne d'induction   

Same as → line of force in a magnetic field.

line; → induction.

linear acceleration
  شتاب ِ خطی   
šetâb-e xatti

Fr.: accélération linéaire   

The rate of change of the → linear velocity with time. It is defined by the expression Δvt and is equal to the → first derivative of the → linear velocity.

linear; → acceleration.

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