An Etymological Dictionary of Astronomy and Astrophysics

فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory



<< < -le Lag lam Lap lar lat lea leg len lev lig lim lin lin liq loa loc lon Lor low lum lun Lym > >>

Number of Results: 458
Lagrangian method
  روش ِ لاگرانژی   
raveš-e Lâgrânži

Fr.: méthode lagrangienne   

Fluid mechanics: An approach in which a single fluid particle (→ Lagrangian particle) is followed during its motion. The physical properties of the particle, such as velocity, acceleration, and density are described at each point and at each instant. Compare with → Eulerian method.

Lagrangian; → method.

Lagrangian multiplier
  بستاگر ِ لاگرانژ   
bastâgar-e Lagrange

Fr.: multiplicateur de Lagrange   

Math.: A constant that appears in the process for obtaining extrema of functions of several variables. Suppose that the function f(x,y) has to be maximized by choice of x and y subject to the constraint that g(x,y)k. The solution can be found by constructing the → Lagrangian function  L(x,y,λ) = f(x,y) + λ[k - g(x,y)], where λ is the Lagrangian multiplier.

Lagrangian point; → multiplier.

Lagrangian particle
  ذره‌ی ِ لاگرانژی   
zarre-ye Lâgrânži

Fr.: particule lagrangienne   

Fluid mechanics: In the → Lagrangian method, a particle that moves as though it is an element of fluid. The particle concept is an approach to solving complicated fluid dynamics problems by tracking a large number of particles representing the fluid. The particle may be thought of as the location of the center of mass of the fluid element with one or more property values.

Lagrangian; → particle.

Lagrangian point
  نقطه‌های ِ لاگرانژ   
noqtehâ-ye Lagrange (#)

Fr.: points de Lagrange   

On of the five locations in space where the → centrifugal force and the → gravitational force of two bodies (m orbiting M) neutralize each other. A third, less massive body, located at any one of these points, will be held in equilibrium with respect to the other two. Three of the points, L1, L2, and L3, lie on a line joining the centers of M and m. L1 lies between M and m, near to m, L2 lies beyond m, and L3 on the other side of M beyond the orbit. The other two points, L4 and L5, which are the most stable, lie on either side of this line, in the orbit of m around M, each of them making an equilateral triangle with M and m. L4 lies in the m's orbit approximately 60° ahead of it, while L5 lies in the m's orbit approximately 60° behind m. See also → Trojan asteroid; → Roche lobe; → equipotential surface; → horseshoe orbit.

Lagrangian; → point.

daryâcé (#)

Fr.: lac   

A body of fresh or salt water entirely surrounded by land.

From O.Fr. lack, from L. lacus "pond, lake," related to lacuna "hole, pit," from PIE *lak- (cf. Gk. lakkos "pit, tank, pond," O.C.S. loky "pool, cistern," O.Ir. loch "lake, pond").

Daryâcé, from daryâ "sea" Mid.Pers. daryâp variant zrah; O.Pers. drayah-; Av. zrayah- "sea;" cf. Skt. jráyas- "expanse, space, flat surface" + -cé diminutive suffix, from Mid.Pers. -cak, variants -êžak (as in kanicak "little girl," sangcak "small stone," xôkcak "small pig"), also Mod.Pers. -ak.

Lamb shift
  کیب ِ لمب   
kib-e Lamb

Fr.: décalage de Lamb   

A tiny change in the energy levels of the hydrogen atom between the states 2S1/2 and 2P1/2, which creates a shift in the corresponding spectral lines. The 2P1/2 state is slightly lower than the 2S1/2 state, contrarily to the Schrodinger's solution.

Named after Willis Eugene Lamb, Jr. (1913-), an American physicist who discovered this effect in 1951, and won the Nobel Prize in physics in 1955 "for his discoveries concerning the fine structure of the hydrogen spectrum;" → shift.


Fr.: lambda   

The eleventh letter of the Greek alphabet. In lower case, λ, it denotes → wavelength. It is also used in the → Bayer designation system to identify a specific star in a → constellation. See also → lambda point.
In upper case, Λ, it represents the → cosmological constant or → dark energy.

From Phoenician lamedh.

Lambda Bootis star
  ستاره‌ی ِ لامبدا گاوران   
setâre-ye lâmbda Gâvrân

Fr.: étoile lambda du Bouvier   

The prototype of a small class of stars (A-F types) which have weak metallic lines (indicating that they are depleted in metals heavier than Si, but with solar abundances of C, N, O, and S). Moreover, they have moderately large rotational velocities and small space velocities. Lambda Boo stars may be pre-main-sequence objects, or they may be main sequence stars that formed from gas whose metal atoms had been absorbed by interstellar dust.

Named after the prototype, the star → Lambda (λ) of constellation → Bootes; → star.

lambda cold dark matter model
  مدل ِ لامبدا-ماده‌ی-سرد ِ- تاریک   
model-e lâmbdâ-mâde-ye-sard-e-târik

Fr.: modèle ΛCDM   

The → standard model of → Big Bang that incorporates both → dark matter and → dark energy. See also → cold dark matter (CDM).

lambda, → cosmological constant; → cold; → dark; → matter; → model.

lambda point
  نقطه‌ی ِ لامبدا   
noqte-ye lâmbdâ

Fr.: point lambda   

The temperature (roughly 2.17 K) at which → liquid helium (→ helium I) becomes → superfluid (→ helium II).

The name was given by the Dutch physicist Willem Hendrik Keesom (1876-1956), who discovered the behavior of helium near this transition point and successfully solidified helium in 1926 (under an external pressure of 25 atmospheres). The name was originally suggested by Paul Ehrenfest (1880-1933), who was inspired by the shape of the → specific heat curve, which resembles the Gk. letter → lambda; → point.


Fr.: lambert   

A centimeter-gram-second (cgs) unit of luminance (or brightness) equal to 1/π candle per square centimeter. Physically, the lambert is the luminance of a perfectly diffusing white surface receiving an illuminance of 1 lumen per square centimeter.

Johann Heinrich Lambert (1728-1777), German scientist and mathematician; → law.

Lambert's cosine law
  قانون ِ کوسینوس ِ لامبرت   
qânun-e cosinus-e Lambert

Fr.: loi en cosinus de Lambert   

The intensity of the light emanating in any given direction from a perfectly diffusing surface is proportional to the cosine of the angle between the direction and the normal to the surface. Also called → Lambert's law.

lambert; → cosine; → law.

Lambert's law
  قانون ِ لامبرت   
qânun-e Lambert

Fr.: loi de Lambert   

Same as → Lambert's cosine law.

lambert; → law.

Lambertian disk
  گرده‌ی ِ لامبرتی، دیسک ِ ~   
gerde-ye Lamberti, disk-e ~

Fr.: disque lambertien   

A → planetary or → satellite disk with → Lambertian surface. Such a disk has the same → surface brightness at all angles.

lambert; → disk.

Lambertian surface
  رویه‌ی ِ لامبرتی   
ruye-ye Lamberti

Fr.: surface lambertienne   

A surface whose → luminous intensity obeys → Lambert's cosine law. Such a source has a → reflectance that is uniform across its surface and uniformly emits in all directions from all its points. It appears equally bright from all viewing directions. Lambertian surface is a very useful concept for the approximation of radiant power transfer.

lambert; → surface.

varaqé (#)

Fr.: lame, lamina   

A thin plate, layer, or flake.

From L. lamina "thin plate or layer, leaf."

Varaqé "sheet, plate," from varaq "a leaf of tree or of paper," from Ar. waraq, from Pers. barg "leaf" (Tabari, Gilaki valg, balg; Kurd. belg, balk, Semnâni valg); Mid.Pers. warg "leaf;" Av. varəka- "leaf;" cf. Skt. valká- "bark, bast, rind;" Russ. volokno "fibre, fine combed flax."


Fr.: laminaire   

Composed of, or arranged in, laminae, sheets.

lamina + → -ar.

laminar boundary layer
  لایه‌ی ِ کرانی ِ ورقه‌ای   
lâye-ye karâni-ye varaqe-yi

Fr.: Couche limite laminaire   

In a fluid flow, layer next to a fixed boundary. The fluid velocity is zero at the boundary but the molecular viscous stress is large because the velocity gradient normal to the wall is large. → turbulent boundary layer.

laminar; → boundary; → layer.

laminar flow
  تچان ِ ورقه‌ای   
tacân-e varaqe-yi

Fr.: écoulement laminaire   

A flow in which the particles of fluid are moving orderly, and in which adjacent layers or laminas glide smoothly over another with little mixing between them. A laminar flow may rapidly transform into a → turbulent flow for large → Reynolds numbers.

laminar; → flow.

laminar sublayer
  زیرلایه‌ی ِ ورقه‌ای   
zirlâye-ye varaqe-yi

Fr.: sous-couche laminaire   

A layer in which the fluid undergoes smooth, nonturbulent flow. It is found between any surface and a turbulent layer above.

laminar; sublayer, from → sub- + → layer.

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