An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

One of the two laws governing → refraction of light when it enters another transparent medium: a) The → incident ray, normal to the surface, and refracted ray, all lie in the same plane. b) → Snell's law is satisfied.

→ law; → refraction.

The ordinary Newtonian energy conservation equation when expressed in expanding cosmological coordinates. More specifically, it is the relation between the → kinetic energy per unit mass associated with the motion of matter relative to the general → expansion of the Universe and the → gravitational potential energy per unit mass associated with the departure from a homogeneous mass distribution. In other words, it deals with how the energy of the → Universe is partitioned between kinetic and potential energy. Also known as → cosmic energy equation. In its original form, the Layzer-Irvine equation accounts for the evolution of the energy of a system of → non-relativistic particles, interacting only through gravity, until → virial equilibrium is reached. But it has recently been generalized to account for interaction between → dark matter and a homogeneous → dark energy component. Thus, it describes the dynamics of local dark matter perturbations in an otherwise homogeneous and → isotropic Universe (P. P. Avelino and C. F. V. Gomes, 2013, arXiv:1305.6064).

W. M. Irvine, 1961, Ph.D. thesis, Harvard University; D. Layzer, 1963, Astrophys. J. 138, 174; → equation.

A → cross correlation technique for computing average profiles from thousands of → spectral lines simultaneously. The technique, first introduced by Donati et al. (1997, MNRAS 291,658), is based on several assumptions: additive → line profiles, wavelength independent → limb darkening, self-similar local profile shape, and weak → magnetic fields. Thus, unpolarized/polarized stellar spectra can indeed be seen as a line pattern → convolved with an average line profile. In this context, extracting this average line profile amounts to a linear → deconvolution problem. The method treats it as a matrix problem and look for the → least squares solution. In practice, LSD is very similar to most other cross-correlation techniques, though slightly more sophisticated in the sense that it cleans the cross-correlation profile from the autocorrelation profile of the line pattern. The technique is used to investigate the physical processes that take place in stellar atmospheres and that affect all spectral line profiles in a similar way. This includes the study of line profile variations (LPV) caused by orbital motion of the star and/or stellar surface inhomogeneities, for example. However, its widest application nowadays is the detection of weak magnetic fields in stars over the entire → H-R diagram based on → Stokes parameter V (→ circular polarization) observations (see also Tkachenko et al., 2013, A&A 560, A37 and references therein).

→ least; → square; → deconvolution.

An improvement of → Schwarzschild's criterion for convective instability, which includes effects of chemical composition of the gas. In the Ledoux criterion the gradient due to different molecular weights is added to the adiabatic temperature gradient.

After the Belgian astrophysicist Paul Ledoux (1914-1988), who studied problems of stellar stability and variable stars. He was awarded the Eddington Medal of the Royal Astronomical Society in 1972 (Ledoux et al. 1961 ApJ 133, 184); → criterion.

The → differential equation of the form: d/dx(1 - x²)dy/dx) + n(n + 1)y = 0. The general solution of the Legendre equation is given by y = c₁P_n(x) + c₂Q_n(x), where P_n(x) are Legendre polynomials and Q_n(x) are called Legendre functions of the second kind.

Named after Adrien-Marie Legendre (1752-1833), a French mathematician who made important contributions to statistics, number theory, abstract algebra, and mathematical analysis; → equation.

A mathematical operation that transforms one function into another. Two differentiable functions f and g are said to be Legendre transforms of each other if their first derivatives are inverse functions of each other: df(x)/dx = (dg(x)/dx)^-1. The functions f and g are said to be related by a Legendre transformation.

→ Legendre equation; → transformation.

1) The act of making or enacting laws.
2) A law or a body of laws enacted (Dictionary.com).

From Fr. législation, from L.L. legislationem, from legis latio, "proposing (literally 'bearing') of a law," → legislator.

Qânungoz&acric;ri "act or process followed by the qânungoz&acric;r", → legislator.

Same as → Lorentz contraction.

→ length; → contraction.

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.

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.

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").

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.

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.

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.

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.

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.

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.

The process of recognizing the spectral lines in a spectrum.

→ line; → identification.

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.

Same as → line of force in a magnetic field.

→ line; → induction.