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

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

→ linear; → acceleration.

Taking the first term in the Taylor series as an approximation to a mathematical function at a given point. → first approximation.

→ linear; → approximation.

A measure of how well data points fit a straight line. When all the points fall on the line it is called a perfect correlation. When the points are scattered all over the graph there is no correlation.

→ linear; → correlation.

An equation in which the → dependent variable y and all its differential coefficients occur only in the first degree. A linear differential equation of → order  order n has the form:
f_n(x)y⁽ⁿ⁾ + f_n-1(x)y^(n-1) + ... + f₁(x)y^' + f₀(x)y = Q(x),
where f₀(x), f₁(x), ..., f_n(x) and Q(x) are each continuous functions of x defined on a common interval I and f_n(x)≠ 0 in I. A linear differential equation cannot have, for example, terms such as y² or (y^')^1/2. See also: → homogeneous linear differential equation; → nonhomogeneous linear differential equation.

→ linear; → differential; → equation.

An equation composed of first degree variables and representing a straight line.

→ linear; → equation.

A function expressed by a → first degree equation that can be graphically represented in the → Cartesian coordinate plane by a → straight line.

→ linear; → function.

Assumption that the variations in the plasma parameters, due to the presence of waves, are small (to the first order) as compared to the undisturbed parameters. This makes it possible to linearize equations by dropping out second order (and higher) nonlinear terms.

→ linear; → perturbation; → theory.

Of an electromagnetic radiation, a → polarization in which the electric vibrations are confined to one plane along the direction of propagation. Also called → plane polarization. See also → circular polarization.

→ linear; → polarization.

In statistics, a regression method that establishes a linear relationship between two random variables.

→ linear; → regression.

A process of reduction to linear form by appropriate change of variables or by approximation.

Verbal noun of → linearize.

A differential equation that has been derived from an original nonlinear equation.

Linearized, p.p. of → linearize; → differential; → equation.

A large, usually tawny-yellow cat, Panthera leo, native to Africa and southern Asia, having a tufted tail and, in the male, a large mane (dictionary.com).

M.E., from O.Fr., variant of leon, from L. leon- (stem of leo), from Gk. leon.

Mid.Pers. šagr "lion;" cf. Parth. šarg; Sogd. šarγu "lion;" its O.Pers. and Av. forms are not extant. Šir may be cognate with Skt. kēsarin- "lion; literally maned," from kēsar- "mane;" PIE *kaisar- "mane; hairs." If so, šir could be related to PIE *kaisaraka-, provided that the the initial *kai- is dropped and *saraka- has transformed in *sarg, šarg, šir.

1) The act or process of liquefying or making liquid.
2) The state of being liquefied.

→ liquefy; → -tion.

→ Einstein equivalence principle.

→ local; → position; → invariance.

The condition of the wave-function of an electron if it is confined to a small region of a large system rather than being extended through the system.

Verbal noun of → localize.

Same as → reddening coefficient.

→ logarithmic; → extinction.

A → probability distribution in which the natural logarithm (logX) of the → random variable (X) has a → Gaussian distribution.

→ logarithm; → normal distribution.

The decrease in the length of a body moving in the direction of its length as measured by an observer situated in that direction. The shortening factor is [1 - (v/c)²]^1/2, where v is the relative velocity and c light speed.

→ Lorentz; → contraction.

A set of linear equations that expresses the time and space coordinates of one → reference frame in terms of those of another one when one frame moves at a constant velocity with respect to the other. In general, the Lorentz transformation allows a change of the origin of a coordinate system, a rotation around the origin, a reversal of spatial or temporal direction, and a uniform movement along a spatial axis. If the system S'(x',y',z',t') moves at the velocity v with respect to S(x,y,z,t) in the positive direction of the x-axis, the Lorentz transformations will be: x' = γ(x - vt), y' = y, z' = z, t' = γ [t - (vx/c²)], where c is the → velocity of light and γ = [1 - (v/c)²]^-1/2. For the special case of velocities much less than c, the Lorentz transformation reduces to → Galilean transformation.

→ Lorentz; → transformation.

The quality of an instrument that lacks sufficient resolution for a specific observation. This is a relative quality, but presently a resolution below about 1 arcsecond.

→ low; → resolution.