An Etymological Dictionary of Astronomy and Astrophysics
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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.

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.

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.

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.

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.

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.

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 Ω_p = Ω ± κ/m, where Ω_p 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 Ω_p = 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.

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.

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.