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

Transition from an excited energy state to a lower energy level, as in spectral line formation or particle emission from an atomic nucleus.

From → de- + → excitation.

A method for estimating the real orientation of a field and/or related velocities/separations from two-dimensional images.

→ de-; → projection.

A ten-sided → polygon. A decagon that is equilateral and equiangular is called a regular decagon. Each angle of a regular decagon is 144°. The central angle subtending any side is 36°. The ratio of the radius to the side of a regular decagon forms the → golden ratio.

→ deca-; → -gon.

A constant of proportionality occurring in the formula expressing spontaneous → decay of → radionuclides. The number of atoms decaying is given by N = N₀e^-kt, where N₀ is the number of nuclei in the given volume of the substance at instant t = 0, N is the number of nuclei at t, and k is decay constant. Decay constant is related to → half-life by τ = ln2/k, roughly 0.693/k.

→ decay; → constant.

The act or process of moving, or of causing to move, with decreasing speed. Sometimes called negative acceleration.

Verbal noun of decelerate.

A parameter designating the rate at which the expansion of the Universe would slow down owing to the braking gravitational effect of the matter content of the Universe. It is expressed by: q(t) = -R(t)R ^..(t)/R ^.2(t), where R(t) represents the size of the Universe at time t. See also → expansion parameter; compare with → acceleration parameter.

→ deceleration; → parameter.

A fraction expressed by using → decimal representation, as opposed to a vulgar fraction. For example, 2/5 is a vulgar fraction; 0.40 is a decimal fraction.

→ decimal; → fraction.

A representation of a real number between 0 and 1, as written a = d₁d₂d₃ ..., where each d_i is one of the digits 0, 1, 2, ..., 9. This means that a = d₁ x 10^-1 + d₂ x 10^-2 + d₃ x 10^-3+ ...

→ decimal; → representation.

The act or process of deciding. → rule of decision.

Verbal noun of → decide.

The act of declaring; announcement.
2) Something that is announced or proclaimed.

Verbal noun of → declare.

The → inflection of nouns, pronouns, or adjectives for case, number, and gender.

M.E. declenson, declynson (with suffix later assimilated to -sion), from O.Fr. declinaison, from L. declinatio, → declination

→ declination.

1) Angular distance (symbol δ) from the celestial equator. One of the coordinates, with → right ascension, that defines the position of a heavenly body in the equator system. Declination is measured positively (+) north or negatively (-) south of the celestial equator from 0° to 90°.
2) Angle between geographic North and magnetic North (symbol D).

Verbal noun of → decline; → -tion.

The axis to which the telescope tube is fastened in an → equatorial mounting.

→ declination; → axis.

For a telescope with an → equatorial mounting, a graduated circle attached to the → declination axis that shows the → declination to which the telescope is pointing.

→ declination; → circle.

The process or the state of breaking down a physical entity or an organic material.

From → de- + → composition.

1) A gradual decrease in surrounding → pressure, especially the controlled decrease in pressure that divers undergo after working in deep water or breathing compressed air.
2) Computers: The restoration of compressed electronic data to their full size.

→ de- + → compression.

A mathematical operation that allows to restore the original input signal, such as an astronomical image or spectrum, to its state before being affected by the → atmospheric turbulence and the → transfer function of the instrument. → convolution.

From → de- + → convolution.

An algorithm used to improve the resolution of an image particularly when the convolving function is well defined. Also called deconvolution code.

→ deconvolution; → algorithm.

Verbal form of → deconvolution.

From → de- + → convolve.

A physical process that is contrary to accretion, referring to the outward direction of the mass flow from a star to its environment.

From → de- "reversal, removal," + -cretion, from L. accretionem (nom. accretio, gen. accretionis) "a growing larger," from stem of accrescere, → accretion.