An Etymological Dictionary of Astronomy and Astrophysics
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A mathematical function that describes the relationship between two or more variables in a set of data.

→ regression, → function.

The curve representing a → linear regression. It is a line drawn through a set of data that summarizes the relationship between the → variables being studied.

→ regression, → line.

The slow motion of the → nodes of the Moon's orbit in the opposite direction to the Moon's movement. This westward motion, caused by perturbations of other bodies, mainly the Earth and Sun, has a rate of 19.35 degrees per year, making one rotation in 18.6 years.

→ regression; → node.

A function of a complex variable which is single-valued in a domain and which has a finite derivative at every point.

→ regular; → function.

The reflection of light when the reflecting surface is very smooth and the reflected rays bounce off parallel to each other. Opposite of → diffuse reflection; same as → specular reflection.

→ regular; → reflection.

1) The act of regulating; the state of being regulated.
2) A law, rule, or other order prescribed by authority.
3) Electronics: The degree by which a voltage source is held near its no-load amplitude when a load is applied.

Verbal noun of → regulate.

1) The act or process of reionizing a gas; an instance of this action.
2) The → ionization of → neutral hydrogen atoms formed during → recombination epoch in the → early Universe. See also → reionization epoch.

→ re-; → ionize.

An early epoch in the → Universe's history, but after the → recombination epoch, when the → first stars formed and their → ultraviolet light began to ionize the → neutral hydrogen gas that filled the Universe. The epoch of reionization is estimated to last between → redshifts of 12 to 6 (or when the Universe had between 2 and 5% of its age). Reionization marks the end of the → Dark Age in cosmic history.

→ reionization; → epoch.

1) To become → ionized again.
2) To → ionize again.

→ re-; → ionize.

General: A connection or association between two or more things.
Math.: A property that associates two quantities in a definite order, as equality or inequality.

Verbal noun of → relate.

Same as → predicate symbol.

→ relation; → symbol.

Of or pertaining to relations.

→ relation + → -al.

A type of data structure in which data are represented as tables in which no entry contains more than one value.

→ relational; → data; → structure.

A database management system in which a relational data structure is used.

→ relational; → system.

The aberration of light for an object moving with → relativistic speed. In contrast to the classical case, the → Lorentz transformation between the → rest frame of the observer and that of the object must be used. Relativistic aberration is expressed by the equation: cos φ' = (cos φ - )/(1 - cos φ.v/c), where φ is the classical → aberration angle, v the speed of the Earth, and c the → speed of light. It can also be expressed as: tan φ' = -(c/v)(1 - v²/c)^1/2.

→ relativistic; → aberration.

A → general relativistic phenomenon in which the line joining the → apsides of an orbit gradually rotates in a → prograde direction. Also called the → orbital precession and → perihelion precession. Although the general relativistic precession of Mercury's orbit is extremely small, the similar precession observed in the orbit of the → binary pulsar PSR B1913+16 amounts to 4.23 degrees per year, i.e. 36,000 times greater than the → advance of perihelion of Mercury.

→ relativistic; → precession.

The change in the direction of the → rotation axis of a → pulsar in a → binary pulsar. In such a system, → geodetic precession leads to a relativistic → spin-orbit coupling, analogous of → spin-orbit coupling in atomic physics. In consequence, the pulsar spin precesses about the total → angular momentum, changing the relative → orientation of the pulsar toward Earth (Damour & Ruffini, 1974). As a result, the angle between the pulsar → rotation axis and our → line of sight changes with time, so that different portions of the emission beam can be observed leading to changes in the measured pulse profile. In extreme cases, the precession may even move the beam out of our line of sight and the pulsar may disappear as predicted for PSR 1913+16 for the year 2025.

→ relativistic; → spin; → precession.

1) The evolution of the properties of a physical system which has been disturbed and which regains its equilibrium condition once the disturbing action has ceased. Relaxation is the response of the system to the perturbation. The time required by the system to regain its condition of minimum energy is called the → relaxation time.
2) Math.: In numerical analysis, a method of solving equations in which the errors, or residuals, resulting from an initial approximation are reduced by succeeding approximations until finally all are within the toleration limit.

Verbal noun of → relax.

The characteristic length of time that is required for a system undergoing → relaxation to move to its equilibrium state. If the system follows an exponential law G = G₀ exp(-t / τ), the relaxation time is the time required for G to obtain the fraction 1/e of its initial value G₀.

→ relaxation; → time.

A mathematical process used in quantum physics that avoids infinite terms by carefully defining fundamental quantities such as mass and charge.

→ re-; → normalization.