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

In → propositional logic, a → sentence without any → connectives. See also → molecular proposition.

→ atomic; → proposition.

A change in the → energy level or → state of an → atom in which a → quantum of energy is either gained or lost. See also → forbidden transition; → permitted transition; → semiforbidden transition.

→ atomic; → transition.

1) The act or faculty of attending, especially by directing the mind to an object.
2) Observant care; consideration (Dictionary.com).

→ attend; → -tion.

The falling off of the energy density of radiation with distance from the source, or with passage through an absorbing or scattering medium.

Verbal noun of → attenuate.

The fraction of a beam of → X-rays or → gamma rays that is absorbed or scattered per unit thickness of the → absorber. The linear attenuation coefficient, denoted by the symbol μ, appears in the equation I(x) = I₀e^-μx, where I(x) is the intensity at depth of x cm and I₀ is the original intensity.

→ attenuation; → coefficient.

The ratio of the radiation intensity after traversing a layer of matter to its intensity before.

→ attenuation; → factor.

The act or capability of attracting. A physical force (gravitational, electric, magnetic, etc.) exerted by material bodies.

Attraction, n. from → attract.

1) The act of attributing.
2) Something attributed.

Verbal noun of → attribute.

The → electromagnetic radiation emitted in planetary atmospheres involving the → aurora phenomenon.

→ auroral; → emission.

Computers: The process by which a user's identity is checked within the network to ensure that the user has access to the requested resources.

Verbal noun of → authenticate.

1) In radio astronomy, a process performed by an → autocorrelator.
2) In statistics, a linear relation between values of a random variable over time.
3) In electronics, a technique used to detect cyclic activity in a complex signal.

Autocorrelation, from → auto- "self" + → correlation.

Xod-hambâzâneš, from xod- "self" + hambâzâneš, → correlation.

A mathematical function that describes the correlation between two values of the same variable at different points in time.

→ autocorrelation; → function.

The autocovariance function (ACF) is defined as the sequence of covariances of a stationary process.
A mathematical function that expresses the autocovariance of a series in terms of the interval of separation.

→ autocovariance; → function.

Of a body traveling from A to B, the change of → velocity divided by the time interval: ā = (v₂ - v₁) / (t₂ - t₁).

→ average; → acceleration.

The technique of looking slightly to the side of a faint object being studied while continuing to concentrate on the object. The technique helps bring out details which otherwise would be missed by looking directly at an object. The reason is that the portion of the eye's retina that best detects dim light (fovea) is located all around the edges rather than the center.

Averted p.p. from avert, from O.Fr. avertir, from L. avertere "to turn away," from → ab- "from, away" + vertere "to turn" (cognate with Pers. gardidan "to turn"); → vision.

Negâh, → vision; kaž "averted."

The angle at which a planet's axis of rotation is tilted, with respect to that of the planet's orbit.

→ axial; → inclination.

1) A change in the orientation of the → rotation axis of a non-spherical, spinning body caused by → gravity. A rotating top will precess in a direction determined by the → torque exerted by its → weight. The precession → angular velocity is inversely proportional to the spin angular velocity, so that the precession is faster and more pronounced as the top slows down.
2) In particular, the gradual shift in the orientation of Earth's rotation axis which traces out a conical path once every ~ 26,000 years. The cause of the Earth's precession is the → equatorial bulge of the Earth, brought about by the → centrifugal force of the → rotation, which has changed the Earth from a perfect sphere to a slightly flattened one, thicker across the equator. The attraction of the Moon and Sun on the bulge is the reason which makes the Earth precess.

→ axial; → precession.

A hypothetical weakly-interacting → boson of small mass required by models of → particle physics in order to solve the strong CP problem and explain a number of observed astrophysical/cosmological phenomena, including → dark matter, and the dimming of → type Ia → supernovae (→ accelerating Universe). Photons traveling in the → intergalactic medium would in part turn into axions in the presence of magnetic fields. The transformed photons are not detected on Earth and therefore supernovae would appear fainter even if the Universe is not accelerating.

Axion, first coined by Frank Wilczek (2004 Nobel Prize in Physics) apparently after a brand of washing detergent! The reason seems to be the idea that the particle will iron out a wrinkle in the → standard model of fundamental particles and forces while solving the problem of the Universe's → missing mass.

Same as → rotation axis and → rotational axis.

→ axis; → rotation.

A map projection on which the → azimuths of all points are shown correctly with respect to the center. A plane tangent to one of the Earth's poles is the basis for polar azimuthal projection.

→ azimuthal; → projection.