An Etymological Dictionary of Astronomy and Astrophysics
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Any of basic physical quantities, such as mass, length, time, electric charge, and temperature in terms of which all other kinds of quantity can be expressed.

→ physical; → dimension.

A real periodic variation in the rotation rate of a celestial object, as distinct from a → geometrical libration. In particular, slight oscillations in the → Moon's rotation caused by the → gravitational attraction of the Earth on the → equatorial bulge of the Moon's near side. The Moon's physical libration is about 0.03° in longitude and about 0.04° in latitude.

→ physical; → libration.

A kind of → adsorption in which the forces involved are → intermolecular  → van der Waals forces. Same as → physical adsorption. See also → chemisorption.

Physi-, from → physical; → sorption.

A function f(x) in an interval if :1) the interval can be divided into a finite number of pieces in each of which f(x) is continuous, and 2) the limits of f(x) as x approaches the boundary point of each piece are finite. In other words, a piecewise continuous function is one that is made up of a finite number of continuous pieces.

→ piecewise; → continuous; → function.

An → aberration of a → lens  → system in which → magnification increases with → distance from the → optical axis, whereby → horizontal and → vertical lines bend inward toward the → center of the → field. Opposite of → barrel distortion.

Pincushion, from pin, from M.E. pinne, O.E. pinn "peg;" cf. D. pin, Ger. Pinne; perhaps from L. pinna "feather, quill" + cushion, M.E. cuisshin, O.Fr. coissin (Fr. coussin) a variant of V.L. *coxinum, either from L. coxa "hip, thigh," or from L. culcita "mattress;" → distortion.

Cowlegi, → distortion; bâleštaki, adj. of bâleštak, diminutive of bâlešt, variant bâleš "cushion, pillow," Mid.Pers. bâlišn, bâlên "cushion, pillow;" Av. barəiš- "pillow, cushion;" cf. Skt. barhis- "straw, a bed or layer of kusa grass strewed over the sacrificial ground."

An unstable nuclear particle of mass intermediate between that of a proton and an electron; also called π meson.

From pi (meson) + → -on.

A mathematical notation system in which the → numerals get different values depending on their position relative to the other numerals. Same as → positional notation and → positional number system.

→ place; → value; → notation.

The distribution of radiation with wavelength for a blackbody, given by → Planck's radiation law.

→ Planck; → distribution.

Same as → Planck's blackbody formula.

→ Planck; → function.

An equation that expresses the energy radiated per unit area per unit time per unit wavelength range by a blackbody as a function of temperature. It is expressed by → Planck's blackbody formula.

→ Planck; → radiation; → law.

In a → linearly polarized light, a plane perpendicular to the → plane of vibration and containing the direction of propagation of light. It is also the plane containing the direction of propagation and the magnetic vector (H) of the electromagnetic light wave.

→ plane; → polarization.

For a rotating object, the plane → perpendicular to the → rotation axis.

→ plane; → rotation.

In a → linearly polarized light, a plane perpendicular to the → plane of polarization and containing the direction of propagation of light. It is also the plane containing the direction of propagation and the electric vector (E) of the electromagnetic light wave.

→ plane; → polarization.

Same as → linear polarization.

→ plane; → polarization.

The difference between the true position of a planet and its apparent position, due to the time required for light to travel the distance from the planet to Earth. Correction for planetary aberration is necessary in determining orbits.

→ planetary; → aberration.

The motion of the → ecliptic plane caused by the gravitational influence of the other planets, mainly → Jupiter. The observational effect of planetary precession is similar to that of the → lunisolar precession. But planetary precession causes the → equinoxes to move along the ecliptic in the opposition direction (eastward) from that of luni-solar precession (westward) and at a much slower rate: 0''.12 per year. Same as → precession of ecliptic.

→ planetary; → precession.

A space observatory under development by the → European Space Agency for launch around 2024. Its objective is to detect and characterize → exoplanets by means of their → transit signature in front of a very large sample of → bright stars, and measure the seismic oscillations (→ asteroseismology) of the parent stars orbited by these planets in order to understand the properties of the exoplanetary systems.

→ planetary; → transit; → oscillation; → star.

The oscillatory motion of electrons in a neutral plasma around their equilibrium position while the ions remain at rest. When electrons are displaced by any kind of perturbation with respect to ions, a pair of positive and negative charged regions is formed. The Coulomb force accelerates back the displaced electrons, which get kinetic energy. The electrons overshoot their original positions by the same amount as their first displacement.

→ plasma; → oscillation.

Permanent → deformation of a → solid subjected to a → stress.

→ plastic; → deformation.

A star in the constellation → Taurus and a member of the → Pleiades star cluster. Pleione is a blue-white B-type → main sequence → dwarf with a mean apparent magnitude of +5.09. It is a variable star and its brightness varies from magnitude +4.77 to +5.50. It is approximately 380 light-years from Earth.

Pleione was an Oceanid nymph. She lived in a southern region of Greece called Arcadia, on a mountain named Mount Kyllini. She married Atlas and gave birth to the Hyades, Hyas and the Pleiades.