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

The time required for a complete revolution of the Earth's pole on the celestial sphere as the result of → precession. A Platonic year is equal to 25 800 years.

Of or pertaining to Gk. philosopher Plato, from Gk. Platon "broad-shouldered," from platys "broad." → year.

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.

A → supernova remnant which has a filled center rather than being a shell. The internal region is "filled" by energetic particles streaming from a rotating → pulsar. The → Crab Nebula is the archetypal plerion.

Plerion, from Gk. pleres "full," akin to Pers. por "full," → poly-.

A → radioactive → chemical element, symbol Pu. → Atomic number 94; → mass number of most stable isotope 244; → melting point 640 °C; → boiling point 3,235 °C. It was first synthesized in 1940 by American chemists Glenn T. Seaborg, Edwin M. McMillan, Joseph W. Kennedy and Arthur C. Wahl in the → nuclear reaction: ₉₂U²³⁸ + ₀n¹→ ₉₃Np²³⁹ + β^- (23.5 minutes) → ₉₄Pu²³⁹ + β^- (2.36 days). The → half-life of ₉₄Pu²³⁹ is 2.44 × 10⁴ yr. Plutonium-239 is a → fissile isotope.

The name derives from the planet → Pluto. It was selected because it is the next planet in the solar system beyond the planet → Neptune and the element plutonium is the next element in the → periodic table beyond → neptunium.

The constant 2.512, which is the 5th → root of 100 (2.512⁵ = 100); the ratio between two successive stellar → magnitudes.

→ Pogson's relation; → ratio.

The equation that expresses the → magnitude  → difference between two objects in terms of the → logarithm of the → flux  → ratio:
I₁/I₂ = 2.5^{(m₂ - m₁)}, or
m₂ - m₁ = 2.5 log(I₁/I₂),
where m is → apparent magnitude, I flux, and log the logarithm to base 10.

Named after Norman Robert Pogson (1829-1891), the English astronomer, who introduced the magnitude scale in 1856; → relation.

The motion of a torque free rotating rigid body in space, in general whose angular velocity vector precesses regularly about the constant angular momentum factor.

After Louis Poinsot (1777-1859), French physicist and mathematician. He was the inventor of geometrical mechanics, showing how a system of forces acting on a rigid body could be resolved into a single force and a couple.

The two-dimensional intensity distribution about the image of a point source.

→ point; → spread; → function.

A → probability function that characterizes → discrete  → random events occurring independently of one another within some definite time or space. It may be regarded as an approximation of the → binomial distribution when the number of events becomes large and the probability of success becomes small. The Poisson distribution is expressed by: f(x) = (λ^xe^-λ)/x!, where λ is the mean number of successes in the interval, e is the base of the → natural logarithm, and x is the number of successes we are interested in.

Named after Siméon Denis Poisson (1781-1840), French mathematician, who developed the application of Fourier series to physical problems and made major contributions to the theory of probability and to the calculus of variations; → distribution.

An equation (∇²φ = 4πGρ) which relates the gravitational (or electromagnetic) potential to the mass density (or charge density).

→ Poisson distribution; → equation.

A chemical bond where the electrons are shared unequally between atoms. The atom that is more electronegative will pull the electrons closer to itself.

→ polar; → bond.

An equation for a curve written in terms of the → polar coordinates.

→ polar; → equation.

The irregularly varying motion of the Earth's pole of rotation with respect to the Earth's crust.

→ polar; → motion.

1) Optics: A process or state in which the directions of the electric or magnetic fields of an → electromagnetic radiation change in a regular pattern. Light can be polarized by a variety of ways, involving the following processes: reflection, transmission, double refraction, and scattering. See also → unpolarized light; → linear polarization; → circular polarization; → elliptical polarization. The study of the polarization of light from astronomical sources can yield unique information in particular related to the properties of magnetic fields.
2) Electricity: A process or state in which the → dipole moments of → polar molecules in a → dielectric material get aligned under the action of an external electric field. Actually thin layers of bound charges with opposite signs appear on the surfaces of the dielectric.
3) Electricity: The accumulation of ions, produced during electrolysis, on the electrodes of a cell, increasing the resistance of the cell.

Verbal noun of → polarize.

Same as → polarizing angle and → Brewster angle.

→ polarization; → angle.

Same as → bound charge.

→ polarization; → charge.

→ degree of polarization.

→ polarization; → degree.

The ratio expressed by P = (I_par - I_per) / (I_par + I_per), where I_par and I_per are the light intensities with the electric field vector respectively parallel and perpendicular to the incident beam.

→ polarization; → fraction.

The introduction of harmful substances or light into the natural environment as a consequence of human activities. → light pollution.
The act of polluting or the state of being polluted.

M.E., from O.Fr., from L.L. pollutionem "defilement," from L. polluere "to soil, defile," from pol-, variant of por- "forth, forward, before" + -luere "smear," related to lutum "mud," and to lues "filth; plague, pestilence;;" cf. Gk. luma "filth, dirt, disgrace;" O.Ir. loth "mud, dirt;" Lith. lutynas "pool, puddle;" Pers. âludan, as below.

Âludegi, from âludan, âlây-"to pollute, soil, stain;" Mid.Pers. âlutan; from prefixed Proto-Iranian *ā-rūta-, from rav- "to stain, soil;" Mid.Pers. Manichean Parthian rwd "rascal;" PIE base *leu- "dirty; to soil;" cf. L. lues, as above.

A radioactive chemical element; symbol Po. Atomic number 84; mass number of most stable isotope 209; melting point 254°C; boiling point 962°C.

The name derives from Poland, the native country of Marie Sklodowska Curie. It was discovered by Pierre and Marie Curie in 1898, from its radioactivity.