An Etymological Dictionary of Astronomy and Astrophysics

To trespass, especially on another’s game preserve, in order to steal animals or to → hunt; to take game or fish illegally (Dictionary.com).

Etymology (EN): M.E., from M.Fr. pocher “to thrust, poke,” from O.Fr. pochier “poke out, gouge, prod,” related to poke (v.), from a Germanic source (compare M.H.G. puchen “to pound, beat, knock,” Ger. pochen, Middle Dutch boken “to beat”) related to poke (v.).

Etymology (PE): Beškaridan, from beškar(d), bišgar(d) “hunter, fowler; chase; game; place for hunting,” variant of šekâr, → hunt.

A person who trespasses on private property, especially to catch fish or game illegally (Dictionary.com). See also → hunter.

See also: → poach; → -er.

The illegal taking of wildlife, in violation of local, state, federal or international law.

See also: → poach; → -ing.

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

See also: → 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.

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

In an → isolated system, any initial state will occur again in the course of the → evolution of the system over a sufficiently long but finite → time.

See also: → Poincaré sphere; → recurrence; → theorem.

A representation that permits an easy visualisation of all different states of → polarization of a vector wave. The equator represents → linear polarization; the north pole corresponds to right-circular and the south pole to left- → circular polarization.

See also: Named after Henri Poincaré (1854-1912), French mathematician and theoretical physicist, and a philosopher of science; → sphere.

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.

See also: 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.

1a) General: A sharp or tapering end, as of a dagger; a projecting part of anything.
1b) Physics: Position or time of occurrence, as in boiling point, freezing point, etc.
1c) Math.: A dimensionless geometric element whose location in space is defined solely by its coordinates.

2. To direct a telescope toward a particular position on the sky.

Etymology (EN): M.E. point(e); O.Fr. point “dot, mark, place, moment;” L. punctum noun use of neuter p.p. of pungere “to prick, pierce.”

Etymology (PE): 1) Noqté, loan from Ar. Pandé, variants in classical dictionaries pindé, pendé, fand “a point, dot, mole, freckle;” cf. Skt. prānta- “point, tip, border,” from pra “before, forward,” → pro-, + ánta- “end, limit, term;” Pali, panta- “remote, solitary;” Prakrit panta " last;" Sindhi pandu “border of a garment;” Lahnda pand, pad “end, top of sugar cane.”

2. Ãmâjidan, verb from âmâj “aim, goal,” from Proto-Iranian base *āma-, from prefix *ā- + *ma- “to measure;” cf. Av. mati- “point, tip;” O.Pers./Av. mā(y)- “to measure;” Pers. mun/mân “measure,” as in Pers. terms pirâmun “perimeter,”
   âzmun “test, trial,” peymân “measuring, agreement,” peymâné “a measure; a cup, bowl;” cf. Skt. mati “measures,” matra- “measure,” Gk. metron “measure,” L. metrum; PIE base *me- “to measure.”

A hypothetical object which can be thought of as infinitely small.

See also: → point; → mass.

A source of radiation at a great distance from the observer; an ideal source of infinitesimal size.

See also: → point; → source.

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

See also: → point; → spread;
→ function.

The two stars that form the front of the Big Dipper’s bowl, away from the handle.
More specifically, the stars Dubhe (α Ursae Majoris) and Merak (β Ursae Majoris). A line through β to α passes close to the North Star and they are used for finding it.

Etymology (EN): → point + -er.

Etymology (PE): Dorahnemâ, literally “the two guides,” from do “two” + rah, râh “way, path” (from Mid.Pers. râh, râs “way, street,” also rah, ras “chariot;” from Proto-Iranian *rāθa-; cf. Av. raθa- “chariot;” Skt. rátha- “car, chariot,” rathyā- “road;” L. rota “wheel,” rotare “to revolve, roll;” Lith. ratas “wheel;” O.H.G. rad; Ger. Rad; Du. rad;
O.Ir. roth; PIE *roto- “to run, to turn, to roll”) + nemâ agent noun of nemudan “to show” (Mid.Pers. nimūdan, nimây- “to show,” from O.Pers./Av. ni- “down; into” (Skt. ni “down,” nitaram “downward,” Gk. neiothen “from below,” cf. E. nether, O.E. niþera, neoþera “down, downward, below, beneath,” from P.Gmc. *nitheraz,
Du. neder, Ger. nieder; PIE *ni- “down, below”) + māy- “to measure;” cf. Skt. mati “measures,” matra- “measure;”
Gk. metron “measure;” L. metrum; PIE base *me- “to measure”).

The act or process of directing a telescope. → point.
The direction in the sky to which the telescope is pointed. Pointing also describes how accurately a telescope can be pointed toward a particular direction in the sky.

See also: Verbal noun of → point.

A mathematical model that reproduces the diurnal rotation of the Earth and is used to direct a telescope toward a precise position on the sky.

See also: → pointing; → model.

The unit of viscosity in the c.g.s. system, equal to 1 dyne.s/cm². Symbol: P

See also: Poise, from Jean-Louis-Marie Poiseuille (1797-1869), a French physiologist and physician who studied the flow of liquids through tubes and developed a method for measuring blood pressure.

In fluid dynamics, the law that the rate of flow of a liquid through a horizontal tube of uniform radius is directly proportional to the pressure of the liquid and the fourth power of the radius of the tube and is inversely proportional to the viscosity of the liquid and the length of the tube.

See also: Named after Jean-Louis-Marie Poiseuille (1797-1869), a French physiologist and physician who found the law in 1844; → law.

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.

See also: 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).

See also: → Poisson distribution; → equation.

1. Of or pertaining to the pole of any sphere, a magnet, an electric cell, etc.
   

2. A subclass of → cataclysmic variables, the prototype being AM Herculis. Polars are short-period systems in which a → late-type → main sequence star transfers mass to a highly magnetized → white dwarf.
   The strong magnetic field (10-70 million → gauss) prevents the formation of an → accretion disk, locks both stars in synchronous rotation and guides the accreting matter to accretion spots which are the source of intense X-ray radiation (material impacts on to the white dwarf where it is radiated away).

See also: 1) Adj. of → pole.

2. From polar(ization) + (st)ar, because of their → circularly polarized light.

The process or the state of making a → telescope’s → polar axis → parallel to the → Earth’s → rotation axis, that is with the → true North or → South  → celestial pole. When this is accomplished, the sky’s motion can be cancelled out simply by turning the axis (either by hand or with a motor → drive) at the same rate as the rotation of the Earth, but in the opposite direction.

See also: → polar; → alignment.

The axis of an → equatorial mounting that is parallel to the Earth’s axis, and consequently points to the celestial pole.

See also: → polar; → axis.

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

See also: → polar; → bond.

1. Either of the regions around the poles of the Earth that are permanently covered with ice.
   

2. Either of the two regions around the poles of the planet Mars, consisting of frozen carbon dioxide and water ice.
   

3. An area of a → pulsar’s surface from where open magnetic field lines emanate.

See also: → polar; → cap.

An imaginary parallel circle on the celestial sphere or on the Earth at a distance of 23°.5 from either poles.

See also: → polar; → circle.

A coordinate system in which the position of any point (M) in a plane is specified by two coordinates: 1) ρ, which expresses the distance from a fixed point (the pole, denoted O), and 2) the number φ, which is the angle formed by
the line segment OM and a fixed reference line passing through the pole.

See also: → polar; → coordinate.

An area in the Earth’s → magnetosphere, where the → magnetosheath plasma has direct access to the → ionosphere.

See also: → polar; → cusp.

In polar regions, the portion of the year when the Sun is continuously in the sky. Its length changes from twenty hours at the Arctic/Antarctic Circle (latitude 66°33’ N or S) to 186 days at the North/South Pole.

See also: → polar; → day.

The angular distance of an object from a celestial pole. It is equal to 90° minus the object’s declination.

See also: → polar; → distance.

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

See also: → polar; → equation.

Solar faculae occurring in regions of high heliographic latitudes. They are smaller than the main-zone faculae; their shape is point-like or oval. Their lifetimes range from a few minutes to some hours, but the decisive difference from the main-zone faculae lies in their activity cycle. When spots and faculae of the main zone are at minimum, the polar faculae have their maximum activity, and vice versa.

See also: → polar; → facula.

A molecule in which the centers of positive and negative charge distribution do not converge and therefore has a mostly positive charge on one side and a mostly negative charge on the other. Different atoms around a central atom will always be polar molecules. Some polar molecules are H₂O, HF, COS, and CH₃Cl. Polar molecules are characterized by a → dipole moment.

See also: → polar; → molecule.

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

See also: → polar; → motion.

In polar regions, the portion of the year when the Sun does not rise above the horizon. Its length changes from twenty hours at the Arctic/Antarctic Circle (latitude 66°33’ N or S) to 179 days at the North/South Pole.

See also: → polar; → day.

A spacecraft orbit that passes over, or close to, the geographic poles of the Earth or some other solar system object.

See also: → polar; → orbit.

A satellite that revolves around the Earth in an almost north-south orbit, passing close to both poles. The orbits are sun synchronous, allowing the satellite to cross the equator at the same local time each day.
These satellites orbit at a height of 830-880 km and
take about 100 minutes to complete a turn around the Earth.

See also: → polar; → orbit; → satellite.

A coronal feature of the Sun, which appears as
long, thin streamers that project outward from the Sun’s north and south poles

See also: → polar; → plume.

1. The → solar wind occurring at high latitudes during low → solar activity as a fast (around 750 km s^-1) and relatively steady flow. A remarkable feature of the polar wind is the ubiquitous presence of an intense flow of → Alfvénic fluctuations.
   

2. One of the two main wind components in → B[e] stars. The pole is hotter than the equator due to the → gravity darkening effect
   leading to a relatively fast, low-density wind driven by Fe IV lines.
   The other component is the → equatorial wind. The mechanism put forward to explain this wind morphology is the rotationally induced → bistability mechanism.
   

3. An extremely cold wind blowing from the north or south polar regions of the Earth.

See also: → polar; → wind.

Of or relating to → polarimetry.

See also: → polarimetry; → -ic.

The measurement of the → polarization state of light, usually through the use of a polarimeter.

See also: From polari, from → polarization + → -metry.

The brightest star associated with the → north celestial pole. Polaris, also called the Pole Star, is a → triple system lying at about 433 → light-years (133 → parsecs) from the Earth. It is not exactly located on the Earth → rotation axis, because an → angular distance of 42 arc-minutes (about 1.4 lunar diameter) separates it from the true north pole. The main star, Polaris Aa (→ visual magnitude about 2), is a variable → pulsating star of type → Cepheid. It is a hot, blue F7 Ib → supergiant star having a → luminosity about 1,260 times that of the Sun. It has a mass of 5.4 Msun, a radius of 37.5 Rsun, and a → surface temperature of 6,015 K.

The close companion Ab (apparent magnitude 9.2) is only 0’’.17 (about 18.5 → astronomical units) from Polaris A. It was discovered in 1929 through examining the spectrum of Polaris A. It orbits Aa every 29.59 years. Ab is a → main sequence star of → spectral type F6 V. It has a mass of 1.26 Msun, a radius of 1.04 Rsun, and a luminosity of 3 Lsun.

The third component, Polaris B (visual magnitude 8.7), is separated from A by 18.2 arc sec, corresponding to approximately 2,400 AU. It
was first noticed by William Herschel in 1780. Polaris B is a main sequence star of type F3 V with a mass 1.39 Msun, a radius 1.8 Rsun, a luminosity of 3.9 Lsun, and a surface temperature of 6,900 K.

Due to the → precession of equinoxes, the direction that Earth’s axis points at changes slowly with time. Hence, Polaris has not always been, nor will it always be, the Pole Star. Polaris is actually drawing closer to the pole and in 2100 it will be as close to it as it ever will come, just 27.15 arc-minutes or slightly less than the Moon’s apparent diameter. It will continue its reign as the North Star for many centuries to come.

Historically, around 400 B.C., during Plato’s time the nearest star to the Pole star was → Kochab (β Ursae Minoris).

Some 4,600 years ago, when the Egyptians constructed the Pyramids, the Pole star was → Thuban (α Draconis).

In 2,000 years the star → Errai (γ Cephei) will become the Pole Star. And around the year 14,000, Earth’s axis will point reasonably close to the star → Vega, the fifth brightest star in the sky.

Etymology (EN): Mod.L. short for stella polaris “the pole star,” → polar.

Etymology (PE): Setâre-ye qotbi “polar star,” from setâré, → star, + qotbi, → polar.
Jodey, from Ar. Judaiy.
Mix-e gâh, Mid.Pers. name of the star, literally “the nail (peg) of time/place,” from mix, → nail,

• gâh “time; place,” → origin.

1. Physics: The condition, in a system, of having opposite characteristics at different points, especially positive or negative with respect to electric charge or magnetic properties.
   

2. Chemistry: A property of a molecule caused by unsymmetrical charge distribution.

See also: From → polar + → -ity.

The time during which the Earth’s magnetic field was of a single polarity; an interval of time between reversals of Earth’s magnetic field.

See also: → polarity; → epoch.

A specific event in the history of Earth’s magnetic field. Usually used in reference to a specific → polarity reversal.

See also: → polarity; → event.

1. A change in the → polarity of Earth’s magnetic field in which the north magnetic pole becomes the south magnetic pole and vice versa. Also known as geomagnetic reversal or magnetic reversal. Earth’s magnetic field has reversed many times in the past and the time intervals between these changes are known as → polarity epochs.
   

2. The change in the polarity of the → solar magnetic field with the ending of each → solar activity cycle so that the North magnetic pole becomes the South and vice versa.

See also: → polarity; → reversal.

Able to be polarized.

See also: → polaize; → -able.

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.

See also: Verbal noun of → polarize.

Same as → polarizing angle and → Brewster angle.

See also: → polarization; → angle.

Same as → bound charge.

See also: → polarization; → charge.

→ degree of polarization.

See also: → 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.

See also: → polarization; → fraction.

To cause → polarization. To undergo polarization.

See also: Infinitive of → polarization.

Of or pertaining to a medium or physical phenomenon that exhibits → polarization.

See also: P.p. of → polarize.

→ Electromagnetic radiation in the optical region which has undergone → polarization.

See also: Past participle of → polarize; → light.

An optical device capable of transforming natural light into polarized light, usually by selective transmission of polarized rays.

See also: Agent noun of → polarize.

The angle of → incidence for which the reflected light is completely polarized. Also called the → Brewster angle and → polarization angle. See also → Brewster’s law.

See also: Polarizing, adj. of → polarize; → angle.

A filter that polarizes light passing through it.

See also: Polarizing, adj. of → polarize;
→ filter.

A prism that is used to produce or analyze plane-polarized light.

See also: Polarizing, adj. of → polarize;
→ prism.

1. Either extremity of the axis of the Earth or of any spherical body.
   

2. Either of the two regions or parts of an electric battery, magnet, or the like, at which the magnetic flux density is concentrated.
   

3. → pole of a mirror.
   

4. The fixed point in a system of polar coordinates that serves as the origin.

Etymology (EN): M.E., from L. polus “end of an axis, the sky,” from Gk. polos “pivot, pole,” polein “move around;” from PIE base *kwel- “to turn, move around, sojourn, dwell;” cognate with Pers. carx “every thing performing a circulatory motion; a wheel; a cart;” Av. caxra- “wheel,” caraiti “he moves, approaches;” cf. Skt. cakra- “wheel, circle; cycle,” carati “he moves, wanders;” Gk. kyklos “circle, wheel” (loaned L.L. cyclus); L. colere “to dwell in, to cultivate, move around,” colonus “farmer, settler;” O.E. hweol “wheel;” Rus. koleso “wheel.”

Etymology (PE): Qotb, from Ar. quTb.

The point where the → principal axis passes through the mirror.

See also: → pole; → mirror.

A star that lies along the Earth’s → rotation axis. The term usually refers to the star → Polaris, which is the current → North Celestial Pole star. The → South Celestial Pole is not currently associated with any bright star. See also: → North Pole Star, → South Pole Star.

See also: → pole; → star.

Mechanics: For a rotating rigid body not subject to external torque, the closed curve described on the → inertia ellipsoid by the intersection with this ellipsoid of an axis parallel to the angular velocity vector and through the center.
Astro.: The path described by the North pole of the Earth in a reference frame attached to the Earth. The rotation axis of our planet is not fixed with respect to the Earth’s crust. It describes a motion within a square with an amplitude reaching 20 m.

Etymology (EN): Polhode, from Fr. polhodie, coined by Louis Poinsot (1777-1859), a French mathematician and physicist, the inventor of geometrical mechanics, from → pole + Gk. hodos “way.”

Etymology (PE): Qotbrâh, from gotb, → pole, + râh
“way, path” (from Mid.Pers. râh, râs “way, street,” also rah, ras “chariot;” from Proto-Iranian *rāθa-; cf. Av. raθa- “chariot;” Skt. rátha- “car, chariot,” rathyā- “road;” L. rota “wheel,” rotare “to revolve, roll;” Lith. ratas “wheel;” O.H.G. rad; Ger. Rad; Du. rad;
O.Ir. roth; PIE *roto- “to run, to turn, to roll”).

1. A definite course of action adopted for the sake of expediency, facility, etc.
   

2. A course of action adopted and pursued by a government, ruler, political party, etc. (Dictionary.com).

Etymology (EN): M.E. policie “government, civil administration,”
from O.Fr. policie “political organization, civil administration,” from L.L. politia “the state, civil administration,” from Gk. politeia “state, administration, government, citizenship,” from polites “citizen,” from polis “city, state.”

Etymology (PE): Kârâh, contraction of kâr râh, or râh-e kâr literally “way of doing, ~ ~ action,” from râh, → way, + kâr “doing, action, → work.”

A person responsible for making policy, especially in government (Dictionary.com).

See also: → policy; → make; → -er.

To make smooth and glossy by rubbing with something.

Etymology (EN): M.E. polishen, from O.Fr. poliss-, pr.p. stem of polir “to polish,” from L. polire “to polish, make smooth,” of unknown origin.

Etymology (PE): Nasu kardan (Dehxodâ) “to polish, smooth, brighten,” from nasu “smooth, bright” (mirror, sword), Mid.Pers. Manichean Parthian nswg “tender,” from Proto-Iranian *ni-sau-, from prefix ni- “down,” → ni- (PIE),

• *sau- “to rub;” cf. Mod.Pers. sâbidan, variants sâyidan, pasâvidan “to touch, to rub;” Khotanese sauy- “to rub;” Sogdian ps’w- “to touch.”

A stage in the process of mirror making after grinding that puts a highly finished, smooth and apparently amorphous surface on a mirror.
→ figuring; → grinding.

See also: Verbal noun of → polish.

1. Shrewd or prudent in practical matters; tactful; diplomatic.
   

2. Contrived in a shrewd and practical way.
   

3. Political (Dictionary.com).

Etymology (EN): M.E. politik, from M.Fr. politique “political,” from L. politicus “of citizens or the state, civil, civic,” from Gk. politikos “of citizens, pertaining to the state and its administrators,” from polites “citizen,” from polis “city.”

Etymology (PE): Kârâhi, adj. of kârâh, → policy.

Of, pertaining to, or concerned with → politics.

See also: → politic; → -al.

A person who studies the structure and theory of government and seeks practical and theoretical solutions to political problems.

Etymology (EN): → political; → scientist.

Etymology (PE): Kârâhikdân, literally “politics scientist,” from kârâhik, → politics, + dân “knower,” present stem of dânestan, → science.

1. A person who is active in party politics.
   

2. A person who holds a political office.
   

3. A person skilled in political government or administration; statesman or stateswoman (Dictionary.com).

Etymology (EN): From politic + -ian an agent noun suffix.

Etymology (PE): Kârâhigar, from kârâhi-, contraction of kârâhik, → politics, because of the k-g succession, + -gar, → -or.

1. The science or art of → political  → government.
   

2. The practice or profession of conducting political affairs.
   

3. Political → principles or → opinions (Dictionary.com).

Etymology (EN): From adj. → politic modeled on Aristotle’s ta politika “affairs of state,” the name of his book on governing and governments; → -ics.

Etymology (PE): Kârâhik, from kârâh, → policy, + -ik, → -ics.

Powdery grains that contain the male reproductive cells of most plants (Dictionary.com).

Etymology (EN): From L. pollen “mill dust; fine flour,” related to polenta “peeled barley,” and probably to Gk. poltos “pap, porridge.”

Etymology (PE): Gardé, from gard “powder, dust.”

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.

Etymology (EN): 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.

Etymology (PE): 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.

The brightest star in the constellation → Gemini despite its designation as β. Also known as HR 2990 and HD 62509. Pollux lies about 4 degrees apart from the blue star → Castor with which it has no physical connection. Pollux is an orange-red star with a → visual magnitude 1.14 (B - V = +1.00) located about 34 → light-years away. It is a → giant star of → spectral type K0 III, with a mass of 2 Msun, a radius of about 10Rsun, a luminosity of 43 Lsun, and an → effective temperature of 4666 K. Pollux has a large planet, → Pollux b, with a mass of at least 2.3 times the mass of Jupiter.

Etymology (EN): Pollux, Gk. Polydeucus is one of the “Heavenly Twins,” brother to → Castor, sons of Leda and Zeus.

Etymology (PE): Pollux, loan from L., as above. It has a too long Ar. name: Al-Ra’s-at-Tau’am-al-Mo’akhkhar (الرأس‌التوأم‌المؤخر) “the head of the posterior twin,” translating the Gk. mythological figure.

An → extrasolar planet orbiting the bright star → Pollux (β Gem). It lies approximately 34 → light-years away in the constellation → Gemini. Called also → Thestias, it has a mass of at least 2.30 → Jupiter mass, and orbits Pollux at a distance of about 1.64 → astronomical units once every 590 days.

See also: → Pollux.

1. In → protoplanetary disk models, the magnetic field whose large-scale lines of force depart away from the → accretion disk, in the direction of the rotation axis.
   

2. In a → tokamak, the magnetic field produced by a current flowing in the → plasma vertically to the toroid plane. → toroidal magnetic field.

Etymology (EN): → pole; → -oid; → magnetic field.

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

See also: 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.

A prefix denoting “much, many.”

Etymology (EN): From Gk. poly-, combining form of polus “much,”
cognate with L. plus, Pers. bol-, as below; from PIE base *pel- “to be full.”

Etymology (PE): Bol- “much; very; many,” as in examples bolkâmé “desirous” (literally “with much desire”), bolhavas, variant porhavas “very desirous, libidinous” (literally “with much lust”); variant of por “full, much, very,”
Mid.Pers. purr “full;” O.Pers. paru- “much, many;” Av. parav-, pauru-, pouru- “full, much, many,” from par- “to fill;” PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru- “much, abundant;” Gk. plethos “great number, multitude;” O.E. full.
Cand-, from cand “how many, how much; so many; several; some; much;” Mid.Pers. cand “how many?, so many; much; some;” O.Pers. yāvā “as long as;” Av. yauuant- [adj.] “how great?, how much?, how many?,” yauuat [adv.] “as much as, as far as;” cf. Skt. yāvant- “how big, how much;” Gk. heos “as long as, until.”

Having or exhibiting a variety of colors.

See also: → poly-; → chromatic.

1. Chemistry: A family of → organic molecules composed of carbon and hydrogen atoms (→ hydrocarbons) in which → carbon atoms appear in multiple loops (polycyclic) with strong chemical → bonds that exist between them (aromatic). PAHs are formed during the incomplete burning of coal, oil and gas, garbage, or other organic substances like tobacco or charbroiled meat. As a pollutant, they are of concern because some compounds (benzo(a)pyrene) have been identified as tending to cause cancer.
   

2. Astro.: Strong → infrared emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 microns, which are attributed to PAH molecules in the form of → dust grains with radii smaller than 10 Å. PAHs are common toward various sources characterized by a strong → ultraviolet radiation field (such as → active galaxies, → reflexion nebulae, → planetary nebulae, → H II regions).
   In particular, they trace the presence of → massive stars. The PAH features are thought to result from the fluorescent emission process of PAH molecules and ions excited by ultraviolet radiation of nearby stars. PAHs are likely produced by → photoevaporation of → very small grains.

See also: → poly-; → cyclic; → aromatic; → hydrocarbon.

A kind of → deism according to which multiple deities created the Universe, but do not interact with the physical world. → polytheism.

See also: → poly-; → deism.

A one-dimensional closed figure consisting of a series of points, each of which is called a → vertex, and the line segments, called → sides, joining the vertices. Polygons of three sides are called → triangles, and of four sides → quadrilaterals.

See also: → poly- + → -gon.

1. Geometry: A three-dimensional figure formed by several → polygons.
   

2. A set P ⊆ ℜⁿ is a polyhedron if there is a system of finitely many unequalities Ax ≤ b such that P = {x ∈ ℜⁿ | Ax ≤ b}.

See also: → poly-; → hedron.

1. A mathematical expression involving a sum of powers in one or more variables multiplied by coefficients.
   

2. Consisting of or characterized by two or more terms or names.

See also: From → poly- + (bi)nomial, contraction of
→ nominal.

An equation of the form a₀ + a₁x + a₂x² + … + a_nxⁿ, where a₀ … a_n are → real numbers and a_n≠ 0. Same as → algebraic equation.

See also: → polynomial; → equation.

Diversity of meanings.

Etymology (EN): From N.L. polysemia, from L. polysemus “with many significations” (from Gk. polysemos, from → poly- + sem, from sema “sign,” → semantic, + adj. suffix -os), + -ia.

Etymology (PE): Bolcemi, from bol, → poly-, + cem, → meaning, + noun suffix -i.

The doctrine of or belief in more than one god or in many gods (Dictionary.com). → polydeism.

See also: → poly-; → theism.

In astrophysics, a gaseous sphere in hydrodynamic equilibrium in which the pressure and density are related by the equation P = Kρ^(n+1)/n at each point along the radius, where K is a constant and n is the → polytropic index. The constant K depends upon the nature of the polytrope. Before the advent of computing technology, the theory of polytropes played an important role in physically modeling the structure of stars. → Lane-Emden equation.

Etymology (EN): Polytrope, from → poly- + trope, from → -tropic.

Etymology (PE): Bolgašt, from bol-, → poly-, + gašt “change, alteration,” → -tropic.

1. Math.: Describing a function which has different values for one variable.
   

2. Thermodynamics: Pertaining to pressure and volume change that maintains specific heat. → polytropic process.

Etymology (EN): → poly- + → -tropic.

A change in the → pressure or → volume of a → gas in a → polytropic process.

See also: → polytropic; → change.

A gas capable of undergoing a → polytropic process.

See also: → polytropic; → gas.

A number appearing in the equation describing a → polytropic process.

See also: → polytropic; → index.

A thermodynamic process that obeys the relation: PVⁿ = C, where P is pressure, V is volume, n is any real number, called the → polytropic index, and C is a constant.

If n = 0, then P = C and it is an → isobaric process.
If n = 1, then for an → ideal gas PV = NkT = C and it is an → isothermal process.

If n = γ, → adiabatic index, then for an ideal gas it is an → adiabatic process.

See also: → polytropic; → process.

1. Chem.: Having more than one → valence.
   

2. → polyvalent logic.

See also: → poly-; → valence.

A system of logic with more than two → truth values, as opposed to → classical logic. A polyvalent logic may have a continuous scale of values with → true and → false as limiting → extremes.

See also: → polyvalent; → logic.

1. Any combination of resources put together to be shared in community, such as
   memory pool, storage pool.
   

2. To combine into a common fund, as for a joint enterprise.

Etymology (EN): 1) M.E., O.E. pol, akin to Du. poel, O.H.G. pfuol, Ger. Pfuhl “puddle.”

2. Verb from M.Fr. poule (literally “hen”) “the receptacle for the stakes played for in certain games with cards, the collective stakes of the players at these games,” from M.L. pulla “hen,” from L. pullus “young animal,” related to L. putus, putillus “small boy,” puer “son, boy;” Av. puθra- “son;” O.Pers. puça- “son;” Mod.Pers. pur, pesar “son, boy;” cf. Skt. putrá- “son, child, young of an animal.”

Etymology (PE): Kul “pond, pool, reservoir,” variants kulâb, qulé, farqar, related to kulidan “to dig, excavate,” Mid.Pers. kwl “pit, sink, cavity.”

The act or result of putting resources into a pool or common stock by agreement.

See also: Verbal noun of → pool.

Lacking an expected supply of something specified.

Etymology (EN): M.E. pov(e)re, from O.Fr. povre, from L. pauper “poor,” perhaps a compound of paucus “little” and parare “to get.”

Etymology (PE): Kam “little, few; deficient, wanting; scarce” (Mid.Pers. kam “little, small, few;” O.Pers./Av. kamna- “small, few,” related to
keh “small, little, slender” (related to kâstan, kâhidan “to decrease, lessen, diminish,” from Mid.Pers. kâhitan, kâstan, kâhênitan “to decrease, diminish, lessen;” Av. kasu- “small, little;” Proto-Iranian *kas- “to be small, diminish, lessen”) kamdâr, literally “having little possession,” from kam

• dâr “having, possessor,” from dâštan “to have, to possess,” Mid.Pers. dâštan; O.Pers./Av. root dar- “to hold, keep back, maintain, keep in mind;” Skt. dhr-, dharma- “law;”
  Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE *dher- “to hold, support.”

1. Accepted, followed, used, or done by many people.
   

2. Of, relating to, or coming from most of the people in a country, society, or group (Merriam-Webster.com).

Etymology (EN): M.E., from M.Fr. populier, from L. popularis “belonging to the people, general, common; devoted to or accepted by the people; democratic,” from populus “people,” → population.

Etymology (PE): Mardom-pasand, from mardom, → people, + pasand agent noun from pasandidan “to approve, cherish;” Mid.Pers. passandidan “to like, approve, appreciate;” ultimately from Proto-Ir. *pati-sand- “to appreciate; “from *sand- “to appear, seem (good);” cf. Av. sənd- “to appear; seem (good);” O.Pers. θand- “to seem;” Skt. chand- “to appear, to please;” L. censo “I approve, judge.” Mardomâné from mardom + -âné suffix with several significations: similarity, manner, ability, suitability, property, possession, etc., from Mid.Pers. -ânag.

The act of popularizing.

See also: → popularize; → -tion.

1. To make popular; make attractive to the general public.
   

2. To make (a scientific or academic subject) accessible to the general public by presenting it in an understandable form (OxfordDictionaries.com).

See also: → popular; → -ize.

1. To inhabit; live in; be the inhabitants of.
   

2. To furnish with inhabitants, as by colonization (Dictionary.com).

See also: Infinitive, corresponding to → population.

Statistics: Any finite or infinite set of individuals, items, or data
subject to a statistical study. → disk population; → halo population; → population inversion; → Bose-Einstein distribution.

Etymology (EN): Verbal noun of populate, from M.L. populatus, p.p. of populare “to inhabit,” from L. populus “people.”

Etymology (PE): In the IE languages the concepts of “full, many, multitude” and “people, group, herd, flock” are related. In Pers. several variants of por “full, much, many” denote “group, population,” as in Lori, Qâyeni bor “group, tribe, herd,” Torbat-Heydariyeyi, Qomi borr “heap, bundle, group,” Qomi borreh “group, assemblage of people,” Pashtu parrak “flock, herd,” Urdu para “flock, herd,” Lârestâni baila “group, tribe,” Tabari balik “herd, flock;” other examples from
literary Pers. bâré “herd, flock,” parré “a rank or file of soldiers, a circular disposition of troops.” Therefore, porineš “population,” verbal noun of porinidan “to populate,” infinitive of porin “populous,” from por “mutitude, many, full”

• -in attribution suffix. Por, from Mid.Pers. purr “full;” O.Pers. paru- “much, many;” Av. par- “to fill,”
  parav-, pauru-, pouru- “full, much, many;”
  PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru- “much, abundant;”
  Gk. polus “many,” plethos “great number, multitude;” O.E. full.

A member of a class of relatively young stars, containing a large fraction of → metals, found mainly in the disk of the Galaxy.

See also: → population; I, Roman number 1; → star.

A member of a population of relatively old stars, containing a small fraction of → metals, found mainly in the → halo of the Galaxy and in → globular clusters.

See also: → population; II, Roman number 2; → star.

A member of the first generation of stars, formed out of pristine gas, enriched by → primordial nucleosynthesis alone. The material from which these stars formed consisted mostly of hydrogen and helium. Because neutral hydrogen clouds were free of dust, their cooling mechanism was drastically ineffective. As a result, these star forming clouds had a much higher temperature than in the present epoch, and their → Jeans mass was much higher. Therefore, these first generation of stars were principally massive, with a typical mass scale of order of about 100 M_sun.
Population III stars started forming about 300 million years after the → Big Bang at → redshifts
between 50 and 6, when the Universe had between 1 and 5% of its present age. These stars were probably responsible for the → reionization of the Universe. Given their high mass, they lived only a few million years ending with either a → pair-instability supernova phase or a direct collapse to a → black hole.
Population III stars thus initiated the chemical enrichment of the Universe and opened the way to more normal modes of star formation, namely → Population II. Some models predict a bimodal → initial mass function for the first stars, allowing also for solar mass stars. See also → extremely metal-poor star.

See also: → population; III, Roman number 3; → star.

In physics, specifically statistical mechanics, the state of an atomic or molecular system in which the number of members in an excited state is larger than those in lower energy states. → optical pumping;
→ inverted population.

See also: → population; → inversion.

1. A small hole such as a space especially in a rock, soil, etc. → porous dust grain, → porosity.
   

2. A minute orifice in an animal or plant, as in the skin or a leaf.
   

3. Sun: A small → sunspot that does not have a → penumbra. Pores are up to about 2,500 km across and are lighter than a sunspot’s → umbra. They appear and disappear at the surface of the Sun where sunspots are rarely
   formed, e.g. at low latitudes. Their lifetimes can be as short as 1/2 hour and as long as several days.

Etymology (EN): From L. porus “a pore,” from Gk. poros “a pore,” literally “passage, way,” from PIE root *per- “to lead, pass over.”

Etymology (PE): Lik, from Gilaki lik “hole,” variants luk, luke, liuk, luxa, Tabari luk, li, Sangesari, Semnâni lu, Aftari lo.

1. General: A → dimensionless number characterizing a porous medium, expressed by the ratio of the volume occupied by the pores to the total volume of the medium.
   

2. Geology: The percentage volume of hollow spaces in a rock, sediment, or soil.
   

3. Dust grains: The quantity usually defined as:

P = (V_v/V_t) = (1 - V_s/V_t), where V_v and V_s are the volumes of vacuum and of the solid matter making up the particle, and V_t is the total volume of the particle within some defined surface.

See also: → porous; → -ity.

Full of pores.

Etymology (EN): From M.Fr. poreux, from M.L. porosus, from L. porus “opening,” → pore.

Etymology (PE): Porlik, from por “full; many,” → full,

• lik, → pore.

A type of → interstellar dust grain made up of an aggregate of components with a hollow structure. Various processes operating in interstellar and → circumstellar media are believed to produce inhomogeneous and porous dust grains. Porous grains can produce more → extinction per unit mass than their combined individual dust components. They are generally cooler than compact grains (see, e.g., Iati et al. 2001, MNRAS 322, 749).

See also: → porous; → dust; → grain.

A → binary star in the constellation → Virgo comprising two yellow-white → main sequence stars of +3.6 and +3.7 magnitude and spectral types F0V. Their orbital period is about 170 years and
they are about 40 light-years away.

See also: Porrima after the Roman goddess of childbirth.

A World Wide Web site that functions as an entry point to the Internet and is accessed through a browser.

Etymology (EN): M.E. portale “city gate, porch,” from M. L. portalis (adj.) “of a gate,” from L. porta “gate, door.”

Etymology (PE): Dargâh, from Mid.Pers. dargâh “doorway,” from dar “door” (O.Pers. duvara-; Av. dvar-; cf. Skt. dvár-; Gk. thura, L. fores; O.E. duru; E. door; Lith. dvaras “court-yard;” PIE *dhwer-/*dhwor- “door, gate”) + gâh “place; time” (Mid.Pers. gâh, gâs; O.Pers. gāθu-; Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gátu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come”).

1. A part of any whole, either separated from or integrated with it.
   

   2. To divide into or distribute in portions or shares.

Etymology (EN): M.E. porcion, from O.Fr. porcion “part, portion, share,” from partion- “share, part, piece,” related to pars, → part.

Etymology (PE): Pâreš, from pâr “part, portion, piece” (variants pâré, parré “portion, segment (of an orange),” pargâlé, “piece, portion; patch;” (dialects Kermâni pariké “portion, half;” Tabari perik “minute quantity, particle;” Lârestâni pakva “patch;” Borujerdi parru “patch”);
Mid.Pers. pârag “piece, part, portion; gift, offering, bribe;” Av. pāra- “debt,” from par- “to remunerate, equalize; to condemn;” PIE *per- “to sell, hand over, distribute; to assigne;”
Gk. peprotai “it has been granted;” L. pars, as above; Skt. purti- “reward;” Hitt. pars-, parsiya- “to break, crumble”) + -eš suffix.

1a) Condition with reference to place; location; situation.

1b) A place occupied or to be occupied. → mean position.

2a) To put in a particular or appropriate position; place.

2b) To determine the position of; locate.

Etymology (EN): M.E. posicioun, from O.Fr. posicion, from L. positionem “act or fact of placing, position, affirmation,” from positus, p.p. stem of ponere “to put, place.”

Etymology (PE): Neheš, verbal noun from nehâdan “to place, put; to set;” Mid.Pers. nihâtan, from ne-, ni- “down; into,” → ni- (PIE),

• dâ- “to put; to establish; to give,” dadâiti “he gives;” cf. Skt. dadâti “he gives;” Gk. didomi “I give;” L. do “I give;” PIE base *do- “to give”).
  Neheš-dâdan, from neheš “position,” + dâdan “to give, yield, put,” → datum.

The convention for measuring angles on the sky in astronomy (Abbreviated as PA). It is the direction of an imaginary arrow in the sky, measured from north through east: 0° = north, 90° = east, 180° = south, and so on to 359° and back to 0°. Applied to a binary system it is the direction of a secondary body or feature from a primary, measured in the system. .

See also: → position; → angle.

In single dish astronomy, an observing mode in which the telescope is moved between the object position and a user defined reference position. The aim is to eliminate unwanted signals in the baseline. → beam switching; → frequency switching.

See also: → position; → switching.

Relating to or determined by position.

See also: → position; → -al.

The branch of astronomy that is used to determine the location of objects on the celestial sphere, as seen at a particular date, time, and location on the Earth. Same as → spherical astronomy.

See also: → positional; → astronomy.

A system of representing → numbers in which the → position of a → digit
in a string of digits affects its value. The decimal system is a positional notation for expressing numbers. Same as → place-value notation and → positional number system.

See also: → positional; → notation.

A → number system in which the value of each digit is determined by which place it appears in the full number. The lowest place value is the rightmost position, and each successive position to the left has a higher place value. In the → number system conversion, the rightmost position represents the “ones” column, the next position represents the “tens” column, the next position represents “hundreds”, etc. The values of each position correspond to powers of the → base of the number system. For example, in the usual decimal number system, which uses base 10, the place values correspond to powers of 10. Same as → place-value notation and → positional notation. See also → number system conversion.

See also: → positional; → number; → system.

The act or process of putting in a particular position or determining the psition of.

See also: Verbal noun of → position; → -ing.

1. General: Explicitly stated, stipulated, or expressed.
   

2. Capable of being measured, detected, or perceived.
   

3. Math.: Noting a quantity greater than zero.
   

4. Physics: Having an electrical charge of the same polarity as that of a proton.
   

5. Photography: Having colors or values of dark and light corresponding to the subject.
   

6. Philosophy: Relating to the theory that knowledge can be acquired only through direct observation and experimentation, and not through metaphysics or theology. → positivism.
   

7. Law: Conclusive and beyond doubt or question; irrefutable.
   

8. Opposite of → negative.
   

See also:
→ positive charge, → positive correlation, → positive feedback, → positive skewness, → positiveness, → positivism.

Etymology (EN): M.E., from O.Fr. positif, from L. positivus “placed, settled; positive” (opposed to naturalis “natural”), from posit-, from positus “placed, put,” p.p. of ponere “to put, place, set” + -ivus a suffix of adjectives.

Etymology (PE): Dâhidâr, from dâhid- + -âr. The first component is the past stem of *dâhidan “to put, create, determine,” variant of dehidan, dâdan “to give;” from Mid.Pers. dâdan, dahidan “to give; to create;” O.Pers. dā- “to give, grant, yield;” Av. dā- “to give, grant; put; create; determine;” dāhi “he would give/put” (single second person, subjunctive transitive), dadāiti “he gives;” cf. Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
L. dare “to give, offer,” facere “to do, to make;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do.” The second component -âr, accusative suffix; on the model of gereftâr “captive, involved (in trouble),” didâr “exposed to view.”

A charge having sign opposite to that of the electron.

See also: → positive; → charge.

Same as → direct correlation.

See also: → positive; → correlation.

A → feedback process in which the → output reacts on the → input so as to increase the initial → effect.

See also: → positive; → feedback.

Of a distribution function, a skewness in which the right tail (tail at the large end of the distribution) is more pronounced than the left tail (tail at small end of the distribution).
→ negative skewness.

See also: → positive; → skewness.

The quality or state of being positive; positivity.

See also: → positive + → -ity.

Any doctrine that excludes a priori affirmations and admits only positive truth, i.e. factual knowledge gained through observation.

See also: From Fr. positivisme, from positif, → positive, in its philosophical sense of “imposed on the mind by experience;” → -ism.

The → antiparticle of the → electron, which has the same → mass, → spin, and → electric charge as the electron, but the charge is → positive. Positrons may be generated by
→ radioactive decay or by → pair production from energetic → gamma ray photons.

See also: From posi(tive), → positive + (elec)tron→ electron.

A short-lived bound state of a positron and an electron.

See also: From → positron + -ium (as in barium, titanium), from N.L., from L. neuter suffix.

1. To have as belonging to one; have as property; own.
   

   2. To have as a faculty, quality, or the like.
      

   3. Of a spirit, especially an evil one) to occupy, dominate, or control (a person) from within (Dictionary.com).

Etymology (EN): M.E. possesen, from M.Fr. possesser, “to have and hold, take, be in possession of,” from L. possess-, p.p. stem of possidere “to have and hold, be master of, own,” probably a compound of potis “having power, powerful, able,” from PIE root *poti- “powerful; lord;” from which also derived Skt. patih “master, husband,” Gk. posis, Lithuanian patis “husband” + sedere, from PIE root *sed-, “to → sit.”

Etymology (PE): Dâreštidan, infinitive and back formation from dârešt, → possession.

1. The act or fact of possessing; the state of being possessed.
   

   2. A thing possessed (Dictionary.com).

Etymology (EN): → possess; → -tion.

Etymology (PE): Dârešt, verbal noun of dâštan “to have, possess” (on the model of konešt, from kardan; xoršt, from xordan; bâlešt, from bâlidan; râmešt, from râmidan; (Lori) zenešt, from zadan; (Nowdân, Fârs) perešt, from paridan); Mid.Pers. dâr-, dâštan “to have, hold, preserve;” O.Pers./Av. dar- “to hold, keep back, maintain, keep in mind;” Skt. dhr- “to hold, keep, preserve,” dharma- “what is established or firm; law;” Gk. thronos “elevated seat, throne,” L. firmus “firm, stable,” Lith. daryti “to make,” PIE *d^her- “to hold, support.”

1. Of or relating to possession or ownership.
   

   2. having or showing an excessive desire to possess, control, or dominate.
      

   3. Denoting an inflected form of a noun or pronoun used to convey the idea of possession, association, etc.; the possessive case (Dictionary.com).

Etymology (EN): → possess; → -tion.

Same as → genitive case.

See also: → possessive; → case.

The state or fact of being possible. Something possible.

Etymology (EN): M.E. possibilite, from L.L. possibilitas, → possible

• → -ity.

Etymology (PE): ŠÃ¢yani, from šÃ¢yan, → possible.

1. Capable of happening or likely to happen in the future.
   

2. Capable of being real, present, or true.

Etymology (EN): M.E., from L. possibilis “that may be done,” from posse
“to be able” + -ibilis “-able.”

Etymology (PE): ŠÃ¢yan, from Mid.Pers. šÃ¢yan “possible,” from stem šÃ¢y- “to be able, possible, to be worthy,” relatd to Pers. šÃ¢yad “perhaps” (literally, “it is fitting”), šÃ¢yestan “to be appropriate,” šÃ¢yân “fitting, suitable, possible;” šÃ¢h “king;” Zazaki šinây, šÃ¢yiš “to be able;” Gazi šÃ¢- “to be able;” Abyâne-yi) ešÃ¶/šo-; Naini šÃ¢/ši- “to be able;” Av. xša- “to be able; rule.”

1a) An online message that is submitted to a message board or electronic mailing list.

1b) Text, images, etc., that are placed on a website.

2a) To affix (a notice, bulletin, etc.) to a post, wall, or the like.

2b) To place (text, images, etc.) on a website (Dictionary.com).

Etymology (EN): M.E., from O.E. post “pillar, doorpost,” and O.Fr. post “post, upright beam,” both from L. postis “post, doorpost.”

Etymology (PE): Barnemâ, literally “display, show off,” from bar- “on; up; upon; in,” → on-, + nemâ, present stem of nemudan “to show,” → display.

A prefix, meaning “behind, after, later, subsequent to, posterior to.”

Etymology (EN): From L. post (adverb and preposition) “behind, after, afterward,” cognate with Gk. (Arcadian and Cyprian dialects) pos “toward, on, at;” Skt. paśca “behind, after, later.”

Etymology (PE): Pasâ-, from pas
“behind” (e.g.: pas-e pardé “behind the curtain”), variant pošt “back; the back; behind;”
Mid.Pers. pas “behind, before, after;” O.Pers. pasā “after;” Av. pasca “behind (of space); then, afterward (of time);” cf.
Skt. paścā “behind, after, later;” L. post, as above;
O.C.S. po “behind, after;” Lith. pas “at, by;”
PIE *pos-, *posko-.

A star in a short-lived evolutionary stage evolving from the → asymptotic giant branch toward higher → effective temperatures. The majority of low and intermediate mass stars (1 to 8 → solar masses) are believed to pass through this stage on their way to becoming → planetary nebulae.

See also: → post-; → asymptotic giant branch.

A star that has evolved off the → main sequence.

See also: → post-; → main sequence; → star.

→ post-Newtonian formalism.

See also: → post-; → Newtonian; → expansion.

An approximate version of → general relativity that applies when the → gravitational field is → weak, and the matter → velocity is → small.

Post-Newtonian formalism successfully describes the gravitational field of the solar system. It can also be applied to situations involving compact bodies with strong internal gravity, provided that the mutual gravity between bodies is weak.

It also provides a foundation to calculate the → gravitational waves emitted by → compact binary star systems, as well as their orbital evolution under radiative losses.

The formalism proceeds from the Newtonian description and then, step by step, adds correction terms that take into account the effects of general relativity. The correction terms are ordered in a systematic way (from the largest effects to the smallest ones), and the progression of ever smaller corrections is called the → post-Newtonian expansion.

See also: → post-; → Newtonian; → formalism.

The stage following a nova outburst, when the star has returned to a quiescent state.

See also: → post-; → nova.

An evolved star whose → planetary nebula has dissipated.

See also: → post-; → planetary; → nebula; → star.

Any of a number of trends or styles in architecture, philosophy, literature, and art developed in the latter part of the 20th century often in reaction to
→ modernism. In philosophy, postmodernists claim that value systems are concoctions of human partial knowledge rather than systems reflecting universal objective truth. The most influential early postmodern philosophers include Jean Baudrillard, Jean-François Lyotard, and Jacques Derrida.

See also: The term postmodernism was first coined by architects to designate an architectural response against the earlier Bauhaus style, which was
characterized by box-like apartment buildings, the absence of ornamentation and harmony between the function of a building and its design; → post- + → modernism.

To put off to a later time; defer.

Etymology (EN): From L. postponere “put after; neglect; postpone,” from → post- “after” + ponere “to put, place,” → position.

Etymology (PE): Pas afkandan, literally “to throw after,” “to postpone” (Dehxodâ), from pas- “after,” → post-, + afkandan “to throw,” → stopword.

1a) Math.: An unproved → assumption taken as basic in a mathematical system, and from which (in combination with other → postulates) the propositions of the system are derived, or in terms of which the propositions are proved. In modern usage, postulate is synonymous with → axiom.

1b) Physics: A fundamental principle. For example, the two postulates of special relativity of Einstein are: 1) The laws of physical phenomena are the same when studied in terms of two → reference frames moving at a constant velocity relative to each other. 2) The → velocity of light in free space is the same for all observers and is independent of the relative velocity of the source of light and the observer. See also → Planck postulate.

2a) General: To ask, demand, or claim. To claim or assume the existence or truth of, especially as a basis for reasoning or arguing.

2b) Math.: To assume as a postulate.

Etymology (EN): From L. postulatum “petition, thing requested,” noun use of neuter of p.p. of postulare “to ask, request, demand,” akin to poscere “to request.”

Etymology (PE): Farâvas (on the model of piš-nahâd “proposition,” and farâ-nemudan “to exhibit, expose”), from farâ-, → pro-, + vas. The second component from Av. vas- “to will, desire, wish, long for,” vasəmi “I wish,” vasna- “will, favor,” ušti- “desire, wish, will,” vasô, vasə “at one’s will;” cf. O.Pers. vasiy “at will, greatly, utterly,” vašna- “will, favor;” Mid.Pers. vasnâd “because, on account of.” This word is extant in several Modern Iranian dialects: Tabari vessen “to wish, desire,” Gilaki vâssan “to wish, desire,” vâsti “desire; for, because, on account of;” Kurd. wistin “to desire, wish,” Lâri avessa “to desire;” Nâyini vas “to like;” colloquial Tehrâni vâsé “for;” Lori, Malâyeri biza “pregnancy craving;”
cognate with Skt. vaś- “to wish, want, desire,” váśa- “wish, desire,” vasēna “for, because;” Gk. ekon “voluntary;” PIE base *uek- “to wish.”

A silvery-white metallic chemical element; symbol K (from
L. kalium “potash”). Atomic number 19; atomic weight 39.0983; melting point 63.25°C; boiling point 760°C.

Etymology (EN): Coined by the English chemist Sir Humphrey Davy (1778-1829), who first isolated it in 1807 from electrolysis of caustic potash (KOH); from Mod.L. potassa, Latinized form of E. potash + -ium.

1. A latent ability that may or may not be developed; possibility.
   

2. Physics: The → work required to → move a unit positive → charge, unit magnetic pole, or an amount of → mass respectively from → infinity (i.e. a place infinitely distant from the causes of the field) to a designated point. Gravitational potential is always negative, but the electric or magnetic potentials may be positive or negative.
   

3. (adj.) Capable of being or becoming, as opposed to → actual.
   

See also:
→ chemical potential, → electric scalar potential, → electromagnetic potential, → equipotential surface, → excitation potential, → gravitational potential energy, → ionization potential, → kinetic potential, → magnetic vector potential, → potential barrier, → potential density, → potential difference, → potential energy, → potential energy curve, → potential field, → potential gradient, → potential well, → potentiality, → retarded potential, → scalar potential, → thermodynamic potential, → Yukawa potential.

Etymology (EN): From L.L. potentialis “potential,” from L. potentia “power,” potis “powerful, able, capable;” cognate with Av. paiti- “lord, husband;” Mod.Pers. -bad (sepah-bad “general, commander of an army”); Skt. páti-
“master, husband;” Gk. posis “husband;” Lith. patis “husband.”

Etymology (PE): Tavand, from tav- + -vand. The first component tav- is the stem of tavân “power, strength,” tavânestan “to be powerful, able;” variants tâv, tâb, (dialects) tew “power;” Mid.Pers. tuwan “power, might;” O.Pers. tav- “to have power, to be strong, to be able,” tauman- “power, strength,” tunuvant- “powerful;”
Av. tauu- (tu-) “to be able, strong,” tavah- “power,” təviši- “strength” (Mod.Pers. tuš “power, ability”);
Skt. tavⁱ- “to be strong, to have authority,” tavas-, tavisa- “strong, energetic,” tavisi- “power, strength;” Gk. taus, saos “healthy;” L. tumere “to be swollen;” PIE *teu- “to swell, be strong.” The second component -vand
a suffix of adjectives and agent nouns, → actual.
Note: Tavand used as both noun and adjective, such as honarmand (n.) and mard-e honarmand (adj.).

Region in a field of force in which the potential is such that a particle, which is subject to the field, encounters opposition to its passage.

See also: → potential; → barrier.

Of a fluid parcel at pressure P, the density that it would acquire if adiabatically brought to a reference pressure.

See also: → potential; → density.

Between two points, the work done in taking the unit test object from one point to the other. Potential is a scalar quantity.

See also: → potential; → difference.

Of a system, the work done in changing the system from some standard configuration to its present state. Thus, if a body of mass m is raised vertically through a height h, the work done, mgh, is the increase in potential energy.

See also: → potential; → energy.

A plot that displays the → potential energy of a moving body as a function of its position. It is explained by the → conservation of energy and the conversion of potential energy into → kinetic energy and vice versa.

See also: → potential; → energy; → curve.

A field that has a → potential. A continuous → vector fieldA in a domain D is a potential field in D if and only if its → work around every closed curve C contained in D is zero: ∫A.ds = 0.
Examples include the → gravitational field and the → electrostatic field.

See also: → potential; → field.

At a point, the rate of change of potential V, with distance x, measured in the direction in which the variation is a maximum. The intensity F of the field is proportional to the potential gradient, but is oppositely directed: F = -dV/dx.

See also: → potential; → gradient.

Region in a → field of force in which the potential decreases abruptly, and in the surrounding region of which the potential is larger.

See also: → potential; → well.

1. The state or quality of being potential. Something potential.
   

2. According to Aristotle, what has the possibility of having
   → form, in contrast to → actuality; the power to effect change.

See also: → potential + → -ity.

With a possibility of becoming actual; possibly.

See also: → potential; → -ly.

An asteroid that could make a threatening close approach to the Earth. In technical terms a PHA is defined as having an → absolute magnitude of 22 or brighter and an → Earth Minimum Orbit Intersection Distance (MOID) of less than 0.05 → astronomical unit or 7.5 million km.

See also: → potentially; → hazardous; → asteroid.

An instrument for measuring electrical quantities by balancing an unknown potential difference against a known potential difference.

See also: From potenti(al), → potential + -o

• → -meter.

1. Physics: The → rate of doing → work on transferring → energy, as expressed in ergs/sec, watts, etc. Same as → mechanical power.
   

2. Math.: An → exponent to which a given quantity is raised. The expression xⁿ is therefore known as “x to the n-th power.”

Etymology (EN): M.E. pouer(e), poer(e), from O.Fr. povoir, noun use of the infinitive in O.Fr., “to be able,” from V.L. *potere, from L. potis “powerful, able, capable;”
cognate with Av. paiti- “lord, husband;” Mod.Pers. -bad (sepah-bad “general, commander of an army”); Skt. pái-
“master, husband;” Gk. posis “husband;” Lith. patis “husband.”

Etymology (PE): Tavân “power, strength,” tavânestan “to be powerful, able;” Mid.Pers. tuwan “power, might,” from O.Pers./Av. base tav- “to have power, to be strong, to be able,” Av. tavah- “power,” təviši- “strength,” Mod.Pers. tuš, tâb “power, ability,” O.Pers. tauman- “power, strength,” tunuvant- “powerful,” Skt. tu- “to be strong, to have authority,” tavas-, tavisa- “strong, energetic,” tavisi- “power, strength.”

A function of the form f(x) = xⁿ, where n is a → real number.

See also: → power; → function.

A mathematical relationship between two quantities expressed by a
→ power function.

See also: → power; → law.

A series in which the terms contain regularly increasing powers of a variable. In general, a₀ + a₁x + a₂x²

• … + a_nxⁿ, where a₀, a₁, etc. are constants.

See also: → power; → series.

Same as → spectral density.

See also: → power; → spectral; → density.

The plot that gives the portion of a signal’s power falling within given frequency bins. The most common way of generating a power spectrum is by using a discrete Fourier transform.

See also: → power; → spectrum.

For a → random variable X, any → distribution which has the form: P(X ≥ x) = (k/x)^α, where x is a value in the range defined for X, k > 0 is a parameter termed location parameter, and α > 0 is the → slope parameter.

See also: → power; → law; → distribution.

An → elliptical galaxy whose → surface brightness can be approximated by a single → power law at small radii (r ≤ 10-20’’). More modern interpretations have emphasized that these profiles can be better understood as the inward continuation of the galaxy’s overall → Sersic profile, usually modified by an additional, nuclear-scale stellar component (S. P. Rusli et al., 2013, AJ 146, 160).

See also: → power; → law; → galaxy.

The amount of electromagnetic energy flowing through unit area, perpendicular to the direction of energy propagation, per unit time, given by (c/2 π)[E x H]. → Poynting’s theorem.

See also: → Poynting’s theorem; → vector.

A loss of → orbital angular momentum by tiny ring particles associated with their absorption and re-emission of → solar radiation. Also known as the → Poynting-Robertson effect (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → Poynting-Robertson; → drag.

The effect of → solar radiation on a small (centimeter-sized) particle in → orbit around the Sun that causes it to lose velocity and fall gradually into the Sun. The particle → absorbs solar radiation and → radiates the energy → isotropically in its own frame. The particle thereby preferentially radiates (and loses → angular momentum) in the forward direction in the → inertial frame of the Sun (aberration effect). This leads to a decrease in the particle’s angular momentum and causes it to spiral sunward. In contrast, the → Yarkovsky effect is anisotropic; the object may be accelerated or decelerated.

See also: → Poynting’s theorem; Howard Percy Robertson (1903-1961), American physicist and mathematician; → effect.

The space through which electromagnetic radiation passes is filled with electric and magnetic fields at right angles to each other and to the direction of propagation of the radiation. The rate of energy transfer is given by the Poynting vector.

See also: In honor of John Henry Poynting (1852-1914), English physicist; → theorem.