An Etymological Dictionary of Astronomy and Astrophysics

A feature in a stellar spectrum showing strong hydrogen and helium emission lines with absorption lines on their blueward wing. This line profile indicates a strong outflow of matter from a star. → inverse P Cygni line profile.

See also: After → P Cygni star.

1. A variable star in the constellation → Cygnus, and one of the most luminous stars in the Milky Way Galaxy. It is a → Luminous Blue Variable (LBV)
   star of → spectral type B2 Ia located about 7000 light-years away. It was unknown until the 17th century, when it suddenly brightened to third magnitude. It briefly disappeared and returned, and today has a magnitude of about 4.8.
   

2. A member of a class of high-luminosity, early-type stars, in which spectral lines have a characteristic → P Cygni profile.

See also: P, letter of alphabet; Cygni, pertaining to → Cygnus; → star.

Acoustic waves trapped inside stars, which act as a resonating cavity, exhibiting millions of oscillation modes or standing waves. Same as → pressure mode. P-mode oscillations in the Sun have frequencies in the 0.2-5.5 mHz range. They are particularly intense in the 2-4 mHz range, where they are often referred to as solar “5-minute oscillations.”
See also → pulsation mode.

See also: P, referring to pressure; → mode.

Same as → parity symmetry.

See also: → parity; → symmetry.

→ compressional wave.

See also: P, referring to → pressure; → wave.

A prefix meaning thick.

Etymology (EN): From Fr. pachy-, from L., from Gk. pachys “thick,” akin to Av. bazah “high, deep,” Baloci baz “thick, dense,” ON bingar “heap,” Latvian biezs “dense, thick.”

Etymology (PE): Bazâ-, from Av. bazah, Baloci baz, as above; cf. Waxi bâj “thickness,” variant dabz, → concentrated.

The difference between the isotopic mass of a nuclide and its mass number divided by its mass number. The packing fraction is a measure of the stability of the nucleus.

Etymology (EN): Packing, from the verb pack “to put together in a pack,” from the noun pack, from M.E. pak, packe, from M.D. pac or perhaps M.L.G. pak; → fraction.

Etymology (PE): Barxé, → fraction; anbâštegi quality noun of anbâštan, anbârdan “to fill, to replete;” Mid.Pers. hambāridan “to fill;” from Proto-Iranian *ham-par-, from prefix ham- + par- “to fill;” cf. Av. par- “to fill,”
parav-, pauru-, pouru- “full, much, many;”
O.Pers. paru- “much, many;” Mid.Pers. purr “full;” Mod.Pers. por “full, much, very;”
PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru- “much, abundant;”
Gk. polus “many,” plethos “great number, multitude;” O.E. full.

1. Something consisting of or regarded as having two parts or pieces joined together. → pair annihilation; → pair production.
   
2. Two identical, similar, or corresponding things.

Etymology (EN): M.E., from O.Fr. paire, from L. paria “equals,” neuter plural of par (genitive paris) “a pair, counterpart, equal.”

Etymology (PE): 1) Joft “pair, couple,” Lori, Laki jeft, Qâyeni jof, Tabari jeft, Mid.Pers. yuxt “pair, couple,” Av. yuxta- “a team of horses,” from yaog- “to yoke, harness, put to; to join, unite,” infinitive yuxta, Mid.Pers. jug, ayoxtan “to join, yoke,” Mod.Pers. yuq “yoke;” cf. Skt. yuga- “yoke,” Gk. zygon “yoke,” zeugnyanai “to join, unite,” L. jugare “to join,” from jugum “yoke,”
P.Gmc. *yukam, E. yoke; PIE *yeug- “to join.”
Hamâl, from ham- “together, with; same, equally, even” (Mid.Pers. ham-, like L. com- and Gk. syn- with neither of which it is cognate. O.Pers./Av. ham-, Skt. sam-; also O.Pers./Av. hama- “one and the same;” Skt. sama-; Gk. homos-; originally identical with PIE numeral *sam- “one,” from *som-. The Av. ham- appears in various forms: han- (before gutturals, palatals, dentals) and also hem-, hen-) + suffix -âl, → -al.

Mutual destruction of a particle and its antiparticle, such as an electron-positron pair, when they collide. The total energy of the two particles is converted into energy as gamma rays. It is the inverse process to → pair production.

See also: → pair; → annihilation

An instability arising from the → pair production inside a → massive star leading to energetic → supernova explosions. The pair instability occurs when, late in the star’s life, the core reaches a sufficiently high temperature after → carbon burning, a condition in which the pair production can take place. The pairs of electron and positron annihilate to form a neutrino and an anti-neutrino. Consequently, the pressure drops and the outer layers fall in onto the core. The temperature and pressure increase rapidly and eventually titanic nuclear burning causes an extraordinary explosion with energies higher than 10⁵¹ erg. See also → pair-instability supernova and → pulsational pair-instability supernova.

See also: → pair; → instability.

The creation of an → elementary particle and its → antiparticle from a → boson. For example, the formation of an → electron and a → positron in the interaction of high-energy → gamma ray photons, having at least 1.02 Mev, with an → atomic nucleus (γ → e^-

• e⁺). The → rest masses of the electron and positron being 0.51 MeV each, the excess energy will be carried away by these two particles. Pair production is the inverse process to → pair annihilation.

See also: → pair; → production

→ pair instability

See also: → pair; → instability.

A special type of → supernova that would result from the → pair instability in → supermassive stars with a mass range between 140 and 260 Msun in a low → metallicity environment. Such objects descended from the → Population III stars in the early history of the Universe. Such supernovae are the most powerful thermonuclear explosions in the Universe.
Pair-instability supernovae may have played an important role in the synthesis of → heavy elements. Moreover, the energetic feedback of the processed elements to their surroundings could have affected the structure and evolution of the early Universe (See, e.g., Fryer et al. 2001, ApJ 550, 372;
Heger & Woosley 2002, ApJ 567, 532). See also → pulsational pair-instability supernova.

See also: → pair; → instability; → supernova.

In nuclear physics, the extra binding energy associated with pairs of nucleons of the same kind. This quantity expresses the fact that nuclei with odd numbers of neutrons and protons have less energy and are less stable than those with even numbers of neutrons and protons.

See also: Pairing, verbal noun of → pair; → energy.

The roof of the → mouth, separating the oral and nasal cavities. → hard palate; → soft palate.

Etymology (EN): M.E., from O.Fr. palat and directly from L. palatum “roof of the mouth.”

Etymology (PE): Kâm “roof of the mouth,” of unknown origin.

Lacking in color, not bright, weak in color or shade.

Etymology (EN): M.E., from O.Fr. paile “pale, light-colored,” from L. pallidus “pale, pallid, colorless,” from pallere “be pale, grow pale.”

Etymology (PE): Pâl, from p(ox) + âl, a combination of (Delijân) pox “pale” + (Kurd) âl “pale.”

A prefix meaning “old, ancient” especially in reference to former geologic time periods, e.g. → paleoclimatology, → paleolithic, and
→ paleomagnetism.

Etymology (EN): From Gk. palaio-, combining form of palaios “old, ancient,” from palai “long ago, far back,” from PIE root *kwel- “to turn, move about,” also “far”

Etymology (PE): Pârin, pâriné “ancient,” also “last year” (contraction of pâr sâl), related to pir “old;” Mid.Pers. pir “old, aged, ancient;” Av. parô (adv.) “before, before (of time), in front (of space);” cf. Skt. puáh, combining form of puras “before (of time and place), in front, in advance.”
Dirin, diriné “old, ancient, of times past,” from dir “old, antique; a long while” + suffix -in. The first component
from Mid.Pers. dêr, variants dagr, drâz “long”
(Mod.Pers. derâz “long,” variant Laki, Kurdi derež);
O.Pers. darga- “long;” Av. darəga-, darəγa- “long,” drājištəm “longest;” cf. Skt. dirghá- “long (in space and time);” L. longus “long;” Gk. dolikhos “elongated;” O.H.G., Ger. lang; Goth. laggs “long;” PIE base *dlonghos- “long.” The second component -in adj. suffix, from Mid.Pers. -ên (as in âhênên, zarrên), from Av. -aēna- (ayanhaēna- “made of iron,” zaranaēna- “golden”); cf. Skt. -ēna-.

The climate during a past geologic time or before historical records.

See also: → paleo-; → climate.

The study of past → climates throughout → geologic and → historic time (paleoclimates), and the causes of their variations.

See also: → paleo-; → climatology.

A period of → geologic time lasting about 42 million years, roughly from 65 to 23 million years ago. The Paleogene is most notable as being the time in which mammals evolved from relatively small, simple forms into a large group of diverse animals in the wake of the → Cretaceous-Paleogene extinction event that ended the preceding → Cretaceous period. Birds also evolved considerably during this period, changing into roughly modern forms.

See also: Literally “ancient birth,” from → paleo- + -gene, → gene.

Of, pertaining to, or characteristic of the cultures of the Old Stone Age, marked by the earliest known chipped stone tools. The period continued from about 750,000 years ago, until the beginning of the Mesolithic Age, about 15,000 years ago.

See also: → paleo-; lithic, from Gk. lithos “stone.”

The study of natural remanent magnetization in order to determine the intensity and direction of the Earth’s magnetic field in the geologic past.

See also: → paleo-; → magnetism.

The study of ancient life through → fossils.

Etymology (EN): From Fr. paléontologie, from paléo-, → paleo-, + onto-, from Gk. ont- “being,” pr.p. of einai “to be,” + → -logy.

Etymology (PE): Pârin-šenâsi, from pârin-, → paleo- + šenâsi, → -logy.

A technical scale that categorizes the → impact hazard of a → near-Earth object (NEO). It compares the threat of a given NEO to the so-called background threat of all NEOs of the same size or larger. In this way, the probability of the → impact itself as well as the time until the predicted impact are considered. The scale is → logarithmic and continuous. A Palermo scale of -2 indicates that the predicted event is only 1% as likely as a random background event between now and the time of predicted impact. A value of 0 indicates that the risk is the same as the risk from the background threats. A value of +2 indicates an event that is 100 times more likely than the background hazard. The Palermo scale is defined in the paper “Quantifying the risk posed by potential Earth impacts” by Chesley et al. (2002), Icarus 159n 423. See also → Torino scale.

See also: Named after Palermo, in recognition of the Palermo Observatory, where in 1801 the first and largest asteroid (→ Ceres) was discovered by the Italian astronomer Giuseppe Piazzi (1746-1826); → scale.

A silvery white metal which belongs to the → platinum group elements, symbol Pd. → Atomic weight 106.4, → atomic number 46, → melting point 1554.9 °C, → boiling point 2963 °C. It is used in alloys and as a catalyst.

See also: Named 1803 by discoverer William Hyde Wollaston (1766-1828),
after the asteroid → Pallas, which was discovered at about the same time.

A class of → iron meteorite containing → olivine crystals.

See also: Named after the German naturalist Peter Pallas (1741-1811), who first studied such a type of meteorites.

An observatory located atop Palomar Mountain about 65 km north-northeast of San Diego, California. It is a center of astronomical research owned and operated by the California Institute of Technology (Caltech). The Observatory is home to three active research telescopes: the 200-inch (5.1-meter) Hale Telescope, the 48-inch (1.25-meter) Samuel Oschin Telescope, and the 60-inch (1.5-meter) telescope. Research at Palomar Observatory is pursued by a broad community of astronomers from Caltech and other domestic and international partner institutions. The famous Hale Telescope proved instrumental in cosmological research. It was the largest instrument of its kind until 1976.

See also: Palomar, a mountain ridge in the Peninsular Ranges in northern San Diego County whose highest elevation is 1,871 m; → Observatory.

A photographic atlas of the northern hemisphere and a portion of the southern hemisphere created at Mount → Palomar Observatory in southern California. The original survey was completed in 1954 using the 48-in Schmidt (Oschin) Telescope. The square photographic plates were 35.5 cm (14-inch) on a side, each encompassing roughly 6 × 6 degrees of the sky. The survey was originally intended to cover the entire sky from +90 degrees declination down to -24 degrees (plate centers) in 879 regions, using both red and blue sensitive emulsions, and including stars to magnitude +22.

Ultimately the survey was extended to -30 degrees (both red and blue), an additional 57 regions. Finally, the Whiteoak Southern Extension was added in 1962 (red plates only), with another 100 plates which extended the set down to a declination of -42 degrees plate center.

See also: → Palomar Observatory; → sky; → survey.

The innermost of Saturn’s known satellites, orbiting within the Encke Division in the A Ring at a distance of 133,583 km. Also know as Saturn XVIII. It orbits Saturn every 0.575 days and its diameter is about 20 km. Pan was discovered in 1990 from Voyager photos taken in 1981.

See also: In Gk. mythology, Pan was the god of woods, fields, and flocks, having a human torso and head with a goat’s legs, horns, and ears.

A prefix meaning “all, whole,” used as a general formative (panorama; pantelegraph; pantheism; pantonality), and especially in terms implying the union of all branches of a group (Pan-Christian; Pan-Hellenic; Pan-Slavism).

Etymology (EN): From Gk. pan-, combining form of pas (neuter pan) “all, every,” of unknown origin.

Etymology (PE): Pân- loan from Gk., as above.
Sarâsar- “all, entirely, the whole,” literally “from beginning to end; from one end to the other,” from sar “head” + -â- epenthetic vowel + sar. The main word sar is related to soru, sorun “horn”
(karnâ “a trumpet-like wind instrument,” variant sornâ “a wind instrument”);
Mid.Pers. sar “head,” sru “horn;” Av. sarah- “head,” srū- “horn, nail;” cf. Skt. śiras- “head, chief;” Gk. kara “head,” karena “head, top,” keras “horn;”
L. cornu “horn,” cerebrum “brain;”
P.Gmc. *khurnaz (Ger. Horn, Du. horen; cognate with E. horn, as above, from PIE *ker- “head, horn;”
O.E. horn “horn of an animal,” also “wind instrument;”
E. horn); PIE base *ker- “head, horn, top, summit.”
Hamé-, → all.

A system for wide-field astronomical imaging developed and operated by the Institute for Astronomy at the University of Hawaii. Its goal is to survey the sky for moving or variable objects on a continual basis, and also produce accurate astrometry and photometry of already detected objects. It is situated at Haleakala Observatories near the summit of Haleakala in Hawaii. Pan-STARRS1 (PS1) was the first part of Pan-STARRS. The survey used a 1.8 meter telescope and a 1.4 Gigapixel camera to image the sky in five broadband filters (g, r, i, z, y). The PS1 consortium is made up of astronomers and engineers from 14 institutions and six countries. The survey was completed in April 2014. The Pan-STARRS Project is now focusing on building PS2.

See also: Short for Panoramic Survey Telescope and Rapid Response System.

A thin, flat cake of batter fried on both sides on a griddle or in a frying pan (Dictionary.com).

Etymology (EN): From M.E., from pan “a usually broad, shallow, and open container for domestic use, as for cooking;” O.E. panne + cake, from M.E., from O.N. kaka “cake,” cognate with M.E. kechel “little cake;” G. Kuchen;

Etymology (PE): Lavâš “a sort of thin flattened bread.”

A model of galaxy formation in which regions of primordial gas as massive as clusters of galaxies began to collapse into thin sheets (pancakes). Within the pancakes, smaller regions of gas later collapsed to form individual galaxies.

See also: → pancake; → model.

A star strongly compressed due to the → tidal force of a → massive black hole. The intense → gravity of the → black hole pulls harder on the nearest part of the star, creating an imbalance. When the star penetrates the → tidal radius, first it becomes cigar-shaped, then the squeezing of the tidal forces flattens the star in its orbital plane to the shape of a → pancake. Next the star rebounds, and as it leaves the tidal radius, it starts to expand. A little further on its orbit the star finally breaks up into gas fragments. This flattening would increase the → density and → temperature inside the star enough to trigger intense nuclear reactions that would tear it apart (Brassart & J.-P. Luminet, 2008, Astron. Astrophys. 481, 259).

See also: → pancake; → star.

One of the inner moons of Saturn and the outer shepherd moon for the F-ring. It was discovered in 1980 from Voyager 1 photos and is also known as Saturn XVII.

See also: In Gk. mythology Pandora was the very first woman who was formed out of clay by the gods.
She was bestowed upon humankind by Zeus as a punishment for Prometheus’ theft of fire. Entrusted with a box containing all the ills that could plague people, she opened it out of curiosity and thereby released all the evils of human life.

1. One of the divisions of a window or the like, consisting of a single plate of glass in a frame. A plate of glass for such a division.
   

2. A panel, as of a wainscot, ceiling, door, etc (Dictionary.com).

Etymology (EN): M.E. pane, pan “strip of cloth, section,” from M.Fr. pan,
from O.Fr. pan “section, piece, panel,” from L. pannum “piece of cloth, garment;” cf. Goth. fana “piece of cloth,” Gk. penos “web,” O.E. fanna “flag.”

Etymology (PE): Poš, from Baluci poc “cloth, clothing,” from puš-, pušidan “to cover, to wear,” → envelope.

1. A distinct portion, section, or division of a wall, wainscot, ceiling, door, shutter, fence, etc., especially of any surface sunk below or raised above the general level or enclosed by a frame or border.
   

2. A comparatively thin, flat piece of wood or the like, as a large piece of plywood.
   

3. A group of persons gathered to conduct a public discussion, judge a contest, serve as advisers, be players on a radio or television game, or the like (Dictionary.com).

Etymology (EN): M.E., from O.Fr. panel “a piece (of anything),” diminutive of pan “piece of cloth or the like,” ultimately from L. pannus “piece of cloth.” The sense of “a small group of people called on to discuss, judge, advice on a particular matter” is from 1570s.

Etymology (PE): Pošel, from poš, → pane, + -el, → -al.

Hypothetical super-continent that existed from about 300 to 200 million years ago. It has since broken up and the fragments have drifted to become the configuration of Earth’s present-day continents.

Etymology (EN): From Gk. → pan- “all” + gaia, variant ge “earth, land, ground, soil.”

Etymology (PE): Pânzam, from → pan- + zam, variants
zamin, zami “earth, ground,” from Mid.Pers. zamig “earth;”
Av. zam- “the earth;” cf. Skt. ksam; Gk. khthôn, khamai “on the ground;” L. homo “earthly being” and humus “the earth” (as in homo sapiens or homicide, humble, humus, exhume);
PIE root *dh(e)ghom “earth.”
Pânžé, loan from Fr.

The hypothesis that life exists and is distributed throughout the Universe in the form of “seeds” that develop in the right environment. The oldest record of this idea may be traced back to the ancient Greek philospher Anaxagoras, who lived in the fifth century B.C.

Etymology (EN): N.L., from Gk. panspermia “mixture of all seeds,” from → pan- + -sperm, a combining form of sperma “seed” + -ia a noun suffix.

Etymology (PE): Pân-dâne-vari, sarâsar-dâne-vari, from pân-, sarâsar-, → pan-, + dâné “seed, grain” (Mid.Pers. dân, dânag “seed, corn,” Av. dânô- in dânô.karš- “carrying grains; an ant,” Skt. dhânâ- “corn, grain,” Tokharian B tâno “grain,” cf. Lith. duona “corn, bread”)

• -var suffix of possession, variant -ur (Mid.Pers. -uwar, -war; from O.Pers. -bara, from bar- “to bear, carry”)
• -i noun suffix.

1. The doctrine that God is the transcendent reality of which the material universe and human beings are only manifestations: it involves a denial of God’s personality and expresses a tendency to identify God and nature.
   

2. Any religious belief or philosophical doctrine that identifies God with the universe (Dictionary.com).

See also: → pan-; → theism.

1. A thin sheet made from fibrous material (wood pulp, rags, straw, etc.) suitable for writing or printing on.
   

2. A written or printed document or the like.

Etymology (EN): From M.E. papire, from L. papyrus “paper,” from Gk. papyros “any plant of the paper plant genus,” may be of Egyptian origin.

Etymology (PE): Kâqaz “paper,” probably a transliteration of old Chinese gu zhi; cf. Sogd. kāγaδā “paper,” Skt. kakali, kakari, Marathi kagad, Tamil kagidam, Malayalam kayitam (Y. Kumar, 2005, A History of Sino-Indian Relations).

1. A prefix occurring in loanwords from Gk. with the meanings “beside, alongside of, by, beyond.”
   
2. Chemistry: A combining form designating the para (1, 4) position in the benzene ring.
   Short for → parahydrogen.

Etymology (EN): From Gk. para-, from para (preposition) “beside, near, from, against, contrary to,” cognate with Av. parā, as below;
L. pro “before, for, in favor of,” per- “through;” Goth. faur “along;” O.E. for- “off, away.”

Etymology (PE): Pârâ-, parâ-, from O.Pers. parā (adv.) “along; forth;” Av. parā (adv.) “at first, in the first place; in former times, formerly;” also “away, aside;” cf. Skt. pur&#257: “before, formerly;” cognate with Gk. para, as above.

The → water molecule in which the → nuclear spins of the constituent → hydrogen atoms are → antiparallel (→ parahydrogen). See also: → ortho-water.

See also: → para-; → water.

A plain curve obtained by slicing a cone with a plane parallel to one side of the cone. A parabola can be considered an ellipse with an infinite major axis. It is one of the types of conic sections.

Etymology (EN): N.L., from Gk. parabole “comparison, application,” literally “a throwing beside,” from → para- + bole “throwing,” related to ballein “to throw.”

Etymology (PE): Sahmi, of unknown origin.

Having the form of a parabola.

See also: of or pertaining to → parabola.

An antenna comprising a parabolic reflector with a receiving and/or transmitting element positioned at or near its focal point.

See also: → parabolic; → antenna.

A concave mirror that has the form of a paraboloid of revolution.

See also: → parabolic; → mirror.

An orbit whose overall shape is like a parabola; it is the limiting case between an elliptical orbit (eccentricity less than 1) and a hyperbolic orbit (eccentricity larger than 1). The speed necessary to form a parabolic orbit is known as the escape velocity.

See also: → parabolic; → orbit.

The speed necessary to form a parabolic orbit around a gravitational center. It is also the minimum speed necessary to escape from the gravitational pull of a body.

See also: → parabolic; → velocity.

A solid formed by the revolution of a parabola about its axis.

See also: From parabol(a), → parabola, + → -oid.

1. General: A typical example of something.
   

2. Grammar: A set of word forms giving all of the possible inflections (declensions, conjugations) of a word.
   

3. Philosophy of science: A framework of concepts, results, and procedures within which subsequent work is carried out, as defined by the influential Thomas Kuhn (1922-1996) in his classic The Structure of Scientific Revolutions (1962). “Normal science” proceeds within such a framework or paradigm. When a paradigm has to be changed (→ paradigm shift),
   there comes about a crisis and then scientific revolution. For example, Newton’s mechanical paradigm
   was replaced by the paradigm of Einstein’s relativistic universe.
   Each paradigm is an interpretation of the world, rather than an objective explanation.
   For criticism of Kuhn’s views, see, e.g., Stanford Encyclopedia of Philosophy.
   

4. Term now occurring frequently in every kind of discourse, usually to mean something like “way of thinking” or “approach to a problem.”

Etymology (EN): L.L. paradigma “pattern, example,” especially in grammar, from Gk. paradeigma “pattern, model,” from paradeiknynai “to exhibit, show side by side,” from → para- “beside” + deiknynai “to show,” related to
dokein “to appear, seem, think,” cognate with Av. daēs- “to show” (Mod.Pers. andišé, andiš “thought, think,” see below); Skt. diś- “to show, point out,” diśati “he shows;” L. dicere “to utter;” PIE base *deik- “to show, to pronounce.”

Etymology (PE): Parâdiš, pârâdiš, from parâ-, pârâ, → para- + diš “to show,” as in andiš, andišidan “to → think” (related to dis, disé, → form);

Sogd. andiš “to seem,” andêš “to show,” andêšik “appearing;” ultimately from
Av. daēs- “to show,” s-aorist forms dāiš-, dôiš-, diš-, akin to Gk. deiknynai “to show,” as above, dike “manner, custom;” Skt. diś- “to show, point out;” L. dicere “to utter, say;” O.H.G. zeigon, Ger. zeigen “to show;” O.E. teon “to accuse,” tæcan “to teach.”

1. Philosophy of science: A process of revolutionary change in scientific → paradigms, whereby established scientific ideas are replaced by new ones. For instance, Copernicus’ evidence that the Earth revolved around the Sun caused a paradigm shift in astronomy.
   

2. The idea of paradigm shift has been transferred from the sciences to other areas of society and culture, referring to a fundamental reorganization of how people think about an entire topic.

See also: → paradigm; → shift; → change.

A statement, proposition, or situation that seems self-contradictory or absurd but in reality is or may be true. → Fermi paradox; → faint early Sun paradox; → twins paradox; → paradox of youth.

Etymology (EN): From L. paradoxum “contrary to expectation,” from Gk. paradoxon, from neuter of adj. paradoxos “contrary to common opinion, unbelievable,” from → para- “contrary to” + dox(a) “opinion, belief” + -os adj. suffix. The main component dox, from dokein “to appear, seem, think,” is cognate with Av. daēs- “to show;” Skt. diś- “to show, point out,” diśati “he shows;” L. dicere “to utter;” PIE base *deik- “to show, pronounce solemnly.”

Etymology (PE): Pârâdaxš (on the model of Gk. paradoxos), from pârâ-, → para-, + daxš, from Av. daxš- “to reveal, instruct, point out,” fradaxštar- “teacher,” *daxšārə “revelations;” Mod.Pers. daxš “task, effort;” cf. Skt. daks- “to be able,” dáksa- “able, expert.”

The observed presence of young stars in the immediate vicinity of the → supermassive black hole (SMBH), → Sgr A*, residing in the center of our Galaxy. The stellar population within 1 pc of the SMBH contains a variety of young and → massive stars orbiting the SMBH. Some of them are only about 20 Myr old and get as close as a few light-days to the SMBH, while from 0.1 to 0.4 pc even younger stars are found with ages of 3-7 Myr. The presence of these stars so near to the SMBH is a paradox. Their → in situ formation should be almost impossible,
since the environment is too hostile for these stars to form. Indeed the strong → tidal influence of the SMBH should hamper their formation. On the other hand, the scenario considering their → migration from other places does not seem to be adequate. The time required for the migration
from > 1 pc by dynamical friction would exceed their inferred ages unless the migration rate were somehow accelerated. This apparent contradiction was termed “paradox of youth” by Ghez et al. (2003, ApJ 586, L127). See also Genzel et al. (2010, Rev.Mod.Phys. 82, 3121, also at astro-ph/1006.0064).

See also: Youth, the condition of being → young; → paradox.

Molecular hydrogen in which the nuclei (protons) of the two hydrogen atoms contained in the molecule have spins in opposite directions. → orthohydrogen

See also: → para-; + → hydrogen.

Of or pertaining to a parallax.

See also: Adj. form of → parallax.

Of an object in the sky, the angle between the → celestial pole, the object, and the → zenith. Since parallactic angle
describes the orientation on the sky of the object for a particular observer, it can be an important quantity in some observations.

See also: → parallactic; → angle.

The path on the sky of the apparent position of a star as seen from the Earth, due to the Earth’s annual motion around the Sun.

See also: → parallactic; → ellipse.

An irregularity in the Moon’s motion caused by the Sun’s gravitational attraction, which sets the Moon ahead or behind its normal orbital position. The Moon is about 2 arcminutes ahead of its expected position at first quarter, and a similar amount behind at last quarter.

See also: → parallactic; → inequality.

The proper motion of a star due to the effect of the Sun’s motion relative to the → local standard of rest.

See also: → parallactic; → motion.

The apparent → shift of a nearby object’s → position in relation to more distant ones when the nearby object is observed from different → viewing angles. See also → stellar parallax.

Etymology (EN): From M.Fr. parallaxe, from Gk. parallaxis “change, alteration,” from parallassein “to alter, make things alternate,” from → para- “beside” + allassein “to change,” from allos “other;” → alias.

Etymology (PE): Didgašt, literally “view change,” from did “sight, view; eye,” from didan “to see” (Mid.Pers. ditan “to see, regard, catch sight of, contemplate, experience;” O.Pers. dī- “to see;” Av. dā(y)- “to see,” didāti “sees;” cf.
Skt. dhī- “to perceive, think, ponder; thought, reflection, meditation,” dādhye; Gk. dedorka “have seen”) + gašt “change, alteration,” past stem of gaštan, gardidan “to turn, to change” (Mid.Pers. vartitan; Av. varət- “to turn, revolve;” Skt. vrt- “to turn, roll,” vartate “it turns round, rolls;” L. vertere “to turn;” O.H.G. werden “to become;” PIE base *wer- “to turn, bend”).

The angular displacement associated with → parallax.

See also: → parallax; → angle.

1. Said of two or more things, such as lines or planes, that are equally distant from one another at all points.
   
2. Electricity: An arrangement of the components in an electric circuit so that the same voltage is applied to each component. Compare → series.
   

Etymology (EN): From M.Fr. parallèle, from L. parallelus, from Gk. parallelos “parallel,” from para allelois “beside one another,” from → para- “beside” + allelois “each other,” from allos “other,” → alias.

Etymology (PE): Parâsu, from parâ-, → para-, + su “direction, side,” from Mid.Pers. sôk “direction, side.”

The → moment of inertia of a body about any given axis is the moment of inertia about a parallel axis through the center of mass, plus the moment of inertia about the given axis if the mass were located at the center of mass. same as → Steiner’s theorem.

See also: → parallel; → axis; → theorem.

A small circle on the celestial sphere whose plane is parallel to the celestial horizon. Same as → almucantar.

Etymology (EN): → parallel; → altitude.

Etymology (PE): → almucantar.

A solid figure whose six bases are → parallelograms, opposite pairs being identical and parallel.

Etymology (EN): From Gk. parallelepipedon, from parallelos, → parallel + epipedon “plane surface,” from neuter of epipedos “flat,” from → epi- + pedon “ground,” cognate with L. ped-, pes, → foot.

Etymology (PE): Parâsuruyé, from parâsu, → parallel, + ruyé, → surface. Lowzivâr, from lowzi, → rhombus + -vâr, → -oid.

A four-sided → polygon whose opposite sides are parallel. A parallelogram all of whose angles are right angles is a → rectangle.

Etymology (EN): From Fr. parallélogramme, from L. parallelogrammum, from Gk. parallelogrammon “bounded by parallel lines,” from parallelos, → parallel, + gramme “line,” related to graphein “to write, draw” → -graph.

Etymology (PE): Parâsubar, from parâsu, → parallel, + bar, → side.

A paramagnetic substance, which possesses → paramagnetism.

See also: → para- + → magnet.

Relative to or characterized by → paramagnetism.

See also: → para- + → magnetic

The property of a substance that possesses a → magnetic permeability greater than that of a vacuum but significantly less than that exhibited by
→ ferromagnetism. In the absence of an external magnetic field the atomic → magnetic moments of the substance are randomly oriented and thus cancel each other out with no net total magnetic moment. Moreover the coupling between neighboring moments is weak.
However, when a magnetic field is applied magnetic moments align with the direction of the field and so the magnetic moments add together. Therefore paramagnetic substances affect external fields in a positive way, by attraction to the field resulting in a local increase in the magnetic field.
The → magnetization vanishes when the field is removed.

See also: → para- + → magnetism

General: Any of a set of physical properties whose values determine the characteristics or behavior of something. → impact parameter; → ionization parameter.
Math.: A constant or variable term in a function that determines the specific form of the function but not its general nature, as a in f(x) = kx, where k determines only the slope of the line described by f(x).

Etymology (EN): Mod.L. parametrum, from Gk. → para- + metron “measure,” → meter.

Etymology (PE): Parâmun, from parâ-, → para-,

• mun/mân “measure,” as in Pers. terms pirâmun “perimeter,” âzmun “test, trial,”
  peymân “measuring, agreement,” peymâné “a measure; a cup, bowl,”
  from O.Pers./Av. mā(y)- “to measure;” cf. Skt. mati “measures,” matra- “measure;” Gk. metron “measure;” L. metrum; PIE base *me- “to measure.”

Any of a set of equations that defines the coordinates of the dependent variables
of a curve or surface in terms of one or more independent variables or parameters.

See also: → parametric; → equation.

An → ESO observatory, located on Cerro Paranal in the Atacama Desert, northern Chile, at 2,635 m altitude. It is about 120 km south of the town of Antofagasta and 12 km inland from the Pacific Coast. The Paranal Observatory hosts the
→ Very Large Telescope (VLT) with four 8.2 m telescopes. Each telescope provides one → Cassegrain and two → Nasmyth focus stations for facility instruments. One Nasmyth focus is available for visitor instruments. In addition each telescope is equipped with a → coudé focus station from which the light can be coherently combined in the interferometric focus. ESO also operates four 1.8 m Auxiliary Telescopes (ATs), that are used as an
interferometric array (VISA) and a 4 m infrared survey telescope (VISTA).
Currently, more than 10 instruments including two
interferometric instruments (MIDI, AMBER) are operational and offered for science observations.

See also: Paranal, the name of the mountain, in the Quechua language meaning “whirlwind;” → observatory.

An astronomical and meteorological calendar written by ancient Greeks from 450 B.C. during at least three centuries. The parapegma was an inscribed stone for public use. It had holes beside the inscription, in which a peg could be inserted next to the appropriate day. The term was later applied to purely written forms of such calendars, or almanacs; plural form parapegmata.

See also: From → para- “next to” + pegma “something that fastens something else together,” from pegnyein “to stick.”

Characteristic of optical analyses that are limited to infinitesimally small apertures. Also called first-order or Gaussian optics.

Etymology (EN): → para-; → axial.

Etymology (PE): From par(â)-, → para-, + âseyi→ axial.

A ray that lies close to and almost parallel to the optical axis and behaves according to paraxial equations.

See also: → paraxial, → ray.

A catalog of stars in the → Orion Nebula created by P. P. Parenago in 1954 (Publ. Astr. Inst. Sternberg, Band 25, p. 393-437, Moskau).

See also: Pavel Petrovich Parenago (1906-1960), a Soviet astronomer.

1. General: A father or a mother. A precursor, or
   → progenitor.
   

2. Physics: A → nuclide that upon → radioactive decay or → disintegration yields a specific nuclide (the → daughter). See also → parent cloud, → parent element, → parent galaxy, → parent molecule.

Etymology (EN): From O.Fr. parent, from L. parentem (nominative parens) “father or mother, ancestor,” from parere
“to bring forth, give birth to, produce,” from PIE base *per- “to bring forth”

Etymology (PE): Permâr, literally “father-mother” (as in Sogd. māt-pitri “parent”),
from Gilaki per, → father, + Gilaki mâr, → mother.

Usually of a newborn star, the molecular cloud in which the star has formed.

See also: → parent; → cloud.

A radioactive element that spontaneously decays into a new substance. The product of this decay is known as a “daughter” element.

See also: → parent; → element.

Of a high redshift supernova, the galaxy in which the event has occurred.

See also: → parent; → galaxy.

The molecule initially produced when a comet nucleus sublimates, soon changed to different daughter molecules because of solar radiation.

See also: → parent; → molecule.

An atmospheric optical phenomenon, seen as a bright spot sometimes appearing at either side of the → Sun, often on a luminous ring or → halo and at the same angular elevation as the Sun. Parhelia are caused by the → refraction and → reflection of → sunlight by → ice crystals suspended in the Earth’s → atmosphere. Also called mock Sun or sundog.

Etymology (EN): From Gk. parhelion “a mock Sun,” from → para- “beside” + helios “sun,” → helio-.

Etymology (PE): Parâhur, from parâ-, → para-,

• hur “sun,” → helio-.

1. General: Equality, as in amount, status, or character; equivalence; correspondence; similarity; analogy. Opposite of disparity.
   

2. Physics: The principle of space-inversion invariance.
   An operation that reverses the algebraic sign of the coordinate axes used to describe a system, i.e. (x, y, z) → (-x, -y, -z). The parity principle is important in quantum mechanics because the wave functions which represent particles can behave in different ways upon transformation of the coordinate system.
   The parity is 1 (or even) if the wave function of the system is unchanged by an inversion of the coordinate system; it is -1 (or odd) if the wave function is changed
   only in sign. Parity is conserved in strong interactions, but not in weak ones.
   

3. Math.: The attribute, of an integer, of being even or odd. Thus, it can be said that 8 and 12 have the same parity, whereas 5 and 16 have opposite parity.
   

See also: → charge-parity symmetry, → even parity, → parity conservation, → parity symmetry, → parity violation.

Etymology (EN): From M.Fr. parité, from L.L. paritas “equality,” from L. adj. par “equal.”

Etymology (PE): Hamâli, quality noun of hamâl, → pair (equivalent 2).

In quantum mechanics, the condition of parity in strong and electrodynamic interactions, where it remains constant and does not change with time. In other words, parity conservation implies that Nature is symmetrical and makes no distinction between right- and left-handed rotations or between opposite sides of a subatomic particle. Thus, for example, two similar radioactive particles spinning in opposite directions about a vertical axis should emit their decay products with the same intensity upward and downward. Same as → parity symmetry.

See also: → parity; → conservation.

The invariance of physical laws under a transformation that changes the sign of the space coordinates. Parity symmetry is sometimes called mirror symmetry. It is known that the parity symmetry is violated in some weak interactions, while it is well preserved in all other three interactions (gravitational, electromagnetic, strong). Same as → P-symmetry and → parity conservation.

See also: → parity; → symmetry.

In quantum mechanics, the condition of → parity in the → weak interaction. For example, the emitted → beta particles
in → radioactive decay of → cobalt-60 nuclei are not equally distributed between the two poles of cobalt-60. More beta particles emerge from one pole than the other, and it would be possible to distinguish the mirror image nuclei from their counterparts.

See also: → parity; → violation.

A type of instability found in some astrophysical phenomena involving → magnetic fields; it arises if a gas layer is supported by the horizontal magnetic fields against → gravity. Also called → magnetic buoyancy. Briefly, this instability works as follows. Consider a uniform disk of gas which is coupled to a magnetici field that is parallel to the disk. Suppose that the disk is gravitationally stratified in the vertical direction, and is in dynamical equilibrium under the balance of gravity and pressure (thermal and magnetic). Now consider a small perturbation which causes the field lines to rise in certain parts of the disk and sink in others. Because of gravity, the gas loaded onto the field lines tends to slide off the peaks and and sink into the valleys. The increase of mass loads in the valleys makes them sink further, while the magnetic pressure causes the peaks to rise as their mass load decreases. Consequently, the initial perturbation is amplified, causing the production of density fluctuations in an initially uniform disk. The characteristic scale for the Parker instability is ~4πH, where H is the scale height of the diffuse component of the disk. For the Milky Way, where H ~ 150 pc, this scale is about 1-2 kpc. Numerical simulations show that the density contrast generated by the Parker instability is generally of order unity before the instability saturates. This implies that the Parker instability on its own may not be able to drive collapse on large scales. Nevertheless, it may trigger gravitational instability in a marginally unstable disk and/or induce strong motions in the medium, thereby acting as a source of turbulence on large scales (see, e.g., Houjun Mo, Frank van den Bosch, Simon White, 2010, Galaxy Formation and Evolution, The University Press, Cambridge, UK).

See also: First studied by E. N. Parker, 1966, ApJ 145, 811; → instability.

A NASA space mission launched on August 12, 2018 to study the outer corona of the Sun at very close distances. Parker Solar Probe is the first space mission to penetrate into solar corona as close as about 10 solar radii. It will approach the Sun to this distance 25 times. Approaching the Sun to such distances is a big technological challenge.

The main goals of the mission are to answer these questions: Why is the solar corona so hotter than the solar surface? How is the solar wind accelerated? How are the energetic particles produced and transported?

See also: Named after the physicist Eugene Newman Parker (1927-), who proposed the existence of the solar wind and did pioneering work for its interpretation.

A catalog of 397 radio sources between declinations +20° and +27° which were compiled from a finding survey made at 635 MHz with the 64m radio telescope at the Australian National Radio Astronomy Observatory, Parkes, N.S.W. and published in 1968. Originally abbreviated PSR, this catalog, which is also called PKS,
replaces and improves on four earlier lists (1964 to 1966).

See also: Parkes, proper noun; selected, p.p. of → select; → region.

A basic unit of astronomical distances, corresponding to a → trigonometric parallax of one second of arc. In other words, it is the distance at which one → astronomical unit (the mean radius of the Earth’s orbit) subtends an angle of 1 arcsecond.
1 pc = 3.2616 → light-years = 206 265 astronomical units = 30.857 x 10¹² km.

See also: From parallax + second.

An optical phenomenon resulting from the refraction and reflection of moonlight within ice crystals in cirrus cloud; also known as paraselene,
mock Moon or moondog. It is the lunar counterpart of the → parhelion.

Etymology (EN): From Gk. para- “beside,” → para-, + selene “moon,” from Gk. selene “moon,” related to selas “light, brightness, flame.”

Etymology (PE): Pârâ-, → para-, + mâh, → moon.

A theorem relating the → Fourier coefficients to the function that they describe. It states that:

(1/L) ∫ |f(x)|²dx (integrated from x₀ to x₀ + L) = (a₀/2)² + (1/2) Σ (a_r² + b_r²) (summed from r = 1 to ∞).

In other words, the sum of the moduli squared of the complex Fourier coefficients is equal to the average value of |f(x)|² over one period.

See also: Named after Marc-Antoine Parseval (1755-1836), French mathematician; → theorem.

1. A portion or division of a whole that is separate or distinct; piece, fragment, fraction, or section; constituent.
   

2. A section or division of a literary work (Dictionary.com).

Etymology (EN): M.E., from O.Fr. part “share, portion; character; dominion; side, path,” from L. partem (nominative pars) “a part, piece, a share, a division; a party or faction,” related to portio “share, portion,” from PIE root *per- “to assign, allot;” cf. Pers. pâr, pâré “piece, part, portion, fragment;” as below.

Etymology (PE): Pâr, variant pâré “piece, part, portion,” 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 assign;”
Gk. peprotai “it has been granted;” L. pars, as above; Skt. purti- “reward;” Hitt. pars-, parsiya- “to break, crumble.”

Being such in part only; not total or general; incomplete.
See also:
→ partial derivative, → partial differential equation, → partial eclipse, → partial ionization zone, → partial lunar eclipse, → partial solar eclipse, → partial truth.

Etymology (EN): M.E. parcial, from O.Fr. parcial, from M.L. partialis “pertaining to a part,” from L. pars, → part; → -al.

Etymology (PE): → part; → -al.

The derivative of a function of two or more variables, e.g., z = f(x,y), with respect to one of the variables, the others being considered constants (denoted ∂z / ∂x).

See also: → partial; → derivative.

A type of differential equation involving an unknown function (or functions) of several independent variables and its (or their) partial derivatives with respect to those variables.

See also: → partial; → differential; → equation.

An eclipse that is not total. → partial lunar eclipse, → partial solar eclipse.

See also: → partial; → eclipse.

One of several zones of the stellar interior where increased → opacity can provide the → kappa mechanism to drive → pulsations. See also → Kramers’ law. In these zones where the gases are partially ionized, part of the energy released during a layer’s compression can be used for further ionization, rather than raising the temperature of the gas. Partial ionization zones modulate the flow of energy through the layers of the star and are the direct cause of → stellar pulsation. The partial ionization zones were first identified by the Russian astronomer Sergei A. Zhevakin (1916-2001) in the 1950s. In most stars there are two main ionization zones. The hydrogen partial ionization zone where both the ionization of neutral hydrogen (H ↔ H⁺ + e^-) and the first ionization of helium (He ↔ He⁺ + e^-) occurs in layers with a characteristic temperature of 1.5 x 10⁴ K. The second, deeper zone is called the He⁺ partial ionization zone, and involves the second ionization of helium (He⁺↔ He⁺⁺ + e^-), which occurs deeper at a characteristic temperature of 4 x 10⁴ K. The location of these ionization zones within the star determines its pulsational properties. In fact if the → effective temperature of the star is ≥ 7500 K, the pulsation is not active, because the ionization zones will be located very near to the surface. In this region the density is quite low and there is not enough mass available to drive the oscillations. This explains the blue (hot) edge of the instability strip on the → H-R diagram. Otherwise if a star’s surface temperature is too low, ≤ 5500 K, the onset of efficient convection in its outer layers may dampen the oscillations. The red (cool) edge of the instability strip is believed to be the result of the damping effect of convection. He⁺ ionization is the driving agent in → Cepheids. See also → gamma mechanism.

See also: → partial; → ionization; → zone.

A → lunar eclipse when the Earth’s → umbra passes over only part of the Moon, causing only moderate darkening of the full Moon. See also → penumbral lunar eclipse.

See also: → partial; → lunar; → eclipse.

A → solar eclipse when only the → penumbra of the Moon touches the Earth.
The → umbra passes either just above the North Pole or just below the South Pole, missing the Earth.

See also: → partial; → solar; → eclipse.

A → truth value in → fuzzy logic where it can range between “completely true” and “completely false.”

See also: → partial; → truth.

To take part, be or become actively involved.

Etymology (EN): From L. paticipatus p.p. of partcipare “to share,” from particeps “partaking, sharing,” from part-, pars “part,” → partial, + capere “to take,” → concept.

Etymology (PE): Pârgertidan, from pâr “part,” → partial, + gertidan “to take,” → concept.

An act or instance of participating. The fact of taking part.

See also: Verbal noun of → participate.

A lexical form derived from a verb, that has some of the characteristics and functions of both verbs and adjectives. In most Indo-European languages participles are used to express participation in an action (present participle) or relate to a completed action (past participle). They can also appear in attributive form as adjectives.

Etymology (EN): M.E., from O.Fr. participle, variant of participe, from L. participium, literally “a sharing, partaking,” from particeps “sharing, partaking,” → participate.

Etymology (PE): Pârgerté, from pârgert present stem of pârgertidan, → participate, + nuance suffix -é.

1. A unit of → matter smaller than the → atom or its main components.
   The term particle also includes any (currently hypothetical) new particles that might be discovered, such as the supersymmetric partners of the → quarks and → leptons and → bosons.
   

2. Mechanics: A material body (a body possessing mass) the size of which can be neglected in investigating its motion.
   

3. Grammar: In some languages, a word that has a grammatical function but does not fit into the main parts of speech (i.e. → noun, → verb, → adverb, → preposition). For example, in English, off, on, out, and up in call off, pass on, rule out, and grow up, respectively.
   

See also:
→ alpha particle, → antiparticle, → astroparticle physics, → beta particle, → charged particle, → elementary particle, → energetic solar particle, → exchange particle, → Lagrangian particle, → nanoparticle, → particle horizon, → particle nature, → particle physics, → relativistic particle, → resonance particle, → sink particle, → Smoothed Particle Hydrodynamics, → strange particle, → subatomic particle, → test particle, → virtual particle, → wave-particle duality.

Etymology (EN): From L. particula “little bit or part,” diminutive of pars (genitive partis), from PIE base *per- “to assign, allot;” cf. Mid.Pers. pârag “gift, offering, bribe;” Mod.Pers. pâreh “gift” (→ partial); Gk. porein “to provide, give, grant,” peprotai “it has been granted;” Skt. purtá- “gift, pay, reward.”

Etymology (PE): Zarré, from Ar. dharrat “particle.”
Pârul, from pâr, → part, + -ul, → -ule.

For an observer at a given epoch t₀, the boundary between the observable and the unobservable regions of the → Universe. Therefore, the distance to the particle horizon at t₀ defines the size of the → observable Universe. Same as → cosmic horizon.

See also: → particle; → horizon.

A general term to describe → light involving the following phenomena: → reflection, → refraction, and → photoelectric effect. Compare → wave nature.

See also: → particle; → nature.

The branch of physics that deals with the smallest known structures of matter and energy
in order to understand the fundamental particles and forces of nature.

See also: → particle; → physics.

1. (adj.) Of or pertaining to a single or specific person, thing, group, class, occasion, etc., rather than to others or all; special rather than → general.
   

2a) (n.) An individual or distinct part, as an item of a list or enumeration.

2b) (n.) Logic: An individual or a specific group within a general class (Dictionary.com).

Etymology (EN): M.E., from O.Fr. particuler and directly from L.L. particularis “of a part, concerning a small part,” from L. particula, → particle, + -ar, → -al.

Etymology (PE): Pâruli, adj. from pârul, → particle; pârulé, from pârul + nuance adj. -é.

An → astrolabe that serves only a limited number of → latitudes.

See also: → particular; → astrolabe.

Of partial differential equations, the solution which can be obtained from the general solution by particular choice of the arbitrary functions. → general solution; → singular solution.

See also: → particular; → solution.

1. Of or relating to minute separate → particles.
   

2. A particulate → substance.

Etymology (EN): From Mod. L. particulatus, from particula, → particle.

Etymology (PE): From pârul, → particle, + suffix -é or -mand.

Meteorology: A complex → mixture of → microscopic → particles and → liquid droplets suspended in the → atmosphere, especially pollutants.

See also: → particulate; → matter.

1a) A division into or distribution in portions or shares.

1b) A separation; something that separates or divides; a part, division, or section.

2a) The act or process of dividing something into parts. The state of being so divided.

2b) Math.: → integer partition; → set partition.

2c) Computers: → disk partition.

3. To divide into parts or portions. To divide or separate by a partition.

Etymology (EN): M.E., from O.Fr. particion, from L. partition- “division, partition, distribution,” from p.p. stem of partire “to part, divide,” from PIE root *per- “to grant, allot,” related to → part and akin to pake, as below.

Etymology (PE): Parke, from Kermâni parke “piece, part, fragmant,” related to pâr “→ part;” parkidan, infinitive from parke; parkeš, verbal noun of parkidan.

See → integer partition.

See also: → partition; → function.

The ratio of the → concentration of a → solute in a single definite form in the stationary phase to its concentration in the same form in the other phase at equilibrium.

See also: → partition; → ratio.

A chemical separation process whereby a solute is distributed between two phases.

See also: → partition; → -ing.

The act or fact of being partitioned.

See also: → partition; → -ment.

In particle physics, a constituent of the hadron originally postulated in the theoretical analysis of high-energy scattering of particles off hadrons. In modern usage, the term parton is often used to mean a quark or a gluon.

See also: Coined by the American physicist Richard Feynman (1918-1988), from part, from → particle + → -ion

A fraction of a whole number in units of 1/1000,000.
It is usually used to describe chemical concentrations, very small amounts of pollutants in air, water, body fluids, and uncertainty. For example 30 ppm is 3 x 10^-5 or 0.003%.

See also: → part; → per; → million.

The → SI unit of → pressure, that of one → newton per → square → meter. Since 1 Pa is a small pressure, hPa (→ hectopascals) are more widely used. 1 Pa = 10 dyn cm^-2, = 1.02 x 10^-5 kgf cm^-2 = 10^-5 bars = 9.87 x 10^-6 atm = 7.50 x 10^-3 torr (mm Hg).

See also: In honor of Blaise Pascal (1623-1662), French mathematician, physicist, and religious philosopher for his contribution in the study of hydrodynamics and hydrostatics, in particular establishing the principle of the barometer.

An experiment carried out by Blaise Pascal in 1646 to demonstrate the hydraulic pressure. A long and narrow vertical pipe was connected to the content of a closed wooden barrel already full of water. He poured a small quantity of water into the pipe, whereby the height of the fluid within the pipe sharply increased. Due to the increase in hydrostatic pressure and → Pascal’s law, the barrel could leak and even burst.

See also: → pascal (Pa); M.E. barel, from M.Fr. baril, O.Fr. barril; → experiment

A change in the pressure of an enclosed incompressible fluid is conveyed undiminished to every part of the fluid and to the surfaces of its container.

See also: Named after Blaise Pascal (1623-1662), French mathematician, physicist, and religious philosopher for his contribution in the study of hydrodynamics and hydrostatics, in particular establishing the principle of the barometer.

An array of numbers in the shape of a triangle, having a 1 at the top and also at the ends of each row. Each number is obtained by summing the two adjacent numbers to it in the preceding row. Each row is a set of → binomial coefficients. In the expansion of (x + y)ⁿ, the coefficients of x and y are given by the n-th row of Pascal’s traingle.

See also: → pascal; → triangle.

The spectral series associated with the third energy level of the hydrogen atom. The series lies in the infrared, with Pα at 18,751 Å, and
Paschen limit at 8204 Å.

See also: In honor of Friedrich Paschen (1865-1947), German physicist; → series.

An effect on spectral lines obtained when the light source is located in a strong magnetic field. The strong field disrupts the coupling between the orbital and spin angular momenta, resulting in a different pattern of splitting.

See also: Named for the German physicists Friedrich Paschen (1865-1947) and Ernst E. A. Back (1881-1959); → effect.

The fifteenth of Jupiter’s known satellites, orbiting at 23,660,000 km from Jupiter; also known as Jupiter VIII. Its diameter is 36 km and orbital period 744 days.

See also: In Gk. mythology, Pasiphae was the wife of Minos and mother, by a white bull, of the Minotaur.

An act or instance of passing from one place, condition, etc., to another; transit. Same as → transit.

Etymology (EN): M.E, from O.Fr. passage, from passer “to go by;” originally “a road, passage.”

Etymology (PE): Gozar “passage, transit, passing,” from gozaštan “to pass, cross, transit,” variant gozâštan “to put, to place, let, allow;” Mid.Pers. widardan, widâštan “to pass, to let pass (by);” O.Pers. vitar- “to pass across,” viyatarayam “I put across;” Av. vi-tar- “to pass across,” from vi- “apart, away from” (O.Pers. viy- “apart, away;” Av. vi- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”) + O.Pers./Av. tar- “to cross over;” → trans-.

The range of wavelengths that are transmitted by a filter. Same as → band-pass.

Etymology (EN): Pass from O.Fr. passer, from V.L. *passare “to step, walk, pass,” from L. passus “step, pace;” cf. Pers. pâ “foot,” pey “step;” → band.

Etymology (PE): Gozar “passage, transit, passing,” from gozaštan “to pass, cross, transit,” variant gozâštan “to put, to place, let, allow;” Mid.Pers. widardan, widâštan “to pass, to let pass (by);” O.Pers. vitar- “to pass across,” viyatarayam “I put across;” Av. vi-tar- “to pass across,” from vi- “apart, away from” ( O.Pers. viy- “apart, away;” Av. vi- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”) + O.Pers./Av. tar- “to cross over”); bând, → band.

Tending not to participate actively; not working or operating.

Etymology (EN): From L. passivus “submissive; capable of feeling or suffering,” from passus, p.p. of pati “to experience, undergo, suffer.”

Etymology (PE): Akâr “not working, not doing,” from → a- negation prefix + kâr “work,” varaint of kar, present stem of kardan “to do, to make” (Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make”).

A cumulus cloud that is no longer dynamically connected with the atmospheric boundary layer via updrafts or downdrafts.

See also: → passive; → cloud.

An electronic component which contains no source of power, in contrast to active components.

See also: → passive; → component.

A galaxy lacking optical emission-line activity (e.g., [O II], Hα, [O III]) and showing only stellar absorption lines (e.g., the 4000 Å → calcium break, Mg I, Na I). Also called passively evolving galaxy.

See also: → passive; → galaxy.

An electronic system which emits no energy, and is not detectable.

See also: → passive; → system.

Same as → passive galaxy.

See also: → passive; → evolve; → galaxy.

An arbitrary string of characters chosen by a user or system administrator and used to authenticate the user when he attempts to log on, in order to prevent unauthorised access to his account (foldoc.org).

See also: Pass, → passage; → word.

Of, having existed in, or having occurred during a time previous to the present; bygone (Dictionary.com).

Etymology (EN): M.E.; variant spelling of passed, p.p. of pass, from O.Fr. passer, from V.L. *passare “to step, walk, pass,” from L. passus “step, pace;” cf. Pers. pâ “foot,” pey “step,” → foot.

Etymology (PE): Gozašté, p.p. of gozaštan “to pass,” → passage.

A → participle that indicates a completed action or state.

See also: → past; → participle.

1. A small piece of material used to mend a tear or break, to cover a hole, or to strengthen a weak place.
   

   2. A piece of material used to cover or protect a wound, an injured part, etc.
      

   3. A small piece, scrap, or area of anything (Dictionary.com). → bald patch; → patchy turbulence.

Etymology (EN): M.E. pacche; of uncertain origin.

Etymology (PE): Pac, from Bašgaridi (Giroft) pac “patch; fragment,” probably from *parka-, from *par- “piece” + diminutive/relation suffix -ka; cf. pâré “piece, part, portion;” parré “portion, segment (of an orange);” pargâlé “piece, portion; patch;” Kermâni pariké “portion, half;” Tabari perik “minute quantity, particle;” Lârestâni pakva “patch;” Borujerdi parru “patch;” (Fin Bandar Abbâs) park “patch;” (Tâti) pajina “patch, darn;” → part.

A → turbulence that is not continuous in space, but is separated by regions of stability and → laminar flow.

See also: Patchy adj. of → patch; → turbulence.

A shallow dish-like crater with irregular, sometimes scalloped rims, on
the surface of a planet.

See also: From L. patera “abroad, shallow dish” (used for drinking, primarily in a ritual context).

1. A route, course, or track along which something moves.
   

2. A course of action, conduct, or procedure.
   

3. Math.: A continuous curve that connects two or more points.
   

4. In → graph theory, a → graph whose → vertices can be arranged in a sequence.

Etymology (EN): O.E. paþ, pæþ; cf. O.Fris. path; M.Du. pat; Du. pad;
O.H.G. pfad; Ger. Pfad “path.”

Etymology (PE): Pah “path, way,”
from O.Pers. paθi- “path, way;” Av. paθ-, variants paθi-, paθā-, pantay- (with conversion of -θ- to -h-, as in ciθra-/cehr, xšaθra-/šahr/šâh, vərəθraγna-/bahrâm, → Mars);
Mid/Mod.Pers. pand “path, advice, counsel;” Khotanese pande “road, path;” Ossetic fœndœg “path, road;”
cf. Skt. pánthā- “road, path, course;” Gk. patos “path, way,” pontos “sea;” L. pons “bridge, path;” P.Gmc. *finthanan “to find;” E. find; PIE base *pent- “to go, to tread.”
Râh, → way.

The path followed by an individual particle of fluid over an interval of time. It contrasts with the → streamline that represents an instantaneous picture of the motion of particles.

See also: → path; → line.

The path (up to 320 km wide) that the Moon’s shadow traces on the Earth during a total solar eclipse.

See also: → path; → totality.

1. A person who makes or finds a way, especially through unexplored areas or fields of knowledge.
   

2. An airplane, or a person dropped from a plane, sent into a target area to illuminate the area for succeeding aircraft (Dictionary.com).
   

3. → LISA pathfinder.

See also: → path; → finder.

1. A regular or repetitive form, order, or arrangement; a distinctive form. → diffraction pattern; → interference pattern.
   

2. In the → density wave model, a region of enhanced stellar density (arm or bar) in the → Galactic disk.
   → pattern speed.

Etymology (EN): M.E. patron, from O.Fr. patron, from M.L. patronus
“patron saint, lord, master, model, pattern,” from pater “father.”

Etymology (PE): Olgu “pattern,” from Turkish ülgü “form, mold, model.”

In models of → galactic structure, the → angular velocity of a → spiral arm or a → bar, assumed to be a pattern rotating as a solid body in the galaxy. Pattern speed, Ω_P , is one of the most important parameters of the → density wave theory responsible for the spiral arms. Pattern speed determines the location of the → corotation resonance.

See also: → pattern; → speed.

A quantum mechanical principle according to which no two identical
→ fermions can share the same → quantum state. Also known as → exclusion principle.

See also: In honor of Wolfgang Pauli (1900-1958), Austrian theoretical physicist, who formulated the principle in 1925; → exclusion; → principle.

The Peacock. A → constellation in the → southern hemisphere, at about 19h 30m → right ascension, 65° south → declination. Abbreviation: Pav; genitive: Pavonis.

Etymology (EN): From L. pavo “peacock;” paupulo “peacock’s sound;” Gk. taos “peacock;”

Etymology (PE): Tâvus, from Ar., ultimately from Gk. taos “peacock.”

The round edible seed of a widely cultivated plant, Pisum sativum, of the legume family.

Etymology (EN): False singular from M.E. pease (plural pesen), from O.E. pise (West Saxon), piose (Mercian) “pea,” from L.L. pisa, variant of L. pisum “pea,” from Gk. pison “the pea.”

Etymology (PE): Noxod “pea,” from Mid.Pers. naxôd, naxvat “pea.”

1. The normal, non warring condition of a nation, group of nations, or the world.
   

   2. (often initial capital letter) An agreement or treaty between warring or antagonistic nations, groups, etc., to end hostilities and abstain from further fighting or antagonism.
      

   3. A state of mutual harmony between people or groups, especially in personal relations.

Etymology (EN): From M.E. pes, pais, pees, from Anglo-Norman peis, pes, from O.Fr. pais “peace, reconciliation, silence,” from L. pacem (nominative pax) “compact, agreement, treaty of peace, tranquility,” ultimately from PIE root *pag- “to fasten.”

Etymology (PE): Râman, related to râmeš “tranquility, rest,” ârâm “quiet, peaceful,” ârâmidan “to become calm, calmed, rest,” Mid.Pers. râm “peace,” râmenidan “to give peace, pleasure,” râmišn “peace, pleasure;” Av. ram- “to stay, rest;” cf. Skt. ram- “to stop, stand still, rest, become appeased;” Gk. erema “quietly, gently;” Goth. rimis “rest;” Lith. rāmas “rest.”

• suufix -an.

The pointed top of a mountain or ridge. The pointed top of anything. → Gamow peak; → peak wavelength.

Etymology (EN): Perhaps from M.L.G. pék “pick, pike.”

Etymology (PE): Setiq “summit, top,” from *us-tig, from us-, → ex-,

• tig “pointed,” related to tiq “blade,” tiz “sharp,” tež, tej, tij; Mid.Pers. tigr, têz, têž “sharp,” O.Pers. tigra- “pointed,”
  tigra.xauda- “pointed helmet (epithet of Scythians),” Av. tiγra- “pointed,” tiγray- “arrow,” tiži.arštay- “with the pointed spear,” cf. Skt. tikta- “sharp, pungent, bitter,” tejas- “sharpness, edge, point or top of a flame;” PIE base *st(e)ig- “to stick; pointed.” Cognates in other IE languages: Gk. stizein “to prick, puncture,” stigma “mark made by a pointed instrument,” L. in-stigare “to goad,” O.H.G. stehhan, Ger. stechen “to stab, prick,” Du. stecken, O.E. sticca “rod, twig, spoon,” E. stick.

The → bolometric luminosity of a → supernova corresponding to the highest brightness in its → light curve. The peak luminosity occurs after the → supernova explosion; it is directly linked to the amount of radioactive ⁵⁶Ni produced in the explosion and can be used to test various explosion models. Following → Arnett’s rule, one can derive the ⁵⁶Ni mass from the peak luminosity of a → Type Ia supernova.

See also: → peak; → luminosity.

The wavelength at which the radiant intensity of a source is maximum.

See also: → peak; → wavelength.

The pod or the enclosed edible seed of the plant, Arachis hypogaea, of the legume family: the pod is forced underground in growing, where it ripens (Dictionary.com). → box-peanut bulge.

Etymology (EN): From pea, → green pea galaxy + nut O.E. hnutu, akin to L. nux, → nucleus.

A secretion consisting mainly of calcium carbonate, CaCO₃, produced by various mollusks.

Etymology (EN): M.E. perle, from O.Fr. perle, M.L. perla of unknown origin.

Etymology (PE): Morvârid “pearl;” Mid.Pers. murwârid, murgârid; cf. Sogd. marγārit, marrγārt; Khotanese mrāhe. Gk. margarites “pearl” may be a loanword from Iranian.

A partially carbonized vegetable matter, usually mosses, found in bogs and used as fertilizer and fuel.

Etymology (EN): M.E. pete, of unknown origin.

Etymology (PE): Turb, from Fr. tourbe, from Germanic turba; cf. O.Fris. turf, O.H.G. zurba, Ger. Torf, O.E. turf, tyrf “slab of soil and grass,” E. turf.

Geology: A → sedimentary particle that is between 4 and 64 mm in size. Pebbles are larger than → granules but smaller than → cobbles. Pebbles have typically been rounded by → abrasion during sedimentary transport (geology.com/dictionary).

Etymology (EN): From M.E. pibel, from O.E. *papol, *pyppel, *pæbbel, of unknown origin.

Etymology (PE): Rig, from Mid.Pers. rik “pebble, sand.”

A planet formation concept that concerns the → accretion of small objects of negligible gravitational mass (ranging from centimeters up to meters in diameter) onto large, gravitating bodies: → planetesimals, → protoplanets, or → planets. In a more narrow sense, pebble accretion is an accretion process where (gas) drag and gravity play major roles. Simply put, this means that the pebble has to be aerodynamically small and the planet to be gravitationally large (C. W. Ormel, in Formation, Evolution, and Dynamics of Young Solar Systems, p. 197, eds. M. Pessah, O. Gressel, Springer, 2017).

See also: → pebble; → accretion.

In astronomy, designating an object with special properties that deviates from others of its type.

Linguistically related terms: → particular, → special.

Etymology (EN): From L. peculiaris “of one’s own (property),” from peculium “private property,” from pecu “flock, farm animals, cattle,”
pecunia “money, property;” cf. Av. pasu-, fšu- “sheep;” Mid.Pers. pâh, pasvīk “cattle;” Mid./Mod.Pers. šu/ša in šupân/šabân “cattle keeper, shepherd;” Pers. dialects Laki and Tâti pas “sheep;” Skt. paśu- “cattle;” Goth. faihu “money, fortune;” O.E. feoh “cattle, money;” Ger. Vieh “cattle;” Lith. pekus “cattle;” PIE base *peku- “cattle.”

Etymology (PE): Afd “peculiar; strange” (Dehxodâ), from Mid.Pers. afd, awd “peculiar, strange.”

An irregular galaxy that has an abnormal shape (neither elliptical, spiral, nor lenticular) and/or has another unusual characteristic.

See also: → peculiar; → galaxy.

1. The true motion of a star with respect to the Local Standard of Rest. → proper motion.
   

2. The motion of a cosmological object other than the apparent recession caused by the expansion of the Universe.

See also: → peculiar; → motion.

A star with a spectrum that cannot be conveniently fitted into any of the standard → spectral classifications.

See also: → peculiar; → star.

1. Velocity with respect to the Local Standard of Rest.
   

2. Any velocity a galaxy has with respect to us that is not a Hubble law velocity due to the expansion of space.

See also: → peculiar; → velocity.

The quality or condition of being peculiar.

See also: Noun form of → peculiar.

The Winged Horse. A large constellation in the northern hemisphere at 22h 50m right ascension, 20° north declination. The stars → Markab, → Scheat, and → Algenib form three corners of the famous Great → Square of Pegasus, which is completed by the star → Alpheratz from neighboring → Andromeda.
Abbreviation: Peg; Genitive: Pegasi.

Etymology (EN): In Gk. mythology, Pegasus is the winged horse that was fathered by Poseidon with Medusa. When the head of Medusa was cut off by Perseus, the horse sprang forth from her pregnant body. Pegasus aided Perseus in his fight against both the Chimera and the Amazons.

Etymology (PE): Pegâsus, from Gk., as above; asb-e bâldâr “Winged Horse,” referring to the Gk. mythology, from asb→ horse; bâl, → wing; 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”).

To make worse; to cause to deteriorate. To endow (a word) with a less favorable meaning.

Etymology (EN): Back formation from → pejorative.

Etymology (PE): Pastârdan, literally “to render low, vile, bring down” from past “low, vile, abject,” → platykurtic, + ârdan, short for âvardan, “to cause or produce; to bring,” → cause.

1. Depreciation; a lessening in worth, quality, etc.
   

2. Semantic change in a word to a lower, less approved, or less respectable meaning.

See also: Verbal noun of → pejorate.

1. Having negative connotations; tending to disparage or belittle.
   

2. A pejorative word, expression, etc.

Etymology (EN): From Fr. péjoratif, from L.L. peiorat-, p.p. stem of peiorare “make worse,” from L. peior “worse,” related to pessimus “worst,” pessum “downward, to the ground,” from PIE *ped-yos-, comparative of root *ped- “to walk, stumble, impair,” → foot.

Etymology (PE): Pastârandé agent noun from pastârdan, → pejorate.

An → H II region, also known as IC 5067 and IC 5070, about 2,000 → light-years away in the constellation → Cygnus. It is part of a much larger, complex star-forming region also containing the larger and bright → North America Nebula.

See also: So named because of its resemblance to a pelican on long exposure images. M.E. pellican; O.E. pellicane, from L.L. pelecanus, from Gk. pelekan “pelican:” → nebula.

When an electric current is sent through the junction between two different conductors or semiconductors, a quantity of heat is liberated or absorbed, depending on the direction of the current. The heat is proportional to the total electric charge crossing the junction. This effect is due to the existence of an electromotive force at the junction.

See also: Named after Jean-Charles Peltier (1785-1845), French physicist and watchmaker,
who discovered the effect in 1834; → effect.

1a) A slender tube of wood, metal, plastic, etc., containing a core or strip of graphite, a solid coloring material, or the like, used for writing or drawing.

1b) Anything shaped or used like a pencil.

2. A narrow set of lines, light rays, or the like, diverging from or converging to a point (Dictionary.com). → pencil of light.

Etymology (EN): M.E. pencel, from M.Fr. pincel, from L. penicillus “painter’s brush or pencil,” diminutive of peniculus “little tail,” diminutive of penis “tail;” → light.

Etymology (PE): Medâd “pencil,” of unknown origin.
Kelk “quill, pen,” originally “hollow reed.”
Bâriké, from bârik, → narrow, + nuance suffix -é; nur, → light.

A beam of radiant energy in the form of a narrow cone or cylinder.

See also: → pencil; → beam.

A small part of the → Vela supernova remnant with a narrow appearance. The Pencil Nebula measures about 0.75 → light-years across,
is about 5 light-years long, and lies about 800 light-years from Earth. It is moving through the → interstellar medium at about 650 000 kilometres per hour.

See also: → pencil; → nebula.

A small bundle of → rays of light. See also → beam of light.

Etymology (EN): M.E. pencel, from M.Fr. pincel, from L. penicillus “painter’s brush or pencil,” diminutive of peniculus “little tail,” diminutive of penis “tail;” → light.

Etymology (PE): Bâriké, from bârik, → narrow, + nuance suffix -é; nur, → light.

In its simple form, a device consisting of a body suspended from a fixed point on the end of a string to move to and fro by the action of gravity and acquired momentum. The period of oscillation for small amplitudes of swing is determined by the formula T = 2π √(l/g).

Etymology (EN): From Mod.L. pendulum, noun use of neuter of L. pendulus “hanging down,” from pendere “to hang.”

Etymology (PE): Âvang, related to âvixtan, âviz- “to hang” (akin to bixtan, biz- “to shake, to sort out, to sift”); Mid.Pers. âwixtan “to hang” (Sogdian wyc “to move, shake;” Chorasmian wc- “to tremble, shake;” Ossetic wigyn “to shake,” awynzyn “to hang”), from prefixed (â-) Proto-Iranian base *uij-, *uic- “to shake, swing;” cf. Av. vij- “to shake, swing,” vaējant- “swinging;” cf. Skt. vej- “to tremble, wince.”

The time required for the plane of a freely suspended → Foucault pendulum to complete an apparent rotation about the local vertical. It is given by T = 23.9344 / sin φ in hours, where φ represents the latitude of the place. For Paris it is 31h 47m 38s; for the poles it 23.9344 h and for the equator it is ∞ since the plane of pendulum does not turn.

See also: → pendulum; → day.

A hypothetical means of extracting energy from a rotating black hole. If a particle spirals into the ergosphere of a black hole in a direction counter to the rotation of the black hole, and if the particle then breaks up into two fragments inside the ergosphere, one of the fragments can escape with energy greater than the energy of the original particle.

See also: Named after Roger Penrose, English physicist (1931-), who devised the process; → process.

A collapsing object whose radius is less than its Schwarzschild radius must collapse into a singularity.

See also: → Penrose process; → theorem.

A diagram involving → formal compactification of → space-time used in → general relativity to describe the causal properties of the space-time. Only two of the space dimensions are shown and horizontal lines represent space, while vertical lines belong to time. The → null geodesicss are at 45°, which facilitates the visualization of → light cones. The major feature of Penrose-Carter diagram is representing the whole space-time on a finite surface, while putting → spacelike and → timelike infinities at finite distance.

See also: Named for Roger Penrose (1931-) and Brandon Carter (1942-) who introduced it independently; → diagram.

Prefix denoting five, fivefold (e.g. pentacyclic, pentahedron, pentahydrate).

Etymology (EN): From Gk. pent-, penta-, combining forms from pente “five;” cognate with Pers. panj, E. five, as below.

Etymology (PE): Panj, from Mid.Pers. panj, Av. panca; cf.
Skt. pánca; Gk. pente; L. quinque; O.E. fif, from P.Gmc. *fimfe (O.S. fif, O.H.G. funf);
from PIE base *penk^we “five.”

1. The portion of a shadow in which light from an extended source is partially but not completely cut off by an intervening body; the area of partial shadow surrounding the umbra.
   

2. The lighter part of a shadow during an eclipse. An observer standing in the penumbra of a Solar eclipse will see a partial eclipse.
   

3. The lighter zone surrounding the dark area (→ umbra) of a → sunspot. It consists of fine, light filaments which are approximately radial for sunspots with circular outline, but irregular for complex spots.

Etymology (EN): N.L., from L. pæne “almost” + → umbra “shadow.”

Etymology (PE): Nimsâyé, from nim “mid-, half” (Mid.Pers. nêm, nêmag “half;” Av. naēma- “half;” cf. Skt. néma- “half”)

• sâyé, → shadow; →umbra.

Of or relating to a → penumbra.

See also: → penumbra; → -al.

A lunar eclipse that occurs when the Moon passes through the Earth’s → penumbra, but misses the darker umbral shadow. Because the Moon is only partially dimmed, a penumbral eclipse is not impressive. Total penumbral eclipses are rare, and when these occur, that portion of the Moon which is closest to the umbra can appear somewhat darker than the rest of the Moon.

See also: → penumbral; → eclipse.

Any plant of the genus Paeonia, having showy, pink, red, or white, globular flowers. Originally cultivated in the temperate regions of Asia, China, Europe, and the northwest parts of North America.

Etymology (EN): M.E., from M. Fr. peonie, pioiné, from L.L. peonia, from L. pæonia, from Gk. paionia,
perhaps from Paion, physician of the gods.
The plant was recognized to possess many remedial properties, and was thus given an association with the gods.

Etymology (PE): Payoniyâ, loan from Gk, as above.

A peony-shaped nebula around the → Wolf-Rayet star WR 102ka as shown by the → Spitzer Space Telescope image at 24 μm. The formation of this infrared bright nebula is attributed to the recent evolutionary history of WR 102ka during → LBV-type eruptions and/or its strong → stellar wind
(see, e.g., A. Barniske et al. 2008, A&A 486, 971).

See also: → peony; → star.

1. Persons indefinitely or collectively; persons in general.
   

2. Human beings, as distinguished from animals or other beings (Dictionary.com).

Etymology (EN): M.E. peple, from O.Fr. peupel “people, population, crowd; mankind,” from L. populus “a people, nation; a multitude, crowd,” of unknown origin.

Etymology (PE): Mardom “man, human being, mankind, people;” from Mid.Pers. mardôm “human being, man, mankind, people” (from *mard-tohm
“seed of man”), from mard “man, humanity” (cognate with mordan “to die”); O.Pers. martiya-; Av. marəta- “mortal, man,” maša- “mortal;” cf. Skt. márta- “mortal, man;” Gk. emorten “died;” L. mortalis “subject to death.”

1. A prefix meaning “through, thoroughly, utterly, very."
   

2. Chemistry: A prefix used in the names of inorganic chemical compounds containing an element in its highest oxidation state or containing a large or the largest possible proportion of an element.

Etymology (EN): From L. per “through, during, by means of, on account of,” cognate with Pers. par-, pirâ-, as below; cf. Gk. → peri- “around, about, beyond;” O.C.S. pre-; Russ. pere- “through;” Lith. per “through;” Goth. fair-; Ger. ver-; O.E. fer-.

Etymology (PE): Par-, per-, variants of pirâ- “around, about,” from Mid.Pers. pêrâ; O.Pers. pariy “around, about;” Av. pairi “around, over,” per- “to pass over, beyond;”
cf. Skt. pari; Indo-Iranian *pari- “around;” cf. Gk. peri “around, about,
beyond;” L. per “through,” as above; PIE base *per- “through, across, beyond.”

1. To become aware of, know, or identify by means of the senses.
   

2. To recognize, discern, envision, or understand (Dictionary.com).

Etymology (EN): M.E. perceiven, from Anglo-Fr. *perceivre,
from O.Fr. percoivre “perceive, notice, see; understand,” from L. percipere “obtain, seize entirely, take possession of,” from L. → per- “thoroughly” + capere “to take,” → concept.

Etymology (PE): Âgertidan, from â- intensive prefix + gertidan “to take, seize,” → concept.

The mental product of the act of perceiving, as distinguished from the thing perceived.

See also: → perception.

The process of recognizing or identifying something. Usually employed of sense perception, when the thing which we recognize or identify is the object affecting a sense organ.

See also: Verbal noun of → perceive.

Any salt of perchloric acid.

See also: → per-; → chlorate.

A colorless liquid, HClO₄, that reacts explosively with organic matter or other reducible materials.

See also: → perchlorate; → -ic; → acid.

1a) Corresponding to an ideal standard or abstract concept.

1b) Exactly fitting the need in a certain situation or for a certain purpose.

2. To bring to perfection or completion. See also: → perfect cosmological principle, → perfect cube, → perfect gas, → perfect lens, → perfect number, → perfect power, → perfect set, → perfect square.

Etymology (EN): From M.E. parfit, from O.Fr. parfit, from L. perfectus “completed,” p.p. of perficere “accomplish, finish, complete,” from per- “through, thoroughly, utterly, very”

• facere “to make, do, perform;” cognate with Pers. dâdan “to give”
  (O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives;” Skt. dadáti “he gives;” Gk. tithenai “to place, put, set,” didomi “I give;”
  L. dare “to give, offer;” Rus. delat’ “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do;” PIE base *dhe- “to put, to do”).

Etymology (PE): Farsâxt, farsâz, literally “thorougly made,” from far- intensive prefix “much, abundant; elegantly” (Mid.Pers. fra- “forward, before; much; around;”
O.Pers. fra- “forward, forth;” Av. frā, fərā-, fra- “forward, forth; excessive;” cf. Skt. prá- “before; forward, in fron;” Gk. pro “before, in front of;” L. pro “on behalf of, in place of, before, for;” PIE *pro-)

• sâxt, sâz “made,” short for sâxté, sâzidé “made; make, construction, structure; style,” present stem of sâxtan, sâzidan “to build, make, fashion; to adapt, adjust, be fit” (from
  Mid.Pers. sâxtan, sâz-, Manichean Parthian s’c’dn “to prepare, to form;” Av. sak- “to understand, to mark,” sâcaya- (causative) “to teach”).

The → assumption, adopted by the → steady-state theory, that all observers, everywhere at all times, would view the same large-scale distribution of matter in the → Universe in all regions and in every direction.
In contrast to the → cosmological principle, the perfect cosmological principle adds the assumption that the Universe does not change with time on the large scale.

See also: → perfect; → cosmological;
→ principle.

An → integer of the form n³, where n is a → positive number. In other words, a → perfect power when k = 3.

See also: → perfect; → cube.

A hypothetical gas with molecules of negligible size that exert no intermolecular forces. Also called → ideal gas.

See also: → perfect; → gas.

A theoretical, ideal lens capable of producing perfect images. Used as a lens design and analysis tool to image collimated output from an afocal system.

See also: → perfect; → lens.

An → integer that is equal to the → sum of its → positive  → divisors, not including itself. For example 6, because its positive divisors are 1, 2, and 3, and 1 + 2 + 3 = 6. Two other examples are 28 and 496.

See also: → perfect; → number.

A → positive integer that can be expressed as an integer → power of another positive → integer. For example, n is a perfect power if there exist natural numbers m> 1, and k> 1 such that m^k = n. → perfect cube, → perfect square.

See also: → perfect; → power.

A set P is called perfect if P = P’, where P’ is the derived set of P.

See also: → perfect; → set.

An → integer of the form n², where n is a → positive number. In other words, a
→ perfect power when k = 2.

See also: → perfect; → square.

Capable of becoming or of being made perfect.

See also: → perfect; → -able.

The state or quality of being or becoming perfect.

See also: → perfect; → -tion.

1. Any of various doctrines holding that religious, moral, social, or political perfection is attainable.
   

2. A personal standard, attitude, or philosophy that demands perfection and rejects anything less (Dictionary.com).

See also: → perfection; → -ism.

1. A person who adheres to or believes in → perfectionism.
   

2. A person who demands perfection of himself, herself, or others (Dictionary.com).

See also: → perfection; → -ist.

1. To carry out; → execute; do.
   

2. To go through or execute in the proper, customary, or established manner (Dictionary.com).

Etymology (EN): M.E. parformen, from Anglo-Fr. performer, from O.Fr. parfornir “to do, carry out, finish, accomplish,” from par- “completely,” → per-,

• fornir “to provide.”

Etymology (PE): Pergâlidan, from Kurd. (Sanandaj) pergâl “work, doing; order, command,” ultimately from Proto-Ir. *parikar-, from *pari- “through, throughout; thoroughly” (O.Pers. pariy “around, about;” Av. pairi “around, over”)

• *kar- “to do;” Pers. kar-, variants kâr, gar, gâr, → work.

1. The act of performing a ceremony, play, piece of music, etc.
   

2. The execution or accomplishment of work, acts, feats, etc.
   

3. A musical, dramatic, or other entertainment presented before an audience (Dictionary.com).

See also: → perform; → -ance.

1. Relating to or of → performance, especially of dramatic or artistic performance.
   

2. A → performative utterance.

See also: → perform; → -ive.

A sentence or expression which is not only describing a given reality, but actually does or accomplishes something. For example “I now declare you husband and wife” (when uttered by the authorized officiator during a marriage ceremony).

See also: → performative; → utterance.

1. Someone who performs in front of an audience, for example an actor or musician.
   

2. Someone who does something with a particular amount of success (Macmillan Dictionary).

See also: → perform; → -er.

A prefix meaning “about, around” (perimeter, periscope), “enclosing, surrounding” (pericardium), and “near” (perigee, perihelion).

Etymology (EN): From Gk. peri “around, about, beyond;” cognate with Pers. pirâ-, as below.

Etymology (PE): Pirâ-, variant par- “around, about,” from Mid.Pers. pêrâ; O.Pers. pariy “around, about,” Av. pairi “around, over,” per- “to pass over, beyond;”
cf. Skt. pari; Indo-Iranian *pari- “around;” PIE base *per- “through, across, beyond;” cf. Gk. peri “around, about,
beyond;” L. per “through.”

The point in an orbit of a body where it is closest to its primary; opposite of → apoapsis.

See also: → peri- + → apsis.

In a binary system, the point in the orbit where a component is nearest the other component; opposite of → apastron.

See also: → peri- + astron; → astro-.

The point in the orbit of one component of a binary system which is closest to the center of mass of the system; opposite of → apocenter.

See also: → peri- + → center.

The point in the orbit of a satellite around the Moon closest to the Moon; opposite of → apocynthion.

Etymology (EN): → peri- + Gk. Cynthia “goddess of the Moon;”
→ Cynthian.

Etymology (PE): Pirâmâh, from pirâ-, → peri-, + mâh→ moon.

Of or pertaining to a → perigalacticon.

See also: → peri-; → galactic.

The point in an object’s orbit around a galaxy when the object lies closest to the galactic center; opposite of → apogalacticon.

See also: → peri- + → galaxy.

The point in the orbit of a body revolving around the Earth at which it is nearest to the Earth; opposite of → apogee.

Etymology (EN): From Fr. périgée, from L. perigæum, from Gk. perigaion “near of the earth,”
from → per- “near” + gaia/ge “earth.”

Etymology (PE): Pirâzam, from pirâ-, → peri-, + Av. zam- “the earth,” Mid.Pers. zamig, Mod.Pers. zami, zamin “the earth;” cf. Skt. ksam, Gk. khthôn, khamai “on the ground,” L. homo “earthly being” and humus “the earth” (as in homo sapiens or homicide, humble, humus, exhume);
PIE root *dh(e)ghom “earth”.

A solar or lunar eclipse that takes place when the Moon is at the → perigee of its orbit. The maximum duration of a solar perigee eclipse is 5h 14m (between first and the fourth contact). The maximum duration of a lunar perigee eclipse, between the two exterior contacts of the Moon with the penumbra, is 5h 16m, the maximum totality being 1h 40m (M.S.: DSE).

See also: → perigee; → eclipse.

The → full Moon when our natural satellite is at its closest approach to the Earth. Perigee full Moons are as much as 14% larger and 30% brighter than → apogee full Moons. Also called perigee-syzygy full Moon, super full Moon, and → supermoon.

The Supermoon on November 14, 2016, was the closest (356,523 km) a Full Moon has been to Earth since January 26, 1948. The next time a Full Moon is even closer to Earth (356,448 km) will be on November 25, 2034.

See also: → perigee; → full; → moon.

→ perigee full Moon.

See also: → perigee-; → syzygy; → full; → moon.

The nearest point to the Sun in an orbit around the Sun; opposite of → aphelion.

Etymology (EN): Perihelion, from L. perihelium, from → peri- + helios “sun,” cognate with L. sol, Skt. surya, Av. hvar-, Mod.Pers. xor, hur, O.H.G. sunna, Ger. Sonne, E. sun; PIE *sawel- “sun”.

Etymology (PE): Pirâhur, from pirâ-, → peri-, + hur “sun,” as above;

→ advance of perihelion.

See also: → advance of perihelion.

The distance between the → Sun and an → object in orbit around it when they are at their closest approach.

See also: → perihelion; → distance.

→ advance of perihelion.

See also: → perihelion; → precession.

The → point in the → orbit of a → satellite of → Jupiter that is nearest to Jupiter’s center. → apojove.

See also: → peri- + jove, from L. Jovius “Jupiter,” → Jovian.

1. Exposure to injury, loss, or destruction; grave risk; jeopardy; danger.
   

2. Something that causes or may cause injury, loss, or destruction (Dictionary.com).

Etymology (EN): M.E., from O.Fr. peril “danger, risk,” from L. periculum “an attempt, trial; risk, danger,” related to experiri “to try,” cognate with Gk. peria “trial, attempt,” empeiros “experienced;” O.Ir. aire “vigilance;” Goth. ferja “watcher;” O.E. fær “danger; fear.”

Etymology (PE): Sij “trouble;” Mid.Pers. sêj “danger, trouble,” sêjômand “dangerous;” Av. iθyejah- “trouble, danger,” iθyejahvant- “full of danger, hazardous;” cf. Skt. tyajas- “difficulty, danger.”

The point in a lunar orbit that is nearest to the moon. Same as → pericynthion.

Etymology (EN): From → peri- + lune, from L. luna “moon;” → lunar.

Etymology (PE): Pirâmâh, → apocynthion.

The total outer boundary of a two-dimensional figure.

See also: → peri- + → -meter.

Physics: The duration of one complete cycle of an oscillation;
the reciprocal of the frequency.
Astro.: The amount of time an astronomical body takes to complete an orbit.

Etymology (EN): From M.E. periode, from M.Fr., from M.L. periodus “recurring portion, cycle,” from L. periodus “a complete sentence,” from Gk. periodos “rounded sentence, cycle, circuit, period of time,” literally “going around,” from → peri- “around” + hodos “way, journey;” cognate with L. cedere “to go, yield.”

Etymology (PE): Dowré, from dowr, from Ar. daur “age, time; revolution.”

The rate at which the rotation period of a → pulsar changes over time. This quantity, dP/dT, can range from as small as 0.05 picoseconds per year (1.5 x 10^-21 seconds per second) to as large as about 10 milliseconds per year (4.2 x 10^-10 seconds per second). For the → Crab pulsar, the period derivative is 4.2 x 10^-13 s s^-1, implying a decrease in the star’s → rotation energy of about 4.5 x 10³⁸ erg s^-1. Period derivative is a very important parameter for the determination of the pulsar age.

See also: → period; → derivative.

A → correlation between the periods and luminosities of → Cepheid variable stars: Cepheids with longer periods are intrinsically more luminous than those with shorter periods. The relation was discovered by Henrietta Leavitt in 1912 when studying Cepheids in the → Small Magellanic Cloud.

Once the period of a Cepheid variable is determined from observations, the period-luminosity relation can be used to derive its luminosity. Since
luminosity is a function of → distance, the distance can then be calculated with the luminosity. The period-luminosity relation is an invaluable tool for the measurements of distances out to the nearest galaxies and thus for studying the structure of our own Galaxy and of the Universe.

See also: → period; → luminosity; → relation.

A relation that gives a rough estimate of the oscillation period of a → pulsating star as a function of its mean density. This relation is obtained by considering how long it would take a sound wave to travel across the diameter of a model star: Π ≅ (3π/2γGρ)^1/2, where ρ is the mean density, γ the ratio of → specific heats (C_p/C_v), and G the → gravitational constant. This relation shows that the pulsation period of a star is inversely proportional to the square root of its mean density. And this is the reason why the pulsation periods decrease along the → instability strip from the luminous, very tenuous → supergiants to the faint, very dense → white dwarfs.

See also: → period; → mean; → density; → relation.

Recurring at regular intervals of time.

See also: Adjective of → period.

A comet with a period of less than 200 years. Also called short-period comet.

See also: → periodic; → comet.

A function f(x) if for all x, f(x + P) = f(x), where P is a positive constant. The least value of P > 0 is called the period of f(x).

See also: → periodic; → function.

Any motion that recurs in identical forms at equal intervals of time.

See also: → periodic; → motion.

Arrangement of the → chemical elements in the → periodic table.

See also: → periodic; → system.

An arrangement of the → chemical elements in order of their → atomic numbers in such a way as to demonstrate periodic similarities and trends in physical and chemical properties. Elements with similar properties are arranged in the same column (called a group), and elements with the same number of → valence electrons, or number of electrons in the outer shell, are arranged in the same row (called a period). Under the latest recommendations from IUPAC (the International Union of Pure and Applied Chemistry), the groups are labelled 1 to 18 from left to right (1988, Pure and Applied Chemistry 60, 431). Also called Mendeleev’s table.

The periodic table was introduced by Dmitri Ivanovich Mendeleev (1834-1907) in 1869, who originally arranged them in order of their → atomic weights. Using the table, it was possible for Mendeleev to correct some of the atomic weights
(e.g. that of beryllium) and to predict the properties of a number of elements yet to be discovered (e.g. gallium, scandium, and germanium). The British physicist Frederick Soddy (1877-1956) showed that the loss of an → alpha particle reduces the nuclear charge by two and hence lowers the atomic number by two and the position of the element in the periodic table by two groups.

See also: → periodic; → table.

In perturbation theory used in celestial mechanics, a term that indicates a bounded disturbance which recurs regularly. → secular term.

See also: → periodic; → term.

An oscillatory motion in which each point is repeatedly displaced at equal time intervals.

See also: → periodic + → wave.

Same as → periodic.

See also: → periodic; → -al.

A variable → supergiant star with typical periods of the order of 10 to 100 days and amplitudes less than a few tenths of a magnitude. PVSGs are thought to be pulsating → g modes, caused by a density inversion, arising from an → opacity bump, most likely from Fe, H, and/or He.

See also: → periodical; → -ly; → variable; → supergiant.

A state or condition characterized by regular repetition in time or space.

See also: → periodic + → -ity.

A plot for examining frequency-domain data in an equi-spaced → time series. The periodogram is the → Fourier transform of the → autocovariance function. The periodogram method relies on the definition of the → power spectral density .

See also: → period; + euphonic infix -o-; → -gram.

Pertaining to, situated in, or constituting the periphery.

See also: Adj. of → periphery.

In a charge-coupled device, the detection of charge collected by the transport register rather than by the image-sensing elements.

See also: → peripheral; → response.

In optics, the ability to see over large angles of view.

See also: → peripheral; → vision.

The external surface or boundary of a body. The circumference or perimeter of any closed figure.

Etymology (EN): From, M.E., from O.Fr. periferie, from L.L. peripheria, from Gk. peripheria “circumference, outer surface,” literally “a carrying around,” from peripheres “rounded, moving round,” peripherein “to carry or move round,” from → peri- “round about”

• pherein “to carry;” cognate with Pers. bordan “to carry, lead,” as below.

Etymology (PE): Pirâbar, from pirâ-, → peri-, + bar present stem of bordan “to carry, lead” (Mid.Pers. burdan,
O.Pers./Av. bar- “to bear, carry,” barəθre “to bear (infinitive),” Skt. bharati “he carries,” Gk. pherein, L. fero “to carry;” PIE base *bher- “to carry”).

An optical instrument for viewing objects which are above the eye-level of the observer, or are placed so that direct vision is blocked.

See also: → peri-; + → -scope.

Layer of soil or rock, at some depth beneath the surface, in which the temperature has been continuously below 0°C from a few to several thousands of years. It exists where summer heating fails to reach the base of the layer of frozen ground.

Etymology (EN): From perma(nent) + → frost.

Etymology (PE): Mândbašm, from mând, → permanent,

• bašm, → frost.

Lasting or remaining without essential change.

Etymology (EN): Permanent, from M.Fr. permanent, from L. permanentem “remaining,” pr.p. of permanere “endure, continue, stay to the end,” from per- “through” + manere “stay,” cognate with Pers. mândan, as below; → gas.

Gas which cannot be liquefied by pressure alone; gas above its critical temperature.

See also: → permanent; → gas.

A piece of magnetic material which, having been → magnetized, retains a substantial proportion of its → magnetization indefinitely.

In permanent magnets the magnetic field is generated by the internal structure of the material itself. Atoms and crystals constituting materials are made up of electrons and atomic nuclei. Both the nucleus and the electrons themselves act like little magnets. There is also a magnetic field generated by the orbits of the electrons as they move about the nucleus. So the magnetic fields of permanent magnets are the sums of the nuclear spins, the electron spins and the orbits of the electrons themselves. In many materials, the magnetic fields are pointing in all sorts of random directions and cancel each other out and there is no permanent magnetism. But in certain materials, called → ferromagnets, all the spins and the orbits of the electrons will line up, causing the materials to become magnetic.

Many permanent magnets are created by exposing the magnetic material to a very strong external magnetic field. Once the external magnetic field is removed, the treated magnetic material is now converted into a permanent magnet.

Overheating a permanent magnet causes the magnet’s atoms to vibrate violently and disrupt the alignment of the atomic domains and their dipoles. Once cooled, the domains will not realign as before on their own and will structurally become a temporary magnet (MagLab Dictionary).

See also: → permanent; → magnet.

Storage capacity which does not depend on a continuous supply of power, e.g. disks, magnetic tapes, etc.

See also: → permanent; → memory.

The degree of → magnetization of a material that responds linearly to an applied magnetic field. → magnetic permeability.

Etymology (EN): From permeable, from L. permeabilis “that can be passed through, passable,” from L. permeare “to pass through,” from per- “through”

• meare “to pass,” from PIE base *mei- “to change; to go, move.”

Etymology (PE): Tarâvâyi quality noun of tarâvâ “permeable,” from tarâvidan “to exude, trickle, ooze; to drop,” probably from Proto-Iranian *tra-vaxš-. The first component *tra- “across, over, beyond,” → trans-. The second component *vaxš-, cf. Av. uxš-/vaxš- “to sprinkle,” present tense stem uxš-; cf. Skt. uks- “to sprinkle, moisten,” uksati “spinkles, wets;” Gk. hygros “wet, moist, fluid;” L. uvidus “watery, humid, damp.” Tarâvidan may be a back formation from *tarâvaš.

The act of permitting. Authorization granted to do something.

See also: verbal noun of → permit; → -tion.

To allow to be done or occur.

Etymology (EN): From M.Fr. permetre, from L. permittere “give up, allow, allow to pass through,” from per- “through;” (from PIE base *per- “through, across, beyond;” cf. Gk. peri “around, about, beyond;” O.Pers. pariy “around, about,” Av. pairi “around, over;” Skt. pari; Indo-Iranian *pari- “around;” Mod.Pers. par-, pirâ- “around, about”) + mittere “let go, send.”

Etymology (PE): Parzâmidan, literally “to allow to go through,” infinitive of parzâm, from par- “through,” from O.Pers. pariy “around, about,” Av. pairi “around, over,” cognet with L. per-, as above

• zâm stem of Mid.Pers. zâmenidan “to let go, lead, send,”
  Mod.Pers. gâm “step, pace,” âmadan “to come;”
  O.Pers./Av. gam- “to come; to go,” Av. jamaiti “goes;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step,” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come,” E. come; PIE base *gwem- “to go, come.”

Allowed. → permitted line; → permitted transition.

See also: Past participle of → permit.

An ordinary spectral line emitted by atoms undergoing energy transitions that are allowed by the selection rules of quantum mechanics. → forbidden lines.

See also: → permitted; → line.

A transition between two quantum mechanical states that does not violate the quantum mechanical selection rules.

See also: → permitted; → transition.

A measure of the ability of a material to transmit (or “permit”) an electric field. Permittivity is defined as the ratio of the flux density produced by an electric field in a given dielectric to the flux density produced by that field in a vacuum. In → SI units, permittivity is measured in → farads per meter. The constant ε₀ is known as the permittivity of free space; its value is about 8.854 x 10^-12 F/m.

See also: State or quality noun from → permit.

Math.: A rearrangement of the elements of a set in a particular order. The number of permutations of n objects is equal to n! (→ factorial n). For example, there are 24 permutations of letters A, B, C, and D (4! = 1 × 2 × 3 × 4). The number of permutations of n objects taken r at a time is denoted by ⁿP_r and equals n! / (n - r)!. For example, the number of permutation of A, B, C, and D taken two at a time is 12. If n objects are of k different kinds, with r₁ alike of one kind, permutations of n objects equals n! / r₁! r₂! … r_k!, where r₁ + r₂ + … r_k = n.

Etymology (EN): Verbal noun of → permute.

Etymology (PE): Permuteš, verbal noun of → permute.
Jâygašt , from jâ “place”
(from Mid.Pers. giyag “place;” O.Pers. ā-vahana- “place, village;” Av. vah- “to dwell, stay,” vanhaiti “he dwells, stays;” Skt. vásati “he dwells;” Gk. aesa (nukta) “to pass (the night);”
Ossetic wat “room; bed; place;” Tokharian B wäs- “to stay, wait;” PIE base ues- “to stay, live, spend the night”) + gašt past tense of gaštan, gardidan “to turn, to change” (Mid.Pers. vartitan; Av. varət- “to turn, revolve;” Skt. vrt- “to turn, roll,” vartate “it turns round, rolls;” L. vertere “to turn;” O.H.G. werden “to become;” PIE base *wer- “to turn, bend”).

Etymology (EN): M.E., from L. permutare “to change throughout,” from per- “through”

• mutare “to change,” from PIE base *mei- “to change, go, move;” cf. Av. miθô “inverted, false,” miθaoxta-
  “wrong spoken;” Skt. methati “changes, alternates,
  joins, meets,” mith- “to alternate, meet,” mithás “opposite, in opposition;” L. meare
  “to go, pass,” mutuus “done in exchange;” Goth.
  maidjan “to change;” E. prefix mis- (in mistake).

Etymology (PE): Permutidan, from permute, as above.

A line or plane at right angles to another line or plane. Two curves are said to be perpendicular if their tangent lines are mutually perpendicular. → normal; → vertical

Etymology (EN): From M.E. perpendiculer(e), from
O.Fr. perpendiculiere, from L. perpendicularis “vertical, as a plumb line,” from perpendiculum “plumb line,” from perpendere “balance carefully,” from per- “thoroughly” + pendere “to weigh, to hang.”

Etymology (PE): Pâlâr “pillar, column, main beam.”

The → moment of inertia of a plane object (→ lamina) about an axis perpendicular to the plane is equal to the sum of the moments of inertia about any two perpendicular axes in the plane. Thus if x and y axes are in the plane, I_z = I_x + I_y.

See also: → perpendicular; → axis; → theorem.

Lasting an indefinitely long time; eternal; permanent. → perpetual motion; → perpetual calendar.

Etymology (EN): M.E. perpetuall, from O.Fr. perpetuel, from L. perpetualis “permanent,” from perpetuus “continuous, universal,” from perpetis, genitive of Old L. perpes “lasting;”

Etymology (PE): Hamišegi from hamišé “always;” Mid.Pers. hamišag, from anôšag “immortal,” from Av. an-aoša-, from negation prefix → an- + aošah- “death; ruin; corruption” (Mid.Pers. hôš “death;” Mod.Pers. hôš, huš “death; mind; intellect”); cf. derivatives Sogd. nôšé “immortal,” nôšak “always;” Mod.Pers. nôš, nuš “the water of immortality; sweet; honey.”

A chart or mechanical device that indicates the day of the week for
any given date over a period of many years.

See also: → perpetual; → calendar.

The motion of a hypothetical machine which, once set in motion, will go on for ever without any losses due to → friction or other forms of → dissipation of energy and without receiving any external energy.

See also: → perpetual; → motion.

A → meteor shower, one of the three most active of the year, which occurs between July 25 and August 20 with the greatest activity between August 8 and 14, peaking about August 12. The Perseids appear as the Earth’s orbit around the → Sun crosses the dusty tail of the comet → Swift-Tuttle, as first explained by Giovanni Schiaparelli (1835-1910), an Italian astronomer.

Usually about 50 meteors can be seen per hour, but in outburst years (such as in 2016) the rate can be between 150-200 meteors an hour.

See also: → Perseus + -ids.

The Hero. A rich constellation in the northern hemisphere between → Auriga and → Cassiopeia, at 3h 20m right ascension, 45° north declination. Beta (β) Persei, or → Algol, is a famous → eclipsing binary star. Abbreviation: Per; genitive: Persei.

Etymology (EN): The most celebrated of the Greek heroes, the son of Zeus and Danae, who slew the Gorgon Medusa, and afterward saved Andromeda from a sea monster.

Etymology (PE): Perseus, from Gk., as above, instead of the Arabicized form Barsâvuš (برساووش).

A → radio source in the constellation → Perseus, identified with the supergiant elliptical galaxy → NGC 1275.

See also: → Persus.

One of the spiral arms of the Galaxy. It is about 5000 light-years farther from the center than the local Orion Arm, in which the Sun lies.

See also: → Persus; → arm.

A → galaxy cluster of about 12,000 members about 250 million → light-years (→ redshift z = 0.0176) away, covering 4° of sky in the constellation → Perseus. It is dominated by elliptical galaxies. At its center lies the → radio source→ Perseus A. Also known as Abell 426 (→ Abell catalog).

See also: → Perseus; → cluster.

A long, dense chain of galaxies with a length of almost 300 million → light-years, constituting one of the largest known structures in the → Universe. At the left end of the supercluster lies the massive → Perseus cluster (A426), one of the most massive clusters of galaxies within 500 million light-years.

See also: → Perseus; → Pisces; → cluster.

Same as → Iranian calendar.

Etymology (EN): Persian, adj. of Persia, from O.Pers. Pārsa.

Etymology (PE): Irâni adj. of Irân, from Mid.Pers. Ãrân “(land of) the Aryans,” pluriel of êr “noble, hero,” êrîh “nobility, good conduct;” Parthian Mid.Pers. aryân; O.Pers. ariya- “Aryan;” Av. airya- “Aryan;” cf. Skt. ārya- “noble, honorable, respectable.”

1. To continue steadfastly or firmly in some state, purpose, course of action, or the like, especially in spite of opposition, remonstrance, etc.
   

2. To last or endure tenaciously (Dictionary.com).

Etymology (EN): M.Fr. persister, from L. persistere “abide, continue steadfastly,” from → per- “thoroughly” + sistere “come to stand, cause to stand still,” → resist.

Etymology (PE): Paristâdan from par-, → per-,

• istâdan “to stand,” → resist.

The act or fact of persisting; the quality of being persistent.

See also: Verbal noun from → persist.

Persisting, especially in spite of opposition, obstacles, discouragement, etc.; persevering (Dictionary.com).

See also: Adjective from → persist.

1. A human being, whether man, woman, or child as distinguished from an animal or a thing.
   

2. Philosophy: A self-conscious or rational being.
   

3. Grammar: Any one of three forms of verbs and pronouns used to denote the speaker, the person addressed, or somebody else being referred to.
   

4. Law: A living human being or a group, either or both having legal rights and responsibilities.
   

See also:
→ personal, → personal equation, → personality.

Etymology (EN): M.E. persone, from O.Fr. persone “human being,” from L. persona “human being,” originally “character in a drama, mask,” possibly borrowed from Etruscan phersu “mask,” from Gk. prosopa
“face; mask” + -na a suffix.

Etymology (PE): Tanum, from O.Pers. and Av., related to Mod./Mid.Pers. tan “body, person,” O.Pers. tanūš “body,” tanūm [acc.sg.] “(to) onself;” Av. tanū- “body, person, self,” tanūm [acc.sg.]; cf. Skt. tanūš- “body, self;” PIE base *ten-uh- “body.”

1. A person of distinction or importance.
   

2. A dramatic, historical, or fictional → character in a play, story, etc.

Etymology (EN): M.E. “form, appearance, stature, figure, air, and the like, of a person,” from O.Fr. personage “size, stature; a dignitary,” from M.L. personagium, from persona, → person.

Etymology (PE): Tanumsâ, from tanum, → person, + -sâ, contraction of -âsâ, suffix of “form, type, similarity.”

Of or pertaining to a particular person; individual; private. → personal equation.

See also: → person; → -al.

A systematic observational error due to the characteristics of the observer.

See also: Personal, adj. of → person; → equation.

The sum total of the physical, mental, emotional, and social characteristics of an individual (Dictionary.com).

See also: → personal; → -ity.

1. To make → personal or → individual; specifically, to mark as the property of a particular → person.
   

2. Same as → personify.

Etymology (EN): → personal; → -ize.

Etymology (PE): Tanumidan, from tanum, → person, + -idan, → -ize; tanumârdan, → personify.