An Etymological Dictionary of Astronomy and Astrophysics

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.

1. The attribution of human nature or character to animals, inanimate objects, or abstract notions, especially as a rhetorical figure.
   

2. The representation of a thing or abstraction in the form of a person, as in art.
   

3. The person or thing embodying a quality or the like; an embodiment or incarnation (Dictionary.com).

See also: Verbal noun of → personify.

1. To conceive of or represent as a person or as having human qualities or powers.
   

2. To be the embodiment or personification of (Merriam-Webster.com).

See also: → person + -ify, → -fy.

A body of persons employed in an organization or place of work (Dictionary.com).

Etymology (EN): From Fr. personnel (as contrasted with matériel), from
O.Fr. personel, → personal.

Etymology (PE): Tanumgân, from tanum, → personal,

• -gân multiplicity suffix.

1. The technique or art of drawing three-dimensional objects on a flat surface so that to give the right impression of their relative sizes and distances. A drawing so made.
   

2. Of, or pertaining to the art of perspective.

Etymology (EN): From M.Fr. perspective, from M.L. (ars) perspectiva “science of optics,” from feminine of perspectivus “of sight, optical” from L. perspectus, p.p. of perspicere “to inspect, look through,” from → per- “through” + specere “to look at,” → prospect.

Etymology (PE): Pargâsmandi, from pargâsmand, from pargâs, from par-, → per-, + gâs, “to look at,” → prospect,

• -mand possession suffix, from Mid.Pers. -omand; O.Pers./Av. -mant; cf. Skt. -mant or -ik, → -ic.

To cause a small → deviation in the → behavior of a → physical system, e.g. in the → orbit of a planet.

Etymology (EN): O.Fr. perturber, from L. perturbare “to confuse, disorder, disturb,” from per- “through” + turbare “disturb, confuse,” from turba “turmoil, crowd,” turbidus “muddy, full of confusion.”

Etymology (PE): Parturidan, from par-, related to pirâ- (cf. Av. per- “to pass across, through”) + turidan “to run away, be very much ashamed,” tur “withdrawal, flight;” Lori, Laki tur
“restive, disobedient,” Laki turyâyen “to get angry, lose one’s temper,”
probably cognate with L. turba, as above.

1. Any departure introduced into a steady state of a system.
   The magnitude is often assumed to be small so that the resulting terms in the dependent variables may be neglected. The term “perturbation” is therefore sometimes used as synonymous with “small perturbation.”
   

2. Gravitational effect of a third body that causes an alteration in the orbit of a body going around its primary.
   

See also:

→ linear perturbation theory, → method of small perturbationsn → perturbation equation, → perturbation method, → primordial curvature perturbation, → scalar perturbation, → secular perturbation, → tensor perturbation, → vector perturbation.

See also: Verbal noun of → perturb.

Any equation governing the behavior of a → perturbation.

See also: → perturbation; → equation.

Approximate method of solving a difficult problem if the equations to be solved depart only slightly from those of a problem already solved.

See also: → perturbation; → method.

A celestial body that causes a perturbation in the orbit of another body.

See also: Agent noun of → perturb; → body.

A prefix denoting 10¹⁵.

See also: Of unknown origin.

A combining form meaning “rock,” “stone.” Also, petri-, and petr- when before a vowel.

Etymology (EN): From Gk. petro-, combining form of petra “rock.”

Etymology (PE): Sang “rock,” → stone.

Natural mixture of liquid hydrocarbons and other organic compounds that include crude oil, refined products obtained from the processing of crude oil, and natural gas liquids.

Etymology (EN): M.L. petroleum literally “rock oil,” from L. petra “rock,” from Gk. → petro-, + oleum “oil.”

Etymology (PE): Naft, from Mid.Pers. npt “moist, damp;
naphtha,” nmb “moisture,” from which derives Mod.Pers. nam “humidity, moisture;” Av. napta- “moist,” nabah- “cloud; sky;”
cf. Skt. nábhas- “moisture, cloud, mist;” PIE base *nebh- “cloud, vapor, fog, moist, sky” (Gk. nephos “cloud, mass of clouds,” nephele “cloud;” L. nebula “mist,” nimbus “rainstorm, rain cloud;” O.H.G. nebul; Ger. Nebel “fog;” O.E. nifol “dark”). The link between “water, moisture” and “naphta” is suggested to be
the natural gas or oil seepages surfacing through water. In fact many of the Zoroastrian fire-temples were located in areas which contained large petroleum leakages, such as those in Khuzestân
and at Surakhany near Baku.

The branch of → geology that deals with → rocks: their classification, composition, structure, occurrence, and conditions of origin.

See also: → petro-; → -logy

An astrophysical source which accelerates → cosmic rays up to energies of several petaelectronvolts. For example, in the → Galactic center, cosmic ray → protons reach such energies. The source of such particles is a matter of research (→ HESS collaboration, 2016, Nature 531, 476).

See also: PeVatron, from PeV (→ peta-

• → electronvolt); + euphonic affix -a-; + → -tron.