An Etymological Dictionary of Astronomy and Astrophysics

An area, position, or portion of space. → mean place

Etymology (EN): O.E. from O.Fr. place, from M.L. placea “place, spot,” from L. platea “courtyard, open space, broad street,” from Gk. plateia (hodos) “broad (way),” feminine of platus “broad;” cognate with Av. pərəθu- “broad;” Skt. prthú- “broad, wide;” Lith. platus “broad;” Ger. Fladen “flat cake;” O.Ir. lethan “broad;” PIE base *plat- “to spread.”

Etymology (PE): 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”).

An area, position, or portion of space. → mean place

Etymology (EN): O.E. from O.Fr. place, from M.L. placea “place, spot,” from L. platea “courtyard, open space, broad street,” from Gk. plateia (hodos) “broad (way),” feminine of platus “broad;” cognate with Av. pərəθu- “broad;” Skt. prthú- “broad, wide;” Lith. platus “broad;” Ger. Fladen “flat cake;” O.Ir. lethan “broad;” PIE base *plat- “to spread.”

Etymology (PE): 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”).

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

See also: → place; → value; → notation.

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

See also: → place; → value; → notation.

A bright cloud-like feature that appears in the vicinity of a sunspot. Plages represent regions of higher temperature and density within the chromosphere. They are particularly visible when photographed through filters passing the spectral light of hydrogen or calcium.

Etymology (EN): From Fr., from It. piaggia, from L.L. plagia “shore;”
noun use of the feminine of plagius “horizontal;” frpm Gk. plagios “slanting, sideways” from plag(os) “side” + -ios adj. suffix.

Etymology (PE): Plâž, loan from Fr., as above.

A bright cloud-like feature that appears in the vicinity of a sunspot. Plages represent regions of higher temperature and density within the chromosphere. They are particularly visible when photographed through filters passing the spectral light of hydrogen or calcium.

Etymology (EN): From Fr., from It. piaggia, from L.L. plagia “shore;”
noun use of the feminine of plagius “horizontal;” frpm Gk. plagios “slanting, sideways” from plag(os) “side” + -ios adj. suffix.

Etymology (PE): Plâž, loan from Fr., as above.

Any of the → feldspar minerals consisting of a mixture of → sodium and → calcium  → aluminium  → silicates in triclinic crystalline form. Members of the plagioclase group are the most common rock-forming minerals. They are important or dominant minerals in most → igneous rocks of the → Earth’s crust.

See also: From Ger. Plagioclase, from Gk. plagio- a combining form meaning “oblique,” + clase a suffix used in the formation of compound words that denote minerals with a particular cleavage, as specified by the initial element.

Any of the → feldspar minerals consisting of a mixture of → sodium and → calcium  → aluminium  → silicates in triclinic crystalline form. Members of the plagioclase group are the most common rock-forming minerals. They are important or dominant minerals in most → igneous rocks of the → Earth’s crust.

See also: From Ger. Plagioclase, from Gk. plagio- a combining form meaning “oblique,” + clase a suffix used in the formation of compound words that denote minerals with a particular cleavage, as specified by the initial element.

An extent of flat land not noticeably diversified with mountains, hills, or valleys.

Etymology (EN): M.E. from O.Fr. plain, from L. planum “level ground, plain.”

Etymology (PE): Dašt, from Mid.Pers. dašt “plain, open ground.”

An extent of flat land not noticeably diversified with mountains, hills, or valleys.

Etymology (EN): M.E. from O.Fr. plain, from L. planum “level ground, plain.”

Etymology (PE): Dašt, from Mid.Pers. dašt “plain, open ground.”

1a) A scheme or method of acting, doing, proceeding, making, etc., developed in advance.

1b) A representation of a thing drawn on a plane, as a map or diagram.

2a) To arrange a method or scheme beforehand for.

2b) To make plans for (Dictionary.com).

Etymology (EN): M.E., from Fr. plan “ground plan, map,” literally “plane surface,” from L. planum “level or flat surface,” noun use of adjective planus “level, flat” (from PIE root *pele- “flat; to spread;”

Gk. plassein “to mold,” plasma “something molded or created;” L. planus “flat, level, even, plain, clear;” Lith. plonas “thin;” O.C.S. polje “flat land, field,” Russ. polyi “open;” O.E. feld, M.Du. veld “field.”

Etymology (PE): Pišgâr, literally “beforehand written, painted in advance,” from piš- “before, in front,” → pre-, + gâr present stem of negâridan, negâštan “to paint, write,” → graph.

1a) A scheme or method of acting, doing, proceeding, making, etc., developed in advance.

1b) A representation of a thing drawn on a plane, as a map or diagram.

2a) To arrange a method or scheme beforehand for.

2b) To make plans for (Dictionary.com).

Etymology (EN): M.E., from Fr. plan “ground plan, map,” literally “plane surface,” from L. planum “level or flat surface,” noun use of adjective planus “level, flat” (from PIE root *pele- “flat; to spread;”

Gk. plassein “to mold,” plasma “something molded or created;” L. planus “flat, level, even, plain, clear;” Lith. plonas “thin;” O.C.S. polje “flat land, field,” Russ. polyi “open;” O.E. feld, M.Du. veld “field.”

Etymology (PE): Pišgâr, literally “beforehand written, painted in advance,” from piš- “before, in front,” → pre-, + gâr present stem of negâridan, negâštan “to paint, write,” → graph.

Variant of → plano-.

See also: → plano-.

Variant of → plano-.

See also: → plano-.

Short for Max Planck (1858-1947), German physicist, great authority on thermodynamics and creator of the quantum theory.

Short for Max Planck (1858-1947), German physicist, great authority on thermodynamics and creator of the quantum theory.

A physical constant that determines the energy of quantum as a function of its frequency; symbol h. Also called → Planck’s constant.

On 16 November 2018, the International Bureau of Weights and Measures (BIPM) voted to redefine the kilogram by fixing the value of the Planck constant, thereby defining the kilogram in terms of the second and the speed of light. Starting 20 May 2019, the new value is exactly

6.626 070 15 × 10^-34 J s. The → reduced Planck constant, ħ = h / 2π, is also called the → Dirac constant.

See also: → Planck; → constant.

A physical constant that determines the energy of quantum as a function of its frequency; symbol h. Also called → Planck’s constant.

On 16 November 2018, the International Bureau of Weights and Measures (BIPM) voted to redefine the kilogram by fixing the value of the Planck constant, thereby defining the kilogram in terms of the second and the speed of light. Starting 20 May 2019, the new value is exactly

6.626 070 15 × 10^-34 J s. The → reduced Planck constant, ħ = h / 2π, is also called the → Dirac constant.

See also: → Planck; → constant.

Same as → blackbody curve.

See also: → Planck; → curve.

Same as → blackbody curve.

See also: → Planck; → curve.

The density corresponding to a → Planck mass in a cubic region of edge length given by the
→ Planck length: ρ_P = c⁵/(ħG²) ≅ 5.16 x 10⁹³ g cm^-3, where c is the → speed of light, ħ is the → reduced Planck’s constant, and G is the → gravitational constant.

See also: → Planck; → density.

The density corresponding to a → Planck mass in a cubic region of edge length given by the
→ Planck length: ρ_P = c⁵/(ħG²) ≅ 5.16 x 10⁹³ g cm^-3, where c is the → speed of light, ħ is the → reduced Planck’s constant, and G is the → gravitational constant.

See also: → Planck; → density.

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

See also: → Planck; → distribution.

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

See also: → Planck; → distribution.

The unit of energy in the system of Planck units. E_P = √ (ħ c⁵/G) ≅ 1.22 x 10¹⁹ GeV.
It can also be defined as E_P = ħ / t_P, where t_P is the Planck time. This is an extraordinarily large amount of energy on the subatomic scale and particle accelerators have yet to produce a particle with this magnitude of energy. Understanding the properties of a subatomic particle that contains the Planck Energy is helpful in developing a Unified Field Theory which encompasses the realms of Quantum Theory and Relativity, although this too has evaded complete scientific understanding.

See also: → Planck; → energy.

The unit of energy in the system of Planck units. E_P = √ (ħ c⁵/G) ≅ 1.22 x 10¹⁹ GeV.
It can also be defined as E_P = ħ / t_P, where t_P is the Planck time. This is an extraordinarily large amount of energy on the subatomic scale and particle accelerators have yet to produce a particle with this magnitude of energy. Understanding the properties of a subatomic particle that contains the Planck Energy is helpful in developing a Unified Field Theory which encompasses the realms of Quantum Theory and Relativity, although this too has evaded complete scientific understanding.

See also: → Planck; → energy.

The first 10^-43 seconds of the Universe’s existence, when the size of the Universe was roughly the Planck length and
during which quantum effects of gravity were significant. Also called Planck epoch. Our understanding of the Planck era is poor because theory which encompasses both quantum mechanics and general relativity is needed to be
developed.

See also: → Planck; → era.

The first 10^-43 seconds of the Universe’s existence, when the size of the Universe was roughly the Planck length and
during which quantum effects of gravity were significant. Also called Planck epoch. Our understanding of the Planck era is poor because theory which encompasses both quantum mechanics and general relativity is needed to be
developed.

See also: → Planck; → era.

Same as → Planck’s blackbody formula.

See also: → Planck; → function.

Same as → Planck’s blackbody formula.

See also: → Planck; → function.

The size limit, l_P = √ (ħ G/ c³), about 10^-33 cm, at which Einstein’s notions of space-time are supposed to break down, and space is predicted to become “foam like.”

See also: → Planck; → length.

The size limit, l_P = √ (ħ G/ c³), about 10^-33 cm, at which Einstein’s notions of space-time are supposed to break down, and space is predicted to become “foam like.”

See also: → Planck; → length.

1. The unit of mass in Planck’s system of physical units, m_P = √ (ħ c/ G) = 2.176 × 10^-8 kg. It is also the mass of a → black hole whose → Compton wavelength is comparable to its → Schwarzschild radius.
   

2. The fundamental unit of mass in the theory of gravitation. It is the mass of hypothetical particles where gravitational attraction for eachother would be as strong as the electric force between two electrons separated by the same distance (Steven Weinberg).

See also: → Planck; → mass.

1. The unit of mass in Planck’s system of physical units, m_P = √ (ħ c/ G) = 2.176 × 10^-8 kg. It is also the mass of a → black hole whose → Compton wavelength is comparable to its → Schwarzschild radius.
   

2. The fundamental unit of mass in the theory of gravitation. It is the mass of hypothetical particles where gravitational attraction for eachother would be as strong as the electric force between two electrons separated by the same distance (Steven Weinberg).

See also: → Planck; → mass.

The postulate that the energy of oscillators in a blackbody is quantized by E = nhν, where n = 1, 2, 3, …, h is Planck’s constant, and ν the frequency.

See also: → Planck; → postulate.

The postulate that the energy of oscillators in a blackbody is quantized by E = nhν, where n = 1, 2, 3, …, h is Planck’s constant, and ν the frequency.

See also: → Planck; → postulate.

A European Space Agency (ESA) mission to map the full sky in the 30 GHz to 1 THz range and to measure the → anisotropies of the → cosmic microwave background (CMB) with a sensitivity set by fundamental limits, i.e. photon noise and contamination by foregrounds. It was launched on 14 May 2009, together with the → Herschel Satellite. Its observing position is a halo orbit around the L2 → Lagrangian point, some 1.5 million km from Earth. Its → Gregorian-like off-axis telescope has an effective aperture of 1.5 m and images the sky on two sets of feed-horns. The Low Frequency Instrument (LFI, at frequencies 30, 44, 70 GHz) amplifies with High Electron Mobility Transistors cooled at 20 K the radiation collected by 13 horns. The High Frequency Instrument (HFI, at 100, 143, 217, 353, 545,
857 GHz) detects the shorter waves with 52 bolometers cooled at only 0.1 degree above the → absolute zero. Both instruments can detect both the total intensity and polarization of photons. The Planck mission is intended to provide maps with a sensitivity of a few micro-Kelvin and an angular resolution down to 5 arcmin, which is considered as a major improvement with respect to the Wilkinson Microwave Anisotropy Probe (→ WMAP) launched by the NASA in 2002. Planck will provide a major source of information relevant to several cosmological and astrophysical issues, such as testing theories of the early Universe and the origin of cosmic structure. It will also provide to astronomers 9 complete maps of the full sky at wavelengths from 0.3 mm to 1 cm, that complement the 4 maps taken by the → IRAS satellite in the 2.5 to 100 micrometer range.

See also: → Planck; → satellite.

A European Space Agency (ESA) mission to map the full sky in the 30 GHz to 1 THz range and to measure the → anisotropies of the → cosmic microwave background (CMB) with a sensitivity set by fundamental limits, i.e. photon noise and contamination by foregrounds. It was launched on 14 May 2009, together with the → Herschel Satellite. Its observing position is a halo orbit around the L2 → Lagrangian point, some 1.5 million km from Earth. Its → Gregorian-like off-axis telescope has an effective aperture of 1.5 m and images the sky on two sets of feed-horns. The Low Frequency Instrument (LFI, at frequencies 30, 44, 70 GHz) amplifies with High Electron Mobility Transistors cooled at 20 K the radiation collected by 13 horns. The High Frequency Instrument (HFI, at 100, 143, 217, 353, 545,
857 GHz) detects the shorter waves with 52 bolometers cooled at only 0.1 degree above the → absolute zero. Both instruments can detect both the total intensity and polarization of photons. The Planck mission is intended to provide maps with a sensitivity of a few micro-Kelvin and an angular resolution down to 5 arcmin, which is considered as a major improvement with respect to the Wilkinson Microwave Anisotropy Probe (→ WMAP) launched by the NASA in 2002. Planck will provide a major source of information relevant to several cosmological and astrophysical issues, such as testing theories of the early Universe and the origin of cosmic structure. It will also provide to astronomers 9 complete maps of the full sky at wavelengths from 0.3 mm to 1 cm, that complement the 4 maps taken by the → IRAS satellite in the 2.5 to 100 micrometer range.

See also: → Planck; → satellite.

1. A general term for anything roughly the size of the → Planck length.
   

2. Similarly, an energy on the order of the → Planck energy.

See also: → Planck; → scale.

1. A general term for anything roughly the size of the → Planck length.
   

2. Similarly, an energy on the order of the → Planck energy.

See also: → Planck; → scale.

Same as → blackbody spectrum.

See also: → Planck; → spectrum.

Same as → blackbody spectrum.

See also: → Planck; → spectrum.

The time representing the → Planck length divided by the → speed of light. It is expressed by: t_P = (Għ/c⁵)^1/2≅ 5.4 x 10^-44 s, where G is the → gravitational constant, ħ is the → reduced Planck’s constant, and c is the → speed of light. At the Planck time, the mass density of the Universe is thought to approach the → Planck density.

See also: → Planck; → time.

The time representing the → Planck length divided by the → speed of light. It is expressed by: t_P = (Għ/c⁵)^1/2≅ 5.4 x 10^-44 s, where G is the → gravitational constant, ħ is the → reduced Planck’s constant, and c is the → speed of light. At the Planck time, the mass density of the Universe is thought to approach the → Planck density.

See also: → Planck; → time.

A set of → natural units in which the normalized units are:
the gravitational constant, Planck’s constant, the speed of light, the Coulomb constant, and Boltzmann’s constant.

See also: → Planck; → unit.

A set of → natural units in which the normalized units are:
the gravitational constant, Planck’s constant, the speed of light, the Coulomb constant, and Boltzmann’s constant.

See also: → Planck; → unit.

A formula that determines the distribution of intensity of radiation that prevails under conditions of thermal equilibrium at a temperature T: B_v = (2hν³ / c²)[exp(hν / kT) - 1]^-1 where h is Planck’s constant and ν is the frequency.

See also: → Planck; → blackbody;
→ formula.

A formula that determines the distribution of intensity of radiation that prevails under conditions of thermal equilibrium at a temperature T: B_v = (2hν³ / c²)[exp(hν / kT) - 1]^-1 where h is Planck’s constant and ν is the frequency.

See also: → Planck; → blackbody;
→ formula.

→ Planck constant.

See also: → Planck; → constant.

→ Planck constant.

See also: → Planck; → constant.

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

See also: → Planck; → radiation; → law.

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

See also: → Planck; → radiation; → law.

1. (n.) a flat or level surface.
   

2. (adj.) Of or pertaining to planes or plane figures.

Etymology (EN): 1) From L. plantum “flat surface,” noun use of adj. planus “flat, level, plain.”
2) From L. planus, as above.

Etymology (PE): 1) Hâmon, variant of hâmun “plain, level ground;” Mid.Pers. hâmôn “level, flat;” Proto-Iranian *hāma-van-, from
*hāma- “same, equally, even; together, with” (Mod.Pers./Mid.Pers. ham-; O.Pers./Av. ham-; cf. 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-) + -van- suffix.
2) Taxt “flat;” Mid.Pers. taxtag “tablet, plank, (chess)board.”

1. (n.) a flat or level surface.
   

2. (adj.) Of or pertaining to planes or plane figures.

Etymology (EN): 1) From L. plantum “flat surface,” noun use of adj. planus “flat, level, plain.”
2) From L. planus, as above.

Etymology (PE): 1) Hâmon, variant of hâmun “plain, level ground;” Mid.Pers. hâmôn “level, flat;” Proto-Iranian *hāma-van-, from
*hāma- “same, equally, even; together, with” (Mod.Pers./Mid.Pers. ham-; O.Pers./Av. ham-; cf. 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-) + -van- suffix.
2) Taxt “flat;” Mid.Pers. taxtag “tablet, plank, (chess)board.”

A two-dimensional geometric figure. The points of the figure lie entirely in a plane.

See also: → plane; → figure.

A two-dimensional geometric figure. The points of the figure lie entirely in a plane.

See also: → plane; → figure.

A mirror whose reflective surface is neither concave nor convex.

See also: → plane; → mirror.

A mirror whose reflective surface is neither concave nor convex.

See also: → plane; → mirror.

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

See also: → plane; → polarization.

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

See also: → plane; → polarization.

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

See also: → plane; → rotation.

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

See also: → plane; → rotation.

An imaginary plane that is perpendicular to the → line of sight.

See also: → plane; → sky.

An imaginary plane that is perpendicular to the → line of sight.

See also: → plane; → sky.

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

See also: → plane; → polarization.

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

See also: → plane; → polarization.

Same as → linear polarization.

See also: → plane; → polarization.

Same as → linear polarization.

See also: → plane; → polarization.

Light exhibiting → plane polarization. Same as → linearly polarized light.

See also: → plane; → polarized; → light.

Light exhibiting → plane polarization. Same as → linearly polarized light.

See also: → plane; → polarized; → light.

A wave whose wavefronts of constant phase are infinite parallel planes normal to the direction of propagation.

See also: → plane; → wave.

A wave whose wavefronts of constant phase are infinite parallel planes normal to the direction of propagation.

See also: → plane; → wave.

An approximation used in many stellar atmosphere models that depict the
atmosphere as being only one-dimensional and bounded at the top and bottom by horizontal plane surfaces normal to the direction of gravity.

See also: → plane; → parallel; → atmosphere.

An approximation used in many stellar atmosphere models that depict the
atmosphere as being only one-dimensional and bounded at the top and bottom by horizontal plane surfaces normal to the direction of gravity.

See also: → plane; → parallel; → atmosphere.

A piece of glass with plane parallel surfaces used to admit light into an optical system and to exclude dirt and moisture.

See also: → plane; → parallel; → plate.

A piece of glass with plane parallel surfaces used to admit light into an optical system and to exclude dirt and moisture.

See also: → plane; → parallel; → plate.

An object with a mass greater than an → asteroid, but smaller than that of a → brown dwarf.

See also: Short for planetary mass object; → planetary; → mass; → object.

An object with a mass greater than an → asteroid, but smaller than that of a → brown dwarf.

See also: Short for planetary mass object; → planetary; → mass; → object.

1. A celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. → dwarf planet.
   
2. → extrasolar planet.

Etymology (EN): From O.E., from O.Fr. planete (Fr. planète), from L.L. planeta (plural form planetae), from Gk. planetes (single form) “wandering,” from (asteres) planetai “wandering (stars),” from planasthai “to wander,” of unknown origin.

Etymology (PE): Sayyâré, from Ar. saiyârat “walker, traveller.”

1. A celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit. → dwarf planet.
   
2. → extrasolar planet.

Etymology (EN): From O.E., from O.Fr. planete (Fr. planète), from L.L. planeta (plural form planetae), from Gk. planetes (single form) “wandering,” from (asteres) planetai “wandering (stars),” from planasthai “to wander,” of unknown origin.

Etymology (PE): Sayyâré, from Ar. saiyârat “walker, traveller.”

A hypothetical large planet in the far outer → solar system the gravitational effects of which would explain the unexpected orbital configuration of a group of → trans-Neptunian objects (TNOs). Trujillo & Sheppard (2014) noticed a clustering of the → argument of perihelion of bodies lying beyond ~150 → astronomical unit (AU), and attributed this to a hypothetical super-Earth body lying at several hundred AUs. Batygin & Brown (2016) showed numerically and analytically how the apsidal and nodal clustering of the distant TNOs arises as a result of resonant and secular dynamical effects from a distant perturber. They identified a range of semimajor axes (400-1500 AU) and eccentricities (0.5-0.8) for which a distant planet can explain the → orbital elements of the distant TNOs. The predicted planet would have a mass of 10 Earths (approximately 5,000 times the mass of → Pluto), a diameter of four times Earth and a highly elliptical orbit with an → orbital period of approximately 15,000 years.

See also: → planet; → nine.

A hypothetical large planet in the far outer → solar system the gravitational effects of which would explain the unexpected orbital configuration of a group of → trans-Neptunian objects (TNOs). Trujillo & Sheppard (2014) noticed a clustering of the → argument of perihelion of bodies lying beyond ~150 → astronomical unit (AU), and attributed this to a hypothetical super-Earth body lying at several hundred AUs. Batygin & Brown (2016) showed numerically and analytically how the apsidal and nodal clustering of the distant TNOs arises as a result of resonant and secular dynamical effects from a distant perturber. They identified a range of semimajor axes (400-1500 AU) and eccentricities (0.5-0.8) for which a distant planet can explain the → orbital elements of the distant TNOs. The predicted planet would have a mass of 10 Earths (approximately 5,000 times the mass of → Pluto), a diameter of four times Earth and a highly elliptical orbit with an → orbital period of approximately 15,000 years.

See also: → planet; → nine.

1. A device that produces a representation of the heavens by the use of a number of moving projectors.
   
2. The building or room in which such a device is housed.

Etymology (EN): From → planet + -arium “a place for.”

Etymology (PE): Âsmânnemâ, literally “sky displayer,” from âsmân “sky” (Mid.Pers. âsmân “sky, heaven;” O.Pers. asman- “heaven;” Av. asman- “stone, sling-stone; heaven;” cf. Skt. áśman- “stone, rock, thunderbolt;” Gk. akmon “heaven, meteor, anvil;” Akmon was the father of Ouranos (Uranus), god of sky; Lith. akmuo “stone;” Rus. kamen; PIE base *akmon- “stone, sky.”
The link between the “stone” and “sky” concepts indicates that the sky had once been conceived as a stone vault by prehistoric Indo-Europeans) + nemâ “displayer,” from nemudan “to show” (Mid.Pers. nimūdan, nimây- “to show,” from O.Pers./Av. ni- “down; into” (Skt. ni “down,” nitaram “downward,” Gk. neiothen “from below,” cf. E. nether, O.E. niþera, neoþera “down, downward, below, beneath,” from P.Gmc. *nitheraz,
Du. neder, Ger. nieder; PIE *ni- “down, below”) + māy- “to measure;” cf. Skt. mati “measures,” matra- “measure;”
Gk. metron “measure;” L. metrum; PIE base *me- “to measure”).
Âsmânxâné, literally “sky house,” from âsmân

• xâné “house” (Mid.Pers. xânak, xân, xôn; cf. L. cunae “cradle;” Gk. kome “village;” Pers. Aftari dialect kiye “house, home;” PIE base *kei- “bed; to lie, to settle; beloved” (other cognates: P.Gmc. *khaim-; O.E. ham “dwelling, house, village;” E. home; Ger. Heim; L. civis “townsman;” Fr. cité; E. city; Skt. śiva- “auspicious, dear”).

1. A device that produces a representation of the heavens by the use of a number of moving projectors.
   
2. The building or room in which such a device is housed.

Etymology (EN): From → planet + -arium “a place for.”

Etymology (PE): Âsmânnemâ, literally “sky displayer,” from âsmân “sky” (Mid.Pers. âsmân “sky, heaven;” O.Pers. asman- “heaven;” Av. asman- “stone, sling-stone; heaven;” cf. Skt. áśman- “stone, rock, thunderbolt;” Gk. akmon “heaven, meteor, anvil;” Akmon was the father of Ouranos (Uranus), god of sky; Lith. akmuo “stone;” Rus. kamen; PIE base *akmon- “stone, sky.”
The link between the “stone” and “sky” concepts indicates that the sky had once been conceived as a stone vault by prehistoric Indo-Europeans) + nemâ “displayer,” from nemudan “to show” (Mid.Pers. nimūdan, nimây- “to show,” from O.Pers./Av. ni- “down; into” (Skt. ni “down,” nitaram “downward,” Gk. neiothen “from below,” cf. E. nether, O.E. niþera, neoþera “down, downward, below, beneath,” from P.Gmc. *nitheraz,
Du. neder, Ger. nieder; PIE *ni- “down, below”) + māy- “to measure;” cf. Skt. mati “measures,” matra- “measure;”
Gk. metron “measure;” L. metrum; PIE base *me- “to measure”).
Âsmânxâné, literally “sky house,” from âsmân

• xâné “house” (Mid.Pers. xânak, xân, xôn; cf. L. cunae “cradle;” Gk. kome “village;” Pers. Aftari dialect kiye “house, home;” PIE base *kei- “bed; to lie, to settle; beloved” (other cognates: P.Gmc. *khaim-; O.E. ham “dwelling, house, village;” E. home; Ger. Heim; L. civis “townsman;” Fr. cité; E. city; Skt. śiva- “auspicious, dear”).

Of, pertaining to, or resembling a planet or planets.

See also: → planet; → -ary.

Of, pertaining to, or resembling a planet or planets.

See also: → planet; → -ary.

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

See also: → planetary; → aberration.

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

See also: → planetary; → aberration.

A hot envelope of gas ejected from a central evolved star before becoming a → white dwarf. At the end of the → asymptotic giant phase the pulsating → red giant star is surrounded by an extended shell formed by the material ejected from it. As the evolved star contracts, its → effective temperature rises considerably. When it reaches about 30,000 K, the radiated photons become energetic enough to ionize the atoms in the nebula. The nebula becomes then visible in the optical. It shines essentially in a few → emission lines, produced by cascades during recombination or by collisional excitation with electrons. The central stars of planetary nebulae, → CSPNe, are
typically 0.6 to 0.8 solar masses. They have → main sequence masses in the range 1 to 8 solar masses, with an average mass of 2.2 solar masses for a standard
→ initial mass function. Thus a total of about 1.6 solar masses is in average lost during the → AGB and planetary nebula phases. The life-time of planetary nebulae
is relatively short. A typical planetary nebula lasts only a few 10,000 years.

See also: → planetary; → nebula. The name comes from the fact that these objects appear as planetary disks in a low-resolution telescope. The first planetary nebula, designated NGC 7009 or the → Saturn Nebula, was discovered in 1782 by the German-born English astronomer William Herschel (1738-1822), who described it as “planetary nebula.”

A hot envelope of gas ejected from a central evolved star before becoming a → white dwarf. At the end of the → asymptotic giant phase the pulsating → red giant star is surrounded by an extended shell formed by the material ejected from it. As the evolved star contracts, its → effective temperature rises considerably. When it reaches about 30,000 K, the radiated photons become energetic enough to ionize the atoms in the nebula. The nebula becomes then visible in the optical. It shines essentially in a few → emission lines, produced by cascades during recombination or by collisional excitation with electrons. The central stars of planetary nebulae, → CSPNe, are
typically 0.6 to 0.8 solar masses. They have → main sequence masses in the range 1 to 8 solar masses, with an average mass of 2.2 solar masses for a standard
→ initial mass function. Thus a total of about 1.6 solar masses is in average lost during the → AGB and planetary nebula phases. The life-time of planetary nebulae
is relatively short. A typical planetary nebula lasts only a few 10,000 years.

See also: → planetary; → nebula. The name comes from the fact that these objects appear as planetary disks in a low-resolution telescope. The first planetary nebula, designated NGC 7009 or the → Saturn Nebula, was discovered in 1782 by the German-born English astronomer William Herschel (1738-1822), who described it as “planetary nebula.”

The study of the structure, composition, as well as physical and chemical properties of the planets of the solar system, including their atmospheres and their immediate cosmic environment.

See also: → planetary; → physics.

The study of the structure, composition, as well as physical and chemical properties of the planets of the solar system, including their atmospheres and their immediate cosmic environment.

See also: → planetary; → physics.

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

See also: → planetary; → precession.

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

See also: → planetary; → precession.

→ Interplanetary dust and other small particles organized into thin, flat rings encircling a planet. The most spectacular planetary rings known are those around → Saturn, but the other three → giant planets
of the solar system (→ Jupiter, → Uranus, and → Neptune) have their own ring systems.

See also: → planetary; → ring;
→ system.

→ Interplanetary dust and other small particles organized into thin, flat rings encircling a planet. The most spectacular planetary rings known are those around → Saturn, but the other three → giant planets
of the solar system (→ Jupiter, → Uranus, and → Neptune) have their own ring systems.

See also: → planetary; → ring;
→ system.

The branch of astronomy that deals with the science of planets, or planetary systems, and the solar system.

See also: → planetary; → science.

The branch of astronomy that deals with the science of planets, or planetary systems, and the solar system.

See also: → planetary; → science.

A system composed of a star and all the celestial bodies bound to it by gravity, especially planets and their natural satellites.

See also: → planetary; → system.

A system composed of a star and all the celestial bodies bound to it by gravity, especially planets and their natural satellites.

See also: → planetary; → system.

The passage of an → inferior planet against the disk of the Sun, as viewed from Earth. Mercury and Venus pass in front of the Sun only when they are close to one of their → orbital nodes, at → inferior conjunction. For Mercury this occurs at the beginning of November (the → ascending node) or at the beginning of May (the → descending node), while for Venus it takes place at the beginning of December (the ascending node) or at the beginning of June (the descending node). See also → transit of Mercury, → transit of Venus.

See also: → planetary; → transit.

The passage of an → inferior planet against the disk of the Sun, as viewed from Earth. Mercury and Venus pass in front of the Sun only when they are close to one of their → orbital nodes, at → inferior conjunction. For Mercury this occurs at the beginning of November (the → ascending node) or at the beginning of May (the → descending node), while for Venus it takes place at the beginning of December (the ascending node) or at the beginning of June (the descending node). See also → transit of Mercury, → transit of Venus.

See also: → planetary; → transit.

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

See also: → planetary; → transit; → oscillation; → star.

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

See also: → planetary; → transit; → oscillation; → star.

Any of numerous small solid bodies in a → protoplanetary disk that in some cases clump together to form → planets but in other cases remain relatively small and become → asteroids and → comets. Similarly, → Kuiper Belt Objects are probably the remnants of the planetesimals that formed the planets.

Etymology (EN): From → planet + (infinit)esimal, → infinitesimal.

Etymology (PE): Xordesayyâré, from xordé “small, minute; crumbs,” from
xord “minute, little, small”
(from Mid.Pers. xvart, xôrt “small, insignificant;” Av. ādra- “weak, dependent;” Skt. ādhrá- “small, weak, poor,” nādh “to be oppressed;” Gk. nothros “sluggish;” PIE base *nhdhro-) + sayyâré, → planet.

Any of numerous small solid bodies in a → protoplanetary disk that in some cases clump together to form → planets but in other cases remain relatively small and become → asteroids and → comets. Similarly, → Kuiper Belt Objects are probably the remnants of the planetesimals that formed the planets.

Etymology (EN): From → planet + (infinit)esimal, → infinitesimal.

Etymology (PE): Xordesayyâré, from xordé “small, minute; crumbs,” from
xord “minute, little, small”
(from Mid.Pers. xvart, xôrt “small, insignificant;” Av. ādra- “weak, dependent;” Skt. ādhrá- “small, weak, poor,” nādh “to be oppressed;” Gk. nothros “sluggish;” PIE base *nhdhro-) + sayyâré, → planet.

An older equivalent for → asteroid.

See also: → planet + → -oid.

An older equivalent for → asteroid.

See also: → planet + → -oid.

Same as → planetary science.

See also: → planet; + → -logy.

Same as → planetary science.

See also: → planet; + → -logy.

Variant of → plano-, as in → planisphere.

See also: → plane.

Variant of → plano-, as in → planisphere.

See also: → plane.

A projection or representation of the whole or a part of a sphere on a plane. In particular, a polar projection of the celestial sphere or the Earth on a plane.

See also: → plani-; → sphere.

A projection or representation of the whole or a part of a sphere on a plane. In particular, a polar projection of the celestial sphere or the Earth on a plane.

See also: → plani-; → sphere.

Of or relating to → planisphere.

See also: → planisphere + → -ic..

Of or relating to → planisphere.

See also: → planisphere + → -ic..

The most common form of the → astrolabe in which both the → celestial sphere and the observer’s horizon are projected on to one or more plane surfaces by means of the stereographic projection. See also → universal astrolabe and → particular astrolabe.

See also: → planispheric; → astrolabe.

The most common form of the → astrolabe in which both the → celestial sphere and the observer’s horizon are projected on to one or more plane surfaces by means of the stereographic projection. See also → universal astrolabe and → particular astrolabe.

See also: → planispheric; → astrolabe.

The business strategy of deliberately designing products so they have a limited effective lifespan. Its objective is to incite the consumer to abandon the currently owned item and buy another one.

See also: Planned, p.p. of “to → plan;” → obsolescence.

The business strategy of deliberately designing products so they have a limited effective lifespan. Its objective is to incite the consumer to abandon the currently owned item and buy another one.

See also: Planned, p.p. of “to → plan;” → obsolescence.

A combining form meaning “flat, plane,” used in the formation of compound terms. Also plani- and plan- before a vowel, → plane.

See also: → plane.

A combining form meaning “flat, plane,” used in the formation of compound terms. Also plani- and plan- before a vowel, → plane.

See also: → plane.

A → divergent lens composed of one concave surface and one plane surface. This type of lens has a negative focal point and produces a → virtual image.

See also: → plano-; → concave; → lens.

A → divergent lens composed of one concave surface and one plane surface. This type of lens has a negative focal point and produces a → virtual image.

See also: → plano-; → concave; → lens.

A → convergent lens that has one flat side and one convex side. Plano-convex lenses focus parallel rays of light to a positive point, thus forming → real images.

See also: → plano-; → convex; → lens.

A → convergent lens that has one flat side and one convex side. Plano-convex lenses focus parallel rays of light to a positive point, thus forming → real images.

See also: → plano-; → convex; → lens.

A living organism, in the Kingdom Plantae, with cellulose cell walls and lacking a nervous system or powers of voluntary motion.

Etymology (EN): M.E. plaunte; O.E. plante; L. planta “shoot, sprig, scion.”

Etymology (PE): Giyâh “plant;” dialectal Bašâkardi gidâ(h), gida; Av. gaodāyu-, literally “cattle nourisher,” from gao- “cattle, bull, ox,” → Taurus, + dāyu- “nourisher,” from dā(y)- “to feed” (Gershevitch 1962); Mod.Pers. dâyé “wet nurse;” cognate with L. femina “woman, female,” → feminism; PIE base *d^heh(i)- “to suck, suckle.”

A living organism, in the Kingdom Plantae, with cellulose cell walls and lacking a nervous system or powers of voluntary motion.

Etymology (EN): M.E. plaunte; O.E. plante; L. planta “shoot, sprig, scion.”

Etymology (PE): Giyâh “plant;” dialectal Bašâkardi gidâ(h), gida; Av. gaodāyu-, literally “cattle nourisher,” from gao- “cattle, bull, ox,” → Taurus, + dāyu- “nourisher,” from dā(y)- “to feed” (Gershevitch 1962); Mod.Pers. dâyé “wet nurse;” cognate with L. femina “woman, female,” → feminism; PIE base *d^heh(i)- “to suck, suckle.”

A → binary system consisting of two → massive stars, which are → supergiants of → spectral types O7.5 and O6. The two components are so close together that they orbit each other with a period of 14.4 days only. The Plaskett’s star is a → double-line binary. The estimated masses of the components are 43 (Plaskett A) and 51 (Plaskett B) → solar masses. The lower mass component is optically brighter than the other star. Also known as HR 2422 and HD 47129 (See, e.g., Bagnuolo et al. 1992, ApJ 385, 708).

See also: Named after the Canadian astronomer John S. Plaskett (1865-1941), who made a detailed spectroscopic study of this star in 1922.

A → binary system consisting of two → massive stars, which are → supergiants of → spectral types O7.5 and O6. The two components are so close together that they orbit each other with a period of 14.4 days only. The Plaskett’s star is a → double-line binary. The estimated masses of the components are 43 (Plaskett A) and 51 (Plaskett B) → solar masses. The lower mass component is optically brighter than the other star. Also known as HR 2422 and HD 47129 (See, e.g., Bagnuolo et al. 1992, ApJ 385, 708).

See also: Named after the Canadian astronomer John S. Plaskett (1865-1941), who made a detailed spectroscopic study of this star in 1922.

In physics, a gas in which an important fraction of the atoms is ionized, so that the electrons and ions are separately free. Plasma, often described as the fourth state of matter, occurs at extremely high temperatures, as in the interiors of stars, in fusion reactors, and in the interstellar medium ionized by hot stars.

Etymology (EN): From L.L. plasma, from Gk. plasma “something molded or formed,” from plassein “to mold,” originally “to spread thin.”

Etymology (PE): Plâsmâ, loan from Fr.

In physics, a gas in which an important fraction of the atoms is ionized, so that the electrons and ions are separately free. Plasma, often described as the fourth state of matter, occurs at extremely high temperatures, as in the interiors of stars, in fusion reactors, and in the interstellar medium ionized by hot stars.

Etymology (EN): From L.L. plasma, from Gk. plasma “something molded or formed,” from plassein “to mold,” originally “to spread thin.”

Etymology (PE): Plâsmâ, loan from Fr.

In plasma physics, a dimensionless quantity which is the ratio of the plasma thermal pressure to the magnetic pressure: β = nkT/(B²/2μ₀). When β is much smaller than 1, like in the → solar corona, the magnetic forces dominate over the plasma forces.

See also: → plasma; β, Gk. letter of alphabet.

In plasma physics, a dimensionless quantity which is the ratio of the plasma thermal pressure to the magnetic pressure: β = nkT/(B²/2μ₀). When β is much smaller than 1, like in the → solar corona, the magnetic forces dominate over the plasma forces.

See also: → plasma; β, Gk. letter of alphabet.

An alternative cosmology, initially conceived by Hannes Alfvén in the 1960s, that attempts to explain the development of the visible Universe through the interaction of electromagnetic forces on astrophysical plasma. Like the steady state model, plasma cosmology hypothesizes an evolving Universe without beginning or end.

See also: → plasma; → cosmology.

An alternative cosmology, initially conceived by Hannes Alfvén in the 1960s, that attempts to explain the development of the visible Universe through the interaction of electromagnetic forces on astrophysical plasma. Like the steady state model, plasma cosmology hypothesizes an evolving Universe without beginning or end.

See also: → plasma; → cosmology.

The natural frequency at which internal oscillations of a plasma occur. The plasma frequency is proportional to the square root of the electron density. → plasma oscillation.

See also: → plasma; → frequency.

The natural frequency at which internal oscillations of a plasma occur. The plasma frequency is proportional to the square root of the electron density. → plasma oscillation.

See also: → plasma; → frequency.

(Geophysics): A layer of plasma located on the night-side of Earth, inside the magnetosphere and along its boundary. Under the action of electromagnetic forces, plasma contained in the mantle drifts equator-ward, along the tail axis.

See also: → plasma; → mantle.

(Geophysics): A layer of plasma located on the night-side of Earth, inside the magnetosphere and along its boundary. Under the action of electromagnetic forces, plasma contained in the mantle drifts equator-ward, along the tail axis.

See also: → plasma; → mantle.

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

See also: → plasma; → oscillation.

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

See also: → plasma; → oscillation.

The study of the physical properties of the various forms of plasmas and their processing.

See also: → plasma; → physics.

The study of the physical properties of the various forms of plasmas and their processing.

See also: → plasma; → physics.

The ionized component of a comet’s tail, driven nearly straight away from the Sun by the solar wind. Also called ion tail and gas tail.

See also: → plasma; → tail.

The ionized component of a comet’s tail, driven nearly straight away from the Sun by the solar wind. Also called ion tail and gas tail.

See also: → plasma; → tail.

The sharp outer boundary of the plasmasphere, at which the plasma density decreases by a factor of 100 or more.

Etymology (EN): → plasma + pause “break, cessation, stop,” from M.Fr. pause, from L. pausa “a halt, stop, cessation,” from Gk. pausis “stopping, ceasing,” from pauein “to stop, to cause to cease.”

Etymology (PE): → plasma + marz “frontier, border, boundary,” from Mid.Pers. marz “boundary;” Av. marəza- “border, district,” marəz- “to rub, wipe;” Mod.Pers. parmâs “contact, touching” (→ contact), mâl-, mâlidan “to rub;” PIE base *merg- “boundary, border;” cf. L. margo “edge” (Fr. marge “margin”); P.Gmc. *marko;
Ger. Mark; E. mark, margin.

The sharp outer boundary of the plasmasphere, at which the plasma density decreases by a factor of 100 or more.

Etymology (EN): → plasma + pause “break, cessation, stop,” from M.Fr. pause, from L. pausa “a halt, stop, cessation,” from Gk. pausis “stopping, ceasing,” from pauein “to stop, to cause to cease.”

Etymology (PE): → plasma + marz “frontier, border, boundary,” from Mid.Pers. marz “boundary;” Av. marəza- “border, district,” marəz- “to rub, wipe;” Mod.Pers. parmâs “contact, touching” (→ contact), mâl-, mâlidan “to rub;” PIE base *merg- “boundary, border;” cf. L. margo “edge” (Fr. marge “margin”); P.Gmc. *marko;
Ger. Mark; E. mark, margin.

In the magnetosphere, a region of relatively cool (low energy) and dense plasma that may be considered an outer extension of the ionosphere with which it is coupled. Like the ionosphere, the plasmasphere tends to co-rotate with the Earth.

See also: → plasma + → sphere.

In the magnetosphere, a region of relatively cool (low energy) and dense plasma that may be considered an outer extension of the ionosphere with which it is coupled. Like the ionosphere, the plasmasphere tends to co-rotate with the Earth.

See also: → plasma + → sphere.

Physics: The quasiparticle resulting from the → quantization of → plasma oscillations. Plasmons are collective oscillations of free electrons inside or on surfaces of materials.

See also: From plasm-, from → plasma + → -on.

Physics: The quasiparticle resulting from the → quantization of → plasma oscillations. Plasmons are collective oscillations of free electrons inside or on surfaces of materials.

See also: From plasm-, from → plasma + → -on.

The research area dealing with the interaction of → plasmons and → photons and the technology of its practical use. Plasmonics represents one of the most active
research fields at the interface of → nanotechnology and → optics.

See also: → plasmon; → -ics.

The research area dealing with the interaction of → plasmons and → photons and the technology of its practical use. Plasmonics represents one of the most active
research fields at the interface of → nanotechnology and → optics.

See also: → plasmon; → -ics.

1a) Capable of being → deformed continuously and permanently without → rupture.

1b) Capable of being molded.

1c) Made or consisting of a plastic.

2. → plastic substance.

Etymology (EN): From L. plasticus, from Gk. plastikos “that may be molded, pertaining to molding,” from plassein “to mold.”

Etymology (PE): Šukâyand, literally “moldable,” from šuk, contraction of šuka (Dehxodâ) “a mold in which ingots are cast” + âyand agent noun form of âmadan “to come; to become,” → elastic.

1a) Capable of being → deformed continuously and permanently without → rupture.

1b) Capable of being molded.

1c) Made or consisting of a plastic.

2. → plastic substance.

Etymology (EN): From L. plasticus, from Gk. plastikos “that may be molded, pertaining to molding,” from plassein “to mold.”

Etymology (PE): Šukâyand, literally “moldable,” from šuk, contraction of šuka (Dehxodâ) “a mold in which ingots are cast” + âyand agent noun form of âmadan “to come; to become,” → elastic.

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

See also: → plastic; → deformation.

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

See also: → plastic; → deformation.

A substance which remains → deformed after an applied → force ceases to act.

See also: → plastic; → substance.

A substance which remains → deformed after an applied → force ceases to act.

See also: → plastic; → substance.

The property which enables a material to be → deformed
permanently without → rupture during the application of a → force. An → elastic material becomes plastic above its → yield point.
See also → elasticity, → ductility.

See also: → plastic; → -ity.

The property which enables a material to be → deformed
permanently without → rupture during the application of a → force. An → elastic material becomes plastic above its → yield point.
See also → elasticity, → ductility.

See also: → plastic; → -ity.

1. Optics: Any crystalline material whose length is much less than its measured diameter. → half-wave plate.
   
2. A flat piece of material on which a picture or text has been produced.
   
3. Any of the large movable segments into which the Earth’s lithosphere is divided. → plate tectonics.

Etymology (EN): M.E., from O.Fr. plate “thin piece of metal,” from M.L. plata “plate, piece of metal,” perhaps via V.L. *plattus, from Gk. platys " flat, broad."

Etymology (PE): 1) Tiqé, from tiq “blade,” related to tiz “sharp,” variants tig, 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.
2); 3) loanword from Fr., as above.

1. Optics: Any crystalline material whose length is much less than its measured diameter. → half-wave plate.
   
2. A flat piece of material on which a picture or text has been produced.
   
3. Any of the large movable segments into which the Earth’s lithosphere is divided. → plate tectonics.

Etymology (EN): M.E., from O.Fr. plate “thin piece of metal,” from M.L. plata “plate, piece of metal,” perhaps via V.L. *plattus, from Gk. platys " flat, broad."

Etymology (PE): 1) Tiqé, from tiq “blade,” related to tiz “sharp,” variants tig, 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.
2); 3) loanword from Fr., as above.

The celestial coordinates of the center of the field of an astronomical photographic plate.

See also: → plate; → center.

The celestial coordinates of the center of the field of an astronomical photographic plate.

See also: → plate; → center.

The scale factor for converting linear measure on a photographic plate to angular measurement on the sky.

See also: → plate; → scale.

The scale factor for converting linear measure on a photographic plate to angular measurement on the sky.

See also: → plate; → scale.

The theory supported by a wide range of evidence in which the Earth’s crust is composed of several large, thin,
relatively rigid plates that move relative to one another.
The interaction of the plates at their boundaries causes seismic and tectonic activity along these boundaries. See also → continental drift.

See also: → plate; → tectonics.

The theory supported by a wide range of evidence in which the Earth’s crust is composed of several large, thin,
relatively rigid plates that move relative to one another.
The interaction of the plates at their boundaries causes seismic and tectonic activity along these boundaries. See also → continental drift.

See also: → plate; → tectonics.

1. An extensive area with a fairly level surface raised sharply above adjacent land.
   

2. A period, phase, or state of little or no growth or decline in something. → Spite plateau.

Etymology (EN): From Fr. plateau, from O.Fr. platel “flat piece of metal, wood, etc.,” diminutive of plat “flat surface or thing,” → plate.

Etymology (PE): Taxtâl, from taxt “flat;” Mid.Pers. taxtag “tablet, plank, (chess)board” + -âl, → -al.

1. An extensive area with a fairly level surface raised sharply above adjacent land.
   

2. A period, phase, or state of little or no growth or decline in something. → Spite plateau.

Etymology (EN): From Fr. plateau, from O.Fr. platel “flat piece of metal, wood, etc.,” diminutive of plat “flat surface or thing,” → plate.

Etymology (PE): Taxtâl, from taxt “flat;” Mid.Pers. taxtag “tablet, plank, (chess)board” + -âl, → -al.

1. A small platelike body.
   

   2. One of a variety of forms taken on by → snowflakes.
      

   3. A tiny cell occurring in the blood of vertebrates and involved in clotting of the blood. Formerly called thrombocyte.

See also: → plate; → -let.

1. A small platelike body.
   

   2. One of a variety of forms taken on by → snowflakes.
      

   3. A tiny cell occurring in the blood of vertebrates and involved in clotting of the blood. Formerly called thrombocyte.

See also: → plate; → -let.

A silvery metallic → chemical element which is tenacious, malleable, and ductile; symbol Pt. → Atomic number 78; → atomic weight 195.08; → melting point 1,772°C; → boiling point 3,827±100°C; → specific gravity 21.45 at 20°C; → valence +2 or +4. It has several short-lived → radioactive isotopes, with the exception of ¹⁹⁰Pt whose → half-life is 6.0 x 10¹¹ years.

Etymology (EN): From Sp. platina diminutive of plata “silver,” from O.Fr. → plate “sheet of metal.”
The first known reference to platinum can be found in the writing of Italian physician, scholar, and poet Julius Caesar Scaliger (1484-1558). He reported seeing the metal during a visit to Central America in 1557. Aborigines
knew of no use for the metal and regarded it as a nuisance in their search for gold and silver. By the end of the 17th century, the Spanish conquistadors started developing the river soil looking for gold and found some gray looking beads together with the gold. They called those little silver beads platina “small silver.” They became known as platina del Pinto “granules of silvery material from the Pinto River,” a tributary of the San Juan River in the Chocó region of Colombia. The first complete description of platinum was given in 1735 by the the Spanish explorer and naval officer Antonio de Ulloa (1716-1795).

Etymology (PE): Pelâtin, loanword from Fr.

A silvery metallic → chemical element which is tenacious, malleable, and ductile; symbol Pt. → Atomic number 78; → atomic weight 195.08; → melting point 1,772°C; → boiling point 3,827±100°C; → specific gravity 21.45 at 20°C; → valence +2 or +4. It has several short-lived → radioactive isotopes, with the exception of ¹⁹⁰Pt whose → half-life is 6.0 x 10¹¹ years.

Etymology (EN): From Sp. platina diminutive of plata “silver,” from O.Fr. → plate “sheet of metal.”
The first known reference to platinum can be found in the writing of Italian physician, scholar, and poet Julius Caesar Scaliger (1484-1558). He reported seeing the metal during a visit to Central America in 1557. Aborigines
knew of no use for the metal and regarded it as a nuisance in their search for gold and silver. By the end of the 17th century, the Spanish conquistadors started developing the river soil looking for gold and found some gray looking beads together with the gold. They called those little silver beads platina “small silver.” They became known as platina del Pinto “granules of silvery material from the Pinto River,” a tributary of the San Juan River in the Chocó region of Colombia. The first complete description of platinum was given in 1735 by the the Spanish explorer and naval officer Antonio de Ulloa (1716-1795).

Etymology (PE): Pelâtin, loanword from Fr.

One of the six metals → platinum (Pt), → iridium (Ir), → osmium (Os), → palladium (Pd), → rhenium (Rh), and → ruthenium (Ru) that are grouped together in the → periodic table. They are relatively hard and resistant to corrosion and are used in jewellery and in some industrial applications. All are resistant to chemical attack.

See also: → platinum; → group; → element.

One of the six metals → platinum (Pt), → iridium (Ir), → osmium (Os), → palladium (Pd), → rhenium (Rh), and → ruthenium (Ru) that are grouped together in the → periodic table. They are relatively hard and resistant to corrosion and are used in jewellery and in some industrial applications. All are resistant to chemical attack.

See also: → platinum; → group; → element.

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

See also: Of or pertaining to Gk. philosopher Plato, from Gk. Platon “broad-shouldered,” from platys “broad.” → year.

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

See also: Of or pertaining to Gk. philosopher Plato, from Gk. Platon “broad-shouldered,” from platys “broad.” → year.

A frequency distribution with negative → kurtosis, which has a smaller “peak” around the mean than the corresponding normal distribution. → leptokurtic.

Etymology (EN): From Gk. platys “flat”, → plate + → kurtosis.

Etymology (PE): Past-afrâšté, literally “lowly elevated,” from past “low; plain” (Mid.Pers. past; proto-Iranian *pasta- “fallen,” from *pat- “to fall,” cf. Av. pat- “to fall; to fly; to rush,” patarəta- “winged;” Mid.Pers. opastan “to fall,” patet “falls;” Mod.Pers. oftâdan “to fall,” oft “fall;” Skt. patati “he flies, falls,” pátra- “wing, feather, leaf;” Gk. piptein “to fall,” pterux “wing;” L. penna “feather, wing;” O.E. feðer “feather;” PIE base *pet- “to fly, rush”) + afrâšté “raised, elevated, erect,” p.p. of afrâštan, → kurtosis.

A frequency distribution with negative → kurtosis, which has a smaller “peak” around the mean than the corresponding normal distribution. → leptokurtic.

Etymology (EN): From Gk. platys “flat”, → plate + → kurtosis.

Etymology (PE): Past-afrâšté, literally “lowly elevated,” from past “low; plain” (Mid.Pers. past; proto-Iranian *pasta- “fallen,” from *pat- “to fall,” cf. Av. pat- “to fall; to fly; to rush,” patarəta- “winged;” Mid.Pers. opastan “to fall,” patet “falls;” Mod.Pers. oftâdan “to fall,” oft “fall;” Skt. patati “he flies, falls,” pátra- “wing, feather, leaf;” Gk. piptein “to fall,” pterux “wing;” L. penna “feather, wing;” O.E. feðer “feather;” PIE base *pet- “to fly, rush”) + afrâšté “raised, elevated, erect,” p.p. of afrâštan, → kurtosis.

Pleasing, agreeable, or enjoyable; giving pleasure.

See also: M.E., from O.Fr. plaisant, from plaisir, → pleasure.

Pleasing, agreeable, or enjoyable; giving pleasure.

See also: M.E., from O.Fr. plaisant, from plaisir, → pleasure.

1. To act to the pleasure or satisfaction of.
   

2. To be the pleasure or will of (Dictionary.com).

Etymology (EN): From M.E. plesen, plaisen, from O.Fr. plaisir “to please, give pleasure to, satisfy,” from L. placere “to be acceptable, be liked, be approved,” related to placare “to soothe, quiet.”

Etymology (PE): Âzušidan, from prefix â- + zušé, → pleasure, + infinitive suffix -idan.

1. To act to the pleasure or satisfaction of.
   

2. To be the pleasure or will of (Dictionary.com).

Etymology (EN): From M.E. plesen, plaisen, from O.Fr. plaisir “to please, give pleasure to, satisfy,” from L. placere “to be acceptable, be liked, be approved,” related to placare “to soothe, quiet.”

Etymology (PE): Âzušidan, from prefix â- + zušé, → pleasure, + infinitive suffix -idan.

1. The state or feeling of being pleased.
   

2. Enjoyment or satisfaction derived from what is to one’s liking; gratification; delight.
   

3. Worldly or frivolous enjoyment (Dictionary.com).

Etymology (EN): M.E., from O.Fr. plesir, plaisir “enjoyment, delight, desire,” from plaisir “to please,” from L. placere “to please, give pleasure.”

Etymology (PE): Zusé, from Av. zuš- “to take pleasure;” related to O.Pers. daušta- “friend,” Mid.Pers. dôš- “to love, like, choose,” dôšišn “pleasure, liking;”
Parthian zwš “love;” Mod.Pers. dôst, dust “friend;” cf. Skt. jos- “to like, enjoy;” Gk. geuomai “to taste;” L. gusto “I taste;” gustus “taste, enjoyment.”

1. The state or feeling of being pleased.
   

2. Enjoyment or satisfaction derived from what is to one’s liking; gratification; delight.
   

3. Worldly or frivolous enjoyment (Dictionary.com).

Etymology (EN): M.E., from O.Fr. plesir, plaisir “enjoyment, delight, desire,” from plaisir “to please,” from L. placere “to please, give pleasure.”

Etymology (PE): Zusé, from Av. zuš- “to take pleasure;” related to O.Pers. daušta- “friend,” Mid.Pers. dôš- “to love, like, choose,” dôšišn “pleasure, liking;”
Parthian zwš “love;” Mod.Pers. dôst, dust “friend;” cf. Skt. jos- “to like, enjoy;” Gk. geuomai “to taste;” L. gusto “I taste;” gustus “taste, enjoyment.”

A prominent → open cluster in the constellation → Taurus, popularly called the Seven Sisters. It is a very young cluster of several hundred stars (with spectral types B6 and later), spanning over 1.5 degrees on the sky and about 400 → light-years distant. Six members of the cluster are visible to the → naked eye, the brightest one being → Alcyone. The cluster contains extensive nebulosity, consisting of dust clouds that reflect the light of the → embedded stars. Other designations: M45, NGC 1432.

Etymology (EN): In Gk. mythology, the seven daughters of Atlas and Pleione, transformed by Zeus into seven stars, from L., from Gk. Pleiades, perhaps literally “constellation of the doves,” from a shortened form of peleiades, plural of peleias “dove,” from PIE base *pel- “dark-colored, gray.”

Etymology (PE): Parvin, variants Parv, Parvé, Paran, Parand, Parviz, Kurd. Pêrû, Pashtu Pêrûne, Baluchi Panvar; Mid.Pers. Parwiz. According to Bartholomae, it originates from
Av. Paoiryaēini-, the first component paoirya- denoting “first.” Alternatively, the first element in Parvin may be related to Av. parav-, pauru-, pouru- “full, much, many”
(Mod.Pers. por “full, much, very;” Mid.Pers. purr “full;” O.Pers. paru- “much, many,”
from par- “to fill;” PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru- “much, abundant;” Gk. polus “many,” plethos “great number, multitude;” O.E. full), denoting “many, numerous,” because the object contains several stars.

A prominent → open cluster in the constellation → Taurus, popularly called the Seven Sisters. It is a very young cluster of several hundred stars (with spectral types B6 and later), spanning over 1.5 degrees on the sky and about 400 → light-years distant. Six members of the cluster are visible to the → naked eye, the brightest one being → Alcyone. The cluster contains extensive nebulosity, consisting of dust clouds that reflect the light of the → embedded stars. Other designations: M45, NGC 1432.

Etymology (EN): In Gk. mythology, the seven daughters of Atlas and Pleione, transformed by Zeus into seven stars, from L., from Gk. Pleiades, perhaps literally “constellation of the doves,” from a shortened form of peleiades, plural of peleias “dove,” from PIE base *pel- “dark-colored, gray.”

Etymology (PE): Parvin, variants Parv, Parvé, Paran, Parand, Parviz, Kurd. Pêrû, Pashtu Pêrûne, Baluchi Panvar; Mid.Pers. Parwiz. According to Bartholomae, it originates from
Av. Paoiryaēini-, the first component paoirya- denoting “first.” Alternatively, the first element in Parvin may be related to Av. parav-, pauru-, pouru- “full, much, many”
(Mod.Pers. por “full, much, very;” Mid.Pers. purr “full;” O.Pers. paru- “much, many,”
from par- “to fill;” PIE base *pelu- “full,” from *pel- “to be full;” cf. Skt. puru- “much, abundant;” Gk. polus “many,” plethos “great number, multitude;” O.E. full), denoting “many, numerous,” because the object contains several stars.

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

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

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

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

The earliest Epoch of the Quaternary Period, beginning about 1.6 million years ago and ending 10,000 years ago. Commonly known as the “Ice Age,” a time with episodes of widespread continental glaciation.

See also: From Gk. pleisto(s), superlative of Polys “much,”
cognate with Pers. por, → full,

• -cene from Gk. kainos “new, recent.”

The earliest Epoch of the Quaternary Period, beginning about 1.6 million years ago and ending 10,000 years ago. Commonly known as the “Ice Age,” a time with episodes of widespread continental glaciation.

See also: From Gk. pleisto(s), superlative of Polys “much,”
cognate with Pers. por, → full,

• -cene from Gk. kainos “new, recent.”

The property of certain crystals of exhibiting different colors when viewed from different directions under transmitted light. This is because the degree with which
certain birefringent crystals transmit polarized light is different, depending on whether the ray is ordinary or extraordinary. Pleochroism is the general term for both dichroism, which is found in uniaxial crystals (crystals with a single optic axis), and trichroism, found in biaxial crystals (two optic axes).

Etymology (EN): From pleochro(ic), from pleo- prefix meaning “more,” from Gk. pleion “more,” cognate with Pers. por, → full,

• chroic, from chroos “skin, color” + -ism.

Etymology (PE): Candfâmi, from cand “so many, much; how many, how much” (O.Pers. yāvā “as long as;” Av. yauuant- [adj.] “how great?, how much?, how many?,” yauuat [adv.] “as much as, as far as;” cf. Skt. yāvant- “how big, how much;” Gk. heos “as long as, until”) + fâm “color,” + -i noun suffix.

The property of certain crystals of exhibiting different colors when viewed from different directions under transmitted light. This is because the degree with which
certain birefringent crystals transmit polarized light is different, depending on whether the ray is ordinary or extraordinary. Pleochroism is the general term for both dichroism, which is found in uniaxial crystals (crystals with a single optic axis), and trichroism, found in biaxial crystals (two optic axes).

Etymology (EN): From pleochro(ic), from pleo- prefix meaning “more,” from Gk. pleion “more,” cognate with Pers. por, → full,

• chroic, from chroos “skin, color” + -ism.

Etymology (PE): Candfâmi, from cand “so many, much; how many, how much” (O.Pers. yāvā “as long as;” Av. yauuant- [adj.] “how great?, how much?, how many?,” yauuat [adv.] “as much as, as far as;” cf. Skt. yāvant- “how big, how much;” Gk. heos “as long as, until”) + fâm “color,” + -i noun suffix.

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

See also: Plerion, from Gk. pleres “full,” akin to Pers. por “full,” → poly-.

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

See also: Plerion, from Gk. pleres “full,” akin to Pers. por “full,” → poly-.

The latest Epoch of the Tertiary Period, beginning about 5.3 million years ago and ending 1.6 million years ago.

See also: From plio-, varaint of pleo-, from Gk. pleon “more,” cognate with Pers. por, → full, + -cene from Gk. kainos “new, recent.”

The latest Epoch of the Tertiary Period, beginning about 5.3 million years ago and ending 1.6 million years ago.

See also: From plio-, varaint of pleo-, from Gk. pleon “more,” cognate with Pers. por, → full, + -cene from Gk. kainos “new, recent.”

The British name of a group of seven stars (→ asterism) lying inside the Northern constellation → Ursa Major. Same as → Big Dipper.

Etymology (EN): M. E. plough, plouw, from O.E. ploh, plog “plow, plowland.”

Etymology (PE): Haft barâdarân “the seven brothers,” from haft “seven” (Mid.Pers. haft, Av. hapta, cf. Skt. sapta, Gk. hepta, L. septem, P.Gmc. *sebun, Du. zeven, O.H.G. sibun, Ger. sieben, E. seven; PIE *septm)

• barâdarân, plural of barâdar “brother” (Mid.Pers. brad, bardar, O.Pers./Av. brātar-, cf. Skt. bhrátar-, Gk. phrater, L. frater, P.Gmc. *brothar; PIE base *bhrater- “brother”).
  Haftowrang, Mid.Pers. haptôiring, from Av. haptôiringa- “with seven marks,” from hapto- “seven,“as above, + iringa- “mark,” cf. Skt. linga- “mark, token, sign.”

The British name of a group of seven stars (→ asterism) lying inside the Northern constellation → Ursa Major. Same as → Big Dipper.

Etymology (EN): M. E. plough, plouw, from O.E. ploh, plog “plow, plowland.”

Etymology (PE): Haft barâdarân “the seven brothers,” from haft “seven” (Mid.Pers. haft, Av. hapta, cf. Skt. sapta, Gk. hepta, L. septem, P.Gmc. *sebun, Du. zeven, O.H.G. sibun, Ger. sieben, E. seven; PIE *septm)

• barâdarân, plural of barâdar “brother” (Mid.Pers. brad, bardar, O.Pers./Av. brātar-, cf. Skt. bhrátar-, Gk. phrater, L. frater, P.Gmc. *brothar; PIE base *bhrater- “brother”).
  Haftowrang, Mid.Pers. haptôiring, from Av. haptôiringa- “with seven marks,” from hapto- “seven,“as above, + iringa- “mark,” cf. Skt. linga- “mark, token, sign.”

A small mass of lead or other heavy material, as that suspended by a line and used to measure the depth of water or to ascertain a vertical line (Dictionary.com). → plumb line.

Etymology (EN): M.E. plumbe, from O.Fr. *plombe, plomee “sounding lead,” from L. plumbum “lead (the metal), lead ball,” of unknown origin, related to Gk. molybdos “lead.”

Etymology (PE): Gulé “ball, sphere,” a variant of golulé, → bullet.

A small mass of lead or other heavy material, as that suspended by a line and used to measure the depth of water or to ascertain a vertical line (Dictionary.com). → plumb line.

Etymology (EN): M.E. plumbe, from O.Fr. *plombe, plomee “sounding lead,” from L. plumbum “lead (the metal), lead ball,” of unknown origin, related to Gk. molybdos “lead.”

Etymology (PE): Gulé “ball, sphere,” a variant of golulé, → bullet.

A cord with a weight attached to one end, used to verify a true vertical alignment or to find the depth of water.

Etymology (EN): → plumb; → line.

Etymology (PE): Šâqul, variants šâhul, sâhul, probably from sahi

• suffix -ul, variant -âl. The first element sahi “upright, right,” variants (Tabari, Torbat-Heydariyeyi: šax) “right, upright, straight, level,” colloquial Pers. šaq (o raq = râst) “upright, erect.” For the second element → -âl.

A cord with a weight attached to one end, used to verify a true vertical alignment or to find the depth of water.

Etymology (EN): → plumb; → line.

Etymology (PE): Šâqul, variants šâhul, sâhul, probably from sahi

• suffix -ul, variant -âl. The first element sahi “upright, right,” variants (Tabari, Torbat-Heydariyeyi: šax) “right, upright, straight, level,” colloquial Pers. šaq (o raq = râst) “upright, erect.” For the second element → -âl.

A structure or form that is like a long feather. → polar plume.

Etymology (EN): From M.E., from O.Fr. plume, from L. pluma “feather, down,” from PIE base *pleus- “feather, fleece.”

Etymology (PE): Parr “feather,” variant bâl “wing,” Mid.Pers. parr “feather, wing,” bâl; Av. parəna- “feather,” Skt. parnam, cf.
O.H.G. farn “fern,” PIE pornom “feather.”

A structure or form that is like a long feather. → polar plume.

Etymology (EN): From M.E., from O.Fr. plume, from L. pluma “feather, down,” from PIE base *pleus- “feather, fleece.”

Etymology (PE): Parr “feather,” variant bâl “wing,” Mid.Pers. parr “feather, wing,” bâl; Av. parəna- “feather,” Skt. parnam, cf.
O.H.G. farn “fern,” PIE pornom “feather.”

1a) Act of plunging.

1b) The act of descending or dipping suddenly; (steep) fall, slide from vertical.

2. Geology: The vertical angle between the fold axis or any inclined line in a geologic structure and the horizontal plane.
   

3. To cast oneself, or fall as if cast, into water, a hole, etc.

Etymology (EN): M.E., from M.Fr. plung(i)er, from O.Fr. plongier “plunge, sink into; plunge into, dive in,” from V.L. *plumbicare “to heave the lead,” from L. plumbum “lead,” → plumb.

Etymology (PE): 1, 3) Širjé, probably deformation of sarjé, literally “head jump” (nose dive), from sar, → head, + jé, from jahidan, → jump.

2. Laxšé, noun from laxšidan “to slide,” variant laqzidan.

1a) Act of plunging.

1b) The act of descending or dipping suddenly; (steep) fall, slide from vertical.

2. Geology: The vertical angle between the fold axis or any inclined line in a geologic structure and the horizontal plane.
   

3. To cast oneself, or fall as if cast, into water, a hole, etc.

Etymology (EN): M.E., from M.Fr. plung(i)er, from O.Fr. plongier “plunge, sink into; plunge into, dive in,” from V.L. *plumbicare “to heave the lead,” from L. plumbum “lead,” → plumb.

Etymology (PE): 1, 3) Širjé, probably deformation of sarjé, literally “head jump” (nose dive), from sar, → head, + jé, from jahidan, → jump.

2. Laxšé, noun from laxšidan “to slide,” variant laqzidan.

1. Consisting of, containing, or pertaining to more than one thing.
   

2. Grammar: A form of nouns, pronouns, and verbs
   that refers to more than one thing or (in languages that recognize the dual form) more than two.

Etymology (EN): M.E., from O.Fr. plurel “more than one,” from L. pluralis “of or belonging to more than one,” from plus (genitive pluris) “more,” → plus.

Etymology (PE): Bišâl, from biš “much, more; great,” → plus,

• -âl, → -al.

1. Consisting of, containing, or pertaining to more than one thing.
   

2. Grammar: A form of nouns, pronouns, and verbs
   that refers to more than one thing or (in languages that recognize the dual form) more than two.

Etymology (EN): M.E., from O.Fr. plurel “more than one,” from L. pluralis “of or belonging to more than one,” from plus (genitive pluris) “more,” → plus.

Etymology (PE): Bišâl, from biš “much, more; great,” → plus,

• -âl, → -al.

1. A state of society in which various religious, ethnic, racial, and political groups are allowed to thrive in a single society.
   

   2. Philo.: A belief that there are more than two kinds of principles, as contrasted to the → dualism and → monism.

See also: → plural; → -ism.

1. A state of society in which various religious, ethnic, racial, and political groups are allowed to thrive in a single society.
   

   2. Philo.: A belief that there are more than two kinds of principles, as contrasted to the → dualism and → monism.

See also: → plural; → -ism.

The state or fact of being plural or numerous.

See also: → plural + → -ity.

The state or fact of being plural or numerous.

See also: → plural + → -ity.

1. (prep.) With the addition of.
   

2. (adj.) Additional; → positive.
   

3. (n.) An addition; a positive quantity; the → plus sign.

Etymology (EN): L. plus “more,” cognate with Gk. polys “much,” Pers. por, → full.

Etymology (PE): Bišan, from biš “much, more; great” + suffix -an, → minus.
The first component from Mid.Pers. veš “more, longer; more frequently,” related to vas “many, much” (Mod.Pers. bas);
O.Pers. vasiy “at will, greatly, utterly;” Av. varəmi “I wish,” vasô, vasə “at one’s pleasure or will,” from vas- “to will, desire, wish.”

1. (prep.) With the addition of.
   

2. (adj.) Additional; → positive.
   

3. (n.) An addition; a positive quantity; the → plus sign.

Etymology (EN): L. plus “more,” cognate with Gk. polys “much,” Pers. por, → full.

Etymology (PE): Bišan, from biš “much, more; great” + suffix -an, → minus.
The first component from Mid.Pers. veš “more, longer; more frequently,” related to vas “many, much” (Mod.Pers. bas);
O.Pers. vasiy “at will, greatly, utterly;” Av. varəmi “I wish,” vasô, vasə “at one’s pleasure or will,” from vas- “to will, desire, wish.”

The symbol + indicating summation or a positive quantity. The sign is believed to be a shortened form of the L. word et denoting “and” which was the term for addition. The signs + and - first appeared in an arithmetic book by Johannes Widmann entitled Behennde und hübsche Rechnung, published in Leipzig in 1489.

See also: → plus; → sign.

The symbol + indicating summation or a positive quantity. The sign is believed to be a shortened form of the L. word et denoting “and” which was the term for addition. The signs + and - first appeared in an arithmetic book by Johannes Widmann entitled Behennde und hübsche Rechnung, published in Leipzig in 1489.

See also: → plus; → sign.

A → dwarf planet in the → solar system which until 2006 was known as the 9th major planet. Pluto revolves around the → Sun in a highly elliptical orbit at a mean distance of 39.5 → astronomical units once every about 248 years. The orbit → eccentricity is 0.25 (compare with the Earth’s 0.02) yielding a → perihelion distance of 29.66 → astronomical units and an → aphelion distance of 48.87 AU.
Its → orbital inclination is 17 degrees, which is much higher than those of the other planets. Pluto’s mass is 1.308 × 10²² kg, that is 0.00218 Earth mass (0.177 Moon mass), its equatorial radius ib 0.19 Earth radius, and its → rotation period is equal to 6.39 Earth days.

It has five known → satellites, in order of distance from Pluto:

→ Charon, → Styx, → Nix, → Kerberos, and → Hydra.

Pluto’s radius is estimated to be about 1150 km (0.18 Earths).
Pluto is smaller than seven of the solar system’s satellites
(the → Moon, → Io (Jupiter I) , → Europa, → Ganymede, → Callisto, → Titan, and → Triton).

Pluto’s surface has an estimated temperature of 37.5 K and is composed of more than 98% → nitrogen  → ice, with traces of → methane and → carbon monoxide.

See also: In Roman mythology, Pluto is the god of the underworld and Judge of the dead, from L. Pluto, Pluton, from Gk. Plouton “god of wealth,” literally “wealth, riches.” Pluto was the son of Saturn. The alternative Gk. name is Hades.

A → dwarf planet in the → solar system which until 2006 was known as the 9th major planet. Pluto revolves around the → Sun in a highly elliptical orbit at a mean distance of 39.5 → astronomical units once every about 248 years. The orbit → eccentricity is 0.25 (compare with the Earth’s 0.02) yielding a → perihelion distance of 29.66 → astronomical units and an → aphelion distance of 48.87 AU.
Its → orbital inclination is 17 degrees, which is much higher than those of the other planets. Pluto’s mass is 1.308 × 10²² kg, that is 0.00218 Earth mass (0.177 Moon mass), its equatorial radius ib 0.19 Earth radius, and its → rotation period is equal to 6.39 Earth days.

It has five known → satellites, in order of distance from Pluto:

→ Charon, → Styx, → Nix, → Kerberos, and → Hydra.

Pluto’s radius is estimated to be about 1150 km (0.18 Earths).
Pluto is smaller than seven of the solar system’s satellites
(the → Moon, → Io (Jupiter I) , → Europa, → Ganymede, → Callisto, → Titan, and → Triton).

Pluto’s surface has an estimated temperature of 37.5 K and is composed of more than 98% → nitrogen  → ice, with traces of → methane and → carbon monoxide.

See also: In Roman mythology, Pluto is the god of the underworld and Judge of the dead, from L. Pluto, Pluton, from Gk. Plouton “god of wealth,” literally “wealth, riches.” Pluto was the son of Saturn. The alternative Gk. name is Hades.

A → radioactive → chemical element, symbol Pu. → Atomic number 94; → mass number of most stable isotope 244; → melting point 640 °C; → boiling point 3,235 °C. It was first synthesized in 1940 by American chemists Glenn T. Seaborg, Edwin M. McMillan, Joseph W. Kennedy and Arthur C. Wahl in the → nuclear reaction:

₉₂U²³⁸ + ₀n¹→ ₉₃Np²³⁹ + β^- (23.5 minutes) → ₉₄Pu²³⁹ + β^- (2.36 days). The → half-life of ₉₄Pu²³⁹ is 2.44 × 10⁴ yr.

Plutonium-239 is a → fissile isotope.

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

A → radioactive → chemical element, symbol Pu. → Atomic number 94; → mass number of most stable isotope 244; → melting point 640 °C; → boiling point 3,235 °C. It was first synthesized in 1940 by American chemists Glenn T. Seaborg, Edwin M. McMillan, Joseph W. Kennedy and Arthur C. Wahl in the → nuclear reaction:

₉₂U²³⁸ + ₀n¹→ ₉₃Np²³⁹ + β^- (23.5 minutes) → ₉₄Pu²³⁹ + β^- (2.36 days). The → half-life of ₉₄Pu²³⁹ is 2.44 × 10⁴ yr.

Plutonium-239 is a → fissile isotope.

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