An Etymological Dictionary of Astronomy and Astrophysics

El Niño. A significant warming of the ocean surface over the eastern and central equatorial Pacific that occurs at irregular intervals, generally ranging between two and seven years. El Niño conditions, which are often characterized by “warm events,” most often develop after late December during the early months of the year and decay during the following year. → La Nina.

See also: From Sp. El Niño “the child,” i.e. “the Christ Child,” alluding to the appearance of the current near Christmas. The term was originally applied by fishermen of northern Peru.

The thirteenth known moon of Jupiter, discovered in 1905 by Charles Perrine.

See also: In Gk. mythology, Elara was the mother by Zeus of the giant Tityus.

Of, pertaining to, or noting a body having the property of → elasticity. See also → elastic collision, → elastic deformation, → elastic limit, → elastic scattering.

Etymology (EN): From Fr., from Gk. elastos “ductile, flexible,” related to elaunein “to strike, beat out.”

Etymology (PE): Kešâyand, from keš stem of kešidan/kašidan “to pull, drag, draw”
(Av. karš- “to draw, to plough,” karša- “furrow;” cf. Skt. kars-, kársati “to pull, drag, plough,”
Gk. pelo, pelomai “to be busy, to bustle”) + âyand agent form of âmadan “to come; to become,” from Mid.Pers. âmatan (O.Pers. gam- “to come; to go,” Av. gam- “to come; to go,” jamaiti “goes;” O.Iranian *āgmatani; 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 root *gwem- “to go, come”).

A collision between two particles which conserves the total kinetic energy and momentum of the system.

See also: → elastic; → collision.

A deformation of a → solid body in which the change (→ strain) in the relative position of points
in the body disappears when the deforming stress is removed. See also → elastic limit.

See also: → elastic; → deformation.

The smallest → stress beyond which a → solid body can no longer return to its original shape. The material ceases to obey → Hooke’s law. Also called → yield point.

See also: → elastic; → limit.

In a → collision between two → particles, the reaction in which the total → kinetic energy of the system, projectile plus target, is the same before the collision as after.

In the interaction of → electromagnetic waves with particles, the scattering when the → wavelength (→ frequency) of the → scattered light is the same as the → incident light (→ Rayleigh scattering,
→ Mie scattering).

See also: → elastic; → scattering.

A wave that propagates by → elastic deformation of the medium. The → propagation takes place
by a change in shape that disappears when the forces are removed. In other words,
the displaced particles transfer momentum to adjoining particles, and are themselves restored to their original position. A → seismic wave is a type of elastic wave.

See also: → elastic; → wave.

The ability of a body which has been → deformed by an applied → force to return to its original shape when the force is removed. Up to a certain point the material obeys → Hooke’s law. See also → ductility, → plasticity.

See also: → elastic + → -ity.

The joint of the human → arm between the → upper arm and the → forearm.

Etymology (EN): M.E. elbowe, from O.E. elboga, elnboga, from ell + bow. Cognate with Scots elbuck, Du. elleboog, Ger. Ellbogen, Ellenbogen, Dan. albue, Icelandic olbogi, olnbogi “elbow.”

Etymology (PE): Ârenj “elbow,” variants âranj, âran “elbow,” araš “forearm;” Mid.Pers. âranj, O.Pers. arašan- “cubit,” Av. arəθnâ- “elbow,” Skt. aratni- “elbow,” Iranian stem aratan-, araθn-, borrowed from Iranian into General Slavic as aršin “ell.”

Pertaining to, derived from, produced by, or associated with electricity.

Etymology (EN): Term coined in by the English physicist William Gilbert (1540-1603) in treatise De Magnete (1600), from L. electrum “amber,” from Gk. elektron “amber.”

Etymology (PE): Barqi, adj. of barq, → electricity.

A luminous and extremely hot electrical → discharge between two → electrodes when an ionized → plasma is created in the air or gas across the electrodes.

See also: → electric; → arc.

The intrinsic property of matter responsible for all electric phenomena, occurring in two forms arbitrarily designated → negative and → positive.

See also: → electric; → charge.

Physics: A closed path followed by an → electric current;
a number of → conductors interconnected for the purpose of carrying an electric current.

See also: → electric; → circuit.

The → rate at which → electric charge → flows past a given point through a → conductor, measured in → amperes.

See also: → electric; → current.

1. A type of → charge distribution consisting of two charges, a positive and a negative charge of the same magnitude separated by a distance s, which is small compared to the distance r to the point P at which the → electric potential is V and the → electric field intensity is E.The potential falls as the square of the distance (1/r²) and the electric field intensity decreases as the cube of the distance (1/r³).
   

   2. A simple → antenna consisting of a pair of oppositely charged → conductors capable of radiating an → electromagnetic wave in response to the movement of an electric charge from one conductor to the other.

See also: → electric; → dipole.

The flow of electricity through a gas, resulting in the emission of radiation that is characteristic of the gas and of the intensity of the current.

See also: → electric; → discharge.

The effect produced by the existence of an → electric charge in the volume of space that surrounds it. The direction of the field is taken to be the direction of the force it would exert on a positive test charge. The electric field is radially outward from a positive charge and radially in toward a negative point charge.

See also: → electric; → field.

The strength of an electric field at any point as measured by the force exerted upon a unit positive charge placed at that point.

See also: → electric; → intensity.

An electric → charge distribution consisting of more than four → positive and → negative → electric charges located at a small distance from each other. The multipole concept is an extension of the → electric quadrupole. For the generalized multipole characterized by the letter l, the potential at a distance r varies as 1/r^{l + 1} and the field intensity as 1/r^{l + 2}.

See also: → electric; → multipole.

The amount of → work required to move a unit → electric charge from → infinity to a specific point against an → electric field. The → SI unit of electric potential is → joules per → coulomb, otherwise known as → volt.

See also: → electric; → potential.

A potential φ defined so that the → electric field  E is expressed by a combination of its → gradient and the variation of the → magnetic vector potential over time: E = -∇φ -∂A/∂t.

See also: → electric; → scalar; → potential.

Of, relating to, or concerned with electricity; electric.

See also: → electric + → -al.

A measure of a material’s ability to conduct an electrical current. It is the reciprocal of the → resistivity. Conductivity is expressed by σ = ne²l/(2mv), where n is the number of electrons per cm³ volume of the → conductor, e is the → electron charge, l is the → mean free path, m is the → electron mass, and v is the arithmetic mean velocity of thermal motion of electrons at a given temperature.

See also: → electrical; → conductivity.

An arrangement of the various electrical energy sources with interconnected electrical devices.

See also: → electric; → network.

1. The physical phenomena arising from the behavior of → electrons and → protons that is caused by the → attraction of particles with opposite → charges and the → repulsion of particles with the same charge.
   

2. The → science of electric charges and → currents.
   

3. A → flow of electrons that is used to generate → light and → power electric devices.

Etymology (EN): From L. electrum “amber,” from Gk. elektron “amber” + -ity a suffix used to form abstract nouns expressing state or condition.

Etymology (PE): Barq, Pers. term, used also in Ar. and Hebrew (barak “lightening”); variants in Pers.: varq, barx, balk, belak, bala; Lârestâni belak; Tabari, Lahijâni, Semnâni, Sorxeyi, Sangesari belk; Gilaki val; Lori beleyz; Kurd. bilese; Tokharian AB pâlk; Mid/Mod.Pers. bir “lightening,”
Mid.Pers. brâh “brilliance, splendour,” br’z- “to shine, beam,” Mod.Pers. barâz “beauty, grace, elegance;”
Av. brāz- “to shine, beam; splendour,” brazāiti “shines;” cf. Skt. bhrāj- “to shine, beam, sparkle,” bhrajate “shines;” Gk. phlegein “to burn;” L. fulgere “to shine,” fulmen “lightning,” flagrare “to blaze, burn;” O.H.G. beraht “bright;” O.E. beorht “bright;” E. → bright; PIE base *bherəg-; *bhrēg- “to shine; white.”

A combining form denoting → electric or → electricity in compound words, such as → electrostatic, → electrodynamics, → electromagnetic. Also, before a vowel, electr-.

Etymology (EN): From electr(ic) + -o-.

Etymology (PE): Barq-, or barq + -â-, → electric.

1. A conductor by means of which a current passes into or out of a medium. The positive electrode is called anode; the negative electrode is called cathode.
   
2. In a CCD detector, one of a series of parallel conducting plates which run across the device at right angles to the channels and subdivide a channel into pixels. The plates create an electric field within the semiconductor which therefore forms a storage site for photo-generated charges.

See also: Coined by E. physicist and chemist Michael Faraday (1791-1867) from electro-, → electric, + Gk. hodos “way.”

Referring to electrons in motion.

See also: → electro- + → dynamics.

The phenomena, science, and applications of moving electric charges, as contrasted with → electrostatics. More specifically, the branch of physics concerned with the → interaction of → electric currents with → magnetic fields and → electric fields or with other electric currents.

See also: → electro- + → dynamics.

A temporary magnet made by coiling wire around an → iron core. When current flows in the coil, or → solenoid, the iron becomes a → magnet. The electromagnet acts as a magnet only so long as the current is flowing in the solenoid.

See also: → electro-; → magnet.

Of or pertaining to electromagnetism or electromagnetic fields.

See also: → electro- + → magnetic

An → electromagnetic signal associated with the location on the sky and the time of a → gravitational wave event. The electromagnetic signal is predicted by models to be associated with the → merger of a → compact binary star system composed of two → neutron stars (NS) or a neutron star and a → black hole (BH).

Accordingly, the gravitational waves are accompanied by a short-duration → gamma-ray burst (GRB) powered by the → accretion of material that remains in a → centrifugally supported → torus around the BH following the merger.

NS-NS/BH-NS mergers are also predicted to be accompanied by a more isotropic counterpart, commonly known as a → kilonova. Kilonovae are day to week-long thermal, → supernova-like → transients, and are powered by the → radioactive decay of heavy, neutron-rich elements synthesized by the → r-process in the expanding merger ejecta (Li and Paczynski 1998). The first detection of an electromagnetic counterpart to gravitational waves belongs to → GW170817.

See also: → electromagnetic; → counterpart.

Same as → fine-structure constant.

See also: → electromagnetic; → coupling; → constant.

A region of space consisting of coupled electric and magnetic lines of force at each point, generated by time-varying currents and accelerated charges.

See also: → electromagnetic; → field.

The fundamental force that is associated with electric and magnetic fields. One of the four fundamental forces of nature, it is carried by photons.

See also: → electromagnetic; → force.

The production of an → electromotive force in a circuit caused by a variation in the magnetic flux through the circuit. If this variation is produced by a change in the current flowing in the circuit itself, it is called → self-induction. If due to the variation in a current in some other circuit, it is called mutual induction. See also → Faraday’s law of induction.

See also: → electromagnetic; → induction.

The combination of both → electric scalar potential
and → magnetic vector potential.

See also: → electromagnetic; → potential.

Radiation propagating in the form of an advancing wave in electric and magnetic fields. It includes radio waves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

See also: → electromagnetic; → radiation.

Information transmitted by means of a modulated current or an electromagnetic wave and received by telephone, radio, television, etc.

See also: → electromagnetic; → signal.

The range of frequencies over which electromagnetic waves are propagated. → electromagnetic radiation.

See also: → electromagnetic; → spectrum.

The description of combined electric and magnetic fields mainly by → Maxwell’s equations. Same as → electromagnetism.

See also: → electromagnetic; → theory.

The theory describing light as a wave phenomenon resulting from the combination of two electric and magnetic fields vibrating transversely and mutually at right angles. → electromagnetic radiation; → electromagnetic wave; → Maxwell’s equations.

See also: → electromagnetic; → theory; → light.

A wave produced by oscillation or acceleration of an electric charge. → electromagnetic radiation.

See also: → electromagnetic; → wave.

1. The science dealing with the physical relations between → electricity and → magnetism. Same as → electromagnetic theory.
   

2. One of the four fundamental forces of nature, governing the electric and magnetic interaction between particles.

See also: → electro-; → magnetism.

The force, analogous to a pressure, which maintains a flow of electricity through a closed circuit. It is the algebraic sum of the → potential differences acting in the circuit. The unit of electromotive force is the → volt.

Etymology (EN): From → electro- + motive, from M.E., from M.Fr., from
O.Fr. motif, from M.L. motivus “moving, impelling,” from L. motus, p.p. of movere “to move,” → motion; → force.

Etymology (PE): Niru, → force; barqrân, literally “driving electricity,” from barq, → electro- + rân present stem of rândan, → drive.

The → elementary particle that possesses the smallest possible negative → electric charge. This
structureless particle has an intrinsic → spin (1/2), a mass of 9.109 382 91 (40) x 10^-31 kg, and an electric charge of 1.602 176 565(35) × 10^-19 → coulombs, or 4.803 204 51(10) × 10^-10 → esu.

See also: Term first suggested in 1891 by Irish physicist G. J. Stoney (1826-1911); from electr-, from → electric + -on, a suffix used in the names of subatomic particles, probably extracted from → ion.

The amount of energy released or absorbed in the process in which an electron is added to a neutral atom or molecule in gaseous state to form a negative ion.

See also: → electron; → affinity.

A process whereby an → unstable atom becomes stable. In this process, an → electron in an atom’s inner shell is drawn into the → nucleus where it combines with a → proton, forming a → neutron and a → neutrino. The neutrino escapes from the atom’s nucleus. The result is an element change, because the atom loses a proton. For example, an atom of → carbon (with 6 protons) becomes an atom of → boron (with 5 protons). Electron capture is also called K-capture since the captured electron usually comes from the atom’s K-shell. See also → neutronization.

See also: → electron; → capture.

The charge of one electron, e = -1.602 176 × 10^-19→ coulombs or -4.803 204 51 × 10^-10→ statcoulombs.

See also: → electron; → charge.

Of an atom, a form of notation which shows how the electrons are distributed among the various atomic orbital and energy levels. The format consists of a series of numbers, letters and superscripts. For example, 1s² 2s² 2p³ means: 2 electrons in the 1s subshell, 2 electrons in the 2s subshell, and 3 electrons in the 2p subshell.

See also: → electron; → configuration.

A → degenerate matter in which electrons are very tightly packed together, as in a white dwarf, but cannot get closer than a certain limit to each other, because according to quantum mechanics laws (→ Pauli exclusion principle) the lowest energy levels can be occupied by only one electron. Therefore, electrons are forced into high energy states. And the significant pressure created by these high energy electrons supports white dwarf stars against their own gravity.

See also: → electron; → degeneracy.

The number of electrons per unit volume in an ionized medium, like an → H II region, as determined from → emission lines.

See also: → electron; → density.

A diffraction phenomenon resulting from the passage of electrons through matter, analogous to the diffraction of visible light. This phenomenon is the main evidence for the existence of waves associated with elementary particles; → de Broglie wavelength.

See also: → electron; → diffraction.

The mass of an electron, which is 9.109 382 91 × 10^-28 g.

See also: → electron; → mass.

The classical size of the electron given by r_e = e²/m_ec² = 2.81794 × 10^-13 cm, where e and m_e are the → electron charge and → electron mass, respectively, and c is the → speed of light.

See also: → electron; → radius.

Any of up to seven energy levels on which an electron may exist within an atom, the energies of the electrons on the same level being equal and on different levels being unequal. The number of electrons permitted in a shell is equal to 2n². A shell contains n² orbitals, and n subshells.

See also: → electron; → shell.

1. The temperature of electrons in an interstellar ionized nebula (e.g. in → H II regions and → planetary nebulae) as determined by characteristic → emission lines (optical → forbidden lines or → radio recombination lines).
   

2. In the → solar wind, the temperature derived from the mean → thermal agitation of the electrons. More specifically, electric field receivers on board space probes carry out the spectroscopy of the → thermal noise due to the potential fluctuations produced by the thermal agitation of the electrons, yielding the electron temperature in certain conditions (N. Meyer-Vernet, 2007, Basics of the Solar Wind, Cambridge Univ. Press). See also → proton temperature.

See also: → electron; → temperature.

→ electron-volt.

See also: → electron; → volt.

The simultaneous formation of an → electron and a → positron in the → pair production process.

See also: → electron; → positron; → pair.

A → line broadening phenomenon involving the scattering effect of → free electrons on the → radiation transfer in → stellar atmospheres. The scattering of radiation by free electrons plays an important role in the atmospheres of → hot stars, such as → O-types, early → B-types, and → Wolf-Rayet stars. The first detailed study of electron scattering in Wolf-Rayet stars was by Castor et al. (1970), who used electron scattering to explain the broad emission wings of N IV λ3483 in HD 192163. In → P Cygni stars the explanation of the very extended (almost symmetric) wings on the → Balmer lines as caused by electron scattering was first made by Bernat & Lambert (1978). Hillier (1991) showed that significant reduction in the strength of an electron-scattering wing can be achieved in a model of → clumped wind for a lower mean → mass loss rate. This resulted in a better agreement between observations and theoretical predictions. Electron-scattering wings provide diagnostics regarding the presence of density inhomogeneities in → stellar winds (Münch, 1948, ApJ 108, 116; Hillier, 1991, A&A 247, 455).

See also: → electron; → scattering; → wing.

The energy acquired by an electron when accelerated through a → potential difference of 1 volt (1 eV = 1.602 × 10^-12 → ergs = 11605 → kelvins).

See also: → electron; → volt.

1. Of or relating to electrons or to an electron.
   

   2. Of or relating to → electronics or to → devices, → circuits, or systems developed through electronics (Dictionary.com).

See also: → electron; → -ic.

In molecular quantum mechanics, any of → quantum states corresponding to a particular → electron configuration (i.e. an arrangement of the electron(s) in certain → orbitals). The electron configuration with the lowest energy is called the → ground state. All higher energy states are called → excited states.

See also: → electronic; → state.

The → transfer of an → electron from one → energy level to another.

See also: → electronic; → transition.

The science dealing with the development and application of → devices and → systems involving the flow of → electrons in a → vacuum, in → gaseous media, and in → semiconductors (Dictionary.com).

See also: → electron; → -ics.

An instrument for detecting electric charges or → potential differences.

See also: → electro-; → -scope.

Referring to electric charges at rest.

See also: → electro-; → static.

A quantity of electricity at rest on the surface of an insulator or an insulated conductor.

See also: → electrostatic; → charge.

A region of space in which a non-moving → electric charge would be subjected to a force of attraction or repulsion as a result of the presence of another stationary electric charge. The electrostatic field is a special case of the → electromagnetic field.

See also: → electrostatic; → field.

The production of stationary electric charges on an uncharged object
as a result of a charged body being brought near it without touching it. A positive charge will induce a negative charge, and vice versa.

See also: → electrostatic; → induction.

The unit of electric charge in the → cgs system of units. Also called the → statcoulomb. The esu is defined such that if two objects, each carrying a charge of +1 esu, are 1 cm aparat, then they repel each other with a force of 1 → dyne. 1 esu = 3.3356 × 10^-10 → coulombs.

See also: → electrostatic; → unit; → charge.

In a → plasma, a disturbance that is devoid of magnetic field, and hence can be expressed by an electrostatic potential.
The electric field is always parallel to the propagation vector, so that the electrostatic wave is → longitudinal.

See also: → electrostatic; → wave.

The branch of → electricity dealing with the phenomena and properties of stationary → electric charges, as opposed to → electrodynamics. It involves the build-up of charge on the → surface of → objects due to → contact with other surfaces.

See also: → electro- + → statics.

Of, relating to, or being the → unification of → electromagnetism and the → weak interaction.

See also: → electro-; → weak.

A period in the early history of the Universe lasting from 10^-36 to 10^-12 seconds after the → Big Bang. The electroweak epoch begins at the same time as cosmic → inflation is triggered. This is also the time when the → strong force breaks from the → grand unified force and ends with another → phase transition will occur in which the → weak interaction breaks from the → electroweak force.

See also: → electroweak; → epoch.

The force that takes part in an → electroweak interaction.

See also: → electroweak; → force.

The unified description of two of the four fundamental interactions of nature, → electromagnetism and the → weak interaction which would merge into a single force under conditions of extreme temperature (above 10¹⁶ degrees, 10² GeV) prevalent in the early history of the → Universe.

See also: → electroweak; → interaction.

A postulated type of star that could form toward the end of a → massive star’s life, after → nuclear fusion has stopped in its → core, and before the star → collapses into a → black hole. In those → extreme conditions, when → temperature and → density inside the star are very high, → quarks could convert into → leptons. Hence huge amounts of energy can be released, much of which would be in the form of → neutrinos.

See also: → electroweak; → star.

Elegance quality; something elegant.

See also: Noun from → elegant.

Gracefully refined and dignified, as in tastes, habits, or literary style; graceful in form or movement; excellent; fine; superior (Dictionary.com).

Etymology (EN): M.E., from M.Fr., from L. elegantem (nominative elegans) “choice, fine, tasteful,” from eligere “to select, choose.”

Etymology (PE): Qašang “elegant, nicely fitted up” (Steingass), variant šang; cf. Sogd. xšang “beautiful, magnificient, excellent,” maybe related to Av. xšnu- “to entertain, welcome, take care of (a guest),” O.Pers. xšnu- “to be satisfied, glad,” Pers. xošnud “satisfied, content.”

An equation with surprising simplicity that expresses a fundamental result relating several apparently unassociable elements. For example, → Euler’s formula for the particular case of θ = π, and the → mass-energy relation.

See also: → elegant; → equation.

1. General: A component or constituent of a whole or one of the parts into which a whole may be resolved by analysis.
   

2. Astro.: Any of the data required to define the precise nature of an orbit and to determine the position of a planet in the orbit at any given time. → orbital element.
   

3. Chemistry:: One of the 117 presently known substances that cannot be decomposed by chemical reaction into a simpler substance.
   

4. Math.:a) Of a cylinder or cone, the generating line of the surface, taken in any position. b) Of a set, any member of the set.

Etymology (EN): From O.Fr. élément, from L. elementum “rudiment, one of the four elements, first principle,” origin unknown.

Etymology (PE): Bonpâr, from bon “basis; root; foundation; bottom;” Mid.Pers. bun “root; foundation; beginning,” Av. būna- “base, depth,” cf. Skt. bundha-, budhná- “base, bottom,” Pali bunda- “root of tree” + pâr contraction of pâré “piece, part, portion, fragment;” Mid.Pers. pârag “piece, part, portion; gift, offering, bribe;” Av. pāra- “debt,” from par- “to remunerate, equalize; to condemn;” PIE *per- “to sell, hand over, distribute; to assign;” cf. L. pars “part, piece, side, share,” portio “share, portion;” Gk. peprotai “it has been granted;” Skt. purti- “reward;” Hitt. pars-, parsiya- “to break, crumble.”

Onsor from Ar.

An important physical process occurring in stars, which is the relative separation of the various → chemical elements. It is
caused by → gravitational settling and → thermal diffusion, on the one hand, and → radiative levitation on the other. This process, which was described by Michaud (1970) to account for the abundance anomalies observed in → chemically peculiar  → A star, is now recognized as occuring in all types of stars. Its influence on the observed → chemical abundances is extremely variable, however, due to competing macroscopic motions like → convective  → mixing or rotation-induced → turbulence.

In the Sun, no observable abundance anomalies are expected from element diffusion, as the time scale of the process is longer than the solar lifetime. However the small induced → depletion of → helium and → heavy elements by about 20% is detectable through → helioseismology. Such detections are more difficult in stars, as only global → oscillation modes can be detected, in contrast to the Sun, where local oscillations of the surface can be analyzed

(Théado et al., 2005, A&A 437, 553).

See also: → element; → diffusion.

Emission nebulae: The relative amount of a given → chemical element in an ionized nebula with respect to another element, usually → hydrogen. Elemental abundance ratios of → emission nebulae are obtained either by adding the observed → ionic abundances of the element or by using → ionization correction factors. Same as → total abundance.

See also: Elemental, from M.L. elementalis, → element

• -al;
  abundance, from O.Fr. abundance, from L. abundantia “fullness,” from abundare “to overflow,” from L. ab- “away” + undare “to surge,” from unda “water, wave;” → abundance.

A particle which cannot be divided into other constituents. More specifically, a particle whose field appears in the fundamental field equations of the unified field theory of elementary particles, in particular in the Lagrangian. For example, the → electron, the → photon, and the → quark are elementary particles, whereas the proton and neutron are not. The elementary nature of a particle can be revised depending on new observations or theories. Also called → fundamental particle.

Etymology (EN): Elementary, M.E. elementare, from M.F. élémentaire, from L. elementarius, from → element + adj. suffix -arius; → particle.

Etymology (PE): Bonyâdin, from bonyâd “basis, foundation,” variant of bonlâd, from bon “basis; root; foundation; bottom” → element + lâd “root; foundation; reason, cause; wall” + adj. suffix -in.

The time required for → light to cross the classical radius of the electron
(→ electron radius): t_e = r_e/c ≅ 10^-23 s.

See also: → elementary particle; → time.

→ orbital element.

See also: → element; → orbit.

An elongated structure of → interstellar dust and gas which absorbs the radiation from background stars in an → H II region. These structures are
the denser remnants of → molecular clouds from which → massive stars are formed. Elephant trunks are eventually dissipated by the action of the → ionizing radiation and → stellar wind of the associated massive stars. A remarkable example of these structures is displayed by the → HST image of the → Eagle Nebula as → pillars of obscuring matter protruding from the interior wall of a dark molecular cloud. Some → Bok globules may represent the remaining densest fragments of elephant trunks.

Etymology (EN): M.E. elephant, from O.Fr. olifant, from L. elephantus, from Gk. elephas “elephant, ivory,” probably from a non-I.E. language, likely via Phoenician; trunk, from M.E. trunke, O.Fr. tronc, from L. truncus “stem, trunk, stump.”

Etymology (PE): Xortum “the proboscis of an elephant,” loanword from Ar. xartum; fil, pil “elephant,” from Mid.Pers. pil “elephant;” O.Pers. piru- “ivory.”

An elongated dark structure of gas and dust in the → H II region IC 1396. It spans about 5 degrees on the sky in the constellation → Cepheus, about 2400 → light-years from the Earth. The Elephant Trunk Nebula is believed to be site of star formation, containing several very young stars. It is an example of → elephant trunks associated with star forming regions.

See also: → elephant trunk; → nebula.

1. To move or raise to a higher place or position; lift up.
   

2. To raise to a higher state, rank, or office; exalt; promote (Dictionary.com).

Etymology (EN): From L. elevatus, p.p. of elevare “to lift up, raise,” from → ex- “out” + levare “lighten, raise,” from levis “light” in weight, → lever.

Etymology (PE): Bâlâyidan, from bâla “up, above, high, elevated, height,” related to boland “high,” borz, “height, → magnitude.”

The floor below a telescope used to lift observers to the level of the telescope’s eyepiece, since the telescope is tilted at varying angles when it is in use.

See also: Elevating, adj. of → elevate; → floor.

1. The height to which something is elevated or to which it rises.
   

2. An elevated place, thing, or part; an eminence (Dictionary.com).

See also: Verbal noun of → elevate; → -tion.

A → chemical reaction on solid surfaces in which one atom or molecule is → adsorbed on the catalyst surface, and another reacts directly from the gas phase. This type of mechanism may occur preferentially on very small → dust grains, where transient heating events prevent weakly bound species from remaining and in larger grains at high temperatures. Compare with the → Langmuir-Hinshelwood mechanism.

See also: Proposed in 1938 by D. D. Eley (1914-2015), a British chemist and Professor of Physical Chemistry and E. K. Rideal (1890-1974), an English physical chemist.

1. To remove or get rid of, especially as being in some way undesirable.
   

2. Math.: To remove (an unknown variable) from two or more simultaneous equations (Dictionary.com).

Etymology (EN): L. eliminatus, p.p. of eliminare “to thrust out of doors, expel,” from ex limine “off the threshold,” from → ex- “off, out” + limine, ablative of limen “threshold.”

Etymology (PE): Osândan, from Tabari uzitan, huzənniyən, hozənniyan “to expel,” from os- “out,” → ex-, + -ândan suffix of transitive verbs.

1. The act of eliminating; the state of being eliminated.
   Math.: The process of solving a system of simultaneous → equations by using various techniques to remove the → variables successively (Dictionary.com).

See also: Verbal noun of → eliminate; → -tion.

The locus of a point the sum of whose distances from two fixed points is constant.

Etymology (EN): From O.Fr. ellipse, from L. ellipsis “ellipse,” also, “a falling short, deficit,” from Gk. elleipsis “an omission,” from el-, variant of en-, + leip-, stem of leipein “to leave” + suffix -sis.

Etymology (PE): Beyzi, from Ar.

A three-dimensional geometric figure resembling a flattened sphere. It is generated by rotating an ellipse around one of its axes.

See also: From → ellipse + → -oid.

Relating to or having the form of an → ellipse. Same as → elliptical.

Etymology (EN): From Gk. elleiptikos “pertaining to an ellipse,” from elleipein “to fall short, leave out,” from en- “in” + leipein “to leave,” + → -ic.

Etymology (PE): Beyzigun, from beyzi, → ellipse,

• -gun, from
  gun “resembling; manner, fashion; color”
  (Mid.Pers. gônak “kind, species;” Av. gaona- “color”).

That part of → annual aberration proportional to the → eccentricity of the Earth’s orbit.

See also: → elliptic; → aberration.

Pertaining to or having the shape of a geometric ellipse.

See also: → elliptic; → -al.

A galaxy whose structure is smooth without spiral arms and ellipsoidal in shape. Ellipticals are redder than spirals of similar mass. Giant ellipticals contain over 10¹² solar masses, whereas dwarf ellipticals have masses as low as 10⁷ solar masses.

See also: → elliptical; → galaxy.

The → polarization of an → electromagnetic radiation in which the electric vector at any point in the path of the beam describes an ellipse in a plane perpendicular to the propagation direction. Elliptical polarization results from the combination of two perpendicular → linearly polarized waves whose → phase difference is other than 0, 90, or 180°. The form of the ellipse is determined by the amplitudes of the component waves and the phase difference. → Linear polarization and → circular polarization can be considered as limiting cases of elliptical polarization.

See also: → elliptical; → polarization.

Light exhibiting → elliptical polarization.

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

The degree of divergence of an ellipse from a circle.

Etymology (EN): From elliptic-, from elliptical + → -ity.

Etymology (PE): Beyzigi, from beyzi, → ellipse,

• -igi, → -ity.

Same as → Alnath.

See also: → Alnath.

To draw out to greater length; lengthen; extend.

Etymology (EN): From L.L. elongatus “lengthened out,” p.p. of elongare “to make longer, to remove to a distance,” from → ex- “out” + longus “long;” PIE base *dlonghos- “long;” cf. Av. darəga-, darəγa- “long,” drājištəm “longest;” Mod.Pers. derâz “long,” dir “late; long;” Skt. dīrghá- “long (in space and time);” Gk. dolikhos “long;” P.Gmc. *langgaz (Ger. lang;
O.N. langr; M.Du. lanc; Goth. laggs “long;” E. long).

Etymology (PE): DerâzidanDerâzeš “to elongate,” from derâz “long,” Mid.Pers. drâz “long;” Av. darəga-, darəγa- “long,” drājištəm “longest;” PIE *dlonghos- “long,” as above.

Made longer; long and narrow.

See also: Past participle of → elongate.

An → elliptical orbit with a high → eccentricity.

See also: → elongated; → orbit.

1. Increase in length per unit of original length.
   
2. The angular distance of a planet from the Sun as seen from the Earth. An elongation of 0° is called → conjunction; one of 180° is called → opposition; and an elongation of 90° is called → quadrature.

See also: → elongate; → -tion.

A → dimensionless quantity used in → magnetohydrodynamics to describe the relative balance of → Lorentz forces to
→ Coriolis forces. It is given by: Λ = σB²/(ρΩ), where σ s the → electrical conductivity of the fluid, B is the typical → magnetic field strength within the fluid, ρ is the fluid → density, and Ω is the → angular velocity. A typical value for the Earth is Λ ~ 1.

See also: Named after Walter Maurice Elsasser (1904-1991), American theoretical physicist of German origin; → number.

The brightest star in the constellation → Draco, with a visual magnitude of V = 2.23 and color B - V +1.52. It is a cool (4000 K) → giant star of spectral Type K5 III, lying 148 → light-years. Gamma Draconis has
a luminosity 600 times that of the Sun and a diameter 50 times that of the Sun. It crosses the sky near the zenith point for England, a nd this was the reason why James Bradley (1693-1762) observed γ Draconis when he was trying to detect parallax and so calculate the distance.
He found that the star undergoes a yearly shift of a form quite different from that expected from parallax. In a 1728 paper, Bradley announced his discovery and explained the effect as due to the → aberration of starlight . Variant names: Etamin, Etanin; Ettanin, other designations:
HR 6705, HD 164058.

See also: From Ar. At-Tinnin (التنین) “the great serpent,” the Ar. rendition of the Greek constellation → Draco.

A transient upper atmospheric phenomenon occurring over a → thunderstorm in the lower → ionosphere. Elves result from especially powerful electromagnetic radiation pulses that are generated from certain lightning discharges (→ sprite). As the energy passes upwards through the base of the ionosphere it causes the gases to briefly glow for less than a thousandth of a second. This makes elves virtually impossible to see with the naked eye. Elves occur at a height of around 90-95 km, and can expand outward to several hundred kilometers in diameter, like giant expanding doughnuts.

Etymology (EN): Short for: Emission of Light and Very low-frequency perturbations from Electromagnetic pulse sources.

Etymology (PE): From E. elf “(in folklore) a small often malicious fairy; goblin; sprite;”
O.E. elf, ælf, ylfe; cf. O.S. alf, O.N. alfr, Ger. alp, of unknown origin.