An Etymological Dictionary of Astronomy and Astrophysics

A part of the → solar corona defined by the → emission lines of hot gases. These emission lines include the so-called → forbidden lines of the strongly ionized atoms of iron, calcium, and other elements. The E
corona is thinner than the → K corona and the → F corona (M.S.: SDE).

See also: E indicating → emission; → corona.

The → Saturn’s ring, with a width of 300,000 km, lying beyond the → G ring, at 180,000-480,000 km from the center of Saturn.

See also: → ring.

The world’s largest → optical and → near infrared telescope, with a diameter of 42 m, to be built by the → European Southern Observatory (ESO).
→ First light is foreseen for 2018. The telescope has an innovative five-mirror design that includes advanced → adaptive optics to correct for the → atmospheric turbulence, giving exceptional → image quality. The → primary mirror will consist of almost 1000 hexagonal segments each 1.4 m across. The → secondary mirror will be 6 m in diameter. The E-ELT will collect 15 times more light than the largest optical telescopes operating today.

On 26 April 2010, the ESO Council selected Cerro Armazones as the baseline site for the E-ELT. Cerro Armazones is a mountain at an altitude of 3060 m in the central part of Chile’s Atacama Desert, some 130 km south of the town of Antofagasta and about 20 km from Cerro Paranal, home of ESO → Very Large Telescope.

See also: Short for the European Extremely Large Telescope

The time within which the amplitude of an oscillation increases or decreases by a factor e (= 2.71828…).

Etymology (EN): From e the base of the natural, or Napierian, system of logarithms; folding, from -fold suffix meaning “of so many parts,” or denoting multiplication by the number indicated by the stem or word to which the suffix is attached (as in twofold; manifold), from O.E. -feald, related to Ger. -falt; Gk. altos, -plos, -plus; → time.

Etymology (PE): Zamân, → time; e, as above; tâyi noun of tâ multiplicative suffix, also “fold, plait, wrinkle; like, resembling.”

1. A system for sending → messages from one individual to another via telecommunications links between → computers or → terminals using dedicated → software.
   

   2. A message sent by email (Dictionary.com).

Etymology (EN): From electronic, → electronic, + mail, from M.E. male, from O.Fr. male “wallet, bag,” of Germanic source; cf. O.H.G. malaha “wallet, bag,” M.Du. male “bag;”) from PIE *molko- “skin, bag.”

Etymology (PE): Payel, contraction of payâm-e elelktronik “electronic message,” from payâm, → message,

• elektronik, → electronic; cf. Fr. mél “e-mail,” from message + électronique, or courriel “e-mail,” from courrier + électronique.

A → polarization component in the → cosmic microwave background radiation that depends only on → gradient, is independent of → curl and does not have → handedness. In contrast to the → B-mode, the E-mode may be due to both the → scalar perturbations and → tensor perturbations.

See also: E, indicating electric-field like; → mode; → polarization.

The same as → elliptic aberration.

See also: e, → elliptic; → term; → aberration.

Same as → enstatite chondrite.

See also: → enstatite; → type; → chondrite.

A prominent → H II region in the constellation → Serpens lying about 2 kpc away from the Sun. It measures about 30’ across and surrounds the open cluster → M16 (NGC 6611), which contains at least 20 stars of spectral type B0.5 or earlier, including one 05V. At a projected distance from the cluster of about 2 pc, where the H II region has expanded into surrounding → molecular cloud, a striking → elephant trunk morphology or → pillar results. The nebula is the site of ongoing → star formation, especially in these pillar structures. The Eagle Nebula is often erroneously given the cluster’s designation (M16).

Etymology (EN): Eagle, because the nebula’s shape is vaguely reminiscent of an eagle in flight; from M.E. egle, from O.Fr. egle, from O.Prov. aigla, from L. → aquila “black eagle;” → nebula.

Etymology (PE): Miq, → nebula; &#352âhin, → aquila.

The vertebrate organ of hearing, which is also responsible for maintaining equilibrium. It is usually composed of three parts: the outer ear, middle ear, and inner ear.

Etymology (EN): M.E. ere, O.E. eare “ear” (O.N. eyra, Dan. øre, M.Du. ore, Du. oor, O.H.G. ora, Ger. Ohr); cf. Gk. aus; L. auris; Av. usi “both ears;” Lith. ausis; O.C.S. ucho; O.Ir. au; PIE *ous-.

Etymology (PE): Guš “ear” (gušidan “to hear,” niyušidan “to listen”);
Mid.Pers. gôš “ear,” gôšidan “to hear, listen;” O.Pers. gauša-; Av. gaoša- “ear,” gaoš- “to hear;”
cf. Skt. ghosa- “noise, tumult,” ghus- “to sound, cry or proclaim aloud;” PIE *ghous-.

Occurring at or near the beginning of a period of time, process, or sequence of events.

Etymology (EN): M.E. erlich (adj.), erliche (adv.); O.E. ærlice, from ær “soon, ere” (from P.Gmc. *airiz, from PIE *āier- “day, morning,” cf. Av. ayar- “day, day time,” ayarə.drājah- “duration of one day,” ayarə.bara- “day’s ride,” Mid/Mod.Pers. parēr “the day before yesterday,” from *parā.ayer- “the day before”)

• -lice “-ly,” adverbial suffix.

Etymology (PE): Âqâzin adj. of âqâz “beginning,” from Proto-Iranian *āgaHz-, from prefix ā- + *gaHz- “to run, start,” cf. Sogdian āγāz “beginning, start” (Cheung 2007).

A fairly long-lived step in the evolution of → low-mass
and → intermediate-mass stars when helium burning shifts from the center to a shell around the core. At this phase the stellar luminosity is provided almost entirely by → helium shell burning. The He-shell burning generally adds mass to the growing carbon/oxygen core, resulting in → degenerate matter due to its increasing density.

See also: → early; → AGB; → phase.

A period in the evolution of the → solar system when the planets and satellites were in the process of formation.

See also: → early; → solar; → system.

A star near the beginning of the → spectral classification sequence. A star of → spectral type O, B, A, or F0 to F5. Same as → early-type star.

See also: → early; → spectral; → class; → star.

A qualitative term used to describe a phase in the history of the Universe, from the → Big Bang event to the apparition of the first structures (seeds of future galaxies), at a → redshift around 30.

See also: → early; → universe.

In the → Hubble classification, galaxies on the left part of the → Hubble sequence.
Early-type galaxies tend to have redder colors, higher average surface brightnesses, and lower → neutral hydrogen content than → late-type galaxies. This terminology is based on the obsolete and erroneous idea that → elliptical and → lenticular galaxies might be evolutionary precursors to → spiral and → barred spiral
galaxies.

See also: → early; → type; → galaxy.

Hot, luminous stars of → spectral type O, B, A, and F0 to F5. They were originally thought, wrongly, to be at an earlier stage of evolution than → late-type stars. See also → spectral classification.

See also: → early; → type; → star.

The third planet from the Sun. At → perihelion, it is 147,099,590 km from the Sun, and at → aphelion it is 152,096.150 km, whereas its mean distance from the Sun (→ astronomical unit) is 149,598 × 10⁶ km. Its orbital period is 365.2563 days (→ sidereal year) and its → eccentricity 0.017. Other characteristics: → axial inclination 23.44°; rotation period 23.934 h (→ sidereal day); mean density 5.52 g/cm³; mass 5.974 × 10²⁴ kg; → escape velocity 11.18 km/s; average → albedo 0.37.

The Earth’s atmosphere consists of 78% nitrogen, 21% oxygen and 0.9% argon, plus carbon dioxide, hydrogen, and other gases in much smaller quantities. The atmospheric pressure at sea level is about 1,000 mbar. The surface average temperature is 15° C, but it varies, on the average, between -50° C (winter, Siberia) and up to + 40° C (summer, Sahara).

Liquid water covers 71% of the surface. Over 5,000 active volcanoes have been registered throughout man’s known history. The outer layer of the planet, the → lithosphere, is covered with the → crust. In the → upper mantle and beneath the lithosphere, lies → asthenosphere.
Convection in the asthenosphere has caused → plate tectonics motions and continent drifts. The densest layer of the Earth is its → core, about 3000 km to 6400 km beneath the surface, consisting primarily of iron and nickel. This core is believed to be at the origin of the magnetic field, which reaches about 3 × 10^-5 tesla near the equator. It has only one natural satellite, the Moon (M.S.: SDE).

Etymology (EN): M.E. erthe, from O.E. eorðe “ground, soil, dry land;” cf. O.N. jörð, M.Du. eerde, O.H.G. erda, Goth. airþa; from PIE base *er-.

Etymology (PE): Zamin, variant zami “earth, floor, land,” Mid.Pers. zamig, Av. zam- “the earth;” cf. Skt. ksam- “the ground, earth;” Gk. khthôn, khamai “on the ground;” L. homo “earthly being” (as in homo sapiens,
homicide, humble, humus, exhume), humus “the earth;” O.Russ. zemi “land, earth;” PIE root *dh(e)ghom “earth”.

The mass of our planet Earth, which is 5.9736 × 10²⁴ kg (3 × 10^-6 → solar masses), 317.83 times smaller than the → Jupiter mass. The Earth mass is in particular used to describe the mass of → super-Earth  → extrasolar planets.

See also: → Earth; → mass.

The distance from the Earth’s center to its surface, about 6,371 km.

See also: → earth; → radius.

An → asteroid or → comet whose → orbit occasionally brings it relatively close to the Earth. → near-Earth object.

See also: → Earth; → grazer.

A physical system composed on the → Earth and the → Moon in which both objects directly influence each other. The total energy in the Earth-Moon system is conserved. The most notable influence that the two objects have on each other is → tides.

See also:

→ tidal braking, → tidal bulge, → tidal capture, → tidal coupling, → tidal disruption, → tidal force, → tidal friction, → tidal heating, → tidal locking, → tidal radius, → tidal stretching.

See also: → Earth; → Moon; → system.

The innermost part of the Earth consisting of a solid → inner core, mainly composed of → iron, and a → liquid → outer core. The → pressure and → temperature are so extreme that the molten iron solidifies. The temperature at the inner core boundary is expected to be close to the
→ melting point of iron at 330 gigapascal (GPa). From static laser-heated diamond anvil cell experiments up to 200 GPa, using synchrotron-based fast → X-ray diffraction as a primary melting diagnostic, S. Anzellini et al. (2013, Science 340, 484)
conclude that the melting temperature of iron at the inner core boundary is 6230 ± 500 K. This estimation favors a high heat flux at the core-mantle boundary with a possible partial melting of the → mantle. The inner core, 2,400 km in diameter, is suspended in the molten metal of the → outer core, which is about 2,240 km thick. The temperature difference between the mantle and the core is the main engine for large-scale thermal movements, which coupled with the → Earth’s rotation, function as a generator for the planet’s → magnetic field.

See also: → Earth; → core.

The rocky outermost layer of the Earth,
ranging from about 10 to 65 km in thickness.

It is distinguished from the underlying the → Earth’s mantle layer by its more → silicon- and → aluminium-rich composition, lower density, and the lower velocity at which it conducts seismic energy.

It includes → continental crust (about 40 km thick) and → oceanic crust (about 7 km thick). The crust and the topmost layer of the mantle form the → lithosphere.

The five most abundant → chemical elements in the Earth’s crust are, in percentage by weight of the Earth’s crust: → oxygen (O) 46%, silicon (Si) 28%, aluminium (Al) 8%, → iron (Fe) 5%, and → calcium (Ca) 4%.

See also: → Earth; → crust.

A major subdivision of Earth’s internal structure, located beneath the → Earth’s crust and above the central → core. On average, the mantle begins 35 km below the surface and ends at a depth of about 2,900 km. See also → upper mantle and → lower mantle, → asthenosphere, → lithosphere.

See also: → Earth; → mantle.

The natural motion of the Earth around its own axis, which takes place once in a → sidereal day. The Earth rotates toward the → east, in the same direction as it revolves around the Sun. If viewed from the north celestial pole, the Earth turns → counterclockwise. The opposite is true when the Earth is viewed from the south celestial pole. The Earth’s rotation is responsible for the diurnal cycles of day and night, and
also causes the apparent movement of the Sun across the sky. The Earth’s rotation velocity at the → equator is 1,673 km h^-1 or about 465 m s^-1.
More generally, at the → latitude  φ it is given by: v_φ = v_eq cos φ, where v_eq is the rotation velocity at the equator.

The Earth’s rotation is gradually slowing down under the action of the → tides, which are generated by the → gravitational attraction of the → Moon. As the result of this → tidal friction, the day is becoming
longer at a rate of about 2 milliseconds, or 0.002 seconds, per century (or one second every 50,000 years). Moreover, the loss of the Earth’s → rotational angular momentum increases the Moon’s → orbital angular momentum, because the angular momentum of the → Earth-Moon system is conserved. In consequence,
the Moon slowly recedes from the Earth by about 4 cm per year, which leads to increasing its orbital period and the length of a month as well.

See also: → Earth; → rotation.

The illumination of the dark part of the Moon’s disk by the light reflected from the Earth’s surface and atmosphere. Also called → earthshine.

See also: → earth; → light.

Sudden shaking of the → Earth’s surface caused by the passage of a → seismic wave whose mechanical effects can be destructive. See also → starquake.

See also: → earth; → quake.

The visibility of that part of the Moon not illuminated by the Sun. The phenomenon is caused by the solar light reflected by the Earth. It was explained correctly for the first time by Leonardo da Vinci (M.S.: SDE). Same as → earthlight.

See also: → earth; → shine.

The point on the → celestial horizon 90° → clockwise from the → north point. The point where the Sun rises at the → equinoxes.

Etymology (EN): O.E. east, from P.Gmc. *aus-to-, *austra- “east, toward the sunrise” (cf. Du. oost, Ger. Ost, O.N. austr “from the east”), from PIE *aus- “dawn” (cf. Av. uš-, ušah- “dawn,” Skt. usas-, usah- “dawn,” Gk. aurion “morning,” Lith. auszra “dawn,” L. aurora “dawn,” auster “south”).

Etymology (PE): Xâvar “east,” originally “west,” from Mid.Pers. *xvar barân “where the Sun is led,” from xvar “sun” (Av. hū-, hvar- “sun”, cf. Skt. surya-, Gk. helios, L. sol, O.H.G. sunna, Ger. Sonne, E. sun; PIE base *sawel- “sun”) + barân, pr.p. of bar-, bordan “to carry, lead” (Mid.Pers. burdan,
O.Pers./Av. bar- “to bear, carry,” barəθre “to bear (infinitive),” Skt. bharati “he carries,” Gk. pherein, L. fero “to carry;” PIE base *bher- “to carry”)

• time and place suffix -ân.

Lying toward or situated in the east.

Etymology (EN): From → east + -ern an adjective suffix occurring with names of directions.

Etymology (PE): Xâvari from xâvar, → east, + -i adj. suffix.

The position of a planet when it can be seen in the western sky just after sunset.

See also: → eastern; → elongation.

1. Not hard or difficult; requiring no great labor or effort.
   

2. Free from pain, discomfort, worry, or care (Dictionary.com).

Etymology (EN): M.E. aisie, esy, from O.Fr. aisie “comfortable, at ease, rich, well-off,” p.p. of aisier “to put at ease,” from aise “comfort, pleasure, well-being,” of unknown origin.

Etymology (PE): Âsân “easy,” from Mid.Pers. âsân “calm, quiet, at rest, peaceful, easy,” related to âsudan “to rest, repose;” Av. ā- + saē- (saii-) “to lie down, go to sleep,” → holiday.

To listen secretly to a private conversation.

Etymology (EN): Probably back-formation from → eavesdropper.

Etymology (PE): Gušârdan, from gušâr, → eavesdropper,

• -dan infinitive suffix.

A person or thing that secretly listens to or monitors the private conversation or data of others.

Etymology (EN): From M.E. eavesdrop, from O.E. yfesdrype “place around a house where the rainwater drips off the roof,” literally “one who stands on the eavesdrop in order to listen to conversations inside the house,” from eaves “the lower border of a roof that overhangs the wall”

• drip, drop “to fall in small portions.”

Etymology (PE): Gušâr in Tabari “eavesdropper,” from guš, → ear, + -âr probably contraction of dâr (as in dustâr/dustdâr) agent noun from dâštan “to hold, have, possess; appoint, erect,” → property.

The monitoring and/or examining the data that is passed over the network without sender and receiver’s permission and/or knowledge. For example, a user on the Internet could eavesdrop on someone’s phone conversation or e-mail.

See also: Noun from → eavesdrop.

A → tidal current that generally moves seaward and occurs during the part of the tide cycle when sea level is falling.

Etymology (EN): M.E. eb(be); O.E. ebba; cognate with O.Fr. ebba, Du. eb(be), Ger. Ebbe ebb, M.E. ebben, O.E. ebbian, derivative of the noun; akin to → off; PIE base *apo- “off, away;” → tide.

Etymology (PE): Bâzkeš, from bâz- a suffix denoting “reversal, repetition, opposition,” → re-, + keš present stem of kešidan “to draw, drag, carry,” → tide.

1. An orbit that has a high → eccentricity, i.e. is highly elliptical.
   

2. In Ptolemy’s geocentric model, a → deferent which is slightly off-center from the Earth.

See also: → ex-; → center; → -ic.

Of a planetary orbit, the angle measured from the perihelion position, to the center of the circumscribing auxiliary circle, to the projected position of the planet on the circle.
→ anomaly; → mean anomaly;
→ true anomaly.

See also: → eccentric; → anomaly.

The amount by which the orbit deviates from circularity: e = c/a, where c is the distance from the center to a focus and a the semi-major axis. If e = 0, the orbit is a circle. If e < 1, the orbit is an ellipse, if e > 1 it is a hyperbola, and if e = 1 it is a parabola. The eccentricity is one of the six → orbital elements that define a → Keplerian orbit.

See also: → eccentric; → -ity.

A diffraction grating in which the groves are relatively widely spaced and
serves to provide high resolution and dispersion.

Etymology (EN): Echelle, from Fr. échelle “ladder,” , from O.Fr. eschele, from
L. scala “ladder;” → grating.

Etymology (PE): Turi, → grating; nardé, contraction of nardebân “ladder; échelle.”

A spectrograph that uses an echelle grating to disperse the light.

See also: → echelle grating; → spectrograph.

Acoustics: Effect produced when sound is reflected or thrown back on meeting a solid obstacle.
Radio.:
A wave returned to the transmitter with sufficient magnitude and delay to be distinguished from the directly transmitted wave. In radar, the portion of the energy of the transmitted pulse reflected back to the receiver.

Etymology (EN): From L. echo, from Gk. echo, personified as a mountain nymph, from ekhe “sound.”

Etymology (PE): Pažvâk, literally “return sound,” from paž “back, against, opposite,” varaint pâd- (Mid.Pers. pât-, from O.Pers. paity “agaist, back, opposite to, toward, face to face, in front of;” Av. paiti, akin to Skt. práti “toward, against, again, back, in return, opposite;” Pali pati-; Gk. proti, pros “face to face with, toward, in addition to, near;” PIE *proti)

• vâk “sound,” Mid./Mod.Pers. vâng/bâng “sound, clamour;” Av. vacah- “word,” from vac- “to speak, say;” cf. Mod.Pers. vâžé “word,” âvâz “voice, sound, song” (Skt. vakti “speaks, says,” vacas- “word;”
  Gk. epos “word;” L. vox “voice;” PIE base *wek- “to speak”).

The passage of the shadow of a celestial body over the surface of another. The maximum number of solar and lunar visible eclipses occurring annually is seven; the minimum number is two, both being solar. → Solar eclipses take place when the new Moon is close to an
→ orbital node and on the same longitude with the Sun. At that moment either the → umbra, → antumbra, or the → penumbra touches the Earth’s surface. For an observer located in the umbra the eclipse is total, while for one placed in the antumbra it is annular. → Annular eclipses occur around lunar → apogee. An observer situated in the penumbra sees only a → partial eclipse. A total or annular eclipse can be seen from a band with a width of 270 km at the most, around which, the much larger partiality zone extends. The Moon’s shadow crosses the Earth from west to east at about 3,200 km/h. During
→ total eclipses the Sun’s disk is entirely covered and the → solar corona can be seen. A solar eclipse can last up to 3 h (between the first and the → fourth contacts). Totality has a theoretical maximum duration of 7m 31s, but it is usually shorter. A → lunar eclipse can be seen from any place on Earth where the Moon is above the horizon; it occurs when the full Moon passes through the central dark shadow of the Earth. The Earth’s shadow is much wider than the Moon and this is why the lunar eclipses can last up to four hours (between the first and the fourth contact) (M.S.: SDE).

Etymology (EN): From O.Fr. éclipse, from L. eclipsis, from Gk. ekleipsis “a leaving out, forsaking, an eclipse,” from ekleipein “to forsake a usual place, fail to appear, be eclipsed,” from ek “out,” → ex-,

• leipein “to leave.”

Etymology (PE): Gereft, past stem of gereftan “to obscure, close up; to take, seize, catch; to undergo an eclipse,” from Mid.Pers. griftan, Av./O.Pers. grab- “to take, seize,” cf.
Skt. grah-, grabh- “to seize, take,” graha “seizing, holding, perceiving” (see also → concept); cf. M.L.G. grabben “to grab;” E. grab “to take or grasp suddenly;” PIE base *ghrebh- “to seize”.

The fraction of the Sun’s diameter occulted by the Moon. It is strictly a ratio of diameters and should not be confused with → eclipse obscuration, which is a measure of the Sun’s surface area occulted by the Moon. Eclipse magnitude may be expressed as either a percentage or a decimal fraction (e.g., 50% or 0.50). By convention, its value is given at the instant of → greatest eclipse (F. Espenak, NASA).

See also: → eclipse; → magnitude.

A method for imaging the continuum light distributions of the → accretion disks of → cataclysmic variable stars. It relies on geometrical information contained in eclipse light curves. An alternative method is → Doppler tomography.

See also: → eclipse; → mapping.

The fraction of the Sun’s area occulted by the Moon. It should not be confused with → eclipse magnitude,
which is the fraction of the Sun’s diameter occulted by the Moon. Eclipse obscuration may be expressed as either a percentage or a decimal fraction (e.g., 50% or 0.50) (F. Espenak, NASA).

See also: → eclipse; obscuration, verbal noun from → obscure.

The period during which the Sun is close enough to one of the → lunar orbit nodes so that an eclipse can take place. This time window lasts for 37 days for → solar eclipses and almost 24 days for → lunar eclipses. These seasons occur every 173.31 days. Two eclipse seasons make up an → eclipse year.

See also: → eclipse; → season.

The interval of time (346.620 03 days) between two successive passages of the Sun through the same node of the Moon’s orbit. It takes less than a solar year to complete an eclipse year because the Moon’s orbit and the lunar nodes are slowly regressing.

See also: → eclipse; → year.

A binary star in which one of the two stars passes in front of the other so that the system’s total light periodically fades. The most famous eclipsing binary is → Algol.

See also: → eclipse; → binary.

Same as → eclipsing binary.

See also: → eclipse; → variable.

The Sun’s apparent path in the sky relative to the stars in the course of a year. It is also the projection of the Earth’s orbital plane onto the → celestial sphere. Because of the inclination of the → Earth’s rotation axis, the ecliptic is tilted by about 23.4° with respect to the → celestial equator, an angle known as the → obliquity of the ecliptic. The ecliptic crosses the celestial equator at the → equinoxes.

Etymology (EN): From L. ecliptica linea “path of eclipses,” so called because eclipses happen only when the Moon is near this path, from eclipsis,
→ eclipse.

Etymology (PE): Hurpeh “sun path,” from hur “sun,” variant xor, cognate with Gk. helios, → Sun, + peh “path, way,” from O.Pers. paθi- “path, way;” Av. paθ-, variants paθi-, paθā-, pantay-;
Mid/Mod.Pers. pand “path, advice, councel;” Khotanese pande “road, path;” Ossetic fœndœg “path, road;”
cf. Skt. pánthā- “road, path, course;” Gk. patos “path, way;”
L. pons “bridge, path;” E. find; PIE base *pent- “to go, to tread.”

One of the two coordinates in the → ecliptic system; the angle measured from the ecliptic, positive toward the north.

See also: → ecliptic; → latitude.

One of the two coordinates in the → ecliptic system; the angle measured eastwards along the ecliptic from 0° to 360°, with the origin at the → vernal equinox.

See also: → ecliptic; → longitude.

The plane of Earth’s orbit around the Sun.

See also: → ecliptic, → plane.

A catalog of fields along the → ecliptic observed by the → K2 mission. The catalog is hosted at the → Mikulski Archive for Space Telescopes (MAST).

See also: → ecliptic; → plane; → input; → catalogue.

Either of the two points on the celestial sphere that are 90° above and below the plane of ecliptic. The north ecliptic pole lies in → Draco, and the south ecliptic pole in → Dorado. Due to → precession, the → celestial pole moves in a circle around the ecliptic poles once every 25,800 years.

See also: → ecliptic, → pole.

Coordinate system with the ecliptic as the fundamental plane.

See also: → ecliptic; → system.

A combining form meaning “house, household, environment, nature, natural habitat.”

Etymology (EN): Ultimately from Gk. oikos “house,” cognate with L. villa “country house, farm,” related to vicus “village, group of houses; " cf. Av. vis- “homestead, community;” O.Pers. viθ- “house, royal house, farm;” Mid.Pers. wis “village;”
dialectal Pers. wiš-, vīš- “to set (of Sun),” wīs- “to enter” (Cheung 2007); Skt. viś- “settlement, house, tribe, people;”
Goth. weihs “village;” Lith. viešpats “master of the house;” PIE *ueik’- “to settle (down).”

Etymology (PE): Bum-, from bum “region, land, a mansion or place where one dwells in saftey; nature, disposition;” Mid.Pers. bûm “land, earth, country;” O.Pers. būmi- “place of being/living, land, region;” Av. būmī- “earth,” from bav- “to be, become, take place;” cf. Skt. bhūmi- “land, region;” PIE *bheu- “to be, come into being, become” (cf. Gk. phu- “become,” phuein “to bring forth, make grow;” L. fui “I was” (perf. tense of esse), futurus “that is to be, future;” Ger. present first and second person sing. bin, bist; E. to be; O.Ir. bi’u “I am;” Lith. bu’ti “to be;” Rus. byt’ “to be”).

1. The scientific discipline that is concerned with the relationships between living organisms and their past, present, and future environments.
   

2. The study of the damaging effects of modern civilisation on the environment, with the aim of prevention or reversal through conservation.

See also: → eco- + → -logy.

1. Pertaining to the production, distribution, and use of income, wealth, and commodities.
   

2. Of or relating to the science of economics.
   

3. Pertaining to an economy, or system of organization or operation, especially of the process of production (Dictionary.com).

See also: Adjective, from → economics.

An increase in the output that an economy produces over a period of time.

See also: → economic; → growth.

1. Avoiding waste or extravagance; thrifty.
   

2. → economic.

See also: → economic; → -al.

The science that deals with description and analysis of the production, distribution, and consumption of goods and services.

Etymology (EN): From L. oeconomicus “well ordered,” from Gk. oikonomikos “practiced in the management of a household or family,” from oikonomia, from oiko- “house,” → eco-,

• -nomia “rule, law,” → -nomy; + → -ics.

Etymology (PE): Bum, → eco- + -dât,
→ -nomy, + -ik, → -ics.

1. To practice economy; avoid waste or extravagance.
   

2. To manage economically; use sparingly or frugally (Dictionary.com).

See also: From econom(y), → economy, + → -ize.

1. Thrifty and efficient use of material resources of a community, society, or household; frugality in expenditures.
   

2. An act or means of thrifty saving; a saving.
   

3. The management of the resources of a community, country, etc., especially with a view to its productivity (Dictionary.com).

Etymology (EN): From M.Fr. economie, → economics.

Etymology (PE): Bumdât, back formation from bumdâti, → economic.

The fabrication of a stable, enduring → ecosystem on a lifeless planet. It is the last stage of a → terraforming process.

Etymology (EN): From → eco- “house, dwelling place,” + poiesis a combining form meaning “making, formation; poetry,” from Gk. poesy, from poiein “to make, compose.” Coined by Robert Hall Haynes (1931-1998), a Canadian geneticist and biophysicist.

Etymology (PE): Bum-âfarini, literally “creating environment,” from bum “region, land,” → eco- + âfarini “creation,” from âfaridan, âfarin- “to create,” from Mid.Pers. âfridan, âfrin- “to praise, bless; create,” ultimately from Proto-Iranian āfrīta-, from prefixed frī- “to praise; to like;” cf. Av. frāy- “to satisfy, propitiate, pray;” Skt. prī- “to gladden, show favor to,” prīta- “glad;” Gk. praus “gentle.”

The space around a star in which a planet would experience external conditions that are not incompatible with the existence of life.

Etymology (EN): Ecosphere, from eco-, → ecology,

• → sphere.

Etymology (PE): Bumsepehr, from bum “eco-,” → ecology,

• sepehr, → sphere.

Any geographic area that includes all of the organisms and nonliving parts of their physical environment.

See also: → ecology; → system.

Same as → Eddington parameter.

See also: → Eddington limit; → factor.

The theoretical upper limit of → luminosity at which the → radiation pressure of a light-emitting body would exceed the body’s → gravitational attraction. A star emitting radiation at greater than the Eddington limit would break up. The Eddington luminosity for a non-rotating star is expressed as:
L_Edd = 4πGMm_pcσ_T^-1, where G is the → gravitational constant, M the star mass, m_p the → proton mass, c the → speed of light, and σ_T the → Thomson cross section. It can also be written as L_Edd = 4πGMcκ^-1, where κ is the → opacity. In terms of solar mass, the Eddington limit can be expressed by:
L_Edd = 1.26 × 10³⁸ (M/Msun) erg s^-1. See also → rotational Eddington limit.

See also: Named after Arthur Stanley Eddington (1882-1944), prominent British astrophysicist; → limit.

Same as → Eddington limit.

See also: → Eddington limit; → luminosity.

A → dimensionless parameter indicating the degree to which a star is close to the → Eddington limit. It is expressed as Γ = L / L_Edd = κ L / (4πGMc), where L and M are the star luminosity and mass respectively, κ is the opacity, c the speed of light, and G the → gravitational constant.
At the Eddington limit, Γ = 1, the star would become unbound. Because stellar luminosity generally scales with a high power of the stellar mass (L∝ M^3-4), → massive stars with M larger than 10 Msun generally have electron Eddington parameters of order Γ ≅ 0.1-1.

See also: After Arthur Stanley Eddington (1882-1944), prominent British astrophysicist; → parameter.

A theoretical model in which the → cosmological constant plays a crucial role by allowing an initial phase that is identical to the Einstein static Universe. After an arbitrarily long time, the Universe begins to expand. The difficulty with this model is that the initiation of galaxy formation may actually cause a collapse rather than initiate an → expansion of the Universe.

See also: → Eddington limit; Lemaître in honor of
Georges-Henri Lemaître (1894-1966), a Belgian Roman Catholic priest, who
first proposed the Big Bang theory; → universe.

The time required for the redistribution of → angular momentum due to → meridional circulation. The Eddington-Sweet time for a uniformly → rotating star is expressed as:

τ_ES = τ_KH . GM / (Ω² R³), where τ_KH is the → Kelvin-Helmholtz time scale, R, M, and L designate the radius, mass, and luminosity respectively, Ω the → angular velocity, and G the → gravitational constant. The Eddington-Sweet time scale can be approximated by τ_ES≅ τ_KH / χ, where χ is the ratio of the → centrifugal force to
→ gravity. For the Sun, χ ≅ 10^-5 resulting in an Eddington-Sweet time scale which is too long (10¹² years), i.e. unimportant. In contrast, for a rotating → massive star  χ is not so much less than 1. Hence the Eddington-Sweet circulation is very important in massive stars.

See also: Named after the prominent British astrophysicist Arthur S. Eddington (1882-1944), who was the first to suggest these currents (in The Internal Constitution of the Stars, Dover Pub. Inc., New York, 1926) and P. A. Sweet who later quantified them
(1950, MNRAS 110, 548); → time scale.

A deviation in the steady flow of a fluid causing a vortex-like motion running contrary to the general flow.
Meteo.: A small disturbance of wind in a large wind flow, which can produce turbulent conditions or turbulence.

Etymology (EN): Late M.E., from O.E. ed- “turning” + ea “water;” akin to O.N. itha.

Etymology (PE): Gižâv, from dialectical Kurd. “whirlpool,” from giž “turning,” Pers. gij “giddy-headed, vertigio;” Laki géž “whirlpool,” vagéža “whirlwind”

• âv, variant âb “water” (Mid.Pers. âb “water;” O.Pers. ap- “water;” Av. ap- “water;” cf. Skt. áp- “water;”
  Hitt. happa- “water;” PIE āp-, ab- “water, river;”
  cf. Gk. Apidanos, proper noun, a river in Thessalia; L. amnis “stream, river” (from *abnis); O.Ir. ab “river,” O.Prus. ape “stream,” Lith. upé “stream;” Latv. upe “brook”).

An induced current circulating in masses of metal moving in a magnetic field or located in a changing magnetic field. Also known as → Foucault current.

See also: → eddy; → current.

A macroscopic process that occurs in a → fluid because of the relative motions induced by the non-uniform → turbulent motions of the fluid.
Also known as turbulent → diffusion. Eddy diffusion may occur in an atmosphere if it is unstable against turbulence. It dominates the atmosphere below the homopause. See also → molecular diffusion.

See also: → eddy; → diffusion.

1. A line or border at which a surface terminates.
   

2. The thin, sharp side of the blade of a cutting instrument or weapon (Dictionary.com).
   

3. In → graph theory, any line in a → graph that joins two distinct → nodes. Any pair of → vertices.

Etymology (EN): M.E. egge; O.E. ecg “corner, edge;” cf.
Ger. Eck “corner;” PIE base *ak- “sharp, pointed” (cf. L. acies; Gk. akis “point”).

Etymology (PE): 1) Labé “limb, edge,” from lab “lip;” Mid.Pers. lap; cognate with L. labium; E. lip; Ger. Lefze.

2. Tiqé “blade of a knife or sword,” from tiq “blade,” 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.

A → spiral galaxy oriented edge-on to our view. → face-on galaxy.

Etymology (EN): → edge; on, from O.E. on, variant of an “in, on, into”
(cf. Du. aan; Ger. an; Goth. ana “on, upon”), from PIE base *ano “on” (cf. Av. ana “on;” Gk. ana “on, upon;” L. an-); → galaxy.

Etymology (PE): Kahkašân, → galaxy; pahlunemâ “showing the side,” from pahlu, → side, + nemâ, from nemudan “to show, display,” → display.

To modify or prepare for publication or public presentation by checking,
improving, cutting, rearranging, etc.

Etymology (EN): Back formation from editor or from Fr. éditer, or from L. editus, p.p. of edere “bring forth, produce,” from → ex- “out,” + -dere, combining form of dare “to give,” → datum.

Etymology (PE): Virâstan, from Mid.Pers. virâstan “to arrange, prepare, restore,”
from prefix vi- “apart, away from, out” (Av. vi-; O.Pers. viy- “apart, away;” cf. Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”) + râstan “to arrange, to set in order,” → coordinate.

1. The act or process of editing.
   

2. A version of anything, printed or not, presented to the public.

See also: Verbal noun of → edit.

1. A person who edits material for publication or public presentation.
   

2. A computer program that enables creating and editing text files.

See also: Agent noun of → edit.

1. An article in a newspaper or other periodical or on a website presenting the opinion of the publisher, writer, or editor.
   

2. A statement broadcast on radio or television that presents the opinion of the owner, manager, or the like, of the program, station, or channel (Dictionary.com).

See also: → editor + -i- + → -al.

To give knowledge or develop the faculties and powers of somebody by teaching. instruction.

Etymology (EN): From L. educatus, p.p. of educare “bring up, rear, educate,” from → ex- “out” + ducere “to lead.”

Etymology (PE): Farhixtan, “educate,” literally “drawing forward, pulling up;” from far- perfection prefix, → pro-, + hanjidan “to draw,” → culture.

The act or process of educating.
The knowledge or abilities gained through being educated.

See also: Verbal noun of → educate.

1. Something brought about by a → cause or agent; a result.
   

2. A scientific law, hypothesis, or phenomenon, such as the → Compton effect, → Coriolis effect, → Doppler effect, → diamond ring effect, → photoelectric effect, and so on.

Etymology (EN): From O.Fr. effect, from L. effectus “accomplishment, performance,” from stem of efficere “to work out, accomplish,” from → ex- “out” + facere “to do, to make,” from PIE base *dhe- “to put, to do;” cf. Mod.Pers. dâdan “to give,”
Mid.Pers. dâdan “to give,” O.Pers./Av. dā- “to give, grant, yield,” dadāiti “he gives;” Skt. dadáti “he gives,” Gk. didomi “I give,” tithenai “to put, set, place;” L. dare “to give, offer;” Rus. delat “to do;” O.H.G. tuon, Ger. tun, O.E. don “to do.”

Etymology (PE): Oskar, from os-, → ex-, + kar-, kardan “to do, make;” Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build,” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make.”

Actually in operation or in force; concrete; real.

Etymology (EN): M.E., from L. effectivus “practical,” from effect(us), p.p. of efficere, → effect + -ivus “-ive.”

Etymology (PE): Oskarmand, from oskar, → effect + -mand possession suffix.

Of an antenna, the ratio of the extracted power by the antenna to the power per unit area (power density) in the incident wave.

See also: → effective; → aperture.

The effective value of the → Eddington parameter in a non-homogeneous system (porous opacity).

See also: → effective; → Eddington limit; → parameter.

The focal length of an imaging system, which consists of several lenses or mirrors.

See also: → effective; → focal length.

In a → rotating star, the sum of the → gravity and the → centrifugal acceleration. The effective gravity is a function of the rotation velocity (Ω) and the → colatitude (θ). At the pole (θ = 0°) and the equator (θ = 90°) the effective gravity is radial. See also → total gravity.

See also: → effective; → gravity.

Of a galaxy, the distance from its center within which half of the total luminosity is included.

See also: → effective; → radius.

A measure of the surface temperature of a star derived from the total emitted energy,
assuming that the star is a → blackbody emitter (→ Stefan-Boltzmann law, → Planck’s radiation law). See also
→ brightness temperature; → color temperature.

See also: → effective; → temperature.

The degree to which goals are achieved and the extent to which posed problems are solved. Compare → efficiency.

See also: → effective; → -ness.

Power or capacity to produce a desired effect; → effectiveness.

Etymology (EN): From L. efficacia “efficacy, efficiency,” from efficax “powerful, effectual, efficient,” from stem of efficere “accomplish,” → effect.

Etymology (PE): Oskarmandi, → effectiveness.

1. The state or quality of being efficient; competence. Compare → effectiveness.
   

Of a machine, engine, or device, the ratio of the physical quantity which may be stored, transferred, or transformed to the total input quantity.

Etymology (EN): L efficientia, from efficient-, → effect,

• -ia “-y,” an E. suffix of adjectives.

Etymology (PE): Kârâyi, from kârâ “efficient,” from kâr, → work + â present stem of âmadan “to come,” from Av. ay- “to go, to come,” aēiti “goes,” O.Pers. aitiy “goes,” Skt. e- “to come near,” eti “arrival,” Gk. eimi “I go,” L. eo “I go,” Tokharian AB i-; PIE *ei- “to go, to walk.”

Outward flow of a → liquid. Something that → flows out.

Etymology (EN): L. effluxus, p.p. of effluere “to flow out,” from → ex- “out” + fluere “to flow,” → flux.

Etymology (PE): Zošâr, from zo- “out of, from,” → ex-, + šâr, → flux.

1. An oval or round object laid by a female bird, reptile, fish, or invertebrate, usually containing a developing embryo. The eggs of birds are enclosed in a chalky shell, while those of reptiles are in a leathery membrane.
   

   2. Biology: The female reproductive cell in animals and plants; an ovum (OxfordDictionaries.com).

Etymology (EN): M.E., from Old Norse egg, cognate with O.Saxon, M.Du., Du., O.H.G., Ger. Ei, probably from PIE *owyo-/*oyyo- “egg;” source of Pers. xâg, as below.

Etymology (PE): Toxm, → seed.
Xâg “egg,” Lori, Laki xâ, Pash. hâ “egg,” Ossetic ajk “egg,” Bojnurdi hek “egg,” Khotanese āhaa- “egg;” variant xâyé “egg; testicle;” Mid.Pers. xâyak “egg;” Av. aēm/aiam “egg;” cf. Gk. oion, L. ovum; Goth. ada; O.E. æg; Ger. Ei; PIE *owyo-/*oyyo- “egg.”

The reappearance of a celestial body after an eclipse, an occultation, or a transit; same as emersion. → ingress.

Etymology (EN): From L. egressus, from egredi “to go out,” from → ex- “out”

• -gredi, comb. form of gradi “to walk, go, step;” from PIE *ghredh- (cf. Lith. gridiju “to go, wander,” O.C.S. gredo “to come”).

Etymology (PE): Osgâm “going out,” from os- “out,” → ex-, + gâm “step, pace,” Mid.Pers. gâm, O.Pers. gam- “to come; to go,” Av. gam- “to come; to go,” jamaiti “goes,” Mod.Pers. âmadan “to come,” 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.”

Same as → extreme horizontal branch star.

See also: → extreme horizontal branch star.

1. Math.: An → eigenvector for a linear → operator on a → vector space whose vectors are → functions. Also known as proper function.
   

2. Quantum mechanics: A → wave function corresponding to an → eigenvalue. Eigenfunctions represent the stationary → quantum states of a system.

Etymology (EN): From Ger. Eigenfunktion, from eigen- “characteristic, particular, own” (from P.Gmc. *aigana- “possessed, owned,” Du. eigen, O.E. agen “one’s own”) + → function.

Etymology (PE): Viž-karyâ, from viž, contraction of vižé “particular, charcteristic” + karyâ, → function. Vižé, from Mid.Pers. apēcak “pure, sacred,” from *apa-vēcak “set apart,” from prefix apa- + vēcak, from vēxtan (Mod.Pers. bixtan) “to detach, separate, sift, remove,” Av. vaēk- “to select, sort out, sift,” pr. vaēca-, Skt. vic-, vinakti “to sift, winnow, separate; to inquire.”

Quantum mechanics: A dynamical state whose state vector (or wave function) is an → eigenvector of an → operator corresponding to a specified physical quantity.

See also: → eigenfunction; → state.

1. Math.: The one of the → scalars λ such that T(v) = λv, where T is a linear → operator on a → vector space, and v is an → eigenvector.
   

2. Quantum mechanics: The specified values of → quantized energy for which the → Schrodinger equation is soluble, subject to the appropriate → boundary conditions.

See also: → eigenfunction; → value.

Math.: A nonzero vector v whose direction is not changed by a given linear transformation T; that is, T(v) = λ v for some scalar λ.

See also: → eigenfunction; → vector.

A → cardinal number between → seven and → nine.

Etymology (EN): M.E. eighte, from O.E. eahta, æhta, related to O.Norse atta, Swed. åtta, Du. acht, O.H.G. Ahto, Ger. acht; Pars. hašt, as below, from PIE *okto(u) “eight.”

Etymology (PE): Hašt, from Mid.Pers. hašt; Av. ašta; cognate with Skt. asta; Gk. okto; L. octo (from which It. otto, Sp. ocho, Fr. huit).

A unit of radiation energy sometimes used in the investigation of
photochemical processes. The unit is defined as
N_Ahν, where N_A is → Avogadro’s number and hν is the energy of a → quantum of the radiation. One einstein (or Einstein unit) is the energy per → mole of photons carried by a beam of monochromatic light.

See also: Named for Albert Einstein (1879-1955).

A measure of the probability that a particular atomic transition leading to the formation of an atomic spectral line occurs. The coefficient of spontaneous emission is denoted by A_ij, and the coefficient of stimulated emission by B_ij, i representing the lower level and j is the upper level.

See also: Named after Albert Einstein (1879-1955) who introduced the coefficients in 1916; → coefficient.

An image of a distant quasar (redshift 1.7) formed by a foreground spiral galaxy (redshift 0.039) through gravitational lensing. The image of the quasar is split into four point sources forming a cross at the center of the galaxy.

See also: → Einstein; → cross.

The → equivalence principle as stated by Einstein, on which is based the theory of → general relativity. It comprises
the three following items:

1. The → weak equivalence principle is valid.
   

2. The outcome of any local non-gravitational experiment is independent of the velocity of the freely-falling → reference frame in which it is performed. Also known as → local Lorentz invariance.
   

3. The outcome of any local non-gravitational experiment is independent of where and when in the Universe it is performed. Also called
   → local position invariance.

See also: → Einstein; → equivalence; → principle.

A model for the → specific heat of solids in which the specific heat is due to the vibrations of the atoms of the solids. The vibration energy is → quantized and the atoms have a single frequency, ν. Put forward in 1907 by Einstein, this model was the first application of → quantum theory to the solid state physics. The expression for the specific heat is given by: C_V = 3Rx²e^x/(e^x -1)², where R is the → gas constant, x = T_E/T, T_E = hν/k, h is → Planck’s constant, and k is → Boltzmann’s constant.
T_E is called the → Einstein temperature. This model could explain the temperature behavior of specific heat but not very satisfactorily at low temperatures. It has therefore been superseded by the → Debye model. See also → Dulong-Petit law.

See also: Albert Einstein in 1907; → model.

A notation convention in → tensor analysis whereby whenever there is an expression with a repeated → index, the summation is done over that index from 1 to 3 (or from 1 to n, where n is the space dimension). For example, the dot product of vectors a and b is usually written as: a.b = Σ (i = 1 to 3) a_i.b_i. In the Einstein notation this is simply written as a.b = a_i.b_i. This notation makes operations much easier. Same as Einstein summation convention.

See also: → Einstein; → notation.

In gravitational lens phenomenon, the critical distance from the → lensing object for which the light ray from the source is deflected to the observer, provided that the source, the lens, and the observer are exactly aligned.

Consider a massive object (the lens) situated exactly on the line of sight from Earth to a background source. The light rays from the source passing the lens at different distances are bent toward the lens. Since the bending angle for a light ray increases with decreasing distance from the lens, there is a critical distance such that the ray will be deflected just enough to hit the Earth. This distance is called the Einstein radius. By rotational symmetry about the Earth-source axis, an observer on Earth with perfect resolution would see the source lensed into an annulus, called Einstein ring, centered on its position. The size of an Einstein ring is given by the Einstein radius: θ_E = (4GM/c²)^0.5 (d_LS/(d_L.d_S)^0.5, where G is the → gravitational constant, M is the mass of the lens, c is the → speed of light, d_L is the angular diameter distance to the lens, d_S is the angular diameter distance to the source, and d_LS is the angular diameter distance between the lens and the source. The equation can be simplified to:

θ_E = (0’’.9) (M/10¹¹Msun)^0.5 (D/Gpc)^-0.5.

Hence, for a dense cluster with mass M ~ 10 × 10¹⁵ Msun at a distance of 1 Gigaparsec (1 Gpc) this radius is about 100 arcsec.
For a gravitational → microlensing event (with masses of order 1 Msun) at galactic distances (say D ~ 3 kpc), the typical Einstein radius would be of order
milli-arcseconds.

See also: → Einstein; → radius.

The apparent shape of a background source unsergoing the effect of → gravitational lensing as seen from Earth, provided that the source, the intervening lens, and the observer are in perfect alignement through → Einstein radius.

See also: → Einstein; → ring.

Same as → Einstein model.

See also: → Einstein; → solid.

A cosmological model in which a static (neither expanding nor collapsing) Universe is maintained by introducing a cosmological repulsion force (in the form of the cosmological constant) to counterbalance the gravitational force.

See also: → Einstein;
→ static; arr; universe.

A characteristic parameter occurring in the → Einstein model of → specific heats.

See also: → Einstein; → temperature.

A mathematical entity describing the → curvature of → space-time in → Einstein’s field equations, according to the theory of → general relativity. It is expressed by

G_μν = R_μν - (1/2) g_μνR,
where R_μν is the Ricci tensor,
g_μν is the → metric tensor, and R the scalar curvature. This tensor is both symmetric and divergence free.

See also: Named after Albert Einstein (1879-1955); → tensor.

The time during which a → microlensing event occurs. It is given by the equation t_E = R_E/v, where R_E is the → Einstein radius, v is the magnitude of the relative transverse velocity between source and lens projected onto the lens plane. The characteristic time-scale of → microlensing events is about 25 days.

See also: → Einstein; → time-scale.

Same as → geodetic precession.

See also: → Einstein-de Sitter Universe; → effect.

The → Friedmann-Lemaitre model of → expanding Universe that only contains matter and in which space is → Euclidean (Ω_M > 0, Ω_R = 0, Ω_Λ = 0, k = 0). The Universe will expand at a decreasing rate for ever.

See also: → Einstein; de Sitter, after the Dutch mathematician and physicist Willem de Sitter (1872-1934) who worked out the model in 1917; → Universe.

In → general relativity, the → action that yields → Einstein’s field equations. It is expressed by:
S_{EH</SUB = (1/2κ)∫d⁴x (-g)^1/2R + S^m,
where κ ≡ 8πG and S^m is the matter part of the action.}

See also: → Einstein; → Hilbert space; → action.

→ EPR paradox.

See also: A. Einstein, B. Podolsky, N. Rosen: “Can quantum-mechanical description of physical reality be considered complete?” Phys. Rev. 41, 777 (15 May 1935); → paradox.

A hypothetical structure that can join two distant regions of → space-time through a tunnel-like shortcut, as predicted by → general relativity. The Einstein-Rosen bridge is based on the → Schwarzschild solution of → Einstein’s field equations. It is the simplest type of → wormholes.

See also: Albert Einstein & Nathan Rosen (1935, Phys.Rev. 48, 73); → bridge.

A → thought experiment, involving an elevator, first conceived by Einstein to show the → principle of equivalence. According to this experiment, it is impossible for an observer situated inside a closed elevator to decide if the elevator is being pulled upward by a constant force or is subject to a gravitational field acting downward on a stationary elevator. Einstein used this experiment and the principle of equivalence to deduce the bending of light by the force of gravity.

Etymology (EN): → einstein; elevator, from L. elevator, agent noun from p.p. stem of elevare “to lift up, raise,” from → ex- “out” + levare “lighten, raise,” from levis “light” in weight, → lever.

Etymology (PE): Bâlâbar, → lift.

A system of ten non-linear → partial differential equations in the theory of → general relativity which relate the curvature of → space-time with the distribution of matter-energy. They have the form: G_μν = -κ T_μν, where G_μν is the → Einstein tensor (a function of the → metric tensor),
κ is a coupling constant called the → Einstein gravitational constant, and T_μν is the → energy-momentum tensor. The field equations mean that the curvature of space-time is due to the distribution of mass-energy in space. A more general form of the field equations proposed by Einstein is:

G_μν + Λg_μν = - κT_μν, where Λ is the → cosmological constant.

See also: Named after Albert Einstein (1879-1955); → field; → equation.

The coupling constant appearing in → Einstein’s field equations, expressed by:

κ = 8πG/c⁴, where G is the Newtonian → gravitational constant and c the → speed of light.

See also: → einstein; → gravitational; → constant.

Same as → Einstein model.

See also: → Einstein; → theory; → specific heat.

The laws of physics are the same in all → inertial reference frames and are invariant under the → Lorentz transformation. The → speed of light is a → physical constant, i.e. it is
the same for all observers in uniform motion. Einsteinian relativity is prompted by the → Newton-Maxwell incompatibility. See also: → Galilean relativity, → Newtonian relativity.

See also: → Einstein; → relativity.

A radioactive metallic → transuranium element belonging to the → actinides; symbol Es. → Atomic number 99, → mass number of most stable
→ isotope 254 (→ half-life 270 days). Eleven isotopes are known. The element was first identified by A. Ghiorso and collaborators in the debris of first hydrogen bomb explosion in 1952.

See also: Named after Albert Einstein, → einstein + → -ium.

To throw out material, for example by a massive star through stellar wind, or by a volcano in eruption.

Etymology (EN): From L. ejectus, p.p. of eicere “to throw out,” from → ex- “out” + -icere, comb. form of jacere “to throw.”

Etymology (PE): Ešândan, from Hamadâni ešândan “to throw out;” Pashto aestal, wištal “to throw, project;”
Laki owštan “to throw, to shoot (with bow and arrow);” Lori šane “throwing,” šane kerde “to throw;” Av. ah- “to throw,” pres. ahya- “throws,” asta- “thrown, shot,” astar- “thrower, shooter;” cf. Khotanese ah- “to throw, shoot,” Skt. as- “to throw, shoot,” ásyati “throws,” ásana- “throw, shot.”

Material, in solid, liquid, or gaseous form, thrown out by a body, especially as a result of → volcanic eruption, → meteoritic impact, or → supernova explosion. See also: → ejecta blanket, → supernova ejecta.

Etymology (EN): Plural of L. ejectus, → eject.

Etymology (PE): Ešânâk “that which is ejected,” from šân present
stem of šândan→ eject + suffix -âk.

Of an → impact crater, the ejecta that after the → impact event settles back to the Earth’s surface. The ejecta blanket is thick near the → crater rim and thin outward from the crater.

See also: → ejecta; → blanket.

Act or instance of ejecting; the state of being ejected.

See also: Verbal noun of → eject.

A kind of viscous → boundary layer in a rotating fluid system. Such a layer forms over a flat bottom that exerts a frictional → stress against the flow, bringing the velocity gradually to zero within the layer above the bottom. An Ekman layer occurs also on the fluid surface whenever there is a horizontal frictional stress, for example along ocean surface, when waters are subject to wind stress.

See also: Named for Vagn Walfrid Ekman (1874-1954), Swedish oceanographer, who studied the phenomenon originally in his doctoral thesis (1902) and later developed it (1905, 1906); → layer.

A → dimensionless quantity that measures the strength of → viscous forces relative to the → Coriolis force in a rotating fluid.
It is given by E_k = ν/(ΩH²), where ν is the → kinematic viscosity of the fluid, Ω is the → angular velocity, and H is the depth scale of the motion. The Ekman number is usually used in describing geophysical phenomena in the oceans and atmosphere. Typical geophysical flows, as well as laboratory experiments, yield very small Ekman numbers. For example, in the ocean at mid-latitudes, motions with a viscosity of 10^-2 m²/s are characterized by an Ekman number of about 10^-4.

See also: → Ekman layer; → number.

A cosmological model in which the → Big Bang is not the beginning of the → Universe, but a transitory phase in a more global scenario. The ekpyrotic Universe model is fundamentally different from the → standard cosmology and offers radically different explanations for the cosmological problems (→ homogeneity, → isotropy, → flatness, → magnetic monopoles, etc.). In this highly speculative model → space-time has five dimensions, four spatial and one temporal. Two three-dimensional → branes, one visible and one hidden, collide following the contraction of the extra dimension. The contraction produces a blue shift effect that converts gravitational energy into brane kinetic energy. Some fraction of this kinetic energy is converted into matter and radiation that can fuel the Big Bang. The resulting temperature is finite, so the hot Big Bang phase begins without a → singularity. Apart from being speculative, this model suffers from several fine tunings (J. Khoury et al. 2001, Phys. Rev. D64, 123522 (hep-th/0103239); P. J. Steinhardt & N. Turok, 2002, Phys. Rev. D65, 126003 (hep-th/0111098), and references therein).

Etymology (EN): Ekpyrotic is inspired by the ancient Stoic doctrine according to which the world ends in a supreme conflagration, called ekpyrosis, and then reborns from the fire (palingenesis), only to be destroyed again at the end of the new cycle; ekpyrosis, from Gk. ek- “out of,” → ex-, + → pyro- combining form of pyr, → fire, + -sis a suffix used to form nouns of action, process, state, condition, such as thesis, analysis, catharsis; → Universe.

Etymology (PE): Giti, → Universe; âtašzâd, literally “born out of fire,” from âtaš, → fire,

• zâd “born,” from zâdan “to bring forth,” → generate.

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.

To flow out, issue, or proceed, as from a source or origin; come forth; originate; arise (Dictionary.com).

Etymology (EN): From L. emanatus, p.p. of emanare “flow out,” figuratively “arise from, proceed from.”

Etymology (PE): Runemudan “to appear, come out,” literally “to show face,” from ru “face,” → surface, + nemudan “to show,” → display.

An act or instance of emanating; something that emanates or is emanated.

See also: → emanate; → -tion.

A newborn star which is tightly enveloped by a surrounding cloud of gas and dust.

Etymology (EN): Ebedded p.p. of embed, from en- + bed from O.E. bed “bed,” from P.Gmc. *badjam “sleeping place dug in the ground” (O.H.G. betti; Ger. bett); PIE base *bhedh- “to dig, pierce;” cf. Gk. bothyros “pit;” L. fodere “to dig,” fossa “ditch;” → star.

Etymology (PE): Setâré, → star; forupušidé, p.p. of forupušidan, from
foru- “below; beneath; down, downwards;” Mid.Pers. frôt “down, downwards;” O.Pers. fravata “forward, downward;” Skt. pravát- “a sloping path, the slope of a mountain” + pušidan “to cover, conceal, clothe; to wear clothes;” Mid.Pers. pôšidan, pôš- “to cover, put on, wear;” cf. Mid.Pers. pôst; Mod.Pers. pust “skin, hide;” O.Pers. pavastā- “thin clay envelope used to protect unbaked clay tablets;” Skt. pavásta- “cover;” Proto-Indo-Iranian *pauastā- “cloth.”

The act of embodying; the state or fact of being embodied.

Etymology (EN): verbal noun of → embody.

1. To give a concrete form or body to an idea or quality.
   

2. To cause to become a body or part of a body.

Etymology (EN): From en- “in” + → body

Etymology (PE): Tanigândan, from Mid.Pers. tanig “bodily, corporal,” from tan “body,” → if and only if, + -ig, → -ik,

• -ândan suffix of transitive verbs.

1. In several → lunisolar calendars, an → intercalary month employed to preserve a seasonal relationship between the Lunar and Solar cycles. For example, in the → Hebrew calendar the extra month,
   called Adar Alef, was inserted after Shvat
   so that the month of Nissan (month of spring) does not begin in winter.
   

2. In ancient Iranian → solar calendar the additional whole month of 30 days employed every 120 years to compensate for the left-over quarters of days in a calendar year of 365 days (120 years × 0.25 days = 30 days).

Etymology (EN): M.E. embolisme, from M.L. embolismus “intercalation,” from Gk. emballein “to throw into, to insert,” from em- “in”

• ballein “to throw” (source of the medical term embolism “the obstruction (of an artery, etc.) by a clot of blood, bubble of air, etc.”); → month.

Etymology (PE): Behizak, from Mid.Pers. vihezagig or vihezakik
“movable,” from vihezag “movement, progression,” from vihez- “to move, progress;” mâh, → month.

In ancient calendars, a year that contains an → embolismic month.

See also: → embolismic month; → year.

1. Move out of or away from something and become visible.
   

   2. To rise or come forth from or as if from water or other liquid (Dictionary.com).

See also: → ex-, + merge, → submerge.

1. The process of becoming visible after being concealed.
   

   2. The escape of an insect or other invertebrate from an egg, cocoon, or pupal case.
      

   3. The process of coming into existence or prominence (OxfordDictionaries.com).

See also: → emerge; → -ence.

1. Coming into being or notice.
   

   2. Philo.: (of a property) arising as an effect of complex causes and not analyzable simply as the sum of their effects.
      

   3. Ecology: Of or denoting a plant which is taller than the surrounding vegetation, especially a tall tree in a forest (OxfordDictionaries.com).

See also: → emerge; → -ent.

Optics: The → light ray leaving a → medium, in contrast to the → incident ray. If the medium has parallel sides, → angle of incidence and → angle of emergence</i are equal as emergent ray and incident ray are parallel to each other.

See also: → emergent; → ray.

Astro.: Same as → egress.

See also: → emerge.

A person who emigrates, as from his or her native country or region (Dictionary.com).

See also: → emigrate + -ant a suffix forming adjectives and nouns from verbs.

To leave one country or region to settle in another; migrate.

See also: → ex-; → migrate.

An act or instance of emigrating; a body of emigrants; emigrants collectively.

See also: Verbal noun of → emigrate.

In an atom, a characteristic amount of energy radiated (as line emission) when an electron moves from an outer orbit to an inner orbit around the nucleus, corresponding to the lost energy of the electron.

Etymology (EN): From L. emissionem “a sending out,” from emiss-, stem of emittere “send forth,” from → ex- “out” + mittere “to send.”

Etymology (PE): Gosil, variant gosi “sending away, dismission;” Mid.Pers. wisé “to despatch” (Parthian Mid.Pers. wsys- “to despatch;” Buddhist Mid.Pers. wsydy “to despatch;” Sogdian ‘ns’yd- “to exhort”), from Proto-Iranian *vi-sid- “to despatch, send off,” from prefix vi- “apart, away, out,” + *sid- “to call.”

In a spectrum, a combination of several closely spaced, often unresolved, → emission lines occurring across a limited range of wavelengths.

See also: → emission; → band.

Electromagnetic flux emitted by a source per unit volume per unit time.

See also: → emission; → coefficient.

Any emission mark in the spectrum of a celestial object resulting from line formation or of unknown origin.

See also: → emission; → feature.