An Etymological Dictionary of Astronomy and Astrophysics

A conspicuous band of molecular → CH (methylidine) at 4300 Å, which is present in the spectra of late-type G-K stars.

See also: G refers to → G type stars in the spectra of which this feature is strong. → band.

Waves trapped inside stars, whose restoring force is the → buoyancy. Same as → gravity mode. See also: → oscillation modes;
→ p mode; → f mode.

See also: g referring to gravity; → mode.

The → Saturn’s ring, with a width of 8,000 km, lying before the → F ring, at 164,000-172,000 km from the center of Saturn.

See also: → ring.

A member of a class of stars to which the Sun belongs. The G-type stars on the → main sequence have → surface temperatures of 5,300-6,000 K and therefore appear yellow in color. G type → giant stars (such as → Capella) are almost 100-500 K colder than the corresponding main sequence stars.
G type → supergiants have temperatures of 4,500-5,500 K. The spectrum of early type G stars, such as the Sun (G2), is dominated by ionized lines of calcium (→ H and K lines, mainly) and neutral metals. In later type G stars the molecular bands of → CH molecules
and → CN molecules become visible. The main sequence and giant
stars have masses of ~ 1 solar mass, while the supergiants are of ~ 10 solar masses. The luminosities of G-type giants are almost 30-60 times greater than that of the Sun, whereas the supergiants are 10,000-30,000 times more luminous.

See also: G, from the → Harvard classification; → star.

A yellowish star whose surface temperature is about 6000 K and its spectrum is dominated by H and K lines of ionized calcium (Ca II 3968 Å and 3934 Å).

See also: G from the alphabetical sequence of spectral types; → type;
→ star.

A relatively uncommon → carbonaceous carbonaceous asteroid whose spectrum contains a strong → ultraviolet  → absorption feature below 0.5 μm (→ Tholen classification). In the → SMASS classification it corresponds to the Cg and Chg types , depending on the presence or absence (respectively) of the absorption feature at 0.7 μm. The most remarkable “asteroid” in this type is → Ceres (now classified as a → dwarf planet).

See also: → type; → asteroid.

Same as → G star.

See also: → G star; → type.

A → European Space Agency  → astrometry mission launched on 19 December 2013. Gaia’s goal is to create the largest and most precise three-dimensional chart of the → Milky Way galaxy by providing unprecedented positional (position on the sky and distance to the Sun) and annual → proper motion measurements for about one billion stars in our Galaxy and throughout the → Local Group. Moreover, the third component of the velocity, the → radial velocity, will be obtained for all stars down to V = 17 mag. Similarly, multi-color photometry will be carried out on all stars down to V = 20 mag. Gaia will achieve the planned astrometric requirements
by repeatedly measuring the positions of all objects down to V = 20 mag with final accuracies of about 20 microarcsec at 15 mag. It will provide distances accurate to 20% as far as the → Galactic Center. The satellite is expected to be launched in 2012 and be placed in a → Lissajous orbit around the Sun-Earth → Lagrangian point L2. Gaia is a much more advanced version of the → Hipparcos mission.

See also: Initially, GAIA was the short for Global Astrometric Interferometer for Astrophysics. Although subsequently the interferometer option was abandoned, the acronym was maintained in lower case.

1. A measure of the → amplification of an electronic device, usually expressed as the ratio of → output power to → input power.
   
2. → antenna gain.

Etymology (EN): From M.Fr. gain, from O.Fr. gaaigne, from guaaignier “to obtain,” from Germanic *waidanjan “to hunt, plunder,” also “to graze, pasture,” from P.Gmc. *wartho “hunting ground” (cf. Ger. weide “pasture, pasturage”); PIE base *weiə- “to go after something, strive after.”

Etymology (PE): Bahré, from bahr “part, portion, share, lot;” Av. baxəδra- “portion,” from bag- “to attribute, allot,” → division.

1. Of or pertaining to a → galaxy.
   
2. Usually with capital G, pertaining to our galaxy, the → Milky Way.

See also: Adjective of → galaxy.

The point in the → Galactic plane that lies directly opposite the → Galactic center. It lies in the constellation → Auriga at approximately R.A. 05h 46m, Dec. +28° 56'.

See also: → galactic; → anticenter.

An elongated bar-shaped structure composed of stars present in some spiral galaxies. About two-third of such galaxies contain bars that cross their centers. Bars, like → spiral arms, result from a → density wave
in which stars take very elliptical orbits.
They form when the → galactic disk dominates the → galactic bulge, → Ostriker-Peebles criterion. Bars play an extremely important role in a galaxy’s evolution. The gravity from a bar is the mechanism that drives → interstellar gas from the outer parts of a → spiral galaxy inward toward the central regions, and into the galactic nucleus itself. This causes tremendous bursts of star formation. Therefore, a majority of massive stars are born in such starbursts in the nuclei of galaxies. Bars may also channel the material that falls into black holes within active galactic nuclei, releasing enormous power in radiation and particles from tiny regions at the centers of some galaxies. Bars disappear as galactic centers grow more massive (after some 2 to 8 Gyr).

See also: → galactic; → bar.

The central → galaxy bulge of the → Milky Way.

See also: → galactic; → bulge.

1. The rotational center of the → Milky Way galaxy located in the direction of the → Sagittarius constellation at a distance of 7.62 ± 0.32 kpc (2005, ApJ 628, 246). Its equatorial coordinates (J2000 epoch) are: R.A. 17h45m40.04s, Dec. -29° 00’ 28.1’’.
   The Sun orbits around the Galactic center once every 200 million years at a speed of 220 km per second. It is believed that there is a → supermassive black hole at the Galactic center.
   

2. The innermost region of a → spiral galaxy characterized by high number of stars per unit volume. The center may contain a → supermassive black hole.

See also: → galactic; → center.

One of the three massive clusters located toward the → Galactic center: → Quintuplet cluster, → Arches cluster, → Central cluster. Heavily extinguished by the presence of dust clouds and only accessible at infrared (and longer) wavelengths or in X-rays, each of these clusters has a population of more than a hundred → massive stars. The three clusters are similar in most respects, each containing about 10⁴ solar masses in stars. The Arches cluster is younger than the two others.

See also: → galactic; → center; → cluster.

1. Same as → open cluster.
   
2. same as → cluster of galaxies.

See also: → galactic; → cluster.

A system of astronomical coordinates using → latitude (b^II) measured north and south from the → Galactic equator and → longitude (l^II), measured from the → Galactic Center in the sense of increasing → right ascension from 0 to 360 degrees. In the old system (l^I,b^I), the Galactic center was at l^I = 327°41’. Same as → galactic system.

See also: → galactic; → coordinate.

The flattened component of a → spiral galaxy which is composed of stars and concentrations
of dust and molecules. → Star formation takes place mainly in the disk.

See also: → galactic; → disk.

The study of the → motions of the → stars, → gas, and → dark matter in a → galaxy to explain the main → morphological and → kinematical features of the galaxy.

See also: → galactic; → dynamics.

The great circle in the sky defined by the place of the → Galactic plane or the → Milky Way. At an angle of about 62°, the Galactic equator intersects the celestial equator at two points located in the constellations → Monoceros and → Aquila.

See also: → galactic; → equator.

A region of the Galaxy whose boundaries are set by its calm and safe environment and access to the chemical materials necessary for building terrestrial planets similar to the Earth. → circumstellar habitable zone; → habitable zone.

See also: → galactic, → habitable;
→ zone.

A roughly spherical aggregation of → globular clusters, as well as the oldest stars and unseen mass that surrounds the Galaxy.

See also: → galactic, → halo.

In the → Galactic coordinate system, the angle between the line of sight to an object and the → Galactic equator. Galactic latitude, usually represented by the symbol b^II, ranges from +90 degrees to -90 degrees.

See also: → galactic; → latitude.

In the → Galactic coordinate system, the angle between the → Galactic Center and the projection of the object on the → Galactic plane.
Galactic longitude, usually represented by the symbol l^II, ranges from 0 degrees to 360 degrees.

See also: → galactic; → longitude.

A concentration of stars and gas in the innermost region of a galaxy, sometimes extending over thousands of light-years from the center of the galaxy.

See also: → galactic; → nucleus.

→ galactic-scale outflow.

See also: → galactic; → outflow.

The plane in which the → disk of a → spiral galaxy, such as our → Milky Way, lies.

See also: → galactic; → plane.

The point on the sky, north or south, at which the galaxy’s rotation axis would meet the celestial sphere.

See also: → galactic; → pole.

A diffuse radio signal that originates outside the solar system. It is strongest in the direction of the Galactic plane.

See also: → galactic; → radio;
→ noise.

The revolving of the gaseous and stellar content of a galaxy around its central nucleus. The rotation is not uniform, but differential. One revolution of the Sun within our own Galaxy takes about 220 million years, or one cosmic year.

See also: → galactic; → rotation.

The discrepancy between observed galaxy → rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material.

See also: → galactic; → rotation; → problem.

The global shape and the arrangement of the various parts or constituents of a galaxy.

See also: → galactic; → structure.

Same as → galactic coordinates.

See also: → galactic; → system.

An outflow of hot gas, analogous to the → solar wind, from a galaxy that has recently undergone a high → burst of star formation or has an → active galactic nucleus. Galactic winds are streams of high speed charged particles
blowing out of galaxies with speeds of 300 to 3,000 km s^-1. In the case of starbursts, galactic winds are powered by → stellar winds driven by → massive stars and → supernova explosions. Galactic winds contain a mixture of extremely hot metal-enriched supernova ejecta and cooler entrained gas and dust.
Outflowing material has been observed at great distances from galaxies (10 to 100 kpc). In some cases they escape the galaxy potential well and pollute the → intergalactic medium with → heavy elements. A prominent example is the → superwind of the starburst galaxy M82.

See also: → galactic; → wind.

The regions near the Galactic plane where there is low absorption of light by interstellar clouds so that some external galaxies may be seen through them.

See also: → galactic; → window.

The time taken for the Sun to revolve once around the center of the Milky Way, amounting to about 220 million years.

See also: → galactic; → year.

The enormous amounts of → mass and → energy released from active galaxies into the → intergalactic medium. → Supermassive black holes,
believed to exist at the centres of active galaxies (→ active galaxy), → accrete matter and liberate huge quantities of energy. The energy output is often observed as → active galactic nuclei (AGN) outflows in a wide variety of forms, e.g. → collimated  → relativistic jets and/or huge overpressured cocoons in → radio, → blueshifted broad → absorption lines in the → ultraviolet and → optical, → warm absorbers and ultrafast outflows in → X-rays, and → molecular gas in → far infrared.

Moreover, the processes of → star formation and → supernova explosions release mass/energy into the surroundings. This → stellar feedback heats up, ionizes and drives gas outward, often generating large-scale outflows/→ winds. Galactic outflows are observed at low redshifts reaching a velocity as large as 1000 km s^-1 and at high-z up to z ~ 5, sometimes extending over distances of 60-130 kpc.

Galactic-scale outflows may be a primary driver of galaxy evolution through the removal of cool gas from star-forming regions to a galaxy’s → halo or beyond.

See also: → galactic; → scale; → outflow.

Of or relative to the center of a galaxy.

See also: From galacto-, combining form of → galaxy

• centric, adj. of rarr; center.

The distance from the center of a galaxy.

See also: → galactocentric; → distance.

1. Generally, a large body of → gas, → dust, and → stars held together by their mutual → gravitational attraction and ranging in mass from about 10⁶ to 10¹³ Msun. If a galaxy also contains
   → dark matter its mass will be much larger.
   Galaxies are grouped into three main categories: → spiral galaxy, → elliptical galaxy, and → irregular galaxy
   (→ Hubble classification).
   

2. With capital G, the galaxy to which our Sun belongs; → Milky Way galaxy.
   

See also:
→ active galaxy, → Andromeda galaxy, → barred spiral galaxy, → biased galaxy formation, → binary galaxy, → blue compact dwarf galaxy, → broad-line radio galaxy, → bulge of a galaxy, → Cartwheel Galaxy, → compact galaxy,

→ core-halo galaxy, → disk galaxy, → dwarf elliptical galaxy, → dwarf galaxy, → dwarf irregular galaxy, → dwarf spheroidal galaxy, → early-type galaxy, → edge-on galaxy,

→ face-on galaxy, → field galaxy, → flocculent spiral galaxy, → galaxy bimodality, → galaxy cluster, → galaxy formation, → galaxy harassment, → galaxy main sequence, → gas-poor galaxy, → gas-rich galaxy, → grand design spiral galaxy, → green pea galaxy, → halo of galaxy, → halo of the Galaxy, → Haro galaxy, → host galaxy, → hypergalaxy, → infrared galaxy, → Irr I galaxy, → Irr II galaxy, → isolated galaxy, → late-type galaxy, → lensing galaxy, → lenticular galaxy, → low surface brightness galaxy, → luminous infrared galaxy, → Lyman break galaxy, → Markarian galaxy, → metagalaxy, → metal-deficient galaxy, → metal-poor galaxy, → parent galaxy, → passive galaxy, → passively evolving galaxy, → peculiar galaxy, → primordial galaxy, → progenitor galaxy, → protogalaxy, → radio galaxy, → receding galaxy, → retired galaxy, → ring galaxy, → Sagittarius Dwarf Elliptical Galaxy, → Sagittarius Dwarf Irregular Galaxy, → satellite galaxy, → Sculptor Dwarf Elliptical Galaxy, → Seyfert galaxy, → shell galaxy, → Sombrero galaxy, → starburst galaxy, → strong arm spiral galaxy, → submillimeter galaxy, → superthin galaxy, → superwind galaxy, → tidal dwarf galaxy, → Triangulum galaxy, → ultraluminous infrared galaxy, → violent galaxy, → weak arm spiral galaxy, → Whirlpool galaxy, → Wolf-Rayet galaxy.

Etymology (EN): From L.L. galaxias “Milky Way,” from Gk. galaxis (adj.), from gala (genitive galaktos) “milk.”

In Gk. mythology, Jupiter, hoping to immortalize his infant son Hercules (who was born to a mortal woman), placed the baby on Hera’s breast. Her milk spilled up, forming the Milky Way. A painting by Italian artist Jacopo Tintoretto (c. 1518-1594), called “The Origin of the Milky Way,” depicts the legend describing how the Milky Way was formed.

Etymology (PE): Kahkešân, short for (râh-e) kahkešân literally “the (path of the) chaff-draggers” or “trail of chaff,” from kah, kâh “chaff, straw, hay” (Mid.Pers. kâh “chaff, straw;” cf. Pali kattha- “a piece of wood;” Skt. kastha- “stick;” Gk. klados “twig;”
O.Ir. caill “wood;” Ger. Holz “wood;” E. holt; PIE *kldo-)

• kešân pr.p. of kešidan/kašidan “to carry, draw, protract, trail, drag” (Mid.Pers. kešidan “to draw, pull;” Av. karš- “to draw; to plow,” karša- “furrow;” cf. Skt. kars-, kársati “to pull, drag, plow;”
  Gk. pelo, pelomai “to move, to bustle;” PIE base k^wels- “to plow”). The term (râh-e) kahkešân may be a popular corruption of Mid. Pers. (râh-i) Kâwôsân “the path of Kâwos” referring to the Persian mythological king Kay Kâwôs, who built an eagle-propelled throne to fly to China, as recounted in the Dênkard and the Shâhnâmé.

The division of galaxies into a “red sequence” and a “blue sequence” in the → color-magnitude diagrams of galaxies involving large statistical surveys. In both sequences, redder galaxies tend to be brighter. The blue sequence is truncated at the red magnitude ~ -22, while the red sequence extends to brighter magnitudes. The division between the two classes of galaxies is associated with a critical stellar mass ~ 3 × 10¹⁰ Msun. Galaxies below the critical mass are typically blue, star forming spirals and reside in the field. Galaxies above the critical mass are dominated by red spheroids of old stars and live in dense environments (Kauffmann et al, 2003, MNRAS 341, 33 & 54).

See also: → galaxy; → bimodality.

A → spheroidal region at the center of a → spiral galaxy which mostly contains → old stars.
Galactic bulges are generally classified into two types: → classical bulges and → pseudo-bulges.

See also: → galaxy; → bulge.

An aggregation of galaxies, made up of a few to a few thousand members, which may or may not be held together by its own gravity. Same as → cluster of galaxies.

See also: → galaxy; → cluster.

The study dealing with the processes that gave rise to galaxies in a remarkably → early Universe. See also
→ structure formation, → protogalaxy

See also: → galaxy; → formation.

Frequent, high speed galaxy → encounters within → galaxy clusters. Harassment can disturb the morphologies of the galaxies involved, often inducing a new
→ burst of star formation. Asymmetrical galaxies,
→ warps, → bars, and → tidal tails can all be produced through galaxy harassment.

See also: → galaxy; → harassment.

The dominant member of the → Virgo cluster of galaxies, which contains some 2,000 galaxies. Also known as NGC 4486, it has an → apparent visual magnitude 9.6. Discovered in 1781 by Charles Messier, this → elliptical galaxy is located 55 million → light-years away from Earth in the constellation → Virgo.

M87 is the home of several thousand billion stars, a → supermassive black hole (SMBH) and a family of roughly 15,000 → globular clusters. For comparison, our → Milky Way galaxy contains only a few hundred billion stars and about 150 globular clusters.

M87 is characterized by a prominent kiloparsec scale → relativistic jet emitted by the central SMBH. As gaseous material from the center of the galaxy → accretes onto the black hole, the energy released produces a stream of subatomic particles that are accelerated to velocities near the → speed of light.

See also: → galaxy; → Messier catalog.

A scaling relation between the → star formation rate (SFR) in galaxies and the total stellar mass (M) of the galaxies. This relation, colloquially called the “galaxy main sequence,” extends over several orders of magnitudes in M and out to → high redshifts, with a modest scatter of ~ 0.3 dex which includes both intrinsic scatter and measurement uncertainties. The existence of such tight scatter at all observed epochs suggests
that most galaxies assembled their stellar mass fairly steadily rather than predominantly in → starburst episodes, implying that → mergers have a sub-dominant contribution to the global star formation history (Wuyts et al., 2011 ApJ 742, 96).

See also: → galaxy; → main; → sequence.

An unusually strong wind.

Etymology (EN): Gale, from gaile “wind,” origin uncertain, perhaps from O.N. gol “breeze,” or O.Dan. gal “bad, furious.”

Etymology (PE): Tondbâb “gale,” from tond “swift, rapid, brisk; fierce, severe,” Mid.Pers. tund “sharp, violent;” Sogdian tund “violent;” cf. Skt. tod- “to thrust, give a push,” tudáti “he thrusts;” L. tundere “to thrust, to hit” (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere “to thrust or bore through,” from per- + tundere, as explained); PIE base *(s)teud- “to thrust, to beat” + bâd, → wind.

Of or pertaining to Galileo Galilei (1564-1642), Italian physicist and astronomer.

Same as → Galilean relativity.

See also: → Galilean; → invariance.

Same as → Galilean satellites.

See also: → Galilean; → moon.

Same as → inertial reference frame.

See also: → Galilean; → reference; → frame.

The principle according to which the fundamental laws of physics are the same in all frames of reference moving with constant velocity with respect to one another (→ inertial reference frames). Same as → Galilean invariance and → Newtonian relativity.

See also: → Galilean transformation, → Einsteinian relativity.

See also: → Galilean; → relativity.

The four largest and brightest satellites of → Jupiter, that is: → Io (Jupiter I), → Europa, → Ganymede, and → Callisto.

See also: Galileo, who had discovered them, called them Sidera Medicæa “Medicean Stars” in honor of the Medici family. → Galilean Moons; → satellite.

The method of relating a measurement in one → reference frame to another moving with a constant velocity with respect to the first within the → Newtonian mechanics. The Galilean transformation between the coordinate systems (x,y,z,t) and (x’,y’,z’,t’) is expressed by the relations: x’ = x - vt, y’ = y, z’ = z. Galilean transformations break down at high velocities and for electromagnetic phenomena and is superseded by the → Lorentz transformations.

See also: → Galilean; → transformation.

A space mission whose main goal was to explore → Jupiter and its moons and rings.
The spacecraft was launched on October 19, 1989, arrived at Jupiter in December 1995.
It disappeared on September 21, 2003, after eight years orbiting Jupiter, when mission controllers crashed it into → Jupiter’s atmosphere.

On December 7, 1995, Galileo’s probe dived into Jupiter’s atmosphere, and measured atmospheric pressure, density, and composition, and explored the planet’s → radiation belts. Galileo had two parts: an orbiter and a descent probe that parachuted into Jupiter’s atmosphere. The orbiter sent back hundreds of pictures of the four large → Galilean satellites of Jupiter (→ Io, → Europa, → Ganymede, and → Callisto).

It made many discoveries during its eight years looping around Jupiter. It found evidence for layers of salt water below the surface on Europa, Ganymede, and Callisto, and measured high levels of volcanic activity on Io. When → Shoemaker-Levy slammed into Jupiter in 1994, Galileo had the only direct view of the → comet striking Jupiter’s atmosphere. Galileo determined that → Jupiter’s rings are formed from dust hurled up by → meteorite impacts on planet’s inner moons. Measurements by the orbiter’s → magnetometer revealed that Io, Europa, and Ganymede have metallic cores, while Callisto does not. Also, Galileo discovered that Ganymede possesses its own → magnetic field; it is the first moon known to do so. The orbiter also found that the Galilean satellites all have thin atmospheres.

During it’s trip from Earth to Jupiter, Galileo passed by and studied two asteroids: → Gaspra in 1991 and → Ida in 1993, around which it discovered → Dactyl, the first moon orbiting an asteroid (windows2universe.org).

See also: → Galileo; → mission.

In the absence of air resistance, any two bodies that are dropped from rest at the same moment will reach the ground at the same time regardless of their mass.

See also: Galileo (1564-1642) was the first to determine, at the start of the seventeenth century, the law of constant acceleration of free-falling bodies. → law; → fall;
→ body.

An electrolytic cell capable of producing electric energy by electrochemical reaction.

See also: → galvanism; → cell.

A pair of dissimilar conductors, commonly metals, in electrical contact.

See also: → galvanism; → couple.

The direct electric current that flows between metals or conductive nonmetals in a → galvanic couple.

See also: → galvanism; → current.

1. The production of electricity from a chemical reaction.
   

2. The therapeutic application of electricity to the human body.

See also: From Fr. galvanisme, after Luigi Galvani (1737-1798), the Italian physiologist, who demonstrated (1790) muscular action due to contact with dissimilar metals.

The coating of steel or iron with → zinc, either by immersion in a bath of molten zinc or by electrolytic deposition from a solution of zinc sulfate, to give protection against corrosion.

See also: Verbal noun of → galvanize.

1. To coat a metal with → zinc by dipping into molten zinc or by electrolytic deposition.
   

2. To stimulate by application of an electric current.

Etymology (EN): From Fr. galvaniser, from galvanisme, → galvanism.

Etymology (PE): Gâlvânidan, from Gâlvâni, → galvanism,

• -idan, → -ize.

A prefix denoting galvanic or galvanism in compound words, such as → galvanometer, → galvanoplasty.

See also: Galvano-, from → galvanism.

An instrument for measuring or detecting small → direct currents, usually by the mechanical reaction between the magnetic field of the current and that of a magnet.

See also: → galvano- + → -meter.

A process used for covering an object with a thin layer of metal by electrochemical means.

Etymology (EN): → galvano- + -plasty a suffix meaning “molding, formation, surgical repair, plastic surgery,” from Gk. -plastia, from plastos “molded, formed,” from plassein “to mold.”

Etymology (PE): Gâlvânopuši, from gâlvâno-, → galvano-, + puši “covering, coating,” from pušidan “to cover; to put on” (Mid.Pers. pôšidan, pôš- “to cover; to 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”).

1. An amusement or pastime.
   

2. The material or equipment used in playing certain games.
   

3. A competitive activity involving skill, chance, or endurance on the part of two or more persons who play according to a set of rules, usually for their own amusement or for that of spectators (Dictionary.com).

Etymology (EN): M.E. gamen. O.E. gaman “game, joy, fun, amusement;” cf. O.Fris. game “joy, glee,” O.N. gaman, O.H.G. gaman “sport, merriment,” D. gamen, Sw. gamman.

Etymology (PE): Bâzi, from Mid.Pers. wâzig “game, play,” related to bâzidan “to play,” bâxtan/bâz- “to loose (in game);” Proto-Ir. *uāz- “to play, contend;” cf. Skt. vāja- “contest, war, gain, reward” (Cheung 2007).

1. The third letter of the Greek alphabet (γ, Γ).
   

2. Symbol used to denote the ratio of the principal specific heats C_P/C_V of a gas, where C_P is the specific heat at constant pressure and C_V that measured at constant volume.
   

3. Unit of magnetic field intensity, equal to 10^-5 gauss.
   

4. The distance of the Moon’s shadow axis from Earth’s center in units of equatorial Earth radii. It is defined at the instant of → greatest eclipse when its absolute value is at a minimum (F. Espenak, NASA).

See also: The third letter of the Gk. alphabet, from Gk. gamma, from Phoenician gimel.

A bright, third → magnitude (3.22) → giant star of → spectral type K1, also called → Errai, HR 8974, HIP 116727, and HD 222404. γCephei has a → surface temperature of 4920 K a mass of 1.40 Msun, a → luminosity 10.6 solar, and a radius 4.8 solar. Its distance is estimated to be 45 → light-years. γ Cephei will become the → Pole Star in about 2,000 years. γ Cephei has a low mass → companion (B), a main → main sequence star of spectral type M4 V with a mass of 0.4 Msun. It orbits the → primary star every 67.5 years. An → extrasolar planet. (γ Cephe b) has been discovered orbiting the main star.

See also: Gamma, as in → Bayer designation; → Cepheus.

The star → Sadr.

See also: Gk. letter → gamma; Cygni, genitive of → Cygnus.

A type of → radioactivity in which some unstable atomic nuclei dissipate excess energy by a spontaneous electromagnetic process, usually accompanied by → alpha decay or → beta decay.

See also: → gamma; → decay.

A process which reinforces the → kappa mechanism in a → partial ionization zone. Because the temperature in the partial ionization zone is lower than in the adjacent stellar layers, heat tends to flow into the zone during compression, prompting further ionization.

See also: γ, after the smaller ratio of → specific heats caused by the increased values of C_p and C_v; → mechanism.

An → electromagnetic wave with a typical → wavelength less than 10^-2Å (10^-12 m), corresponding to frequencies above 10¹⁹ Hz and photon energies above 100 → keV.

See also: → gamma; → ray.

The study of → gamma rays from → extraterrestrial → sources, especially → gamma-ray bursts.

See also: → gamma ray; → astronomy.

An intense discharge of → gamma rays,
which range in duration from tenth of a second to tens of seconds and occur from sources widely distributed over the sky. The radio wave → afterglow from the → burst can last more than a year, making long-term observations of the sources possible.

The favored hypothesis is that they are produced by a relativistic jet created by the merger of two → compact objects (specifically two → neutron stars or a neutron star and a → black hole). Mergers of this kind are also expected to create significant quantities of neutron-rich radioactive species, whose decay should result in a faint → transient, known as a → kilonova, in the days following the burst. Indeed, it is speculated that this mechanism may be the predominant source of stable → r-process elements in the Universe. Recent calculations suggest that much of the kilonova energy should appear in the → near-infrared spectral range, because of the high optical opacity created by these heavy r-process elements (Tanvir et al., 2017, Nature 500, 547).

See also: → gamma rays; → burst.

The → object or → phenomenon at the origin of a → gamma-ray burst.

See also: → gamma ray; → burster.

1. An astronomical object that emits → gamma rays.
   
2. A radioactive material that emits gamma rays in a form that can be used in medical imaging.

See also: → gamma ray; → source.

The closest → Wolf-Rayet star, located at 336 → parsecs. Also known as HR 3207, HD 68273, and WR 111. γ² Velorum is composed of a → WC8 component in a → close binary system with an → O star in a 78.5 day orbit (see, e.g., Lamberts et al., 2017, arXiv: 1701.01124).

See also: Gamma, as in → Bayer designation; Velorum, genitive of → Vela.

In nuclear physics, a potential barrier near the surface of the nucleus that inhibits the release of alpha particles.

See also: Gamow, after George Gamow (originally Georgiy Antonovich Gamov), the Ukrainian born theoretical physicist and cosmologist, who discovered quantum tunneling; → barrier.

The constraint on the → baryon number density at T ~ 10⁹ K in the early → expanding Universe. Gamow recognized that a key to the element buildup is the reaction n + p ↔ d + γ. Deuterium needs to be produced in sufficient abundance for higher elements to form, but if all → neutrons are immediately locked up into → deuterium, no higher elements can form either. The Gamow condition is expressed by n_b<σv>t ~ 1, where n_b is the baryon number density, σ is the cross section for the reaction at relative → velocity v, and t the expansion time-scale for the → Universe. This means that the time-scale for the above reaction is comparable to the expansion time. From this condition the baryon number density at the start of element buildup is found to be n_b ~ (σvt)^-1 ~ 10¹⁸ cm^-3 at T = 10⁹ K (P. J. E. Peebles, 2013, Discovery of the Hot Big Bang: What happened in 1948, arXiv.1310.2146).

See also: → Gamow barrier; → condition.

In nuclear fusion, the product of the Maxwell-Boltzmann distribution with the tunnelling probability of the nuclei through their Coulomb barrier. This is the energy region where the reaction is more likely to take place: at higher energies, the number of particles becomes insignificant while at lower energies the tunnelling through the Coulomb barrier makes the reaction improbable.

See also: → Gamow barrier; → peak.

The seventh and largest of → Jupiter’s known satellites. This → Galilean satellite has a diameter of
5270 km, slightly larger than Mercury, a mass about 1.48 × 10²³ kg
(about 2 Earth Moons); an → orbital period of 7.155 days, and an → eccentricity of e = 0.0015. It was discovered by Galileo and Marius in 1610. The mean → surface temperature of Ganymede is -160 °C. It is the only moon known to have a → magnetosphere.

See also: In Gk. mythology, Ganymedes, a unusually beautiful prince of Troy who was abducted to Olympus by Zeus and made the cup-bearer of the gods.

An empty space or interval; interruption in continuity; a break or opening, as in a fence, wall. → Encke gap.

Etymology (EN): Gap, from O.N. gap “chasm,” related to gapa “to gape.”

Etymology (PE): Gâf, variant kâf “split, slit,” stem of kâftan, kâvidan “to split; to dig,” Mid./Mod.Pers. škâf- škâftan “to split, burst,” Proto-Iranian *kap-, *kaf- “to split;” cf. Gk. skaptein “to dig;” L. cabere “to scratch, scrape,” P.Gmc. skabanan (Goth. skaban;
Ger. schaben; E. shave). PIE base (s)kep- “to cut, to scrape, to hack.”

A variable → red supergiant star of → spectral type M2 Ia in the → constellation → Cepheus. Also called → Mu Cephei. Its → apparent magnitude is usually about 4.5 and varies from 3.6 to 5.1. It is also a → triple star.

Etymology (EN): Garnet “a deep-red color,” from the more or less transparent, usually red, silicate mineral that has a vitreous luster. So named by William Herschel from its unusual deep reddish tint. From O.Fr. grenat “garnet,” from M.L. granatum, originally an adj., “of dark red color,” probably abstracted from pomegranate, from M.L. pomum granatum “apple with many seeds,” from pome “apple, fruit” + grenate “having grains.”

Etymology (PE): Nârsang, from nâr, from anâr “pomegranate,” from Mid.Pers. anâr “pomegranate” + sang, → stone.

A substance whose physical state is such that it always occupies the whole of the space in which it is contained.

Etymology (EN): Gas, from Du. gas, probably from Gk. khaos “empty space,” → chaos. The term gas was coined by the Belgian physician Jean-Baptiste van Helmont (1579-1644) to designate aerial spirits.

Etymology (PE): Gâz, loanword from Fr.

For a given quantity of an → ideal gas, the product of its → pressure and the → volume divided by the → absolute temperature (R = PV/T).

See also: → gas; → constant.

An equation that links the pressure and volume of a quantity of gas with the absolute temperature. For a gram-molecule of a perfect gas, PV = RT, where P = pressure, V = volume, T = absolute temperature, and R = the gas constant.

See also: → gas; → equation.

A → giant planet composed mainly of → hydrogen and → helium with → traces of → water, → methane, → ammonia, and other hydrogen compounds. Gas giants have a small rocky or metallic core. The core would be at high temperatures (as high as 20,000 K) and extreme pressures. There are four gas giants in our solar system: → Jupiter, → Saturn, → Uranus, and → Neptune. Another category of gas giants is → ice giants. Ice giants are also composed of small amounts of hydrogen and helium. However, they have high levels of what are called “ices.” These ices include methane, water, and ammonia.

See also: → gas; → giant.

A kind of laser where the lasing medium is a gas or a mixture of gases that can be excited with an electric discharge.

The first gas laser to operate successfully was built by A. Javan and William R. Bennette at the Bell Telephone Laboratories.
This laser used a mixture of helium and neon as the active medium and produced a continuous beam rather than a series of pulses. This laser operated in the infrared region of the spectrum at 1.15 micrometres. A few years later Kumar Patel developed the CO₂ laser.

See also: → gas; → laser.

The metallicity derived from observations of the gas component of a galaxy. It is mainly measured from optical → emission lines using primarily oxygen abundances.

The gas → metallicity is one of the most important tools to investigate the evolutionary history of galaxies. The reason is that the gas metallicity of galaxies is basically determined by their star-formation history. Recent observational studies has allowed the investigation of the gas metallicity even in → high redshift beyond z = 1, such as → Lyman break galaxies, submillimeter-selected high-z galaxies, and so on. Such observational insights on the metallicity evolution of galaxies provide constraints on the theoretical understandings of the formation and the evolution of galaxies.

See also: → gas; → metallicity.

An aggregate of several different kinds of gases which do not react chemically under the conditions being considered. A gas mixture constitutes a homogeneous thermodynamical system.

See also: → gas; → mixture.

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

See also: → gas; → tail.

A galaxy which has a relatively low gas content. More specifically, a galaxy whose → baryonic matter is chiefly in the form of stars and has very little → interstellar matter.

See also: → gas; → poor; → galaxy.

A galaxy, usually young, which has a relatively important gas content.

See also: → gas; → rich; → galaxy.

The mass ratio of gas to dust. It amounts to approximately 100 in the → interstellar medium, but may vary in → molecular clouds and → circumstellar disks
due to dust → grain evaporation, → dust settling, → condensation of gas, etc. The gas-to-dust ratio depends on the → metallicity. It is larger in galaxies with lower metallicity.

See also: → gas; → dust; → ratio.

1. Existing in the → state of a gas.
   

2. Pertaining to or having the characteristics of gas.

See also: → gas; → -eous.

An → isotope separation process using the different diffusion speeds of → atoms or → molecules for separation. This process is used to divide → uranium hexafluoride (UF₆) into two separate streams of U-235 and U-238.

Before processing by gaseous diffusion, uranium is first converted from → uranium oxide (U₃O₈) to UF₆. The UF₆ is heated and converted from a solid to a gas. The gas is then forced through a series of compressors and converters that contain porous barriers. Because uranium-235 has a slightly lighter isotopic mass than uranium-238, UF₆ molecules made with uranium-235 diffuse through the barriers at a slightly higher rate than the molecules containing uranium-238. At the end of the process, there are two UF₆ streams, with one stream having a higher concentration of uranium-235 than the other (EVS, a Division of Argonne National Laboratory).

See also: → gaseous; → diffusion.

An → H II region, a → planetary nebula, or a → supernova remnant.

See also: → gaseous; → nebula.

1. (n.) A standard of measure or measurement, size, or quantity.
   

2. Any of a wide variety of devices or instruments used for measuring a parameter or characteristic of an object, such as its dimension, quantity, or mechanical accuracy.
   

3. Physics: One of the family of choices for a constant in the expression of → potential energy in a central force field. Force is connected with potential energy by the relation F = - ∂U/∂r, U = ∫F.dr. The upper limit in the integral can be chosen arbitrarily. The potential energy is usually considered zero at infinity.
   

4. (v.) To determine the exact dimensions, capacity, quantity, or force of; measure.

Etymology (EN): From Fr. jauge “gauging rod,” perhaps from Frankish galga “rod, pole for measuring;” cf. O.N. gelgja “pole, perch;” O.H.G. galgo; Lith. zalga “pole, perch;” Arm. dzalk “pole;” E. gallows; see below.

Etymology (PE): Gaz “a yard for measuring cloth; a length of 24 finger-breadths, or six hands; the tamarisk-tree,” from Mid.Pers. gaz “tamarisk,” may be of the same origin as gauge. In verbal form with kardan “to do, to make” (Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build;” Av. kərənaoiti “he makes;” cf. Skt. kr- “to do, to make,” krnoti “he makes, he does,” karoti “he makes, he does,” karma “act, deed;” PIE base k^wer- “to do, to make”).

A class of elementary particles that includes the gluon, photon, W⁺, W^-, and Z⁰ particles, each having an integral spin.

See also: → gauge; → boson.

The mathematical group associated with a particular set of gauge transformations.

See also: → gauge; → group.

The invariance of any field theory under gauge transformation.

See also: → gauge; → invariance.

A principle underlying the quantum-mechanical description of the three non-gravitational forces. It allows a system to behave in the same way even
though it has undergone various transformations. The earliest physical theory which had a gauge symmetry was Maxwell’s electrodynamics.

See also: → gauge; → symmetry.

A field theory in which it is possible to perform a transformation without altering any measurable physical quantity.

See also: → gauge; → theory.

A change of the fields of a gauge theory that does not change the value of any measurable quantity.

See also: → gauge; → transformation.

A technique in which the thickness, density, or quantity of a material is determined by the amount of radiation it absorbs.

Etymology (EN): Gauging, from → gauge + → -ing, suffix of nouns formed from verbs, expressing the action of the verb or its result.

Etymology (PE): Gazkard, from gaz, → gauge, + kard past stem of kardan “to do, make,” → gauge.

In the atomic theory of spectral line formation, a quantum mechanical correction factor applied to the absorption coefficient in the transition of an electron from a bound or free state to a free state.

See also: Gaunt, after John Arthur Gaunt (1904-1944), English physicist born in China, who significantly contributed to the calculation of continuous absorption using quantum mechanics;
→ factor

The c.s.g. unit of magnetic flux density (or magnetic induction), equal to
1 maxwell per square centimeter, or 10^-4 tesla.

See also: Named after the German mathematician and physicist Carl Friedrich Gauss (1777-1855).

The total electric flux ψ out of an arbitrary closed surface in free space is equal to the net charge within the surface divided by the → permittivity. In differential form: ∇ . E = ρ/ε₀, where ρ is the → charge density and ε₀ the permittivity. The integral form of the law: ∫E . dS = Q/ε₀ (closed surface integral). This is one of the four → Maxwell’s equations.

See also: → gauss; → law; → electricity.

The → magnetic flux through an arbitrary closed surface equals zero. Mathematically, in differential form: ∇ . B = 0 and in integral form: Φ_B = ∫B.dS = 0 (closed surface integral). This is one of the four → Maxwell’s equations. This law expresses the fact that there are no free magnetic poles (→ monopoles) in nature and that all the lines of force of a magnetic field are closed curves.

See also: → gauss; → law;
→ magnetism.

If a → polynomial with → integer coefficients can be → factorized into polynomials with → rational number coefficients, it can be factorized using only integers.

See also: → Gaussian; → lemma.

The total normal induction over any closed surface drawn in an electric field is equal to 4π times the total charge of electricity inside the closed surface. Gauss’s theorem applies also to other vector fields such as magnetic, gravitational, and fluid velocity fields. The theorem can more generally be stated as: the total flux of a vector field through a closed surface is equal to the volume → integral of the vector taken over the enclosed volume. Also known as → divergence theorem, Ostrogradsky’s theorem,
and Gauss-Ostrogradsky theorem.

See also: → gauss; → theorem.

Of or relating to Carl Friedrich Gauss or his mathematical theories of magnetism, electricity, astronomy, or probability. → Gaussian distribution; → Gaussian profile.

See also: → gauss.

A theoretical frequency distribution for a set of variable data, usually represented by a bell-shaped curve with a
mean at the center of the curve and tail widths proportional to the standard deviation of the data about the mean.

See also: → Gaussian; → distribution.

A method of solving a matrix equation of the form A x = b, where A is a matrix and x and b are vectors. The process consists of two steps, first reducing the elements below the diagonal to 0 and second, back substituting to find the solutions.

See also: → Gaussian; → elimination.

The function e^-x2, whose integral in the interval -∞ to +∞ gives the → square root of the → number pi: ∫e^-x2dx = √π. It is the function that describes the → normal distribution.

See also: → Gaussian; → function.

The constant, denoted k, defining the astronomical system of units of length (→ astronomical unit), mass (→ solar mass), and time (→ day), by means of → Kepler’s third law. The dimensions of k² are those of Newton’s constant of gravitation: L ³M ^-1T ^-2. Its value is: k = 0.01720209895.

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

A → complex number whose → real and → imaginary parts are both integers.

See also: → Gaussian; → integer.

The shape of a curve representing a normal distribution.

See also: → Gaussian; → profile.

Math.: The condition of having → Gaussian distribution. The extent to which something is Gaussian.

See also: → Gaussian + → -ity.

1. Law of combining volumes. The volumes of gases used and produced in a chemical reaction, are in the ratio of small whole numbers when measured at constant temperature and pressure.
   

2. For a gas held at constant volume, there is a direct correlation between temperature and pressure: P₁/T₁ = P₂/T₂. Gay-Lussac’s law, → Boyle-Mariotte law, and → Charles’ law were later unified into the → ideal gas law.

See also: Named after Joseph Louis Gay-Lussac (1778-1850), a French chemist and physicist; → law.

A follow-up community network concerned with → gamma-ray burst (GRB)s. It deals with:

1. locations of GRBs and other → transients detected by spacecraft (most in real-time while the GRB is still bursting), and 2) reports of follow-up observations (the Circulars) made by ground-based and space-based optical, radio, X-ray, TeV, and other observers.

The GCN Circulars allow the GRB follow-up community to make optimum use of its limited resources (labor and telescope time) by communicating what has already been done or will soon be done.

See also: → gamma ray; → coordinate; → network.

The prototype of the → L dwarf class. It has a spectral type of L4 V. This object was discovered by Becklin & Zuckerman (1988, Nature 336, 656) as the red companion to a → white dwarf (DA4) lying 104 → light-years away. Its true nature was however recognized several years later (Kirkpatrick et al. 1993, ApJ 406, 701). It has an → effective temperature of 1900 K and a luminosity about 10^-4 times that of the Sun (→ solar luminosity).

See also: GD, referring to Giclas Dwarf, a catalog of white dwarf stars compiled at the Lowell Observatory (Giclas et al. 1980, LowOB 8, 157).

A faint glow of light in the night sky seen exactly opposite the Sun. The gegenschein is sunlight back-scattered off millimeter-sized dust particles along the ecliptic. In comparison with zodiacal light (forward-scattered sunlight), which is triangular in shape and found near the horizon, the gegenschein is roughly circular. Same as counterglow.

Etymology (EN): Gegenschein, from Ger. gegen “counter, opposed to” (O.H.G. gegin, gagan, M.Du. jeghen, E. against, again)

• Schein “glow, shine” (M.H.G. schinen, O.H.G. skinan,
  P.Gmc. *skinanan; E. shine; cf. Mod.Pers. sâyé “shadow;” Mid.Pers. sâyak “shadow;” Av. a-saya- “throwing no shadow;” Skt. chāya- “shadow;” Gk. skia “shade;” Rus. sijat’ “to shine;” PIE base *skai- “bright”).

Etymology (PE): Pâdfrouq “counterglow,” from pâd- “agaist, contrary to” (from Mid.Pers. pât-; O.Pers. paity “agaist, back, opposite to, toward, face to face, in front of;” Av. paiti; cf. 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) + foruq “light, brightness” (related to rôšan “light; bright, luminous;” ruz “day,” afruxtan “to light, kindle;” Mid.Pers. payrog “light, brightness,” rošn light; bright," rôc “day;” O.Pers. raucah-; Av. raocana- “bright, shining, radiant,” raocah- “light, luminous; daylight;”
cf. Skt. rocaná- “bright, shining, roka- “brightness, light;” Gk. leukos “white, clear;” L. lux “light” (also lumen, luna; E. light, Ger. Licht, and Fr. lumière;
PIE base *leuk- “light, brightness”).

A device for detecting ionizing radiations, whether corpuscular
(α-, β-particles), or electromagnetic (X- and gamma-rays). It consists essentially of a fine wire anode (e.g., tungsten) surrounded by a coaxial cylindrical metal cathode, mounted in a glass envelope containing gas at low pressure. A large potential difference (800 to 2000 volts) is maintained between the anode and the cathode. The ionizing particle can enter through a thin glass or mica window. The particle produces ionization of gas molecules. The ions are accelerated by the electric field and produce more ions by collisions, causing the ionization current to build up rapidly. The current, however, decays quickly since the circuit has a small time constant. There is therefore a momentary potential surge which may be amplified and made to actuate a relay to advance a mechanical counter, or to produce a click in a loudspeaker. Same as Geiger-Mulle counter.

See also: Named after Hans Geiger (1882-1945), the German physicist, who invented the instrument. He is also known for his work on atomic theory and cosmic rays; → counter.

To castrate (an animal, especially a horse).

Etymology (EN): M.E. gelden, from O.Norse gelda, ultimately from PIE *ghel- “to cut.”

Etymology (PE): Axtan, variant of âxtan, âhixtan, âhiz- “to draw out; castrate, geld,” → object.

A castrated male animal, especially a horse.

See also: → geld; → -ing.

A bright → gamma-ray source discovered in 1973 in the constellation → Gemini with instruments aboard NASA’s first γ-ray satellite SAS-2.
It was known only as a γ-ray source until it was detected in X-rays by the Einstein Observatory and associated with an optical counterpart of apparent magnitude 25.
Because its luminosity outside of the γ-ray region is extremely low, the nature of this object remained a mystery until the discovery of pulsed emission, by the → ROSAT satellite in 1992, showed that it is a → pulsar. The pulsar period (~237 milliseconds) and its → period derivative (~1.1 × 10^-14 s s^-1) correspond to a → spin-down age of ~340,000 years. Also called PSR J0633+1746 (see Bignami & Caraveo 1996, ARA&A 34, 331 for a review).

See also: An abbreviation for the Gemini gamma ray source. More amusingly, Geminga has been related to the Italian dialectal ghè minga spoken by the involved astronomers. This, in Milanese,
means “it’s not there,” referring to the fact that the source could not be detected in the radio frequencies, one of the ongoing enigmas.

The Twins. A prominent constellation of the northern hemisphere and the third (and northernmost) of the → Zodiac. It lies south and east of → Auriga, west of → Cancer, and north and east of → Orion, at 7h right ascension and +22°
declination. Its brightest stars are → Castor and → Pollux. Abbreviation: Gem; genitive: Geminorum.

Etymology (EN): Gemini, from M.E., from L. gemini, plural of geminus “twin; double;” cf. Av. yəma- “twin;” Skt. yamá-, yamala- “twin, paired;” Persian dialects Kermâni jomoli “twin,” Qâyeni jamal “twin,” Tabari da-cembali “twin;” PIE base *iem- “to hold.”

Etymology (PE): Dopeykar, from do “two” (Mid.Pers. do, Av. dva-; Skt. dvi-; Gk. duo; L. duo ( Fr. deux); E. two; Ger. zwei)

• peykar “figure, form, body” (from Mid.Pers. pahikar “picture, image;” from O.Pers. patikara- “picture, (sculpted) likeness,” from patiy “against” (Av. paiti; Skt. prati; Gk. poti/proti) + kara- “doer, maker,” from kar- “to do, make, build;” Av. kar-; Skr. kr-; cf. Skt. pratikrti- “an image, likeness, model; counterpart”).

A → meteor shower that occurs in the first half of December, with its → radiant in the → constellation → Gemini. Geminids are pieces of debris from the extinct comet → 3200 Phaethon. The shower appears when Earth runs into a stream of the debris every year in mid-December, causing → meteors to fly from that constellation.

See also: → Gemini + → -ids.

The brightest star in the constellation → Corona Borealis. Same as → Alphekka.

Etymology (EN): Gemma, from L. gemma “precious stone, jewel,” originally “bud,” from the root *gen- “to produce”, → generate.

Etymology (PE): Alfakké, → Alphekka.

In some languages (not in English, nor in Persian) a set of two or more grammatical categories (called → masculine, → feminine, and → neuter) into which nouns, pronouns, and adjectives
are divided. For example, French, Spanish, and Italian have two genders, masculine and feminine (shown, for example, in French by the use of le and la, respectively); German and Russian have three genders, masculine, feminine, and neuter. Ancient Iranian languages had three genders, like Sanskrit and Greek.

Etymology (EN): From M.E. gendre, from O.Fr. gendre, from stem of L. genus “race, stock, family; kind, rank, order; species.”

Etymology (PE): Žâné “race, species,” ultimately from Proto-Ir. *zan- “to be born,” cf. Av. za(n)- “to give birth; to be born;” related to Pers. zâdan, akin to L. genus, as above, → generate; the transformation of z into ž, as in ne&#382âd, → race.

The basic unit of hereditary that is an ordered sequence of nucleotides located in a particular position of a particular chromosome. It is the means by which characteristics are transmitted from parents to offsprings.

Etymology (EN): From Ger. Gen, coined 1905 by Danish scientist Wilhelm Ludvig Johannsen (1857-1927), from Gk. genos “race, kind,” genesis “origin,” genea “generation, race;” cognate with L. genus “race, stock;” generare “to bring forth;” Pers. zâdan “to bring forth;” → generate.

Etymology (PE): Žen, loanword from Fr., as above.

(Adj.) 1) Not limited to one class, field, product, service, etc. 2) Relating to the whole or to the all or most. 3) Dealing with overall characteristics, universal aspects, or important elements.
See also:
→ general precession, → general relativity, → generalization, → generalize, → generalized, → generalized coordinates, → generalized forces, → generalized momenta, → generalized velocities, → New General Catalogue (NGC).

Etymology (EN): From L. generalis “relating to all, of a whole class,” from genus “race, stock, kind,” akin to Pers. zâdan, Av. zan-
“to bear, give birth to a child, be born,” infinitive zazāite, zāta- “born;” Mod.Pers. zâdan, present stem zā-
“to bring forth, give birth” (Mid.Pers. zâtan; cf. Skt. jan- “to produce, create; to be born,” janati “begets, bears;” Gk. gignomai “to happen, become, be born;” L. gignere “to beget;” PIE base *gen- “to give birth, beget.”

Etymology (PE): Harvin, from Mid.Pers. harvin “all,” from har(v) “all, each, every” (Mod.Pers. har “every, all, each, any”); O.Pers. haruva- “whole, all together;” Av. hauruua- “whole, at all, undamaged;” cf. Skt. sárva- “whole, all, every, undivided;” Gk. holos “whole, complete;” L. salvus “whole, safe, healthy,” sollus “whole, entire, unbroken;” PIE base *sol- “whole.”

The secular motions of the → celestial equator and → ecliptic. In other words, the sum of → lunisolar precession, → planetary precession, and → geodesic precession.

See also: → general; → precession

The secular displacement of the → equinox on the → ecliptic of date.

See also: → general; → precession; → longitude.

The secular motion of the → equinox along the → celestial equator.

See also: → general; → precession; → right ascension.

Of, relating to, or subject to the theory of → general relativity.

See also: → general; → relativistic.

The theory of → gravitation developed by Albert Einstein (1916) that describes the gravitation as the → space-time
curvature caused by the presence of matter or energy. Mass creates a → gravitational field which distorts the space and changes the flow of time. In other words, mass causes a deviation of the → metric of space-time continuum from that of the “flat” space-time structure described by the → Euclidean geometry and treated in
→ special relativity. General relativity developed from the → principle of equivalence between gravitational and inertial forces.

According to general relativity, photons follow a curved path in a gravitational field. This prediction was confirmed by the
measurements of star positions near the solar limb during the total eclipse of 1919. The same effect is seen in the delay of radio signals coming from distant space probes when grazing the Sun’s surface. Moreover, the space curvature caused by the Sun makes the → perihelion of Mercury’s orbit advance by 43’’ per century more than that predicted by Newton’s theory of gravitation. The → perihelion advance can reach several degrees per year for → binary pulsar orbits. Another effect predicted by general relativity is the → gravitational reddening. This effect is verified in the → redshift of spectral lines in the solar spectrum and, even more obviously, in → white dwarfs. Other predictions of the theory include → gravitational lensing, → gravitational waves, and the invariance of Newton’s → gravitational constant.

See also: → general; → relativity.

→ secretary-general.

See also: → general; → secretary.

The act or process of generalizing; → generalize.
A result of this process; a general statement, proposition, or principle.

See also: Verbal noun of → generalize.

To make general, to include under a general term; to reduce to a general form.
To infer or form a general principle, opinion, conclusion, etc. from only a few facts, examples, or the like.

See also: → general; → -ize.

Made general. → generalized coordinates; → generalized velocities.

See also: P.p. of → generalize

In a material system, the independent parameters which completely specify the configuration of the system, i.e. the position of its particles with respect to the frame of reference. Usually each coordinate is designated by the letter q with a numerical subscript. A set of generalized coordinates would be written as q₁, q₂, …, q_n. Thus a particle moving in a plane may be described by two coordinates q₁, q₂, which may in special cases be the → Cartesian coordinates x, y, or the → polar coordinates r, θ, or any other suitable pair of coordinates. A particle moving in a space is located by three coordinates, which may be Cartesian coordinates x, y, z, or → spherical coordinates r, θ, φ, or in general q₁, q₂, q₃. The generalized coordinates are normally a “minimal set” of coordinates. For example, in Cartesian coordinates the simple pendulum requires two coordinates (x and y), but in polar coordinates only one coordinate (θ) is required. So θ is the appropriate generalized coordinate for the pendulum problem.

See also: → generalized; → coordinate.

In → Lagrangian dynamics, forces related to → generalized coordinates. For any system with n generalized coordinates q_i (i = 1, …, n), generalized forces are expressed by F_i = ∂L/∂q_i, where L is the → Lagrangian function.

See also: → generalized; → force.

In → Lagrangian dynamics, momenta related to → generalized coordinates. For any system with n generalized coordinates q_i (i = 1, …, n), generalized momenta are expressed by p_i = ∂L/∂q^._i, where L is the → Lagrangian function.

See also: → generalized; → momentum.

The time → derivatives of the → generalized coordinates of a system.

See also: → generalized; → velocity.

To bring into existence; create; produce.
Math.: To trace (a figure) by the motion of a point, straight line, or curve.

Etymology (EN): Generate, from M.E., from L. generatus “produce,” p.p. of generare “to bring forth,” from gener-, genus “descent, birth,” akin to Pers. zâdan, Av. zan- “to give birth,” as explained below.

Etymology (PE): Âzânidan, from â- nuance/strengthening prefix + zân, from Av. zan- “to bear, give birth to a child, be born,” infinitive zazāite, zāta- “born;” Mod.Pers. zâdan, present stem zā-
“to bring forth, give birth” (Mid.Pers. zâtan; cf. Skt. jan- “to produce, create; to be born,” janati “begets, bears;” Gk. gignomai “to happen, become, be born;” L. gignere “to beget;” PIE base *gen- “to give birth, beget”)

• -idan infinitive suffix.

1. A coming into being.
   

2. The → production of → energy (→ heat or → electricity).

See also: Verbal noun of → generate.

1. Capable of producing or creating.
   

2. Pertaining to the production of offspring.

See also: → generate; → -ive.

1. A machine for converting one form of energy into another.
   

2. Geometry: That which creates a line, a surface, a solid by its motion.

Etymology (EN): From L. generator “producer,” from genera(re)→ generate + -tor a suffix forming personal agent nouns from verbs and, less commonly, from nouns.

Etymology (PE): Âzângar, from âzân the stem of âzânidan→ generate

• -gar suffix of agent nouns, from kar-, kardan “to do, to make” (Mid.Pers. kardan; O.Pers./Av. kar- “to do, make, build,” Av. kərənaoiti “makes;” cf. Skt. kr- “to do, to make,” krnoti “makes,” karma “act, deed;” PIE base k^wer- “to do, to make”).

Pertaining or according to → genetics or → genes.

See also: From Gk. genetikos, from genesis “origin,”
→ gene; → -ic.

The study of heredity and inheritance, of the transmission of traits from one individual to another, of how genes are transmitted from generation to generation.

See also: From → genetic and → -ics.

The → grammatical case that marks a noun or pronoun typically expressing “possession” or “origin.”

In English, the genitive case of a noun is shown in writing by adding an s together with an appropriately positioned apostrophe or creating it by using the pronoun of.
For instance: “John’s house,” or “the house of John.” A → synthetic language would express the same idea by putting the name “John” in the genitive case.

Also called → possessive case.

Etymology (EN): From O.Fr. genitif or directly from L. (casus) genitivus “case expressing possession, source, or origin,” from genitivus “of or belonging to birth,” from genitus, p.p. of gignere “to beget, produce,” → generate; → case.

Etymology (PE): Dârešti, → possessive; kâté, → case..

1. An exceptionally intelligent person or one with exceptional skill in a particular area of activity.
   

2. Exceptional intellectual or creative power or other natural ability (Oxford Dictionaries).

Etymology (EN): From L. genius “tutelary deity or genius of a person;” originally “generative power,” from gignere “beget, produce,” → generate.

Etymology (PE): Farhuš, from far- intensive prefix “much, abundant; elegantly,” → perfect, + huš, → intelligence. Farhuši, from farhuš

• -i.

The deliberate and systematic extermination of a national, racial, political, or cultural group (Dictionary.com).

Etymology (EN): From Gk. genos “race, kind,” → generate,

• → -cide.

Etymology (PE): Nežâdkoši, from nežâd, → race,

• koši, → -cide.

1. Biology: The usual major subdivision of a family or subfamily in the classification of organisms, usually consisting of more than one species.
   

2. Logic: A class or group of individuals, or of species of individuals.
   

3. A kind; sort; class (Dictionary.com).

Etymology (EN): From L. genus “race, stock, kind, gender;” cognate with Gk. genos “race, kind,” and gonos “birth, offspring, stock,” → generate.

Etymology (PE): Sardé, from Mid.Pers. sardag “sort, kind;” Av. sarrəδa- “kind, type.”

A combining form meaning “the earth,” used in the formation of compound words.

Etymology (EN): Geo-, form Gk. ge “earth, land, ground, soil.”

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

1. Relating to, measured from, or with respect to the center of the Earth.
   

2. Having the earth as a center. → geocentric coordinate system, → Geocentric Coordinate Time, → geocentric cosmology, → geocentric parallax, → geocentric system.

See also: → geo- + → center

• -ic an adjective-forming suffix.

A coordinate system which has as its origin the center of the Earth.

See also: → geocentric; → coordinate;
→ system.

The proper time experienced by a clock at rest in a coordinate frame co-moving with the center of the Earth, i.e. a clock that performs exactly the same movements as the Earth but is outside the Earth’s gravity well. TCG was defined in 1991 by the International Astronomical Union as one of the replacements for Barycentric Dynamical Time (TDB).

See also: → geocentric; → coordinate;
→ time.

A model of the Universe in which the Earth is centrally located and the Sun, planets, and stars revolve around the Earth.

See also: → geocentric; → cosmology.

The angle between the geocentric location vector and the → geodetic equator.

See also: → geocentric; → latitude.

The same as → geodetic longitude.

See also: → geocentric; → longitude.

The difference between the direction of an object as seen from a point on the surface of the Earth and the direction in which it would be seen from the Earth’s center. Also known as → diurnal parallax.

See also: → geocentric; → parallax.

An ancient model of the Universe whereby all the celestial bodies travel around the Earth in circular orbits. Eudoxus of Cnidus (c. 390- c. 337 BC), one of Plato’s pupils, maintained that all objects in the sky are attached to moving crystalline spheres, with the Earth at the centre. This model is often named → Ptolemaic system after its most famous supporter, the Greco-Roman astronomer Ptolemy.

See also: → geocentric; → system.

Of or relating to → geochemistry.

See also: → geochemistry; → -al.

A field of science that is concerned with the relative abundance, distribution, and the movement of → chemical elements in the → Earth’s crust or other → solar system objects.

See also: → geo-; → chemistry.

The outermost part of Earth’s atmosphere, a tenuous halo of hydrogen and some helium extending out to perhaps 15 Earth radii. It emits at the → Lyman alpha line (wavelength 121 nm) caused by → resonant scattering of solar → ultraviolet.

See also: → geo- + → corona.

1. The shortest distance between two points in space (or → space-time). A geodesic on a sphere is an → arc of a → great circle. In the theory of → general relativity, freely falling bodies follow geodesic paths in space-time.
   

2. → geodetic.

Etymology (EN): From Fr. géodésique, → geodesy; → -ic.

Etymology (PE): Kehinrah “shortest path,” from kehin, superlative of keh “small, little, slender” (related to kâstan, kâhidan “to decrease, lessen, diminish,” from Mid.Pers. kâhitan, kâstan, kâhênitan “to decrease, diminish, lessen;” Av. kasu- “small, little;” Proto-Iranian *kas- “to be small, diminish, lessen”)

• râh “path, way, road” (from Mid.Pers. râh, râs “way, street,” also rah, ras “chariot;” from Proto-Iranian *rāθa-; cf. Av. raθa- “chariot;” Skt. rátha- “car, chariot,” rathyā- “road;” L. rota “wheel,” rotare “to revolve, roll;” Lith. ratas “wheel;” O.H.G. rad; Ger. Rad; Du. rad;
  O.Ir. roth; PIE base *roto- “to run, to turn, to roll”).

The shortest line between two points on a surface. Also called → geodesic.

See also: → geodesic; → line.

→ geodesic precession.

See also: → geodesic; → precession

The study and measurement of the shape, size, and curvature of the Earth.

Etymology (EN): From Fr. géodésie, from Gk. geodaisia, from → geo-

• dai(ein) “to divide” + -sia, variant of -ia a noun suffix.

Etymology (PE): Zamin-sanji, from zamin, → geo-, + sanji, from sanjidan “to measure; to compare,” from Mid.Pers. sanjidan “to weigh,”
from present tense stem sanj-, Av. θanj- “to draw, pull;” Proto-Iranian
*θanj-. Other terms from this base in Pers.: lanjidan “to pull up,”
hanjidan, âhanjidan “to draw out,” farhang “education, culture.”

A type of Earth observing satellite used to measure the location of points on Earth’s surface with great accuracy. Their observations help determine the exact size and shape of Earth, act as references for mapping, and track movements of Earth’s crust.

See also: → geodesy; → satellite

Of, relating to, or determined by → geodesy.

See also: → geodesy; → -ic.

A → coordinate system, composed of → geodetic latitude and
→ geodetic longitude on the → Earth’s surface which is dependent on the figure and size of a particular model for the Earth’s surface.

See also: → geodetic; → coordinate.

Any of the adopted values of → geodetic latitude, → geodetic longitude, or → azimuth at a selected location (an initial station) whose astronomical coordinates have already been determined.

See also: → geodetic; → datum.

The plane swept out as the generating ellipse of the → reference ellipsoid rotates about its minor axis.

See also: → geodetic; → equator.

The → acute angle between the → geodetic vertical and the → geodetic equator.

See also: → geodetic; → latitude.

The angle between the plane of the → geodetic meridian and the plane of of the geodetic meridian through the site of the → Airy transit circle at the Royal Greenwich Observatory.

See also: → geodetic; → longitude.

The → ellipse through the point in question which passes through the → geodetic poles.

See also: → geodetic; → meridian.

Any of the small circles on the → reference ellipsoid parallel to the → geodetic equator.

See also: → geodetic; → parallel.

Any of the interaction points of the axis of revolution of the → reference ellipsoid with its surface.

See also: → geodetic; → pole.

A → relativistic effect on the precession motion of a gravitational system due to the → curvature of the → space-time. When a body revolves around a primary, the → rotation axis of the orbiting body follows the curvature of spece-time. Over time the space-time warping causes the spin axis to precess.
In the case of the Earth-Moon system, this means a small → direct motion of the → equinox along the → ecliptic, amounting to 1’’.915 per century. The geodetic precession is given by:
ψ_g = (3/2) k² (1 - e_⊕) n_⊕, where k is the → constant of aberration (in radians), e_⊕ the → eccentricity of the Earth and n_⊕ the mean angular orbital motion of the Earth (in arcsec/cy).
Also called → Einstein-de Sitter effect and → geodesic precession.

See also: → geodetic; → precession

The limiting case of → astronomical refraction when the light path is entirely within the Earth’s atmosphere.

See also: → geodetic; → refraction.

The direction defined by the → normal to the → reference ellipsoid at the point in question.

See also: → geodetic; → vertical.

The intersection of the prolongation of the outward → normal to the → reference ellipsoid at the point in question with the → celestial sphere.

See also: → geodetic; → zenith.

Of or pertaining to → geography.

See also: → geography; → -ic.

A → ccordinate system on the surface of the Earth that defines every location by a set of numbers and letters, indicating the
→ latitude and → longitude.

See also: → geographic; → coordinate; → system.

A synonym for → geodetic latitude or → astronomical latitude.

See also: → geographic; → latitude.

→ north pole.

See also: → geographic; → north; → pole.

→ south pole.

See also: → geographic; → south; → pole.

The science dealing with the areal differentiation of the Earth’s surface, as shown in the character, arrangement, and interrelations over the world of such elements as climate, elevation, soil, vegetation, population, land use, industries, or states, and of the unit areas formed by the complex of these individual elements (Dictionary.com).

See also: → geo-; → -graphy.

The form of the → Earth obtained by taking average sea level surface and extending it across the continents. In other words, the → equipotential surface (“mean sea level”) of the Earth’s → gravitational field. The geoid is considered to represent the true physical figure of the Earth, in contrast to the → reference ellipsoid, which is idealized geometrical figure of the Earth.

See also: → geo-; → -oid.

Of, pertaining to, or based on → geology. Also geological.

See also: From geolog(y), → geology, + → -ic.

The long span of time from the end of the formation of Earth during which our planet underwent its major transformations.

See also: → geologic; → time.

The scientific study of the composition, structure, and physical history of the Earth.

See also: → geo- + → -logy.

Of or pertaining to → geomagnetism.

See also: → geo-; → magnetic.

The natural variations in the → geomagnetic field due to interactions of the Earth’s field and → magnetosphere with energetic particles from the Sun.

See also: → geomagnetic; → activity.

A geophysical event, distinguished from the → magnetic reversal, in which the Earth’s magnetic field departs for a relatively short time from its usual near axial configuration, without establishing a reversed direction. During the excursion the intensity and direction of the Earth’s magnetic field undergo drastic changes. Palaeomagnetic measurements have revealed that since the last full reversal the Earth’s magnetic field has, for brief intervals, deviated from the behavior expected during “normal” secular variation.

See also: → geomagnetic; → excursion.

The magnetic field that is generated within the Earth and extends out around the Earth. The intensity of the magnetic field at the Earth’s surface is about 0.32 → gauss at the equator and 0.62 gauss at the north pole.

See also: → geomagnetic; → field.

A violent disturbance of the Earth’s magnetic field, distinct from regular diurnal variations, following a → solar flare or → coronal mass ejection.

See also: → geomagnetic; → storm.

A branch of geophysics concerned with the study of the Earth’s → geomagnetic field, including its origin, spatial extent, and variations in time.

See also: → geo-; → magnetism.

Of or pertaining to geometry or to the principles of geometry.

See also: Adj. of → geometry

A measure of the → reflectivity of a surface, especially of the solar system bodies (planets, satellites or asteroids). It is the ratio of a body’s brightness at zero → phase angle to the brightness of a perfectly diffusing disk with the same position and apparent size as the body. Geometric albedo depends on the radiation wavelength.
The bolometric geometric albedo refers to reflectivity in all wavelengths. Compare with the → Bond albedo.

See also: → geometric; → albedo.

One of the two factors contributing to → gravitational lensing time delay that arises from the fact that the bent trajectory is longer than the straight one. The other factor is due to the → Shapiro time delay.

See also: → geometric; → delay.

Where the apparent → sea horizon would be if there were no → atmospheric refraction.

See also: → geometric; → horizon.

The middle term in a → geometric progression. Of two terms, the geometric mean is the square root of their product. For example, the geometric mean of 4 and 9 is ± 6. For a series of n terms, it is expressed as: (a₁.a₂. … .a_n)^1/n.

See also: → geometric; → mean.

A branch of physics that deals with reflection and refraction of rays of light without reference to the wave or physical nature of light.

See also: → geometric; → optics.

A → sequence in which the ratio of a term to its predecessor is the same for all terms. In general, the nth term has the form ar^(n-1), where n is a positive integer, and a and r are nonzero constants; r is called the ratio or common ratio. The sum of the first n terms is given by: S_n = a(1 - rⁿ)/(1 - r). Also called → geometric sequence.

See also: → geometric; → progression.

A type of scattering in which the wavelength (of the light or the sound) is much smaller than the size of object causing the scattering.

See also: → geometric; → scattering.

→ geometric progression.

See also: → geometric; → sequence.

Libration resulting from changes in the location of the observer with respect to body. More specifically, a lunar libration motion that results from the Earth based observer seeing the Moon from different directions at different times. There are three types of geometrical libration: → libration in longitude, → libration in latitude, and → diurnal libration. See also → physical libration.

See also: → geometric; → libration.

The branch of mathematics that deals with the nature of space and the size, shape, and other properties of figures as well as the transformations that preserve these properties.

Etymology (EN): From O.Fr. géométrie, from L. geometria, from Gk. geometria “measurement of earth or land,” from → geo- + -metria, from metrein “to measure,” → -metry.

Etymology (PE): Hendesé, Mid.Pers. handâxtan “to measure,” Manichean Mid.Pers. hnds- “to measure,” Proto-Iranian ham-, → com-, + *das- “to heap, amass;” cf. Ossetic dasun/dast “to heap up;” Arm. loanword dasel “to arrange (a crowd, people),” das “order, arrangement,”

The branch of physics that deals with the Earth and its environment, including meteorology, oceanography, seismology, and geomagnetism.

See also: → geo-; → physics.

The study or the application of the influence of political and economic geography on the politics, national power, foreign policy, etc., of a state (Dictionary.com).

See also: → geo-; → politics.

A satellite orbit in the plane of the Earth’s equator and 35,880 km above it, at which distance the satellite’s period of rotation matches the Earth’s and the satellite always remains fixed in the same spot over the Earth.

See also: Geostationary, from → geo- + → stationary; → orbit.

Of or pertaining to the force produced by the rotation of the Earth.

Etymology (EN): From Gk. → geo- + Gk. strophe “a turning,” from strephein “to turn,” from PIE *strebh- “to wind, turn” + → -ic.

Etymology (PE): From zamin-, → geo-, + carxeši, → rotational.

Meteo.:
The balance between the → Coriolis force and the → pressure gradient force. See also → geostrophic flow.

See also: → geostrophic; → balance.

Oceanography: A flow resulting from
→ geostrophic balance. In geostrophic flow water moves along the lines of constant pressure or → isobars. Geostrophic flow is characterized by small → Rossby and → Ekman numbers.

See also: → geostrophic; → flow.

Meteo.: A wind which is balanced by the → Coriolis effect and → pressure gradient force.

An air parcel initially at rest will move from high pressure
to low pressure because of the pressure gradient force. 
However, the air parcel in its movement is
deflected by the Coriolis force, to the right in the northern 
hemisphere and to the left on the southern hemisphere. As the
wind gains speed, the deflection increases until the Coriolis
force equals the pressure gradient force. At this point, the
wind will be blowing parallel to the → <i><a class="linkVoir" href="/terms/isobar/">isobar</a></i>s.

See also: → geostrophic; → wind.

A circular orbit around the Earth identical to a geostationary orbit except that the satellite’s orbit does not necessarily lie in the Earth’s equatorial plane.

See also: → geo-; → synchronous;
→ orbit.

An equatorial mounting in which the declination axis is at the end of the polar axis, which is on top of a pier to raise the telescope to a convenient height.

Etymology (EN): German, from L. Germanus, maybe of Gaulish (Celtic) origin, perhaps originally meaning “noisy” (cf. O.Ir. garim “to shout”) or “neighbor” (cf. O.Ir. gair “neighbor”); → mounting.

Etymology (PE): Barnešând, → mounting; Âlmâni “German,” from Âlmân, from Fr. Allemand “German,”
from P.Gmc. *Alamanniz, probably meaning “all-man” and denoting a wide alliance of tribes. Alternatively, perhaps meaning “foreign men,” cognate with L. alius “the other.”

A noun formed from a verb, denoting an action or state. In English, the gerund is the “-ing” form of a verb when it functions grammatically as a noun in a sentence; it is identical in appearance to the present participle.

Etymology (EN): From L.L. gerundium, from gerundum “to be carried out,” gerundive of gerere “to bear, carry.”

Etymology (PE): Karnâm, short for karvâznâm, from karvâz, → verb, + nâm “name, → noun.”

Giga (billion) → electron volt. A unit of → energy used to describe the total energy carried by a → particle or → photon.

Etymology (EN): → giga- + → electron volt.

A faint false image caused by reflection that is sometimes seen in an optical system. In spectroscopy, a false image of a spectral line formed by irregularities in the ruling of diffraction gratings.

Etymology (EN): Ghost, from O.E. gast “soul, spirit, breath,” from P.Gmc. *ghoizdoz (cf. M.Du. gheest, Ger. Geist “spirit, ghost”), from PIE base *ghois- “to be excited, frightened;” cf. Av. zaēša- “horrible, frightful,” zôiždi&#353ta- “the most loathsome;” Mid./Mod.Pers. zešt “ugly, disgusting;” Goth. usgaisjan “to be afraid;” O.E. gæstan “to frighten.”

Etymology (PE): Parhib “ghost,” Pers. word of Xorâsâni dialect.

A star forming region in the → Large Magellanic Cloud, a satellite of our Galaxy, as pictured by the → Hubble Space Telescope. It spans about 50 light-years and contains several young, → massive stars.

See also: → ghost; → head; → nebula

A person or thing of unusually great size, power, importance. In astronomy, e.g. → giant star, → giant branch, → red giant, → asymptotic giant branch (AGB), → blue supergiant, → blue giant, → gas giant, → giant H II region, → giant impact hypothesis, → giant magnetoresistance (GMR), → giant molecular cloud (GMC), → giant planet, → Li-rich giant, → subgiant, → supergiant.

Etymology (EN): From O.Fr. géant, from V.L. *gagantem, from L. gigas “giant,” from Gk. gigas (gen. gigantos), huge and savage monsters, children of Gaia and Uranus, who fought the Olympians but were eventually destroyed by the gods, probably from a pre-Gk. language.
The Gk. word was used in Septuagint (the Greek translation of the Jewish Scriptures) to refer to men of great size and strength, hence the expanded use in Western languages.

Etymology (PE): Qul “an imaginary hideous demon, supposed to devour men and animals,” Pers. word probably related to Skt. grábha- “a demon causing diseases, one who seizes,”
grahila- “possessed by a demon,” from grah-, grabh- “to seize, take,” graha “seizing, holding, perceiving,” Av./O.Pers. grab- “to take, seize;” Mid.Pers. griftan; Mod.Pers. gereftan “to take, seize;” cf. M.L.G. grabben “to grab,” from P.Gmc. *grab, E. grab “to take or grasp suddenly;” PIE base *ghrebh- “to seize.”
Qulpeykar, from qul, as explained, + peykar “figure, form, body” (from Mid.Pers. pahikar “picture, image;” from O.Pers. patikara- “picture, (sculpted) likeness,” from patiy “against” (Av. paiti; Skt. prati; Gk. poti/proti + kara- “doer, maker,” from kar- “to do, make, build;” Av. kar-; Skt. kr-; cf. Skt. pratikrti- “an image, likeness, model; counterpart”).
Qulâsâ, from qul + suffix of nature, relation -âsâ, → -aceous.
Kalân “great, large, big, bulky.”

A conspicuous family of stars in the → Hertzsprung-Russell diagram composed of red, evolved stars with large sizes. → giant star; → red giant.

See also: → giant; → branch.

An → H II region emitting at least 10⁵⁰ → Lyman continuum photons per second, or about 10 times → Orion nebula. Such an H II region should be
powered by at least one O3V star or by at least a dozen → O-type and tens → B-type stars. Our nearest giant H II region is → NGC 3603. Some other Galactic giant H II regions are: → Lagoon Nebula, M17, W31, W51A, and NGC 3576.

See also: → giant; → H II; → region.

A model for → Moon formation (initially put forward by Hartmann and Davis, 1975, Icarus 24, 504), according to which the → proto-Earth suffered a collision with another → protoplanet near the end of the → accretion process that ejected material into a → circumterrestrial disk, out of which the Moon formed. Also called → canonical model.

The giant impact hypothesis is the leading theory for lunar formation. There are, however, some key observations that cannot be explained using this model. First, the Moon is a large fraction of the mass of Earth (~ 1%) and it is difficult to get enough mass into orbit to form such a massive Moon.

Second, the Moon has a similar bulk composition to the Earth, but it is missing large amounts of more → volatile elements. The model does not properly explain Moon’s distinctive composition.

Finally, Earth and the Moon share virtually the same → isotopic ratios. It is therefore expected that the body that hit the Earth, often called → Theia, would have had a different isotopic ratio than the proto-Earth. In the canonical model, most of the mass of the Moon comes from Theia and so the Moon should have a different isotopic fingerprint than Earth, but it does not.

The type of impact that formed the Moon in the canonical model is dictated by a very strong constraint, the → angular momentum of the Earth-Moon system. It is assumed that the angular momentum of the Earth-Moon system immediately after the Moon formed was the same as it is today. This assumption limits the velocity of the impact, the mass of the impacting bodies, and the angle at which the two bodies collided. It was found that only a grazing impact with a Mars-mass impactor at near the escape velocity can put enough mass into orbit to potentially form a lunar-mass Moon. This is why the canonical model is such a specific type of impact.

However, the angular momentum of the Earth-Moon system could have been reduced over time by competition between the gravitational pull of Earth, the Moon and the Sun. Therefore, the Moon-forming collision could have been much more energetic than the canonical impact.

Simon Lock and Sarah Stewart (2017, J. Geophys. Res. Planets, 122, 950-982) have shown that such high-energy, high-angular momentum impacts can produce a different type of planetary object, → synestias. High-energy impacts vaporize a substantial fraction (~ 10%) of the rock of the impacting bodies and the resulting synestias can be huge, with equatorial radii of more than ten times that of the present-day Earth.

Because the impact-produced synestia was so big, the Moon formed inside the vapor of the synestia surrounded by gas at pressures of tens of bars and temperatures of 3000-4000 K. Fragments of molten rock from the impact collided together and formed a lunar seed orbiting within the vapor of the synestia. The surface of the synestia was hot (2300 K) and the body cooled rapidly. The loss of energy led to the condensation of rock droplets at the surface of the synestia, and a torrential rock rain fell towards the center of the synestia. Some of this rain was revaporized in the hot vapor of the synestia, but some encountered the lunar seed, and the Moon grew.

As the synestia cooled, more of the vapor condensed and the body contracted rapidly. After ten years or so, the synestia shrank inside the orbit of the Moon and the nearly fully-formed Moon emerged from the vapor of the synestia. The synestia continued to cool and became a planet within a thousand years or so of the Moon emerging from the structure.

Without the tight constraint of the angular momentum, impacts that form synestias can put a lot more mass into the outer regions of the synestia than can be put into the disk in the canonical impact. This makes forming a large, lunar-mass Moon much easier.

Moreover, because the Moon formed within the synestia, surrounded by hot vapor, it inherited its composition from Earth but only retained the elements that are more difficult to vaporize. The more volatile elements remained in the vapor of the synestia. When the synestia cooled and contracted inside the Moon’s orbit, it took all the more volatile elements with it.

This model can also help explain the isotopic similarity between Earth and the Moon. The Moon inherited its isotopic fingerprint from the vapor that surrounded it in the outer regions of the synestia. Energetic impacts that form synestias tend to efficiently mix material from the two colliding bodies, and the outer portions of the synestia in which the Moon formed would have had an isotopic composition that was similar to the rest of the synestia. Earth and the Moon therefore share a similar isotopic fingerprint which is made by a mixture of the isotopic compositions of both the bodies that collided.

See also: → giant; → impact; → hypothesis.

A quantum mechanical phenomenon where the resistance of certain materials drops dramatically upon application of a magnetic field in certain structures composed of alternating layers of magnetic and nonmagnetic metals. The basis of the GMR is the dependence of the electrical resistivity of electrons in a magnetic metal on the direction of the electron spin, either parallel or anti-parallel to the magnetic moment of the layers. The 2007 Nobel Prize in physics was awarded to the French physicist Albert Fert (1938-)
and German physicist Peter Grünberg (1939-) for the discovery of GMR.

See also: → giant; magneto- combining form of → magnet; → resistance.

A massive complex of → interstellar gas and → dust, consisting mostly of → molecular hydrogen, that typically stretches over 150 light-years and contains several hundred thousand → solar masses. Giant molecular clouds are the principal sites of star formation. → molecular cloud.

See also: → giant; → molecular; → cloud.

A planet much more massive than Earth. The solar system has four giant planets: → Jupiter, → Saturn, → Uranus, and → Neptune.

See also: → giant; → planet.

A high-luminosity star that has evolved off the → main sequence and lies above the main sequence on the → Hertzsprung-Russell diagram. A member of the → giant branch. → red giant.

See also: → giant; → planet.

An adjective applied to the phase of the Moon (or a planet) when it is more than half full, but less than entirely full.

Etymology (EN): From L.L. gibbous “hunchbacked,” from L. gibbus “hump, hunch;” cf. Mod.Pers. kaž “crooked, bent, being aside;” Skt. kubja- “hump-backed, crooked;” Pali kujja- “bent;” Lith. kupra “hump.”

Etymology (PE): Kuž “humped,” Mid.Pers. kôf “hill, mountain; hump” (Mod.Pers. kuh, “mountain”), kôfik “humpbacked,” O.Pers. kaufa-, Av. kaofa- “mountain;” mâh, → moon.

The probability distribution of the various possible states of a certain → quasi-closed subsystem.

See also: → Gibbs free energy; → canonical; → distribution.

The total energy needed to create a thermodynamic system minus the energy provided the environment. It is defined by G = U + PV -TS, where U is the → internal energy, T the → absolute temperature, S the → entropy, P the → pressure, and V is the final → volume. Same as the → Gibbs function and → thermodynamic potential.

See also: Named after Josiah Willard Gibbs (1839-1903), an American physicist who played an important part in the foundation of analytical thermodynamics; → free; → energy.

Same as → Gibbs free energy.

See also: Named after Josiah Willard Gibbs (1839-1903), an American physicist who played an important part in the foundation of analytical thermodynamics; → function.

A prefix that is used to represent 10⁹ in the SI system.

See also: From Gk. gigas, → giant.

A unit of → frequency, equal to 10⁶ Hz.

See also: → giga-; → hertz.

1. A support component of a gyroscope, which allows the axis to move freely.
   
2. A mechanical mounting frame having two mutually perpendicular axes of rotation.

Etymology (EN): Gimbal, alteration of gemel “twin,” from M.E., gemelles, from O.Fr. gemeles (Fr. jumeau, jumelle), from L. gemellus, diminutive of geminus “twin;” cf. Pers. Kermâni dialect jomoli “twin;” → Gemini.

Etymology (PE): Doqâb, from do “two” (Mid.Pers. do; Av. dva-; cf.
Skt. dvi-; Gk. duo; L. duo; O.E. twa; Ger. zwei) + qâb “frame,” from Turkish.

The prototype of → T dwarfs discovered by Nakajima et al. (1995, Nature 378, 463). This → brown dwarf lies 21.8 → light-years away and orbits
a primary star of type M1 V every about 40 years. It has a temperature of less than 1,200 K, and a mass approximately 20-50 times that of Jupiter. Its luminosity is about 2 x 10 ^-6 that of the Sun.

See also: Gl, referring to the → Gliese catalogue.

An extended mass of ice formed from snow falling and accumulating over the years and moving very slowly, either descending from high mountains, as in valley glaciers, or moving outward from centers of accumulation, as in continental glaciers (Dictionary.com).

Etymology (EN): From Fr. glacier, from O.Fr. glace “ice,” from V.L. glacia, from L. glacies “ice,” probably from PIE root *gel-, → cold.

Etymology (PE): Yaxzâr, from yax, → ice, + -zâr suffix denoting profusion and abundance, as in šurezâr “infertile, salty ground; nitrous earth,” xoškzâr “arid land,” kârzâr “a field of battle; combat,” marqzâr “a place abounding with the grass,” and so forth.

The breaking off of chunks of ice at the terminus, or end, of a glacier. Ice breaks because the forward motion of a glacier makes the terminus unstable. Ice or glacier calving is the formal name for the birth of an → iceberg.

See also: → glacier; → calve.

1. A very harsh, bright, dazzling light.
   

2. A type of → light pollution which is
   a blinding light within the field of vision. It compromises security and safety.

Etymology (EN): M.E. glaren; cognate with M.Du., M.L.G. glaren; akin to glass.

Etymology (PE): Xirtâv, literally “dazzling light,” from xir, from xiré konandé, “dazzling,” from xiré “much, many; obstinate; perverse; unwilling;” + tâv, variant tâb, tâbidan “to shine,” → luminous.

A noncrystalline, inorganic mixture of various metallic oxides fused by heating with glassifiers such as silica, or boric or phosphoric oxides.

Etymology (EN): From O.E. glæs, from W.Gmc. *glasam (M.Du. glas, Ger. Glas), from PIE base *gel-/*ghel- “to shine, glitter.”

Etymology (PE): Šišé “glass;” Mid.Pers. šišag.

A mass of glass ready to be shaped into a telescope mirror. Same as → glass disk.

Etymology (EN): → glass; blank, from O.Fr. blanc “white, shining,” from Frank. *blank “white, gleaming,” of W.Gmc. origin (cf. O.E. blanca “white horse”), from P.Gmc. *blangkaz, from PIE *bhleg- “to shine.”

Etymology (PE): Gerdé, → disk; šišé→ glass.

Same as glass blank.

See also: → glass; → disk.

A filter used in → broad-band photometry. The → bandwidth ranges usually between 30 and 100 nm.

See also: → glass; → filter.

A coating of ice, generally clear and smooth, formed on exposed objects by the freezing of a film of supercooled water deposited by rain, drizzle, fog, or possibly condensed from supercooled water vapor. Also called glaze ice, verglas, and (especially British) glazed frost.

Etymology (EN): Glaze, from → glass.

Etymology (PE): Hasar “ice,” probably related to Av. isu- “icy, chilly,” aexa- “ice, frost,” Mod.Pers. yax “ice;” cf.
O.E. is “ice,” from P.Gmc. *isa-; Du. ijs, Ger. Eis, E. ice.

A compilation of all known stars within the solar neighborhood with accurately known distances. The first version, Catalogue of Nearby Stars, published in 1957, contained nearly 1000 stars located within 20 pc of Earth, listing their known properties. Gliese published an updated version
in 1969, extending the range out to 22 pc. He published the second edition of the catalog in 1979 in collaboration with Hartmut Jahreiss. The combined catalog is now commonly referred to as the Gliese-Jahreiss catalog.

See also: Wilhelm Gliese (1915-1993), a German astronomer who worked at the Heidelberg observatory; → catalog.

A defect or malfunction in a machine or plan.
A brief or sudden interruption or surge in voltage in an electric circuit.
A sudden change in the rotation period of a pulsar.

Etymology (EN): Glitch, from Yiddish glitsh “slippery area;” cf. glitshn, Ger. glitschen “to slip, slide.”

Etymology (PE): Geles, from Lori gelese “to fall down, to slide.”

Pertaining to the whole → world; worldwide; → universal.

Etymology (EN): → globe; → -al.

Etymology (PE): → universal.

A coordinate positioning tool, using a combination of satellites
that can rapidly and accurately determine the → latitude, → longitude, and the → altitude of a point on or above the Earth’s surface. The GPS is based on a constellation of 24 Earth-orbiting satellites at an altitude of about 26,000 km. The system is a direct application of the thories of → special relativity and → general relativity.

See also: → global; → positioning; → system.

An increase in the average → temperature of the Earth’s → atmosphere that brings about climatic changes.

See also: → global; → warming.

A spherical body; sphere.
The planet Earth (usually preceded by the). A sphere on which is depicted a map of the Earth (terrestrial globe) or of the heavens (celestial globe).

Etymology (EN): M.E. globe, from M.Fr. globe, from L. globus “round body, ball, sphere,” cognate with Pers. guy, see below.

Etymology (PE): Guy “ball, sphere,” variants golulé, gullé, goruk, gulu, gudé; cf. Skt. guda- “ball, mouthful, lump, tumour,” Pali gula- “ball,”
Gk. gloutos “rump,” L. glomus “ball,” globus “globe,” Ger. Kugel, E. clot; PIE *gel- “to make into a ball.”

A spherical aggregate of stars made up of thousands to a few million stars which is an
orbiting satellite of a galaxy. There are over 150 globular clusters orbiting our galaxy. Globular clusters are gravitationally → bound systems, highly concentrated to the center (up to a few 10³ stars per cubic → light-years), with a volume ranging from a few dozen up to more than 300 light-years in diameter.
They are generally old and → metal-poor and are among the first
objects to be formed in a galaxy. There is also strong evidence that they form in major galaxy interactions and → mergers. The stars in a globular cluster are thought to have a common origin and thus a single age and → chemical abundance; with some exceptions such as → Omega Centauri and NGC 2808, which exhibit multiple populations. The presence of various sub-populations within a globular cluster is interpreted as indicating distinct epochs of mass → accretion and/or major → star formation. The Milky Way hosts about 200 globular clusters. They are spherically distributed about the → Galactic Center up to a radius of 350 light-years, with a maximum concentration toward the Galactic center. All but the smallest → dwarf galaxies possess globular clusters. Some galaxies, e.g. M87, contain several thousands of them. There are, however, important differences. While all the globular clusters in our Galaxy and in → M31 are old (ages of about 10 billion years, at least), there are galaxies, such as the two → Magellanic Clouds and → M33, that host much younger globular clusters (ages of a few billion years, or less).

Etymology (EN): Globular, from → globule + -ar, variant of → -al; → cluster.

Etymology (PE): Xušé, → cluster; guysân “shaped like a globe,” from guy, → globe + -sân “manner, semblance” (variant sun, Mid.Pers. sân “manner, kind,” Sogdian šôné “career”).

Generally, a small spherical mass, especially a small drop of liquid.
A dense spherical cloud of dust that absorbs radiation; → Bok globule.

Etymology (EN): From → globe + → -ule.

Etymology (PE): Guycé, fro guy, → globe, + -cé diminutive suffix, from Mid.Pers. -cak, variants -êžak (as in kanicak “little girl,” sangcak “small stone,” xôkcak “small pig”), also Mod.Pers. -ak.

A colored aureole that is visible around the shadow of an observer’s head, appearing on top of a cloud situated below the observer. A glory is caused by the same optics as a rainbow plus diffraction. → heiligenschein.

Etymology (EN): From O.Fr. glorie, from L. gloria “great praise or honor,” of uncertain origin.

Etymology (PE): Šokuh, from Mid.Pers. škôh “magnificience, majesty, dignity; fear.”

The opening at the upper part of the → larynx, between the → vocal cords.

Etymology (EN): From Gk. glottis “mouth of the windpipe,” from glotta, Attic dialect variant of glossa “tongue.”

Etymology (PE): Câknây, literally “trachea’s slit,” from câk “slit, fissure,” → rift, + nây, → trachea.

A covering for the hand made with a separate sheath for each finger and for the thumb (Dictionary.com). → mitten, → mitt.

Etymology (EN): M.E.; O.E. glof; cognate with O.Norse glofi.

Etymology (PE): Dastkeš, from dast, → hand, + keš, from kešidan / kašidan “to draw, protract, to support,” → galaxy.

1a) A light emitted by or as if by a substance heated to luminosity; incandescence.

1b) Brightness of color.

2a) To emit bright light and heat without flame; become incandescent.

2b) To shine like something intensely heated.

2c) To exhibit a strong, bright color; be lustrously red or brilliant (Dictionary.com).
→ afterglow, → airglow, → counterglow, → nightglow, → skyglow.

Etymology (EN): M.E. glowen, from O.E. glowan “to shine as if red-hot,” ultimately from PIE *ghlo-.

Etymology (PE): Foruz-, foruzidan, afruxtan
“to light, kindle;” related to foruq “light, brightness” (Mid.Pers. payrog “light, brightness”); rôšan “light; bright, luminous;” ruz “day;” Mid.Pers. rošn light; bright," rôc “day;” O.Pers. raucah-; Av. raocana- “bright, shining, radiant,” raocah- “light, luminous; daylight;”
cf. Skt. rocaná- “bright, shining, roka- “brightness, light;” Gk. leukos “white, clear;” L. lux “light,” also lumen “light, window,” luna “Moon;” E. light; Ger. Licht; Fr. lumière; PIE base *leuk- “light, brightness.”

The hypothetical particle, in the → quantum chromodynamics theory, that carries the force between → quarks. There are eight independent types of gluon.

See also: From glue (O.Fr. glu, from L.L. glus “glue,” from L. gluten “glue”) + → -on.

The organic compound with the formula HOCH2-CHO. It is the simplest → sugar and the first intermediate product in the formose reaction that begins with formaldehyde (H2CO) and leads to the (catalyzed) formation of sugars and ultimately ribose, the backbone of RNA, under early Earth conditions. The presence of glycolaldehyde is therefore an important indication that the processes leading to biologically relevant molecules are taking place. However, the mechanism responsible for its formation in space is still unclear. Glycolaldehyde has been detected toward the → Galactic Center cloud Sgr B2,
in the high-mass → hot molecular core G31.41+0.31, and more recently in the gas surrounding a young binary star with similar mass to the Sun (IRAS 16293-2422). See Jorgensen et al. 2012, astro-ph/1208.5498, and references therein.

See also: From glycol, from glyc(erin) + (alcoh)ol + → aldehyde.

1. A rod oriented in such a way that its shadow, cast by the Sun’s rays, shows the hours on a → sundial; a style.
   
2. A device used in ancient times consisting of a vertical shaft used to measure the altitude of the Sun and hence to determine the time of day.

Etymology (EN): From L. gnomon, from Gk. gnomon “carpenter’s square, rule; indicator,” literally “one who discerns,” from gignoskein “to know, think, judge,” cognate with L. gnoscere, noscere “to come to know” (Fr. connaître; Sp. conocer);
O.Pers./Av. xšnā- “to know, learn, come to know, recognize;” Mid.Pers. šnâxtan, šnâs- “to know, recognize,” dânistan “to know;” Mod.Pers. šenâxtan, šenâs- “to recognize, to know,” dânestan “to know;” Skt. jñā- “to recognize, know,” jānāti “he knows;” P.Gmc. *knoeanan; O.E. cnawan, E. know; Rus. znat “to know;” PIE base *gno- “to know.”

Etymology (PE): Bâhu “stick, staff; arm (from the elbow to the shoulder),” related to bâzu “arm,” Mid.Pers. bâzûk “arm;” Av. bāzu- “arm;” cf. Skt. bāhu- “arm, forearm,” also “the shadow of the gnomon on a sundial; the bar of a chariot pole;” Gk. pechys “forearm, arm, ell;” O.H.G. buog “shoulder;” Ger. Bug “shoulder;” Du. boeg; O.E. bôg, bôh “shoulder, bough;” E. bough " a branch of a tree;" PIE *bhaghu- “arm.”

The projection of a spherical surface onto a plane through a point. A gnomonic → map projection displays all great circles as straight lines, and therefore indicates the shortest path between two points. Small circles are projected as conic sections.

See also: → gnomon; → -ic; → projection.

A domesticated ruminant mammal (Capra hircus) having backward curving horns and a beard especially in the male, raised for its wool, milk, and meat (TheFreeDictionary.com).

Etymology (EN): M.E. got, O.E. gat “she-goat;” cf. O.Saxon get, O.Norse geit, Dan. gjed, Du. geit, Ger. Geiss, Goth. gaits “goat,” from PIE *ghaid-o- “young goat.”

Etymology (PE): Boz “goat;” Mid.Pers. buz; Av. buza-; cf. Skt. bukka-; O.Ir. bocc; O.H.G. boc; Bret. bouc’h).

1. The Being perfect in power, wisdom, and goodness who is worshipped as creator and ruler of the Universe.
   

2. (lowercase) A being or object believed to have more than natural attributes and powers and to require human worship (Merriam-Webster.com). See also: → fingers of God

Etymology (EN): M.E. from O.E. akin to O.H.G. got, Ger. Gott, O.N. guð, Goth. guþ, from PIE *g^heuH- “to call upon;” cf.
Av. zu- “to call, invoke;” O.Pers. (upa)zu- “to proclaim;”
Skt. hu-, variant hve- “to call upon, invoke,” huta- “invoked,” an epithet of Indra, from root *gheu(e)- “to call, invoke.”

Etymology (PE): Xodâ, xodây “god, lord, master;” Mid.Pers. xwadây “king, master;” Av. xvadāta- “autonomous” (darego.xvadāta- “highly autonomous”),
from xva-, → self- + dā- “to give, grant, yield” (Pers. dâdan, → datum); cf. Skt. svadhā- “inherent power, habitual power, self-placed,” from sva- “self,”

• dhā- “to place, fix, maintain”

In numerical analysis and fluid dynamics, a conservative scheme for solving → partial differential equations based on utilizing the solution of the local → Riemann problem at each time step.

See also: Suggested by Sergei K. Godunov (1929-) in 1959, Math. Sbornik, 47, 271, translated 1969, US Joint Publ. Res. Service, JPRS 7226; → method.

A yellow, → ductile  → metal which occurs naturally in veins and alluvial deposits associated with → quartz or → pyrite; symbol Au (L. aurum “shining dawn”). → Atomic number 79; → atomic weight 196.9665; → melting point 1,064.43 °C; → boiling point 2,808 °C; → specific gravity 19.32 at 20 °C.

Like other → chemical elements the gold found on Earth has an → interstellar origin. However, the new-born Earth was too hot and most of the molten gold, mixed with → iron, sank to its center to make the core during the first tens of millions of years. The removal of gold to the → Earth’s core should have left the Earth’s crust
depleted of gold. Nevertheless, the precious metal is tens to thousands of times more abundant in the → Earth’s mantle than predicted. One explanation for this over-abundance is the → Late Heavy Bombardment. Several hundred million years after the core formation a flux of → meteorites enriched the → Earth’s crust with gold (Willbold et al., 2011, Nature 477, 195).

Etymology (EN): M.E., from O.E. gold, from P.Gmc. *gulth- (cf. O.H.G. gold, Ger. Gold, Du. goud, Dan. guld, Goth. gulþ), from PIE base *ghel-/*ghol- “yellow, green;” cf. Mod.Pers. zarr “gold,” see below.

Etymology (PE): Talâ “gold,” variants tala, tali.
Zarr “gold;” Mid.Pers. zarr; Av. zaranya-, zarənu- “gold;” O.Pers. daraniya- “gold;” cf. Skt. hiranya- “gold;” also Av. zaray-, zairi- “yellow, green;” Mod.Pers. zard “yellow;” Skt. hari- “yellow, green;” Gk. khloe literally “young green shoot;” L. helvus “yellowish, bay;” Rus. zeltyj “yellow;” P.Gmc. *gelwaz; Du. geel; Ger. gelb; E. yellow.

Every number greater than 2 is the sum of two → prime numbers. Goldbach’s number remains one of the most famous unsolved mathematical problems of today.

See also: Named after the German mathematician Christian Goldbach (1690-1764); → conjecture.

1. The number giving the position of any year in the lunar or → Metonic cycle of about 19 years. Each year has a golden number between 1 and 19. It is found by adding 1 to the given year and dividing by 19; the remainder in the division is the golden number. If there is no remainder the golden number is 19 (e.g., the golden number of 2007 is 13).
   

2. Same as → golden ratio.

See also: Golden, adj. of → gold; → number.

If a line segment is divided into a larger subsegment (a) and a smaller subsegment (b), when the larger subsegment is related to the smaller exactly as the whole segment is related to the larger segment, i.e. a/b = (a + b)/a. The golden ratio, a/b is usually represented by the Greek letter φ. It is also known as the divine ratio, the golden mean, the → golden number, and the golden section. It was believed by Greek mathematicians that a rectangle whose sides were in this proportion was the most pleasing to the eye. Similarly, the ratio of the radius to the side of a regular → decagon has this proportion. The numerical value of the golden ratio, given by the positive solution of the equation φ² - φ - 1 = 0, is φ = (1/2)(1 + √5),
approximately 1.618033989. The golden ratio is an → irrational number. It is closely related to the → Fibonacci sequence.

See also: → golden; → ratio.

A → geochemical classification scheme in which → chemical elements on the → periodic table are divided into groups based on their → affinity to form various types of compounds: → lithophile, → chalcophile, → siderophile, and → atmophile. The classification takes into account the positions of the elements in the periodic table, the types of electronic structures of atoms and ions, the specifics of the appearance of an affinity for a particular → anion, and the position of a particular element on the → atomic volume curve.

See also: Developed by Victor Goldschmidt (1888-1947); → classification.

An extremely faint and broad ring (in fact two rings) of tiny particles around → Jupiter lying just outside the main ring.

Etymology (EN): Gossamer “a film of cobwebs floating in air in calm clear weather; an extremely delicate variety of gauze, used esp. for veils,” from M.E. gossomer, from gos “goose” + somer “summer.” Possibly first used as name for late, mild autumn, a time when goose was a favorite dish, then transferred to the cobwebs frequent at that time of year; → ring.

Etymology (PE): Halqé, → ring; tanté “cobweb, spider’s web,” from tanidan “to spin, twist, weave” (Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to spin, stretch;" tanoti “stretches,” tantram “loom;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
PIE base *ten- “to stretch”), Pers. târ “string,” tur “fishing net, net, snare,” and tâl “thread” (Borujerdi dialect) belong to this family; variants tanta “cobweb,” tanadu, tafen, kartané, kârtané, kâtené,
Pashtu tanistah “cobweb;” cf. Skt. tantu- “cobweb, thread, string.”

1. Talk about other people’s private or personal matters often including remarks that are unkind or untrue.
   

   2. Engage in gossip.

Etymology (EN): From M.E. gossib, godsib “a close friend or relation, a confidant,” from
O.E. godsibb, “godfather, godmother,” literally “a person related to one in God,” from god “→ God” + sibb “a relative,” → sibling. In M.E. the sense was “a close friend with whom one gossips,” hence “a person who gossips,” later “idle talk.”

Etymology (PE): Gotré, from Shirâzi gotré “idle talk, nonesence,” cf. (Qatrân, Damâvand) gotâré “loquacious,” related to goftan “to say, tell,” → promise.