An Etymological Dictionary of Astronomy and Astrophysics

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.