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geodesic precession pišâyân-e kehinrahi Fr.: précession géodésique → geodesic; → precession |
geodetic precession pišâyân-e kehinrahi Fr.: précession géodésique 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) k^{2} (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. → geodetic; → precession |
geodetic refraction šekast-e zamin-sanjik Fr.: réfraction géodésique The limiting case of → astronomical refraction when the light path is entirely within the Earth's atmosphere. → geodetic; → refraction. |
geomagnetic excursion zocâr-e zamin-meqnâti Fr.: excursion géomagnétique 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. → geomagnetic; → excursion. |
geometric progression farâyâsi-e hendesi (#) Fr.: progression géométrique 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 - r^{n})/(1 - r). Also called → geometric sequence. → geometric; → progression. |
geometrical libration roxgard-e hendesi (#) Fr.: libration géométrique 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. |
geostationary orbit madâr-e zamin-istvar Fr.: orbite géostationnaire 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. Geostationary, from → geo- + → stationary; → orbit. |
giant H II region nahiye-ye H II-ye qulpeykar Fr.: région H II géante An → H II region emitting at least 10^{50} → 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. |
Gibbs canonical distribution vâbâžeš-e hanjârvâr-e Gibbs Fr.: distribution canonique de Gibbs The probability distribution of the various possible states of a certain → quasi-closed subsystem. → Gibbs free energy; → canonical; → distribution. |
Gibbs function karyâ-ye Gibbs Fr.: fonction de Gibbs Same as → Gibbs free energy. Named after Josiah Willard Gibbs (1839-1903), an American physicist who played an important part in the foundation of analytical thermodynamics; → function. |
Global Positioning System (GPS) râžmân-e nehešdâd-e jahâni Fr.: système de positionnement par satellites 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. → global; → positioning; → system. |
gnomonic projection farâšâneš-e bâhu-yi Fr.: projection gnomonique 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. → gnomon; → -ic; → projection. |
Goldschmidt classification radebandi-ye Goldschmidt Fr.: classification de Goldschmidt 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. Developed by Victor Goldschmidt (1888-1947); → classification. |
graduation padâkeš, padâk dehi, padâk giri Fr.: graduation 1) Marking the scale of an instrument, e.g. the stem of a thermometer is graduated in
degrees. Verbal noun of → graduate. |
grain coagulation mâseš-e dâné Fr.: coagulation des grains Sticking together of micron- to centimetre-sized grains occurring in the interstellar and protoplanetary environments to form larger grain agglomerates. → grain; → coagulation. |
grain evaporation boxâreš-e dâné Fr.: évaporation des grains Conversion of dust grains into smaller grains due to high environmental temperatures. → grain; → evaporation. |
grain formation diseš-e dâné Fr.: formation des grains The process by which dust grains are assembled or produced. |
granulation dâne-bandi Fr.: granulation The mottled appearance of the solar → photosphere, caused by → convective cells, resembling → granules, which rises from the interior of the Sun. Each granule has a mean size of about 1,000 km and an upward velocity of about 0.5 km/sec. Granules are separated by intergranular walls about 400 K colder. They emerge from the fragments of the preceding granules and their lifetimes are about 20 minutes. From → granule + -ation a combination of -ate and -ion, used to form nouns from stems in -ate. Dâne-bandi, from dâné, → grain, + bandi verbal noun of bastan, vastan "to bind, shut;" O.Pers./Av. band- "to bind, fetter," banda- "band, tie" (cf. Skt. bandh- "to bind, tie, fasten;" PIE *bhendh- "to bind;" Ger. binden; E. bind). |
gravitation gerâneš (#) Fr.: gravitation 1) The universal phenomenon of attraction between material bodies.
→ Newton's law of gravitation. Verbal noun of → gravitate. |
gravitational gerâneši (#) Fr.: gravitationnel Of or relating to or caused by → gravitation. Adj. of → gravitation. |
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