An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

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

→ general; → precession; → right ascension.

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

→ 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.

→ general; → relativity.

→ secretary-general.

→ general; → secretary.

→ geodesic precession.

→ geodesic; → precession

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.

→ geodetic; → precession

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

→ geodetic; → refraction.

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.

→ geometric; → progression.

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.

→ giant; → H II; → region.

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.

→ giant; magneto- combining form of → magnet; → resistance.

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.

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

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.

M.E. glaren; cognate with M.Du., M.L.G. glaren; akin to glass.

Xirtâv, literally "dazzling light," from xir, from xiré konandé, "dazzling," from xiré "much, many; obstinate; perverse; unwilling;" + tâv, variant tâb, tâbidan "to shine," → luminous.

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.

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

The change in the wavelength or frequency of electromagnetic radiation in a gravitational field predicted by general relativity.

→ gravitational; → redshift.

A simplifying assumption in the models of stellar atmosphere, according to which the absorption coefficient has the same value at all wavelengths.

→ gray; → atmosphere.

Unusual or considerable in degree, power, intensity, number, etc.

O.E. great "big, coarse, stout," from W.Gmc. *grautaz (cf. Du. groot, Ger. groß "great").

Bozorg "great, large, immense, grand, magnificient;" Mid.Pers. vazurg "great, big, high, lofty;" O.Pers. vazarka- "great;" Av. vazra- "club, mace" (Mod.Pers. gorz "mace"); cf. Skt. vájra- "(Indra's) thunderbolt," vaja- "strength, speed;" L. vigere "be lively, thrive," velox "fast, lively," vegere "to enliven," vigil "watchful, awake;" P.Gmc. *waken (Du. waken; O.H.G. wahhen; Ger. wachen "to be awake;" E. wake); PIE base *weg- "to be strong, be lively."

A hypothesized large concentration of mass (about 10¹⁶  → solar masses), some hundred million → light-years from Earth, in the direction of the → Centaurus → supercluster, that seems to be affecting the motions of many nearby galaxies by virtue of its gravity.

→ great; → attractor.

A circle on a sphere whose plane passes through the center of the sphere.

→ great; → circle.

One of a series of dark spots on → Neptune similar in appearance to Jupiter's → Great Red Spot. It was discovered in 1989 by NASA's Voyager 2 space probe. Also known as GDS-89. The dark, oval spot had initial dimensions of 13,000 × 6,600 km, about the same size as Earth. Although it appears similar to Jupiter's spot, which is an → anticyclonic storm, it is believed that the Great Dark Spot is an atmospheric hole similar to the hole in Earth's → ozone layer ozone layer. Moreover, unlike Jupiter's spot, which has lasted for hundreds of years, the lifetimes of Great Dark Spots appear to be much shorter, forming and disappearing once every few years or so. Based on pictures taken by Voyager and since then with the → Hubble Space Telescope, Neptune appears to spend somewhat more than half its time with a Great Dark Spot. Around the Great Dark Spot, winds were measured blowing up to 2,400 km an hour, the fastest in the solar system.

→ great; → dark; → spot.

An anticyclonic storm on the planet Jupiter akin to a hurricane on Earth, but it is enormous (three Earths would fit within its boundaries) and it has persisted for at least the 400 years that humans have observed it through telescopes.

→ great; → red; → spot.