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

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

→ 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.

→ 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 → isobars.

→ 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.

→ 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.

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.

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.

From L.L. gerundium, from gerundum "to be carried out," gerundive of gerere "to bear, carry."

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.

→ 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.

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," zôiždišta- "the most loathsome;" Mid./Mod.Pers. zešt "ugly, disgusting;" Goth. usgaisjan "to be afraid;" O.E. gæstan "to frighten."

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.

→ 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.

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.

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.

→ 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.

→ 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.

→ 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.

→ 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.

→ giant; → molecular; → cloud.

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

→ 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.

→ 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.

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."

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.

→ 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.

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