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gravity assist yâri-ye gerâneši Fr.: gravidéviation An important astronautical technique whereby a → spacecraft takes up a tiny fraction of the → orbital energy of a planet it is flying by, allowing it to change → trajectory and → speed. Since the planet is not at rest but gravitating around the Sun, the spacecraft uses both the orbital energy and the gravitational pull of the planet. Also known as the slingshot effect or → gravitational slingshot. More specifically, as the spacecraft approaches the planet, it is accelerated by the planet's gravity. If the spacecraft's velocity is too low, or if it is heading too close to the planet, then the planet's → gravitational force will pull it down to the planet. But if its speed is large enough, and its orbit does not bring it too close to the planet, then the gravitational attraction will just bend the spacecraft's trajectory around, and the accelerated spacecraft will pass rapidly by the planet and start to move away. In the absence of other gravitational forces, the planet's gravity would start to slow down the spacecraft as it moves away. If the planet were stationary, the slow-down effect would be equal to the initial acceleration, so there would be no net gain in speed. But the planets are themselves moving through space at high speeds, and this is what gives the "slingshot" effect. Provided the spacecraft is traveling through space in the same direction as the planet, the spacecraft will emerge from the gravity assist maneuver moving faster than before. → gravity; assist, from M.Fr. assister "to stand by, help, assist," from L. assistere "assist, stand by," from → ad- "to" + sistere "to cause to stand," from PIE *siste-, from *sta- "to stand" (cognate with Pers. istâdan "to stand"). Yâri "assistance, help; friendship," from yâr "assistant, helper, friend," from Mid.Pers. hayyâr "helper," hayyârêh "help, aid, assistance," Proto-Iranian *adyāva-bara-, cf. Av. aidū- "helpful, useful." |
gravity brightening rowšaneš-e gerâneši Fr.: embrillancement gravitationnel → gravity; → brightening. |
gravity darkening târikeš-e gerâneši Fr.: assombrissement gravitationnel The darkening, or brightening, of a region on a star due to localized decrease, or increase, in the → effective gravity. Gravity darkening is explained by the → von Zeipel theorem, whereby on stellar surface the → radiative flux is proportional to the effective gravity. This means that in → rotating stars regions close to the pole are brighter (and have higher temperature) than regions close to the equator. Gravity darkening occurs also in corotating → binary systems, where the → tidal force leads to both gravity darkening and gravity brightening. The effects are often seen in binary star → light curves. See also → gravity darkening exponent. Recent theoretical work (Espinosa Lara & Rieutord, 2011, A&A 533, A43) has shown that gravity darkening is not well represented by the von Zeipel theorem. This is supported by new interferometric observations of some rapidly rotating stars indicating that the von Zeipel theorem seems to overestimate the temperature difference between the poles and equator. |
gravity darkening coefficient hamgar-e târikeš-e gerâneši Fr.: coefficient de l'assombrissement gravitationnel According to the → von Zeipel theorem, the emergent flux, F, of total radiation at any point over the surface of a rotationally or tidally distorted star in → hydrostatic equilibrium varies proportionally to the local gravity acceleration: F ∝ g_{eff}^{α}, where g_{eff} is the → effective gravity and α is the gravity darkening coefficient. See also the → gravity darkening exponent. → gravity; → darkening; → coefficient. |
gravity darkening exponent nemâ-ye târikeš-e gerâneši Fr.: exposant de l'assombrissement gravitationnel The exponent appearing in the power law that describes the → effective temperature of a → rotating star as a function of the → effective gravity, as deduced from the → von Zeipel theorem or law. Generalizing this law, the effective temperature is usually expressed as T_{eff}∝ g_{eff}^{β}, where β is the gravity darkening exponent with a value of 0.25. It has, however, been shown that the relation between the effective temperature and gravity is not exactly a power law. Moreover, the value of β = 0.25 is appropriate only in the limit of slow rotators and is smaller for fast rotating stars (Espinosa Lara & Rieutord, 2011, A&A 533, A43). |
gravity mode tarz-e gerâni, mod-e ~ Fr.: mode gravité Same as → g mode |
gravity wake kel-e gerâni Fr.: sillage de gravité Transient → streamers which form when → clumps of particles begin to collapse under their own → self-gravity but are sheared out by → differential rotation. This phenomenon is believed to be the source of → azimuthal asymmetry in → Saturn's → A ring (Ellis et al., 2007, Planetary Ring Systems, Springer). |
gravity wave mowj-e gerâni Fr.: onde de gravité 1) A wave that forms and propagates at the free → surface
of a body of → fluid
after that surface has been disturbed and the fluid particles
have been displaced from their original positions.
The motion of such waves is controlled by the restoring force of gravity rather
than by the surface tension of the fluid. |
gravo-turbulence gerâni-âšubnâki Fr.: gravo-turbulence The interplay between supersonic turbulence and self-gravity in star forming gas. Gravo-, from grav-, from → gravity + epenthetic vowel -o- + → turbulence. |
gray xâkestari (#) Fr.: gris (n.) A color between white and black. (adj.) Having a neutral hue. M.E., O.E. græg, from P.Gmc. *græwyaz; cf. O.N. grar, O.Fris. gre, Du. graw, Ger. grau; Frank. *gris, Fr. gris. Xâkestari, "ash-colored," from xâkestar "ashes," from Mid.Pers. *xâkâtur, from xâk "earth, dust" + âtur "fire," varaint âtaxš (Mod.Pers. âtaš, âzar, taš), from Av. ātar-, āθr- "fire," singular nominative ātarš-; O.Pers. ātar- "fire;" Av. āθaurvan- "fire priest;" Skt. átharvan- "fire priest;" cf. L. ater "black" ("blackened by fire"); Arm. airem "burns;" Serb. vatra "fire;" PIE base *āter- "fire." |
gray (Gy) gray Fr.: gray An SI unit of absorbed radiation dose. One gray is equivalent to an energy absorption of 1 → joule/kg. It has replaced the → rad (rd), an older standard. One gray is equivalent to 100 rad. Named for Louis Harold Gray (1905-1965), British radiologist and the pioneer of use of radiation in cancer treatment. |
gray atmosphere javv-e xâkestari, havâsepher-e ~ Fr.: atmosphère grise A simplifying assumption in the models of stellar atmosphere, according to which the absorption coefficient has the same value at all wavelengths. → gray; → atmosphere. |
gray body jesm-e xâkestari (#) Fr.: corps gris A hypothetical body which emits radiation at each wavelength in a constant ratio, less than unity, to that emitted by a black body at the same temperature. |
graze barmažidan (#) Fr.: raser, frôler, effleurer To touch or rub lightly in passing. Perhaps special use of graze "to feed on grass," from M.E. grasen, O.E. grasian. Barmažidan, from Choresmian parmž "to touch, to rub," variants barmajidan, majidan, parmâsidan, Mid.Pers. pahrmâh- "to touch, to feel;" ultimately from Proto-Ir. *pari-mars-, from *Hmars-, *Hmarz- "to touch, rub, wipe;" probably related to marz "border, frontier," mâlidan "to rub, polish." |
grazer barmžandé Fr.: rasant A thing that grazes. Agent noun of → graze. |
grazing 1) barmažandé; 2) barmaž Fr.: 1) rasant; 2) rasage, frôlement, effleurement 1) Describing something that grazes. → grazing angle,
→ grazing occultation. |
grazing incidence fotâd-e barmažandé Fr.: incidence rasante Light striking a surface at an angle almost perpendicular to the normal. → grazing-incidence telescope. |
grazing occultation forupušâneš-e barmažandé Fr.: occultation rasante A special type of occultation that occurs when the star appears to pass tangentially on the → edge of the → Moon. → grazing; → occultation. |
grazing-incidence telescope teleskop bâ fotâd-e barmažandé Fr.: télescope à incidence rasante A telescope design used for focusing → extreme ultraviolet, → X-rays, and → gamma rays by means of → grazing incidence. Such short wavelengths do not reflect in the same manner as at the large incidence angles employed in optical and radio telescopes. Instead, they are mostly absorbed. To bring X-rays to a → focus, one has to use a different approach from → Cassegrain or other typical → reflecting telescopes. In a grazing-incidence telescope, incoming light is almost → parallel to the → mirror surface and strikes the mirror → surface at a very → shallow angle. Much like skipping a stone on the water by throwing it at a low angle to the surface, X-rays may be → deflected by mirrors arranged at low incidence angles to the incoming energy. Several designs of grazing-incidence mirrors have been used in various → X-ray telescopes, including → plane mirrors or combinations of → parabolic and → hyperbolic surfaces. To increase the collecting area a number of mirror elements are often nested inside one another. For example, the → Chandra X-ray Observatory uses two sets of four nested grazing-incidence mirrors to bring X-ray photons to focus onto two → detector instruments. → Bragg's law; → X-ray astronomy. → grazing incidence; → telescope. |
great bozorg (#) Fr.: grand 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." |
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