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

The velocity at which the envelope of a → wave packet propagates, v_gr = dω/dk, at k₀ (the central value of k). The group velocity can be equal to, larger, or smaller than the → phase velocity.

→ group; → velocity.

The act or process of uniting into groups.
A collection of things assembled into a group.
The occurence of several astronomical objects, usually of the same category, in a region of the sky.

Verbal noun of → group.

To increase by natural development, as any living organism or part by assimilation of nutriment; increase in size or substance (Dictionary.com).

From M.E. growen, O.E. growan; cf. Du. groeien, O.H.G. grouwan; PIE base *ghre- "to grow, become green," from which is also derived grass.

Ruyidan, rostan "to grow," from Mid.Pers. rôditan, rustan "to grow;" Av. raod- "to grow, sprout, shoot," with fra- "to grow up, shoot forth;" cf. Skt. ruh- "to grow, develop, ascend, climb," rohati "grows," rudh- "to grow, sprout, shoot," rodhati "grows."

The act or process, or a manner of growing; development; gradual increase. → curve of growth; → grain growth.

Ruyeš, verbal noun of → grow; rost, past stem of ruyidan, → grow, used as verbal noun.

The Crane. A constellation in the Southern Hemisphere , located at 22h 30m right ascension, -45° declination. Its brightest star, of magnitude 1.7 and spectral type B7. Abbreviation: Gru; genitive: Gruis

From L. grus "crane;" akin to Gk. geranos "crane;" Welsh garan; Lith. garnys "heron, stork;" O.E. cran; E. crane. Named by Johann Bayer in 1603.

Dornâ "crane," from Turkish, a bird of the family Gruidae.

1a) An opinion that one reaches or to which one commits oneself on the basis of probability alone or in the absence of any evidence whatever.
1b) The act of forming such an opinion.
2a) To arrive at or commit oneself to an opinion about (something) without having sufficient evidence to support the opinion fully.
2b) To estimate or conjecture about correctly (Dictionary.com).

M.E. gessen, perhaps from Scandinavian; cf. Sw., Dan., Norw. gissa, M.L.G. gissen, M.Du. gessen, related to verb get.

Hads, from Ar. Hads "intuition, conjecture, surmise."

1) To → control or direct the → motion or course of.
2) Something that directs the motion or points the way. → autoguider, → guide star, → guider, → lightguide, → waveguide.

M.E. giden (v.), from O.Fr. guider "to guide, lead," from Frankish *witan "show the way" (cf. Ger. weisen "to show, point out," wissen "to know;" O.E. witan "to see"). Cognate with Pers. bin- "to see" (present stem of didan "to see"); Mid.Pers. wyn-; O.Pers. vain- "to see;" Av. vaēn- "to see;" Skt. veda "I know;" Gk. oida "I know," idein "to see;" L. videre "to see;" PIE base *weid- "to know, to see."

Râh bordan "to guide, conduct," from râh "path, → way," + bordan "to carry, lead," → vector.

A relatively bright star conveniently located in the → field of view used for → guiding.

→ guide; → star.

Same as → guiding telescope.

A → technique used in astronomical → observations to keep the → telescope→ tracking in pace with the → rotational motion of the → Earth. Guiding consists of maintaining the → image of a star motionless during the observation. See also → guiding accuracy, → guiding telescope, → offset guiding, → autoguiding.

Verbal noun of → guide.

The accuracy (expressed in arcseconds) with which a → telescope follows the → rotational motion of the → Earth.

→ guiding; → accuracy.

In the → epicyclic theory of → galactic rotation, the center of the → epicycle.

→ guide; → center.

A telescope which is attached to a second telescope being used for photographic purposes. The guiding telescope, mounted parallel to the optical axis of the main telescope, is used by the observer to keep the image of a celestial body motionless on a photographic plate.

→ guiding; → telescope.

A nebula resembling a guitar produced by a → neutron star, which is travelling at a speed of 1600 km per sec! The neutron star leaves behind a "wake" in the → interstellar medium, which just happens to look like a guitar (only at this time, and from our point of view in space). The Guitar Nebula is about 6.5 light years away, in the constellation → Cepheus, and occupies about an arc-minute in the sky, corresponding to about 300 years of travel for the neutron star.

Guitar, ultimately from Gk. kithara "cithara," a stringed musical instrument related to the lyre, perhaps from Pers. sehtar "three-stringed," from sé "three" + târ, → string. → nebula.

A deep → inlet of the sea almost surrounded by land, with a narrow mouth; a large deep → bay (OxfordDictionaries.com).

M.E. go(u)lf, from O.Fr. golfe, from It. golfo, from Gk. Gk. kolpos "bay, gulf of the sea," originally "bosom."

Xalij, ultimately from Proto-Ir. *garika-, from *gar- "to soak, moisten" (+ relation suffix -ika-, → -ics; notably the variant Tabari -ij, as in Yušij); cf. Gilaki *xal-, xâlə "stream, brook" (as in the stream names Cam.xâlə, Zât.xâlə, Hašu.xâlə, etc.), Tabari câl in Câlus (name of a river in Mâzandarân); Laki cal.ow "marsh;" Tabari kela, kila "stream;" Baluci kor "river;" Iranian rivers Kor (in the Fârs Province), Kârun and Karxé (both in Xuzestân); in classical Pers. literature kul, kul.âb "pond, reservoir," (prefixed far-) far.qar "a minor stream derived from a more considerable one; the bed of a river when almost dry, the small quantity of water remaining in such a river" (Steingass); âqâridan, âqeštan, farqâridan "to moisten, wet, macerate;" Dari Yazd qeriz "saliva;" Râvar, Bardsir, Kerman geriz "saliva;" Laki xur "swamp;" Tâti xer "cloud;" Bandare Jâski gerâh "moisture;" Ossetic I. qaryn/qard "to permeate, seep through (of liquid);" Shughni (prefixed) ažär- "to soak, wet;" Roshani (prefixed) nižêr-/nižêrd "to soak, wet;" Skt. gal- "to drip;" O.H.G. quellan "to well, to gush;" Ger. Quell, Quelle "source." Note that, according to the classical Pers. dictionary Borhân-e Qâte' compiled in India (17th century), xalij is a Pers. word and not Arabic. Similarly, M.A. Emâm-Shushtari, in his "Dictionary of Persian Words in Arabic," remarks that the Arabic root XLJ is irrelevant to the "gulf" sense.

A trench or ravine worn away by running water in the earth.

Gully, a variant of M.E. golet "water channel," from O.Fr. goulet, dim. of goule "throat, neck," from L. gula; cf. Mod.Pers. galu "throat," geri, geribân "collar," gerivé "low hill," gardan "neck;" Mid.Pers. galôg, griv "throat," gartan "neck," Av. grīvā- "neck;" Skt. gala- "throat, neck," Gk. bora "food;" L. vorare "to devour;" PIE base *g^wer- "to swallow, devour."

Âbkand, literally "dug by water," from âb "water" (Mid.Pers. âb "water;" O. Pers. ap- "water;" Av. ap- "water;" cf. Skt. áp- "water;" Hitt. happa- "water;" PIE āp-, ab- "water, river;" cf. Gk. Apidanos, proper noun, a river in Thessalia; L. amnis "stream, river" (from *abnis); O.Ir. ab "river," O.Prus. ape "stream," Lith. upé "stream;" Latv. upe "brook") + kand, contraction of kandé, p.p. of kandan "to dig" (Mid.Pers. kandan "to dig;" O.Pers. kan- "to dig," akaniya- "it was dug;" Av. kan- "to dig," uskən- "to dig out" (→ ex- for prefix us-); cf. Skt. khan- "to dig," khanati "he digs").

An immense emission nebula about 40° across lying toward the southern constellations → Vela and → Puppis. It contains the → Vela pulsar and the → Vela supernova remnant, and seems to be created by an outburst of ionizing radiation that accompanied a → supernova explosion.

Named after its discoverer, the Australian astronomer Colin Stanley Gum (1924-1960); → nebula.

The continuum trough observed in the spectra of high redshift quasars (z> 6) at the blue wing of their Lyman-alpha emission line (1216 Å). It is explained by the scattering of the radiation of the quasar by intergalactic neutral hydrogen on the line of sight. Because of the cosmological expansion, the quasar line is redshifted with respect to the continuum trough. The Gunn-Peterson opacity increases rapidly with redshift. It is interpreted as a strong evidence for the reionization of the Universe around z = 6.

After James E. Gunn and Bruce A. Peterson who predicted the effect in 1965; → effect.

The first → gravitational wave event detected in association with an → electromagnetic counterpart. On 2017 August (12h 41m 04s UTC) the gravitational event GW170817 was observed by → Laser Interferometer Gravitational-Wave Observatory (LIGO) and the → Virgo interferometer. 1.7 seconds later the Earth-orbiting Fermi and INTEGRAL observatories detected a → gamma-ray burst (GRB 170817A). The gravitational wave data were used to attribute the event to the → merger of → neutron stars in a → neutron star binary system. The component masses range 1.17-1.60 → solar masses (Abbott et al., 2017, Physical Review Letters 119, 161101). The source was rapidly localized to a region of 31 deg² using data from all three detectors. The analysis of the gravitational wave data suggested a distance of 40 (± 8) Mpc for the event. 45 min after sunset in Chile and 10 hours after the GW trigger, astronomers (Coulter et al. 2017, GCN 21529) located the → electromagnetic counterpart of the gravitational wave event in the → lenticular galaxy (S0) → NGC 4993 offset 10.6 arcseconds north-east from center (corresponding to 2.0 kpc). Follow-up observations revealed an optical-infrared → transient known as → kilonova that lasted a few days, as predicted by models of neutron star merger. Kilonova is powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the → r-process). The merger ejected 0.03-0.05 → solar masses, including high opacity → lanthanides. Kilonovae are believed to be cradles of production of rare → chemical elements like → gold and → platinum (Pian et al., 2017, Nature doi:10.1038/nature24298).

GW, short for → gravitational wave; 170817 detection date, 2017 August 17.

To move in a circle or spiral, or around a fixed point; to revolve in or as if in a spiral course. Close concepts: → revolve; → rotate; → spin. See also: → gyro-.

From L. gyratus, p.p. of gyrare "to turn around," from L. gyrus "circle," from Gk. gyros "circle, ring;" PIE base *geu- "to bend, curve."

Leridan, from Lori, Laki lerr "revolving, whirling, turning" (lerr dâye "to make rotate, to stir a liquid," lerese "to rotate, turn"), variant xer "circular, round" (xer dâyen "to make turn"), maybe cognate with Gk. gyros "circle, ring," as above; variants in Mod.Pers. lulé "rolled-up, wound-up; tube," lulé kardan "to roll up, wind up;" Hamadâni lul "spiral, twisted;" Kurd. lûl "curly (of hair)," garda-lûl, ~ lûlân "wind that whirls dust."