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

An experiment performed in 1971 using four atomic → cesium clocks transported in jet airplanes eastward and westward around the Earth to verify the → time dilation predicted by the theory of → special relativity.

J.C. Hafele and R. E. Keating, 1972, Science 177, 166; → experiment.

A transition metal found in zirconium ores. This silvery, ductile metal is used in control rods for nuclear reactors and in tungsten filaments and electrodes. Symbol: Hf; Atomic mass: 178.49; Atomic number: 72; melting point 2230°C; boiling point 4602°C.

Hafnium, from N.L. Hafn(ia) "Copenhagen" + -ium. Hafnium was first observed by the French chemist Georges Urbain in 1911 in rare earth samples. Subsequently, the Danish physicist Nils Bohr predicted hafnium's properties using his theory of electronic configuration of the elements.

A dimensionless number characterizing the importance of → viscous force in a → forced flow.

named after the German hydraulic engineer Gotthilf H. L. Hagen (1797-1884); → number.

The interference fringes seen with thick plates near normal incidence.

W. K. von Haidinger (1798-1871), Austrian mineralogist and geologist; → fringe.

A showery precipitation in the form of nearly spherical or irregular → pellets of ice having a diameter of up to 50 mm or more. Hail is associated with → thunderstorm cells that have strong currents of rising air and relatively great → humidity content. Hail can only form in cumulonimbus clouds. Water droplets, after the formation, freeze and begin to fall downward through the cloud, but the wind blows them back upward. As the droplets begin to fall back down again, they collect more water which also freezes, so the drop becomes bigger. Then the wind blows them back up again. This occurs several times, but eventually the frozen droplets become too big and heavy and fall as hail. See also → sleet.

From M.E. haghel, hayl; O.E. hægl, hagol; cf. O.H.G. hagal, Ger. Hagel "hail;" probably from PIE *haghlo- "pebble"; cf. Gk. kakhlex "round pebble;" Pers. Lori hogela "(big) stone."

Tagarg, from *takaraka, *tancaraka- "dense, condensed," from Proto-Ir. base *tanc- "become narrow, dense, constrict," cf. Pers. tanjidan, "to squeeze, → compress," tang "narrow, constricted;" Shahmirzadi tāž/tižd; Sariqoli tož/tižd "to pull, drag;" Pashto tat "close, thick;" Skt. tanákti "it coagulates," takrá- "buttermilk;" M.Irish techt "coagulated;" Lith. tánkus "thick;" PIE *tenk- "to twist together, become thick" (H. W. Bailey, 1979).

1) Any of the numerous fine filaments growing from the skin of humans or animals.
2) A mass or aggregate of hairs covering the human head. The mean diameter of human hair is about 100 μm (from 17 to 181 μm)
3) An outgrowth of the epidermis of a plant.
→ crosshairs; → no hair theorem; → Berenice's Hair.

M.E. heer; O.E. hær; cf. O.H.G. har, Du. haar, Ger. Haar "hair;" PIE base *kaisaro- "hair," from *ker(s)- "to bristle;" cf. Skt. kesara- "hair, mane (of a horse or lion)."

Mu(y) "hair;" Mid.Pers. môy "hair."
Gis, gisu, from Mid.Pers. ges, gesuk "hair, lock, tress," Av. gaêsa- "curly hair," gaêsav-, gaêθav- "curly, curly-haired," cf. Gk. kaite "bushy hair, mane," O.Ir. gaiset, PIE *ghaites "curly hair."

1) In a cathode-ray tube, the glow surrounding a bright spot that appears on the fluorescent screen as the result of the screen's light being reflected by the front and rear surfaces of the tube's face.
2) The effect in which a halo appears around the image of a bright object recorded on a photographic film or plate. It is produced by the fan-like pattern of light reflected through the emulsion by the medium's backing material.

Halation, from hal(o), → halo + -ation a combination of -ate and -ion, used to form nouns from stems in -ate.

Hâlegiri, from hâlé, → halo + giri, verbal noun of gereftan "to take, seize" (Mid.Pers. griftan, Av./O.Pers. grab- "to take, seize," cf. Skt. grah-, grabh- "to seize, take," graha "seizing, holding, perceiving," M.L.G. grabben "to grab," from P.Gmc. *grab, E. grab "to take or grasp suddenly;" PIE base *ghrebh- "to seize").

The leader and → follower spots have opposite polarities on either side of the equator. This reverses after the ~11 year → solar cycle. Also called Hale-Nicholson's law.

Named after George Ellery Hale (1868-1938), American astronomer; → law.

One of two equal or approximately equal parts of a divisible whole.

M.E., from O.E. h(e)alf "side, part," from P.Gmc. *khalbas "something divided" (cf. M.Du. half, Ger. halb, Goth. halbs "half").

Nim, nimé "half," from Mid.Pers. nêm, nêmag "half;" Av. naēma- "half;" cf. Skt.. néma- "half."

The moon when, at either quadrature, half its disk is illuminated.

→ half; → moon.

The length of time required for half of a given quantity of → radioactive material to → decay.

→ half; → life.

The angle between extreme points of the main lobe of an antenna pattern where the sensitivity of the antenna is half the value at the center of the lobe. This is the nominal resolving power of the antenna system.

→ half; → power; → beam; → width.

The thickness of material required to reduce the intensity of an → X-ray beam to one half of its initial value. The HVL is an indirect measure of the photon energies of a beam.

→ half; → value; → layer; → attenuation.

A plate of optical material whose thickness is such that the phase difference between the extraordinary and ordinary rays after passing through the place is exactly one-half cycle. It can serve to rotate the plane of polarization of a light beam.

→ half; → wave; → plate.

A retrograde irregular satellite of Neptune discovered in 2002. Also called Neptune IX. Halimede is about 62 kilometres in diameter.

In Gk. mythology, one of the Nereids, the fifty daughters of Nereus and Doris.

The most famous comet orbiting the Sun once about every 75 years. The last time it appeared was in 1986, and it is predicted to return in 2061. Its earliest recorded sighting is traced back to 240 BC in China. In 1705 Edmond Halley used Newton's new theory of gravitation to determine the orbits of comets from their recorded positions in the sky as a function of time. He found that the bright comets of 1531, 1607, and 1682 had almost the same orbits. He concluded that these appearances must belong to a single recurring comet, and predicted its return for 1758. Halley's comet is the first known → periodic comet, hence its → designation 1P/Halley.

Named after the English astronomer Edmond Halley (1656-1742), who first computed its orbit and predicted its return in 1758; → comet.

1) Meteo.: Rings or arcs that seem to encircle the sun or moon and are the result of the refraction of light through the ice crystals that make up cirrus clouds.
2) → halo of galaxy.

Halo, from L. (acc.) halo, from Gk. halos "ring of light around the sun or moon."

Hâlé, loanword from Ar.

The → probability distribution of the → number of galaxies that a host → dark matter halo of a given mass contains. HOD is a powerful theoretical frame to populate dark matter halos with luminous galaxies. More specifically, it describes the bias between galaxies and dark matter by specifying (a) the probability P(N|M) that a halo of → virial mass M contains N galaxies of a particular class and (b) the relative spatial and velocity distributions of galaxies and dark matter within halos.

→ halo; → occupation; → distribution.

The diffuse, nearly spherical cloud of stars and → globular cluster s that surrounds a → spiral galaxy.

→ halo; → galaxy.

The → halo of galaxy belonging to our → Milky Way.

→ halo; → galaxy.