Fr.: principe de Huygens
Every point of a → wavefront may be considered as a center of a secondary disturbance which gives rise to spherical wavelets, and the wavefront at any later instant may be regarded as the envelope of these wavelets. This statement suffices to account for the laws of → reflection and → refraction, and the approximately straight line propagation of light through large apertures, but it fails to account for → diffraction, the deviations from exact straight line propagation of light. Huygens' principle was later extended by Fresnel and led to the formulation of → Huygens-Fresnel principle, which is of great importance in the theory of diffraction.
Fr.: principe Huygens-Fresnel
A development of → Huygens' principle stating that every point on a → wavefront acts, at a given instant, as a source of outgoing secondary spherical waves. The secondary wavelets mutually interfere and the resulting net light amplitude at any position in the outgoing wavefront is the vector sum of the amplitudes of all the individual wavelets. Using this principle, Fresnel calculated with a high accuracy the distribution of light in → diffraction patterns. The Huygens-Fresnel principle was put on a firm theoretical basis by Kirchhoff and expressed as an integral derived from the → wave equation.
A young, nearby cluster of stars (spectral types A1-K) visible to the naked eye in the constellation → Taurus about 150 light-years away. Its individual stars (more than 200) appear to spread out in space. → Aldebaran is a foreground star in that region of the sky.
In Gk. mythology, a group of nymphs and sisters of Hyas, or else his daughters, and when Hyas died while hunting, killed by a lion or a boar, they grieved his death exceedingly, and turned into the stars called Hyades.
Huâdes, from Gk., as above.
Composed of two distinct races, breeds, varieties, species, or heterogeneous elements.
Hybrid, from L. hybrida, from ibrida "a crossbred animal," of unknown origin.
xorgereft-e doragé (#)
Fr.: éclipse solaire hybride
A rare type of solar eclipse where at some places the eclipse is annular while at other places it appears total. This duality comes about when the vertex of the Moon's umbral shadow pierces Earth's surface at some points, but falls short of the planet along other portions of the eclipse path. Hybrid eclipses are also known as → annular-total eclipses. In most cases, hybrid eclipses begin as annular, transform into total, and then revert back to annular before the end of their track. In rare instances, a hybrid eclipse may begin annular and end total, or vice versa (F. Espenak, NASA).
Fr.: pulsateur hybride
A star on the upper → main sequence which simultaneously exhibits low-order pressure mode (→ p mode) and high-order gravity mode (→ g mode) characteristic of β Cephei and → slowly pulsating B stars, respectively. Examples include: γ Pegasi, HD 43317, and HD 50230.
Fr.: étoile hybride
hidr- (#), âb- (#)
1) The Water Serpent. The longest and largest constellation in the sky,
stretching almost 7 hours of right ascension, and covering over 1300 square degrees,
from → Canis Minor to → Libra.
It lies south of → Cancer, → Leo,
and → Virgo, and is best seen in the northern hemisphere during
the months of February through May. Its brightest star is → Alphard.
Abbreviation: Hya; Genitive: Hydrae.
Hydra, from L. hydrus, from Gk. hydra "water-snake;" cf. Av. udra- "otter;" Skt. udrá- "otter;" O.H.G. ottar "otter;" O.E. otor, ottor; Ger. Otter; E. otter; Lith. udras, udra "otter;" akin to → water. In Gk. mythology, this constellation represents the gigantic nine-head water-snake which haunted the swamps of Lerna. Herakles was sent to destroy her as one of his twelve labours, but for each of her heads that he decapitated, two more sprang forth. So with the help of Iolaos, Herakles applied burning brands to the severed stumps, cauterizing the wounds and preventing regeneration. In the battle he also crushed a giant crab (→ Cancer) beneath his heel which had come to assist Hydra.
Hudrâ, from Gk. hydra, as above. → Hydrus (
Hydra I cluster
Fr.: amas de l'Hydre
A relatively poor → galaxy cluster at about 50 Mpc containing a pair of bright galaxies near its centre: NGC 3309 and NGC 3311. Also known as Abell 1060 (→ Abell catalog), Hydra I is the prototype of an evolved and dynamically relaxed cluster, being dominated by early-type galaxies and having a regular core shape.
hidridan, âbidan (#)
To combine chemically with → water.
hidridé, âbidé (#)
Combined with → water molecules.
P.p./adj. of → hydrate.
hidreš, âbeš (#)
The process of combining with → water.
Verbal noun of → hydrate.
Operated, moved, or employing water or other liquids in motion.
From Gk. hydraulikos organon "water organ," from → hydro- "water" + aulos "musical instrument, hollow tube."
Fr.: bélier hydraulique
A device, which uses the energy of water flowing by gravity intermittently through a pipe to force a small portion of the water to a height greater than that of the source.
A binary compound containing hydrogen and another element, such as CH, OH, and HCl.
→ hydr- + -ide.
Hidrur, loan from Fr.
hidro- (#), hidr- (#), âb- (#)
A combining form (hydr- before a vowel) originally meaning "water," but also "liquid, gas." In chemical nomenclature, often denotes a compound of hydrogen.
Gk. hydro-, combining form of hydor "water," cognate with Skt. udá- "water;" Khotanese ūtcā "water;" Hittite uātar; L. unda "wave;" O.C.S., Rus. voda; Lith. vanduo; P.Gmc. *watar (cf. Du. water; O.H.G. wazzar; Ger. Wasser; Goth. wato; O.E. wæter; E. water); from PIE base *wed- "water; wet."
Hidro-, loanword from Gk., as above. Âb- "water," from 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."
Any of a class of compounds containing only → hydrogen and → carbon. Hydrocarbons are organic compounds found in coal, petroleum, natural gas, and plant life. They are used as fuels, solvents, and as raw materials for numerous products such as dyes, pesticides, and plastics. Petroleum is a mixture of several hydrocarbons.
asid siyânidrik (#)
Fr.: acide cyanhydrique
Same as → hydrogen cyanide.
Of or pertaining to → hydrodynamics.