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

1) A state of urgency or eagerness; haste
2) (often followed by up) To move, proceed, or act with haste (Dictionary.com).

Of uncertain origin.

Tâveš, from Tâleši tâveš "hurry, haste," tâvisté "to hurry up;" variant Kurd. (Mahâbâd) tus "hasty."

Christiaan Huygens (1629-1695), an outstanding Dutch mathematician, astronomer, physicist, and horologist. → Huygens Division, → Huygens Region, → Huygens' principle, → Huygens-Fresnel principle.

In the system of → Saturn's rings, the gap at the inner edge of the → Cassini division at a distance of 117,680 km from the center of the planet with a width of 285-400 km.

→ Huygens; → division.

The inner bright part of the → Orion Nebula, from which most of the radiation is emitted. It is about 5' across corresponding to 0.7 pc (for a distance of 440 pc). See O'Dell (2001, ARAA 39, 99).

Named after the Dutch astronomer Christiaan → Huygens (1629-1695), who sketched the appearance of the Orion Nebula. His drawing, the first such known sketch, was published in Systema Saturnium in 1659. First named such by O. Gingerich (1982, Ann. NY Acad. Sci. 395, 308); → region.

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.

→ Huygens; → principle.

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.

→ Huygens; → Fresnel diffraction; → principle.

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.

Doragé, literally "of two veins, races," from do, → two, + ragé, from rag "vein; lineage; race," → vessel.

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).

→ hybrid; → eclipse.

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.

→ hybrid; → pulsator.

A hypothetical object composed of a → quark matter in the central regions, enveloped by ordinary → hadronic matter.

→ hybrid; → star.

→ hydro-.

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.
2) The third satellite of → Pluto discovered in 2005 by astronomers using the → Hubble Space Telescope images. Also called Pluto III (P3). It has an estimated diameter of between 60 and 170 km and an → orbital period of 38.2 days. It orbits the → barycenter of the → Pluto-→ Charon system at a distance of about 65,000 km.

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 (آبمار)

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.

→ Hydra; → cluster.

To combine chemically with → water.

From → hydr-, → hydro- + -ate a verbal suffix.

Hidridan, from Gk. → hydr-, âbidan, from âb, → water, + -idan infinitive suffix.

Combined with → water molecules.

P.p./adj. of → hydrate.

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."

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.

→ hydraulic; → ram.

A binary compound containing hydrogen and another element, such as CH, OH, and HCl.

→ hydr- + -ide.

Hidrur, loan from Fr.