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

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.

The process of combining with → water.

Verbal noun of → hydrate.

Fluid mechanics: A → partial differential equation which describes the motion of an element of fluid subjected to different forces such as pressure, gravity, and frictions.

→ hydrodynamic; → equation.

A → nuclear reaction where hydrogen (H) nuclei combine to form helium (⁴He) nuclei. Same as the → proton-proton chain.

→ hydrogen; → fusion.

Chemistry: The → positively charged hydrogen atom, H⁺, formed by removal of the orbital electron. Same as → proton.

→ hydrogen; → ion.

The process of combining or exposing to → hydrogen.

The equation describing the → hydrostatic equilibrium in a star, expressed as: dP/dr = -GMρ/r², where P and M are the mass and pressure of a spherical shell with thickness dr at some distance r around the center of the star, ρ is the density of the gas, and G the → gravitational constant.

→ hydrostatic; → equation.

Any of the six functions sinh, cosh, tanh, coth, csch, and sech that are related to the → hyperbola in the same way the → trigonometric functions relate to the → circle. Many of the formulae satisfied by the hyperbolic functions are similar to corresponding formulae for the trigonometric functions, except for + and - signs. For example: cosh²x - sinh²x = 1. See also: → hyperbolic cosine, → hyperbolic sine. Hyperbolic functions were first introduced by the Swiss mathematician Johann Heinrich Lambert (1728-1777).

→ hyperbolic; → function.

An → atomic transition involving a → hyperfine structure.

→ hyperfine; → transition.

The sixteenth of → Saturn's known → natural satellites. It is shaped like a potato with dimensions of 410 x 260 x 220 km and has a bizarre porous, sponge-like appearance. Many of the sponge holes or craters have bright walls, which suggests an abundance of → water  → ice. The crater floors are mostly the areas of the lowest → albedo and greatest red coloration. This may be because the average temperature of roughly -180 °C might be close enough to a temperature that would cause → volatiles to → sublimate, leaving the darker materials accumulated on the crater floors. Hyperion is one of the largest bodies in the → Solar System known to be so irregular. Its density is so low that it might house a vast system of caverns inside. Hyperion rotates chaotically and revolves around Saturn at a mean distance of 1,481,100 km. It was discovered by two astronomers independently in 1848, the American William C. Bond (1789-1859) and the British William Lassell (1799-1880).

Hyperion, in Gk. mythology was the Titan god of light, one of the sons of Ouranos (Heaven) and Gaia (Earth), and the father of the lights of heaven, Eos the Dawn, Helios the Sun, and Selene the Moon.

Meteo.: The process by which a layer of ice builds up on solid objects that are exposed to freezing precipitation or to supercooled fog or cloud droplets.

→ ice; → accretion.

1) An act or instance of identifying; the state of being identified.
2) Psychology: The process of associating the self closely with other persons and assuming their characteristics or views unconsciously or partially so.
3) In psychological theories, a phase of recognition, the way in which the child's personality is formed by modeling it on a chosen parent.

Verbal noun of → identify.

Idâneš, verbal noun of idânidan→ identify.

Recognizing the lines in the spectrum of a star, nebula, galaxy, etc. using a calibration template.

→ identification; → line.

Math.: Any function f for which f(x) = x for all x in the domain of definition.

→ identity; → function.

1) The act or fact of igniting; state of being ignited.
2) The process of setting something on fire.

Verbal noun of → ignite.

The minimum temperature to which a fuel must be heated in order to initiate self sustained combustion independent of another heat source.

→ ignition; → temperature.

General: An act or instance of illuminating. The fact or condition of being illuminated.

Illumination, from O.Fr. illumination, from L. illuminationem (nom. illuminatio), from illuminare "to throw into light," from → in- "in" + lumen "light," cognate with Pers. rowšan, as below.

Rowšanâyi, noun of quality and state from rowšan "bright, clear," related to ruz "day," foruq "light," and afruxtan "to light, kindle," rowzané "window, aperture;" Mid.Pers. rôšn "light; bright, luminous," rôc "day," rocânak "window;" O.Pers. raucah-, Av. raocana- "bright, shining, radiant," raocah- "light, luminous; daylight;" akin to Skt. rocaná- "bright, shining," roka- "brightness, light;" Gk. leukos "white, clear;" L. lux "light," also lumen "light, window," luna "Moon;" E. light; Ger. Licht; Fr. lumière; PIE base *leuk- "light, brightness."

1) A false sense impression of something actually present.
2) Something that produces a false or misleading impression of reality. → optical illusion.
3) An instance of being deceived.

From O.Fr. illusion "a mocking," from L. illusionem (nom. illusio) "a mocking, irony," from illudere "to mock at," literally "to play with," from in- "at" + ludere "to play," from ludus "a game, play;" PIE base *leid- "to play, jest."

Safmân, literally "wrong thought, ~ idea," from saf "wrong" (Tâleši), → false, + mân "thought," → idea.

The meeting of light rays emanating from an object after crossing an optical system.

→ image; → formation.