An Etymological Dictionary of Astronomy and Astrophysics
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Based on chemical composition, the Earth atmosphere is divided into two broad layers: the homosphere and the → heterosphere. The homosphere extends from the surface of the Earth up to the height of about 90 km. It is characterized by an almost homogeneous composition of nitrogen (78%), oxygen (21%), argon (10%), carbon dioxide as well as traces of constituents like dust particles, → aerosols and cloud droplets.

→ homo-; → shere.

A series of → spectral lines in the → infrared spectrum of → neutral hydrogen emitted by electrons in → excited states transitioning to the level described by the → principal quantum number  n = 6. It begins at 12368 nm (Hu α 12.37 microns) and has been traced to 3281.4 nm (3.28 microns).

Named after Curtis J. Humphreys (1898-1986), American physicist; → series.

An empirical upper → luminosity boundary in the → H-R diagram. It consists of two sections, a sloping part and a horizontal part. The sloping part, which decreases with decreasing → effective temperature, corresponds roughly to the → Eddington limit. The horizontal part is the temperature-independent upper luminosity limit for late-type → hypergiants. It is thought that → massive stars above the Humphreys-Davidson limit encounter an → instability, possibly due to the opacity-modified Eddington limit, and experience high → mass loss episodes which prevent their evolution to cooler temperatures. → Luminous Blue Variable stars are examples of this high mass loss phase.

Named after Roberta M. Humphreys and Kris Davidson, who first dealt with this limit (1979, ApJ 232, 409); → limit.

The study, measurement, and description of depths and currents in open seas, lakes, estuaries, and rivers.

→ hydro- + → -graphy.

A term denoting the water portion of the Earth's surface.

→ hydro-; → sphere.

An instrument that records the hygrometer's measure of water vapor.

→ hygro- + → -graph.

The method used to detect very brief flashes of → Cherenkov radiation generated by the → cascade shower of → relativistic charged particles produced when a very high-energy → gamma ray (in the range 50 GeV to 50 TeV) strikes the atmosphere at a typical altitude of 10 km. Owing to this technique, it possible to discriminate cosmic gamma rays from the cosmic ray background and to determine their energy and source direction. More specifically, the incoming gamma-ray photon undergoes → pair production in the vicinity of the nucleus of an atmospheric molecule. The electron-positron pairs produced are of extremely high energy and immediately radiate in a → bremsstrahlung process. This radiation is itself extremely energetic, with many of the photons undergoing further pair production. A cascade of charged particles ensues which, due to its extreme energy, produces a flash of Cherenkov radiation lasting between 5 and 20 nano-seconds. The total area on the ground illuminated by this flash corresponds to many hundreds of square meters, which is why the effective area of IACT telescopes should be large.

→ imaging; → atmospheric; → Cherenkov; → technique.

The condition which exists when two waves of the same frequency pass through their maximum and minimum values in a correlated or synchronized way.

→ in; → phase.

Hamfâz, from ham- "together," → com- + fâz→ phase.

The global network of all the world's communications, databases, and sources of information.

→ information; → sphere.

The value of the phase corresponding to the origin of time. Same as the → epoch angle.

→ initial; → phase; → angle.

A dynamical model developed in France since 2003 which calculates the motion of → Solar System bodies with the highest accuracy. It integrates data obtained by NASA and ESA space missions. See, for example, A. Fienga et al. 2011, arXiv:1108.5546.

INPOP, short for Intégration Numérique Planétaire de l'Observatoire de Paris; → ephemerides.

The → photoelectric effect whereby photons absorbed by a solid (→ semiconductor) raise electrons from a lower to a higher → energy level (from → valence band to → conduction band). See also → external photoelectric effect.

→ internal; → photoelectric; → effect.

The region of the Earth's upper atmosphere containing a small percentage of free electrons and ions produced by photoionization of the constituents of the atmosphere by solar ultraviolet radiation.

→ ion + → sphere.

A mechanical device, consisting of thin overlapping plates, designed to smoothly vary the effective diameter of a lens, thereby controlling the amount of light allowed through.

→ iris; → diaphragm.

A one-to-one → mapping between two → sets, which preserves the relations existing between elements in its → domain. An isomorphism is a → bijective morphism.

→ iso-; → morphism.

A line drawn through all points on a weather map having the same amount of → cloud cover.

From Gk. → iso- + nephos "cloud," cognate with Pers. nam "humidity, moisture;" Av. napta- "moist," nabās-câ- "cloud," nabah- "sky;" Skt. nábhas- "moisture, cloud, mist;" L. nebula "mist," nimbus "rainstorm, rain cloud;" O.H.G. nebul; Ger. Nebel "fog;" O.E. nifol "dark;" from PIE *nebh- "cloud, vapor, fog, moist, sky."

Izoabr, from izo-, → iso-, + abr → cloud.

Of or relating to → isophotes.

→ isophote; → -al.

The size attributed to a galaxy corresponding to a particular level of → surface brightness. The reason is that galaxies do not have sharp edges.

→ isophotal; → radius.

A line joining points with the same surface brightness on a plot or in image of a celestial object such as a nebula or galaxy.

Isophote, from → iso- + a combining form of Gk. phos (gen. photos) "light."

Izošid, from izo-, → iso-, + šid "light, sunlight," from Mid.Pers. šêt "shining, radiant, bright;" Av. xšaēta- "shining, brilliant, splendid, excellent."

A quantum mechanical → tunnel effect allowing the flow of a continuous current across two weakly coupled → superconductors which are separated by a very thin insulating barrier.

Named after the British physicist Brian David Josephson, who predicted the existence of the effect in 1962; → effect.