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

The act of a person or thing that loads. See also: → download, → upload.

Verbal noun of → load.

The transport of the magnetic field by a fluid. It is given by the term ∇ x (v x B) in the → induction equation.

→ magnetic; → advection.

Radiation emitted by a rotating magnet.

→ magnetic; → dipole; → radiation.

Specifically defined data elements that describe how and when a particular set of data was collected, and how it is formatted. Metadata is used to organize, manipulate, and work with data when it is not necessary or desired to actually deal with the data itself. The reason is that the metadata is usually far smaller and easier to work with than the data that it represents.

→ meta-; → data.

The decrease in the → abundances of → heavy elements in a → disk galaxy as a function of distance from the center. Radial metallicity gradients are observed in many galaxies, including the → Milky Way and other galaxies of the → Local Group. In the case of the Milky Way, several objects can be used to determine the gradients: → H II regions, → B stars, → Cepheids, → open clusters, and → planetary nebulae. The main diagnostic elements are oxygen, sulphur, neon, and argon in photoionized nebulae, and iron and other elements in Cepheids, open clusters, and stars. Cepheids are probably the most accurate indicators of abundance gradients in the Milky Way. They are bright enough to be observed at large distances, so that accurate distances and spectroscopic abundances of several elements can be obtained. Average abundance gradients are generally between -0.03 → dex/kpc and -0.10 dex/kpc, with a a flattening out of the gradients at large galactocentric distances (≥ 10 kpc). The existence of these gradients offers the opportunity to test models of → chemical evolution of galaxies and stellar → nucleosynthesis.

→ metallicity; → gradient.

Thermal radiation with a temperature of 2.73 K that is apparently uniformly distributed in the Universe. It is believed to be a redshifted remnant of the hot radiation that was in thermal equilibrium with matter during the first hundred thousand years after the Big Bang. Same as → cosmic microwave background (CMB) radiation.

→ microwave; → background; → radiation.

Electromagnetic radiation carried by → microwaves.

→ microwave; → radiation.

A → mural instrument used to determine stellar positions. The quadrant consisted of a wall supporting a 90° graduated arc carefully oriented to the meridian. By means of a movable arm equipped with a sight, the altitude of a star could be determined at meridian passage.

→ mural; → quadrant.

The point on the celestial sphere blocked from view by Earth and diametrically opposite to the → zenith (سرسو).

From M.L. nadir, from Ar. nazir "opposite to," contraction of nazir as-samt (نظیر‌السمت), literally "opposite of the zenith," from nazir "opposite" + samt "zenith."

Pâsu, literally "direction of the foot," from pâ "foot, step" (from Mid.Pers. pâd, pây; Khotanese fad; Av. pad- "foot;" cf. Skt. pat; Gk. pos, genitive podos; L. pes, genitive pedis; P.Gmc. *fot; E. foot; Ger. Fuss; Fr. pied; PIE *pod-/*ped-) + su "direction, side" (from Mid.Pers. sôk "direction, side").

A federal agency of the United States government founded in 1958 for civil aeronautical research and space exploration, superseding the National Advisory Committee for Aeronautics (NACA). Its goals include improving human understanding of the universe, the solar system, and Earth and establishing a permanent human presence in space. NASA is headquarted at Washington, D.C., and operates several research, development, and test facilities, as follows alphabetically: 1) Ames Research Center; 2) Dryden Flight Research Facility at Edwards, California, used for flight testing and as a landing site for the Space Shuttle; 3) Glenn Research Center at Cleveland, Ohio, concerned with aircraft and rocket propulsion; 4) Goddard Space Flight Center; 5) Jet Propulsion Laboratory; 6) Johnson Space center; 7) Kennedy Space Center; 8) Langley Research Center at Hampton, Virginia, which carries out research in aeronautics and space technology; 9) Marshall Space Flight Center; 10) the Space Telescope Science Institute; 11) Stennis Space Center, near Bay St Louis, Mississippi, for testing rocket engines; and 12) Wallops Flight Facility on Wallops Island, Virginia, which manages NASA's sounding rocket and scientific balloon programs.

→ national; → aeronautics; → space; administration, verbal noun of administer, from M.E. amynistre, from O.Fr. aministrer, from L. administrare "to serve, carry out, manage," from → ad- "to" + ministrare "to serve," from minister "servant, priest's assistant," from minus, minor "less," hence "subordinate," + comparative suffix *-teros.

Sâzmân, → organization; fazâyi pertaining to fazâ, → space; Âmrikâ "United State of America."

The broadening of any spectral line due to the fact that excited levels have mean lives, which, by virtue of the uncertainty principle, implies a spread in the energy values.

→ natural; → line; → broadening.

A device consisting of a series of equal → pendulums in a row used to demonstrate the laws of → conservation of momentum and → conservation of energy.

→ Newton; → cradle.

Radiation having waves that are out of phase in space and/or time; radiation which is not → coherent.

→ non-; → coherent; → radiation.

A type of stellar pulsation in which waves run in different directions on and beneath the surface of a star.

→ non-; → radial; → pulsation.

An process in which an excited state returns to the ground state without emitting radiation. → radiative process.

→ non-; → radiative; → process.

The electromagnetic radiation whose characteristics do not depend on the temperature of the emitting source. In contrast to → thermal radiation, it has a different spectrum from that of → blackbody radiation. The three common types of non-thermal radiation in astronomy are: → synchrotron radiation, → bremsstrahlung radiation, and → maser  → stimulated emission.

→ non-thermal; → radiation.

A group of eight units or figures.
Chemistry: An element, atom, or group having a valence of eight.

From Gk. oktad- (stem oktás) "group of eight," from okt-→ oct- + -ad a prefix denoting a group or unit comprising a certain number, sometimes of years (e.g. dyad; triad).

The puzzle of why the night sky is not as uniformly bright as the surface of the Sun if, as used to be assumed, the Universe is infinitely large and filled uniformly with stars. It can be traced as far back as Johannes Kepler (1571-1630), was discussed by Edmond Halley (1656-1742) and Philippe Loys de Chéseaux (1718-1751), but was not popularized as a paradox until Heinrich Olbers took up the issue in the nineteenth century. This paradox has been resolved by the → Big Bang theory. In a Universe with a beginning, we can receive light only from that part of the Universe close enough so that light has had time to travel from there to here since the Big Bang. The night sky is dark because the galaxies are only about ten billion years old and have emitted only a limited amount of light, not because that light has been weakened by the expansion of the Universe (P. S. Wesson et al., 1987, ApJ 317, 601).

Formulated in 1826 by Heinrich Wilhelm Olbers (1758-1840), German physician and amateur astronomer, who discovered the asteroids Pallas and Vesta as well as five comets; → paradox.

A → Lindblad resonance expressed by: Ω_p = Ω + κ/m.

→ outer; → Lindblad resonance.

The part of observing time at a telescope which is not directly used for science, such as the time spent for detector read-out, changing instruments, focusing, etc.

→ over-; → head; → time.

Bâlâ "up, above, high, elevated, height" (variants boland "high, tall, elevated, sublime," borz "height, magnitude" (it occurs also in the name of the mountain chain Alborz), Laki dialect berg "hill, mountain;" Mid.Pers. buland "high;" O.Pers. baršan- "height;" Av. barəz- "high, mount," barezan- "height;" cf. Skt. bhrant- "high;" L. fortis "strong" (Fr. and E. force); O.E. burg, burh "castle, fortified place," from P.Gmc. *burgs "fortress;" Ger. Burg "castle," Goth. baurgs "city," E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg; PIE base *bhergh- "high") + sar, → head.