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

A large on-line astronomical database, developed at the Centre de Données astronomiques de Strasbourg, → CDS in France. It provides a large collection of astronomical data, including cross-identifications, bibliography, and measurements for astronomical objects outside the solar system.

SIMBAD, short for Set of Identifications, Measurements, and Bibliography for Astronomical Data.

The radiative power per unit area in all wavelengths from the Sun received by the Earth at its average distance from the Sun. Its mean value is called the → solar constant. The solar irradiance changes over a year by about 6.6% due to the variation in the Earth/Sun distance. Moreover, solar activity variations cause irradiance changes of up to 1%.

→ solar; → irradiance.

All the constituents making up the Sun's emission: photons, electrons, protons, neutrinos, and atomic nuclei.

→ solar; → radiation.

The → radiation pressure of solar photons, which pushes a comet's dust outward to form a → dust tail.

→ solar; → radiation; → pressure.

Plural form of → solar radius.

→ solar; → radii.

A unit of length, representing the radius of the → Sun, used to express the size of stars in astrophysics. It is equivalent to: 695,700 km, 0.00465047 → astronomical units, 7.35355 × 10^-8 → light-years, and 2.32061 → light-seconds.

→ solar; → radius.

A digging tool with a flat blade attached to a shaft so that it can be pushed into the ground with the foot.

M.E., from O.E. spadu; cognate with Gk. spathe "blade of a sword or oar."

Bil "spade," variants Kurd. bêr, Baluci bard, Gabri bard(a); Mid.Pers. bêl, bêr; Proto-Iranian *barda- metathesis of *badar-; cf. Av. vadar- "weapon" (Gershevitch 1962).

A meteor occurring occasionally, and not associated with any known meteor shower.

Sporadic, from M.L. sporadicus "scattered," from Gk. sporadikos "scattered," from sporas (genitive sporados) "scattered," from spora "seed, a sowing;" related to sporos "sowing," and speirein "to sow," from PIE *sper- "to strew;" → meteor.

Šahâb, → meteor; gahgâhi "from time to time," from gah, gâh "time; place" (Mid.Pers. gâh, gâs "time;" O.Pers. gāθu-; Av. gātav-, gātu- "place, throne, spot;" cf. Skt. gâtu- "going, motion; free space for moving; place of abode;" PIE *gwem- "to go, come").

1a) To draw, stretch, or open out, especially over a flat surface, as something rolled or folded (often followed by out).
1b) To stretch out or unfurl in the air, as folded wings, a flag, etc.
2) An act or instance of spreading; expansion, extension, or diffusion (Dictionary.com).

M.E. spreden, from O.E. sprædan "to spread, extend," cf. Dan. sprede, O.Swed. spreda, M.Du. spreiden, O.H.G. and Ger. spreiten "to spread," from PIE root *sper- "to strew."

Gostardan "to spread; to diffuse, to expand," from Mid.Pers. wistardan "to extend; to spread;" Proto-Iranian *ui.star-; Av. vi- "apart, away from, out" (O.Pers. viy- "apart, away;" cf. Skt. vi- "apart, asunder, away, out;" L. vitare "to avoid, turn aside") + Av. star- "to spread," starati "spreads" (cf. Skt. star- "to spread out, extend, strew," strnati "spreads;" Gk. stornumi "I spread out," strotos "spread, laid out;" L. sternere "to spread;" Ger. Strahlung "radiation," from strahlen "to radiate," from Strahl "ray;" from M.H.G. strāle; from O.H.G. strāla "arrow," stripe; PIE base *ster- "to spread").

A flow in which the characterizing conditions, such as → streamlines or velocity at any given point, do not change with time.

→ steady; → flow.

Tacân, → flow; pâyâ "steady, constant," from pâyidan "to stand firm, to be constant, steady," from Mid.Pers. pattây-, pattutan "to last, endure, stay."

A → cosmological model according to which the → Universe has no beginning and no end and maintains the same mean density, in spite of its observed expansion, by the continual creation of matter throughout all space. The theory was first put forward by Sir James Jeans in about 1920 and again in revised form in 1948 by Hermann Bondi and Thomas Gold. It was further developed by Sir Fred Hoyle to deal with problems that had arisen in connection with the alternative → Big Bang model. Observations since the 1950s have produced much evidence contradictory to the steady state theory and supportive of the Big Bang model. More specifically, the steady state theory attributed the → cosmic microwave background to → thermal radiation from → dust clouds, but this cannot account for a single → blackbody spectrum. Moreover, the steady state theory lacked a plausible mechanism for the creation of matter in space. See also → perfect cosmological principle.

→ steady; → state; → theory.

The solid angle subtended at the center of a sphere by an area on its surface numerically equal to the square of the radius. → square degree.

From ste(reo)-, → stereo- + → radian.

That part of the → electromagnetic radiation with a → wavelength beyond 300 → microns.

→ submillimeter; → radiation.

A condition in which there is an excess of the actual temperature gradient over the → adiabatic temperature gradient corresponding to the same pressure gradient. A region with superadiabatic temperature gradient is convectively unstable. → Hayashi forbidden zone.

→ super-; → adiabatic; → temperature; → gradient.

The → radius of a → supernova remnant (SNR) when, at the end of the → free expansion phase, the mass of the swept-up → shell equals that of the ejected gas from the → supernova explosion. It is given by R_SW = (3M_e / 4πρ₀)^(1/3), where M_e is the ejected mass and ρ₀ is the initial density of the → interstellar medium.

Sweep, from M.E. swepen, from O.E. swapan "to sweep;" cognate with Ger. schweifen; → up; → radius.

Šo'â', → radius; rubeš, noun from ruftan, rubidan "to sweep," → scan.

The electromagnetic radiation emitted by high-energy particles that are moving in magnetic fields, as in a synchrotron particle accelerator. The acceleration of the moving charges causes the particles to emit radiation. Radio galaxies and supernova remnants are intense sources of synchrotron radiation. Characteristics of synchrotron radiation are its high degree of polarization and nonthermal spectrum.

→ synchrotron; → radiation.

The aquatic larva of → frogs and → toads, having internal gills and a tail (TheFreeDictionary).

From M.E. taddepol, from tadde "toad" + polle "head," from M.L.G. or M.Du. pol "head, top."

Vagiz, from Tabari vag "→ frog" + -iz, -ize diminutive suffix (variants -ce, -že, -ak, Mid.Pers -cak).

A shorter → horseshoe orbit confined to the → Lagrangian points L4 or L5.

→ tadpole; → orbit.

A physical quantity that describes the rate of change of temperature with displacement in a given direction from a given reference point. Same as → thermal gradient.

→ temperature; → gradient.

1) A group of four.
2) A sequence of four → total lunar eclipses occurring at approximately six month intervals. Tetrads are relatively rare events. The first → total eclipse of a tetrad occurred on 15 April 2014, the next eclipse of the series will happen on 8 October 2014 and the next two eclipses on 4 April and 28 September 2015, respectively. The previous tetrad happened in 2003-2004 and the next one will be in 2032-2033.

From Gk. tetrad-, stem of tetras "group of four."

Cahârgân, from cahâr, → four, + -gân suffix of plurality.