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

Irregularly shaped → dust grains that occur in the → interstellar medium. They are mostly composed of carbon and/or silicates and measure a fraction of micron across.

→ interstellar; → grains.

A type of → interstellar dust grain made up of an aggregate of components with a hollow structure. Various processes operating in interstellar and → circumstellar media are believed to produce inhomogeneous and porous dust grains. Porous grains can produce more → extinction per unit mass than their combined individual dust components. They are generally cooler than compact grains (see, e.g., Iati et al. 2001, MNRAS 322, 749).

→ porous; → dust; → grain.

A → refractory → nanoparticle embedded in → meteorites and → interplanetary dust particles whose → isotopic ratios suggest formation earlier than the Solar System.

→ presolar; → grain.

The main rainbow that forms between about 40° and 42° from the → antisolar point (or about 50° from the → antisolar point), as viewed by the observer. The light path involves → refraction and a → single  → reflection inside the water → droplet. If the drops are large, 1 millimeter or more in diameter, red, green, and violet are bright but there is little blue.

As the droplets get smaller, red weakens.

Rainbows are not seen in midday since the whole 42° circle is below the horizon at most latitudes. So rainbows tend to be seen most in the later afternoon when a thundershower has passed and the Sun is illuminating from the west.

→ primary; → rainbow.

Water that is condensed from the aqueous vapor in the atmosphere and falls to earth in drops greater than 0.5 mm in diameter.

M.E. rein; O.E. regn; cf. O.S. regan; O.N. regn; M.Du. reghen; Ger. Regen; maybe cognate with Pers. (Tabari) rag "thunder;" (Šahmirzâdi, Semnâni, Sorxe-yi) rak "thunder;" (Gilaki) râk "cloudburst;" L. rigare "to wet, moist;" PIE *reg- "rain, damp."

Bârân, from bâridan "to rain;" Mid.Pers. vârân "rain," vâritan "to rain;" Av. vār- "rain; to rain;" cf. Skt. vār- "rain, water; to rain;" L. urinari "to plunge under water, to dive;" Gk. ourein "to urinate;" PIE base *uer- "water, rain, river."

A precipitation consisting of rain and partially melted snow. It usually occurs when the temperature of the air layer near the ground is slightly above freezing. Called sleet in British English speaking countries, but not in the United States where the term has a different meaning in meteorology.

→ rain; → snow; → mix.

Šaliv, of dialectal origin, Kurd. šalêwa "rain and snow mixed," Aftari šelâp, Qasrâni šelâb with the same meaning, Tabari šalâb "strong cloudburst." The first element šal, šel, šor, šâr, âbšâr, šâridan "to flow." The second element iv, êw, âp, âb, → water.

Any cloud from which rain falls.

→ rain; → cloud.

Zafâk "rain cloud" (Dehxodâ); Mid.Pers. zafâ.

A color effect produced by the → refraction and → internal reflection of sunlight passing through a mist of tiny spherical water → droplets in the air. The effect is visible only when the observer has his back to the Sun. It appears as a colored band at about 138° from the Sun, hence 42° from the → antisolar pint. In other words, 42° is the angle between the direction of the → incident sunlight and the → line of sight. The → primary rainbow is caused from one reflection inside water droplets; the red color appears on the top and violet on the bottom. At solar elevations higher than 42° the bow is entirely below the → horizon and therefore invisible in the sky. A full rainbow is actually a complete circle, but from the ground we see only part of it. From an airplane, in the right conditions, one can see an entire circular rainbow. A → secondary rainbow appears if the sunlight is reflected twice inside the water droplets. Secondary rainbows are fainter, and the order of the color is reversed, with red on the bottom and violet on the top. See also: → Alexander's dark band, → supernumerary rainbow.

From → rain; → bow.

The → obtuse angle between sunlight and the → line of sight. Rainbow angle = 180° minus → scattering angle. For the → primary rainbow it is 138°, and for the → secondary rainbow 130°.

→ rainbow; → angle.

The sunlight incident on a tiny spherical droplet of water.

→ rainbow; → ray.

The total liquid product of precipitation or condensation from the atmosphere, as received and measured in a rain gauge.

→ rain + → fall.

Bâreš verbal noun of bâridan "to rain," bâridan "to rain;" Mid.Pers. vârân "rain," vâritan "to rain;" Av. vār- "rain; to rain;" cf. Skt. vār- "rain, water; to rain;" L. urinari "to plunge under water, to dive;" Gk. ourein "to urinate;" PIE base *uer- "water, rain, river."

To hold back from action; keep in check; repress; to limit or hamper the activity, or effect of.

M.E. restreynen, from O.Fr. restreindre "press, push together; curb, bridle;" from L. restringere "to draw back tightly, confine," from → re- "back" + stringere "draw tight," → strain.

Bâzdâštan, from bâz- prefix denoting "reversal, opposition," → re-, + dâštan "to hold," → property.

A mixture of the → Cassegrain telescope with a very short → focal length and of a Schmidt design (due to the presence of the → corrective plate), used mainly in → amateur astronomy. The main advantage of this telescope is its compact design. However, Schmidt-Cassegrain telescopes produce fainter images with less contrast than other telescope designs with similar → aperture sizes. This is due to the comparatively large → secondary mirror required to reflect the light back the → eyepiece.

→ Schmidt telescope; → Cassegrain telescope.

A fainter rainbow appearing about 10° above the → primary rainbow, as viewed by the observer. The secondary rainbow is about twice as wide, and has its colors reversed.

→ secondary; → rainbow.

Change of volume and/or shape of a body, or part of a body, due to an applied → stress. When a body is deformed by such a force, through compression or distension, the strain is the ratio of the dimensional change to the original or un-strained dimension. The strain may be a ratio of lengths, areas, or volumes. See also → shear.

M.E. streinen (v.), from O.Fr. estreindre "to bind tightly, clasp, squeeze," from L. stringere "to bind or draw tight," from PIE base *strenk- "tight, narrow; pull tight, twist;" cf. Gk. strangein "twist;" Lith. stregti "congeal;" O.H.G. strician "mends nets;" Ger. stramm, Du. stram "stiff."

Šepil "squeeze; fondness" (Dehxodâ) of unknown origin.

An additional faint arc or series of arcs just below the → primary rainbow. Supernumerary bows are caused by → interference and are more common toward the top of the bow.

→ supernumerary; → rainbow.

1) A series or sequence of objects or events.
2) A line of coaches or wagons coupled together and drawn by a railway locomotive.

M.E., from O.Fr. train "tracks, path, trail; act of dragging," from trainer "to pull, drag, draw," from V.L. *traginare, from *tragere "to pull," back-formation from tractus, p.p. of L. trahere "to pull, draw."

Qatâr "a row of camels," loan from Ar.; teran, loan from Fr., as above.

A special type of carbonaceous → interstellar dust grains with a size ranging from 10 to 150 Å and consisting of tens to hundreds of atoms. In contrast to → big grains, VSGs are not in → thermal equilibrium. They can be heated to very high temperatures (~ 1000 K) by the absorption of a single photon. It is thought that VSGs are clusters of → PAH.

→ very; → small; → grain.

A series of successive waves spaced at regular intervals.

→ wave; → train.