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

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."

General: Slope or inclination away from the perpendicular or the horizontal; departure from a reference base.
Aeronautics: The angle measured between the tip edge of an aircraft or missile wing or other lifting surface and the plane of symmetry.
Geology: The inclination of an ore shoot, or other linear geologic structure, from the horizontal, as measured in the plane of the associated veins, faults, or foliation.

Rake, etymology unknown.

Varkeš "slope" in Gilaki dialect. It can also be literally interpreted as "departure from a surface, a side, depart away" from var, variant bar, "side, surface" + keš present stem of kešidan "to pull, drag."

1) A male sheep.
2) The → constellation  → Aries.
3) Any of various → machines that hits something again and again to force it into a position.

M.E. ram, from O.E. ramm "male sheep," also "battering ram," earlier rom "male sheep," a W.Gmc. word of unknown origin (cf. M.L.G., M.Du., Du., O.H.G. ram). The verb meaning "to beat with a heavy implement" is first recorded c.1330.

Quc "ram, horned male sheep," loan from Turkish.
Garând "ram," in Baluci, cognate with Pers. barré, → lamb.
Qucvâr "resembling a ram," with -vâr a suffix meaning "resembling, like" (from Mid.Pers. -wâr; Av. -vara, -var; cf. Skt. -vara).

The pressure exerted on a body moving through a → fluid medium. For example, a → meteor traveling through the Earth's atmosphere produces a → shock wave generated by the extremely rapid → compression of air in front of the → meteoroid. It is primarily this ram pressure (rather than → friction) that heats the air which in turn heats the meteoroid as it flows around the meteoroid. The ram pressure increases with → velocity according to the relation P = (1/2)ρv², where ρ is the density of the medium and v the relative velocity between the body and the medium. Similarly, → ram pressure stripping produces → jellyfish galaxies. Same as → dynamic pressure.

→ ram; → pressure.

A process proposed to explain the observed absence of gas-rich galaxies in → galaxy clusters whereby a galaxy loses its gas when it falls into a cluster. There is a tremendous amount of hot (~ 10⁷ K) and tenuous (~ 10^-4 cm^-3) gas (several 10¹³ → solar masses) in the → intracluster medium (ICM). Ram pressure stripping was first proposed by Gunn & Gott (1972) who noted that galaxies falling into clusters feel an ICM wind. If this wind can overcome the → gravitational attraction between the stellar and gas disks, then the gas disk will be blown away. The mapping of the gas content of spiral galaxies in the → Virgo cluster showed that the → neutral hydrogen (H I) disks of cluster spiral galaxies are disturbed and considerably reduced. Their molecular gas, more bound to the galaxy, is less perturbed, but still may be swept out in case of very strong ram pressure. These observational results indicate that the gas removal due to the rapid motion of the galaxy within the intracluster medium is responsible for the H I deficiency and the disturbed gas disks of the cluster spirals (e.g., J. A. Hester, 2006, ApJ 647:910).

→ ram; → pressure; → strip.

Same as → Raman scattering.

Named after the Indian physicist Sir Chandrasekhara Venkata Raman (1888-1970), who discovered the effect; recipient of the 1930 Nobel Prize in Physics; → effect.

The scattering of monochromatic light (visible or ultraviolet) by molecules in which the scattered light differs in wavelength from the incident light. It is caused by the light's interaction with the vibrational or rotational energy of the medium's scattering molecules.

→ Raman effect; → scattering.

The small circular patch of light visible in the back focal plane of an eyepiece.

Named after Jesse Ramsden (1735-1800), English maker of astronomical instruments; → disk.

An eyepiece consisting of two planoconvex lenses of the same focal length, placed with the convex sides facing each other and with a separation between the lenses of about two-thirds of the focal length of each.

Named after Jesse Ramsden (1735-1800), English maker of astronomical instruments; → eyepiece.

1) General: Made or occurring without a definite pattern, plan, or system; haphazard arrangement as if due to pure chance.
2) Statistics: Of or characterizing a process of selection in which each item of a population has an equal probability of being chosen.
See also:
→ random access memory, → random error, → random experiments, → random noise, → random sample, → random structure, → random thermal motion, → random variable , → independent random variables, → random walk, → randomization, → randomize, → randomness.

M.E. raundon, random "impetuosity, speed," from O.Fr. randon "rush, disorder, impetuosity," from randir "to run fast."

Kâturé originally "dazzled, confused," variants katré "disorderly, ragged, tattered, babble, meaningless or incoherent speech," katreyi "disorderly, at random;" maybe from kat- "to fall;" → case.

In computer technique, a configuration of memory cells that hold data for processing by a central processing unit (CPU). The term random derives from the fact that the CPU can retrieve data from any individual location, or address, within RAM.

→ random; → access; → memory.

The fluctuating part of the overall error that varies from measurement to measurement. Normally, the random error is defined as the deviation of the total error from its mean value; opposite of → systematic error.

→ random; → error.

Statistics: Experiments in which results will not be essentially the same even though conditions may be nearly identical.

→ random; → experiment.

Unpredictable noise comprising large numbers of frequent, transient impulses occurring at statistically random time intervals. Thermal noise is a form of random noise.

→ random; → noise.

A sample selected at random from a population.

→ random; → sample.

Crystalline arrangement in which equivalent positions are not necessarily occupied by atoms of a single kind.

→ random; → structure.