An Etymological Dictionary of Astronomy and Astrophysics
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The ratio of the buoyancy force to the viscous force in a medium. This dimensionless number is used to estimate when convection commences in a fluid. It depends on the density and depth of the fluid, the coefficient of thermal expansion, the gravitational field, the temperature gradient, the thermal diffusivity, and the kinematic viscosity. Convection usually starts when Ra is 1000 or more, while heat transfer is entirely by conduction when Ra is less than 10.

→ rayleigh; → number.

The scattering of light by → particles of size small compared with the → wavelength of light. The intensity of the light scattered by unit volume of the medium at an angle θ to the direction of propagation of the incident light is: I_θ = 8 π⁴α² N I₀ (1 + cos²θ)/(R²λ⁴), where α is the → molecular polarizability, N is the number of scattering molecules, I₀ is intensity of the incident light, λ is the wavelength, and R is the distance from the scatterer. The fourth power dependence on wavelength means that blue light is much more strongly scattered than red light from a medium containing very fine particles. The air molecules, mostly → nitrogen (78%) and → oxygen (21%) are some 1,000 times larger than → visible light wavelengths. This accounts for the bluish appearance of smoke and of clear sky when the observation is not along the direction of illumination. The setting Sun, seen through a considerable thickness of atmosphere appears reddish because long wave radiation predominates in the transmitted light.
Historically, John Tyndall first discovered this phenomenon in 1859 (→ Tyndall effect), but Lord Rayleigh studied it in more detail in 1871.

→ rayleigh; → scattering.

A criterion for the instability of a basic swirling flow with an arbitrary dependence of angular velocity Ω(r) on the distance r from the axis of rotation. This states that in → inviscid fluids: Ω(r) < 0 for instability, where Ω = (1/r³) (d/dr)(r⁴Ω⁴).

→ Rayleigh; → criterion.

A classical law approximately describing the intensity of radiation emitted by a → blackbody. It states that this intensity is proportional to the temperature divided by the fourth power of the wavelength (8πkT/λ⁴). The Rayleigh-Jeans law is a good approximation to the experimentally verified Planck radiation formula only at long wavelengths. At short wavelengths it runs into a paradox named the → ultraviolet catastrophe.

→ Rayleigh; → Jeans; → law.

The part of → electromagnetic spectrum approximated by the → Rayleigh-Jeans law.

→ Rayleigh; → Jeans; → spectrum.

A type of hydrodynamical instability between two fluids of different densities, which occurs when the heavy fluid lies above the lighter fluid in a gravitational field. More generally a material interface is said to be Rayleigh-Taylor unstable whenever the fluid acceleration has an opposite direction to the density gradient.

→ rayleigh; → Taylor number; → instability.

A temperature scale in which the → freezing point and the → boiling point of → water are set to 0 and 80 degrees respectively. See also → Celsius scale, → Fahrenheit scale, → Kelvin scale, → Rankine scale.

Named after René Antoine Ferchault de Réaumur (1683-1757), who first proposed it in 1730; → scale.

A catalog of → H II regions in the → southern  → Milky Way based on observations obtained at Mount Stromlo Observatory (Australia). It contains 181 → H-alpha emission objects characterized by their positions, dimensions, and estimated brightness.

Rodgers,A.W., Campbell, C.T., Whiteoak, J.B., 1960, MNRAS, 121, 103; → catalog.

A prefix meaning "again, anew" to indicate repetition (as rebuild, retell), "back, backward" to indicate "withdrawal" or "backward motion" (react, recall), or sometimes merely intensive (refine, resolve).

M.E., from O.Fr., from and L. re- "again, back."

Vâ-, variant bâz-, prefix denoting "reversal, opposition; separation; repetition; open; off; away," from Mid.Pers. abâz-, apâc-; O.Pers. apa- [pref.] "away, from;" Av. apa- [pref.] "away, from," apaš [adv.] "toward the back;" cf. Skt. ápāñc "situated behind."

1) To act in response to an agent, influence, or stimulus.
2) To act in a reverse direction or manner; to act in opposition, as against some force.
3) Chem.: To undergo a → chemical reaction.

→ re-; → act.

The opposition to the flow of alternating current caused by the inductance and capacitance in a circuit rather than by resistance.

From → react + -ance a suffix of nouns.

Vâžirâyi state noun of vâžirâ agent noun of vâžiridan→ react.

1) General: A person or thing that reacts.
2) A substance on the left side of a → chemical equation.

From → react + -ant.

1) General: Action in response to some influence, event, stimulus.
2) An equal and opposite force exerted by a body against a force acting upon it. → principle of action and reaction.
3) A → nuclear reaction; → chain reaction.
4) → chemical reaction.

Verbal noun of → react.

The speed with which a → chemical reaction takes place. In other words, the change in → concentration of a → substance divided by the → time interval during which the change is observed.

→ reaction; → rate.

Tending to react. Pertaining to or characterized by reaction.

From → react + -ive a suffix of adjectives expressing tendency, disposition, function, connection, etc.

Vâžirandé verbal adj. of vâžiridan→ react; vâžireši, adj. of vâžireš→ reaction.

1) General: The quality or condition of being reactive.
2) Physics: A measure of the deviation from the condition at which a → nuclear reactor is critical.
3) Chem.: The relative → capacity of an → atom, → molecule, or → redical to undergo a → chemical reaction with another atom, molecule, or → compound.

From → reactive + → -ity.

1) A device for introducing reactance into an electrical circuit (e.g. a capacitor).
2) Same as → nuclear reactor.

From → react + → -or.

The part of a → nuclear reactor in which → nuclear fission takes place and huge quantities of heat energy are generated.

→ reactor; → core.

In computing and data processing, the number of words, characters, fields, etc. sensed by an input sensing device per unit of time.

Reading, verbal noun of read, from M.E. reden, O.E. rædan, redan "to counsel, read;" cf. Du. raden, Ger. raten "to advise, counsel, guess;" akin to Skt. rādh- "to succeed, accomplish;" Gk. arithmos "number amount;" L. ratio; Pers. rây, râ "because of, for the sake of;" → reason; → rate.

Nerx, → rate; xâneš verbal noun of xândan "to read; to sing; to call;" Mid.Pers. xwân- "to resound; to call;" Av. x^van- "to sound," Proto-Iranian *huan- "to call;" cf. Skt. svan-, sváranti "to sound, make a sound, sing;" L. sonus "sound," sonare "to sound;" O.E. swinn "music, song," PIE base *suen- "to sound" (Cheung 2007).

The noise added in the process of reading a detector such as a CCD.

→ reading rate; → noise.