An Etymological Dictionary of Astronomy and Astrophysics

1a) A group of persons related by common descent or heredity.

1b) A population so related.

2. A contest of speed, as in running, riding, driving, or sailing (Dictionary.com).

Etymology (EN): 1) From M.Fr. race “race, breed, lineage, family,” from It. razza, (cf. Sp. and Port. raza), of unknown origin.

2. M.E. ras(e), from O.N. ras “running, race,” cognate with O.E. ræs “a running, a rush, a leap, jump.”

Etymology (PE): 1) Nežâd, literally “born,” ultimately from Proto-Ir. *nizat-, cf. Av. nizənta- “born,” from → ni- + *zan- “to give birth, to be born,” cognate with âzâd, → free; see also → generate.

2. Tâz, present stem of Tâxtan/tâz- “to run, rush upon, assault;” → surge.

1. A belief or doctrine that inherent differences among the various human races determine cultural or individual achievement, usually involving the idea that one’s own race is superior and has the right to rule others.
   

2. A policy, system of government, etc., based upon or fostering such a doctrine; discrimination (Dictionary.com).

See also: → race; → -ism.

A unit of energy absorbed from ionizing radiation, equal to 100 → ergs per gram, or 0.01 → joules per kilogram, of irradiated material. Rad has been replaced by → gray (Gy).

See also: Shortened form roentgen absorbed dose;
→ roentgen.

An emitting/receiving device in which the echo of a pulse of microwave radiation is used to detect and locate distant objects.

See also: From ra(dio) d(etecting) a(nd) r(anging).

Graphic display of measurements by a → radar of mineral deposits on a planetary surface.

See also: → radar; → chart.

Emanating from a common central point; arranged like the radii of a circle.

See also: From L. radialis, from → radius;
→ -al.

The process whereby a → disk star changes its → galactocentric distance. Radial migration involves → angular momentum transfer, resulting from → resonances created by transient → density waves such as → bars or → spiral arms in → galactic disks. According to → galactic dynamics models, → churning is the main cause of radial migration. Radial migration of stars plays an important role in shaping the properties of galactic disks.

See also: → radial; → migration.

A motion away from or toward a central point or axis.

See also: → radial; → motion.

Any of short-lived (generally lasting less than 24 hours) radial features that periodically appear over the outer half of → Saturn’s → B ring, when the ring tilt angle is small. These features revolve at the same rate as the planet’s → magnetic field and maintain their shape over much of the course of their existence even though they extend tens of thousands of kilometers across the rings. It is believed that the tiny particles that make up these spokes are electrically charged and temporarily “frozen” into the planet’s magnetic field (Ellis et al., 2007, Planetary Ring Systems, Springer).

See also: → radial; → spoke.

The component of a three-dimensional velocity vector of an object directed along the line of sight. It is measured by examining the Doppler shift of lines in the spectrum of astronomical objects.

See also: → radial; → velocity.

A curve describing the variation of the radial velocity of a star, due to the Doppler effect,
under the gravitational effect of a secondary body (companion or exoplanet). The amplitude of these variations depends upon the mass of the secondary and its distance from the star.

See also: → radial velocity; → curve.

The technique based on the analysis of the → radial velocity curve, used to detect the presence of an invisible secondary around a host star. This method holds the majority of exoplanet discoveries.

See also: → radial velocity; → method.

A unit of angular measure; one radian is that angle with an intercepted arc on a circle equal in length to the radius of the circle.

See also: From radi(us) + -an an originally adj. suffix.

1. Generally, the → radiant energy per unit → solid angle per unit of → projected area of the → source. It is usually expressed in → watt per → steradian per → square  → meter (W m^-2 sr^-1). Same as steradiancy.
   

2. Of any particular → wavelength within the interval covered by a → spectral line, the → energy per unit → surface per steradian, per wavelength; denoted I_λ. The term radiance is often loosely replaced by “→ intensity.” The radiance of the whole line is given by I = ∫ I_λ dλ. The radiance of an
   → emission line depends, among other things, upon the → number of → atoms per unit area in the → line of sight (the → column density) in the → upper level of the line.

Etymology (EN): From radia(nt), → radiant, + → -ance.

Etymology (PE): Tâbešmandi, noun from tâbešmand “possessing radiation,” from tâbeš, → radiation, + -mand a suffix denoting possession; Mid.Pers. -ômand suffix forming adjectives of quality.

1. Sending out rays of light; bright; shining.
   
   See also: → radiant energy, → radiant flux, → radiant intensity.
   

2. The point in the sky from which → meteors
   in a → meteor shower appear to radiate or come. See also: → radiant drift.

Etymology (EN): M.E., from M.Fr. radiant, from L. radiantem (nominative radians) “shining,” pr.p. of radiare “to shine, radiate,” → radiation.

Etymology (PE): 1) Tâbandé, tâbeši adj. from tâbidan,
→ radiate.
2) Tâbsar, from tâb “light, radiation,” → radiation,

• sar “head, top, summit, point,” → head.

The apparent slow motion of the → radiant of a → meteor shower from night to night against the background stars due to the Earth moving in its orbit around the Sun.

See also: → radiant; → drift.

The energy that is transmitted in the form of → radiation, in particular as → electromagnetic radiation.

See also: → radiant; → energy.

Rate of flow of energy as → radiation.

See also: → radiant; → flux.

A measure of the amount of radiation emitted from a point expressed as the radiant flux per unit solid angle leaving this source.

See also: → radiant; → intensity.

To send out → energy, such as → heat or → light, in the form of → rays or → waves.

Etymology (EN): From L. radiat(us), p.p. of radiare “to shine, to beam”

• -ate verbal suffix.

Etymology (PE): Tâbidan, variants tâftan “to shine,” tafsidan “to become hot;” Mid.Pers. tâftan “to heat, burn, shine;” taftan “to become hot;” Parthian t’b “to shine;”
Av. tāp-, taf- “to warm up, heat,” tafsat “became hot,” tāpaiieiti “to create warmth;” cf. Skt. tap- “to heat, be/become hot; to spoil, injure, damage; to suffer,” tapati “burns;” L. tepere “to be warm,” tepidus “warm;” PIE base *tep- “to be warm.”

The emission of any → rays, → waves, or → particles from a source; usually applied to the → emission of → electromagnetic energy.

See also: Verbal noun of → radiate.

A ring-shaped region in the → magnetosphere of a planet in which charged particles are trapped by the planet’s magnetic field. The radiation belts surrounding Earth are known as the → Van Allen belts.

See also: → radiation; → belt.

Same as → radiation density constant.

See also: → radiation; → constant.

Damping of a system which loses energy by → electromagnetic radiation.

See also: → radiation; → damping.

The constant related to the total energy radiated by a → blackbody and defined as: a = 4σ/c, where σ is the → Stefan-Boltzmann constant and c the → speed of light. Its value is a = 7.5657 x 10^-15 erg cm^-3 K^-4. Same as → radiation constant.

See also: → radiation; → density; → constant.

The epoch in the history of the Universe, lasting from the → Big Bang until about 400,000 years later, when the temperature had dropped to 10⁹ K and the rate of electron-positron → pair annihilation exceeded the rate of their production, leaving radiation the dominant constituent of the Universe. The radiation era was followed by the → matter era.

See also: → radiation; → era.

1. The portion of an → electromagnetic field outside the → induction field where there is a power flow of both → magnetic and → electric components in a well-defined relationship.
   

2. → interstellar radiation field

See also: → radiation; → field.

The mean distance traveled by a photon or particle in a given medium before its energy is reduced by a factor e due to its interaction with matter.

See also: → radiation; → length.

Same as → antenna pattern.

See also: → radiation; → pattern.

The → momentum carried by → photons to a surface exposed to → electromagnetic radiation. Stellar radiation pressure on big and massive objects is insignificant, but it has considerable effects on → gas and → dust particles. Radiation pressure is particularly important for → massive stars. See, for example, → Eddington limit, → radiation-driven wind , and
→ radiation-driven implosion. The → solar radiation pressure is also at the origin of various physical phenomena, e.g. → gas tails in → comets and → Poynting-Robertson effect.

See also: → radiation; → pressure.

An illness resulting from excessive exposure to ionizing radiation. The earliest symptoms are nausea, vomiting, and diarrhea, which may be followed by loss of hair, hemorrhage, inflammation of the mouth and throat, and general loss of energy.

Etymology (EN): → radiation; sickness, M.E. siknesse, seknesse; O.E. sēocnesse, from seoc + suffix -ness.

Etymology (PE): Bimâri “sickness, infirmity, disease,” from bimâr “sick, infirm, afflicted;” Mid.Pers. vêmâr “sick, ill;” maybe by corruption of Proto-Iranian *amavayā-bara- “bearing illness;” cf. Av. amavayā- “pain, suffering, affliction;” Skt. ámīvā- “pain, grief, distress” + *bara- “bearing;” cf. Av. bar- “to bear, carry;” Mod.Pers. bar-, bordan “to bear, carry, lead.” Alternatively, from *vi-mar-, prefixed *mar- “to die;” cf. Av. mar- “to die;” Mod.Pers. mir-, mordan “to die;” Skt. mar- “to die;” cognate with Gk. emorten “died;” L. morior “to die;”
tâbeši related to tâbeš, → radiation.

The components of radiation arranged in order of their wavelengths, frequencies, or quantum energies. For particle radiation they are arranged in order of their kinetic energies.

See also: → radiation; → spectrum.

The temperature of a source calculated assuming that it behaves as a → blackbody that radiates with the same intensity at the same frequency. Compared to the → effective temperature, the radiation temperature is measured over a narrow region of the → electromagnetic spectrum.

See also: → radiation; → temperature.

→ radiative transfer.

See also: → radiation; → transfer.

→ radiative transfer equation.

See also: → radiation; → transfer; → equation.

An early epoch in the history of the → Universe when the radiation → density parameter was Ω_r≈ 1, while other density parameters had negligible contributions. A radiation-dominated Universe is characterized by R/R₀ ∝ t^1/2, where R is the → cosmic scale factor and t is time. According to the → Big Bang model, the radiation-dominated phase was followed by the → matter-dominated phase.

See also: → radiation; → dominate; → Universe.

A hydrodynamic process occurring in star forming regions where a neutral cloud (→ clump) is subjected to the intense ultraviolet radiation of a newly-born → massive star. The gas within the layer exposed to the radiation is ionized and forms an → ionization front at the front surface. The increased pressure due to temperature rise at the top layer drives an → isothermal  → shock front into the clump, which compresses the neutral gas ahead of it, below the surface. A density → gradient builds up leading rapidly to the formation of a condensed core. With further concentration of the gas, the hydrogen density in the center of the core increases drastically, reaching 10⁸ cm^-3 about 4 x 10⁵ years after the first impact of the ionizing radiation on the clump, according to current models (e.g. Bertoldi 1989, ApJ 346, 735; Miao et al. 2006, MNRAS 369, 143, and references therein). The core can develop further to form a → cometary globule or → collapse under its self-gravity,
eventually giving rise to new → low-mass stars (→ triggered star formation). In the process, the whole clump accelerates away from the initial ionizing star. Indeed, the gas evaporated off the side of the clump facing the ionizing star can create a rocket effect accelerating the clump away from the star (with a velocity of up to 5 km s^-1), while losing part of its initial mass.

See also: → radiation; driven, p.p. of → drive; → implosion.

The → mass loss experienced by a → massive star due to the effect of → radiation-driven wind.

See also: → radiation; driven, p.p. of → drive; → mass; → loss.

The loss of matter from the → photosphere due to the acceleration imparted to the outer layers of the star by photons created inside the star. The coupling between radiation and matter creates a → radiative acceleration that may exceed the → gravity. This mechanism is particularly important in → massive stars, since the luminosity is high and therefore the number of energetic ultraviolet photons important. Same as → line-driven wind.

See also: → radiation; → drive; → wind.

A process in which a molecule relaxes without emitting a → photon.

See also: → radiation; → -less; → relaxation.

Of or pertaining to radiation.

See also: Adjective of → radiation.

The acceleration imparted to matter by → radiation pressure.

See also: → radiative; → acceleration.

The slowing down of a star’s rotation due to radiative momentum transfer caused by emission of electromagnetic radiation.

See also: → radiative; → braking.

Capture of a free electron by an ion with the subsequent emission of photons;
also called → radiative recombination.

See also: → radiative; → capture.

A collision between charged particles in which part of the kinetic energy is converted into electromagnetic radiation.

See also: → radiative; → collision.

The process by which temperature decreases due to an excess of emitted radiation over absorbed radiation.

See also: → radiative; → cooling.

The process when the energy difference between the excited and non excited states of an atom is taken away by radiation.

See also: → radiative; → decay.

A process of → radiative transfer in which photons are repeatedly absorbed and re-emitted by matter particles.

See also: → radiative; → diffusion.

A → radiative zone occupying the outer parts of a star.

See also: → radiative; → envelope.

The balance between radiative emission and radiative absorption in a specified system.

See also: → radiative; → equilibrium.

The radiative energy put back to the environment through an astrophysical process. For example, in the process of → star formation 
→ accretion disks form around → protostars. The material in the disk spirals inward and on to the protostar, provided that there is an efficient mechanism to redistribute → angular momentum outward in the disk. During this process → gravitational energy is transformed into radiation due to → viscous dissipation in the disk and at the → accretion shock around the protostar. This radiation heats the region around the protostar and may → suppress subsequent → fragmentation and further star formation. Thus, radiative feedback plays a critical role in regulating the stellar → initial mass function.

See also: → radiative; → feedback.

The radiative energy per unit time and unit area.

See also: → radiative; → flux.

The process by which temperature increases due to an excess of absorbed radiation over emitted radiation.

See also: → radiative; → heating.

A physical process occurring in → stellar atmospheres whereby → radiation pressure selectively pushes certain → chemical elements outward, leading to an atmospheric overabundance of such elements. See also → gravitational settling.

See also: → radiative; → levitation.

For a → supernova remnant (SNR), same as the → snowplow phase.

See also: → radiative; → phase.

An process in which an excited state loses its absorbed energy by emission of radiation. → non-radiative process.

See also: → radiative; → process.

The process by which an ionized atom binds a free electron in a → plasma to produce a new atomic state with the subsequent radiation of photons.

See also: → radiative; → recombination.

A → shock wave in which the → time-scale for → cooling is much shorter than the appropriate → dynamical or → evolutionary time-scale of the system that drives the shock. Radiative shock waves are believed to play a key role in a variety of different astrophysical environments, including → magnetic cataclysmic variables, → jets from → young stellar objects, → accretion in → T Tauri stars, → colliding stellar winds, and → supernova remnants.

See also: → radiative; → shock.

The process by which the → electromagnetic radiation passes through a medium that may contain any combination of → scatterers, → absorbers, and → emitters.

See also: → radiative; → transfer.

The equation that describes the → radiative transfer. It states that the → specific intensity of radiation I_σ during its propagation in a medium is subject to losses due to → extinction and to → gains due to → emission:

dI_σ/dx = - μ_σ . I_σ + ρ . j_σ, where x is the coordinate along the → optical path, μ_σ is the → extinction coefficient, ρ is the mass → density, and j_σ is the → emission coefficient per unit mass.

See also: → radiative; → transfer; → equation.

A transition between two states of an atomic or molecular entity, the energy difference being emitted or absorbed as photons.

See also: → radiative; → transition.

The region of a star in which the energy generated by → nuclear fusion in the core is transferred outward by → electromagnetic radiation and not by → convection. Such zones occur in the interior of
low-mass stars, like the Sun, and the envelopes of → massive star.
The radiative zone of the Sun starts at the edge of the core of the Sun, about 0.2 solar radii, and extends up to about 0.7 radii, just below the → convective zone.

See also: → radiative; → zone.

Same as → radiation-driven wind

See also: → radiative; → drive; → wind.

1. Math.: The indicated root of a quantity, as denoted by an expression written under the → radical sign.
   

2. Chemistry: A group of atoms present in a series of compounds which conserves its identity in the course of chemical changes which affect the rest of the molecule.
   For example, the
   → cyano radical and → hydroxyl group.

Etymology (EN): M.E., from L.L. radicalis “of or having roots,” from → radix “root.”

Etymology (PE): → root.

Of two circles, the straight line containing all points P such that the lengths of the tangents from P to the two circles are equal.

See also: → radical; → axis.

The symbol √ placed before a number or quantity to indicate the extraction of the square root. The value of a higher (the n-th) root is indicated by a raised positive digit (n) in front of the symbol, as in ³√ (cube root). The first known occurrence of this symbol was in the book Die Cross, published in 1525, by the German mathematician Christoff Rudolff.

See also: → radical; → symbol.

Plural form of → radius.

See also: → radius.

1. An apparatus for receiving or transmitting radio broadcasts.
   

2. Pertaining to the → radio waves of the → electromagnetic radiation.
   

3. As a prefix, indicating → radition or → radioactivity.

Etymology (EN): 1); 2) Short from radiophone and radio-telegraphy.
3) Combining form of → radiation.

Etymology (PE): Râdio, loan from Fr., as above; partow→ ray.

A large number of narrow filaments in → radio continuum
occurring toward the → Galactic Center,
about 15 to 20 arc-minutes (some 50 parsecs in projection) north of → Sgr A*. The radio Arc is the prototype of → non-thermal filaments (NTFs)
and resolves into a set of more than a dozen vertical filaments with lengths of about 30 pc distributed symmetrically with respect to the → Galactic equator (Yusef-Zadeh et al. 1984, Nature 310, 557). Among more than 100 NTFs found in the Galactic center region, the Arc is the only one known to show inverted spectrum with a → spectral index α = +0.3 (Law et al. 2008, ApJS 177, 515, and references therein). This implies a very hard energy spectrum of particles for a source of → synchrotron radiation.

See also: → radio; → arc.

The branch of astronomy that deals with the study of the Universe by means of → radio waves.

See also: → radio; → astronomy.

A burst of emission in the radio frequencies of the electromagnetic spectrum.

See also: → radio; → burst.

A → continuum emission with frequencies in the radio range of the electromagnetic spectrum.

See also: → radio; → continuum; → emission.

The representation in → radio wavelengths of an astronomical object that has emission in other parts of the → electromagnetic spectrum.

See also: → radio; → counterpart.

→ Electromagnetic radiation carried by → radio waves.

See also: → radio; → emission.

Total radiation in radio wavelengths going out from the 2π solid angles of a hemisphere. → flux.

See also: → radio; → flux.

The flux of → radio waves that falls on a → detector per → unit → surface area of the detector per unit → bandwidth of the radiation per unit → time. The unit is → jansky.

See also: → radio; → flux; → density.

The → electromagnetic radiation with the frequency range between 3 → kiloherz (kHz) to 300 → gigahertz (GHz). See also → radio wave.

See also: → radio; → frequency.

A galaxy that is extremely luminous at radio wavelengths between 10 MHz and 100 GHz. The radio luminosity of a strong radio galaxy (10³⁷-10³⁹ watts) can be up to a million times greater than the radio output of an ordinary galaxy and up to a hundred times greater than the optical luminosity of a galaxy such as the Milky Way. The optical counterparts of radio galaxies are usually an → elliptical galaxy. Radio galaxies often exhibit jet structure from a compact nucleus. They typically display two → radio lobes that are often approximately aligned with the jets observed in the optical and that may extend for millions of → light-years.

See also: → radio; → galaxy.

An → interferometer designed for radio frequencies.

See also: → radio; → interferometer.

An → astrophysical jet appearing in the radio wavelengths of the electromagnetic spectrum.

See also: → radio; → jet.

An extended region of diffuse radio emission, often dumbbell shaped, that surrounds a
radio galaxy.

See also: → radio; → lobe.

The electromagnetic noise at radio wavelengths.

See also: → radio + → noise.

A → pulsar that emits → pulses in → radio waves. The bulk of discovered pulsars are radio pulsars. There is a small number of pulsars that emit at optical wavelengths, X-ray wavelengths, and gamma-ray wavelengths.

See also: → radio; → pulsar.

A → recombination line whose wavelength lies in the radio range of the electromagnetic spectrum. Radio recombination lines are the result of electronic transitions between high energy levels (n > 50) in an atom or ion.

See also: → radio; → recombination line.

A signal in the → radio frequency region of the → electromagnetic spectrum.

See also: → radio; → signal.

A condition in which all fixed or mobile equipments capable of radio transmitting in an area are kept inoperative, typically for the duration of some sensitive time period. The term has both military and civilian applications.

See also: → radio; → silence.

A point or small portion of the sky giving stronger radio emission than the sky in its vicinity.

See also: → radio; → source.

That part of the → electromagnetic spectrum that includes → radio waves.

See also: → radio; → spectrum.

A star which is a source of emission at radio frequencies. Radio stars include pulsars, flare stars, binary star systems in which
mass is transferred from one component to the other, and some
X-ray stars.

See also: → radio; → star.

Strong radio frequency radiation from the Sun, occurring in association with eruptions of solar flares or other causes of solar activity.

See also: → radio; → storm.

The image of the Sun obtained from its electromagnetic radiation in radio frequencies. The apparent size of the radio Sun depends of the frequency of the signal, since different radio frequencies originate from various atmospheric layers of the Sun.

See also: → radio; → sun.

A map or series of images of a region of sky obtained in → radio wavelengths of the → electromagnetic radiation.

See also: → radio; → survey.

A telescope whose receiver is sensitive to → radio waves.

See also: → radio; → telescope.

The → electromagnetic radiation with the longest → wavelengths (and lowest energies), ranging from 0.3 mm to several km. Radio waves form a very broad category, which includes the
→ submillimeter waves (with a wavelength of 0.3-1 mm) and → microwave regions (1 mm to several cm).

See also: → radio; → wave.

The → electromagnetic radiation with a wavelength ranging from 1 mm to several 100 km. See so → radio wave.

See also: → radio; → wavelength.

A range of electromagnetic radiation in the radio frequencies to which the Earth’s atmosphere is transparent.

See also: → radio; → window.

A quasar that has the same characteristics as a → radio-quiet quasar with the addition of having strong radio emissions.

Etymology (EN): → radio; loud, from O.E. hlud “making noise;” cf. M.Du. luut, Du. luid, O.H.G. hlut, Ger. laut “loud;” → quasar.

Etymology (PE): Kuâsâr, → quasar; râdioyi, adj. of → radio; xorušân “shouting aloud, roaring,” from xorušidan “to shout, cry aloud, roar;” Mid.Pers. xrôšitan “to shout.”

A type of quasar with weak radio emission. These types of quasars have strong emissions in both the optical and X-ray spectra. Within the optical spectrum, both broad and narrow emission lines are present. Their host is usually an elliptical galaxy, but less commonly, it might be a spiral.
→ radio-load quasar.

Etymology (EN): → radio; quiet, M.E., from O.Fr. quiete, from L. quies (genitive quietis) “rest, quiet;” → quasar.

Etymology (PE): Kuâsâr→ quasar; → radio; ârâm “quiet” (Mid.Pers. râm “peace,” râmenidan “to give peace, pleasure,” râmišn “peace, pleasure;” Av. ram- “to stay, rest;” cf. Skt. ram- “to stop, stand still, rest, become appeased;” Gk. erema “quietly, gently;” Goth. rimis “rest;” Lith. rãmas “rest”).

Possessing, or pertaining to, → radioactivity.

See also: → radio + → active.

Determining the age of an object from the → radioactive decay of its constituting material. The technique consists of comparing the → abundance ratio of a → radioactive isotope to its → decay product. This will yield the number of half-lives that have occurred since the sample was formed. More specifically, if an object is made up of 50 % decay product then it has gone through 1 → half-life. 75% decay product equals 2 half-lives, 87.5% decay product equals 3 half-lives, 93.76% decay product equals 4 half-lives, and so on. For example, the decay product of → uranium-238 (²³⁸U) is → lead-206 (²⁰⁶Pb).
The half-life of ²³⁸U is 4.5 billion years. Hence, if the sample has gone through two half-lives, it is 9 billion years old. See also: → radiocarbon dating.

See also: → radioactive; → dating.

Spontaneous emission by a nucleus of photons or particles.

See also: → radioactive; → decay.

A → nuclide that is radioactive.

See also: → radioactive; → isotope.

A → nuclide that disintegrates by emitting radiation and transforms into another nuclide.
Same as → radionuclide.

See also: → radioactive; → nuclide.

The radioactive by-products from the operation of a nuclear reactor or from the reprocessing of depleted nuclear fuel. Also known as nuclear waste.

See also: → radioactive; → waste.

The spontaneous disintegration of certain atomic nuclei, which is accompanied by the emission of either α- or β- particles and/or a γ rays.

See also: → radio; → activity.

A → radioactive → isotope of → carbon, especially → carbon-14.

See also: → radio; → carbon

A radioactive dating technique, applied to organic materials, which
measures the content of the radioactive isotope of carbon ¹⁴C.
The radioactive carbon isotopes created by the impact of cosmic rays with the nitrogen atoms of the atmosphere find their way, via carbon dioxide and photosynthesis, into living material. When an organic material dies it ceases to acquire further ¹⁴C atoms, and its ¹⁴C fraction declines at a fixed exponential rate due to the radioactive decay of ¹⁴C. Comparing the remaining ¹⁴C fraction of a sample to that expected from atmospheric ¹⁴C allows the age of the sample to be estimated.

See also: → radiocarbon; → dating.

An element, such as uranium, whose isotopes are all radioactive.

See also: → radio; → element.

The process of producing an image on a sensitive surface by radiation such as X-rays or gamma rays passing through an object.

See also: → radio + → -graphy.

A synonym for radionuclide.

See also: → radio; → isotope.

The study and use of radiation and radioactive substances for the treatment of diseases.

See also: → radio; + → -logy.

The dissociation of molecules by radiation, for example in a reactor core, when the water used for cooling breaks down into hydrogen and oxygen.

See also: From → radio + -lysis a combining form denoting “breaking down, loosening, decomposition,” → analysis.

An instrument that measures the intensity of radiant energy.

See also: → radio; → -meter.

A dating method that uses measurements of certain radioactive isotopes to calculate the ages in years (absolute age) of rocks and minerals.

See also: → radiometer; → dating.

The detection and measurement of radiant energy, either as separate wavelengths or integrated over a broad wavelength band, and the interaction of radiation with matter in such ways as absorption, reflection, and emission.

Etymology (EN): → radio + → -metry.

Etymology (PE): Tâbeš-sanji, from tâbeš, →radiation, + -sanj→ -metry.

A radioactive → nuclide.

See also: → radio + → nuclide.

A meteorological instrument that is carried aloft by a balloon to measure and send back information on atmospheric temperature, pressure, and humidity via radio to a ground receiving system.

Etymology (EN): → radio + sonde, from Fr. sonde “sounding line.”

Etymology (PE): → radio; gomâné “a probe, a shaft sunk in order to ascertain the depth of the water when making a subterraneous canal,” from Proto-Iranian *vi-mā-, from vi- “apart, away from, out” (cf. Av. vi-; O.Pers. viy- “apart, away;” Skt. vi- “apart, asunder, away, out;” L. vitare “to avoid, turn aside”) + mā- “to measure” (cf.
O.Pers./Av. mā(y)- “to measure;” Mod.Pers.
mâ/mun/mân “measure,” as in Pers. terms âzmâ- “to test;”
pirâmun “perimeter,” âzmun “test, trial,”
peymân “measuring, agreement,” peymâné “a measure; a cup, bowl;”
PIE base *me- “to measure;”
cf. Skt. mati “measures,” matra- “measure;” Gk. metron “measure;” L. metrum).

A radioactive metallic chemical element; symbol Ra. Atomic number 88; atomic weight 226.0254; melting point 700°C; boiling point 1,140°C. Discovered in 1898 by Marie Sklodowska Curie in an ore of pitchblende. In 1911 Curie and André Debierne successfully isolated radium by electrolysis.

See also: N.L., from L. rad(ius)" ray, beam" → radius

• -ium a noun suffix.

Of a circle, any straight line segment extending from the center to a point on the circumference.
Of a sphere, any straight line segment extending from the center to a point on the surface.
Of a regular polygon, the radius of the circle circumscribed about the polygon.

Etymology (EN): From L. radius “staff, spoke of a wheel, beam of light,” of unknown origin.

Etymology (PE): Šo’â’, loan from Ar.

Same as → Larmor radius.

See also: → radius; → gyration.

Math.: In a system of polar or spherical coordinates, a line joining a point to the origin.
Astro.: A line drawn from a central body to a satellite object in any position in its orbit.

See also: → radius; → vector.

The base of a number system; thus 2 is the radix of the binary system, 10 the radix of the decimal system, 12 the radix of the duodecimal system.

Etymology (EN): From L. radix “root;” akin to Gk. rhiza “root;”
cf. O.N. rot “root,” O.E. wyrt “plant, herb;” E. radish.

Etymology (PE): Pâyé “basis, foundation; step,” from pâ “foot, step” (from
Mid.Pers. pâd, pây; 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-).

A gaseous radioactive chemical element; symbol Rn. Atomic number 86; mass number of most stable isotope 222; melting point about -71°C; boiling point -61.8°C. Radon was discovered in 1900 by the German chemist Friedrich Ernst Dorn and it was first isolated in 1910 by the Scottish chemist William Ramsay and the English chemist Robert Whytlaw-Gray. The longest half-life associated with this unstable element is 3.8 day.

See also: The name indicates its origin from → radium. It had first been called radium emanation or just emanation (with chemical symbol Em) because it was a decay product of radium. Ramsay subsequently suggested the name “niton” (with chemical symbol Nt), which means “shining” in Latin. It was finally changed to radon in 1923.

A flat structure made up of a collection of logs or planks fastened together for floating or transportation on water.

Etymology (EN): M.E. rafte, rafter, from O.N. raptr “log.”

Etymology (PE): Sal “raft,” probably related to PIE base *sel-, *swel- “beam, board,” cf. Gk. selma “beam;” O.E. syll “beam, large timber,” O.N. svill “framework of a building,” M.L.G. sull, O.H.G. swelli, Ger. Schwelle “sill,” and also akin to Mid.Pers. sard “ladder,” Pers. dialectal variants (Lârestâni) se, si “ladder,” (Gilaki, Tâleqâni) sardi, (Qazvini) sorda, (Hamedâni) serda, (Kâšâni) sart, sârda, serde, and others all meaning “ladder.”

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

Etymology (EN): 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.”

Etymology (PE): 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.

Etymology (EN): → rain; → snow; → mix.

Etymology (PE): 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.

Etymology (EN): → rain; → cloud.

Etymology (PE): 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.

See also: 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°.

See also: → rainbow; → angle.

The sunlight incident on a tiny spherical droplet of water.

See also: → rainbow; → ray.

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

Etymology (EN): → rain + → fall.

Etymology (PE): 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.

Etymology (EN): Rake, etymology unknown.

Etymology (PE): 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.

Etymology (EN): 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.

Etymology (PE): 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.

See also: → 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).

See also: → ram; → pressure; → strip.

Same as → Raman scattering.

See also: 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.

See also: → Raman effect; → scattering.

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

See also: 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.

See also: 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.

Etymology (EN): M.E. raundon, random “impetuosity, speed,” from O.Fr. randon “rush, disorder, impetuosity,” from randir “to run fast.”

Etymology (PE): 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.

See also: → 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.

See also: → random; → error.

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

See also: → 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.

See also: → random; → noise.

A sample selected at random from a population.

See also: → random; → sample.

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

See also: → random; → structure.

The agitated motion of molecular, atomic, or → subatomic particles in all possible directions at any temperature, except at → absolute zero, where → thermal motion would cease.

See also: → random; → thermal; → motion.

A quantity that takes different real values as a result of the → outcomes of a → random event or experiment involving specified probabilities.

See also: → random; → variable.

The trajectory consisting of a series of successive moves
in which the direction and size of each move is randomly determined.

See also: → random; → walk.

Arrangement of data in such a way as to simulate chance occurrence.

See also: Verbal noun of → randomize.

To arrange or select in a random manner in order to reduce bias and interference caused by irrelevant variables.

See also: Verbal form of → random.

The property of being random.

See also: State, condition noun of → random.

1. Physics: The maximum distance a projectile travels.
   

2. Math.: The → set of values that actually comes out of a
   → function. The range is a → subset of the → codomain.
   

3. Statistics: The interval between the largest and smallest values in a statistical distribution.

Etymology (EN): M.E., from O.Fr. range “range, rank,” from rangier “to place in a row, arrange,” from reng “row, line.”

Etymology (PE): 1) Bord past stem of bordan “to carry, transport” (Mid.Pers. burdan,
O.Pers./Av. bar- “to bear, carry,” barəθre “to bear (infinitive),” Skt. bharati “he carries,” Gk. pherein, L. fero “to carry;” PIE base *bher- “to carry”).
2) Gostaré, from gostar, gostardan “to expand; to spread; to diffuse” (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”).

Position, in a series arranged in order, on the basis of some principle of arrangement, with reference to the other items or values in the series.

Etymology (EN): M.E., from O.E. ranc “proud, overbearing, showy,” from O.Fr. renc, ranc, rang “row, line;” cf. Dan. rank “right, upright,” Ger. rank “slender,” O.N. rakkr “straight, erect,” perhaps from PIE *reg- “to stretch, straighten,” cognate with Pers. râst, → right.

Etymology (PE): Rotbé, loan from Ar. ratbat “rank.”

A temperature scale in which the degree intervals are the same size as in the → Fahrenheit scale, but 0 is set at absolute zero, -459.69 °F. Therefore, 1 degree Rankine is equal to exactly 5/9 → kelvin.
It was formerly used by engineers in English-speaking countries, but is now obsolete. See also → Celsius scale,
→ Kelvin scale, → Reaumur scale.

See also: Named for the British physicist and engineer William John Rankine (1820-1872); → scale.

Hydrodynamics → conservation laws (which can be extended to → magnetohydrodynamics, MHD) which describe the physical conditions of material across a → shock front. A fluid is completely described by its velocity, density, pressure, specific heat ratio, and magnetic field (in the MHD case). Mass, momentum, and energy fluxes are conserved in the shock, leading to the Rankine-Hugoniot relations. Also called Rankine-Hugoniot jump conditions. See also → jump condition.

See also: Named after William John Rankine, → Rankine scale, and Pierre Henri Hugoniot, → Hugoniot curve; → condition.

The → vapor pressure of an ideal → solution is dependent on the vapor pressure of each chemical component and the → mole fraction of the component present in the solution. This means that the addition of → solute to a liquid lessens the tendency for the liquid to become a → solid or a → gas. For example, the addition of → salt to water causes the water to freeze below its normal → freezing point (0°C) and to boil above its normal → boiling point (100°C).

See also: After François-Marie Raoult (1830-1901), the French chemist who studied
the physical properties of chemical solutions; → law.

Occurring within a short time; happening speedily; moving or acting with great speed; swift (Dictionary.com).

Etymology (EN): From L. rapidus “tearing away, seizing, swift,” from rapere “to hurry away, seize, plunder;”

Etymology (PE): Tond “swift, rapid, brisk; fierce, severe” (Mid.Pers. tund “sharp, violent;” Sogdian tund “violent;” cf. Skt. tod- “to thrust, give a push,” tudáti “he thrusts;” L. tundere “to thrust, to hit” (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere “to thrust or bore through;”
PIE base *(s)teud- “to thrust, to beat”).

An object with technical designation MXB 17302335 which is characterized by erratic and extremely intense → X-ray emissions. The Rapid Burster is a → binary system comprising a → low-mass star as its → primary and a → secondary  → neutron star. The → gravitational attraction of the neutron star strips its → companion of some of its gas, which then forms an → accretion disk and spirals toward the neutron star. The Rapid Burster is located at a distance of 110 kpc in the highly reddened → globular cluster Liller 1. It is a → low-mass X-ray binary (LMXB) and a recurrent → X-ray transient. So far less than 200 LMXBs have been detected in the → Galaxy and the → Magellanic Clouds. All produce a persistent flux of X-rays, the result of a release of → gravitational potential energy. Approximately 40 of them also exhibit → Type I bursts which are due to → thermonuclear flashes on the surface of a neutron star. The Rapid Burster is unique among the LMXBs in that it produces X-ray bursts in quick succession. These are called → Type II bursts, and they result from a spasmodic release of gravitational potential energy, which is due to some unknown → accretion disk instability (Lewin et al., 1996, ApJ 462, L39).

See also: → rapid; → burster.

A chemically peculiar star characterized by the presence of high-frequency non-radial oscillations, with periods that range between about 4 and 16 min. These variations have periods from about 5 to 20 minutes and low amplitudes (B < 10 mmag). They are consistent with acoustic (→ p mode) pulsations of low degree and high radial overtone.

See also: → rapid; → -ly; → oscillate; → -ing; → Ap star.

A part of a river where the current runs very swiftly. See also → torrent.

Etymology (EN): → rapid.

Etymology (PE): Tondâb, from tond, → rapid, + âb, → water.

Not occurring very often; not found in large numbers.

Etymology (EN): M.E., from O.Fr. rere “sparse” from L. rarus “thinly sown; not thick; having intervals between.”

Etymology (PE): Perz, in several Iranian languages and dialects “little, few, small, minute part of any thing” (Khonsâri: perz, Qatrâni: perzema,
Birjandi: porzu, Farâmarzâni: pesk, Kurd petik, Tabari: perik “minute quantity, particle”); maybe ultimately from Proto-Ir. *paraka- “small part,” → part.
Kamyâb “rare, difficult to find,” from kam “little, few; deficient, wanting; scarce”
(Mid.Pers. kam “little, small, few,” O.Pers./Av. kamna- “small, few”);

• yâb present stem of yâftan, yâbidan “to find, discover; to obtain, acquire”
  (Mid.Pers. ayâftan, ayâpênitan “to reach, attain;” Manichean Mid.Pers. ‘y’b “to attain;” Parthian, Sogdian (+ *pati-) pty’b “to reach, obtain;” Av. ap- “to reach, overtake,” apayeiti “achieved, reached;” Skt. âp- “to reach, gain,”
  âpnoti “reaches, gains;” Gk. hapto, haptomai “to touch, cling to, adhere to;” L. apiscor “touch, reach;” PIE base *ap- “to take, reach”).

Any of the group of metallic → chemical elements with → atomic numbers between 57 and 71 inclusive. The name is an inappropriate terminology, since they are neither rare nor earth; preferred name → lanthanide.

See also: → rare; → earth; → element.

Another name for → inert gas.

Etymology (EN): M.E., from O.Fr. rere “sparse,” from L. rarus “loose, wide apart, thin, infrequent;” &rar; gas.

Etymology (PE): Kamyâb “rare, difficult to find,” from kam “little, few; deficient, wanting; scarce”
(Mid.Pers. kam “little, small, few,” O.Pers./Av. kamna- “small, few”);

• yâb present stem of yâftanyâftan, yâbidan “to find, discover; to obtain, acquire”
  (Mid.Pers. ayâftan, ayâpênitan “to reach, attain;” Manichean Mid.Pers. ‘y’b “to attain;” Parthian, Sogdian (+ *pati-) pty’b “to reach, obtain;” Av. ap- “to reach, overtake,” apayeiti “achieved, reached;” Skt. âp- “to reach, gain,”
  âpnoti “reaches, gains;” Gk. hapto, haptomai “to touch, cling to, adhere to;” L. apiscor “touch, reach;” PIE base *ap- “to take, reach”).

The state of being rarefied, less dense.
An instantaneous reduction in density of a gas resulting from passage a sound wave;
opposite of → compression.

Etymology (EN): M.E. rarefien, from M.Fr. rarefier, from L. rarefacere “make rare,” from rarus “loose, wide apart, thin, infrequent.”

Etymology (PE): Verbal noun from âlar present stem of âlaridan→ rarefy + -š, a suffix.

A pressure wave in a fluid generated by rarefaction. It travels in the opposite direction to that of a shock wave in the medium.

See also: → rarefaction; → wave.

A gas whose pressure is much less than a reference pressure.

See also: Past participle of → rarefy; → gas.

1. To make less dense.
   
2. To become less dense; become thinned.
   Related concepts: → attenuate, → dilute, → decompress.

Etymology (EN): M.E. rarefien, from M.Fr. rarefier, from rare, combining form of rarus “loose, wide apart, thin, infrequent.”

Etymology (PE): Alaridan, infinitive of âlar, from intensive/nuance prefix â- + lar “thin, meagre” (Dehxodâ), Lori, Laki larr, larrek “thin cow or sheep,” Kurd. lerr “thin, lean,” variants of laqar (Torbat-Heydariyé), lâqar “lean, meagre, slender; weak.”

The brightest star in the constellation → Hercules. It is a → red supergiant of type M5 (surface temperature about 3300 K) lying at a distance of 380 light-years. Rasalgethi is a variable star with a mean magnitude of V = 3.48. It has a fifth magnitude companion 5’’ away. This secondary is itself a double that consists of a 4 solar-mass class G5 giant star with a temperature about that of the Sun and a 2.5 solar-mass F2 dwarf star (somewhat hotter than the Sun) in orbit around each other separated by 0.4 AU with a 52 day period.

See also: Rasalgethi, from Ar. Ra’s al-Jathi (رأس‌الجاثی) “the Kneeler’s Head,” in reference to an early name for the constellation, the figure of the man seen upside down, his head toward the south, from ra’s “head” + jathi “kneeling.”

The brightest star (V = 2.08) in the constellation → Ophiuchus. Rasalhague is a
→ giant star of type A5 lying 47 light-years from Earth. It has a faint, very close companion only 0’’.5 away, 7 A.U., that orbits with a period of 8.7 years.

See also: Rasalhague, from Ar. Ra’s al-Hayyah (رأس‌الحیه), literally the “head of the snake,” from ra’s “head” + Hayyah “snake,” referring to the creature in Gk. mythology.

Any of various long-tailed rodents resembling mice but larger, especially one of the genus Rattus (TheFreeDictionary.com).

Etymology (EN): From M.E. rat, rotte, from O.E. ræt, of unknown origin.

Etymology (PE): Gerz, from (Lori, Laki) gerza “rat, big mouse,”
Gilaki (Lâhijân, Gâleš, Bandar-Anzali, etc.)
gerze “rat; mouse,” variant (Mâzandarân) gal “mouse,” of unknown origin.

The amount of change of some quantity during a time interval divided by the length of the time interval.

Etymology (EN): M.E. rate “monetary value,” M.Fr. rate “price, value,” from M.L. rata (pars) “fixed (amount),” from L. rata “fixed, settled,” p.p. of reri “to reckon, think,” → reason.

Etymology (PE): Nerx “rate, price, tariff.”

Math.: The quotient of two quantities arrived at by dividing one by the other.

Etymology (EN): From L. ratio “reckoning, calculation,” also “reason,” from rat-, p.p. stem of reri “to reckon, calculate,” also “think,” → reason.

Fr. rapport, back-formation from rapporter “bring back,” from → re- “back, again” + apporter “to bring,” from L. apportare “to bring,” from → ad- “to” + portare “to carry.”

Etymology (PE): Vâbar, on the model of Fr. rapport “bringing back,” as above, from vâ-
prefix meaning “back to the original place, again,” → re-,

• bar present stem of bordan “to carry, lead” (Mid.Pers. burdan,
  O.Pers./Av. bar- “to bear, carry,” barəθre “to bear (infinitive),” Skt. bharati “he carries,” Gk. pherein, L. fero “to carry;” PIE base *bher- “to carry”).

1. Having or exercising the ability to → reason.
   
2. Consistent with or based on reason; logical; in accordance with sound reasoning.
   
3. Math.: Capable of being expressed as a quotient of integers.

Etymology (EN): M.E. racional, from O.Fr. racionel, from L. rationalis “of or belonging to reason, reasonable,” from ratio (genitive rationis) “reckoning, calculation, reason,” from rat-, pp. stem of reri “to reckon, calculate; consider, think.”

Etymology (PE): 1) Râyani, of or pertaining to râyan, → reason.
2) Xeradâné, xeradmandâné, of or pertaining to xerad “understanding, judgement, intellect, wisdom;” Mid.Pers. xrad “reason, intellect, intelligence, wisdom, understanding;” O.Pers. xraθu- “wisdom;” Av. xratu- “intelligence, undestanding, wisdom; will, purpose, council;” cf. Skt. krátu- “power, will-power;” Gk. kratos “power, strength.”

3) vâbari, of or pertaining to vâbar, → ratio.

Any number that can be expressed as a ratio of two integers, providing the second number is not zero.

See also: → rational; → number.

1. The fundamental reason or reasons serving to account for something.
   

2. A statement of reasons.
   

3. A reasoned exposition of principles (Dictionary.com).

Etymology (EN): L. neuter of rationalis.

Etymology (PE): Râyanâl, from râyan, → reason, + -âl, → -al.

A philosophical doctrine that holds that → reason alone, unaided by experience, can arrive at basic truth regarding the world.

Etymology (EN): From → rational + -ism a Gk. suffix used in the formation of nouns denoting action or practice, state or condition, principles, doctrines, and so forth.

Etymology (PE): Râyan, → reason; xerad, → rational; bâvari, from bâvar “belief” (Mid.Pers. wâbar “beleif;” Proto-Iranian *uar- “to choose; to convince; to believe;” cf. Av. var- “to choose; to convince” varəna-, varana- “conviction, faith;” O.Pers. v(a)r- “to choose; to convince;” Skt. vr- “to choose,” vara- “choosing”).

Any of several large, corvine birds having lustrous, black plumage and a loud, harsh call, especially Corvus corax, of the New and Old Worlds (Dictionary.com).

Etymology (EN): O.E. hræfn, hrefn; hræfn, cognate with O.Norse hrafn, Dan. ravn, Du. raaf, Ger. Rabe “raven,” ultimately from PIE root *ker- imitative of harsh sounds (source also of L. crepare “to creak, clatter,” cornix “crow,” corvus “raven;” Gk. korax “raven,” korone “crow;” O.C.S. kruku “raven;” Lith. krauklys “crow,” related to Pers. kalâq, as below.

Etymology (PE): Kalâq, ultimately from Proto-Iranian*karak-, from *kar- “raven’s harsh sound;” cf. Pers. qâr, qârqâr “croak,” cognate with L. corvus, Gk. korax “a raven,” Skt. kâkah, E. raven, as above and → crow.

Data that are unprocessed or not yet subjected to analysis.

Etymology (EN): Raw, from M.E., from O.E. hreaw “uncooked, raw;” cf. M.Du. rau, O.H.G. hrawer, Ger. roh; PIE base *krowos “congealed, bloody” cf. Skt. kravis- “raw flesh,” krura- “bloody, raw, hard;” Gk. kreas “flesh;” L. crudus “not cooked,” cruor “thick blood;” Lith. kraujas, O.C.S. kruvi “blood;” → data.

Etymology (PE): Dâdehâ→ data; xâm “crude, uncooked;” Mid.Pers. xâm “crude, raw” (Khotanese hāma- “raw;” Ossetic xom “raw;” Pash. ôm “raw”); cf. Skt āmá- “raw, uncooked;” Gk. omos “raw, uncooked.”

1. Any of the lines or streams in which light appears to radiate from a luminous body. → light ray.
   

2. The straight line normal to the wave front in the propagation of radiant energy.
   

3. A stream of material particles all moving in the same straight line.

Etymology (EN): M.E. raie, raye, from O.Fr. rai “ray, spoke,” from L. → radius “ray, spoke, staff, rod.”

Etymology (PE): Partow, ultimately from Proto-Iranian *pari-tap- “to shine around, radiate away.” The first component *pari- “around, about;” cf. Mod.Pers. par-, pirâ- “around, about,” from Mid.Pers. pêrâ; O.Pers. pariy “around, about,” Av. pairi “around, over,” per- “to pass over, beyond;”
Skt. pari; Gk. peri “around, about,
beyond;” L. per “through;” PIE base *per- “through, across, beyond.” The second component *tap- “to shine, radiate;” cf. Mod.Pers.
tâbidan, variants tâftan “to shine,” tafsidan “to become hot;” Mid.Pers. tâftan “to heat, burn, shine;” taftan “to become hot;” Parthian t’b “to shine;”
Av. tāp-, taf- “to warm up, heat,” tafsat “became hot,” tāpaiieiti “to create warmth;” cf. Skt. tap- “; to heat, be/become hot; to spoil, injure, damage; to suffer,” tapati “burns;” L. tepere “to be warm,” tepidus “warm;” PIE base *tep- “to be warm.”

In → rainbows, a sun ray directly reflected from the surface of a water → droplet.

See also: → ray; → class.

In → rainbows, a sun ray transmitted directly through a water → droplet.

See also: → ray; → class.

In → rainbows, a sun ray that emerges from a water → droplet after one internal reflection. Rays of class 3 give rise to the → primary rainbow.

See also: → ray; → class.

In → rainbows, a sun ray that emerges from a water → droplet after two internal reflections. Rays of class 4 give rise to the → secondary rainbow.

See also: → ray; → class.

A c.g.s. unit of light intensity used in astronomy and physics to measure the brightness of the night sky, auroras, etc. One rayleigh (R) represents
the light intensity of one million photons of light emitted in all directions per square centimeter of receiver per second; or, in SI units, 795.775 x 10⁶ photons per square meter per steradian (m^-2·sr^-1). A dark night sky has a light intensity of roughly 250 R.

See also: In honor of the English mathematician and physicist Lord Rayleigh (1842-1919), surname of John William Strutt, Third Baron Rayleigh,
whose research ranged over several fields of physics.

1. A straight line that connects the points corresponding to the initial and final states on a graph of pressure versus specific volume for a substance subjected to a → shock wave. The slope of the Rayleigh line is proportional to the square of shock speed. Steeper Rayleigh lines correspond to higher shock speeds. See also → Hugoniot curve.
   

2. In → Raman scattering, the spectral line in scattered radiation which has the same frequency as the corresponding incident monochromatic radiation.

See also: → rayleigh; → line.

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.

See also: → 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.

See also: → rayleigh; → scattering.

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.

See also: → Rayleigh; → Jeans;
→ law.

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

See also: → 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.

See also: → rayleigh; → Taylor number; → instability.

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).

See also: → Rayleigh; → criterion.