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

The act of revoking; annulment.

Verbal noun of → revoke.

1) To take back or withdraw; annul, cancel, or reverse; rescind or repeal.
2) To bring or summon back (Dictionary.com).

M.E., from O.Fr. revoquer, from L. revocare "rescind, call back," from → re- "back," + vocare "to call," from vox, → voice.

Vâvacidan, from vâ-, → re-, + vacidan "to call," → convoke.

The movement of a celestial body which is in orbit around another. It is often measured as the → orbital period.

Verbal noun of → revolve.

To move in a an orbit around another object. Compare with → rotate.

M.E. revolven, from L. revolvere "to turn, roll back," from → re- "back, again" + volvere "to roll."

Gardidan variants gaštan "to turn, to change," dialectal gel, gelidan, gellidan "to turn;" gardun "wheel; chariot;" (+ → ni-) navardidan, navard- "to twist; to fold; to stroll;" Mid.Pers. wardidan, walt- "to turn; twist;" Av. varət- "to turn, revolve;" cf. Skt. vart- "to turn, roll," vartate "it turns round, rolls;" L. vertere "to turn;" O.H.G. werden "to become;" PIE base *wert- "to turn, bend."

A dimensionless quantity that governs the conditions for hydrodynamic stability and the occurrence of turbulence in fluids. It is defined by the ratio, R, of the inertial force (ρ u²) and the viscous force (μ u / L), i.e. R = L u ρ/μ, where L is a typical dimension of the system, u is a measure of the velocities that prevail, ρ the density, and μ the kinematic viscosity. At low Reynolds numbers the flow is steady, since the viscous forces are predominant in controlling the flow. At a critical value of R, corresponding to a critical velocity, the flow becomes turbulent.

Named after Osborne Reynolds (1842-1912), a British physicist who pioneered the study of turbulent flows; → number.

Same as → red horizontal branch star.

→ red; → horizontal; → branch; → star.

The second largest satellite of Saturn, also known as Saturn V. It has a diameter of 1530 km and orbits Saturn at a mean distance of 527,040 km in 4.518 days. Rhea is composed primarily of water ice with rock making up less than 1/3 of its mass. It was discovered by Cassini in 1672.

In Gk. mythology Rhea was the sister and wife of Cronus (Saturn) and the mother of Demeter, Hades (Pluto), Hera, Hestia, Poseidon (Neptune), and Zeus (Jupiter).

A metallic chemical element; symbol Re. Atomic number 75; atomic weight 186.207; melting point about 3,180°C; boiling point about 5,625°C. Rhenium is a very dense, high-melting, silver-white metal; specific gravity 21.02 at 20°C. Of the elements, only carbon and tungsten have higher melting points and only iridium, osmium, and platinum are more dense. It was discovered by x-ray spectroscopy in 1925 by the German chemists, Walter Noddack, Ida Tacke and Otto Berg.

The name derives from L. rhenus for "the Rhine river in Germany".

A branch of physics dealing with the way matter flows and deforms.

From rheo- "flow, current, stream," from Gk. rheos "stream," cognate with Pers. rud "river," → Eridanus, + → -logy.

Ravâné, from ravân "flowing, running," pr.p. of raftan "to go, walk; to flow;" (Mid.Pers. raftan, raw-, Proto-Iranian *rab/f- "to go; to attack") + šenâsi, → -logy.

Relating to a constraint or system that contains time explicitly. For example, a pendulum with an extensible string of length l rheonomous, the condition of constraint is: x² + y² = l²(t), where l(t) is the length of the string at time t.

From Gk. rheo-, from rheos "a flowing, stream, current," → rheology, + -nomous, → -nomy.

A → yellow hypergiant in the constellation Cassiopeia, classified as F8-G2 Ia0pe. Rho Cas is about 11,650 light-years away from Earth, yet can still be seen by the naked eye, as it is 550,000 times as luminous as the Sun. It is in fact one of the most luminous stars in our Galaxy. ρ Cas is also one of the only seven currently known yellow hypergiants in the Milky Way. It is variable and fluctuates around magnitude 5 but dimmed to 6th magnitude on 1946. Its last eruption happened in 2000-2001, during which the star ejected about 10 percent of a solar mass, dimmed by more than a full magnitude, and changed its spectral type from late F to early M. During this outburst, the star surface cooled from 7,000 to 4,000 K.

Rho, a Gk. letter of alphabet used in → Bayer designation; Cassiopeiae, genitive of → Cassiopeia.

A complex region of molecular and dust clouds containing emission and reflection nebulae near the star ρ Oph in the constellation → Ophiuchus. It is one of the closest star forming regions, some 400 light-years distant. Recent studies using the latest X-ray and infrared observations reveal more than 300 young stellar objects within the large central cloud. Their median age is only 300,000 years.

Rho, Gk. letter; → Ophiuchus; → cloud.

A metallic chemical element; symbol Rh. Atomic number 45; atomic weight 102.9055; melting point about 1,966°C; boiling point 3,727±100°C; specific gravity 12.41 at 20°C. Rhodium was discovered in 1803 by the English chemist and physicist William Hyde Wollaston during experiments on crude platinum ore.

The name derives from Gk. rhodon "rose" because of the "rose color of dilute solutions of its salts."

Shaped like a rhombus.

From rhomb, → rhombus, + → -ic.

A quadrilateral having all sides equal and all angles oblique.

L.L. rhombus, from Gk. rhombos "rhombus, spinning top," from rhembesthai "to spin, whirl."

Lowzi, resembling a lowz "almond."

1) An ordered recurrent alternation of strong and weak elements in the flow of sound and silence in speech; a particular example or form of rhythm.
2) The aspect of music comprising all the elements (as accent, meter, and tempo) that relate to forward movement.
3) A regularly recurrent quantitative change in a variable biological process (Merriam-Webster.com).

From L. rhythmus "movement in time," from Gk. rhythmos "measured flow or movement, rhythm; proportion, symmetry; arrangement," related to rhein "to flow," from PIE root *sreu- "to flow"

Ritm, loan from Fr.

The simplest curvature invariant for a → Riemannian manifold. It is derived from the → Ricci tensor R_μν ≡  R^α_μαν by contracting indices. Taking the trace of the Ricci tensor gives the Ricci scalar: R ≡ R_μνg^μnu; = R^μ_ν = R^αμ_αμ. Also called → scalar curvature.

→ Ricci tensor; → scalar.

A → rank 2, → symmetric tensor R_μν that is a contraction of the → Riemann curvature tensor R^λ_μνλ. More specifically, R_μν ≡ Σ (λ) R^λ_μνκ = R^λ_μνκ. Closely related to the Ricci tensor is the → Einstein tensor, which plays an important role in the theory of → general relativity.

Named after the Italian mathematician Gregorio Ricci-Curbastro (1853-1925); → tensor.

Having large amounts of something specified. → metal-rich environment, → rich cluster; → enrich, → enrichment, → richness, → poor.

M.E., from O.E. rice "wealthy, powerful" (cf. Du. rijk, Ger. reich "rich"), from PIE base *reg- "move in a straight line," hence, "to direct, rule" (cf. Mod.Pers./Mid.Pers. râst "right, straight;" O.Pers. rāsta- "straight, true," rās- "to be right, straight, true;" Av. rāz- "to direct, put in line, set," razan- "order;" Skt. raj- "to direct, stretch," rjuyant- "walking straight;" Gk. orektos "stretched out;" L. regere "to lead straight, guide, rule," p.p. rectus "right, straight;" Ger. recht; E. right).

Por "full, much, very, too much" (Mid.Pers. purr "full;" O.Pers. paru- "much, many;" Av. parav-, pauru-, pouru-, from par- "to fill;" PIE base *pelu- "full," from *pel- "to be full;" cf. Skt. puru- "much, abundant;" Gk. polus "many," plethos "great number, multitude;" O.E. full); pordâr, literally "having much possession," from por + dâr "having, possessor," from dâštan "to have, to possess," → property.

A → galaxy cluster with a particularly large number of galaxies.

→ rich; → cluster.