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Robertson-Walker metric metrik-e Robertson-Walker (#) Fr.: métrique de Robertson-Walker The mathematical description of the interval (→ space-time
separation) between → events ("points" in space-time)
in a → homogeneous and
→ isotropic → Universe.
It results from an exact solution of
→ Einstein's field equations
of → general relativity.
Under the assumptions, the
Robertson-Walker interval is expressed by: Named after Howard Percy Robertson (1903-1961), American mathematician and physicist, and Arthur Geoffrey Walker (1909-2001), British mathematician and physicist, for their contributions to physics and physical cosmology; → metric. |
robot robot (#) Fr.: robot A machine that does mechanical, routine tasks on command. From Czech, coined by Karel Čapek in the play R.U.R. (1920), from the base robot-, as in robota "compulsory labor," robotník "peasant owing such labor," from robotiti "to work, drudge." Robot, loan from E., as above. |
robust tanâvar (#) Fr.: robuste The quality of a model when it is insensitive to small discrepancies in assumptions. From L. robustus "strong and hardy," literally "as strong as oak," from robur, robus "hard timber, strength," also "a special kind of oak," named for its reddish heartwood, from L. ruber, → red. Tanâvar "robust, stout, corpulent," from tan "corpus, body," → if and only if + âvar contraction of âvarandé agent noun of âvardan "to bring; to cause, produce," → collect. |
Roche limit hadd-e Roche Fr.: rayon de Roche The smallest distance at which a → satellite under the influence of its own → gravitation and that of a central mass about which it is describing a → Keplerian orbit can be in equilibrium. This does not, however, apply to a body held together by the stronger forces between atoms and molecules. At a lesser distance the → tidal forces of the → primary body would break up the → secondary body. The Roche limit is given by the formula d = 1.26 R_{M} (ρ_{M}/ρ_{m})^{1/3}, where R_{M} is the radius of the → primary body, ρ_{M} is the → density of the primary, and ρ_{m} is the density of the secondary body. This formula can also be expressed as: d = 1.26 R_{m} (M_{M}/M_{m})^{1/3}, where R_{m} is the radius of the secondary. As an example, for the Earth-Moon system, where R_{M} = 6,378 km, ρ_{M} = 5.5 g cm^{-3}, and ρ_{m} = 2.5 g cm^{-3} is 1.68 Earth radii. Named after Edouard Albert Roche (1820-1883), the French astronomer who first calculated this theoretical limit in 1848; → limit. |
Roche lobe lap-e Roche Fr.: lobe de Roche The region around a star in a → binary system within which orbiting material is gravitationally bound to that star. The point at which the Roche lobes of the two stars touch is called the → inner Lagrangian point. → equipotential surface. → Roche limit; → lobe. |
Roche lobe overflow (RLOF) sarriz-e lap-e Roche Fr.: débordement du lobe de Roche A process in a → binary system when a star fills its → Roche lobe, often by becoming a → giant or → supergiant during the later stages of → stellar evolution. When the star expands, any material that passes beyond the Roche lobe will flow onto the binary → companion, often by way of an → accretion disk. This occurs through the → inner Lagrangian point where the gravity of the two stars cancels. The RLOF is responsible for a number of phenomena including → cataclysmic variables, → Type Ia supernovae, and many → X-ray binary systems. → Roche lobe; → overflow. |
rock bard (#), sang (#) Fr.: roche 1) A large mass of → stone forming a hill, cliff,
promontory, or the like. M.E. rokk(e), from O.Fr. ro(c)que, roche (cf. Sp., Provençal roca, It. rocca, M.L. rocca, V.L. *rocca, of uncertain origin. Bard (Dehxodâ) "rock, stone," used in a large part of Western Iran, specifically in Lori and Kurd., related to Kurd. pal "rock, stone;" cf. Gk. poros "rock." Sang, → stone. |
rock crystal bolur-e sangi, bard-bolur Fr.: cristal de roche Pure natural crystalline form of → silica, SiO_{2}, which is transparent and colorless. |
rocket roket Fr.: fusée A projectile driven by reaction propulsion that carries its own propellants.
→ missile = mušak ( From It. rocchetto "a rocket," literally "a bobbin," diminutive of rocca "a distaff," with reference to its shape. |
rocket astronomy axtaršenâsi bâ roket Fr.: astronomie par fusée The study of celestial bodies in the wavelengths that are almost completely absorbed by the atmosphere, by using a rocket to carry instruments above 250 km to measure the searched for phenomena. |
rocketry roket šenâsi, roketgari Fr.: fuséologie The science of rocket design, development, and flight. → rocket + -ry a noun suffix. Roket šenâsi, from roket, → rocket, + šenâsi, → -logy; roketgari with suffix -gari, from -gar, → -or. |
rockoon roklon Fr.: fusée-sonde lancée à partir d'un ballon A rocket launched from a balloon at a pre-determined height and fired by a ground-controlled radio relay when some particular event, e.g. a solar flare, occurs. From rocket + balloon. From roket + bâlon, → ballon astronomy. |
Roemer's measurement andâzegiri-ye Rømer Fr.: mesure de Rømer The first successful measurement of the → speed of light carried out by the Danish astronomer Ole Rømer in 1675 at Paris Observatory. Astronomers knew that the mean period of revolution for Jupiter's innermost satellite → Io (Jupiter I) was 42.5 hours. During this period Io was sometimes eclipsed by Jupiter. Astronomers expected that if Io was visible at some time it must be visible 42.5 hours later. But Ole Rømer discovered that there were many irregularities in Io's orbital period. Sometimes Io appeared too early and other times too late in relation to the expected times. The irregularities repeated themselves precisely at a one-year interval, which meant that they must be connected to the Earth's rotation around the Sun. Rømer attributed this difference in time to the additional distance the light from Io had to travel at different times, and used this information to calculate the speed of light. He found that it takes light 22 minutes to traverse the Earth's orbital diameter; the correct figure was later determined to be 16 minutes and 40 seconds. Rømer was able to measure the speed of light to be 230,000 km s^{-1}. Although this figure was very close to the currently accepted value of 300,000 km s^{-1}, it was rejected by the scientific community of the time, who assumed it to be much too high a figure. Ole Rømer (1664-1710); → measurement. |
roentgen rontgen (#) Fr.: roentgen A unit of radiation exposure defined as a charge release rate of 258 micro-coulombs per kilogram of air. Named after the German physicist Wilhelm Konrad Röntgen (1845-1923), one of the early investigators of radioactivity. |
roentgenium rontgeniom (#) Fr.: roentgenium An artificially produced radioactive chemical element; symbol Rg. Atomic number 111; mass number of most stable isotope 272; melting point, boiling point, specific gravity, and valence unknown. Named after the German physicist Wilhelm Konrad Röntgen (1845-1923), one of the early investigators of radioactivity. |
rogue velgard (#) Fr.: vagabond A vagabond or tramp. A dishonest or unprincipled person. Perhaps short for obsolete roger "begging vagabond." Velgard "vagabond, roamer, tramp." |
rogue planet sayyâre-ye velgard Fr.: planète vagabonde Same as → free-floating object. |
roll cloud abr-e lule-vâr Fr.: nuage en rouleau A low, horizontal, tube-shaped, and relatively rare type of → arcus cloud. M.E. scroll, inscribed scroll, register, cylindrical object < OF ro(u)lle M.E. rolle, from O.Fr. roule, rolle, from M.L. rotulus "a roll of paper," from L. rotula "small wheel," diminutive of rota "wheel;" → cloud. Abr, → cloud; lule-vâr "tube like," from lulé "tube, pipe," related to lulidan "to roll, rotate; to stir, vibrate" + -vâr suffix of resemblance. |
Rolle's theorem farbin-e Rolle Fr.: théorème de Rolle If a function f(x) is → continuous on an interval [a,b] and is → differentiable at all points within this interval, and vanishes at the end points x = a and x = b, that is f(a) = f(b) = 0, then inside [a,b] there exists at least one point x = c, a < c < b, at which the derivative f'(x) vanishes. Named after Michel Rolle (1652-1719), a French mathematician; → theorem. |
Roman calendar gâhšomâr-e Rumi Fr.: calendrier romain Any of several → lunar calendars used by Romans before the advent of the → Julian calendar in 46 B.C. The original Roman calendar, which had 10 months and 304 days, went back to the Greek calendar, although Romulas, the ruler of Rome, is given credit for starting the Roman calendar. Originally, the Roman calendar started the year in March with the → vernal equinox. The Roman calendar went through several changes from 800 B.C. to the Julian calendar. The 800 B.C. calendar had 10 months and a winter period, with a year of 304 days. In this calendar, the first month, March, was followed by Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, December, and Winter. The months starting with and following Quintilis all used the Latin numbers for names. Finally, for political reasons, the Romans made a change around 150 B.C. when they started using January as the beginning of their calendar year. Around 700 B.C. the 304 day calendar was expanded to 355 days by adding the months of February and January to the end of the year. Later in 450 B.C., January was moved in front of February. Finally, in 150 B.C. the Romans began to use January as the beginning of the calendar year. This calendar was replaced by the Julian calendar in 46 B.C. From L. Romanus "of Rome, Roman," from Roma "Rome," of uncertain origin. |
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