teleskop-e Ritchey-Chrétien, durbin-e ~
Fr.: télescope Ritchey-Chrétien
A type of → Cassegrain telescope in which the → primary mirror is a → hyperboloid. It is designed to eliminate → coma and → spherical aberration, thus providing a relatively large field of view as compared to a more conventional configuration.
Named after the American astronomer George Ritchey (1864-1945) and the French optician Henri Chrétien (1879-1956); → telescope.
Ritz combination principle
parvaz-e miyâzeš-e Ritz
Fr.: principe de combinaison de Ritz
An empirical rule discovered before the advent of quantum mechanics which states that it is possible to find pairs of spectral lines, which have the property that the sum of their wavenumbers is also an observed spectral line.
1) A person who is competing for the same object or goal as another, or who
tries to equal or outdo another; competitor.
From L. rivalis "a rival, adversary; neighbor," originally, "one who uses a stream in common with another," from riv(us) "stream, brook," + -alis, → -al.
Hamâvard "a rival; an adversary in a combat," from ham- "together," → com-, + âvard "battle, struggle," variants nabard, nibard, nâvard "fight, struggle, war," ultimately from Proto-Ir. *part- "to fight, to struggle."
1) The action, position, or relation of a rival or rivals; competition.
Noun from → rival.
A large natural stream of water flowing in a definite course.
M.E., from O.Fr. rivere, riviere, from V.L. *riparia "riverbank, seashore, river," noun use of feminine of L. riparius "of a riverbank."
Rud, from Mid.Pers. rôd "river," O.Pers. rautah- "river;" cf. Skt. srotas- "river," sru- "to flow;" Pali sota- "stream, flood;" Gk. rhoos "a stream, a flowing," from rhein "to flow;" O.E. stream; Ger. Strom; PIE base *sreu- "to flow."
setâre-ye Ap-ye tond navandé
Fr.: étoile Ap à oscillation rapide
Same as → rapidly oscillating Ap star
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.
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.
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.
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 RM (ρM/ρm)1/3, where RM is the radius of the → primary body, ρM is the → density of the primary, and ρm is the density of the secondary body. This formula can also be expressed as: d = 1.26 Rm (MM/Mm)1/3, where Rm is the radius of the secondary. As an example, for the Earth-Moon system, where RM = 6,378 km, ρM = 5.5 g cm-3, and ρm = 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.
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 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.
bard (#), sang (#)
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.
bolur-e sangi, bard-bolur
Fr.: cristal de roche
Pure natural crystalline form of → silica, SiO2, which is transparent and colorless.
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.
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.
roket šenâsi, roketgari
The science of rocket design, development, and flight.
→ rocket + -ry a noun suffix.
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.
Fr.: retard de Rømer
A time delay caused by the light travel across a → dynamical system. The finite → speed of light causes a delay, for example, between the → primary eclipse and the → secondary eclipse in → binary systems.
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.