Fr.: Nébuleuse de l'Anneau
A bright → planetary nebula in the constellation → Lyra, also called M57 or NGC 6720. In small telescopes it has the appearance of a slightly elliptical luminous ring around a central hot star (15th magnitude). The radius is one-third of a → light-year, and the nebula is about 2,000 light-years away.
ring opening angle
zâviye-ye gošâyeš-e halqé
Fr.: angle d'ouverture des anneaux
Of → Saturn, the angle between the line of sight and the ring plane. Also known as elevation angle, tilt angle.
Zâviyé, → angle; gošâyeš "opening," verbal noun from gošudan, gošâdan "to open up, loose, let free;" gošâd "opened; ample, broad;" Mid.Pers. wišâdan "to let free;" Khotanese hīyā "bound;" O.Pers. višta "untied, loosened," vištāspa- "with loosened horses" (personal name); Av. višta "untied," ā-hišāiiā "holds fettered," hita- "fastened, tied on, put to;" cf. Skt. sā- "to bind, fasten, fetter," sitá- "bound," ví-sita- "untied;" halqé, → ring.
Fr.: système d'anneaux
Fr.: desexcitation finale
The last stage of → merger between two → black holes undergoing → inspiral. At the end of the evolution of a → binary black hole system, the black holes get close enough to → merge together into a single, larger black hole (→ black hole merger). The resulting black hole is at first distorted and asymmetric, but in the ringdown process the black hole's vibrations decay due to → gravitational radiation leaving finally a quiescent, spinning black hole.
M.E. ring, from O.E. hringan; akin to O.Norse hringja "to ring;" → down.
1) A small ring.
Fr.: ondulation, ride
A wave on a fluid surface, of sufficiently short wavelength, in which gravity is the dominant influence.
Of unknown origin, perhaps frequentative of rip (v.) "to tear apart."
Cinâv, literally "water wrinkle," from cin "fold, plait, wrinkle" + âv, variant of âb, → water.
1) barâmadan (#); 2) barâmad (#)
Fr.: 1) se lever; 2) lever
M.E. risen (v.); O.E. risan; cf. O.N. risa, Goth. urreisan "to rise," O.H.G. risan "to rise, flow," Ger. reisen "to travel."
Barâmadan, from bar- "up; upon; on; in; into; at; forth; with; near; before; according to" (Mid.Pers. abar; O.Pers. upariy "above; over, upon, according to;" Av. upairi "above, over," upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;" L. super-; O.H.G. ubir "over;" PIE base *uper "over") + âmadan "to come, to occur, to become" (Mid.Pers. âmatan; O.Pers. gam- "to come; to go," Av. gam- "to come; to go," jamaiti "goes;" Proto-Iranian *āgmatani; Skt. gamati "goes;" Gk. bainein "to go, walk, step;" L. venire "to come;" Tocharian A käm- "to come;" O.H.G. queman "to come;" E. come; PIE root *gwem- "to go, come").
barâmad (#), barâyeš (#)
The act of rising; the appearance of a celestial body above the horizon. Opposite of → setting.
Verbal noun of → rise.
Exposure to the chance of injury or loss; a hazard or dangerous chance (Dictionary.com).
From Fr. risque, from It. risco, riscio (modern rischio), from riscare "to run into danger," of uncertain origin.
Risk, loan from Fr.
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.