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

Of → Saturn, the angle between the line of sight and the ring plane. Also known as elevation angle, tilt angle.

→ ring; → open; → 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.

→ planetary ring system.

→ ring; → system.

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.
2) Any of the thin or narrow rings that compose the major → Saturn's rings.

→ ring; → -let.

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) Of a heavenly body, to ascend above the horizon. → set.
2) An act or instance of rising; appearance above the horizon. → moonrise, → sunrise

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").

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.

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.

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.

Named after Walther Ritz (1878-1909), a Swiss theoretical physicist; → combination; → principle.

1) A person who is competing for the same object or goal as another, or who tries to equal or outdo another; competitor.
2) A person or thing that is in a position to dispute another's preeminence or superiority (Dictionary.com).

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.
2) An instance of this (Dictionary.com).

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."

Same as → rapidly oscillating Ap star

→ rapidly oscillating Ap star

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:
ds² = c²dt² - R²(t) [dr²/(1 - kr²) + r²dθ² + r²sin²θ dθ²)].
Same as Friedmann-Lemaître-Robertson-Walker metric. Compare → Minkowski metric.

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.

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.

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.

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.

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.