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

1) Severely exact or accurate; precise.
2) Logic, Math.: Logically valid. See also: → accurate, → exact, → precise.

M.E., from M.L. rigorosus, from rigor "stiffness, rigor," from rigere "to be stiff."

Farsaxt, from far- intensive prefix "much, abundant; elegantly" (Mid.Pers. fra- "forward, before; much; around;" O.Pers. fra- "forward, forth;" Av. frā, fərā-, fra- "forward, forth; excessive;" cf. Skt. prá- "before; forward, in fron;" Gk. pro "before, in front of;" L. pro "on behalf of, in place of, before, for;" PIE *pro-) + saxt "hard, strong, firm, secure, solid, vehement, intense" (Mid.Pers. saxt "hard, strong, severe;" Av. sak- "to understand or know a thing, to mark;" cf. Skt. śakta- "able, strong," śaknoti "he is strong," śiksati "he learns").

A → selection rule obeyed by → discrete transitions. Among them are: rigorous selection rules for → electric dipole transitions (→ permitted) requiring: 1) ΔJ must be 0 or ± 1 with J = 0 ↔ 0 forbidden. 2) ΔM_J = 0, ± 1. 3) → Parity change, i.e. even ↔ odd.

→ rigorous; → selection; → rule.

A long, narrow, straight or sinuous trench or valley with steep walls and roughly parallel sides on the surface of the Moon.

From Ger. Rille "a small rivulet or brook."

Borrowed from E., as above.

The outer edge, border, margin, or brink of something, especially of a circular object.

M.E., from O.E. -rima (in compounds, as in særima "seashore"); cognate with O.Norse rimi "raised strip of land, ridge"

Labé "limb, edge," from lab "lip;" Mid.Pers. lap; cognate with L. labium, E. lip; Ger. Lefze.

A long fissure on the surface of a planet or Moon; plural form rimae.

From L. rima "fissure, slit."

Loan from E., as above.

The coordinates that describe the → Minkowski space-time in a → hyperbolic version of → polar coordinates. If the coordinates in an → inertial frame is denoted by (t,x), the Rindler coordinates (η,ξ) are defined by: t = (1/α) e^αξ sinh (αη) and x = (1/α) e^αξ cosh (αη), where α is some positive constant.

Named after Wolfgang Rindler (1924-), Austrian physicist; → coordinates.

A portion of the → space-time described by → Rindler coordinates.

→ Rindler coordinates; → wedge.

1) A circular band of something. → gossamer ring.
2) A resonant sound like that of a bell or bells.

O.E. hring "circular band" (cf. O.N. hringr, Ger. Ring), literally "something curved," from PIE base *(s)ker- "to turn, bend."

1) Halqé, loan from Ar. Halqat "ring, hoop, circle."
2) Zang "bell."

A galaxy with a ring-like appearance around the central luminous center. The ring consists of massive, relatively young bright stars. It is believed that ring galaxies result from the head-on collision of two different galaxies.

→ ring; → galaxy.

Of → Saturn, the angle measured with respect to the sub-observer point (a line connecting the observer to Saturn) in the direction of the orbital motion.

→ ring; → longitude.

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; → nebula.

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.