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

1) In → Ptolemaic system, a circular → orbit of a body around a point that itself orbits circularly another point. Such a system was formulated to explain some → planetary orbits in terms of → circular motions in a → geocentric cosmology.
2a) Math.: A circle that rolls, externally or internally on another circle, generating an → epicycloid or → hypocycloid.
2b) In → galactic dynamics models describing the → spiral arms, a → perturbation of simple circular orbits. → epicyclic theory.

→ epi-; → cycle.

1) Falak-e tadvir, from Ar. falak al-tadwir, from falak "sphere" + tadwir "causing to turn in a circle."
2) → epi-; → cycle.

Of or pertaining to an → epicycle.

→ epicycle; → -ic.

In the → epicyclic theory of Galactic rotation, the frequency at which a star in the → Galactic disk describes an ellipse around its mean circular orbit. The epicyclic frequency relates to the → Oort's constants. In the solar neighborhood the epicyclic frequency is about 32 km s^-1 kpc^-1.

→ epicyclic; → frequency.

In a → disk galaxy, the motion of a star about the orbital → guiding center when it is displaced radially. See also → epicyclic frequency, → epicyclic theory.

→ epicyclic; → oscillation.

The theory that describes the Galactic dynamics, that is the orbits of stars and gas clouds in the → Galactic disk, as well as the spiral → density wave. Formulated by Bertil Lindblad (1895-1965), the epicyclic theory assumes that orbits are circular with small deviations. Star orbits are described by the superposition of two motions: i) a rotation of the star (epicenter) around the Galactic center at the circular angular velocity, Ω, and ii) a retrograde elliptical motion at → epicyclic frequency, κ. The epicyclic motion in the Galactic plane occurs in a retrograde sense to conserve → angular momentum. In general Ω and κ are different and, therefore, orbits do not close. However, seen by an observer who rotates with the epicenter, orbits are closed ellipses.

→ epicyclic; → theory.

A curve traced by a point of a circle that rolls on the outside of a fixed circle. This curve was described by the Gk. mathematicians and astronomer Hipparchus, who made use of it to account for the apparent movement of many of the heavenly bodies.

→ epi-; → cycloid.

The fifth of → Saturn's known satellites. It has a mean radius of 55 x 69 km and orbits its planet at a mean distance of 151,422 km. It shares the same → horseshoe orbit with → Janus. Epimetheus was discovered by Richard L. Walker in 1966. Also known as Saturn XI.

In Gk. mythology, brother of → Prometheus and → Atlas, and husband of → Pandora. His task was to populate the Earth with animals.

A → morphism f : Y → X if, for any two morphisms u,v : X → Z, u f = v f  implies u = v.

→ epi-; → morphism.

1) An incident in the course of a series of events.
2) An incident, scene, etc., within a narrative, usually fully developed and either integrated within the main story or digressing from it (Dictionary.com).

From Fr. épisode from Gk. epeisodion "addition," noun use of neuter of epeisodios "coming in besides," from → epi- "in addition" + eisodos "a coming in, entrance" (from eis"into" + hodos "way," → period).

Apyâ, literally "coming in besides," from api-, → epi-, + â- present stem of âmadan "to come," → rise.

1) Pertaining to or of the nature of an episode.
2) Divided into separate or tenuously related parts or sections.
3) Occurring sporadically or incidentally (Dictionary.com).

→ episode; → -ic.

A branch of philosophy that investigates the possibility, origins, nature, methods, and limits of human knowledge.

From Gk. episteme "knowledge," from Ionic Gk. epistasthai "to understand," literally "overstand," from → epi- "over, near" + histasthai "to stand;" cognate with Pers. istâdan "to stand," → standard; PIE base *sta- "to stand."

From šenaxt, → knowledge, + -šenâsi, → -logy.

1) The date for which → orbital elements or the positions of celestial objects are calculated. Specifying the epoch is important because the apparent positions of objects in the sky change gradually due to → precession and → nutation, while orbital elements change due to the gravitational effects of the → planets. The → standard epoch used in ephemerides (→ ephemeris) and stellar catalogues at present is January 1, 2000, 12h (written also as 2000.0). See also: → Julian epoch.
2) Same as → cosmological epoch, such as → current cosmological epoch, → electroweak epoch, → epoch of thermalization, → recombination epoch, → reionization epoch.
3) A period of time usually marked by some distinctive development or series of events. See also: → polarity epoch, → epoch angle.

From M.L. epocha, from Gk. epokhe "pause, cessation, fixed point," from epekhein "to pause, take up a position," from epi- "on" + ekhein "to hold, to have;" cf. Av. hazah- "power, violence, superiority;" Skt. sahate "he masters," sáhas- "power, violence, might;" Goth. sigis; O.H.G. sigu; O.E. sige "victory;" PIE base *segh- "to hold."

Zime, from Mid.Pers. zim "time, year, winter," from Av. zyam-, zayan- "winter," probably related to zaman "time" + nuance suffix -é.

Same as the → initial phase angle.

→ epoch; → angle.

→ reionization epoch.

→ epoch; → reionization.

The period during the → early Universe before the → recombination era when the photons were hot enough to ionize hydrogen. The density was so high that the interactions between → matter and → radiation were very numerous. Therefore, matter and photons were in constant contact and their → temperatures were the same. As a result, the radiation became → thermalized, i.e. the → electromagnetic spectrum of the radiation became that of a → blackbody, a process called → thermalization. Since the time of recombination the photons of → cosmic background radiation have been free to travel uninhibited by interactions with matter. Thus, their distribution of energy is a perfect → blackbody curve, as predicted by the → Big Bang theory and shown by several observations, such as → Cosmic Background Explorer (COBE), → Wilkinson Microwave Anisotropy Probe (WMAP), and → Planck Satellite.

→ epoch; → thermalization.

A thought experiment developed in 1935 by A. Einstein (1879-1955), Boris Podolsky (1896-1966), and Nathan Rosen (1909-1995) to demonstrate that there is a fundamental inconsistency in → quantum mechanics. They imagined two physical systems that are allowed to interact initially so that they will subsequently be defined by a single quantum mechanical state. For example, a neutral → pion at rest which decays into a pair of → photons. The pair of photons is described by a single two-particle → wave function. Once separated, the two photons are still described by the same wave function, and a measurement of one → observable of the first system will determine the measurement of the corresponding observable of the second system. For example, if photon 1 is found to have → spin up along the x-axis, then photon 2 must have spin down along the x-axis, since the final total → angular momentum of the two-photon system must be the same as the angular momentum of the initial state. This means that we know the spin of photon 2 even without measuring it. Likewise, the measurement of another observable of the first system will determine the measurement of the corresponding observable of the second system, even though the systems are no longer physically linked in the traditional sense of local coupling (→ quantum entanglement). So, EPR argued that quantum mechanics was not a complete theory, but it could be corrected by postulating the existence of → hidden variables that furthermore would be "local". According to EPR, the specification of these local hidden parameters would predetermine the result of measuring any observable of the physical system. However, in 1964 John S. Bell developed a theorem, → Bell's inequality, to test for the existence of these hidden variables. He showed that if the inequality was satisfied, then no local hidden variable theory can reproduce the predictions of quantum mechanics. → Aspect experiment.

A. Einstein, B. Podolsky, N. Rosen: "Can quantum-mechanical description of physical reality be considered complete?" Phys. Rev. 41, 777 (15 May 1935); → paradox.

As great as; like or alike in quantity, degree, value.

From L. æqualis "uniform, identical, equal," from æquus "level, even, just," of unknown origin, + -alis, → -al.

Hamug, from Mid.Pers. hamôg "equal, like," from ham "the same; together; also" (O.Pers./Av. ham-; cf. Skt. sam-; also O.Pers./Av. hama- "one and the same;" Skt. sama-; Gk. homos-; originally identical with PIE numeral *sam- "one," from *som-) + suffix -og/-ok/-uk, as in nêrog "force" (from nar "man, male"), nêvakôk "good, nice" (from nêvak "good, beautiful, nice, favorable"), mastôk "drunk" (from mast "drunk, drunken"), câpuk "quick; active," sapuk "light, brisk."

1) The state or quality of being equal.
2) Math.: A → statement of two → mathematical objects being equal. Like → equations, equalities are written as two mathematical objects connected by the → equality sign.

M.E. from L. aequalitat-, stem of aequalitats, → equal + -ity.

Hamugi noun of hamug, → equal.

Same as → equals sign.

→ equality; → sign.

The act of making equal or uniform.
Electronics: The reduction of frequency or phase distortion by introducing networks which compensate for the particular type of distortion over the required frequency band.

Noun of equalize.