Fr.: méridien des éphémérides
A fictitious meridian that rotates independently of the Earth at the uniform rate implicitly defined by → Terrestrial Dynamical Time (TDT).
Fr.: seconde des éphémérides
The length of a tropical second (1/31,556,925.97474 of the tropical year) on 1900 January 0.5 → ephemeris time.
ephemeris time (ET)
Fr.: Temps des éphémérides
The uniform time-scale used as the independent variable
to calculate the orbits in the solar system prior to 1984. Ephemeris Time was adopted in
1960 to deal with irregularities in the → Earth's rotation
that had been found to affect the
course of mean solar time. The definition of Ephemeris Time is based on Newcomb's analytical
theory of the Earth's motion around the Sun (Newcomb 1898), according to which the geometric
mean longitude of the Sun with respect to the Earth-Moon barycenter is expressed by:
Fr.: transit au méridien des éphémérides
The passage of a celestial body or point across the → ephemeris meridian.
Prefix meaning "upon, at, close upon (in space or time), on the occasion of, in addition."
Gk. epi- "upon, at, close upon (in space or time), on the occasion of, in addition," cognate with O.Pers./Av. apiy-, aipi- "upon, toward, along; also; however;" Skt. api "also, besides."
Prefix api-, from O.Pers./Av. apiy-, aipi-, as above.
1) falak-e tadvir (#); 2) apicarxé
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
orbits in terms of → circular
motions in a → geocentric
Of or pertaining to an → epicycle.
Fr.: fréquence épicyclique
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.
Fr.: oscillation épicyclique
Fr.: théorie épicyclique
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.
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.
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.
A → morphism f : Y → X if, for any two morphisms u,v : X → Z, u f = v f implies u = v.
1) An incident in the course of a series of events.
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).
1) Pertaining to or of the nature of an episode.
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."
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.
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.
Fr.: angle de phase initial
Same as the → initial phase angle.
epoch of reionization (EoR)
Fr.: époque de réionisation
epoch of thermalization
Fr.: époque de thermalisation
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.