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

A fictitious meridian that rotates independently of the Earth at the uniform rate implicitly defined by → Terrestrial Dynamical Time (TDT).

→ ephemeris; → meridian.

The length of a tropical second (1/31,556,925.97474 of the tropical year) on 1900 January 0.5 → ephemeris time.

→ ephemeris; → second.

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:
L = 279° 41' 48".04 + 129 602 768".13 T + 1''.089 T²,
where L refers to the → mean equinox of date while T measures time from noon 1900 January 0 GMT in Julian centuries of 36525 days. Ephemeris Time is therefore defined as the instant near the beginning of the calendar year A.D. 1900 when the mean longitude of the Sun was 279° 41' 48''.04, at which instant the measure of ET was 1900 January 0, 12h precisely. In this system the fundamental unit was the → ephemeris second, which was defined so that the → tropical year at the epoch 1900.0 should be exactly 31 556 925,9747 seconds of ephemerides. Ephemeris Time was inconvenient in many ways and was supeseded with the → Terrestrial Dynamical Time (TDT), whose fundamental unit is the SI second.

→ ephemeris; → time.

The passage of a celestial body or point across the → ephemeris meridian.

→ ephemeris; → transit.

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) 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.