An Etymological Dictionary of Astronomy and Astrophysics

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.

Etymology (EN): 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.”

Etymology (PE): Zime, from Mid.Pers. zim “time, year, winter,” from Av. zyam-, zayan- “winter,” probably related to zaman “time” + nuance suffix -é.