epoch

(ĕp'ək, ē'pŏk')
n.

1. 1. A particular period of history, especially one considered remarkable or noteworthy.
   2. A notable event that marks the beginning of such a period. See synonyms at period.
2. A unit of geologic time that is a division of a period.
3. Astronomy. An instant in time that is arbitrarily selected as a point of reference.

[Medieval Latin epocha, measure of time, from Greek epokhē, a point in time.]

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1. An epoch is the beginning of a distinctive period in the history of something or someone, whereas an era is a period of history characterized by particular circumstances or by a particular series of events. A period is a more general term for a distinct portion of time in relation to a person's life or to human history and an aeon is an immeasurable length of time often used in more rhetorical contexts. In geology, the three terms epoch, era, and period have special meanings: era denotes the largest unit of time, a period is a division of this, and an epoch is a subdivision of a period.

2. The adjective epoch-making is first recorded in the 19th century (Coleridge had used epoch-forming in 1816), and is now widely used to mean 'remarkable, significant' as well as (more appropriately) 'historic':

This was an epoch-making moment in the history of Egypt, like the day a dam bursts—N. Barber, 1984

There were a large number of epoch-making events whose 40th anniversaries were celebrated towards the end of last year, as readers will undoubtedly recall—Times Educational Supplement, 2004.

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The time and date at which an astronomical observation is made (epoch of observation), or the date for which orbital elements (epoch of 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. Data given in star catalogs and ephemerides are referred to a standard epoch, also known as a fundamental epoch. Prior to 1984, coordinates of star catalogs were commonly referred to as Besselian epochs. However, from 1984 on, Julian Dates have been used, denoted by the prefix J. The current standard epoch is J2000.0. It will be superceded in half a century by J2050.0.

For more information on epoch, visit Britannica.com.

[Etymology: Gk: ‘pause’] time A specific point in time or the interval between two such points, but not as such a measure of time. In astronomy it is usually the single point, otherwise it generally has the extended meaning; in geology it can be millions of years. By contrast, an epoch within the initial cosmic ‘big bang’ is less than a microsecond.

geology The fourth-largest unit (following period) of the geochronologic scale, and the tertiary one into which it, excluding the pre-Cambrian, is divided: examples include Pliocene, Pleistocene, and Holocene. Typical size is about 15 million years.

The starting date from which time is measured as a number of days or minutes or seconds, etc. In computer applications, epochs are used to maintain a time reference as a single number for ease of computation. Otherwise, depending on the granularity of time desired, every point in time would have to be stored with some of or all of the components of the time hierarchy, including year, month, day, hour, minute, second, millisecond, microsecond and nanosecond. Following are the various epochs in use. See also EPOC.

  System          Epoch            Measured in

  Unix            Jan. 1, 1970     Seconds
  Java            Jan. 1, 1970     Milliseconds

  Windows files   Jan. 1, 1601     Ticks (100 ns)
  Windows dates   Jan. 1, 0001     Ticks (100 ns)

  Mac             Jan. 1, 2001     Seconds
  Earlier Mac     Jan. 1, 1904     Seconds

  Excel           Dec. 31, 1899    Days
  DB2             Dec. 31, 1899    Days

  Unununium       Jan. 1, 2000     Microseconds

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noun

A particular time notable for its distinctive characteristics: age, day, era, period, time (often used in plural). See time.

[Unix: prob.: from astronomical timekeeping] The time and date corresponding to 0 in an operating system's clock and timestamp values. Under most Unix versions the epoch is 00:00:00 GMT, January 1, 1970; under VMS, it's 00:00:00 of November 17, 1858 (base date of the U.S. Naval Observatory's ephemerides); on a Macintosh, it's the midnight beginning January 1 1904. System time is measured in seconds or ticks past the epoch. Weird problems may ensue when the clock wraps around (see wrap around), which is not necessarily a rare event; on systems counting 10 ticks per second, a signed 32-bit count of ticks is good only for 6.8 years. The 1-tick-per-second clock of Unix is good only until January 18, 2038, assuming at least some software continues to consider it signed and that word lengths don't increase by then. See also wall time. Microsoft Windows, on the other hand, has an epoch problem every 49.7 days — but this is seldom noticed as Windows is almost incapable of staying up continuously for that long.

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IN BRIEF: A memorable event. Also: A division of geologic time.

We do not choose our historical epoch, or the country of our birth, or the immediate circumstances of our upbringing. — Joseph Epstein.

Tutor's tip: The "epic" (a long poem or other writing telling of a nation's heroic acts) adventure tale takes place during the "epoch" (a period of time in history or in geology) after the Bronze Age.

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For a list of words related to epoch, see:

• Processes, Phenomena, Events, and Techniques - epoch: smallest major unit of geologic time; subdivision of period
• Branches, Laws, Theories, and Techniques - epoch: time at which positions of celestial bodies are observed
• Periods of Time - epoch: significant time period marked by historical events; geologic time division smaller than period

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See words rhyming with "epoch."

  See crossword solutions for the clue Epoch.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - epoke, periode, tid

Nederlands (Dutch)
tijdvak, periode, belangrijke datum of gebeurtenis, mijlpaal

Français (French)
n. - époque, période, (fig) date

Deutsch (German)
n. - Epoche

Ελληνική (Greek)
n. - εποχή, χρονική περίοδος

Italiano (Italian)
epoca

Português (Portuguese)
n. - época (f), período (m)

Русский (Russian)
эпоха, эра, век

Español (Spanish)
n. - época, era, período

Svenska (Swedish)
n. - epok

中文（简体）(Chinese (Simplified))
新纪元, 时期, 时代

中文（繁體）(Chinese (Traditional))
n. - 新紀元, 時期, 時代

한국어 (Korean)
n. - 신기원, 지층 구성 기간

日本語 (Japanese)
n. - 新紀元, 画期的なでき事, 時代, 世

العربيه (Arabic)
‏(الاسم) دور, عهد‏

עברית (Hebrew)
n. - ‮עידן, מאורע חשוב‬

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