epoch

1. Astronomy a precise date to which information, such as coordinates, relating to a celestial body is referred

2. Geology a unit of geological time within a period during which a series of rocks is formed

3. Physics the displacement of an oscillating or vibrating body at zero time

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

epoch

An arbitrary fixed date or instant of time that is used as a reference datum, especially for stellar coordinates and orbital elements. For example, the coordinates right ascension and declination are continuously changing, primarily as a result of the precession of the equinoxes. Coordinates must therefore refer to a particular epoch, which can be the time of an observation, the beginning of the year in which a series of observations of an object was made, or the beginning of a half century. The standard epoch specifies the reference system to which coordinates are referred. Coordinates of star catalogs commonly referred to the mean equator and equinox of the beginning of a Besselian year. Since 1984 the Julian year has been used: the current standard epoch, designated J2000.0, is 2000 Jan. 1.5; it is exactly one Julian century removed from the standard epoch of 1900 Jan 0.5. Epochs for the beginning of a year now differ from the standard epoch by multiples of the Julian year. A standard epoch is usually retained for 50 years.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

epoch

[′ep·ək]

(astronomy)

A particular instant for which certain data are valid; for example, star positions in an astronomical catalog, epoch 1950.0.

(geology)

A major subdivision of a period of geologic time.

(physics)

time

epoch

(operating system)

(Probably from astronomical timekeeping) A term used originally in Unix documentation for the time and date corresponding to zero in an operating system's clock and timestamp values.

Under most Unix versions the epoch is 1970-01-01 00:00:00 GMT; under VMS, it's 1858-11-17 00:00:00 (the base date of the US Naval Observatory's ephemerides); on a Macintosh, it's 1904-01-01 00:00:00.

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 0.1 * 2**31-1 seconds, or 6.8 years. The one-tick-per-second clock of Unix is good only until 2038-01-18, assuming at least some software continues to consider it signed and that word lengths don't increase by then. See also wall time.

epoch

(editor)

(Epoch) A version of GNU Emacs for the X Window System from NCSA.

This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)

epoch

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/Linux      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

No USB Drives in 1969

When this USB drive was formatted, the date was reset to the epoch. The 7:00pm tells us it was done on the East Coast, five hours behind the UTC time of January 1, 1970 (see above). This info comes from the Mac that the drive was plugged into.

Copyright © 1981-2025 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.

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References in periodicals archive

An epochal change is called for in the way we all engage with the climate crisis.

Surviving climate change: the struggle to avert global catastrophe

Lynn describes how giant corporate institutions such as Wal-Mart have all but banished the free market from much of our economy--a staggering, epochal transformation that has, however, eluded the attention of economists, whose theories obstruct their observation of the actual world.

From the executive editor

Epochal issues can change an electorate's mood or historical patterns; eight years ago, anger over the drawn-out impeachment of Bill Clinton inspired voters to add more Democrats to Congress, despite the "rule" of the sixth-year slump.

Peace on the border: why anti-immigration conservatives fell flat in 2006

One increasingly sees in recent published work (and in dissertations-in-progress) the 1750-1850 periodization, for example, in which independence from Spain (1821) is still taken seriously, but as a watershed, in the sense that one needs to see which way political processes flow on either side, rather than as an epochal disjunction.

The Time of Liberty: Popular Political Culture in Oaxaca, 1750-1850

The ultimate answer from President Grant's Washington about whether federal troops could establish order and enforce the laws--laws that would have allowed blacks to vote--was depressing and epochal: No.

The savage south: lessons of an American insurgency

This book describes the epochal struggles of that era: the Montgomery bus boycott; the Selma to Montgomery march; the marches in Washington; the sanitation workers' strike in Memphis that cost King his life.

The Autobiography of Martin Luther King, Jr

Cate doesn't do justice to Nietzsche's epochal notion of the potentially infinite perspectives for viewing everything; and he abruptly dismisses as "untenable" Nietzsche's claim that humans can't be held morally responsible for their actions, because they aren't free to begin with, and in the seamless continuum of existence there are, strictly speaking, no atomistic acts at all, just as there are no separate "things" in the naively realistic sense.

Poor Fritz!

In theorizing 9/11 as an epochal turning point and postmodernity as a completed historical phase, Luperini trenchantly positions his view within the debate surrounding the various ideologies of modernity and postmodernity.

Romano Luperini. La fine del postmoderno

First, it refers to arguments regarding epochal changes in economic relations and organization which position culture as integral to economic organizations and institutions, and related changes in the type and relations of consumption.

Cultural Economy: Cultural Analysis and Commercial Life

This book provides a fascinating picture of two towering figures in the development of jazz: one, Paul Whiteman, is now largely overlooked in favor of Louis Armstrong, typically seen as the "King of Jazz." But it was not always so--that title was once given to Whiteman--and Joshua Berrett makes a compelling case for the contributions of the classically trained Whiteman, a musician whose efforts to promote "symphonic jazz" led to the commissioning of George Gershwin to create his epochal Rhapsody in Blue.

Louis Armstrong & Paul Whiteman: Two Kings of Jazz

He went on to a brilliant career that included at least one other epochal paper, his 1916 discussion of gravitational fields in "The Foundation of the General Theory of Relativity." When Hitler and the Nazis came to power in Germany in 1933, Einstein emigrated to the United States, where he joined the newly formed Institute for Advanced Studies at Princeton.

Albert Einstein

He'd like to take on Stravinsky's epochal music himself, but doesn't feel ready yet.

Christopher Williams: transforming the strange to the beautiful