epoch
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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)
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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.
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)This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)