Epoch (astronomy): Difference between revisions - Wikipedia


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{{Short description|Moment in time used as a reference point in astronomy}}

{{Use mdy dates|date=August 2018}}

In [[astronomy]], an '''epoch''' or '''reference epoch''' is a [[instant|moment]] in time used as a reference point for some time-varying astronomical quantity. It is useful for the [[celestial coordinates]] or [[orbital elements]] of a [[Astronomical object|celestial body]], as they are subject to [[Perturbation (astronomy)|perturbations]] and vary with time.{{sfn|Soop|1994|p=}} These time-varying astronomical quantities might include, for example, the [[mean longitude]] or [[mean anomaly]] of a body, the node of its orbit relative to a [[reference plane]], the direction of the [[apogee]] or [[Perihelion and aphelion|aphelion]] of its orbit, or the size of the [[major axis]] of its orbit.

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==Epoch versus equinox==

Astronomical data are often specified not only in their relation to an epoch or date of reference but also in their relations to other conditions of reference, such as coordinate systems specified by "[[equinox (celestial coordinates)|equinox]]", or "equinox and [[equator]]", or "equinox and [[ecliptic]]" –– when these are needed for fully specifying astronomical data of the considered type.

===Date-references for coordinate systems===

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== Epoch of the day ==

Over shorter timescales, there are a variety of practices for defining when each day begins. In ordinary usage, the [[Civil time|civil day]] is reckoned by the [[midnight]] epoch, that is, the civil day begins at midnight. But in older astronomical usage, it was usual, until January 1, 1925, to reckon by a [[noon]] epoch, 12 hours after the start of the civil day of the same denomination, so that the day began when the mean sun crossed the [[meridian (astronomy)|meridian]] at noon.{{sfn|Wilson|1925|pp=1-21–2}} This is still reflected in the definition of J2000, which started at noon, Terrestrial Time.

In traditional cultures and in antiquity other epochs were used. In [[ancient Egypt]], days were reckoned from sunrise to sunrise, following a morning epoch. This may be related to the fact that the Egyptians regulated their year by the [[heliacal rising]] of the star [[Sirius]], a phenomenon which occurs in the morning just before dawn.{{sfn|Neugebauer|2004|p=1067}}

In some cultures following a [[lunar calendar|lunar]] or [[lunisolar calendar]], in which the beginning of the month is determined by the appearance of the New Moon in the evening, the beginning of the day was reckoned from sunset to sunset, following an evening epoch, e.g. the [[Hebrew calendar|Jewish]] and [[Islamic calendar]]s{{sfn|Neugebauer|2004|pp=1067-10691067–1069}} and in Medieval Western Europe in reckoning the dates of religious festivals,<ref>[[Bede]], ''The Reckoning of Time'', 5, trans. Faith Wallis, (Liverpool: Liverpool University Press, 2004), pp. 22-2422–24. {{ISBN|0-85323-693-3}}</ref> while in others a morning epoch was followed, e.g. the [[Hindu calendar|Hindu]] and [[Buddhist calendar]]s.

==See also==

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{{notelist|refs=

{{efn|name=NumExp|Examples of this usage are seen in: {{harvnb|Simon|Bretagnon|Chapront|Chapront-Touze|1994|pp=663-683663–683}} }}

{{efn|name=clock24|This article uses a 24-hour clock, so 11:59:27.816 is equivalent to 11:59:27.816&nbsp;a.m.}}

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==External links==

* [http://aa.usno.navy.mil/faq/docs/TT.html What is Terrestrial Time?] {{Webarchive|url=https://web.archive.org/web/20060806040555/http://aa.usno.navy.mil/faq/docs/TT.html |date=August 6, 2006 }} -– U.S. Naval Observatory

* [http://aa.usno.navy.mil/faq/docs/ICRS_doc.html International Celestial Reference System, or ICRS] {{Webarchive|url=https://web.archive.org/web/20060805073908/http://aa.usno.navy.mil/faq/docs/ICRS_doc.html |date=August 5, 2006 }} -– U.S. Naval Observatory

* [http://www.iers.org/MainDisp.csl?pid=46-25776 IERS Conventions 2003 (defines ICRS and other related standards)]