Editing Epoch

{{Short description|Reference point from which time is measured}}
{{wiktionary|epoch}}
{{Other uses||Epoch (disambiguation)}}
In [[chronology]] and [[periodization]], an '''epoch''' or '''reference epoch''' is an [[instant]] in time chosen as the origin of a particular [[calendar era]]. The "epoch" serves as a reference point from which time is measured.

The moment of epoch is usually decided by congruity, or by following conventions understood from the epoch in question. The epoch moment or date is usually defined from a specific, clear event of change, an ''epoch event''. In a more gradual change, a [[Point of no return|deciding moment]] is chosen when the ''epoch criterion'' was reached.{{clarify|reason=Presumably this sentence is trying to say that when there is no clear change to pick as the epoch, because there was gradual change instead, an instant where there is a [[point of no return]] is picked as the epoch. But if this is so, it is expressed poorly, and also needs a citation that this is a practice that people use.|date=February 2025}}{{cn|date=February 2025}}

== Calendar eras ==
=== Pre-modern eras ===
* The [[Yoruba calendar]] (''Kọ́jọ́dá'') uses 8042 BC as the epoch, regarded as the year of the creation of [[Ile-Ife]] by the god [[Obatala]], also regarded as the creation of the earth.
* ''[[Anno Mundi]]'' [lit. "Year of the World"] (years since the creation of the world) is used in 
** the [[Byzantine calendar]] (5509 BC).
** the [[Hebrew calendar]] (3761 BC).<ref>{{cite web |url=http://www.myjewishlearning.com/article/counting-the-years/ |title=Counting the Jewish Years |website=myjewishlearning.com |last=Solomin |first=Rachel M. |access-date=2016-12-27 |archive-date=2020-02-12 |archive-url=https://web.archive.org/web/20200212083037/https://www.myjewishlearning.com/article/counting-the-years/ |url-status=live }}</ref><ref>{{cite web |last=Lee |first=Scott E. |date=2006 |url=http://www.rosettacalendar.com/calendars.html |title=Overview of Calendars |website=rosettacalendar.com |access-date=2006-10-23 |archive-date=2020-08-20 |archive-url=https://web.archive.org/web/20200820000454/https://www.rosettacalendar.com/calendars.html |url-status=live }}</ref>
* The [[Mesoamerican Long Count Calendar]] uses the creation of the fourth world in 3114 BC.
* [[Olympiad]]s, the ancient Greek era of four-year periods between [[Ancient Olympic Games|Olympic Games]], beginning in 776 BC.
* ''[[Ab urbe condita]]'' ("from the foundation of [[Rome|the city]]"), used to some extent by [[Roman calendar]]s of the Roman imperial period (753 BC).
*  [[Buddhist calendar]]s tend to use the epoch of 544 BC (date of [[Buddha]]'s ''[[parinirvana]]'').
* The term [[Hindu calendar]] may refer to a number of traditional Indian calendars. A notable example of a Hindu epoch is the ''[[Vikram Samvat]]'' (58 BC),<ref>{{Cite book| edition = 3rd| publisher = Cambridge University Press| isbn = 978-0-521-70238-6| last1 = Dershowitz| first1 = Nachum| author-link1 = Nachum Dershowitz | last2 = Reingold| first2 = Edward M.| author-link2 = Edward Reingold  |title = Calendrical Calculations|title-link= Calendrical Calculations | date = 2008 | page = [https://archive.org/details/calendricalcalcu00ders/page/n321 289]}}</ref> also used in modern times as the national calendars of [[Nepal]] and [[Bangladesh]].
* The [[Julian calendar|Julian]] and [[Gregorian calendar|Gregorian]] calendars use as epoch the [[Incarnation (Christianity)|Incarnation of Jesus]] as calculated in the 6th century by [[Dionysius Exiguus]].<ref>{{cite book |last1=Blackburn |first1=B |last2=Holford-Strevens |first2=L |date=2003 |title=The Oxford Companion to the Year: An exploration of calendar customs and time-reckoning |publisher=Oxford University Press |chapter=Incarnation era |page=881}}</ref> (Subsequent research has shown that this moment is about four years after the best estimate for the [[date of birth of Jesus]].) This epoch was applied retrospectively to the Julian calendar, long after its original creation by [[Julius Caesar]]. 
* The epoch of the [[Islamic calendar]] is the ''[[Hijra (Islam)|Hijra]]'' (AD 622).  The year count in this calendar shifts relative to the solar year count, as the calendar is [[lunar calendar|purely lunar]]: its year consists of 12 [[lunation]]s and is thus ten or eleven days shorter than a solar year. This calendar denotes "lunar years" as ''Anno Hegiræ'' ([since] the year of the ''[[Hijra (Islam)|Hijra]]'') or ''AH''. This calendar is used in [[Sunni Islam]] and related sects.
* The epoch of the official [[Iranian calendars|Iranian calendar]] is also the ''Hijra'', but it is a [[solar calendar]]; each year begins at the Northern spring equinox. This calendar is used in [[Shia Islam]] and related sects.

=== Modern eras ===
* The [[Bahá'í calendar]] is dated from the [[March equinox|vernal equinox]] of the year the [[Báb]] proclaimed his religion (AD 1844). Years are grouped in ''Váḥids'' of 19 years, and ''Kull-i-Shay'' of 361 (19×19) years.<ref>{{cite book |first=E. G. |last=Richards |chapter=Calendars |editor-first1=S. E. |editor-last1=Urban |editor-first2=P. K. |editor-last2=Seidelman  |title=Explanatory Supplement to the Astronomical Almanac |location=Mill Valley, CA |publisher=University Science Books |date=2013 |pages=616–617 | edition=3rd}}</ref>
* In [[Thailand]] in 1888 King [[Chulalongkorn]] decreed a National Thai Era dating from the founding of [[Bangkok]] on April 6, 1782. In 1912, New Year's Day was shifted to April 1. In 1941, [[Prime Minister of Thailand|Prime Minister]] [[Plaek Pibulsonggram|Phibunsongkhram]] decided to count the years since 543 BC. This is the [[Thai solar calendar]] using the Thai Buddhist Era. Except for this era, it is the Gregorian calendar.
* In the [[French Republican Calendar]], a calendar used by the French government for about twelve years from late 1793, the epoch was the beginning of the "Republican Era", September 22, 1792 (the day the [[French First Republic]] was proclaimed, one day after the Convention abolished the ''[[Ancien Regime]]'').
* The [[Indian national calendar]], introduced in 1957, follows the [[Saka era]] (AD 78).
* The [[Minguo calendar]] used by officials of [[Taiwan]] and [[Republic of China (1912–49)|its predecessor]] dates from January 1, 1912, the first year after the [[Xinhai Revolution]], which overthrew the [[Qing dynasty|Qing Empire]].
* [[North Korea]] uses a system that starts in 1912 (= [[Juche]] 1), the year of the birth of its founder [[Kim Il-Sung]].
* The [[Era Fascista|Fascist Era]] dates to [[Mussolini]]'s [[March on Rome]] in 1922, and was in use only in countries under hegemony of the Fascist regime of Benito Mussolini. It has been defunct since the fall of the [[Italian Social Republic]] in 1945. 
* In the scientific [[Before Present]] system of numbering years for purposes of [[radiocarbon dating]], the reference date is January 1, 1950 (though the specific date January 1 is quite unnecessary, as radiocarbon dating has limited precision).<ref>{{cite web |url=http://www.c14dating.com/agecalc.html |website=c14dating.com |title=Radiocarbon dating – Age calculation |last=Higham |first=Thomas |author-link1=Thomas Higham (archaeologist) |publisher=Thomas Higham (archaeologist) |access-date=December 31, 2009 |archive-date=June 10, 2007 |archive-url=https://web.archive.org/web/20070610195000/http://www.c14dating.com/agecalc.html |url-status=live }}</ref><ref>{{cite journal |last=Stuiver |first=Minze |author2=Polach HA |year=1977 |title=Discussion; reporting of C-14 data. |journal=Radiocarbon |publisher=University of Arizona |volume=19 |issue=3 |pages=355–363 |doi=10.1017/S0033822200003672 |bibcode=1977Radcb..19..355S |s2cid=56572650 |url=https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/493/498 |access-date=October 5, 2018 |archive-date=August 17, 2020 |archive-url=https://web.archive.org/web/20200817043941/https://journals.uair.arizona.edu/index.php/radiocarbon/article/view/493/498 |url-status=live |doi-access=free }}</ref>
* Different branches of [[Freemasonry]] have selected different years to date their documents according to a Masonic era, such as the ''[[Anno Lucis]]'' (A.L.).
* The [[Holocene calendar]] uses 10,000 BC as the epoch, the beginning of the [[Holocene epoch]] on the [[geological time scale]].

=== Regnal eras ===
{{Further|Regnal era}}
The official [[Japanese era name|Japanese system]] numbers years from the accession of the current [[Emperor of Japan|emperor]], regarding the calendar year during which the accession occurred as the first year. A similar [[Chinese era name|system]] existed in [[China]] before 1912, being based on the accession year of the emperor (1911 was thus the third year of the [[Xuantong]] period). With the establishment of the [[Republic of China (1912–1949)|Republic of China]] in 1912, the republican era was introduced. It is still very common in [[Taiwan]] to date events via the republican era. The People's Republic of China adopted the common era calendar in 1949 (the 38th year of the Chinese Republic).

=== Fictional eras ===
* Events in the [[Star Wars universe|''Star Wars'' universe]] are conventionally dated using an epoch of the [[Battle of Yavin]].
* Events in the [[Avatar: The Last Airbender (franchise)|Avatar: The Last Airbender universe]] are conventionally dated using an epoch of the genocide of the air nomads.

== Other applications ==

An [[Epoch (computing)|epoch in computing]] is the time at which the representation is zero. For example, [[Unix time]] is represented as the number of seconds since 00:00:00 UTC on 1 January 1970, not counting [[leap second]]s. 

An [[Epoch (astronomy)|epoch in astronomy]] is a reference time used for consistency in calculation of positions and orbits. A common astronomical epoch is J2000, which is noon on January 1, 2000, [[Terrestrial Time]].

An epoch in [[Geochronology]] is a [[time period]], typically in the order of tens of millions of years. The current epoch is the [[Holocene]].

== See also ==

* {{Annotated link |Dating creation}}
* {{Annotated link|Era}}
* {{Annotated link |Geologic time scale}}
* {{Annotated link |Lunisolar calendar}}
* {{Annotated link |Metonic cycle}}
* {{Annotated link |Saros (astronomy)}}
* {{Annotated link |Timekeeping on Mars}}

== References ==
{{Reflist}}

{{Chronology}}
{{Authority control}}

[[Category:Calendar eras]]
[[Category:Calendaring standards]]
[[Category:Chronology]]