Inchronology andperiodization, anepoch orreference epoch is aninstant in time chosen as the origin of a particularcalendar 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, anepoch event. In a more gradual change, adeciding moment is chosen when theepoch criterion was reached.[clarification needed][citation needed]
TheYoruba calendar (Kọ́jọ́dá) uses 8042 BC as the epoch, regarded as the year of the creation ofIle-Ife by the godObatala, also regarded as the creation of the earth.
The termHindu calendar may refer to a number of traditional Indian calendars. A notable example of a Hindu epoch is theVikram Samvat (58 BC),[3] also used in modern times as the national calendars ofNepal andBangladesh.
TheJulian andGregorian calendars use as epoch theIncarnation of Jesus as calculated in the 6th century byDionysius Exiguus.[4] (Subsequent research has shown that this moment is about four years after the best estimate for thedate of birth of Jesus.) This epoch was applied retrospectively to the Julian calendar, long after its original creation byJulius Caesar.
The epoch of theIslamic calendar is theHijra (AD 622). The year count in this calendar shifts relative to the solar year count, as the calendar ispurely lunar: its year consists of 12lunations and is thus ten or eleven days shorter than a solar year. This calendar denotes "lunar years" asAnno Hegiræ ([since] the year of theHijra) orAH. This calendar is used inSunni Islam and related sects.
The epoch of the officialIranian calendar is also theHijra, but it is asolar calendar; each year begins at the Northern spring equinox. This calendar is used inShia Islam and related sects.
TheBahá'í calendar is dated from thevernal equinox of the year theBáb proclaimed his religion (AD 1844). Years are grouped inVáḥids of 19 years, andKull-i-Shay of 361 (19×19) years.[5]
InThailand in 1888 KingChulalongkorn decreed a National Thai Era dating from the founding ofBangkok on April 6, 1782. In 1912, New Year's Day was shifted to April 1. In 1941,Prime MinisterPhibunsongkhram decided to count the years since 543 BC. This is theThai solar calendar using the Thai Buddhist Era. Except for this era, it is the Gregorian calendar.
In theFrench 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 theFrench First Republic was proclaimed, one day after the Convention abolished theAncien Regime).
In the scientificBefore Present system of numbering years for purposes ofradiocarbon dating, the reference date is January 1, 1950 (though the specific date January 1 is quite unnecessary, as radiocarbon dating has limited precision).[6][7]
Different branches ofFreemasonry have selected different years to date their documents according to a Masonic era, such as theAnno Lucis (A.L.).
The officialJapanese system numbers years from the accession of the currentemperor, regarding the calendar year during which the accession occurred as the first year. A similarsystem existed inChina before 1912, being based on the accession year of the emperor (1911 was thus the third year of theXuantong period). With the establishment of theRepublic of China in 1912, the republican era was introduced. It is still very common inTaiwan 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).
Anepoch 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 countingleap seconds.
Anepoch 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 inGeochronology is atime period, typically in the order of tens of millions of years. The current epoch is theHolocene.
^Blackburn, B; Holford-Strevens, L (2003). "Incarnation era".The Oxford Companion to the Year: An exploration of calendar customs and time-reckoning. Oxford University Press. p. 881.
^Richards, E. G. (2013). "Calendars". In Urban, S. E.; Seidelman, P. K. (eds.).Explanatory Supplement to the Astronomical Almanac (3rd ed.). Mill Valley, CA: University Science Books. pp. 616–617.
