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
deciding moment is chosen when the epoch criterion was reached.
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.
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),[3] also used in modern times as the national calendars of
Nepal and
Bangladesh.
The
Julian and
Gregorian calendars use as epoch the
Incarnation of Jesus as calculated in the 6th century by
Dionysius Exiguus.[4] (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 (AD 622). The year count in this calendar shifts relative to the solar year count, as the calendar is
purely lunar: its year consists of 12
lunations 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) or AH. This calendar is used in
Sunni Islam and related sects.
The epoch of the official
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
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.[5]
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 MinisterPhibunsongkhram 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).
North Korea uses a system that starts in 1912 (=
Juche 1), the year of the birth of its founder
Kim Il-Sung.
The
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).[6][7]
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 official
Japanese system numbers years from the accession of the current
emperor, regarding the calendar year during which the accession occurred as the first year. A similar
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 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).
Other applications
An
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 seconds.
An
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 period of time, typically in the order of tens of millions of years. The current epoch is the
Holocene.
^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.