Epoch

Last updated January 08, 2024

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.
Anno Mundi (years since the creation of the world) is used in the Byzantine calendar (5509 BC).
Anno Mundi (years since the creation of the world) as used in the Hebrew calendar (3761 BC).^[1]^[2]
The Mesoamerican Long Count Calendar uses the creation of the fourth world in 3114 BC.
Olympiads, the ancient Greek era of four-year periods between Olympic Games, beginning in 776 BC.
Ab urbe condita ("from the foundation of the city"), used to some extent by Roman calendars of the Roman imperial period (753 BC).
Buddhist calendars 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),^[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 Minister 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 its predecessor dates from January 1, 1912, the first year after the Xinhai Revolution, which overthrew the 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 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 Holocene calendar uses 10,000 BC as the epoch, the beginning of the Holocene epoch on the geological time scale.

Regnal eras

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.

Related Research Articles

The terms anno Domini (AD) and before Christ (BC) are used when designating years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin and means "in the year of the Lord" but is often presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ". The form "BC" is specific to English, and equivalent abbreviations are used in other languages: the Latin form, rarely used in English, is Ante Christum natum (ACN) or Ante Christum (AC).

A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A date is the designation of a single and specific day within such a system. A calendar is also a physical record of such a system. A calendar can also mean a list of planned events, such as a court calendar, or a partly or fully chronological list of documents, such as a calendar of wills.

There are various types and subtypes of the Chinese calendar and horology, which over a millennium plus history has produced many variations. The topic of the Chinese calendar includes various traditional types of the Chinese calendar, of which particularly prominent are, identifying years, months, and days according to astronomical phenomena and calculations, with generally an especial effort to correlate the solar and lunar cycles experienced on earth—but which are known to mathematically require some degree of approximation. Typical features of early calendars include the use of the sexagenary cycle-based ganzhi system's repeating cycles of Heavenly Stems and Earthly Branches. The logic of the various permutations of the Chinese calendar has been based in considerations such as the technical from mathematics and astronomy, the philosophical considerations, and the political, and the resulting disparities between different calendars is significant and notable. Various similar calendar systems are also known from various regions or ethnic groups of Central Asia, South Asia, and other areas. Indeed, the Chinese calendar has influenced and been influenced by most parts of the world these days. One particularly popular feature is the Chinese zodiac. The Chinese calendar and horology includes many multifaceted methods of computing years, eras, months, days and hours.

An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy, a calendar era used for a given calendar, or the geological eras defined for the history of Earth.

The Hebrew calendar, also called the Jewish calendar, is a lunisolar calendar used today for Jewish religious observance and as an official calendar of Israel. It determines the dates of Jewish holidays and other rituals, such as yahrzeits and the schedule of public Torah readings. In Israel, it is used for religious purposes, provides a time frame for agriculture, and is an official calendar for civil holidays alongside the Gregorian calendar.

The Hijri calendar, also known in English as the Muslim calendar and Islamic calendar, is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to determine the proper days of Islamic holidays and rituals, such as the annual fasting and the annual season for the great pilgrimage. In almost all countries where the predominant religion is Islam, the civil calendar is the Gregorian calendar, with Syriac month-names used in the Levant and Mesopotamia but the religious calendar is the Hijri one.

Intercalation or embolism in timekeeping is the insertion of a leap day, week, or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.

A leap year is a calendar year that contains an additional day compared to a common year. The 366th day is added to keep the calendar year synchronized with the astronomical year or seasonal year. Because astronomical events and seasons do not repeat in a whole number of days, calendars that have a constant number of days in each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("intercalating") an additional day, a leap day, or month, a leap month, into some years, the drift between a civilization's dating system and the physical properties of the Solar System can be corrected.

A lunar calendar is a calendar based on the monthly cycles of the Moon's phases, in contrast to solar calendars, whose annual cycles are based on the solar year. The most widely observed purely lunar calendar is the Islamic calendar. A purely lunar calendar is distinguished from a lunisolar calendar, whose lunar months are brought into alignment with the solar year through some process of intercalation – such as by insertion of a leap month. The details of when months begin vary from calendar to calendar, with some using new, full, or crescent moons and others employing detailed calculations.

A lunisolar calendar is a calendar in many cultures, combining lunar calendars and solar calendars. The date of Lunisolar calendars therefore indicates both the Moon phase and the time of the solar year, that is the position of the Sun in the Earth's sky. If the sidereal year is used instead of the solar year, then the calendar will predict the constellation near which the full moon may occur. As with all calendars which divide the year into months there is an additional requirement that the year have a whole number of months. In some case ordinary years consist of twelve months but every second or third year is an embolismic year, which adds a thirteenth intercalary, embolismic, or leap month.

The Metonic cycle or enneadecaeteris is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. The recurrence is not perfect, and by precise observation the Metonic cycle defined as 235 synodic months is just 2 hours, 4 minutes and 58 seconds longer than 19 tropical years. Meton of Athens, in the 5th century BC, judged the cycle to be a whole number of days, 6,940. Using these whole numbers facilitates the construction of a lunisolar calendar.

In astronomy, the new moon is the first lunar phase, when the Moon and Sun have the same ecliptic longitude. At this phase, the lunar disk is not visible to the naked eye, except when it is silhouetted against the Sun during a solar eclipse.

A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar. The main other types of calendar are lunar calendar and lunisolar calendar, whose months correspond to cycles of Moon phases. The months of the Gregorian calendar do not correspond to cycles of the Moon phase.

<span class="mw-page-title-main">Thai solar calendar</span> Legal calendar in Thailand

The Thai solar calendar was adopted by King Chulalongkorn in 1888 CE as the Siamese version of the Gregorian calendar, replacing the Thai lunar calendar as the legal calendar in Thailand. Years are now counted in the Buddhist Era (B.E.): พุทธศักราช, พ.ศ.RTGS: Phutthasakkarat which is 543 years ahead of the Gregorian calendar.

Calendar reform or calendrical reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design.

A calendar era is the period of time elapsed since one epoch of a calendar and, if it exists, before the next one. For example, it is the year 2024 as per the Gregorian calendar, which numbers its years in the Western Christian era.

The Buddhist calendar is a set of lunisolar calendars primarily used in Tibet, Cambodia, Laos, Myanmar, India, Sri Lanka, Thailand and Vietnam as well as in Malaysia and Singapore and by Chinese populations for religious or official occasions. While the calendars share a common lineage, they also have minor but important variations such as intercalation schedules, month names and numbering, use of cycles, etc. In Thailand, the name Buddhist Era is a year numbering system shared by the traditional Thai lunar calendar and by the Thai solar calendar.

The Holocene calendar, also known as the Holocene Era or Human Era (HE), is a year numbering system that adds exactly 10,000 years to the currently dominant numbering scheme, placing its first year near the beginning of the Holocene geological epoch and the Neolithic Revolution, when humans shifted from a hunter-gatherer lifestyle to agriculture and fixed settlements. The current year by the Gregorian calendar, AD 2024, is 12024 HE in the Holocene calendar. The HE scheme was first proposed by Cesare Emiliani in 1993, though similar proposals to start a new calendar at the same date had been put forward decades earlier.

<span class="mw-page-title-main">Thai calendar</span> Solar and lunar calendars used in Thailand

In Thailand, two main calendar systems are used alongside each other: the Thai solar calendar, based on the Gregorian calendar and used for official and most day-to-day purposes, and the Thai lunar calendar, used for traditional events and Buddhist religious practices.

References

↑ Solomin, Rachel M. "Counting the Jewish Years". myjewishlearning.com. Archived from the original on 2020-02-12. Retrieved 2016-12-27.
↑ Lee, Scott E. (2006). "Overview of Calendars". rosettacalendar.com. Archived from the original on 2020-08-20. Retrieved 2006-10-23.
↑ Dershowitz, Nachum; Reingold, Edward M. (2008). Calendrical Calculations (3rd ed.). Cambridge University Press. p. 289. ISBN 978-0-521-70238-6.
↑ 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.
↑ Higham, Thomas. "Radiocarbon dating – Age calculation". c14dating.com. Thomas Higham (archaeologist). Archived from the original on June 10, 2007. Retrieved December 31, 2009.
↑ Stuiver, Minze; Polach HA (1977). "Discussion; reporting of C-14 data". Radiocarbon. University of Arizona. 19 (3): 355–363. Bibcode:1977Radcb..19..355S. doi: 10.1017/S0033822200003672 . S2CID 56572650. Archived from the original on August 17, 2020. Retrieved October 5, 2018.

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[1] Solomin, Rachel M. "Counting the Jewish Years". myjewishlearning.com. Archived from the original on 2020-02-12. Retrieved 2016-12-27.

[2] Lee, Scott E. (2006). "Overview of Calendars". rosettacalendar.com. Archived from the original on 2020-08-20. Retrieved 2006-10-23.

[3] Dershowitz, Nachum; Reingold, Edward M. (2008). Calendrical Calculations (3rd ed.). Cambridge University Press. p. 289. ISBN 978-0-521-70238-6.

[4] 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.

[5] 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.

[6] Higham, Thomas. "Radiocarbon dating – Age calculation". c14dating.com. Thomas Higham (archaeologist). Archived from the original on June 10, 2007. Retrieved December 31, 2009.

[7] Stuiver, Minze; Polach HA (1977). "Discussion; reporting of C-14 data". Radiocarbon. University of Arizona. 19 (3): 355–363. Bibcode:1977Radcb..19..355S. doi: 10.1017/S0033822200003672 . S2CID 56572650. Archived from the original on August 17, 2020. Retrieved October 5, 2018.

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