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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. Comparable terms are epoch, age, period, saeculum, aeon (Greek aion) and Sanskrit yuga.

Contents

1 Etymology 2 Use in chronology

2.1 Geological era 2.2 Cosmological era 2.3 Calendar
Calendar
eras 2.4 Regnal eras 2.5 Historiography

3 See also 4 Notes

Etymology[edit]

Look up era in Wiktionary, the free dictionary.

The word has been in use in English since 1615, and is derived from Late Latin
Late Latin
aera "an era or epoch from which time is reckoned," probably identical to Latin æra "counters used for calculation," plural of æs "brass, money". The Latin word use in chronology seems to have begun in 5th century Visigothic Spain, where it appears in the History
History
of Isidore of Seville, and in later texts. The Spanish era is calculated from 38 BC, perhaps because of a tax (cfr. indiction) levied in that year, or due to a miscalculation of the Battle of Actium, which occurred in 31 BC. Like epoch, "era" in English originally meant "the starting point of an age"; the meaning "system of chronological notation" is c.1646; that of "historical period" is 1741. Use in chronology[edit] In chronology, an era is the highest level for the organization of the measurement of time. A calendar era indicates a span of many years which are numbered beginning at a specific reference date (epoch), which often marks the origin of a political state or cosmology, dynasty, ruler, the birth of a leader, or another significant historical or mythological event; it is generally called after its focus accordingly as in Victorian era. Geological era[edit] Main article: Era (geology) In large-scale natural science, there is need for another time perspective, independent from human activity, and indeed spanning a far longer period (mainly prehistoric), where geologic era refers to well-defined time spans. The next-larger division of geologic time is the eon. The Phanerozoic
Phanerozoic
Eon is subdivided into eras.[1] There are currently three eras defined in the Phanerozoic; the following table lists them from youngest to oldest (BP is an abbreviation for "before present").

Era[2][3] Beginning (millions of years BP) End (millions of years BP)

Cenozoic 66.038 N/A

Mesozoic 252.17 66.038

Paleozoic 542 252.17

The older Proterozoic
Proterozoic
and Archean
Archean
eons are also divided into eras. Cosmological era[edit] For periods in the history of the universe, the term "epoch" is typically preferred, but "era" is used e.g. of the "Stelliferous Era". Calendar
Calendar
eras[edit] Main article: Calendar
Calendar
era Calendar
Calendar
eras count the years since a particular date (epoch), often one with religious significance. Anno mundi
Anno mundi
("year of the world") refers to a group of calendar eras based on a calculation of the age of the world, assuming it was created as described in the Book of Genesis. In Jewish religious contexts one of the versions is still used, and many Eastern Orthodox
Eastern Orthodox
religious calendars used another version until 1728. Hebrew year 5772 AM began at sunset on 28 September 2011 and ended on 16 September 2012. In the Western church Anno Domini
Anno Domini
(=AD = CE), counting the years since the birth of Jesus on traditional calculations, was always dominant. The Islamic calendar, which also has variants, counts years from the Hijra or emigration of the Islamic prophet Muhammad
Muhammad
from Mecca
Mecca
to Medina, which occurred in 622 CE. The Islamic year is some days shorter than 365; January 2012 fell in 1433 AH ("After Hijra"). For a time ranging from 1872 to the Second World War, the Japanese used the imperial year system (kōki), counting from the year when the legendary Emperor Jimmu
Emperor Jimmu
founded Japan which occurred in 660 BC. Many Buddhist calendars count from the death of the Buddha, which according to the most commonly used calculations was in 545-543 BCE or 483 BCE. Dates are given as "BE" for "Buddhist Era"; 2000 CE was 2543 BE in the Thai solar calendar. Other calendar eras of the past counted from political events, such as the Seleucid era and the Ancient Roman ab urbe condita ("AUC"), counting from the foundation of the city. Regnal eras[edit] Main article: Regnal year The word era also denotes the units used under a different, more arbitrary system where time is not represented as an endless continuum with a single reference year, but each unit starts counting from one again, as if time starts again. The use of regnal years is a rather impractical system, and a challenge for historians if a single piece of the historical chronology is missing, and often reflects the preponderance in public life of an absolute ruler in many ancient cultures. Such traditions sometimes outlive the political power of the throne, and may even be based on mythological events or rulers who may not have existed (for example Rome numbering from the rule of Romulus and Remus). In a manner of speaking the use of the supposed date of the birth of Christ as a base year is a form of an era. In East Asia, each emperor's reign may be subdivided into several reign periods, each being treated as a new era. The name of each was a motto or slogan chosen by the emperor. Different East Asian countries utilized slightly different systems, notably:

Chinese eras Japanese era Korean eras Vietnamese eras

A similar practice survived in the United Kingdom
United Kingdom
until quite recently, but only for formal official writings: in daily life the ordinary year A.D. has been used for a long time, but Acts of Parliament were dated according to the years of the reign of the current Monarch, so that "61 & 62 Vict c. 37" refers to the Local Government (Ireland) Act 1898 passed in the session of Parliament in the 61st/62nd year of the reign of Queen Victoria. Historiography[edit] "Era" can be used to refer to well-defined periods in historiography, such as the Roman era, Elizabethan era, Victorian era, etc. Use of the term for more recent periods or topical history might include Soviet era, and "musical eras" in the history of modern popular music, such as the " Big Band
Big Band
era", "Disco era", etc. See also[edit]

Periodization List of time periods List of archaeological periods

Notes[edit]

^ Short, N.M. (2009). "Geologic Time" Archived 2005-04-18 at the Wayback Machine. in Remote Sensing Tutorial Archived 2009-10-27 at the Wayback Machine.. NASA. ^ Lide, D. R. (1990). Handbook of Chemistry and Physics. Boca Raton: CRC Press. pp. 14-16. ^ "International Stratigraphic Chart". International Commission on Stratigraphy. Archived from the original on 30 May 2014. 

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Chronology

Key topics

Archaeology Astronomy Geology History Paleontology Time

Eras Epochs

Calendar
Calendar
eras

Human Era Ab urbe condita Anno Domini / Common Era Anno Mundi Byzantine era Seleucid era Spanish era Before Present Hijri Egyptian Sothic cycle Hindu units of time
Hindu units of time
(Yuga) Mesoamerican

Long Count Short Count Tzolk'in Haab'

Regnal year

Canon of Kings Lists of kings Limmu

Era names

Chinese Japanese Korean Vietnamese

Calendars

Pre-Julian / Julian

Pre-Julian Roman Original Julian Proleptic Julian Revised Julian

Gregorian

Gregorian Proleptic Gregorian Old Style and New Style dates Adoption of the Gregorian calendar Dual dating

Astronomical

Lunisolar Solar Lunar Astronomical year numbering

Others

Chinese sexagenary cycle Geologic Calendar Hebrew Iranian Islamic ISO week date Mesoamerican

Maya Aztec

Winter count
Winter count
(Plains Indians)

Astronomic time

Cosmic Calendar Ephemeris Galactic year Metonic cycle Milankovitch cycles

Geologic time

Concepts

Deep time Geological history of Earth Geological time units

Standards

Global Standard Stratigraphic Age (GSSA) Global Boundary Stratotype Section and Point (GSSP)

Methods

Chronostratigraphy Geochronology Isotope geochemistry Law of superposition Luminescence dating Samarium–neodymium dating

Chronological dating

Absolute dating

Amino acid racemisation Archaeomagnetic dating Dendrochronology Ice core Incremental dating Lichenometry Paleomagnetism Radiometric dating

Radiocarbon Uranium–lead Potassium–argon

Tephrochronology Luminescence dating Thermoluminescence dating

Relative dating

Fluorine absorption Nitrogen dating Obsidian hydration Seriation Stratigraphy

Genetic methods

Molecular clock

Linguistic methods

Glottochronology

Related topics

Chronicle New Chronology Periodization Synchronoptic view Timeline Year zero Circa Floruit Terminus post quem ASPRO chronology

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