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An era is a span of time defined for the purposes of

geological eras defined for the history of Earth.[1]

Comparable terms are

age, period, saeculum, aeon (Greek aion)[2] and Sanskrit yuga.[3]


The word has been in use in English since 1615,[4] and is derived from 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".[5][6]

The Latin word use in chronology seems to have begun in 5th century Visigothic Spain, where it appears in the History of Isidore of Seville,[7] and in later texts. The Spanish era is calculated from 38 BC, Before Christ,[8][9] 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.[10]

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.[11]

Use in chronology


reference date (epoch),[12] 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;[13] it is generally called after its focus accordingly as in "Victorian era

Geological era

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.[13]
The next-larger division of geologic time is the
before present

Era[16][17] 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 and Archean eons are also divided into eras.[18][19]

Cosmological era

For periods in the

Stelliferous Era".[20]

Calendar eras

Calendar eras count the years since a particular date (epoch), often one with religious significance.

CE), counting the years since the birth of Jesus on traditional calculations, was always dominant.[24]


Hijra or emigration of the Islamic prophet Muhammad from Mecca to Medina, which occurred in 622 AD.[25] The Islamic year is some days shorter than 365; January 2012 fell in 1433 AH ("After Hijra").[26]

For a time ranging from 1872 to the Second World War, the Japanese used the imperial year system (kōki),[27] counting from the year when the legendary Emperor Jimmu founded Japan, which occurred in 660 BC.[28]


Buddha, which according to the most commonly used calculations was in 545–543 BCE or 483 BCE.[29] Dates are given as "BE" for "Buddhist Era"; 2000 AD was 2543 BE in the Thai solar calendar.[29]

Other calendar eras of the past counted from political events, such as the Seleucid era[30] and the Ancient Roman ab urbe condita ("AUC"), counting from the foundation of the city.[31]

Regnal eras

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.

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.[33] The name of each was a motto or slogan chosen by the emperor. Different East Asian countries utilized slightly different systems, notably:

A similar practice survived in the 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

Queen Victoria.[35]


"Era" can be used to refer to well-defined periods in historiography, such as the

Use of the term for more recent periods or topical history might include

See also


  1. ^ "Era | definition in the Cambridge English Dictionary". Cambridge Dictionary. Retrieved 11 December 2023.
  2. ^ " - The world's favorite online thesaurus!". 6 December 2023. Retrieved 11 December 2023.
  3. ^ "Yuga". Unabridged (Online). n.d. Retrieved 11 December 2023.
  4. ^ "Time Traveler by Merriam-Webster: Words from 1615". Retrieved 11 December 2023.
  5. ^ Peón, Baltasar (1863). Estudios de cronología universal (in Spanish). Imprenta Nacional.
  6. JSTOR 593870
  7. ^ Hispalensis, Isidorus (1773). Isidori Hispalensis Historia de regibus Gothorum, Vandalorum et Suevorum (in Latin).
  8. ^ Cheney, Carl D.; Jones, Michael (2000). A Handbook of Dates: For Students of British History (Rev. ed.). Cambridge University Press. p. 2.
  9. .
  10. ^ "Actium, 31 BC: the beginning of the end for Mark Antony and Cleopatra". HistoryExtra. Retrieved 11 December 2023.
  11. ^ "Definition of EPOCH". 22 November 2023. Retrieved 11 December 2023.
  12. .
  13. ^ a b "The Geological Society of London - How are Geological Periods Determined?". Retrieved 11 December 2023.
  14. . p. 4.
  15. ^ 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.
  16. ^ Lide, D. R. (1990). Handbook of Chemistry and Physics. Boca Raton: CRC Press. pp. 14–16.
  17. ^ "International Stratigraphic Chart". International Commission on Stratigraphy. Archived from the original on 30 May 2014.
  18. ^ "Proterozoic Eon | Oxygen Crisis, Animals, & Facts | Britannica". Retrieved 11 December 2023.
  19. ^ "Archean Eon | Atmosphere, Timeline, and Facts | Britannica". Retrieved 11 December 2023.
  20. ^ "Big Bang Timeline- The Big Bang and the Big Crunch - The Physics of the Universe". Retrieved 11 December 2023.
  21. ^ "Anno mundi | Jewish Calendar, History & Origins | Britannica". Retrieved 11 December 2023.
  22. ^ "Hebrew Date Converter - September 28, 2011 after sunset / 1st of Tishrei, 5772". Retrieved 11 December 2023.
  23. ^ "Hebrew Date Converter - September 16, 2012 after sunset / 1st of Tishrei, 5773". Retrieved 11 December 2023.
  24. ^ "Chronology - Christian History, Dates, Events | Britannica". Retrieved 11 December 2023.
  25. ^ "Islamic calendar | Months, Definition, & Facts | Britannica". 14 November 2023. Retrieved 11 December 2023.
  26. ^ "Hijri to Gregorian Date Converter - Islamic Date Converter". IslamicFinder. Retrieved 11 December 2023.
  27. .
  28. ^ Gubbins, John Harrington. (1922). The Making of Modern Japan, p. 71; Mossman, Samuel. (1873). New Japan, the Land of the Rising Sun, p. 462.
  29. ^ a b "Calendar systems and their role in patent documentation |". Retrieved 11 December 2023.
  30. ^ Denis C. Feeney, Caesar's Calendar, University of California Press, Berkeley 2007, p. 139.
  31. ^ .
  32. ^ "Regnal Years - The University of Nottingham". Retrieved 11 December 2023.
  33. ^ "Calendar systems and their role in patent documentation |". Retrieved 11 December 2023.
  34. .
  35. ^ "Chapter Five: Table of regnal year of English Sovereigns". Sweet & Maxwell's Guide to Law Reports and Statutes (Fourth ed.). London: Sweet & Maxwell's Guide. 1962. pp. 20–33.
  36. ^ "Historiography | NMU Writing Center". Retrieved 11 December 2023.
  37. ^ "Big Bands and the Swing Era". Acoustic Music. Retrieved 11 December 2023.
  38. ^ "Disco | Origins, Genres & Cultural Impact | Britannica". 9 December 2023. Retrieved 11 December 2023.

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