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Era

The meaning of «era»

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.

The word has been in use in English since 1615, 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".

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, 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.

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".

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 Eon, for example, 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").

The older Proterozoic and Archean eons are also divided into eras.

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 eras count the years since a particular date (epoch), often one with religious significance. 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 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 (AD also written 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 from Mecca to Medina, which occurred in 622 AD. The Islamic year is some days shorter than 365; January 2012 fell in 1433 AH ("After Hijra").

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