Era

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

.

Comparable terms are

.

Etymology

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, Before Christ, 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

In

".

Geological era

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

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 and Archean eons are also divided into eras.

Cosmological era

For periods in the

".

Calendar eras

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

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

The

, which occurred in 622 AD. 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 founded Japan, which occurred in 660 BC.

Many

.

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

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

). 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 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

.

Historiography

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

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

disco era

", etc.

See also

• Periodization
• List of time periods
• List of archaeological periods
• Epoch – Reference point from which time is measured

References