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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.
2 Use in chronology
2.1 Geological era
2.2 Cosmological era
2.4 Regnal eras
3 See also
Look up era in Wiktionary, the free dictionary.
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.
Use in chronology
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 is subdivided into eras. There are
currently three eras defined in the Phanerozoic; the following table
lists them from youngest to oldest (BP is an abbreviation for "before
Beginning (millions of years BP)
End (millions of years BP)
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 = 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
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
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
Seleucid era and the Ancient Roman ab urbe condita ("AUC"),
counting from the foundation of the city.
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:
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 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.
"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 era", "Disco era", etc.
List of time periods
List of archaeological periods
^ 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.
Ab urbe condita
Anno Domini / Common Era
Hindu units of time
Hindu units of time (Yuga)
Canon of Kings
Lists of kings
Pre-Julian / Julian
Old Style and New Style dates
Adoption of the Gregorian calendar
Astronomical year numbering
Chinese sexagenary cycle
ISO week date
Winter count (Plains Indians)
Geological history of Earth
Geological time units
Global Standard Stratigraphic Age (GSSA)
Global Boundary Stratotype Section and Point (GSSP)
Law of superposition
Amino acid racemisation
Terminus post quem