An
era is a commonly used word for long period of time. When used in science, for example geology,
eras denote clearly defined periods of time of arbitrary but well defined length, such as for example the
Mesozoic era from 252 Ma–66 Ma, delimited by a start event and an end event. When used in social history, eras may for example denote a period of some monarch's reign. In colloquial language, eras denote longer spans of time, before and after which the practices or fashions change to a significant degree.
Uses
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.
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. Such rather impractical system — a nightmare for historians once a single piece of the puzzle is missing — is the use of
regnal years, which often reflects the preponderance in public life of the absolute ruler in many ancient cultures, while such tradition sometimes outlives the political power of the throne.
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. was used since long, 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.
Geological era
In 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, but only the current eon, the
Phanerozoic, is subdivided into eras. There are currently three eras defined in the geological record; the following table lists them from youngest to oldest (BP is an abbreviation for "before present").
Cosmological era
In astronomy the periods are even longer, to cover the entire existence of the universe (in the order of 13.7 billion years), but usually just denoted in numerical units, as there is no significant link to any earthly reality, our planet being astronomically insignificant (except as the only known observation point).
Religious era
The term Christian era refers to
Anno Domini, the time when Jesus was born, while the expressions Jewish era and Islamic era indicates the time since those said religions began.
Colloquial "eras"
In common speech and various contexts, the term era is also used, by extension, for any (as a rule relatively long) period in history with a name, often relating to common characteristic(s), even if this is not the normal way to organize time. The most relevant type are politic periods, for example: the Roman era, the
Elizabethan era, the
Victorian era (dynastic criteria, only formally correct within the British realm/empire/Commonwealth) and the
Soviet era, or comparable literary notions like the Biblical era.
The word era is also popularly used to denote the passing of — often shorter — periods that are only defined in terms of a specific discipline of sphere of life, such as the prominence of an artistic style, or more specifically in
music, see
musical eras, described in
History of music, such as the
Big Band era,
Disco era. An event such as the death of
Frank Sinatra is poetically called the
end of an era.
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's 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.
