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An era is a commonly used word for a long period of time. When used in science, for example geology, an era denotes a clearly defined period 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. When era is extended to a calendar system, it is known as a calendar era. In Sanskrit or Indian culture eras are known as Yugas.
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.
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. 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 present").
|Era||Beginning (millions of years BP)||End (millions of years BP)|
Cosmological era 
In astronomy the periods are even longer, to cover the entire existence of the universe (in the order of 13.8 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).
Calendar eras 
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 will end 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 Muhammad from Mecca to Medina, which occurred in 622 CE. The Islamic year is some days shorter than 365; January 2012 fell in 1433 AH ("After Hijra").
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.
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 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 Regulus). In a manner of speaking the use of the supposed date of the birth of Christ as a base year is a form of a Regnal 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.
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.
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.
- Short, N.M. (2009). "Geologic Time" in Remote Sensing Tutorial. NASA.
- Lide, D. R. (1990). Handbook of Chemistry and Physics. Boca Raton: CRC Press. p. 14-6.
See also 
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