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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.


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 .

Geological 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").

Era [2] [3] Beginning (millions of years BP)End (millions of years BP)
Cenozoic 66.038N/A
Mesozoic 252.1766.038
Paleozoic 542252.17

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

Cosmological era

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

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").

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 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 AD was 2543 BE in the Thai solar calendar.

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 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.

See also

Related Research Articles

<i>Anno Domini</i> Western calendar era

The terms anno Domini (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin and means "in the year of the Lord", but is often presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ".

<i>Ab urbe condita</i> Ancient Roman year-numbering system

Ab urbe condita, or Anno urbis conditae, often abbreviated as AUC in either case, is a convention that was used in antiquity and by classical historians to refer to a given year in Ancient Rome. Ab urbe condita is rendered into idiomatic English as "from the founding of the City", while anno urbis conditae likewise translates to "in the year since the City's founding". Therefore, the traditional year of the foundation of Rome, 753 BC, would be written AUC 1, while AD 1 would be AUC 754. The foundation of the Empire in 27 BC would be AUC 727.

Chinese calendar Lunisolar calendar from China

The traditional China calendar, or Former Calendar, Traditional Calendar or Lunar Calendar, is a lunisolar calendar which reckons years, months and days according to astronomical phenomena. It is defined by GB/T 33661-2017, "Calculation and promulgation of the Chinese calendar", issued by the Standardisation Administration of China on 12 May 2017.

Geologic time scale System that relates geological strata to time

The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth's history. The table of geologic time spans, presented here, agree with the nomenclature, dates and standard color codes set forth by the International Commission on Stratigraphy (ICS).

The PaleozoicEra is the earliest of three geologic eras of the Phanerozoic Eon. It is the longest of the Phanerozoic eras, lasting from 541 to 251.902 million years ago, and is subdivided into six geologic periods : the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Paleozoic comes after the Neoproterozoic Era of the Proterozoic Eon and is followed by the Mesozoic Era.

The Precambrian is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.

The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 541 million years to the present, and began with the Cambrian Period when animals first developed hard shells preserved in the fossil record. Its name was derived from the Ancient Greek words φανερός and ζωή, meaning visible life, since it was once believed that life began in the Cambrian, the first period of this eon. The term "Phanerozoic" was coined in 1930 by the American geologist George Halcott Chadwick (1876–1953). The time before the Phanerozoic, called the Precambrian, is now divided into the Hadean, Archaean and Proterozoic eons.

An epoch, for the purposes of chronology and periodization, is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.

Chronology Science of arranging events in order of occurrence

Chronology is the science of arranging events in their order of occurrence in time. Consider, for example, the use of a timeline or sequence of events. It is also "the determination of the actual temporal sequence of past events".

A calendar era is the year numbering system used by a calendar. For example, the Gregorian calendar numbers its years in the Western Christian era. The instant, date, or year from which time is marked is called the epoch of the era. There are many different calendar eras.

Kings of Judah Wikimedia list article

The Kings of Judah were the monarchs who ruled over the ancient Kingdom of Judah. According to the biblical account, this kingdom was founded after the death of Saul, when the tribe of Judah elevated David to rule over it. After seven years, David became king of a reunited Kingdom of Israel. However, in about 930 BCE the united kingdom split, with ten of the twelve Tribes of Israel rejecting Solomon's son Rehoboam as their king. The tribes of Judah and Benjamin remained loyal to Rehoboam, and re-formed the Kingdom of Judah, while the other entity continued to be called the Kingdom of Israel, or just Israel.

A regnal year is a year of the reign of a sovereign, from the Latin regnum meaning kingdom, rule. Regnal years considered the date as an ordinal, not a cardinal number. For example, a monarch could have a first year of rule, a second year of rule, a third year of rule, and so on, but not a zeroth year of rule.

Egyptian chronology timeline

The majority of Egyptologists agree on the outline and many details of the chronology of Ancient Egypt. This scholarly consensus is the so-called Conventional Egyptian chronology, which places the beginning of the Old Kingdom in the 27th century BC, the beginning of the Middle Kingdom in the 21st century BC and the beginning of the New Kingdom in the mid-16th century BC.

Unit of time measurement unit for time

A unit of time or midst unit is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern definition, from the National Institute of Standards and Technology is:

The Holocene calendar, also known as the Holocene Era or Human Era (HE), is a year numbering system that adds exactly 10,000 years to the currently dominant numbering scheme, placing its first year near the beginning of the Holocene geological epoch and the Neolithic Revolution, when humans transitioned from a hunter-gatherer lifestyle to agriculture and fixed settlements. The current year by the Gregorian calendar, AD 2020, is 12020 HE in the Holocene calendar. The HE scheme was first proposed by Cesare Emiliani in 1993.

The Spanish era, sometimes called the era of Caesar, was a calendar era commonly used in the states of the Iberian Peninsula from the 5th century until the 15th, when it was phased out in favour of the Anno Domini (AD) system. The epoch of the Spanish era was 1 January 38 BC. To convert an Anno Domini date to the corresponding year in the Spanish era, add 38 to the Anno Domini year, such that Era 941 would be equivalent to AD 903. A date given in the Spanish era always uses the word era followed by a feminine ordinal number. This contrasts with the AD system that uses the masculine anno (year): i.e., era millesima octava versus anno millesimo octavo.

Tenji period

The Tenji period is a brief span of years during the Asuka period of Japanese history. The Tenji period describes a span of years which were considered to have begun in the 1322nd year of the Yamato dynasty.

Eonothem chronostratigraphic unit representing the totality of rock strata laid down during a certain eon of the geologic time scale

In stratigraphy and geology, an eonothem is the totality of rock strata laid down in the stratigraphic record deposited during a certain eon of the continuous geologic timescale. The eonothem is not to be confused with the eon itself, which is a corresponding division of geologic time spanning a specific amount of years, during which rocks were formed that are classified within the eonothem. Eonothems have the same names as their corresponding eons, which means during the history of the Earth only four eonothems were formed. Oldest to newest these are the Hadean, Archean, Proterozoic, and Phanerozoic. A rock stratum, fossil or feature present in the "upper Phanerozoic" eonothem would therefore have originated within the "later Phanerozoic" eon. In practice, the rock column is discontinuous:

In stratigraphy, paleontology, geology, and geobiology an erathem is the total stratigraphic unit deposited during a certain corresponding span of time during an era in the geologic timescale.

A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place.


  1. 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.
  2. Lide, D. R. (1990). Handbook of Chemistry and Physics. Boca Raton: CRC Press. pp. 14-16.
  3. "International Stratigraphic Chart". International Commission on Stratigraphy. Archived from the original on 30 May 2014.