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(1) In chronology, a fixed point in time that is connected with a real or legendary event and from which a series of years is reckoned. The term “era” is also applied to the system of chronological notation itself. The Christian era, or Common Era, for example, counts the years from a generally accepted date in the Christian religion: the birth of Christ. The peoples of ancient times used different eras that took as their starting point a real or mythical event or the founding of a ruling dynasty. The era of Nabonassar of Babylon, for example, began in 747 B.C.; in ancient Rome, time was reckoned from the founding of Rome (ab urbe condita), considered to have taken place in 753 B.C.; and the Muslim era (the Hegira) began in A.D. 622, the year in which, according to legend, Muhammad fled from Mecca to Medina.
Some eras are reckoned from a point in time chosen for astronomical reasons, which may be combined with religious considerations; examples include those systems that start from an assumed date of the creation of the world. In Judaism this date is 3761 B.C; in the Russian Orthodox Church it is 5508 B.C. In the same category is the Hindu Kali Yuga, which began in 3102 B.C. The Julian period, which was introduced at the end of the 16th century, provided a convenient system of reckoning for astronomical and chronological calculations; it began in 4713 B.C. (see and CALENDAR).
(2) A major historical period that differs fundamentally from the preceding period.