ERA: This word (Lat. æra and era) denotes a sequence of years reckoned from a definite point of time, wherein every particular year has its fixed position by numerical rotation; and the point of time from which the era proceeds is termed its epoch. The word is first used by Isidore of Seville (q.v.) in the beginning of the seventh century; and attempts have accordingly been made to derive it from the Gothic. It has been correlated with the German Jahr and English "year"; but this is not at all certain, and many deem the more probable origin to be from the Latin æra (plural of æs in the sense of "counters," "reckoning").
The Christian Era. Among Christian peoples, the era now generally in use is that which has for its epoch the birth of Jesus Christ; that is, the years are reckoned "after the birth of Christ." This era was certainly brought into general use and probably was invented by Dionysius Exiguus (q.v.). That is to say, when (in 525) he was making a continuation of the ninety-five year Easter-table of Cyril of Alexandria from its expiration in the year 531 after Christ, he did not designate the separate years of this Easter cycle, as Cyril had done, as so many years after the Diocletian persecution, but as so many years ab incarnation Domini. He says, "We have been unwilling to connect our cycle with the name of an impious persecutor [Diocletian], but have chosen rather to note the years from the incarnation of our Lord Jesus Christ." For the first year of his computation, Dionysius assumed 754 of the City of Rome according to Varro's calculation, following probably some reckoning already known by his age. By incarnatio Domini, however, he understood, consistently with the phraseology then in vogue, not the birth of Jesus, but his conception; that is, the day of the annunciation to Mary (celebrated on Mar. 25; see ANNUNCIATION, FEAST OF THE). At the same time, he did not begin his era with this day, but with the first of January preceding--in other words, with the beginning of the year as it stood accepted in the calendar of Julius Cæsar. Hence "the first of January, 754, of the City of Rome according to Varro," is the epoch of the era of Dionysius. This was afterward misunderstood; incarnatio coming to be identified with nativitas [and Dec. 25 being the accepted day of Christ's birth (see CHRISTMAS)], people supposed that according to the reckoning of Dionysius, Jesus was born on Dec. 25, 753 A.U.C.--although Dionysius began his era a week after its proper epoch. Others supposed that Mar. 25, 753, or Dec. 25, 754, was the date of the incarnatio according to Dionysius (cf. the works on chronology, e.g., C. L. Ideler, Handbuch der mathematischen und technischen Chronologie, ii. 381 sqq. Berlin, 1826).
Consistently with the practise of making incarnatio synonymous with nativitas, this era was also designated a nativitate Domini, post Christum natum, or as now quite generally "after the birth of Christ." Other designations are anni circumcisions, anni domini nostri Jesu Christi, anni Christi gratiæ, anni gratiæ, etc.; still again, anni salutis, anni orbis redempti, etc. The designation anni trabeationis was doubtless originally intended to signify so many years after Christ's crucifixion, but is also applied to years after the birth of Christ.
It can not be doubted that Jesus was not born in the year 754 A.U.C. Dionysius, or his authority, must have been in error. King Herod, who commanded the slaughter of the children at Bethlehem (Matt. ii. 16-18) died 750 A.U.C., and Jesus must surely have been born before the death of Herod. But, if we consider the great difficulties of all chronological calculations, and especially the inadequate auxiliary apparatus that was available for Dionysius, no reproach is due him for his mistake. On the other hand, no one can seriously think of attempting to alter the Christian era to accord with the correct date of the birth of Jesus, even if this date could be accurately determined. The era is commended by its convenience, especially since the practise has arisen of reckoning backward as well as forward from its epoch; that is, of dating events before its inception, according to years before the birth of Christ (ante Christum natum). This custom came about at a comparatively late date; the well-known historian and chronologer J. C. Gatterer of Göttingen about 1780 dated events before the birth of Christ in "years of the world."
Other Eras. World eras, the epoch of which is the year of the creation of the world, have been prevalent in great number. To mention only two, a rather wide vogue was enjoyed by the world era of Panodorus, who reckoned 5,904 years from Adam to the year 412 A.D. (about which time he lived); his years began with Aug. 29, corresponding to the First of Thoth, or the Egyptian new year. Afterward, this era is usually termed the Antiochian, sometimes the Alexandrian. Its new year was also transferred to Sept. 1, in which case the eight latter months of its year 5493 are the eight former months of the year one of our chronology. More important than this is the Byzantine world era, which long served as the standard of computation in the Eastern Empire, in Russia, among the Albanians, Servians, and Modern Greeks. It counts sixteen years in excess of the Antiochian era, though likewise beginning the year with Sept. ; its year 5509 began with Sept. 1 of the year one before Christ. This era was in use in Russia till 1700; whence it originated appears not to be known.
Attempts to compute the year of the creation of the world on the basis of figures supplied in the Old Testament (the ages of the patriarchs, etc.), have been made by chronologists almost down to the present time. Scaliger and Calvisius hold the year one of our era to be the year 3950 from the creation; Petavius, the year 3984; Usher, the year 4004; Frank, 4182. Historians once used one or another of these systems in dating events, especially for the time before Christ; thus Gatterer, mentioned above, computed, in his earlier works, according to the world era of Petavius; in his later ones, according to that of Frank.
Of the eras employed in the Christian Church, two others may be mentioned briefly. The one is the Diocletian, already cited above, which originated in Egypt. Its epoch is the First of Thoth (Aug.29 of the Julian calendar), of 284 A.D. It numbers the years from the accession of Diocletian, though the first year of Diocletian is not reckoned from the day of his proclamation (Sept. 17), but, in accordance with a generally observed custom, from the new year's day of this year. As this era gained circulation in the Christian Church, it came to be termed, by way of reminder that Diocletian had cruelly persecuted the Christians, æra martyrum. The same era continued in observance, to some extent, as late as the eighth century. Besides this, the Spanish era was prevalent in Spain from the beginning of the fifth century, and in particular among the West Goths. Its epoch is the year 716 A.U.C., or 38 B.C. It is used, among others, by Isidore of Seville in his Historia Gothorum, and traces of its observance occur into the twelfth century.
The New Year. All these chronological systems had to yield, step by step, to that of Dionysius; and for a long time past, it has been the custom throughout Christendom to compute in years after (and before) the birth of Christ. In the light of this simple and unequivocal reckoning, it was not advantageous to forego the uniform practise of beginning the year with Jan. 1, as Dionysius had done in agreement with the Roman calendar. As a matter of fact, Jan. 1 appears to have maintained its place as the beginning of the year in civil life everywhere, nor have any calendars been found with a different initial date; moreover, Jan. 1 was named new year's day (see NEW YEAR'S FESTIVAL). Nevertheless other initial dates came into official use; especially Mar. 25 and Dec. 25 were favorite dates for beginning the year in the Middle Ages and down to modern times. [In England the change from Mar. 25 was made by act of 1751.] In the case of Mar. 25, we have still to distinguish between the calculus Pisanus, which computed from Mar. 25 before our new year, and the calculus Florentinus which computed from Mar. 25 after our new year. Other new year's dates are Mar. 1, Sept. 1, and the Saturday before Easter. Luther computed the year from Dec. 25; so that, for instance, the dating of a letter die innocentum 1530 denotes, by our mode of reckoning, Dec. 25, 1529. More detailed information as to these new year's dates is to be sought in text-books of chronology; a good synopsis is furnished by H. Grotefend in Taschenbuch der Zeitrechnung (Hanover, 1898), pp. 11 sqq.
Bibliography: Besides the work of Ideler, mentioned in the text, consult Ideler, Lehrbuch der Chronologie, Berlin, 1829; H. Grotefend, Zeitrechnung der deutschen Mittelalter und der Neuzeit, vols. i.-ii., 2d part, Hanover, 1891-98; idem, Taschenbuch der Zeitrechnung, ib. 1898; F. Ruhl, Chronologie des Mittel alters und der Neuzeit, Berlin, 1897; F. K. Ginzel, Handbuch der mathematischen und technischen Chronologie, vol. i., Leipsic, 1906. The literature under CHRONOLOGY may also be consulted. A voluminous literature might be cited, but it is composed largely of treatment of special topics bearing not too directly upon the subject.