ÆRA, in Chronology, a fixed point of time from whence any number of years is begun to be counted.
It is sometimes also written in ancient authors Æra. The origin of the term is contested, though it is generally allowed to have had its rise in Spain. Sepulveda supposes it formed from A. ER. A. the notes or abbreviations of the words, annus erat Augusti, occasioned by the Spaniards beginning their computation from the time their country came under the dominion of Augustus, or that of receiving the Roman calendar. This opinion, however ingenious, is rejected by Scaliger, not only on account that in the ancient abbreviations A never stood for annus, unless when preceded by V for vis; and that it seems improbable they should put ER for erat, and the letter A, without any discrimination, both for annus and Augustus. Vossius nevertheless favours the conjecture, and judges it at least as probable, as either that of Isidore, who derives æra from æra, the "tribute-money," wherewith Augustus taxed the world: or that of Scaliger himself, who deduces it likewise from æra, though in a different manner. Æra, he observes, was used among the ancients for an article or item in an account; and hence it came also to stand for a sum or number itself. From the plural æra, came by corruption æra, æram, in the singular: much as Qstia, Qstiam, the name of a place, from Qstia, the mouths of the Tyber.
The difference between the terms æra and epoch is, that the æras are certain points fixed by some people, or nation; and the epochs are points fixed by chronologists and historians. The idea of an æra comprehends also a certain succession of years proceeding from a fixed point of time, and the epoch is that point itself. Thus the
Ararium the Christian ara began at the epoch of the birth of Jesus Christ. See CHRONOLOGY, where the different Eras, &c. are enumerated and explained.