- Greek: chronos, time
The science of time measurement. Mathematical chronology determines units to be employed in measuring time; historical chronology fixes in the general course of time the position of any particular occurrence or its date. The first requisite is the era, a fixed point of time. The Christian era was introduced c.527 by Dionysius Exiguus, a Scythian monk resident at Rome. He made its starting point the birth of Christ, which he reckoned to have been in the year 753 from the foundation of Rome, making this the first year of his era. This year and those following are designated as A.D. (Anno Domini, in the year of the Lord), and the years before Christ are designated as A.C. (< lang="la">ante Christum, before Christ) or B.C. This system was not general in Europe until after 1000. The Greeks dated events from 776 B.C., by Olympiads (periods of four years intervening between the Olympic games). The Romans frequently reckoned from the traditional foundation of their city (753 B.C.). They designated years by A.U.C. (ab urbe condita, from the founding of the city), or by the name of the consul in office. The later custom of dating by the regnal years of the Roman emperors was imitated by popes and other rulers. The indications of papal documents consist of conventional periods of 15 years, beginning in the reign of Constantine. When Julius Cregar reformed the calendar in 45 B.C., he fixed 1 January as New Year’s Day. In Rome until the reform of the calendar, 1582, the year began with Christmas Day, a custom still continued in papal bulls. The Julian system of time measurement was inaccurate and by the 16th century was 10 days in arrear. The New Style (N.S.) was introduced by Pope Gregory XIII, who ordained that 10 days in October 1582, should not be counted, that the year should begin 1 January, and that three leap years should be omitted in every four centuries. In 1583 the Julian Period, containing about 7980 years, was introduced by Joseph Scaliger for reckoning events in one sequence, especially in astronomy. The Mohammedans date from the Hegira (flight, of Mohammed from Mecca), 16 July 622. At the French Revolution a new system of chronology was introduced: the Era of Liberty began in 1789 and was replaced by the Republican Era in 1792. This calendar was abolished by Napoleon in 1806. The order of the days of the week has never been interrupted in the Christian period and is the same in the Old Style and in the New. Various methods have been devised for ascertaining upon what day of the week any given date falls. The best known is that of Dominical Letters.