The Roman adaptation of the Egyptian solar calendar introduced by Julius Caesar, with the technical aid of the Alexandrian astronomer Sosigenes, in 46 B.C.; that year was extended to 445 days by intercalation in order to bring the year into line with the solar year. While the divided the solar year of 365.25 days into 12 months of 30 days each, with 5, or in every fourth year, 6, intercalary days added after the last day of the twelfth month, the Romans, in their Julian calendar, retained the 31 days of March, May, Quintilis (July), and October, and the 28 days of February, as they had been in the older Roman calendar, but increased the other months, which until then all had 29 days, by one day (June, April, September, November) or two days (January, Sextilis [August], December), in order to have an annual total of 365 days.

The intercalary month previously inserted periodically, at the discretion of the pontifex maximus, after 23 February was replaced by the intercalary day inserted every fourth year after 23 February, and in such a year the 24 February (ante diem sextum Kalendas Martias) was counted twice, the intercalary day being ante diem bis sextum Kalendas Martias, hence the expression annus bissextilis for “leap year.” In the first thirty-six years of the Julian calendar’s use, the extra day was intercalated every three years instead of every four, by mistaken interpretation of the original prescription, and in 9 B.C. prohibited the intercalation of the extra day until A.D. 8. The vernal equinox was placed on 25 March, and the year began on 1 January.

The Julian calendar remained in general use in the Western world until it was replaced by the Gregorian calendar, itself a reform of the Julian calendar, in various countries between 1582 and 1924. It is still used for the calculation of and the movable feasts dependent on Easter in the Chalcedonian Orthodox churches. (For its correlation to the Alexandrian calendar used by Copts and Ethiopians, see CALENDAR, COPTIC and CALENDAR, GREGORIAN.)

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