BISSEXTILE, or leap-year, a year con sisting of 366 days, and happening every fourth year, by the addition of a day in the month of February, when that year consists of 29 days. And this is done, to recover the 6 hours which the sun takes up nearly in his course more than the 365 days commonly allowed for it in other years.
The day thus added was by Julius Cx sar appointed to be the day before the 24th of February, which among the Ro mans was the 6th of the calends, and which on this occasion was reckoned twice ; whence it was called the bissex tile. By the statute The anno bissextile, 21 Hen. III. to prevent misunderstand ings, the intercalary day and that next be fore it are to he accounted as one day.
To find what year of the period any given year is, divide the given year by 4, then if 0 remains it is leap year ; but if any thing remain, the given year is so ma ny after leap year. But the astronomers concerned in reforming the calendar in 1582, by order of Pope Gregory XIII. observing that in four years the bissextile added 44 minutes more than the sun spent in returning to the same point of the ecliptic ; and computing that in 133 years these supernumerary minutes would form a day ; to prevent any changes be ing thus insensibly introduced into the seasons, directed, that in the course of 400 years there should he three sextiles retrenched ; so that every centesimal year, which is leap year according to the Julian account, is a common year in the Gregorian account, unless the number of centuries can be divided by 4 without a So 1600 and 2000 are bissex tile ; but 1700, 1800, and 1900 are com mon years.
The Gregorian computation has been received in most foreign countries ever since the reformation of the calendar in 1582, excepting some northern countries, as Russia, &e. And by act of parliament, passed in 1751, it commenced in all the dominions under the crown of Great Britain in the year following ; it being or dered by that act, that the natural day next following the 2nd of September should be accounted the 14th; omitting the intermediate 11 days of the common calendar. The supernumerary day in leap years being added at the end of the month February, and called the 29th of that month.