Bissextile n. Leap year; every fourth year, in which a day is added to the month of February on account of the excess of the tropical year (365 d. 5 h. 48 m. 46 s.) above 365 days. But one day added every four years is equivalent to six hours each year, which is 11 m. 14 s. more than the excess of the real year. Hence, it is necessary to suppress the bissextile day at the end of every century which is not divisible by 400, while it is retained at the end of those which are divisible by 400.
... simplify the calendar, this accumulating fraction of five hours, forty-eight minutes, forty-six seconds (about a quarter day) is added every four years to a bissextile year (leap-year), and thus we have uneven years of three hundred and sixty-five, and three hundred and sixty-six days. Every year of which the figure is divisible by four is a leap-year. By adding a quarter day to each year, there is a surplus ... — Astronomy for Amateurs • Camille Flammarion Read full book for free!
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