Epact (Gr. en-anTdg, added, inserted, excessive), a word formerly employed to denote the difference in time obtained by comparing any two periods, as the 11 days by which the solar exceeds the lunar year. Now it denotes generally the number of days between the last new moon and the first day of the next year. These epacts are counted in Roman figures from I. to XXVIII. When a new moon falls on the first of January, then the epact is 0, that of the next year XI., and that of the third XXII. But in continuing this calculation, 30 is subtracted from every number exceeding 30. The epact for the fourth jTear is therefore reduced to III.; then for the following two years 11 is added again, and from the sum obtained for the 7th year, being greater than 30, that number is again subtracted. Every 19th year the epact is XXVIII., after which the series is repeated. The epact does not give the exact age of the moon. In the Julian calendar the correction of the error in the lunar cycle is made at the end of 300 years; in the Gregorian calendar this error is assumed to amount to one day in 312£ years.

The epacts are used to determine Easter Sunday, on which the dates of all the other changeable feasts of the church depend.