Epact

One feature which comes down from the earliest times, but which survives even in the printed calendars of our existing Breviary and Missal, is the insertion against each day of the "Epact" and the "Dominical Letter".

The Catholic Encyclopedia, Volume 3: Brownson-Clairvaux 1840-1916 1913

To take an example: the Epact of 1696 (its Golden Number being 6) is

The Catholic Encyclopedia, Volume 5: Diocese-Fathers of Mercy 1840-1916 1913

From this arrangement it is evident that if, for example, the Epact of a year is X, the new moons will occur in that year on the days before which the Epact X is placed in the calendar.

The Catholic Encyclopedia, Volume 5: Diocese-Fathers of Mercy 1840-1916 1913

Golden Number of 2459 is 9, and Epact of ninth year of Lunar Cycle after 2400 is

The Catholic Encyclopedia, Volume 5: Diocese-Fathers of Mercy 1840-1916 1913

The first day after 8 March to which the Epact XXVI is prefixed in the Church calendar is 4 April: consequently the paschal new moon in the year 2459 will occur on 4

The Catholic Encyclopedia, Volume 5: Diocese-Fathers of Mercy 1840-1916 1913

Since the Epact of 2459 is XXVI, the new moons of this year will occur on the days before which XXVI is placed in the church calendar (e.g. in the Breviary).

The Catholic Encyclopedia, Volume 5: Diocese-Fathers of Mercy 1840-1916 1913

The accompanying table [below] gives the Epact of every year from 1 B.C. to A.

The Catholic Encyclopedia, Volume 5: Diocese-Fathers of Mercy 1840-1916 1913

According to the Metonic Cycle, the new moon can never occur twice on the same date in the same nineteen years (the case is exceedingly rare even in the purely astronomical calendar); consequently, whenever the two Epacts XXV and XXIV occur in the same nineteen years, the new moons of the year whose Epact is

The Catholic Encyclopedia, Volume 5: Diocese-Fathers of Mercy 1840-1916 1913

XXVI, and since this Epact is found opposite 4 February in the Church calendar we know that in 1696 the new moon happened on that date and that consequently 23 February was the twentieth day of the calendar moon.

The Catholic Encyclopedia, Volume 5: Diocese-Fathers of Mercy 1840-1916 1913

XXV are indicated in the months of 29 days by Epact XXVI, with which the number 25 is for this object associated in the Church calendar.

The Catholic Encyclopedia, Volume 5: Diocese-Fathers of Mercy 1840-1916 1913