Epact

Epact "in chronology, is the excess of the solar month above the lunar synodical month; or of the solar year above the lunar year of twelve synodical months; or of several solar months above as many synodical months; or of several solar years above as many periods, each consisting of 12 synodical months. The menstrual epact is the excess of the civil calendarmonth above the lunar month. For a month of 31 days, this epact is 1 day 11 hours 15 minutes 57 seconds, if we suppose new moon to occur on the first day of the month. The annual epact is the excess of the solar year above the lunar. As the Julian solar year is (nearly) 365 days, and the Julian lunar year is (nearly) 354 days, the annual epact is nearly 11 days. The epact for two Julian years is, therefore, nearly 22 days; for three years, 33 days; and so on. When, however, the epact passes 30 days, 30 falls to be deducted from it, as making an intercalary month. For three years, then, the epact is properly 3; and for 4 years, adding 11 days, it is 14 days; and so on. Following the cycle, starting from a new moon on the 1st of January, we find that the epact becomes 30 or 0 in the 19th year. The epact for the 20th year is again 11; and so on. The years in the cycle are marked by Roman numerals I, II, III, etc., called the Golden Numbers; and a table of the Julian epacts exhibits each year in the cycle with its golden number and epact. As the Gregorian year, SEE CALENDAR, differs from and is in advance of the Julian by 11 days (the number lost on the Julian account before the Gregorian computation of time was introduced in England), and as 11 days is the difference between the solar and lunar years, it follows that the Gregorian epact for any year is the same with the Julian epact for the year preceding 1."