epact

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The period of time necessary to bring the solar calendar into harmony with the lunar calendar.

Century Dictionary and Cyclopedia

1. n. The excess of a solar over a lunar year or month. Hence, usually A number attached to a year by a rule of the calendar to show the age, in days completed and commenced, of the calendar moon at the beginning of the year—that is, on January 1st in the Gregorian, Victorian, and early Latin calendars, or March 22d in the Dionysian calendar, or old style. A rule for the epact has been attached to every calendar of the Western churches, except the German Evangelical calendar of a. d. 1700-1779. The epact usually increases by 11 from one year to the next, 30 being subtracted from the sum when the latter exceeds 30 (a circumstance which indicates 13 new moons in the year); but in some years the increase is 12 instead of 11, and this is called a leap of the moon. In the Gregorian calendar the increase is sometimes only 10. In the earliest calendars the leaps of the moon took place every 12 years, and later every 14; but since the adoption of the Victorian calendar in the fifth century, they have taken place every 19 years. To find the epact in old style, divide the number of the year by 19, take 11 times the remainder after division, divide the product by 30, and the remainder after this division is the epact. When there is no remainder, some chronologers make the epact 29, but 30 is preferable. This epact shows the age of the calendar moon on March 22d, by means of which the age on every other day can be calculated, by allowing alternately 29 and 30 days to a lunation. This would also agree with the age of the mean moon were the calendar perfect. The intercalary day of leap-year necessarily removes the calendar moon one day from the mean moon in certain years; and the error of the 19-year period accumulates to one day every 310 years, so that to approximate more closely to the age of the moon the epact should be increased by 2 for every 300 years from the middle of the fifth century. It should also be increased by 1 for leap-years and years following leap-year. The Gregorian epact exceeds the Dionysian by 1 in the sixteenth and seventeenth centuries, agrees with it in the eighteenth and nineteenth centuries (but instead of 30 an asterisk, *, is written), and falls short of it by 1 in the twentieth and twenty-first centuries. This irregularity is because the Gregorian epact receives a solar correction, being a deduction of 1, at the advent of every century-year not a leap-year, and a lunar correction, being an addition of 1, every 300 years beginning with A. P. 1800 until seven such corrections have been applied, when 400 years elapse before a new series of seven corrections commences. This is called the cycle or period of epacts. The Gregorian epact shows the age of the calendar moon on January 1st. This will rarely differ by more than one day from the real moon.

Wiktionary

1. n. the time (number of days) by which a solar year exceeds twelve lunar months; it is used in the calculation of the date of Easter

GNU Webster's 1913

1. n. (Chron.) The moon's age at the beginning of the calendar year, or the number of days by which the last new moon has preceded the beginning of the year.