Epact

From the Catholic Encyclopedia

(Gr. épaktai hemérai; Lat. dies adjecti).

The surplus days of the solar over the lunar year; hence, more freely, the number of days in the age of the moon on 1 January of any given year. The whole system of epacts is based on the Metonic Lunar Cycle (otherwise known as the Cycle of Golden Numbers), and serves to indicate the days of the year on which the new moons occur.

THE CHURCH LUNAR CALENDAR

It is generally held that the Last Supper took place on the Jewish Feast of the Passover, which was always kept on the fourteenth day of the first month of the old Jewish calendar. Consequently, since this month always began with that new moon of which the fourteenth day occurred on or next after the vernal equinox, Christ arose from the dead on Sunday, the seventeenth day of the so-called paschal moon. It is evident, then, that an exact anniversary of Easter is impossible except in years in which the seventeenth day of the paschal moon falls on Sunday. In the early days of Christianity there existed a difference of opinion between the Eastern and Western Churches as to the day on which Easter ought to be kept, the former keeping it on the fourteenth day and the latter on the Sunday following. To secure uniformity of practice, the Council of Nicæa (325) decreed that the Western method of keeping Easter on the Sunday after the fourteenth day of the moon should be adopted throughout the Church, believing no doubt that this mode fitted in better with the historical facts and wishing to give a lasting proof that the Jewish Passover was not, as the Quartodeciman heretics believed, an ordinance of Christianity.

As in the Julian calendar the months had lost all their original reference to the moon, the early Christians were compelled to use the Metonic Lunar Cycle of the Greeks to find the fourteenth day of the paschal moon. This cycle in its original form continued to be used until 1582, when it was revised and embodied in the Gregorian calendar. The Church claims no astronomical exactness for her lunar calendar; we shall show presently the confusion which would necessarily result from an extreme adherence to precise astronomical data in determining the date of Easter. She wishes merely to ensure that the fourteenth day of the calendar moon shall fall on or shortly after the real fourteenth day but never before it, since it would be chronologically absurd to keep Easter on or before the Passover. Otherwise, as Clavius plainly states (Romani Calendarii a Gregorio XIII P.M. restituti explicatio, cap. V, § 13, p. 85), she regards with indifference the occurrence of the moons on the day before or after their proper seats and cares much more for peace and uniformity than for the equinox and the new moon. It may be mentioned here that Clavius's estimate of the accuracy of the calendar, in the compilations of which he took such a leading part, is extremely modest, and the seats assigned by him to the new moons tally with strict astronomical findings in a degree which he seems never to have anticipated. The impossibility of taking the astronomical moons as our sole guide in finding the date of Easter will be best understood from an example: Let us suppose that Easter is to be kept (as is at least implied by the British Act of Parliament regulating its date) on the Sunday after the astronomical full moon, and that this full moon, as sometimes happens, occurs just before midnight on Saturday evening in the western districts of London or New York. The full moon will therefore happen a little after midnight in the eastern districts, so that Easter, if regulated strictly by the paschal full moon, must be kept on one Sunday in the western and on the following Sunday in the eastern districts of the same city. Lest it be thought that this is carrying astronomical exactness to extremes, we may say that, if Easter were dependent on the astronomical moons, the feast could not always be kept on the same Sunday in England and America. Seeing, therefore, that astronomical accuracy must at some point give way to convenience and that an arbitrary decision on this point is necessary, the Church has drawn up a lunar calendar which maintains as close a relation with the astronomical moons as is practicable, and has decreed that Easter is to be kept on the Sunday after the fourteenth day of the paschal moon as indicated by this calendar.

METONIC LUNAR CYCLE OR CYCLE OF GOLDEN NUMBERS

In the year now known as 432 B.C., Meton, an Athenian astronomer, discovered that 235 lunations (i.e. lunar months) correspond with 19 solar years, or, as we might express it, that after a period of 19 solar years the new moons occur again on the same days of the solar year. He therefore divided the calendar into periods of 19 years, which he numbered 1, 2, 3, etc. to 19, and assumed that the new moons would always fall on the same days in the years indicated by the same number. This discovery found such favour among the Athenians that the number assigned to the current year in the Metonic Cycle was henceforth written in golden characters on a pillar in the temple, and, whether owing to this circumstance or to the importance of the discovery itself, was known as the Golden Number of the year. As the 19 years of the Metonic Cycle were purely lunar (i.e. each contained an exact number of lunar months) and contained in the aggregate 235 lunations, it was clearly impossible that all the years should be of equal length. To twelve of the 19 years 12 lunations were assigned, and to the other seven 13 lunations, the thirteenth lunation being known as the embolismic or intercalary month.

Length of the Lunations

The latest calculations have shown that the average duration of the lunar month is 29 days, 12 hours, 44 mins., 3 secs. To avoid the difficulty of reckoning fractions of a day in the calendar, all computators, ancient and modern, have assigned 30 and 29 days alternately to the lunations of the year, and regarded the ordinary lunar year of 12 lunations as lasting 354 days, whereas it really lasts some 8 hours and 48 mins. longer. This under-estimation of the year is compensated for in two ways: (1) by the insertion of one extra day in the lunar (as in the solar) calendar every fourth year, and (2) by assigning 30 days to six of the seven embolismic lunations, although the average lunation lasts only about 29.5 days. A comparison of the solar and lunar calendars for 76 years (one cycle of 19 years is unsuitable in this case, since it contains sometimes 4, sometimes 5, leap years) will make this clearer:

Therefore 940 calendar lunations (since 19 years equal 235 lunations) contain 27,759 days (29 d., 12 hrs., 44 mins., 3 secs. times 940 equals 27,758 d., 18 hrs., 7 mins.). But 940 lunations averaging 29.5 days equal only 27,730 days. Consequently, if we assign 30 and 29 days uninterruptedly to alternate lunations, the lunar calendar will, after 76 years, anticipate the solar by 29 days. The intercalation of the extra day every fourth year in the lunar calendar reduces the divergent to 10 days in 76 years i.e. 2.5 days in 19 years. The divergence is removed by assigning to the seven embolismic months (which would otherwise have contained 7 times 29.5, or 206.5, days) 209 days, 30 days being assigned to each of the first six and 29 to the seventh.

THE MANNER OF INSERTION OF THE EMBOLISMIC MONTHS

As the Gregorian and Metonic calendars differ in the manner of inserting the embolismic months, only the former is spoken of here. It has just been said that seven of the 19 years of the lunar cycle contain a thirteenth, or embolismic, month, consisting in six cases of 30 days and in the seventh of 29 days. Granted that the first solar and lunar years begin on the same day (i.e. that the new moon occurs on 1 January), it is evident that, as the ordinary lunar year of 12 lunations is 11 days shorter than the solar, the lunar calendar will, after three years, anticipate the solar by 33 days. To the third lunar year, then, is added the first embolismic month of 30 days, reducing the divergence between the calendars to three days. After three further years, i.e. at the end of the sixth year, the divergence will have mounted to 36 (3 X 11 + 3) days, but, by the insertion of the second embolismic lunation, will be reduced to six days. Whenever, then, the divergence between the calendars amounts to more than 30 days, an embolismic month is added to the lunar year; at the end of the nineteenth lunar year, the divergence will be 29 days, and, as the last embolismic month consists of 29 days, it is clear that after the insertion of this month the nineteenth solar and lunar years will end on the same day and that the first new moon of the twentieth (as of the first) year will occur on 1 January. The divergence, therefore, at the end of the 19 successive years of the lunar cycle is: 11, 22, 3, 14, 25, 6, 17, 28, 9, 20, 1, 12, 23, 4, 15, 26, 7, 18, and 0 days.

CYCLE OF EPACTS

We have defined an epact as the age of the moon on 1 January, i.e. at the beginning of the year. If, then, the new moon occurs on 1 January in the first year of the Lunar Cycle, the Epact of the year is 0 or, as it is more usually expressed, *; and since the lunar year always begins with the new moon, it is clear that the divergence between the solar and lunar calendars, of which we have just been speaking, gives the Epacts of the succeeding years. Thus, after the first year, the divergence between the calendars amounts to 11 days; therefore, the new moon occurs 11 days before 1 January of the second solar year, which is expressed by saying that the Epact of the second solar year is XI. Granted, then, that the new moon occurs on 1 January in the first year of the Lunar Cycle, the epacts of the 19 years are as follows: