60. A formula for finding it
THE RECKONING OF TIME
crating it as holy and illustrious, they utterly disdain, as if this [day] did not belong to Eastertide at all.
Along with other defenders of catholic faith and action, Theophilus, bishop of the Church of Alexandria, defeated these people by plain reasoning. Writing /449/ to the Emperor Theodosius the Elder, he said:
But it sometimes happens that some folk fall into error concerning the fourteenth Moon of the ¢rst month, if this Moon should fall on a Sunday.
When this happens, the fast would have to end on Saturday, when the thirteenth Moon is shown to arrive, and [thus] we start to do what is contrary to the Law. Hence it is appropriate to issue a vigorous warning that whenever the fourteenth Moon falls on a Sunday, we should rather postpone
Easter until the following week, and this for two reasons: ¢rst, lest when the thirteenth Moon is found on a Saturday we end our fast [then], which does not make sense, nor does the Law prescribe it, particularly since the light of that Moon is still seen to be imperfect in its own sphere; and then, lest we be obliged to fast on Sunday, as this constitutes the fourteenth
Moon, thereby doing something unseemly and illicit, for this is the custom of the Manichean sect. Therefore, since we ought not to fast when the fourteenth Moon falls on Sunday, nor is it reasonable to end our fast if the thirteenth Moon occurs on Saturday, we assert that [Easter] ought tobeputo¡untilthefollowingweek,asisevidentfromwhatisimmediately above, but nonetheless without perpetrating any prevarication concerning the calculation of Easter from this delay. For just as the number 10 includes 9, so also whenever the fourteenth Moon falls on a Sunday, because fasting is not permitted on that day, it is necessary to defer
Easter to the following week. But no harm is done to Easter because of this, because the rest of the days which follow are contained [within it].61
60. A FORMULA FOR FINDING IT
The skilful computist ought to memorize the fourteenth Moons of the ¢rst month, just as he memorizes the annual lunar epacts. But if anyone wishes to ¢nd it by means of a formula, let him look at what the epacts are of the year /450/ he wishes to compute, and if they are 14 or 15, let him know that the fourteenth Moon will come on the 11th or 12th kalends of April [22 or 21 March]. For as has often been mentioned, the 11th kalends of this month is the proper date of all the epacts. If the 61 Prologus Theophili 3 (223^225).