Jewish months follow the moon, the feast necessarily comes each year at a different period, and in order to calculate this time correctly a computation is made of the number of years after which the moons will come on exactly the same days as before. This term of years is called a 'cycle.' If in any year Easter falls say on the last day of March, it will again fall on that day when the number of years in the cycle have gone by. The calculation requires a considerable amount of astronomical knowledge, and a great many different numbers have been proposed. The Metonic cycle, called after its inventor Meton of Athens (B.C. 432), was a period of nineteen years. The Jewish cycle, followed by the early Christians, was one of eighty-four years. The famous Hippolytus (A.D. 230) proposed a cycle of one hundred and twelve years. The Alexandrians, after the Council of Nicæa, fell back on the old Metonic cycle of nineteen years; but their adhesion to it was not constant. Theophilus of Alexandria (A.D. 380) proposed a cycle of four hundred and thirty-seven years, and Cyril of Alexandria (A.D. 412), one of ninety-five years. Meantime the Church of Rome had mostly followed the eighty-four year period, sometimes called the cycle of Anatolius (A.D. 284), although really of much older date than his time. Finally a cycle of five hundred and thirty-two years was proposed by Victorius (A.D. 463), and this in the end received general acceptance. It is now generally known as the cycle of Dionysius Exiguus (A.D. 527), and is practically the cycle used at the present day.
When Christianity was first preached in Ireland the eighty-four year cycle of Anatolius was in use. The Irish Church therefore continued to use it, and when the Church of Rome changed it for a better
