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The science of determining (Lat. computare ) time. In ecclesiastical usage it covers the ensemble of rules by which the date of Easter is reckoned. The earliest-known ecclesiastical computus is the Easter table of hippolytus for 222233, which was engraved by his admirers on his statue (now in the vestibule of the Vatican Library). The next known is the De pascha computus, written in 243 by an African writer (pseudo Cyprian) to correct a three-day error arising from Hippolytus's choice of a 16-year lunar cycle (Patrologia latina 4:93974). More important, however, is the computus (Patrologia latina 10:20932) of anatolius, bishop of Laodicea. This introduced, about 258, the Alexandrian 19-year cycle (Golden Number), which later, through the Liber de paschate of dionysius exiguus in 526, became standard. Since the Gregorian Calendar of 1582, however, a more practical system has obtained, based on the epact, i.e., the age of the moon on January 1. Other influential computists were bede (De ratione computi, Patrologia latina 90:579600); William duranti the elder (123096), bishop of Mende [bk. 8 of his Rationale divinorum officiorum (ed. Lyons 1672) 468485]; Helperic of Auxerre (9th century); john de sacrobosco; roger bacon; and robert grosseteste.

A knowledge of the computus was until recently part of every cleric's training. Charlemagne imposed the computus in his schools; a passage attributed in Gratian's Decretum (Corpus iuris canonici D.38 c.5) to Augustine but which is mostly from Bede's Penitentiale, specifies a computus as one of the books without which a priest "scarcely is worthy of the name." Ready-made computus tables, listing epact and Golden Number, dominical letters, and Indications, preface present-day Missals, Breviaries, and Martyrologies.

See Also: liturgical calendar i, catholic.

Bibliography: a. bucherius, De doctrina temporum commentarius (Antwerp 1634). n. nilles, De computo ecclesiastico (2d ed. Arras 1864); Kalendarium manuale utriusque ecclesiae orientalis et occidentalis, 2 v. (Innsbruck 189697). c. h. turner, "The Paschal Canon of Anatolius of Laodicea," English Historical Review 10 (1895) 699710. j. mayr, "De computo ecclesiastico," Zeitschrift für katholische Theologie 77 (1955) 301330. a. cordeliani, "Les Traités de comput du haut moyen âge (5261003)," Archivum latinitatis medii aevi 17 (1943) 5172. e. hufmayr, Die pseudocyprianische Schrift "De pascha computus," (Augsburg 1896). m. noirot, Catholicisme 2:143031.

[l. e. boyle]