Bryn Mawr Classical Review

Bryn Mawr Classical Review 2009.05.57

Alden A. Mosshammer, The Easter Computus and the Origins of the Christian Era. Oxford Early Christian Studies.   Oxford/New York:  Oxford University Press, 2008.  Pp. xi, 474.  ISBN 9780199543120.  $130.00.  

Reviewed by Gregory Halfond, Framingham State College (
Word count: 1446 words

It has been nearly ten years since a computer glitch dubbed Y2K by the media raised millennial fears (and hackles) throughout much of the developed world. While talking heads debated the severity of the problem and the possibilities of an apocalyptic crash of the world's computer systems, others raised the question whether, strictly speaking, the year AD 2000 marked the beginning of a new millennium, or whether that distinction belonged to 2001. As Alden Mosshammer observes in his new monograph, The Easter Computus and the Origins of the Christian Era, this debate caused a remarkable amount of attention to be directed towards Dionysius Exiguus, the sixth-century putative inventor of Anno Domini dating. Skeptics of the apocalyptic implications of a new millennium noted that Dionysius' dating of the nativity was quite likely wrong. They were by no means the first. The Venerable Bede had observed back in the early eighth century that Dionysius' dating of Christ's birth contradicted biblical, annalistic, and patristic evidence, which favored the years BC 2/3. Most modern scholars thus have assumed that Dionysius willfully broke with tradition, and calculated his own (incorrect) nativity date.

Mosshammer effectively undermines this accusation in a lengthy, highly technical, and meticulously-argued study, presented as a monograph not as a handbook.1 That he largely succeeds is a testament to his clearly extensive knowledge of antique calendrical traditions. Nevertheless, the exposition of his findings is not by any means reader-friendly, and even has the potential to limit their diffusion. There are too many occasions when Mosshammer's otherwise admirable attention to detail distracts the reader from his larger argument. Additionally, his lengthy, often multi-page, summaries of scholarly debates belong in the footnotes, as they likely will be of interest only to specialists. While Mosshammer's intention of allowing opposing views to be heard is admirable and while these discussions are models of sharp critical analysis, stylistically they confuse and interrupt his narrative. And, while our author surely cannot be held responsible for the complexity of his subject matter, which Mosshammer himself acknowledges can be "highly technical and abstruse" (5), he makes no concessions to his readers. These stylistic critiques are in no way intended to diminish Mosshammer's accomplishment. His work is not only an important contribution to the study of early Christian computus, it also clarifies Dionysius Exiguus' role in the definition of the chronological periodization scheme that defines our own time in relation to antiquity. However, it is a legitimate concern that such contributions will go underappreciated by readers unable to penetrate Dr. Mosshammer's difficult work.

Mosshammer divides his study into four sections: "Contexts," "The Easter Tables of Dionysius Exiguus," "Paschal Calculations in Early Christianity," and "The Origin of the Christian Era." Part I provides a necessary, albeit difficult, introduction to the chronological systems of antiquity, including Olympiads, Diocletian dating, regnal years, and indictions. Mosshammer additionally provides a brief, but helpful, discussion of the calendrical origins of Easter, whose intimate relationship with the Passover feast -- celebrated by the sacrifice of the paschal lamb on the fourteenth day of the lunar month of Nisan-- made it necessary to graft a lunar-based feast onto a solar calendar. Today, Easter falls on the first Sunday after the first full moon occurring on or after the vernal equinox (25 March in the Roman calendar), but standardization of observance was lacking in the centuries prior to the Council of Nicaea (325), which formulated this rule. Sunday observance, as Mosshammer shows, was by no means requisite: worshipers in Asia Minor, known by their critics as Quartodecimans, observed Pascha as a commemoration of Christ's passion on the fourteenth day of the month, the date of the lamb's sacrifice. Mosshammer convincingly argues that the attendees of Nicaea "apparently or implicitly endorsed the rule of the equinox, even if it published no rule as such" (52). Although the Council did not formally adopt a nineteen-year cycle, as Dionysius later claimed, for early Christians such a cycle made the calculation of the date of Easter an easier task. Ninety-five-year paschal tables, such as the one constructed by Dionysius, could thus be calculated, as after five nineteen-year cycles the Paschal moon returned to the table's launch-date.

In Part II, Mosshammer examines in detail Dionysius and his table, which Dionysius completed in AD 525. Dionysius addressed his Easter table to a certain Bishop Petronius, who had requested the table with the ninety-five-year table of Cyril of Alexandria nearing its end. Dionysius claimed that his use of Anno Domini dating was his sole innovation; otherwise he followed Cyril's cycle. Dionysius' table, like its predecessor, consisted of five nineteen-year cycles, beginning with the last nineteen-year cycle of Cyril. To his table, Dionysius appended argumenta, nine or ten mathematical formulae, including formulae for calculating the Anno Domini from the indiction. Following Charles W. Jones, Mosshammer makes the important point that regardless of whether the Roman Church requested the table's construction, it had no official standing in the Western Church (61).

In Part III, the longest and most wide-ranging section of his study, Mosshammer examines the context and (primarily eastern) models for Dionysius' table, and the extent to which he was more follower than innovator. Mosshammer begins by tracing the use of epacts, i.e. the age of the moon measured in days, in Easter calculations. Mosshammer credits Demetrius of Alexandria (AD 189-232) with first employing epacts for the purpose of paschal calculations, basing his conclusion primarily on later Coptic (and derivative Ethiopian) sources, which he acknowledges have their problems (112). Epacts allowed Demetrius to construct an Easter table based on an eight-year cycle. The oldest surviving table, a sixteen-year chart credited to Hippolytus of Rome (third century), while adapted to Roman customs, is based on this Alexandrian eight-year cycle. It was the third-century polymath, Anatolius of Laodicea, who introduced the nineteen-year cycle with the new moon on 22 March and an equinoctial date of 21 March (25 Phamenoth in the Egyptian calendar). According to Anatolius' methodology, Easter could fall no earlier than the fifteenth day of the moon. Over the course of the fourth-century, as Mosshammer shows, Anatolius' work served as a model for subsequent Alexandrian tables, augmented by the employment of Diocletian dating, which "arose in response to the introduction of the consular year and the loss of Egypt's special status as a royal domain" (177). Around the early fifth-century, Annianus of Alexandria introduced the 532-year period to Easter calculation. In his Pachoualion, he calculated a new base date for Creation (Sunday, 25 March, 5492), and adjusted Anatolius' epact so as to coordinate the cycle with the Alexandrian civil year. Annianus' work provided much of the theoretical basis for Cyril of Alexandria's paschal list, which commenced in the year AD 399.

In Part IV, Mosshammer turns to the central question of the origins of Dionysius' Christian era. His principal argument in this section is that Dionysius was not the originator of his infamous nativity dating. Mosshammer observes that Pandorus, a contemporary of Annianus, had used a similar chronology. Pandorus had rejected Annianus' calculation that the Passion had taken place in the nineteenth year of Tiberius and the Resurrection in the twentieth, and dated both events to the nineteenth year. Pandorus, Mosshammer argues, was following Julius Africanus (late second-early third century), who dated the Resurrection to 25 March, AD 31. Africanus' dating implies a Nativity date of 25 March, 1 BC (cosmic era 5501). Pandorus, Mosshammer argues, probably was not Dionysius' direct source, just as he was not the ultimate source for this chronological scheme. Pandorus was merely "reasserting" the dating of Julius Africanus against Annianus (421). Anatolius too borrowed from Julius Africanus in composing his nineteen-year cycle, and Julius' Christian era was implicit in his table. Despite his reliance on the work of Annianus, Cyril of Alexandria did not adopt the latter's Christian era, but rather preferred the 'traditional' Alexandrian nativity date. Cyril's table, of course, served as the foundation for that of Dionysius. Thus, Mosshammer concludes, "The Christian era of Dionysius is no more than the cosmic year 5501 in the system of Julius Africanus, adapted first to the Alexandrian civil year, then to the Roman [civil] or indictional year" (435).

Although the history of Christian computus certainly does not end with Dionysius, Mosshammer concludes his study with the sixth-century monk, leaving aside the fascinating question of the transmission of Dionysius' table and his Anno Domini dating in subsequent centuries.2 However, Mosshammer's contribution to our understanding of Christian chronology is a significant one, clarifying the extent to which the system that Dionysius introduced to the West was of eastern origin, and its adoption into a Roman context. In doing so, Mosshammer redeems the much-maligned Dionysius from the charges of his critics, both medieval and modern.


1.   For students, a fine introduction is Arno Borst, The Ordering of Time, trans. Andrew Winnard (Chicago: University of Chicago Press, 1993). Briefer, but slightly more technical, is the introduction by Faith Wallis to her translation of Bede, The Reckoning of Time, (Liverpool: Liverpool University Press, 1999), xxxiv-lxiii.
2.   A problem discussed by Charles W. Jones, "The Victorian and Dionysiac Paschal Tables in the West," Speculum 9 (1934), 408-421; Charles W. Jones, "Two Easter Tables," Speculum 12 (1938), 204-205.

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