The subject matter of this book is the history of the calculation of the date of the Christian festival of Easter. Alden Mosshammer has provided a masterful exposition of the detailed knowledge of astronomy and scriptural interpretation required for this task on the part of leading Egyptian clergymen at the end of the fourth and in the first half of the fifth century. Two introductory chapters cover the historical context and the textual tradition. Then follow critical editions, translations and commentary on the Prologues on Easter. Mosshammer's view of the paschal calculations attributed to Cyril is that they cannot be his work (hence the printing of the patriarch's name in square brackets). The book concludes with a short chapter on the anonymous text Ratio solis uel lunae and two appendices on ‘The 95-year Easter Table of Cyril’ and ‘The Roman 84-Year Cycle’.

The complex issues treated in these texts result from the effort on the part of the Early Church to combine several time-measuring systems with which to determine the date of Easter. Put briefly, the aim was to celebrate on the first Sunday after the first full moon after the spring equinox. Theophilus of Alexandria, in the fourth and fifth sections of his Prologue, argued that this combination of weekly, lunar and solar cycles would assure the strict observance of the Old Testament commands to keep the month of new fruits (Exodus xii.1–20; Deuteronomy xvi.1). Reports about but not direct evidence from the Council of Nicaea indicate that this type of calculation was aimed at moving away from celebrating in the Jewish way – that is keeping the 14th of Nisan regardless of the day of the week – as some Christians had done since at least the end of the second century (Eusebius, Historia ecclesiastica 5.23–4; Life of Constantine 3.17–18). In this Eusebian interpretation, the three-fold calculation provided Christians with a rationale for a joint celebration and, simultaneously, facilitated their separation from Judaism. From the point of view of the Alexandrian patriarchs, having a system to work out a date in advance was also helpful for the announcement of Easter to which their annual festal letters were dedicated.

As history continued, however, the joint celebration proved to be a rather long-term goal. The divisive nature of the Easter question was thus felt with particular force by the members of the seventh-century court at Northumbria, who found themselves one year with their king feasting while his queen and her entourage were still fasting (Bede, Ecclesiastical history of the English people 3.25). Mosshammer has rendered an invaluable service to members of both the scholarly and the faith communities who wish to understand how such events could take place. His book will be a prize possession in every research library. Readers will find it helpful to approach his text alongside his earlier monograph The Easter computus and the origins of the Christian era (Oxford 2008) and Leofranc Holford-Strevens's The history of time: a very short introduction (Oxford 2005).