Chronology, like mathematics when I studied it in my northern grammar school decades ago, can be divided into the two categories of pure and applied. In a learned and weighty tome Alden Mosshammer describes and analyses Christian computations of Easter down to and beyond the Easter table compiled by the monk Dionysius in Rome in 525 (p. 8), taking within his purview a plethora of Paschal calculations and eras, including the Byzantine, Armenian and Georgian eras of the world. In doing this, Mosshammer has performed an inestimable service for scholars who lack his profound knowledge of ancient astronomical calculations and calendaric matters; for his clear exposition of how the various chronological systems and calendars known in the Graeco-Roman world worked (pp. 10–55) deserves to replace Elias Bickerman's handbook as an introduction to the chronology of the ancient world. Although there are venerable standard German handbooks of technical and astronomical chronology, which contain much of the information that Mosshammer lays out, he has not only brought them up-to-date by drawing on recent scholarship, but also made a vast amount of indigestible material much more accessible. The most significant result that Mosshammer has established by hacking his way through this thicket of chronological and calendaric systems is to deprive Dionysius Exiguus of any credit for the one achievement for which he is widely remembered.
All calendars based on both the solar year and lunar months, as are the Julian and Gregorian, confront the problem that the solar year averages 365·24219 days in length, while the lunar month averages 29·53059 days. Only once every nineteen years, therefore, does a whole number of lunar months correspond at all closely to a whole number of complete solar years (235 lunar months being equal in length to nineteen years plus two hours and about three minutes). Hence a pressing problem for early Christians, who wished to know the exact date of the next Easter. Should they take over the Jewish calculation for Passover and use it to fix the date of Easter each year? (Christ was crucified at Passover on either 14 or 15 Nisan: the Gospels notoriously differ over the exact day, but the 14 Nisan of John has traditionally prevailed over the 15 Nisan of the Synoptic Gospels.) But following the Jewish calendar came to seem improper to all except the Quartodecimans by the end of the second century, so that it became necessary to formulate new rules in order to perform a calculation independent of the Jewish calendar and the annual Jewish calculations of the date of Passover. The earliest explicitly attested Christian computation of the dates of future Easters is the Paschal Table that Eusebius reports that Hippolytus compiled in 222/223 (HE 6.22). Over the following centuries many others composed Easter tables of their own, most of them deriving through intermediaries from Anatolius of Laodicea, who adapted a pre-existing nineteen-year cycle to the Alexandrian civil year in the late third century and used this nineteen-year cycle to compose a ninety-five-year list of Easters running from 4 April 258 to Easter 352 which came into effect at Easter 277.
Mosshammer shows that Dionysius did not invent the Christian era of bc/ad (bce/ce) in Rome in the early sixth century, but derived it from earlier scholars; specifically, he argues, the Christian era associated with the name of Dionysius was ‘no more than the cosmic year 5501 in the system of Julius Africanus’, which Anatolius had adopted and adapted, and the Christian era of Africanus was transmitted to Dionysius along with the Alexandrian nineteen-year Easter cycle (435–7). Since I am not an expert in theoretical or technical chronology, reading Mosshammer's exposition, which seems so cogent, leaves me puzzled by a discrepancy at the heart of his argument, a minor one to be sure, but nevertheless a discrepancy where there ought to be an exact correspondence. Whereas Mosshammer states categorically that Africanus dated the Incarnation to 25 March anno mundi 5501=1 bc (p. 418), the outline of ‘Africanus’ Chronological System' in the recent collaborative edition of Julius Africanus' Chronographiae by Martin Wallraff and others (GCS, N.F. 15, Berlin–New York 2007) puts the Incarnation in his year 5500 and the death of Herod in 5501 (facing p. 350, cf. 274–5 T 92).
It is unfortunate that Mosshammer has woven into his exposition of different eras and Christian computations of Easter occasional discussions of problems in applied chronology. These problems are primarily historical and can often only be solved by the deployment of historical skills and knowledge that Mosshammer does not possess. The clearest example concerns the dating of the birth of Christ. Mosshammer takes seriously the absurd claim of Ernest L. Martin, which he cites at second hand from the unreliable revised edition of Jack Finegan's classic Handbook of biblical chronology (Princeton 1964; rev. Peabody, Ma 1998) that Herod the Great might have lived on for fully three years beyond the Passover season of 4 bc, that the census in Luke's Gospel can be dated to 2 bc, and that Christ may have been born in 1 bc (p. 321: ‘Finegan … found himself persuaded by a set of recent arguments that Herod died in 1 bc’; p. 322: ‘Finegan (1998: pp. 305–6) … argues for a universal registration … that he dates to 2 bc’). The evidence that Herod can have died no later than the first half of the month Nisan in 4 bc and that his successors numbered their regnal years from 5/4 bc is both very varied and completely decisive: it is fully set out in T. D. Barnes, ‘The date of Herod's death’, JTS n.s. xix (1968), 204–9, refuting an earlier attempt to redate Herod's death to 1 bc, and E. Schürer, History of the Jewish people in the age of Jesus Christ (175 BC–AD 135), i, rev. G. Vermes and F. Millar (Edinburgh 1973), 326–8 n. 165. Neither of these appears in Mosshammer's extensive bibliography.
In conclusion, it must be emphasised that the fact that Mosshammer is so clearly out of his depth in tackling problems of applied or historical chronology cannot diminish his achievement in elucidating Christian calculations of the date of Easter, in establishing a close link between these calculations and the Christian era which we still use, and in proving that this era was first fixed (give or take a year), not by a sixth-century monk of otherwise modest attainments, but by Julius Africanus, one of the great scholars of the ancient world.
