Anno Domini and the Computus

Bede did not invent Anno Domini dating, but he made it the framework of historical narrative. Dionysius Exiguus had devised the Christian era for his Easter tables around 525; Bede was the first major historian to carry it systematically through a work of history, fixing the convention the West still uses, and in The Reckoning of Time (725) he gave the Latin Middle Ages its definitive textbook of the computus — the science of time-reckoning by which the date of Easter is found. This file treats both achievements: the dating era he popularized and the calendrical science he mastered.

Tradition: Pan-Orthodox (pre-Schism; Doctor of the Church; the Father of English History)

The Era Bede Inherited

The system of numbering years from the Incarnation was not Bede’s invention, and he never claimed it was. Its author was Dionysius Exiguus (“Dionysius the Humble”), a Scythian monk working at Rome, who around 525 drew up a continuation of the Alexandrian Paschal tables and chose to number his years not by the era of the persecuting emperor Diocletian — the “era of the Martyrs” then in use in the Alexandrian computus — but from what he reckoned to be the year of the Lord’s Incarnation, anni Domini nostri Iesu Christi (Wallis 1999, introduction, lvii–lxiii; Brown 2009, ch. 5). Dionysius’s purpose was strictly computistical: he needed a continuous count of years to carry the nineteen-year lunar cycle forward, and the Incarnation era served that table. He showed no interest in re-dating history at large, and for nearly two centuries his era remained a tool of the Easter calculators rather than a way of telling time.

What Bede did was different in kind. He took the era out of the Paschal table and made it the spine of a narrative of the English Church and people. In the Ecclesiastical History of the English People (completed 731) he dates events throughout ab incarnatione Domini — “in the year of the Lord’s Incarnation” — beginning the work proper with the arrival of Julius Caesar in Britain “in the year 60 before the Incarnation of the Lord” and carrying the count through the whole of the narrative to his own day (HE 1.2; 5.23–24; Colgrave & Mynors 1969). He had already used the Dionysian reckoning in his chronological works; but it is the History that turned a computist’s convenience into a historian’s instrument, and it is from Bede’s example, transmitted with the immense popularity of that book, that Anno Domini dating spread through the Latin West (Wallis 1999, lxviii–lxx; Harrison, in DeGregorio 2010).

The First Historian to Date by the Incarnation

The claim must be stated carefully. Isolated documents had used the Incarnation era before Bede, and the era was in liturgical and computistical circulation. But Bede was the first writer to employ it systematically and as the organizing chronology of a major historical work — to make the year of the Lord, rather than regnal years, indictions, consulships, or the year of the world, the common measure against which all events are set (Wallis 1999, lxviii; Brown 2009, ch. 5). The History still records regnal years and indictions where its sources gave them, and Bede was perfectly capable of reckoning by a king’s reign; but he subordinates these to the Incarnation count, so that the reader can hold the whole sweep of the narrative on a single continuous scale. This is the decisive innovation, and its consequence is the dating system used in this very file.

Bede also reached, tentatively, for the other half of the convention. For events before the Incarnation he sometimes reckoned ante incarnationem Domini, “before the Incarnation of the Lord” — as in his dating of Caesar’s invasion sixty years before Christ (HE 1.2; Colgrave & Mynors 1969). This is not yet the developed “Before Christ” of later usage, and Bede did not build a continuous backward count; but the instinct to extend the era in both directions from the fixed point of the Incarnation is already present in him, anticipating the BC reckoning that the West would later formalize (Wallis 1999, lxviii–lxix). The fixed point itself Bede took on Dionysius’s authority. He did not re-derive the date of the Incarnation, and the modern recognition that Dionysius’s calculation is off by some years — that the Nativity, on the Gospel data about Herod, falls a few years “before Christ” — is no charge against Bede, who was faithfully transmitting the best chronology of his age [Disputed: the precise error in Dionysius's epoch is itself estimated variously, most commonly at four to six years, because the underlying date of Herod's death is debated; the point is not material to Bede's use of the era].

The Computus: The Reckoning of Time

Bede’s mastery of dating rested on a deeper competence: the computus, the body of calendrical science by which the Church found the moveable date of Easter and ordered its year. He treated it first in the short De temporibus (On Times, 703) and then, at full length and with lasting authority, in De temporum ratione (The Reckoning of Time, 725), the work that became the standard medieval textbook of the subject (Wallis 1999, introduction, xv–xxxiv; Brown 2009, ch. 4). The Reckoning of Time is not a miscellany but a structured science. It moves outward from the smallest unit to the largest: from the divisions of time — the day, the week, the month, the year — through the technical machinery of the lunar and solar calendars, to the calculation of Easter, and finally to the ages of the world and the end of time (the latter treated in the companion file).

Bede begins with the natural and conventional divisions of time and the means of counting them, including the old practice of finger-reckoning, before turning to the day and its parts, the week, and the months in both their Roman and English forms — it is here that he preserves the names of the Anglo-Saxon months and the heathen festivals from which some derive (The Reckoning of Time 1–15; trans. Wallis 1999). He treats the solar year and its seasons, the equinoxes and solstices, and the reason the civil calendar must be reconciled with the courses of sun and moon. The intellectual temper throughout is mathematical and observational: Bede reasons from cycles and remainders, sets out his calculations to be checked, and corrects his sources where observation or arithmetic requires it (Wallis 1999, xviii–xxxiv; Wallis, in DeGregorio 2010).

The Lunar and Solar Cycles

The heart of the computus is the reconciliation of two incommensurable cycles — the solar year of roughly 365¼ days and the lunar month of roughly 29½ days — so that the date of a feast defined by the moon (Easter) can be predicted within the calendar defined by the sun. The instrument is the nineteen-year cycle (the cycle Bede inherited through the Alexandrian and Dionysian tradition), at the end of which the phases of the moon return to nearly the same dates in the solar calendar (The Reckoning of Time 19–20, 45–46; trans. Wallis 1999). Across these nineteen years the lunar and solar reckonings are kept in step by a fixed scheme of intercalation: twelve of the years are “common,” of twelve lunar months, and seven are “embolismic,” receiving a thirteenth, embolismic month to make up the accumulating difference between lunar and solar time (The Reckoning of Time 45; trans. Wallis 1999).

Bede sets out the supporting apparatus with care: the epact, the age of the moon on a fixed day of the year, by which the moon’s phase for any date can be found; the concurrents, the numbers that track the weekday of a fixed date so that the calculator can locate the Sunday; and the saltus lunae, the “leap of the moon” — the single day that must be subtracted once in the nineteen-year cycle to keep the lunar reckoning from drifting out of true (The Reckoning of Time 20, 42, 50; trans. Wallis 1999). These are not antiquarian curiosities but the working gears of the calendar, and Bede explains each as a problem in arithmetic with a determinate solution, insisting that the reader understand the reason of the reckoning and not merely copy the tables (Wallis 1999, commentary on chs. 42–50).

The Calculation of Easter

All of this converges on the single question that had divided the Churches of the British Isles and that the Synod of Whitby (664) had settled for the English in favor of the Roman-Alexandrian reckoning: when is Easter? The rule, as Bede states and defends it, requires three conditions to be met together. Easter is the Sunday — the Lord’s Day, on which the Resurrection fell — that follows the first full moon (the paschal full moon) that falls on or after the spring equinox, the equinox being fixed for computistical purposes at 21 March (The Reckoning of Time 6, 30, 59–61; trans. Wallis 1999). Each element carries weight: the equinox anchors the feast to the renewal of the year and forbids an Easter that falls before spring; the paschal full moon ties it to the Passover moon of the Gospel; and the requirement of the Lord’s Day fixes it to the day of the Resurrection. The errors against which the reckoning guards — celebrating before the equinox, or with the Jewish fourteenth of the moon rather than on the following Sunday — were precisely the points of the old British and Quartodeciman disputes (HE 3.25; 5.21; Colgrave & Mynors 1969).

Bede insists that all three conditions stand together and that none may be relaxed for convenience: an Easter set by the moon but before the equinox would celebrate the new creation while the old year still held; an Easter on the fourteenth of the moon itself, rather than on the Lord’s Day that follows, would keep the Passover but not the Resurrection. The whole art of the computus is to find, for any year, the one Sunday in which all three are satisfied at once — and to find it not by guesswork but by the cycles, so that every church may keep the feast on the same day (The Reckoning of Time 59–61; trans. Wallis 1999). Bede gives the fullest statement of the theology and computus of Easter not only in The Reckoning of Time but in the great letter of Abbot Ceolfrith to King Nechtan of the Picts, which he reproduces in the History (HE 5.21; Colgrave & Mynors 1969). And he did more than expound the rule: he computed it. To the Dionysian Paschal cycle, which ran in tables of nineteen years gathered into a great cycle of 532 years (the product of the nineteen-year lunar cycle and the twenty-eight-year solar cycle, after which the whole pattern of Easter dates repeats), Bede appended a continuation, carrying the table forward so that the date of Easter could be found for centuries to come (The Reckoning of Time 65; Wallis 1999, commentary; Brown 2009, ch. 4). The Paschalion he served is the same in principle as the one the Orthodox Church keeps to this day (see The Church Calendar).

The Tides and the Temper of the Computist

The scientific cast of Bede’s mind shows nowhere more clearly than in his treatment of the tides, which he discusses in the course of expounding the moon’s influence on the natural world. He observed — and stated with a precision unusual for his time — that the tides follow the moon, rising and falling with its course, and he went further: he recognized that the moment of high water differs from place to place along a coast, so that the tide that is full at one port is not yet full at another. This is the principle he called the establishment of the port, the “law of the sea-coast,” and his account of the lunar governance of the tides and their local variation is one of the genuine pieces of original observation in early medieval science (The Reckoning of Time 29; trans. Wallis 1999; Wallis 1999, commentary on ch. 29; Wallis, in DeGregorio 2010).

The tides are characteristic of the whole work. Bede is not content to repeat what Pliny or Isidore had said; he reasons from what can be observed on the Northumbrian shore, distinguishes the general law from its local qualification, and submits the result to measurement. This is the same temperament that made him the historian who insisted on naming his sources and the computist who asked his readers to follow the reckoning and not merely trust the table. In an age more often content with authority, Bede combined reverence for his sources with a steady willingness to check them against number and observation — and it is this that earned The Reckoning of Time its long reign as the West’s textbook of time, and earned Bede, alone among Englishmen, the title Doctor of the Church (Brown 2009, ch. 4; Wallis 1999, xv–xxxiv).

References

Bede. The Reckoning of Time (De temporum ratione). Translated, with introduction, notes, and commentary, by Faith Wallis. Translated Texts for Historians 29. Liverpool: Liverpool University Press, 1999. (Cited by chapter; the divisions of time, chs. 1–15; the solar year and tides, chs. 27–29; the nineteen-year cycle, epacts, concurrents, and saltus lunae, chs. 19–20, 42–50; embolismic months, ch. 45; the equinox and the rule of Easter, chs. 6, 30, 59–61; the great Paschal cycle and Bede’s continuation, ch. 65. Wallis’s introduction, lvii–lxx, on Dionysius Exiguus and the spread of Anno Domini.)
Bede. Ecclesiastical History of the English People. Edited and translated by Bertram Colgrave and R. A. B. Mynors. Oxford Medieval Texts. Oxford: Clarendon Press, 1969. (Cited by book.chapter; Caesar’s invasion dated before the Incarnation, 1.2; the Easter controversy and Whitby, 3.25; Ceolfrith’s letter to Nechtan on the computus of Easter, 5.21; Bede’s chronological recapitulation, 5.23–24.)
Brown, George Hardin. A Companion to Bede. Anglo-Saxon Studies 12. Woodbridge: Boydell Press, 2009. (The computistical works and the Anno Domini era, chs. 4–5.)
DeGregorio, Scott, ed. The Cambridge Companion to Bede. Cambridge: Cambridge University Press, 2010. (Faith Wallis on Bede and science and the computus; Kenneth Harrison and others on the chronology of the History.)