A Medieval Easter Table

To get some sense of how Easter was calculated after Dionysius by people who lacked computers (or even Arabic numerals), I here transcribe an actual table from a manuscript written in 1004. I will reproduce it as exactly as I can, including abbreviations.
  ANNI DNI INDICT EPACTE CCVRR CICLLVN XIIIIma LVNA DIES DOM POST LVNA IPSIUS B M iiii ii xxvi vi xiii v id aprl xvi kl aprl xxi M v iii vii vii xv iiii kl aprl kl aprl xvii END M vi iiii xviii i xvi xv kl mai xi kl mai xviii M vii v Nvlla ii xvii Non aprl xvii id Aprl xv B M viii vi xi iiii xviii viii kl aprl v kl aprl xvii M viiii vii xxi v xviiii Id april xx kl mai xviii M x viii iii vi i iiii non aprl v id aprl xxi M xi viiii xiiii vii ii xi kl aprl viii kl aprl xvii B M xii x xxv ii iii iiii id aprl id aprl xvii M xiii xi vi iii iiii iii klaprilis non aprl xx OGD M xiiii xii xvii iiii v xiiii kl mai vii kl mai xii

So, what does it all mean? Let's start with the column headings, which in this case actually come in the middle of the table. Expanding the abbreviations, the headings mean: anni domini (years of the lord); indictiones (indictions); epactæ (epacts); concurrentes; cicli lunae (lunar cycles), 14ma Luna (the 14th moon); dies dominica post (the Sunday afterwards); luna ipsius (this moon).

The indiction we have already seen. It plays no direct role in the calculation of Easter, but note that the cycle remains consistent with that given by Dionysius. The epacts indicate the age of the moon (i.e. days into the lunar month) on March 22, the earliest possible date of Easter Sunday. The concurrentes give the day of the week (the so-called ferial numbers) of March 24th. The lunar cycles track the Metonic, 19-year cycle. Later in the Middle Ages, this cycle will be determined by the numerus aureus, the golden number, so called because it is the key to figuring out the date of Easter. Note, however, that this lunar cycle, while it has the same practical effect as the golden number, is not exactly the same. For example, 1010, which has a golden number 4, is listed as the first year of the lunar cycle. The 14th moon is the 14th day of the lunar month, i.e., the full moon. The Sunday afterwards is Easter. The "moon itself" is the age of the moon, i.e., the day of the lunar month, on Easter.

Apart from the numbers and dates, the other abbreviations in the margin are B, for bisextilis, i.e. a leap-year; END for endecadas and OGD for ogdoadas mark the subdivisions of the Metonic cycle. The first is a period of 11 years, the second of 8. They coordinate the insertion of lunar leap.

Translated Medieval Easter Table
Year Indct Epct 3/24 Gldn# Full Moon Easter Moon on Easter
L 1004 2 26 6 14 April 9 March 17 21
1005 3 7 7 15 March 29 April 1 17
1006 4 18 1 16 April 17 April 21 18
1007 5 0 2 17 April 5 April 6 15
L 1008 6 11 4 18 March 25 March 28 17
1009 7 22 5 19 April 13 April 17 18
1010 8 3 6 1 April 2 April 9 21
1011 9 14 7 2 March 22 March 25 17
L 1012 10 25 2 3 April 10 April 13 17
1013 11 6 3 4 March 29 April 5 20
1014 12 17 4 5 April 18 April 25 21

If you check these years (for example with the ChurchCalendar applet) you will find only one (March 17) is wrong. This one is an obvious scribal blunder. Easter must come after the full moon. The scribe wrote 16 Kalends of April when he should have written 16 Kalends of May, which is April 16, the correct date. Further, if you calculate the week day of the 24th, you will find that all of them match up correctly (1 = Sunday, etc.).

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