The Epact for a Julian year can be calculated by taking the remainder of
11 times the Golden Number divided by 30.
So, possible Epact values are 0 to 29.
For a Gregorian year a correction has to be made.
For the first Gregorian years (1582 and up) the correction was -9 (or +21).
Every time a leap day was skipped in a Century year, the correction changed.
The following table lists the corrections for years up to 2499.
| years | correction |
|---|---|
| 1582-1699 | -9 or +21 |
| 1700-1799 | -10 or +20 |
| 1800-1899 | -11 or +19 |
| 1900-2099 | -12 or +18 |
| 2100-2199 | -13 or +17 |
| 2200-2299 | -14 or +16 |
| 2300-2499 | -15 or +15 |
In stead of making the calculation mentioned above and then making the proper correction, one can find the Epact (for the given years) in the next table.
| Golden Number | Epact | |||||||
|---|---|---|---|---|---|---|---|---|
| 0001-1582 | 1583-1699 | 1700-1799 | 1800-1899 | 1900-2099 | 2100-2199 | 2200-2299 | 2300-2499 | |
| I | 11 | 2 | 1 | 0 | 29 | 28 | 27 | 26 |
| II | 22 | 13 | 12 | 11 | 10 | 9 | 8 | 7 |
| III | 3 | 24 | 23 | 22 | 21 | 20 | 19 | 18 |
| IV | 14 | 5 | 4 | 3 | 2 | 1 | 0 | 29 |
| V | 25 | 16 | 15 | 14 | 13 | 12 | 11 | 10 |
| VI | 6 | 27 | 26 | 25 | 24 | 23 | 22 | 21 |
| VII | 17 | 8 | 7 | 6 | 5 | 4 | 3 | 2 |
| VIII | 28 | 19 | 18 | 17 | 16 | 15 | 14 | 13 |
| IX | 9 | 0 | 29 | 28 | 27 | 26 | 25 | 24 |
| X | 20 | 11 | 10 | 9 | 8 | 7 | 6 | 5 |
| XI | 1 | 22 | 21 | 20 | 19 | 18 | 17 | 16 |
| XII | 12 | 3 | 2 | 1 | 0 | 29 | 28 | 27 |
| XIII | 23 | 14 | 13 | 12 | 11 | 10 | 9 | 8 |
| XIV | 4 | 25 | 24 | 23 | 22 | 21 | 20 | 19 |
| XV | 15 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| XVI | 26 | 17 | 16 | 15 | 14 | 13 | 12 | 11 |
| XVII | 7 | 28 | 27 | 26 | 25 | 24 | 23 | 22 |
| XVIII | 18 | 9 | 8 | 7 | 6 | 5 | 4 | 3 |
| XIX | 29 | 20 | 19 | 18 | 17 | 16 | 15 | 14 |
For example: the Epact for the year 1999 (Golden Number V) is 13 [11 * 5 mod 30 - 12 = 13].
Using the Epact, one can find the New Moon dates.