THE DATE OF EASTER
In 2008, Easter Sunday fell on March 23rd -- almost as early as it is possible for the day to fall. Inevitably, we were treated to article after article in the press explaining how the date of Easter is calculated; these articles interspersed with the complaints and pleas for a fixed Easter.
While a fixed Easter would probably simplify all manner of industries -- holidays, DIY and sports to name but a few --
personally we rather like the 'randomness' of the date. It seemed to us that it would be a public service for the folks of Nordelph were we to note down the calculations and the history of this movable feast.
The 'simple' definition of the date of Easter is that it is 'the first Sunday after the first full moon after the spring equinox'. This is the 'simple' definition because -- as you will see -- nothing here is quite what it seems, albeit for very good reasons. And even this definition is a bit asymmetric: the full moon can be 'on or after' the equinox, while Easter is strictly 'after' the full moon -- if the full moon itself falls on a Sunday, Easter is delayed by a week!
One disadvantage of the definition above is that, at the time it was first pressed into use, around AD325, astronomical skills weren't up to the forecasting of events like the equinox, which were observed rather than predicted. So, rather than the 'true' equinox, the formula uses the fixed date of March 21st. Curiously, many people even today are unaware that the equinox can vary from this date by two to three days in either direction -- there is a folk myth that the equinox is always the 21st: and indeed in the calculation of Easter, it is!
This 'artificial' equinox avoids another problem, which also affects the full moon.
The moment when the moon is full is just that -- a moment, not a day. As such, it occurs at different times (even different days) at different places around the world. The 'moon' that is used in this prediction is an artificial one, based on what is known as the 'metonic cycle' and far too complicated to go into here!
Searching the World Wide Web turns up a variety of algorithms to do the calculation for you. Here's one such, with the workings for this year and next.
| Method | 2008 | 2009 |
|---|---|---|
| Calculate remainder when year is divided by 19 | 13 | 14 |
| Multiply by 11 | 143 | 154 |
| Subtract from 225 | 82 | 71 |
| If this number is greater than 50, subtract 30 repeatedly until it is less than 51 | 82-(2*30) =22 |
71-(30) =41 |
| If the number is now greater than 48, subtract 1 (call this 'D') | 22 | 41 |
| Add the year plus 1 | 2031 | 2051 |
| Divide the year by 4 and ignore any remainder | 2008/4 =502 |
2009/4 =502r1 |
| Add this to the previous number | 2031+502 =2533 |
2051+502 =2553 |
| Calculate remainder when this is divided by 7 (call this 'E') | 6 | 5 |
| Calculate D + 7 - E | 23 | 43 |
This last number is the date in March on which Easter Sunday falls. Where this number is greater than 31, as it is in 2009, that indicates a date in April -- just subtract 31. Easter 2009 is (we very much hope) on Sunday 12th April.