When is Easter?
A little background
So there I was, thinking how cool it would be to make an plugin for my mail
program that would "date" the letter in true Medieval style, something like:
The second Tuesday before Lent, Anno Domini 1998
I grabbed a dusty copy of "The Lives of the Saints", and started making a list
of reference points, thinking clever thoughts like: "The reference day doesn't
just have to be the nearest holy day, it could be the most relevant. You could
have settings for nationality, gender or trade which would favor the
appropriate saints as reference points!" While many days had specific dates
(Candlemas: Feb 2), many of the relevant days were given in terms of Lent,
Holy Week, Advent, etc. Those in turn were all based on Easter, and Easter
was based on... well... something complicated.
It was here that I came across the soon-to-cause-much-strife term "moveable
feast". If you search for this term in
Google
you will get 40% references to Hemmingway, 50% references to various arriviste
cookbooks, and the occasional link to something related to the
original
meaning.
The Easter Algorithm
The definition of Easter draws from three calendars of antiquity: the Solar
calendar, the Lunar calendar, and the 7-day Biblical week.
- [Solar] Start at the vernal equinox, March 21
- [Lunar] Go forward to the next full (Paschal) moon, which may be March 21.
- Go forward one day
- [Biblical] Go forward to the next Sunday.
- That Sunday is Easter.
Some Notes on the Algorithm
- There are indeed formulas available for locating the Paschal moon
given a year, but these are not definitive. Gauss' formula is:
20 March + (((year % 19) * 19 + 16) % 30)
- The Church is the definitive source in this matter, although
they have not deviated from the above formula in many years.
- In some years in some places, Easter may occur on the same day as the
local astronomical full moon.
- The above means that Easter can be as early as March 22 (when the
Paschal moon is March 21 and March 22 is a Sunday) and as late as April 25.
(When there is a full moon on March 20, and April 18 is a Sunday) March 21 to
April 25 is a 35 day spread: 28 days of swing for a full moon offset plus 7
mode for Monday->Sunday offset.
Accumulated by: Jeff Evarts