In my case I derive it by looking it up on the web. ;-)
I know it's something to do with the full moon after 21 March or something and the difference between (21 March)Gregorian and (21 March)Julian accounts for the difference while the fact that the relevant full moon can occur at different points within that span accounts for why the two Easters are not always the same distance apart.
And yes, it's directly (and explicitly, not coincidentally) tied to Passover.
Okay, now to ask Wikipedia for the bits I couldn't remember...
[...] using the Gregorian calendar, Easter always falls on a Sunday between March 22 and April 25 inclusively. [...] In Eastern Christianity, using the Julian calendar, Easter also always falls on a Sunday from March 22 to April 25 inclusive, which on the Gregorian calendar, due to the 13 day difference between the calendars between 1900 and 2099, are dates from April 4 to May 8 inclusive.
[...]
In applying the ecclesiastical rules, the various Christian Churches use 21 March as their starting point from which they find the next full moon, etc. However because Eastern Orthodox and Oriental Orthodox Churches use the Julian Calendar as their starting point, while Western Christianity uses the Gregorian Calendar, the end point, the date for Easter, may diverge.
ISTR there having been some argument over whether the date also had to be shifted to prevent Easter from happening before Passover, but I can't find the reference (and as you can see, Western Easter was before Passover this year).
I did find another explanation (http://catholicism.about.com/od/holydaysandholidays/f/Calculate_Date.htm) which may or may not be easier to follow:
The Council of Nicaea (A.D. 325) set the date of Easter as the Sunday following the 14th day of the paschal full moon, which is the full moon whose 14th day falls on or after the vernal (spring) equinox.
We know that Easter must always occur on a Sunday, because Sunday was the day of Christ's Resurrection. But why the 14th day of the paschal full moon? Because that was the date of Passover in the Jewish calendar, and the Last Supper (Holy Thursday) occurred on the Passover. Therefore, Easter was the Sunday after Passover.
The Church does not use the exact date of the paschal full moon but an approximation, because the paschal full moon can fall on different days in different time zones, which would mean that the date of Easter would be different depending on which time zone you live in.
Likewise, the Church sets the date of the vernal equinox at March 21, even though it can occur on March 20. Both approximations allow the Church to set a universal date for Easter.
Still, Easter isn't celebrated universally on that date. While Western Christians use the Gregorian calendar (the calendar that's used throughout the West today, in both the secular and religious worlds) to calculate the date of Easter, the Eastern Orthodox continue to use the older, astronomically inaccurate Julian calendar. Currently, March 21 on the Julian calendar falls on April 3 in the Gregorian calendar. Therefore, for the Orthodox, the Sunday following the 14th day of the paschal full moon has to fall after April 3, hence the discrepancy in the date of Easter.
BTW, while (AFAIK) all Orthodox churches celebrate Easter on the Julian calendar, some also celebrate Christmas on the Julian calendar (so there are 'old calendar' and 'new calendar' Orthodox churches). I wrote about (http://dglenn.livejournal.com/854780.html) how celebrating both Christmases makes Christmas last nearly a month instead of just twelve days, a while ago. :-)
Hm, I find that interesting in that even the timing of Easter has been messed with.
Passover is tied to the the lunar calendar and the date of the full moon and the vernal equinox because the moon was needed to illuminate the doorposts and later to give light to the Jews as they fled Egypt. It was not tied to a date in any man-made calendar as such. (Although, IIRC in the Jewish calendar it was Nisan 15, making the last supper Nisan 14 and the blood of the lamb used on the doorposts the blood shed in preparing the last meal eaten before Passover took place).
Another complicating factor is that at the time of the Council of Nicaea, the system by which intercalary months are put into the Jewish calendar was not widely understood -- at least not by the Catholic community -- and so trying to figure out when to place a feast synchronized to Passover required reverse-engineering the scheme by which Passover's dates in the Julian calendar can occur. And as there are several rules that aren't obvious if you don't know how the calendar works, it's remarkable the work came out as close to agreeing as it did.
This may amuse; I once worked with someone who had written a computer program (way back when it was Fortran and punch cards) to calculate the dates of Easter which worked on track for many years in advance and then somewhere around the year 2100 suddenly failed catastrophically when it came up with a "negative Easter".
(no subject)
I know it's something to do with the full moon after 21 March or something and the difference between (21 March)Gregorian and (21 March)Julian accounts for the difference while the fact that the relevant full moon can occur at different points within that span accounts for why the two Easters are not always the same distance apart.
And yes, it's directly (and explicitly, not coincidentally) tied to Passover.
Okay, now to ask Wikipedia for the bits I couldn't remember...
ISTR there having been some argument over whether the date also had to be shifted to prevent Easter from happening before Passover, but I can't find the reference (and as you can see, Western Easter was before Passover this year).
I did find another explanation (http://catholicism.about.com/od/holydaysandholidays/f/Calculate_Date.htm) which may or may not be easier to follow:
BTW, while (AFAIK) all Orthodox churches celebrate Easter on the Julian calendar, some also celebrate Christmas on the Julian calendar (so there are 'old calendar' and 'new calendar' Orthodox churches). I wrote about (http://dglenn.livejournal.com/854780.html) how celebrating both Christmases makes Christmas last nearly a month instead of just twelve days, a while ago. :-)
(no subject)
Passover is tied to the the lunar calendar and the date of the full moon and the vernal equinox because the moon was needed to illuminate the doorposts and later to give light to the Jews as they fled Egypt. It was not tied to a date in any man-made calendar as such. (Although, IIRC in the Jewish calendar it was Nisan 15, making the last supper Nisan 14 and the blood of the lamb used on the doorposts the blood shed in preparing the last meal eaten before Passover took place).
Easter and Passover should be coincident.
(no subject)
Another complicating factor is that at the time of the Council of Nicaea, the system by which intercalary months are put into the Jewish calendar was not widely understood -- at least not by the Catholic community -- and so trying to figure out when to place a feast synchronized to Passover required reverse-engineering the scheme by which Passover's dates in the Julian calendar can occur. And as there are several rules that aren't obvious if you don't know how the calendar works, it's remarkable the work came out as close to agreeing as it did.
(no subject)