Easter's Date?

[mongrel_8]

Recently I was curious as how the date of Easter was picked because it seems to vary from late March to April quite often. According to this site:
http://www.christiananswers.net/q-eden/easter-date.html

"First, find the vernal equinox, or first day of spring (about March 21-22), on a calendar that lists basic astronomical data. Then look for the next full moon, usually indicated in a corner box of the calendar. Easter will then fall on the following Sunday.

According to this rule, the latest possible date for Easter is April 25, next occurring in 2038. The earliest is March 22, in 2285. Most often, Easter comes during the first week of April."

So to check I got out my calendar. It shows that the Full Moon is on April 7, a Saturday. So according to the quote above Easter should be on Sunday, April 8. However, that is Palm Sunday and Easter doesn’t occur till the following Sunday, April 15.

What gives?

[Beadalin]

Here’s a fun link to an Easter Date Calculator.

Determining the date for Easter is more complicated than the first Sunday after the full moon after the spring equinox (there’s a mouthful). The problem is that Saturday doesn’t reach the whole world at the same time, which means that there can be varying dates for Easter (the instant the moon reaches full may occur on Saturday in Europe but Sunday in America). The now widely accepted calculator is based on an “Ecclesiastical Moon,” which calls for the full moon to arrive on the same date around the world and sets the equinox on March 21. Only Eastern Orthodox church calculates Easter based on the real full moon.

And an interesting bit from Lair Gauche:

With this definition, the date of Easter can be determined in advance without further astronomical knowledge. But the sequence of dates varies significantly from year-to-year with Easter Sunday being as early as March 22 and as late as April 25. In fact, the exact sequence of Easter dates repeats itself approximately every 5,700,000 years in our Gregorian calendar.

To sum up, the real full moon falls on a Saturday this year, and so to avoid the possibility of Easter being celebrated early, Easter will fall on the following Sunday.

[DSYoungEsq]

Simple enough: your calendar is incorrect. April 8 is the day on which the full moon falls, as measured by GMT, though it might be on April 7 local time in the US (I’m not going to look it up). Thus, Easter is on the first Sunday after that.

[lno]

Obligatory link to the Catholic Encyclopedia’s article.

To quote a snippet (which reads like stereo instructions):

But for such a feast it was necessary to know the exact calendar date of Christ’s death. To know this day was very simple for the Jews; it was the day after the 14th of the first month, the 15th of Nisan of their calendar. But in other countries of the vast Roman Empire there were other systems of chronology. The Romans from 45 B.C. had used the reformed Julian calendar; there were also the Egyptian and the Syro-Macedonian calendar. The foundation of the Jewish calendar was the lunar year of 354 days, whilst the other systems depended on the solar year. In consequence the first days of the Jewish months and years did not coincide with any fixed days of the Roman solar year. Every fourth year of the Jewish system had an intercalary month. Since this month was inserted, not according to some scientific method or some definite rule, but arbitrarily, by command of the Sanhedrin, a distant Jewish date can never with certainty be transposed into the corresponding Julian or Gregorian date (Ideler, Chronologie, I, 570 sq.). The connection between the Jewish and the Christian Pasch explains the movable character of this feast. Easter has no fixed date, like Christmas, because the 15th of Nisan of the Semitic calendar was shifting from date to date on the Julian calendar.

[donkeyoatey]

Check this site mongrel
http://www.chariot.net.au/~gmarts/easter.htm
This sums it up:
This is right!
Easter Sunday is the Sunday following the Paschal Full Moon (PFM) date for the year. In June 325 A.D. astronomers approximated astronomical full moon dates for the Christian church, calling them Ecclesiastical Full Moon (EFM) dates. From 326 A.D. the PFM date has always been the EFM date after March 20 (which was the equinox date in 325 A.D.).

[panamajack]

The link provided by beadalin (given again since it’s so useful ) :Ecclesiastical Calendar

From there, here’s Butcher’s Method for calculating the date of Easter in the Gregorian calendar(valid for a 5,700,000 year cycle ) :

a=year%19
b=year/100
c=year%100
d=b/4
e=b%4
f=(b+8)/25
g=(b-f+1)/3
h=(19a+b-d-g+15)%30
i=c/4
k=c%4
l=(32+2e+2i-h-k)%7
m=(a+11h+22l)/451
Easter Month =(h+l-7m+114)/31 [3=March, 4=April]
p=(h+l-7*m+114)%31
Easter Date=p+1 (date in Easter Month)

And for intricate details on an Easter-date algorithm, see the Easter Date FAQ. Don’t say I didn’t warn you, though.

(another method, from the FAQ) :

For people who want code, Fred Avolio sent this in to the MailJC list a
few years ago:

#!/bin/sh

eas.sh - find the date of Easter for a given year.

Usage: eas.sh <year>

echo $* ‘[ddsf[lfp[too early
]Pq]s@1583>@
ddd19%1+sg100/1+d34/12-sx85+25/5-sz54/lx-10-sdlg1120+lz+lx-30%
d[30+]s@0>@d[[1+]s@lg11<@]s@25=@d[1+]s@24=@se44le-d[30+]s@21>@dld+7%-7+
[March ]smd[31-[April ]sm]s@31<@psnlmPpsn1z>p]splpx’ | dc | sed ‘N
y/
/ /’
Believe it or not, it works.

Anyone who can derive the algorithm from the code is officially entitled
to post to comp.unix.wizards.

[Protesilaus]

The symbols from Butcher’s Algorithm in panamajack’s post should be interpreted as follows (from the same page).

In the text below, / represents an integer division neglecting the remainder, while % is division keeping only the remainder. So 30/7=4 , and 30%7=2 .

[rocking_chair]

to throw a bit of a wrench into the works, the orthodox church’s date for pascha can be the same date as everyone else, one week apart, or up to 5 weeks apart.

the following rule is given in hapgood’s service book:

" the first sunday after the full moon which happens upon or next after the 21st of march; and if the full moon happen upon a sunday, easter day is the sunday after. the council of nicea (ad 325) added this bit to the calculation: the christian passover shall NEVER either precede or coincide with the jewish passover, but must always follow it. the date cannot fall earlier than march 23rd (julian) or later than april 23rd (julian)."

this year is the first time since 1990 that easter falls on the same day in the eastern and western churchs. the nicean rule that is not followed in the western churchs is the reason for the dates sometimes being 5 weeks apart. to do calculations in the oc you have to check the julian calendar, gregorian calendar, and jewish calendar and land on a day that works with all three. this year, if i count correctly, passover ends at sundown saturday the 14th, so we squeek by, having easter on sunday the 15th.