Ancient customs set Easter’s date

Do you know why the date of Easter changes each year? Experts say the day set aside for one of the most important Christian observances is determined by an ancient celebration, varying calendars and cycles of the moon.
   
Teman Knight, an associate in the office of evangelism at the Alabama Baptist State Board of Missions who holds a doctorate in Old Testament, explained that Easter is associated with Passover, which Jesus observed the night before He died. 
   
“The early church debated on whether or not to celebrate Easter on the same day of the month each year or on the Sunday after Passover,” he said. “They finally agreed to always keep it on Sunday every year.”
   
William Keel, an astronomy professor at the University of Alabama, said tying Easter to the date of Passover also helped leaders keep the observance during the right time of year.
   
“The early church fathers decided it was really only appropriate to celebrate Easter on a Sunday because of the resurrection and (because) Sunday was becoming more and more important,” he said. “Since the same date is not on Sunday every year, they needed to find some way to have it on a Sunday but keep it as accurate in the time of year as they could.”
   
Knight said that the date was fixed by the Council of Nicaea in A.D. 325 to be the Sunday after the Paschal, or Passover, moon after the vernal, or spring, equinox. This full moon occurs on or about March 21.
   
One problem with this rule was the difference in the calendars used at that time, which caused some groups to celebrate Easter on different days. Knight said three different calendars were used in Old Testament times and most were lunar calendars.
   
According to Keel, lunar calendars are based on the cycles of the moon, which change slightly each year. This makes calculating the exact time of the full moon and spring equinox, in advance, extremely difficult and time consuming.
   
“The Orthodox church calculates it a little differently,” Keel said. “One of the differences is that the full moon — according to which the date of Western Easter is calculated — is not a ‘real’ moon. It does not suffer the slight cyclical changes in the length of the month. It’s a ‘calendar’ moon. For the purposes of a worldwide date of Easter, they (the early church) would define this ‘fictitious’ (calendar) moon that has all of its full phases at the same time.” 
   
He added that the current rule for determining the date of Easter is “the first Sunday after the calendrical full moon after the 21st of March.” Depending on the date of the full moon, Keel said, Easter could happen as early as March 22 or as late as April 25. 
   
He explained that Orthodox Easter is calculated by the Julian calendar, which does not completely coordinate with the Gregorian calendar used by Western churches.
   
“They use the same kind of rule except that March 21 happens almost two weeks later than our calendar,” Keel said. 
   
“The Julian calendar inserts a leap year every four years, which gets the calendar off one day per century. The Gregorian calendar skips a leap year once a century to remain accurate. Right now, the two calendars are almost two weeks apart.”
   
The Julian calendar goes back to Julius Caesar. The Gregorian calendar was named for Pope Gregory XIII who, in 1582, set a group of scholarly monks off doing a reworking of the calendar to bring all the calendars and particularly the date of Easter back in line with the astronomical equinox, Keel said.
   
Since the Julian calendar became widely accepted about 85 years ago, he said the Julian calendar is now used “only to calculate when events in the Orthodox liturgical calendar occur, most notably Easter.”