The date of Easter has long been a subject of dispute. It was one of the issues at the Synod of Whitby, which took place in 664. This year, Easter is particularly early. The Orthodox Easter and the Jewish Passover are a month later. This discrepancy dates back to 1582 and the replacement of the Julian calendar by the more accurate Gregorian calendar. The Orthodox churches mostly retained the Julian calendar, which is now twelve days late. Thus they are celebrating Christmas when everyone else is celebrating the Feast of the Epiphany. Only occasionally do the two occur at the same time.
The western churches fix Easter Sunday to the first Sunday after the first full moon following the vernal equinox, the latter being on 19th, 20th or 21st March. If a full moon occurs just before the vernal equinox, an additional lunar month is effectively inserted before Easter, which will then occur at a late date and correspond with the Orthodox Easter and the Jewish Passover.
It would in principle be an excellent expression of Christian unity if all the churches celebrated Easter at the same time, which logically and for the sake of historic authenticity would correspond with the Jewish Passover. However, this brings us to the Jewish calendar, which is a lunar calendar. A lunar year is 353 days, ie 11 days short.of a solar year, and additional months are therefore added to correct the shortfall. The extra month in the Jewish leap year is called Adar Sheni - the second Adar, and seven of these occur within a 19 year cycle, on the 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years.
This 19 year cycle is quite accurate but it is not perfect. The average length of the Jewish year over a 19 year cycle is about 365 days, 5 hr, 55 min, 25.4 sec. This is 6 min, 40.2 sec longer than the current average value of the solar year (though that is changing very slowly) and 6 min, 13.4 sec longer than the average value of the year in the Gregorian calendar (which is itself slightly too long).
As a result, the average date of the first day of Passover is getting later by one day in about 216 years compared to the Sun and 231 years compared to the Gregorian calendar. Thus it is already on average a few days later than 2000 years ago.
In a 19 year cycle, Pesach currently occurs a month later than on the first Full Moon after the vernal equinox in the 8th, 11th and 19th year of each 19 year cycle, such as 2005, 2008 and 2016. This accounts for the discrepancy.
Thus an important question regarding an agreed date for Easter is whether the desire for unity should outweigh considerations of astronomical accuracy?
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