This year, Western Christians and Greek Orthodox believers, who use different liturgical calendars, celebrate Easter, or Pascha one week apart, with the former being on April 9th and the latter on April 16th.
In 2017, Easter came at the same time for both denominations, something that will happen again in 2025.
However, for purely astronomical reasons, the difference between the celebration of Easter for the two denominations will be getting wider and wider.
As a results of this widening gap, from 2700 onward, the celebration of Easter for the Greek Orthodox Church and the Western Christian churches will never coincide again.
Altogether, in the whole of the 21st century, the celebration of Easter will be held on the same day 31 times, but during every forthcoming century, this will happen more and more rarely.
The last time Easter celebrations will coincide is estimated to be in 2698. From then on, Orthodox and western Christians will never celebrate the Resurrection of Christ together again.
Easter and the Western calendar
The First Ecumenical Synod in 325 AD decided that Easter would be celebrated on the first Sunday after the first full moon of spring.
If this full moon occurs on Sunday, then it will be celebrated on the folllowing Sunday. Thus, Easter would never coincide with Jewish Passover.
At the same time, the celebration of Easter was clearly associated with an astronomical phenomenon, the spring equinox, and the first full moon of spring.
In order to calculate the date of Easter, the date of the first full moon and then the first Sunday after the full moon had to be determined.
The First Ecumenical Synod instructed the Patriarch of Alexandria to inform the other churches of Easter day after the date of the first full moon was calculated with the help of the astronomers in the Egyptian city.
The calendar that was in force at the time of the First Ecumenical Synod was the Julian, which Julius Caesar himself had instituted in 45 BC with the help of the Alexandrian astronomer Sosigenis.
The latter, based on calculations of Hipparchus (who a century prior had estimated with impressive accuracy that the solar year had a duration of 365.242 days), established a calendar with 365 days, and every fourth year (the “leap year”), another day was added.
However, the Julian calendar contained a minor error because the duration of the solar year is in fact 365.242199 days. Every four years, this small error reaches 45 minutes, and every 129 years the minutes add up to a day. As a result, the spring equinox has been arriving earlier and earlier.
Thus, while the spring equinox at the time of Christ occurred on March 23rd, by the year 1582 AD it occurred on March 11th.
At the time, Pope Gregory II instructed astronomers Christoforos Klavios and Luigi Lilios to reform the calendar.
October 5, 1582 was recalculated as October 15, 1582 to correct the error that had accumulated over the past 11 centuries, and the spring equinox returned to March 21st, as it had been during the First Ecumenical Synod.
The new, or Gregorian, calendar was adopted by the Catholic states of Europe in the next five years, and by the Protestants much later.
Due to an even stronger reaction by the Orthodox Church to the Gregorian Calendar, the Julian Calendar remained in force in all Orthodox States until the 20th century.
Greece and the Gregorian calendar
In Greece, the Julian Calendar was finally replaced by the Gregorian on February 16, 1923 with the date changing to March 1st on that day.
That is, 13 days have been removed since 1923, because of the 10-day error between the Gregorian and the Julian calendars since 1582 and another three days for the difference between the adoption of the Gregorian calendar by the West and its adoption by the Greeks three and a half centuries later.
In 1924, the Greek Orthodox Church accepted the ecclesiastical calendar, which would be identical to the civil calendar and would apply to fixed holidays. It would, however, not apply to the Easter celebration or to holidays which are not fixed but are, instead, still calculated on the basis of the Julian or old calendar.
The difference in the celebration of Easter between Orthodox and Western Christians is not only based on the error of the Julian Calendar but also on the error of the so-called “Metonic cycle,” named after Greek astronomer Meton of Athens of the 5th century BC.
The Metonic cycle is a period of close to 19 years, which is almost a common multiple of the solar year and the synodic (lunar) month.
The Metonic cycle was used by the Christian astronomers of Alexandria on the basis of which the Orthodox Church continues to count the dates for future spring full moons.
On the 13 days of the Julian spring equinox, the error of the 19-year Metonic cycle, which from 325 AD to the present time amounts to four to five days, must be added. As a consequence, the Metonic (or Julian) full moon is calculated four to five days later than the actual one.
The Greek Orthodox Church continues to use the Old Julian Calendar and the Metonic cycle to determine the date of Pascha.
Thus, Orthodox Pascha is often celebrated not on the first Sunday after the full moon but on the following full moon or after the second full moon instead of the first Sunday after the first spring full moon, as the Nice Synod had decided.
Catholics and other Christians celebrate Easter according to the rule of the First Ecumenical Synod, but their Spring Equinox and the spring full moon are calculated according to the new Gregorian calendar, also taking into account the Metonic error.
So the Gregorian-Catholic full moon is much closer to the astronomical one (often coinciding or having only one day difference) than the Julian-Orthodox.
It is fairly common for Orthodox and other Christians to celebrate Easter together when both the Gregorian and the Julian-Metonic Easter moon fall from Sunday to Saturday of the same week (as long as it is after April 3rd and two full moons). This would consequently result in Easter celebrations for both Christian denominations on the following Sunday.
However, after 2700, due to the accumulation of the Metonic error for almost seven centuries, the Julian and the Gregorian full moon will never coincide in the same week again, so there will be no common Easter again after this year.