This year, Catholic and Greek-Orthodox Christians — who use different liturgical calendars — celebrated Easter one week apart, with the former being on April 1 and the latter April 8. In 2017 the celebration coincided exactly.
In 2017, Easter came at the same time for both denominations, something that will happen again in 2025. Until 2020, Catholic and Greek Orthodox Easters will continue to fall one week apart.
However, for purely astrological reasons, the difference between the celebration of Easter for the two denominations will be getting wider by some years.
And from 2700 and on, the celebration of Easter for the Greek Orthodox Church and the Catholic Church will never coincide again.
Altogether, in the whole 21st century, the celebration of Easter will be common 31 years, but 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 Catholic 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 the next Sunday. Thus, the Christian Easter would never coincide with the Jewish Passover. At the same time the celebration of Christian 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 found. The First Ecumenical Synod instructed the Patriarch of Alexandria to inform the other churches the 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 one that Julius Caesar had instituted in 45 BC with the help of the Alexandrian astronomer Sosigenis. The latter, based on the calculations of Hipparchus (who a century ago had estimated with impressive accuracy that the solar year had a duration of 365.242 days), established a calendar with 365 days, and in every fourth year (the “leap year”) one more day was added.
However, the Julian Calendar had a small 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 the minutes add up to a day. As a result, the Spring Equinox is coming more and more early.
Thus, while the Spring Equinox at the time of Christ occurred on March 23, in 1582 AD it had reached March 11.
Then Pope Gregory II instructed astronomers Christoforos Klavios and Luigi Lilios to make a calendar reform. Oct. 5, 1582 was renamed Oct. 15 to correct the error that had accumulated over the past 11 centuries and the Spring Equinox to return to March 21, 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 replaced by the Gregorian on Feb. 16, 1923, with the date changing to March 1st. That is, 13 days have been removed since 1923, because of the 10-days 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 be identical with the civil calendar and to apply for immovable holidays, but not for the Easter Calendar and for the mobile holidays, which are still calculated on the basis of the Julian or Old Calendar.
But the difference in the celebration of Easter between Orthodox and Catholics 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 Metanic 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 Easter. Thus, Orthodox Easter is often celebrated not on the first Sunday after the full moon, but on the next or after the second full moon, instead of the first Sunday after the first Spring full moon, as the Nice Synod had decided.
Catholics 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 that Orthodox and Catholic 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 3 and two full moons), so the next Sunday is Easter common for both.
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.