Easter is the most important holiday in Christianity. How is its date determined and why is it different every year? And why does it often not coincide with that of the Catholic Easter?
According to the Ecumenical Council of Nicaea of 325 AD, Easter should be celebrated on the first Sunday after the first full moon after the vernal equinox, based on mathematical calculations, occurring on or after March 21. If the full moon is on a Sunday, Easter is celebrated on the following Sunday. It seems like a clear and simple rule, but it turns out that it is not exactly so, writes nauka.offnews.
ASTRONOMICAL VERSUS CHURCH DATES
Although Easter is liturgically associated with the beginning of spring in the Northern Hemisphere (the March equinox) and the full moon, its date is not based on the actual astronomical dates of these events.
The vernal equinox is the time when the day is equal in length to the night. According to astronomy, this is the moment when the center of the Sun in its apparent motion along the ecliptic (the great circle of the celestial sphere along which the apparent annual motion of the Sun occurs) crosses the celestial equator. The vernal equinox falls on March 21 or 22. Therefore, Easter can be on any day starting from March 22.
The other factor determining the date of Easter is the full moon. It happens when the Earth is between the Moon and the Sun and in a plane passing through the Moon, the Earth and the Sun and perpendicular to the plane of the already mentioned ecliptic (a special case of a full moon is a lunar eclipse, when the Moon, the Earth and the Sun line up so exactly in a line that the shadow Earth covers the Moon).
In the case of equinoxes and full moons, the complexity lies in the fact that it is necessary to take into account both the orbit of the Earth around the Sun (the equinox) and the orbit of the Moon around the Earth (the full moon).
So there are two variables in the ecclesiastical and astronomical calendars that can coincide, but often don’t:
- The church has accepted March 21 as the date of the vernal equinox, regardless of time zone, while the true date of the equinox varies between March 19 and March 22, and the date also depends on the time zone.
- The date of Easter is linked to the date of the Paschal full moon, which is based on mathematical approximations following a 19-year cycle called the Metonic cycle, used by the Babylonians since at least the early 5th century BC.
The Metonic cycle is used to reconcile the lunar and solar calendars. If we add up the 12 lunar months, we get 11 days less than the modern calendar year, but 235 lunar months equal exactly 19 years.
And so we have the beginning of spring, which falls somewhere between March 19 and March 21, and the cycle of the Moon’s orbits around the Earth, which lasts more than 27 days. For this reason, the astronomically determined date of Easter can vary between March 22 and April 25, but other, theological factors also intervene.
The astronomical spring in 2024 began on March 20 at 5:06 a.m. for Bulgaria. The first full moon after the astronomical spring equinox is on March 25. The following Sunday is April 7th, so according to the Nicene Rule Easter should then have been the Sunday after the full moon on March 25th. But Catholics and Protestants celebrate Easter in 2024 on March 31, 2024, and Orthodox – on May 5. Why?
CATHOLIC AND ORTHODOX EASTER
The Nicene formula is valid for both the Western Roman Catholic and Eastern Orthodox churches.
The Eastern Churches have also fixed their “vernal equinox” on March 21, but on the Julian calendar, a date that corresponds to April 3 on the Gregorian calendar, which the Catholic Church follows.
Naturally on March 21st and April 3rd the Moon is in different phases – the Julian calendar is 13 days behind the Gregorian calendar – this is roughly half a month.
During this period there may or may not be a full moon – the probability is 50:50. Therefore, the Western and Eastern Orthodox Easter may coincide approximately half of the time over the years, and in the rest, the Eastern Orthodox is a week later, and sometimes more.
At the same time, Passover (the Jewish holiday commemorating the exodus from Egypt) is calculated according to the new moon, and between the new moon and the full moon, the difference is also half a month.
The difference between Catholics and Orthodox in the dates of the celebration of Easter is that the Orthodox observe the additional requirement that Easter does not coincide with the Jewish Passover and that it is chronologically more correct that Easter must be celebrated after the Jewish Passover.
In the year 2024, the Jewish holiday Pesach or Passover is celebrated from sundown on April 22nd and ends on the night of April 30th. Therefore, Catholics and Protestants celebrated Easter on March 31, and the Orthodox Church postponed it by one lunar month, waiting for the Jewish Passover to pass.
And so this year, for the Orthodox, Easter falls on the Sunday after the full moon on April 24, i.e. May 5.
To summarize, the date of Orthodox Easter is influenced by four factors:
1. The vernal equinox
2. The first full moon after the vernal equinox
3. The other, which is not related to astronomy, but only to religious tradition, is Sunday. The first Sunday after the first full moon after the first equinox is counted. There is an important clarification: if the desired full moon falls on a Sunday, then Easter is celebrated on the following Sunday.
4. The difference between the pan-European calendar and the Julian (“Orthodox”) calendar leads to a change in the calculation for 13 days ahead (once it was 12, someday it will be 14, etc.). That is, if according to the Julian (“Orthodox”) calendar Easter falls on one of the days from March 22 to April 25, then in the modern calendar it is the period from April 4 to May 8.
5. The other difference is the compliance with the Jewish Passover of the Orthodox Church, which waits for the Jewish holiday to pass and declares the Sunday after the full moon as Easter.
Source: Calculating the Easter Date
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