The Orthodox Easter Sunday of 2020, and how its date is determined, makes for an interesting story, which starts 2452 years ago ( Yes, it all started “Before Christ!”)

432 BC: Back then in Athens, an astronomer and mathematician of renown named Meton, after precise measurements – using the tools of the era – and incredibly careful calculations, determined that there was a relationship between the year (circle of the Earth around the Sun) and the number of rotations of the Moon around the Earth. His figure was that 19 years equals “exactly” 235 full Lunar Months. This Cycle repeats every 19 years, so it has been named the “Metonian Cycle.” (It’s interesting that, according to experts, the famous “Antikythera Mechanism,” estimated as being built between 60 and 200 BC, contains this Cycle.) As may be expected, that ancient level of observation accuracy left a small error. We’ll come back to Meton later.

45 BC: Moving forward about 387 years, during the times when Rome ruled the known world, they came up with a calendar, whose basic scheme has survived to the present time. Knowing that the actual year could not be divided exactly in so many full days and that it was off by about one day every four years, they simply added an extra day onto the end of February. So it was decreed in 45 BC by Julius Caesar (who was then essentially the Dictator of Rome and fooling around with the Greek Cleopatra,) and so it was done. (He also managed to get one of the months – Julius – named after himself, and the calendar took his name, too – the rogue!) We’ll come back to the “Julian” calendar later.

325 AD: Moving forward again, 370 years past Julius, and now back in the early days of Christianity, the Council of Nicaea in 325 AD set forth certain rules, among them when to observe religious events. It was decreed that Easter would be held on the first Sunday after the first Full Moon following the Vernal Equinox, except if it fell on Jewish Passover, in which case it would be the Sunday after that. (This exception was in order to follow the proper time-sequence in the scriptures: Jesus traveled to Jerusalem to participate in the Passover, and then was crucified and resurrected.) This decree is still being followed today. In the same year, probably as part of the Council’s organization of the religious calendars, top Alexandrian astronomers updated their observations of the Equinox and the Moon and tabulated the dates of the Full Moon from that date forward. However, to create their tabulations, although starting with their updated observations, they used Meton’s formula (with its small error.) We’ll come back to the Council of Nicaea later.

 The “Julian” calendar had worked well, but the known error was slowly, but surely, building up – the calendar was slightly longer in time than the actual circle of the Earth around the Sun. That tiny inaccuracy amounted to only 10 minutes per year, insignificant, yet over the centuries, it built up and began creating problems. The calendar no longer matched the actual seasons.

1582 AD: Moving forward some 1257 more years, in 1582, the Catholic Pope Gregory XIII decreed that, to bring the calendar back to its proper place to match the observable universe, an adjustment of 10 days had to be made. That is, e.g., what was then, say, March 10 by the “Julian” calendar, would become March 20. (The intervening days would…vanish.) The new calendar, which we still use today, is named after him, as the “Gregorian Calendar.”

1923 AD: It took a very long time for everyone to accept the “Gregorian” edict. The Catholic countries, of course, adopted it almost immediately – after all, it was their Pope that decreed it – but the rest of the Christian world “took a bit longer.” Part of the resistance was probably a general reaction of the Eastern world against anything from the West, part was the faithfuls’ discomfort in changing traditional holidays to a new date to which they could not relate. Russia, for example, changed to the Gregorian only after the (atheist) Communist revolution in 1918, that did away with the clergy’s influence. Greece did it 341 years later, in 1923, (dropping 13 days, not 10, because the difference had increased over the extra years since 1582.) The Greek Orthodox Church accepted the civil change but maintained the “Old” calendar for religious moving holidays. (Thus came the current split, in terms of the dates of religious celebrations of the Modern vs. “Paleoimerologites” (“Old-calendarists”) in Greek society.)

2020 AD: We can now move forward to understand the Christian Easter dates and why they differ:

The Catholic Easter of 2020 follows the formula, as set by the Council of Nicaea: Using the modern (Gregorian) calendar, the actual (current, observable) Vernal Equinox falls on April 3, the next actual full Moon falls on April 7, and the next Sunday (Easter Sunday) falls on April 12. This date occurs after the beginning of Jewish Passover, which begins on April 8, so no exception is necessary.

The Orthodox Easter of 2020 follows the same basic formula as set by the Council of Nicaea. However, in full understanding that this was inaccurate but practical and convenient, it was decreed that Vernal Equinox would be considered fixed at (“Julian”) March 21. Also, it is still using the tabulated “Metonian” full Moon dates, as created by the Alexandrian astronomers of the Council of Nicaea’s period.

But the “Metonian” table, over the intervening 1695 years since its creation in 325 AD, has also accumulated an error of about five days. As a result of this, and using strictly the “Julian” calendar and the Alexandrian/Metonian table, adjusted for those five days, the 2020 date of a Full Moon falls on March 30. In 2020, the “fixed” Vernal Equinox is on March 21, the next Full Moon falls on March 30, so all is well, and since the next Sunday is April 6 (which, adding the current difference of 13 days, becomes “Gregorian” April 19) – so, that is the Orthodox Easter Sunday!

As we know, sometimes the two Easter dates coincide, and sometimes they can be weeks apart. That is because of the changing actual dates of the Vernal Equinox, the “fixed” Orthodox date of the Vernal Equinox, the actual Moon phases, the “Metonian” Moon phases, the adjustments to the Metonian table, the dates of the Jewish “Passover,” and several other considerations, too complex to mention here. But those are other calculations, for another time. But, it all started with Meton, 2452 years ago! 

Happy and Healthy Easter, 2020!

The following Orthodox Easter is on May 2, 2021 (“Gregorian” calendar.)