Easter 2019: Sunday 24 March, or not?


Firitia Velt is an astronomer who lives at Fasi moe Afi in Nuku‘alofa, Tonga.

By Firitia Velt.

Celebrating Easter on the right date has always been a source of worry for the Christian churches. Because what is the right date?

In early centuries, one tried to follow the Jewish Passover, but that was wrought with problems. Our calendar, or rather the Roman calendar, as it was in use at that time, is based on the yearly movement of the sun, while the Hebrew calendar follows the moon.

The moon year and sun year are not equal, and as such one sees that, for example, the new moon, which heralds the start of a new Hebrew month, can happen on any day of the month in our calendar.

So, where Passover is on a fixed day in the Hebrew calendar, namely 14 Nissan, it cannot be in ours. Now the 14^th of any Hebrew month is usually a full moon.

Furthermore, with the Hebrew year starting around the autumn equinox, Nissan being the 7^th month is therefore around the spring equinox (of the northern hemisphere, that is March).

So, Easter had to be on a Sunday somewhere around full moon in spring. Which still leaves a lot of room for interpretation.

Easter rules

It was not until the famous first council of Nicea in 325 AD that the conclave set the rules of how Easter should be determined. But since they only set the rule and did not reveal how in practice it should be calculated, it took a few centuries before the Roman calendar gave way to the Christian era as we know it now, and the rules for the calculation crystallised into what we have today.

The Sunday following the full moon, which falls on or after the equinox, will give the lawful Easter.

Unfortunately, the early Christian calendar had the same defects as the Roman calendar introduced by Julius Caesar, and therefore also known as the Julian calendar: it was a little bit too long. By consequence the real start of the astronomical spring, fixed on 21 March, came a little bit earlier every year, or the date of 21 March drifted towards the summer.

Gregorian calendar

It was not until the 16^th century that astronomy and mathematics had progressed far enough to pinpoint the problems and in 1582 Pope Gregorius XIII decreed that a new calendar should be introduced, now known as the Gregorian.

By that time Martin Luther had been dead for almost 40 years: that is to say the Protestant Reformation was in full swing, and the protestant churches had no intention of following papal decrees.

It would take more than a century before most protestant countries agreed that in this case the Catholics were not that bad, and the whole of western Europe switched to the Gregorian calendar that we have today.

But the Great Schism of 1054, where east and west Europe split into the Orthodox church and Catholic church, was not so easily repaired. And still is not. Even though for civil purposes countries like Russia have adopted the Gregorian calendar, in its Orthodox church the Julian calendar is still used. They differ now by 13 days. So, 24 March in our calendar is 11 March for them. As you may have expected, the two churches also disagree about the real date of Easter. After all 11 March is before the 21^st, so it cannot be Easter.

Calculating Easter 2019

It is important to state that the Easter date is not obtained from the actual observation of equinox and full moon, but that a formula is used. The calculation of Easter in the Julian calendar is relatively simple, but it is found wanting. In the Gregorian calendar a much more elaborate calculation is used for a better answer, but still not perfect.

Let us see how things work out for this year, 2019.

The equinox is on 20 March 23:50 IT (Israel time). Once scientists had understood that different places on earth experience different times, it was more or less silently agreed that in this case Jerusalem time should be used. Tongan time (TOT) is 11 hours in advance from that, so for us it will be 10:50 on 21 March when the equinox passes, and it will be an autumn equinox to us.

Full moon is on 21 March 3:43 IT, or 14:43 TOT. Clearly the full moon happens after the equinox so the following Sunday, 24 March should be Easter.

Except that it is not.

The ecclesiastical moon and sun used in the computus (Latin term for the Easter calculation) are average positions which can be a few hours off from their real counterparts. And in this case, apparently, they dictate that the full moon supposedly occurs before the equinox. By consequence the paschal full moon will not be until next month, bringing Easter to 21 April instead. Therefore, Easter will be late this year; although it could still be later, as it was in 1943 on 25 April. But that is rare; equally rare is an early Easter on 22 March.

Easter dates

In the table below we see some typical examples. Listed are the dates of the full moon (in Jerusalem), the date of the Jewish Passover, Easter Sunday if real Astronomical data were used, and the actual Gregorian and Julian Easter dates. (All expressed in the Gregorian calendar). The equinox date does not need to be included, because the ecclesiastical equinox is fixed on 21 March.

We see that the Jewish Passover is quite accurate with the moon, often on the right date or no more than 1 day off. Except this year when they too miss a whole month.

The Gregorian calendar is also not too bad except, again, this year. The Julian calendar, because of its lesser accuracy, usually is a week late. Sometimes a whole month. And sometimes, by accident, the Orthodox and Catholic churches agree.

The Gregorian calculation is so complex that it takes 5.7 million years to cycle. So, Easter in 5702019 will be celebrated on 21 April again, if there will be anybody around to do so that is - which is doubtful. And even if there are people who will care about Easter, they will have done away with the formulas millions of years ago, since they would no longer be valid. Because despite how accurate the formulas are to include the irregular movements of the moon or at least most of them; as they are now they do not take into account that the earth and moon orbits slowly and change over time, making them sufficiently different over periods as short as some ten thousand years.