To be honest I have never thought much about why Easter moves from year to year; I just accepted it as a fact. However, when Pamela was asked the question recently, I thought I would try and find out. I already knew that, although Easter is ostensibly about the death of Christ, the origins are in fact pagan as are the origins of Christmas.
What I didn’t know was that the ancient Egyptians and Hebrews used different calendars. The Egyptians had one based on the movement of the sun, which was passed on through the Romans and Christian culture to become the modern world's standard. The Jews had one based on the phases of the moon – as Islam does, which is why the month of Ramadan moves round the calendar and takes places at different times of the year each year, with Muslims waiting for sightings of the moon before they know what day it will begin.
Easter is one of the festivals which tries to harmonise the solar and lunar calendars. As a general rule, Easter falls on the first Sunday, following the first full moon after 21 March. But not always.
Easter is the time when Christians celebrate the Resurrection of Christ. Now, according to the gospels, he was killed three days before the Resurrection, around the time of the Jewish Passover. So Christians wanted to have their feast day around the same time as the Jewish festival which was fixed by the first full moon following the vernal equinox – the spring day when night and day are exactly the same length.
That is where the problem arises, because a solar year (the length of time it takes the earth to move round the sun) is 365 days, 5 hours, 49 minutes, and 12 seconds whereas a lunar year is 354.37 days. Calculating one against another is seriously complicated.
There have been various attempts to reconcile this, including the famous saltus lunae (the moon's jump) whereby one of the 30-day months in the lunar cycle gets arbitrarily shortened to 29 days. But the solar and lunar years diverge by 11 days every year. Scores of formulae have been devised to try to reconcile the two as a method of marking time.
It had been decided that the vernal equinox would occur on 20 March (as it did at Nicaea) but that too varies. It was on 21 March in 2007. So the old church fathers got round this by redefining what a full moon is.
To complicate matters further, an astronomical full moon, like an astronomical equinox, is not a day but a moment in time – which can be observed happening on different days depending on which side of the international date line you stand. And waiting for an event to happen made it impossible to plan ahead.
So it was decided that the Paschal Full Moon (PFM) would not be an astronomical moon but an ecclesiastical full moon. These could be set down ahead of time, which is what happened from 325 AD. Astronomers approximated astronomical full moon dates for the church, calling them Ecclesiastical Full Moon (EFM) dates. Thus Easter was defined as the Sunday after the first EFM after 20 March. And that date was the appointed vernal equinox, regardless of whether it was or not. So we have a notional full moon following a notional equinox.
The earliest that Easter can be is on the 22nd March, as it was in 1761 and 1818, but that won't happen again until 2285. Its latest possible date is the 25th April but we haven't had that since 1943 and won't again until 2038. The commonest date is 19 April though the full cycle of Easter dates only repeats after 5,700,000 years.
Both governments and churches have tried fix the date for Easter. Secularists have suggested that Easter should fall on the second Sunday of April each year. The World Council of Churches in 1997 suggested replacing the current equation-based system with direct astronomical observation.
However, even where there is notional agreement, implementation is another matter. In Britain, an Easter Act was passed in 1928 fixing the holiday as "the first Sunday after the second Saturday in April". The law remains on the statute book but it has never been enforced.
So it seems we are stuck with the issue of Easter changing from year to year.