Easter moves because it is pinned to the sky rather than to a calendar date. Wait for the spring equinox, find the next full moon, and Easter is the Sunday that follows it. The full moon lands on a different day each year, so Easter does too.
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That rule gives Western churches a window of just over a month. The earliest Easter can fall is 22 March, and the latest is 25 April. The commonest date is 19 April.
Two things in the sky
Dr Marek Kukula, public astronomer at the Royal Observatory Greenwich, puts it plainly. The date depends on the spring equinox and the full moon, and astronomy sits at the centre of the sum, he told the BBC.
The trouble is that neither fits neatly into a calendar. A solar year is not exactly 365 days, and a lunar cycle is not exactly 28. Kukula says people have spent thousands of years adjusting calendars to match the sky, and they still do not match. So working out the equinox and each full moon is never quite tidy.
Why the moon at all
The answer is Passover. The Crucifixion took place at Passover, so early Christians fixed Easter by the Jewish calendar, whose months follow the moon and wander against the solar year. Passover itself fell on the first full moon after the spring equinox.
The rules then grew in stages. Easter had to be a Sunday, then a Sunday after Passover rather than on it, and never before the equinox. By the end of the 2nd century some churches kept Easter on the day of Passover itself, while others kept the Sunday after.
The Council of Nicaea set the general rule in 325. Different traditions still went on doing the sum in different ways. The BBC traces the tangle to a blend of Egyptian, Roman and Hebrew reckoning, with the Egyptian solar calendar passing to Rome and then to Christians.
The king and queen with two Easters
In England the clash came to a head in Northumbria. Irish missionaries had brought one method of calculation from the north, and Roman missionaries another from Kent. King Oswiu followed the Irish custom, while his queen kept the Roman one. One year the king was celebrating Easter Sunday while she was still observing Lent.
In 664 the king called a synod at Whitby, at the abbey led by the abbess Hild. Bishop Colman of Lindisfarne argued the Irish case, and Wilfrid, a Northumbrian trained on the continent, argued the Roman one. Dr Michael Carter, a historian at English Heritage, says Wilfrid’s appeal to St Peter, keeper of the keys of heaven, swayed Oswiu. The synod chose the Roman method.
The monk Bede recorded the arguments in detail in his Ecclesiastical History of the English People. He studied computus, the calculation of Easter, which drew on his astronomy as well as his theology. Bede also gave English speakers a short way to remember the rule.
The calendar built for Easter
The sum also shaped the calendar still in use today. Pope Gregory brought in the Gregorian calendar in 1582, largely to make Easter earlier and easier to calculate. The Roman Catholic Church’s need to date its feasts also kept it interested in astronomy, and it opened its first observatory in 1774.
Not every church does the sum the same way, even now. In the year the BBC asked the question, Western churches kept Easter on 27 March, while Eastern churches kept it on 1 May.
Could the date be fixed?
Talk of a fixed date has come and gone. In 2016 the Anglican Communion said it meant to fix an annual date for Easter, working with the Catholic, Orthodox and Coptic churches. Justin Welby, the Archbishop of Canterbury, said he hoped for agreement within five to 10 years.
Some in the Church of England argued for keeping it moveable. The Conversation names the Very Reverend Michael Sadgrove, a former dean of Durham, as one of them. Bede’s tomb stands in that same cathedral.
