Why does the date of Easter vary by more than a month?

Because 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.

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Why do we still have to use both solar and lunar calendars?

Easter is the time when Christians celebrate the Resurrection of Christ. 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.

The problem comes 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.

How many ways are there of calculating Easter?

Dozens. In the 2nd century a Roman called Hippolytus devised an eight-year cycle. A century on, 84-year tables were introduced, which were still in use in parts of the British Isles as late as 931. There was a ruling by the First Council of Nicaea in 325 that Easter should be celebrated on the Sunday after the first full moon following the spring equinox but this was ignored for centuries, with even the church in Rome using its own methods until the 6th century. More recently there have been Mallen's Method, Marcos Montes Method, Carter's Method, Oudin's Method... and on it goes.

There is another complicating factor. 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.

Why are there so many different definitions of a full moon?

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 they 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.

Is this why Easter is on a different day in Eastern Europe?

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No. That's because the Orthodox church sticks to the calendar promulgated by Julius Caesar but which the West abandoned in the 16th century. But it is all linked to trying to harmonise solar and lunar calendars.

By 1582 the Julian calendar (which made the year too long by several minutes) was out of sequence with the equinoxes by 10 days and with Caesar's original dates by 14 days. So Pope Gregory XIII decreed a new calendar which dropped leap years when they happened at the end of a century where the number was not divisible by 400 – thus 2000 was a leap year but 2100 won't be. He also decreed that the world would jump from 5 October to 14 October.

The English, afraid that this was some kind of popish plot, refused to come on board until 1753, by which time English calendars were 11 days out. When the change was finally imposed here innumerate protesters, assuming that real days were being stolen from them, rioted demanding "Give us back our 11 days!"

The Orthodox Easter is still a Julian one and usually follows ours by a week or so and can even stretch into May.

So is this the earliest Easter can get?

No. It can be on 22 March, as it was in 1761 and 1818, but that won't happen until 2285. Its latest possible date is 25 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.

Can't the date be fixed by modern astronomy?

Astronomers have tried to reconcile the lunar and solar cycles since the time of the ancient Greek stargazer Meton of Athens who came up with the 19-year Metonic cycle. And the ancient Egyptians knew that the heliacal rising of Sirius was a more accurate predictor of the flooding of the Nile than solar dates were.

But more accurate astronomy only makes things more complicated. We now know that a solar year is 365.2422 days and a vernal equinox year is 365.2424 days and a sidereal year is 365.25636042 days – none of which fit exactly with the 365.2425 days of the Gregorian calendar.

Then there is the precession of the equinoxes by which the earth wobbles like a spinning top. Its poles shift in relation to certain stars in a 25,800-year complete one-wobble cycle. And the tidal drag between the Earth and the Moon and Sun, which is affected by melting glaciers and sea-level rise, increases the length of the day and of the month.

Can't we just pluck a fixed date out of the air and agree on it?

Both governments and churches have tried to do that. 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.

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. There are too many contradictory influences brought to bear. It seems that Easter is set to remain the original moveable feast.