Elizabeth Amelotte, left, and her cousin Grace Mattern show each other their finds at the East Greenwich Historical Preservation Society Easter egg hunt at the Varnum House Museum.

The Providence Journal / John Freidah

Easter hasn’t come this early since 1913 and won’t come this early again until 2160.

Because of the way the holiday’s date is calculated, it always falls between March 22 and April 25.

So how is the date for Easter determined?

It sounds like a straightforward question, but it’s a question that might have sparked a revolution in science and touches on Christianity, Judaism, astronomy and mathematics.

Back in 325, the Christian church decided to separate Easter from the Jewish holiday of Passover; previously Passover and Easter — the Christian Passover — were celebrated together. The church wanted to end that practice.

So at the First Nicaean Council, in 325, church officials decided that Easter would be celebrated on the first Sunday after the first full moon after the March equinox.

That same year, the church calculated dates for full moons well into the future. It’s those dates, and not necessarily the date that a full moon actually occurs, that’s used to calculate when Easter will fall.

So the first Sunday after the first full moon after the March equinox does not literally refer to a full moon, but to the date on which scholars predicted a full moon would fall.

And there’s more.

The church does not use the real date of the equinox, which can differ by a few days from year to year. Instead, Easter calculations always use the date of the equinox in the year 325: March 20.

So maybe a better definition is the first Sunday after the first predicted full moon date after March 20.

Nearly 1,200 years later, the church realized it had a problem. The calendar year was too long and dates — such as March 20 and, by association, Easter — were moving forward through the seasons.

So church officials asked for help from scientists, including Nicholas Copernicus.

“There is a record of Copernicus being asked,” said William Krieger, professor of philosophy of science at the University of Rhode Island. “There is a record that there was a response. There is, of course, no record of what that response was.”

It might have been a historical side note, but then again it might have been a clear case of cause and effect, but shortly after being asked for help, Copernicus began work on the Commentariolus, a brief work that he never published but that outlined the arguments he would make in his seminal work, On the Revolutions of the Heavenly Spheres.

Commentariolus laid out his theory of heliocentricity — that the sun, not Earth, was the center of the known universe.

“People could argue that [Commentariolus] is either the impetus for the Gregorian calendar and/or that it was the impetus for the heliocentric model that would change the way we look at astronomy,” Krieger said.

“It could have been either very pro-church or anti-church. Or both.”

Seventy years after the church sent out its SOS and 40 years after Revolutions was published, the church had effectively solved its calendar problem by introducing a new one.

The Gregorian calendar dropped 10 days and added a leap year for accuracy, and we still use it today.

So how do we calculate Easter? We still use a method devised in 325.

There are a number of resources online that offer tables containing the projected full moon dates used to determine Easter, along with a fairly complex algorithm that will give an Easter date far into the future.

But on any given year, Easter is the first Sunday after the first full moon (as it was predicted in 325) after March 20.

Orthodox Easter is determined the same way, Krieger said, but since the church still uses the Julian Calendar, the dates do not always match.

Today is Easter, and whether or not you celebrate it, maybe now you can find it on a calendar.

bjeffers@projo.com