How an ancient astronomical error affects Easter

The date of the Easter celebration changes every year, but it's actually based on an ancient misunderstanding of astronomy.

How a misunderstanding of astronomy can make us wait up to a month to celebrate Easter.

Easter Sunday is supposed to fall on the first Sunday after the full moon that follows the spring equinox...but that isn't always the case. (iStock)

Easter weekend brings a lot of things to mind — religion, family, food, bunnies and Easter eggs, to name a few. The moon usually isn't on that list, but it actually has a big influence over the date on which the holiday weekend falls.

In fact, an ancient misunderstanding of astronomy can make us wait up to a month to celebrate Easter, depending on the year.

For hundreds of years, churches have looked to the sky to determine when they should celebrate Easter, trying to align it with the Vernal, or Spring, equinox. The holiday is a time of renewal and rebirth for the Christian church, so celebrating around the dawn of Spring made sense.

But way back when people were determining the rules for when the date would fall, humanity's understanding of the sky was a little lacking. Robert Cockcroft, a McMaster University astronomer and physicist, explains.

"Back in 325 A.D. these rules were made, but they couldn't accurately predict astronomical events because they didn't understand it all, so they set up estimations," Cockcroft said.

The church decided Easter would be celebrated on the first Sunday after a full moon that falls either on or after the Spring equinox.

The Spring equinox, Cockcroft explained, is the point in the year when the path of the sun crosses the line representing the projection of the Earth's equator, meaning it's making its way back to the northern hemisphere and spring is on the way.

Unfortunately, when they were making the rules nearly 1,700 years ago, they didn't realize the date of the equinox changes slightly every year. It can occur on March 19, 20 or 21 (most often on the 20), but the church fixed the date for the equinox at March 21. 

Church officials also followed an ecclesiastical calendar to determine when the full moon would fall — the 14th of a "lunar month" — which isn't always the case, either.

"Those understandings of the words 'full moon' and 'Vernal equinox' are not the astronomical definition of those terms, which is where the complication arises," Cockcroft said.

Out of sync

Most years the church's definitions are pretty much in harmony with the actual astronomical events taking place. This year, for example, the equinox occured at 11:02 a.m. on March 20 and the first full moon to follow occurred early Wednesday, so Sunday's date is accurate.

But some years, the discrepancies mean the two dates fall out of sync: six years from now, the church's Easter date falls almost a full month after the astronomical Easter date.

"I can imagine it would be confusing during the years when they don't agree," Cockcroft said.

And it gets even more complicated depending on where in the world you're celebrating Easter.

Western Christianity follows the Gregorian calendar — that's the civic calendar we follow — to determine the date. Eastern Christianity follows the Julian calendar, which is 13 days out of sync.

Therefore, Easter can fall on any Sunday between April 4 and May 8 for Eastern Christians; this year it's May 5.

Scientific errors

Cockcroft explained it's not just churches that can get caught up in old traditions that don't mesh with new understandings; it happens all the time in science.

"This is how science works as well. We look for a pattern and categorize it, then realize later on those categories are incorrect, but we're kind of stuck with them," he said, pointing to the categorization of stars as an example.

"Rather than label stars hottest to coolest from A to Z, we label the hottest O, followed by B, then A, F. It's because it's arranged differently than when we first made the rules."

On the bright side, Easter may change from year to year and hemisphere to hemisphere, but at least the cream eggs stay the same.