When is Easter? That’s easy—Easter is on Sunday, April 12, 2020.

In fact, Easter does not have a specific date.

Unlike Christmas Day, it changes every year depending on something that most people don’t often associate with the Christian religion—the moon.

For the world’s 1.8 billion Muslims, the exact date of Ramadan is determined by the lunar cycle—with the physical sighting of the crescent moon the day after a New Moon the critical factor. In 2020, that’s Thursday, 23 April—the evening after the next New Moon—unless the crescent isn’t sighted, in which case it will begin the evening after when the New Moon is higher in the post-sunset sky.

In China, Lunar New Year is celebrated by a billion people; it always occurs in late January or during one of the first three weeks of February. In 2020, it occurred on Saturday, 25 January—the day of the New Moon.

Easter, too, is a lunar festival. Here’s how it works.

How our moon determines the exact date of Easter Sunday

Easter is celebrated by the world’s two billion Christians on the first Sunday after the first full moon occurring on or after the vernal equinox. That’s the rule. Since the equinox occurred on March 20, 2020 and the “Super Pink Moon” appeared on April 7, 2020, so the date of Easter Sunday was automatically set for Sunday, April 12, 2020. It’s been that way since the year 325 A.D. according to EarthSky.

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How the church determines the exact date of Easter Sunday

Although its 99% a lunar festival in terms of the date it’s held, Easter isn’t just determined by the moon’s phases. The ecclesiastical authorities fix the vernal equinox at March 21, thereby limiting the dates when Easter can occur.

Why is Easter determined by the lunar calendar?

Clocks are a modern invention, as is the convention of global calendars standard time. “Prior to the Gregorian calendar, which is our standard calendar now, calendars were set by the phases of the moon because we didn’t have clocks,” said Dr. Jackie Faherty, Senior Scientist and Senior Education Manager jointly in the Department of Astrophysics and the Department of Education at the American Museum of Natural History.

The Gregorian calendar is a solar calendar that evolved from a lunar calendar. “Standards of time only began when people wanted to set train timetables, and this idea that we have a universal time that we can all get from looking at our watch is something we take for granted,” said Faherty.

What is a lunar calendar?

A lunar calendar is a calendar based on the monthly cycles of the Moon’s phases, though the lunar month lasts 29.5 days. A lunar year lasts 354.3 days, so it all gets all out of whack with the solar calendar, where a years—the time it takes for the Earth to orbit once around the sun—lasts 365.25 days.

Our calendar’s astronomical origins

“Much of our calendar comes from astronomy,” said Faherty, stating that Monday is named after the Moon. “It’s a constant and observable feature that you can get your own understanding of, with its changing phases a great way of tracking the seasons.”

Our concepts of time, and the language we use to talk about it, owe a lot to the Moon and the Sun.“There’s so much to our keeping track of our time that’s related to astronomy, and so much in the early days was set and understood by the Moon—so that was the way that many religious calendars were calculated, and still are today,” said Faherty.

So when you’re handling an Easter egg this Sunday, consider the moon—and maybe this year even use it to describe the Moon’s elliptical, egg-shaped orbit around the Earth.

Wishing you clear skies and wide eyes.