How Do Astronomers Pinpoint Easter Sunday?

Easter Sunday can fall on any day between March 22 and April 25. Who decides when it is, and how?

It turns out there's no more "deciding" about it. The exact date of Easter is already known for every year until 4099 AD, thanks to precise observations of the movements of the Sun, moon and Earth.

The tradition of observing the cosmos goes back thousands of years. In 325 AD, Christian theologians declared that Easter should fall on the first Sunday after the first full moon following the vernal equinox , which, in the northern hemisphere, is the spring day when there are equal hours of daylight and darkness. For a long time scientists struggled to predict the important date in advance by studying the revolutions of the heavenly bodies -- a particularly difficult pursuit considering these scientists assumed the Sun moved around the Earth.

Then, in 1651, astronomer Giovanni Cassini punched a pinhole in the roof of the San Petronio cathedral in Bologna, Italy. By watching the Sun's image -- light that projected through the hole onto the cathedral floor -- shrink and expand as it traveled across the stone, he deduced that the distance between the Sun and Earth varied throughout the year. After much careful study, Cassini was able to prove that Earth revolved around the Sun in an elliptical (non-circular) orbit. He also made a reasonably accurate calculation of the distance between the Sun and the Earth .

Calculating Easter also requires knowledge of the exact length of a year. If Earth completed its orbit in exactly 365 days as our calendar implies, the equinox would fall on exactly the same day each year and calculating the date of Easter would be easy. But, as astronomers eventually found out, a full orbit actually takes about 365.25 days. The extra quarter of a day means that the equinoxes occur 6 hours later each year, which pushes the date of the equinox -- and Easter -- around from year to year.

Today, the orbits of the Sun, moon and Earth are calculated to extreme accuracy using complex mathematics, observational telescopes and computation, and future Easter dates are set.

"The tools used may have changed, and accuracy improved beyond measure, but the urgent desire of European astronomers to predict accurately the movements of celestial bodies has altered little since the days when Cassini sat on a cold stone floor watching sunbeams," the European Space Agency wrote in an Easter press release.

Follow Natalie Wolchover on Twitter @nattyover.

Natalie Wolchover

Natalie Wolchover was a staff writer for Live Science from 2010 to 2012 and is currently a senior physics writer and editor for Quanta Magazine. She holds a bachelor's degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Along with the staff of Quanta, Wolchover won the 2022 Pulitzer Prize for explanatory writing for her work on the building of the James Webb Space Telescope. Her work has also appeared in the The Best American Science and Nature Writing and The Best Writing on Mathematics, Nature, The New Yorker and Popular Science. She was the 2016 winner of the  Evert Clark/Seth Payne Award, an annual prize for young science journalists, as well as the winner of the 2017 Science Communication Award for the American Institute of Physics.