How do they know it’s Easter? Ever wondered how the exact dates of the Easter break are chosen? Easter Sunday can fall anytime between 22 March and 25 April and, thanks to European observations of the Sun that go back many centuries, the exact date can be predicted as far ahead as 4099 AD.
Back in 325 AD, it was declared that Easter should be celebrated on the first Sunday after the first full Moon following the vernal equinox (the Spring day in the northern hemisphere when the hours of daylight and darkness are equal). Over the next few centuries, theologians and scientists struggled with the problem of calculating these vital dates years in advance. Although they often studied the skies in some detail to help them work out future calendars, this was particularly difficult when working on the premise that the Sun moved around the Earth. Then, in 1651, Giovanni Cassini installed a pinhole camera in the roof of the San Petronio cathedral in Bologna in Italy.
Previous studies of the movement of the Sun indicated the urgent need for calendar reform in the sixteenth century and the introduction of the new Gregorian calendar, that is still in use today. But it was Cassini who installed the most accurate observatory at San Petronio, and made ample use of it to monitor the accuracy of the new calendar. Cassini’s observations allowed exact calculations of future equinoxes on the Gregorian calendar to be made in advance, thus helping to solve the ‘When’s Easter?’ problem.
For more: ESA Portal - Focus On - Early European astronomers determined Easter dates
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