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Science - Pinning down the date

What it's all about

Exactly when we celebrate Easter varies. But why does the date move around so much, asks James Williams?

The answer is found partly in theology, partly in astronomy. In AD325 the First Council of Nicaea, a group of Christian Bishops, set the date of the holiday we now call Easter. It was determined as being "On the Sunday which follows the 14th day of the Moon which reaches this age on 21 March or immediately after that." But given that the date of Easter is determined by astronomical events, should it not be easy to predict and understand? Well, not quite.

Easter always falls on a Sunday - the name is derived from the Old English term Sunnandaeg, meaning "sun's day". That is our first astronomical link. But which Sunday? Well, the rule is the one that follows the 14th day of the moon "which reaches this age on 21 March" or thereabouts. The 21 March is obviously a significant date, when the earth is tilted neither towards nor away from the sun. The vernal equinox gives us equal lengths of day and night, as does the autumnal equinox on or around 22 September. For Easter, however, we have to create another equinox: the ecclesiastical equinox on 21 March.

The moon here means the full moon. So Easter is the Sunday closest to the 14th day after the full moon that is nearest to 21 March. Hence, the date of Easter varies between 22 March and 25 April.

What else?

Andrew Jackson's Astronomy Masterclass, named outstanding teaching resource of the year in the 2011 TES Schools Awards, offers a series of interactive lessons to ignite young imaginations.


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