Science - Pinning down the date

8th March 2013 at 00:00
Look to the sun and the moon to shed light on Easter

Exactly when we celebrate Easter can vary. It is literally a moveable feast. But why does the date move around so much?

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." So that's clear, then. (By the way, in AD325 the calendar was the Julian calendar, not the one we use today, the Gregorian calendar.) 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". The 21 March is obviously a significant date. But why? Well, the vernal equinox - the date when the earth is tilted neither towards nor away from the sun - occurs on or around 20 March. The vernal equinox gives us equal lengths of day and night; the same thing happens at 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. The result of all this is that the date of Easter varies between 22 March and 25 April.

As for eggs and rabbits, their connection to Easter comes from the goddess of spring, Eostre (or Eastre), the rabbit being a symbol of fertility and the egg a symbol of new life.

So, when you are eating your chocolate, it is worth remembering that the timing of Easter is determined by the sun and the moon and a calculation that dates back nearly 1,700 years.

James Williams is a lecturer in science education at the University of Sussex's School of Education and Social Work. Follow him on Twitter at @edujdw

What else?

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


Alongside Easter-themed science comes Easter-themed maths. Get pupils to turn their talents to TESSD's Easter Percentages activity. Suitable for differentiation and as a starter or plenary.


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