Easter Sunday has a different date each year, but how exactly is it calculated?
The rules date from the year 325, and the First Council of Nicaea. Christian bishops met to agree on various aspects of their church and, among other things, decided how Easter would be reckoned.
Although the calculation is a bit obscure at first, it is actually quite simple in practice. The short version is this: It’s the first Sunday after the first full moon that occurs on or after March 21. For example, in 2013, the first full moon after March 21 is on the 27th (Wednesday), so Easter Sunday follows a few days later on the 31st.
A few more points may help clarify:
- A full moon that occurs on March 21 is valid for computation.
- easter must come later the full moon, so if the full moon is on a Sunday, then Easter is the Next Sunday.
- That means the earliest possible date for Easter is March 22, and this can only happen when the full moon rises on Saturday March 21.
About twice a year, the day and night are about the same length due to Earth’s tilt toward the Sun. This is called the equinox, and in spring (in the Northern Hemisphere), it’s around March 20. However, it varies from year to year and can be as early as March 19 or as late as March 21.
At the First Council of Nicaea in AD 325, the Christian bishops met to try to fix some things in the church. One of the big issues was determining a date for Passover, which until then was based on the Jewish calendar, in particular the Passover feast.
The Synoptic Gospels actually describe the Last Supper as a Passover meal. However, some believed that relying on the Jewish calendar for this date was unreliable, as a month before the March equinox was sometimes chosen.
To overcome these difficulties, they decided on their own independent calculation of the date. The starting point was the spring equinox, but they slightly incorrectly identified it as being fixed on March 21.
Over the centuries, inaccuracies in the calendar pushed the actual equinox further and further back, until Pope Gregory XIII introduced a new system, including leap years, which became known as the Gregorian calendar we use today. This kept the date of the vernal equinox constant over time.
With a fixed date for the March equinox, calculating Easter became much more predictable, and this system remains with us today.
