When is Easter?

Thanks to Phil Krause for this great post on how to find out when Easter is.

EasterIsland02Easter is the oldest and most important festival of the Christian Church, celebrating the resurrection of Christ.  Easter is a moveable feast, in that it is not fixed in relation to the civil calendar.  The first council of Nicaea in AD 325 established the date of Easter as the first Sunday after the first full moon (the Paschal Full Moon) following the Northern hemisphere spring equinox.

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An equinox occurs twice a year (around 20 March and 22 September), when the plane of the Earth’s equator passes the centre of the Sun.  At this time the tilt of the Earth’s axis is inclined neither away from nor towards the Sun.  The name “equinox” is derived from the Latin aequus (equal) and nox (night), because around the equinox, night and day are about equal length.

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When Julius Caesar established his calendar in 45 BC he set 25 March as the spring equinox.  Since a Julian year (365.25 days) is slightly longer than the actual year, the calendar drifted with respect to the equinox, such that the equinox was occurring on 21 March in AD 300 and by AD 1500 it had reached 11 March.  This drift induced Pope Gregory XIII to create a modern Gregorian calendar.  The Pope wanted to restore the edicts concerning the date of Easter of the Council of Nicaea of AD 325.  Today, the spring equinox normally appears around the 20 March each year.

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Because the days of the week vary each year and the date that we see a Full Moon  varies as well, Easter has quite a variable date.  If the spring equinox falls on Friday 20 March and there is a Full Moon on Saturday 21 March, then Easter Sunday will fall on its earliest possible date of Sunday 22 March. The last time this occurred was 1818 and the next time will be 2285.

The time interval between similar lunar phases (the synodic month) averages about 29.53 days.  Therefore, in those lunar calendars in which each month begins on the new moon, the full moon falls either the 14th or 15th of the lunar month.  Because calendar months have a whole number of days, lunar months may be either 29 or 30 days long.

If the spring equinox is on Saturday 20 March and the full moon is on the same day then the next Full Moon could be 30 days later on Monday 19 April which gives rise to the latest possible date for Easter on Sunday 25 April. The last time this occurred was 1943 and the next time will be 2038.

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This year the spring equinox is on Thursday 20 March but because we will only just have had a full moon on 16 March we will have to wait until the next one on Tuesday 15 April, therefore Easter will be on the following Sunday 20 April, quite late.  Whereas last year Easter was quite early with the equinox falling on Wednesday 20 March, the next full moon was on Wednesday 27 March, the following Sunday 31 March was Easter Day.  So the earliest and latest full moon only happens every once in a Blue Moon; but that’s another story.  Happy Easter everyone!

Thanks again Phil.

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