The Timing of Easter

by Deb on April 11, 2011

Easter Teach/Learn

This post is part of the Easter Teach/Learn, if you have an Easter post, join us.

Easter is a moveable holiday, so how do they work out when it falls? It’s even more complicated than most people think, and a fascinating demonstration of how European cultures have changed and interacted.

The Solar Calendar

Most people know it is the first Sunday after the first full moon after the March equinox, except it’s not the real full moon and not the real equinox. It’s the ecclesiastical full moon and equinox, which don’t always line up with the real thing. The equinox is when the sun crosses the equator and is at a slightly different time every year, in Western Europe it generally falls on March 20th. But for the ecclesiastical calendar it is always March 21st, so saying ‘after the March equinox’ they really mean ‘after March 21st.’

Western Europe and those of us who inherited their calendar use the Gregorian calendar. This is the calendar that took out some of the leap years on centuries, so there are only 97 leap years in 400 years.  It was brought in because the previous Julian calendar had too many leap years and was moving the equinox further and further forward. In 1582 when it was introduced the equinox was on March 11th. Unfortunately it wasn’t easily adopted, mainly for politicoreligious reasons. It was introduced by Pope Gregory XIII and many of the newly protestant, eastern and orthodox countries and churches would not accept a papal innovation. Some orthodox churches still use the Julian calendar to calculate Easter, giving them a completely different date.

The Lunar Calendar

Full MoonThe full moon is even more interesting, because it is based on a lunar calendar rather than solar. Many ancient cultures, especially hunter-gatherers and pastoralists, used a lunar calendar because the changes of the moon are very obvious wherever you are and provide a simple method of timekeeping. Unfortunately it doesn’t match with the solar calendar that gives us the year, which is important for things like when to plant crops.

A lunar month is slightly over 29 days long, if you use the actual phases of the moon you are left with a choice of 12 months with a year of 348 days or 13 months with a year of 377 days. Neither of them is a good choice if you need to set yearly events such as feasts or for agriculture. So most lunar calendars, including the ecclesiastical calendar, have a system of months with 29 and 30 days that get them closer to 365. The ecclesiastical lunar year is 354 days long, which means it is still out by 11 days. To get it back into sync many cultures with lunar calendars use something called the Metonic cycle,  which means out of 19 years 7 of the years will have a leap month and be 13 months long. There also needs to be a day correction 8 times in 2500 years. Simple?

Unfortunately when you sync up with the year, you get out of sync with the moon. So the ‘full moon’ in the rules for determining Easter isn’t actually the full moon, it is the 14th day of the lunar month as defined by the ecclesiastical calendar, which may or may not have anything to do with the sky.

What is such an untidy system doing at the heart of Christianity’s most important festival?

The Christian Easter celebration is based on the Jewish Passover, and is a symbolic equation of Jesus’ sacrifice with the sacrifice of the Passover lambs. In the early Christian church they actually asked the local Jewish community to determine the date. However this caused problems for several reasons, including the way it moved around and the fact that different communities sometimes used different days. In addition the church wanted to establish their own identity and not be dependent on Jews for the date of such an important feast.

Passover of course commemorates the Exodus, in which the Jews were freed from slavery in Egypt and made their way north. At this point in their history they were nomadic pastoralists, driving their flocks of sheep with them. This is the reason they had a lunar calendar, still used today.

In most Christian cultures Easter is actually named as Passover, as it was in the early Bibles. Both Christianity and Judaism began around the Mediterranean and Middle East and were taken to northern Europe, where they met well established local religions. One of the Germanic celebrations of Spring was called Eostre, the obvious root of our Easter.

But that’s a completely different story.

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{ 5 comments… read them below or add one }

Suzi April 11, 2011 at 8:19 am

Hi there, I really liked ‘the timing of Easter’ post. I love teaching my kids all the scientific things you show on the blog. I am a Christian and was well impressed with your write up. I think it’s entertaining to find out the origin of modern day festivities and goings on. Christmas is another corker. Actually a Spring occurrence with lots of lovely pagan influences from those early ‘converts’ to Catholicism. Nice how they compromised :o)

What’s your take on Lunar planting? I’m skeptical but whenever I’ve done it and documented it I’ve noticed the plants are stronger..


Veronica April 11, 2011 at 8:22 am

Complicated much?

Thankyou for the explanation!
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Cass April 17, 2011 at 12:33 pm

I am still confused. I would love a couple of examples of the calculation.


Deb April 17, 2011 at 3:11 pm

Since the full moon is defined as the 14th day of the lunar month, and the solstice is defined as March 21st, Easter will be the third Sunday of the lunar month that starts on or after March 8th (14 days before March 22nd).

The metonic cycle is just a trick to get the lunar and solar years to line up for humans when they don’t in nature, so the easiest way is to get it from a chart. There’s a table about 2/3 of the way down this page with the beginnings of the lunar months (US style dates). 2011 has 13 lunar months, one starting on the 6th of March, the next is on the 4th of April. 14 days after that would be the 18th of April, and the next Sunday is the 24th, which is Easter this year.

Next year has 12 lunar months, with one beginning on March 25th. 14 days later would be the 8th of April which is a Sunday, and that’s Easter. In 2013 a lunar month begins on the 14th of March, making the full moon the 28th. The next Sunday is March 31st.


Deb @ Living Montessori Now April 18, 2011 at 1:23 pm

Fascinating! I’d never actually studied the entire explanation for the moveable date of Easter before. Thanks for doing the research for us! 🙂
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