Being neither Christian nor Pagan I’m not inclined to pay much heed to Easter (except of course for the days off work and the chocolate eggs) but it does present an opportunity to talk about a little astronomy. Did you know, for example, that the date of Easter changes each year, and that it depends on the phase of the Moon?
The Venerable Bede, a Northumbrian Monk, wrote, in his book “The Reckoning of Time” (725CE):
“The Sunday following the full Moon which falls on or after the [Spring] equinox will give the lawful Easter”
So that all seems fairly straight forward: wait till the Spring Equinox; wait till the next full Moon; wait till the next Sunday; and voila, Easter.
However complications arise because:
(a) the ecclesiastical date of the Spring Equinox does not always match the astronomical date; and
(b) the ecclesiastical date of the Full Moon does not always match the astronomical date.
Why is this?
Put simply the ecclesiastical Spring Equinox always falls on 21 March, whereas the astronomical Spring Equinox, that is the point in the year when the Sun passes from the southern to the northern celestial sphere, can occur on 19, 20 or 21 March (and occurs most often on 20 March), and so the starting date for the computation of Easter is often out.
Also the ecclesiastical date for the Full Moon takes no real account of the actual phase of the Moon, instead it refers to the 14th day of the lunar month in which the New Moon occurs between 8 March and 5 April, which can differ from the date of the Full Moon by up to three days. This is referred to as the Paschal Lunar Month, the word “Paschal” deriving from the Hebrew word for Passover, a Jewish festival commemorating the Exodus.
The table below shows how these two dates vary over the next decade, and how that effects the date of Easter.
Date of Ecclesiastical Spring Equinox
Date of Astronomical Spring Equinox *
Date of Paschal Full Moon on or after A **
Date of Astronomical Full Moon on or after B
|Date of Sunday on or after C (Easter)||Date of Sunday on or after D
|2011||21 March||20 March||18 April||18 April||24 April||24 April|
|2012||21 March||20 March||8 April||6 April||8 April||8 April|
|2013||21 March||20 March||30 March||27 March||31 March||31 March|
|2014||21 March||20 March||15 April||15 April||20 April||20 April|
|2015||21 March||20 March||4 April||4 April||5 April||5 April|
|2016||21 March||20 March||24 March||23 March||27 March||27 March|
|2017||21 March||20 March||12 April||11 April||16 April||16 April|
|2018||21 March||20 March||1 April||31 March||1 April||1 April|
|2019||21 March||20 March||19 April||21 March||21 April||24 March|
|2020||21 March||20 March||9 April||8 April||12 April||12 April|
* Despite these dates being all the same, the date of the astronomical Vernal Equinox can vary between 19, 20 and 21 March, but occurs most often on 20 March. Over the next 100 years it will fall on 19 March only 4 times, and on 21 March only 20 times.
** The dates of the Lunar Months (there are 13 Lunar Months in one Solar Year) repeat on a 19 year cycle, called the Metonic Cycle. 2011 in the 17th year in that cycle; in 2014 we’re back to year 1. The dates for the Paschal New Moon can be found in calculation tables.
So you can see that, despite the fact that the Ecclesiastical Spring Equinox date doesn’t match the Astronomical Spring Equinox date on any year over the next decade (and only matches it in 20 out of the next 100 years), plus the fact that the date of the Paschal Full Moon often doesn’t match the date of the astronomical Full Moon, the dates of Easter calculated using the Ecclesiastical and the Astronomical methods only differ on one year in the next decade – 2019CE, when those who are so inclined will celebrate Easter on 21 April, but when, according to Bede, the Sunday after the full Moon after the equinox actually occurs on 24 March!
Maybe I’ve got too much time on my hands…
[Please note that throughout this post I have calculated and referred to the dates of Gregorian Easter, that celebrated by the western Christian churches. Eastern Orthodox Christians calculate Easter using the Julian calendar, which is currently 13 days ahead of the Gregorian calendar, i.e. on 21 March (the Spring Equinox) the Gregorian calendar reads 3 April.]