diamond geezer

 Friday, March 21, 2008

Extreme
Early
Easter

(full moon 18:50 GMT, 21/03/07)

The Easter story, according to the Bible, is intrinsically linked to the Jewish festival of Passover. And Passover is a lunar festival, linked to the date of the full moon. So Easter has to be linked to the date of the full moon too. Here's how the date of Easter is determined...
Easter Day is the first Sunday after the first full moon after the spring equinox.
To get an early Easter you need an early full moon. Like we have this year. This year the spring equinox was yesterday, the next full moon is today, and so Easter is on Sunday. It's not very often that there's a full moon almost immediately after the equinox and then a Sunday very soon afterwards. And that's why Easter is bloody early this year. Happy with that? Good, please stop reading now.

OK, now for the real, rather more complicated explanation. You knew it wouldn't be that simple, didn't you? One reason for this is because the church's method of determining Easter doesn't use the real spring equinox, it always uses March 21st. You can thank the Council of Nic├Ža in 325AD for that one. And Easter calculations don't use the date of the real full moon either, they use something called the Paschal Full Moon instead. These form an artificial list of full moon dates, applicable globally, almost but not quite linked to the real ones. So the actual rule for determining the date of Easter is this...
Easter Day is the first Sunday after the Paschal Full Moon on or after March 21st.
Let's take a more detailed look at these mysterious Paschal Full Moons. The ancient church didn't want to have to mess around with actual astronomical data, because that was quite difficult to predict accurately in those days, and leap years kept getting in the way. So they took advantage of a convenient lunar coincidence called the Metonic Cycle and used this to help. The phases of the Moon may not repeat on the same day every year, but they do repeat (give or take 2 hours) on the same day every 19 years. So if you know the dates of the spring full moons over a 19 year period, you can repeat that list every 19 years afterwards. Simple?

Table to find the date of
the Easter Full Moon
Golden
Number

Years

Full Moon
I1976 1995 2014April 14
II1977 1996 2015April 3
III1978 1997 2016March 23
IV1979 1998 2017April 11
V1980 1999 2018March 31
VI1981 2000 2019April 18
VII1982 2001 2020April 8
VIII1983 2002 2021March 28
IX1984 2003 2022April 16
X1985 2004 2023April 5
XI1986 2005 2024March 25
XII1987 2006 2025April 13
XIII1988 2007 2026April 2
XIV1989 2008 2027March 22
XV1990 2009 2028April 10
XVI1991 2010 2029March 30
XVII1992 2011 2030April 17
XVIII1993 2012 2031April 7
XIX1994 2013 2032March 27
Here's a table showing the 19 possible Paschal Full Moons. I've listed all the years from 1976 to 2032, but you can trawl back as far as 1900 and forward as far as 2199 should you want to. Remember, these are all artificial full moons for calculation purposes only.

Every year on the same line has the same full moon date. For example, the Paschal Full Moon in 1995 fell on April 14th, the same as 19 years earlier in 1976 and 19 years later in 2014. These years are described as having a Golden Number of 1 (which is the remainder when dividing the year by 19, plus 1). Don't worry about that. All you need to know is that every year with the same Golden Number has the same Paschal Full Moon. Every year with a Golden Number of 1 (including 1900 and 2185, for what it's worth) has its post-equinox full moon on April 14th.

Scanning down the table you'll see that the first post-equinox full moon in the year 2000 was on April 18th, and that's the latest date possible. It's years with a Golden Number of 6 that have the very latest Easters. And the Paschal full moon this year is on March 22nd, which is the earliest date possible. It's years with a Golden Number of 14 that have the very earliest Easters. Well, near enough, but I'll come back to the extra complications on Sunday.

And one more thing. During the 20th, 21st and 22nd centuries the Paschal Full Moon never falls on March 21st itself, which means that Easter Day can never fall as early as March 22nd. But in other centuries the table of Paschal Full Moons is different. Throughout the 19th century, for example, March 21st was linked to a Golden Number of 14 (that's because 1900 wasn't a leap year, so the pattern skipped a day). And hey presto, in 1818 there actually was an Easter Day on March 22nd. Similarly there'll be another March 22nd Easter in the 23rd century, in 2285. But between 1900 and 2199, March 23rd is the earliest Easter we can possibly get. It really is an utterly extreme Easter this year. Sorry, is anybody still reading? I did warn you to stop earlier, didn't I?

• Easter Sunday - geek level dates and tables
• Computus - calculating the date of Easter
• Extreme Early Easters - a 1768 perspective


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