diamond geezer

 Thursday, March 21, 2019

If you know a bit about Easter, you'll know it's defined as the first Sunday after the first full moon after the spring equinox.

In which case...
The spring equinox was yesterday - Wednesday March 20th
»» The next full moon is today - Thursday March 21st
»»»» The next Sunday is March 24th, which must be Easter
Except Easter isn't March 24th, it's still a whole month away on April 21st.

So what's going on, and why is Easter 2019 late when it should be early?

Firstly, we need to look at the date of the spring equinox.

If you thought it was always March 21st, you're out of date. The spring equinox was last on March 21st in 2007 and won't be on March 21st again until next century. It is true that March 21st was the usual date for the spring equinox during the 20th century, but the usual date during the 21st century is March 20th. Here's why.

The spring equinox occurs whe the Sun is overhead at the equator, crossing from the southern to the northern hemisphere, and its precise time varies from year to year. It takes the Earth 365 days, 5 hours and 49 minutes to orbit the Sun, so every year the spring equinox shifts almost 6 hours later than the year before. That's approximately 24 hours later every 4 years, which is then cancelled out by the presence of a February 29th a few weeks before the next spring equinox occurs. But this still leaves the calendar 11 minutes short of reality every year, and this tiny difference shifts the spring equinox approximately three-quarters of an hour earlier every 4 years.

This table shows how both the time and the date of the spring equinox change over a 28-year period. All times are GMT (which is important, because in other time zones the dates might be one day earlier or one day later).

Time of Spring Equinox (GMT)
March 20th (blue)   March 21st (green)

Look across the rows to see how the times jump approximately six hours later each year. Look at the final column in each row to see that the latest equinoxes always occur in years immediately before a leap year. And look down the columns to see how the times nudge about 45 minutes earlier every 4 years. That's why 2007 was the very last occasion this century that the spring equinox occurred on March 21st. It's also why in 2044 the times in the first column of this table will retreat past midnight pushing the spring equinox back one further day onto 19th March (initially for leap years only).

Possible dates of the spring equinox (GMT)
20th March or 21st March
1876-1899: 20th March only
1900-1911: 21st March only
1912-2007: 20th March or 21st March
2008-2043: 20th March only
2044-2099: 19th March or 20th March
2100-2135: 20th March or 21st March
2136-2175: 20th March only
2176-2199: 19th March or 20th March
[cycle repeats every 400 years, approximately]

The first day of spring therefore always falls on 19th, 20th or 21st March, with the 20th more common than the other two dates. This century there are only two March 21sts, both passed, and we'll only have twenty March 19ths. All the other spring equinoxes are on March 20th, including every year from 2008 to 2043.

But in the year 325 AD, when the rules for fixing the date of Easter were drawn up, March 20th wasn't the date that was picked. The First Council of Nicaea preferred a fixed, constant system independent of equinoctial cycles, so agreed on a constant date of March 21st, because that was more appropriate at the time.

The spring equinox may have been yesterday, but the official rules assume it's today. That's the first part of the reason why Easter 2019 is late rather than early.

Next, the dates of full moons.

These repeat, pretty much exactly, every 19 years. For example in 2000, 2019, 2038 and 2057 the March full moon falls on March 21st - precisely 19 years apart. This pattern was first spotted by a Greek philosopher called Meton 2500 years ago, hence is called the Metonic cycle. We now know he wasn't completely spot on, and that in fact 19 years contain 234.997 full moons, so the dates do move very slightly. But the ecclesiastical body which set the rules for determining Easter decided it was close enough, and came up with a list of 19 Full Moon dates to repeat every 19 years.

These are known as Paschal Full Moons. They don't necessarily match with the dates of actual full moons but are designed to be damned close, and are the full moons used to calculate the date of Easter.

n.b. Technically the date used isn't the date of the full moon but '14 days after the date of the new moon', because this best mimics the Hebrew Calendar, upon which the date of Passover is based. But let's not get bogged down in Quartodecimanism.

n.b. We no longer use the original full moon dates because the Julian calendar in use in 325 AD has been replaced by the Gregorian Calendar, which deleted several days to drag the calendar back in sync with the seasons. To match solar reality three leap days are now removed every four centuries: specifically years ending in 00 are only leap years if divisible by 400. A consequence of this is that a different set of 19 full moon dates is required every time a leap day is deleted, e.g. the list for the 19th century is different to the list for the 20th. But the list for the 20th century is the same as the list for the 21st century because February 29th 2000 actually existed, so the data which follows is correct for the period 1900-2099.

Table to find the date of
the Paschal Full Moon


Full Moon
I1995 2014 2033April 14
II1996 2015 2034April 3
III1997 2016 2035March 23
IV1998 2017 2036April 11
V1999 2018 2037March 31
VI2000 2019 2038April 18
VII2001 2020 2039April 8
VIII2002 2021 2040March 28
IX2003 2022 2041April 16
X2004 2023 2042April 5
XI2005 2024 2043March 25
XII2006 2025 2044April 13
XIII2007 2026 2045April 2
XIV2008 2027 2046March 22
XV2009 2028 2047April 10
XVI2010 2029 2048March 30
XVII2011 2030 2049April 17
XVIII2012 2031 2050April 7
XIX2013 2032 2051March 27
Here's a table showing the current dates of the 19 possible Paschal Full Moons.

Every year on the same line has the same full moon date. For example, the Paschal Full Moon in 2014 fell on April 14th, the same as 19 years earlier in 1995 and 19 years later in 2033. 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 how it's calculated, just know that the Golden Number repeats every 19 years, and that every year with the same Golden Number has the same Paschal Full Moon.

For example, every year with a Golden Number of 19 has its post-equinox full moon on March 27th. Easter is then the first Sunday after that, which could be March 28th or any day up to April 3rd. In 2013 it was March 31st, in 2032 it'll be March 28th and in 2051 April 2nd.

Scanning down the table you'll see that the earliest possible post-equinox full moon is March 22nd, which occurs in years with a Golden Number of 14. Years with a Golden Number of 14 deliver the earliest Easters of all. Meanwhile the latest possible post-equinox full moon is April 18th, which occurs in years with a Golden Number of 6. Years with a Golden Number of 6 deliver the latest Easters of all.

We're in a year with a Golden Number of 6 right now. Easter is the first Sunday after April 18th, which this year is Sunday April 21st, which is definitely on the late side. But in 19 years time, in 2038, we'll get the ultimate late Easter. The first Sunday after April 18th will be Sunday April 25th, which is the latest it can be, a date last matched in 1943 and not equalled again until 2190.

Easter on 21st April: last happened 1946 & 1957, next happens 2019 & 2030
Easter on 22nd April: last happened 1973 & 1984, next happens 2057 & 2068
Easter on 23rd April: last happened 1916 & 2000, next happens 2079 & 2152
Easter on 24th April: last happened 1859 & 2011, next happens 2095 & 2163
Easter on 25th April: last happened 1886 & 1943, next happens 2038 & 2190

So, to finally return to the question I posed right back at the start, why is Easter 2019 late when it should be early?

It's because Easter isn't defined by the actual movement of the sun and moon but by a set of rules which very closely approximate to it.

Easter is not the first Sunday after the first full moon after the spring equinox.
The spring equinox was yesterday - Wednesday March 20th (21:58 GMT)
»» The next full moon is today - Thursday March 21st (01:43 GMT)
»»»» The next Sunday is March 24th, which would be Easter, but isn't
Forget the true spring equinox. Focus instead on the first Sunday after the first full moon after 21st March.
The 'spring equinox' is today - Thursday 21st March
» Today's full moon therefore doesn't count
»» The next full moon is four weeks away - Friday April 19th
»»»» The next Sunday is April 21st, which turns out to be Easter
And to be really precise forget that too, because the true Easter definition is the first Sunday after the Paschal full moon.
This year's Golden Number is 6
»» The Paschal Full Moon (from the table) is Thursday April 18th
»»»» The next Sunday is April 21st, which is indeed Easter
If the First Council of Nicaea had set their rules slightly differently, today's full moon would have made Easter really early. But they didn't, making Easter really late.

And this is why you have a month to wait for your chocolate egg, and why the weather ought to be better when the bank holiday weekend finally comes round.

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