Date of Easter--Interesting Random Facts

Many years ago, when I was teaching English in Japan, a Japanese colleague once asked me when Easter is every year.

I paused, thought about it, and realized I had no idea how the date of Easter was determined.  I had just always checked the calendar every year to see when Easter was.  I knew that sometimes it came early, and sometimes it came late, but I had no idea why.

It was one of those moment where I thought to myself: "I wonder why I've never been curious about this before.  It's strange how you can go your whole life, and just take certain things for granted, without ever being curious about it until somebody asks you."

So, I Googled it, expecting that there would be a really simple answer.  And instead I got this really complicated answer about how there was some sort of mathematical formula to determine the date of Easter every year.  I attempted to read some of it, only partially understood it, and eventually went back to my Japanese colleague and said, "It appears that the way the date of Easter is decided is really complicated."

That was over 20 years ago, so let's go over to Wikipedia now, and see if I can make any more sense of this.

Wikipedia says: 

Easter is celebrated on the first Sunday after the Paschal full moon (a mathematical approximation of the first astronomical full moon, on or after 21 March – itself a fixed approximation of the March equinox).

Uh, okay, that actually doesn't sound too complicated, right?  The first Sunday after the first full moon after the March equinox.

But what exactly is a "Paschal full moon"?  How is that different than a regular full moon?  And what do they mean "a mathematical approximation of the first astronomical full moon"?

Okay, well let's click on the link, then, and go over to the Paschal full moon page:

The paschal full moon is the ecclesiastical full moon of the northern spring and is used in the determination of the date of Easter. The name "paschal" is derived from "Pascha", a transliteration of the Aramaic word meaning Passover. The date of Easter is determined as the first Sunday after the "paschal full moon" that falls on or after March 21. (March 21 is the ecclesiastical equinox, the date fixed by the Gregorian reform of the calendar as a fixed reference date for the Spring Equinox in the Northern hemisphere; the actual Equinox can fall on March 19, 20 or 21). This "full moon" does not currently correspond directly to any astronomical event, but is instead the 14th day of a lunar month, determined from tables. It may differ from the date of the actual full moon by up to two days.^{[3][better source needed]}

The calculations to determine the date of the paschal full moon can be described as follows:

• Nineteen civil calendar years are divided into 235 lunar months of 30 and 29 days each.
• This period of 19 years (the metonic cycle) is used because it produces a set of civil calendar dates for the ecclesiastical moons that repeats every nineteen years while still providing a reasonable approximation to the astronomical facts.
• The first day of each of these lunar months is the ecclesiastical new moon. Exactly one ecclesiastical new moon in each year falls on a date between March 8 and April 5, both inclusive. This begins the paschal lunar month for that year, and thirteen days later (that is, between March 21 and April 18, both inclusive) is the paschal full moon.
• Easter is the Sunday following the paschal full moon.

Hmmm, um, this is getting a bit more complicated.  I'm not sure I follow all of this.

Okay let's go back to the main Date of Easter Page to see if we can make any more sense of this:

The saltus and the seven extra 30-day months were largely hidden by being located at the points where the Julian and lunar months begin at about the same time. The extra months commenced on 1 January (year 3), 2 September (year 5), 6 March (year 8), 3 January (year 11), 31 December (year 13), 1 September (year 16), and 5 March (year 19).^[31][32] The sequence number of the year in the 19-year cycle is called the "golden number", and is given by the formula

GN = (Y mod 19) + 1

That is, the year number Y in the Christian era is divided by 19, and the remainder plus 1 is the golden number. (Some sources specify that you add 1 before taking the remainder; in that case, you need to treat a result of 0 as golden number 19. In the formula above we take the remainder first and then add 1, so no such adjustment is necessary.) ^[g]

Cycles of 19 years are not all the same length, because they may have either four or five leap years. But a period of four cycles, 76 years (a Callippic cycle), has a length of 76 × 365 + 19 = 27,759 days (if it does not cross a century division). There are 235 × 4 = 940 lunar months in this period, so the average length is 27759 / 940 or about 29.530851 days. There are 76 × 6 = 456 usual nominal 30-day lunar months and the same number of usual nominal 29-day months, but with 19 of these lengthened by a day on leap days, plus 24 intercalated months of 30 days and four intercalated months of 29 days. Since this is longer than the true length of a synodic month, about 29.53059 days, the calculated Paschal full moon gets later and later compared to the astronomical full moon, unless a correction is made as in the Gregorian system (see below).

...and I give up!

And this is before you even get to the part of the Wikipedia page outlining the various algorithms that are used to calculate the date of Easter.

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Or is Wikipedia making this more complicated than it needs to be?  I tried to find some Youtube videos explaining this, and the 2or 3 Youtube videos that I watched just say that Easter is on the first Sunday after the first full moon after the spring equinox.  Example below: 

So are these Youtube videos oversimplifying?  Or is Wikipedia over-complicating?

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I mentioned above that I first tried to look this up 20 some years ago.  When I was searching on Google 20 years ago, one little factiod that I came across, and that got stuck in my head, was that Easter was the only holiday that was based on both the solar calendar and the lunar calendar.  I don't remember where I read that, but I read it somewhere.  So there you go.

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In Asia, Easter was always noticeable by its absence.  As I noted back in 2004:

In Japan, Easter is a holiday that is notable by its absence. Most Western holidays have been imported to Japan in one form or another. Christmas isn't officially celebrated, but the decorations are everywhere. Valentines day is arguably bigger in Japan than in the states. Halloween is a non-event, but Japanese people know it exists, and us JETs are usually asked do some sort of Halloween English lesson in October.

But for Easter, nothing. In fact the past two years, Easter came and went without me even realizing it.

In later years, I would learn the same was true in Cambodia and Vietnam as well.  It's strange but, for whatever reason, Easter seemed to be the only major Western holiday that never got imported over into Asia.