This year, it’s April 4th. But last year, it was April 12th… and back in 1818, it was on March 22nd.

It’s always on a Sunday – and often, it happens twice in a year.

Yes, we’re talking about Easter! This biblical holiday is a “moveable feast”, which means it doesn’t land on the same day every year – and the calculation to determine it is so famous, it even has its own name: Computus.

Today we’ll dive into this interesting innovation from the past. You’ll learn about the history of Computus, the curious calculation that decides its date, and even get a chance to try a simple version of the equation yourself!

Let’s start at the beginning…

When was the first Easter?

 

The first “Easters” were actually celebrated by Jewish Christians, who timed it with Passover. Then a more fully formed Christian festival, called Pascha, began to appear in the mid-2nd century.

Finally, in the year 325, the Easter we know today was formally defined by the First Council of Nicaea. They created two rules: first, that Easter should be independent from the Jewish calendar; and second, that Christians should celebrate it on the same day. However, they didn’t specify how to decide the date… which resulted in controversy that continues to this day.

Eventually, it was decided that Easter should be observed on the first Sunday following the first Paschal full moon after the spring equinox.

Err… what was that? The first what after the huh?! Couldn’t they have chosen something simple like “the first Sunday of April” or “March 30th”?

Well, there’s actually a reason for this complicated equation…

Why is Easter on a different day every year?

 

Even though it’s a Christian holiday, Easter is actually based on the Jewish calendar – because Easter evolved from Jewish Christians and was tied to Passover, which is determined by the Paschal full moon (which is the 14th day in a lunar month).

This complicates things right away, because the Jewish calendar is based on lunar cycles, while Christian calendars are based on solar cycles. A solar calendar is 365 days, while a lunar calendar is only 354 days – so the dates don’t match up.

To make matters even more complicated, there are actually two Easters! Western Christians use the Gregorian calendar, while Eastern Christians use the Julian calendar to “translate” the date from the Jewish calendar.

This means the Eastern Easter (which can occur between April 4th and May 8th) is typically later than the Western Easter (which can occur between March 22nd and April 25th)… though both Easters sometimes fall on the same date, as they did in 2014 and 2017. Whew!

Clearly, an innovative solution was required – so a complex equation called “Computus” was developed over the years, which religious scholars use to determine each year’s Easter. Let’s see how it works!

What is Computus?

 

Computus refers to the mathematical process of determining the first Sunday after the first Paschal full moon falling on or after 21 March – a.k.a. Easter.

To do this, scholars need to determine the “epact” for each year. Remember how the lunar calendar and solar calendars are off by 11 days? These “extra days” are called epacts, and they are added up each year until they reach 30 – at which point they’re “cashed in” for an extra month in the lunar year (to keep the dates synchronized).

In other words, Computus helps us translate the lunar calendar to the solar calendar so we can find the correct date of the Paschal full moons. Then, all we need to do is find the first one after March 21st and voilá – Easter is the following Sunday! Easy, right?!

Ready to give Computus a try?

While the actual formula for Computus is very complex, there are some simple “shortcut” versions you can use yourself. They typically only work for certain periods (due to changes in astronomical cycles), but they’re still quite handy.

For example, you can use the formula below to determine the date of Easter (for Western Christians) in any year between 1900 and 2199. Let’s try it!

1. First, divide the year by the number 19.

For example: 2021 / 19 = 106.368

2. Multiply the number before the decimal point by 19.

106 x 19 = 2014

3. Subtract that number from the current year.

2021 – 2014 = 7

4. Add 1.

7 + 1 = 8

5. Look up this number in the following list. Easter is on the first Sunday after the date given.

0 — March 27
1 — April 14
2 — April 3
3 — March 23
4 — April 11
5 — March 31
6 — April 18
7 — April 8
8 — March 28
9 — April 16
10 — April 5
11 — March 25
12 — April 13
13 — April 2
14 — March 22

In this case, our number was 8. That means Easter should be on the first Sunday after March 31st – which it is!

Will Easter’s date ever be simplified?

 

The debate over Easter’s date rages on. As you can see, the whole situation is quite confusing, and there are advocates for both simplifying it and unifying the date between Eastern and Western Christans.

In fact, the Second Vatican Council even agreed to change it to a fixed date in 1963… but they haven’t taken any action to make the change yet.

So for now, we’re stuck with Computus – one of the most famous mathematical equations ever devised. But hey, at least we have the formula above to help us out over the next 179 years!