The Mostly Logical, Kind Of Long Reason Easter Hops Around On The Calendar

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Ever notice how Easter moves around a lot on the calendar? Yeah. So did my boss. While trying to come up with Easter topics to write about, he posed this one. I scoffed. How did people not know it was because of the equinox? I thought. As it turns out, my sarcastic enthusiasm on the matter quickly came back and slapped me. The subject of why Easter is where it is on our schedules just so happens to be a pretty complex affair.

To solve the mystery, I started with the calendars. I knew we use the Gregorian but that was the extent of my knowledge on the subject – and it is a subject. Seriously, don’t be fooled by the simplicity of the word “calendars” – the history of them and how they’re figured out could easily be an entire college course. In 1582 CE, the Gregorian calendar was made official by Pope Gregory XIII. As you well know, this lovely system neatly organizes our years into increments of 365 days with the exception of leap years, which have 366. The Gregorian is a solar calendar, meaning that the dates indicate the position of the Earth on its revolution around the sun.

But before 1582, the Julian calendar was being used (and had been since 45 BCE). This calendar was pretty accurate and relatively similar to the Gregorian in that it had 365 ? days, which was achieved with a leap year every 4 years. That said, the Julian calendar was the standard during the First Council of Nicaea, which is when Christian bishops were brought together by Emperor Constantine I in order to come to a consensus on how Christianity was to be represented. Anyway, one of the major things on their agenda was to come up with a date for Easter. They established that the first Sunday after the Paschal Full Moon was when the holiday would be celebrated. (The Paschal Full Moon has basically nothing to do with the actual full moon. Instead it’s the 14th day of a lunar month and can differ from the actual full moon by up to two days.)

Why did they do this? Well, they didn’t want to get all the holidays jumbled. Right before Easter is Passover, which is greatly celebrated in Judaism, which is based on the Hebrew calendar, which is a lunisolar system. (It has 12 lunar months and one solar year.) Passover is always celebrated on the 15th of Nisan, the Hebrew month that coincides with March-April. So, it was decided that Easter Sunday would be the first Sunday after the first full moon that followed the vernal equinox – which is officially the Gregorian’s March 21st. That way, the two holidays were always about a week or so apart.

Yeah, it is a lot to process. But that’s not even the half of it.

It gets tricky. To calculate this Pascal Full Moon, you need to be aware of two really important concepts: the Golden Number and the Epact. (Isn’t it amazing what ancient people were capable of?) The Golden Number is also called the Metonic cycle and is officially known as computus. This cycle is based around the relationship between the moon’s phases and the days of the year, which repeat every 19 years. So, every year under this cycle is given a number 1 through 19, depending on when the new moon falls. Under the Julian calendar, this Golden Number was the solution to calculating the Pascal Full Moon and was expressed thus:

Golden Number = (year mod 19) + 1

After calculating this, you’d plug it into a table that would give you the date of the full moon. Clean and simple. But the world wanted accuracy. After hundreds of years, the Julian was cast uncerimoniously aside and the Gregorian moved in. With the introduction of the modern day calendar, the Golden Number alone wasn’t enough to calculate the full moon. Something more had to be added. And that’s where the Epact comes in.

The Epact measures the age of the moon (or rather the number of days that have come and gone since the full moon). Before the reform, a pretty simple relationship existed between the Epact and the Golden Number:

Julian Epact = (11(Golden Number – 1)) mod 30

After the introduction of the Gregorian calendar, this formula was expanded, creating a way to find the Epact. Here it goes:

Gregorian Epact = Julian Epact – S + L + 8

This is when:

S = (3 x century)/4

This is the Solar Equation and is the difference between the two calendars.

L = (8 x century)/ 25

This is the Lunar Equation and is the difference between the Julian calendar and the Metonic cycle.

And that 8 that’s tacked on the end calibrates the equation to equal the age of the moon on New Year’s Day.

The Gregorian Epact is then looked up in a lovely algorithm, which you can find here (along with most of the research I poured through to bring you this information).

Being slightly skeptical of these formulas (shame on me) I decided to give it a try by plugging in the information to calculate the Epact for this year, 2013. Thus:

Gregorian Epact = Julian Epact – S + L + 8

Golden Number = (2013 mod 19) + 1

= 19

Julian Epact = (11(19-1)) mod 30

= 18

S = (3(21))/4

= 16

L = (8(21) +5)/25

= 7

Gregorian Epact = 18 – 16 + 7 + 8

= 17

The results? Perfect. According to the algorithm, the Epact of 17 correlates to March 27th, which is, of course, the first full moon after the equinox in 2013.

Longest most complicated blog post ever? It’s quite possible. But it was so worth it, right? …Right? Okay, now go impress your friends.

Questions? Comments? Lemme know!

About the Author:

” is the Gourmet Scribe at, one of the top suppliers of gift baskets in the nation, and currently resides in Manchester, New Hampshire.

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