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Editorial - That magic lunisolar number

Editorial That magic lunisolar number

Ever wondered how the date of Easter is set?

Inquiring minds want to know: Why is Easter not on a fixed calendar date?

If you grew up around religious people (still relatively common) or simply have a penchant for calendar-related esoterica (less common), you’ll be familiar with the idea of a movable feast — an ecclesiastical holiday that has no fixed calendar date. The original movable feast is Easter, based on which the dates of all other movable feast days such as the entire Lenten season (Shrove Tuesday, Ash Wednesday, Maundy Thursday etc), Ascension and Pentecost are determined.

So how is the date of Easter derived? There are a few ways. One option is for you to acquire a Patek Philippe Calibre 89, the first watch with an indication of the date of Easter. Or, if you don’t have seven-figure sums of money lying around, you can work it out mathematically.

That magic lunisolar number

Calibre 89 © Patek Philippe

To put it as briefly as possible, Easter is celebrated on the first Sunday that follows the Paschal Full Moon. This of course brings up more questions than it answers, because what on earth is the Paschal Full Moon? My friends, the Paschal Full Moon is the first full moon to take place on or after the spring equinox. This is like watching clowns get out of a tiny car — an endless series of questions following one innocuous line of inquiry. The equinox is commonly understood as one of the two times in a year when the hours of day and night are exactly equal, but more precisely, it’s when the sun crosses the celestial equator. The celestial equator is the imaginary line projected outwards from latitude 0.

Hands up, those of you who regret asking about the date of Easter. Now put those hands down, because I’m not done.

You can easily determine the date of Easter from year to year, using the method outlined above. You wait for the spring equinox, figure out when the next full moon is, and — boom! — the upcoming Sunday is Easter. I bet you all can’t wait to impress your friends with this new party trick. Determining the date of Easter in advance, however, takes a bit more than that.

Ever heard of the Metonic cycle? It’s a system of unification that reconciles the 29.53-day lunar month and the 365.25-day solar year. If you feel like all the Easter information above is already more than you can digest, the Metonic cycle is going to be that final Lindt chocolate bunny that sends you into a coma. That’s why I’m not going into the details (believe me, I totally could).

Just the essentials — the Metonic cycle is a 19-year unit of time that is approximately the lowest common multiple of the lunar month and the solar year. Five centuries before Christ was born, some Greek dude called Meton figured out that 19 solar years is more or less 235 lunar months. (Fun fact, and this is a very loose definition of “fun” — the Babylonians actually worked it out way before Meton, but he gets the credit for whatever reason.)

To make a long story short (or less long anyway), the date of the Paschal Full Moon recurs every 19 years. Determining Easter in any particular year merely requires the knowledge of the full 19-year sequence of Paschal Full Moon dates and looking at the calendar for the relevant year. It’s a great number, 19, one of my favourites. It’s the age where you’re old enough to know better, but still young enough to get away with pretty much anything. Like how I just got away with a crash course in lunisolar calendrical systems on a luxury lifestyle website.


Lecture 1 Comment(s)

14 April 2020

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