How Do We Figure the Date of Easter?
Yesterday, I was preparing some things for Easter when I came across a chart for which day Easter will fall each year. I’ve seen these charts before, but this was the first time I saw one with an explanation for how to figure when Easter will fall. I had always heard that it had something to do with the moon or something, but no one could ever give a really good answer as to how it’s figured. Check this out:
“Easter is always the first Sunday after the Full Moon happens upon or next after the twenty-first of March; and if the Full Moon happens on a Sunday, Easter is the Sunday after.”
So there’s the formula for figuring the date of Easter. But why the first Full Moon after March 21st? Does that date hold some significance or was it chosen at random? I did some more research.
Apparently, Easter (and the church holidays that all relate to the date of Easter – Ash Wednesday, Pentecost, Trinity Sunday, Palm Sunday, Ascension Day, etc.) weren’t set on the Gregorian calendar, the one we use today. They were determined using the lunisolar calendar (as is the Hebrew calendar).
It was apparently an issue back in 325 at the Council of Nicaea. They decided that Christians should all celebrate Easter on the same Sunday. They didn’t specify which one, but gave the privilege of annually announcing the date of Easter to a bishop. Councils continued to meet, as councils do, and took it upon themselves to determine a date. Since the church used a lunisolar calendar, it figured the dates using the moon.
The rule has since the Middle Ages been phrased as Easter is observed on the Sunday after the first full moon on or after the day of the vernal equinox. However, this does not reflect the actual ecclesiastical rules precisely. The reason for this is that the full moon involved (called the Paschal full moon) is not an astronomical full moon, but an ecclesiastical moon. The difference is that the astronomical vernal equinox is a natural astronomical phenomenon, while the ecclesiastical vernal equinox is a fixed March 21. Easter is determined from tables which determine Easter based on the ecclesiastical rules described above, which approximate the astronomical full moon.
A couple of interesting little tidbits:
- In 2008, Easter was the earliest it will ever fall in our lifetimes (March 23). The earliest it can fall is March 22, which it did in 1818 and won’t do again until 2285. The next time it will fall on March 23 will be 2160.
- The last time it fell the latest it can fall (April 25) was 1943. Next time will be 2038.
- The dates in the cycle repeats after exactly 5,700,000 years, with April 19 being the most common date (happening 220,400 times)
It’s interesting history, although not something I want to spend a lot of time trying to figure out because my head might explode. I’m just glad they put it on calendars so I don’t have to figure it out on my own!