When Calendars Collide

Reader Chana writes:

A few years ago there was a very strange situation where Easter actually fell before Passover. Someone explained the whys and wherefores to me, but the truth is that I didn’t understand it at all. I mean, huh? Your explanation above is so clear (really), I thought maybe you’d like to tackle this one as well.

That’s a bit of a toughie, Chana. The year was 2008 and everybody — Christians and Jews alike — was confused. Only an astronomer can properly explain how it happened that way, but suffice to say that the Catholic Church has revised its timekeeping methods since 325 A.D. when Christians and Jews were more or less on the same calendar, a lunar one, meaning a calendar that’s dictated by the cycles of the moon.

The Christian calendar changed at the Council of Trent in 1563, when Pope Gregory switched Christendom from the lunar Julian calendar to the solar Gregorian calendar. More on that here. The Gregorian Calendar is not totally accurate either, but it has allowed the Church to set the date of Easter ever since according to what’s called a “Paschal Full Moon” which is theoretical full moon, not an actual one. It’s an approximation of the real full moon that occurs after the spring equinox. It can be off from the actual full moon by up to two days.

I should insert here that the Catholic spring equinox has also been theoretical since the Council of Trent, when it was officially set at March 20th. And if it sounds foolish for human beings to “set” an equinox, remember that back in the Middle Ages there was no way to know precisely when an equinox was occurring unless you happened to know an astronomer. Add to that the fact that an equinox can vary depending on where you’re standing on the planet, and you can see why people of the time welcomed a fixed date that they didn’t have to worry about.

But back to calendars. The thing about a solar calendar is that it’s shorter than a lunar calendar. There are fewer “leap days” in it, which is to say, days you strike or skip over to make up for the fact that man-made calendars are at variance from the actual movement of celestial bodies. Under the old Julian Calendar, a leap day happened every 100 years or so, which doesn’t sound like much but over time really causes confusion (see above link).

The Gregorian Calendar corrected for leap days while at the same time putting the Roman Church on a new standard. The Eastern Orthodox Church never moved from the Julian Calendar, which is why to this day the Roman and Eastern Orthodox Church calendars are off by about ten days, with Orthodox feasts happening later than ours. The Hebrew Calendar, like the Eastern Orthodox calendar, is also traditionally lunar.

It takes a lot of complicated formulas to reconcile all this, but me, I think of it this way: the solar Gregorian Calendar tends to push dates backward relative to the traditional lunar calendars. As a result, depending on how early the Paschal Full Moon is, it can push the date of Easter so far back that it can actually come before Passover. That’s my own mental shorthand for what went on that year. I think that’s more or less right, but I welcome comments from anyone out there with a more thorough knowledge of the subject!