On Easter and Astronomy

All through this week and leading up to Sunday the Christian blogosphere has been posting about Easter and Passion related themes. For my Easter reflection I'd like to participate, but in my own way. I want to reflect on how to be a good Christian you need to be a great astronomer.

This topic came to mind as this week my sons, nephews and I did a little backyard astronomy with my 4.5 inch Dobsonian telescope (a wee thing, but good for backyard fun). We were looking at the full moon. It dawned on me, as we were looking at the moon, that this particular full moon was the Paschal Moon, the moon we use to select the date of Easter.

Easter, we all know, is a moveable feast. Notoriously so. What holiday makes you ask "When is _____ this year?" as much as Easter? But it's even worse than this. The dating of Easter is one of the great church controversies dividing the Western and Eastern churches.

Today we date Easter this way: Easter shall be the first Sunday after the first full moon (the Paschal moon we were looking at) after the spring equinox.

(Two notes: First, the equinox here is the Northern spring equinox, for the Southern hemisphere it is the autumn equinox. Second, the phrase "full moon" is vague. A "full" moon is relative to your position on earth. But this rule is close enough for our purposes. My point: See how good at astronomy you have to be to get Easter right?)

This year the Paschal Moon came just one day after the spring equinox which was also early this year (due to the leap year). This is why Easter is so very early this year. The last time Easter was this early was in 1913. And it won't be this early again in our lifetimes.

So you get my point: You have to be a pretty good astronomer to get Easter right. You need to be able to note the equinox (i.e., the point where the sun is at one of two opposite points on the celestial sphere where the celestial equator and ecliptic intersect), the full moon, and manage the calendar (those leap years and such). The math behind all this is called computus and it requires, as best as I can tell, an advanced degree in mathematics and planetary astronomy.

The ancient Easter controversies have to do with calendars. Here's an abstract of that story.

The events surrounding the death, burial, and resurrection of Jesus occurred during the Jewish Passover celebrations. Obviously, this association is of great theological import. So the early church was keen to keep a close association between Easter and Passover. But the Jewish calendar is a lunar calendar, fixing the celebration on the 15th day of the month of Nisan. This places Passover in the spring in the Northern Hemisphere. Thus, Easter is always in the spring.

However, after Constantine, as Christianity came to be centered on Rome, the Roman solar calendar came to dominate the celebrations of the church. It was unwieldy to manage a lunar calendar for religious feasts (e.g., Passover and Easter) and a solar calendar for civic functions. Plus, the 15th of Nisan doesn't always fall on the same day of the week. This complicated things for the church who wanted to associate the first day of the week, Sunday, with Easter. I mean, wouldn't it be odd to celebrate Easter Wednesday?

So the early church made a compromise. It switched to a solar calendar but kept Easter in the spring, keeping it seasonally associated with Passover (and still, vestigially, following the phases of the moon), as well as locking it onto a Sunday. Problem solved!

Well, not so fast. This early solution was based on the Julian calendar. Julius Caesar introduced this calendar to Rome after seeing the advantages of the Egyptian solar year. I guess we have Cleopatra to thank for this.

(Side history: Most ancient societies used the moon as a clock. The moon's regular phases make it ideal for this purpose. The trouble is, a lunar year doesn't sync well with the solar year, and it's the sun that's in charge of agricultural events. The Egyptians were one of the first civilizations to break with the moon and go with the sun, a much more difficult astronomical task. Most scholars think that the Egyptians were able to break with the moon because their lives were governed by another non-moon time-keeping device: The floods of the Nile.)

The Julian calendar gives us a year of 365 days. Which is remarkably close to the actual solar year. But the match is not exact. A solar year is closer to 365 and 1/4th days long. So with the Julian calendar you are drifting against the solar year by 1/4 day each year. This drift is not much to notice on a year to year basis but over a century your calendar is drifting about 25 days, almost a full month. Eventually, if you date Easter by the Julian calendar Easter ends up being in the dead of winter. And Santa starts showing up in Hawaiian shirts.

Now this Julian drift can and was dealt with by adding in days here and there to catch the calendar up with the sun. But these were regional and post hoc measures. This calendar thing just had to get fixed once and for all. And only precise astronomy--nailing the exact length of the solar year---could help. Eventually, all the frustrations and science fell into the lap of Pope Gregory XIII who, in 1582, created the Gregorian calendar. It is the Gregorian calendar that gives us the leap years which correct every four years for the 1/4 day drift (but again, it is much more complicated than this as a solar year isn't exactly 365 1/4th days. See how good at astronomy you have to be to be a good Christian?). The Gregorian calendar now governs most of the world. Problem solved!

Well, not exactly. The Eastern Orthodox church didn't go along with the Gregorian calendar reforms. They stayed with the Julian calendar. Consequently, to this day, there are two Easters in Christendom, each celebrated on different days.

So, when exactly is Easter? Well, you could track with Passover. But if you do so you might not be celebrating Easter on Sunday. Or, you could track with the Julian calendar like the Orthodox. The trouble with this is that you'll be basing Easter on a calendar taken from ancient pagan Egypt and celebrating a Christian holy day on a day of the week that carries a pagan name: The Sun's Day. The Gregorian calendar fares no better.

So when is Easter? Hard to say. But I do know this. You'd have to be very, very good at astronomy to know.


I hope you have a blessed and happy Easter! And let me encourage you to enjoy some backyard astronomy when you can. Saturn is in the east now, but growing fainter as the days pass...