The first sunday after the first full moon after the vernal equinox

This year the vernal equinox happened on different dates on the east and west coasts of the US. The equinox happened at 05:14 UTC, 20 Mar¹. But that’s 12:14AM EDT or 9:14PM PDT 19 Mar. Now suppose, just suppose that the full moon were at 5:14 UTC, Sun, 8 Apr. Then the full moon would happen on Sunday on the east coast and on Saturday on the west coast. Would Easter happen a week earlier on the west coast than the east?

So when does “Sunday” begin? What timezone should it be measured in?

Would it be the dateline? Jerusalem? Rome? Constantinople? Canterbury? Nicaea? For the sake of argument, I’m going to pretend it is Jerusalem. Which is GMT+3 hours.

Except it isn’t. Back in 325 at the council of Nicaea, there was no concept of mean time or timezones or daylight savings, so presumably “Sunday” was decided by apparent solar time (possibly at Jerusalem). Now mean solar time in Jerusalem is GMT+2:20 or there-abouts (depending on exactly WHERE in Jerusalem you choose).

Suppose you chose the wrong part of Jerusalem? It’s big. Every mile east or west would mean an error of roughly four seconds (4.1 seconds) in determining the start of a new day. So with just a mile error, if the full moon were at 23:59:58 Sat. you might think it happened on Sunday. Or vice versa.

I assume the day begins at midnight. But did it in 325? Did they use the Jewish idea that the day begins at sundown?

But that’s mean solar time, not apparent. Of course in April the difference between mean and apparent solar time is slight (it usually reaches 0 in the middle of the month somewhere) but on most potential “Easter” days there will be a discrepancy. This means that even if you had a moon calendar accurate for Jerusalem’s mean solar time there will be some full moons which will be on the wrong day because of the difference between mean and apparent time.

Now people like to calculate Easter in advance. Unfortunately we can’t predict the earth’s rotation very accurately and the time lords who determine UTC can throw in a leap second at the end of any month (though so far it has come either 31 Dec or 30 June). In fact at about 6 months in the future the prediction of when a “day” begins can be off by as much as a second.

So some day we’ll have a full moon which is predicted to be at 23:59:59.5 on Saturday, but which is actually at 00:00:00.5 on Sunday. And people would have to reschedule Easter for the following week, and we’d get two Palm Sundays.

The same confusion over the the earth’s rotation means that the vernal equinox can’t be predicted accurately either. So someday, if the full moon is very close to the equinox the predicted order will be wrong and Easter will be off by a whole month (lunar cycle). Possibly coming a month earlier, possibly a month later than expected.

The timing of the full moon, and the earth’s rotation round the sun have their own uncertainties, but my understanding is that these are much less than those involved in the rotation of the earth.


I’ve been puzzling this riddle for years and I could never find anything about timezones in the calculation of Easter. And it turns out that just about everything I thought I knew was wrong.

The council of Nicea did not decide how to determine Easter — it just said that everyone should use the same day. And Easter isn’t decided by “the first sunday after the first full moon on or after the vernal equinox,” but by a peculiar algorithm decided on by the church which approximates the above statement.²

Interesting to see that in the 18th century Germany and Sweden used a method based on Kepler’s data which produced a different sunday 4 times in that century.

It’s sort of like the Hitchhiker’s Guide which “make[s] the reassuring claim that where it is inaccurate, it is at least definitively inaccurate. In cases of major discrepancy it was always reality that’s got it wrong.” [10th fit of the radio series]

Hmm. In 1928 parlement decided that Easter should be “the first sunday after the second saturday of April”.

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