A note on the date of Easter

By Br. Guy Consolmagno  |  3 Apr 2018

This entry is part 43 of 98 in the series Diary

Horst Rademacher,  a seismologist at U C Berkeley, wrote to friends of mine there last weekend, asking about the date of Easter:

May I bother you with a question, which is probably trivial for astronomers like you to answer. However neither Peggy nor I could find an answer anywhere. Here is the question:

Tomorrow (Apr 1) is Easter. According to the classic definition Easter always falls on the first Sunday after the first full moon after the vernal equinox. That makes sense, because today is full moon, which is the first full moon after the beginning of spring. And tomorrow is Sunday, hence Easter.

However, today’s full moon occurred at 5:37 am PDT. Let’s assume, the full moon would have occurred at 5:37 pm PDT. Applying the definition above from a purely California perspective, tomorrow would still be Easter. However, if we were in the Netherlands, in Germany or in the Vatican for that matter, this assumed full moon would occur the next day at 2:37 CEDT. But because the next day (tomorrow) is a Sunday, it could not be be Easter, because Easter always falls on the first Sunday AFTER the first full moon. Hence Easter would fall on the next Sunday……

So here is the question: Do you know in which time zone the full moon is measured in order to define Easter? If you don’t know, does your friend, the astronomer in the Vatican know?

Sorry to bother you with something that esoteric…

Being their “friend at the Vatican” it got passed on to me, and here was my reply:
Nothing esoteric at all about this. The problem was well known by the time of the age of exploration, the 16th century, and it formed an important part of the Gregorian reform of the calendar in 1582!

The dates of Easter calculated by Fr. Chrisopher Clavius in 1602, based on the new Gregorian calendar.

The short answer is this: in the Gregorian calendar, Easter is no longer defined as the first Sunday after the first full Moon of spring. Instead, it is determined by a totally arbitrary formula that approximates this definition, getting it right for most but not all of the years in the 19 year Metonic lunar cycle.

This article by Fr. Juan Casanovas explains it in much more detail:
In other words, we no longer need to rely on the Moon to set our religious holidays. In principle we can define such holidays on Mars with its two moons…or anywhere else. (Even if nuclear waste stored on the Moon’s far side explodes, knocking the Moon out of orbit and sending it, as well as the 311 inhabitants of Moonbase Alpha, hurtling uncontrollably into space – a la the 1970’s TV show, “Space 1999”…)
(Thanks to Horst for asking the question, and for giving me permission to quote him and cite him here!)

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