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Blame the Babylonians: A king prepares to sacrifice an animal, c.1800 BC, on a Babylonian cylinder seal

This year Western Christianity celebrates Easter on the same day as the Eastern Orthodox Church, the Sunday following the Jewish Passover. This is just as it should be, but usually is not. Why? The answer takes us first to the Council of Nicaea in 325 AD, and then to the great calendar change in 1582.

Nicaea (modern Iznik in Turkey), a prosperous Hellenistic city named after the Greek word for victory, was where the Roman Emperor Constantine set the course for the new religion. A great gathering of Christian fathers from all parts of the Mediterranean world finally settled a free-for-all that had led to numerous doctrinal disputes. The emperor paid the expenses and the prelates engaged on theological matters for a month before his arrival, providing him with a thick sheaf of papers. Opening the formal session seated on a gold throne, he ordered the bishops to set aside their arguments, dumped the papers in a brazier, and told his audience it was this council that would now establish a uniform doctrine for Church teaching and practice.

They settled on a co-equal relationship between God the Father and God the Son, banning the Arian heresy; they created the Nicene Creed, and determined the date of Easter. According to Mark’s gospel, Mary Magdalene, Mary the mother of Jesus and Salome came to the tomb on a Sunday, the first day of the week, to find Christ’s body missing. This became the day of Resurrection, so Easter had to be a Sunday. And since the date of the Crucifixion was associated with Passover, which starts on a springtime full moon, some bishops had dated Easter using the Jewish calendar. No longer. They would now all use the calendar introduced by Julius Caesar, with March 21 as the spring equinox, and the rule for Easter was simple: the first Sunday following the first full moon that occurred on or after March 21. If that date was both a Saturday and a full moon — which for us last occurred in 1818 — then Easter would be March 22, and the latest Easter could be was April 25, which it was in 1943.

Using this rule, Easter is often the Sunday following Passover but not always, because the Jewish calendar is lunar. Some years have 12 lunar months, some 13. They alternate according to a rule created for the Babylonian calendar. The Babylonians had calculated that 19 seasonal years is approximately 235 lunar months, and set up a 19-year cycle using a fixed pattern of 12 short years and seven long years.

The rule for Passover is simple: it starts on the 15th day of the month of Nissan, which is necessarily a full moon because Jewish lunar months start with the first crescent. On our modern calendar Passover varies from very late March to late April, but the “paschal full moon” for Christianity is sometimes a month earlier.

The word paschal derives from Hebrew pesach meaning Passover, and in many languages the same word is used for both Passover and Easter — Cáisc in Irish, and Páskha in Russian for instance — while French distinguishes Pâque and Pâques for Passover and Easter. English and the Germanic languages are outliers here, the word Easter coming from a spring festival to a Germanic dawn-goddess (Eastre in standard West Saxon). But names aside, the Christian Church also uses the 19-year cycle to calculate the date of Easter, so all should have worked well. But it didn’t.

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