Trying to change the calendar to accommodate religious festivals has been a centuries-old minefield
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.
Scholars in the Middle Ages noticed that the spring equinox had shifted from March 21st, and urged a calendar change. Easter was a vital Christian festival, and part of the Church year was entirely dependent on it. Leading up to it is Lent — the name refers to the lengthening of days — and counting Easter Sunday as day one, the 50th day is Pentecost, named after the Greek word for 50. So it was important to get it right.
In the 13th century Sacrobosco (John Holywood) at the University of Paris proposed omitting one leap day every 288 years. Great minds such as Robert Grosseteste and Roger Bacon worked on the problem, and Bacon’s contemporary Giovanni Campano, a mathematician and astronomer in Lombardy, wrote a treatise to Pope Urban IV calling for accurate determinations of the equinox and full moon. Regiomontanus (Johann Müller of Königsberg) was invited to the Vatican by Pope Sixtus IV in 1476 to advise on calendar reform, but died shortly after arrival. The Council of Trent in 1563 planned to tackle the problem but failed, and it fell to Pope Gregory XIII to undertake a major calendar reform in 1582.
He removed ten days from that year (October 4 was followed by October 15), setting the equinox back to March 21 for the following year. Furthermore, century years would no longer be leap years unless divisible by 400: thus 1600 and 2000 were leap years, but 1900 was not. This created the Gregorian calendar we use today, but Protestants protested and the reform only occurred in Roman Catholic regions.
Britain did not change until 1752, by which time we had to lop 11 days from the calendar rather than ten. “Give us back our 11 days” was the popular refrain, because if you believe the date of your death is preordained you have indeed lost 11 days of your life. When Russia and Greece both changed in the 20th century they had to remove 13 days.
Abandoning old calendars is very hard indeed. Julius Caesar managed it in 44 BC because the Roman republican calendar was a frightful mess, subject to annual political interference, and had got 80 days out of phase with the seasonal year. It was a great moment, and the Eastern Orthodox Church still calculates Easter using the Julian calendar. Unfortunately the rigid lunar calculations now give a false date for the paschal full moon, which often sets the two Easters a week apart, and the 13-day shift sometimes puts them into different months.
In 1923, at a congress of Eastern Orthodox Churches, the Serbian scientist Milutin Milankovitch, well known for his work explaining the Ice Ages using long-term variations in the earth’s orbit, proposed a very accurate revised Julian calendar. Several churches adopted it, while leaving the Easter calculations unchanged, but even national pride could not persuade the Serbian church to follow suit. To change a calendar you need to be an emperor — or a Pope.