The Christian festival of Easter, which celebrates the death and resurrection of Jesus Christ, is celebrated at some point in March or April. As the date varies, it is what is termed a ‘moveable feast’. In Period there were a number of different ways of calculating this, and the calculations varied across Europe. How it has changed was simultaneously a matter of politics and of astronomy.

According to the canonical Gospels, Jesus was crucified on a Friday on or near to the Jewish festival of Passover and was resurrected on the third day, this being the Sunday following. This directly links the date of the celebration of the resurrection to the date of Passover, which occurs on the 14th day of the Jewish month of Nisan.

The Jewish calendar is a lunar calendar, where the first day of the month occurs at the first sighting of the crescent moon, meaning that the full moon falls on the 14th day of the month. Lunar calendars are based solely on the phases of the moon, which takes approximately 29.53 days. For convenience lunar months or lunations are defined as either 29 days (a ‘hollow’ month) or 30 days (a ‘full’ month) alternating throughout the year. Twelve lunations makes up 354 days, making a lunar year 11 or 12 days short of a solar year. This means that a pure lunar calendar (such as the Islamic calendar) will have each month in a given year start 11 or 12 days earlier than it started the previous year. All twelve months cycle through the entire solar year over a period of 33-34 years.

Although it is a lunar calendar, the Jewish calendar also has an extra feature that occasionally inserts a leap month every two or three years in order to keep the months in line with the seasons of the solar year. 14 Nisan is therefore kept close to, but later than, the spring equinox on March 19th-21st.

During the first two centuries CE the resurrection was often celebrated on the third day after the Passover, particularly as Christianity started as a Jewish sect that maintained the Jewish festivals. By the third century CE the Christian church had split, with the Church in Jerusalem and the east sticking to the Jewish calendar while the churches in Rome and Alexandria had moved to always celebrate Easter on a Sunday, as that was the day of the week on which it was believed to have occurred. A resolution to this conflict was first suggested at the Council of Arles in 314, where it was announced that all churches should celebrate Easter on the same date, which was to be announced each year by letters from the Bishop of Rome using the Julian calendar first introduced to the city by Julius Caesar in 45 BCE.

The First Council of Nicea in 325 CE strengthened this statement, partly in an attempt to unify the young Church but also in order to disconnect Christianity from Judaism. From that point on the date of Easter would be defined only by the Church. Unfortunately the Council failed to provide a calculation method for date of Easter, and this took several centuries and led to a variety of controversies. The quest to accurately identify the date of Easter became a major driving force of observational astronomy throughout the Middle Ages and beyond.

Through the second to fourth centuries CE several attempts were made to create an accurate set of tables that would allow priests to easily identify the correct date without having to perform complex astronomical calculations. An early step was for the Church of Alexandria to designate March 21st as the ecclesiastical date for the spring equinox, ignoring the actual astronomical event. This Alexandrian method defined Easter Sunday as the Sunday after the first full moon on or after the ecclesiastical equinox, and was accepted throughout the Church at the end of the fourth century CE. It was absorbed into the Julian calendar in around 440, and by this point the method of calculating the date of Easter had become known as the computus.

The computus was used to generate Paschal (relating to Easter) tables throughout the fourth to seventh centuries, some of them more accurate than others. Dionysius Exiguus (who introduced the idea of AD and BC in 525 CE) provided a good set of tables while those of Victorius of Aquitaine had a number of problems. The Dionysius tables were adopted by much of the Church by the eighth century CE.

Except, of course, in the British Isles, where a local computus had the full moon occurring early. This led to a report of King Oswiu of Northumberland celebrating Easter a week earlier than his southern wife Queen Eanflæd, who followed the Dionysius tables, was still fasting for Palm Sunday. The Irish Synod of Magh-Lene in 630 and the Synod of Whitby in 664 finally brought the northern isles into line with mainstream Church practice.

Good though the Dionysius tables were for their time, by 725 when Bede described the system in detail he could already show that the ecclesiastical equinox was drifting away from the astronomical equinox by a significant amount due to the fractions of days and months that were not accounted for using the simple 29/30 day lunation and 365.25 day solar year. In spite of this, these tables continued in use within the Catholic Church until the Gregorian reform in the late sixteenth century. Most of the Eastern Orthodox Churches did not implement the Gregorian reform and continue to use the Julian calendar to this day, leading to the two rites normally celebrating Easter Sunday on different dates.

Bede estimated that the difference between the ecclesiastical and astronomical equinoxes was 2-3 days. By around 1200 Roger Bacon estimated it at 7-8 days. Increases in astronomical knowledge and the accuracy of astronomical observations by the fifteenth century meant that it was becoming obvious that reform was needed, as the equinox occurred on March 10th or 11th rather than the 21st.

The Council of Trent in 1545 decreed that the spring equinox must be returned to where it had been at the Council of Nicea. This led to a commission being set up, and its results were sent to a number of astronomers and mathematicians in 1577 for review and comment. Some of the experts argued that the true astronomical equinox should always be used, but this was rejected in favour of easier-to use tables that used the existing computus (with added features for rare lunar/solar month interactions) and a change to the calendar to remove three leap days every 400 years. Previously leap years had occurred every fourth year but now century leap years would only occur if the year was divisible by 400.

In 1582 the Papal Bull Inter gravissimas was proclaimed by Pope Gregory XIII and Catholic Europe gradually moved from the Julian calendar to the Gregorian one. Phillip II of Spain decreed on September 29th that Spain and her provinces would adopt the new calendar, so that Julian October 4th was followed by Gregorian 15th October. By the end of the year France had also changed over. Protestant countries such as England viewed this as a Catholic plot and refused to implement the change, some for several centuries.

The new calendar restored, in part, the link between the astronomy and Easter, and provided a mechanism to ensure that the link remained over far longer periods that had previously been possible. It was not a complete restoration, however, as the date of Easter is still defined by the artificial ecclesiastical equinox rather than the astronomical one.