Have you ever wondered why Easter never falls on the same date each year? If so, you’re not alone – the mathematician John Wallis, one of the earliest Fellows of the Royal Society, gave two lectures on the topic to the Society at Gresham College in April and May 1680. These lectures were never published, but the manuscripts are preserved in the Royal Society’s archives, and here are some photos:

 

Title page of Royal Society MS/366/2/7.

 

The Christian festival commemorating the resurrection of Jesus was soon established as an annual celebration, but the date varied among different groups of Christians. In 325AD Emperor Constantine of Rome called a Council at Nicaea to settle the date, separating it from the Jewish calendar and its computations for Passover. Easter would henceforth be on the first Sunday after the first ecclesiastical full moon following the vernal equinox on 21 March. The result is that Easter Sunday never falls on the same date in two successive years, and it may fall on any of the days between 22 March and 25 April inclusive. The ecclesiastical calendar for the entire year is controlled by a movable Easter.

The whole computation process was complicated by the reform of the Julian calendar by Pope Gregory XIII in 1582, whereby 10 days were lost to make the solar year more accurate. This was not universally adopted – England did not ‘go Gregorian’ until 1752, and many Eastern churches still use the Julian calendar. This led to considerable discussion about the correct date of Easter, and how to establish it.

John Wallis’s concerns over the accuracy of dates of the ecclesiastical full moon, given the differences between the Julian and Gregorian calendars, led him to a draw up his own tables to work out the date of Easter in any given year:

 

to find Easter for ever

 

John Wallis’s calculations, from Royal Society MS/366/2/7.

 

Wallis was a mathematician who was also skilled with ciphers. During the Civil War, he deciphered captured coded letters for the Parliamentarians, and work for Oliver Cromwell’s side led to his appointment as the Savilian Professor of Geometry at Oxford in 1649. He also, however, signed the remonstrance against the execution of Charles I; this, together with his denial that he had deciphered important royal letters, may have eased the path to the reconfirmation of his Savilian post by Charles II after the Restoration. He wrote on theology, logic and English grammar, but is best known for his mathematical works, from Arithmetica Infinitorum in 1656 (said to have influenced Isaac Newton) to the Treatise of Algebra both Historical and Practical (1685) and Institutio Logicae (1687).

 

Wallis_P138

Portrait of John Wallis by Gerard Soest, 17th century © The Royal Society

 

Wallis was not the only Fellow who was concerned with establishing the correct date for Easter. George Parker, 2nd Earl of Macclesfield and President 1752-64, communicated to the Royal Society on 10 May 1750 his ‘Remarks upon the solar and the lunar years, the cycle of 19 years, commonly called the golden number, the epact and a method of finding the time of Easter as it is now observed in most parts of Europe.’ This was at the time when discussion was taking place about changing to the Gregorian calendar, and in his speech in Parliament Macclesfield supported the Act which came into force in 1752.

You can see the original of the manuscript shown above, and our portraits of Wallis and Macclesfield, if you visit the Royal Society Library or come on one of our building tours. We’re normally open Monday to Friday, but we’ll be closed on Good Friday and Easter Monday, reopening on Tuesday 29 March. Happy Easter!

 

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