The astronomer Johannes Stöffler and the reform of Easter

This year marks the fifth centenary of the publication of the Calendarium Romanum magnum, a carefully crafted ensemble of astronomical tables and detailed supplementary treatises that qualifies as one of the most impressive manifestations of the mathematical culture of the Northern Renaissance. Its author, the Swabian astronomer Johannes Stöffler (1452–1531), had spent several decades of his career as a parish priest before being appointed to the University of Tübingen’s newly established chair of mathematics in 1507. Besides teaching maths and astrology to illustrious sixteenth-century figures such as Philip Melanchthon, Johannes Schöner, and Sebastian Münster, Stöffler’s contemporary fame rested chiefly on his astrological prognostications and skills as an instrument maker, but also on the great success of his Almanach nova, first published in 1499 in collaboration with Jacob Pflaum, which recorded the daily positions of the planets for a period of 33 years. 

Stöffler’s abilities as an astronomical calculator come to the fore very clearly in the 290-page Calendarium Romanum magnum (1518), which he dedicated to the astrology-enthused ruler of the Holy Roman Empire, Maximilian I. The scope of the Calendarium’s tables was more circumscribed than one’s average set of ephemerides, however, being restricted for the most part to the positions and syzygies (i.e. conjunctions and oppositions) of the Sun and Moon. Its introductory chapters demonstrated in great detail how the information contained in these tables could be harnessed for various purposes: keeping time, administering medical remedies, predicting eclipses, and, most important of all, calculating the dates of the mobile feast days.

Stöffler was well aware that the latter topic had been the cause of ongoing controversy within the Roman Church. Only a few years prior to the publication of his Calendarium, the Fifth Lateran Council (1512–1517) had been busy making preparations for a reform of the dating of Easter Sunday. This project came at the tail end of four centuries of increasingly vehement complaints about the calendrical cycles used by the Church, which were no longer in tune with the courses of the Sun and Moon. This failure caused frequent violations of the rules governing the date of Easter Sunday, which was defined as the first Sunday after the first full moon to fall on or after the vernal equinox. Eager to put an end to this perceived scandal, the Lateran Council and its presiding pope (Leo X) set up a commission charged with solving the problem. Part of its mission was to solicit advice from the outside, by sending letters to rulers, bishops, universities, and scholars from all over Europe. The list of experts who wrote back to Rome with their suggestions includes Nicholas Copernicus, whose annotated personal copy of the Calendarium Romanum magnum is still extant and has even been used to sample the astronomer’s DNA.

Another contributor to this project was Stöffler himself, who submitted his report on the calendar problem in 1515. This report is lost, but its contents can be reconstructed from the Calendarium. In it, Stöffler expresses the usual set of worries about the scandalous state of the ecclesiastical calendar, which he and many of his contemporaries feared was liable to undermine the authority of the Church and expose it to the ridicule of unbelievers. Where he deviated from the mainstream were his plans for how to solve the problem. Rather than attempting to improve the existing calendrical cycles with a slight bit of nip and tuck, Stöffler declared it best to leave the whole business of calculating Easter to astronomers, who could use their own tables and algorithms to predict the time of the vernal equinox and the true full moon. For this solution to work, the Church and its astronomers had to base all relevant calculations on a common meridian, as this was the only way to ensure that Christians on the newly discovered West Indies would celebrate the feast on the same date as those in India or the Holy Land. Stöffler recommended placing this reference meridian in the city of Rome, the centre of Latin Christianity.

It is no secret that the Roman Church never went along with Stöffler’s proposal, but instead opted for a fully cyclical computation of Easter in the guise of the Gregorian calendar. Introduced in October 1582 under the auspices of Pope Gregory XIII, the Gregorian calendar has found numerous admirers, but about as many critics over the past 436 years of its run. Complaints have targeted its bewilderingly complicated structure — the Gregorian dates of Easter Sunday return in the same order only after 5.7 million years! — but also the compromises its inventors made with regard to astronomical accuracy. In some parts of the Christian world, in particular the Orthodox East, the Gregorian reform of Easter reckoning has never been adopted and proposals of the sort voiced by Stöffler five centuries ago still remain relevant to the contemporary discussion. The possibility of a new reform that would lead to an ecumenical date of Easter was raised as recently as June 2015 by Pope Francis, who declared his Church’s willingness to give up the Gregorian calculation for the sake of Christian unity. Non-Catholic supporters of this plan include the Archbishop of Canterbury, Justin Welby, who in January 2016 expressed his preference for an Easter fixed to the second or third Sunday in April. His solution would no doubt be welcomed by businesses, school administrations, and other institutions with an interest in stabilizing holiday schedules. It would not have been acceptable to Johannes Stöffler, however, whose work emphasized the historical-theological underpinnings that tie Easter to its Jewish precursor, Passover, and hence to the phases of the Moon. It remains to be seen whether such objections will continue to hold sway in the twenty-first century.

Featured image credit: time calendar saturday weekend by Basti93. CC0 via Pixabay.