A Rare Manuscript Up for Sale at Raab Shows How the Middle Ages Calculated Easter

The Raab Collection announced that it is offering for sale a rare and powerful medieval treasure: a cipher, or mnemonic device, decipherable by just the educated few, from a fifteenth-century religious text, written in Latin. This illuminated manuscript, which comes from Flanders or Luxembourg, uses a complex series of letters and phrases to help the reader determine what remains a moveable feast: Easter. In medieval times, its date had to be calculated using a combination of mathematics and astrology (computus), lunar phases, and the date of Passover.   

In 1408, an Englishman named John de Foxton wrote a encyclopedic work called “Liber Cosmographiae,” which included a line of poetry accompanied by several unrelated letters, written in red, that act as a mnemonic device for figuring out the date of Easter. The manuscript, which consists of several surviving pages from a once-larger book, is one of the rare computational texts that include this cipher. It was likely commissioned by a member of clergy who used his prayerbook both for daily devotions as well as the more complex matters of the Christian calendar. 

The above section shows a portion of the mnemonic used to calculate Easter

A full description of the manuscript, valued at $16,500, is available here. Nathan Raab, principal at The Raab Collection and author of The Hunt for History (Scribner, 2020), is available for interview and commentary. 

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