A Rare Medieval Computational Cipher, a Computus Text, Along with a Poetic Device to Aid in Its Use, Employed by a 15th Century Clergyman in the Calculation of Easter

Records show only a small number of such mathematical devices having reached the public market

 

The manuscript bears markings of subsequent owners, including a poem from the 16th century to Saint Basil, normally associated with the East

Prior to 1420, devotional books such as Books of Hours or Breviaries were made to order and reflected the particular interests of the patron. A breviary, usually formatted in a writing block of two columns (as opposed to its single column relative, the Book of Hours), contains excerpts of psalm, gospel, hymns, and prayers, to guide the reader in his or her daily prayer at the fixed Canonical Hours. Because these books were created at this time for a specific use, they could be catered to that use, including the use of alternate texts. This would also be the case for clergymen.

One of the most important branches of medieval learning was computus, the science of calculating times and dates using a combination of mathematics and astronomy. The use of poetry to transmit scientific material would have made it easier for students to memorize. These calculations were often applied to the tricky and at times controversial calculation of the date of Easter, a highly variable and occasionally contentious date relying on lunar calendars, the date of the Jewish celebration of Passover, and whether or not the observer is on the Julian or Gregorian calendar. Easter remains celebrated on different days in the West than the East.

In 1408, a Yorkshire man named John de Foxton (ca. 1369-1450) wrote an encyclopaedic work called Liber Cosmographiae, now housed at Cambridge, Trinity College, MS. R.15.21. John’s extensive work includes a poem on fol. 19v. The poem includes the line “Qu[a]erit amor Christi multos dum regnat iniqus” (The love of Christ seeks many, while the unjust reign). It appears in a very limited number of other manuscripts. It is a cipher. The poem also includes seemingly unrelated letters, written in red, above each of the words. These letters would accompany the poetic text, making the poem into a memory-based (mnemonic) device to figure out the date of Easter (John de Foxton’s Liber Cosmographiae: An Edition and Codicological Study, 1988, p. xxiii-xxiv).

Such computational texts, which can combine a description of the calculation process, along with the paired poetic text / letters, are occasionally found in texts of this period but finding them on the market would be very uncommon.

Within the first decade of the fifteenth century, an ecclesiastical man, a member of the clergy, had a breviary commissioned for him.

[Breviary, in Latin, ca. 1410, Flanders or Luxembourg] 2-, 3-, and 4- line initials in blue and pink with white penwork, shellgold illumination; 23 partial borders consisting of a pink and blue bar extending the length of the text, illuminated with gold and often terminating in penwork floral sprays.

Within this book, which he would use for his daily prayers, he had an unusual selection incorporated — a paraphrase of a computus text, along with the Foxton poetic mnemonic, computistical text to further solidify his study and understanding of the dating of Easter. The letters of the Foxton text were added, though in a slightly different structure.

Combined, these instructions describe the dating of easter and include an alphabet written out to help the ciphering. The text also mentions a Charles (Karolus) who says [or teaches] on the matter in “m cccc lvxxiii”. The year, 1478, is a scribal error, inserting an extra century, for what should have been m ccc lvxxiii. On November 29, 1378, Charles of Luxembourg, better known as the Holy Roman Emperor died. This portion of text of the ecclesiastic’s breviary is a paraphrase and therefore not published as is in any source we could determine.

A curious manuscript likely refering to the computistical ideas of Charles IV, the Holy Roman Emperor, influenced by the English cultural diffusion of John de Foxton’s mnemonic and cipher, a bespoke production for a clergy man interested in this complex medieval science of dating Easter. A later 16th century prayer to Saint Basil has been added in space left after the text on fol. 399v by a reader.

A note on location and paleography

Several of the saints’ names in the liturgy have been copied with the French spelling conventions. While the illumination in the later, liturgical portion of the text appear at first glance to be French— for example the pale yellow touches to capitals to draw the readers eyes— the hand lacks some of the sophistication of French manuscript production one would expect. In a Book of Hours, the blue tipped, three-petalled flowers would certainly indicate Bruges, but this gets diffused further away from the city in other liturgical books. The spiky seed pods that flourish the borders are indicative of Flanders and were often emulated by English artists. During this time, books were commonly being produced in the Low Countries for English audiences, creating a fusion of paleographic and illustration styles. The paleography and orthography reveal the mystery. The hook-like descenders on the letter h go far beyond the baseline, which is a distinctly Flemish feature. The spelling of “ewangelia” on the verso of the folio numbered in modern pencil as 367 proves the Germanophone claim moving the manuscript firmly into the liminal sphere of BeNeLux (Belgium, Netherlands, Luxembourg) region, with its changing boarders and political statuses.

See also:

Neues Archiv der Gesellchaft für ältere deutsche Geshichtskunde: zur Beförderung einer Gesammtausgabe der Quellenschriften deutscher Geschichten des Mittelalters, Sechster Band [Vol. 6], Hannover: Hahnsche Buchhandlung, 1881.

J. Van den Gheyn, Catalogue des manuscrits de la Bibliothèque Royale de Belgique: Tome Deuxieme [Vol. 2], Bruxelles: Henri Lamertin, 1902.

Friedman, John, “John Siferwas and the Mythological Illustrations in the Liber Cosmographiae of John de Foxton,” Speculum, (58:2), 1983. Pp. 391-418.

Friedman, John, “The Cipher Alphabet of John de Foxton’s Liber Cosmographiae,” Scriptorium (36:2), 1982. Pp. 219-235.