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Byrhtferth’s Diagram; Computus Diagrams
From the Thorney Computus
Cambridgeshire, England; ca. 1102–10
Saint John’s College, Oxford, MS 17
This volume is an album of texts and graphic material organized around computus (literally, “computation”), the medieval science of reckoning time and fixing the dates of ecclesiastical feasts, particularly Easter. Byrhtferth’s digram is a visual meditation on the cosmic and religious resonances of computus, its subject the harmony of the twelve months and four elements, of time and the material world. The tables on the opposite page show a series of diagrams used for determining lunar cycles, days of the week, and divination diagrams based on numerical values assigned to the letters.