Abstract

This article investigates a little-known computational and mnemonic device invented in c. 1348 by the English Carmelite friar John of Northampton, the details of which are known from a treatise written in or before 1394 by Richard Maidstone, a theologian and fellow member of the Carmelite Order. John’s anulus took the form of a metal finger ring whose wearer could use the complex arrangement of its alphanumeric inscriptions to make a range of calendrical calculations as well as predict the times of the mean conjunctions of the sun and moon. In addition to reconstructing the principles by which this unusual instrument functioned, the article concludes by placing the anulus in the wider context of the computistical and astronomical sciences practised in fourteenth-century Europe.