This article presents an edition and translation of a Latin treatise on the uses of an astronomical instrument, extant in two manuscripts originating in Oxford in the mid-14th century. The accompanying commentary highlights the links between the treatise and the oldest surviving equatorium, at Merton College, Oxford. The article also explains the workings of the instrument, discusses the treatise’s striking didactic presentation of theoretical explanation and practical instruction, and notes some remarkable variation in language and structure between the two copies of the treatise.