Histories of medieval astronomy have brought to light a rich textual tradition, of treatises and tables composed and computed, transmitted and translated across Europe and beyond. These have been supplemented by fruitful inquiry into the material culture of astronomy, especially the instruments that served as models of the heavens, for teaching and for practical purposes. But even now we know little about the practices of medieval astronomers: how they obtained and passed on their knowledge; how they drew up and used mathematical tables; how they drafted the treatises in which they found words to express their ideas and inventions for their particular audiences. This thesis uses a case study approach to elucidate these medieval astronomical practices.

Long thought to be a holograph manuscript in the hand of Geoffrey Chaucer, the Equatorie of the Planetis (Peterhouse, Cambridge MS 75.I) has recently been identified as the work of John Westwyk (d. c. 1400), a Benedictine monk of Tynemouth Priory and St Albans Abbey. His draft description of the construction and use of an astronomical instrument, with accompanying tables, provides an opportunity to reconstruct the practices of an unexceptional astronomer.

The first chapter of this thesis reconstructs Westwyk’s astronomical reading and understanding, through an examination of the other manuscript that survives in his hand: a pair of instrument treatises by the outstanding monastic astronomer Richard of Wallingford. I show how Westwyk copied this manuscript in a monastic context, learning as he annotated texts and recomputed tables. In the second chapter I discuss the purposes of planetary instruments such as equatoria, their place among other astronomical instruments, and the physical constraints and possibilities experienced by their makers. Through this discussion I assess the craft environment in which Westwyk came to write his own instrument-making instructions. Chapters three and four assess Westwyk’s language, explaining the basis for his choice to write a technical work in the vernacular, and analysing how his innovative use of Middle English furthered his didactic objectives. In the final chapter, I undertake a technical reassessment of the Equatorie treatise, an integrated analysis of the instrument with the somewhat neglected tables that Westwyk compiled alongside it. The thesis thus applies a range of methodologies to examine the practices and products of a single inexpert astronomer from all angles. It aims to show what an in-depth case study approach can offer historians of the medieval sciences.