Shadows in Medieval Optics, Practical Geometry, and Astronomy: On a Perspectiva Ascribed to Thomas Bradwardine

Abstract

In examining the roles of the shadow (umbra) in medieval science, this paper analyses a hitherto unstudied early fourteenth-century optical treatise with the incipit Perspectiva cum sit una (PCSU), which, on the basis of medieval evidence, may arguably be attributed to Thomas Bradwardine. The third part of this treatise, on shadows, presents the doctrine of three shadow shapes – a doctrine which was popular in pre-modern optics and astronomy and was important in explaining eclipses – as well as the theory of umbra recta and versa, parallels of (co)tangent functions, which were essential for (instrumental) measurements. While the bulk of the treatise draws on John Peckham’s Perspectiva communis, an extensive analysis of medieval canons to astronomical tables, manuals of practical geometry and texts on instruments leads us to Campanus of Novara’s Practica quadrantis as the chief source of the last chapter of PCSU. Finally, the paper reflects on whether the light-centred conception of optics embodied in the PCSU may echo an alternative current to the otherwise predominantly sight-centred approach in pre-modern optics.