Bi-Daily Venus in the Medieval Thought of William of Conches: Explaining an Uncommon Celestial Event by Circumsolarity

Abstract

Bi-Daily Venus (BDV) is an uncommon event, occurring when an observer can view that planet twice in a single day – in the twilight of dawn and dusk. That William of Conches knew of this phenomenon in twelfth-century France is surprising given the dearth of BDV accounts. Yet following unnamed sources, he discussed BDV in three of his works: Philosophia Mundi, Glosae Super Boetium, and Dragmaticon. From the appearance of the first of these volumes (ca. 1125) to the last (ca. 1145), William’s reasoning for BDV’s occurrence changed from mild scepticism of given explanations to acceptance of Venus’s sometimes circumsolar height above the sun as the correct interpretation. This paper examines William’s evolving BDV discussions, considers what planetary height meant to William, presents an explanatory diagram, and concludes that William understood Venus to be within the ecliptic plane when BDV occurs – in contrast to our modern explanation for BDV.