Among the characteristic features of Georg Peurbach's influential Theoricae novae planetarum (1454) was a model of the motion of the fixed stars, based on the Alfonsine Tables, that involved both an eighth sphere undergoing trepidation and a ninth sphere subject to linear precession. Instead of being an innovation on Peurbach's part, the model in question can be shown to have been derived from an obscure treatise De triplici motu octave spere written in 1450 by the Paduan astrologer Nicolò Conti (d. 1468). The main purpose of this article is to highlight relevant parallels between the two works, especially in their use of diagrams, and to present evidence of personal ties between Conti and Peurbach, which explain how the model in question was transmitted from Padua to Vienna. In addition, it discusses the astrological impetus behind Conti's physical model, which was based on the notion that the slow motion of the fixed stars influences the course of history.
