‘Undeterred by Aristotle’s demonstrations’: parallax and cometary distance in a forgotten epistolary treatise of 1265

A manuscript now in Bamberg preserves the only surviving fragment of a thirteenth-century treatise on comets or 'new stars', which was written as a letter addressed by an unknown Dominican author to the Master General of his order, John of Vercelli. The present article offers the first discussion of this forgotten work, which was composed in the year after the Great Comet of 1264. Although most of the text has been lost, the inclusion of a geometrical diagram in the manuscript makes it possible to reconstruct a crucial part of its overall argument. The Dominican author was openly critical of the Aristotelian doctrine of comets as atmospheric phenomena and considered the possibility that reliable distance estimates might instead place such objects in the celestial realm. His geometrical investigation of this question is historically significant for containing the earliest known analysis of the effect of cometary distance on its observable parallax, thus anticipating aspects of Johannes Regiomontanus's seminal <i>16 Problems</i> on comets.