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Giovanni Borelli and the Comets of 1664–65

On 18 December 1664, Giovanni Borelli (1608–79), a mathematician and philosopher of nature working for the Grand Ducal Court in Tuscany, excitedly wrote from Pisa to his patron, Prince Leopoldo de’ Medici, in Florence. Borelli recounted how early that morning he had heard about the sighting of a comet just above the horizon. Borelli did not have time to observe the comet through a telescope. Instead he simply recorded its latitudinal and longitudinal positions, as well as its apparent rectilinear path across the background of the constellations. In the months that followed, Borelli tracked the comet’s progress, published a pseudonymous book interpreting the comet’s movements according to a heliocentric model, and was involved in a dispute with French astronomer, Adrien Auzout (1622–91), about cometary observations and their significance. Furthermore, while he moved into a makeshift observatory and began to consider the mechanical causes of planetary motion, Borelli continued to work on cometary movements, even constructing a mechanical model of the 1664–65 comet. This was, therefore, a busy period in Borelli’s career in which he engaged in disputes and constructed new astronomical theories. The aim of this paper is to examine the changes in Borelli’s cometary theory, as well as his broader cosmological beliefs, during this period in 1664 and 1665. This will add to our understanding of how astronomical claims were constructed and negotiated in seventeenth-century Europe, especially Italy. Borelli’s excitement at seeing the comet was matched by astronomers in various parts of Europe, including Gian Domenico Cassini (1625–1712) in Rome, Adrien Auzout and Pierre Petit (1598–1667) in France, Christiaan Huygens (1629–95) in Leiden, Johannes Hevelius (1611–87) in Danzig, Robert Hooke (1635–1703) in London, and Geminiano Montanari (1633–87) in Bologna. This comet, the first seen with such brightness in Europe since 1618, immediately stirred up so much interest amongst European astronomers because of the philosophically sensitive issues that cometary theory continued to raise in the midto late seventeenth century. Those issues were: the origins of comets — how they are formed; the path traced by comets — whether they have a linear, circular or elliptical trajectory; and the location of comets — their distance from the Earth. The importance of these issues rested on questions to do with the structure and organization of nature that had been asked since Antiquity, and especially since the end of the sixteenth century. This included the organization of the cosmos, the elements and movements within it, and the causes for those movements. Such speculation formed the foundations of natural philosophical thought during the Scientific Revolution. During the High Middle Ages, Scholastics followed Aristotle’s lead when it came JHA, xl (2009)