‘Poor Borelli!’ exclaimed Alexandre Koyre at the end of his wonderful and by now classic study of Borelli's ‘celestial mechanics’. Koyre frankly admitted that Borelli lacked Newton's genius and intellectual audacity. However, in his story Borelli deserved a place between Kepler and Newton for his ‘imperfect but decisive’ unification of terrestrial and celestial physics. This framework finds a powerful justification in Borelli's extensive usage of Keplerian astronomy and in Newton's references to Borelli's work on the Medicean planets, Theoricae mediceorum planetarum (Florence, 1666), both in his correspondence with Edmond Halley, with regard to a controversy with Robert Hooke, and in Philosophiae naturalis principia mathematica (London, 1687). Newton's own copy of Borelli's work with signs of his reading – the famous dog-earings – is preserved in the library of Trinity College, Cambridge. The magnitude of Newton's achievement has haunted Borelli's work ever since Principia mathematica appeared in print. For example, although Christiaan Huygens was sent a copy of Borelli's book by Prince Leopold in 1666, his marginal annotations in his own copy of the book were written after he had read Newton's masterpiece, as if Huygens had felt the need to read the book again after 1687. From then onwards Borelli's work has almost inevitably appeared in a new light that has coloured its subsequent readings.

My ambition in this paper is to provide a fresh reading of Borelli's work by reconstructing its circumstances of composition, establishing a comparison with a relevant strictly contemporary source, and attending to the immediate reception of Theoricae. Borelli's work was written with an eye to a composite audience including Roman Jesuits, Sicilian intellectual circles and Leopold de' Medici's correspondents across the Alps, such as the Copernican astronomer Ismael Boulliau in Paris. Borelli was aware that his Sicilian readers were likely to have different concerns from those of Roman Jesuits or the Medici. Thus the task of charting the reception of his work is a formidable one. For a variety of reasons, including availability of sources, limitations of space, taste and competence, my analysis of Borelli's work on the Medicean planets is limited to a few themes and is not only no less partial than Koyre's, but in many respects it relies on it. If readers feel stimulated to re-read Koyre's text, one of the aims of this paper will have been fulfilled.