THOMAS DIGGES, THE DEVELOPMENT OF THE RELATIVITY OF MOTION, AND ITS RELATION TO THE NATURAL PHILOSOPHY OF GIORDANO BRUNO

Since the time of ancient Greek scientists, the appearance of similar physical phenomena for two observers in relative motion has been generally exemplified by means of a moving ship. This proved useful in discussions of the rotation of Earth. We briefly track the evolution of this ideal experiment from the time of the exposition in Euclids’ <italic>Optics</italic> up to Middle and Early Modern Ages, with emphasis on the contribution of Oresme and its use in Copernicus’ <italic>De Revolutionibus</italic>. Giordano Bruno discussed the motion of a ship in <italic>The Ash Wednesday Supper</italic>, published in 1584, reporting the free fall of a plummet from the mast as a test of the dynamical equivalence of the two reference systems. We show that this phenomenon was already considered and published by Thomas Digges, the leading Copernican scientist in England, in 1576. We argue that during his long visit in London from 1583 to 1585, Bruno adopted this example from this source and therefore he cannot be considered an original contributor to the classic relativity principle, which was clearly established by Galilei in 1632.