For long it has been accepted that Kepler ’proved’ three empirical laws of planetary motion, and that Newton depended upon these in ’establishing’ his law of universal gravitation. As Professor Wilson demonstrates, the truth is more complicated - but more interesting. The question of observational evidence therefore forms the theme of this volume. The first articles trace the evolution of Kepler’s ideas and reconstruct the steps in his journey. Their conclusion is that observational error inevitably prevented any satisfactory direct verification of Kepler’s first law so, as Kepler himself recognised, his results rested upon hypothesis. The final articles present a similar study of Newton’s thoughts on gravitation and planetary motion: again, as Newton left it, the theory he propounded can be considered no more or less than a hypothesis. In between Professor Wilson examines the attitudes of mid-17th-century astronomers to Kepler’s ideas, and in particular, the achievements of Jeremiah Horrocks: he died in 1640, at the age of only twenty-two, but his improvements in Keplerian astronomy were of great importance for Newton’s future work.
