Newton as Geodesist: The Problem of the Earth's Figure and the Argument for Universal Gravitation*

Isaac Newton’s Philosophiæ Naturalis Principia Mathematica is usually remembered for its theoretical advances in mechanics and its empirical argument for universal gravitation. This argument, in turn, is taken to rest on Newton’s empirical predictions of astronomical phenomena in the Principia’s third book. For all its merits, this focus on astronomy has led many historians, physicists, and philosophers to overlook an equally important empirical problem in the Principia: deriving the earth’s figure and the variation of gravity on its surface. My talk aims to rectify this situation. After presenting a new reconstruction of Newton’s derivations of the earth’s figure and surface gravity, I illustrate why these results were of primary importance to his argument for universal gravitation. I close by briefly summarising the problem’s subsequent development throughout three centuries of research in physical geodesy and gravitational physics.