The Shape of Earth: An Early Test of Newtonian Physics

Abstract

During the early decades of the 18th century, Newtonian physics was still new. Much effort was expended in testing its validity. One arena in which evidence could be found was the shape of Earth. Was it perfectly spherical? On the observational side there were two hints. In 1671 Jean Richer had measured the rate of ticking of a pendulum clock near the equator and found it to swing more slowly than in Europe: the conclusion was that gravity was weaker towards the equator. And in 1718 Jacques Cassini announced that he had analyzed surveying data over a large region of France and found that they suggested a prolate shape (elongated at the poles). But the precision required to draw such a conclusion with confidence was very great, and there was considerable doubt as to whether it had actually been achieved.