ESHS/HSS Annual Meeting

Pendulum Length, Gravitational Variation, and the Shape of the Earth: Cassini’s Interpretation of Richer’s Observations

• In Event: Scientific Disputes, Cosmological Theories from Scheiner to Cassini: Archival Sources in the 17th Century 1

English Abstract

In the 1670s, Gian Domenico Cassini oversaw numerous scientific expeditions organized by the Académie Royale des Sciences: he prepared detailed instructions for overseas observations, trained scholars, and inspected the instrumentation. One of the most significant French expeditions was to Cayenne, led by Jean Richer (?-1696), which contributed directly to Cassini’s accurate determination of the Earth–Sun distance, the Astronomical Unit (AU). The expedition pursued several other objectives, including a lesser-known one: la longueur du pendule à secondes, that is, the comparison of pendulum oscillations at different locations. Using two precision clocks made by the King’s watchmaker, Isaac II Thuret, Richer detected a discrepancy of 1 ¼ lignes (about 2.8 mm) between the lengths of the seconds pendulum in Cayenne and in Paris.
Although secondary to the expedition’s other objectives, this observation proved crucial: in horological terms, a pendulum calibrated at the poles would run slower at the equator. This discovery prompted further expeditions, as academic debate focused not only on the mathematical techniques and instruments used, but also on the broader cosmological question of the Earth’s shape—a matter of great interest to Cassini. Drawing on archival material as well, the presentation will examine how Cassini approached the Earth’s shape problem and the precise form of planetary orbits, and will discuss how Newtonian mechanics resolved these debates. The principle that gravitational force decreases with the inverse square of the distance between bodies led to a key conclusion: locations near the equator lie farther from the Earth’s center than places such as Paris, and therefore experience weaker gravity. This discovery laid the foundation of a branch of geophysics devoted to studying and measuring the Earth’s gravitational field—a field that has undergone significant technological advances and is now well understood, thanks to modern space missions.

Authors

• Marcella Brusa, University of Bologna (Italy)

• Luca Ciotti, University of Bologna (Italy)

• Elisabetta Rossi, University of Bologna and SISFA (Italian Society for the History of Physics and Astronomy)