Pierre Bouguer and Isaac Newton: The Shape of the Earth
At the same time, in England, Isaac Newton published the Principia. He presented the theory of universal gravitation and pondered the precise shape of the globe. If the Earth rotates on itself in twenty-four hours, the centrifugal force must slightly flatten it at the poles and make it swell at the equator. Starting from a seemingly modest observation, the difference in pendulum period between the equator and Cambridge, Newton carried out an extraordinary thought experiment and estimated the extent of this flattening. For the first time, a physical theory aimed to predict the shape of the planet.
The Rivalry Between France and England Becomes Scientific
The first French measurements, however, seem to indicate the opposite: the Earth would be elongated at the poles. Controversy arises. Behind the scientific debate looms an ancient rivalry between France and England, nations close, competing, and yet intimately linked by history. To settle the matter, the Academy of Sciences decides to send two expeditions: one to Lapland, near the Arctic Circle, and the other to the equator. Measurements must be taken again, compared, calculated. The conditions are extreme, the instruments fragile, and uncertainties numerous. But the verdict is clear: the Earth is indeed slightly flattened at the poles. French scientists, through their own observations, confirm Newton’s prediction.
An Adventure That Never Ends
The story does not end with the Newtonian ellipse. The actual Earth is neither perfectly spherical nor exactly regular. Internal masses, reliefs, oceans introduce irregularities grouped under the name “geoid”. In the 19th century, mathematicians like Henri Poincaré question the stability of rotating bodies and develop concepts that also shed light on the understanding of planetary forms. Today, thanks to satellites and spatial altimetry, the average surface of the oceans is known with centimeter precision. From Eratosthenes’ stone to orbital instruments, it is an intellectual and technical quest that continues.
This scientific and political adventure is retraced in an exhibition jointly organized by the Royal Society and the Academy of Sciences. Initially presented in London, it is now hosted in Paris at the Mazarine Library of the Institute of France. Manuscripts, maps, instruments, field notebooks, correspondences testify to these debates, these voyages, and these calculations. It shows how sometimes rival scientists have learned over time to confront their theories with reality, measure their errors, and engage in dialogue beyond borders.
At a time when some still doubt the roundness of the Earth, it is useful to revisit this long and complex history. It reminds us that science is neither a dogma nor a revealed truth but a patient, collective, often conflictual, and always open construction process. Understanding how we have measured the Earth is understanding how knowledge is constructed. And perhaps today, it is also a way to rebuild confidence in knowledge.
