At the same time, in England, Isaac Newton published the “Principia.” In it, he explains the theory of universal gravitation and ponders the exact 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 cause it to bulge at the equator. From what initially seemed like a simple 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 aims to predict the shape of the planet.
The rivalry between France and England becomes scientific
French measurements at first seem to indicate the opposite: the Earth would be elongated at the poles. A controversy ensues. Beyond the scientific debate, an ancient rivalry between France and England emerges, close nations, competitors, yet intimately linked by history. To settle the dispute, 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, the uncertainties plentiful. But the verdict is in: the Earth is indeed slightly flattened at the poles. French scholars, through their own observations, confirm Newton’s prediction.
This adventure does not end
The history does not end with the Newtonian ellipsoid. The Earth itself is neither perfectly spherical nor exactly regular. Internal masses, reliefs, oceans introduce irregularities grouped under the term “geoid.” In the 19th century, mathematicians, such as Henri Poincaré, pondered the stability of rotating bodies and developed concepts that also shed light on planetary shapes. Today, thanks to satellites and space altimetry, the average surface of the oceans is known with centimetric precision. From Eratosthenes’ measuring sticks to orbital instruments, it is an intellectual and technical quest that continues.
This scientific and political adventure is retraced by 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 bear witness to these debates, journeys, and calculations. It shows how sometimes rival scholars learned over time to confront their theories with reality, measure their errors, and engage in dialogue across borders.
At a time when some still doubt the roundness of the Earth, it is not useless 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. Understanding how we have measured the Earth is understanding how knowledge is constructed. And perhaps it is also a way, today, to rebuild trust in knowledge.
