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How did 18th-century surveys in Lapland and Peru support the idea that Earth is flattened at the poles?
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geoid
Deflection of the vertical from the geoid to the spheroid.
To resolve a scientific debate regarding the shape of the Earth, the French Academy of Sciences organized two expeditions in the 1730s to measure the length of a meridian degree using triangulation. This was necessary because earlier measurements by the Cassinis in France had suggested that Earth was a prolate spheroid (elongated at the poles), which contradicted Isaac Newton's theoretical derivation that Earth should be an oblate spheroid (flattened at the poles).1
The expeditions provided the following evidence:
- Lapland Expedition (1736): Led by Pierre-Louis Moreau de Maupertuis, this team measured the length of a meridian degree in the north. When compared to the corresponding value near Paris, the results proved that the Earth was indeed flattened at the poles.12
- Peru Expedition (1735): Led by Pierre Bouguer and Charles-Marie de La Condamine, this group measured a degree near the Equator. Their findings, finalized in 1743, confirmed the results from Lapland, supporting the conclusion that the Earth is an oblate spheroid.1
Although these early measurements contained errors, they were in the correct direction to validate Newtonian theory.1