Reference ellipsoid
In practice this was shown by Pierre Bouguer and Alexis-Claude Clairaut half a century later. The two had done an expedition to Peru and Lapland (1735-1741). Their measurement of the meridian arc lead to the definition of the metre. This was the ten-millionth part of the idealized distance between the pole and the equator, 1791. Because of different errors in measurement, it turned out to be 0.022% too short, and was redefined twice, in 1793, and 1799. The value of 1799 is still the official definition but it is about 0.197‰ too short.