|Jean Richer geodesy
Jean Richer was the first to observe that the force of gravity changes over Earth’s surface. Cassini sent Richer to French Guiana to observe Mars during its perihelic opposition. There Richer also measured the length of the seconds pendulum and was surprised to find that it was shorter than in Paris.
Isaac Newton later established that gravity between two objects varies as the inverse square of the distance between them and Newton was the first to conclude that Earth’s surface near the equator is farther from Earth’s center than in Paris. Earth’s shape is an oblate spheroid. A cross-section from pole to pole is a bumpy ellipse.
Local topography deviates from the earth’s ideal oblate spheroid although by very small degrees. Local persons sometimes deviate from let’s call it the normal, not the ideal, thoroughly and dramatically.