1671 Gravimetry

The book of science

Tom Sharp

Jean Richer geodesy Gravimetry


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.

Earth’s shape

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.

The diameter of the earth at its equator is forty-three kilometers greater than the diameter at its poles. This seems like a good distance; however, the top of Mount Everest deviates from the ideal radius of the earth only 0.14%. When calculating the local g, one has to remove the effects of altitude and local topography.

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