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.

## Deviations

- 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.See also in

The book of science:Pendulum clock—Christiaan HuygensPlanetary distances—Giovanni Domenico CassiniPrincipia Mathematica—Isaac NewtonKater’s pendulum—Christiaan Huygens, Gaspard de Prony, Henry KaterGeneral relativity—Albert EinsteinReadings on wikipedia: