French astronomer whose observations of the planet Mars from Cayenne, French Guyana, in 1671–73 contributed to both astronomy and geodesy. The French government sent Richer to Cayenne to measure the parallax of Mars at its perigee. Comparison of his observations with those made elsewhere made it possible to determine the distances of Mars and the Sun from the Earth, leading to the first reasonably accurate calculation of the dimensions of the solar system.

Richer's observations also led to a discovery about the shape of the Earth. Through experimentation, Richer discovered that the beat of a pendulum is slower at Cayenne than at Paris, which is at a different latitude. He concluded that gravity must be weaker at Cayenne than at Paris and thus that Cayenne must be farther than Paris from the centre of the Earth. Sir Isaac Newton and Christiaan Huygens used this discovery to prove that the Earth is not a sphere but is actually an oblate spheroid.