Jean Richer (1630–1696) was a French astronomer and assistant (élève astronome) of Giovanni Domenico Cassini.
Between 1671 and 1673 he traveled to Cayenne at the request of the French Academy of Sciences to observe Mars during its perigee. This led to an estimate of the distance between sun and Mars.
While there he also measured the length of the seconds pendulum, that is a pendulum with a swing of one second, and found it was 1.25 lignes (2.8 millimeters) shorter than at Paris. This was due to the increase of gravitational force with latitude, due to the oblate shape of the Earth. He thus became the first person to observe a change in gravitational force, beginning the science of gravimetry.
He died at Paris in 1696.
1. ^ Poynting, John Henry; Joseph John Thompson (1907). A Textbook of Physics: Properties of Matter, 4th Ed.. London: Charles Griffin & Co.. pp. 20. http://books.google.com/books?id=TL4KAAAAIAAJ&pg=PA20.
Retrieved from "http://en.wikipedia.org/"