Jean Richer

Jean Richer (1630–1696) was a French astronomer and assistant (élève astronome) at The French Academy of Sciences, under the direction of Giovanni Domenico Cassini.

Between 1671 and 1673 he performed experiments and carried out celestial observations in Cayenne, French Guiana, at the request of the French Academy. His observations and measurements of Mars during its perihelic opposition, coupled with those made simultaneously in Paris by Cassini, led to the earliest data-based estimate of the distance between Earth and Mars, which they then used to calculate the distance between the Sun and Earth (the astronomical unit).

While there he also measured the length of a seconds pendulum, that is a pendulum with a half-swing of one second, and found it to be 1.25 lignes (2.8 millimeters*) shorter than at Paris. Isaac Newton later commented that if, as he had proposed, the force of gravity decreases with the inverse square of the distance between objects, the obvious conclusion to be drawn from Richer's work is that near-equatorial Cayenne is further from the center of the earth than Paris, where the first such measurements had been taken. Thus the earth could not be spherical, as had earlier been presumed, but rather bulges at and near the equator. It could be said that Richer was the first person to observe a change in gravitational force over the surface of the earth, beginning the science of gravimetry.