Jean Richer

Jean Richer, who lived from 1630 to 1696, was a French astronomer and an assistant, an eleve astronome, at the French Academy of Sciences under the direction of Giovanni Domenico Cassini. Between 1671 and 1673 he carried out experiments and celestial observations at Cayenne in French Guiana at the request of the Academy. His observations of Mars near its perihelic opposition, coupled with those made at the same time in Paris by Cassini, gave the earliest data-based estimate of the distance between Earth and Mars and so of the astronomical unit. At Cayenne he also found that a seconds pendulum had to be about 2.256 millimetres shorter than at Paris, making him the first person to observe a change in gravitational force over the Earth's surface. Newton read this as evidence that the Earth bulges at the equator, while Huygens ascribed it to centrifugal force. His results were published in 1679, he was later assigned to an engineering project in Germany, and the rest of his life is undocumented.