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.
Richer's 1673 return to Paris was duly celebrated, and when his data were reduced, the findings for which we remember him could be made public. However, publication was delayed, for unknown causes, until 1679, when a work entitled Observations Astronomiques et Physiques Faites en L'Isle de Caïenne par M. Richer, de l'Academie Royale des Sciences. was released under Richer's name. Not long thereafter, he was assigned to an engineering project in Germany. The remainder of his life is undocumented. Most biographers believe that he died at Paris in 1696.
- note: this does not agree with the difference of 2.5 minutes observed as quoted on the pendulum page... This number gives a time difference of 8.1 minutes (~5.2 if the world's lowest, in Mexico City, is assumed (still a factor of 2x above quoted))
- Poynting, John Henry; Joseph John Thompson (1907). A Textbook of Physics: Properties of Matter, 4th Ed. London: Charles Griffin & Co. p. 20.