Jean Richer is remembered for having determined that pendulums beat more slowly at the equator than at higher latitudes, a discovery that initiated the famous controversy between Newtonians and Cartesians over Earth's shape. Also, his observations of Mars were used by Giovanni Domenico Cassini (1625-1712) to make the first accurate estimate of the size of the solar system.
The details of Richer's early life have been lost, but when the Académie des Sciences was organized in 1666, he was admitted as an élève astronome (assistant astronomer). In March of 1670 he was selected to carry out astronomical measurements in the East Indies in conjunction with measurements to be made simultaneously in Paris. During this trip he was also responsible for testing Christiaan Huygens's (1629-1695) marine clocks constructed for the purpose of determining longitude at sea.
Last minute changes resulted in the expedition being diverted to New England. Bad weather early on stopped Huygens's clocks, thus preventing the collection of any useful horological data. In New England Richer took tidal measurements at different sites and accurately determined the latitude of the French fort at Penobscot Bay. This was the most precise astronomical measurement made in the Western Hemisphere to that time. The expedition was back in France by September, and Richer reported his results to the Académie in January 1671.
Notwithstanding Huygens's unjustified imputation of Richer's abilities in the failure of his marine clocks, the Académie was suitably impressed with Richer's performance to select him for their next project, an expedition to Cayenne off the coast of French Guiana. The primary goals of this expedition were to accurately determine the motions of the plants and Sun, assess existing tables of refraction, and determine the parallax of Mars. Additionally, it was hoped that the uniformity in length of the seconds pendulum at all latitudes could be established, thus making it possible to determine a universal standard of linear measurement. The expedition departed in February 1672 and arrived at Cayenne in April. Due to an illness Richer left the expedition early, departing in May 1673.
His lunar and planetary observations corroborated the accuracy of existing astronomical tables. He also carried out extensive solar observations and accurately determined the obliquity of the ecliptic and time of the solstices and equinoxes. All of these measurements helped establish the accuracy of Cassini's tables of refraction. Richer also made observations of Jupiter and its satellites that allowed him to fix the longitude of Cayenne—a result of vital importance for the proper reduction of the his measurements.
Richer took careful observations of the meridian altitude and meridian transit times of Mars and certain nearby fixed stars. Cassini and others made corresponding measurements in France. By reducing Richer's observations to the Paris meridian and comparing several sets of corresponding measurements, Cassini determined the horizontal parallax of Mars and then the astronomical unit. His value of 87 million miles (140 million km) was the first fairly accurate estimate of the mean Earth-Sun distance.
Richer also observed that pendulums beat slower in Cayenne than in France. This was interpreted as a decrease in the gravitational attraction at the equator, suggesting that points along the equator were further from Earth's center than at higher latitudes. Isaac Newton (1642-1727) argued that his gravitational theory adequately accounted for Richer's results because it predicted Earth was an oblate spheroid—bulging equator and flattened poles. This contradicted the Cartesian view that Earth is a prolate spheroid—elongated along the polar axis. Expeditions to Peru (1734-44) and Lapland (1736) later settled the issue decisively in Newton's favor.
Richer was elected to full membership in the Académie in 1679. He died in Paris in 1696.
STEPHEN D. NORTON