A measurement error of 1.25 lignes in a seconds pendulum provided the first empirical evidence that Earth is not a perfect sphere. Jean Richer recorded this discrepancy during a mission to Cayenne between 1671 and 1673, shifting the understanding of terrestrial gravity and validating theories regarding the planet's equatorial bulge.
Academy Appointments and Early Work
Born in Omonville-la-Petite in 1630, Richer built his scientific career within the formal structures of seventeenth-century France. He served as an astronomer and student at the French Academy of Sciences from 1666 until 1673. Under the supervision of Giovanni Domenico Cassini, he participated in observational programs designed to standardize geodetic and astronomical measurements.
Twenty questions, eight minutes on the clock, and a percentile measured against everyone who has taken it. No sign-up.
Take the IQ test →The Cayenne Expedition
The French Academy of Sciences dispatched Richer to Cayenne to perform celestial observations and physical experiments. His primary achievement involved measuring the position of Mars during its perihelic opposition. By coordinating these observations with simultaneous data collected by Cassini in Paris, he generated the earliest data-based estimate for the distance between Earth and Mars, which enabled a new calculation of the distance between the Sun and Earth.
Geodesy and Gravimetry
While in Cayenne, Richer observed that a seconds pendulum was 1.25 lignes shorter than its counterpart in Paris. He confirmed this by comparing the oscillation of a free-decaying pendulum against a mechanical clock. This result suggested that gravity varies across the Earth's surface, a finding Isaac Newton later used to argue that the planet bulges near the equator. Others, such as Christiaan Huygens, attributed the phenomenon to centrifugal force reducing apparent gravity at lower latitudes.
Final Years
Richer returned to Paris in 1673, though his findings remained unpublished until 1679, when the Academy released his report titled Observations Astronomiques et Physiques Faites en L'Isle de Caïenne par M. Richer, de l'Académie Royale des Sciences. Following his return, he transitioned into engineering work under Sébastien Le Prestre de Vauban. Documentation of his activities remains sparse until his death in Paris in 1696.
Fast facts
- Born: 1630, Omonville-la-Petite
- Died: 1696, Paris
- Employer: French Academy of Sciences
- Expedition site: Cayenne
- Scientific specialty: Geodesy
- Primary finding: Seconds pendulum variation
- Collaborator: Giovanni Domenico Cassini
Questions readers ask
What did Jean Richer discover in Cayenne?
He discovered that a seconds pendulum beat slower in Cayenne than in Paris, indicating that gravitational force changes across the surface of the Earth.
How did Richer contribute to the study of the solar system?
His coordinated observations of Mars with Cassini in Paris allowed for the first data-based estimate of the distance between Earth and Mars.
Achievements
- Held posts at French Academy of Sciences and Sébastien Le Prestre de Vauban
- Fields of research: astronomical observatory, astronomy, geodesy and geodetic astronomy
