Jean Richer

In 1670 Richer was sent by the Académie to La Rochelle to measure the heights of the tides there at both the spring and vernal equinoxes. Also in 1670 he set out on a voyage to Canada (France controlled parts of the country). On the voyage he had the task of testing two clocks made by Huygens. Accurate clocks were important in determining longitude. However there was a storm and Huygens's clocks stopped.

On his return Richer reported the failure of the clocks to Huygens and to the Académie. Huygens accused Richer of incompetence but this was certainly untrue. Richer had made many important observations on the voyage and the problem with Huygens's clocks was certainly not his fault.

In 1671 Richer was sent on an expedition to Cayenne, French Guyana by the French Government. His first task there was to measure the parallax of Mars and the observations were to be compared with that taken at other sites to compute the distance to the planet. This data enabled the scale of the solar system to be computed, the first reasonably accurate results to be found.

Richer's second important work was to examine the periods of pendulums at different points on the Earth. He examined the period of a pendulum while on the expedition to Cayenne, French Guyana and found that the pendulum beat more slowly than in Paris. From this Richer deduced that gravity was weaker at Cayenne, so it was further from the centre of the Earth than was Paris.

Richer published his observations in his only written work Observations astronomiques et physiques faites en l'isle de Caienne.

Newton and Huygens used Richer's gravity data to show that the Earth is an oblate sphere.

In 1673 Richer returned to Paris where he was given the title of 'royal engineer' and undertook work on fortifications.

Article by: J J O'Connor and E F Robertson

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