Jean Richer

Astronomer and navigator, Jean Richer (1630 - 1696) is named with the Academy of Science in 1666.

Sent on mission to Cayenne, in 1672 it studies there the planet Mars in conjunction with the work of Jean-Dominique Cassini with Paris, which makes it possible to determine the Parallaxe this Astre.

During a opposition of Mars (i.e. when Mars, the Ground, and the Sun are aligned, in this order), of simultaneous measurements in Paris, in Cayenne, and in England take place in 1672 (Article of Yael Nazé, to see p 27): the distance Ground-Sun, deduced from the measurement of the parallax, arises to a few 140 million km… It is about one moment " historique" ignored: the size of the Solar system, such as we imagined it hitherto, dilates factor 20 abruptly! Indeed, since Ptolémée, one believed that the Sun was to 7 million km of the Earth! (Johannes Kepler had had a presentiment of well that this distance, that one calls nowadays the astronomical Unité (ua) was at least 3 times larger…).

In Guyana, Richer observes also which a pendulum in Paris beats more quickly than in Cayenne. Huygens and Newton will deduce from it that the Ground is a Sphère flattened by the Centrifugal force with 20 km of bulge to the equator. Like that contradicted measurements of Cassini, it follows a polemic that the famous voyages of Maupertuis in Lapland as those of Bouguer in Ecuador make it possible to solve in 1736.

Nowadays, the shape of the Earth is known with a few centimetres close and is always in evolution on this scale by the effect of the Plate tectonics .

The studies of Richer on the pendulum will be continued by those of Halley to Sainte-Hélène.

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