Richer , Jean

(1630–1696) French astronomer

Richer is a rather anonymous figure who is known only through his work with others. In 1671 he went on a scientific mission to Cayenne, where it was intended that he should observe meridian transits of the Sun and also measure the distance of Mars from any nearby stars while Giovanni Cassini performed similar observations in Paris. Apart from the obvious advantage of having two observers making the same measurements with a long base line, there was also the hope that Cayenne, 5° from the equator, would provide better viewing and that, because of its equatorial position, meridian sightings would be subject to less atmospheric refraction than in Paris.

But more important discoveries were to be made. The observations of Mars were made successfully, allowing Cassini to use the parallax obtained to give a distance of the Sun from the Earth of 86 million miles (138 million km). This was not particularly accurate – being 7% out – but at least it was properly determined and could easily be improved. What, however, really surprised Richer was that his pendulum timings were all slightly different from what they would have been in Paris; the pendulum was running slow, even at sea level. Cassini, at first, seems to have doubted the competence of Richer's observations but was eventually convinced. It was this work that provided Newton with the essential information in working out the size and shape of the Earth. If the pendulum slowed down then there must be a smaller gravitational force operating on it. The only way this could reasonably happen was if Cayenne was further from the center of the Earth than Paris; that is, if the Earth bulged at the equator, or to be precise, is an oblate spheroid.