Person: Richer, Jean
Jean Richer was a French mathematician and astronomer who worked for the French Academy of Sciences on the longitude problem.
Mathematical Profile (Excerpt):
- Almost all that is known about Richer relates to two scientific expeditions he made for the Académie des Sciences.
- In February 1670, Richer was paid for experiments and astronomical observations he had carried out for the Académie des Sciences during in the previous months.
- Richer and Meurisse, following their luggage and instruments, went to La Rochelle, from where the ship was to depart, and while there Richer measured the height of the tides at the vernal equinox.
- Eight days later the ship sailed from La Rochelle for Madagascar but neither Richer nor Meurisse were on board.
- A rival method for determining the longitude to that proposed by Huygens using marine pendulum clocks had been proposed by Jean Deshayes.
- There is no definite proof that Jean Deshayes did in fact sail on the 'Saint Sébastien' for he is not mentioned again.
- Soon after leaving port the ship ran into a heavy storm and, before the conditions improved, both of Huygens' clocks that Richer was to test had stopped.
- Richer did, however, make some remarkably good determinations of latitude on the voyage.
- Arriving in Canada, Richer measured the latitude of the French fort at Pentagoûët as 44° 23' 20".
- This observation is remarkable, first because Richer had the confidence to give the reading in degrees, minutes and seconds where all measurements at the time were only given in degrees and minutes, second because it is remarkably close to the correct value of 44° 23' 25".
- Some historians have claimed that Richer must have been lucky to get all his readings as close as he did, but since his measurements are consistently good it is only fair to assume that he was an observer of extraordinary high ability.
- On his return Richer from Acadia reported the failure of the clocks to Huygens and to the Académie.
- By September 1670 Richer was back in Paris following his visit to Acadia.
- Preparation began for a scientific expedition to Cayenne, French Guyana, and, after slow but steady progress, Richer's passport was issued in September 1671.
- He was to lead the expedition, assisted by Meurisse, and after the equipment and supplies were made ready in October, Richer discussed the final details of the research to be undertaken with Giovanni Cassini in the Paris Observatory in November.
- Giovanni Cassini, and also Jean Picard, hoped that a station close to the equator, where the planets and the sun were nearer the observer's zenith, would practically eliminate the effects of refraction.
- The expedition left La Rochelle on 8 February 1672, later than intended but this was fortunate since it meant that observations of Mars would be made by Richer at the time of the planet's closest approach to the Earth.
- Richer made careful observations of the planet during August, September and October of 1672.
- Giovanni Cassini observed the planet from the Paris Observatory and later used his own data and that of Richer to compute the parallax of Mars.
- Later astronomers failed to obtain results which were as accurate as those obtained by Richer and Cassini and it was only in 1769, nearly 100 years later, that a more accurate value for the distance of the Earth to the sun was obtained using data from transits of Venus.
- Richer's second important work on this expedition to Cayenne was to calculate the length of the seconds pendulum in Cayenne and compare this with the length of the seconds pendulum in Paris.
- And, first of all, in the year 1672, M Richer took notice of it in the island of Cayenne; for when, in the month of August, he was observing the transits of the fixed stars over the meridian, he found his clock to go slower than it ought in respect of the mean motion of the sun at the rate of 2m 28s a day.
- These experimental results by Richer supported Newton's theoretical claims that the Earth was flattened at the poles.
- Richer measured the positions of many southern hemisphere stars not visible from Paris.
- It had been intended that Richer carry with him on the voyage a new remodelled marine clock designed by Huygens.
- Huygens, who unfairly blamed Richer for the failure of the earlier trial of his marine clocks, said he was glad that the clock was not ready in time for Richer to test it! We should mention that Richer's assistant, Meurisse, died in Cayenne in 1673 shortly after Richer, who himself had become ill, left Cayenne on 25 May 1673 and returned to France earlier than he had intended.
- After Richer returned to Paris, his service to the Academy of Sciences was terminated.
- In 1679 Richer was elected to full membership of the Academy of Sciences.
Born 1630, France. Died 1696, Paris, France.
View full biography at MacTutor
Tags relevant for this person:
Astronomy, Physics
Thank you to the contributors under CC BY-SA 4.0! 
- Github:
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- non-Github:
- @J-J-O'Connor
- @E-F-Robertson
References
Adapted from other CC BY-SA 4.0 Sources:
- O’Connor, John J; Robertson, Edmund F: MacTutor History of Mathematics Archive
