HOME

TheInfoList



OR:

Jean Richer (1630–1696) was a French
astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either obse ...
and assistant (''élève astronome'') at the French Academy of Sciences, under the direction of Giovanni Domenico Cassini. Between 1671 and 1673 he performed experiments and carried out celestial observations in
Cayenne Cayenne (; ; gcr, Kayenn) is the capital city of French Guiana, an overseas region and department of France located in South America. The city stands on a former island at the mouth of the Cayenne River on the Atlantic coast. The city's mot ...
, French Guiana, at the request of the French Academy. His observations and measurements of Mars during its
perihelic opposition Mars has an orbit with a semimajor axis of 1.524 astronomical units (228 million km), and an eccentricity of 0.0934.Jean Meeus, ''Astronomical Formulæ for Calculators''. (Richmond, VA: Willmann-Bell, 1988) 99. Elements by F. E. Ross The planet o ...
, coupled with those made simultaneously in Paris by Cassini, led to the earliest data-based estimate of the distance between Earth and Mars, which they then used to calculate the distance between the Sun and Earth (the
astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits ...
). While there he also measured the length of a seconds pendulum, that is a pendulum with a half-swing of one second, and found it to be 1.25 ''
ligne The ''ligne'' ( ), or line or Paris line, is a historic unit of length used in France and elsewhere prior to the adoption of the metric system in the late 18th century, and used in various sciences after that time. The ''loi du 19 frimaire an ...
s'' (2.8 millimeters*) shorter than at Paris. His methodNewton, Principia Mathematica: "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." was to compare the oscillation of a freely decaying pendulum with the time kept by another mechanical clock and astronomical observations. Isaac Newton later commented that if, as he had proposed, the force of gravity decreases with the inverse square of the distance between objects, the obvious conclusion to be drawn from Richer's work is that near-equatorial Cayenne is further from the centre of the earth than Paris, where the first such measurements had been taken. Thus the earth could not be spherical, as had earlier been presumed, but rather bulges at and near the equator ( Equatorial bulge). It could be said that Richer was the first person to observe a change in gravitational force over the surface of the earth, beginning the science of gravimetry. Richer's 1673 return to Paris was duly celebrated, and when his data were reproduced, the findings for which we remember him could be made public. However, publication was delayed, for unknown causes, until 1679, when a work entitled '' Observations Astronomiques et Physiques Faites en L'Isle de Caïenne par M. Richer, de l'Académie Royale des Sciences,'' was released under Richer's name.Re-published as: Not long thereafter, he was assigned to an engineering project in Germany. The remainder of his life is undocumented. Most biographers believe that he died at Paris in 1696. *note: To obtain the relative difference in the pendulum's frequency from this number, note that Richer gives the Paris pendulum's length as 3 feet 8 lines and 1/3. Since a line is 1/12th of an inch, the 1 line and a quarter discrepancy is a 0.284% difference in length, and thus a 0.142% difference in frequency, in fair agreement with Newton's claim of a 2 and a half minutes per day difference.


See also

* Seconds pendulum


References


External links


Jean Richer
at MacTutor History of Mathematics archive * http://galileo.rice.edu/Catalog/NewFiles/richer.html {{DEFAULTSORT:Richer, Jean 1630 births 1696 deaths 17th-century French astronomers