Picard, Jean

Picard, Jean, 1620–82, French astronomer, noted for having made the first accurate measurement of a degree of the earth's meridian. The figures he established were of great value to Newton in his calculation of the force of gravitation. Picard, who had previously been the prior of Rillé, in Anjou, went to Paris to occupy the chair of astronomy in the Collège de France in 1655. He determined (1671) the latitude and longitude of Tycho Brahe's observatory at Ven (now Landskrona, Sweden) in order to be able to use Brahe's observations of the positions of heavenly bodies. To him is due in great part the establishment of the Paris Observatory and of the Connaissance des temps, the first five volumes (1679–83) of which he wrote.

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Picard, Jean 

Born July 21, 1620, in La Flèche, Anjou; died July 12, 1682, in Paris. French astronomer. Member of the Académie des Sciences in Paris (1666).

A student of the French materialist philosopher P. Gassendi, Picard became professor of astronomy at the Collège de France in 1655. In 1669 and 1670, on a commission from the academy, he measured the length of the arc of the meridian between Paris and Amiens to be 1° 22′ 55″. In this measurement, he used the method of triangulation and was the first to measure angles by means of optical tubes with reticles instead of diopters. According to his measurements, the length of a degree of a meridian is equal to 111.21 km-the length is 111.18 km according to modern data. The data Picard obtained on the earth’s dimensions were used by I. Newton for numerical confirmation of the law of universal gravitation.

Picard invented the method of determining the right ascensions of heavenly bodies by observing, with the aid of a pendulum clock, the times when the bodies cross the meridian. For this purpose, he proposed in 1669 the fixing of a quadrant in the meridian, but this was not done at the Paris Observatory until 1683. In 1671 he conducted an excavation of Tycho Brahe’s observatory Uraniborg in Denmark. In 1672, together with G. Cassini, he performed observations at Paris of the planet Mars with the aim of determining the solar parallax. Picard founded the astronomical journal La Connaissance des Temps in 1679; he was compiler and editor of the journal’s first four volumes.

WORKS

Degré du méridien entre Paris et Amiens. Paris, 1740.
Mesure de la Terre. Paris, 1671.
Voyage d’Uranibourg.… Paris, 1680.

REFERENCES

Delambre, J. B. J. Histoire de l’astronomie moderne, vol. 2. Paris, 1821.
Boquet, F. Histoire de l’astronomie. Paris, 1925.
Defossez, L. Les Savants du XVII siècle et la mesure du temps. Paris, 1946.