Jean Picard

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.