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Astronomer, b. at La Flêche, 21 July, 1620; d. at Paris, 12 Oct., 1682. He was a priest and prior of Rillé in Anjou. As a pupil of Gassendi he observed with him the solar eclipse of 25 Aug., 1645. In 1655 he succeeded his master as professor of astronomy at the College de France. His principal achievement was the accurate measurement of an arc of a meridian of the earth, the distance from Sourdon, near Amiens, to Malvoisine, south of Paris, in 1669-70. His result, 57060 toises (a toise = about 6.4 ft.) for the degree of arc, has been found to be only 14 toises too small. He applied telescopes and micrometers to graduated astronomical and measuring instruments as early as 1667. The quadrant he used had a radius of 38 inches and was so finely graduated that he could read the angles to one quarter of a minute. The sextant employed for determining the meridian was 6 feet in radius. In 1659 he was able to observe stars on the meridian during day-time and to measure their position with the aid of cross-wires at the focus of his telescope. In order to make sure that his standard toise should not be lost, like those used by others before him, he conceived the idea of comparing it with the length of the simple pendulum beating seconds at Paris, and thus made it possible to reproduce the standard at any time.
Picard is regarded as the founder of modern astronomy in France. He introduced new methods, improved the old instruments, and added new devices, such as the pendulum clock. As a result of Picard's work, Newton was able to revise his calculations and announce his great law of universal gravitation. The discovery of the aberration of light also became a possibility on account of Picard's study of Tycho Brahe's observations. In 1671 he received from Bartholinus at Copenhagen an exact copy of Tycho's records and then went with Bartholinus to the Island of Hveen in order to determine the exact position of Tycho's observatory at Uranienborg. He was modest and unselfish enough to recommend the rival Italian astronomer Cassini to Colbert and Louis XIV for the direction of the new observatory at Paris. Cassini, on the contrary, proved envious, ignoring Picard's insistent recommendations of a mural circle for accurate meridional observations, until after the latter's death.
Picard was among the first members of the Academy. He also started the publication of the annual "Connaissance des temps" in 1679 (Paris, 1678), and continued the same until 1683. Since then it has been published continuously. His "Mesure de la terre" was brought out in 1671, Paris.
WOLF, Geschichte der Astronomie (Munich, 1879); DELAMBRE, Hist. de l'astr. mod., II (Paris, 1821), 567-632.
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