Picard, Jean (1620–1682)

French astronomer. He was professor of astronomy at the Collège Royal de France from 1655, and was a founding member of the Academy of Sciences in 1666. In 1667 he was one of the first astronomers to use cross-wire sights in a telescope, thus establishing a new standard of accuracy in positional measurements. In 1671 he published his measurements of an arc of the meridian of Paris, giving a value of the Earth's radius that was important in verifying Newton's theory of gravitation.