1620-1682

French Astronomer

Jean Picard was a French astronomer who was the first to accurately measure the length of the arc of the meridian, the imaginary line running across the Equator between the North and South Pole. Picard's historic measurement allowed him to do something magnificent—compute the size of Earth. His observations helped English physicist and mathematician Sir Isaac Newton (1642-1727) to verify his theory of gravitation.

Early in his career Picard observed stars on the meridian during the day and measured their position using cross-wires at the focus of his telescope. Since many of his colleagues had lost their standard default measurements, Picard devised a method of comparing his with the length of a simple pendulum beating seconds at Paris. His ingenuity allowed him to reproduce the standard at any time.

Shortly after, the French astronomer applied telescopes and micrometers to graduated astronomical and...