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 measuring instruments. In 1669-70 he made his historic observations using a specially designed telescope and Willebrord Snell's (1591-1626) theory of triangulation. By measuring the angles of a series of triangles extending from Paris northward, he determined latitude, a term used in mapping to locate a place north or south of the Equator. Latitude is expressed by angular measurements ranging from 0° at the equator and 90° at the poles. Picard was among the first to apply scientific methods during mapmaking. He produced a map of the Paris region, then went on to join a project to map France.
Picard is regarded as the founder of modern astronomy in France. He studied for the priesthood at the Jesuit college at Le Flèche and later received a masters in astronomy from the University of Paris. Ten years after observing the solar eclipse of August 1645 he became professor of astronomy at the Collège de France, Paris. In 1666 he became one of the first members of the Academy of Royal Sciences.
Shortly after joining the Academy, Picard visited the observatory of noted astronomer Tycho Brahe (1546-1601) on Hven Island in Sweden. His goal was to precisely determine the observatory's location so Brahe's astronomical observations of could be directly compared with others. Picard later visited the Paris Observatory, where he collaborated with rival Italian astronomer Jean-Dominique Cassini (1625-1712), Ole Römer (1644-1710), and, slightly later, mathematician Philippe de la Hire (1640-1718). Putting aside his own ambitions, Picard recommended Cassini to King Louis XIV for the direction of the new observatory at Paris.
One of Picard's less publicized discoveries occurred during his study at Tycho Brahe's observatory. While taking measurements on the observatory mountaintop, Picard observed what later became the single most important concept behind neon signs. The French astronomer discovered that a faint glow appeared in the mercury-filled tube (barometer) he used to measure atmospheric pressure. When he shook the tube, the glow intensified. The effect, called barometric light, caused quite a stir in the scientific community, although the actual cause of the light was not well understood.
In 1679 Picard began publication of the first national almanac, the Connaissance des temps (Knowledge of Time or the Celestial Motions). He authored the first five volumes, which contained tables for the crude determination of longitude for the position of celestial bodies. Since then it has been published continuously.
Picard is also credited with the introduction of telescopic sights and the introduction of the pendulum clock.
KELLI A. MILLER