Jean Picard

Jean Picard Solar Clock, on the Sorbonne Front

Jean Picard (21 July 1620 in La Fléche - 12 July 1682 in Paris) was a French astronomer, geodesist and priest. He studied at the Henry Le Grand Jesuit College.

He is considered the founder of modern geodesy: he was the first to measure an arc of one degree of the terrestrial meridian by means of triangulation with instruments equipped with reticle astronomical lenses. He deduced the radius of the Earth, supposedly spherical, with a precision hitherto unequaled (he obtained 6328.9 km, currently 6357 km, that is, an error of 0.44%). His work also focused on the search for a standard of universal length and on leveling methods to supply water to the fountains of the Palace of Versailles.

In astronomy, he made numerous observations and measurements —in field work, for geography and the future triangulated map of France, of which he would be the initiator— and in the observatory, in pure astronomy. He established a new method to determine the equatorial coordinates of the stars by passing through the meridian, published ephemeris and in 1679 the first Astronomical Yearbook in French. As such, Picard is considered to be at the origin of the development of precision astronomy.

He was also interested in the gnomonic and the diopter.

Acknowledgments

• In 1736, Jacques Cassini, on the orders of King Louis XV, placed a monument on the site of the northern sight;
• In 1740, the Académie des sciences erected two pyramids along the route to Paris: in Villejuif and in Juvisy-sur-Orge, in memory of the works of Picard: these two monuments are symbolically placed at the ends of the base of their Paris-Amiens triangulation used to measure the Earth;
• The moon crater Picard bears this name in his memory.
• a French observation satellite of the Sun, Picard, receives its name in its honor; the measurements of the solar diameter of Picard, taken during the minimum of Maunder, stodavía are used to evaluate the influence of the solar constant in global warming.

Published Works

• Mesure de la terre (1671): BnF, Gallica.
• Traité du nivellement (1684): BnF, Gallica.

In the Mémoires de l'Académie royale des sciences, [1]:

• Volume VI, [2]:
  • Pratique des grands cadransp. 481;
  • Fragments de dioptrique, p. 550.
• Volume VII-I, [3]:
  • Voyage d'Uranibourgp. 193;
  • Observations Astronomiques diverses, dans le Royaume, p. 358-429.