géodésie

Celebrated astronomer and geologist Jean Baptiste Joseph Delambre on this day in 1792 undertook his commission to precisely define the metre, a universal measure defined as one ten-millionth of the distance from the North Pole to the equator, organising an expedition to measure the length of the meridian arc (distance—the two cities being on the same line of longitude) between Dunkirk and Rodez, by Toulouse in the south of France, mathematically extrapolating from that value, and then from Rodez to Barcelona’s Fortress of Montjuïc. The survey mission took six years beset by technical set-backs, bouts of yellow fever and the French Revolution, including several unfortunate incarcerations by Royalist elements. Precise measurements were taken with a device called a repeating circle ( cercle répétiteur ), invented by machinist Etienne Lenoir originally for Jean-Charles de Borda and improved for Delambre and team. Finally in 1799, the metre was formally defined as 0.514074 Parisien toise (from the Latin tender—that is the span of the outstretched arms, six feet) or three feet and eleven lignes—a historical unit that was approximately one twelfth of an inch and still used by watchmakers to size casings and in button-manufacturing.