SECTORS AND TOISES ASSAULT THE FIGURE DE LA TERRE

International audienceAt the end of the 17th century a controversy arose concerning the shape of the Earth. Is its shape like a mandarin orange or like a lemon? In essence, that was the question. The first opinion was advocated by Newtons Principia, published in 1687. The second came from Cassini Is measurements made in 1683, running from Picards southern point along a meridian arc up to Bourges, in the center of France, and (1700/1701) with his son Cassini II, up to the Pyrenees; the data obtained were compared with Picards measurements from 1669/1670. Both measurements were made by the geodetic method of triangulation, employing sectors and, for the length reference, the toise. But which sectors and which toise were used? That is a second question. Later, mostly by La Hire, the northern part of Picards arc was extended to the frontier by 1718. The results were published in 1720 and 1723 by Cassini II. Voltaire and others supported Newtons shape and the Académie Royale des Sciences decided to send two expeditions, in 1735 (close to the equator) and in 1736 (as close as possible to the North Pole). After the return of the second one, in 1737, credit for the correct shape was given to Newton and new measurements were performed, mostly by Lacaille in 1739/1740. In turn, these were published by Cassini III in 1744. So, which were the instruments and the toise? That is the last question and the end of the controversy