GEODETIC ARCS, PENDULUMS AND THE SHAPE OF THE EARTH

By measuring the length of an arc of meridian near Paris and the latitudes of its extremities, Jean Picard obtained for the first time in 1671 a good value for the dimensions of the Earth. Many such measurements of arcs of meridian were then made during almost three centuries in order to refine Picard's one and to determine the shape of the Earth and the value of its flattening, after they had demonstrated that it is approximately an oblate ellipsoid. Measurements of parallel arcs contributed in a lesser way to these results. Both kinds of arcs also made the frame of accurate geographic maps. Gravity was measured in many places of the world from the length of a pendulum oscillating in one second (the seconds pendulum), and contributed strongly to the determination of the shape of the Earth. After 1960, space geodesy and the GPS have made obsolete all this previous work, which is now only of historical interest. This paper summarizes the history of this past activity, which has involved a considerable number of astronomers and geodesists, civilian or military, often working in difficult conditions, sometimes within important international collaborations.