Measuring the size of the Earth. Surveyors measuring angles for triangulation at night, during the 1669-1670 survey to determine the size of the Earth. The other corners of the triangulation triangle are marked by beacon fires (top left and top right). This survey was carried out by the French astronomer Jean Picard (1620-1682) using 13 triangulation triangles from Paris to Sourdon, along the Paris Meridian. By using telescopes and micrometers, the result was the most accurate of the time. This artwork, by French illustrator Sebastien Leclerc (1637-1714), is a frontispiece vignette from Picard's book Mesure de la Terre (1671).