French Geodesy in 19th and 20th Centuries. Context, actors and mathematical practices on the field

International audienceGeodesy in the nineteenth century was a branch of astronomy concernedwith the determination of the mathematical figure of the Earth, but alsowith questions of earth-science (cartography, geology and geophysics). Thisscience played a crucial role in military and colonial endeavors, andprovides an example of the way in which mathematical techniques weredeveloped and mobilized in the service of the state. Who were the main practitioners of 19th-century geodesy, and whatsignificance should we attribute to geodetic operations, such as themeasurement of a meridian or parallel arc, or the determination of theastronomical position of geodetic stations? Geodesists were often notscientists but military officers, and historians have tended to treatthem as followers rather than innovators. I will examine examples of geodesy as material and theoretical knowledgemobilized in the field or developed in scientific institutions such asthe French Bureau des longitudes, where academicians, military officersand precision instrument makers worked together. One of the Bureau'sdirectors, Henri Poincaré, considered geodesy to be useful for theFrench State, not only for cartography, but also for the generaleducation of students at the Ecole polytechnique. Poincaré also helpeddetermine latitudes, the gravitational constant, and the nature ofgravitational action. On a larger scale, my aim is to illuminate the evolutions in geodeticscience before WWI