Peser la terre, peser les mondes de la «balance à peser les mondes» de Cavendish (1798) aux repetitions didactiques a l'université

An historical overview is given from the first XVIIIth century experiments «to determine the density of the Earth» to the recent determinations of the newtonian «big G» constant. The dominant role played in this story by the gravitational torsion balance including recent measurements is emphasized through a detailed study of Cavendishs gravitational balance and its important technical, methodological and publishing innovations. In the framework of our «historical experiments » approach of Science and History of Science Teaching, the use of a modern pedagogical version of the Cavendish balance in a university classroom is described. The relationship of the «weighing the Earth» challenge with Astronomy is finally shortly evoked