Reconstruction of the pendulum designed and demonstrated by Leon Foucault in the winter of 1851, hung from the 22-story dome at the Pantheon in the 5th district of Paris. Foucault's experiment proved for the first time, using evidence available without astronomical observation, that the Earth rotates on its axis. The pendulum swings freely from a beam high above, while the movement of the Earth is measured by the movement of the table below (about two millimeters each time the pendulum swings). Digital photo by Eric Francis. First of cover photo trio, 14th April 2005.