Fizeau–Foucault apparatus unit. At the time, Foucault was working at the Paris Observatory under Urbain le Verrier. It was le Verrier's belief, based on extensive celestial mechanics calculations, that the consensus value for the speed of light was perhaps 4% too high. Technical limitations prevented Foucault from separating mirrors R and M by more than about 20 meters. Despite this limited path length, Foucault was able to measure the displacement of the slit image (less than 1 mm) with considerable accuracy. In addition, unlike the case with Fizeau's experiment (which required gauging the rotation rate of an adjustable-speed toothed wheel), he could
Foucault pendulum vector diagrams the each figure represents the range of swing of the pendulum as viewed from above and normalized to a standard orientation. The smaller arrows depict the magnitude of the relative velocity vector for the point on the Earth's surface in-line with the pendulum bob projected to the center of the Earth (the magnitude is shown since the scematic is two-dimensional, not three-dimensional). The pendulum bob is always affected by the force of gravity directed towards the center of the Earth. The force associated with the connection of the pendulum to a support structure directs the pendulum bob along the swing