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 of the North Pole, equal to zero since zero distance is traveled in relation to the spinning Earth. Even though the surface velocities are different the angular velocities underneath the pendulum swing are all the same for this orientation. For the 45° North pendulum with latitudinal swing (Figure 3B) the support point of the pendulum swing is moving along with the direction of rotation and the surface velocity vectors on either side of the swing are not balanced. The rotation of the Earth is observable in relation to the pendulum swing because a change in relationship to the surface occurs.