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 would not be observable in relation to the pendulum swing with this type of connection. At the equator the rotation of the Earth is not observable in relation to the pendulum swing because the plane of the pendulum swing is moving along with the rotation of the Earth and no net change in relationship occurs. It is shown by the diagrams that the surface velocity vectors of the Earth underneath the swing of the pendulum are either balanced in the same direction or included within the same plane as the pendulum swing. For the equatorial pendulum with a swing in