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
Panthéon cross of the dome, which was retained in compromise, is again visible during the current major restoration project. In 1851, physicist Léon Foucault demonstrated the rotation of the Earth by constructing a Foucault pendulum beneath the central dome. The original sphere from the pendulum was temporarily displayed at the Panthéon in the 1990s (starting in 1995) during renovations at the Musée des Arts et Métiers. The original pendulum was later returned to the Musée des Arts et Métiers, and a copy is now displayed at the Panthéon. It has been listed since 1920 as a "monument historique" by the French