The Foucault pendulum is named after French physicist Léon Foucault, who first demonstrated it in Paris in 1851. This device consists of a large pendulum that is free to swing in any direction. Foucault demonstrated that the plane of the swing appeared to slowly rotate over time. The change of direction was the result of the Earth’s rotation beneath the pendulum – although the amount of rotation depends on latitude. Because the pendulum cannot change the direction of its swing on its own, it continues to swing in the same plane because of inertia. With Earth’s rotation underneath the pendulum, the pendulum appears to rotate relative to the ground. It is a simple and visually striking way of demonstrating Earth’s rotation. When Foucault first demonstrated this device, it was met with wonder by some and skepticism by others.

Up to the mid-nineteeth century, it was a commonly accepted belief that the Earth rotated on its axis. This had been based on the astronomical observations of previous astronomers and scientists, such as Copernicus, Galileo, and Newton. These conclusions, however, were based on theory and not direct observation. The Foucault pendulum changed all that by offering direct, visible proof of Earth’s rotation that can be observed by anyone. It visually validates the Earth's rotation, something that cannot be directly observed. Moreover, it also illustrates how sophisticated mathematics or astronomical tools are not always necessary when trying to understand scientific phenomena.