National Museum of American History
1300 Constitution Ave., NW
Modeled after French physicist J. B. L. Foucault's 1851 pendulum, the museum's pendulum demonstrates the axial rotation of the earth. The 240-pound brass, hollow bob is suspended by a 54-ft. steel cable from the ceiling of the 4th floor. Although the pendulum's vertical plane seems to change, in fact it remains fixed. What is actually moving is the floor, which rotates under the pendulum because of the Earth's rotation. As the Earth moves, the red markers—arranged in a circle around an inlaid compass rose—move into the path of the pendulum, and the bob knocks them over one by one.