The French physicist Jean-Bernard-Léon Foucault provided one of the best proofs of the planet's rotation. He first performed his pendulum experiment in 1851. Foucault suspended a steel wire from the dome of the Pantheon (a public building in Paris). He then attached a heavy weight to the end of the wire. A pin was fastened on the end of the weight. The pinpoint rested on a circular layer of sand on the floor.
Foucault set the pendulum swinging. The pendulum moved to and fro, in the same plane. The pin at the end of the weight began to trace lines in the sand. As the pendulum continued to swing, the lines followed different directions.
There could be only one explanation. The pendulum did not change direction. Therefore, it must be the ridge of sand that was turning. The sand rested on the floor of the Pantheon, and the Pantheon itself rested on Earth. Foucault concluded that Earth itself must be rotating.
Foucault's device is known as the Foucault pendulum. One of these pendulums has been erected in the General Assembly Building of the United Nations in New York City. Another interesting effect due to the rotation of Earth can be produced with a camera.
Focus a camera on the North Star and leave the shutter open for several hours. The photographed stars will not appear as points -they will be curved lines. This is because Earth, on which the camera rests, has been rotating on its axis.
The alternation of day and night is also due to the rotation of Earth about its axis. As the planet turns, a given place on its surface will be in sunlight or in darkness. It depends on whether the surface is facing toward the Sun or away from the Sun. In addition, Earth's rotation effects air currents. Air currents turn toward the right in the Northern Hemisphere, and to the left in the Southern. This phenomenon is called the Coriolis effect.
