How did Jean Foucault proved that the earth is rotating?

A2. Foucalt invented the Foucalt Pendulum, by observing that a long massive pendulum would rotate gradually, its plane of oscillation. Of course, the plane of oscillation is not moving - it is the earth that is moving.

I remember a fine Foucalt Pendulum display at the Smithsonian in Washington D.C. Well worth the 10 min or so to watch its progression.

At the poles, the period of a full rotation of the pendulum's plane will be 24 hours. At the Equator, the plane of the pendulum will remain fixed.

A1. He is wrongly credited with inventing the "Gyroscope". It also is used to prove that the Earth Rotates.