Just inside the front doors of the Griffith Observatory is the Rotunda, with a Foucault ("Foo-Co") Pendulum on permanent display.  Originally devised by French Physicist Leon Foucault in 1851, as the first way to visibly see the rotation of the Earth.  Every eight minutes the pendulum will knock down another pin on the floor.  Pendulums like this are on display in science museums around the world, including the original public display in the Pantheon of Paris.  Here in Los Angeles, it takes 42 hours for the circle in the floor to move one complete revolution around the pendulum swinging above it.