This cool contraption demonstrates the Earth's rotation and is one of the largest in the world. The bronze ball is suspended by a cable that swings in a constant direction while the Earth is turning beneath it. The pendulum is mounted to a bearing in the ceiling that doesn't turn with the building as it rotates with the Earth. As the day goes on, the pendulum knocks over pegs that are set up in the pendulum pit showing the Earths rotation. Showing my geeky side, but i think that's kinda cool.

For more info check http://ashblagdon.com/griffith-park-observatory/