question: How were Earth bound observations used? answer: Earth motion paragraph: So how do we know that the Earth rotates on its axis? Before this century, all evidence collected about Earths motion came from Earth-bound observations. From Earth-bound observation it is easy to see why, historically, the Earth was stationary. It was the Sun and the stars that orbited the Earth. One simple experiment can be used to show that is not the case. It is the Earth that is rotating causing the cycles we see in the sky. The experiment was designed by Lon Foucault, a French scientist, in the year 1851. He hung a heavy iron weight from a long wire. He pulled the weight to one side and then released it. The weight swung back and forth in a straight line. If Earth did not rotate, the pendulum would never change direction as it moved back and forth. But it did not continue to swing in the same direction. It was thought to change because the Earth was rotating beneath it. Figure 2.49 shows how this might look. 