[KMag]

The pendulum shows you the net rotation of the pendulum about the Earth's axis of rotation, the rotation of the Earth about the Sun, the Sun about the center of the Milky Way, the Milky Way about the gravitational center of our local cluster, etc., etc.

Having not known, I think I would have strongly guessed that the rotation of the Earth about its own axis would be by far the dominant factor, but it's still good to measure.