As the Earth Turns

The Science Museum in London has a marvellous long swinging pendulum that appears to change its direction during the day. Some books say that the pendulum keeps swinging in the same plane in space. Is this true in London? How do we explain the shift in the position of the pendulum during the day?

At the north pole the direction of the swing of the pendulum does not change in space, so the pendulum shows the Earth turning beneath it in 24 hours. Here in London it is not so simple; the direction of the pendulums swing moves around the circle in about 30.5 hours. As we travel towards the Equator the direction of swing moves more slowly, until at the Equator no movement is seen. In the southern hemisphere the direction of swing moves in the opposite direction.
— Description of Foucault's pendulum at the Science Museum, South Kensington, London