You let the pendulum swing back and forth, and in 24 hours it completes a full circle. But the pendulum itself does not rotate, as there is no sideways force that would affects its linear swinging motion. So it must be the ground beneath the pendulum that rotates.

The pendulum is fixed to the ceiling with a nearly frictionless ball joint that allows the pendulum to swing unaffected even when the building itself around the pendulum is slowly rotating.

I think this is super cool.