Complex sentence: The experimental apparatus consists of a tall pendulum free to swing in any vertical plane: the plane of swing appears to rotate relative to the Earth; in fact the pendulum maintains a fixed orientation in space while the Earth rotates under it once a sidereal day.
Split into simpler sentences: The experimental apparatus consists of a tall pendulum free to swing in any vertical plane. The actual plane of swing appears to rotate relative to the Earth; in fact the plane is fixed in space while the Earth rotates under the pendulum once a sidereal day.