From the perspective of an Earth-bound coordinate system (the measuring circle and spectator are Earth-bounded, also if terrain reaction to Coriolis force is not perceived by spectator when he moves), using a rectangular coordinate system with its x -axis pointing east and its y -axis pointing north, the precession of the pendulum is due to the Coriolis force (other fictitious forces as gravity and centrifugal force have not direct precession component, Euler's force is low because Earth's rotation speed is nearly constant). Consider a planar pendulum with constant natural frequency ω in the small angle approximation. There are two forces acting on the pendulum bob: the restoring force provided by gravity and the wire, and the Coriolis force (the centrifugal force, opposed to the gravitational restoring force, can be neglected). The Coriolis force at latitude φ is horizontal in the small angle approximation and is given by F c , x = 2 m Ω Ω d y d t sin ⁡ ⁡ φ φ F c , y = − − 2 m Ω Ω d x d t sin ⁡ ⁡ φ φ where Ω is the rotational frequency of Earth, F c, x is the component of the Coriolis force in the x -direction and F c, y is the component of the Coriolis force in the y -direction.