Is the Coriolis effect an optical illusion'?

The difference between the derivations of the Coriolis effect on a rotating turntable and on the rotating Earth is discussed. In the latter case a real force, the component of the earth's gravitational attraction, non-parallel to the local vertical, plays a central role by balancing the centrifugal force. That a real force is involved leaves open, not only the question on the inertial nature of the inertial oscillations', but also the way we tend to physically conceptualize the terrestrial Coriolis effect.