an illusion known as the wagon-wheel effect. The illusion can often be seen in old westerns, when the spoked wheel of a moving stagecoach appears to be rotating in reverse. The effect results from a mismatch between the wheel’s rotation rate and the frame rate of the camera that’s filming it. If a wheel spoke travels more than one-half but less than a full rotation between movie stills, the spokes will look like they’re rotating backward.
