Why does a cat sway its head from side to side when staring at its prey?

When a cat is preparing to pounce on its prey it sometimes sways its head rhythmically from side to side. This is a device employed by many predators blessed with binocular vision. The head-sway is a method of checking the precise distance at which the prey is located. If you sway your own head from side to side you will see how, the closer an object is, the more it is displaced by the lateral movements. The cat does this to refine its judgment, because when the rapid pounce forward is made it must be inch-accurate or it will fail.