Why do wheels look like they are spinning backwards in a movie?

A movie camera looks like this inside: www.google.com/search?tbm=isch&q=maltese+cross+movie+camera

The camera takes thirty pictures per second. When the film is projected on a screen, the eye thinks it is seeing moving images, but it actually is seeing thirty still images per second. So if the spokes happen to be moving at that exact rate then the image on screen appears not to be moving at all, and if it is slightly off then they appear to move backwards. Digital cameras work differently, so the effects are different. Like an airplane propeller with a dozen blades all parallel.