As people above have said, it's the same as wheels on film, commonly referred to as the wagon wheel effect.http://en.wikipedia.org/wiki/Wagon-wheel_effect Abusive comment hidden. (Show it anyway.)
This site is just as pretty, but a whole lotta more disturbing.http://www.georgehutchins.com/ Abusive comment hidden. (Show it anyway.)
http://en.wikipedia.org/wiki/Wagon-wheel_effect