Please explain the “Wagon Wheel Effect.” How can the wheel appear to move forw…

Please explain the “Wagon Wheel Effect.” How can the wheel appear to move forward, then backward, then stop, just by viewing it differently? — J, Davenport, IA

This effect is the result of viewing a series of stop-action frames in rapid sequence as a movie or video. Even though a wagon wheel is turning forward, its orientation during sequential frames of a movie may make it appear to be stopped or turning backward. For example, if the wagon wheel completes exactly one full turn between each frame of the movie, the wheel will appear to be stopped—its orientation in each frame will be the same. If it completes slightly less than one full turn between each frame, it will appear to be turning backward! As you can see, a tiny change in wheel rotation rate, from slightly more than one full turn per frame to slightly less than one full turn per frame, is enough to make the wheel appear to switch from turning forward, to stopped, to turning backward. So it’s no wonder that the wheels appear to change speeds abruptly from no apparent reason.