Cogwheels Motion

Movie #1: Silvanus P. Thompson, in “Light visible & invisible” (1897) noted that when a cogwheel pattern moves along a CW path, it shows an illusory counter-CW rotation. (I admit that these movies do not work all that well. Try jiggling a paper version.)

Movie #2: Low contrast cogs seem to rotate CW. I don’t know why.

Movie #3: Hedgehogs with outward-pointing teeth seem to rotate CW.

Movie #4: Simulation & explanation. When only the outer tips of hedgehog teeth are visible, they rotate strongly CW. When only the inner teeth of cogwhels are visible, they rotate strongly CCW.