Following on from my post about the Illusion of the Year competition, this is just looking back at
an amazing optical illusion that was voted the winner of the competition in 2006.
The movie below shows an animation of a black and white-striped disk rotating in the middle of a "starfield" of dots. As the disk rotates, it appears at times to speed up, and at other times to slow down, or even stop moving for a moment.
But in fact, the disk is rotating at one constant velocity throughout. The background, however, is swinging back and forth and accelerating from being stationary to a maximum angular velocity greater than that of the disk, and then back down to a velocity of zero again, so that the starfield swings in the same direction as the disk on some occasions, and in the opposite direction on others. When the two move in the same direction, the disk appears to slow down or stand still.
This movie is a replication (created with our FlyFly software) of the Frozen Rotation Illusion, first described by Dürsteler in 2006. The illusion has been interpreted by Dürsteler as due to our assumption that surrounds are stable and predictable, and hence can serve as a reference point for the motion of the object in the centre (the disk). Consequently, when the centre and surround are moving at approximately the same speed, the centre appears to move more slowly than it actually does (and even appears to stand still, as here), and when they move in opposite directions, the speed of the centre appears greater than it really is.
For an extended demonstration of the Frozen Rotation illusion and several of its variants, see the video by Dürsteler at http://illusionoftheyear.com/2006/05/the-freezing-rotation-illusion/, and also the publication associated with this phenomenon.