Freezing Rotation Illusion
In the neighbouring demo you see (1) an inner rotating part (the text) placed on (2) a rotating disk made up of coloured dots. The outer disk does not rotate continuously, but ‘swings’ back and forth. What is the motion of the central text? Is is a smooth rotation?
What to do
If you use the ‘hide’ button (top) it becomes obvious that the central rotation is completely smooth. When the outer disk is visible, it exerts an influence on the perception of the central disk’s rotation.
You can experiment with the speed of the inner and outer parts and the frequency of the swing.
The buttons on the bottom right allow you to add compensatory motion to the inner part, to what is known as ‘to null’ the effect. For slow and medium speed swings, for me ≈33% counter rotation without phase shift largely nullifies the ‘freezing’. More research is necessary ;-).
Max Dürsteler’s lucid comments (→PDF) need no further elaboration.