The circles in this illusion are separated by 30 degrees. By altering the degrees and frequency by which the reel rotates, you will notice that in some cases that the reel will seem to change direction or be stationary with the alternate colored circles moving. The illusion of the reel being still with the tail moving occurs at 30 and 60 degress of rotation. Since each dot is spaced by 30 degrees, the circles' new position is that of it's counterpart's old, giving the effect of stationarity.