A Visual Explanation of the Illusion That Makes a Windmill Appear to Spin in the Opposite Direction

Researchers Dr. Michael Pickard and Gurpreet Singh of the University of Sunderland submitted to 2017 Best Illusion of the Year Contest a simple but brilliant animation that explains in two different scenarios why windmills appear to spin in the opposite direction while staring at them.

In this illusion, by removing alternate dots or using alternate light/dark colours or creating open spaces, the dot pitch is changed. Thus both conditions are present, giving rise to the appearance of motion in both directions.