He is following a simple rule: Keep the angle of the falling ball constant in your vision and adjust your running speed accordingly.

WSJ: Matt Ridley on Gut Feelings and the Writings of Gerd Gigerenzer | Mind & Matter

That, in turn, depended on the angle and amount of sunlight falling on Voyager, and the amount of heat generated by its various components as they were switched on and off.

ECONOMIST: The long arm of the celestial repairman