In pursuit of a new explanation, Kaufman and Rock performed a set of experiments using a device that could place a disk of light in the sky so that it was “exactly like looking through a window at the moon.”7 They considered eye-elevation both outdoors and indoors, the moon’s color and brightness, and the presence of terrain. The first two had no effect, but the third was crucial: They concluded that if an observer’s view of the terrain is obstructed, the illusion vanishes.

This observation supported the apparent distance hypothesis: The presence of terrain increased the subjective sense of distance to the moon, so that while it was actually the same size, the perceptual system would “conclude” that it was a more distant object and thus inflate its size. Boring had rejected the apparent distance hypothesis because people reported that the horizon moon looks closer, not farther away. As the behavioral physiologist J.T. Enright points out, this size-distance paradox seemed to require some kind of disconnect between our conscious and subconscious perception of distance: We expand the moon to be larger given that it seems farther away, but then report that it seems closer. “The subconscious impression of distance, once it has determined apparent size, must remain irretrievably locked in the subconscious,” he wrote. “These seem to be unresolvable paradoxes.”

But Kaufman’s data clearly pointed to the importance of terrain, as did other experiments.8 Kaufman even called the astronaut Ed Lu when he was overhead in the International Space Station to ask him if he saw the moon illusion up in space: He said no. “There’s nothing there but the curvature of the Earth,” Kaufman told me. “You got no distance.” Plus, he told me, he’s asked test pilots (“these guys have eyes like eagles”) if they see the moon illusion. They told him “sure, but only when we get down low.”