The Moon Illusion

Two moons, drawn the same size: one low over rooftops, one high and alone overhead. That’s the popular belief, drawn plainly, and to its credit the board says outright that nobody has fully settled why it’s so convincing. This lesson measures both moons properly, then spends two thousand years failing to fully explain the gap it finds.

Before you start. You need one everyday fact: things look smaller the farther away they are, which is why a plane shrinks to a speck at altitude and fills your view on the runway. Every number below is worked out for you as it comes up.