The Moon looks much larger near the horizon than high in the sky, even though its size never changes.

The Moon Illusion has been documented since ancient times, with records appearing in Aristotle's Meteorologica around 350 BC. Despite centuries of study, scientists still debate the exact cause, though the Ebbinghaus illusion and the Ponzo illusion are leading theories. These psychological effects suggest that our brains perceive objects as larger when they are surrounded by smaller objects or placed within a context of depth. When the Moon is near the horizon, it is viewed alongside terrestrial objects like trees and hills that provide a sense of distance.One prominent explanation is the 'apparent distance' theory. This suggests that the brain perceives the sky as a flattened dome rather than a perfect sphere. Because the horizon feels further away than the zenith (the point directly overhead), the brain compensates by enlarging the Moon to account for that perceived distance. This is a survival mechanism designed to help humans judge the size of objects in a 3D environment.Experimental data confirms that the physical size of the Moon does not change. Measurements taken with instruments like the theodolite show that the angular diameter of the Moon remains approximately 0.5 degrees regardless of its position. In fact, due to the Earth's rotation, the Moon is actually about 1.5 percent smaller when it is on the horizon because it is slightly further away from the observer than when it is overhead. Photography also proves the illusion, as the Moon's image on a sensor remains the same size throughout the night.