Why Does the Moon Look Larger Near the Horizon?

A full Moon rising behind trees or buildings can look enormous. Several hours later, when it is high in the sky, it seems to have shrunk. Careful measurements show that the Moon has barely changed size at all.

A photograph taken with the same camera settings will show nearly the same lunar diameter in both positions. You can also hold a small object at arm's length and compare it with the Moon throughout the evening. The apparent transformation will disappear when measured.

The atmosphere does not magnify the rising Moon. Refraction can actually compress it slightly in the vertical direction. The Moon is also a little farther from an observer when it is on the horizon than when Earth's rotation carries that observer closer to it.

The effect is known as the Moon illusion, and scientists still debate its complete explanation. The leading ideas involve how the brain interprets distance and size. Near the horizon, the Moon appears beside trees, mountains, and buildings that provide familiar clues about scale. The sky near the horizon may also be perceived as farther away than the sky overhead. If the brain judges the Moon to be unusually distant while its visual size remains unchanged, it may interpret the Moon as physically larger.

Looking through a narrow tube removes much of the surrounding landscape and often weakens the illusion.

The enormous rising Moon is therefore not produced in the sky. It is constructed by a brain trying to understand an object that does not behave like anything else along the horizon.