What is the Moon Illusion? Why Does the Moon Look Really Big?

The Moon is not in fact any larger on the horizon, it simply seems larger. This phenomenon is referred to as the "Moon illusion" and is more pronounced at around full Moon when the maximum area of its disc is illuminated. The moon illusion has been known since the Aristotle first documented the illusion in the 4th century BC.

In reality, the Moon has roughly the same apparent diameter of around half a degree, whether or not it is looming over the horizon or riding high in the sky. One reason behind the Moon illusion arises from our perception of the shape of the celestial sphere above us; instead of a hemisphere, we perceive the sky to be shaped like a flattened dome. Consequently, the lower the Moon is in the sky, the further away and bigger it is perceived to be; when the Moon is high in the sky we perceive it to be nearer to us and subsequently smaller in apparent size.

Few people appear to be immune to the Moon illusion, despite the fact that the viewer may be fully aware that for any given evening there is actually no appreciable difference in the Moon's apparent diameter, irrespective of its height above the horizon.