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

The Moon illusion makes the Moon appear much larger when it is closer to the horizon. Find out what causes the illusion in this article.

Why does the Moon appear so large when it rises or sets? This trick that our brains play on us is known as the Moon illusion. Photographs prove that the Moon is the same size near the horizon as it is high in the sky, yet this is not what the human eye perceives. As a result, it is an optical illusion caused by the way our brains absorb visual information.

The Moon is not 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 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.

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

The Moon Really Is More Yellow Near the Horizon

There is one significant difference in the Moon's appearance when it is low in the sky. When compared to when it's high overhead, it has a more yellow or orange tint. This is again due to the Moon's light travelling a greater distance through the atmosphere. More of the shorter, bluer wavelengths of light are dispersed as it travels a longer distance, leaving more of the longer, redder wavelengths. Dust or pollution in the atmosphere can further intensify the yellow and orange colour.

The Harvest Moon is the most famous example of a large, golden yellow Moon, however, any time the moon is near the horizon it will appear larger and orange.