Moon illusion

The moon illusion is an optical illusion in which the moon appears larger near the horizon than it does while higher up in the sky. This optical illusion also occurs with the sun and star constellations.

A false explanation

It is common belief that the moon actually does appear larger near the horizon due to a magnification effect caused by the Earth's atmosphere. This is not true. Although the atmosphere does change the color of the moon, it does not magnify or enlarge the moon.

Proving that it's just an illusion

Here is a way to prove that the moon actually is the same size in different areas of the sky. When the moon is near the horizon, hold a small coin at arms length and, with one eye closed, position it next to the moon and compare the size of the coin relative to the size of the moon. Then, when the moon is higher in the sky, position the coin next to it again, and you will see that the moon looks the same size compared to the coin as when it was near the horizon. Another way is to hold your fist at arms length and compare it to the size of the moon in various positions through the sky.

What causes the illusion?

Humans perceive the size of a distant object by comparing it to other objects visibly near it. In the case of the moon, when it is close to the horizon, we judge its size in comparison to distant trees or buildings. However, once the moon gets further up into the sky, there are no other objects which can be used for size comparison, so the ability to perceive its true size is lost. This is why the moon appears larger near the horizon than it does when high in the sky. In actuality, it is exactly the same size in both places.

External links

• http://www.lhup.edu/~dsimanek/3d/moonillu.htm
• Explanation of the Moon illusion and many other gestalt effects