Moon Illusion

About 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. It has been known since ancient times, and recorded by numerous different cultures.

A popular belief, stretching back at least to Aristotle in the 4th century BC, holds that the Moon appears larger near the horizon due to a real 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 it. In fact, the Moon appears about 1.5% smaller when it is near the horizon than when it is high in the sky, because it is further away by up to 1 Earth radius and also because of atmospheric refraction, which makes the image of the Moon slightly smaller in the vertical axis. (Note that between different full moons, the Moon's angular diameter can vary from 33.5 arc minutes at perigee to 29.43 arc minutes at apogee — a difference of over 10%.)

The angle that the full Moon subtends at an observer's eye can be measured directly with a theodolite to show that it remains constant as the Moon rises or sinks in the sky (discounting the very small variations due to the physical effects mentioned). Photographs of the Moon at different elevations also show that its size remains the same.