Visual curiosities: the moon illusion—paradoxes of apparent size

23/10/2024

When the moon rises or sets, it appears unusually large. This is one of the oldest known perceptual illusions, though experts still lack consensus on its cause.

Popular belief often attributes this phenomenon, first described by Aristotle, to an optical magnification effect by the atmosphere. However, repeated photography reveals that no real magnification occurs—the moon’s size is identical at the horizon and at its zenith. Why, then, do most observers perceive a larger moon when it’s near the horizon?

The illusion persists even when a drawing shows two identical moons, one on the horizon and one overhead. The answer is complex.

The moon illusion likely falls into the category of apparent size illusions, similar to the Ponzo and Ebbinghaus illusions.

The Ponzo illusion

In this illusion, two identical horizontal lines are placed between two converging lines, like railroad tracks appearing to recede into the distance. The upper “crossbar” is perceived as farther away and, therefore, would need to be longer to create an identical retinal image size as the nearer, lower line.

With the moon at the horizon, we perceive it as larger because it appears farther away than when at its zenith. In fact, when photographed with a telephoto lens, the moon looks quite large compared to trees or buildings in the same line of sight. Since the moon’s visual angle remains the same with distance while the angle of distant terrestrial objects diminishes, this leads us to believe the moon must be larger.

The Ebbinghaus illusion

The Ebbinghaus illusion showcases another aspect of relative size perception. It involves two groups of circles: both central circles are identical, but one is surrounded by smaller circles and the other by larger circles. This makes the central circle surrounded by smaller circles appear larger, and vice versa.

Numerous variations of this illusion highlight the importance of context—such as the presence, absence, relative size, and proximity of surrounding objects—in our size perception.

Some argue that both the Ponzo and Ebbinghaus illusions relate to objects within stereoscopic vision range, whereas the moon lies far beyond that. Psychologist A. Trehub suggests that due to evolutionary needs, we devote more visual resources to horizontal, nearby spaces and fewer to very high or very low areas.

This may explain why objects at eye level appear larger than those viewed from above or below, or why we perceive the celestial vault as flattened, making the horizon seem farther away, as well as the low moon, which we would automatically perceive as “large” in a compensatory way. However, a majority of subjects respond that the moon on the horizon “seems closer”… The question remains open.

Professor Rafael I. Barraquer, Medical Director, Barraquer Ophthalmology Centre