Visual Curiosities: The Moon Illusion: Paradoxes of Apparent Size

10/23/2024

When the moon rises or sets, it appears very large to us. This is one of the oldest known perceptual illusions, but there is still no consensus among experts about its cause.

Popular opinion usually attributes this phenomenon, already described by Aristotle, to the effect of optical magnification caused by the atmosphere. But from repeated photographs it is easy to see that such an increase does not actually exist: the size of the star at the horizon and at the zenith is the same. Why then do almost all observers believe that they see a large moon when it is low?

The illusion occurs even when faced with a picture with the moon on the horizon and the same one above. The answer is not simple.

In any case, the Moon illusion seems to belong to the group of illusions regarding apparent size, such as the Ponzo or Ebbinghaus illusions.

Ponzo Illusion

The first consists of two identical horizontal lines that are located between two other converging lines, like railroad tracks, actually parallel, extending towards the horizon. Thus, the top “bar” will be further away and therefore must be longer to create an image on the retina of the same size as the closer image below.

In the case of the moon on the horizon, it will appear larger because it will appear to be further away than at the zenith. In fact, when photographed with a telephoto lens, the star appears very large compared to objects such as trees or buildings in the same line of sight. Since the Moon’s viewing angle does not change with distance, but the viewing angle of distant terrestrial objects decreases, this leads us to believe that the Moon must be larger.

Ebbinghaus illusion

The Ebbinghaus illusion shows us another side to the perception of relative size. It consists of two groups of circles: the central ones are the same in both, but surrounded in one case by smaller circles, and in the other by larger ones. This makes the central circle surrounded by smaller circles appear larger than vice versa.

There are many variations emphasizing the importance of context, presence or absence, relative size, proximity, etc. of nearby objects in this aspect of vision.

However, it has been argued that the Ponzo and Ebbinghaus illusions refer to objects at a distance at which stereoscopic vision signals operate, and the Moon is far beyond. According to psychologist A. Trehub, as a result of our evolutionary needs, we devote more visual resources to the neighboring space and at the horizontal level and less to everything that is very high or very low.

This could explain why the size of objects at eye level is perceived as larger than the size of the same objects when looking up, or that the vault of the sky appears flattened and therefore the horizon appears further away, as well as the low moon we see. will be automatically perceived as “large” in a compensatory manner. However, most subjects answer that the moon on the horizon “seems closer”… The question remains open.

Professor Rafael I. Barraquer, Medical Director of the Barraquer Eye Center