The Moon Illusion

On May 5^th the world experienced what is known as a “supermoon”. This refers to a time at which the moon is both full and at its perigee – that is, the point in its orbit at which it is closest to the Earth. The result is that the moon looks bigger and brighter than normal, and there have been some beautiful pictures taken around the world.

Looking at these photos, I started wondering why the moon – whether “super” or not – looks so big when it is near the horizon compared to when it is higher in the sky. A quick look on the internet revealed that I was far from the only one wondering this. In fact, the “Moon Illusion”, as it is termed, has apparently been pondered for centuries. My initial belief, based on my very cursory knowledge of physics, that this phenomenon had something to do with refraction of light through the atmosphere was quickly dispelled by Wikipedia. Rather it seemed that the Moon Illusion was an actual psychological illusion. However, the writers of the page frustratingly left me hanging as to the mechanisms behind the illusion.  So like any good student, I turned to SOLO.

A surprising amount of literature has been conducted on the phenomenon in the past 100 years. In fact, it seems that no-one is completely sure why the phenomenon occurs, and it continues to be studied to this day. Two major theories regarding the Moon Illusion have emerged from the literature. The older of these is called the apparent-distance theory. This theory basically states that we see the moon as larger when it is at the horizon because we perceive the distance to a horizon moon as greater than that to an elevated moon. We know from experience that the horizon is far away, and environmental cues such as buildings or trees getting smaller and converging into the distance accentuate the vast distance to the horizon. However the lack of such cues in the sky makes points in the sky seem closer. Thus although the image of the moon on our retina is always the same size, it seems bigger at the horizon because we perceive it as further away – and we know that the further away things are, the smaller they appear. This takes a bit of thinking to get one’s head around.

The apparent-distance theory: The moon appears big at the horizon because environmental cues accentuate how far away it is (diagram: from http://lasp.colorado.edu)

However, this cannot be the whole story. When you ask people to judge the distance to a horizon moon or an elevated moon, they say that the horizon moon is closer, inconsistent with the apparent-distance theory. Thus a newer theory suggests that the perceived size of the moon depends on the context in which it seen. That is, people do not infer the size of the moon from its perceived distance, but rather the opposite – they use environmental cues to infer the size of the moon, and subsequently calculate its distance (which is why they perceive a horizon moon as closer). So when the moon is at the horizon, it seems large because you compare it to much smaller objects such as trees and houses. When the moon is elevated, however, it is situated in the middle of a dark expanse of sky, and there are no cues with which to compare it. This has been termed the relative-size theory.

The relative-size theory: The moon appears big when it is low because it can be easily compared with objects in the environment (photo: Bob King, from http://astrobob.areavoices.com)

But the relative-size theory alone also appears insufficient to explain the Moon Illusion. When participants view inverted scenes of the moon at the horizon or in the sky, as if they were bending over and looking at the moon between their own legs (go on, try it!), the effect of the Moon Illusion diminishes.  So when viewing these images, participants see the horizon moon as closer in size to the elevated moon, compared to upright images. These images contain all the same information regarding the size of the moon compared to objects in the environment, so the relative-size theory can’t account for this diminished effect. However, turning the image upside-down impairs the ability to judge distance because of the unusual perspective. This implies that these distance judgments are important in the Moon Illusion.

So it seems that we are still not sure exactly why the Moon Illusion occurs.  However, if you are to encounter a scientist out at Port Meadow in the middle of the night, head between legs and staring at the sky, show a litte respect. They are attempting to answer one of the most ancient questions of the science of perception.