The moon illusion

"So why do you think the moon looks larger on the horizon?"

The question was posed by Rob Cockcroft, a graduate student who was also a presenter at the University's McCallion Planetarium. He was about to present a show dedicated to the moon and thought it possible he would be asked this question by one of the audience.

I blinked and looked up from my sandwich. "It does?"

I realized immediately I had just failed as an Astronomer. Yet the truth of the matter was that if an astronomical object appeared too large in my simulations, I had probably messed up the units in my calculation and caused the Universe to expand too slowly. Actual objects were not really my thing.

Fortunately the rest of the lunch table were more use.

"Isn't it because the moon is closer to other objects, such as trees, when it is low in the sky?" another graduate student asked. "Compared to those the moon will appear bigger."

This was a logical guess and one Rob himself had held until he had looked into the matter. The effect is known as the Ponzo Illusion and it is an optical effect that causes the human mind to judge the size of objects based on their background. Simple diagrams such as the ones shown here easily demonstrate this effect exists, but it is not the cause for the moon illusion, which has been proven to occur even on a featureless plain such as the ocean.

Another popular myth is the moon illusion is caused by the distortion of light in the Earth's air. It is true that as the moon sinks towards the horizon, you view it through a thicker layer of atmosphere, but the bending of light this produces actually causes the moon to appear smaller, not bigger.

In fact, Rob's investigations turned up no definite reason for the moon to appear larger; it seems no one knows for sure. The most common explanation, however, is that our brains have a view of the sky that is not a perfect hemisphere (like a planetarium) but a squished, shallower arc, more like a soup bowl. This causes us to believe that an object on the horizon is further away from us than when it was directly overhead. Since the moon's size actually hasn't changed, our brains assume that is must be larger at the horizon since it is apparently at a greater distance.

Quite why our brains would do this is a mystery. One argument suggests that we have evolved to be better at judging the size and proportion of objects that are close to us, since we are far more likely to be eaten by them than something at the bottom of a cliff or high in the sky. No one though, is entirely sure and some people reportedly don't experience the moon illusion at all. For those super interested, a good description of the moon illusion, including the flaw in the soup-bowl sky theory can be read here.

As I finished my lunch, I wondered if I was one of those rare people unaffected by the moon illusion. That would turn my folly at not knowing about it into the product of superior perception, not incompetence. It was a long shot, but I made a mental note to check next time there was a full moon.