Look at this image and ask yourself which vertical red line would be longer if measured with a ruler on the photograph.
Phil Plait uses this image to give an explanation of the ‘moon illusion’, that refers to the phenomenon that a full moon that lies low on the horizon appears to be much larger than when it is high in the sky a few hours later, even though the measured size in the same. (To test this for yourself, roughly measure the diameter of the moon by keeping a pencil (or coin or any small object) at arms length straight in front of you and see how much of it corresponds to the diameter of the moon. Do this when the moon is low and again when it is high. It will be the same.)
There are a variety of explanations as to the origins of this illusion. One of the most popular is that this effect is due to the fact that when the moon is low, we gauge its size by comparing it with that of terrestrial objects like trees, buildings, etc. that it is close to or partially obscured by, whereas when it is high in the sky, we see it as it ‘really’ is. This is not particularly convincing because it does not really explain why the proximity of other objects makes something appear larger. It merely adds information that may or may not be relevant. But it is the usual explanation that is proffered.
But Plait says that this explanation is wrong, and that the effect is due to something called the Ponzo Illusion, which is that our brain figures size based on how far away we think an object is and it is the adjusted size that we ‘see’. If we think that something is far away, our brains make us ‘see’ it as larger than if we think it is nearby.
So what has all this got to do with the moon illusion? According to Plait, studies have repeatedly shown that the mental image that people have of the sky is not that of an inverted hemispherical bowl over our heads but is more like that of a bowl with a flattened bottom, a shallow dish really. This model of the sky may have been created by the fact that objects such as clouds and planes and birds look larger when they are directly overhead than when they are on the horizon, because in the latter situation they really are farther away. Hence our brain thinks of the sky directly above us as much closer than the sky near the horizon. Hence the moon near the horizon appears larger than when it is directly overhead because our brain thinks that the moon on the horizon is further away. It has nothing to do with proximity to buildings and trees.
Back to the above image of the red lines. The two red lines are actually the same length in the photograph. We think the one on the right looks longer because we know that if we had been actually in the London subway station where the photo was taken, it would be longer. The clues come from the wall tiles converging in the distance. The right line would be five times as long as the one on the left but appears reduced in size because it is farther away. So when we look at the photo, our brain automatically factors this in and makes the line on the right seem longer.
Is this the right explanation for the moon illusion? I don’t know, though it does seem plausible. It depends crucially on the assumption that we have a mental image of the sky as a shallow dish. I don’t know that I have consciously thought of it that way because I know that there is no such thing as ‘the sky’ except as an imaginary construct, and besides which I know that the moon’s ‘motion across the sky’ is itself an illusion caused by the Earth’s rotation and hence the distance to the moon cannot be changing. But that is a conscious modern view, based on scientific knowledge gleaned over the last few hundred years. It may be that what is hardwired in our brains are beliefs that date back to deep evolutionary times where the sky may have been believed to be a shallow dish and the moon tracked along it.
So although my conscious, scientific mind knows that the size of the moon must be the same on the horizon and overhead, it does not have the power to overrule the brain’s hardwiring that determines what I see. In other words, what I ‘see’ is not purely caused by physical visual input but is the result of unavoidable brain processing of that input over which I have little or no control. After all, even after measuring them and finding them equal, I still see the red line on the right as longer than the one on the left. I just can’t help it.
In reading about the Ponzo Illusion, I was reminded of something that lent support for it. In the book Your Inner Fish by Neil Shubin, whose team made the spectacular discovery of the fossil Tiktaalik in 2006, the author describes an experience in which they were looking for fossils in that part of Greenland that lies within the Arctic Circle. It is a flat, bleak, and barren region with no houses or trees or any of the familiar objects that normally give us a sense of distance or scale. The biggest danger to people in those areas is due to polar bears that can be highly dangerous. People who travel to those regions are given extensive training on how to avoid attracting them and defend themselves against them and warned to keep a sharp look out. As a result, he writes that after being dropped off at their site by plane,
The first thought is of polar bears. I can’t tell you how many times I’ve scanned the landscape looking for white specks that move… In our first week in the Arctic, one of the crew saw a moving white speck. It looked like a polar bear about a quarter mile away. We scrambled like Keystone Kops for our guns, flares, and whistles until we discovered that our bear was a white Arctic hare two hundred feet away. (p. 17)
Shubin writes that this error in perception was caused by anxiety making them see things. It is true that they were ‘seeing things’ but it may not have been due to anxiety, at least not exclusively. It could have been the Ponzo Illusion kicking in, with their brains misreading visual clues as to the distance of the hare from them.