The mind sees and the mind hears. The rest is blind and deaf.
(Epicharmus, 450 BC)
Can you always trust what you see? What happens when our minds wrongly interpret the information from the eyes, leading to our perception of something being different from the reality? This is what is happening in my favourite optical illusion: the horizon moon.
Ever wondered why the moon looks so much bigger as it appears over the horizon than when it's at its highest point in the sky? There are varying theories for why this happens; some are extremely complicated and none fully explain the phenomenon. But here is a rough summary of what could be happening.
Firstly, the moon’s 27.3 day orbit around the Earth is ecliptical (oval), and this results in its distance from the Earth varying between 363,300 and 405,500 km. This does mean that the moon will appear 11% larger in the sky when it is at its closest (perigee) than at its furthest away (apogee). But remember, the moon’s changing position in the sky over the course of a night isn’t caused by the moon rotating round the Earth, but the rotation of the Earth on its axis. This means that the distance of the moon from the Earth is constant throughout the night and it doesn’t explain the difference in apparent size of the moon when it is over the horizon.
So could it be due to the way that the light reflected from the moon travels through the atmosphere on its way to our eyes? After all, when it's near the horizon, the light has to travel further through the atmosphere than when its directly above us. It turns out that refraction (bending) of light does alter how we see the moon, but not to make it appear larger at the horizon, although it will make it appear sort of squashed when it is low in the sky.
But if we have ruled out the possible physical causes of this phenomena, what is left? Could it be all in our minds?
First of all, lets prove that the image of the moon projected on our retinas doesn’t vary during one rotation of the Earth. If you were to hold a coin at arm’s length and compare it to the horizon moon and the moon at its highest point in the sky, you’d see that it doesn’t actually change in size. Also, if you were to photograph it, the moon would be the same size in a picture taken at any point during the night, regardless of how it looked to the naked eye.
It could have something to do with size constancy. Imagine you are walking towards an elephant. The actual image of the elephant will fill up more and more of your visual field as you get closer. But you don’t perceive the elephant to be growing in size as you approach. This is because our brains take into account how far away the elephant is to tell us it is huge even when it's hundreds of metres away despite it looking tiny in comparison with, for example, our own hand. You can test this out by staring at a light bulb or a bright circle of paper and then staring at a white wall. The image burnt onto your retina is of fixed size but the circle will seem much bigger if you look at a distant wall than if you look at a piece of paper right in front of you.
Judging how far away something is depends largely on the surroundings. When the moon is on the horizon, you are seeing it surrounded by reference objects such as trees, buildings and the contours of the land. Try this: next time you see the massive horizon moon, try looking at it through a cardboard tube – the size illusion should disappear. Many people take this as evidence that the reason the moon looks so large at the horizon is because we are comparing it to smaller objects, such as trees and buildings, making it appear larger in comparison.
But it isn’t only the surroundings that cause the moon illusion, the way our brain judges distances plays a big role. Visualise the sky as a dome over the top of us as if we’re trapped in a snow globe. Our brains don’t judge distances exactly right and we don’t actually tend to imagine the sky as a perfect dome but instead underestimate vertical distances compared with horizontal distances and our mind sees the sky as a sort of flattened bowl. This all means that when you see the moon directly overhead, you think it is closer than when it is at the horizon. And, just like the light bulb demonstration above, thinking that the moon is far away makes the mind tell you that it is bigger than when its above you in the sky. If you fancy a bit of yoga, you could try bending over and looking at the horizon moon through your legs. Does it still look so large? Being upside-down confuses your brain and makes it harder to judge distances and use other objects as a size reference.
But this still doesn’t make complete sense. Most people would say the horizon moon is closer than the moon directly overhead. How can our subconscious mind be thinking it is further away and must therefore be bigger, while our conscious brain is telling us that its closer? Is it our brain trying to rationalise something that doesn’t make sense? Our knowledge tells us that the moon isn’t growing so could we be trying to understand the tricks our mind are playing on us by thinking that it is closer? It's not just how your brain perceives something, but how you then try to understand it based on what you have learnt. Past experience and how we expect something to look probably plays a bigger part than you would expect. It makes my head hurt just thinking about it.
