A large, yellow supermoon against a black sky

Is it the moon that’s super… or is it you?

Well, a bit of both, actually.

Supermoons are captivating in a way that few visual phenomena can match. After all, who hasn’t found themselves staring at the sky saying, “wow, look how big the moon is tonight!” But its increased size is only partially down to its closeness to the earth. The special kit that truly brings the drama is simply your very own magnificent eyes and brain.

When the moon travels round the earth, it doesn’t follow the shape of it. Instead, it goes around in an ‘elliptical orbit’, which means that there are times when it’s closer to the earth than others. What we call a ‘supermoon’ is the point when the moon is both full and at its closest point to the earth, making it appear larger and brighter. So, yes, the supermoon is closer to the earth, but only about 15% bigger and brighter than a regular full moon. And that’s not really enough to be particularly noticeable to the human eye. Which begs the question… what’s happening to make it look so gigantic?

Our brilliant brains

Everything we see in the world is a result of our brain doing a really good job of interpreting information that originates from the eyes. The ‘input’ (light reflected from the thing we’re looking at) travels along the optical nerve, through the thalamus and into the visual cortex, where it undergoes a whole lot of mental processing and becomes ‘what we see’. All very straightforward so far. However, there’s a lot going on in there, including plenty of information that has already been stored. For example, when we see the same thing multiple times, our brains are already familiar with what to expect and quickly attempts to fill in the blanks. This is called ‘top-down processing’ and is simply our brains being super-efficient, anticipating what is coming and then working with any new information to create the final picture.

A fun example of top-down processing in action is called The Stroop Effect, where people are asked to identify the colours that words are printed in, but the words are also colours (the word ‘blue’ printed in black, the word ‘yellow’ printed in red etc). If the two match, it’s fine – what the brain sees and what it expects are working in harmony – but if they don’t, then the brain finds this tricky and slows down. What it expects is not what it is given.

Two diagrams that illustrate the Ponzo Illusion (top) and the Ebbinghaus Illusion (bottom). The top diagram represents two grey ladders, side by side with two orange bars across them. The first looks like the orange bars are of different widths, the second demonstrates that they are the same width by connecting them with two lines. The bottom left shows two orange circles, one surrounded by larger grey circles, the other surrounded by smaller grey circles. On the bottom right, the same diagram shows that the two orange circles are the same size by connecting them with two lines.
Top: The Ponzo Illusion Bottom: The Ebbinghaus Illusion

When what we get isn’t what we see

What has any of this got to do with supermoons? Well, the unexpected size of the moon actually has little to do with how our eyes work, so the term ‘optical’ illusion (which is often used when talking about supermoons) is, in reality, a bit misleading. Like ‘The Stroop Effect’, what’s happening is more to do with our brains attempting to efficiently make sense of the information they have been presented with. However, neuroscience still hasn’t quite nailed the precise reason for the illusion of size in the case of the supermoon, but there are some really interesting theories to consider:

The Ebbinghaus Illusion puts two circles of identical size near to each other, but one is surrounded by large circles and the other is surrounded by small circles. The second central circle will appear larger than the first central circle. In this way, the theory is that the supermoon appears huge because it may be surrounded by objects that create this comparison illusion.

The Ponzo Illusion is another theory that’s often thrown into the hat. It’s demonstrated by two straight lines that converge upwards (and seem to be disappearing into the distance) with a pair of equally sized bars across the top and bottom. However, the top bar appears to be bigger than the bottom one because the mind makes a flawed adjustment to compensate for the perceived perspective. In the context of the supermoon, this theory suggests that the brain sees the moon as further away than any reference points in the horizon and makes a similar adjustment.

A final explanation is the Flat Sky Theory. Again, around size perception, in that the human brain perceives the sky above not as a dome, but as a flattened dome, with its zenith close by and horizon far away. When the brain sees the moon against this flat sky, it appears far bigger on the horizon than at the top of the dome.

And if you want to elude the illusion?

Some people say that looking at the moon from between your legs will put it back into the correct perspective, but you might find that a little… inconvenient. Instead, try looking at the moon between your thumb and forefinger, as though you’re holding it. Or view it in isolation from the rest of the surroundings by looking through a cardboard tube.

One thing is certain; our incredible, complex brains are constantly processing the world around us as speeds that are nothing short of extraordinary. Each sense – and our senses in combination – provides our brains with more data than it’s possible for us to comprehend. So, visual illusions, rather than being considered a ‘fail’ on the part of our brains are actually a tremendous opportunity for science and our endless fascination with the supermoon is actually a starting point, if you will, for understanding and learning more and more about what it means to be human.

Written by Marie-Anne Leonard

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