The Earth is flat. So is the sky.

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The Earth is flat. So is the sky. At least our brains think they are.

It's a flat Earth

You are probably thinking that your mind knows that the Earth is round. And if you spend a couple of minutes thinking about it, you will probably remember some of the proofs that you learned in school. However, weather you want or not, your brain believes that the Earth is flat and it acts accordingly. And I don’t mean just here, around you, for a couple of metres, where you wouldn’t be able to feel the curbature of the planet.

Your brain believes that the Earth is flat as far as your eyes can see, and, what is even more strange, it is convinced that the sky is just as flat. And I will give you an example so that you can test this every evening.

If you take a look at the moon in the evening, around dusk, it seems much bigger (and maybe closer) that it looks late during the night, when the moon is high in the sky. And I am not talking about different eveninings, when the moon might really be closer or further from the Earth, I am talking about the same evening. You can even notice the craters better and clearer. That’s the way it is, you can see it every evening – every evening when you can see the moon – and if you are not convinced you can test this any time. This should be the easiest part of all this story and, if you have any doubts about it, it would be best to check this first and only then read the rest of story.

The part that was the most complicated for me was verifying that it is not true. Meaning during one evening, at any hour, the size of the moon, as we see it, is the same. And the trouble with this is that it quite difficult to be convinced of this, exactly because of the optical illusion that I am referring to. You can check this by taking pictures at different times and comparing them, like the time-lapse photo just lower, or by holding a coin at arm’s length and making sure that the relative size of the moon compared to that coin remains the same throughout the evening.

The reason for this optical illusion seems to be that for us, the size of an object is given by the brain, it is not just linked to the size of the object as it seen by the eyes. And the brain understands that, because of perspective, object that are further seem smaller, and objects that are closer seem larger. On a flat surface, like the Earth (or not like the Earth?), the parallel lines of the train tacks seem to get closer to one another. It is because of this that two lines of the same length seem to be larger or smaller if they are drawn higher of lower on the tracks. This optical illusion is called the Ponzo illusion, named after the Italian psychologist Mario Ponzo, which was the first to demonstrate it (as far as we know).
And now comes the weird part. Our brain believes that the sky is also flat, or at most it is slightly curved, and the brain does the same thing as with the Earth, but in an imaginary mirror. For this reason, when the moon is lower in the sky, closer to the horizon, it seems to be larger, and then, when it goes high in the sky, it seems smaller.

The proof for the moon illusion is not generally accepted. There are other theories why this illusion occurs and why the moon seems larger when it is closer to the horizon. The illusion was noticed from the Ancient Greece and people are still looking for an accepted theory. There are also a number of people (aproximated in some articles to 5%) that do not notice this optical illusion and for which the moon seems to be the same size at any time of the night. From my point of view these exceptions only strengthen the theory that this illusion is formed by the brain.

We have been taught a lot of facts. Some of them we believed, some of them we did not. Some we learned by ourselves. However, there are some facts that, no matter if they are proven to be true or false (really – no matter), our brain knows. And no matter what we tell our brain about those facts it will keep believing what it knows and what it wants to know.

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