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WHY IS THE MOON BIGGER AT THE HORIZON?

Is it an astronomical phenomenon or is your brain being duped?

By Mark Thompson 
Thu Jul 15, 2010 09:19 PM ET 
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Moonset

A moonset behind the Very Large Telescope (VLT) at the European Southern Observatory (ESO), Paranal, Chile. Is the moon the right size? Click to enlarge this image. 
G.Gillet/ESO
We’ve all been there, done it and got the t-shirt; glanced out the window and noticed a stunning, slightly orange-colored yet huge moon peeping over the horizon.
Maybe you’ve even tried to take a photograph of it only to be disappointed when confronted with a tiny image of the moon’s disk. That’s not how you remembered it… right?
You my friend have been duped, not by the strange effects of the Earth’s atmosphere but by one of nature’s very own optical illusions.
It’s fair to say at this point that the refractive properties (ability of the gas in the atmosphere to bend light) doslightly change the shape of the moon but the effects aren’t huge and it’s not this that is responsible for the “Moon Illusion.”
In fact, you can prove yourself that it’s just an illusion: take a long stick plus a small coin and when the moon is low, hold the piece of wood and attach the coin to it so the coin just covers the moon in the sky. Then wait until the moon is a lot higher later that night and check. The moon’s size hasn’t changed at all.
Lots of people have asked me over the years why the Moon is bigger when it’s nearer the horizon than when it’s high up in the sky. The answer is really quite simple and it lies in the way that the eye/brain combination interprets distance clues.
For example, take a look at this picture:
As you look at it, your brain interprets the path and brick wall heading off into the distance. Now turn your attention to the elephants. The one at the back looks bigger than the one at the front, right? Wrong, they are all identical.
The actual image that forms on the back of your eye shows each elephant is the same size. That is until your brain gets in on the act. Because you perceive the path heading off into the distance, you also assume the elephants are at different distances.
However, because the image in your eye of each elephant is identical, your brain decides that the elephant at the ‘back’ must be bigger than the elephant at the ‘front’ to allow it to form the same sized image. This illusion is named after its Italian discoverer Mario Ponzo who first demonstrated it in 1913.
Ponzo Illusion: 1 – You: 0.
So how does this apply to the moon? There are plenty of clues to distance on the horizon, other than the fact that we know the horizon is a long way away. Even clouds or flocks of birds are typically further away when near the horizon so we assume anything near the horizon is at a great distance, even the moon is perceived to be further away than when higher in the sky (due to the lack of distance clues).
Just like the elephants in the picture, the image of the moon in our eyes are identical regardless of whether it is high up or low down but the brain does the same thing again. It assumes that if both images of the moon are the same size because one is down by the horizon and thus further away, it must actually be a lot bigger to form the same size image! So our brains interpret the moon as looking bigger.
Apparently if you turn with your back to the moon bend forward and look at it through your legs, because you see everything upside down and different to usual, your distance clues are all gone and the moon looks its normal size. I’ve never tried that myself though, so I’ll leave that up to you.
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