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) do slightly 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.