The moon illusion is one of the most commonly experienced illusions in real life. Have you ever noticed that the full moon when viewed just above the horizon, appears to be much larger than when it is viewed overhead? Well, the moon does not get bigger & smaller, it always remains the same size. It is only our Perception of it's size that changes.
This is because we "know" that things on the horizon are very far away, but we have little or no reference points for comparison when we look straight overhead. For instance an airplane on the horizon would appear as just a tiny speck, but as it passes overhead the image of the plane gets larger. It would seem that so much of our world is interpreted this way that we are not equipped to cope with an object like the moon which stays the same size whatever position it occupies in the sky. And so our brain "interprets" the image that it "sees", and tells us that the moon is larger than it really is.
It is much the same phenomenon that fools our brain when we view the "Ames Room".
The next time there is a full moon you might want to try this experiment: Hold a piece of paper with a small hole in it up to one eye and look at the moon on the horizon with the other eye closed. The moon will not look as big. Then open your other eye. You will see two moons of different sizes!!!!
Another way to demonstrate the principle can be done right now on your computer. Click the link below to see an image of a circle. Stare at the dot in the center for 30-60 seconds, then look at the white space below the picture, then look away at a distant wall and the "moon" will appear to be much larger. y
