Have you ever seen the moon when it looks closer to the Earth than usual, appearing gigantic just over the horizon? If so, you've witnessed the moon illusion. The fact is, it's the same size and distance from the Earth as always, but our minds perceive it to be closer and larger than usual. One theory about moon illusion is that it's due to a phenomenon called size constancy: Your mind sees clues about the distance of objects and associates sizes to them accordingly. Sometimes, images lack visual clues about their distances, causing us to perceive objects as bigger or smaller. Size constancy is always happening. If you look down the street and see a compact car, it can look bigger than a truck much farther away, but you still know the truck is bigger. It's possible that when the moon is along the horizon, our minds perceive it to be much closer to us than when it appears high in the sky. Size constancy causes our minds to compensate for this perceived distance, making the moon seem bigger.
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