One of my favorite illusions involves the moon. It’s common knowledge that the moon looks huge when close to the horizon, but appears smaller when higher in the sky. Many theories have attempted to explain this phenomenon. (Feel free to read about them on Wikipedia. However, the article wasn’t written in layman’s terms and can be confusing.)
The moon is actually the same size regardless of where it is in the sky. To prove this just take a picture of it at the horizon and at other positions and compare the size in the pictures. It is not an illusion caused by the atmosphere, as commonly believed. The illusion occurs at the brain level and deals with the brain’s inability to judge large distance differences.
To understand this you have to realize that the brain manipulates image sizes in an attempt to match up the image with the relative distance from the viewer. Here’s an example. Have someone lay down on their back on the floor. Take a picture of them by aiming the camera so that their feet are in the foreground and their head is in the background. Notice how small the person’s head is in the picture. Now assume the same position you were in to take the picture, but this time without the camera. Notice that the person’s head doesn’t look as small.
Your brain realizes that their head isn’t that far away from their feet, so it “enlarges” the head to match the perceived distance. The camera, on the other hand, doesn’t realize the head is only about 5 feet away from their feet and fails to match up the head size with the short distance. Now imagine the horizon is the “feet” and the moon is the “head”. Your brain doesn’t realize that the moon is actually 200,000 miles away. It sees it as being just beyond the horizon. Therefore, it “enlarges” the image of the moon just like it did with the person’s head.