The moon is always the same size. It seems like an easy enough concept to grasp, but it doesn’t explain why the moon doesn’t always look like it’s the same size. I’m sure you’ve all seen it. You go out early on the night of a full moon, and the moon looks HUGE. In high school, I remember watching the Full Moon rise during a football game. I watched as the huge orange (yes, orange) circle rose slowly from the eastern horizon, brightly illuminated behind a telephone pole. By the time the game ended, the moon was quite a bit higher in the sky and appeared quite a bit smaller. I’ve had a lot of trouble grasping this “Moon Illusion,” but an article that Dr. G sent me recently (found here) has helped clear up the confusion a bit.
From Susquehanna University in Pennsylvania, Joseph Antonides and Toshiro Kubota have proposed a new theory to explain the moon illusion. According to Antonides and Kubota, the illusion is the brain’s way of dealing with a discrepancy in our binocular vision and our perceptual model of the world. Our two eyes give our brain an image of a distant moon. Our perceptual model of the world complicates this picture when the moon is near the horizon. Near the horizon, trees and telephone poles appear relatively close, so when the moon is poised behind them, our perceptual model tells us that the moon is also close. High in the sky, with no trees or skyscrapers in the way, the moon appears as it is–distant, both from our perceptual model and our binocular vision. Only near the horizon do the two conflict, and that is when we see a “larger” moon.
Seeing is believing, so I’ll need to test this out for myself before I’ll completely come to terms with the fact that the beautiful huge, orange moon on the horizon is exactly the same size as the beautiful small, white moon high in the sky. It seems you can test this out by viewing the moon upside down. Some night soon, you may find me outside, standing on my head, remarking “It really is the same size, after all!”
The Solar System 