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Why does the moon look close some nights and far away on other nights? – Gabriel H., age 7, Providence, Rhode Island
Some nights the Moon seems really close and bigger than usual.
One summer evening when I was a child, I remember being mystified and then startled at a huge round shape slowly creeping up behind my friend Nancy’s house, which sat on a hill on the other side of our village.
At some point I suddenly realized it was the Moon, and I ran yelling through the garden to tell my dad and get him to come and see. It was bigger than a house, deep orange in color and surely of great significance. My dad muttered something about perspective and went back to gardening or playing the piano.
Unconvinced, I kept watching the Moon. Later, once the Moon had risen higher in the sky, it was back to looking like its usual self.
Welcome to what astronomers like me call the Moon illusion.
It can be hard to believe that it is just an illusion when the Moon looks huge, but it is true. You can actually test the illusion yourself and even capture it with a camera.
The two Moons in this edited image are the same size, but the one near the horizon on the right side looks bigger because of the Moon illusion.
Heeheemalu/Wikimedia Commons, CC BY-SA
A trick of the mind
Astronomers have discussed the Moon illusion for centuries, and there are some facts they all agree on.
People mainly notice the Moon looking bigger and closer when it is full and near the horizon. This is because your mind judges how big or small an object like the Moon is by comparing it with other, familiar things.
Imagine you are standing outside close to your house. Your house will look big, and if the Moon rises next to it, the Moon will look normal. If you look at a house from far away, though, the house looks very small.
The illusion comes from the fact that the Moon is so far away that no matter where you are on Earth, the Moon always looks the same size. It is actually the things your mind compares the Moon with – a house, a mountain or anything else – that look bigger or smaller depending on how far away from them you are. So when the Moon rises next to a distant house or a faraway mountain, the Moon looks enormous.
The two orange circles in the center of the gray circles are the same size, but they look different because of the different sizes of the circles surrounding them.
Phrood/Wikimedia Commons
Photographers use this trick to take spectacular images of distant objects with the Moon behind them. People often experience the Moon illusion on vacations when they go to wide-open spaces. This may be why big Moons become powerful memories of happy times.
