The Exploratorium: seeing | looking at the moon/3

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Why does the moon seem to change size as it travels through the sky?
To most people, the moon looks substantially bigger when it is close to the horizon than when it is overhead. Of course, the moon itself doesn’t grow or shrink as it moves through the sky. But it may come as a surprise that the image of the moon (its measurement on a photograph) also doesn’t change in size.

In fact, the apparent change in size happens in your brain. While people have argued for centuries about exactly why this occurs, it seems to have to do with our expectations about the size and distance of things in the sky. Most things that pass over us—birds, clouds, and planes, for instance—are quite a bit closer when over our heads, so their image is larger; conversely, objects on the horizon are farther away, and therefore project a smaller image. But the size of the moon’s image stays virtually the same whether it is overhead or on the horizon, your brain, expecting it to be larger when overhead, sees it as relatively smaller. You expect a smaller image on the horizon, and therefore the moon looks bigger.

Things to do:
When a full moon has just risen (in the early evening), step outside with a clear piece of plastic or glass and a grease pencil. Hold it at arm’s length in front of the moon and mark its outline with the pencil. Wait a few hours and then go out and view the moon again. Though it will probably look smaller to you, when you hold up the marked piece of plastic the same distance from your eye, the moon should fit perfectly within the boundary you marked earlier. next

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