The Moon Illusion

Most of us agree that the Moon can appear to be bigger when it is low on the horizon, and much smaller when it is high in the sky. This difference in apparent size is no small thing; about one and a half times bigger is pretty extreme! This is the Moon Illusion, and scientists have been trying to explain it for centuries.

The Moon's distance from Earth is fairly constant; an average distance of about 385,000 km. So we cannot explain the Moon Illusion by blaming huge distance changes in its orbit. The size of the Moon is also constant; about 3,476 km, so we should see the Moon the same no matter where it happens to be.

Some have tried to blame the illusion on the Earth's atmosphere. They argue that when the Moon is low on the horizon we are looking through more of the atmosphere, and a thicker atmosphere acts like a lens which magnifies the Moon. "Sorry Charlie!" say atmospheric scientists. There just isn't enough atmosphere to cause such an effect. It is interesting (and a bit scary) to note that the height of Earth's breathable atmosphere over a standard-sized classroom globe is only about 1/4-inch!

The Moon illusion is still being hotly debated in the scientific world, and there are undoubtedly many more explanations than are outlined here.

The illusion may be related to Tichener's size comparison illusion. We cannot help but compare objects with other nearby objects. When the Moon is near the horizon we see it with familiar nearby terrestrial objects such as houses, trees, mountains, and hills. We are so familiar with the relative sizes of these objects that we cannot help judging them against each other, and also against the apparent size of the Moon. It looks bigger! When the Moon is high in the sky it is less influenced by the Tichener effect, and thus appears closer to its true size.

I have included an Interactive Demonstration which demonstrates the apparent difference in sizes due to the Moon Illusion. Please note that this is not the actual moon illusion. However it is an accurate representation of the visual effect. The minimum and maximum moon sizes in the demonstration are based on the apparent size differences (± 1.5) as reported by most observers. I believe this is what the Moon might look like to an observer on the ground witnessing the illusion.

Read More About It. Visit to Donald E. Simanek's web page, The Moon Illusion, An Unsolved Mystery. Also check out this Science News article, The Moon Moon Also Rises—And Assumes New Sizes. And finally here is another online article, Explaining the moon illusion.

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