Have you ever been riding in a car in the evening and noticed something huge and yellow behind the trees and buildings in the east--and then realized it was the Moon? Especially in the fall, a pumpkin-colored harvest moon, looming up over the horizon, looks enormous and even spooky. It's not hard to imagine a broomstick-riding witch flying across.

But picture this Halloween scene when the moon is high in the sky, small and white, and it's just not the same.

Scientists say that the horizon Moon appears up to twice as big as the overhead Moon to most of us. People have been arguing over why for more than a thousand years. Astronomers, psychologists, and nonscientists all have their theories. In 1989, researchers even published a book of such explanations, called "The Moon Illusion."

You can prove to yourself that the Moon is actually the same size no matter where it is in the sky, using a key or a ruler. Note the Moon's width at the horizon, and later, compare it to the Moon's width overhead.

You may even make the size illusion vanish. Some suggest bending over and looking at the horizon moon upside down, in a kind of lunar yoga. Others recommend looking at the moon through a cardboard tube that blocks out landscape features.

What causes the illusion? In the past, some textbooks stated that the Moon appears larger at the horizon because dense air near the ground refracts (bends) moonlight, causing a magnifying-glass effect. This theory, scientists now say, is not really a contender: While there is refraction, it doesn't magnify the Moon's image, and would actually tend to make the Moon appear squashed.

Most agree that the illusion is a matter of perception--a trick of the brain. Some argue that the horizon Moon looks bigger because it is framed by smaller objects like trees, houses and hills, making it huge by comparison. However, that doesn't explain why the moon looks so big rising over the flat expanse of the ocean. (And it also doesn't explain why the pretend moon in a planetarium appears to be the same size at the horizon AND overhead.)

Several complicated theories involving the brain's visual system also try to explain the moon paradox. Here's a simplified version of one popular explanation: The brain perceives the sky (and Moon) above us as closer than the sky (and Moon) at the horizon. When an object is perceived to be nearer, the brain may compensate by making it look smaller to us. Likewise, an object thought to be farther away will be seen as larger.

(To see how perceived distance makes an object look bigger or smaller, visit the website www.howstuffworks.com/question491.htm.)

For now, The Moon Illusion remains one of nature's loveliest unsolved mysteries.