The Moon seems larger when it is near the horizon than when it is high in the sky, a phenomenon called the Moon illusion. Although recognized for centuries—the horizon Moon was important to early civilizations that functioned according to the Moon’s cycle—this ancient phenomenon has only recently been explained.

Early astronomers believed the Moon at the horizon was physically closer to Earth than when it was high in the sky, and the closeness meant a larger Moon. However, Newton’s description of the Moon’s orbit showed the contrary to be true. The Moon is actually closest to the observer at its zenith, or when it is high in the sky, but the difference is so small that it is negligible anyhow. Others theorized that the Moon illusion was caused by refraction when light rays passed through more of Earth’s atmosphere. Today, scientists guess that the illusion occurs not externally, but through a trick of our brains.

Optical illusions play a big role in the appearance of the Moon. When the Moon is placed as a backdrop against objects of known heights—such as trees, cars, or buildings—it appears larger than when it is isolated in the sky. In one experiment, researchers asked participants to view the horizon Moon through a cardboard tube, which caused background objects to disappear. They found the Moon seemed to shrink to a size similar to the zenith Moon.

The Ebbinghaus illusion describes this perceived effect. Two circles of identical size are placed near each other. One circle is surrounded by smaller circles, and the other circle is surrounded by larger circles. Although we know the original circles are identical, we perceive the circle surrounded by smaller circles as larger than the neighboring circle surrounded by larger circles. We also view the Moon as we do other objects, like clouds and birds, that recede into the skyline. We expect them to look smaller as they get farther away. In what is known as the Ponzo illusion, our brain tricks us into thinking the Moon is getting smaller as it rises in the sky, and in our minds only, farther away from Earth. But the Moon is still mostly the same distance away in its orbit. Nothing has changed in its size or its distance from our planet. To practice this, draw two identical parallel lines horizontally across a photo of a receding railroad track. The line closest will appear smaller than the line farther away, because as the tracks recede into the horizon, they become smaller, and your brain expects the line to do the same.

Still not convinced? Try taking a picture of the large Moon at the horizon. The camera doesn’t suffer from the same visual cues that make the Moon appear as massive as in real life. This illusion is not unique to the Moon—the Sun and stars show the same properties. And while interesting to consider, the Moon illusion offers little insight into astronomy and the atmosphere. Instead, it proves an example of optical illusion.