The Moon Illusion Explained: The Optical Illusion You Can See Every Day

The full moon in October is approaching and one of the best times to view it is while the moon is rising or setting because it appears larger in the night sky at these times. But why does this phenomenon, known as the "moon illusion," occur?

The moon illusion is a psychological trick of our brains that makes the moon seem larger when it is close to the horizon and smaller when it's higher in the sky, according to Susanna Kohler, a spokesperson for the American Astronomical Society.

"Most of us have had the experience of seeing a giant moon sitting low on the horizon one night—but this seemingly large size isn't real," Kohler told Newsweek. "In reality, the moon's apparent size doesn't change as it rises."

You can test this by holding one finger out at arm's length in front of the moon. You should notice that your fingernail will just cover the width of the full moon, whether the moon is close to the horizon or high up in the sky.

There is also another way to test this using a camera, Gianluca Masi, an astronomer with the Virtual Telescope Project, told Newsweek.

"This effect has been tested many times, taking images of the rising or setting full
moon—when our satellite looks 'big' behind trees or buildings—and comparing
them with images taken a few hours later or earlier, with the same full moon
overhead," Masi said.

"The lunar disk is the same size in both images, provided of course that the same imaging setup was used. This clearly shows it is an 'illusion.' Needless to say, the fascinating moon illusion has captured our imagination since antiquity and several attempts have been made to explain it."

The full Harvest Moon above New York
The full Harvest Moon sets behind the Statue of Liberty as the sun rises on September 10, 2022, in New York City. The moon illusion is a psychological trick of our brains that makes the moon seem larger when it is close to the horizon and smaller when it’s higher in the sky. Gary Hershorn/Getty Images

Surprisingly, it is still not entirely clear why this illusion occurs, although it is likely something to do with the shift in perspective, according to Kohler.

"One factor may be that when we view the moon next to small, foreground objects like buildings or trees, it tricks us into thinking the moon looks bigger than when it's overhead and surrounded by only empty expanses of sky," Kohler said.

"Another possibility is that our brains expect distant objects near the horizon—which lie behind and beyond foreground objects—to be farther away than those overhead. This can cause us to inflate how we perceive the size of the horizon object. There are other potential explanations as well. But the illusion definitely lies in our minds."

You may also have noticed that the moon appears to have a reddish hue near the horizon. But unlike the moon illusion, this is not a trick of the brain and instead can be explained by the way that light passes through the Earth's atmosphere.

"When the moon lies low in the sky instead of overhead, light from it must travel a greater distance through Earth's atmosphere to reach us," Kohler said. "On that journey, the shorter, bluer wavelengths of light are scattered away, leaving mostly longer, redder wavelengths of light that reach us."

"More dust or pollution in the atmosphere can increase this effect, making the moon look even redder as it's rising or setting against the horizon."

The next full moon, known as the Hunter's Moon, will occur on October 9, reaching peak illumination at around 4:54 p.m. ET. At this time it will be below the horizon in North America, so skygazers will have to wait until after sunset for it to appear.

Full moons are a lunar phase that occurs roughly once every month when the moon is located opposite the sun in space, with the Earth in between. As a result, every full moon rises around the time of sunset and sets around the time of sunrise.