We’ve all seen a full moon looming large shortly after it rises, when it’s still hugging the horizon. Scientists say that large moon is an illusion, a trick your brain is playing. It’s called the moon illusion. Its causes aren’t precisely known, but the video above, from AsapSCIENCE, offers some explanation. The main one is that, when the moon is near the4 horizon, you’re gazing at it in the company of many familiar visual reference points: trees, buildings, mountains and so on. Your brain automatically compares the moon to these reference points. But when the moon is higher up, there’s nothing to compare it to, and, as ASAPScience says:
The moon seems smaller against the vastness of the night sky.
By the way, there’s a second phenomenon that the moon is subject to when it’s seen near the horizon. That is, a low moon often appears red or orange in color. That reddish color is not an illusion. It’s a true physical effect, caused by the fact that – when the moon is low in the sky – you’re seeing it through a greater thickness of Earth’s atmosphere than when it’s overhead. The atmosphere filters out the bluer wavelengths of white moonlight (which is really reflected sunlight). Meanwhile, it allows the red component of moonlight to travel straight through to your eyes. So a low moon is likely to look red or orange to you.
How do people get those photos of extra big moons seen near a horizon? They’re the result of photographic tricks and techniques, which you can read about here.
Bottom line: A full moon, in particular, might look big seen near a horizon. But all full moons seen near a horizon look big, due to a psychological effect called the moon illusion.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.