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.
By the way, a large moon seen low in the sky might also appear red or orange in color. And 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.