Why does the Moon look big on the horizon? (Intermediate)

Please answer this question--we're arguing about it at work. Why does the moon appear to be different sizes at different times? Sometimes when the moon is rising, it seems huge, and then seems to get smaller as it gets higher in the sky. Is this an illusion because of the moon's orbit, the gasses in the atmosphere and the curvature of the earth magnifying the moon, or is it all relative to having a comparison/perspective as it rises--such as having the mountains/trees to compare it's size?? Or is it something entirely different? We'll be anxiously waiting for your explanation..............

No, it's not caused by either the orbit of the moon and the atmosphere of the Earth, or by comparison to other objects as the moon rises, although these are both commonly given as reasons.

The Moon-Horizon Illusion is a cognitive illusion, meaning that it is an illusion due to the processing of information in our brains. No one really knows why the brain always interprets the Moon this way, but one thing is for sure, though: it is not due to comparison with other objects. Prove this to yourself in two different ways: first, by looking at the Moon on the horizon while standing on your head: it looks normal-sized! Second, look at the Moon rising over the ocean where there are no objects for comparison. It still looks big.

So it's something entirely different. It's a psychological effect that we still do not fully understand.

May 2002 addition by Karen: Take a look at The Bad Astronomy Page on the Moon Illusion. I encourage you to follow the link to this page which offers a very thorough discussion of why the Moon and Sun appear bigger on the horizon.

This page was last updated on July 18, 2015.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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