Have you noticed that when the full moon is close to the horizon that it appears larger than when it is high in the sky. This is known as "The Moon Illusion", I have heard of this illusion before but I hadn't thought about it in a while, that is until the last full moon.
I was driving up to my house early and I was absolutely amazed at how large the moon looked rising next to my house. I know that it wasn't any larger than normal but on this night the illusion was captivating.
Below is an excellent photo depicting this illusion.
Why does this happen?
Moonrise over the Lincoln Memorial
Richard Nowitz/National Geographic
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Before we talk about why this happens, I need to establish with you the fact that the moon is no larger on the horizon than it is high in the sky. If you actually measure the size of the moon at different points across the night sky you will find that there is no appreciable difference in the size of the moon through its path.
Take the below photo (Astronomy Picture of the Day) as evidence that the moon appears the same size no matter where it is in the sky.
Moonrise Over Seattle
Credit & Copyright: Shay Stephens
So why is this illusion so compelling? I think NASA sums up the current theories nicely when they say "After all these years, scientists still aren't sure why." Below is a list of sites that explain or debunk the various theories. I'll let you decide who might be right. I just plan on enjoying the view when I see the moon low on the horizon and leave the theorizing to others.
The Moon Illusion Explained -- According to Don McCready, a Professor Emeritus of Psychology at the University of Wisconsin, the Moon Illusion is caused by oculomotor micropsia/macropsia.
Summer Moon Illusion -- The lowest-hanging full moon in 18 years is going to play tricks on you this week.
The Moon Illusion: An Unsolved Mystery -- a nice overview of the Moon Illusion and its possible causes.
New Thoughts on Understanding the Moon Illusion -- from Carl J. Wenning, Physics Department, Illinois State University
Experiment in Perception: The Ponzo Illusion and the Moon Explaining the Moon Illusion -- from the Proceedings of the National Academy of Sciences