Moon above Seattle. Image credit: Shay Stephens
Have you ever noticed the the Moon seems to change in size. When the Moon is low on the horizon, shortly after it rises, it looks enormous and often red. And then later on in the evening, when the Moon has moved much higher in the sky, it looks much smaller. What’s going on here? Is the Moon really changing in size? No, the Moon has remained the same size the whole time, it’s just your brain that’s getting fooled – this is known as the Moon illusion.
The does change in size, but only a little bit. That’s because the Moon’s orbit around the Earth is elliptical. At its most distant point, the Moon is about 12% smaller than when it’s at its closest point. This is because it’s about 50,000 km further away. But that change in size and distance happens over the course of the Moon’s 27-day orbit around the Earth.
It’s easy to see that this is an illusion. When the Moon is low on the horizon, think about its size for a second. Then, put your hand out and cover up the Moon with your smallest finger. The top joint on your pinky finger should be just large enough to cover up the Moon. Then, later on in the night, when the Moon is overhead, do the same experiment. You should find that the Moon is neatly covered up by the small joint of your pinky finger.
So why does the Moon illusion happen? Scientists think that the human brain sees the Moon as larger when it’s near to the horizon, and close to things we know the size of, like buildings and trees. When the Moon is overhead, we see it surrounded by large amounts of dark sky, so it’s hard to see how big it is in comparison to other objects. Even thought the Moon is the same size it always was, our brains think it looks much bigger and larger when it’s on the horizon.
Here’s an article we did on Universe Today about the Moon illusion.
Want to learn more about the Moon illusion? Here’s a great article from Science@NASA, and a very interesting investigation into the topic..
You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?
References:
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/961201a.html
http://science.nasa.gov/science-news/science-at-nasa/2008/16jun_moonillusion/
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