Super Moon, or just a super Illusion?
Some evenings I step outside and I just can’t stop myself from admiring the moon. It seems huge, even for a full moon. As if it had come a little bit closer to us. But has it? Or what else makes our moon look bigger in the sky?
Every so often the media tells us to look out for a Supermoon. But what exactly is that? Astrologer Richard Nolle defined a Supermoon as a Full or New Moon that is less than 359,000 km away from Earth [1].
The average distance from Earth to the Moon is around 384.400 km. But the orbit that our Moon takes around the Earth each month is not a perfect circle. The Moons distance actually varies from around 357.000 km to 407.000 km. The point where the Moon is closest to Earth is called its Perigee, and where it is farthest away its Apogee.
So, if there is a Full (or New) Moon on a night where the Moon is close to its closest to earth, this is called a Supermoon. On those nights the Moon is seen bigger than its average by about 6%. That is not a lot!
If I stay outside watching the moon rise up higher and higher in the sky, at some point I will notice that it is not so big anymore. It seems to have shrunk to its usual size within hours. Apparently it was never actually bigger or closer. What may have caused the Moon to appear bigger as long as it was near the horizon? Could it be, that the Moons light is being refracted by the Earths atmosphere? Indeed it is! [2]
When you are looking at the Moon close to the horizon, then you are also looking through a lot of air. Much more than when you look at it high up in the sky. And the more air there is, the more the light is getting refracted. But this effect doesn’t make the Moon look bigger! Due to the way the light is refracted in the atmosphere the Moon is getting a squished look - flat at the top and bottom, and broader around the middle. It’s the same as looking at a spoon through a glass of water.
On the right: a spoon next to a glass of water. On the left: the same spoon at the back of the glass, its image refracted by the water.
How strong we can see this effect depends on many factors, like the pressure and temperature of the air that we are looking through. It does not look the same everywhere, and it is not what we usually see when we are admiring a huge full moon rising. What we see is in fact an illusion. [3]
There are many possible explanations for the Moon Illusion, but no one knows for sure what is causing it. One theory suggest, that it is the proximity to the things on earth, like houses and trees, that makes our brains think that the Moon is so much closer and bigger. It seems smaller again, when there is just empty space all around it. Speaking against this theory is the fact, that the Moon Illusion is also seen by people on air planes, who do not have houses or trees close by to make a comparison.
Another theory assumes, that we perceive the horizon to be further away than things over our heads. So if the Moon is further away, but it’s relative size stays the same, then it must be bigger!
If you want to check for yourself that the Moon Illusion is just an illusion, you can try the following: Look at the Moon directly at first. Then look at it again through some kind of narrow opening. For example, a cardboard tube, or you can just pinch it between your thumb and forefinger. The illusion will probably go away.
Sadly, another way to make the illusion disappear is by taking a picture of it. You may be wondering now, why to Moon is so big in those photos of mine. The answer is simple and not very spectacular. I used photo blending to make the Moon look as big as I had perceived it to be.
The upper two pictures are the photos that I actually took. And this one is the end product after blending them together to make the Moon look as big and bright as I had seen it. Or maybe a little bit bigger ; )
