Attack of the Gigantic Moons! Sort of”¦

On Saturday, people on earth were able to see a full supermoon, a moon so large even werewolves have got to wear shades. For those who missed the chance of seeing a gigantic glowing ball in the night sky, never despair: another supermoon will be coming around in 2013. If that is too far in the future for comfort, there is another way to see a gigantic moon and it requires an optical illusion.

Comparison of regular moon and supermoon. Photo from wikicommons, created by marcoaliaslama.

A supermoon is not an optical illusion. It happens when a full moon occurs when the moon is closest to the earth in its orbit. When an object  is closest to the earth, it is called “perigee.” When three celestial bodies are all in a straight line, it is called “syzygy.” This means that a supermoon’s technical name is perigee-syzygy because it occurs when the moon is closest to the earth and the sun, moon, and earth are all aligned. Sounds like the basis for some sort of creepy thriller/horror movie, right? With monsters and stuff, yeah?

There are folktales and urban legends that suggest that horrible things happen under the watchful gaze of the supermoon. Absolutely no scientific evidence supports this idea, but it tickles my fancy to think that supermoons create super-werewolves that fight their animal urges so instead of attacking innocent people, they attack  innocent double bacon cheeseburgers.

If my version of reality was actually reality, it would be a huge shame to miss a supermoon. Fortunately, there is another way of experiencing moon-magic: watch the moon when it is close to the horizon. For most people, a moon hanging out on the horizon should look a lot bigger than a moon hanging out at the top of the sky. There is absolutely nothing that makes the moon actually larger at the horizon than at the top of the sky, so this phenomenon is called the “moon illusion.”

Example of the Ponzo Illusion. Photo from wikicommons, created by NASA.

The moon illusion is not actually fully understood, but evidence suggests that it is at least in part due to the “Ponzo illusion.” The Ponzo illusion was first described by the Italian researcher Mario Ponzo. He suggested that we use cues from an object’s background to determine its size. In the picture to the right, the yellow lines are the same size, but the one in the background looks substantially bigger.

Something similar happens with the moon based on how we see the horizon and the rest of the sky. See, in general, people see the sky as sort of a flattened dome, with the horizon being further out than the top of the sky. Imagine, for example, a cloudy day. For many, those clouds seem closer than the horizon. So when the moon gets thrown in at the horizon, it appears larger based on the Ponzo illusion and our own view of a flattened dome sky. It seems further away and thus bigger than a moon hanging out overhead.

So if you’re bummed about missing the supermoon, get an approximation by going outside and staring intently at a horizon-hovering moon. Go, go! Before it is too late!

8 replies on “Attack of the Gigantic Moons! Sort of”¦”

Thank you for the explanation! I didn’t quite catch the moon at its fullest point on the horizon on Saturday because of clouds RIGHT where the moon was rising. (Clear sky everywhere except in front of the moon. I was so mad!) What I did get to see was still absolutely breathtaking. Pictures didn’t do it justice at all.

I have known how to spell and pronounce “syzygy” since I was 10 years old, but I never knew what it meant until now. Unfortunately, my non-science-minded self thought the supermoon was something else entirely, and was sorely disappointed when it didn’t result in superpowers for me.

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