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.

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.”

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!