Special Edition of the Spaced Out Challenge: The "Supermoon" Illusion

A bit of media hype has been made over the so-called "supermoon" visible tomorrow night. While it is the largest in apparent size for the year, it will only be about 6 or 7% larger than the average full moon. Still, it makes for a great opportunity to get out there and enjoy our natural satellite.

Amplifying the hype over the supermoon is it's apparent embiggination when rising from the eastern horizon shortly after sunset. Sky and Telescope's excellent supermoon article explains this odd phenomena, known as the Moon Illusion, with two possible explanations:

The so-called Moon Illusion causes the Moon to appear bigger when viewed along the horizon and can fool people into believing the supermoon myth.

Nobody quite understands why the Moon illusion happens, but here are two of the theories psychologists think might explain this effect.

One theory claims that our brain perceives the distance to the sky directly overhead as closer than the distance to the horizon. We think the Moon must be bigger along the horizon because we perceive the distance to the horizon as farther away. Don't buy the theory? Look at the picture [below]. The top line seems larger than the bottom one. But measure for yourself and, surprisingly, you'll find the lines are the same size.

Charles Stangor's Introduction to Psychology
Psychologists have also suggested that an overhead Moon looks smaller because it’s surrounded by empty space, while a Moon near the horizon appears larger juxtaposed against trees and buildings. The picture [below] shows two orange circles of identical size. But the orange circle on the right appears larger than the one at the left because of its relative size to the surrounding circles.

from New World Encyclopedia
So the supermoon might look bigger than normal if you see it in the evening when the Moon's just rising, but the real size difference isn't big enough to notice.

If you are going to whip out the scope tomorrow night to enjoy the fully illuminated moon, be sure to take a polarizing/moon filter with you, as the reflected sunlight amplified in a telescope could be painful to look at. As always, clear skies to you and keep looking up! The regular astronomy post will appear Sunday, continuing our exploration of the Summer Milky Way.

posted by CAC at 08:08 PM

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