The Moon Illusion, Phoenix, AZ. ©Erin Matlock
Last night I headed out for a hike around sunset to catch The Moon Illusion. This phenomenon occurs once or twice a year when the moon is at its closest point to us – and is full.
Doing a search on the web turns up quite a few different explanations for the illusion – and a great deal of debate. There is one agreement, though. It is a fantastic sight! I love optical illusions!
The following is background information provided by Nasa with links at the conclusion for further resources. I’ve also included some photos I took of the event.
Sky watchers have known for thousands of years that low-hanging moons look unnaturally big. At first, astronomers thought the atmosphere must be magnifying the Moon near the horizon, but cameras showed that is not the case. Moons on film are the same size regardless of elevation: example. Apparently, only human beings see giant moons.
The moon just above the horizon. ©Erin Matlock
Are we crazy?
After all these years, scientists still aren’t sure. When you look at the Moon, rays of moonlight converge and form an image about 0.15 mm wide on the retina in the back of your eye. High moons and low moons make the same sized spot, yet the brain insists one is bigger than the other. Go figure.
A similar illusion was discovered in 1913 by Mario Ponzo, who drew two identical bars across a pair of converging lines, like the railroad tracks pictured right. The upper yellow bar looks wider because it spans a greater apparent distance between the rails. This is the “Ponzo Illusion.”
The Ponzo Illusion. Image credit: Dr. Tony Phillips/NASA
Some researchers believe that the Moon Illusion is Ponzo’s Illusion, with trees and houses playing the role of Ponzo’s converging lines. Foreground objects trick your brain into thinking the Moon is bigger than it really is.
But there’s a problem: Airline pilots flying at very high altitudes sometimes experience the Moon Illusion without any objects in the foreground. What tricks their eyes?
Maybe it’s the shape of the sky. Humans perceive the sky as a flattened dome, with the zenith nearby and the horizon far away. It makes sense; birds flying overhead are closer than birds on the horizon. When the moon is near the horizon, your brain, trained by watching birds (and clouds and airplanes), miscalculates the Moon’s true distance and size.
Author: Dr. Tony Phillips | Credit: Science@NASA
Sunset on the other side of the mountain. ©Erin Matlock