When you are watching the full "blue" moon rising over the eastern horizon Friday at 8:30 p.m., you will experience an illusion that sky watchers have experienced since ancient times. This celestial phenomenon is known as the "moon illusion" and is also true for the sun and constellations whenever they hover near the horizon.
Stargazers have known for thousands of years that a low-hanging moon or sun peeking through trees and houses or over mountain ridges looks unnaturally big and inflated. At first, astronomers thought the atmosphere must be magnifying the moon near the horizon, but cameras showed that is not the case. Photographs of the moon are the same size regardless of elevation. The moon is really the same size (0.5 degrees wide) no matter where it appears in the sky.
Researchers still aren't sure what causes this horizon illusion with the moon and sun. 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-size spot, yet the brain insists one is bigger than the other.
One popular explanation for the illusion is that when the moon is rising or setting, distant trees and houses might make the moon seem bigger than it is. However there's a problem with this theory. Airline pilots flying at very high altitudes sometimes experience the moon illusion without any objects in the foreground.
Scientists believe that the moon-horizon illusion is a cognitive illusion, meaning that it is an illusion due to the processing of information in our brains. No one really knows why the brain always interprets the moon this way, but one thing is for sure: It is not due to comparison with other objects. It's something entirely different. It's a psychological effect that we still do not fully understand.