When the moon hits your eye, it could be an illusion

Jonas Bahrmasel, 4, a preschooler at Schaumburg's Elizabeth Blackwell Elementary School, asked, "Why does it seem like the moon is following us?"

Sometimes it does seem like the moon is following us. And have you ever noticed that sometimes the moon looks gigantic - like a huge ball that practically takes up the whole sky? Other times, as it hangs in space, it's merely as big as your thumb.

The idea of the moon following us has to do with distance and size.

"Imagine driving down a highway. Objects that are close to you, like streetlights, appear to rush by," said Geza Gyuk, director of the Adler Planetarium in Chicago. "Objects slightly farther away, say a tree off in a field, change their position more slowly. The farther away an object is the less it appears to move due to your motion."

A streetlight is about 10 feet tall, and a tree might be 30 feet tall. But the moon is huge - about a quarter of the Earth's size at 2,160 miles across. And it's far away - about 250,000 miles. The size and the distance make it seem like the moon just doesn't budge.

"No matter how much one moves on the Earth, the position of the moon in your field of view isn't going to change appreciably. This is why, as one walks or drives along, the moon appears to follow you even as the much closer trees and houses and mountains drift back," Gyuk said.

Sometimes, when the moon rises, it looks absolutely humongous. Later in the evening that same moon seems to go back to its original size. Even astronauts and pilots notice this when they travel in space and fly through the earth's atmosphere.

This is called the moon illusion, and it has to do with vision. Your eyes don't actually see. Your brain interprets light signals and creates a picture. Your brain can play tricks on you. Scientists believe that the brain "sees" images that are visible on the horizon - like a huge, rising moon - as seeming bigger than they actually are. But once that moon ascends to a spot higher in the sky, the brain sees it as it really is.