Science Puzzle
Why Does the Moon Look Bigger Near the Horizon?
A full Moon rising just above the rooftops can look enormous, glowing and golden, almost close enough to touch. A few hours later, high in the sky, the same Moon looks much smaller.
Why does the Moon look so much bigger when it's near the horizon?
The Answer
It's an illusion. Measure the Moon low and high, and it's the same size both times.
Hold your arm out straight and cover the rising Moon with the nail of your little finger. Try again when the Moon is high in the sky: the same nail covers it just as well. Photographs show the same. In fact, the rising Moon is very slightly further away than the overhead Moon, by up to the radius of Earth, so its image is a tiny bit smaller. The air near the horizon bends its light slightly too, but that only squashes the Moon a little from top to bottom; it doesn't magnify it.
So why does it look bigger? People have argued about the Moon illusion for more than 2,000 years, and there's still no single agreed answer. A leading idea is that your brain treats the sky as a flattened dome, with the horizon much further away than the sky overhead. When the Moon sits near the horizon, beyond trees and houses, your brain judges it to be further away. Something that looks the same size but seems further away must be bigger, so your brain makes it look bigger.
You can weaken the illusion by looking at the rising Moon through a cardboard tube, which hides the landscape, or even by bending over and looking at it upside down between your legs.
The principle: Perception. What we see is the brain's interpretation of the image on the eye, and clues about distance can change how big something appears.