Relative size is a perceptual clue which allows you to determine how close objects are to an object of known size. Sometimes our perceptions are faulty. Humans use relative size to judge the size of the moon. This is why the moon seems bigger and closer when it is near the horizon than when it is high in the sky. We use buildings, trees and other objects of known size to make judgements about the size and closeness of the moon. When the moon is higher in the sky there are no known objects or frame of references to judge the moon's closeness, and thus, appears smaller and farther away. This is called the lunar illusion.