The moon illusion is an optical illusion in which the moon appears larger when it is closer to the horizon than when it is higher in the sky. This phenomenon has been noticed and pondered over since ancient times when people first looked to the sky. It was considered a mystery until psychological research has illuminated what may be causing our differing perceptions of the moon. Angular size seems to be the main influence on what causing the moon illusion. The perception of the moon size depends on the angle at which it is being viewed. When the moon is closer to the horizon it is surrounded by objects that are much smaller than the moon. Because it is so close to these smaller objects (such as trees and buildings) it appears huge in comparison. When the moon is high in the sky it is only surrounded by the vastness of the sky so it appears much smaller in comparison.