Why does the Moon look bigger on the horizon?

The Moon always covers about half a degree of sky, whether it is rising or high overhead. In fact, when it sits on the horizon it is roughly one Earth radius, about 6,400 kilometres, farther from you than when it is overhead, so it appears about 1.5 percent smaller. Yet almost everyone sees it as far larger, a puzzle known as the Moon illusion that was discussed by Aristotle and Ptolemy. Leading explanations say the brain judges the horizon Moon as more distant because it sits beyond trees and buildings, and then inflates its size to compensate, similar to the Ponzo illusion. You can break the spell by viewing the Moon through a rolled-up tube of paper, which removes the surrounding clues.