A trick of the moonlight

摘要

When the Moon is near the horizon it appears to be larger and closer than when it is high in the sky. Yet light reflected from the Moon to the eye of an observer on Earth provides the same stimulus, regardless of the elevation of the Moon. This phenomenon is known as the 'Moon illusion'. A similar illusion is observed for the setting Sun and for celestial distances between star points at different elevations. The Moon illusion is arguably the oldest unsolved problem in modern science. It is mentioned in cuneiform script on clay tablets from the royal library of Nineveh and Babylon, dating from before the sixth century BC, and in a collection of Chinese legends ascribed to the Taoist philosopher Lieh-tzu, dating from the fifth century BC.