The Moon Illusion

Few would argue that the Moon is more wondrous and enchanting than when it is near the horizon, appearing closer and looming larger than when it is high in the sky. But this is an illusion-the "Moon Illusion," to be precise: the distance traveled by light reflected from the moon to the eye of an observer is essentially the same regardless of the Moon's elevation. A similar illusion is observed for the setting sun and for celestial distances between star points at different elevations.

But the Moon Illusion is not only captivating to behold; it also happens to be perhaps the oldest unsolved problem in science. References to it can be found on clay tablets from the royal library of Nineveh and Babylon, dating from before the sixth century BC, as well as in a collection of Chinese legends ascribed to Lieh Tzu dating from the fifth century BC. Many of history's leading scientists and mathematicians have analyzed the phenomenon: da Vinci, Kepler, Descartes, to name but a few.

For most of recorded history, the illusion was thought to be a consequence of physical processes. For example, Aristotle in the third century BC and Ptolemy in the second century AD incorrectly attributed the illusion to the magnifying properties of the atmosphere. Alhazen (Ibn al-Haytham) related the illusion to the flattened appearance of the dome of the sky.

In the nineteenth century, it became clear that the Moon Illusion is psychological, a consequence of the cognitive processes underlying the human brain's visual space perception. Many theories have since been offered to explain the illusion, but there is still little agreement among researchers.