A solution to moon illusion

January 21, 2000

A father-and-son team believe they have cracked an age-old mystery of the heavens: why does the moon appear much bigger when it is rising or setting near the horizon than it does when it is high in the sky? The answer is like something from an optical illusion parlour game, writes Steve Farrar.

The so-called moon illusion has been discussed and debated for centuries. Many have noted the effect but there is no physical phenomenon at work. The moon on the horizon or elevated is the same size and distance from a viewer on Earth.

Lloyd Kaufman, professor emeritus at New York University, and his son, James Kaufman, a physicist at IBM's Almaden Research Center at San Jose, California, report their answer in the latest issue of the Proceedings of the National Academy of Sciences.

The two scientists believe the illusion occurs because the brain interprets the horizon moon as being much further away than the elevated moon. The apparent distance to the moon determines its perceived size.

"Understanding such a pervasive and historic phenomenon as the moon illusion is central to scientists' quest to understand how our brains perceive space and distance," said Professor Kaufman.

"Our latest results leave no doubt that perceived distance information plays a primary role in creating the moon illusion."

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