Observing the Moon: The Moon Illusion

Jack Kramer

Many people see a full Moon right above the horizon and note that it looks surprisingly huge. I've heard some who have called into local radio stations all excited about the moon's sudden increase in size. Actually, it has nothing to do with the moon being bigger or brighter. Such perceptions have been dubbed the "Moon illusion." And it's been recognized since the second century AD, when the Greek astronomer Ptolemy pondered this celestial curiosity. Once the moon has risen high in the sky, it seems to shrink back to normal size.

Recently, Lloyd Kaufman, a psychologist, and his son James H. Kaufman, a physicist, put the illusion to the test. They performed two experiments in which subjects adjusted the distance of a simulated moon near a horizon and high in the sky. They confirmed exactly what Ptolemy concluded. There is no doubt that perceived distance information plays a primary role in creating the moon illusion. The illusion arises because seeing an object across miles of "filled space" makes it look farther, bigger, and more impressive than when there are no visual cues to its great distance. The nature of the Moon itself plays no role. This same illusion works its magic on other objects in the night sky; it also makes constellations seem larger when they're near the horizon.

Published in the May 2000 issue of the NightTimes