The moon also rises—and assumes new sizes

People have long noted that a full moon seems to shrink and expand in size depending on its heavenly position. A moon looks larger near the horizon than perched high in the sky, although its distance from Earth remains the same.

Scientists have yet to unravel the so-called moon illusion. Experiments now indicate that a horizon moon grows larger because it lies near visual markers of depth and distance on Earth’s landscape that make it look much farther away than a higher moon.

Psychologist Lloyd Kaufman of Long Island University in Brookville, N.Y., and his son, physicist James H. Kaufman of the IBM Almaden Research Center in San Jose, Calif., conducted the new research.

“Our results leave no doubt that perceived-distance information plays a primary role in creating the moon illusion,” Lloyd Kaufman says.

Since 1960, he has championed a theory that the brain automatically estimates distances to objects and then interprets their size accordingly. For example, the perceived distance to a far-off vehicle allows one to recognize it as a full-size car rather than a child’s toy.

Similarly, he argues, visual cues on the terrain leading to a horizon moon indicate that it lies at a vast distance from the observer. This distance estimation inflates its perceived size. An elevated moon draws on weaker distance cues, so observers perceive it as closer and, thus, smaller.

An alternative set of theories argues that viewers use visual cues to determine the moon’s size before noting its distance. The visual system processes the image of an elevated moon more quickly than that of a horizon moon, one of several cues that makes the elevated version look smaller, and thus more distant, in this scenario.

Most researchers have manipulated the moon’s perceived size in laboratory sessions but not its perceived distance. Lloyd Kaufman and James Kaufman directly measured perceived distance to the moon.

They projected stereoscopic images of moons from a computer display with a screen and a movable mirror. Lloyd Kaufman then had six men sit on a hillside and look at moons projected against the midmorning sky.

Each volunteer positioned one moon image so that it appeared halfway between himself and another projected moon. All of them placed the halfway point much farther away for the horizon moon, as predicted by perceived-distance theory, the researchers report in the Jan. 4 Proceedings of the National Academy of Sciences.

Moreover, as participants moved either a horizon or an elevated moon closer, they reported that it appeared to become smaller. The presence of terrain behind an approaching horizon moon led to a more rapid reduction in its perceived size, again supporting perceived-distance theory.

“This is the strongest evidence so far that the perceptions of size and distance are both involved in the moon illusion,” says psychologist Julian Hochberg of Columbia University.

However, the researchers have yet to show that perceived distance causes this impression, Hochberg holds.