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Fooled by the moon: the moon sometimes makes us think it's closer than it is in reality.

APRIL FOOLS' DAY arrived in February for Apollo 14 astronauts Alan Shepard and Edgar Mitchell. Disoriented by the absence of distinct landmarks and unable to assess distances accurately in the alien environment of the Moon, they couldn't find the rim of Cone Crater in the Fra Mauro highlands as they explored the lunar surface in 1971. Although they negotiated the crater's slope for more than an hour and actually approached within 20 meters (65 feet) of its lip, the astronauts didn't know exactly where they were and had to return to the lunar module before achieving their goal.

Earth's atmosphere cues us to distance through softening the appearance and altering the color of the distant horizon, but the airless Moon doesn't offer the same hints. In Cone Crater's vicinity, Shepard explained, "It's so crystal clear up here it just looks a lot closer than it is."

Here on Earth, too, the Moon sometimes fools us into thinking it's closer than it is. Nearly everyone has noticed that the Moon looks bigger along the horizon than when it's high in the sky. Rising or setting with the illusion of a larger size, it cons us into believing that it's nearer to us.

Actually, the Moon is a little farther away when it appears on the horizon. As Earth's rotation lifts the Moon's position higher in the sky, our separation diminishes slightly, but the change is too small to notice. For all practical purposes, the angular size of the Moon remains the same each night whether it's measured near the horizon or well above it.

The Greek philosopher Aristotle wrote the oldest known account of the Moon's horizon intrigue in the 4th century BC. The air, he asserted in Meteorology, magnifies the Moon. He must have let the air out of that explanation, however, for later in On Sense and the Sensible he disputed the idea. Other ancient authorities, like Posidonius in the 2nd century BC, also attributed lunar enlargement to atmospheric optics. Earth's atmosphere does alter the appearance of the Moon, but it flattens the disk through refraction. It doesn't magnify the Moon.

Today the Moon illusion is usually explained as a consequence of the eye and brain comparing the lunar disk with adjacent terrestrial references--like trees, buildings, or mountain peaks. When the Moon is elevated, those familiar horizon features fall away, and the illusion fails.

Although we are afflicted by misperception when the Moon is low, we have a cure. Turn your back to that horizon-huge Moon, bend over, and look at it between your legs. In this posture the Moon doesn't loom as large as you left it when you were still respectably right-side up. It shrinks for your inverted eye, allegedly because the upturned landscape no longer looks so familiar.

This explanation is neither sufficient nor necessary. When the Moon floats upon the open sea without nearby landmarks, it still looks big. Those who have thought more carefully, and in many positions, about how we perceive the Moon's disk insist that there's more to the story.

In The Mystery of the Moon Illusion (Oxford University Press, 2002), two academic psychologists, Helen Ross and Cornelis Plug, remind us that the size illusion distorts other things in the sky. The Sun looks enlarged near the horizon, and familiar star patterns, like Orion and the Big Dipper, also project a greater profile. So whatever is fooling people, it affects more than just the Moon.

In their comprehensive catalog and critical review of Moon-illusion research, Ross and Plug assess exactly how much larger the Moon appears on the horizon and indicate that most people feel that the Moon looks about 50 percent larger when it hugs the landscape. Keeping the company of familiar references and being visible over recognizable terrain are, they say, part of the Moon's trick. But the Moon's misdirection also relies on three additional tactics: the atmosphere's ability to alter the color and clarity of more distant scenery, the eye's and brain's response to lifted sight-line and adjusted posture, and our perception of the sky as a flattened vault.

When we locate and map celestial objects, we imagine the sky to be a hemispherical dome, but we actually perceive it differently. The horizon usually seems to be farther away than the point directly overhead. Some people claim that we unconsciously conclude that the Moon is closer to us when it is high and farther when it is low. Because it actually has the same angular size in both locations, we make the lower, farther Moon bigger in our mind's eye. This is said to compensate for our sense that the same Moon is farther away. Our eyes "see" that it's the same size, but our brain "knows" that it's more distant. It should appear smaller, but it doesn't. So our brain tells us that it has to be bigger to be the same size, and this is what we see.

Ross and Plug regard the effort to understand the Moon illusion via a flattened-dome sky as a fool's errand, but since we do see the sky that way they acknowledge that the flat-sky illusion deserves an explanation.

In an article published in the September 2001 Griffith Observer, astronomer Arthur Young allied the Moon illusion with our unconscious inference of a flattened sky, and his explanation for the flattened shape seems promising. According to Young, clouds form our sense of the horizon's distance. Although the clouds on the horizon are roughly 50 times farther away than those overhead, their geometric regularity, physical uniformity, and altitude allow foreshortening to fool us into thinking that the cloud horizon is closer than it is--only six times farther than the zenith, according to Young's calculations.

Others have also looked at clouds from both sides. As early as the 17th century, Dutch astronomer Christiaan Huygens recalled the sky's illusions when describing that things overhead seem "to be no farther away than the clouds that float above our heads." He added, "So now when two bodies of equal magnitude cover the same visual angle, we always judge as larger the one taken to be farther away."

Still, our effort to understand the Moon illusion has not diminished our expectations. We are so primed to see a big Moon rising that we're easy marks for the fabricators who use Adobe Photoshop to paste oversize Moons onto tourist postcards. Years ago, Fritz Leiber, one of America's most literate science-fiction and fantasy writers, and I regularly exchanged postcards featuring errant Moons--sometimes too large, often backward, and usually both.

Even those most attuned to the illusions of the Moon can't always avoid moonshine. The huge Moon that moonlights the dust jacket of The Mystery of the Moon Illusion is a wrong-way Moon that turns the sky inside out. No Moon is foolproof.

E. C. Krupp rushes in and fools around with the Moon at Griffith Observatory in Los Angeles.
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Author:Krupp, E.C.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Apr 1, 2006
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