Special Feature—City of Stars

Full Moon Over a New York Cityscape

The Moon in the Mind’s Eye Peter Miller; Yankee Image

The full Moon always looms large when seen close to Earth’s horizon. Yet no matter where you find the Moon in the sky, a dime held at arm’s length will cover it completely. This Moon-near-horizon illusion is entirely in the mind, yet psychologists have not achieved consensus about its cause. What we do know, however, is that your depth cues for the Moon are somehow confused when familiar terrestrial objects sit adjacent to it. Your brain mistakenly places the Moon on your horizon rather than at its true distance of a quarter-million miles away. The phenomenon is most dramatic above urban skylines and is significantly diminished when the Moon is above a smooth horizon such as that of the open ocean.

One way to overcome the illusion is to turn your back on the Moon, bend all the way forward, and look at it upside down through your open legs. (Make sure nobody else is looking when you attempt this.) All objects on the horizon—be they buildings, trees, or hills—will now appear upside down and unfamiliar. Your brain, no longer tricked, places the Moon where it ought to be, and at the size it ought to be.

People who try to photograph this famous Moon illusion are consistently disappointed. When the prints come back, they show only a small, uninteresting glowing dot on the sky. In fact, nearly all book or newspaper photographs that show a large Moon over a horizon are composites, like the one pictured here, produced by superimposing a large cutout of the full Moon on a skyline image. Without this trick, the only way to get the picture you want is to place yourself far, far away from your skyline so that the size of the objects on the horizon diminishes perceptibly but the Moon does not. You then use a fat telephoto lens (at least 300mm) to zero in on the scene, which now matches what the brain perceives from close up.

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