Why moon hits your eye like a big pizza pie

This just in for tonight's eclipse: Scientists now know why the moon

It's a Y2K lunar celebration: Last month, the full moon moved in close to shine as bright as it can get. Tonight, it will treat us to a total eclipse.

All of North and South America will have grandstand seats as the moon slips into, through, and out of the darkest part of Earth's shadow over a 3-hour, 24-minute period beginning at 10:01 p.m. EST. Its light will start to dim about an hour earlier.

To add perspective to the show, a father-son research team has nailed down the reason we think the moon is larger than it really is when it's on the horizon. As it turns out, it has nothing to do with the atmosphere. Nor is it an optical illusion that makes us think the moon is closer. Actually, it's an optical illusion that makes us think the moon is farther away.

That notion - that the moon appears bigger because we think it's farther away - is so counterintuitive that scientists discounted it for centuries. But physicist Lloyd Kaufman at IBM's Almaden Research Center in San Jose, Calif., and his psychologist father, James Kaufman, at Long Island University in Brookville, N.Y., say they have proof that ancient astronomers got it right when they posited just such a theory long ago.

According to data released earlier this month, the illusion occurs because of the way the brain and eyes perceive distant objects. The brain takes clues from the intervening landscape to judge how far away an object is. The eyes, however, judge distance by the size of objects, measuring how big an angle they take up in our field of vision.

Our visual system combines this information to adjust our perception so objects keep their right proportions. This is why we don't mistake a distant truck for a toy.

When the moon is close to the horizon, the eyes see the moon covering the correct angle of vision, but the brain misinterprets the intervening terrain and thinks the moon is farther away than it is. Therefore, the brain constructs a larger image to reconcile the problem (see graphic).

By contrast, there are no terrain clues to misinterpret when we see the moon in the sky, so the brain gets the distance right.

The Kaufmans had volunteers view the horizon or the sky through a screen onto which a computer projected lunar images. The subjects could adjust the display so that a moon image was at a greater or lesser perceived distance. In all cases, "when a moon of constant angular size was moved closer, it was also perceived as growing smaller," the Kaufmans reported in a paper that appeared in the Proceedings of the National Academy of Sciences.

"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," says James Kaufman. For the most part, though, that's still a mystery.

Tonight's full moon will replay the illusion, but it will be the proper size when it enters Earth's shadow. The eclipse will be visible even in well-lit areas, although city lights will probably wash out some of the subtle colors. Also, when the moon is dark, the Milky Way and many stars will be visible in dark viewing areas. Binoculars will be useful, and there's no danger in looking at the moon through them.

(c) Copyright 2000. The Christian Science Publishing Society