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Last Updated: Friday, 24 June, 2005, 12:45 GMT 13:45 UK
Why does the moon look so big now?

For the past few nights the moon has appeared larger than many people have seen it for almost 20 years. It is the world's largest optical illusion, and one of its most enduring mysteries.

It can put a man in space, land a probe on Mars, but Nasa can't explain why the moon appears bigger when it's on the horizon than when it's high in the night sky.

The mystery of the Moon Illusion, witnessed by millions of people this week, has puzzled great thinkers for centuries. There have even been books devoted to the matter.

Not since June 1987 has the moon been this low in the sky, accentuating the illusion even further.

But opinion differs on why there is such an apparent discrepancy in size between a moon on the horizon and one in the distant sky.

Can the Moon Illusion be explained? Here are two theories

Two main theories dominate. The first, known as the Ponzo Illusion - named after Mario Ponzo who demonstrated it in 1913 - suggests that the mind judges the size of an object based on its background.

Ponzo drew two identical bars across a picture of railway tracks which converge as they recede into the distance (see pop-up, right). The upper bar looks wider because it appears to span the rails, as opposed to the lower bar, which sits between the rails.

In the same way, with a low-lying moon the trees and houses, which are familiar foreground reference points, appear smaller against the moon, which appears bigger than it really is.

Sceptics of this theory point to airline pilots who also see the illusion, although they have no ground reference points.

Alternatively, there's the theory that the brain perceives the sky as a flattened dome rather than the true hemisphere it really is.

Try for yourself

The theory runs that we believe things immediately overhead, flying birds for example, are closer than birds on the horizon. When the moon is on the horizon, the brain therefore miscalculates its true size and distance.

Then there are those who scoff that this is an illusion at all. They, at least, can be proved wrong. Hold a coin up to a low-lying moon to and compare differences in size. Any difference will remain exactly the same, as one traces the trajectory of the moon through the night.

Indeed, it's said that by viewing a low moon though a rolled up piece of paper, to block out the surroundings, the illusion immediately vanishes.

But experts have yet to agree on either or, indeed, any explanation. For the moment at least, the real reason for the Moon Illusion remains up in the air.

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