Wednesday night. 11.30pm. A wonderful evening, shirtsleeve warm. The river splashes by; a mouse squeaks and rushes in the long grass, and the profanities of three youths returning from the pub drift abrasively through the gentle stillness.

Behind, the sun is still setting over Darlington. The horizon is yellowy pink, fading into translucent blue before petering out overhead into darkness. A single grey cloud breaks up the colour show. It lies west to east. Its slight arc makes it look like a whale.

In front, the full moon. The biggest, brightest full moon to have shone on this country since June 1987.

It rises behind a large tree, but it burns so fiercely through the branches that it looks like someone blowing on the embers of a yellowy-orange fire.

It is a day after the summer solstice when the moon is at its lowest in the sky. It is also the point of "lunar standstill" on the moon's 18.6 year wobbly cycle which makes it appear even lower in the sky.

I move to get a clearer view. It is huge. As big as a frying pan. It is 2,160 miles wide.

Even at this distance - 240,000 miles - you can see its face: its hollow staring eyes, its lop-sided, toothy grin and its nobbly, warty, drinker's nose.

A bat flaps in front of it, a tiny black silhouette - wings outstretched - flying across a fiery plate. Just like that moment in ET when he heads for home on a pushbike.

In moving, I notice that the single grey cloud has been blown from a whale into a fiercesome shark with a horrible protruding nose, a wide gaping mouth and woolly teeth.

In moving, I alert the three youths.

They lurch over, all swear words and swagger. "What yous lookin' at?" asks one from beneath a baseball cap.

I point. They look. "Mint moon, mate, " says one in genuine awe. "Cool."

Two of them take out their camera phones. They are too drunk to photograph. The moon dances in the dark sky, and they dissolve into laughter.

"Hey, why's it so big?" asks one, regaining his composure.

And I try to explain that it isn't, that this is the greatest optical illusion on earth. We hold our arms, stick up our thumbs and squint through one eye. In front of our eyes, the moon looks as big as a planet, but is, we agree, not even as big as our thumbnails. And when it rises high in the sky, it will look as small as a pebble yet will still be exactly the same size on our thumbnails.

"Huh?"

They're right. It's a question that's been troubling man for 3,000 years since the Babylonians were the first to be optically illused.

It could be that trees and houses on the horizon give the moon some perspective that isn't there when its high in the sky on its own.

But then pilots above the clouds where there are no trees or houses also report moon illusion.

More likely, we see the sky as a squashed dome. We think the horizon is far, far away whereas we think the point above our heads is quite close.

Which is true: birds flying overhead are closer than birds on the horizon.

Therefore, our brain miscalculates the size of a moon on the horizon in comparison to the one overhead.

I leave them pondering. By now, the single grey cloud over Darlington has been blown from the shape of a shark into that of a fat, juicy slug. With a tiny jockey on its back. Look carefully - you can see the reins.

I forget who had been drinking.

Published: 25/06/05