[Polmear (Tim)]

It can be almost impossible to grasp the true size of the Solar System. We can use a planetarium program and zoom out and zoom out and go, "Oh, Earth is just a dot now," or we can throw around millions and billions of kilometres but it really doesn't mean anything to our brains that are wired to judge the distance to the nearest tree, as the sabre toothed tiger comes leaping towards us. Just rarely, however, we might catch a tantalising glimpse of how immense the Solar System actually is.

Some time ago I was was outside and I happened to look up at the moon. It was in the west and slightly older than a waning quarter. The sun was in the east at approximately the same angle of elevation above the horizon. What captured my attention was that the illumination of the moon didn't look right. From my position and the apparent positions of the moon and sun, drawing an imaginary line from the sun to the moon, I should have been able to see a gibbous moon. But the moon was a quarter moon at best.

Now it has long been commented that owing to a freak of nature, the moon and sun look to be almost identical in size, from Earth. When I look at the moon and sun in the sky, the illusion is that they also seem to be the same distance away, making an isosceles triangle with the observer as the third corner. That's all very comforting but quite wrong, of course.

When I projected another imaginary line from the moon, normal to the lunar hemisphere being illuminated, it seemed to point off up and away, somewhere in space, as if the sun was actually in a different position. The sun (apparently) could not possibly be shining on the far side of the moon which was to the west of my observing position, since the sun was in the east.

That's when I had my 'Total Perspective Vortex' moment. The position of the sun and the illumination of the moon could only be reconciled if the sun was some unimaginably hideous distance away and unimaginably, hideously larger. Solving the puzzle of the moon's phase gave me a sudden insight into the true scale of the Solar System. Like the cold, implacable forces of the subatomic universe this insight was just a little bit frightening. And then I went inside.

Next time there's a quarter moon or crescent in the sky with the sun on the opposite side of the meridian, why not see if you can catch a bit of total perspective.