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By Marcus Chown
it's the moon, stupid London - June 17, 1999 - Why does the Moon look the same size as the Sun in the sky? This coincidence, which makes the spectacle of total eclipses possible, has been crying out for an explanation. Now an astronomer in Seattle has proposed one.

If he's right, there is a surprising connection between the conditions required for a total eclipse and for the emergence of intelligent life.

The coincidence in the apparent sizes of the Moon and the Sun occurs because the Sun, though 400 times bigger than the Moon, is also 400 times farther away. In fact, the Moon sometimes looks a shade bigger than the Sun, which is essential for a "perfect eclipse" when the sky is dark enough for you to see the Sun's faint outer atmosphere, or corona.

Because tidal effects cause the Moon to slowly recede from the Earth, perfect eclipses have been visible only for about 150 million years and will continue for only another 150 million years, about 5 per cent of the current age of the Earth. Furthermore, Earth is the only planet in our Solar System where a perfect eclipse is visible, although there are 64 other moons.

So are we just extraordinarily lucky? Guillermo Gonzalez of the University of Washington in Seattle thinks not. He points out that our distance from the Sun, and hence its apparent size, is a necessary condition for us to be here. "If we were a little nearer or farther from the Sun, the Earth would be too hot or too cold and so uninhabitable," says Gonzalez.

At the same time our existence depends on an unusually large moon since its pull stops the Earth wobbling around too much on its axis and causing wild and catastrophic swings in climate like those on Mars. Our Moon, which is unusually large compared to those in almost all other planet-moon systems, probably formed from molten material blasted from the Earth during the impact of a giant body more than 4 billion years ago.

In the current issue of Astronomy & Geophysics (vol 40, p 3.18), Gonzalez points out that the way the Moon formed means it started off very close to the Earth and has taken several billion years to move far enough away until it precisely covers the Sun during an eclipse. "The timescale is very similar to that of the appearance of intelligent life," he says. "It is therefore not such a big coincidence that we are around at the time when it is possible to see total eclipses."

Gonzalez's explanation has generated much interest among astronomers, though most remain cautious. "The timescale argument of Gonzalez needs more checking," says John Barrow of the University of Sussex.

If Gonzalez is right, then all extraterrestrials, wherever they are, are likely to live on planets like ours that experience total eclipses. But since an unusually large Moon is rare, he says, this suggests that both ETs and total eclipses are very rare indeed.

  • Astronomy a University of Washington
  • New Scientist
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