The late Enoch Powell, well known for his almost plausible, outrageous views, was once described as being "driven mad by the remorselessness of his own logic". I am fearful that the same fate may befall Dr Guillermo Gonzalez of the University of Washington. He has made the task of finding extraterrestrial life well nigh impossible.
Gonzalez's conclusion is related to an event awaited on August 11th this year. On that day, as we all know, there will be an eclipse of the sun, visible as a total eclipse along a narrow track that runs over Cornwall and down through central Europe. And the fact that a total eclipse can occur at all is due to a strange astronomical coincidence.
The sun is about 400 times bigger than our moon, but because it also 400 times further away from us, the two bodies appear in the sky as almost identical in size. As a result, during an eclipse, the moon covers the whole sun, but only just. Earth is the only planet in our galaxy where a "perfect eclipse" like this occurs; there are 64 moons in the solar system, and in all cases the apparent size is either so big that it totally obscures the sun, or too small to cover the solar disc.
It was not always so on Earth either. Our moon probably started out as a lump of molten material blasted from Earth by some celestial collision about four billion years ago. Since then it has been gradually receding from the Earth, and the right distance for a perfect eclipse was reached only 150 million years ago, almost yesterday in the language of cosmology. Moreover the four billion years that it has taken the moon to reach this perfect spot is similar to the length of time it has taken intelligent life on Earth to evolve to the equally perfect state we know today.
Nor, according to Gonzalez, are the two events entirely disconnected. We owe the existence of life on Earth to our distance from the sun; closer would be too hot, and further away too cold. In addition, our unusually large moon regulates our climate, by preventing with its gravitational pull fluctuations that would otherwise result from the Earth wobbling too much as it spins upon its axis.
The conclusion is that if we want to find intelligent life elsewhere in the universe we should look for planets capable of experiencing a "perfect" total eclipse. I am not quite sure that the logic is as remorseless as it ought to be, but at least it narrows down the search considerably.