What makes astronomers believe that there might be life out there? There are no hard facts - but much circumstantial evidence. Earth is an average planet circling a normal star, at just the right distance for life to be comfortable. Our star, the Sun, is one of 200 billion in our galaxy. The chemicals of life - carbon, nitrogen, hydrogen and oxygen - are among the most common in the cosmos, stardust from long-dead stars.
Astronomers can try to put numbers into this picture. Thirty years ago, Frank Drake - one of the pioneers of Search for Extraterrestial Intelligence (Seti) - devised an equation that sought to quantify the number of intelligent civilisations in our galaxy, taking in factors such as the number of stars with planets, and the number of planets on which intelligent life is likely to have evolved.
Today, the Drake Equation is still the best guide we have to the number of extraterrestrial civilisations - but that number depends on estimating the different factors involved. Pessimists come up with an answer of just one civilisation - us. But most astronomers believe that the evidence points to millions. And some of them may be trying to contact us.
How might they do it? Discounting possibilities such as travelling to us by starship (almost certainly too wasteful on energy), or shooting laser beams in our direction (difficult to detect), it is likely that ET would choose the cheapest way we know to communicate - by radio.
With this in mind, some astronomers have been attempting to listen in for alien signals since the Sixties. All have heart-stopping stories to tell about the times they thought they had found one. But none has turned out to be real: terrestrial and military radio interference loom larger than whispers from space.
Part of the problem, too, is the 'needle in the haystack' syndrome - exactly where do you tune your dial to hear ET? Fortunately, computers and electronics make it possible today to scan millions of frequencies simultaneously, and innovative designs of ultrafast microchips allow for billions of operations a second.
And this new technology is the basis of Nasa's new Seti search. On 12 October 1992, the 500th anniversary of the day Columbus set foot on American soil, two radio telescopes, equipped with state-of-the-art detectors, will be pointed towards the same part of the sky. One will listen in to signals coming from a nearby star similar to our Sun. The other will slowly scan the sky in the vicinity. In the first three minutes of observing, these radio telescopes will clock up more data than in all the previous 30 years of searching.
This is just the beginning. Over the next 10 years, these telescopes - one in the Mojave Desert in California, the other in the jungles of Puerto Rico - will be joined by many others all over the world in the quest for Seti. The 34-metre diameter dish at Goldstone, in the Mojave Desert, will scan the whole sky 31 times for signals. The Arecibo radio telescope in Puerto Rico - at 305 metres across, the biggest radio telescope in the world - will tune in to 800 nearby sun-like stars, searching for signals from their planets.
What if we do detect an unambiguously alien signal? The first priority will be to confirm it with other telescopes. Then the nature of it will be analysed: is it just a beacon - or a detailed message?
There will then be the question of response. A few prominent scientists think it would be sheer folly to return the call: if the aliens have evolved like us, then they are bound to be unfriendly. The majority, however, believe that contact with an advanced intelligence would be the best possible thing to happen to the human race, even perhaps offering us a short cut in helping to solve our global problems. The matter of our reply - should we choose to make one - is already the subject of detailed international protocol.
What if, after 10 years, the search comes up with no fruit? The Seti researchers are still optimistic, recognising all too clearly that they are 'new to the game'. But suppose the quest goes on for a hundred, a thousand, a million years and finds nothing? Astronomers are reluctant to admit that Earth could be the only inhabited planet in the universe. 'But if we searched that long and found nothing - knowing there were suitable planets - we'd be at a crossroads,' said one Seti researcher. 'Maybe we'd realise there's something that science isn't telling us.'
The two planets that have been with us for the past few months - Saturn and Mars - have been joined by a third: Venus. Look low in the west after sunset to spot this, our planetary twin in size. By the end of the month, Venus will be setting more than an hour after the Sun. As the year draws to a close, Venus will continue to draw away from the Sun, brightening as it swings towards the Earth in its orbit. By mid-December, it will rival the Christmas Star itself.
Faithful Saturn - a fixture in our evening skies since June - is still brightening up a very barren area of the southern sky, among the stars in the triangular-shaped constellation of Capricornus, but it is growing gradually fainter.
The final planet in this evening trio is Mars, which rises mid- month at about 10.30pm among the stars of Gemini. Last month, it was of equal brightness with Saturn, but now it is noticeably brighter. A new US space probe to Mars - the Mars Observer - was launched at Cape Canaveral on Friday. When it arrives in just under a year, its cameras will be able to 'see' details on the red planet as small as a family car.
This month, bits of Halley's comet will stream into the atmosphere - so if you missed the real thing in 1985-86, here is an acceptable substitute. As Halley's comet tramps around the solar system, suffering alternate boiling and freezing, it loses matter from its icy surface: tiny grains of 'soot' that litter its orbit. Every October, we plough into some of that soot, and the grains shoot to Earth at high speed. They burn up by friction, and we see them as streaks of light in the sky - shooting stars, or meteors. The October meteors appear (through perspective) to emanate from the constellation of Orion, and so are known as the Orionoids. This year's Orionoids are worth looking out for, because moonlight will not interfere. If you watch the sky after midnight on 21 or 22 October, you should be able to count up to 20 meteors an hour.
The 'flying' constellations of last month have moved over, to be replaced by a whole bunch with 'watery' connections. Pisces (the fishes), Cetus (the whale), Aquarius (the water-bearer), Capricornus (the sea-goat) and Piscis Austrinus (the southern fish) are all faint, straggly constellations, and they are all low in the south.
Why the association with water? One possibility is that the Sun moves through this area of the sky in the early northern spring. This is also the rainy season for lands in the near east and around the Mediterranean, where, thousands of years ago, the constellation- makers lived. They may well have decided to use the rains as an aide- memoire to navigators.
Certainly, these constellations are ancient. Cetus was the sea- monster assigned to gobble up Andromeda; Aquarius, to the ancient Babylonians, was a man pouring water from a jar; Capricornus, the goat with the fish's tail, is typical of the amphibious creatures so common in old legends; and Pisces - two fishes with tails entwined - is a symbol as old as the hills.
Yet Piscis Austrinus may be the daddy of them all. The constellation depicts a fish drinking from Aquarius's jar, and in legend, Piscis Austrinus is the father of the more famous zodiacal fishes. Star- gazers today relish the challenge of spotting its brightest star, Fomalhaut - the most southerly first-magnitude star visible from the latitude of the UK.
Diary (times BST till 25th)
3 3.12pm: Moon at first quarter
11 7.03pm Full Moon
19 5.12am Moon at last quarter
22 Maximum of Orionoids meteor shower
25 2am End of British Summer Time - put clocks back to GMT
The authors are preparing a television programme, 'ET: Please call us', for Channel 4's Equinox series, due to be shown on 20 December.Reuse content