Leading Article: Eternity is an ever-expanding idea. And so is that Dome

If it is portents of the millennium you want, how about Patrick Moore's Sussex observatory being torn apart by a tornado? Only last week the celebrity stargazer said he expected UFO-watchers to be working overtime as the end of the century drew near. "The millennium will send them all bonkers," he said. That will teach him to be sceptical. It will be frogs and boils next. Followed by the flying saucers themselves.

Luckily, we have scientists on hand to explain that tornadoes are surprisingly common in Britain, it is just that they hardly ever hit houses. A layer of stable cold air, a low-lying mass of warm air, a thunderstorm and - whoosh! It spins, touches the ground, and causes havoc. This visitation has nothing to do with judgment-day, the wrong kind of offering or extraterrestrial visitors. At least not mechanically, anyway.

Same with earthquakes, plagues, floods or eclipses. Imagine what would happen in Cornwall at 12.10pm on 11 August 1999, when it goes dark for two minutes, if we had not been told in advance that there was going to be a total eclipse of the sun. Just three months before the calendar flicked over to a very round number, it could have caused millennial mayhem.

Since the last millennium, science has replaced religion as the body of knowledge that explains the world. Scientists are now the explainers, predictors and magicians of our society, and they are a more democratic priesthood than the ones they replace. The increasingly esoteric specialisation of their research has been balanced by the huge expansion of the business of popularising science. Science is popular, and popular science is a growth sector in publishing and journalism.

Yet the millennium itself remains the point at which religion meets arithmetic, which could be regarded as the start of science. Paradoxically, they got the arithmetic more right the first time round. The first millennium was regarded as running up to the end of the year 1000, whereas the second millennium is going to be a year short, ending by common usage on 31 December 1999. This is an indicator of the decline of religious ritual, because if people really wanted to mark the 2000th anniversary of the birth of Jesus Christ, they would want to get the date right.

But most people know that no one really knows when exactly Jesus was born, and that we will essentially be commemorating a number - a number that owes its origin to our Christian heritage but which has a wider meaning.

The Christian heritage causes trouble nonetheless. The Bishop of Oxford has renewed the churches' call for a "spiritual" content for the millennium celebrations, and this week the representatives of various other faiths demanded to be let into the Dome too. Then, inevitably, the atheists wanted a look-in. But frivolous demands from the National Secular Society for an "Inquisition Pavilion", a "Witch Burning Experience" and a history of the Crusades "including Muslim heads impaled on spears" are hardly designed to promote tolerance and mutual understanding.

While the spokesmen of organised religion (and organised irreligion) seem to emphasise differences between people and to look backwards, it is scientists who are striving to unite our explanations of everything and to look to the future.

The real millennial story of the week, then, was the report in today's newspaper that astronomers have concluded that the universe will expand for ever. After our civilisation's first two millennia, we have another 100 million to look forward to. After that, give or take an aeon or two, all the energy in the universe will eventually dissipate and leave it a cold, dark, empty place "with nothing left but rocks". So do not cancel the milk, yet.

The ever-expanding universe is an important conclusion, but does it mean anything practical to the majority of us who would not know a black hole from a mint with the same? The simple answer is, no. It seems that there is not enough stuff in the universe to exert the gravitational pull that would reverse its expansion and cause a "big crunch" to mirror the "big bang" of 15 billion years ago. So it will go on and on expanding, although because space and time get a bit bent at the edges (this is where scientists start to run out of language and brainpower to explain and imagine) there is never anything beyond it into which it expands. And, even if the universe were "closed", the big crunch would be an unimaginable time coming.

The fact that the universe is "open" hardly seems a subject on which a newspaper can have an opinion. We could hardly declare that the universe has expanded, is expanding and ought to be diminished.

But it does matter that the universe is, and always will be, flying apart. It shapes how humanity sees itself. Scientists conduct conversations and arguments at the boundaries of human knowledge, and the metaphors and way of understanding that they employ enter the common consciousness. Big bang, quantum leap, light year. The "open" universe brings us face to face with eternity. When the world did not end in 1001, there was ushered in one of the greatest periods of church-building in Europe's history. If only our growing understanding of the awesome mysteries of the universe would inspire a similar optimism in two years' time.