It's back: the number they couldn't ban

It's not just buildings, phone bills and student overdrafts that are getting larger. Numbers are following suit. You don't blink at millions, you think you're used to billions. But are you ready for the trillion - coming soon to a number near you?

Think about it. Millions have become commonplace - people win them on a Saturday night. Billions are quite usual too - most people can name a billionaire (Bill Gates of Microsoft usually), and don't find it surprising to hear that the National Health Service spends billions of pounds.

But the late astronomer Carl Sagan - often lampooned for a phrase that he never uttered, about "billions and billions" of stars in our galaxy - noted a new trend before he died, which he describes in his latest book.

"While the popularity of millions and billions has not entirely faded, these numbers are becoming somewhat small-scale, narrow-visioned and passe," he wrote in Billions and Billions, published next Thursday.

"A much more fashionable number is now on the horizon, or closer. The trillion is almost upon us."

It's a number that will be familiar to older readers as the British billion. 1,000,000,000,000 - one million million, the number that never quite caught on.

The trouble was that amid the inflation of the 1970s a word was needed to describe the gap between the million (1,000,000) and the British billion. Though such a word - "milliard" - did exist, it sounded too like both its cousins to survive the test of TV and radio speech. And so the American billion (as it was known in Britain) came in and stole the show. Soon, that was the standard measure for a billion: a thousand million.

But, as Dr Sagan points out,billions are no longer enough.

"World military expenditures are now nearly $1 trillion a year. The total indebtedness of all developing nations to Western banks is pushing $2 trillion (up from $60 billion in 1970). The annual budget of the US government is also approaching $2 trillion. The US national debt is around $5 trillion... All the plants on Earth weigh a trillion tonnes."

The trouble with these big numbers, he admits, is that we're not very good at imagining them. Humans can generally only remember seven things at once. They can easily handle the idea of a group of about 100.

But billions? Trillions? If you want to have a go, imagine that, counting from zero to one trillion at one number per second, itwould take you 32,000 years.

And beyond that? There are plenty of numbers waiting - such as the quadrillion and quintillion. For the moment, though, the trillion looks untroubled.

'Billions and Billions', by Carl Sagan, Headline Books, pounds 18.99