Americans Jack Lissauer and John Chapman calculate that our present moon was once flanked by two smaller moons called Trojans. Each of these moonlets was about one kilometre across and all three would have been visible at the same time. If you can imagine our present moon at 12 o'clock in the night sky, the Trojans would shine at 10 and 2 o'clock. The astronomers think the two extra moons hung in our skies in gravitationally stable zones for hundreds of millions of years before breaking up or drifting off into space.

This all happened long ago, after a Mars-sized planet called Theia collided with a semi-molten Earth, knocking off our three moons.

Working back through the Bible, a succession of Divines calculated that the world is 4000 or 5000 years old.

Working from the oldest meteorite known to science, a relic of the cataclysmic collision between molten planets, Joel Baker, now at Victoria University, calculates that the world is much older.

If Dr Baker's calculations are right the Bible is out by 4,566,195,000 years. Christchurch wit and Dominion Post columnist Joe Bennett suggests that if we want to be truly realistic and accurate in our civil dealings we should date our cheques Day/Month/ 4,566,200,000.

The Earth's first two billion years were hellish as we suffered endless volcanic upheavals and, with no atmosphere to protect us, we were seriously blitzed by meteorites and comets.

Geologists call it the Hadean period. About 4.4 billion years ago we got a protective atmosphere, the Earth cooled and oceans developed.

A Macedonian bus driver in Sydney once challenged me to explain where the Earth's water came from. I must admit that at the time I was flummoxed but have since made it my business to find out. Spanish astronomer Jose Oro once suggested that as the Earth cooled, it was bombarded by numberless comets (which consist of little more than ice).

Oro's speculations got a cool reception at the time, but recent space probes confirm that comets are like flying icebergs and that the chemical isotopes of cometary water match those in our ocean waters. Next time I see the bus driver I'll let him know.

Seems, too, that the sun was fainter in those early days. Four billion years ago it was 25 to 30 per cent cooler than now, and our orbit round the sun was more elliptical.

But back to our three moons. They used to be much closer to the Earth and their lunar months were only a week long. Our moon would have looked much larger and the two lost moonlets would have appeared like extremely bright accompanying stars.

Had the moonlets remained in our sky, our earthling mythologists, romantic lyricists, bards and musicians would have told very different stories, sung different songs and declaimed different poems . . .

With what bright steps, O Moons, ye climbst the skies. . .