Philippa Stevenson: Happy birthday to Planet Earth

As birthday parties go it was pretty darn successful. The gathering was held a tad late - Tuesday night when the actual day was the previous Saturday - but a festive air was struck, happy returns wished for and the brightly iced cake went down a treat.

No candles, mind. Not when you need 6009 of them. Not when you're tucking your tongue as well as cake in your cheek to wish Planet Earth happy birthday.

Nowadays, the best scientific estimates put the Earth's age at about 4500 million years, but one of the best-known early efforts at estimating the planet's birth date was a 1650 chronology by Irish Archbishop James Usher.

His calculation used the Bible lists of who begat whom since Adam and Eve, assigned 25 years per generation and added in six days for creation, on the biblical grounds that "one day is with the Lord as a thousand years, and a thousand years as one day". The result was that Earth's first day was October 22, 4004BC. The hour of creation, Usher suggested, was about nightfall.

In honour of the attempt to date Earth, if not its accuracy, this week's Cafe Scientifique - an initiative by Waikato University's science and engineering school to take science to everyday places - was devoted to the birthday.

Over the dessert plates and between the glasses in a Hamilton cafe, Waikato earth sciences research fellow Penny Cooke led us party-goers on a fascinating romp through 4.5 billion years of swirling and coalescing space debris, molten lava, long-lived minerals and never-say-die bacteria.

In case we were inclined to scoff at what now seem decidedly odd efforts to date the Earth, Cooke stressed that every one was a serious and genuine attempt in its day and most were considered sensible at the time.

Several religions had a go at the date. Ancient Hindu traditions of renewal cycles lasting 4320 million years were, said Cooke, "not a bad estimate" but most other calculations estimated the Earth at only a few thousand years old.

Leaping ahead (in human terms) to 1748AD, we learned that French diplomat Benoit de Maillet's study of changing sea levels, though ultimately impractical, put the Earth at 2 billion years old.

Then came the move to measuring processes. For instance, in 1862 Lord Kelvin calculated that if the Earth had formed from a white-hot molten globe of iron it would take 98 million years to cool to modern temperatures. That theory was knocked out when radioactivity in minerals was discovered and the realisation dawned that the Earth might not be cooling down but heating up.

My favourite example from Cooke's tour of dating theories was Edmund Halley's effort to figure out how long it would take to turn fresh water into the salty seas. Halley, who had more luck spotting comets, felt confounded by not knowing how salty the sea was before his day.

Our own Ernest Rutherford also weighed in, in 1906 offering the use of radioactivity to date fossil sequences and coming up with a minimum age for the Earth of 497 million years.

Recently, I came across a shop stocking rocks impregnated with the fossilised remains of - I was told - 110-million-year-old creatures. It was amazing to study the delicate lines of bones fused into the chip of stone and to imagine the fish-like animal alive and flicking round in mud eons before humans existed.

How would it be to hold the oldest rocks on Earth? Or as Cooke and her fellow scientist do, slice them up, study them under microscopes and subject them to radiometric dating, which has determined that rocks found near Canada's Great Slave Lake are 4030 million years old.

Closer to home, in Western Australia, single zircon crystals have radiometric ages of as much as 4300 million years, making the tiny crystals the oldest materials found on Earth so far.

Our home, though, is a relative newcomer. New Zealand may have been around for only 100 to 200 million years.

Like the morning after a birthday bash, such notions make the head pound. It was with some relief that Hamilton's last Cafe Scientifique of the year ended with everyone scoffing a dinosaur-shaped birthday cake thoughtfully provided by the NZ Skeptics.

But the cafe that serves up science over the barstools has also gone on the road and since mid-year has packed them in the Bay of Plenty. The year's final cafe will be on the human genome at Tauranga on November 7.

* Yesterday, the NZ Association of Scientists awarded Penny Cooke and fellow Cafe Scientifique co-ordinator Alison Campbell, a Waikato University biological sciences lecturer, its 2005 science communicator award.