I wouldn’t mind sharing a pint or three with Patrick Wyse Jackson. As he works in Dublin it’s guaranteed to be a fine evening regardless, but his recent book The Chronologers’ Quest: The Search for the Age of the Earth* also shows that he probably can’t be beaten as a storehouse of interesting tales from the early history of geology, and the pioneers who aided its growth into a mature analytical science. The Chronologer’s Quest is his account of an important aspect of this development: the struggle to determine the age of the Earth.
Those looking for a textbook treatment of this subject should probably look elsewhere, because this is as much about the people responsible for the vital discoveries and insights – their lives, their background and the cultural and philosophical environment that they worked in – as it is about the science. If you’re not already familiar with the bare bones of the narrative – which begins with myths and biblical genealogies being considered primary evidence, and ends with radioactive decay products as the clock of choice – you might occasionally lose sight of it amidst all the anecdotes. But if (like me) you thought you knew the story quite well, you may well be surprised at all the things you didn’t know.
Jackson starts with a brief discussion of early creation myths, but the story really kicks off in 17th century Europe, where it turns out that Archbishop Ussher of 4004 BC fame (to whom Jackson is distantly related) was only one of many attempting to work out a consistent Biblical chronology, and hence establish how many years had passed since Creation Week. From the perspective of modern science, it seems surprising that this line of inquiry should have attracted so much scholarly attention, but at the time our probing of the natural world had yet to provide any real competitor to the Bible’s account of Earth history. Reflecting the somewhat contradictory nature of the source material, the proposed dates ranged between about 5,000 and 3,000 BC; the 4004 BC estimate survived its contemporaries due to its association with the King James Bible rather than any obvious superiority, and despite his modern infamy it is far from certain that Ussher was the source. He was certainly not responsible for the laughably precise estimate of “9 o’clock on October 23rd”. Such are the whims of history.
But even as Ussher and his contemporaries were perfecting their estimates, and long before James Hutton saw “no vestige of a beginning – and no prospect of an end”, a few inquiring minds were noting evidence that our planet was a little older than a few thousand years, and laying the foundations for the concept of “deep time”. Some of the names – Halley, Hooke – will probably be familiar even if their specific ideas are not, whilst others – Edward Lhywd, Benoit de Maillet (who proposed way back in 1718 that the Earth was 2 billion years old, in a book which was not published for a further 30 years) – won’t. For me it was interesting to discover that Hutton’s ideas were not a complete bolt from the blue, as is often implied.
From the end of the 18th century, the work of Hutton and Lyell meant that the idea of an ancient Earth became well established amongst the geological fraternity, but working out exactly how ancient was a bit of a problem – none of the proposed methods (sediment accumulation rates, ocean salinity) would give consistent answers. Plus, physicists such as Lord Kelvin were loudly claiming that both the Sun and the Earth were too warm to be as old as the rocks seemed to say. The discovery of radioactivity solved both problems, although it turns out the physicists had another go at cutting down geological time following the discovery that the Universe was expanding, and lost again. Some people never learn, it seems.
Nowadays, the actual age of the Earth – 4.55 billion years – is well established. It’s sobering to think about how recently we didn’t know that – it’s a result younger than relativity, and more recent than quantum physics. The tale of how we came to know is more fascinating, and more complicated, than I imagined.
*In the interests of full disclosure, I should state that this review comes via a free copy sent to me by Cambridge University Press, and I’m always well-disposed towards people who send me free books…