The Chronologer's Quest

Most of us have seen or heard of those who challenge the age of the Earth, from the undue pressure on the NPS, to the assertions that the Earth is "really" just 6000 or so years old. But how did we arrive at the present figure of 4.55 billion years?

"The Chronologers' Quest: The Search for the Age of the Earth" (Patrick Wyse Jackson), gives a nice and comprehensive account of the project to date the earth, and the means used to do it, from early modern theological approaches like the famous Ussher's (and Jackson has some corrections to make to Gould's essay on the topic), through to the guy who finally achieved it.

There is a too-brief and sketchy review of creation myths in various religions, before Jackson moves on to the western tradition that began in the 16th century. The early players are familiar to me, although I know them from a different context.

There's a nice review of the ways in which Ussher and his colleagues calculated the age of the earth from the biblical narrative. John Swan calculated 5,632 years in 1635, and earlier Cooper's Chronology in 1560 had listed a range of dates from 5,041 BC to 8,522 BC. Bp Ussher wasn't the only chronologer of the earth in his time. The techniques used by Ussher are discussed in detail. But the "nine o'clock in the morning of the 23rd of October" is both Ussher's and John Lightfoot's. Ussher calculated the date, but Lightfoot gave the hour. However, Lightfoot gave 12 September, not 23 October. Thomas Burnett, in the famous Telluris Theoria Sacra (Sacred Theory of the Earth 1681, 1689) proposed all kinds of interesting speculations (such as the Flood occurred because the earth was then flat), but remained orthodox with respect to the earth's age.

Later, in the seventeenth century, Edward Lhwyd counted the number of boulders that fell in a Welsh valley each year, and came up with 200,000-300,000 years, 50 times greater than the biblical dates presented by theology, which ranged from 14,000 to the famous 6000. Edmund Halley, of comet fame, tried to infer from the rate of salination of the oceans, but his method was flawed. Benôit de Maillet, in a pseudonymous piece named Telliamed (published 1748) has his Indian philosopher character say the earth is 2 billion years old, but this was not scientific inference.

After Steno and Hooke tried to establish methodological protocols for geology, there were a number of developments in geology, leading to the theories of Werner, who thought all was caused by the actions of water (Neptunism), and Desmarest, who thought it was due to the actions of volcanoes (Vulcanism). Into this came James Hutton, who proposed that there were relatively uniform secondary causes that caused deposition, and gave an indefinite age to the earth. Hutton later influenced Lyell, who influenced Darwin.

The first real attempt to do this scientifically was the great Buffon, who had an entire, and somewhat speculative, cosmology, based on the cooling properties of iron and clay. He came up with 75,000 years, which upset many French theologians at the Sorbonne, who made him retract it. Newton had proposed something similar earlier, and Buffon was a great admirer of Newton, and modeled his work on Newton's.

Things got interesting in the 19th century. I won't rehearse Jackson's narrative of Segdwick, Murchison, Lyell, Agassiz and others, and the rise of sedimentation as a dating mechanism (giving dates from 3 million to 1,584 million years). What is most interesting to me is the way the debate changed in the 20th century. This was when radioactivity became both a reason for an old earth, and a way to measure it.

Lord Kelvin (then Professor Thompson) tried to estimate the age of the earth like Buffon had, using the principles of thermodynamics he himself had helped formulate. He got ages ranging from 20 to 40 million years, giving Darwin no small problems. Ironically, Darwin's son George was both Thompson's assistant, and contributed to the debate by a minimum of 56 million years based on tidal slowing.

Shortly before Thompson's death in 1907, radium was discovered. Here was a source of heat, and Ernst Rutherford noted that it greatly increased the age of the earth. Moreover, the products of radioactive decay could be accurately estimated in age by the ratio of decay from older materials. Dates ranging from 400 to 2,200 million years followed shortly from Bertram Boltwood at Yale. Arthur Holmes published figures from 1,640 million years in 1911, to around 3,000 million years in 1935. The final hero of the story is the fellow who gave us the date we have today, based on lead isotopes : 4.55by ± 70 million years. His name was Clair Patterson, and he published in 1956.

This is an excellent book, full of personal anecdotes about the major players, and I recommend it, especially as an adjunct to the much more detailed book by G. Brent Dalrymple, The Age of the Earth.