BC and all that

In reality, Ussher's was just one of many such calculations. He had the prestige of being a renowned scholar, Professor of Divinity and Vice-Chancellor of Trinity College, Dublin. He used not only Old Testament sources but also contemporary knowledge of the Julian calendar, devised by the Renaissance scholar Joseph Justus Scaliger, astronomical calculation and extra-biblical sources. Scaliger's Julian period was taken to have started on a hypothetical day, January 1st, 4713 BC, which he thought predated all known historical events. According to Ussher's 1650 Annals of the Old Covenant from the First Origin of the World, the world began the night before Sunday October 23rd in year 710 of the Julian calendar - 4004 years before the birth of Christ.

However, as long ago as the fifth century, biblical scholars had noticed that there were problems with the Old Testament account, such as the creation of light before the creation of the Sun (Genesis 1:3, 16) and the fact that the moon is not light emitting but light reflecting. It was also long recognised that there are different components to the Genesis account. In addition, growing scholarly interest in the classical world and the Middle East, during the eighteenth century, led to a reassessment of prehistory and a questioning of the Old Testament chronology. Nevertheless, for the majority of believers the idea that the creation took just six days also became firmly entrenched in western culture. However, by the end of the eighteenth century, there were a number of more scientific attempts to calculate the Earth's age.

The French natural philosopher and experimenter Comte de Buffon mustered a number of lines of evidence to support his ideas about the cooling of the Earth and the time it had taken to do so; in other words, the age of the Earth. The existence of the bones and tusks of elephants in far northern latitudes such as Siberia indicated to Buffon that in earlier times the Earth must have been as hot as Africa. By 1778 Buffon, as we have seen, divided Earth history into seven epochs, echoing the days of creation in the Genesis narrative, with man only appearing in the last epoch, which suggested to Buffon that there was therefore a substantially long period of prehistory or prehuman time.

Buffon also tried an experimental approach, building on ideas developed by Isaac Newton. Heating a number of iron cannonballs of different sizes to white heat, Buffon timed how long they took to cool. He found Newton's conjecture that the cooling time was proportional to the diameter of the sphere was correct. However, Buffon disagreed with Newton's calculation for the cooling time of an iron ball the size of the Earth and reckoned that it would have taken 96,670 years and 132 days, almost twice as long as Newton's calculation.

Buffon was not entirely satisfied with the result. He was aware that the Earth was made of other materials with different cooling rates and that the Sun contributed heat to the Earth, which would have prolonged its cooling time. By 1779, he arrived at his final calculation of 74,832 years for the age of the Earth. Buffon reckoned that some 60,000 years had passed before the Earth was cool enough to be first inhabited by life and then steamy jungles had stretched as far north as Siberia. Not for another 10,000 years was it fit for humans. According to Buffon:

thus we are persuaded, independently of the authority of the sacred books, that man has been created last, and that he arrived to take the sceptre of the earth only when it was found worthy of his empire.

Despite Buffon's work and that of Newton before him, Ussher's 4004 BC date was still widely accepted and even printed in Bibles as an historic truth. Not surprisingly, the notion of a 6ooo-year-old Earth persisted into the nineteenth century and beyond as an established 'fact' for some fundamentalist Christians. It was still being quoted in the infamous 1925 Scopes trial between American creationists and evolutionists in Dayton, Ohio.

By the latter part of the eighteenth century, James Hutton had shown from his study of stratification and rates of sedimentation that Ussher's 6000 years was not nearly enough time for observed rates of geological processes. Hutton, a Scottish philosopher of nature, proclaimed from his examination of strata that he could see 'no vestige of a beginning, no prospect of an end'. By the latter part of the nineteenth century, Lyell and Darwin were using rates of erosion and deposition to estimate that the Earth was hundreds of millions of years old. In the first edition of his book On the Origin of Species, Darwin estimated that it had taken 300 million years for the erosion of Weald, which involved just the relatively recent (geologically speaking) strata of the Cretaceous and Tertiary systems.

Darwin considered that evolution had proceeded by a succession of gradual changes and splitting of descendent species from common ancestors. Consequently, he needed as much time as he could get for all the changes that must have taken place to evolve backboned animals from single-celled protists and within the vertebrates to have evolved humans from fish. And then again, he argued that the origin of life must have preceded the first protists by a huge amount of time. By the 1850s Darwin knew that fish fossils had been found in Old Red Sandstone (Devonian) strata. He also knew that Devonian strata were overlain by many miles of younger stratified rocks and must therefore be very ancient, so how much more ancient must the origin of protists have been?

Such geological efforts were severely criticised by the eminent Belfast-born physicist William Thomson, Lord Kelvin. In 1862, Kelvin dismissed Darwin's published calculation - 'what then are we to think of such geological estimates as 300,000,000 years for the "denudation of the Weald"?' Furthermore, Kelvin argued that as the age of the Earth was constrained by the age of the cooling Sun, it was to be expected that its inhabitants 'cannot continue to enjoy the light and heat essential to their life, for many years longer, unless sources now unknown to us are prepared in the great storehouse of creation'.

William Thomson, Lord Kelvin, 1824-1907, Belfast-bom Scottish physicist and inventor, trained in Glasgow, Cambridge and Paris. He returned to Glasgow as professor (1846-99). One of the greatest scientists of his day, Thomson devoted considerable effort to calculating the age of the Earth and was highly critical of the estimates made by geologists. Created baronet in 1892.

Scientifically, Kelvin was a strict empiricist and despised 'woolly thinking' and vague speculation without hard data, of the kind that Darwin was using. In addition, Kelvin was no friend of evolutionary theory. Like Buffon before him, Kelvin thought that he could use physics to discover how long it had taken for the Earth to cool to its present temperature from the initial heat of its formation. Knowledge of the common rocks of the Earth, their melting points and conductivity had advanced enormously since Buffon's days. Kelvin realised that a simple iron ball model was inadequate and that most of the rocks of the Earth seemed to be comprised of silicate minerals, whose ability to conduct heat was very different from that of iron.

Kelvin's preliminary calculations gave the age of the Sun as around 20 million years and that of the Earth somewhere between 200 and 1000 million years, but he wanted to refine his calculations and did not publish these figures. He was aided by a new theory of heat conduction developed by the French mathematician Joseph Fourier (1768-1830) and assumed that the initial temperature of the molten rock from which the Earth cooled was 3871 degrees C (7000 degrees F). Kelvin was informed by John Phillips that temperature increases downwards into the interior by 0.55 degrees C (1 degree F) for every 15 or so metres (45-60 ft). From this, in 1862 Kelvin calculated that it had taken 98 million years to reach its present state. But he also stated that if his assumptions did not hold then the Earth's age was likely to range anywhere between 20 and 400 million years old.

However, Kelvin was not at all happy with this result because it seemed to be far too long. His concern arose from the prevailing view that the Sun was probably no more than some 40 million years old and the age of the Earth should not be greater than that of the Sun. Over the following decade Kelvin constantly reworked his calculations and each time managed to revise the age downwards to 40 and then later to around 20 million years (in 1893), which was in line with the prevailing ideas about the age of the Sun. Such was Kelvin's influence that his measures were readily accepted by most scientists, apart from some geologists who still thought that they were a serious underestimate.

As the role of radioactivity in maintaining the internal heat of the Earth was not known, Kelvin's measure was indeed a serious underestimate. It is often stated that the discovery of radioactivity and its role in maintaining the internal temperature of the Earth finally broke the straitjacket of Kelvin's influence. But this view fails to take into account Kelvin's dogmatism in linking the age of the Earth to that of the Sun. Ideas about solar energy and the Sun's compositional homogeneity were not overthrown by the discovery of radioactivity. Not until the recognition of thermonuclear fusion in the 1930s was the paradox fully resolved. Nevertheless, it was the discovery of radioactivity and the subsequent development of 'radiometric dating' that eventually made it possible to break Kelvin's stranglehold on the generally accepted age of the Earth.

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