Firmly entrenched and supported by the church, Ussher's date seemed impregnable. However, some skeptics put forward their own versions. A French Calvinist and lawyer, La Peyrere in 1641 postulated that there had been people on Earth before Adam. This was heresy and of course was banned by Cardinal Richelieu, the prime minister of France. Essentially a pragmatic and fair man, he however realised what effect La Peyrereses manuscript might have in France. La Peyrere kept this freedom, but he did not desist with his theories, eventually in 1655 invoking the wrath of the Catholic Church. He was arrested, interred and forced to recant his heretical views - in short he got off lightly. Enter another player on this grand stage - a Jesuit missionary, Farther Martino Martini, who was carrying out missionary work in China.
On telling the Chinese that mankind had been destroyed by God in a great deluge, they greeted his pronouncements with great hilarity and disbelief. Their own history stretched well beyond the so-called date of the Flood, and they had records to prove it. A reversal of roles took place, with Martini realising that the ancient Chinese chronology posed a serious challenge to the authority of the bible. In 1654 he returned to Europe, where he published an account of Chinese history, receiving of course the usual disbelief and hostility which marked all other controversial ideas perpetuated at the time. However arrest and persecution was out of the question as Martini had returned to China to further his missionary work. Europe continued her dyed-in-the-wool approach, but slowly thinking was evolving, with crucial intellectual shift away from biblical textual authority towards an enquiry based on scientific principles. Throughout Europe the cry was the same: rocks, not books were believed to hold the secrets of the past. And indeed they did, and still do, and it was left to the natural philosophers to prove the age of the world.
One of the more interesting notions of the 17th Century was that the Earth had been created fully formed; flat, beautiful, unblemished, with no disease, famine, mountains or deserts to blemish her perfect face. A golden age had existed before the Flood, and age when all God's blessings were poured out onto the world - in short a perfect God had created a perfect world. But now things were in decline - the Earth had become old and wrinkled, volcanoes and deserts were carbuncles and scars on the face of the Earth. An old prophecy that the world would last only 6000 years was being confirmed by nature. The thinking was that the Earth had run most of its days, and doomsday was not far off, perhaps only 350 years in the future (taking of course the date of 4004 BC as the day of creation).
However this idea of an ageing Earth brought the notion of a world in flux and a constant state of change to the fore, and it was Rene Descartes who lead the break with the literal interpretation of the bible. Reason was the reason de etre. Paris, 1625, and our clever and sociable Descartes had become the leading light of Parisian intellectual life - a time when libertine free thinkers were making their mark. Caution was always necessary and a weather eye was kept open for gathering storm clouds in the direction or Rome. One Giulio Vanini, a 'heretic' doctor, had had his tongue cut out before being strangled and burned six years before for voicing some anti religious views. Inevitably the storm broke and Descartes moved to Holland, a country which has always had a tradition of tolerance and liberalism which endures to this day. He began his philosophical treatise, The World, which was to be a completer revision of philosophy which he eventually published in 1641.
However it was a watered-down version of his original manuscript as it was a bad time for liberal thinkers. Galileo had recently been arrested and forced to recant his heretical views that the Earth went around the Sun. Descartes' view of the world was essentially that a few simple laws governed the universe, and these laws were what created the complex world around us. The world had the same relationship to God as a clock had to the clock maker - once it had been carefully constructed and set in motion, there was little more involvement from the creator. His ultimate achievement was to remove God from the day to day running of the world. Until then the belief was that God was intimately involved in the day to day, minute to minute running of the world. God may have created the world, but it was then governed by the laws of nature.
Newton then swept away Descartes' imprecise theories with his Principia - the laws governing the motions of the heavenly bodies were now defined in minute detail. However his seems to have wasted his intellect and the latter years of his life trying to tie the history of ancient kingdoms and biblical chronology into the record of astronomical events - eclipses and the arrival of comets - but with extremely limited success.
His friend Edmond Halley then came up with the theory that the age of the Earth could be estimated by the amount of salt in the sea. By measuring the rate of increase of the sea's salinity, and assuming that salt had been washed into the oceans at a constant rate, it would be possible to work backward to a time when the seas contained no salt at all, which would then give the age of the Earth. In 1715, this was a fantastic idea, but no one, least of all Edmond Halley, knew how to measure increases in salinity year on year, and he concluded that the observations would need to be made a century apart to be meaningful. However Methuselah, in spite of his famed longevity, was no longer alive, so this was impossibility.
Georges Louis Leclerc Buffon was the next to estimate the age of the Earth. He was well connected with the French court and had the ear of several of the crowned heads of Europe. Clever, hard working, and somewhat eccentric, he proposed the idea that the Earth's age could be estimated by the rate at which it cooled.
He heated up metal spheres and then estimated the rate of heat loss as they cooled, and then extrapolated this to the larger Earth, settling on the age of 75 000 to 168 000 years. This was a quantum leap from the 6000 year old theory and he nearly got excommunicated for his troubles. Being a politically astute animal, he recanted his heresies but then proceeded to repeat his assertions for the rest of his days.
Charles Darwin, apparently unhappy with creating controversy, but brave enough to do so anyway, suggested that the Earth was 300 million years old. This date proved so contentious that he actually withdrew it from his Origin of the Species. The great Lord Kelvin, who towered over Victorian science like a colossus, took Darwin to task for some of his assertions, and declared that the Earth cold be no older than 24 million years, which were downward revisions of an original figure of 400 million. However, in spite of Kelvin's phenomenal intellect, he was not aware of the invisible heat source deep with the Earth - radioactivity - which drives the engine of our planet. Radioactivity is a concept to which we have become accustomed to thanks to the popular press, nuclear power and its use in the medical field. But is Kelvin's day it was still unknown, and it was thanks to the pioneering work by Marie and Pierre Curie that radioactive material was first recognised and isolated. Earnest Rutherford then provided the world with an explanation of how it all worked, and of the energies released in the process - which then provided a mechanism for keeping the interior of the Earth at elevated temperatures. Other great names of nuclear physics come from this time - Bohr, Heisenberg, Thompson, Chadwick and of course Einstein. The properties of the atom and the elements became increasingly understood as well as the chemistry and isotopes of radioactive elements.
This understanding of radioactive half lives became fundamental to the dating of rocks, Professor Arthur Holmes of Durham University took up the cudgels and pioneered the methods which are used to this day. The technique was based on Rutherford's 1904 work, when he discovered that atoms decay from one element into another at a rate predictable enough to allow them to be used as clocks.
Holmes measured the rate of decay of uranium to lead, and used this to calculate the age of the Earth. In spite of a lack of funds and sophisticated equipment, he announced in 1946 that the Earth was at least 3 billion years old. His methods met with praise, but his date was not. But he was far closer to the mark than Kelvin had been.
Building on Holmes' pioneering work, Clair Patterson of the University of Chicago took up the challenge. The problem with dating the Earth is that one needs rocks almost that old, and these are rare thanks to the ongoing cycling of crust due to plate tectonics. In addition one needs uranium-bearing crystals within these rocks. Patterson made the assumption that if all the lead on Earth came from radioactive decay, then it would be easy to find the age of the Earth. The more lead a rock contained, the older it had to be. In reality all the lead on Earth did not come from uranium and it was impossible to separate out the 'primordial' lead - that which had been around from the creation of the Earth, from that derived from radioactive decay. To sidestep this thorny issue, another brilliant assumption was made, that meteorites were left over building material dating back to the early days of the solar system, and thus their lead content would be the same as that of the early Earth. Perhaps more importantly they contained no uranium to upset the clock. None of the lead in meteorites then would have come from radioactive decay. Harrison Brown, Patterson's doctoral supervisor said this to him:
"Pat, you just go in and get an iron meteorite - I'll get it for you. We'll get the lead out of the iron meteorite. You measure its isotopic composition and you stick it into the equation. And you'll be famous, because you have will have measured the age of the Earth."
It took Patterson 7 years to build a completely lead free laboratory and to obtain a result, which dated the Earth at 4550 million years. After three hundred years we had at last a date for the beginning of the World. And that figure still stands to this day.
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