Medieval scholars, applying mental arithmetic to the clues provided in their Biblical manuscripts, arrived at very firm conclusions as to how long the Earth was in existence. Give or take a century or two, the world was 6,000 years old. But towards the end of the 18th century, Scottish geologist, James Hutton, pointed out the obvious. Certain natural processes, like the cutting out of river channels and the erosion of mountains, needed millions, not thousands, of years for their completion. And some years later Charles Darwin reached a similar conclusion; he needed many, many millions of years to accommodate his evolutionary theories.
Various ways were tried to improve on Bible-based estimates of the Earth's age. One was to look at the accumulation of salt in the oceans; rivers steadily wash salt into the sea, and since only fresh water leaves it by evaporation, the salt concentration rises with time. It was reckoned that the time required for rivers to endow the oceans which their present 3 per cent concentration of salt was of the order of a billion years.
Yet another method, suggested by the Scottish physicist, Lord Kelvin, was to calculate the rate at which the Earth was currently losing heat; armed with that figure it should be possible to estimate the length of time that had had passed since the Earth was just a molten sphere. This too produced an estimate of something like a billion years.
But both methodologies were flawed. Sea salt for example, is recycled, so any simple calculation based on it would be an underestimate. And the flaw in the thermal method was exposed with the discovery of radioactivity in the 1890s.
Radioactivity generates heat. The cooling of the planet comprises only part of the heat radiating from the Earth at any given time, the remainder being due to the radioactive decay of many radioactive elements in the rocks on, or near, the surface of the planet. Since much of the heat being lost by the Earth was "new" heat generated close to the surface, rather than "old" residual heat from a once molten planet, calculations based Kelvin's method were inaccurate.
But radioactivity also provided the solution. Radioactive elements decay at a constant rate, being transformed in the case of uranium, for example, into lead. By comparing the amount of uranium in a rock sample with the amount of the lead resulting from decay, it is possible to calculate the length of time for which the process has been taking place. Such methods provided our present estimate of the age of the Earth - a full 4.6 billion years.