How old is the Earth?
There is no direct way to find out the age of the Earth. In the modern era, the predictably steady decay of radioactive material in rocks allows them to be dated. But even that is little help here. The Earth is so geologically active that it has no rocks lying around from the time of its formation. Instead, the Earth has to be dated more by analogy with meteorites and other objects thought to have formed at around the same time. This yields an age for the Earth of about 4.54 billion years.
But in the past, the lack of hard data has not deterred scientists from coming up with their own estimates, based on some ingenious lateral thinking.
For example, it is obvious that the Earth and the Sun formed by condensing from some much larger and less dense mass. Lord Kelvin, one of the greatest scientists of the nineteenth century, was quick to see that this condensation must release energy in the form of heat. And as befits the man who gave his name to the scientific scale of temperature (hence the K), Kelvin saw that this was the high road to determining the age of both objects. He worked out that for the Sun to be as hot as it is today, it could not be more than a few million years old. He then applied the same logic to the Earth, using figures for the heat flow through the crust, and worked out that the Earth could not be more than 40 million years old.
These figures were published to the distress of a Victorian England just gening used to the idea of evolution by natural selection made famous by Charles Darwin. Darwin thought that the Earth had to be ten times as old as Kelvin had “proved”.
As the saying goes, idiots do pretty stupid things, but to make a terrible mess you need someone really clever. A few years after Kelvin's figures were published, radioactivity was discovered and it turned out to be the key to the problem. Part of the Earth's inner heat comes from the decay of uranium and thorium atoms (mainly), while the Sun's comes from fusing hydrogen into helium.
Before you scoff, think of it from Kelvin's point of view. At that time, even the idea of atoms was by no means universally accepted, much less the concept of them decaying to make the Earth warm inside and fusing to make the stars shine. And in any case, his estimate was far closer than the few thousand years favoured by earlier scholars such as the Bishop of Armagh in Ireland, James Ussher, who calculated that the Creation had occurred on the evening of 23 October 4004 BC. Kelvin attempted to apply science to the problem and was out by a factor of 100, but Ussher's answer was 750,000 times too short.
However, Kelvin will go down in history as a lesson that distinguished scientists can be dead wrong. He announced in August 1896 that he had “not the smallest molecule of faith in aeronautical navigation”. The Wright Brothers took to the air seven years later.