Since time immemorial, humans have wondered about their world. Philosophers speculated on the age of the Earth thousands of years ago. The Brahmins of India believed that the Earth was eternal, while the Judeo-Christian world believed firmly that the Earth was 6000 years old. Today 
we know by scientific evidence that the Earth is approximately 4.8 billion years old, but how did we get to that figure?

      The Judeo-Christian belief that the Earth was 6000 years old is still argued by some fundamentalists though they are a tiny minority. They steadfastly base their beliefs on the fact that the Old Testament in their bible is an accurate and literal history of the world. This belief was given a boost in 1642 by  John Lightfoot, a distinguished Greek scholar and Vice Chancellor of Cambridge University who got very specific and said that the moment of creation was 9:00 AM, September 17th , 3928 BC.  Its hazy just how he arrived at this figure but it had something to do with adding up all those "begats" in the ancestries of people mentioned.

      This belief was reinforced for a few hundred years and the Church would tolerate no nonsense to the contrary. In 1749, the distinguished French scholar Comte de Buffon proposed that the 6 days of creation may have been 6 long epochs of time and that the Earth's surface had been shaped and reshaped by processes still going on. The Church took great exception to this and threatened Buffon to recant and publicly accept the Old Testament age of 6000 years. No doubt remembering the fate of Galileo (who lived most of his life under house arrest for proposing the Earth went around the Sun,) and Giordano Bruno, who was burned at the stake for proposing the same theory and adding that he believed there was life elsewhere in the universe, Buffon complied.

     The scientific inquiry into the age of Earth began around the mid-18th century. Scientists used evidence such as rock layers in riverbeds - and estimates of the planet's original heat -- to try to determine Earth's age. In 1862, Lord Kelvin calculated how long Earth may have taken to cool from its original molten state. He concluded that Earth was born 20 to 40 million years ago.

     Then scientists learned about radioactivity -- the spontaneous decay of atoms of one chemical element into another. Heat is released during this process, and this heat keeps the Earth from cooling as fast as Kelvin calculated. Following this, radioactive dating came into the picture. It was this that enabled geologists  to reach the 4.8 billion year age for the Earth.

     Radioactive elements (such as uranium and strontium)  are unstable and decay at a predictable and measurable rate. For example, Uranium 238 decays into lead 206. If we compare the ratio of uranium 238 to lead 206 we can tell how much time has passed since the sample was pure. Through this method we are able to date rocks, meteorites and fossils.

     This method actually tells us the minimum age of the Earth, since the Earth was molten before it solidified into those rocks we date. For Earth rocks, the oldest are about 3.9 billion years old. We reach 4.8 billion years by estimating how long planetary formation and rock solidification takes. Our world is a very old world indeed!