Kelvin's Earth Age Estimate
flickr photo, user tom clearwood
In the 19th century, Sir William Thomson, Lord Kelvin, was the first person to use physics to calculate the ages of the Earth and sun. Though he estimated these bodies were about 50 times younger than we now think they are, the calculations themselves were breakthroughs. [50 Amazing Facts About Planet Earth]
Lord Kelvin based his calculation on the idea that Earth began as a hot, molten ball, and has slowly cooled over time. He attempted to calculate how long it would have taken for our planet to get to its current temperature gradient. His numbers were off partly because scientists had not yet discovered radioactivity, so he couldn't include it in his calculation. Radioactive elements in Earth, such as uranium and thorium, are an additional source of heating inside our planet.
But Livio says this wasn't Kelvin's biggest blunder — even if he had included radioactivity, his Earth age estimate would have remained nearly the same. Rather, Kelvin made the larger mistake of ignoring the possibility that unknown mechanisms might have transported heat throughout Earth.
"He assumed that heat is transported with precisely the same efficiency throughout the entire depth of the Earth," Livio said. Even after others suggested that heat could be transported more efficiently deep inside Earth, Lord Kelvin dismissed the possibility. "Kelvin was used to being right far too many times. It was pointed out to him but he never really accepted it."