This slots into the wider story about how science found out about the age of the earth, which is fascinating! In the 1860s Lord Kelvin presented[1] one of the

This slots into the wider story about how science found out about the age of the earth, which is fascinating! In the 1860s Lord Kelvin presented[1] one of the first physical arguments in favor of the earth being millions of years old. His argument was that we could predict the age of the earth by measuring the temperature gradient at the surface of the planet (which is related to the amount of heat that is being radiated to space). He modeled the earth as a cooling solid sphere and using the heat equation[2] he computed that if the earth started at the temperature of molten rock then it would take some tens of millions of years to cool down to the current surface temperatures. Conveniently, this estimate matched contemporaneous theories about the sun being powered by gravity. If one assumed that the sun was slowly shrinking while releasing that gravitational potential energy as light, it also ended up with an age of some tens of millions of years. (Remember: back then neither radioactive decay nor nuclear fusion had been discovered yet). The major flaw in Lord Kelvin's calculations were only discovered in the 1890's when Irish mathematician John Perry suggested that modelling the earth as a cooling solid sphere would not be appropriate if the earth's core were actually liquid. The key difference is that a liquid core transfers heat much more efficiently, due to convection. In the presence of a liquid core the surface of the earth receives more heat from the interior and takes longer to cool down to the present temperatures, compared with an earth that has a solid core. Once Perry corrected Kelvin's model to account for convection the age of the earth jumped to the billions of years that we now know it has. It took a while for the theory of the convective mantle to become widely accepted but it was ultimately proven correct by radiometric dating. [1] https://en.wikipedia.org/wiki/Age_of_the_Earth#Early_calculations https://en.wikipedia.org/wiki/Age_of_the_Earth#Early_calcula... [2] https://en.wikipedia.org/wiki/Heat_equation https://en.wikipedia.org/wiki/Heat_equation