Used by Lord Kelvin to estimate the age of the Earth in 1863 (first extra-Biblical estimate): Given an initial temperature and a present temperature, we know the rate of heat loss, so can calculate an age.
Energy on Earth is not quite in equilibrium 2 nd Law of Thermodynamics: Heat flows from warmer bodies (Earth) to cooler bodies (Space) Led to a huge conflict with Darwin et al. His age: 20-100 million years; not enough time for evolution or some observed geological phenomena.
Energy as a System But a scientist’s results are only as good as the data and the assumptions. What did Kelvin not know about? Convection in the mantle Heat from the radioactive decay of atomic nuclei