This estimate was actually reduced over his lifetime to between 20 Ma and 40 Ma and eventually to less than 10 Ma. Perry, in particular, a noted physicists and former assistant to Kelvin, showed that cooling calculations using different but equally likely assumptions and data resulted in ages for the Earth of as much as 29 Ga.Of course, later scientists, like John Perry and T. After this came to light, Kelvin admitted that he might just as well have set his original upper limit on the age of the Earth at 4,000 Ma instead of 400 Ma.What is especially telling about this whole story is the conclusion of the absolute truth of the conclusion based on premises that are weak, or at least not adequately demonstrated.
Of course, Kelvin formed his estimates of the age of the Sun without the knowledge of fusion as the true energy source of the Sun.
Without this knowledge, he argued that, "As for the future, we may say, with equal certainty, that inhabitants of the Earth cannot continue to enjoy the light and heat essential to their life, for many million years longer, unless sources now unknown to us are prepared in the great storehouse of creation."The same is true of the basis of Kelvin's estimate of the age of the Earth.
There are some circumstances that can affect this rate such as magnetic fluctuations etc...
But in general, this rate is felt by the vast majority of mainstream scientists to be a fundamental constant. al., published a paper suggesting that the decay rate of radioactive elements is related to the Earth's distance from the Sun.
The isochron dating method theoretically overcomes the need to know the initial ratio of parent and daughter isotopes. For now, we will look at those methods that do fall under the above assumptions.