Other factors and basic assumptions must also be considered.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.
Potassium - Argon and Argon - Argon dating are based on the current understanding that radioactive Potassium-40 decays to the stable form, Argon-40 with a half-life of approximately 1.25 billion years.
There are some circumstances that can affect this rate such as magnetic fluctuations etc...
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.
Of course, this was a close as Kelvin ever came to publicly recanting his position.
was published, the earth was "scientifically" determined to be 100 million years old. In 1947, science firmly established that the earth was 3.4 billion years old.