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.

In short, the assumption that decay rates are immune to outside influences isn't as solid as it once appeared to be.

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...

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.

It was based on the idea that no significant source of novel heat energy was affecting the Earth.

Interweaving the relative time scale with the atomic time scale poses certain problems because only certain types of rocks, chiefly the igneous variety, can be dated directly by radiometric methods; but these rocks do not ordinarily contain fossils.

Some, like Robert Gentry, have even argued that Radio-halos from rapidly decaying radioactive isotopes in granite seem to indicate that the granites were formed almost instantly.

Both the physical geologists and paleontologists could point to evidence that much more time was needed to produce what they saw in the stratigraphic and fossil records.

As one answer to his critics, Kelvin produced a completely independent estimate -- this time for the age of the Sun.

These two independent and agreeing dating methods for of the age of two primary members of the solar system formed a strong case for the correctness of his answer within the scientific community.

This just goes to show that just because independent estimates of age seem to agree with each other doesn't mean that they're correct - despite the fact that this particular argument is the very same one used to support the validity of radiometric dating today.

Of course, the detected variation is no more than 0.2% of the published rates, but this paper is still quite interesting since such a correlation was never suspected before.