His result was in close agreement with his estimate of the age of the earth.The solar estimate was based on the idea that the energy supply for the solar radioactive flux is gravitational contraction.Based on these assumptions he at first suggested an age of the Earth of between 100 Ma and 500 Ma.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.
He believed this even though he did admit that some heat might be generated by the tidal forces or by chemical action.
However, on the whole, he thought that these sources were not adequate to account for anything more than a small faction of the heat lost by the Earth.
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.
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.