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Of course, this was a close as Kelvin ever came to publicly recanting his position.

Later, after radioactivity had been proven to be a significant source of the Earth's internal heat, he did privately admit that he might have been in error.

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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.

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

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.

Based on these assumptions he at first suggested an age of the Earth of between 100 Ma and 500 Ma.

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.

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.

In other words, the decay rates show annual changes that closely reflect the Earth's distance from the Sun (see illustration).

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.

The solar estimate was based on the idea that the energy supply for the solar radioactive flux is gravitational contraction.