Apatite fission track dating
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.
If magnetic fluxuations or other influencing forces are strong enough, radiometric decay rates could be much more significantly effected.
Other factors and basic assumptions must also be considered.
The solar estimate was based on the idea that the energy supply for the solar radioactive flux is gravitational contraction.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.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.