Scientific definition of radiometric dating
giving a lower limit for the age of the solar system.
It is hypothesised that the accretion of Earth began soon after the formation of the calcium-aluminium-rich inclusions and the meteorites.
Thus the age of the oldest terrestrial rock gives a minimum for the age of Earth, assuming that no rock has been intact for longer than the Earth itself.
The physicist Hermann von Helmholtz (in 1856) and astronomer Simon Newcomb (in 1892) contributed their own calculations of 22 and 18 million years respectively to the debate: they independently calculated the amount of time it would take for the Sun to condense down to its current diameter and brightness from the nebula of gas and dust from which it was born.
Their values were consistent with Thomson's calculations.
However, they assumed that the Sun was only glowing from the heat of its gravitational contraction.
The process of solar nuclear fusion was not yet known to science.