Radioactive parent (P) atoms decay to stable daughter (D) atoms e.g.
the carbon isotope C-14 decays to nitrogen-14 and the uranium isotope U-235 decays to the lead isotope Pb-207.
These estimates give 4.4-4.5 billion years for moon rock, and 4.54 billion years for iron metreorites.
These techniques utilize the physical parameters of the earth, such as ice cores, annual lake sediments, and astronomical cycles.
Mathematically, P = Po exp(-lambda T), where lambda = the decay constant and T = the period of decay. a zero initial number of D atoms, where P(or D) = the current number of P(or D) atoms, it follows that the age of a rock or mineral is computed as T=(1/lambda)ln(1 D/P).