Age of the Earth & Uranium235 Dating
    U
235 spontaneously decays via emission of α, β, & γ particles from its atomic nuclei
                                  changing to lead 207 with a half-life of 713 million years 
    U
238 spontaneously decays with emission of α, β, & γ particles
                                  changing to lead 206 with a half-life of 4.5 billion (4.5 Ga - Giga years)

To determine the age of rocks (etc...) one measures the ratio of parent isotope (U235) to decay product (Pb207). Assuming the rock formed at an initial time, say at the origin of Earth,   then we would know the number of half-lives that have occurred since then, and the age of the rock, and thus the age of the Earth.

Unfortunately, we often don't know the origin of the initial rock, but, an assumption can be made that all rocks formed about the same time, say as the solar system formed, thus meteorites, which formed at the same time as the Earth, have a U235/Pb207 ratio suggesting their age to be 4.6 billion years old, and we therefore conclude that the oldest solid rocks on the planet Earth are about 4.6 Ga.
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