How do we know how old the Earth is?
Principally from radiometric dating of meteorites, which give an age of approximately 4.54 billion years — and the reason meteorites are used rather than Earth rocks is itself the interesting part.
Why not date Earth rocks directly? Because the Earth recycles its surface. Plate tectonics subducts and destroys old crust, erosion removes material, and metamorphism resets many mineral clocks. The oldest known Earth rocks are around four billion years old, and the oldest individual mineral grains slightly older — but these are minimum ages, not the age of the planet.
Why meteorites work. Certain meteorites are undifferentiated material left over from the formation of the solar system, never incorporated into a body large enough to melt and reset. They have sat essentially unchanged since the solar system formed, so dating them dates that formation — and the Earth accreted from the same material at the same time.
How radiometric dating works. Certain isotopes decay to daughter products at a constant rate, described by a half-life. Measuring the ratio of parent to daughter gives the time elapsed since the mineral formed and became a closed system.
Why it is reliable:
Decay rates are constant, unaffected by temperature, pressure or chemical state — established experimentally across extreme conditions.
Multiple independent isotope systems with very different half-lives — uranium-lead, rubidium-strontium, samarium-neodymium, potassium-argon — give concordant ages on the same samples. Independent clocks based on different physics agreeing is far stronger evidence than any single measurement.
Isochron methods detect and correct for the presence of initial daughter isotope, which addresses the main theoretical objection.
The historical work. Clair Patterson established the modern figure in the 1950s using lead isotopes in meteorites — work that also led him to identify widespread lead contamination from petrol and to campaign successfully for its removal, an unusual scientific legacy.
Corroborating evidence: the age of the oldest stars, models of stellar evolution, and the age of lunar samples, all consistent.
Earlier estimates failed because they lacked radioactivity — Kelvin's calculation from cooling rates gave far too young an age because he did not know about internal heating from radioactive decay.