Earth's age — 4.568 billion years, with a margin of error of only about 70 million years — wasn't precisely determined until 1956, when geochemist Clair Patterson used U-Pb radiometric dating on the Canyon Diablo iron meteorite. The key insight behind this method is that meteorites, Earth, and the rest of the solar system all formed together from the same original solar nebula at essentially the same time — meaning a meteorite's age, once precisely measured, effectively reveals Earth's age too, even though no single piece of Earth's own original crust has survived intact.

This approach elegantly sidesteps a major problem: Earth's surface has been so thoroughly reshaped by plate tectonics, erosion, and volcanism that no rock from Earth's actual formation still exists to date directly. By dating meteorite material instead — material that has remained essentially unchanged since the solar system's formation — Patterson found a reliable proxy for Earth's true age.

In 1862, physicist Lord Kelvin had calculated Earth's age at only 20 to 400 million years, based on how long it would take Earth to cool from a molten state — a dramatic underestimate that was wrong specifically because Kelvin had no knowledge of radioactive decay, which continuously generates internal heat and kept Earth cooling far more slowly than his calculations assumed. Patterson's later, far more accurate radiometric approach didn't just correct this error — his research into lead isotopes also revealed something entirely unexpected: dramatic, widespread industrial lead contamination throughout the environment, a discovery that directly contributed to major public health reforms, including the phase-out of leaded gasoline.