Geologist Ralph Harvey and science historian Mott Greene explain the principles of radiometric dating and its application in determining the age of Earth, in this video segment from A Science Odyssey. As the uranium in rocks decays, it emits subatomic particles and turns into lead at a constant rate. Measuring the uranium-to-lead ratios in the oldest rocks on Earth gave scientists an estimated age of the planet of at least 4.5 billion years.