To a first approximation, the carbon isotope ratio in the atmosphere and in all living organisms is fixed.

As long as the organism is alive, it maintains this same carbon-14 to carbon-12 ratio. But as soon as it dies, exchange with the atmosphere ceases. The slow decay of carbon-14 causes the isotope ratio to diminish by half every 5570 years. It is a simple matter in principle, although careful work is required to prevent contamination, to determine the actual carbon isotope ratio in any ancient sample of wood or other preserved organic matter, and to calculate from this how long ago the specimen died. In this way, surprisingly accurate chronologies can be constructed for the past 10,000 years. Radiocarbon dating has become one of the most important tools of archaeologists and palaentologists.

Similar dating methods can be used for geologic time spans by picking isotopes with longer half-lives. Potassium-40 decays with a half-life of 1.3 billion years to argon-40. Uranium-238 decays to lead-206 in a series of steps, of which the slowest has a half-life of 4.5 billion years, and rubidium-87 decays with a half-life of 47.5 billion years to strontium-87. Dating rock samples from the Earth and moon, and meteorite fragments, via potassium-argon, uranium-lead, and rubidium-strontium methods, has been an important tool in the study of our solar system.