The discovery of "deep time", the idea that time in our universe is measured in billions of years, goes back at least to the speculations of the German philosopher Immanuel Kant (1724-1804), the French mathematician and astronomer Pierre Simon Laplace (1749-1827) and the Scottish naturalist James Hutton (1726-1797). Kant and Laplace proposed the "nebular hypothesis" of the origin of the solar system, whereby the planets formed from condensation out of matter orbiting the early Sun. Obviously such an origin implies enormous amounts of time. Hutton proposed immensely long time spans to explain how the observable rates of erosion and deposition, and the ongoing volcanic activity, can be made responsible for the origin of great valleys, thick sediment sequences and mountain ranges, and all other features of Earth's surface.
Early estimates of actual time spans, in the 19th century, ran to 100 million years and more. Of course, such estimates did not sit well with those who reckoned geologic time by studying the genealogies given in the Old Testament, which summarizes the history of the Jewish people. (In the 1650s Archbishop James Ussher of Ireland, one of the more careful and distinguished scholars of time, put the beginning of Earth at 4004 B.C., a surprisingly “precise” estimate.) An estimate of 100 million years or more proved unacceptable to the famous British physicist William Thomson (1824-1907), better known as Lord Kelvin. Kelvin, then the world's expert on thermodynamics, calculated (in the late 1800s) an age of 20 to 40 million years for the Earth, assuming that the planet cooled from an originally molten condition. This age seemed reasonable, considering that the Sun should have burned out if it were much older, and that a planet should be younger than its star.
The discoveries by Becquerel and the Curies not only set the stage for the finding that mass equals energy, but also led straight into the field of radioactive dating.
The principle of radioactive dating is simple: The content of radioactive elements in a given material decreases with time, as the element gives off radiation (alpha, beta or gamma) and thereby changes its natureby turning it into a "daughter" element. By measuring the amount of a radioactive element and its daughter element (or elements) an estimate can be made about how much of the original amount of radioactive element remains. From the rate of decay (measured by determining the activity) a "half-life" is estimated for each radioactive element. This is the time it takes for one-half of the element in question to "decay" into a daughter element. By using different decay series with different half-lives (uranium-238 to lead-206; uranium-235 to lead-207; thorium-232 to lead-208; rubidium-87 to strontium-87; potassium-40 to argon-40) the estimates can be double-checked and greatly refined (see figure above).