The accumulating evidence pointed to an extraordinary new idea: that the history of Earth goes back much, much further than any human memory.
In 1788, Scottish geologist James Hutton published his "Theory of Earth," which introduced the world to the idea of "deep time." The implications of the treatise were revolutionary: Not only was Earth not young, but it was not static, Hutton said.
The United States Geological Survey(USGS) website has a lot of indepth material about how the age of the Solar System was determined.
The basics of it are that all material radioactively decays into a stable isotope.
Although researchers have determined the ages of rocks from other planetary bodies, the actual experiments—like analyzing meteorites and moon rocks—have always been done on Earth.
Now, for the first time, researchers have successfully determined the age of a Martian rock—with experiments performed on Mars.
The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.
These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay.
Radiometric dating is based upon the fact that some forms of chemical elements are radioactive, which was discovered in 1896 by Henri Becquerel and his assistants, Marie and Pierre Curie.A century later, William Smith realized that rock layers at distant locations came from the same time period.He created a catalog of strata (which all got colorful names such as Lias Blue and Ditto White) and argued that each one represented a distinct time in Earth's history — a principle known as fossil succession.Over time, radioactive isotopes change into stable isotopes by a process known as radioactive decay.Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years.