Finding the age of rocks based on the presence of naturally occurring long-lived radioactive isotopes of several elements in certain minerals and rocks. Measurements of rock ages have enabled geologists to reconstruct the geologic history of the Earth from the time of its formation 4.6 × 10⁹ years ago to the present. Age determinations of rocks from the Moon have also contributed to knowledge of the history of the Moon, and may someday be used to study the history of Mars and of other bodies within the solar system.