A Brief History of Astronomy
Earth and its Moon
The age of Earth (and by inference, the age of most other objects in the solar system) is also not directly known. But related evidence can be studied, in this case, by the technique of radioactive dating. Various elements (the parent element) are unstable and decay to produce another (the daughter) element. The time in which one-half of a parent sample decays into its daughter product is known as the half-life (t1/2): it takes 4.5 billions years, for example, for one-half of a sample of uranium-238 (the form of uranium with 238 nuclear particles) to become lead-206. Alternatively, uranius-235 decays much quicker, with one-half of a sample becoming lead-207 in 710 million years.
After one half-life, the parent/daughter ratio is one-half; after two half-lives, the ratio is (1/2)2 = 1/4, three half-lives, (1/2)3 = 1/8, and so forth. Chemical analysis of a rock sample thus yields the present abundance ratios and an age for the formation of the rock. The oldest Earth rocks (which are rare due to the recycling of surface materials by plate tectonics) have an age of 3.8 × 109 years, which is a lower limit to the age of the planet and the solar system. A more correct estimate of the age of the solar system is based on materials that have been unaltered since their original formation. Applying radioactive dating to a specific class of meteorites believed to be unaltered since their formation yields consistent dates for their origin of 4.6 ±- 0.1 × 109 years. This solution is adopted as the age of Earth and the solar system.
