In the 18th and 19th centuries, geologists (scientists who study Earth and its history) used indirect methods to estimate Earth’s age.
In 1892, William Thomson (later known as Lord Kelvin) calculated Earth to be 100 million years old by assuming that it began as a molten ball and cooled off to its current temperature. In 1896, Thomson’s calculation was confirmed to be invalid when radioactivity was discovered, meaning that heat was continuously produced from radioactive elements under Earth’s surface.
Understanding radioactivity has allowed scientists to date Earth materials such as rocks from Earth’s crust and meteorites that are known to have formed at the same time as Earth. By considering the age of rocks and fossils, scientists created a timeline of geological history of 4.6 billion years.