Earth's age is something that people have been arguing about, at times bitterly, for a long, long time. Back in 1654, a scholar named John Lightfoot, whose calculations were based upon the Bible's Book of Genesis, proclaimed that Earth had been created at precisely 9 a.m. Mesopotamian time, on Oct. 26, 4004 B.C.E. In the late 1700s, a scientist named the Comte de Buffon heated up a small replica of the planet that he had created and measured the rate at which it cooled, and based upon that data, estimated that Earth was about 75,000 years old. In the 19th century, the physicist Lord Kelvin used different equations to set Earth's age at 20 to 40 million years [source: Badash].
But all that was trumped in the late 1800s and early 1900s by the discovery of radioactivity, which was soon followed by calculation of the rates at which various radioactive substances decay [source: Badash]. Earth scientists have used that knowledge to determine the age of Earth's rocks, as well as samples from meteorites and rocks brought back from the moon by astronauts. For example, they've looked at the state of decay of lead isotopes from rocks, and then compared that to a scale based on calculations of how lead isotopes would change over time. From that, they've been able to determine that Earth formed approximately 4.54 billion years ago with an uncertainty of less than 1 percent [source: U.S. Geological Survey].