What were some early ideas about the Earth's age?

 What were some early ideas about the Earth's age?

In 1644, John Lightfoot (1602-1675) calculated the age of the Earth using the Bible. Based on this, he determined that our planet began at 9 a.m. on September 17,3928 B.C.E. Not long after, another attempt was made by James Ussher (1580-1655), Archbishop of Armagh, Ireland. He determined that the Earth began on October 23, 4004 B.C.E., based on the genealogies and ages of people he found in the Bible. It wasn't until the latter part of the 18th centul)' that someone took a different approach to calculating the Earth's age. French naturalist Jean-Baptiste Lamarck (1744-1829) believed the Earth was much older, although he couldn't give a convincing reason how or why this would be true.

By the 19th century, many scientists knew fossils found in rock layers could not have come from animals that lived a mere 6,000 years ago. But the only method used to date rocks at the time was relative dating, in which the age of a rock was estimated based on how deep it was beneath the surface. The verification that some rocks were hundreds of millions of years old didn't come until the turn of the 20th century, when French physicist Antoine Becquerel (1852-1908) discovered radioactivity. Scientists could then use radioactive techniques to determine the decay rate of certain isotopes within rocks (absolute dating), allowing more precise measurements of overall rock ages and determining a more accurate age of the Earth. (For more information about relative and absolute dating, see "Fossils in the Rocks.")