During the late 1800s and early 1900s, various attempts were made determine the age of Earth. Although some of What ods appeared promising at the time, none proved reliable. dates these scientists were seeking was a numerical date. Such an specify the actual number of years that have passed since event occurred-for example, the extinction of the dinosaurs about 65 million years ago. Today, our understanding of radioac- tivity allows to accurately determine numerical dates for rocks that represent important events in Earth's distant past. We will study radioactivity later in this chapter. Prior to the discovery of radioactivity, geologists had no accurate and dependable method of numerical dating and had to rely solely on relative dating.