Scientists use the geologic time scale so they can keep a record of the Earths major history events. The geologic time scale has four major eras called the Precambrian which is longest era from today and the longest time. Then you have the Paleozoic which is the next longest and the second longest era. Then the Mesozoic which is the third longest from todays era and the second shortest era. Then the shortest era and today's current era the Cesozoic era. Scientists used rock layers and index fossils and placed them in order from oldest to youngest to form the geologic time scale. They also use radioactive dating. Another thing that scientists do that is hard is they have to figure out when an era starts and ends. One way is they study the fossil record and find major changes in life.