When thinking about the age of the Earth, it is helpful for scientists to divide such a long time span into units. This is known as the geologists time chart. The largest of these units are called eons. Eons are then divided into eras, the eras into periods, and the periods into epochs. These units highlight what organisms were alive as well as what the climate was like during a particular time. These units are much like the months, weeks and days of the year.