The geologic time scale is divided into several different units. There are eon, era, period, epoch. They all have different lengths, and some are very long, while some are much shorter. The eon is the longest of them all spending for over half a billion years, or even more than a billion. The era stretches for few hundreds of millions of years. The period lasts for tens of millions of years, while the epoch is stretching for several millions of years. The reason why they are with different lengths is that they are marked by certain important events in the past. Some are marked by significant changes in the climate, some by extinction of species, some by big disasters, some by changes on the surface of the Earth, some by the changes of the atmosphere.