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In geology, a geologic eon is part of Earth's existence, made up of a number of eras, which are of different lengths. A geologic era is made up of two or more shorter times called geologic periods. A geologic period is, in turn made up of yet shorter times called geologic epochs.
Geologists have given names to all of the eons, eras, periods and epochs that they have found by looking at rocks. The names given to each eon describe something about that time, or the rocks that were made at that time.
For example, we are now in the Phanerozoic Eon, which comes from the Greek words for life that we can see, that is, there were animals and plants big enough to see without a lens. This era started about 545 million years ago.