An eon is a unit of time used in geology and astronomy to describe a very long period of time. In astronomy, an eon refers to one billion years. In geology, the time span of 4.5 billion years is divided into smaller segments or units called eons, eras, periods, epochs, and ages. Geologic eons are part of Earths existence, made up of a number of eras 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 epochs. The geologic time scale currently has four named eons: Hadeon, Archaean, Proterozoic, and Phanerozoic.