The term geon (for geological eon) refers to large, geologic units of time. Geologists traditionally subdivide Earth history into a hierarchy of named intervals: eons, eras, periods, etc. (e.g., the Jurassic Period of the Mesozoic Era). Historians subdivide the history of human activity into intervals that are comparatively much shorter. In both geological and historical scales, the divisions of equal rank are characteristically of unequal duration, and the identification of a particular interval is primarily based on its fossil, artifact, or cultural content (e.g., Carboniferous, Neolithic, Dark Ages, Ming Dynasty). Both scales are calibrated against numerical ages obtained separately.
An alternative way of referring to the past is to use a scale with intervals of equal duration. We speak of a given decade, century, or millennium. For the enormously long geologic time frame, it is advantageous to use corresponding large, equal time intervals encompassing the events and processes that have shaped our planet. The development of mountain ranges, ocean basins, and continents takes tens to hundreds of millions of years, and large time units thus are convenient for discussing long-term trends. Astronomers use light years and parsecs to deal with huge distances, rather than kilometres. Geologists have geons to refer to large specified time intervals of Earth history. The geon scale is also applicable to other planets with different histories, and to the universe itself.
Two usages of geon have been introduced in geology: