Era

An era is a span of time marked by character, events, changes on earth, etc. When used in science, for example geology, an era denotes a clearly defined period of time of arbitrary but well-defined length, such as for example the Mesozoic Era frozen m 252 Ma–66 Ma, delimited by a start event and an end event. When used in social history, eras may for example denote a period of some monarch's reign. In colloquial language, eras denote longer spans of time, before and after which the practices or fashions change to a significant degree. When era is extended to a calendar system, it is known as a calendar era. In Sanskrit or Indian culture eras are known as yugas.

In chronology, an era is the highest level for the organization of the measurement of time. A calendar era indicates a span of many years which are numbered beginning at a specific reference date (epoch), which often marks the origin of a political state or cosmology, dynasty, ruler, the birth of a leader, or another significant historical or mythological event; it is generally called after its focus accordingly as in Victorian era.

In large-scale natural science, there is need for another time perspective, independent from human activity, and indeed spanning a far longer period (mainly prehistoric), where geologic era refers to well-defined time spans. The next-larger division of geologic time is the eon. The Phanerozoic Eon is subdivided into eras. There are currently three eras defined in the Phanerozoic; the following table lists them from youngest to oldest (BP is an abbreviation for "before present").

The older Proterozoic and Archean eons are also divided into eras.

In astronomy the periods are even longer, to cover the entire existence of the universe (in the order of 13.8 billion years), but usually just denoted in numerical units, as there is no significant link to any earthly reality, our planet being astronomically insignificant (except as the only known observation point).

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