Eon

An eon (also spelled "aeon") is the largest subdivision of the geologic time scale used to divide Earth's history. Geologists divide Earth's history into ranks: eon, era, period, epoch, and age. The eon is the highest rank, and each can span hundreds of millions to billions of years.

The term originates from ancient Greek, meaning "a long period" or "an age without end." For this reason, the word is also used in everyday speech to refer to an immensely long and indefinite span of time. In other contexts, such as philosophy and ancient Gnostic religion, "aeon" was used with a different sense related to existence or spiritual force.

Geological use

Earth's history, spanning approximately 4.5 billion years, is divided into four major eons: Hadean, Archean, Proterozoic, and Phanerozoic. The first three are collectively called the Precambrian, the period before hard-bodied organisms appeared abundantly in the fossil record. The Phanerozoic is the current eon, beginning roughly 539 million years ago.

The boundaries between eons are based on major changes: shifts in Earth's continental configuration, increases in atmospheric oxygen, and the evolution of life. The International Commission on Stratigraphy updates these age estimates as new scientific evidence emerges, so the numerical dates are subject to minor revision.

Measurement and usage

In some scientific fields, particularly astronomy and physics, the term "aeon" is occasionally used as a precise unit of measurement equal to one billion years (1 Ga). This usage is not universal across all literature, and it is important to verify the meaning from context.

Other meanings

The name "Eon" has also been used by companies, literary works, music, and individuals. For this reason, encyclopedia articles in multiple languages sometimes maintain a disambiguation page under the title "Éon" or "Eon" to distinguish between these different uses.