Time Words: Era, Epoch, and Eon

By Maeve Maddox

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Sports writers are fond of saying that the retirement of someone or other marks “the end of an era.”

What is an era? And is it different from an epoch? What about an eon?

All three words denote a period of time. All three have specialized meanings for geologists. Here are their most common meanings and connotations.

Both era and epoch denote measurement.

In the sports writer’s usage, an era is a period in the history of a sport. It is a time during which a particular player, manager, or feature may be seen to typify the sport: the Babe Ruth era, the Casey Stengel era, the era of steroid use.

In the historical sense, an era can be a period of time marked by a specific beginning date: the Roman era (beginning with the traditional founding date of 750 BCE.), the Christian era (beginning with the birth of Jesus of Nazareth, now believed to have been 4 BCE), or the Muslim era (beginning with Mohammed’s flight from Mecca to Medina in 622 CE).

An epoch, (not to be confused with epic), like an era, is a period of time. An epoch is longer than an era and can cover more than one lifetime. It is marked by some significant development or series of developments: the feudal epoch, the epoch of exploration.

An eon is a very long time indeed. It is the longest period of geological time. Geologists subdivide an eon into eras. A geological era is subdivided into periods, epochs, and stages.

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17 Responses to “Time Words: Era, Epoch, and Eon”

  • cynthia

    I’m a little confused. Since “An epoch is longer than an era”, how come “A geological era is subdivided into epochs”?

  • Daniel Scocco

    Hmmm, good questions indeed, let’s wait Maeve to clarify.

  • Maeve

    As I say above, ” All three [words] have specialized meanings for geologists.” Geologists have to have some system in order to talk about the immense units of time they deal with. In their system an era is longer than an epoch.

    In non-geological usage an epoch is generally felt to be longer than an era because the epoch is seen to precede the era. The epoch marks the beginning of something significant to human beings, for example, the earliest use of fire for cooking.

    Here’s how H.W.Fowler puts it:
    An epoch is the date of an occurrence that starts things going under new conditions.
    An era is the time during which the conditions started at an epoch continue.

  • Daniel Scocco

    Got it!

  • tim

    Thanks for the post!
    Just one question.
    I read/heard about the word, “aeon”. Is that just a different way of spelling “eon”? Or is it a different word all together?

  • Maeve

    Aeon is the same word. It suggests an even longer period to me!

  • tim

    Aha! Thank you!

  • cynthia

    Got it. Thanks, Maeve!

  • Joe

    Is there a deffinite amount of years associated with eon or is it different with whoever meashures with it?

  • Maeve

    number of years in an eon — here’s definition 4 in OED:

    . Geol. and Astr. One thousand million years.

  • Patrick

    What is the actual measurement in time of an eon?

  • xXdRaKeXdArSenXx

    Uhmm..I thought Super eon is the longest unit of time in geologic time scale were as divided into two major Eon the precambrian and Cambrian..please correct me if what I know was incorrect

  • Breed7

    Once again, Maeve presents contradictory and incorrect information in an article. Amazing how someone with so little actual knowledge of the English language can pretend to be such an authority.

    An epoch is NOT generally considered to be a longer period of time than an era.

    An epoch is a period of time that can be defined by an event, and can be as brief as the event itself. An era is a period of time between epochs. These are not the geological definitions, but the generally-accepted uses for everyday English.

  • Maeve

    If I have misdefined “epoch,” you are certainly right to call me on it. To imply that I habitually disseminate incorrect information without providing me with specific examples is less than helpful. I’d be grateful if you would send me a list of the articles that you feel contain contradictory and incorrect information. My email is maevemaddox-at-gmail.com/.

  • Shipp

    An aeon is exactly 1,000,000,000 years, just as an epoch is a measure of time for a specific event. Such as the ‘Thirty Year War’, or when Queen Elizabeth rose to power. I think neither of you are incorrect, because both definitions coincide within the meaning of the word. It’s merely a subjective manner in which the writer chooses to use the word.

  • David

    Why has this article still not been revised? It is common knowledge that an Era is longer than an Epoch as an Epoch is a subdivision of an Era. You should clarify that an Epoch may, at times, exceed the length of an Era but generally speaking is not considered to.

  • Dale A. Wood

    Era, Epoch, and Eon — Duh?
    Since Queen Elizabeth I was mentioned, her time is called “the Elizabethan Age”. We now lived in the second Elizabethan Age.
    There were the “Victorian Age”, and “The Age of the Dinosaurs”, too.
    Also, the “Era of the Pharaohs”, the “Epoch of the Mongol Khans”, the “Epoch of the Tsars”, and the “Era of the cave man”.
    I think that the usage is often idiomatic.
    “The Stone Age”, “The Bronze Age”, “The Iron Age”.
    You have also discounted (ignored) the possibility of exaggeration, e.g. “I had to wait for eons at the dentist’s office.”
    e.g. “It seemed like Franklin D. Roosevelt was the President for an eon.”
    “The Georgian Age”, which also included the reign of William IV, because he was surrounded by George I, George II, George III, and George IV.

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