(A)eon, era, epoch, age

Because I’m a recreational (and occasionally professional) pedant, I was curious if I was using the word “epoch” right in a recent paper. I dug around and got my answer.  For my own and others’ future reference, here’s what I found:

In general or popular writing, the terms era, epoch, and age can be used, without error, synonymously to refer to a particular span of time.

Writers who wish to respect their more precise and historical meanings or who need to respect jargon, can follow these rules:

“Eon” is the preferred spelling; “aeon” is a variant that may appear affected.

In general, an eon is a very long time, comparable to the age of the universe.

An epoch is a fixed point in time (like the zero date of a calendar, or the moment a world-changing event occurred), especially one that marks the beginning of a new era. One can “make an epoch” by doing something that changes things forever.

An era follows an epoch and is defined by it. For instance, the “Christian Era” is the time since AD 1, with Christ’s birth (roughly) making the epoch.

An age is basically an era, but seems to be a bit more general, not necessarily needing an epoch. “The Tudor Age” would then just refer to “the time when the Tudors were in charge,” while “The Victorian Era” carries the connotations of the many things that distinguished the time when Queen Victoria was alive.  This distinction might be too subtle to honor.

In geology jargon, time is divided into eons, then further divided into eras, periods, epochs, and finally stages.

In astronomy jargon, an epoch is the moment of an observation. It most commonly comes up in ephemerides, giving the moment in time that a certain object had or will have certain coordinates or orbital parameters. Not to be confused with “equinox” which specifies the Celestial coordinate system one is using.  “Age” should probably be avoided except to refer to how old something is, to avoid confusion.

And now you know.

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