Admittedly, I enjoy learning about recent history: visual arts and music and that sort of thing. Learning about how we _think_ the planet was billions of years before humans even existed is kinda boring to me. However, I fully believe the saying that goes something like, “Those who do not learn from history are doomed to repeat it,” so I’m gonna get educated, as I am well-adapted to the typical level of oxygen in the air in 2026.
Ok, so, I read that the earth is roughly 6.something billion years old. I know that we say “it’s the year twenty twenty-six!” (Or whatever). But to even refer to now as 2026 is to use the Gregorian Calendar, which is the calendar we use today. And I just looked it up and learned that during Pope Gregory XIII’s tenure as pope, in 1582, we began using the Gregorian Calendar, which modified the Julian Calendar slightly. I didn’t look into it deeply, but I saw that the changes, in part, involved leap years. My guess is that the Gregorian Calendar brought the calendar into better harmony with moon cycles — which themselves relate to gravity, so I fully suspect moon cycles in the early days of earth were different than moon cycles today.
In any event, that isn’t really pertinent here.
Rather, I want to talk about eons. Wtf is an eon, anyway? Well, I’m glad you asked. An eon is short for “chronostratigraphic eonothem” and is the largest geochronological time unit. These are standard international units of the geologic time scale and are published by the (wait for it!) International Commission on Stratigraphy on the International Chronostratigraphic Chart (hard to not laugh at this whole idea, much less a regulatory body for it — but I sincerely see the utility of standards!).
So what are the formally-defined eons, anyway? There exist four formally defined eons: the Hadean, Archean, Proterozoic, and Phanerozoic. Hadean was first, and we are currently living in the Phanerozoic eons.
Eons are different than eras, with eras being narrower and typically fitting inside eons, I think.