system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5, how did we determine the length of a second?

Title says it all. What defines a second, and how was this unit of measurement established/discovered with relation to the atomic measurement we currently use.
gpt
Timekeeping is a very ancient system.  Ancient Mesopotamians used a base-12 system for most mathematics so they divided up the day into 12 hours of day and 12 of night.

They also did a lot of counting on their hands so they sub-divided each hour into a full hand of 12s (12x5) to get 60 minutes.

Then again to get 60 seconds.

So the *original* definition of a “second” is 1/86400th of a day.

Accurately determining what that means and how to reproducibly measure it was much more challenging, and wasn’t formalized in any useful way until much more recently.