system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did the first clocks accurately measure seconds?

I understand it might be actually easy for hours, but seconds? Bonus points if you explain how to accurately measure seconds without a clock.
gpt
Well, the short answer is they weren't super accurate at all. Even modern clocks are often a little off from one another, and there's not too much you can do about that with analog, safe technology. However, "clockwork" mechanisms such as springs and pendulums are able to output stored energy at a remarkably consistent rate, so the deviations in movement between clocks built to the same standard could be kept low enough that you only needed to adjust it every day or so.

As for how to measure seconds without a clock, seconds are based on the decay rate of cesium-133, so by measuring that decay in a sample you can get an extremely precise measure of seconds. Obviously that's very expensive and potentially dangerous though, so you're better off just using "one thousands" or "mississippis" like everyone else.