system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is cesium used to define a second, as opposed to other atoms that might be more common like Hydrogen or even Oxygen? Also how do we know that's equal to one second if seconds are arbitrary?

Also I didn't know what to flair this sorry. Figured maybe physics tech or math but I wasnt sure.

Edit: apparently it was physics. *shrug*
gpt
Cesium is used because it is incredibly stable. I.e. it oscillates at a very very specific frequency. In this case you just need to count the oscillations. 

The commonality of an atom is really irrelevant here for a simple reason

You only need to define a second really once and in a few specific places in the world. It's not like you need cesium in every watch, and clock on earth. 

\> **Also how do we know that's equal to one second if seconds are arbitrary?** 

You answered that yourself. It's arbitrary. We defined it to be that value, period.

 We've been refering to a length of time equal to 1/60 of a minute which is 1/60 of an hour which is ABOUT 1/24th of a day for a couple thousand years. 

We (humans) made up that unit. So when we wanted to do more advanced science, we were actually able to measure a unit of time like that more and more accurately and precisely. 

At that point we needed to come up with a universal definition that anyone with the right tools can "set their watch to". So we observed how long we thought a second should be, counted some cesium oscillations and said great that's a second.

It's kind of like saying, why is the colour of the sky called blue. How do we know it's blue. Because one day we said it is.