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.
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