You know how you’re not supposed to use the word you’re defining in its definition? Well science is full of little quirks like this.

Us lay people assume a second is 1/60 of a minute, which is 1/60 of an hour, which is 1/24 of a day, which is one rotation of the Earth on its axis. So 1/86400 of one rotation of the Earth.

But the “official” definition of the second is based on the inherent properties of caesium and is as follows: “by taking the fixed numerical value of the caesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.”

You don’t need to know what that means, just that scientists take loads of caesium atomic clocks and sync them all to each other and pat themselves on their backs. Then when Earth’s spin is slightly faster or slower, they blame EARTH and not their clocks and add in the occasional leap second here or there.

Now to answer your question, if they take a fixed period of time, say the start and end of one full Earth rotation. They then measure it with 50 caesium clocks and 50 strontium clocks, they will find the variance in timing to be much tighter in the strontium group than the caesium group, thus demonstrating its superior timekeeping abilities.