The resonant frequency of the atom is not arbitrary, it is a physical constant. That's the key and the point: It allows to measure exactly 1 second anywhere in

The resonant frequency of the atom is not arbitrary, it is a physical constant. That's the key and the point: It allows to measure exactly 1 second anywhere in the universe because the resonant frequency of that atom is the same anywhere in the universe. Compare with a previous definition of the second as a fraction of the solar day... A solar day isn't in fact constant and, in any case, once you leave earth it means nothing at all.