Nowadays we have a lot of machines. But in the earlier times we have only some pendulum clocks. And their precission was worse then the precission of sunclocks.
Today we have the precision of mechanic clock so more.
How we can obtain a machine (clock) with preccision of 1 000 period of Cs139 ? In the beginning we had clock with precission of plenty period of Cs139 and nowadys we have the one.