Chicken or egg

[RichardHarris123]

No not literally chicken or egg. 

I wanted to post this in the tools section but there's not a  relevant sub section. 

Someone was asking about micrometer calibration, for which you need gauge blocks, so my question is, which came first ?

I order to make accurate blocks you need to accurately measure them, how did the manufacturer know the measuring equipment was correct? 

Same with time, before atomic clocks, if I produced a watch that I claimed was the most accurate, what was it measured against? 

[rjenkinsgb]

Up to about 1920, the rotation of the Earth was used as the time standard.

The first clock more accurate than that was electromechanical, used as a standard from 1921 to the 40s.

https://en.wikipedia.org/wiki/Shortt–Synchronome_clock

 

Not sure about the length standards..