[time-nuts] Primary Time Standards

[Mike S]

At 10:26 PM 7/13/2011, Jim Palfreyman wrote...
>But why is it that Caesium Clocks and Hydrogen Masers have an 
>adjustment
>facility?

Cs defines the second, but only at the physically impossible 
temperature of absolute zero. Relativistic effects make it so the 
second is a different length at different altitudes.

So, when one wants to track global "mean" time, individual clocks need 
adjustment.

Which brings up another question - what altitude (=geoid?) is TAI 
defined for? I don't know enough about general relativity, but think 
the latitude would make a difference, too?