NavList:

Frank:

I used the example of the old physical artifact kilogram because the definition of TAI (and time standards based off of it such as UTC) isn't "atomic clocks" but "these particular atomic clocks."  Given a winning lottery ticket, anybody can set up an SI frequency standard, make observations to determine TT, and reckon what TAI should be, but that wouldn't truly be TAI any more than a metal cylinder of the same height, diameter and alloy as CGPM's artifact would truly be a kilogram.  Communications technology makes comparing your clock to Actual TAI faster and easier than comparing your artifact to Actual Kilogram was, but it still ultimately relies on a chain of certification rather than reproduceability.

Measuring UT1 would make the rotating earth an artifact, but it's at least one we all have direct access to.  "Seconds" would be smeared, but they would be smeared for everyone at the same time and at the same rate. 

And if we do get to a point where not everybody has direct access to the earth, to use the moon as an analogy, I think earth-based observers are better able to measure the moon's rotation than they are to guess what an idealized clock at a particular selenographic coordinate on the rotating and heterogenously massive moon would say over the long-term.