And now I'm wondering how accurate someone could make a reference signal from a physical pendulum like this—or what strange quirks you could introduce into a system using such a 'weighty' clock source!
Here is a world record setting pendulum clock from a few years ago:
http://leapsecond.com/pend/clockb/
Precision pendulum clocks are a very deep rabbit hole.
> The Shortt was the first clock to be a more accurate timekeeper than the Earth itself; it was used in 1926 to detect tiny seasonal changes in the Earth's rotation rate. Shortt clocks achieved accuracy of around a second per year
If my math is right, thats about 30ppb