[tvb]

Many pendulum clocks are capable of well under 1 ppm accuracy, which is better than many quartz oscillators. Here is a tour of clock precision by orders of magnitude:

http://leapsecond.com/ten/

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.

[zokier]

See also https://en.wikipedia.org/wiki/Shortt%E2%80%93Synchronome_clo...

> 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