See also https://en.wikipedia.org/wiki/Shortt%E2%80%93Synchronome_clock https://en.wikipedia.org/wiki/Shortt%E2%80%93Synchronome_clo... > The Shortt was the fi

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