Actual answer: The VLBI radio telescope Celestial Reference System measures the Earth's rotational position to 10 microarcseconds, which is good to within two t

Actual answer: The VLBI radio telescope Celestial Reference System measures the Earth's rotational position to 10 microarcseconds, which is good to within two thirds of a millionth of a second of sidereal time-of-day. The position of distant quasars is very stable. https://en.wikipedia.org/wiki/International_Celestial_Reference_System_and_its_realizations https://en.wikipedia.org/wiki/International_Celestial_Refere...