How do they measure the rotation down to milliseconds? What reference do they use since everything in the solar system is moving?
I would assume it to be the stars in relation to Polaris.
“How do they measure the rotation down to milliseconds? What reference do they use since everything in the solar system is moving?”
Right!! AND how well CALIBRATED is the device measuring it?? ;-)
Atomic clocks, and multiple stellar reference points. Both are quite capable of this level of accuracy, and likely even more accurate.