Radio astronomy is incredibly precise. This is particularly true when they’re used in combination through a process known as Very Long Baseline Interferometry (VLBI). It is so precise that by observing quasars we can measure not only changes in Earth’s rotation, but also tectonic drift between radio telescopes.
The way VLBI works is by measuring how long it takes for fluctuations in a quasar’s light to reach different radio telescopes. Since light travels at a constant rate, the difference in arrival time can be used to determine the difference in distance. Using an array of telescopes you can determine not only the distance between the telescopes, but the precise location of the signal. VBLI can determine the distance between antennas within millimeters, and the position of a radio source to within a fraction of a milliarcsecond.
Because quasars are so distant, they can be treated as fixed points of reference in space. This makes them useful for determining the motion of objects relative to them, such as the precise motion of Saturn, or the Magellanic clouds. But because they are fixed points in the sky, any shift in the location or timing of quasars must necessarily be due to a change in the position of the telescopes themselves, or the rotation of the Earth.
What’s amazing about this technique is that it uses objects billions of light years away to measure millimeter shifts in position on the Earth.