Polar motion is a change in the earths orientation in relation to the spin axis. The earths mantle and crust form a very thin skin over the molten outer core. The mantle and crust move and wobble about the axis as a result of the gravitational forces of the sun and the moon as well as the earths internal geological processes, the atmosphere and the oceans. In doing so, the pole traces a spiral path on the surface within about 6 metres of the pole true location.
Variations in Time - Length of day (UT1)
Forces within the earth, its atmosphere and oceans along with the gravitational pull of the sun and moon act to influence the earths rotation. VLBI's precise measurements allow us to very accurately calculate the rotational speed of the earth and observe its changes.
Variations in Time
If we were to use VLBI observations to repeatedly measure the distance between 2 radio telescope located on different continents we could determine speed and direction in which they were moving relative to one another.
Imagine, we can watch geology happen!