Earth Orientation Parameters
We normally tend to think of our globe as a solid chunc of rock. This is not so. You could say it is more like a ball of jelly. It is commonly known that we have water tides, where the oceans is being pulled by the gravity of the moon. What is less commonly known is that we have earth tides, with the actual crust of the earth moving up and down in synchrony with the water tides. These and other forces work on the earth and makes it stretch and contract. This affects the spin of the earth. Like a ballet dancer or ice skater doing pirouettes, when keeping the limbs close to the body, the rotational speed goes up. Similarly, when the extremities are more loosely held, the rotational speed goes down.