As Earth spins and orbits the sun, it wobbles on its axis. This plot shows how the Earth’s spin axis wandered over a period of several years. In order to accurately assess the Earth’s changing orientation, observations must be made from the Earth of the quasars that compose the celestial reference frame. From these observations we derive Earth Orientation Parameters (EOP).
The specific components of EOPs include:
x and y components of polar motion, as shown in this graphic.
UT1: that is, the day based on the rotation of the earth. Noon is defined as when the Greenwich Meridian (0° longitude) passes under the sun.
Schematic diagram showing precession and nutation
Celestial offsets from precession and nutation. These are essentially wobbles within wobbles.
The USNO predicts Earth Orientation Parameters out to 180 days for autonomous systems.