These rotations are called precession, nutation, and "intrinsic rotation". This provides an absolute reference for the determination of universal time, precession, and nutation. You can think of precession as the average and nutation as the instantaneous. The "true" or instantaneous obliquity includes the nutation. Thus, precession is the change in and nutation is the change in . The calculation of precession and nutation was described in Chapter 6 of Urban & Seidelmann. The "true" or instantaneous obliquity includes this nutation. The period of oscillation of the nutation is 18.6 years. This observed free nutation is called Chandler wobble. Some important effects are Earth's nutation, rotation, atmospheric refraction, tectonic displacement and tidal waves. This oscillation is called "nutation". The polar motion is primarily due to free core nutation and the Chandler wobble. These oscillations, in both precessional speed and axial tilt, are known as the nutation. Eddy current nutation dampers have flown on spin stabilized satellites to reduce and stabilize nutation. This motion is called Euler's free nutation.