Nutation
Small periodic nodding oscillations superimposed upon Earth's long-term rotational axis precession.
Governing formula
Principal Nutation Period T = 18.6 years, Amplitude = 9.2″
SI unit
Arcseconds (17.2″ in longitude, 9.2″ in obliquity)
In depth
Caused by gravitational torque variations exerted by the Moon and Sun as the Moon's orbital node precesses around the ecliptic with an 18.6-year period.
Examples in the real world
The 18.6-year principal nutation cycle shifting Earth's apparent celestial pole position.