Earth’s axis of rotation nutates because it is subject to varying torques from Sun and Moon – the former varying because of the eccentricity of Earth’s orbit, and the latter because of both the eccent...Earth’s axis of rotation nutates because it is subject to varying torques from Sun and Moon – the former varying because of the eccentricity of Earth’s orbit, and the latter because of both the eccentricity and inclination of the Moon’s orbit. This means that the equinox does not move at uniform speed along the ecliptic, and the obliquity of the ecliptic varies quasi-periodically. These two effects are known as the nutation in longitude and the nutation in the obliquity.
