Precession. The change in the orientation of the earth’s rotational axis. Technically, it applies to any astronomical body. What we’ve been referring to in class as precession is the axial sort, the Earth’s axis is tilted about 23.5 degrees from the ecliptic plane (where the orbits of the plants are in the same plane as the Sun).
Precession is caused by the torque from the gravitational pull of the Sun and the Moon on the Earth, which is trying to pull the Earth’s axis perpendicular to the ecliptic plane. If the Earth were a perfect, uniform sphere, there would be no precession!
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