• Precession (進動) is a change in the orientation of the rotational axis of a rotating body.
• Astronomical nutation (章動) is a phenomenon which causes the orientation of the axis of rotation of a spinning astronomical object to vary over time.
• Precession of the ecliptic (黃道歲差): gravitational forces of the planets affect the Earth’s orbit.
• Precession of the equator (赤道歲差): Sun and Moon’s gravitational fields produce a very small torque.
• Combined effects: general precession (總歲差).