Nutation and Precession


Credit: NASA's Goddard Space Flight Center Conceptual Image Lab
Note: The speed of the motion is greatly exaggerated for clarity.

Nutation and precession are the periodic and long-term motion of the Earth's spin axis in space. The direction of the Earth's axis in space, shown in orange, changes over time with respect to extremely distant objects, such as quasars. When measured with respect to these distant objects, the spin axis appears to trace a circle in the celestial sphere over a long period of time (approximately 26,000 years). This precession motion is driven by the gravity of the Moon and the Sun acting on the Earth's equatorial bulge. However, because the Moon orbits the Earth once a month, in a tilted, elliptical orbit, the spin axis also undergoes a smaller set of nutation motions on much shorter time scales (days to years). This is why the line traced by the spin axis appears "bumpy" when viewed up close.