However, because the Moon orbits the Earth once a month, in a tilted, elliptical orbit, the spin axis also undergoes a smaller set of motions on much shorter time scales (days to years). This motion is driven by the gravity of the Moon and the Sun acting on the Earth's equatorial bulge. 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). The direction of the Earth's axis in space, shown in orange, changes over time with respect to extremely distant objects, such as quasars.

Nutation and Precession: Nutation and precession are the periodic and long-term motion of the Earth's spin axis in space. VLBI_procession_nutation_Image_thm.png (80x40)

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