Polar_motion_overhead_earth_frame_youtube_720.mov (1280x720) Polar_motion_overhead_earth_frame_prores_720.mov (1280x720) The motion of the spin pole with respect to the geographic poles fixed to the Earth's crust is called polar motion. Viewed from the perspective of someone on Earth, the spin axis instead appears to spiral away from the geographic poles and then spiral back. Over time, the geographic poles appear to spin away from the spin axis when viewed from space and then back again. Polar Motion - Three Views: Polar motion describes the motion of the Earth's spin axis (shown in orange) with respect to the geographic north and south poles (shown in blue). Polar_motion_three_shot_ipod_lg_thm.png (80x40) Polar_motion_three_shot_ipod_lg_web.png (320x180) Polar_motion_three_shot_prores_ (960x540) Polar_motion_three_shot_appletv.m4v (960x540)

This is why the line traced by the spin axis appears 'bumpy' when viewed up close. 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)

VLBI_procession_nutation_Image_web.png (320x179) VLBI_procession_nutation_Image_searchweb.png (320x180) VLBI_procession_nutation_Image_print.jpg (1024x573)

VLBI_procession_nutation_Image.png (1024x573)

Animated spinning earth