A small, periodic wobble superimposed on Earth's precession, caused primarily by the Moon's gravitational pull on Earth's equatorial bulge. This creates an 18.6-year oscillation in the position of the celestial poles.
From Latin 'nutatio' meaning 'nodding,' describing the nodding motion of Earth's axis. The phenomenon was discovered in 1728 by British astronomer James Bradley, who noticed small periodic variations in star positions that couldn't be explained by precession alone.
Nutation makes Earth's axis nod like a spinning top that's slowing down, but the 'nodding' takes 18.6 years to complete one cycle! This tiny wobble moves the celestial poles by about 17 arcseconds - roughly the width of a quarter seen from a mile away.
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