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nutation
A periodic variation in precession: in other words, a wobble on top of the main wobble of Earth's axis. The chief cause of nutation is the Moon moving in an orbit that is inclined (by 5°) to the ecliptic. This lunar nutation amounts to a +/- 9" back-and-forth jiggling of Earth's poles every 18.6 years (the time it takes for the Moon's orbit to walk around to the same relative position again). The net result is that, instead of describing a perfectly circular path in the sky, every 25,800 years or so, due to precession, the precessional path of Earth's axis is more like the crinkly shape of a cooky-cutter.
Related category
• CELESTIAL MECHANICS
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