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| The Nutation Cycle is caused by a very slight elliptical nodding of the Earth's axis, which is super- imposed on the precessional motion due to the pull of the Moon on the Earth. This shows up as the north node’s retrograde (clockwise) movement around the ecliptic circle, taking 18.6133 tropical years to complete one cycle of the ecliptic circle from spring equinox to spring equinox. |