What causes the change in the axial tilt of the earth? Why does it change?
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That is because the Earth's orbit also precesses. It does so as a result of perturbations by other planets. This precession is a complicated quasiperiodic Spirograph pattern -- its orbit's north pole moves by a few degrees over something like 70,000 years. It makes the Earth's obliquity vary a few degrees with an approximate period of 41,000 years.
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