The Earth goes through periodic wobbles, known and nutation. This is due mainly to outside gravitational forces pulling on the Earth’s equatorial bulge. The Sun and Moon pull on Earth’s bulge, making the Earth wobble. However, for casual astronomical observations, these effects are negligible. The Earth’s obliquity and longitude have a period of 18.6 years, the time it takes for the Moon to circle through its nodes, and lesser wobbles of six months and two weeks, which are due to the orbit of the Earth around the Sun and the Moon’s orbit around the Earth, respectively.
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