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Why Does Earth's Axis of Rotation Wobble?

Robert Feingold, Highlands Ranch, Colorado
BlueMarble
Image created by Reto Stockli with the help of Alan Nelson, under the leadership of Fritz Hasler
The wobble of Earth’s axis is called precession. While most of us are familiar with our planet’s primary motions — rotation and revolution —  few are aware of precession. This characteristic is vividly demonstrated by a spinning top or gyroscope whose tilted axis of rotation gyrates in a circular motion, sweeping out a cone-shaped area in the process.
 
You can cause a top or gyroscope to precess by gently tapping it while it’s spinning. The “tap” that causes Earth’s axis to precess is gravity from the Sun and Moon and, to a much lesser extent, the planets. Gravity alone, however, won’t do the trick. Earth has a slight equatorial bulge, and it’s this bulge that gets the gravitational tug. In other words, if the planet were a perfect sphere, there would be no precession.
 
While a top or gyroscope might precess several times each second, a single precession of Earth’s axis takes nearly 26,000 years. The effects — changes in our “pole” stars, a shift in the celestial coordinates of right ascension and declination, and a westward drift of the solstices and equinoxes — are so gradual that most changes take decades to notice. No wonder precession is one of Earth’s “unknown” motions. — Glenn Chaple, Contributing Editor
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