From Bowditch - "American Practical Navigator" Vol 1 1977 p 362.
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The axis of the earth is undergoing a processional motion similar to that of a top spinning with its axis tilted. In about 25,800 years the axis completes a cycle and returns to the position from which it tarted. Since the celestial equator is 90 degrees from the celestial poles, it too is moving. The result is a slow westward movement of the equinoxes and solstices, which has already carried them about 30degrees, or one constellation, along the ecliptic from the positions they occupied when named more than 2,000 years ago.
Since sidereal hour angle is measured from the vernal equinox, and declination from the celestial equator, the coordinates of celestial bodies would be changing even if the bodies themselves were stationary. This westward motion of the equinoxes along the ecliptic is called precession of the equinoxes (fig. 1419a). The total amount, called general precession, is about 50".27 per year (in 1975). It may be considered divided into two components, precession in right ascension (about 46".10 per year) measured along the celestial equator, and precession in declination (about 20"'04 per year) measured perpendicular to the celestial equator. The annual change in the coordinates of any given star, due to precession alone, depends upon its position on the celestial sphere, since these coordinates are measured relative to the polar axis while the processional motion is relative to the ecliptic axis.
Due to precession of the equinoxes, the celestial poles are describing circles in the sky. The north celestial pole is moving closer to Polaris, which it will pass at a distance of approximately 28' about the year 2102. Following this, the polar distance will increase, and eventually other stars, in their turn, will become the Pole Star. [ a diagram indicates Vega will be the pole star in AD 14,000] Similarly, the south celestial pole will some day be marked by stars of the false Southern Cross.
The precession of the earth's axis is the result of gravitational forces exerted principally by the sun and moon on the earth's equatorial bulge. The spinning earth responds to these forces in the manner of a gyroscope. Regression of the nodes introduces certain irregularities known as nutation in the processional motion.
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ET