In the second century B.C. the Greek astronomer Hipparchus (190?-?120 B.C.) compared the positions of the principal stars in the zodiac with those which astronomers had noted 100 years earlier. He found that the vernal equinox had shifted westward about 1o along the ecliptic. What he discovered is now called the precession of the equinoxes.
The Earth is not a perfect sphere; it bulges slightly around the equator. The gravitational attraction of both the Sun and the Moon tries to pull the Earth's equatorial bulge into the Earth's and the Moon's orbital planes. Acting like a spinning top, the rotating Earth resists this pull. The result, therefore, of the Sun's and Moon's attraction and the Earth's resistance is that the Earth's axis of rotation moves slowly westward around the pole of the ecliptic. Because of this, the points of intersection between the celestial equator and the ecliptic shift westward along the ecliptic at a rate of about 50 minutes of arc per year or over 1o per century. This causes the equinoxes to precess completely around the ecliptic in 25,800 years.
The Earth's axis retains its tilt of 23.5o throughout the cycle of precession. Today, the axis is directed toward a point on the sky less than 1o from the star Polaris. At the time of ancient Egypt the axis pointed in the direction of the star Thuban (Alpha Draconis), which was then only a few degrees from the pole around which the heavens rotated. By A.D. 14,000, the very bright star Vega (Alpha Lyrae) will be the "North Star" and mark the approximate position of the north celestial pole for our descendants.