Figure 1-19Precession and the Path of the North Celestial Pole (a) The gravitational pulls of the Moon and the Sun on Earth’s equatorial bulge cause Earth to precess. (b) The situation is analogous to the motion of a gyroscope. The top of the gyroscope shows the motion of Earth’s North Pole or South Pole, while the point on which the gyroscope spins represents the center of Earth. As the gyroscope spins, Earth’s gravitational pull causes the gyroscope’s axis of rotation to move in a circle—to precess. (c) As Earth precesses, the north celestial pole slowly traces out a circle among the northern constellations. At the present time, the north celestial pole is near the moderately bright star Polaris, which serves as the pole star. The total precession period is about 26,000 years.