We imagine the stars in the sky as distributed on a spherical surface—the celestial sphere. The Earth's axis of rotation intersects the celestial sphere at two points, with the one corresponding to the Earth's North Pole called the North Celestial Pole. Near the North Celestial Pole lies the Ursa Minor constellation, and the brightest star in it, Ursa Minor α, is known as the "North Star". It is very close to the current position of the North Celestial Pole, with a distance of less than 1°, thus becoming a natural landmark for indicating the north. The Ursa Minor constellation is relatively fixed, but the direction of the Earth's axis of rotation in space is not constant. Because the Earth is not a perfect sphere, its equatorial part is slightly protruding, while the polar parts are slightly flattened. Moreover, the Earth's axis of rotation makes an angle of about 66.5° with the plane of the Earth's orbit (the ecliptic plane), and the Sun's gravity towards the Earth is along the ecliptic plane, making an angle of about 23.5° with the Earth's equatorial plane, and the Moon's gravity towards the Earth is also in this direction. This results in a pulling force that pulls the protruding part of the Earth's equator into the ecliptic plane. However, the Earth is rotating, and the inertia of its rotation causes the Earth's axis of rotation to rotate around an axis perpendicular to the ecliptic plane passing through the Earth's center, forming a cone. This movement of the Earth's axis of rotation is called precession.
The precession of the Earth's axis causes the North Celestial Pole to rotate around the North Celestial Pole in the constellations, namely the precession of the equinoxes. Due to the precession of the Earth's axis of rotation, the position of the North Celestial Pole relative to the stars on the celestial sphere traces a large circle with a period of about 26,000 years. The radius of this circle corresponds to the angle between the Earth's equatorial plane and the ecliptic plane, which is about 23.5°. Therefore, the fact that we see Ursa Minor α very close to the North Celestial Pole today is just a temporary coincidence. Four or five thousand years ago, the North Celestial Pole was near the star Alpha Draconis (called "Youshu" in ancient China), but Youshu was an unremarkable 4th-magnitude star, so it was a bit of a stretch to let it play the role of the "North Star". Therefore, in the Zhou and Qin dynasties, our ancestors used the star Betelgeuse in Ursa Major (a second-magnitude star) as the North Star. As time passed, the North Celestial Pole gradually moved towards Ursa Minor α, making it an increasingly authentic "North Pole" star. However, this trend will not last forever. Around 2100, when the angular distance between the North Celestial Pole and Ursa Minor α reaches its minimum (about 28′), the North Celestial Pole will slowly move away from Ursa Minor α. After 12,000 years, the North Celestial Pole will move to the vicinity of Vega, which will become the new North Star. It is a very bright star, and it will be much easier to find than the current North Star.
Corresponding to the changes of the North Celestial Pole, the precession of the Earth's axis of rotation will also cause the precession of the equinoxes. The path of the Earth's equatorial plane extending outward and intersecting the celestial sphere is called the celestial equator; the path of the ecliptic plane extending outward and intersecting the celestial sphere is called the ecliptic. The intersection of the ecliptic and the celestial equator on the celestial sphere is called the vernal equinox. Due to the effect of precession, the vernal equinox moves along the ecliptic from east to west by about 50″ each year. Due to the movement of the vernal equinox, the position of the Sun in the sky at the same time each year will shift slightly. Over time, it will be found that the position of the Sun relative to the background of the stars is moving at the same time each year. This movement is called the precession of the equinoxes.