What is precession?

As we all know, when the gyro rotates, it hardly rotates in parallel with the ground, and it will shake up and down or even stop because of the gravity imbalance on both sides of the gyro. The earth is similar to a top. It rotates around its own axis, and the direction of rotation is not fixed for some reasons, which leads to the "old film". Simply put, precession is the phenomenon of vernal equinox displacement caused by precession of the earth's rotation axis.

Under the gravity of the sun and the moon, the spatial orientation of the earth's rotation axis is unstable. While moving around the center of the earth, the axis perpendicular to the ecliptic plane is also moving slowly and continuously. Seen from the northern hemisphere, it rotates clockwise (seen from the northern hemisphere) for about 25,800 years, depicting a conical surface. This also makes vernal equinox rotate once every 25,800 years along the ecliptic, which is opposite to the apparent motion direction of the sun every year and moves about 50.3 "to the west every year. This long-term precession of the earth's axis is called the precession of the sun and the moon.

In addition, the planets also have a certain gravitational effect on the earth, which makes the orbital plane of the earth constantly change its position, which not only changes the intersection angle between the ecliptic plane and the equatorial plane, but also makes the vernal equinox slightly displace along the equator, in the opposite direction to the precession of the sun and the moon. This effect is called planetary precession. The age difference of the planets makes vernal equinox move eastward about 0. 13 "every year along the equator. If the precession of the sun and the moon is combined with the precession of the planet, the coordinates of celestial bodies such as right ascension and red tail will change, and we call the change within one year precession. In addition, according to the general theory of relativity, rotating objects such as the earth's rotation axis will also produce relativistic precession in narrow space, which is called geodesic precession. The precession of the earth is 1.98 ". The earth's orbit around the ecliptic does not coincide with its equator. We call the points where they intersect the vernal equinox and the autumn equinox. The time when the sun returns from the vernal equinox to the vernal equinox is expressed by the year of return.

Because of precession, all celestial bodies will change with time. It is speculated that Vega will become the North Star in 1 10,000 years. Because of precession, the zodiac 12 in ancient Greece added a Ophiuchus to 13. In addition, the leap every other year in the solar calendar is also related to precession.