What Is Precession?

Precession in astronomy means that the rotation axis of a celestial body points to a slow and continuous change in space due to gravity. For example, the direction of the Earth's rotation axis drifts gradually, tracking the top of its wobble, sweeping out a cone (known as a New Year or Plato Year in astrology) at a period of about 26,000 years. The term "precession" is usually only for long-term movements, and other changes in nutation and pole shifts on the ground axis are much smaller.

Precession refers to the long-term precession of the Earth's rotation axis, causing the vernal equinox to move westward along the ecliptic, causing the return year to be shorter than the stellar year. The reason is that the earth is an ellipsoid, and the equatorial part is uplifted; the equatorial plane and the ecliptic plane do not coincide. The sun, moon, and other planets exert additional gravitational forces on their equatorial uplifts, causing changes in the equatorial plane's tendency, the precession of the Earth's axis. As a result, the vernal equinox moves westward along the ecliptic by about 50 .2 each year and about 26,000 years. It makes the return year slightly shorter than the stellar year, which is called precession in ancient China. It is only caused by the effect of sun and moon The westward shift of the vernal equinox does not change the angle of the yellow-red intersection, which is called "sun and moon precession"; the action of other planets changes the angle of the yellow-red intersection at the same time, and it is called "planetary precession."

Every time the sun moves on the ecliptic for a return year, it is a slight distance worse than returning to the starting point one year ago, so the winter solstice moves backward (west) every year. This is the "precession" ^[2]

Historically, the precession of the earth is called

The effects of precession are frequent changes in Polaris. At present, Gou Chenyi is very suitable for indicating the north pole of the celestial body, because Gou Chenyi's apparent brightness (2.0, etc.) is sufficient to bear this heavy responsibility, and it is only less than half a degree away from the true north pole.

On the other hand, the right axis of the Draco (alpha star Draco), the polar star of 3,000 BC, 3.67 and so on are far less luminous (only one fifth of Gou Chenyi's), in today's cities Almost no longer visible under the light pollution.

The bright Weaver actress in Lyra has also served as the North Star in the past (12,000 BC, and will re-appoint in 14,000 BC), but never approached within 5 ° of the North Pole.

When Gou Chen became the North Star again in 27,800 AD, because his self-movement would be slightly farther from the North Pole, approaching 23,600 BC was the time when he was closest to the North Pole.

At this stage, it is difficult to find the south pole of the celestial sphere, because that area is plain and unremarkable. The Antarctic constellation named Antarctic star is just a dark star of magnitude 5.5 that is barely visible to the naked eye under ideal conditions. But in the 80th to 90th centuries, Tian Antarctica would pass a pseudo cross.

This situation can also be seen from the star chart, with the South Pole pointing toward the Southern Cross. After about 2,000 years, the Southern Cross can point to the South Pole. Because the South Pole is approaching the Southern Cross, the result is that the subtropical region from the northern hemisphere will no longer be able to see this constellation as clearly as the ancient Greek era.

Precession nutation

Nutation refers to the phenomenon that the rotation axis of an object rotates around the other axis (referred to as the "precession axis") in physics. Also in astronomy, it refers to the oscillation phenomenon of the earth's rotation axis when the earth's rotation axis rotates around the zodiac axis. The sun and the moon are sometimes south of the equator and sometimes north of the equator, so the direction of gravity on the earth is constantly changing. Due to the attraction of the celestial bodies such as the sun and the moon, especially the moon, to the equatorial protruding parts of the earth, in the long-term precession of the Earth's axis around the yellow pole (26,000 years), there are also many different periods and different amplitudes. A small amplitude change, in which a short-period swing is added to its average position, that is, a period with a period less than or equal to the moon's intersection period of 18.61 years and an amplitude of 9.211 "(referred to as the" nutation constant "), referred to as a chapter The motion pattern is represented by a wave-like curve behavior along the precession circular orbit of the earth. The nutation can be divided into Huang Jing nutation and intersection angle nutation according to the movement mode, and can be divided into solar nutation, moon nutation and Monthly nutation ^[4] .

Precession

Polar shift is short for polar migration. Refers to the movement of the rotating earth and magnetic poles on the ground over time. There are two types of movement: the first is periodic movement, the north pole moves counterclockwise, and the south pole moves clockwise. There are two types of polar shift cycles: The period is about December, the amplitude is about 0.1 , and an elliptical motion along a long axis of about 4m. This motion is related to large-scale seasonal changes such as ocean currents and atmospheric circulations on the earth The period is 14 months, the amplitude is about 0.2 , and it moves along a circle with a radius of about 7m. It is also called the Zhangdler movement. The period is called the Zhangdler cycle. This movement is related to the movement of matter inside the earth. The second is long-term drift. There are two types: Long-term drift. According to the observation data from the International Latitude Station since 1900, it is concluded that there is a linear drift of about 0.003 / a in the direction of the pole along the west longitude 81.5 °. The maximum amplitude of the pole shift does not exceed 0.4 , in the range of 24m2 Move inside. Due to polar shifts, latitude, longitude, and centrifugal forces tremble in various parts of the earth and affect crustal movement. Due to the internal instability of the earth, the polar trajectory is not a smooth spiral curve and often has sudden turns. Slow long-term polar movement-basically moving along a radial circle to a relatively fixed direction, and its moving rate is about 10 cm / a ^[5] .