Through the observation data of the moon, sun and planets, the analysis of ancient eclipses and the study of ancient coral fossils, we can get the rotation of the earth in geological period. More than 600 million years ago, there were about 424 days on the earth in a year, indicating that the speed of the earth's rotation at that time was much faster than now. 400 million years ago, there were about 400 days in a year, while 280 million years ago it was 390 days. Studies have shown that every hundred years, the earth's rotation slows down by nearly 2 milliseconds (1 millisecond = one thousandth of a second), which is mainly caused by tidal friction. In addition, due to tidal friction, the angular momentum of the earth's rotation becomes smaller, which leads to the moon moving away from the earth at a speed of 3 ~ 4 cm per year, which makes the period of the moon's revolution around the earth longer. In addition to tidal friction, the possible changes in radius of the earth, the coupling between the core and the mantle inside the earth and the changes in the distribution of materials on the earth's surface will also cause long-term changes in the earth's rotation.
In addition to the above-mentioned long-term deceleration, the earth's rotation speed also changes irregularly, which can also be confirmed by the analysis of astronomical observation data. Among them, the so-called "ten-year scale" change with a period of nearly ten years or even decades and the so-called "interannual change" with a period of 2-7 years have been studied more. The amplitude of the ten-year scale change can reach about 3 milliseconds. The real mechanism of this change is still unclear, and the most likely reason is the coupling between the core and the mantle. The interannual variation range is 0.2 ~ 0.3 milliseconds, which is equivalent to one tenth of the ten-year scale variation range. This interannual variation is quite consistent with the abnormal variation of seawater temperature in the equatorial eastern Pacific during El Ni? o event, which may be related to the global atmospheric circulation. However, the real reason for this consistency is currently in the stage of further exploration. In addition, the irregular changes of the earth's rotation include periodic changes from days to months, and the amplitude of this change is about 1 millisecond.
The periodic changes of the earth's rotation mainly include annual cycle, monthly cycle, semi-monthly cycle and near Sunday and semi-Sunday cycle. The annual cycle change, also known as seasonal change, was discovered in the 1930s. It shows that the earth's rotation slows down in spring and accelerates in autumn, with a periodic change of half a year. The annual variation range is 20 ~ 25 ms, which is mainly caused by the seasonal variation of wind. The half-year variation range is 8 ~ 9 milliseconds, which is mainly caused by the solar tidal action. In addition, the amplitude of the moon cycle and the half-moon cycle is about 1 millisecond, which is caused by the tidal force of the moon. The diurnal and semidiurnal tides of the earth's rotation have only been discovered and confirmed in the last decade, and the amplitude is only about 0. 1 millisecond, which is mainly caused by the diurnal and semidiurnal tides of the moon.
Earth revolution
1543, Copernicus, a famous Polish astronomer, first put forward the concepts of the earth's rotation and revolution in his book "On the Operation of Celestial Bodies". The earth's orbit is elliptical, its half-length diameter is 149597870 km, its eccentricity is 0.0 167, and its average orbit speed is 29.79 km per second. The angle between the orbital plane of revolution (ecliptic plane) and the equatorial plane of the earth is 23 27', which is called the ecliptic angle. The rotation of the earth produces the diurnal variation on the earth, and the revolution of the earth and the existence of the angle of intersection between the yellow and the red cause the alternation of the four seasons.
Seen from the earth, the sun moves counterclockwise along the ecliptic, and the ecliptic and the equator have two intersections on the celestial sphere, which are 180 apart. Among them, the point where the sun passes through the celestial equator from south to north along the ecliptic is called vernal equinox, and the other point which is away from vernal equinox 180 is called the autumnal equinox. For people living in the northern hemisphere, when the sun passes through the vernal equinox and the autumnal equinox respectively, it means that it is already spring or autumn. The point at which the sun reaches the northernmost point through the vernal equinox is called the summer solstice, and the other point with a difference of 180 is called the winter solstice. Every year around June 22nd and February 22nd, the sun passes through summer solstice and winter solstice. Similarly, for people living in the northern hemisphere, when the sun approaches the summer solstice and the winter solstice, it enters the astronomical Xia Dong season. The above situation is just the opposite for people living in the southern hemisphere.
Polar motion
The pole shift, referred to as the pole shift, is the movement of the earth's rotation axis in the earth itself. 1765, Euler predicted the existence of the pole shift for the first time by mechanical methods. 1888, Kuestner discovered the pole shift from the observation of latitude change. American astronomer Zhang Dele pointed out in 189 1 that the pole shift consists of two main periodic components: one is the annual period, and the other is the period of nearly 14 months, which is called Zhang Dele's period. The former is mainly the forced swing of the earth caused by the annual movement of the atmosphere, and the latter is the free swing of the earth caused by the non-rigid body of the earth. The amplitude of the pole shift is about 0.4 arcsecond, which is equivalent to the range of 12× 12 square meters on the ground.
Due to the pole shift, the latitude and longitude of every point on the ground will change. 1899, the international latitude service was established to organize optical astronomical telescopes all over the world, specializing in latitude observation and polar shift determination. With the development of observation technology, since the late 1960s, satellite Doppler observation, laser moon measurement, laser satellite measurement, very long baseline interferometry and global positioning system (GPS) have been started internationally, and the measurement accuracy has been improved by several orders of magnitude.
According to the astronomical observation data of nearly a hundred years, it is found that the pole shift contains all kinds of complex movements. In addition to the above-mentioned anniversary cycle and Chandler cycle, there are various short-period pole shifts in secular polar motion and half a month or so in Zhou Yue. Among them, secular polar motion shows that the earth's pole moves westward at the speed of 3.3 ~ 3.5 milliangular seconds per year in the direction of about 70 ~ 80. It is mainly the crustal rebound caused by the melting of ice sheets in North America, Greenland and Northern Europe after the ice age, which leads to the change of the earth's moment of inertia. Other periodic pole shifts are mainly related to the tidal action of the sun and moon and the action of the atmosphere and ocean.
Precession and nutation
Under the action of external force, the direction of the earth's rotation axis in space does not keep a fixed direction, but changes constantly. Among them, the long-term movement of the earth's axis is called precession, while the periodic movement is called nutation. Precession and nutation cause changes in the positions of the celestial pole and vernal equinox relative to the stars. In the second century BC, the ancient Greek astronomer Hippocrates first discovered precession when compiling a catalog containing 1022 stars. Astronomers in the Jin Dynasty in China independently discovered the precession phenomenon based on the observation of the transit of stars on the winter solstice. According to the "History of Song Dynasty", "Yu:' Yao had a short star in winter, but now more than 2,700 years ago, on the East Wall, it got worse every year, so we can know where it is'". This is the origin of the term precession.
Newton was the first to point out that the cause of precession was the attraction of the sun and the moon to the equatorial uplift of the earth. Under the gravity of the sun and the moon, the earth's rotation axis draws a cone around the yellow pole in space, which takes about 26,000 years, and the radius of the cone is about 23. This long-term movement of the earth's axis caused by the sun and the moon is called precession of the sun and the moon. In addition to the gravity of the sun and the moon, the earth is also attracted by other planets in the solar system, which causes the position of the orbital plane of the earth's movement, that is, the ecliptic plane, to change constantly, thus making vernal equinox have a small displacement along the equator, which is called planetary precession. The age difference of the planets makes the vernal equinox advance eastward along the equator by about 0. 13 arc seconds every year.
While the earth's rotation axis moves precession around the yellow pole in space, it is accompanied by many short-period changes. British astronomer Bradley discovered nutation after analyzing the observation data of 1748 for 20 years. The change of the position of the lunar orbital plane (white plane) is the main reason for nutation. At present, astronomers have analyzed that there are as many as 263 nutation periods, among which the nutation term of the main nutation period, 18.6, is the one with the largest amplitude. The main reason is that the ascending intersection of the white road moves westward along the ecliptic, and it takes about 18.6 years to make a circle. Therefore, the gravitational effect of the moon on the earth also has the same periodic change. On the celestial sphere, it shows that the celestial pole moves around the precession of the yellow pole and around its average position, with a period of 18.6 years. Similarly, the gravitational effect of the sun on the earth changes periodically, causing nutation in the corresponding period.