Earth motion

Through the observations of the moon, sun and planets and the ancient Lunar eclipse , Solar eclipse Through the analysis of data and the study of ancient coral fossils, the rotation of the Earth in geological times can be obtained. More than 600 million years ago, there were about 424 days in the Earth's year, indicating that the Earth's rotation rate was much faster then than it is today. 400 million years ago, there were about 400 days a year, and 280 million years ago there were 390 days. Research shows that every hundred years, the Earth's rotation period slows down by nearly 2 milliseconds (1 millisecond = thousandth of a second), which is mainly caused by Tidal friction Caused by. In addition, due to tidal friction, the angular momentum of the Earth's rotation becomes smaller, which causes the moon to move away from the Earth at a speed of 3 to 4 centimeters per year, making the moon circle Earth revolution The period becomes longer. In addition to tidal friction, Earth radius Possible changes to the earth's interior Earth's core and mantle Coupling, changes in the distribution of materials on the Earth's surface can also cause Earth rotation Long-term change. Sidereal day 23:56 minutes and 4 seconds; It takes the sun 24 hours and three minutes and 56 seconds to orbit.

a1 -- a2 is a day and night (24 hours) with a rotation of 360 degrees and a revolution of about 0.9856 degrees.

The first person to prove experimentally that the Earth rotates was Foucault, who used the Foucault pendulum to prove that the Earth rotates.

Irregular variation "interannual variation"

Earth rotation rate In addition to these long-term slowdowns, there are also irregular changes that increase and decrease in time, which can also be confirmed in the analysis of astronomical observation data, in which the so-called "decadal scale" changes ranging from the period of nearly ten years to decades and the so-called "interannual variation" with a period of two to seven years have been more studied. The magnitude of the decadal scale change can reach about ±3 milliseconds, and the real mechanism causing this change is not clear, the most likely reason is the core-mantle coupling, and some scholars believe that it is related to the change of the moment of inertia of the Earth's rotation. The range of interannual variation is 0.2 to 0.3 milliseconds, which is equivalent to the decadal scale Amplitude of change One tenth of it. This interannual variation is related to El Nino event During the eastern equatorial Pacific Sea water temperature The abnormal changes have considerable consistency, which may be associated with global Atmospheric circulation Relevant. The real reasons for this consistency, however, are still being explored. In addition, Earth rotation The irregular variation also includes the variation in the period of days to months, which is about ±1 millisecond in magnitude.

Periodic changes in the Earth's rotation

Earth rotation the Periodic change It mainly includes the variation of annual cycle, monthly cycle, semi-monthly cycle, and near-Sunday and semi-Sunday cycle. Annual periodic variations, also known as seasonal variations, were discovered in the 1930s and are characterized by slower rotation in the spring and faster rotation in the fall, with semi-annual variations. The amplitude of the annual variation is 20 to 25 milliseconds and is mainly caused by seasonal changes in the wind. The semi-annual variation has an amplitude of 8 to 9 milliseconds and is mainly caused by solar tidal action. In addition, the amplitudes of the monthly and semilunar cycle variations are about ±1 millisecond and are caused by lunar tidal forces. Diurnal and semi-diurnal variations in Earth's rotation have only been discovered and confirmed in the last decade, with amplitudes of only about 0.1 milliseconds, mainly caused by the moon's diurnal and semi-diurnal tidal interactions. The Earth's rotation speed also has long periodic variations, including periods of 78,52,22, and 11 years.

In 1543, the famous Polish astronomer Copernicus wrote a book Celestial transport theory It was first fully proposed in the book Earth rotation And the concept of revolution. Earth revolution The orbit is an elliptical, orbital orbit Semi-major axis It's 149597870 km, orbital eccentricity Is 0.0167, with an average orbital speed of 29.79 km/s. Orbital plane of revolution ( Ecliptic plane Equatorial plane with the Earth Crossing Angle Is 23°26', called The intersection Angle of the yellow . The Earth's rotation produces the earth's Diurnal variation The existence of the Earth's revolution and the Angle of the ecliptic causes the alternation of the seasons.

As seen from Earth, the sun moves counterclockwise along the ecliptic, where the ecliptic and the equator are sphere There are two points of intersection 180° apart on the sun from the ecliptic Celestial equator The point from south to north across the celestial equator is called Spring equinox The other point 180° from the vernal equinox is called Autumnal equinox The sun falls on the annual vernal equinox (around March 21) and the Spring Equinox Autumnal equinox (around September 23) through the spring and autumn equinoxes. For people living in the Northern Hemisphere, when the sun passes through the spring and autumn equinoxes, it means that it is spring or autumn. The northernmost point where the sun passes through the equinox is called First point of cancer The other point 180° from it is called December solstice The sun passes the summer solstice around June 22 and the winter solstice around December 22, respectively. Similarly, for people living in the Northern Hemisphere, when the sun is near the summer and winter solstice, astronomically speaking, it has entered the summer and winter seasons. For people living in the southern Hemisphere, the opposite is true.

Polar motion

Pole movement , referred to as Polar motion , yes Earth rotation The movement of axes within the Earth itself. We call the intersection of the Earth's inertial axis and the Earth's surface the inertial pole, and the intersection of the instantaneous axis and the Earth's surface the instantaneous pole. The inertial pole always tracks the instantaneous pole along the logarithmic spiral, making complex movement, which is the ground level movement. In 1765, Euler The existence of the pole shift was first predicted mechanically. In 1888, Kustner of Germany discovered the pole shift from observations of changes in latitude. In 1891, the American astronomer Chandler pointed out that the pole shift consists of two main periodic components: one is an annual cycle, and the other is a nearly 14-month cycle, called Chandler cycle . The former is mainly due to the forced wobble of the Earth caused by the annual motion of the atmosphere, and the latter is due to the Earth's non Rigid body Caused the earth to wobble freely. The amplitude of the pole shift is about ±0.4 Second of arc It is equivalent to a 12×12 square meter area on the ground. Due to the pole shift, the latitude of points on the ground, longitude It will change. It was established in 1899 International latitude service , organizing global optics Astronomical telescope Specializing in latitude observation, measurement Polar motion . With the development of observation technology, from the late 1960s, the international began Satellite Doppler observations , Laser moon survey Laser measurement of human health, Very long baseline interferometry , Global positioning system Measuring the pole shift, the accuracy of the measurement has been obtained Order of magnitude The improvement of... In addition to the 12-month and 14-month cycles, the annual amplitude of the prefectural movement also has a long period of change, the main period is 6-7 years, and there is a period of about 35 years.

Pole shift involves a variety of complex motions

Based on nearly 100 years of astronomical observations, the discovery Polar motion Contains a variety of complex movements. In addition to the aforementioned annual cycle and money Dehler cycle Outside, still exist Secular pole shift , a variety of short period pole shifts around the week, month, half month and day. In the medium and long term, the pole shifts to the west longitude of about 70° to 80° at a rate of 3.3 to 3.5 millimetres per year Second of arc The speed of movement. It is mainly due to North America, Greenland And Northern Europe Ice sheet Caused by melting Ice age After the crust rebounded, causing the Earth Moment of inertia Due to changes. The other polar shifts of various periods are mainly related to the tidal action of the sun and the moon and the action of the atmosphere and the ocean. Under the influence of external forces, the earth's Axis of rotation The orientation in space does not remain in a fixed direction, but constantly changes. among Axis of the earth Long-term motion is called precession, while periodic motion is called nutation. Precession and nutation cause Celestial pole and Spring equinox The change in position relative to the star. Ancient Greek astronomer of the second century BC Hipparchus Precession was first discovered when compiling a catalogue of 1022 stars. Chinese astronomer of Jin Dynasty Yu Xi According to Winter solstice sidereal Transit observation The precession was independently discovered. According to the "History of the Song Dynasty" records: "Yu Xiyun: 'Yao winter solstice short star Subaru, more than 2,700 years today, and in the east wall, you know the difference of every year." precession This is where the term comes from.

Causes of precession

Newton was the first to point out production precession The cause is the attraction of the sun and moon to the bulge of the Earth's equator. Under the gravitational pull of the sun and moon, Earth rotation Axis in space around Ecliptic pole Picture a Conical surface It takes about 26,000 years to circumnavigate, and the radius of the cone is about 23° 1.5. This is caused by the sun and the moon Axis of the earth The long-term motion is called the precession of the sun and moon. In addition to the gravitational action of the sun and the moon, the Earth is also subjected to the gravitational action of other planets in the solar system, which causes the orbital plane of the Earth's movement, that is Ecliptic plane The constant change of position thus makes Spring equinox There is a small displacement along the equator called Planetary precession . The planetary difference moves the equinox about 0.13 east of the equator each year Second of arc . While the earth's rotation axis moves precession around the yellow pole in space, there are also many short period changes. British astronomers Bradre In 1748, after analyzing 20 years of observations of the positions of stars, he discovered nutation Phenomenon. Lunar orbit face White path surface The change of position is the main cause of nutation. Astronomers have analyzed a total of 263 nutation periods, of which the main nutation period term, 18.6 years nutation term, is the largest amplitude term, which is mainly due to the movement of the white path caused by white path Ascending node It moved westward along the ecliptic, resulting in a detour about 18.6 years. Therefore, the gravitational effect of the moon on the Earth is the same Periodic variation , in sphere On which it manifests itself Celestial pole In winding Ecliptic pole As a precession At the same time, it also moves around its average position with a period of 18.6 years. Likewise, the gravitational effect of the sun on the Earth has Periodic change And cause nutation of the corresponding period.