When an asteroid with a diameter of 1000 km is about to hit the earth. ......

earth

earth

One of the nine planets in the solar system. The earth does not occupy a prominent position in the solar system, and the sun is just an ordinary star. However, since human beings have settled and lived on the earth, they have to seek a deeper understanding.

Planet Earth is the third planet in the distance sequence from the sun, and its average distance from the sun is 654.38+49.6 million kilometers, which is called an astronomical unit (A). The earth's orbit is elliptical, with a long radius of 149597870 km, an eccentricity of 0.0 167 and an average speed of 29.79 km/s. ..

The equatorial radius of the earth is about 6378 km, and the polar radius is about 6357 km, with a difference of about 2 1 km. The average radius of the earth is about 637 1 km. The average density of the earth is 5.517g/cm. The scale and other parameters of the earth are shown in the table.

Shape and Size China's ancient understanding of heaven and earth is called Huntian Theory. Zhang Heng of the Eastern Han Dynasty wrote in "Notes on the Huntianyi Instrument": "The celestial body is round like a projectile, and the rehmannia is like a chicken ... The earth wrapped in the sky is still wrapped in a shell." The concept that the earth is round has existed vaguely since ancient times. In 723, Emperor Xuanzong of the Tang Dynasty and Nangong Shuo and others selected 13 positions on the same meridian in present-day Henan, measured the shadow length of the summer solstice and the height of the North Pole, and concluded that the meridian had a length of 3565438+80 steps at 0 (the unit of the Tang Dynasty's degree and length). The modern scale is that the latitude used to be132.3km, equivalent to 7600km in radius of the earth, which is about 20% larger than the modern value. This is the earliest estimate of the earth scale (the Egyptians measured it earlier, but the observation points were not on the same meridian, and the accounting standard of the length unit was unknown, so the accuracy could not be estimated).

Accurate topographic survey was only possible after Newton discovered the law of universal gravitation, and the concept of the shape of the earth gradually became clear. The earth is not a regular sphere. Its surface can be perfectly approximated by a rotating ellipsoid with a small oblateness. The oblateness e is the ratio of the major axis to the minor axis of an ellipsoid, and it is an important parameter to express the shape of the earth. After years of geometric measurement, astronomical measurement and even artificial earth satellite measurement, its numerical value has reached a high accuracy. This ellipsoid is not the real surface of the earth, but a better scientific summary of the ground. It is the same standard of geodesy all over the world, so it is also called reference ellipsoid. According to this reference ellipsoid, the last average value on the meridian circle is11.1.3km, and the last average value on the equator is11.3km. The gravitational potential energy on the reference ellipsoid is equal, so the gravitational acceleration of each point on it can be calculated as follows: G0 = 9.780318 (1+0.0053024sin2j-0.00059sin2j) m/s 2,

Where g0 is the acceleration of gravity when the altitude is zero, and j is the geographical latitude. Knowing the shape of the earth, the gravitational acceleration and gravitational constant g = 6.670×10-1nm2/kg2, we can calculate the mass m of the earth as 5.975x10 24kg.

The scale and other parameters of the earth

Because of the relative stability of the earth's rotation, human life has always taken this as the standard of timing. Simply put, the time for the earth to go around the sun is called one year, and the time for the earth to rotate once is called one day. However, due to the external and internal reasons of the earth, the rotation of the earth is actually very complicated. The complexity of the earth's rotation is manifested in the change of the direction of the rotation axis and the rapid change of the rotation rate.

Among the changes in the direction of the rotation axis, the most important thing is the slow precession of the rotation axis around the ecliptic axis in space, which causes vernal equinox to move westward by 50.256 "precession every year. This is the result of the attraction of the sun and moon to the protruding part of the equator of the earth. Secondly, the position change of the earth's rotation axis relative to the earth itself has caused the latitude change of each point on the ground. This change mainly consists of two parts: one is the forced vibration with an annual cycle and an amplitude of about 0.09 ",which is caused by seasonal changes such as the atmosphere and seawater; The period of the other component is 14 months, and the amplitude is about 0.15 ",which is caused by the internal changes of the earth, and is called a free vibration. In addition, there are some smaller free vibrations.

The change of rotating speed causes the change of day length. There are mainly three kinds: the long-term change is deceleration, and the length of one day is increased by 1 ~ 2 milliseconds every hundred years, which is the result of tidal friction; Seasonal changes can change the maximum length of a day by 0.6 milliseconds, which is caused by meteorological factors; Irregular short-term changes can change the length of a day by up to 4 milliseconds, which is the result of changes in the earth.

Surface morphology and crustal movement The surface morphology of the earth is extremely complex, including endless mountains, vast basins and structures of various scales.

The various forms of the earth's surface are not mainly caused by external forces, but by tectonic movements of the earth's crust. There are at least the following hypotheses about the causes of crustal movement: ① the contraction or expansion of the earth. Many geologists believe that the earth has been cooling and shrinking, resulting in huge stratigraphic folds and fractures. However, observations show that the heat flowing out of the ground is an order of magnitude as that generated by the decay of radioactive materials in the earth. Some people also put forward the argument that the earth is expanding. This question has not been settled. ② crustal equilibrium. At a certain depth below the crust, the load per unit area tends to be equal. The huge height difference on the ground is regulated by the lateral material flow deep underground. (3) Plate Tectonic Hypothesis-The rock stratum with a thickness of about 80-90km at the top of the earth is composed of several huge plates. The interaction and relative movement of these plates have produced all the structural phenomena on the ground. It is not clear where the driving force of plate movement comes from, but many people think that the material convection in the earth plays a decisive role.

Map: world topography

The electromagnetic geomagnetic field does not point due south. 165438+ The "Mengqian Bitan" in China in the 20th century is recorded. The geomagnetic declination is different everywhere. The shape of the real geomagnetic field is very complicated. It has obvious time variation, and the maximum variation range can reach several thousandths or even more of the total geomagnetic field. Changes can be divided into long-term and short-term Long-term changes come from the material movement inside the earth; Short-term changes come from tidal movements in the ionosphere and changes in solar activity. In the geomagnetic field, the so-called basic geomagnetic field is obtained by eliminating short-term changes through statistical averaging or other methods. Using the method of spherical harmonic analysis, it can be proved that more than 99% of the basic geomagnetic field comes from underground, and the part equivalent to the first-order spherical harmonic function accounts for about 80%, which is equivalent to a dipole field with polar coordinates of 78.5 north latitude and 69.0 west longitude. Short-term changes can be divided into two categories: calm changes and disturbance changes. Quiet changes frequently, relatively regularly, with a certain period, and the changing magnetic field intensity can reach dozens of nats; The disturbance changes are sometimes global, and the maximum amplitude can reach several thousand nats, which is called magnetic storm.

The basic magnetic field is not completely fixed, and the magnetic field intensity image drifts westward by 0.2 ~ 0.3 every year, which is called westward drift. This shows that the geomagnetic field may be the result of the material flow inside the earth. At present, it is generally believed that the core is mainly composed of iron and nickel (containing a small amount of light elements), and the conductor moves in the magnetic field to generate current. This coupling of electromagnetic fluid produces a kind of self-excited motor, which produces the geomagnetic field. This is the most accepted origin of the geomagnetic field hypothesis at present.

When magma cools and solidifies into rocks in the geomagnetic field, it is magnetized by the geomagnetic field and retains a little permanent magnetism, which is called thermal remanence. Most magmatic rocks are magnetic, and their direction is the same as the geomagnetic field during diagenesis. The position of the earth's magnetic pole in the diagenetic process can be determined from different rock samples in the same era. However, the positions of geomagnetic poles determined by rock samples of different geological ages are different. This provides strong evidence for the continental drift hypothesis. It is also found that the magnetization direction of diagenetic rocks in some geological ages is just opposite to that of modern geomagnetic field. This is because the geomagnetic field has been reversed many times after the formation of the earth. According to origin of the geomagnetic field hypothesis of self-excited motor, this reversal is understandable. The short-term change of geomagnetic field can induce underground current, which in turn will cause the induced magnetic field on the ground. Underground current is related to the conductivity of underground substances, so the conductivity distribution in the earth can be estimated. But the calculation is complicated and the solution is not single. The consensus that can be obtained now is that the electrical conductivity increases rapidly with the increase of depth near 60 ~ 100 km. At the depth of 400 ~ 700 km, the electrical conductivity has changed obviously, which is equivalent to the transition layer (also called layer C) in the mantle.

Temperature and Energy The radiation energy received by the ground from the sun every year is about 10 Joule, but most of the radiation goes back to space, and only a very small part penetrates into very shallow underground places. The temperature gradient of shallow underground is about 65438 0℃ every 30m, but it varies greatly from place to place. Heat flux can be calculated by the temperature gradient and thermal conductivity of rocks. The global average of heat flowing from the surface is about 6.27 microjoules/cm/s, and the total heat flowing from the surface is about 10.032× 10 20 joules/year.

Part of the energy inside the earth comes from radioactive elements such as uranium, thorium and potassium contained in rocks. In recent years, their contents in rocks have been constantly revised. It is estimated that the annual energy released by long-lived radioactive elements on the earth is about 9.6 14× 10 20 joules, which is very close to the ground heat flow, but this estimate is very rough and contains many unknown factors. The other kind of energy is the gravitational potential energy when the earth was formed, assuming that the earth was accumulated by diffused substances in the solar system. This part of energy is estimated to be 25× 10 32 joules, but in the process of accumulation, a large part of energy disappears in the space outside the earth, and a small part, about 1× 10 32 joules, accumulates as elastic energy of the earth's matter due to the adiabatic compression of the earth. Assuming that the earth is fairly uniform at first, it will release a part of gravitational potential energy, which is estimated to be about 2×10.30 joules, when it evolves into the present layered structure. This will lead to global warming. The earth is turning more and more slowly. Since the formation of the earth, the disappearance of rotational energy is estimated to be about 1.5× 103 1 joule, and there are also energy released by volcanic eruptions and earthquakes, but their orders of magnitude are much smaller.

The temperature gradient near the ground cannot be extrapolated below the depth of several tens of kilometers. The heat transfer mechanism in the deep underground is extremely complex, and it is often impossible to obtain credible results by estimating the temperature distribution in the earth with heat conduction theory. But according to other geophysical phenomena, we can estimate the temperature of some specific depths of the earth. The results are as follows: ① At the depth of 100 km, the temperature is close to the melting point of rocks, which is about1100 ~1200℃; (2) At depths of 400 km and 650 km, the rock undergoes phase transformation, and the temperatures are about 1500℃ and1900℃ respectively; ③ At the core-mantle boundary, the temperature is higher than the melting point of iron, but lower than the melting point of mantle material, about 3700℃; ④ At the junction of the outer core and the inner core, the depth is 5 100 km, the temperature is about 4300℃, and the geocentric temperature is estimated to be similar to this.

Internal structure The layered structure of the earth is basically divided according to the propagation velocity (P and S) of seismic waves. There is obvious lateral inhomogeneity in the upper layer of the earth: the thickness of continental crust is quite different from that of ocean crust, and seawater only covers 2/3 of the ground.

During an earthquake, the source radiates two kinds of seismic waves, P wave and S wave. They travel at different speeds? It takes different time to reach different places on the ground. If the propagation time of p and s varies with the epicentral distance on the ground, the propagation velocities υp and υs of seismic waves at different depths can be calculated.

The stratification in the earth is defined by the velocity distribution of seismic waves. At the bottom of the sea, the top layer of the earth is called the crust, which is about tens of kilometers thick. Below the earth's crust directly to the core, this part is collectively called the mantle. There are many layers inside the mantle. The boundary between the crust and the mantle is an obvious discontinuous surface, which is called M-interface or Moho surface. The depth below the interface is about 80 kilometers, and the speed changes little. This part is called caprock. Further down, the velocity changes little, and this part is called caprock. Further down, the speed dropped significantly, and it didn't rise again until the depth of about 220 kilometers. This part is called the low-speed zone. The depth down to 289 1 km is called the lower mantle. The core-mantle boundary is a very obvious discontinuity. Entering the core, the S wave disappears, so the outer core of the earth is liquid. At the depth of 5 149.5km, the S wave appears again and enters the core.

From the velocity and density distribution of the earth, we can calculate the distribution of two elastic constants, pressure and gravity acceleration. In the mantle, the change of gravity acceleration g is very small, and it only decreases to zero after crossing the boundary of the core and mantle. The boundary pressure of the core-mantle is 1.36 MPa, and the geocentric pressure is 3.64 MPa.

The velocity and density distribution of seismic waves composed of internal materials is a limiting condition of the internal material composition of the earth. About 90% of the earth's core is composed of iron-nickel alloy, but it also contains 10% of lighter substances in Chapter III of the Constitution. It could be sulfur or oxygen. There are still different opinions about the mineral composition of the mantle. Rock minerals in the crust are different from those in the mantle. Volcanic activity and eruption of mantle materials show that peridotite is the main mineral in the mantle. Seismic wave velocity data show that the wave velocity gradient is very large at depths of 400, 500 and 500 kilometers. This can be explained as the result of mineral phase transition. At the depth of 400 kilometers, olivine phase transforms into spinel structure, while pyroxene melts into garnet. At the depth of 500 kilometers in China, pyroxene is also decomposed into spinel and super-chronous structure. At the first depth of 650 km, these minerals are perovskite and oxide structures. At the lowest 200 km of the lower mantle, the density of matter increased significantly. Whether there is iron enrichment in this area is still a controversial issue.

Origin and Evolution The origin and evolution of the earth is actually the origin and evolution of the solar system. The early hypothesis is mainly divided into two schools: the progressive school represented by Kant and Laplace and the radical school represented by G.L.L Buffon. Gradualism holds that the solar system is formed by the gradual cooling of high-temperature rotating gas; The Catastrophic School claims that the solar system is caused by the collision or close attraction between two or three stars. The early hypothesis mainly tried to explain some astronomical facts, such as the regularity of planetary orbits and the difference between inner planets and outer planets. The distribution of angular momentum in the solar system, etc. When fully explaining the above observation facts, both factions encountered insurmountable difficulties.

Since the mid-1940s, people gradually tend to think that the solar system originated from low-temperature solid dust. Early supporters included weizsacker, Schmidt and Yuri. They believe that the planet is not made of high-temperature gas solidification, but of low-temperature solid dust matter accumulation.

When the earth was formed, it was basically a mixture of various stone objects and dust gas. The initial average earth temperature is estimated not to exceed 1000℃. Due to the decay of long-lived radioactive elements and the release of gravitational potential energy, the temperature of the earth is gradually rising. When the temperature exceeds the melting point of iron, the iron in the primitive earth becomes liquid and flows to the central part of the earth due to its high density, thus forming the core. The continuous rise of the temperature inside the earth makes the mantle melt locally, causing chemical differentiation and promoting the formation of the crust.

When the earth was formed, neither the ocean nor the atmosphere existed, and they were all secondary. Because the primitive earth could not keep the atmosphere and water. The ocean is the result of global warming and differentiation. The primitive atmosphere is released from the interior of the earth and is reductive. It was not until the appearance of green plants that free oxygen gradually accumulated in the atmosphere and gradually formed the present atmosphere in the long geological age (see Origin of the Earth).

The age of the earth, if defined as the time from the formation of the primitive earth to the present, can be determined by the radioactive isotopes contained in rocks and minerals. But in doing so, it is inevitable to make some assumptions about the initial state of the earth. According to the accurate analysis of lead isotopes in rocks, minerals and meteorites, it is generally believed that the age of the earth is about 4.6 billion years.

The systematic scientific research on the origin and evolution of the earth began in the middle of the eighteenth century, and many theories have been put forward so far. The popular view now is that the earth, as a planet, originated from the primitive solar nebula 4.6 billion years ago. Like other planets, it has experienced some similar physical evolution processes such as accretion and collision. At the beginning of embryo formation, the temperature was low and there was no layered structure. It is only because of the bombardment of meteorite materials, the heat generated by radioactive decay and the gravitational contraction of the original earth that the temperature of the earth gradually rises. With the increase of temperature, the materials in the earth become more and more plastic, and local melting occurs. At this time, under the action of gravity, substances begin to differentiate, the heavier substances near the surface gradually sink, the lighter substances inside the earth gradually rise, and some heavy elements (such as liquid iron) sink to the center of the earth, forming a dense core (seismic wave observation shows that the outer core of the earth is liquid). The convection of matter was accompanied by large-scale chemical separation, and finally the earth gradually formed the present crust, mantle and core.

In the early stage of the earth's evolution, the primitive atmosphere completely escaped. With the recombination and differentiation of matter, various gases originally in the earth rise to the surface to become the second generation atmosphere; Later, due to the photosynthesis of green plants, it further developed into a modern atmosphere. On the other hand, the internal temperature of the earth rises, which makes the internal crystal water vaporize. With the gradual decrease of surface temperature, gaseous water condenses and falls to the ground to form hydrosphere. About three or four billion years ago, single-celled organisms began to appear on the earth, and then gradually evolved into various organisms until advanced organisms like humans formed a biosphere.

Under the gravity of the earth, a large number of gases gather around the earth, forming a blanket called the atmosphere. The atmosphere moves with the earth; The gravity of the sun and the moon also plays a tidal role in it. The atmosphere has a decisive influence on the physical conditions and ecological environment on the ground. The mass of the earth's atmosphere accounts for about one millionth of the total mass of the earth. The density of the atmosphere decreases with the increase of altitude. 90% of the total atmospheric mass is concentrated in the height range of 15 km above the surface and 99.9% in the height range of 50 km. Above 2000 kilometers above sea level, the atmosphere is extremely thin, and it gradually transitions to interplanetary space, with no obvious upper limit.

The density, temperature, pressure and chemical composition of the earth's atmosphere all change with altitude. The earth's atmosphere can be stratified according to different parameters such as temperature distribution, composition and ionization degree.

According to the distribution of atmospheric temperature with height, it can be divided into:

Troposphere: The lower atmosphere near the surface with obvious convective movement. Its thickness varies with latitude, season and other conditions. The equatorial region is about 16 ~ 18km, the mid-latitude region is about 10 ~ 12km and the polar region is about 7 ~ 8km. Generally speaking, it is thick in summer and thin in winter. The troposphere is most closely connected with the surface and is most affected by the surface conditions. Most water vapor in the atmosphere is concentrated in this layer, forming clouds and precipitation. The upper troposphere is called "tropopause", which is about several hundred meters to 1 ~ 2 kilometers thick. The temperature in the troposphere decreases almost linearly with height, and the top of the troposphere is about MINUS 50 degrees Celsius.

Stratosphere: (also called stratosphere) A layer from the tropopause to a height of 50 kilometers above the ground, where the atmosphere is mainly advection. With the increase of height, the temperature in the layer rises slightly, reaching the highest at a height of about 50 kilometers (about minus 10 ~ minus 20 degrees Celsius).

Intermediate layer: (also known as escape layer), its height is 50 ~ 85km from the surface, and the temperature decreases with the increase of height. At the top of the mesosphere, 85 kilometers away from the surface, the temperature is close to the lowest value, about MINUS degrees Celsius.

Thermosphere: the layer above the middle layer, the temperature rises with the increase of height, reaching about 1 100 degrees Celsius at the top of the thermosphere 500 kilometers away from the surface. The temperature of this layer rises because the atmosphere absorbs a lot of solar ultraviolet radiation. Above the top of the thermosphere is the outer atmosphere. The atmosphere here is extremely thin.

According to the composition of the atmosphere, it can be divided into two layers: the homogeneous layer is about 100 km below the surface (the atmosphere is composed of various gases); The above is a heterogeneous layer. In the homogeneous layer, at a distance of 0/0 ~ 50 kilometers from the surface/kloc-0, the photochemical action of solar ultraviolet radiation produces ozone and forms the ozone layer, and the height of this layer is roughly equivalent to the stratosphere mentioned above. The ozone concentration is the highest at 20 ~ 30 kilometers from the surface, but the ozone content in this part of the atmosphere is still less than one hundred thousand, and various gases are still considered to be evenly mixed. The ozone layer absorbs life-threatening ultraviolet radiation from the sun, making it impossible to reach the surface.

According to the ionization degree of the atmosphere, it can be divided into two layers: from the surface to the layer 80 kilometers away from the surface, the molecules and atoms in the atmosphere are in a neutral state, which is called the neutral layer. At a distance of 80 ~ 1000 km from the surface, atoms in the atmosphere are ionized by solar radiation (mainly ultraviolet radiation) and become a large number of positive ions and electrons, which constitute the ionosphere. Ionization is divided into four layers, and the height and ionization of these layers vary with different times of the day, different seasons of the year and the degree of solar activity. Many interesting astronomical phenomena, such as auroras and meteors, occur in the ionosphere. The ionosphere can also reflect short-wave radio, so that short-wave radio communication can be realized on the ground.

In the near-surface atmosphere, 78% is nitrogen, 265,438+0% is oxygen, and there are other gas components such as carbon dioxide and argon, as well as water vapor. Water vapor is the most unstable component in the atmosphere. In hot and humid places in summer, the content of water vapor in the atmosphere can reach 4%; In dry and cold places in winter, its content can be reduced to 0.065438 0%. In addition to water vapor, there are dust, pollen, volcanic ash, meteor dust and other particles within 3 kilometers of the surface. The primitive atmosphere in the early days of the earth's formation no longer exists, and it has all or most escaped into space. Later, due to the decay of radioactive elements and the so-called "gravitational heating", the earth was in the melting stage, which accelerated the process of gas escaping from the interior of the earth. The gravity of the earth makes these escaping atmospheres gradually accumulate around the earth. This second-generation earth's atmosphere is oxygen-deficient, which is mainly composed of carbon dioxide, carbon monoxide, methane and ammonia, and is called reducing atmosphere. Later, it was mainly the photosynthesis of green plants, followed by the radiation of the sun to decompose water into free oxygen, thus turning the reduced atmosphere into an oxidized atmosphere dominated by nitrogen and oxygen. Some scientists have concluded that oxygen has existed for at least 2.5 billion years by analyzing the sediments in hematite. Since then, the atmosphere has been rich in free oxygen.

The earth is an anisotropic body with a layered structure, and the composition, density and temperature of each layer are different. People mainly study the internal structure of the earth through seismic waves. The propagation speed of seismic waves is closely related to the density and properties of materials in the earth. In media with different properties and states, the propagation speed of seismic waves varies greatly. According to the data of seismic wave propagation velocity in different parts of the earth, the structure of the earth can be analyzed. The analysis shows that there are two discontinuities on the earth, which divide the earth into three main concentric layers: crust, mantle and core.

The crust is also called layer A, and its thickness is uneven. The average thickness of continental crust is about 30 kilometers (the crust thickness of Qinghai-Tibet Plateau in China can reach more than 65 kilometers), while the average thickness of oceanic crust is only 5 ~ 8 kilometers. The density is 1/2 of the average density of the earth. The composition of the upper continental crust is about between granodiorite and diorite, and the lower rocks may be granulite and amphibole. The oceanic crust is peridotite. As far as we know, most crustal rocks are less than 2 billion years old. This means that the rocks in the earth's crust are not the original crust of the earth, but the materials in the earth were formed after volcanic activity and orogeny.

The material density of the mantle increases from 3.3 grams per cubic centimeter near the crust to 5.6 grams per cubic centimeter near the core, and the propagation speed of seismic waves also increases. The mantle is divided into three layers. Layer b and layer c are called the upper mantle. Further down 2900 kilometers is called layer D, which is the lower mantle. The main composition of mantle materials may be ultrabasic rocks similar to peridotite.

The core is also divided into three layers. E layer is the outer core, which can be liquid. F layer is the transition layer between the outer core and the inner core. The G layer is the kernel and may be solid. Although the core only accounts for 16.2% of the earth's volume, its mass exceeds 3 1% of the earth's total mass according to some scholars' calculations due to its high density (the material density in the center of the core reaches per cubic centimeter 13g, and the pressure may exceed 3.7 million atmospheres). The inner core is mainly composed of metallic substances such as iron and nickel.

The temperature inside the earth rises with depth. According to the propagation of seismic waves, it is known that the mantle is in a solid state, the temperature at the depth of 100 km has reached 1300 degrees Celsius, and the temperature at the depth of 300 km is 2000 degrees Celsius. According to recent estimates, the temperature at the edge of the core is about 4000 degrees Celsius, and the temperature at the center of the earth is 5500 ~ 6000 degrees Celsius. Because the earth's surface is a poor conductor of heat, only a tiny part of the huge heat from the sun can penetrate into the very shallow underground. Therefore, the heat energy inside the earth may mainly come from the earth itself, that is, from the decay of natural radioactive elements.

The acceleration of gravity of the earth also varies with depth. It is generally believed that from the surface to the depth of 2900 kilometers underground, gravity generally increases with the depth and reaches the highest value at 2900 kilometers. From here to the center of the earth, gravity drops sharply and reaches zero at the center of the earth.

The earth keeps rotating from west to east around its axis of rotation, and the phenomenon that all kinds of celestial bodies rise in the east and set in the west is a reflection of the earth's rotation. The earth's rotation is the earliest benchmark for measuring time (see time and its measurement), forming a commonly used unit of time-day. Since the 20th century, an important discovery in astronomy has proved that the earth's rotation speed is uneven, thus shaking the traditional concept of taking the earth's rotation as the measurement time, and the almanac time and atomic time have appeared. So far, it has been found that there are three changes in the earth's rotation speed: long-term deceleration, irregular change and periodic change.

The long-term slowdown of the earth's rotation has increased the length of a day by about 1 ~ 2 milliseconds in a century, and slowed down the time measured according to the earth's rotation cycle by more than two hours in 2000. Through the analysis of the observation data of the moon, sun and planets and the ancient solar eclipse data, it can be confirmed that the earth's rotation has slowed down for a long time. By studying the growth line of ancient coral fossils, we can know the rotation of the earth in geological period. For example, it was found that in the middle Devonian, that is, 370 million years ago, there were about 400 days every year, which was consistent with the astronomical argument that the earth's rotation slowed down for a long time. The main reason for the long-term slowdown of the earth's rotation may be tidal friction. Tidal friction reduces the angular momentum of the earth's rotation and makes the moon farther and farther away from the earth, thus making the period of the moon's revolution around the earth longer. This tidal friction mainly occurs in the shallow sea area. In addition, the expansion and contraction of radius of the earth, the proliferation of the core and the coupling between the core and the mantle may also cause long-term changes in the earth's rotation.

In addition to the long-term slowdown of the earth's rotation speed, there are irregular changes from time to time. This irregular change can also be confirmed in the observation data of the moon, sun and planets, as well as astronomical dating data. According to the changes, it can be roughly divided into three types: relative changes for decades or more; Relative changes from several years to ten years; Relative changes from weeks to months. The first two changes are relatively stable, and the latter one is more drastic. The mechanism of these irregular changes remains inconclusive. The relatively stable change may be caused by the exchange of angular momentum between the mantle and the core or the change of sea level and glaciers. The drastic changes may be caused by the action of the wind.

The seasonal and periodic variation of the earth's rotation speed was discovered in the 1930s. In addition to the annual changes that are slower in spring and faster in autumn, there are also changes in the semi-annual cycle. The amplitude and phase of these changes are relatively stable. The corresponding physical mechanism is also more mature and the views are more consistent. The annual variation range is about 20 ~ 25 ms, which is mainly caused by the seasonal variation of wind. The half-year variation range is about 9 milliseconds, which is mainly caused by solar tides. Due to the continuous improvement of astronomical time measurement accuracy, at the end of 1960s, some tiny short-period changes of the Earth's rotation speed were obtained from the observation data, the periods of which were mainly one month and half a month, and the amplitude was only about 1 millisecond, which was mainly caused by the moon tides.