The earth rotates for about 23: 56 minutes and 4 seconds, and the linear speed of rotation on the equator of the earth is 465 meters per second. The earth's orbit around the sun is elliptical, and the average distance from the sun is1495,700,3000 kilometers. One revolution takes 365.25 days, and the average revolution speed is 29.79 kilometers per second. The intersection of the ecliptic and the equator is 23 degrees and 27 minutes. Because of this angle, the combination of rotation and revolution has produced the alternation of day and night on the earth with uneven length, the change of four seasons and the division of five zones (tropical zone, north-south temperate zone and north-south frigid zone). The speed of the earth's rotation is uneven, with long-term change, seasonal change and irregular change. At the same time, the direction of the earth's rotation axis will change in space and the earth itself due to the gravitational effects of the sun, the moon and the planets, as well as the various effects of the atmosphere, the ocean and the materials inside the earth.
The equatorial radius of the earth is 6378 140m, the polar radius is 6,357km, and the equatorial circumference is 40,076km. The earth is not a right sphere, but a flat sphere, or more like a pear-shaped rotating body. The observation results of artificial earth satellite show that the equator of the earth is also an ellipse, and the inertial centrifugal force generated by the earth's rotation makes the spherical earth gradually expand from the poles to the equator, becoming a slightly flat ellipsoid, and the polar radius is about 2 1 km shorter than the equator radius. The inhomogeneity of material distribution in the earth further causes the irregularity of the surface shape of the earth. The gravitational effect of the sun and the moon on the earth causes tidal phenomena in the earth, the ocean and the atmosphere.
The mass of the earth is 5.976×1027g (or about 6× 102 1 ton), and the average density is 5.52g per cubic centimeter. Any particle on the earth is affected by gravity and inertial centrifugal force, and their resultant force is gravity. Gravity decreases with the increase of height and changes with the change of latitude. Gravity anomalies will occur in some places, reflecting the uneven distribution of materials in the earth. Due to the tidal force of the sun and the moon, the acceleration of gravity of the earth also varies slightly periodically.
The earth can be regarded as a series of concentric layers. There are core, mantle and crust structures in the earth. Outside the earth, there are hydrosphere, atmosphere and magnetosphere, forming a coat around the solid earth. The magnetosphere and atmosphere block the direct bombardment of the ground by ultraviolet rays, X-rays, high-energy particles and countless meteors from space.
The surface area of the earth is about 509.6 million square kilometers, of which more than seven tenths are covered by blue oceans, and lakes and rivers only occupy a small part of the surface waters of the earth. The liquid water layer on the earth's surface, called hydrosphere, has been formed for at least 3 billion years. The earth's surface is composed of various rocks and soils, and the ground is rugged, and low-lying areas flood into oceans and lakes; There are plains and mountains on the land above the water. The total vertical fluctuation of the earth's solid surface is about 20 kilometers, which is the height difference between the top of Mount Everest and the Mariana Trench, and is more than half of the average thickness of the continental crust. The seabed is as uneven and unstable as land. The rocks on the seabed are much younger than those on land. Most rocks on land are less than 2 billion years old. Sedimentary rocks can be seen everywhere on the land, indicating that these places may be oceans in ancient times. Although there are several craters on the earth's surface, it is difficult to find as many craters as the moon, Mars and Mercury. This is because the earth's surface is constantly weathered, eroded and disintegrated by external forces (water and atmosphere) and internal forces (earthquakes and volcanoes).
There is not only vertical movement in the upper part of the earth, but also greater horizontal movement, and the relative position of the ocean and the mainland is also changing during the geological period. Some scientists believe that two ancient continents existed earlier on earth-Gondwana in the southern hemisphere and Lauya in the northern hemisphere. Later, the original continental block was torn by the great force of plate movement, and the fragments gradually drifted to today's position. Scientists further believe that global tectonics is the direct result of the continuous expansion of the seabed.
The top layer of the earth, about tens of kilometers thick, is the lithosphere with great strength, and the bottom layer, hundreds of kilometers thick, is the asthenosphere with low strength. The material in this layer is plastic under long-term stress. The lithosphere floats on the asthenosphere. When the energy (primary heat and radioactive heat) in the earth is released, the uneven distribution of temperature and density in the earth causes the convection movement of mantle materials. Mantle convection material moves to both sides along the cracks in the mid-ocean ridge of the ocean floor, and constantly forms a new ocean floor. In addition, the old ocean floor continues to expand outward, and when it approaches the continental margin, it is inserted under the continental crust under the traction of mantle convection, resulting in a series of tectonic movements in the lithosphere. This convection can make the whole seabed renew once every 300 million years. The lithosphere is separated by some active tectonic belts and divided into several discontinuous units called continental plates. Such as Eurasia plate, America plate, Africa plate, Pacific plate, Australia plate and Antarctic plate. The expansion of the seabed led to the movement of continental plates. The mutual compression of plates has created a huge mountain system, from the Alps through Turkey, the Caucasus to the Himalayas. In some places, the rocks of two plates sink at the same time, resulting in the abyss of the seabed; In addition, plate movement also triggered volcanoes and earthquakes.
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 common 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 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.