Knowledge about the earth

The Earth’s rotation direction: from west to east

Revolving around the sun, direction: same as above.

The combination of the earth's rotation and revolution causes the alternation of day and night and the changes of the four seasons on the earth (the speed of the earth's rotation and revolution is uneven). At the same time, due to the gravitational effects of the sun, moon, and nearby planets, as well as the influence of various factors such as the earth's atmosphere, oceans, and internal materials, the direction of the earth's rotation axis changes in space and within the earth itself.

The inertial centrifugal force generated by the Earth's rotation causes the spherical Earth to gradually expand from the poles to the equator, becoming the current slightly flat ellipsoid, with the polar radius being about 21 kilometers shorter than the equatorial radius.

The Apollo spacecraft saw from the moon that the Earth is composed of a series of concentric layers. The Earth's interior has core (core), mantle (mantle), and crust (crust) structures. On the outside of the Earth are the hydrosphere and atmosphere, as well as the magnetosphere, which form a beautiful mantle surrounding the solid Earth.

The Earth, as a planet, was created in the primitive solar nebula 5.6 billion years ago.

Basic parameters of the Earth:

Equatorial radius: ae = 6378136.49 meters

Polar radius: ap = 6356755.00 meters

Average radius: a = 6371001.00 meters

Equatorial gravity acceleration: ge = 9.780327 meters/second2

Average rotation angular velocity: ωe = 7.292115 × 10-5 rad/second

Oblateness: f = 0.003352819

Mass: M⊕ = 5.9742 ×1024 kg

Gravity constant: GE = 3.986004418 ×1014 m3/s2

Average density: ρe = 5.515 g/cm3

Mass ratio of the Sun to the Earth: S/E = 332946.0

Mass ratio of the Sun to the Earth-Moon system: S/(M+E ) = 328900.5

Revolution time: T = 365.2422 days

Average distance from the sun: A = 1.49597870 × 1011 meters

Revolution speed: v = 11.19 km/ Seconds

Surface temperature: t = - 30 ~ +45

Surface atmospheric pressure: p = 1013.250 millibars

Surface gravity acceleration (equator) 978.0 cm/s 2

Surface gravity acceleration (polar) 983.2 cm/s 2

Rotation period 23 hours, 56 minutes and 4 seconds (mean solar time)

Orbit half the length Diameter 149597870 kilometers

Orbital eccentricity 0.0167

Orbital period 1 sidereal year

Earth angle 23 degrees 27 minutes

Earth Structure of each circle

Earth's ocean area is 361,745,300 square kilometers

The thickness of the earth's crust is 80.465 kilometers

The depth of the mantle is 2808.229 kilometers

The radius of the earth's core is 3482.525 Kilometers

Surface area: 510,067,866 square kilometers

It is only recently that people have a relatively clear understanding of the structure of the earth. The entire earth is not a homogeneous body, but has an obvious spherical structure. The composition, density, temperature, etc. of each sphere of the earth are different. In astronomy, studying the internal structure of the Earth is of great significance for understanding the movement, origin and evolution of the Earth, exploring the structures of other planets, and even the origin and evolution of the entire solar system.

The earth's sphere is divided into two parts: the outer circle of the earth and the inner circle of the earth. The outer circle of the Earth can be further divided into four basic spheres, namely the atmosphere, hydrosphere, biosphere and lithosphere; the inner sphere of the Earth can be further divided into three basic spheres, namely the mantle, liquid outer core and solid inner core. . In addition, there is an asthenosphere between the outer circle of the earth and the inner circle of the earth. It is a transitional layer between the outer circle of the earth and the inner circle of the earth. It is located at an average depth of about 150 kilometers below the ground. In this way, the entire earth consists of eight circles, among which the lithosphere, asthenosphere and inner circle of the earth together constitute the so-called solid earth. The atmosphere, hydrosphere and biosphere in the Earth's outer circle, as well as the surface of the lithosphere, are generally studied using direct observation and measurement methods. The inner circle of the Earth is currently mainly studied using geophysical methods, such as seismology, gravity and the inversion of high-precision modern space geodetic observations. There is a remarkable feature in the distribution of the various layers of the earth, that is, the interior of the solid earth and the high altitude above the surface are basically distributed in parallel up and down, while near the surface of the earth, the layers penetrate each other or even overlap each other. Among them, The biosphere performed most significantly, followed by the hydrosphere.

Atmosphere

The atmosphere is the outermost gas sphere in the Earth's outer circle, which surrounds the oceans and land.

There is no exact upper limit for the atmosphere, and there are still thin gases and elementary particles at an altitude of 2,000 to 16,000 kilometers. There is also small amounts of air underground, in soil and certain rocks, which can also be considered a component of the atmosphere. The main components of the earth's atmosphere are nitrogen, oxygen, argon, carbon dioxide and trace gases with a proportion of less than 0.04%. The total mass of the earth's atmospheric gases is about 5.136×1021 grams, which is equivalent to 0.86 millionths of the earth's total mass. Due to the effect of gravity, almost all gases are concentrated within an altitude of 100 kilometers above the ground, and 75% of the atmosphere is concentrated in the troposphere from the ground to an altitude of 10 kilometers. According to the distribution characteristics of the atmosphere, the troposphere can also be divided into the stratosphere, mesosphere, thermosphere, etc.

Hydrosphere

The hydrosphere includes oceans, rivers, lakes, swamps, glaciers and groundwater. It is a continuous but irregular circle. Looking at the Earth from an altitude tens of thousands of kilometers away from the Earth, you can see the white clouds formed by water vapor in the Earth's atmosphere and the blue ocean covering most of the Earth, which makes the Earth a "blue planet." The total mass of the earth's hydrosphere is 1.66×1024 grams, which is about 1/3600 of the earth's total mass. The mass of ocean water is about 35 times that of water on land (including rivers, lakes, surface rock pores and soil). If there were no undulations in the solid part of the entire Earth, the globe would be evenly covered by a layer of water up to 2,600 meters deep. The atmosphere and hydrosphere combine to form the fluid system on the surface.

Biosphere

Due to the existence of the earth’s atmosphere, the earth’s hydrosphere and the minerals on the surface, a natural environment suitable for biological survival is formed under the suitable temperature conditions on the earth. What people usually call organisms refers to living objects, including plants, animals and microorganisms. It is estimated that there are currently about 400,000 species of plants, more than 1.1 million species of animals, and at least 100,000 species of microorganisms. According to statistics, there are about 500 million to 1 billion species of living things that have survived in geological history. However, during the long evolution of the earth, most of them have become extinct. Existing organisms live in the upper part of the lithosphere, the lower part of the atmosphere and the entire hydrosphere, forming a unique circle on the earth called the biosphere. The biosphere is a unique layer that exists only on Earth among all the planets in the solar system.

Lithosphere

The Earth’s lithosphere cannot be directly observed except for its surface morphology. It is mainly composed of the Earth's crust and the top of the upper mantle in the mantle sphere. It extends from the surface of the solid Earth downward through the first discontinuity (Moho surface) shown by seismic waves at nearly 33 kilometers, all the way to the asthenosphere until. The thickness of the lithosphere is not uniform, with an average thickness of about 100 kilometers. Because the lithosphere and its surface morphology are closely related to modern geophysics and geodynamics, the lithosphere is the most studied, detailed, and thorough part of the solid earth in modern earth science. Since the ocean floor occupies 2/3 of the total area of ​​the earth's surface, and the ocean basin accounts for about 45% of the total area of ​​the ocean floor, its average water depth is 4000 to 5000 meters. A large number of submarine volcanoes are distributed in the ocean basin. Vast undersea hills extend around it. Therefore, the main surface morphology of the entire solid earth can be considered to be composed of ocean basins and continental platforms. The study of them constitutes a "global tectonics" theory that is directly related to lithospheric structure and geodynamics.

Asthenosphere

In the upper mantle about 100 kilometers below the earth's surface, there is an obvious low-velocity layer of seismic waves, which was first proposed by Gutenberg in 1926 , called the asthenosphere, which is located in the upper part of the upper mantle, the B layer. Under the ocean floor, it is located below a depth of about 60 kilometers; in the continental area, it is located below a depth of about 120 kilometers, with an average depth of about 60 to 250 kilometers. Modern observations and research have confirmed the existence of this asthenosphere. It is because of the existence of this asthenosphere that the outer circle of the earth is distinguished from the inner circle of the earth.

Mantle sphere

In addition to a significant discontinuity (called the Moho surface) about 33 kilometers below the surface, seismic waves are under the asthenosphere until The interface at a depth of about 2,900 kilometers inside the earth belongs to the mantle. Since the Earth's outer core is liquid, seismic S waves in the mantle cannot propagate through this interface in the outer core. The velocity of the P-wave curve also decreases sharply at this interface. This interface was discovered by Gutenberg in 1914, so it is also called the Gutenberg surface. It forms the interface between the mantle sphere and the outer core fluid sphere. The entire mantle sphere consists of the upper mantle (layer B at a depth of 33 to 410 kilometers, layer C at a depth of 410 to 1000 kilometers, also called the transition zone layer), the D′ layer of the lower mantle (a depth of 1000 to 2700 kilometers) and the D layer of the lower mantle. 〃 layer (depth of 2700 ~ 2900 kilometers). Geophysical research shows that the D" layer has strong lateral inhomogeneity, and its degree of inhomogeneity can even be compared with that of rock layers. It is not only the thermal boundary layer through which heat from the earth's core is transferred to the mantle, but it is also very likely to be the thermal boundary layer between the core and the mantle. There are chemical layerings of different chemical compositions.

Outer core liquid sphere

Under the mantle is the so-called outer core liquid sphere, which is located at a depth of about 2,900 kilometers to 5,120 kilometers below the surface.

The entire outer core liquid circle may basically be composed of liquid with very small dynamic viscosity. The depth of 2900 to 4980 kilometers is called the E layer, which is completely composed of liquid. The depth layer between 4980 kilometers and 5120 kilometers is called the F layer, which is a thin transition layer between the outer core liquid sphere and the solid inner core sphere.

Solid Inner Circle

Among the eight circles of the earth, the one closest to the center of the earth is the so-called solid inner circle. It is located between 5120 and 6371 kilometers from the center of the earth, also known as G layer. According to the detection and research of seismic wave speed, it is proved that the G layer is a solid structure. The inner layer of the earth is not homogeneous. The average earth density is 5.515 g/cm3, while the density of the earth's lithosphere is only 2.6 to 3.0 g/cm3. Therefore, the density inside the earth must be much greater, and the density also changes significantly with depth. The temperature of the Earth's interior rises with depth. According to recent estimates, the temperature is 1300°C at a depth of 100 kilometers, 2000°C at a depth of 300 kilometers, about 4000°C at the boundary between the mantle and the outer core liquid sphere, and the temperature at the center of the earth is 5500 to 6000° C.

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. But since humans settled and lived on the earth, they had to seek a deeper understanding of it.

Planet Earth The Earth is the third planet in order from closest to the sun. Its average distance from the sun is 149.6 million kilometers. This distance is called an astronomical unit (A). The earth's orbit is elliptical, with a long radius of 149,597,870 kilometers, an eccentricity of 0.0167, and an average orbital motion speed of 29.79 kilometers per second.

The earth's equatorial radius is about 6378 kilometers, and its polar radius is about 6357 kilometers. The difference between the two is about 21 kilometers. The average radius of the Earth is approximately 6371 kilometers. The average density of the earth is 5.517 g/cm. The dimensions and other parameters of the Earth are shown in the table.

Shape and size In ancient China, there was the so-called Huntian theory of the heaven and earth. Zhang Heng of the Eastern Han Dynasty wrote in "Illustrated Notes on the Armillary Sphere": "The celestial bodies are as round as projectiles, and the earth is as yellow as a chicken... The sky is wrapped in the earth and the earth is wrapped in yellow." The concept that the earth is round has vaguely existed in ancient times. . In 723, Emperor Xuanzong of the Tang Dynasty, together with Nangong Shuo and others, selected 13 locations on the same meridian in present-day Henan Province to measure the length of the shadow of the summer solstice and the height of the North Pole, and found that the length of one degree of the meridian was 351 miles and 80 steps (Tang units of degree and length). The modern scale is latitude. One degree is 132.3 kilometers long, which is equivalent to the earth's radius of 7,600 kilometers, which is about 20% larger than the modern value. This is the earliest estimate of the scale of the earth (the Egyptians measured earlier, but the observation points are not on the same meridian, and the accounting standard for length units is unknown, so the accuracy cannot be estimated).

Accurate topographic measurements only became possible after Newton discovered the law of universal gravitation, and the concept of the earth's shape gradually became clear. The earth is not a very regular sphere. Its surface can be well approximated by an ellipsoid of revolution with a small oblateness. The oblateness e is the ratio of the difference between the major and minor axes of the ellipsoid to the major axis. It is an important parameter representing the shape of the earth. After years of geometric measurements, astronomical measurements and even artificial earth satellite measurements, its values ​​have reached a high degree of accuracy. This ellipsoid is not the real surface of the earth, but a better scientific summary of the ground. It is used as the unified standard for geodetic measurements around the world, so it is also called the reference ellipsoid. According to this reference ellipsoid, the average degree on the meridian circle is 111.1 kilometers, and the average degree on the equator is 111.3 kilometers. The gravitational potential energy is equal on the reference ellipsoid, so the gravitational acceleration of each point on it can be calculated. The formula is as follows:

g0=9.780318 (1+0.0053024sin2j

- 0.0000059sin2j) meters/second2, where g0 is the gravity acceleration when the altitude is zero, and j is the geographical latitude. Knowing the shape of the Earth, the acceleration of gravity and the gravitational constant G=6.670×10-11 Newton·m2/kg2, we can calculate the mass M of the Earth to be 5.976×1027 grams.

Rotation Due to the relative stability of the earth's rotation, human life has always used it as a standard for timing. Simply put, the time it takes for the earth to revolve around the sun is called a year, and the time it takes for the earth to rotate once is called a day. . However, due to external and internal reasons, the rotation of the earth is actually very complicated. The complexity of the Earth's rotation is manifested in changes in the direction of the rotation axis and changes in the rotation rate, that is, the length of the day.

Among the changes in the direction of the rotation axis, the most important thing is that the rotation axis slowly precesses around the ecliptic axis in space, causing the precession of the vernal equinox to move westward by 50.256″ every year. This is the result of the attraction of the sun and moon to the protruding part of the earth's equator. Secondly, the position change of the earth's rotation axis relative to the earth itself causes the latitude changes of various points on the ground.

This change mainly has two components: one has a one-year cycle with an amplitude of about 0.09″, which is caused by seasonal changes in the atmosphere and sea water, and is a forced vibration; the other component has a 14-month cycle. The period, with an amplitude of about 0.15″, is caused by changes in the earth’s interior. It is called the Zhangdler Oscillation, which is a free vibration. There are also some smaller free vibrations.

Changes in rotational speed cause changes in day length. There are three main categories: long-term changes are decelerating, causing the day length to increase by 1 to 2 milliseconds every hundred years, which is the result of tidal friction; seasonal changes can make the day length change by up to 0.6 milliseconds, which is caused by meteorological factors;

Irregular short-term changes, which can change the length of the day by up to 4 milliseconds, are the result of changes in the Earth's interior.

Surface morphology and crustal movement The surface morphology of the Earth is extremely complex, with rolling mountains, vast ocean basins, and structures of various scales.

The various forms on the earth's surface are not mainly caused by external forces. They originate from the tectonic movements of the earth's crust. There are at least the following assumptions about the causes of crustal movement: ① The contraction or expansion of the earth. Many geoscientists believe that the Earth has been cooling and shrinking, causing huge folds and fractures in the strata. However, observations show that the heat flowing out of the ground is of the same magnitude as the heat generated inside the earth due to the decay of radioactive materials. There are also arguments that the earth is expanding. This issue is currently undecided. ②Crustal isostasis. At a certain depth below the earth's 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 uppermost rock layer of the earth, which is about 80 to 90 kilometers thick, is composed of several huge plates. The interaction and relative movement of these plates produce all geotectonic phenomena on the ground. It is still unclear where the power of plate movement comes from, but many people believe that the convection of materials within the earth plays a decisive role.

Electromagnetic properties The geomagnetic field does not point due south. It was recorded in China's "Mengxi Bi Tan" in the 11th century. Geomagnetic declination varies from place to place. The shape of the true geomagnetic field is very complex. It has significant temporal changes, with the largest change amplitude reaching several thousandths or more of the total geomagnetic field. Changes can be divided into long-term and short-term. Long-term changes come from the movement of matter 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 after short-term changes are eliminated using 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 originates from the underground, and the part equivalent to the first-order spherical harmonic function accounts for about 80%. This part is equivalent to a dipole field, and its North Pole coordinate is 78.5° north latitude. , 69.0° west longitude. Short-term changes are divided into two categories: calm changes and disturbing changes. Quiet changes occur frequently, are relatively regular and have a certain period, and the changing magnetic field intensity can reach tens of nanometers; disturbing changes are sometimes global, with a maximum amplitude of several thousand nanometers, and are called magnetic storms.

The basic magnetic field is not completely fixed. The image of the magnetic field strength drifts westward by 0.2° to 0.3° every year, which is called westward drift. This indicates that the generation of the geomagnetic field may be the result of material flow within the earth. It is now generally believed that the earth's core is mainly composed of iron and nickel (also containing a small amount of light elements), a conductive fluid, and current is generated when a conductor moves in a magnetic field. This coupling of electromagnetic fluids produces the effect of a self-exciting motor, thus generating the geomagnetic field. This is currently the most accepted hypothesis for the origin of the geomagnetic field.

When magma cools down in the geomagnetic field and solidifies into rock, it is magnetized by the geomagnetic field and retains a little permanent magnetism, which is called thermal remanence. Most igneous rocks are magnetic and are oriented in the same direction as the geomagnetic field when the rocks were formed. The position of the Earth's magnetic poles at the time of rock formation can be determined from different rock specimens of the same age. However, the geomagnetic pole positions determined by rock specimens from different geological eras are different. This provides strong evidence for the continental drift hypothesis. It has also been found that the magnetization direction of rocks formed in certain geological eras is exactly opposite to the direction of the modern geomagnetic field. This is because the geomagnetic field has reversed itself many times since the formation of the Earth. According to the hypothesis of the origin of the geomagnetic field of self-excited motors, this reverse direction is understandable. Short-term changes in the geomagnetic field can induce underground currents, which in turn cause induced magnetic fields on the ground. The underground current is related to the electrical conductivity of underground materials, so the electrical conductivity distribution inside the earth can be estimated from this. However, the calculation is complex and the solution is not simple. The consensus that can be obtained now is that conductivity increases with depth, and increases rapidly near a depth of 60 to 100 kilometers. At a depth of 400 to 700 kilometers, the conductivity changes significantly, which is equivalent to the transition layer (also called the C layer) in the mantle.

Temperature and Energy The radiation energy received by the ground from the sun is about 10 joules per year, but most of it is radiated back to space, and only a very small part penetrates into the shallow underground. The shallow underground temperature gradient is about 1°C for every 30 meters, but it varies greatly from place to place. Heat flow can be calculated from the temperature gradient and the thermal conductivity of the rock. The global average heat flow out from the ground is about 6.27 microjoules/cm second, and the total heat energy flow out from the ground is about 10.032×1020 joules/year.

Part of the energy inside the earth comes from the radioactive elements uranium, thorium, and potassium contained in rocks. Their content in rocks has been constantly revised in recent years. Some people estimate that the energy released by long-lived radioactive elements on the earth every year is about 9.614×1020 joules, which is very similar to the ground heat flow. However, this estimate is extremely rough. , contains many unknown factors. Another energy source is the gravitational potential energy when the Earth was formed, assuming that the Earth was formed by the accumulation of diffuse matter in the solar system. This part of energy is estimated to be 25×1032 joules, but during the accumulation process, a large part of the energy disappears in the space outside the earth. A small part, about 1×1032 joules, is accumulated as the elastic energy of the earth’s materials due to the adiabatic compression of the earth. . Assuming that the Earth was initially quite uniform when it was formed, and later evolved into the current layered structure, this would release a part of the gravitational potential energy, estimated to be about 2×1030 Joules. This will lead to a warming of the Earth. The earth rotates slower and slower. Since the formation of the Earth, the loss of rotational energy is estimated to be about 1.5×1031 Joules, and there is also energy released by volcanic eruptions and earthquakes, but their magnitudes are much smaller.

The temperature gradient near the ground cannot be extrapolated below a depth of tens of kilometers. The heat transfer mechanism deep underground is extremely complex. Estimating the temperature distribution inside the earth based on the theory of heat conduction often fails to produce reliable results. But the temperature at certain depths within the Earth can be estimated based on considerations of other geophysical phenomena. The results are as follows: ① At a depth of 100 kilometers, the temperature is close to the melting point of the rock there, about 1100 to 1200°C; ② At a depth of 400 kilometers and 650 kilometers, the rock undergoes phase change, and the temperature is about 1500°C and 1200°C respectively. 1900℃; ③ At the core-mantle boundary, the temperature is above the melting point of iron, but below the melting point of the mantle material, about 3700℃; ④ At the boundary between the outer core and the inner core, the depth is 5100 kilometers, and the temperature is about 4300℃. The temperature at the center of the Earth is estimated to be similar to this.

Internal Structure The layered structure of the Earth is basically divided by the propagation speed of seismic waves (P and S). There are significant lateral inhomogeneities in the upper layers of the Earth: the thickness of continental crust and oceanic crust are very different, and seawater only covers 2/3 of the ground.

During an earthquake, the earthquake source radiates two types of seismic waves, longitudinal waves P and transverse waves S. They each spread around at different speeds and arrive at different locations on the ground at different times. If the changes in the propagation time of P and S with the distance from the epicenter are recorded on the ground, the propagation speeds υp and υs of seismic waves at different depths underground can be estimated.

The layers inside the earth are defined by the distribution of seismic wave velocity. Under the sea water, the uppermost layer of the earth is called the crust, which is about tens of kilometers thick. The part below the crust directly facing the core is collectively called the mantle. There are many layers inside the mantle. The boundary between the crust and the mantle is an obvious discontinuity, called the M interface or Moho interface. Below the interface, the velocity does not change much to a depth of about 80 kilometers. This part is called the caprock. Further down, the speed does not change much, and this part is called the capping layer. Further down, the speed dropped significantly, and did not pick up again until a depth of about 220 kilometers. This part is called the low speed zone. The depth below up to 2891 kilometers is called the lower mantle. The core-mantle boundary is a very obvious discontinuity. Entering the earth's core, the S wave disappears, so the outer core of the earth is liquid. At a depth of 5149.5 kilometers, the S wave appeared again and entered the earth's core.

The distribution of the two elastic constants, pressure and gravitational acceleration inside the earth can be calculated from the distribution of the earth's velocity and density. In the mantle, the gravitational acceleration g changes very little, and only decreases to zero toward the center of the earth after passing the core-mantle boundary. The pressure at the core-mantle boundary is 1.36 megabar and at the core it is 3.64 megabar.

Internal material composition The velocity and density distribution of seismic waves is a limiting condition for the material composition inside the earth. The Earth's core is about 90% composed of an iron-nickel alloy, but it also contains about 10% of a lighter substance; possibly sulfur or oxygen. There are still divergent opinions on the mineral composition of the Earth's mantle. The rock minerals in the earth's crust are formed by the differentiation of mantle materials. Volcanic activity and eruptions of mantle material indicate that the main mineral in the mantle is peridotite. Seismic wave velocity data show that there is a large gradient in wave velocity at depths of 400, 500, and 500 kilometers. This can be explained as a result of mineral phase changes. At a depth of 400 kilometers, olivine changes into a spinel structure, while pyroxene melts into garnet. At a depth of 500 kilometers, pyroxene also breaks down into spinel and metaquartz structures. At a depth of 650 kilometers, these minerals have perovskite and oxide structures. In the lowermost 200 kilometers of the lower mantle, the density of material increases significantly. Whether this region is enriched in iron elements is still a matter of debate.

Origin and evolution The origin and evolution of the earth are actually the origin and evolution of the solar system. The early hypotheses were mainly divided into two major schools: the gradualist school represented by Kant and Laplace and the catastrophic school represented by G.L.L. Buffon. The gradualists believe that the solar system was formed by the gradual cooling of high-temperature rotating gas; the catastrophicists believe that the solar system was created by the collision or close attraction of two or three stars here and there.

Early hypotheses mainly attempted to explain some astronomical facts, such as the regularity of planetary orbits and the differences between inner and outer planets. Distribution of angular momentum in the solar system, etc. In fully explaining the above observational facts, both groups encountered insurmountable difficulties.

Since the mid-1940s, people have gradually tended to the view that the solar system originated from low-temperature solid dust. Earlier advocates included Weizsäcker, Schmitt and Uri. They believe that planets are not formed by the solidification of high-temperature gases, but by the accumulation of solid dust material with low temperatures.

When the earth was formed, it was basically the accumulation of a mixture of various stone objects, dust, and gas. The initial average temperature of the earth is estimated to be no more than 1000°C above the past time. The Earth's temperature gradually increases due to the decay of long-lived radioactive elements and the release of gravitational potential energy. When the temperature exceeds the melting point of iron, the iron element in the primitive earth turned into a liquid state, and due to its high density, it flowed to the center of the earth, thus forming the earth's core. The temperature inside the Earth continues to rise, causing local melting of the mantle, causing chemical differentiation and promoting the formation of the crust.

Both the oceans and the atmosphere were not present when the earth was formed, but were secondary. Because the primitive earth could not have maintained atmosphere and water. The oceans are the result of warming and divergence within the Earth's interior. The original atmosphere was emitted from the interior of the earth and was reductive. It was not until the emergence of green plants that free oxygen gradually accumulated in the atmosphere, gradually forming the current atmosphere over a long period of geological time (see Origin of the Earth).

Age The age of the Earth, if defined as the time from the formation of the original Earth to the present, can be determined by radioactive isotopes contained in rocks and minerals. However, when doing so, it is still inevitable to make some assumptions about the initial state of the earth. Based on precise analysis of lead isotopes in rock minerals and meteorites, the age of the earth is now generally accepted to be about 4.6 billion years.

The atmosphere is the outermost layer of gas in the Earth's outer circle, which surrounds the oceans and land. There is no exact upper limit for the atmosphere, and there are still thin gases and elementary particles at an altitude of 2,000 to 16,000 kilometers. There is also small amounts of air underground, in soil and certain rocks, which can also be considered a component of the atmosphere. The main components of the earth's atmosphere are nitrogen, oxygen, argon, carbon dioxide and trace gases with a proportion of less than 0.04%. The total mass of the earth's atmospheric gases is about 5.136×1021 grams, which is equivalent to 0.86 millionths of the earth's total mass. Due to the effect of gravity, almost all gases are concentrated within an altitude of 100 kilometers above the ground, and 75% of the atmosphere is concentrated in the troposphere from the ground to an altitude of 10 kilometers. According to the distribution characteristics of the atmosphere, the troposphere can also be divided into the stratosphere, mesosphere, thermosphere, etc.

Our home - Earth

What is the shape of the Earth? Where is she from? As early as 1.7 million years ago, human beings had various beautiful reveries about their homeland, the earth, and weaved many colorful legends. In ancient China, there was the story of Pangu creating the world. When ancient Greek mythology told the story of the creation of the world, it was said that the universe was born from chaos, and the first god to appear was Gaia, the god of the earth. The sky, land, and oceans were all born from her, so people respectfully call her "Mother Earth."

The earth is already a 5,000-year-old longevity star. She originated from "Pangu" when he opened the sky and split the earth. About 5,000 years ago, the sky and the earth were connected and gradually evolved, and various different creatures appeared. The average equatorial radius of the Earth is 6378.14 kilometers, which is 21 kilometers longer than the polar radius.

The internal structure of the Earth can be divided into three layers: crust, mantle and core. Under the influence of the earth's gravity, a large amount of gas gathers around the earth to form an envelope, which is the earth's atmosphere.

The earth is like a top, spinning continuously from west to east along its axis. Her rotation period is 23 hours, 56 minutes and 4 seconds, which is approximately equal to 24 hours. At the same time, the earth also revolves around the sun. Her orbit is elliptical, and the semi-major diameter of the orbit reaches 149,597,870 kilometers. One revolution takes 365.25 days, which is one year.