The earth is one of the eight planets in the solar system (Pluto was listed as a dwarf planet in 2006 because its orbit is different from the other eight planets), ranking third in the order from near to far from the sun. It has a natural satellite-the moon, forming a celestial system-the earth-moon system. As a planet, the earth originated from the primitive solar nebula 4.6 billion years ago. The earth will interact with other celestial bodies in outer space, including the sun and the moon. The earth is home to millions of creatures, including humans. The earth is the only known celestial body with life in the universe. The equatorial radius of the earth is 6378. 137 km, the polar radius is 6356.752 km, the average radius is about 637 1 km, the equatorial circumference is about 40075 km, and 7 1% of the earth is ocean. 29% is land. The reason why the earth looks blue from space is because the sky is another aspect of the earth and is covered by sea water. The earth is a big magnet. Through the north and south poles, the magnetic field can extend all the way to the earth and the high altitude above100000 km. The earth consists of crust, mantle and core, and the temperature of the core varies with depth. At the depth of 637 1km, the temperature is as high as 4500 ~ 5000 degrees Celsius. The earth is not a complete sphere, but in fact it is an ellipsoid. The circumference of the equator of the earth is longer than that of the prime meridian.
Eratosthenes, the first person to calculate the circumference of the earth.
More than 2000 years ago, someone calculated the circumference of the earth with simple measuring tools. This man is Eratosthenes in ancient Greece.
Eratosthenes read widely. He is familiar with geography as well as astronomy. He is also a poet, historian, linguist and philosopher, and once served as the curator of the Alexandria Museum.
Careful Eratosthenes found that in the city of Thain (near Aswan, Egypt), which is about 800 kilometers away from Alexandria, the midday sun in summer can always reach the bottom of the well, so all the upright objects on the ground should have no shadow at this time. However, there is a short shadow standing upright on the ground of Alexandria.
He believed that the shadow of Homo erectus was caused by the angle between the sunshine in Alexandria and Homo erectus. Based on the fact that the earth is a sphere, sunlight is the premise of straight line propagation. Draw two straight lines from the imaginary center of the earth to Thain and Alexander, and the included angle should be equal to the included angle formed by sunlight and Alexander. According to the proportional relationship in similar triangles, knowing the distance between the two places can measure the circumference of the earth. The included angle measured by Eratosthenes is about 7 degrees, which is one fiftieth of the earth's circumferential angle (360 degrees). It is calculated that the circumference of the earth is about 40,000 kilometers, which is almost the same as the actual circumference of the earth (40,076 kilometers). He also calculated that the distance between the sun and the earth is 65.438+0.47 billion kilometers, which is surprisingly close to the actual distance of 65.438+0.49 billion kilometers. This fully embodies Eratosthenes's theory and wisdom.
Eratosthenes was the first person to use the name "geography". Since then, he has replaced the traditional "local chronicles" and written a three-volume monograph. This book describes the shape, size and land-sea distribution of the earth. Eratosthenes also used latitude and longitude net to draw maps, and for the first time, he combined physical principles with mathematical methods to establish mathematical geography.
The origin and evolution of the earth;
1, the formation of the earth. The history of the earth is very long.
According to the radiocarbon dating results, the solar system was formed about 65.008 billion years ago, while the primitive earth was formed about 65.004 billion years ago. Theoretically, the formation of the sun began with the gravitational collapse of a huge hydrogen molecular cloud 6.5 billion years ago, and most of the collapsed mass was concentrated in the center, forming the sun; The rest flatten out while rotating, forming protoplanetary disks, and then forming planets, satellites, asteroids, comets, meteoroids and other small celestial bodies in the solar system. According to the nebula hypothesis, the asteroids that formed the earth originated from a mass with a diameter of one to ten kilometers formed by the accretion and collapse of gas, ice particles and dust. After 1000 to 20 million years of growth, these substances finally formed the original earth. The new earth's surface is an "ocean" composed of magma.
Since Archaean, the surface began to cool and solidify, forming hard rocks, and the gas released by volcanic eruption formed a secondary atmosphere. The initial atmosphere can be composed of water vapor, carbon dioxide and nitrogen. The evaporation of water vapor accelerates the cooling of the surface. After it completely cooled down, the rainstorm lasted for thousands of years, and the rain filled the basin and formed the ocean. Heavy rain not only reduced the water vapor content in the air, but also washed away a lot of carbon dioxide in the atmosphere. Besides, water and ice on asteroids, protoplanets and comets are also one of the sources of water. The dark sun paradox points out that although the early sunshine intensity was only about 70% now, the greenhouse gases in the atmosphere were enough to keep the liquid water in the ocean from freezing.
About 3.5 billion years ago, the earth's magnetic field appeared, which helped to prevent the atmosphere from being stripped by the solar wind. Its outer layer cools and solidifies, and the crust is formed under the action of atmospheric water vapor. There are two models that can explain the formation of land, one is that land keeps growing, and the other is more likely to form land rapidly in the early history of the earth, and then keep it today. The internal heat is continuously lost, which pushes the plate tectonic movement to form the continent. According to the continental drift hypothesis, after hundreds of millions of years, supercontinent experienced three times of separation and integration. About 750 million years ago, Rodinia, the first supercontinent to be tested, began to split, then merged into Pangea 600-450 million years ago, and finally split about 65438+800 million years ago. The earth was in the Pleistocene Great Ice Age, which began 2.58 million years ago. The high latitude region experienced several rounds of freezing and thawing, and it circulated once every 400 years to 1 10,000 years. The last time the mainland was frozen was about 10000 years ago.
2. Life evolution. The earth provides the only environment that can sustain the evolution of known life.
It is believed that high-energy chemical reactions about 4 billion years ago produced self-replicating molecules, and after 500 million years, the same ancestor of all life appeared, and then differentiated into bacteria and archaea. Early life forms developed the ability of photosynthesis, which can directly use solar energy and release oxygen into the atmosphere. The oxygen accumulated in the atmosphere is influenced by the ultraviolet rays emitted by the sun, and ozone (O3) is formed in the upper atmosphere, and then the ozone layer appears. Early life existed in the form of prokaryotes. Endogenous theory holds that in the process of life evolution, some small cells are swallowed into large cells and endogenously become organelles of large cells, thus forming eukaryotic cells with relatively complex structures. Since then, the cells in all parts of the cell community have gradually differentiated into different functions, forming a true multicellular organism. Because the ozone layer absorbed harmful ultraviolet rays from the sun, the land became suitable for life, and life began to multiply on the land. The earliest fossil evidence of known life includes microbial mat fossils in sandstone in Western Australia 3.48 billion years ago and biogenic graphite in metamorphic clastic rocks in Siglinland Island 3.7 billion years ago.
1992, Dr. Joseph Kosiwenko proposed for the first time that during the Neoproterozoic Glacier Glaciation 750 million to 580 million years ago, intense glacial activity made most of the earth's surface under the ice, which was a snowball earth hypothesis. 542 million years ago, the extinction event occurred at the end of Ediacaran period, followed by the explosion of life in Cambrian, and the variety of multicellular organisms (such as trilobites and shrimps) on the earth surged. After the CAMBRIAN explosion, the earth experienced five extinction events of biological clusters. Among them, the Permian-Triassic extinction event that occurred 256,543.8+0 million years ago is the largest species extinction event in the known geological history. The latest extinction event was the Cretaceous-Paleogene extinction event that occurred 66 million years ago. The impact of asteroids made non-bird dinosaurs and other large reptiles extinct, but some small animals escaped, such as mammals as big as shrews at that time. In the past 66 million years, mammals have been constantly differentiated. Millions of years ago, African anthropoid animals (such as primitive people like Tugen) learned to walk upright. In this way, they can use tools better, communicate with each other, get more nutrition and stimulation, and their brains are more and more developed, and finally evolve into humans. With the development of agriculture and civilization, human beings have enjoyed the quality of life that other species on the earth have not achieved, which in turn has affected the earth and the natural environment.
3. Future evolution. 15 to 4.5 billion years later, the inclination of the earth's axis may change by up to 90 degrees.
It is speculated that the development of complex life on the earth's surface is still very young, and the activity can continue to reach its peak and last for about 5 to 654.38 billion years, but if the oxygen in the atmosphere disappears completely, this time will be extended to 2.3 billion years. The fate of the earth in the distant future is closely related to the evolution of the sun. With the continuous nuclear fusion of hydrogen in the sun's core to generate helium, the brightness of the sun will continue to increase slowly, 1 100 billion years later, and will increase by as much as 40% after 3.5 billion years. The speed at which the sun releases heat will also continue to increase. According to the climate model, the earth's surface will eventually be exposed to rising solar radiation, which will have serious consequences. At first, it will only be in the tropics, and then it will reach the polar cap. Over time, the ocean will evaporate and disappear.
The rise of the earth's surface temperature will accelerate the inorganic carbon cycle and reduce the atmospheric carbon dioxide content. After about 500 million to 900 million years, the carbon dioxide content in the atmosphere will gradually decrease to 10ppm. Without the evolution of photosynthesis, C4 plants will have no right to survive. The lack of vegetation will reduce the oxygen content in the earth's atmosphere, and the animals and plants on the earth will be extinct in millions of years. Later, it is predicted that after more than one billion years, the surface water will disappear completely, and the average temperature of the earth will rise to 70℃. Even if the sun remains stable forever, due to the decrease of water vapor emitted from the mid-ocean ridge, about 65.438 billion years later, 27% of seawater will enter the mantle, and the decrease of seawater will make the temperature change dramatically, which is not suitable for complex life.
Five billion years later, the sun evolved into a red giant, at which time complex molecules could not be formed on the surface of the earth. The model predicts that the sun will expand to about 250 times the current radius, about 1 astronomical unit (10.50 billion km), and the fate of the earth is still unclear. When becoming a red giant, the sun will lose 30% of its mass. Therefore, if the influence of tidal force is not considered, when the sun is at its maximum, the earth will move to a position about 1.7 astronomical units (250 million kilometers) away from the sun, and get rid of the fate of falling into the expanding outer atmosphere of the sun; However, even so, the peak brightness of the sun will be 5000 times as high as it is now, and the remaining creatures on the earth will not escape the fate of being destroyed by the sun. A simulation conducted in 2008 showed that the earth's orbit would be attenuated by tidal effect, making it fall into the atmosphere of the sun, which has become a red giant, and eventually be evaporated.
The structure of the earth:
1, form. The shape of the earth is roughly oval.
The earth's rotation makes it slightly flat along the axis running through the poles and slightly uplifted near the equator. Starting from the center of the earth, the equatorial radius of the earth is 43 kilometers (27 miles) higher than the polar radius. Therefore, the farthest place on the earth's surface from the earth's center of mass is not Mount Everest, the highest elevation, but the top of Chimborazo Mountain in Ecuador, which is located on the equator. The average diameter of the earth reference ellipsoid is about 12742 km (79 18 miles), which is about equal to (40000 km)/π. This integer is not a coincidence. The unit of length meter was originally defined as one tenth of the distance from the equator to the North Pole on the meridian passing through Paris, France. The gravitational acceleration value of a sea level at the equator is GA = 9.780m/s 2, that of a sea level at the North Pole is GB = 9.832m/s 2, the global standard value of gravitational acceleration is G = 9.807m/s 2, and the earth's rotation period is 23 hours, 56 minutes and 4 seconds (sidereal day), that is, T = 8.665438+.
Due to local topography, the earth deviates slightly from the ideal ellipsoid, but from the planetary scale, these landforms are very small compared with radius of the earth, and the maximum deviation is only 0. 17%, which is located at the elevation of Mariana Trench10,911m (35797 ft) and 8844m above sea level. If the earth is reduced to the size of a billiard ball, the mountains and trenches on the earth will feel like tiny flaws, while most other areas, including the Great North American Plain and the Deep Sea Plain, will feel smoother. The total area of the earth is about 510/0 million square kilometers, of which land accounts for 29.2% (148.94 million square kilometers) and the rest 70.8% (3.611320,000 square kilometers) is water. The land is mainly in the northern hemisphere, with five continents such as Europe and Asia, Africa, America, Australia and Antarctica, and there are many islands. The oceans include the Pacific Ocean, the Atlantic Ocean, the Indian Ocean, the Arctic Ocean, the Southern Glacial Ocean and their affiliated sea areas. The coastline is * * * 356,000 kilometers. Land lowest point: Dead Sea (-4 18m), world lowest point: Mariana Trench (-1kloc-0/034m), and world highest point: Mount Everest (8848.86m).
2. Chemical composition. The total mass of the earth is about 5.97× 1024 Kg, which is about 6 trillion tons.
The main chemical elements that make up the earth are iron (32. 1%), oxygen (30. 1%), silicon (15. 1%), magnesium (13.9%) and sulfur (2.9%). The remaining 1.2% is other trace elements, such as tungsten, gold, mercury, fluorine, boron and xenon. Due to the stratification of mass (the mass is concentrated in the center), it is estimated that the main chemical element that constitutes the core is iron (88.8%), and other elements that constitute the core include nickel (5.8%) and sulfur (4.5%), as well as trace elements with mass less than 1%. The main minerals that make up the mantle are pyroxene (chemical formula (mg, Fe, Ca, Na) (mg, Fe, Al) (Si, Al) 2O 6) and olivine (chemical formula (mg, Fe)2SiO4).
As for the chemical composition of the earth's crust, oxygen is the most abundant element in the earth's crust, accounting for 46%. Oxygenated compounds in the earth's crust include water, silica, calcium sulfate, calcium carbonate, alumina, etc. However, the most abundant 10 compounds in the crust and most of the compounds that constitute common rocks in the crust are oxygenated compounds. Some rocks are fluoride, sulfide and chloride, but the total content of fluorine, sulfur and chlorine anywhere is usually much less than 1%. Igneous rocks occupying more than 90% of the crust surface are mainly composed of silica and silicate. Geochemist Frank Wigerville Clark calculated on the basis of 1 672 analysis of various rocks, and came to the conclusion that 99.22% of rocks are composed of oxides listed in the following table, and there are other components with less content.
3. Internal structure. Like other terrestrial planets, the interior of the earth can be divided into several layers according to its chemical or physical (rheological) properties.
But the inner core and outer core of the earth are obviously different, which is a feature that other terrestrial planets do not have. The outer layer of the earth is the crust composed of silicate minerals, and the thick solid mantle is below. The boundary between mantle and crust is Moho. The thickness of the earth's crust varies from 6 kilometers on the seabed to 30 to 50 kilometers on land. The colder and harder upper layers of the crust and mantle are collectively called lithosphere, and the plates are also formed in this area. Below the lithosphere is the asthenosphere with low viscosity, and the lithosphere slides just above the asthenosphere. The main changes in the crystal structure of the mantle occur between 465,438+00 and 660 kilometers below the surface, which is the transition zone between the upper mantle and the lower mantle. Below the mantle, there is a core-mantle boundary (Gutenberg discontinuity), which separates the mantle from the core, and then there is a liquid outer core with very low viscosity, and the innermost is a solid inner core. The angular velocity of the inner core rotation may be faster than other parts of the earth, which is about 0. 1-0.5 per year. The radius of inland core is 1220km, which is about 1/5 of radius of the earth.
4. Magnetic field and magnetosphere. There is a static magnetic field in the earth and the surrounding space.
According to the multipole expansion of the static magnetic field, if the earth is approximately regarded as a magnetic dipole, its magnetic moment is 7.9/kloc-0 /×1015t m3, and the geomagnetic axis direction is approximately coincident with the rotation axis, but slightly offset, and the included angle between them is called geomagnetic declination. On the geomagnetic equatorial circle formed by the intersection of the plane bisecting the geomagnetic axis vertically and the earth's surface, the magnetic induction intensity is about 3× 10? 5 T, at the geomagnetic pole formed by the intersection of the geomagnetic axis and the earth's surface, the magnetic induction intensity is about twice that of the geomagnetic equator. According to the generator hypothesis, geomagnetism mainly comes from the movement of conductive fluid composed of iron and nickel in the core. In the outer core of the earth's core, the hot conductive fluid is subjected to geostrophic deflection force in the process of convection from the center to the outside, forming eddy current and generating magnetic field. The magnetic field generated by eddy current will react to the flow of fluid, making the movement of fluid and even the magnetic field generated by it approximately stable. However, due to the instability of convection, the direction of geomagnetic axis will change slowly and irregularly, leading to geomagnetic inversion. The cycle of geomagnetic reversal is not fixed, and it may be reversed several times every one million years. The most recent one happened 780,000 years ago and was called Bruni-Songshan inversion. ?
The influence range of geomagnetism in space is called magnetosphere. Ions and electrons of the solar wind are deflected by the magnetosphere, so they cannot directly attack the earth. The pressure of the solar wind will compress the magnetosphere near the sun to radius of the earth 10, while the area far away from the sun will extend into a tail shape. The solar wind blows into the magnetosphere at supersonic speed, forming an arcuate shock wave, so the speed of the solar wind slows down, and part of the kinetic energy is converted into heat energy, which makes the temperature in the nearby area rise. Above the ionosphere, the low-energy charged particles in the magnetosphere form a plasma layer, and its motion is dominated by the geomagnetic field. Because the rotation of the earth will affect the movement of plasma, the plasma layer will rotate with the earth. Particles with medium energy in the magnetosphere rotate and flow around the earth axis, forming a ring current. Charged particles not only spiral along the magnetic field lines, but also drift directionally under the gradient and curvature of the geomagnetic field, and electrons move eastward and positive ions move westward, thus forming a ring current. Van Allen radiation zone is a double-layer doughnut-shaped radiation zone. The inner layer is mainly composed of high-energy protons and electrons, and the outer layer also contains heavier ions such as helium. These high-energy particles are captured by the magnetic field and move along the magnetic field lines in a spiral form. When a magnetic storm occurs, charged particles will deflect from the outer magnetosphere along the magnetic field lines, enter the ionosphere, collide with atmospheric atoms, and excite and ionize atmospheric atoms. Aurora is indeed produced at high latitudes.
5, the earth's internal heat.
Among the heat generated inside the earth, the accumulated residual heat accounts for about 20%, and the radioactive decay heat accounts for 80%. The main thermogenic isotopes on the earth are potassium -40, uranium -238, uranium -235 and thorium -232. The highest temperature in the center of the earth can reach 6000 C( 10830 F) and the pressure can reach 360 GPa. Because a lot of geothermal energy comes from radioactive decay, scientists speculate that in the early stage of the earth's history, before isotopes with short half-lives are exhausted, more heat may be generated in the earth's interior, which may be twice as much as it is now 3 billion years ago. So at that time, the temperature gradient along the radius of the earth will be greater, the rate of mantle convection and plate tectonics will be faster, and some rocks like Komatite may be generated, which is difficult to generate under the current geological conditions.
6. Plate structure.
The main plates of the earth are the Pacific plate, America plate, Eurasia plate, Africa plate, Antarctica plate and Indian Ocean plate. There are also the Arabian Plate, the Caribbean Plate, the Philippine Sea Plate, the Cocos Plate off the west coast of North America, the Nazca Plate off the west coast of South America and the Scottish Plate in the South Atlantic. The Indo-Australian plate was formed by the fusion of the Australian plate and the Indian plate 50 million to 55 million years ago. Among these plates, the ocean plate moves fast, while the continental plate moves slowly: the Cocos plate belonging to the ocean plate moves 75 mm a year, and the Pacific plate moves 52 to 69 mm a year; The Eurasian continental plate belongs to the continental plate, with an average annual movement speed of about 21mm.
7. surface.
The total surface area of the earth is about 5 1 100 million square kilometers, of which about 70.8% is covered by water, and most of the crust surface (3611300 million square kilometers) is below sea level. The surface of the submarine crust is mountainous, including the global mid-ocean ridge system, as well as submarine volcanoes, trenches, submarine canyons, submarine plateaus and deep-sea plains. The remaining 29.2% (1489,400 square kilometers, that is, 575 1000 square miles) are places not covered by water, including mountains, basins, plains and plateaus. Due to structure and erosion, the earth's surface has undergone a long-term remodeling process. The plate tectonic movement will change the landform, and the erosion of the surface by strong wind, precipitation, thermal cycle and chemical action will also change the landform. Glaciation, coastal erosion, the formation of coral reefs and the impact of large meteorites will all have an impact on the reshaping of landforms.
Humanistic knowledge of the earth:
The total population of the world is the number of people living on the earth at a specific time. According to the US Census Bureau, the world population has been increasing since18th century industrial revolution, and the fastest world population growth rate (higher than 1.8%) appeared in 1950s. By 2020, there will be about 7.8 billion people in the world. It is predicted that the world population will continue to grow, reaching 9.2 billion by 2050, with rapid population growth in developing countries. The population density varies greatly around the world, and most people live in Asia. It is estimated that in 2020, 60% of the global population will live in metropolitan areas, not rural areas.
By the end of 20 15, there were 193 sovereign countries in the world, in addition to 2 observer countries and 72 affiliated and limited recognition countries. Asia (48 countries), Europe (44 countries /2 regions), Africa (53 countries /3 regions), Oceania (14 countries/10 regions), North America (23 countries/13 regions), South America (60 Although some nation-states try to rule the world, no sovereign government has ever ruled the whole earth.
It is estimated that only one eighth of the earth is suitable for human habitation. Three quarters of them are covered by sea water, and one quarter is land. Deserts (14%), mountains (27%) and other terrain unsuitable for human habitation account for half of the total land area. Allert (82° 28' N), located in ellesmere island, Nu Loewit, is the northernmost permanent residence in the world. Amundsen-Scott Antarctic Station (90 S) in Antarctica is the southernmost permanent station in the world, almost completely close to the South Pole.