Information about the moon

The moon is commonly known as the moon, also known as the Moon. It is the only natural satellite of the earth in the solar system. The moon is the most obvious example of a natural satellite. In the solar system, except Mercury and Venus, all other planets are natural satellites. The moon is about 4.6 billion years old. The moon has a layered structure such as shell, mantle and core. The average thickness of the outermost lunar shell is about 60-65 kilometers. Below the lunar crust to the depth of 1000 km is the lunar mantle, which occupies most of the volume of the moon. Below the moon's mantle is the moon's core, whose temperature is about 1000 degrees, which is likely to be in a molten state. The moon is about 3476 kilometers in diameter, which is 3/ 1 1 of the earth. The volume is only 1/49 of the earth's, and the mass is about 735 billion tons, which is equivalent to 1/8 1 of the earth's mass. The gravity on the surface of the moon is almost equal to 1/6 of the gravity of the earth.

There are dark parts and bright areas on the moon. When early astronomers observed the moon, they thought that the dark area was covered by seawater, so they called it "sea". The more famous ones are the sea of clouds, the wet sea and the sea of tranquility. The bright part is the mountains, where mountains are stacked and criss-crossed, and craters are dotted. Bailey Crater is located near the South Pole, with a diameter of 295 kilometers, which can hold the whole Hainan Island. The deepest mountain is Newton crater, which is 8788 meters deep. Besides craters, there are ordinary mountains on the moon. Mountains and valleys overlap, giving you a unique perspective.

The front of the moon always faces the earth. On the other hand, except for the area near the edge of the moon, which is visible in the middle due to Libra movement, most areas on the back of the moon are invisible from the earth. In the era without detectors, the back of the moon has always been an unknown world.

A major feature of the back of the moon is that there are almost no dark lunar surface features such as the moon and the sea. When the probe runs to the back of the moon, it will not be able to communicate directly with the earth.

The moon revolves around the earth for about one lunar month, and moves half a degree relative to the background starry sky every hour, which is similar to the apparent diameter of the moon. Unlike other satellites, the orbital plane of the moon is closer to the ecliptic plane than the equatorial plane of the earth.

Relative to the background starry sky, the time required for the moon to go around the earth (the moon makes one revolution) is called sidereal month; The time between the new moon and the next one (or the time between two identical moon phases) is called the new moon. The reason why Wang Shuo's moon is longer than that of the star moon is that during the earth's operation on the moon, it moved forward a certain distance in its orbit around the sun.

Because the rotation period of the moon is exactly the same as its period of revolution, we can only see that the moon always faces the earth with the same face. From the early days of the moon's formation, the moon was influenced by a certain moment, which led to the slow rotation speed. This process is called tidal locking. Therefore, part of the angular momentum of the earth's rotation is converted into the angular momentum of the moon's revolution around the earth. As a result, the moon is far away from the earth at a speed of about 38 mm per year. At the same time, the earth's rotation is getting slower and slower, and the length of a day is getting longer every year 15 microsecond.

The gravity exerted by the moon on the earth is one of the reasons for the tidal phenomenon. The orbit of the moon around the earth is synchronous, and the so-called synchronous rotation is not strict. Because the orbit of the moon is elliptical, when the moon is at perihelion, its rotation speed can't keep up with the revolution speed, so we can see that the eastern part of the moon reaches 98 degrees east longitude. On the contrary, when the moon is at apohelion, its rotation speed is faster than its revolution speed, so we can see that the western part of the moon passes through 98 degrees Darcy. This phenomenon is called Libra movement. Because the orbit of the moon is inclined to the equator of the earth, when the moon moves in the starry sky, the polar regions will shake about 7 degrees, which is called Libra movement. Furthermore, since the distance between the earth and the moon is only 60 times that of radius of the earth, if the observer observes the moon from sunrise to sunset, the observation point will have the displacement of the diameter of the earth, which can be seen in the area with the longitude of 1 degree. This phenomenon is called Libra movement.

Strictly speaking, the earth and the moon revolve around a concentric center, which is 4,700 kilometers away from the center of the earth (2/3 of radius of the earth's). Because the center of homogeneity is below the surface, the movement of the earth around the center of homogeneity seems to be "shaking". Seen from above the North Pole of the Earth, both the Earth and the Moon rotate clockwise. Moreover, the moon is also clockwise around the earth; Even the earth moves clockwise around the sun.

Many people don't understand why the inclination of the moon's orbit and the inclination of the moon from the axis change so much. In fact, the orbital inclination is relative to the central celestial body (the earth), from the axial inclination to the satellite.

The orbit plane of the moon (ecliptic plane) keeps an included angle of 5. 145396 with the ecliptic plane (earth's orbit plane), while the rotation axis of the moon forms an included angle of 1.5424 with the normal of the ecliptic plane. Because the earth is not a perfect sphere, but bulges at the equator, the white road surface keeps precessing (that is, the intersection with the ecliptic rotates clockwise) and completes one week every 6793.5 days (18.5966). During this period, the angle between the white plane and the equatorial plane of the earth (the equatorial plane of the earth inclines to the ecliptic plane at 23.45) will change from 28.60 (i.e. 23.45+5. 15) to18.30 (i.e. 23.45-5. 15). Similarly, the angle between the rotation axis of the moon and the white plane will be between 6.69 (that is, 5. 15+ 1.54) and 3.60 (that is, 5. 15- 1.54). These changes in the lunar orbit will in turn affect the inclination of the earth's rotation axis, making it swing by 0.002 56, which is called nutation.

The two intersections between the ecliptic plane and the ecliptic plane are called the lunar intersection-the ascending intersection (north point) refers to the point where the moon passes to the north of the ecliptic plane; Descending node (South Point) refers to the point where the moon passes south of the ecliptic. When the new moon is just at the intersection of the moon, a solar eclipse will occur; An eclipse occurs when the full moon is just at the intersection of the moons.

The structure on the back of the moon is quite different from that on the front. The Moon Sea occupies less land and there are many craters. The terrain is uneven, and the longest and shortest radius of the moon are located on the back. Some places are 4 kilometers longer than the average radius of the moon, and some places are 5 kilometers shorter (such as Van der Graf depression). No "lump" was found on the back. The moon shell on the back is thicker than that on the front, with the thickest point reaching 150km, while the thickness of the moon shell on the front is only about 60km.

The moon itself does not shine, but reflects sunlight. The brightness of the moon varies with the angular distance between the sun and the moon and the distance between the earth and the moon. The average brightness is 1/465000 of the brightness of the sun, and the brightness varies from 1/630000 to 1/375000. At the full moon, the average brightness is-12.7 (see). Its average illumination to the earth is 0.22 lux, which is equivalent to the illumination of 100 watt electric lamp at a distance of 2 1 meter. The surface of the moon is not a good reflector. Its average albedo is only 7%, and the other 93% is absorbed by the moon. The albedo of Guangdong Sea is even lower, about 6%. The albedo of the lunar highlands and craters is 17%, and it seems that the mountains are brighter than the moon sea. The brightness of the moon also changes. The following table [] lists the brightness values of different months with full moon brightness of 100. It can be seen that the brightness of the full moon is more than ten times that of the upper and lower chords.

Because there is no atmosphere on the moon, the heat capacity and thermal conductivity of lunar materials are very low, so the temperature difference between day and night on the surface of the moon is very large. During the day, the temperature where the sun shines vertically is as high as+127℃; The temperature can be reduced to-183℃ at night. These values only represent the temperature on the surface of the moon. The temperature in the lunar soil can be measured by radio observation. The longer the wavelength of radio waves used, the deeper the temperature in the lunar soil can be detected. This measurement shows that the temperature in the deep soil on the surface of the moon rarely changes, which is precisely because of the low thermal conductivity of the material on the surface of the moon.

From the propagation of lunar seismic waves, we know that the moon also has layered structures such as shell, mantle and core. The outermost lunar shell is 60 ~ 65km thick. Under the lunar crust to the depth of 1000 km is the lunar mantle, which accounts for most of the lunar volume. Below the lunar mantle is the lunar core. The temperature of the lunar nucleus is about 1000℃, which is probably in a molten state. It is speculated that it may be composed of iron, nickel, sulfur and eclogite.

Lunar data

The average orbital radius is 384,400 kilometers.

Track eccentricity is 0.0549.

Perigee distance is 363,300 kilometers.

The apogee is 405500 kilometers away.

The average revolution period is 27 days, 7 hours and 43 minutes 1 1.559 seconds.

The average speed is1.023km/s.

The inclination of the track varies from 28.58 to 18.28.

(The intersection angle with the ecliptic plane is 5. 145).

Ascending intersection right ascension 125.08.

The perigee angle is 3 18. 15.

Repeat stage/day 19.

The average distance between the moon and the earth is about 384,400 kilometers.

The retrogression period of intersection is 18.438+0 years.

The perigee movement period is 8.85 years.

The annual food consumption was 346.6 days.

Repeated lunar eclipse)181011day.

The average inclination of track and ecliptic is 5 9'

The average inclination between the equator and the ecliptic of the moon is132'

The equatorial diameter is 3476.2 kilometers.

The diameter of the pole is 3472.0km.

Flatness 0.00 12

The surface area is 3.976×10.7 square kilometers.

Flatness 0.00 12

The volume is 2.199×1010 cubic kilometer.

The mass is 7.349× 10 22kg.

3.350 times the average density of water.

Equatorial gravity acceleration1.62m/s2

1/6 of the earth

The escape speed is 2.38km/s.

The rotation period is 27 days, 7 hours and 43 minutes 1 1.559 seconds.

(synchronous rotation)

Rotational speed16.655m/s (on the equator)

The axial inclination varies between 3.60 and 6.69.

(The intersection angle with the ecliptic is 1.5424)

Albedo 0. 12

Apparent magnitude at full moon-12.74

Surface temperature (t) -233~ 123℃ (average -23℃)

Atmospheric pressure 1.3× 10- 10 kPa

Monthly cycle

Name value (d) definition

The moon and the moon are 27.32 1 66 1

The moon is 29.530 588 relative to the sun (moon phase).

The equinox 27.32 1.582 is relative to the vernal equinox.

Perigee month 27.554 550 relative to perigee

The node month 27.2 12 220 is relative to the rising node.

Lunar motion

The moon is the closest celestial body to the earth, and its average distance from the earth is about 38440 1 km. Its average diameter is about 3476 kilometers, slightly larger than the earth's diameter 1/4. The surface area of the moon is 38 million kilometers, which is not as big as our Asia. The mass of the moon is about 735 billion tons, which is equivalent to 1/8 1 of the earth's mass, and the surface gravity of the moon is almost equivalent to 1/6 of the earth's gravity.

Orbital motion of the moon

The moon moves around the earth in an elliptical orbit. The great circle cut by this orbital plane on the celestial sphere is called "White Road". The ecliptic plane is neither coincident with the celestial equator nor parallel to the ecliptic plane, and its spatial position is constantly changing. The period is 173 days.

The rotation of the moon

The moon revolves around the earth and rotates at the same time, with a period of 27.438+066 days, which happens to be a sidereal moon, so we can't see the back of the moon. We call this phenomenon "synchronous rotation", which is almost a universal law in the satellite world. It is generally believed that it is the result of the long-term tidal action of planets on satellites. The libration is a wonderful phenomenon, which enables us to see 59% of the moon. There are mainly the following reasons:

1. In different parts of the elliptical orbit, the rotation speed does not match the revolution angular speed.

2, the intersection of the white road and the equator.

Libra movement

Because the orbit of the moon is elliptical, when the moon is at perihelion, its rotation speed can't keep up with the revolution speed, so we can see that the eastern part of the moon reaches 98 degrees east longitude. On the contrary, when the moon is at apohelion, its rotation speed is faster than its revolution speed, so we can see that the western part of the moon passes through 98 degrees Darcy. This phenomenon is called meridian libration.

lunar eclipse

Astronomical characteristics

Eclipse is a special astronomical phenomenon, which means that when the moon moves to the shadow of the earth, the area between the moon and the earth will be blocked by the earth because of the sunlight, and the moon will be missing.

That is to say, at this time, the sun, the earth and the moon are just (or almost) in a straight line, so the light from the sun to the moon will be blocked by the earth.

As far as the earth is concerned, when an eclipse occurs, the directions of the sun and the moon will be different by 180 degrees, so the eclipse must occur in Wangwang (that is, around the lunar calendar 15). It should be noted that the orbits of the sun and the moon in the sky (called the ecliptic and the ecliptic) are not on the same plane, but have an intersection angle of about 5 degrees, so only when the sun and the moon are located near the intersection of the ecliptic and the ecliptic can they be connected in a straight line to produce an eclipse.

Classification of lunar eclipses

Eclipses can be divided into three types: partial eclipse, total eclipse and penumbral eclipse. When only a part of the moon enters the umbra of the earth, there will be a partial lunar eclipse; When the whole moon enters the umbra of the earth, a total lunar eclipse will occur. As for the penumbral eclipse, it means that the moon just passes through the penumbral region of the earth, which causes the brightness of the moon to be extremely slightly weakened, so it is difficult for the naked eye to see the difference, so it is not noticed by people.

The diameter of the earth is about four times that of the moon. In lunar orbit, the umbra diameter of the earth is still 2.5 times that of the moon. So when the centers of the earth and the moon are roughly on the same straight line, the moon will completely enter the umbra of the earth, resulting in a total lunar eclipse. But if only a part of the moon is always covered by the umbra of the earth, that is, only a part of the moon enters the umbra of the earth, a partial lunar eclipse will occur. There will be no annular eclipse on the moon. Because the moon is much smaller than the earth.

The diameter of the sun is much larger than that of the earth, and the shadow of the earth can be divided into umbra and penumbra. If the moon enters the penumbra, the sunlight will also be blocked. This phenomenon is called penumbral eclipse in astronomy. Because the sun in the penumbra is still very strong, the brightness of the lunar surface is only slightly weakened. In most cases, the penumbra eclipse is not easy to distinguish with the naked eye. In general, because it is not easy to be found, it is not called an eclipse, so there are only two kinds of total solar eclipse and partial solar eclipse.

In addition, because the umbra of the earth is much larger than that of the moon, it also means that in the event of a total lunar eclipse, the moon will completely enter the umbra of the earth, so there will be no eclipse.

The number of solar eclipses occurs twice a year, up to three times, and sometimes none at all. Because under normal circumstances, the moon either passes above the Earth's umbra or leaves from below, rarely or partially passes through the Earth's umbra, so an eclipse will not occur under normal circumstances.

According to the statistics of observation data, the percentage of lunar eclipse, partial eclipse and total eclipse in the penumbra in each century is about 36.60%, 34.46% and 28.94%.

Lunar topography

The topography of the lunar surface mainly includes:

volcanic vent

The name crater was given by Galileo. It is a remarkable feature of the lunar surface, covering almost the whole lunar surface. The largest crater is Bailey crater near the South Pole, with a diameter of 295 kilometers, which is a little larger than Hainan Island. A small crater may even be a hole of dozens of centimeters. There are about 33,000 with a diameter of not less than 65,438+000m m. It accounts for 7- 10% of the lunar area.

A Japanese scholar put forward the classification of craters in 1969, which can be divided into Cravey type (ancient craters are generally unrecognizable, and some craters contain mountains), Copernican type (young craters often have "radial lines", and the inner wall generally has concentric sections and a central peak), Archimedean type (the ring wall is low, which may have evolved from Copernicus type).

female horse

The dark part of the moon seen by the naked eye on the earth is actually a vast plain on the moon. Due to historical reasons, this misnamed name has been preserved.

There are 22 identified moon seas, and some landforms are called "moon seas" or "moon-like seas". Most of the 22 recognized ones are distributed on the front of the moon. Three in the back and four on the side. On the front, the area of the Moon Sea is slightly over 50%, of which the largest "storm sea" covers an area of about 5 million square kilometers, almost the total area of nine French countries. Most of the moon seas are generally round and oval, and most of them are surrounded by some mountains, but some seas are connected together. Besides the sea, there are five lakes with similar topography-Hu Meng, Dead Lake, Fox Xia, Qiuhu and Chun Hu, but some lakes are bigger than the sea. For example, Hu Meng has an area of 70,000 square kilometers, which is much larger than Qihai. The parts of the moon and the ocean that extend to the land are called "bays" and "swamps", and they are both distributed on the front. There are five bays: Luwan, Xiawan, Central Bay, Hongwan and Yue Mei Bay. There are three kinds of marshes: humus, epidemic marshes and dream marshes. Actually, there is no difference between a swamp and a bay.

The topography of the Moon Sea is generally low, similar to the basins on the earth. The moon sea is1-2km lower than the average moon horizon, and the southeast of the lowest sea is even 6000m lower than its surroundings. The albedo of the moon (a physical quantity to measure the ability to reflect sunlight) is also relatively low, so it looks darker now.

Moon, land and mountains

The area above the Moon Sea on the surface of the Moon is called the Moon Land, which is generally 2-3 kilometers above the horizon of the Moon Sea. Because of the high albedo, it looks brighter. On the front of the moon, the area of the moon land is roughly equal to that of the moon sea, but on the back of the moon, the area of the moon land is much larger than that of the moon sea. Isotope determination shows that the moon and land are much older than the moon and ocean, and they are the oldest landforms on the moon.

On the moon, in addition to many craters, there are some mountains similar to those on the earth. Mountains on the moon often borrow the names of mountains on the earth, such as Alps and Caucasus mountains. The longest mountain range is the Apennines, which stretches for 1000 km, but its height is only three or four kilometers higher than the moon horizon. There are also some steep peaks in the mountains, whose height was overestimated in the past. At present, it is believed that the heights of most peaks are similar to those of the earth, and the highest peaks (also near the south pole of the moon) are only 9000 meters and 8000 meters.

On the moon, there are 6 peaks over 6000 meters, 20 at 5000-6000 meters, 80 at 4000-5000 meters, and 200 at 1000 meters.

The mountains on the moon have a common feature: the slopes on both sides are asymmetrical, and the slopes on the seaward side are steep, sometimes cliff-shaped, while the other side is quite flat.

In addition to the high mountains, there are four cliffs hundreds of kilometers long on the moon. Three of them stand out in the Moon Sea, also known as the "Moon Graben".

Lunar radiation pattern

Another major feature of the moon's surface is that some younger craters often have beautiful "radiation patterns", which are bright belts extending in all directions with craters as radiation points, and they pass through mountains, the moon sea and craters almost in a straight line. The length and brightness of radiation patterns are different, and the most striking one is the radiation pattern of Tycho crater. The longest one is as long as 1800 km, especially at full moon. Secondly, Copernicus and Kepler craters also have quite beautiful radiation patterns. According to statistics, there are 50 craters with radiation patterns.

The reason for the formation of radiation pattern is still inconclusive. Essentially, it is closely related to the theory of crater formation. At present, many people tend to say that meteorite impact may make high-temperature debris fly far on the moon, with no atmosphere and little gravity. Other scientists believe that the function of volcano is not ruled out, and the eruption of volcano may also form a flying radiation shape.

Moon Valley (Moon Gorge)

There are many famous rift valleys on the earth, such as the Great Rift Valley in East Africa. There is also such a structure on the surface of the moon-those seemingly winding black cracks are the Moon Valley, and some extend for hundreds of meters to Qian Qian meters, with widths ranging from several kilometers to tens of kilometers. Most of those broad moon valleys appear on the flat land of the moon, while those narrow and narrow moon valleys (sometimes called moon streams) can be seen everywhere. The most famous Moon Valley is the alpine Moon Valley connecting Yuhai and Leng Hai, which is located in the southeast of Plato's crater. It cut off the Alps on the moon, which is very spectacular. The photos taken from space are estimated to be 130 km long and 10- 12 km wide.

Lunar volcano distribution

The surface of the moon is covered with a huge layer of basalt lava (volcanic lava). Early astronomers thought that the dark area on the surface of the moon was a vast ocean, so they called it "mare", which means "sea" in Latin. Of course, this is wrong. These dark areas are actually plain areas composed of basalt lava. In addition to basalt lava structure, there are other volcanic features in the dark area of the moon. For example, the most prominent are the winding grooves, black deposits, volcanic domes and volcanic cones on the surface of the moon. However, these features are not significant, only a small part of volcanic traces on the surface of the moon.

Compared with the earth volcano, the moon volcano can be described as senile. Most lunar volcanoes are between 3 billion and 4 billion years old; Typical dark plain, 3.5 billion years old; The youngest lunar volcano has a history of 1 100 million years. In geological age, the earth's volcanoes belong to youth, generally less than 654.38+ 10,000 years old. The oldest rock formation on earth is only 390 million years old, and the oldest submarine basalt is only 2 million years old. The young earth volcano is still very active, but there are no signs of volcanic and geological activities on the moon recently, so astronomers call the moon an "extinct" planet.

Most volcanoes on the earth are distributed in chains. For example, in the Andes, volcanic chains outline the edge of the lithospheric plate. The mountains on the island of Hawaii show the hot spots of plate activity. There is no sign of plate tectonics on the moon. Typical lunar volcanoes often appear at the bottom of huge ancient impact craters. Therefore, most of the dark areas of the moon are round in appearance. The edge of the impact basin usually surrounds mountains and dark areas.

The dark areas of the moon mainly appear on the far side of the moon. Almost covers the area of 1/3 here. On the far side, the dark area accounts for only 2%. On the other side, the terrain is relatively high and the crust is thick. It can be seen that the main factors controlling lunar volcanism are surface height and crustal thickness.

The gravity of the moon is only 1/6 of that of the earth, which means that the flow resistance of the lunar volcanic lava is smaller than that of the earth, and the lava travels more smoothly. This can explain why the surface of the dark area of the moon is mostly flat and smooth. At the same time, the smooth lava flow is easy to spread, thus forming a huge basalt plain. Coupled with the small gravity, the ejected volcanic ash fragments fall further. Therefore, the eruption of the lunar volcano only formed a wide and flat lava plain, not a volcanic cone similar to the shape of the earth. This is one of the reasons why no large volcanoes have been found on the moon.

Unsolved water on the moon. The dark areas of the moon are completely dry. Water is the most common gas in the earth's lava, and it is one of the important factors that cause the strong eruption of the earth's volcanoes. Therefore, scientists believe that water shortage also has a great influence on lunar volcanic activity. Specifically, if there is no water, the eruption of the lunar volcano will not be so strong, and lava may just flow out of the ground calmly and smoothly.

Origin of the moon

At present, there is no conclusion about the origin of the moon: there are roughly three schools of thought about the origin of the moon, but there is no conclusion. Some scientists believe that the moon, like the earth, was formed by the gas and dust of the universe 4.6 billion years ago; Some people think that the moon is a child of the earth, and it is split from the earth. But the data brought back by Apollo several times show that the composition of the moon and the earth is very different. Many scientists believe that the moon was accidentally sucked into the gravity field many years ago, so it was accidentally brought into Earth orbit. But some people cite celestial mechanics to oppose this statement.

First, the division theory. This is the earliest hypothesis to explain the origin of the moon. As early as 1898, George Darwin, the son of the famous biologist Darwin, pointed out in the article Tides and Similar Effects in the Solar System that the moon was originally a part of the earth, but later, due to the rapid rotation of the earth, some substances on the earth were thrown out, forming the moon after leaving the earth, and the pit left on the earth is now the Pacific Ocean. This view was quickly opposed by some people. They think that it is impossible to throw such a big thing at the speed of the earth's rotation. Besides, if the moon was thrown out by the earth, then the material composition of the two should be the same. However, through the analysis of the rock samples brought back from the moon by Apollo 12 spacecraft, it is found that there is a great difference between them.

Second, the capture theory. This hypothesis holds that the moon was originally just an asteroid in the solar system. Once, because it ran near the earth, it was captured by the gravity of the earth and never left the earth again. There is also a view close to the capture theory, that is, the earth keeps accumulating substances that enter its orbit, and over time, more and more things are accumulated, and finally the moon is formed. However, it has also been pointed out that for a planet as big as the moon, the earth may not have that great power to capture it.

Third, homology theory. This hypothesis holds that the earth and the moon are both floating nebulae in the solar system, which form stars at the same time through rotation and accretion. In the process of accretion, the earth is a little faster than the moon and becomes a "brother". This assumption has also been challenged by objective existence. By analyzing the rock samples brought back from the moon by Apollo 12 spacecraft, people found that the moon is much older than the earth. Some people think that the moon should be at least 7 billion years old.

Fourth, the big collision theory. This is a new hypothesis about the origin of the moon in recent years. 1986 On March 20th, at the symposium on the Moon and Planets held at the Johnson Space Center in Houston, Ben Ci and Slattery of Los Alamos National Laboratory and Cameron of Smith Center for Astrophysics of Harvard University put forward the big collision hypothesis. This hypothesis holds that in the early stage of the evolution of the solar system, a large number of "planetesimals" were formed in interstellar space, and they grew up through collision and accretion. The planetesimals merged to form the primitive earth, and at the same time formed celestial bodies equivalent to 0. 14 times the mass of the earth. In the course of their respective evolution, these two celestial bodies formed a metal core dominated by iron and a curtain shell composed of silicate respectively. Because the two celestial bodies are not far apart, there is a great chance of meeting. Occasionally, small celestial bodies hit the earth at a speed of about 5 kilometers per second. The violent collision not only changed the motion state of the earth and tilted its axis, but also caused small celestial bodies to be smashed, the silicate shell and mantle were heated and evaporated, and the expanding gas and huge speed took a large number of shattered dust away from the earth. These materials flying away from the earth are mainly composed of the mantle of collision bodies, and there are also a small amount of materials on the earth, with a ratio of 0.85:0. 15. When the impactor broke, the metal core separated from the curtain slowed down due to the expanding and flying gas, and was absorbed to the earth after about 4 hours. The gas and dust flying away from the earth are not completely out of the control of the earth's gravity. They accrete and combine with each other to form a completely melted satellite, or form several separated small satellites first, and then gradually accrete to form a partially melted satellite.