Information about the Moon

The moon, commonly known as the moon or the lunar moon, is the earth's only natural satellite and the closest celestial body to the earth.

The moon is on average 384,401 kilometers away from the earth. This distance is about 10 times the circumference of the Earth at the equator. The moon's orbit is elliptical, with an average distance of 363,300 kilometers from perigee and 405,500 kilometers from apogee. The diameter of the moon is 3476 kilometers, which is about 3/11 of the diameter of the earth. The surface area of ??the moon is about 1/14 of the earth's surface area, slightly smaller than the area of ??Asia. The moon's volume is only equivalent to 1/49 of the earth's volume. The mass of the moon is approximately equal to 1/81.3 of the mass of the earth. The average density of lunar material is 3.34 grams per cubic centimeter, which is only equivalent to 3/5 of the density of the Earth. The gravitational acceleration of a free-falling object on the moon is 1/6 of the gravitational acceleration on the earth's surface. The escape velocity on the moon is about 2.4 kilometers per second, which is about 1/5 of the escape velocity on the earth.

While the moon makes an elliptical motion around the earth, it also accompanies the earth in its revolution around the sun, one week per year. The moon is not only under the gravity of the earth, but also affected by the gravity of the sun, so it has a very complex orbital motion. The moon itself does not emit light or is transparent, but it reflects sunlight. Since the relative positions of the sun, earth, and moon are constantly changing, the illuminated part of the moon seen by observers on the earth is also constantly changing, resulting in different visual shapes. This is called the moon phase. The phases of the moon change regularly. The periodicity of the moon's phase changes provides people with a measure of time. The lunar or lunar month is based on the phases of the moon, from which the weeks evolved.

People have known since ancient times that the moon always faces the earth with the same side. This is caused by the fact that the moon's rotation period is exactly the same as the moon's rotation period around the earth, and the same two periods are the result of the long-term effects of tides.

The moon's equatorial plane has an inclination of 6 degrees and 41 minutes with its orbital plane. Because of the existence of this tilt angle and the uneven rotation speed of the moon, during the movement of the moon, observers at a certain point on the ground can still see the edge of the moon oscillating back and forth to some extent. From ground observation, not only half of the moon can be seen, but 59% of the moon can be seen, and the remaining 41% cannot be seen directly.

The shape of the moon is also an oblate sphere with the north and south poles slightly flattened and the equator slightly raised. Its average polar radius is 500 meters shorter than its equatorial radius. The Arctic and Antarctic regions are also asymmetrical. The Arctic region is uplifted and the Antarctic region is depressed by about 400 meters. The center of gravity of the moon does not coincide with the geometric center, and the center of gravity is 2 kilometers away from the earth. This conclusion has been confirmed by the data obtained from the Apollo moon landing.

The mountains on the moon are undulating and densely covered with peaks. In addition, there are various characteristic names such as ocean, sea, bay and lake. In fact, there is no water on the moon. It's just that observers in the early years relied on their imagination and borrowed names from the earth. At most, they only have some morphological similarities.

The most obvious feature on the moon is the crater, usually a bowl-shaped pit structure. The largest ones can have a diameter of more than 100 kilometers, while the smaller ones are just pits. There are more than 30,000 craters with a diameter greater than 1 km, accounting for 7 to 10% of the moon's surface area. Most of the craters are named after famous astronomers or other scholars. There are four craters on the far side of the moon, named after ancient Chinese astronomers Shi Shen, Zhang Heng, Zu Chongzhi, and Guo Shoujing. The largest craters on the moon are: Bailey crater near the South Pole, with a diameter of 295 kilometers; Clave crater, with a diameter of 233 kilometers; Newton crater, with a diameter of 230 kilometers. The central area of ??many craters has a central peak or central peak group, which is as high as 2.5 kilometers.

The dim black spots on the moon that can be seen with the naked eye are called "lunar seas". They are vast plains. On the far side of the moon, the maria occupies about half of the entire hemisphere's surface. There are 22 known lunar maria (including the far side), the largest of which is called Oceanus Procellarum, covering an area of ??about 5 million square kilometers. Mare Imbrium covers an area of ??approximately 900,000 square kilometers. The Sea of ??Tranquility in the center of the moon covers an area of ??approximately 260,000 square kilometers. In addition, the larger ones include Chenghai, Fufuhai, Weihai, and Yunhai. Most of the lunar maria are circular and closed, surrounded by mountains. Some lunar maria extend toward the land and are called bays, while smaller lunar maria are called lakes.

The lunar land is the area on the lunar surface that is higher than the lunar sea, generally 2 to 3 kilometers higher. The lunar continent is mainly composed of light-colored anorthositic rocks with high albedo. The area of ??the lunar continent and the lunar maria on the front side of the moon are roughly equal, while on the far side, the lunar continent area is larger. The age of the formation of the lunar continent has been determined by isotopic age to be 4.6 billion years, which is earlier than the lunar maria. There are also some mountain ranges on the moon, most of which are named after mountains on Earth, such as the Apennines, Caucasus Mountains, Alps, etc. The longest mountain range is 1,000 kilometers long, often 3 to 4 kilometers above the lunar sea. The highest mountain is near the south pole of the moon, reaching a height of 9,000 meters, which is higher than Mount Everest, the highest mountain on Earth. In addition to mountains, there are cliffs hundreds of kilometers long, the longest being the Altai Cliffs.

There are some radiation patterns on the moon, typically those around Tycho crater and Copernicus crater. Tycho crater has 12 radial patterns, which extend radially outward from around the crater. The longest one is 1,800 kilometers long. It can be seen most clearly during the full moon. There is no conclusion on its cause: some say it was caused by a volcanic eruption; others think it was caused by meteorites bombarding the lunar surface.

Long-term astronomical observations and direct inspections of the moon have confirmed that there is no obvious magnetic field around the moon.

The strength of the moon's magnetic field is less than 1/1000 of the Earth's magnetic field. There are no radiation belts on the moon like the Earth and Jupiter. There is no water in any form on the moon, there is no atmosphere at all, and it is almost in a vacuum state. Through lunar rocket exploration, it was found that there are as many as 12 gravity anomaly areas called "gravity tumors" or "mass tumors" on the front of the moon; most of the lunar surface is covered by a layer of lunar dust and debris of varying thicknesses. cover.

The moon does not have a protective layer like the earth's atmosphere. The lunar surface is directly impacted by meteoroids, which will affect the chemical composition, debris size, glass content and recrystallization of lunar rocks to a certain extent. extent. In the early days of the moon, widespread volcanic eruptions erupted, spewing out large amounts of molten lava, forming vast lava plains on the moon.

The moon itself does not emit light, it only reflects sunlight. Its brightness changes with the angular distance between the sun and the moon and the distance between the earth and the moon. Its average brightness is 1/465,000 that of the sun, and its brightness varies from 1/630,000 to 1/375,000. The average brightness of a full moon is -12.7 magnitude. The illumination it gives to the earth is on average equivalent to the illumination of a 100-watt electric lamp at a distance of 21 meters. The moon is not a good reflector. Its average albedo is only 7%, and the remaining 93% is absorbed by the moon. The albedo of the lunar maria is even lower, around 6%. The albedo of the lunar highlands and craters is 17%, making the mountains appear brighter than the lunar maria.

Since there is no atmosphere on the moon, and the heat capacity and thermal conductivity of the lunar surface materials are very low, the temperature difference between day and night on the lunar surface is large. During the day, the temperature in places where the sun shines vertically is as high as 127 degrees Celsius; at night, the temperature can drop to minus 183 degrees Celsius. These values ??only represent the temperature of the moon's surface. Radio observations can be used to measure the temperature in the lunar soil, and the longer the wavelength of the radio wave used, the better it can detect the temperature deeper in the lunar soil. This measurement shows that the temperature deeper in the lunar soil changes very little, which is due to the low thermal conductivity of the lunar material.

From the propagation of moon shock waves, we know that the moon also has a layered structure such as crust, mantle, and core. The outermost lunar crust is 60 to 65 kilometers thick. Below the lunar crust to a depth of 1,000 kilometers is the lunar mantle, which accounts for most of the moon's volume. Below the lunar mantle is the lunar core. The temperature of the moon's core is about 1000 degrees Celsius, and it is likely to be molten.

The structure of the far side of the moon is quite different from that of the front side. The maria occupies less area and has more craters. The terrain is uneven and has great fluctuations. The longest and shortest lunar radii are located on the far side. Some places are 4 kilometers longer than the average radius of the moon, and some places are 5 kilometers shorter (such as Van de Graaf Depression). No "mass tumor" was found on the back. The moon crust on the back is thicker than the front, with a maximum thickness of 150 kilometers, while the thickness of the moon crust on the front is only about 60 kilometers.

There are different opinions on the cause of the moon. There are three main hypotheses, namely the capture theory, the split theory and the homology theory.

Capture theory: The moon may be an asteroid orbiting near the earth, and was later captured by the earth and became a satellite of the earth. Because the average densities of the Moon and the Earth are very different, and their chemical compositions are very different, they may have been formed from different materials in different parts of the original nebula of the Sun. On the other hand, the average density of the moon is very close to that of meteorites and asteroids. Therefore, it is very likely that the asteroid was orbiting the sun, and due to its approach to the earth, the earth's gravity caused it to break away from its original orbit and be captured by the earth. Some people believe that this event occurred 3.5 billion years ago and the entire process took 500 million years. After the moon was captured by the earth, the moon erupted a large amount of magma due to the tidal force of the earth, forming mare basalt.

Split theory: In the early days of the formation of the solar system, the earth and the moon were originally a whole. At that time, the earth was still in a molten state and rotated very fast, with a rotation period of only about 4 hours. Therefore, the period of the sun's tidal action on the earth at this time is 2 hours. This period is exactly equal to the free swing period of the earth, which generates primary oscillation, forming a series of elongated expansion bodies on the equatorial plane, and finally splits to form the moon. The Pacific Ocean is what was left when the moon broke apart. According to calculations, the current total angular momentum of the Earth-Moon system, even if coupled with billions of years of angular momentum loss, is not enough to separate the Earth and the Moon. And the position of the moon is not on the equatorial plane of the earth. These facts are difficult to explain by the split theory.

Homology theory: The Earth and the Moon were formed from the same planetary dust cloud. Their average densities and chemical compositions differ because the metal particles in the original nebula condensed long before the planets formed. When the Earth formed a planet, it started with iron as the main component and iron as the core. The moon was formed from the accumulation of non-metallic materials remaining around the earth after the earth was formed. These three hypotheses for the formation of the moon can more or less explain the composition, density, structure, orbit and other basic facts of the moon. Except that the split theory is generally considered difficult to establish, there is currently no conclusion as to which of the two hypotheses, the capture theory or the homology theory, is more reasonable.

According to studies of various thermal history models of the moon, multiple local meltings have occurred throughout the moon. In the early days of the moon's formation, most of the moon's temperature reached 1,000 degrees Celsius. 4.1 billion years ago, the moon experienced a large-scale magma movement. During the separation of magma, a lunar crust composed of anorthosite was formed, and the remaining part became the highland of the lunar surface.

After the lunar surface was consolidated, local melting occurred in deeper parts, producing a melt composed of sardite. About 4 billion years ago, non-marine basalt rich in radioactive and refractory elements was formed. The anorthosite highlands have been exposed on the lunar surface for a long time and have been continuously impacted by meteorite material, thus being reduced by 1.5 to 2 kilometers. A large number of ancient impact craters have developed on the highlands. Later, the highlands were cut and destroyed by a series of faults. Between 4.1 and 3.9 billion years ago, the moon suffered relatively concentrated impacts from various large meteorites, causing many maria basins, that is, large ring structures, to appear on the lunar surface. The most typical one is the Mare Imbrium event. The maria on the moon were generally formed during a similar period. The general order of formation of the Moon Sea is: Sea of ??Wine, Sea of ??Cheng, Sea of ??Shit, Sea of ??Danger, Sea of ??Rain…. The various lunar maria formed in the Imbrian Period were filled and covered by basalt erupted in the later period about 3.9 to 3.1 billion years ago. According to the determination of isotope age, the approximate filling time order is Imbrium West, Imbium East, Humidity Sea, Crisis Sea, Imbium Sea, Tranquility Sea, Rich Sea, Cheng Sea and Procellarum Ocean. Since then, the outline of the lunar surface has basically been formed. For 3.1 billion years, the evolution of the moon's interior has been in a "stagnant" state, and external forces have played a dominant role in the moon's evolutionary history. Meteors impact the lunar surface, causing lunar craters to continue to form and increase. The radiation patterns of the radiating lunar craters formed during the Eratosthenic period are affected by various effects on the lunar surface, and either become inconspicuous or disappear; while the lunar craters formed during the Copernican period have obvious radiating patterns.