The moon is commonly known as the moon, also known as the lunar moon. The moon is also about 4.6 billion years old. It is closely related to the earth. The moon also has a layered structure such as crust, mantle, and core. The average thickness of the outermost lunar crust is about 60-65 kilometers. 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 lunar core is about 1,000 degrees, and it is likely to be in a molten state. The diameter of the moon is about 3476 kilometers, which is 3/11 of the earth. The volume is only 1/49 of the Earth, and the mass is about 735 billion tons, which is equivalent to 1/81 of the Earth's mass. The gravity of the moon is almost equivalent to 1/6 of the Earth's gravity.
There are dark parts and bright areas on the moon. When early astronomers observed the moon, they thought that the dark areas were covered with sea water, so they called them "sea". The famous ones include the Sea of ??Clouds, the Sea of ??Shit, the Sea of ??Tranquility, etc. The bright part is the mountains, where there are many mountains, criss-crossing mountains, and dotted craters everywhere. Bailey Crater, located near the South Pole, has a diameter of 295 kilometers and can fit the entire Hainan Island into it. The deepest mountain is Newton Crater, which is 8,788 meters deep. In addition to craters, there are also ordinary mountains on the moon. High mountains and deep valleys appear one after another, creating a unique scenery.
The front side of the moon always faces the earth. On the other hand, most of the far side of the moon cannot be seen from the Earth, except for an area near the edge of the moon that is occasionally visible due to the movement of the scales. In the era without detectors, the far side of the moon has always been an unknown world.
One of the characteristics of the far side of the moon is that it has almost no darker lunar surface features such as maria. When the probe reaches the far side of the moon, it will not be able to communicate directly with the earth.
The moon orbits the Earth once in 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 Moon's orbital plane is closer to the ecliptic plane than to the Earth's equatorial plane.
Relative to the background starry sky, the time it takes for the moon to orbit the earth (moon revolution) is called a sidereal month; and the time required between a new moon and the next new moon (or between two identical moon phases) The time is called a synodic month. The synodic moon is longer than the sidereal moon because the earth itself advances a certain distance in its orbit around the sun during the movement of the moon.
Because the moon's rotation period is exactly the same as its revolution period, we can only see that the same side of the moon always faces the earth. Since the early formation of the Moon, the Moon has been affected by a torque that causes its rotation to slow down. 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 moves away from the Earth at a speed of about 38 millimeters per year. At the same time, the Earth's rotation is getting slower and slower, and the length of the day is getting longer by 15 microseconds every year.
The gravitational pull exerted by the moon on the earth is one of the causes of tides. The Moon's orbit around the Earth is a synchronous orbit, and the so-called synchronous rotation is not strictly enforced. Since the moon's orbit is elliptical, when the moon is at perihelion, its rotation speed cannot catch up with its revolution speed. Therefore, we can see the area in the eastern part of the moon reaching 98 degrees east longitude. On the contrary, when the moon is at aphelion, its rotation speed is faster than its revolution speed. The speed is fast, so we can see the area in the western part of the moon up to 98 degrees west longitude. This phenomenon is called Libra movement. And because the moon's orbit is tilted toward the Earth's equator, when the moon moves in the starry sky, its polar regions will wobble by about 7 degrees. This phenomenon is called libra movement. Furthermore, since the moon is only 60 earth radii away from the earth, if an observer observes from moonrise to moonset, the observation point will have a displacement of one earth diameter, and the area on the moon's surface with a longitude of 1 degree can be seen. This phenomenon is called Libra movement.
Strictly speaking, the earth and the moon revolve around the center of homogeneity, which is 4,700 kilometers away from the center of the earth (that is, 2/3 of the radius of the earth). Since the center of gravity is below the surface of the earth, the movement of the earth around the center of mass seems to be "rocking". Viewed from above the Earth's North Pole, both the Earth and the Moon rotate in a clockwise direction; the Moon also orbits the Earth in a clockwise direction; and even the Earth orbits the Sun in a clockwise direction.
Many people don’t understand why the values ??of the moon’s orbital inclination and the moon’s rotation axis have changed so much. In fact, the orbital inclination is relative to the central celestial body (i.e. the Earth), while the rotation axis inclination is relative to the orbital plane of the satellite (i.e. the Moon) itself.
While this definition is customary for general cases (such as the orbits of artificial satellites) and is fairly fixed in value, this is not the case for the Moon.
The moon’s orbital plane (the ecliptic plane) maintains an angle of 5.145 396° with the ecliptic plane (the earth’s orbital plane), while the moon’s rotation axis forms an angle of 1.5424° with the normal to the ecliptic plane. horn. Because the Earth is not a perfect sphere, but rather bulges at the equator, the ecliptic plane is constantly precessing (that is, the intersection with the ecliptic rotates clockwise), completing one cycle every 6793.5 days (18.5966 years). During this period, the angle between the ecliptic plane and the equatorial plane of the Earth (the Earth's equatorial plane is tilted at 23.45° to the ecliptic plane) will change from 28.60° (i.e. 23.45° + 5.15°) to 18.30° (i.e. 23.45°- 5.15°). Similarly, the angle between the moon's axis of rotation and the ecliptic plane will also be between 6.69° (i.e. 5.15° + 1.54°) and 3.60° (i.e. 5.15° - 1.54°). These changes in the moon's orbit will in turn affect the inclination of the earth's rotation axis, causing it to wobble by ±0.002 56°, which is called nutation.
The two intersections of the ecliptic plane and the ecliptic plane are called lunar nodes - the ascending node (north point) refers to the point where the moon passes to the north of the ecliptic plane; the descending node (south point) refers to the moon passing through This point is south of the ecliptic. A solar eclipse occurs when the new moon is exactly on the lunar nodes, and a lunar eclipse occurs when the full moon is exactly on the lunar nodes.
Orbital data
Average orbital radius 384,400 kilometers
Orbital eccentricity 0.0549
Perigee distance 363,300 kilometers
< p>The apogee distance is 405,500 kilometersThe average orbital period is 27 days, 7 hours, 43 minutes and 11.559 seconds
The average orbital speed is 1.023 kilometers/second
The orbital inclination is Change between 28.58° and 18.28°
(The intersection angle with the ecliptic plane is 5.145°)
The right ascension of the ascending node is 125.08°
The perigee argument is 318.15°
Metonic chapter (repeat phase/day) 19 years
Average moon-earth distance ~384 400 kilometers
Nodal retrograde period 18.61 years
Perigee motion period is 8.85 years
Eclipse year is 346.6 days
Saros cycles (repeat eclipses) are 18 years and 10/11 days
Orbit and ecliptic Average inclination angle 5°9'
The average inclination angle between the moon's equator and the ecliptic is 1°32'
Physical characteristics
Equatorial diameter 3,476.2 kilometers
< p>The diameter of the poles is 3,472.0 kilometersThe oblateness is 0.0012
The surface area is 3.976×107 square kilometers
The oblateness is 0.0012
Volume 2.199×1010 cubic kilometers
Mass 7.349×1022 kilograms
3.350 times the average density of water
Equatorial gravity acceleration 1.62 m/s2
< p>1/6 of the EarthEscape velocity 2.38 kilometers/second
Rotation period 27 days, 7 hours, 43 minutes, 11.559 seconds
(synchronous rotation) < /p>
The rotation speed is 16.655 meters/second (at the equator)
The inclination angle of the rotation axis varies between 3.60° and 6.69°
(The angle with the ecliptic is 1.5424° )
Albedo 0.12
Visual magnitude of full moon -12.74
Surface temperature (t) -233~123℃ (average -23℃)
p>
Atmospheric pressure 1.3×10-10 kPa
Moon period
Name Value (d) Definition
Sidereal month 27.321 661 relative to background stars
Synodic month 29.530 588 relative to the Sun (moon phase)
Equinox 27.321 582 relative to vernal equinox
Periphase 27.554 550 relative to perigee
Nodal month 27.212 220 relative to the ascending node
History of human lunar exploration
The first man-made object to reach the moon was the former Soviet Union’s unmanned lander Luna 2. It hit the moon on September 14, 1959. Luna 3 took photos of the far side of the moon on October 7 of the same year. Luna 9 was the first lander to soft-land on the moon. It sent back photos taken on the lunar surface on February 3, 1966. In addition, Luna 10 successfully entered orbit on March 31, 1966, becoming the first man-made satellite on the moon.
During the Cold War, the United States of America and the former Soviet Union always hoped to lead each other in space technology. This space race reached its climax when the first man landed on the moon on July 20, 1969. Neil Armstrong, commander of Apollo 11 of the United States of America, was the first man to set foot on the moon, while Eugene Cernan was the last man to stand on the moon during the Apollo 17 mission in December 1972. members.
The astronauts of Apollo 11 left a 9-inch by 7-inch stainless steel plaque on the lunar surface to commemorate the landing and provide information for other creatures who might discover it.
Six Helios missions and three unmanned Lunar missions (Lunar 16, 20, and 24) brought rock and soil samples from the moon back to Earth.
In February 2004, President George W. Bush of the United States of America proposed sending people to the moon again before 2020. The European Space Agency and the People's Republic of China also have plans to launch probes to the moon. Europe's Smart 1 probe was launched on September 27, 2003, and entered orbit around the moon on November 15, 2004. It will survey the lunar environment and produce X-ray maps of the lunar surface.
The People's Republic of China is also actively carrying out lunar exploration programs and seeking the feasibility of mining lunar resources, especially the helium isotope helium-3, an element that is expected to become the earth's energy source in the future. For information about the People's Liberation Army and the country's lunar exploration program, see the Chang'e Project entry.
Japan and India are also unwilling to follow suit. Japan has initially set out its future lunar exploration missions. Japan's Aerospace Exploration Agency has even begun planning a manned lunar base. India will first launch the unmanned moon orbiter Chandrayan.
Myths and Legends
In the ancient mythology of the Republic of China, there are countless stories about the moon. In ancient Greek mythology, the name of the moon goddess is Artemis. She is the twin sister of the sun god Apollo and she is also the goddess of hunting. The astronomical symbol of the moon is like a crescent moon, symbolizing the bow of Artemis.
Spherical motion
The moon is the earth's only natural satellite and the closest celestial body to us. Its average distance from the earth is about 384,401 kilometers. Its average diameter is about 3476 kilometers, which is slightly larger than 1/4 of the Earth's diameter. The surface area of ??the moon is 38 million kilometers, which is not as large as the area of ??Asia. The mass of the moon is about 735 billion tons, which is equivalent to 1/81 of the mass of the earth. The gravity of the moon is almost equivalent to 1/6 of the earth's gravity.
The Moon’s Orbital Movement
The Moon orbits the Earth in an elliptical orbit. The great circle intercepted by this orbital plane on the celestial sphere is called the "white path". The ecliptic plane does not coincide with the celestial equator, nor is it parallel to the ecliptic plane, and its spatial position is constantly changing.
The cycle is 173 days.
The rotation of the moon
The moon rotates while revolving around the earth, with a period of 27.32166 days, which is exactly one sidereal month, so we cannot see the back of the moon. We call this phenomenon "synchronous rotation", and it is almost a universal law in the satellite world. It is generally believed to be the result of the long-term tidal effect of the planet on the satellite. Libra motion is a wonderful phenomenon that allows us to see 59% of the moon. The main reasons are as follows:
1. In different parts of the elliptical orbit, the rotation speed does not match the revolution angular speed.
2. The angle between the white equator and the equator.
Physical conditions
The main terrains of the moon are:
Craters
This name was given by Galileo Galilei. It is a prominent feature of the lunar surface, covering almost the entire lunar surface. The largest crater is Bailey Crater near Antarctica, with a diameter of 295 kilometers, which is slightly larger than Hainan Island. A small ringed mountain may even be a crater of tens of centimeters. There are approximately 33,000 with a diameter of not less than 1,000 meters. It accounts for 7-10% of the moon's surface area.
A Japanese scholar proposed a crater classification in 1969, which was divided into Clavian type (old craters, generally unrecognizable, and some still have mountains within mountains) Copernican type (young craters, often There are "radiating patterns", the inner wall generally has concentric circular hills, and there is usually a central peak in the center) Archimedean shape (the ring wall is low, possibly evolved from the Copernican type) bowl-shaped and dimple-shaped (small Craters, some less than one meter in diameter).
Moonmarine
The dark parts of the moon seen with the naked eye are actually vast plains on the moon. Due to historical reasons, this unworthy name has been retained until now.
There are 22 lunar maria that have been identified, and there are also some landforms called "lunar maria" or "lunar maria". Most of the 22 recognized ones are distributed on the near side of the moon.
There are 3 on the back and 4 on the edge area. On the front side, the area of ??the lunar maria is slightly more than 50%, and the largest "Ocean Procellarum" covers an area of ??more than 5 million square kilometers, almost the total area of ??nine France. Most lunar maria are roughly circular or oval in shape, and are mostly enclosed by mountains, but there are also some seas that are connected into one piece. In addition to the "sea", there are five "lakes" with similar topography - Meng Lake, Dead Lake, Summer Lake, Autumn Lake, and Spring Lake. However, some lakes are larger than the sea. For example, Meng Lake covers an area of ??70,000 Square kilometers, much larger than Qihai and others. The parts of the lunar sea that extend to the land are called "bays" and "marshes", both of which are distributed on the front. There are five bays: Dew Bay, Summer Bay, Central Bay, Rainbow Bay, and Meiyue Bay; there are three marshes: Rotting Marsh, Plague Marsh, and Meng Marsh. In fact, there is no difference between marshes and bays.
The terrain of the lunar maria is generally low, similar to basins on the earth. The lunar maria is 1-2 kilometers lower than the lunar mean level. Some of the lowest seas, such as the southeastern part of the Imbrium, are even 6,000 meters lower than the surrounding area. rice. The moon's albedo (a physical measure of its ability to reflect sunlight) is also relatively low, so it appears darker.
Lunar land and mountains
The area on the lunar surface above the lunar sea is called the lunar land. It is generally 2-3 kilometers higher than the lunar sea level. Due to its albedo High, so it looks brighter. On the far side of the moon, the landmass is roughly the same area as the maria, but on the far side, the landmass is much larger than the maria. It is known from isotope measurements that the lunar continent is much older than the lunar maria and is the oldest topographic feature on the moon.
On the moon, in addition to the many jagged craters, there are also some mountains similar to those on the earth. The names of mountains on the moon are often borrowed from those on Earth, such as the Alps, Caucasus Mountains, etc. The longest mountain range is the Apennine Mountains, which stretches for 1,000 kilometers, but its height is only three or four thousand meters higher than the level of the lunar sea. There are also some steep peaks on the mountain range, and their heights were estimated to be too high in the past. It is now believed that the height of most mountain peaks is similar to that of the Earth's peaks, and the highest peaks (also near the moon's south pole) are only 9,000 meters and 8,000 meters high.
There are 6 peaks above 6,000 meters on the moon, 20 peaks between 5,000-6,000 meters, 80 peaks between 4,000-5,000 meters, and 200 peaks above 1,000 meters.
Mountains on the moon have a common feature: the slopes on both sides are very asymmetrical. The seaward side has a very steep slope, sometimes in the shape of a cliff, while the other side is quite gentle.
In addition to mountains and mountain groups, there are four cliffs hundreds of kilometers long on the moon. Three of them protrude into the moon sea. This kind of cliff is also called "moon chasm".
Radial patterns on the lunar surface
Another main feature on the lunar surface is that some "younger" craters often have beautiful "radiating patterns". This is a kind of crater-like pattern. It is a bright band extending in all directions from the radiant point. It passes through mountain systems, maria and craters in an almost straight direction. Radiation patterns vary in length and brightness. The most eye-catching is the radiation pattern in Tycho crater. The longest one is 1,800 kilometers long and is particularly spectacular during the full moon. Secondly, the two craters Copernicus and Kepler also have quite beautiful radiation patterns. According to statistics, there are 50 craters with radiating patterns.
The reason for the formation of radial patterns has not yet been determined. In essence, it is closely related to the theory of crater formation. Nowadays, many people are inclined to the theory of meteorite impact, believing that on the moon, which has no atmosphere and little gravity, meteorite impact may cause high-temperature fragments to fly very far. Some other scientists believe that the role of volcanoes cannot be ruled out, and the jets from volcanic eruptions may also form radiation patterns that scatter in all directions.
Valley of the Moon (lunar gap)
There are many famous rift valleys on the earth, such as the East African Rift Valley. There is also this kind of structure on the moon - those large black cracks that appear to be curved are lunar valleys. Some of them extend for hundreds to thousands of kilometers, and their widths range from a few kilometers to tens of kilometers. . Those wider valleys are found mostly in flatter areas of the moon, while those narrower, smaller valleys (sometimes called moon creeks) are found everywhere. The most famous lunar valley is the Alpine Great Lunar Valley, which connects the Mare Imbrium and the Mare d'Arc in the southeast of Plato Crater. It cuts off the Alps on the lunar surface, which is very spectacular. Photos taken from space estimate that it is 130 kilometers long and 10-12 kilometers wide.
Why can’t we see the back side of the moon?
The moon always faces the earth with one side.
This is because the moon's autobiography and revolution periods are the same (synchronous rotation). (27.32166)
To understand this phenomenon, you can do an experiment. Draw a circle and mark the directions of true east, west, north and south. You stand in the center of the circle (representing the earth), find another friend, stand on the circle, let him face forward (that is, without twisting his neck), move counterclockwise along the circle, and ask him to keep moving while moving along the circle. Always face towards the center of the circle, which is you. Then such a process basically simulates the rotation process of the moon and the earth.
Obviously, in such a process, your friend always faces you with one face (front). Let’s understand why in such a process, the revolution period is equal to the rotation period.
Your friend starts from your due north and revolves around you. When he appears due north again, he has completed a revolution cycle. (Similar to the time it takes for the moon to make one revolution around the Earth.)
Let’s take a look at its rotation time. We might as well set the posture when your friend is due north of you and faces due south as the initial posture. Then we can find that when your friend moves counterclockwise to your due west position, his rotation posture rotates 90 degrees counterclockwise. (If your friend does not "rotate" during the process, then when he is at this position, he is not facing you, but still facing the south direction. However, during the actual experiment, your friend is facing the right direction at this position. East, so he rotates 90 degrees counterclockwise from his initial position.
Similarly, when he walks directly south of you, he rotates 180 degrees from his initial position. When he walks due east of you, he rotates 270 degrees relative to the initial posture. When he walks due north of you again, he rotates 360 degrees relative to the initial posture. Period.
Because completing a revolution process just completes a rotation process, so from a time perspective, this rotation period is equal to the revolution period, because during the whole process, your friend always moves with the body and face. Facing you, that is to say, one side of the moon always faces the earth.
Guanghan Palace - Moon
Whenever night falls, a bright moon rises in the night sky, clear. The moonlight shines all over the earth, giving rise to countless emotions and reveries. Literati and poets are particularly fond of the moon. The poet Zhang Ruoxu of the Tang Dynasty wrote "Who on the river first sees the moon, and the moon on the river shines on people at the first time", and the poet Su Shi of the Song Dynasty wrote "The bright moon." "Whenever there is a bright moon in the sky, we can clearly see the dark parts and bright areas on it." When astronomers observe the moon, they think that the dark areas are covered with sea water, so they call them "sea". The famous ones are the sea of ??clouds, the sea of ??moisture, the sea of ??tranquility, etc. The bright parts are mountains, with mountains and mountains crisscrossing them. , there are craters dotted everywhere. The Bailey crater located near the South Pole has a diameter of 295 kilometers and can fit into the entire Hainan Island. The deepest crater is Newton crater, which is 8,788 kilometers deep. In addition to craters, there are also ordinary mountains on the moon. Mountains and deep valleys appear on top of each other, creating a unique scene.
The moon is also about 4.6 billion years old. It is closely related to the earth and has a layered structure such as crust, mantle, and core. The average thickness of the lunar crust is about 60 to 65 kilometers. 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, which has a temperature of about 1,000 degrees. It may be in a molten state. The diameter of the moon is about 3476 kilometers, which is 3/11 of the earth. The volume is only 1/49 of the earth, and the mass is about 735 billion tons, which is equivalent to 1/81 of the earth's mass. The gravity of the moon is almost the same. 1/6 of the Earth's gravity.
Discussion on the Causes
1. Split theory. This is the earliest hypothesis to explain the origin of the moon. As early as 1898, the famous biologist Darwin proposed. His son George Darwin pointed out in the article "Tides and Similar Effects in the Solar System" that the moon was originally a part of the earth. Later, because the earth rotated too fast, part of the earth's material was thrown out, and this material broke away from the earth to form the moon. , and the big hole left on the earth is now the Pacific Ocean. This view quickly received some objections. They believe that with the speed of the earth's rotation, it is impossible to throw something as big as that.
Besides, if the moon was thrown out by the earth, then the material composition of the two should be the same. However, through laboratory analysis of the rock samples brought back from the moon by the "Apollo 12" spacecraft, it was found that the two are very different.
2. Capture theory. This hypothesis holds that the moon was originally just an asteroid in the solar system. Once, it moved near the earth and was captured by the earth's gravity. It has never left the earth since. There is also a view close to the capture theory that the earth continues to accretion matter that enters its orbit together. Over time, more and more things are accreted, and eventually the moon is formed. But some people point out that for a planet as big as the moon, the earth may not have that much power to capture it.
3. The theory of homology. This hypothesis holds that the Earth and the Moon are both floating nebulae in the solar system, which formed stars at the same time through rotation and accretion. During the accretion process, the Earth is a little faster than the Moon and becomes the "elder brother". This assumption has also been challenged objectively. Through laboratory analysis of rock samples brought back from the moon by the Apollo 12 spacecraft, it was discovered that the moon is much older than the earth. Some people believe that the moon is at least 7 billion years old.
4. Big collision theory. This is a new hypothesis about the formation of the moon in recent years. On March 20, 1986, at the Moon and Planets Symposium held at the Johnson Space Center in Houston, Benz and Slattery of the Los Alamos National Laboratory in the United States and Cameron* of the Smith Center for Astrophysics of Harvard University ** also proposed the large collision hypothesis. This hypothesis holds that in the early stages of the evolution of the solar system, a large number of "planet particles" were formed in the interstellar space, and the planet particles grew up through mutual collision and accretion. The planetesimals merged to form a primitive Earth, and also formed a celestial body equivalent to 0.14 times the mass of the Earth. During their respective evolution processes, these two celestial bodies formed a metallic core mainly composed of iron and a mantle and shell composed of silicate. Since the two celestial bodies are not far apart, the chance of encountering them is high. By chance, the small celestial body hit the earth at a speed of about 5 kilometers per second. The violent collision not only changed the motion of the Earth and tilted its axis, but also caused the small celestial body to be shattered by the impact. The silicate shell and mantle were heated and evaporated, and the expanding gas and high speed carried a large amount of crushed dust flying away. Earth. The material flying away from the earth is mainly composed of the mantle of the collision object, and also contains a small amount of material on the earth. The ratio is roughly 0.85:0.15. The metal core separated from the mantle when the impactor broke up was decelerated by the expanding and flying gas, and was accreted to the Earth in about 4 hours. The gas and dust flying away from the earth are not completely separated from the gravitational control of the earth. They are combined by mutual accretion to form a completely molten moon, or they first form several separate small moons, and then gradually accrete to form a part. The big molten moon.
Sphere composition
4.5 billion years ago, the surface of the moon was still a liquid magma ocean. Scientists believe that KREEP, the mineral that makes up the moon, exhibits chemical clues left behind by magma oceans. KREEP is actually a composite of what scientists call "incompatible elements"—materials that couldn't fit into the crystal structure and were left behind, floating to the surface of the magma. For researchers, KREEP is a handy clue to the volcanic history of the moon's crust and to infer the frequency and timing of impacts from comets or other celestial bodies.
The lunar crust is composed of a variety of major elements, including: uranium, thorium, potassium, oxygen, silicon, magnesium, iron, titanium, calcium, aluminum and hydrogen. When bombarded by cosmic rays, each element emits specific gamma radiation. Some elements, such as uranium, thorium and potassium, are inherently radioactive and can emit gamma rays on their own. But regardless of the cause, each element emits different gamma rays. Each element has unique spectral line characteristics and can be measured with a spectrometer.
Until now, humans have not made comprehensive measurements of the abundance of lunar elements. Currently, spacecraft measurements are limited to only part of the lunar surface.
Libra Movement
Since the moon's orbit is elliptical, when the moon is at perihelion, its rotation speed cannot keep up with its revolution speed. Therefore, we can see that the eastern part of the moon reaches 98 degrees east longitude. On the contrary, when the moon is at aphelion, the rotation speed is faster than the revolution speed, so we can see the area in the west of the moon reaching 98 degrees west longitude. This phenomenon is called meridian movement.