球俗称月亮，也称太阴。月球就是最明显的天然卫星的例子。在太阳系里，除水星和金星外，其他行星都是天然卫星。月球的年龄大约也是46亿年，它与地球形影相随，关系密切。月球也有壳、幔、核等分层结构。最外层的月壳平均厚度约为60-65公里。月壳下面到1000公里深度是月幔，它占了月球的大部分体积。月幔下面是月核，月核的温度约为1000度，很可能是熔融状态的。月球直径约3476公里，是地球的3/11。体积只有地球的1/49，质量约7350亿亿吨，相当于地球质量的1/81，月面的重力差不多相当于地球重力的1/6。月球上面有阴暗的部分和明亮的区域。早期的天文学家在观察月球时，以为发暗的地区都有海水覆盖，因此把它们称为“海”。着名的有云海、湿海、静海等。而明亮的部分是山脉，那里层峦叠嶂，山脉纵横，到处都是星罗棋布的环形山。位于南极附近的贝利环形山直径295公里，可以把整个海南岛装进去。最深的山是牛顿环形山，深达8788米。除了环形山，月面上也有普通的山脉。高山和深谷叠现，别有一番风光。
The ball is commonly known as the moon, also known as the Taiyin. The moon is the most obvious example of a natural satellite. In the solar system, except for mercury and Venus, other planets are natural satellites. The moon is also about 4.6 billion years old. It is closely related to the earth. The moon also has layers of crust, mantle and core. The average thickness of the outermost moon shell is about 60-65 km. Below the moon"s crust to a depth of 1000 kilometers is the moon"s mantle, which accounts for most of the moon"s volume. Below the mantle is the lunar core, which is probably in a molten state with a temperature of about 1000 degrees. The diameter of the moon is about 3476 kilometers, which is 3 / 11 of the Earth"s. Its volume is only 1 / 49 of the earth"s, and its mass is about 735 billion tons, which is equivalent to 1 / 81 of the earth"s mass. The gravity of the moon is about 1 / 6 of the earth"s gravity. There are dark parts and bright areas on the moon. When early astronomers looked at the moon, they thought that the dark areas were covered with sea water, so they called them "sea". Famous are the sea of clouds, the sea of dampness, the sea of tranquility, etc. And the bright part is the mountains, where there are many mountains and mountains, and there are many ringed mountains everywhere. Bailey crater, located near the south pole, has a diameter of 295 kilometers and can accommodate the whole Hainan Island. The deepest mountain is Newton crater, which is 8788 meters deep. In addition to the crater, there are ordinary mountains on the moon. There are mountains and deep valleys. There is no other scenery.
月球的正面永远向着地球。另外一面，除了在月面边沿附近的区域因天秤动而中间可见以外，月球的背面绝大部分不能从地球看见。在没有探测器的年代，月球的背面一直是个未知的世界。
The face of the moon is always toward the earth. On the other side, most of the back of the moon can"t be seen from the earth except in the middle of the area near the edge of the moon due to libration. In the absence of a probe, the back of the moon has always been an unknown world.
月球背面的一大特色是它几乎没有月海这种较暗的月面特征。而当探测器运行至月球背面时，它将无法与地球直接通讯。月球约一个农历月绕地球运行一周，而每小时相对背景星空移动半度，即与月面的视直径相若。与其他卫星不同，月球的轨道平面较接近黄道面，而不是在地球的赤道面附近。相对于背景星空，月球围绕地球运行(月球公转)一周所需时间称为一个恒星月；
而新月与下一个新月（或两个相同月相之间）所需的时间称为一个朔望月。朔望月较恒星月长()是因为地球在月球运行期间，本身也在绕日的轨道上前进了一段距离。
One of the features on the back of the moon is that it has little of the darker features of the moon sea. When the probe moves to the back of the moon, it will not be able to communicate directly with the earth. The moon moves around the earth for about one lunar month, and moves half an hour relative to the background sky, which is similar to the apparent diameter of the moon surface. Unlike other satellites, the moon"s orbit plane is closer to the ecliptic than near the earth"s equator. Relative to the background sky, the time required for the moon to move around the earth (the moon"s revolution) is called one star month, while the time required for the new moon and the next new moon (or between two identical phases) is called a new moon. The reason why the new moon is longer than the stellar moon is that the earth moves a certain distance in its orbit around the sun.
因为月球的自转周期和它的公转周期是完全一样的，我们只能看见月球永远用同一面向着地球。自月球形成早期，月球便一直受到一个力矩的影响引致自转速度减慢，这个过程称为潮汐锁定。亦因此，部分地球自转的角动量转变为月球绕地公转的角动量，其结果是月球以每年约38毫米的速度远离地球。同时地球的自转越来越慢，一天的长度每年变长15微秒。
Because the rotation period of the moon is exactly the same as its rotation period, we can only see that the moon always faces the earth in the same direction. Since the early days of the formation of the moon, the moon has been affected by a moment that slows down its rotation, a process called tidal locking. As a result, the angular momentum of part of the earth"s rotation is transformed into the angular momentum of the moon"s revolution around the earth. As a result, the moon is away from the earth at a speed of about 38 mm per year. At the same time, the earth"s rotation is becoming slower and slower, and the length of a day is increasing by 15 microseconds every year.
月球对地球所施的引力是潮汐现象的起因之一。月球围绕地球的轨道为同步轨道，所谓的同步自转并非严格。由于月球轨道为椭圆形，当月球处于近日点时，它的自转速度便追不上公转速度，因此我们可见月面东部达东经98度的地区，相反，当月处于远日点时，自转速度比公转速度快，因此我们可见月面西部达西经98度的地区。这种现象称为天秤动。又由于月球轨道倾斜于地球赤道，因此月球在星空中移动时，极区会作约7度的晃动，这种现象称为天秤动。再者，由于月球距离地球只有60地球半径之遥，若观测者从月出观测至月落，观测点便有了一个地球直径的位移，可多见月面经度1度的地区。这种现象称为天秤动。
The gravity of the moon on the earth is one of the causes of the tide phenomenon. The orbit of the moon around the earth is synchronous. 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 not catch up with the revolution speed. Therefore, we can see the east of the moon is 98 degrees east longitude. On the contrary, when the moon is at the distant sun, its rotation speed is faster than the revolution speed. Therefore, we can see the west of the moon is 98 degrees west longitude. This phenomenon is called libration. Because the orbit of the moon inclines to the equator of the earth, the polar region will shake about 7 degrees when the moon moves in the starry sky. This phenomenon is called libration. Moreover, since the moon is only 60 earth radius away from the earth, if the observer observes from the moon to the moon, the observation point will have a displacement of the earth diameter, which can be seen in the area with a moon longitude of 1 degree. This phenomenon is called libration.
严格来说，地球与月球围绕共同质心运转，共同质心距地心4700千米（即地球半径的2/3处）。由于共同质心在地球表面以下，地球围绕共同质心的运动好像是在“晃动”一般。从地球北极上空观看，地球和月球均以迎时针方向自转；
而且月球也是以迎时针绕地运行；
甚至地球也是以迎时针绕日公转的。
Strictly speaking, the earth and the moon revolve around a common center of mass, 4700 kilometers (2 / 3 of the earth"s radius). Since the common center of mass is below the earth"s surface, the movement of the earth around the common center of mass seems to be "shaking". When viewed from the north pole of the earth, both the earth and the moon revolve in the clockwise direction; moreover, the moon also revolves around the earth in the clockwise direction; even the earth revolves around the sun in the clockwise direction.
很多人不明白为甚么月球轨道倾角和月球自转轴倾角的数值会有这么大的变化。其实，轨道倾角是相对于中心天体（即地球）而言的，而自转轴倾角则相对于卫星（即月球）本身的轨道面。在这个定义习惯很适合一般情况（例如人造卫星的轨道）而且是数值相当固定的，但月球却非如此。
Many people don"t understand why there is such a big change in the inclination of the moon"s orbit and rotation axis. In fact, the orbit inclination is relative to the central object (i.e. the earth), while the rotation axis inclination is relative to the orbit surface of the satellite (i.e. the moon). In this definition, the custom is very suitable for the general situation (such as the orbit of artificial satellite) and the value is quite fixed, but the moon is not.
月球的轨道平面（白道面）与黄道面（地球的公转轨道平面）保持着5。145396°的夹角，而月球自转轴则与黄道面的法线成1。5424°的夹角。因为地球并非完美球形，而是在赤道较为隆起，因此白道面在不断进动（即与黄道的交点在顺时针转动），每6793。5天（18。5966年）完成一周。期间，白道面相对于地球赤道面（地球赤道面以23。45°倾斜于黄道面）的夹角会由28。60°（即23。45°+5。15°）至18。30°（即23。45°-5。15°）之间变化。同样地，月球自转轴与白道面的夹角亦会介乎6。69°（即5。15°+1。54°）及3。60°（即5。15°-1。54°）。月球轨道这些变化又会反过来影响地球自转轴的倾角，使它出现±0。00256°的摆动，称为章动。
The moon"s orbit plane (white plane) and the ecliptic plane (Earth"s orbit plane) maintain 5. The angle between 145396 ° and the normal of the ecliptic is 1. An angle of 5424 °. Because the earth is not perfectly spherical, but rather uplifted at the equator, the white plane is constantly advancing (that is, the intersection with the ecliptic is rotating clockwise), every 6793. 5 days (18). 5966). During this period, the white road faces the equatorial plane of the earth (23. 45 ° inclines to the ecliptic). 60 ° (i.e. 23. 45 degrees +5. 15 °) to 18. 30 ° (i.e. 23. 45 degrees -5. 15 °). Similarly, the angle between the moon"s rotation axis and the white path surface will be between 6. 69 ° (i.e. 5. 15 degrees +1. 54 °) and 3. 60 ° (i.e. 5. 15 degrees -1. 54 degrees). These changes in the orbit of the moon will in turn affect the inclination of the earth"s rotation axis, making it appear ± 0. The oscillation of 00256 ° is called nutation.
白道面与黄道面的两个交点称为月交点－－其中升交点（北点）指月球通过该点往黄道面以北；
降交点（南点）则指月球通过该点往黄道以南。当新月刚好在月交点上时，便会发生日食；
而当满月刚好在月交点上时，便会发生月食。
The two intersections of the ecliptic and the white plane are called the monthly intersection, where the ascending intersection (North Point) refers to the point through which the moon passes to the north of the ecliptic, and the descending intersection (South Point) refers to the point through which the moon passes to the south of the ecliptic. An eclipse occurs when the new moon is just at the point of intersection, and when the full moon is just at the point of intersection.
