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.
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