The diameter of the moon is about 30 times that of the earth. The moon moves around the earth in an elliptical orbit. The great circle cut by this orbital plane on the celestial sphere is called the 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 with a period of 27. Number 32.
There are dark parts and bright parts on the surface of the moon, the bright part is highland, and the dark part is low-lying areas such as plains or basins, which are called the moon land and the moon sea respectively.
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 mountain range, where the mountains are stacked and criss-crossed, dotted with craters, that is, impact craters, which are a low-lying shape of annular uplift.