On the first day of every month in the lunar calendar, when the moon moves between the sun and the earth, it faces the earth with its dark side. It rises and disappears with the sun, so people can't see it. At this time, the moon phase is called "new moon" or "new moon". After the new moon, the moon gradually moved out of the area between the earth and the sun, and then we began to see a small part of the moon illuminated by the sun, shaped like a crescent moon. So the moon phase at this time is called "the waning moon". This "waning moon" can only be seen in the western sky at night. On the eighth day of the eighth lunar month, the moon moved from the earth to 90 degrees east of the sun. At this time, you can see the bright half of the moon in the west, and the moon phase at this time is called "winding". The winding moon can only be seen in the first half of the night, and then disappears in the west at midnight. After winding, the moon becomes round day by day. At this time, the moon phase is called "convex moon". On the fifteenth and sixteenth day of the lunar calendar, the moon runs on the celestial sphere directly opposite the sun, and the distance between the sun and the moon is 180, that is, the earth is located between the sun and the moon. Seen from the earth, the phases of the moon are called "full moon" or "full moon". At dusk, the full moon rises in the east, and at dawn. The moon is getting thinner. It goes through several stages in turn, including the convex moon, the last quarter moon and the waning moon, and finally returns to the position of the new moon. China used to call the waning moon in the second half of the month "waning moon". The first quarter moon and the last quarter moon, the waning moon and the waning moon are similar in appearance, but their appearance time, position and bright direction are different. The waning moon and the first quarter moon appear in the western sky in the evening and the first half of the night respectively. The waning moon and the last quarter moon appear in the eastern sky after dawn and midnight. Their faces to the east, that is, the eastern half, are bright. Because the lunar calendar date in China is arranged according to the lunar phase, the working people in ancient China sometimes rely on it to judge the lunar calendar date and the approximate time at night. The average time for the moon to return to the new moon position is 29.53 days, that is to say, the average change period of the moon phase is 29.53 days, which is the so-called "lunar calendar".