Because the moon itself does not give off light, half of the sphere facing the sun is bright and the other half is dark under the sunlight. Because the position of the moon relative to the earth and the sun has changed, the side irradiated by the sun sometimes faces the earth and sometimes faces away from it;
Sometimes the part of the moon facing the earth is larger, and sometimes it is smaller, so there are different moon phases. Whenever the moon moves between the sun and the earth, and the hemisphere illuminated by the sun faces away from the earth, people can't see the moon on the earth. This day is called "Xin", also called "Shuori", which is the first day of the lunar calendar.
After the new moon, the moon moves along the rotation direction of the earth, and the bright area gradually turns to the earth. On the earth, you can see the moon with a slender silver hook, appearing in the western sky, hunched against the resistance of dusk. This month is called "Emei Month", which is the third and fourth day of the lunar calendar.
Then, the moon is away from the sun day by day in the sky. On the eighth day of the seventh lunar month, half the bright area faces the earth, and people can see half the moon (the convex surface faces west). This month is called "the first quarter moon".
When the moon moves in the opposite direction to the earth, that is, on 15, 16 and 17 of the lunar calendar, the bright areas of the moon are all facing the earth, and we can see a full moon, which is called "full moon" or "full moon".
Extended data:
Sentences describing the moon in Mid-Autumn Festival:
1, oh, it's blue, unpredictable, without a cloud and a little dust, like a crescent moon hanging in the southern sky. You can look at it carefully and feel helpless, making people feel as if a magical sickle flew from somewhere and suddenly stopped there.
2, crescent moon, like a comb hanging in the air. People say that the moon is the kindest, saddest and most susceptible girl. Who has any misfortune and sadness, she always looks at you with pity eyes and sometimes tears! Surely she can't bear to see those unfortunate people at the moment? So I covered half my face; But her dim light still sympathizes through the window glass. The dark room also became gloomy.
3. The moon never shines on the earth like the sun; Nor is it as fast or slow as the wind; Not as lazy as a cloud. The moon always looks at the earth tenderly and "smiles" at the stars.