Why is the moon on the fifteenth and sixteenth circles?

As we all know, there is a saying in China that the moon turns sixteen times on the fifteenth day. This sentence is also very reasonable. Children look up at the moon in the morning, and sometimes they find that the moon on the 16th is indeed more round than that on the 15th. Moon 15 said that when the longitude difference between the moon and the sun is 180, the moon can be seen from the earth, and the full moon can only be seen when the moon and the sun are just opposite. The moon revolves around the earth in an elliptical orbit. It takes an average of 29 days from one full moon to the next, 12 hours and 44 minutes. When looking at the moon, the moon, the earth and the sun are the closest to a straight line, the moon. This time is also the time when the original sin is the heaviest, but because the rotation speed of the moon is fast and slow, the time to enjoy the moon is different every time, and most of them try to get to 16 or even the early morning of 17. This is why people say that the moon ratio of 15 is 16 round.