Although the moon on the 15th is round, there is a folk saying that the moon on the 15th is round. The moon revolves around the earth in an elliptical orbit. It takes an average of 29 days 12 hours and 44 minutes from one full moon to the next. When you look at it, the moon, the earth and the sun are closest to a straight line, so the moon is the roundest and brightest. However, due to the different speed of the moon, the time to reach Wangwang is also different, mostly in the early morning of the 16th or 17th lunar month.
Astronomers explained that the moon on the fifteenth day is sixteen circles, because only when the longitude difference between the moon and the sun is 180 degrees, from the earth, the moon and the sun are in the correct relative position (just like two people face to face), can we see the full moon. However, the moon's orbit around the earth is elliptical, near and far; 360,000 kilometers recently and 400,000 kilometers farthest. Because of "gravity", it walks fast in the near future and slowly in the far future. So often the moon on 16 looks bigger and rounder.
However, because the revolution speed of the moon around the earth is not uniform, sometimes it is fast and sometimes it is slow, so sometimes it will be delayed until the seventeenth, so we can enjoy the full moon.