Because on the fifteenth day of the eighth lunar month, the dry and cold air from the north forced the warm and humid air that had been swirling over most parts of China in summer to retreat to the south, and the clouds in the sky decreased. At the same time, the inclination angle of the sun gradually increases, the sunlight and heat obtained by the ground gradually decrease, and the temperature is getting lower every day. Autumn wind reduces water vapor and makes the air transparent, so the night sky is washed, and the moon is particularly bright and looks more round.

There is a folk saying that there are "fifteen moons and sixteen circles" because the moon orbits the earth in an elliptical orbit, and 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 speed of the moon, the time to reach Wangwang is different every time, mostly in the early morning of the lunar calendar 16, 15 or even 17.