The roundest moment of the moon every month is when the moon, the earth and the sun are close to a straight line.

However, the rotation speed of the moon is not uniform, and it is fast and slow, which leads to a certain error in the time when the three are combined into one every month.

This time is sometimes on the fifteenth and sometimes on the sixteenth.

Therefore, there is a folk saying that the moon is fifteen and the circle is sixteen.

It can be explained that the moon is round on the 15th and 16th.