Under normal circumstances, the moon on the night of the Mid-Autumn Festival is not the roundest in the year. It is usually the fifteenth moon and sixteenth round.

The sixteenth round moon on the fifteenth day is an astronomical phenomenon. The reason is that the standard of the lunar calendar is that the new moon must fall on the first day of the lunar month. Normally, if the New Year falls in the early morning of the first day of the Lunar New Year, then the horoscope should be on the night of the 15th day of the lunar calendar. Therefore, when the new moon appears relatively late on the first day of the lunar calendar, Wang appears on the 16th day of the lunar calendar. Sometimes it can even reach the 17th day of the lunar calendar.

The moon is at its fullest and brightest when it is "looking". On the first day of the lunar calendar, the moon moves between the earth and the sun. The illuminated hemisphere of the moon is facing away from the earth, and we cannot see the moon. It is called the "new moon", also called the "new moon"; on the 15th and 16th day of the lunar calendar, , the bright side of the moon is all facing the earth, so we see the round moon, which is called the "full moon", also called "looking".

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Related reasons

According to the lunar calendar, the day of the new moon is the first day of each month. But on the same first day of the lunar month, the new moon may occur in the early morning or at night, and each synodic month itself has its own length and shortcomings. In this way, the "look" at the fullest moment of the moon can occur as early as the early morning of the 15th and as late as the morning of the 17th. ?

As we all know, the lunar calendar is based on the movement of the moon around the earth, so it corresponds to the moon phase very accurately. However, the lunar calendar is a mixture of the advantages of the lunar calendar and the solar calendar, so there will inevitably be errors.

When people regard the first day of the lunar calendar as "Suo", "Wang" depends on the movement of the moon. Usually, it will appear on the 15th and 16th days of the lunar calendar. The fundamental reason for the delay in the full moon is that the moon's rotation speed around the earth is not constant.