Is there a scientific basis for "the moon on the fifteenth and the circle on the sixteenth"?

This is related to the astronomical phenomena. When the moon and the sun move in opposite directions, we can see a full moon, which is the so-called "king". When the moon moves to the opposite position, half of the moon illuminated by the sun is behind the earth. We can't see the moon. It's called the "new moon".

From "king" to "king", or from "new moon" to "new moon", this is called a new moon. On the lunar calendar, the new moon must be on the first day of the first month, but not necessarily on the fifteenth day of the first month, because after the new moon, it takes an average of 14 days and 18 hours and 22 minutes to watch it. So the moon is usually not in the night of 15. Of course, if the new moon occurs in the early morning of New Year's Day, then the full moon is expected to occur on the night of 15. But the traditional 24 solar terms and festivals are calculated according to the lunar calendar.

According to the lunar calendar, the new moon is the first day of every month. In fact, a new moon means a new January. When the moon reaches the same time as the ecliptic, that is, the phase of the moon at that time. At this time, the ground observer can't see any bright part of the moon. In fact, many observers can predict tides according to the phase of the moon. Therefore, it was difficult to observe the moon at that time.

The length of the new moon differs from its average value by 6 hours, so it can also be postponed to 17 night.

This is the reason why "the moon on the fifteenth day is sixteen circles". Of course, because the speed of the moon's revolution around the earth is uneven, sometimes fast and sometimes slow, it is expected that it will happen on the Mid-Autumn Festival night for several years. This is because the orbit of the moon around the earth is elliptical, so the rotation speed of the moon in different positions is not equal. When the moon is close to the earth, its rotation speed is faster; When the moon is far away from the earth, its rotation speed is slow.

It takes 14 days and 18 hours and 22 minutes on average after the "new moon". The difference between the longest time and the shortest time required for the "new moon" is 13 hours. So there are differences in the time when the full moon appears. If the moon moves slowly before the King, the time from the new moon to the King may be 16- 17 days, so there will be 15 months, or even 17 months. It takes about 14-5 days for the moon to orbit the earth for nearly two weeks, and the longitude of the sun and the moon is only 180 degrees. The moon is like a mirror, completely reflecting sunlight, so that observers in dark areas can see the full moon. At this time, in the lunar calendar, it is called full moon, that is, full moon.