Xinhua News Agency, Nanjing, September 20th (Reporters Qiu Bingqing and Zhu Xiao) The Mid-Autumn Festival is very clear.

The Mid-Autumn Festival is approaching, and viewing the moon is one of the highlights.

This year's Mid-Autumn Festival "15 Moon Fifteen Rounds", the fullest time is 7:55 on September 21.

　　Wang Kechao, head of science popularization at the Purple Mountain Observatory of the Chinese Academy of Sciences, introduced that the moon is the closest celestial body to the earth. From the earth, the different appearances of the moon on different dates are called "moon phases": sometimes it is a narrow crescent, sometimes it is a full moon. When the sky is high...

　　“In our traditional calendar, the month is based on the phase of the moon.” Wang Kechao said, the moment when the moon and the sun’s yellow sutra are the same as the “new”, the new day is the new day, which is the first day of the lunar calendar. The moon cannot be seen on the earth with the naked eye, and the new moon is the starting point of the cycle of the moon phase.

The interval from one new moon to the next is one month in the lunar calendar.

　　At approximately the "midpoint" of each lunar month, there will be a full moon.

The specific moment corresponding to the full moon is called "wang".

Because it contains two important nodes, "Suo" and "Wang", the lunar month is called "Synopsis month", and a synopsis month is about 29.53 days on average.

In order to fit the "length" of the synodic month as much as possible, the lunar month has a large month of 30 days and a small month of 29 days. On average, the number of days per month is close to 29.53 days.

　　When is the moon round?

Wang Kechao said that the full moon does not necessarily appear on the fifteenth of the lunar calendar, but more often on the sixteenth of the lunar calendar, rarely on the seventeenth of the lunar calendar, and occasionally on the fourteenth of the lunar calendar.

Beginning in 2021, the Mid-Autumn Festival for three consecutive years will be the "fifteenth moon and fifteen rounds."

　　"Which day the hope is depends on the time of the new moon and the time interval between the new moons." Wang Kechao said, the average interval between the new moons in January is about 14.77 days.

Suppose that the new moon of a certain lunar month occurs at 0:00:00 on the first day of the lunar calendar, and at the same time that the 14.77 days from this new moon is the horizon of the current month, that is, on the fifteenth of the lunar calendar, there will be the "15th moon fifteenth". round".

　　But the actual situation is often different.

Wang Kechao told reporters that in addition to the first day of the first day of the lunar calendar, the new moon may also occur at noon or at the end of the day.

If the synodic interval is 14.77 days unchanged, as long as the time of the new moon is more than 0.23 days from the beginning of the new sun (0:00:00), the full moon will appear on the sixteenth day of the lunar calendar.

The situation of "the moon of fifteen and sixteen rounds" is more common.

　　In addition, the interval between synodic also changes.

The orbit of the moon around the earth and the orbit of the earth around the sun are approximately elliptical, and they move fast and slow.

The swiftness of the "step" of the earth and moon in a certain period of time determines the interval from new moon to hope. The shortest is about 13.9 days and the longest is about 15.61 days.

This makes it possible for the full moon to occur on the seventeenth or fourteenth of the lunar calendar.

　　For example, Wang Kechao said that if the lunar month has a relatively long interval between the lunar calendar, such as 15.1 days, and the lunar month of the month occurs at the end of the lunar day, the month's outlook is on the seventeenth of the lunar calendar.

Conversely, if the lunar month has a short period of time, such as 13.95 days, and the new month of the month is very close to the beginning of the new day, the month’s outlook will be on the fourteenth of the lunar calendar, and there will be a "fifteenth moon and fourteen rounds". A relatively rare situation.