rare! Mid-Autumn Festival and National Day are the same day, only 4 times in the 21st century

　　Xinhua News Agency, Tianjin, September 19 (Reporter Zhou Runjian) The Mid-Autumn Festival and National Day are approaching. Coincidentally, the two festivals appeared on October 1st at the same time.

Astronomical experts say that in the 21st century, this "coincidence" has only occurred 4 times, which is relatively rare.

　　The Mid-Autumn Festival is fixed on the 15th of August in the lunar calendar every year, but its date in the solar calendar is very variable, and the reason for this change is the "leap month."

　　Zhao Zhiheng, an astronomy education expert and director of the Tianjin Astronomical Society, explained that my country currently uses both the Gregorian calendar (Gregorian calendar) and Xia calendar (lunar calendar).

In the solar calendar, the earth revolves around the sun as a tropical year, with 365 days in a normal year and 366 days in a leap year.

The lunar calendar is a combined yin and yang calendar that takes into account both the return year of the solar calendar that is directly related to the sun and the synodic month that changes phase with the moon.

A synodic month has an average of 29.5306 days, and a total of 12 synodic months is 354 days or 355 days, a difference of about 11 days from the return year, and a total of more than one month in three years.

　　In order to solve the contradiction between the two, the ancients adopted the method of "setting leap", that is, in some lunar calendar years, 13 months are arranged, and there are two identical months.

　　The Gengzi Rat Year is a leap year, and a "leap April" is added.

Affected by the "leap April", since then, the corresponding dates in the Gregorian calendar, such as the Mid-Autumn Festival, will be postponed by 18 days compared with last year.

　　The Gregorian calendar date corresponding to the Mid-Autumn Festival in the Year of the Pig is September 13, 2019. Therefore, the Gregorian calendar date corresponding to the Mid-Autumn Festival in the Year of the Gengzi Rat will naturally fall to October 1, 2020.

　　Zhao Zhiheng said that the Mid-Autumn Festival and the National Day occurred on October 1st at the same time, and only four times occurred in the 21st century. The last time was in 2001, and the other two were in 2031 and 2077.