Today is the beginning of the dog days. Why do the dog days sometimes last 30 days and sometimes 40 days?

"It is the dog days of summer, and the weather is hot as soup." Today (July 15) officially begins the dog days of summer, and this year's "dog days" will last 40 days. From 2015 to 2024, the "dog days" have been 40 days for 10 consecutive years.

The so-called "three dog days" refers to the first dog day, the middle dog day and the last dog day, which are from mid-July to mid-August of the Gregorian calendar every year, right between the Lesser Heat and the End of Heat, when the weather is the hottest. When the heat ends, the weather gradually becomes cooler in the morning and evening.

Yang Jing, a member of the Chinese Astronomical Society and director of the Tianjin Astronomical Society, introduced that the date of the "Three Fu" is determined according to the "Ganzhi calendar" of the ancient Chinese calendar. Ganzhi is the collective name of the heavenly stems and earthly branches. The heavenly stems are composed of "Jia, Yi, Bing, Ding, Wu, Ji, Geng, Xin, Ren, and Gui", a total of 10; the earthly branches are composed of "Zi, Chou, Yin, Mao, Chen, Si, Wu, Wei, Shen, You, Xu, and Hai", a total of 12.

The Heavenly Stems and Earthly Branches are matched in order, with the Heavenly Stems in front and the Earthly Branches in the back. Starting from Jiazi and ending with Guihai, 60 cycles can be recorded without repetition, commonly known as the "Sixty Jiazi". When one cycle ends, it starts from the beginning and a new cycle is recorded, and the cycle continues. Since there are 10 Heavenly Stems, a day with the word "Geng" appears every 10 days, which is called "Geng Day".

According to the traditional Chinese calendar, the first dog days begin on the third Geng day, starting from the summer solstice. The first and last dog days are both fixed at 10 days, but the middle dog days are not fixed, sometimes 10 days, sometimes 20 days.

Why is the duration of the mid-fu different? Yang Jing explained that when there are four Geng days between the summer solstice and the beginning of autumn, the mid-fu is 10 days, and when there are five Geng days, the mid-fu is 20 days.

Take this year as an example. The summer solstice is on June 21. Counting from the summer solstice, the first Geng day is June 25 (Gengshen day), the second Geng day is July 5 (Gengwu day), the third Geng day is July 15 (Gengchen day), the fourth Geng day is July 25 (Gengyin day), the fifth Geng day is August 4 (Gengzi day), and the beginning of autumn is August 7. So this year there are 5 Geng days between the summer solstice and the beginning of autumn, and the middle dog days are 20 days. The last dog days will not begin until the first Geng day after the beginning of autumn, August 14 (Gengxu day).

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Since the beginning of this century, only six years, namely 2004, 2006, 2008, 2010, 2012 and 2014, had a "Three-Fother Days" duration of 30 days. The "Three-Fother Days" duration in the other 18 years was 40 days. Among them, from 2015 to 2024, the "Three-Fother Days" duration was 40 days for 10 consecutive years.

"The beginning of the dog days is generally characterized by 'sauna days'. The public must make targeted responses, pay attention to heatstroke prevention and cooling, and try to spend the summer safely," said Yang Jing.

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Source: Xinhua News Agency

Editor: Wang Mengru

Proofread by Xu Lailinlin