As for the origin of the 24 solar terms, the "Historical Records" explains in great detail.
The work of formulating the calendar began in the Yanhuang era, and it was basically completed after the Emperor Zhuan. However, there was a great flood during the year, which changed the climate to a great extent. The main purpose of the calendar was to guide the agricultural time. Changes in climate, especially in temperature, make the previously referred to theories no longer suitable. So from the beginning of the calendar, the calendar was reformulated.
According to the Historical Records, after Yao succeeded to the throne, the four mountains of destiny formulated the 24 solar terms according to the characteristics of celestial phenomena and climate. Of course, the calendar was also formulated. There are different opinions about the identity of the four mountains, but in fact, there is an answer in the Classic of Mountains and Seas.
During the Great Wilderness era, there lived many mountain gods, and these mountain gods were responsible for restricting the various tribes of the human race and preventing communication between them. At that time, the place where the ancestors of the Chinese nation lived was divided into many mountain systems. Together, these mountain systems form the five directions of the southeast, northwest, and central directions, also known as the five Tibets. Among them, there is a god in charge of these mountain gods, and these gods are called the four mountains. In ancient books, it is recorded that the Shushan clan and the Lieshan clan are all members of the four mountains.
By the time of Yao, Siyue's main work had shifted to the formulation of the calendar. And to also set a specific official name for each person in the four mountains. This official name is to separate the names of Xi He, who managed the sun travel in the Great Wilderness Era, and call them Xi Zhong, Xi Shu, He Zhong, and He Shu respectively.
In order to establish a calendar, it is necessary to observe and record the time when the sun and moon rise and fall, and the position of the stars. Then it comes to the conclusion that there are 366 days in a year. The 366 days are divided into four seasons. The error part is corrected by a leap month. Thus the first calendar after the Flood was developed.
On the basis of the calendar, in order to accurately guide farming and let people know what to do on what day, it is necessary to formulate 24 solar terms, and the four most important of which are the spring equinox, summer solstice, autumn equinox and winter solstice.
Yao ordered Xi Zhong to live in a place called Yanggu in Yuyi, which is on today's Taihang Mountain. Xi Zhong gets up early every day to observe and record the time of sunrise, and record the difference between the time of sunrise each day. The day when the day is as long as the night is determined to be the vernal equinox. From the day of the vernal equinox, the stars of the Seven Nakshatras appear due south when the sun sets, and the time of mid-spring is determined based on this phenomenon.
He also ordered Uncle Xi to go to the far south, and the specific location may be to today's Vietnam. Also observe the sun, moon and stars here. The day with the longest day is determined according to the calendar, and this day is determined to be the summer solstice. From the summer solstice, when the sun sets, the Antares of the Seven Nights of the Dragon appear due south, and at this time midsummer begins.
He also ordered Hezhong to live in the westernmost part of the Chinese nation's sphere of activity. At that time, the westernmost part of the country probably reached today's Yumen Pass, and further west was the Aryan entrenchment. The place where Kazunaka lives is called Matsuya, where he observes the time of sunset. The days that are as long as the night are designated as the autumnal equinox. From the autumnal equinox, when the sun sets, the void of the seven Xuanwu Nakshatras appears due south. That's when the mid-autumn season begins.
He also ordered Uncle He to live in a place called Youdu in the north, and I don't know if this place is today's **. Here, after years of recording, he determined that the day with the shortest day was used as the winter solstice. From the day of the winter solstice, when the sun sets, the Pleiades of the Seven Nakshatras of the White Tiger appear due south at dusk. That's when midwinter comes in.
We don't know how many years it took Siyue to determine these 4 solar terms. However, it is certainly not possible to determine a specific date in three or five years. In short, it is a long process. After these 4 solar terms are determined, the other 24 solar terms can be determined.
The 24 solar terms are entirely formulated for agricultural production. Although it is also inextricably linked to the calendar, for a long time, the calendar and the solar terms were two systems that were not combined. It was not until the Taichu calendar was formulated in the Western Han Dynasty that the 24 solar terms and the calendar were combined.
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