5.8 What solar terms?

The origin of Tomb-Sweeping Day: In March and April, when the spring is bright, pink and green, Tomb-Sweeping Day is the most important festival in China's traditional customs. Tomb-Sweeping Day is now a national grave-sweeping festival. According to Sunday, it is around April 5, and according to the lunar calendar, it is the first half of March. The ancients divided a year into 24 solar terms and used this calendar to sow and harvest. Qingming is one of the twenty-four solar terms, fifteen days after the vernal equinox. According to the saying of "always asking questions": "Everything is long, clean and bright at this time. So it is called Qingming. " Therefore, "Tomb-Sweeping Day" was originally a solar term. Later, the custom of forbidding cold food and fire and beating graves was added, and Tomb-Sweeping Day was formed. After Dayu was told to control the water, people used the word "Qingming" to celebrate that the flood had been eliminated and the world was at peace.

Grave-sweeping is popular in Tomb-Sweeping Day, which is actually the content of the Cold Food Festival in Tomb-Sweeping Day the day before. It is said that cold food originated from Jin Wengong's mourning for Jie Zhitui. During the twenty years of Emperor Xuanzong's reign, he ordered the world to "eat cold food in the ground". Because cold food is associated with Qingming, it gradually spread to sweeping graves in Qingming. Cold food festival-cold food means not getting angry, and you can only eat cold or pre-cooked food. According to legend, this custom originated in the Spring and Autumn Period, when someone in the State of Jin tried to kill his eldest son, Zhong Er.

The loyal minister Jiezhitui (also known as Jiezhitui) escorted Zhong Er to escape, and even when he was hungry and cold, he cut off his own meat for Zhong Er to eat, hoping that he would return home safely in the future, become a monarch, and be diligent and love the people.

After more than ten years, Zhong Er finally returned to China to become a monarch, that is, Jin Wengong, one of the five tyrants in the Spring and Autumn Period, rewarded those who helped him during his exile, but forgot to introduce him. After being reminded by others, he quickly sent someone to invite Jiezhi to come and receive the reward. However, Jie Zhitui and his mother went to live in seclusion in the mountains. Jin Wengong and his courtiers found nothing in the mountains. It is suggested that Yamakaji be released. Jie Zhitui is a dutiful son and will definitely save his mother. However, the fire burned for three days and nights, and there was still no referral. After the fire went out, people found Zhizhi carrying his mother's body under a willow tree. Jin Wengong was so sad and regretful that he buried them under the willow tree. Jin Wengong designated this day in Yamakaji as the Cold Food Festival, stipulating that people are forbidden to use fire and eat cold food for one day to commemorate Jie Zhitui's loyalty.

In 732 AD, Emperor Xuanzong of the Tang Dynasty said: "Cold food is buried in the ground, and the ceremony is unwritten. It has been handed down from modern times and infiltrated into customs. " Now that the Cold Food Festival has disappeared, Tomb-Sweeping Day has become an independent festival. Traditional festivals in China are characterized by attaching importance to human relations and family ties. Sacrifice in the Spring and Autumn Period, thinking about the times and offering sacrifices to ancestors in the Spring and Autumn Period are in line with people's psychology of praying for the autumn newspaper in spring. The Chinese nation has a tradition of worshipping and respecting ancestors since ancient times, respecting ancestors and their fine traditions. This tradition has accumulated into a folk cultural psychology and become people's collective consciousness. People go for an outing in festivals and enjoy the spring scenery. Or homesickness, or friends reunion, in short, close the relationship between people and increase cohesion.

24 solar terms divided the year into 24 equal parts according to the position of the sun on the ecliptic. That is to say, the ecliptic is divided into 24 equal parts, each equal part accounts for 15 of the ecliptic.

Because the time required for the sun to pass through each bisection is almost equal, and the Gregorian calendar dates of the 24 solar terms are roughly the same every year, the Gregorian calendar time of each solar term is relatively fixed.

As one of the 24 solar terms, Qingming must be determined according to the true position of the sun in the sky. It is actually a moment, not a day.

China's traditional calendar is a combination of Yin and Yang, and the criterion is the movement of the sun (day, year and solar terms) and the movement of the moon (month).

In order to let our ancestors know the climate change in cold and summer, decide the progress of farming or serve as a reference for daily life, ancient historians stipulated that the time from the winter solstice of each year to the winter solstice of the next year in a tropical year should be divided into twelve equal parts, which is called Zhongqi; Then, each gas division is called solar terms, which is the source of 24 solar terms. The method of making this solar term is called "flat gas method".

However, because the earth's orbit around the sun is elliptical. Near the sun, the speed of the earth's revolution is faster, but far away, it is slower. For the Central Plains region where this calendar was made at that time, some solar terms could not reflect the real climate. Therefore, since the Qing Dynasty, the vernal equinox has been set at 0 degrees, and every time the sun runs 15 degrees on the ecliptic, it is set as a solar term or neutral atmosphere, so the 24 solar terms have 24 specific moments instead of 24 specific days. This method of making solar terms is called "fixing gas method".

Because the length of a year is determined by the movement of the sun, and the 24 solar terms are also determined by the position of the sun, basically the date of each solar term will not change much every year.

However, the actual length of a tropical year is 365.2422 days, and the Gregorian calendar year is 365 days, so there will be 0.2422 days (equivalent to 5.8 hours) every year, and the specific time of solar terms will be "delayed" by 0.2422 days every year; As a result, it accumulated to 0.9688 days in four years, almost one day. In order to correct it, there is a "leap year" system in the Gregorian calendar, and there will be one more day on February 29 every four years. If the date of a solar term is delayed from midnight to the next day or advanced to the previous day due to leap year, the date of the solar term will change, but the date difference will only be within three days.

Therefore, it is precisely because the length of the tropic year is slightly different from the length of the actual calendar year, and the date of the Gregorian calendar year needs to be revised with the actual length of the tropic year, which will make Tomb-Sweeping Day not be fixed on April 5.

Basically, Tomb-Sweeping Day will be on the 4th or 5th of every year.