Time of spring and autumn equinox, winter and summer solstice

China has had the saying of "24 solar terms" since ancient times, which is the product of ancient farming civilization. Agricultural production is closely related to the rhythm of nature. Ancient ancestors observed the movement of celestial bodies in the farming season, and realized the changing laws of seasons, climate and phenology in a year, and divided the year into 24 solar terms, namely, beginning of spring, rain, fright and phenology. Because the "twenty-four solar terms" reflect the annual movement of the sun, the date of the "twenty-four solar terms" in the Gregorian calendar is relatively fixed, with a difference of only 1-2 days. These include:

The vernal equinox is on March 20-22 of the Gregorian calendar every year. The vernal equinox is an important solar term, which is of great significance in astronomy: the northern and southern hemispheres divide the day and night equally. Since then, the direct position of the sun has continued to move from the equator to the northern hemisphere. The days in the northern hemisphere are getting longer and shorter, and the nights in the southern hemisphere are getting shorter and shorter. During the spring equinox, China has entered a bright spring except the Qinghai-Tibet Plateau, Northeast China, Northwest China and North China. At this time, the climate is mild, the rain is abundant and the sun is shining, and the wintering crops in most parts of China enter the spring growth stage.

The summer solstice is celebrated on June 2 1-22 of the Gregorian calendar every year. The summer solstice is the northernmost day of the sun in a year and the limit of the sun's northward movement. The solstice in summer is the turning point of the sun. After this day, I went back. The direct sunlight point moved southward from the Tropic of Cancer, and the daylight in the northern hemisphere became shorter and shorter day by day. After the summer solstice, the Tropic of Cancer and its northern region, the midday sun altitude angle also began to decrease. At the same time, after the arrival of the summer solstice, the stars in the night sky gradually became summer stars.

The autumnal equinox is celebrated on September 22-24 of the Gregorian calendar every year. The autumnal equinox and the vernal equinox are the same. The sun shines almost directly on the equator, and day and night are almost equal. From this day on, the direct sunlight position continued to move from the equator to the southern hemisphere, and the northern hemisphere began to have shorter days and longer nights, while the southern hemisphere was the opposite. In the autumn equinox, most parts of China have entered a cool autumn.

The winter solstice is celebrated on February 2 1-23 of the Gregorian calendar every year. In all parts of the northern hemisphere, the winter solstice is the shortest day of the year. From winter to the future, the direct sunlight position gradually moves northward, and the daytime in the northern hemisphere gradually becomes longer. The solar term of the solstice means that when it begins to enter cold weather, people begin to count to nine to calculate cold weather. From the winter of solstice, every nine days count as nine days, and so on. Generally speaking, "March 9" is the coldest time of the year. After nine "nine days", the cold passed and the weather was warm.

Twenty-four solar terms are an important part of the long history and culture of the Chinese nation, expressing the unique concept of time between human beings and the natural universe, and containing the long cultural connotation and historical accumulation of the Chinese nation. It not only plays a guiding role in agricultural production, but also affects the food, clothing, housing and transportation of ancient people, and even cultural concepts.