Why do the 24 solar terms adapt to the north and south?

At present, the accuracy of lunar terms is the contribution of western astronomy in the late Ming and early Qing dynasties. In the past thousands of years, in the ancient astronomical system of China, the accuracy of lunar terms has been improving, but there are obvious inaccuracies that cannot be eliminated, and astronomers are very annoyed and frustrated for some unknown reasons. Our solar terms are actually defined according to the movement of the sun. The most typical vernal equinox and autumnal equinox, summer solstice and winter solstice, are the easiest to identify and the earliest to form.

This is the most important time for us to understand the movement of the sun. Of course, these phenomena are not only the solar terms in China, but also the knowledge that all countries in the world must master when learning astronomical calendars. For example, the China lunar calendar attaches great importance to the winter solstice. On the solstice of winter, the sun moves to the far south and begins to return to the north. This is actually the first of the 24 solar terms and an important time benchmark. "Winter solstice" refers to the time when the height of the sun returns from the lowest point to the highest point in the north at noon. We now know that the earth actually goes around the sun in an elliptical orbit, which means that its speed is uneven. Of course, the elliptical orbit is very close to the circle. This is difficult to achieve without very accurate mathematical tools.

Therefore, according to the definition of the 24 solar terms in the sun's orbit, it is difficult to accurately grasp. The traditional lunar calendar in China is actually divided into 24 equal parts by an approximate method, that is, 365.2425 days a year. From winter to Sunday, every day is a solar term. Of course, this is a coercive means. At first, people didn't know there were some mistakes. What is particularly embarrassing is that although the winter time of the solstice is very accurate. The results of other solar terms actually have systematic errors, which do not conform to its original definition (that is, astronomical phenomena that produce these errors). With the progress of astronomical observation technology, people gradually realize the existence of errors. However, within the framework of the traditional history of astronomy in China, such as the universe model in which the sky is round and the earth is round, such as relying on the mathematical method of numerical calculation and algebra replacing geometry, there is no way to deal with these mistakes.

Therefore, although traditional astronomers in China have long known these errors, they don't know how to calculate them more accurately. Until the end of Ming Dynasty, with the help of western missionaries, Xu Guangqi almost completely entered western astronomy. One of the most important figures is the German missionary Don. He helped to adjust the definition of 24 solar terms in the history of astronomy in China, and adjusted the average length of tropical year to 24 equal parts of the length of the solar orbit-the ecliptic length. According to the longitude of ecliptic coordinates, it is a circle, 360 degrees is divided into 24 parts, and every 15 degrees is a solar term. This is the definition of the twenty-four solar terms that we often see in popular science books, such as text messages or websites.