Chapter 41 Section 7 Yixing, Guo Shoujing and Geodesy

During this period, a group of outstanding astronomers emerged in our country, and Yixing and Guo Shoujing were two of them. Yiyi (AD 683-727), commonly known as Zhang Sui, was born in Changle, Weizhou (now Nanle County, Henan). He became a monk when he was 20 years old and lived in seclusion in Songshan, Henan and other places.The main contribution of the party in astronomy is the compilation of the Great Yan Calendar.He accepted the task of compiling the calendar in the ninth year of Kaiyuan (721 AD). From the historical development of astronomy, the party realized that the movement of the sun, moon, and stars has certain laws. Through careful observation, we can understand these laws.However, due to the limited level of human understanding, there will be certain errors between the calculations based on these laws and the actual astronomical phenomena.From the actual measurement, these errors can be corrected, and more correct results can be obtained after repeated observation and correction.Therefore, in order to make the calendar consistent with the actual celestial phenomena, he persisted in making actual observations, thus correcting many mistakes made by predecessors and raising the formulation of the ancient Chinese calendar to a new level.

At the same time, in order to make the calendar applicable to all parts of the country, a group of leaders conducted a large-scale geodetic survey.He also invented a measuring instrument called "complex moment diagram" for measurement.A total of 12 measurement sites were selected, and the distribution range reached the north and south ends of the Tang territory.From the actual measurement, he found that the difference between the north and the south is 351 miles and 80 steps, and the height of the North Pole is one degree different.In the ancient Chinese system, one mile = 300 steps, one step = 5 feet, and the day of a week is 3651/4 degrees. Converted to the current system, that is, the distance between the north and the south is 129.22 kilometers, and the height of the North Pole differs by 1°.This is actually the length of the Earth's meridian at 1°. Compared with the current measured value of 1°, which is 111.2 kilometers long, although the result of one line has a large error, it is the first actual measurement of the meridian in the world after all, and it has great scientific significance. .

After several years of calculations, in 725 A.D., he and his party began to compile the new calendar.After completing the draft of "Da Yan Li" in 727 AD, he died. "Da Yan Li" is divided into seven chapters, which are very systematic from content to structure. Before the calendar was compiled by Western European methods at the end of Ming Dynasty, all calendar revisions were carried out according to its structure. Guo Shoujing (AD 1231-1316), styled Ruosi, was born in Xingtai, Hebei.The "Shoushi Calendar" formulated by him and a group of astronomers in the Yuan Dynasty was the best calendar in ancient China and was used until the early Qing Dynasty.In order to formulate the "Shoushi Calendar", he followed his party and organized another geodetic survey of unprecedented scale.Observation points have been set up in 27 places including Beijing, Taiyuan, Chengdu, and Leizhou, reaching the Paracel Islands in the south and the North Sea near the Arctic Circle in the north.Regardless of the geographical scope and scale, or the observation accuracy, it is far more than one line.He made more outstanding contributions in the manufacture of astronomical instruments, which will be introduced below.In addition, he is a hydraulic engineering expert and geographer.