Chapter 19 Section 1 Guo Shoujing and the Latitude Measurement in the Yuan Dynasty

Guo Shoujing (AD 1231-1316), courtesy name Ruosi, was born in Xingtai, Shunde (now Xingtai City, Hebei).His grandfather, Guo Rong, was knowledgeable and proficient in arithmetic and water conservancy, and had a great influence on Guo Shoujing's growth.Guo Rong also made Guo Shoujing a student of Liu Bingzhong, who was proficient in astronomy, geography, and legal calendar.In the third year of Zhongtong (AD 1262), Guo Shoujing was summoned by Kublai Khan, the ancestor of the Yuan Dynasty, because he was familiar with water conservancy, and was appointed as an official in charge of water conservancy thereafter.In the thirteenth year of Zhiyuan (AD 1276), Guo Shoujing was transferred to the Taishi Bureau (later changed to the Academy) and was responsible for compiling the new calendar. During this period, he organized the "Four Seas Test" and conducted an unprecedented scale of actual measurement of the national latitude.In the seventeenth year of Zhiyuan (AD 1280), the famous Shishi calendar was compiled.Since then, Guo Shoujing also presided over the opening of the canal from Tongzhou to the capital Dadu (now Beijing).

After Guo Shoujing was ordered to compile the new calendar, he realized that "the matter of the calendar lies in the test, and the instrument of the test is not prior to the instrument" (the formulation of the calendar mainly relies on actual measurement and inspection, and the tools of measurement and inspection depend on the instrument) .Because of this, he first devoted himself to the creation of "instruments", that is, more than a dozen special instruments such as simple instruments, high meters, elevation instruments, vertical movement instruments, and scene symbols. Zhengyi and other instruments.These instruments have the advantages of precision, dexterity, and simplicity, and played an important role in this large-scale measurement.

14 officials headed by Guo Shoujing and a large number of surveyors set up 27 observation stations in a vast area of ​​11,000 miles long from north to south and 8,000 miles wide from east to west, and carefully selected seven more important points among them: the South China Sea (now the central part of Vietnam) ), Hengyue (now Hengshan, Hunan), Yuetai (now Kaifeng, Henan), Helin (now Harhelin on the east bank of the upper reaches of the Orkhon River in Mongolia), Tiele (now the Ankara River and Yenisei River area in Russia ), Beihai (now the lower reaches of the Lower Tunguska River in Russia) and Dadu (now Beijing).Except for the capital city, the other six points represent major regions from south to north, and the latitude interval between two adjacent points is 10 degrees.The observation items made at these points are more complete than other points.In order to ensure the reliability of the data, observation stations have been encrypted in the Central Plains, and these stations are concentrated in the range of 40 degrees to 30 degrees north latitude.In order to take into account the differences between East and West, Koryo (now Kaesong, North Korea) and Xiliangzhou (now Wuwei, Gansu) were selected.Comparing the above-ground values ​​of the North Pole of these stations (that is, the horizon height of the North Celestial Pole, that is, the latitude) with the actual latitude measured by them (see the table below), it can be seen that the difference between the observation stations in the Central Plains is relatively small, In particular, the differences between three points in Shaanxi Province, Sichuan Province, four points in Henan Province, and three points in Zhongshu Province are 5', 12', and 23', respectively. Among them, Yuetai (now Yuetaifang, Kaifeng, Henan) ) is completely error-free.Hengyue has the largest difference, up to 2°23', which may be caused by the fact that the ancient and modern observation points are not in one place.It was a very advanced level to achieve such a result with the instruments and technology at that time.

Unfortunately, this major achievement was only used in the compilation of calendars, but not in map surveying. According to the "Yuanwenlei·Guo Shoujing Biography", when he was in Ningxia today, he once wanted to go up the Yellow River by boat to investigate the source of the river.He also measured the ground level within a radius of hundreds of miles from the east of Mengmen along the old course of the Yellow River to determine where the flow can be diverted to reduce flooding and where water can be diverted for irrigation. These are all mapped.He also used the sea level to compare the difference between Dadu and Bianliang (now Kaifeng City, Henan Province). He believed that the water in Bianliang was much higher than the sea level, so the flow rate was quite fast; much slower.This is the earliest written record of the starting plane of elevation measurement with sea level as the level in my country.Guo Shoujing understood the sea level theoretically, and took it as the starting surface for measuring elevation, and made great contributions to geodesy and map drawing.