Surveying of the Meridian
Astronomers in ancient China established the of meridian quite early, they divided the meridian into 90 degrees from the North Pole to the equator, and they knew that the shadow under the sun decreased from north to south. However, since they had not made field surveys they did not know the exact proportions of shadows in relation to their geographical locations. For a long time they believed that there was a difference of one cun (roughly an inch) in the length of shadow in relation to a distance of 1,000 li (about 500 km) from north to south. During the early years (about 604-607) of the reign of Emperor Yangdi of the Sui Dynasty, astronomer Liu Zhuo (544-608) suggested that a
surveyor with mathematical knowledge be invited to conduct surveys on the plains north and south of the Yellow River, making sure that tile direction is from north to south, and measuring the distances of several hundred li with ropes. He said this method could be adopted in studying the earth, the sky and stars. In 607 Emperor Yangdi ordered that various localities measure the length of shadows, but the project failed due to Liu Zhuo's death. More than 100 years later, the Monk Yixing, an astronomer of the Tang Dynasty, carried out the historic mission of measuring the meridian on the ground.
The Monk Yixing (683-727) was also known by the name of Zhang Sui before he became a monk. A native of Changle of Weizhou (now Nanle County of Henan Province), Zhang Sui studied diligently in his childhood and grew to become a learned man. Upright Zhang refused to serve Wu Sansi, a corrupt senior official who tyrannized, and he secluded himself in a temple in the Songshan Mountain and became a monk. He was thus known as the Monk Yixing. In 717 Emperor Xuanzong of the Tang Dynasty invited Yixing to Changan, the capital of the empire, and serve as an advisor of astronomy. There Yixing compiled and popularized the Dayan calendar, extended the application of Liu Zhuo's formula on the variable-speed motion of the sun, and created together with another astronomer Liang Lingzan a number of astronomical devices including a bronze armillary sphere and an ecliptic armillary sphere. With the new devices, Yixing redetermined the positions of more than 150 stars and repeatedly measured the degrees of the twenty-eight lunar constellations from the North Pole of the celestial sphere.
His observations proved that quite a few previous records had been inaccurate. Based on his own observations, Yixing came to the deduction that the stars were constantly changing their positions in the celestial sphere. He thus became the first astronomer in the world to study the movement of stars, about one thousand years before English astronomer Edmund Halley (1656-1742).
As major errors had been found in the forecasts of solar eclipses based on previous calendars, Emperor Xuanzong assigned Yixing to the task of making a better calendar. Yixing decided to base his compilation on field surveys. In 724 he launched and directed a giant project of measuring the meridian through field surveys. The sites selected for the surveys included Linyi (in the central part of present-day Vietnam, at about latitude 18~ North) in the southernmost, and Tiele (in today's Mongolia, at latitude 50~ North) in the northernmost; in between were sites in Annam (in today's Vietnam), Wuling of Langzhou (now Changde of Hunan Province), Xiangzhou (now Xiangfan of Hubei Province), Wujinguan of Shangcai in Caizhou (now Runan of Henan Province), Fugou of Xuzhou (now Fugou of Henan Province), Taiyuetai of Junyi in Bianzhou (now Junxian of Henan Province), Baima of Huazhou (now Huaxian of Henan Province), Taiyuan Fu (now Taiyuan of Shanxi Province), Hengyejun of Weizhou (now Weixian of Hebei Province), Yangcheng (now Gaocheng Town of Dengfeng in Henan Province), and Luoyang (now Luoyang of Henan Province). Of all the results, those by Nangong Yue and others at Baima, Junyi, Fugou and Wujin were most satisfactory.
At those sites that ran for several hundred li from north to south, the surveyors recorded the differences in the length of shadows on the summer solstice, the winter solstice, the vernal equinox and the autumn equinox, measured the distances of the four sites, and surveyed the heights of the North Star (latitudes) at the four sites. By calculating the measurement results they found that for a degree of the height of the North Star (latitude) the corresponding distance on the ground was 351 li and 80 bu, or 129.22 km, 18.02 km longer than the present- day value of 111.2 km, which is the length of one degree of arc of the meridian. The Monk Yixing thus overthrew the erroneous concept of one cun in the shadow corresponded to 1,000 li in distance. Joseph Needham believed this was a pioneer work in the history of astronomy, which was 90 years earlier than what ancient Arab astronomers did in the Euphrates valley.