Yi Xing

Yi Xing (683–727 CE), also known as Zhang Sui, was a prominent Chinese astronomer, mathematician, engineer, and Buddhist monk of the Tang dynasty. He is best known for his contributions to astronomical observation, calendrical science, and cartography.

Yi Xing collaborated with Liang Lingzan to create an armillary sphere, a sophisticated astronomical instrument used for observing and modeling the celestial sphere. He also led a large-scale geodetic survey of China to accurately measure the meridian arc, the first of its kind. This survey, conducted between 723 and 725, aimed to improve the accuracy of the Chinese calendar and its predictions of solar and lunar eclipses. The measurements taken during the survey were used to calculate the length of a degree of latitude, improving existing estimations and providing valuable data for cartography.

Beyond his astronomical work, Yi Xing made significant contributions to mathematics. He is credited with independently discovering a numerical value for the constant pi (π), although his specific methods are not fully understood from surviving records.

Yi Xing's work was instrumental in advancing scientific knowledge during the Tang dynasty and cemented his place as a key figure in the history of Chinese science and technology. His meticulous observations and innovative methodologies significantly impacted the development of astronomy, calendrical systems, and cartography in China.