Yi Xing

Yi Xing (683–727) was a Chinese Buddhist monk, astronomer, and mathematician of the Tang dynasty. He helped create the Dayan calendar and led an early geodetic survey that measured differences in the length of a meridian arc.

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Dayan calendar: A Tang dynasty lunisolar calendar compiled under Yi Xing’s direction and adopted in 729. This calendar was the major practical outcome of Yi Xing’s astronomical program.
Astronomical calendar: A calendar that uses observations or calculations of celestial cycles to organize time. Yi Xing used astronomical models to improve calendrical prediction.
Tang dynasty: A Chinese imperial dynasty that ruled from 618 to 907 and oversaw a major period of cultural and political change. Yi Xing’s career unfolded amid Tang court patronage and state calendrical administration.
Emperor Xuanzong of Tang: The Tang emperor who ruled from 712 to 756 and presided over the Kaiyuan era. Xuanzong’s court supported the calendar project associated with Yi Xing.
Dayan li: The calendrical system compiled under Yi Xing and implemented by the Tang court in 729. Its calculations made Yi Xing’s calendar reform concrete.
Chinese calendar: A lunisolar calendar system coordinating lunar months with the solar year through intercalation. The Dayan calendar revised the Chinese calendrical tradition Yi Xing served.
Armillary sphere: An instrument of nested rings representing celestial circles, used to model or observe the sky. Tang astronomers used such instruments to represent celestial coordinates and motions.
Buddhism in China: The development and practice of Buddhist traditions in China since their introduction from Central Asia and India. Yi Xing’s monastic identity arose within Chinese Buddhism.
Gautama Siddha: An Indian-born astronomer and translator active in Tang China, associated with the Jiuzhi li calendar. His translation work brought Indian astronomical material into the Tang scholarly environment.
Tang geodetic survey: An eighth-century survey across Tang China that compared astronomical observations and distances at multiple locations. Yi Xing directed the survey that produced the meridian measurements linked to his name.