Yixing (Chinese: 一行; pinyin: Yīxíng; Wade–Giles: I-Hsing, 683–727) was a Buddhist monk from the Tang dynasty. He was known for his work as an astronomer, his help in improving the imperial calendar, his knowledge of the Yijing (易經), and his role as a respected figure in Esoteric Buddhism.
In secular matters, Yixing became well-known for changing the imperial calendar and building a celestial globe with a special part that used liquid to move. This part was the first of many similar parts used in Chinese astronomical clocks. In Buddhist communities, he is remembered for translating the Mahāvairocana-sūtra and writing an important explanation of that text, called the Darijing shu 大日經疏 (T 1796). Because of his work in both religious and secular areas, stories began to spread during the late Tang period that portrayed Yixing as a master astrologer who practiced astral magic. These stories led to some astrological works being credited to him.
Life
Yixing, originally named Zhang Sui (张遂), was born in Changle, Weizhou, which is now Nanle in Hebei. Some sources suggest he was born in Julu, now Pingxiang, Hebei. He was a great-grandson of Zhang Gongjin (张公谨; 584–632), a respected official in the Tang dynasty. Yixing came from a family of high status, which may have helped him connect with the Tang court later in life. His early intelligence was noted in the Jiu Tang shu, a historical text that records his early mastery of classical writings, especially in astronomy, calendar science, and traditional Chinese philosophy. After his parents died when he was 21, Yixing left his worldly life and became a Buddhist monk. He studied under several teachers, including Hongjing, Facheng, and Puji at Mount Song, and was part of the Northern Chan tradition. His deep practice of "single-practice samādhi" (一行三昧) led to his monastic name. Evidence suggests he was also a student of Shenxiu. During his early years, Yixing showed great interest in studying the Vinaya, the rules of monastic life, and wrote a now-lost work on Vinaya basics.
In the early 700s, Yixing avoided political conflicts with powerful figures like Wu Sansi, instead traveling across southern regions to seek Buddhist teachings. He studied the "Indian Vinaya" under Wuzhen at Mount Dangyang and received additional lessons from the monk Daoyi. In 717, Emperor Xuanzong invited Yixing to the capital, where he became a respected religious advisor, offering guidance on governance and public welfare. Yixing spent his later years in Chang'an and Luoyang, focusing on religious activities. In 727, after falling ill and undergoing merit-making ceremonies for his recovery, Yixing died while traveling with the emperor. Emperor Xuanzong wrote the inscription for Yixing’s memorial stone and posthumously gave him the title Meditation Master Dahui (大慧禅师).
Yixing’s most important work later in life involved translating and explaining Esoteric Buddhist texts. He worked with the Indian teacher Śubhakarasiṃha to translate the Mahāvairocana-sūtra, serving mainly as an editor and recorder rather than a direct translator, which suggests he had limited knowledge of Sanskrit. The Chinese version of the text is shorter than the original Sanskrit version. Yixing also wrote an influential commentary on the Mahāvairocana-sūtra, preserving Śubhakarasiṃha’s teachings and adding his own notes and references to Chinese Buddhist sources. Two versions of this commentary remain: the Dari jing shu (大日经疏) and the Dari jing yishi (大日经义释). The work combines ideas from Esoteric Buddhism with Huayan philosophy and references important texts like Nāgārjuna’s Madhyamaka-śāstra and the Da zhidu lun.
In addition to working with Śubhakarasiṃha, Yixing studied under Vajrabodhi, from whom he received the Vajraśekhara abhiṣeka initiation and helped with translation efforts. His writings also include a lost work titled Shishi xilu (释氏系录), which likely covered monastic rules and practices. While Yixing is remembered for his work in astronomy and calendar reforms in secular history, his role in introducing Esoteric Buddhism to China is also highly regarded in Buddhist traditions.
Yixing’s fame led to many stories about him. Japanese sources say that when Yixing’s mother was pregnant, a white light appeared on her forehead, later seen on the newborn Yixing. Tang dynasty texts describe him as a remarkable figure, even mentioning him appearing as a spirit with the Daoist title "Heavenly Master." The Jiu Tang shu records Yixing borrowing the Taixuan jing from a Daoist named Yin Chong and impressing him with his quick understanding of its teachings. These stories, while highlighting Yixing’s intelligence, are likely exaggerated accounts meant to enhance his reputation.
Another story claims Yixing sought guidance on number theory from the monk Yijing and eventually visited Guoqing Temple on Mount Tiantai. According to legend, a monk in the temple predicted his arrival, and the waters outside the temple reversed their flow, symbolizing his destined arrival. Though this story appears in Buddhist texts like the Song Gaoseng zhuan, it is more connected to mathematical traditions than Buddhist practice. In the late Tang dynasty, as foreign astrology and astral magic became more popular, Yixing’s image grew to include associations with esoteric rituals involving stars, such as summoning the gods of the 28 lunar stations, as recorded in the Qiyao xingchen bie xingfa. These accounts, blending Esoteric Buddhism with astrology, reflect how Yixing’s legacy was shaped by the cultural and religious trends of his time.
Astronomical work
In the early 8th century, the Tang court assigned Yi Xing to lead an astronomical and geographical survey. This survey had several goals. It was created to collect new data about the stars, which would help predict solar eclipses. It also aimed to fix problems in the calendar system and replace it with a more accurate one.
The survey was important for measuring the length of a meridian arc, which is the distance along a line of longitude. However, Yi Xing did not know the Earth was round, so he did not think of his measurements in this way. This work helped solve confusion from earlier methods that used differences in sunlight shadows at two places to calculate distances.
Yi Xing set up thirteen test sites across the empire, from Jiaozhou in Vietnam (latitude 17°N) to the area near Lake Baikal (latitude 50°N). At each site, three observations were made: one to measure the height of Polaris (the North Star), one for summer shadow lengths, and one for winter shadow lengths. Using this data, scientists determined the latitudes of these places. The Tang dynasty’s calculation for the length of one degree of meridian was quite close to modern measurements. Yi Xing also recognized that the length of a degree of meridian changes and criticized earlier scholars who used the same shadow length estimates for the entire year.
Yi Xing was known for his intelligence. He calculated the number of possible positions on a go board (though he struggled to express numbers without a symbol for zero). He and his colleague, Liang Lingzan, are famous for using the earliest-known escapement mechanism in a water-powered celestial globe. However, Yi Xing’s work built on the knowledge of earlier Chinese engineers, such as Zhang Heng of the Han dynasty, Ma Jun of the Three Kingdoms, and Li Lan of the Southern and Northern Dynasties.
Zhang Heng was the first to use hydraulic power, like a waterwheel and water clock, to rotate his equatorial armillary sphere. His system used a water clock (clepsydra) to move gears connected to a polar-axis shaft, allowing the armillary sphere to rotate as the planets and stars moved. Yi Xing also benefited from the work of Ma Jun’s followers, who used waterwheels to power mechanical toys. Li Lan, an expert in water clocks, helped improve the accuracy of clepsydras, which inspired Yi Xing’s designs. Like Zhang Heng’s earlier water-powered systems and Su Song’s later escapement mechanism in an astronomical clock tower, Yi Xing’s celestial globe relied on water power to function.
Joseph Needham, a British biochemist and historian, noted that Zhang Sixun first used liquid mercury instead of water to power an armillary sphere in 979 AD (because mercury does not freeze in winter). Historical records from the Song dynasty mention why Yi Xing’s armillary sphere did not survive after the Tang dynasty.
Earlier Tang-era texts from the 9th century describe Yi Xing’s contributions to astronomical instruments in the 8th century.
Buddhist scholarship
Yi Xing wrote an explanation of the Mahavairocana Tantra. This work had a great influence on the Japanese monk Kūkai and was very important in helping him create Shingon Buddhism.
In his honor
At the Tiantai-Buddhist Guoqing Temple on Mount Tiantai in Zhejiang Province, a Chinese pagoda called the Memorial Pagoda honoring Monk Yi Xing was built just outside the temple. The tomb of Monk Yi Xing is also located on Mount Tiantai.