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Notes on Yixing

Yixing (683–727) 一行. Originally from Changle 昌樂 in Weizhou 魏州 (the Song Gaoseng Zhuan 宋高僧傳 has it as Ju Lu 鉅鹿). His secular name was Zhang Sui 張遂, being a descendent of the early Tang figure Zhong Gongjin 張公謹

In his youth he extensively studied traditional Chinese sciences as well as Daoist texts under a Daoist adept 道士 named Yin Chong 尹崇. He later ordained and served under monk Puji 普寂 at the age of 24 or 25 before later studying the Vinaya under Wuzhen 悟真 (alternatively Huizhen 惠真). 

In 717 Emperor Xuanzong 玄宗 ordered his uncle to request him to come to court and assist Śubhakarasiṃha 善無畏 in translating the Mahāvairocana Sūtra 大日經 (T 848). There he initially drafted the Kaiyuan era Dayan calendar 開元大衍歷 in 721, which was completed in 727. In these years he compiled a number of works on calendrical science and Buddhism. In 723 he along with Liang Lingzan 梁令瓚 used copper and iron to cast an apparatus for measuring the movements of stars and the moon, demonstrating the movement of fixed stars, to which Xuanzong was pleased before ordering construction of another instrument the armillary sphere 渾天儀

In 724 the Mahāvairocana Sūtra was completed and thereafter Yixing requested further clarification on the text, compiling his commentary the Da Rijing Shu 大日經疏 (T 1796), which was probably completed in 725. 

Yixing also studied esoteric Buddhism under Vajrabodhi 金剛智. Yixing died in 727 at the age of 45, being given the posthumous title of Dahui Chanshi 大慧禪師. His depiction in historical documents varies. The earliest source material about him comes from a recording copied by Kūkai 空海 of his memorial stele erected by Emperor Xuanzong 玄宗, which is preserved in the Shingon Fuhō Den 真言付法傳 or Ryaku Fuhō Den 略付法傳. Here he is remembered as an erudite Buddhist monk, whereas the Jiu Tang Shu 舊唐書 (compiled in 945) lists him under the arts (fang ji 方伎) section and celebrates his accomplishments in astronomy. Both Tendai and Shingon claim him in their respective lineages.

Osabe Kazuo 長部和雄. Ichigyō Zenji no Kenkyū 一行禪師の研究. Kōbe: Kōbe Shōka Daigaku Keizai Kenkyūsho, 1963.