Qútán Xīdá 瞿曇悉達
A Tang-period imperial astronomer of Indian-Buddhist family descent. The surname Qútán 瞿曇 is the standard Chinese rendering of the Sanskrit Gautama — the family name of the Buddha (and of various Indian Brahminical-and-Buddhist families). The personal name Xīdá 悉達 derives from Sanskrit Siddhārtha (the Buddha’s personal name).
The Qú-tán (Gautama) family was one of three Indian-derived hereditary astronomical families that served the Tang Imperial Astronomical Bureau (along with the Jiā-yè 迦葉 / Kaśyapa family and the Jū-mó-luó 拘摩羅 / Kumāra family) — descended from Indian Brahminical astronomers who had relocated to China during the Sui-Tang period and continued to practice their distinctive astronomical-astrological tradition in service of the Chinese imperial court. The Qú-tán family in particular held office in the Tang Tài-shǐ-jiān (Bureau of Astronomy) for several generations.
Qútán Xīdá was active in the early Kāiyuán era (713-741); the Yùhǎi records that in Kāiyuán 6 (718) he was commanded to translate the Jiǔzhí lì 九執厯 — the Navagraha (Nine-Planets) calendar, derived from Indian astrological practice. He held the office of Tàishǐjiān shì (Bureau-of-Astronomy Affairs).
His monumental compilation, the Táng Kāiyuán zhànjīng 唐開元占經 (KR3g0018), in 120 juàn, is the principal Tang-imperial divinatory-astronomical encyclopedia, completed before Kāiyuán 17 (729). The work systematically compiles Hàn-Wèi-Six-Dynasties astronomical-and-divinatory material from 70+ apocryphal-classical (chènwěi 緯書) sources, plus the foundational Jiǔzhí and Líndé calendrical computations. Without the Kāiyuán zhànjīng’s preservation, most of the apocryphal-classical literature would be entirely lost; the work is therefore an indispensable primary source for pre-Tang Chinese cosmology-and-divination.
The work was banned in subsequent imperial periods (the apocryphal-classical material being treated as politically dangerous); it survived in obscure transmission until the late-Míng Wàn-lì-period scholar Zhāng Yīxī 張一熙 recovered it in 1617 (per Zhāng’s identification-note preserved in the Sìkù-recension).