Hé Chéngtiān 何承天 (370–447), native of Dōng-hǎi Tán-xiàn 東海郯縣 (in modern Shāndōng), Liú-Sòng astronomer, calendarist, ritualist, and historian. His standard biography is in Sòng shū 64 (列傳 24, Hé Chéngtiān zhuàn). A child prodigy, he was orphaned young and educated by his maternal uncle Xú Guǎng 徐廣 (the Chē-qí jì-shǐ and known classicist). He held office through the late Eastern-Jìn and Liú-Sòng courts, rising under Liú Yù to Zhèn-jūn cān-jūn 鎮軍參軍 and Dōng-yáng tài-shǒu 東陽太守, and eventually under Sòng Wéndì (r. 424–453) to Tài-zǐ shuài-gēng-lìng 太子率更令 and Zhuó-zuò láng 著作郎 with the formal commission to compile the dynastic history of the Liú-Sòng — which was eventually completed by Shěn Yuē 沈約.
His principal scientific achievement is the Yuánjiā lì 元嘉曆 of 443, his major refinement of the Tài-shǐ lì tropical-year and lunar-month constants and the first Chinese calendar to use a 391-year intercalation cycle (replacing the older 19-year zhāng cycle). The calendar was the standard for the Liú-Sòng and was inherited by the Qí and Liáng dynasties.
He was also an important polemicist in the early-medieval Buddhist-Confucian debate, author of the Dá Zōng Jū-shì shū 答宗居士書 (a critique of Buddhist karma doctrine, preserved in Hóng míng jí 弘明集), engaging directly with Zōng Bǐng 宗炳. In this corpus he appears as the attributed author of KR1d0101 Lǐlùn (originally a 100-juàn compendium of Sānlǐ biàn-lùn, now reduced to a single-juàn extract of Tōngdiǎn citations).
CBDB has no entry for Hé Chéngtiān (pre-Táng coverage is patchy); the dates 370–447 are settled in standard reference works following the Sòng shū.