僧一行改曆與唐玄宗制禮

Monk Yixing’s Calendar Reform and Emperor Xuanzong’s Revival of Rituals

唐開元年間由僧一行主持的改曆及其成果《大衍曆》，在中國古代科技史上佔有重要地位。此次改曆是唐玄宗自開元十年起漸次展開的禮樂運動的組成部分。一行出身於學術與事功並舉的魏郡張氏家族，他受徵入朝，與家世背景及儒學修養有關。改曆的實施與禮典的修撰及郊祀、封禪等儀式活動的籌畫，同由玄宗禮樂事業的中樞機構集賢院負責，一行亦是集賢院的特殊成員。改曆在開元十二、十三年的集中推進，包括測影、制器等活動的展開，與封禪的準備和宣傳密切配合，用於表達復興唐虞聖政的象徵意義。玄宗禮樂運動的整體安排體現出周密的設計和對漢武帝禮儀活動的刻意模仿，曆法是其中的關鍵要素之一。開元十六年《大衍曆》修成之際，玄宗君臣又將改曆塑造爲「握乾符」之瑞應。一行利用經傳和緯書中的數術內容，將一套天命、德運的徵應編織於新曆中。其《大衍曆議》又闡發「應食不食」的祥瑞，不惜貶抑曆數的可靠性，而爲傳統災祥論辯護，體現出以「政教」爲旨歸的立場。一行的天文曆法思想是在玄宗制禮作樂的具體脈絡中知識與政治互動的產物。

The calendar reform led by Buddhist monk Yixing as part of the ritual revival program initiated by Emperor Xuanzong of Tang starting in 722 during the Kaiyuan era, as well as the Dayan calendar it produced, enjoys a significant place in the history of Chinese science and technology. Yixing came from the Zhang family of Wei Prefecture, which was well established intellectually as well as politically, and despite being a Buddhist monk, he was summoned to the court largely because of his family background and Confucian learning. Yixing, as a special member of the institution, worked on the reform at Jixian Academy, the center of Xuanzong’s ritual program, where scholars compiled ritual canons and planned the major ceremonies of jiaosi (“suburban sacrifices”) and fengshan. Intensive progress on the reform, which was concentrated in 724 and 725 including the carrying out of a large-scale gnomon shadow survey and the making of a series of instruments, was closely coordinated with the preparation of fengshan ceremonies, serving as a symbolic display of Xuanzong’s sagely rulership that was modelled on Yao and Shun from high antiquity. The arrangement of Xuanzong’s ritual program as a whole shows a careful design and close resemblance to the ritual activities of Emperor Wu of Han, with the calendar as one of the key elements. Upon the promulgation of the Dayan calendar in 727, Xuanzong and his advisors interpreted the success of the reform as a divine sign of the emperor “holding the Heaven’s tally.” Yixing, though he died a year earlier, had woven numerological structures into the new calendar that are meant to be interpreted as tokens of the heavenly mandate bestowed upon the emperor and the long-lasting reign of the Tang dynasty. His Dayan li yi (Discourses on the Dayan Calendar) elaborates on the idea of viewing non-occurrences of predicted eclipses as Heaven’s propitious response to good rulership. In so doing, he reduced the reliability of astronomical computation and upheld an essentialist understanding of the old theory of interpreting unusual celestial phenomena as correlated to earthly events, displaying a stance deferring to political authority and traditional Confucian teachings. Yixing’s calendrical ideas were thus a result of the interactions between technical knowledge and political discourses in the specific context of Emperor Xuanzong’s ritual program.