Monastic learning and the imperial court
The Old Book of Tang identifies Yixing as Zhang Sui from Changle in Weizhou. He studied historical writings and astronomical subjects before becoming a monk. His teachers included Puji in meditation and Wuzhen at Dangyang in Buddhist discipline, linking his education to monastic communities as well as books.
Xuanzong summoned him to the capital in 717. His biography records discussions of government and an objection to using Princess Taiping's exceptionally lavish arrangements as a precedent for Princess Yongmu's marriage. The court therefore consulted him on practical and ceremonial questions as well as calculation.
Translation and Buddhist commentary
The Song Biographies of Eminent Monks also places Yixing in the transmission of esoteric Buddhism. It records study with Vajrabodhi and collaboration with Subhakarasimha on the translation associated with the Mahavairocana scripture. Yixing prepared explanatory writing connected with this teaching.
Translation involved teachers, interpreters and commentators rather than a text moving between languages without mediation. This work places him within exchanges between Indian and Chinese Buddhist scholars and explains a substantial part of his intellectual activity beyond the calendar project.
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Instruments and the Dayan calendar
In 721, failures in eclipse predictions made with the Linde calendar prompted an imperial commission for a replacement. Yixing examined earlier methods and worked with Liang Lingzan and artisans on an ecliptic observing instrument. Calculation and observation were intended to check one another.
He died in 727 with the calendrical draft prepared. Zhang Yue, Chen Xuanjing and other officials organized its methods and discussions, presented them in 728 and brought the calendar into official use in 729. The New Book of Tang also preserves subsequent arguments about its relationship to the Jiuzhi calendar, showing that acceptance did not end scrutiny of its methods.