An Examination of Zhang Heng's Huntianyi Zhu(浑天仪注): Text, Structure, and Observational Framework

The Huntianyi Zhu(浑天仪注)is an instrument annotation that Zhang Heng(张衡)of the Eastern Han dynasty wrote for the Huntianyi(浑天仪). It has long been understood as a description of the geometric structure of the cosmos. This study focuses on the opening passage of the Huntianyi Zhu, reexamining its subject matter and internal logic through textual analysis, conceptual analysis, textual cross-referencing, and critical review of previous scholarship. The analysis shows that “the Earth system is like a chicken egg” is a statement about the overall structure of the Earth system, in which the shell, the white, and the yolk correspond respectively to the boundary, the medium, and the Earth within the system; “its shape is round like a pellet” serves to define the geometric shape—the two formulations have distinct functions and are not redundant. “The Earth is like the yolk within the egg, dwelling alone in the interior” primarily emphasizes that the Earth is situated within the Earth system, rather than at its center. By drawing on texts such as the Jinshu·Tianwen Zhi(晋书·天文志), the Lingxian(灵宪), and the Loushui Zhuan Huntianyi Zhu(漏水转浑天仪注), this study further argues that Zhang Heng’s Huntian system comprised at least three interconnected systems—the observation system, the Earth-based model, and the astronomical simulation system—together with a methodological perspective governing their verification. This study concludes that the Huntianyi Zhu describes an observable, modelable, and verifiable Earth system.