An interpretation of Yi Xing’s (Tang dynasty) star chart: ecliptic geometry and cosmological thinking

Abstract

This study examines a remarkable passage in the Treatise on astronomy 天文志 (Tianwen zhi) of the New book of Tang 新唐書 (Xin tangshu), which offers a detailed textual description of a star chart traditionally attributed to the eminent Tang dynasty (618–907 CE) astronomer Yi Xing 一行 (683–727 CE). The chart adopts the gaitian 蓋天 (canopy heaven) model and is structured around three concentric circles delineating the twenty-eight lunar lodges 二十八宿 (ershiba xiu) and several prominent constellations. Its most striking feature is the depiction of the ecliptic as nearly elliptical rather than circular, a rare representation that distinguishes it among scientific star charts of ancient China. This paper provides a close analysis of the drawing techniques and geometric configuration of the ecliptic, arguing that accurate reconstruction requires an accurate understanding of the “ecliptic discrepancy” 黃道差數 (huangdao chashu), interpreted here as the difference in polar distance between a point on the ecliptic and its counterpart on the celestial equator at the same polar angle. Under this interpretation, the ecliptic naturally assumes a near-elliptical form. Beyond its geometric analysis, this study also investigates the cosmological assumptions embedded in the chart, suggesting that Yi Xing synthesized elements of both the gaitian and huntian 渾天 (sphere heaven) models into a complementary framework. This pragmatic and inclusive approach, characteristic of Tang dynasty astronomical thought, gradually evolved into a widely accepted view among later generations of calendar makers.