<p>This study examines a remarkable passage in the <i>Treatise on astronomy</i> 天文志 (<i>Tianwen zhi</i>) of the <i>New book of Tang</i> 新唐書 (<i>Xin tangshu</i>), 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 <i>gaitian</i> 蓋天 (canopy heaven) model and is structured around three concentric circles delineating the twenty-eight lunar lodges 二十八宿 (<i>ershiba xiu</i>) 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 (<i>huangdao chashu</i>), 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 <i>gaitian</i> and <i>huntian </i>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.</p>