Eryi ming buzhu 二儀銘補注

Eryi ming buzhu 二儀銘補注 "Additional notes on the inscriptions of the two instruments" is a text on astronomy written during the early Qing period 清 (1644-1911) by Mei Wending 梅文鼎 (1633-1721). The Commentary was finished in 1710. It is included in the series Mei Hu'an Xiansheng lisuan quanshu 梅勿庵先生曆算全書, Yihai zhuchen 藝海珠塵竹集 and Congshu jicheng chubian 叢書集成初編.

The armillary dial (yangyi 仰儀) and simplified armillary sphere (jianyi 簡儀) were two astronomical instruments invented by Guo Shoujing 郭守敬 (1231-1316) during the Yuan period 元 (1279-1368). The armillary dial resembled a shallow, horizontally placed bowl. It used the pinhole imaging principle to project celestial objects, preventing the intense sunlight from directly harming the observer's eyes. This instrument was particularly useful for observing the Sun's shadow, solar eclipses, and the Moon's position. The simplified armillary sphere was a highly refined version of the complex armillary spheres used during the Tang 唐 (618-907) and Song 宋 (960-1279) periods. Guo greatly simplified its structure while maintaining precision, making it an ingenious instrument. Even Jesuit missionaries visiting China were deeply impressed by its sophistication.

The Astronomical Treatise (48-49 Tianwen zhi 天文志) of the official dynastic history Yuanshi 元史 records an inscription for the armillary dial but does not explain the instrument. Conversely, it explains the simplified armillary sphere but does not include the inscription.

In 1710, a friend sent the inscriptions of both instruments to Mei Wending, asking him to interpret their meaning. Mei compared them with the descriptions in the Yuanshi and discovered discrepancies, finding that the inscriptions he received were more accurate and superior to those recorded in official history. This led Mei to wonder whether there had been an official revision after Guo Shoujing's original inscription. Inspired by this question, he wrote commentaries on the two inscriptions.

Mei's interpretations are exceptionally clear and accessible, making it much easier to understand the meaning of the inscriptions. While explaining the text, Mei also takes the opportunity to reiterate his belief in the unity of the Spherical-Heaven (huntian shuo 渾天說) and the Canopy-Heaven Theory (gaitian shuo 蓋天說). At the end of the book, he discusses the practical advantages of the simplified armillary sphere, emphasising its efficiency and usability compared to the more complex traditional armillary sphere (hunyi 渾儀). Additionally, he refutes the claim that Guo Shoujing secretly borrowed from the Islamic calendar system (huili 回曆), defending Guo's originality and innovation.