The life and works of Yu Xi are described in his biography found in the Book of Jin, the official history of the Jin dynasty. He was born in Yuyao, Guiji. His father Yu Cha was a military commander and his younger brother Yu Yu was likewise a scholar and writer. During the reign of Emperor Min of Jin he obtained a low-level position in the administration of the governor of Guiji commandery. He declined a series of nominations and promotions thereafter, including a teaching position at the imperial university in 325 AD, an appointment at the imperial court in 333 AD, and the post of cavalier attendant-in-ordinary in 335 AD.
Works
In 336 AD Yu Xi wrote the An Tian Lun. In it he described the precession of the equinoxes. He observed that the position of the sun during the winter solstice had drifted roughly one degree over the course of fifty years relative to the position of the stars. This was the same discovery made earlier by the ancient Greek astronomer Hipparchus, who found that the measurements for either the sun's path around the ecliptic to the vernal equinox or the sun's relative position to the stars were not equal in length. Yu Xi wrote a critical analysis of the huntiantheory of the celestial sphere, arguing that the heavens surrounding the earthwere infinite and motionless. He advanced the idea that the shape of the earth was either square or round, but that it had to correspond to the shape of the heavens enveloping it. The huntian theory, as mentioned by Western Han dynasty astronomer Luoxia Hong and fully described by the Eastern Han dynasty polymath scientist and statesman Zhang Heng, insisted that the heavens were spherical and that the earth was like an egg yolk at its center. Yu Xi's ideas about the infinity of outer space seem to echo Zhang's ideas of endless space even beyond the celestial sphere. Although mainstream Chinese science before European influence in the 17th century surmised that the Earth was flat and square-shaped, some scholars, such as Song dynasty mathematician Li Ye, proposed the idea that it was spherical like the heavens. The acceptance of a spherical earth can be seen in the astronomical and geographical treatise Gezhicao written in 1648 by Xiong Mingyu. It rejects the square-earth theory and, with clear European influence, explains that ships are capable of circumnavigating the globe. However, it explained this using classical Chinese phrases, such as the earth being as round as a crossbow bullet, a phrase Zhang Heng had previously used to describe the shape of both the sun and moon. Ultimately, though, it was the European Jesuits in China of the 17th century that dispelled the Chinese theory of a flat earth, convincing the Chinese to adopt the spherical earth theory established by the ancient Greeks Anaxagoras, Philolaus, Aristotle, and Eratosthenes. Yu Xi is known to have written commentaries on the various Chinese classics. His commentaries and notes were mostly lost before the Tang dynasty, but the fragments preserved in other texts were collected in a single compendium by Qing-dynasty scholar Ma Guohan.
