In the early 8th century, the Tang court put Yi Xing in charge of a terrestrial-astronomical survey. This survey had many purposes. It was established in order to obtain new astronomical data that would aid in the prediction of solar eclipses. The survey was also initiated so that flaws in the calendar system could be corrected and a new, updated calendar installed in its place. The survey was also essential in determining the measurement of the length of meridian arc-although Yi Xing, who did not know the Earth was spherical, did not conceptualize his measurements in these terms. This would resolve the confusion created by the earlier practice of using the difference between shadow lengths of the sun observed at the same time at two places to determine the ground distance between them. Yi Xing had thirteen test sites established throughout the empire, extending from Jiaozhou in Vietnam — at latitude17°N — to the region immediately south of Lake Baikal — latitude 50°N. There were three observations done for each site, one for the height of polaris, one for the shadow lengths of summer, and one for the shadow lengths of winter. The latitudes were deciphered from this data, while the Tang calculation for the length of one degree of meridian was fairly accurate compared to modern calculations. Yi Xing understood the variations in the length of a degree of meridian, and criticized earlier scholars who permanently fixed an estimate for shadow lengths for the duration of the entire year.
Yi Xing was famed for his genius, known to have calculated the number of possible positions on a go board game. He, along with his associate, the mechanical engineer and politician Liang Lingzan, is best known for applying the earliest-known escapement mechanism to a water-powered celestial globe. However, Yi Xing's mechanical genius and achievements were built upon the knowledge and efforts of previous Chinese mechanical engineers, such as the statesman and master of gear systems Zhang Heng of the Han Dynasty, the mechanical engineer Ma Jun of the Three Kingdoms, and the Daoist Li Lan of the Southern and Northern Dynasties period. It was the earlier Chinese inventor Zhang Heng during the Han dynasty who was the first to apply hydraulic power in mechanically-driving and rotating his equatorial armillary sphere. The arrangement followed the model of a water-wheel using the drip of a clepsydra, which ultimately exerted force on a lug to rotate toothed-gears on a polar-axis shaft. With this, the slow computational movement rotated the armillary sphere according to the recorded movements of the planets and stars. Yi Xing also owed much to the scholarly followers of Ma Jun, who had employed horizontal jack-wheels and other mechanical toys worked by waterwheels. The Daoist Li Lan was an expert at working with water clocks, creating steelyard balances for weighing water that was used in the tank of the clepsydra, providing more inspiration for Yi Xing. Like the earlier water-power employed by Zhang Heng and the later escapement mechanism in the astronomical clock tower engineered and erected by Su Song, Yi Xing's celestial globe employed water-power in order for it to rotate and function properly. The British biochemist, historian, and sinologist Joseph Needham states : In regards to mercury instead of water, the first to apply liquid mercury for motive power of an armillary sphere was Zhang Sixun in 979 AD. During his age, the Song dynasty era historical text of the Song Shi mentions Yi Xing and the reason why his armillary sphere did not survive the ages after the Tang : Earlier Tang era historical texts of the 9th century have this to say of Yi Xing's work inastronomical instruments in the 8th century :
Buddhist scholarship
Yi Xing wrote a commentary on the Mahavairocana Tantra. This work had a strong influence on the Japanese monk Kūkai and was key in his establishment of Shingon Buddhism.
