Yi Xing (, 683–727), born Zhang Sui (), was a Chinese astronomer, Buddhist monk, inventor, mathematician, mechanical engineer, and philosopher during the Tang dynasty. His astronomical celestial globe featured a liquid-driven escapement, the first in a long tradition of Chinese astronomical clockworks. Science and technology Astrogeodetic survey In the early 8th century, the Tang court put Yi Xing in charge of an astrogeodetic 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 arc measurement, i.e., 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 latitude 17°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 determined 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. The escapement and celestial globe Yi Xing was famed for his genius, known to have calculated the number of possible positions on a go board game (though without a symbol for zero as he had difficulties expressing the number). 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 achievements were built upon the knowledge and efforts of previous Chinese mechanical engineers, such as the statesman and master of gear systems Zhang Heng (78–139) of the Han Dynasty, the mechanical engineer