Yi Xing

Yi Xing (Chinese: 一行; pinyin: Yī Xíng; Wade–Giles: I-Hsing, 683–727), born Zhang Sui (Chinese: 張遂), was a Chinese astronomer, mathematician, mechanical engineer and Buddhist monk of the Tang dynasty (618–907). His astronomical celestial globe featured a clockwork escapement mechanism, the first in a long tradition of Chinese astronomical clockworks.

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. 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. This was the same process used by the ancient Greek Eratosthenes (276–196 BC).

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 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.

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