Yi Xing

Yi Xing , born Zhang Sui , was a astronomer, mathematician, , and Buddhist monk of the Tang Dynasty . His astronomical celestial globe was the first to feature a clockwork escapement mechanism, the first in a long tradition of Chinese .

Terrestrial-astronomical survey

In the early 8th century, the Tang court put Yi Xing in charge of a . This survey had many purposes. It was established in order to obtain new astronomical data that would aid in the prediction of solar eclipses. 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. 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 historian Joseph Needham states :


When the first escapement came, in +725 , I-Hsing and Liang Ling-tsan arranged for two jacks to strike the hours, standing on the horizon surface of their sphere or globe.



In this apparatus the constant-level tank of the clepsydra provided the major part of the chronometry delivering water or into the scoops of a water-wheel. A minor part was provided by the adjustability of a steelyard or weighbridge which held up each scoop until it was full or nearly so. The essence of the new invention added by I-Hsing and Liang Ling-tsan in +725 was the parallel linkage device which constituted the ancestor of all escapements.


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 Dynasty :


A jade balancing mechanism is erected behind a curtain, holding and resisting the main scoops . Water pours down rotating the wheel . Lower, there is a cog-wheel with 43 . There are also hooks, pins, and interlocking rods one holding another . Each moves the next without reliance on any human force. The fastest wheel turns round each day through 2928 teeth , the slowest one moves by 1 tooth in every 5 days. Such a great difference is there between the speed of the wheels, yet all of them depend on one single driving mechanism. In precision, the engine can be compared with Nature itself . As for the rest, it is much the same as the apparatus made by I-Hsing. But that old design employed mainly bronze and iron, which corroded and rusted so that the machine ceased to be able to move automatically. The modern plan substitutes hard wood for these parts, as beautiful as jade...


Earlier Tang era historical texts of the 9th century have this to say of Yi Xing's work in astronomical instruments in the 8th century :


One was made in the image of the round heavens and on it were shown the lunar mansions in their order, the equator and the degrees of the heavenly circumference. Water, flowing , turned a wheel automatically , rotating it one complete revolution in one day and night. Besides this, there were two rings fitted round the celestial outside, having the sun and moon threaded on them, and there were made to move in circling orbit . Each day as the celestial turned one revolution westwards, the sun made its way one degree eastwards, and the moon 13 and 7/19 degrees . After 29 and a fraction rotations the sun and moon met. After it made 365 rotations the sun accomplished its complete circuit. And they made a wooden casing the surface of which represented the horizon, since the instrument was half sunk in it. This permitted the exact determinations of the times of dawns and dusks, full and new moons, tarrying and hurrying. Moreover there were two wooden jacks standing on the horizon surface, having one a bell and the other a drum in front of it, the bell being struck automatically to indicate the hours, and the drum being beaten automatically to indicate the quarters.



All these motions were brought about within the casing, each depending on wheels and shafts , hooks, pins and interlocking rods , coupling devices and locks checking mutually . Since showed good agreement with the Tao of Heaven, everyone at that time praised its ingenuity. When it was all completed it was called the 'Water-Driven Spherical Bird's-Eye-View Map of the Heavens or 'Celestial Sphere Model Water-Engine' and was set up in front of the Wu Chheng Hall to be seen by the multitude of officials. Candidates in the imperial examinations were asked to write an essay on the new armillary . But not very long afterwards the mechanism of bronze and iron began to corrode and rust, so that the instrument could no longer rotate automatically. It was therefore relegated to the College of All Sages and went out of use.

In his honor

At the Tiantai-Buddhist Guoqing Temple of Tiantai Mountain in Zhejiang province, there is a Chinese pagoda erected directly outside of the temple, known as the Memorial Pagoda of Monk Yi Xing. His tomb is also located at Tiantai Mountain.