I-Hsing

(c. 681–c. 727) Chinese mathematician and astronomer

I-Hsing was a Buddhist monk around whom many legends have grown. Only a small portion of his work has survived so it is difficult to appreciate it in detail. There is, however, no reason to doubt his involvement in two major astronomical achievements. During the period 723–26, in collaboration with the Astronomer Royal, Nankung Yueh, expeditions were organized to measure, astronomically, the length of a meridional line. Over a distance of 1553 miles (2500km) along this line, simultaneous measurements of the Sun's solstitial shadow were made at nine stations. The estimated length of a degree, on the basis of their measurements, was far too large and it must be supposed that some systematic error in the method of observation was taking place. However, when it is appreciated that research expeditions to determine the length of a meridional degree were not organized in Europe until the 17th century, the amazing nature of I-Hsing's work can be appreciated. He also probably anticipated Su Sung in the use of an escapement in an astronomical clock. It was described in a 13th-century encyclopedia: “Water, flowing into scoops, turned a wheel automatically, rotating it one complete revolution in one day and one night.” This turned various rings representing the motion of the celestial bodies. It was soon reported to be corroded, relegated to a museum, and to have fallen into disuse.