Zhang Heng(78~139), a native of Nanyang in central China's Henan Province, was a mathematician, astronomer and geographer of the Eastern Han Dynasty(25~220).

    Zhang's greatest contributions were in the field of astronomy. In 123 he corrected the calendar to bring it into line with the seasons. In 132 he invented the firstseism scope. It was in the shape of a cylinder with eight dragon heads around the top, each with a ball in its mouth. Around the bottom were eight frogs, each directly under a dragon's head. When an earthquake occurred, ball fell out of a dragon's mouth into a frog's mouth, making a noise.

    Zhang Heng was a supporter of the Hun, the ancient theory, which stated that the earth is inside an egg. He wrote, “The sky is like a hen's egg and the Earth is like the yolk of the egg,  lying alone at the center. The sky is large and the Earth is small. ”

    He also drew a detailed map of the heaven. His chart showed 124 constellations (星群) consisting of a total of 2, 500 stars,  320 ofwhichwere bright stars with names. “This is not including those observed by sailors, ” he wrote, “Of the small stars,  there are eleven thousand five hundred and twenty. ”

    However, Zhang Heng was not satisfied with his paper map of the sky. When his charts were completed, he hired craftsmen to build what may have been the world's oldest three-dimensional models of the heaven. The first globes were made out of bamboo strips, but eventually he had a globe made out of bronze, which was almost five meters in circumference. This in itself would have been a remarkable accomplishment.

    But Zhang Heng took it one step further. He used the power of water to make his globe complete one rotation every year, showing how the positions of the stars changed from one season to the next.