Zhang Heng, Mathematician, Astronomer and Geographer of the Eastern Han Dynasty

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 Heng was the first person in China to construct a rotating celestial globe and also the inventor of a primitive seismograph for measuring earthquakes and the inventor of the odometer, or "mileage cart". His use of the square root of 10 in the searching for pi is one of the earliest approximations known. Beyond that, he excelled in writing and visual arts. In fact, he was also considered one of the four great painters of his era.

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 first seismoscope. His device 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, a ball fell out of a dragon's mouth into a frog's mouth, making a noise.

Zhang Heng was a proponent of the Hun theory, which stated that the earth is inside the sky, just like the yolk is inside an egg. He wrote, "The sky is like a hen's egg, and is as round as a crossbow pellet; the Earth is like the yolk of the egg, lying alone at the center. The sky is large and the Earth small."

He also drew a detailed map of the heaven. His chart showed 124 constellations consisting of a total of 2,500 stars, 320 of which were 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 maps of the sky. When his charts were complete, he hired craftsmen to build what may have been the word's oldest three-dimensional models of the heaven. The first globes were made out of bamboo strips, but eventually he commissioned a bronze version that was almost five meters in circumference. This in itself would have been a singular 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 winter solstice to the next. His invention is known as the water-powered celestial sphere.

By adding a few additional gears, Zhang Heng was able to drive a pillar that demonstrated the waxing and waning of the moon.