Zhang Heng (AD 78-139) was born in Xi'e, Nanyang Prefecture (in today's Henan Province). As a young man, Zhang Heng demonstrated an extraordinary quickness in learning, as well as an open mind. He was well versed in almost all types of ancient learning, but did not want to take for granted everything that the books told him. Instead, in AD 94, he left Xi'e to travel through the country to discover for himself how book knowledge fit into the real world.
In AD 111 Zhang Heng was assigned to a position in the government. Only having to perform mediocre routine tasks, Zhang Heng was able to focus his talents and research skills on astronomy after his office work was done. There were two current theories then concerning the nature of the universe. One of them involved a paradigm in which the heavens resembled an umbrella covering over the earth, like an overturned basin. The other drew a different image, and put a round earth at the center of the universe with all the other heavenly bodies moving around it in a celestial sphere. Scrutiny of both theories led Zhang Heng to believe that the second made more sense. He further developed the theory of a round earth at the center of the celestial sphere and made it the most widely accepted model of the universe.
When Emperor An Di heard about his wisdom in astronomy, in AD 115 he put him in charge of astronomical studies at the royal academy. In 117 Zhang Heng in vented a hydro-powered model of an earth-centered universe.
A further great contribution Zhang Heng made to the history of science was what is believed to be the world's very first earthquake sensor that could register earthquakes and locate their origin.
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