Zhang Heng
张恒是谁
Zhang Heng (张衡) (78AD—139AD) was an astronomer, mathematician, artist and literary scholar（文学学者）in the Eastern Han Dynasty of China.
Born in today’s Nangyang County, Henan, he was a good writer at age 12. At the age of 16, he left home to pursue(从事) his studying the capital city. He spent at least 10 years of his youth in literary studies and writing. He published several well-recognized literary writings. He switched to(转向) astronomy after age 30.
In the year 123 he corrected the calendar to bring it into line with the seasons.
In 132 Zhang Heng invented the first seismograph(地震仪) for measuring earthquakes. His device was in the shape of a cylinder(圆柱体) 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 head. When an earthquake occurred, a ball fell out of the dragon’s mouth into a frog’s mouth, making a noise. He also invented the odometer(里程表).
Zhang Heng was the first person in China to construct a rotating celestial globe.
In one of his publications he also proposed π= 730/232 (or about 3.1466) .