Zhang Heng

Zhang Heng (張衡) (78 AD - 139 AD) was an astronomer, mathematician, artist and literature scholar in the Eastern Han Dynasty of China.

Born in today Nanyang County Henan, he was a good writer at age 12. At age 16, he left home to pursue his study in the capital cities. He spent at least 10 years of his youth in literature studies and writing. He published several well recognized literary writings. He switched to astronomy after age 30. He became a government official at age 38. He took on several positions since then (including 郎中, 太史令, 尚書). When he was a government minister, he cleaned up some corruption in the local government.

In the year 123 he corrected the calendar to bring it into line with the seasons.

In 132, Zhang invented the first seismoscope for measuring earthquakes. He theorized that the universe was like an egg with the stars on the shell and the earth as the yolk. Zhang Heng was the first person in China to construct a rotating celestial globe. He also invented the odometer.

In one of his publications ling2 xian4 (《靈憲》 a summary of astronomical theories at the time), he approximated pi π = 730/232 (or approx 3.1466). In one of his formula for spherical volume calculation, he also used pi as square root of 10 (or approx 3.162).