Zhang Heng (張衡) (AD 78–AD 139), also spelled as Chang Heng, was an astronomer, mathematician, inventor, artist and literary scholar in the Eastern Han Dynasty of China.

Born in what is now Nanyang County Henan, he was an accomplished 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 Chinese calendar to bring it inline with the seasons.

In 132, Zhang invented the first seismograph 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 líng xiàn (《靈憲》 a summary of astronomical theories at the time), he approximated pi as 730/232 (or approx 3.1466). In one of his formulae for spherical volume calculation, he also used pi as square root of 10 (or approx 3.162).