The Chinese astronomer, poet, and mathematician Zhang Heng (78–139) achieved fame and honors during his lifetime for inventing the world’s first seismometer, a device that could pinpoint the location of earthquakes from hundreds of miles away. In earthquake-prone China, the bronze seismometer filled a crucial need by alerting authorities so they could send aid to the stricken region.
According to one famous story, the seismometer detected a quake in February 138, even though nobody in the capital city of Luoyang felt anything. Zhang Heng’s skeptics derided the instrument for ringing what they were sure was a false alarm. Then, a few days later, messengers arrived from more than 350 miles away, carrying news of a devastating temblor.
In addition to his seismometer, Zhang Heng was known for his poetry, which is still included in anthologies of Chinese verse, and the three-dimensional working model of the cosmos that he constructed. He also calculated a more accurate estimate of pi than any other contemporary Chinese scholar had produced.
Zhang Heng was born into a distinguished family in Xi’e, a city in central China. At age seventeen he embarked on a tour of China, collecting material he would use in two of his best-known poems. He was named to a minor government position in 103, where he worked on his poetry and began his studies of astronomy and mathematics.
The emperor An (94–125) promoted Zhang Heng in 111, summoned him to the capital city, and appointed him court astronomer, one of the most senior positions in the imperial government, a few years later. The job required Zhang Heng to record weather and earthquakes, compile the calendar, and predict eclipses and other unusual events.
He unveiled the seismometer in 132 and was promoted for his services. Later that decade, however, he became involved in a dispute with the court eunuchs. He eventually left the court in 136, becoming the governor of Hejian province. He retired in 138 and died the next year.
ADDITIONAL FACTS
- Zhang’s seismometer—Houfeng Didong Yi, or the “instrument for inquiring into the wind and the shaking of the earth”—was reconstructed by a Chinese museum in 2005. The device used a pendulum in the middle of an urn that was lined with eight dragons, each pointing outward in a different direction. When the machine detected a quake, the pendulum dislodged a bronze ball that then dropped out of the mouth of one of the dragons and into the mouth of a metal toad—indicating the direction of the earthquake’s epicenter.
- A rare mineral that was discovered by Chinese scientists in a meteorite that crashed to Earth in 1986 was named zhanghengite in honor of the ancient scholar.
- Zhang Heng improved the estimate of the value of pi offered by previous Chinese mathematicians, but was still off. He estimated pi at 3.1724; the actual value is an irrational number beginning with 3.14159.
