Zhang Heng Invents the Seismoscope

Zhang Heng (AD 78–139), courtesy name Pingzi, from Xi'e in Nanyang (present-day Nanyang, Henan), was an outstanding scientist, writer and inventor of the Eastern Han. He twice served as Grand Astrologer, in charge of astronomy and the calendar. In astronomy he put forward the “celestial sphere theory,” likening the heavens to an eggshell and the earth to the yolk, and built a water-driven armillary sphere that vividly demonstrated the motion of the heavens. He also recorded about 2,500 stars and correctly explained the cause of lunar eclipses. Earthquakes were frequent in the Eastern Han, and to track them Zhang Heng built the “wind-and-earthquake instrument” in AD 132. According to the Book of the Later Han, “Biography of Zhang Heng,” it was cast of fine bronze, shaped like a wine vessel, with eight dragons outside facing the eight directions, each holding a bronze ball in its mouth, with an open-mouthed toad beneath each. Inside were a central pillar and mechanisms: when an earthquake struck in some direction, the dragon facing that way released its ball into the toad's mouth, showing the direction of the quake. It is recorded that once the western dragon released its ball while Luoyang felt no tremor, and people doubted it; a few days later a messenger from Longxi reported that an earthquake had indeed occurred, and all were convinced. The instrument is the world's earliest seismoscope, but the original was lost long ago, and those displayed in museums today are reconstructions based on texts. Zhang Heng was also a gifted writer of rhapsodies, author of the Rhapsody on the Two Capitals. In his honor, a crater on the far side of the Moon and an asteroid are named after him.