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Zhang Heng

province celestial earthquake official

(78–139) Chinese astronomer and geophysicist: invented the earthquake seismograph.

Zhang was born in Nanyang, Henan Province, during the Han Dynasty. He was Imperial Historian and official astronomer.

Zhang recognized that the source of the Moon’s illumination was sunlight and that lunar eclipses were caused by the Earth’s shadow falling upon it. He devised a water-driven celestial globe, which revolved in correspondence with the diurnal motion of the celestial sphere. In mathematics, he calculated p as 365/116 (about 3.1466), a substantial improvement on the hitherto accepted Chinese value of 3. Perhaps his best remembered contribution, however, was to geophysics: in 132 he invented an early seismograph. This was to help him locate and record earthquakes, one of his official duties as Imperial Historian. It was a bronze device almost 2m in diameter, containing a mechanism of pendulums and levers, with eight dragon figures arranged around its circumference. Strong seismic tremors caused a metal ball to be released from the mouth of the dragon facing the direction of the shock wave. It is known that the device registered an earthquake in Gansu Province in 138.

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