Zhang Heng was born in 78 CE in the town of Xi’e, in what is now Henan Province, in Han Dynasty China. At 17, he left home to study literature and train to be a writer. By his late 20s, Zhang had become a skilled mathematician and was called to the court of Emperor An-ti, who, in 115 CE, made him Chief Astrologer.
Zhang lived at a time of rapid advances in science. In addition to his astronomical work, he devised a water-powered armillary sphere (a model of the celestial objects) and invented the world’s first seismometer, which was ridiculed until, in 138 CE, it successfully recorded an earthquake 250 miles (400 km) away. He also invented the first odometer to measure distances traveled in vehicles, and a nonmagnetic, south-pointing compass in the form of a chariot. Zhang was a distinguished poet, whose works give us vivid insights into the cultural life of his day.