Zhang Heng (78-139 CE) was a Chinese polymath of the Han dynasty who worked as an astronomer, mathematician, engineer, cartographer, and poet at the imperial court. He is best known for inventing the world's first seismoscope, a device that could detect the direction of distant earthquakes using a system of dropping balls, and for building an early water-powered armillary sphere to model the motion of celestial bodies. His astronomical and mathematical writings, including an improved estimate of the value of pi, place him among the most accomplished scientific figures of the ancient world, and he is honored in China today as one of its greatest historical scientists.
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