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Zhang Heng (張衡, Pinyin: Zhāng Héng, Wade-Giles: Chang Heng) (78 – 139 AD) was an astronomer, mathematician, inventor, geographer, artist, poet, statesman, and literary scholar of the Eastern Han Dynasty in ancient China. He had extensive knowledge of mechanics and gears, applying this knowledge to several of his known inventions. According to historian Joseph Needham, Zhang Heng was noted in his day for being able to "make three wheels rotate as if they were one (neng ling san lun du zhuan ye)".[1] He is best known for his invention of the world's first water-powered armillary sphere to represent astronomical observation,[2] and the world's first seismometer device, which discerned the cardinal direction of earthquakes from incredibly far distances.[3][4][5] In the realm of early science, he is often compared by historians to his contemporaries in ancient Greece and ancient Rome.