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Zhang Heng

Eastern Han Scientist and Man of Letters, Inventor of the Seismoscope

Those Who Set the World Down in Writing · Scholars & Explorers

The SeismoscopeThe Armillary SphereAstronomy and the Calendar

Who they are

Zhang Heng (78–139) was an Eastern Han scientist, inventor, and man of letters, who in astronomy improved the "armillary sphere," driven by water power to simulate the motion of the heavens, and systematically expounded the "sphere-heaven" theory of cosmic structure; his better-known invention was the "seismoscope," the earliest known earthquake-detecting device in human history, shaped like a wine vessel with dragon heads at eight compass directions around it, each dragon’s mouth holding a bronze ball, so that when an earthquake occurred in some direction the ball in the corresponding direction would fall into the mouth of a toad below, indicating the rough direction of the earthquake — according to the Book of the Later Han this device once successfully detected a distant earthquake felt nowhere locally, and the exact principle of its internal mechanism, the original long lost, is still a subject of ongoing reconstruction by historians of science and technology. He was also an outstanding man of letters, his chief work the Rhapsody on the Two Capitals ornate in diction, one of the important representative works of the great Han rhapsody.

Primary sourcesFan Ye, Book of the Later Han, "Biography of Zhang Heng"

Key stories

The Seismoscope

The seismoscope Zhang Heng designed and made was shaped like a wine vessel, set around with eight dragon heads corresponding to the eight directions, each dragon’s mouth holding a bronze ball, with a precise mechanism inside, so that when the tiny tremor caused by an earthquake occurred in some direction, the bronze ball in the mouth of the corresponding dragon would drop loose into the mouth of a toad below, sounding an alarm — according to the Book of the Later Han, in 136 this device successfully detected an earthquake in the Longxi region felt nowhere in Luoyang at the time, and a messenger arriving days later confirmed the device’s accuracy.

The Sphere-Heaven Theory and the Armillary Sphere

Zhang Heng systematically expounded the "sphere-heaven" cosmology, holding that heaven and earth are shaped like an egg, heaven wrapping the earth like the shell around the yolk, the sun, moon, and stars attached to a rotating celestial sphere — a theory closer to the modern understanding of a spherical universe than the earlier "canopy-heaven" theory (heaven round, earth square); on this basis he designed and made a water-powered "armillary sphere" that could automatically demonstrate the paths of the heavenly bodies, the ingenuity of its mechanical design advanced by the standards of the contemporary world.

Version differences

The exact principle of the internal mechanism of the seismoscope, the original long lost and only the textual description of the Book of the Later Han surviving, has prompted modern scholars to propose several different reconstruction schemes; the specific design details are still an ongoing subject of study in the history of science and technology, with no settled conclusion.

Relationships

Ally

🌌Ptolemy　The two lived in close periods, each independently developing his own theoretical system of cosmic structure in East and West

Echoes today

Below are how modern works borrow or reinterpret this name or story — not the original material. The two differ, so keep them apart.

A historical precursor of the modern seismographZhang Heng’s seismoscope is broadly held to be the earliest known conception of an earthquake-detecting device in human history, and though its principle differs fundamentally from the modern seismograph, it is still often cited by popular-science works as a representative case of "ancient Chinese scientific and technological achievement."

Appears in

The Roman Peace and the Two Ends of the Silk RoadThe Han Woman Historian and Inventor

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