At the Jianguomen Ancient Observatory in Beijing is installed a simulated armillary sphere, an astronomical model with solid rings,
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 all circles of a single sphere used to display relationships among the principal celestial bodies. This is one of the two greatest inventions
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of Zhang Heng,  the great astronomer of the  Eastern Han Dynasty,  the other being  the  seismograph,  a remarkable instrument which 
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indicated the region of an earthquake.