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:Armillary sphere

An armillary sphere is basically a skeletal celestial sphere with a model of the Earth or, later, of the Sun placed in the center. It is useful as a teaching tool and as an analog computer for solving various astronomical problems to a crude degree of accuracy.
 It was invented separately first in ancient China during the 4th century BC and ancient Greece during the 3rd century BC, with later uses in the Islamic world and Medieval Europe.