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An armillary sphere or armilla is an instrument used to measure the positions of celestial bodies in ancient China. It is closely related to an ancient Chinese cosmological theory which argued that the sky is a sphere. An armilla uses concentric circles to simulate the spherical sky. It is used to identify the positions of celestial bodies and to draw astronomical charts. The first armilla appeared in the Warring State period. Luoxia Hong during Han Dynasty was the first astronomer to use the armillary sphere to observe the motions of the five planets celestial objects.