Answer
The armillary sphere is a model of the celestial sphere, consisting of a number of concentric metal rings representing celestial meridians, parallels, and ecliptic. It is believed to have been invented by Hipparchus of Nicaea, a Greek astronomer and mathematician, in the 2nd century BC. The armillary sphere was used by astronomers to study the motions of the stars and planets, and to make calculations about their positions.