An armillary sphere is a model used to represent the celestial sphere and the movements of celestial bodies. It consists of a series of rings that represent the Earth's equator, ecliptic, and other important circles in astronomy. These rings are usually made of metal and are mounted on a central axis, allowing for adjustments to demonstrate the positions of stars and planets. Historically, the armillary sphere was an important tool for astronomers and navigators, particularly during the Renaissance. It helped in understanding the relationship between the Earth and the heavens, serving both educational and practical purposes in the study of astronomy.