An armillary sphere is a model of celestial objects used in astronomy to demonstrate the apparent motion of the stars and planets. It consists of a series of rings that represent the celestial equator, ecliptic, and other important circles in the sky. The rings are usually mounted on a central axis, allowing for adjustments to simulate the movement of celestial bodies. Historically, the armillary sphere was an essential tool for astronomers like Claudius Ptolemy and Nicolaus Copernicus. It helped them understand the structure of the universe and the positions of stars and planets. Today, armillary spheres are often used for educational purposes and as decorative items.