Armillary sphere. This astronomical device shows the circles of the celestial sphere surrounding the Earth (centre). The metal circles surrounding the Earth include the plane of the ecliptic, and the celestial equivalents of the equator, North and South Poles, the Tropics of Capricorn and Cancer, and the meridians and parallels. Small models representing the Sun and Moon are also seen (Moon at lower left, Sun at upper right). The signs of the zodiac are seen on the ring to which the Sun is attached. An armillary sphere is used to demonstrate the motion of the stars around the Earth.