Ptolemaic armillary sphere

An armillary sphere is a model of objects in the sky (the 'celestial sphere') that shows the movement of the stars around the Earth or the Sun. As it has the Earth at the center, this armillary sphere is Ptolemaic. This system is based on the geocentric description of the universe - with Earth at the centre and the Sun, Moon, stars and planets orbiting Earth.

Its six rings represent lines of celestial longitude and latitude, the meridians, the equator, the equinoctial colure and the solstitial colure, the two tropics, the North Pole and the South Pole. Everything is framed within a large circle that represents the horizon. This external ring is engraved with the months of the year and the signs of the zodiac.