This Ptolemaic or geocentric armillary sphere was used to measure the position and the apparent movement of stars around the Earth. When the centre is the Sun, the sphere is called Copernican or heliocentric.

It is made of a series of concentric rings (armilla in Latin), one inside the other: the main meridian ring that joins the north and south poles; the equinoctial and solstitial colures; the celestial horizon; the ecliptic with the solar houses divided into astronomic months (zodiac); the celestial equator; the polar caps and the tropics of Cancer and Capricorn. In the centre, is the sphere that represents the Earth.