Since antiquity, astronomers have studied the movement of the stars, striving to reproduce the appearance of the heavens by means of suitable models. One of the oldest solutions to this problem is globes, in which the constellations are shown on the surface of a metal sphere engraved with the main celestial circles: the horizon, the meridian, the equator, the tropics, and the ecliptic. In armillary spheres, the relationships between these circles and the planetary movements are displayed through a combination of concentric rings revolving around the axis of the sphere. Astrolabes, on the contrary, display the projection of the celestial sphere on a flat surface. But the astrolabe reproduces the circumpolar movement of the stars only for a given latitude. The one exception to this rule is the universal astrolabe, which allows a reading of the sky at all latitudes. The astrolabe gave rise to important astronomical instruments such as the quadrant—a sort of astrolabe folded twice on itself—and the torquetum, in which the circles of the celestial sphere are placed on different planes to facilitate the reading of the heavenly bodies' astronomical coordinates—namely, their altazimuth, equatorial, and ecliptic coordinates.